(Scheme 36, bottom reaction), but changing the solvent (benzene, toluene and cyclohexane), concentration of the zirconocene-PMe₃ adduct (0.1 M, 0.3 M), reagent ratio (complex 6 : Py-F₅ = 1:1, 3:1, 5:1), lowering the temperature to 0 °C, and using pre-cooled solvent and reagents (–40 °C) had only little influence on the reaction outcome. At –40 °C the system was unreactive.

To shed more light on the mechanism of this reaction, a control experiment on the reactivity of trimethylphosphine towards pentafluoropyridine was carried out. Addition of an equimolar amount of PMe₃ to a solution of Py-F₅ in C₆D₆ or THF-d₈ at 20 °C triggered the formation of Py-F₄, F₂PMe₃, the 4-methylenetetrafluoropyridine-substituted difluorophosphorane 9 and a phosphonium salt 13, along with unreacted PMe₃ (Scheme 37). The red solid was not observed.

<figure><img src="image_125.png" /><figcaption>**Scheme 37.** Reactivity test of Py-F₅ (1 equiv.) with PMe₃ (1–3 equiv.) in C₆D₆ or THF-d₈ at 20 °C.</figcaption></figure>

The spontaneous reaction of PMe₃ with Py-F₅ casts doubt on a Zr-mediated C–F bond activation for this substrate. The treatment of the mixture produced by the 1:1 PMe₃/Py-F₅ reaction in C₆D₆ with the cyclohexyne complex 6, according to Scheme 38, resulted in the consumption of 8 and 9 and the appearance of the zirconocene-fluoride 7 after 3 hours at 20 °C, as evidenced by the ¹⁹F NMR spectrum. Three equivalents of 6 (vs. Py-F₅) were needed to consume all 8 and 9 to afford 7 rather selectively.

<figure><img src="image_126.png" /><figcaption>**Scheme 38.** Reaction of 6 (3 equiv.) with the compounds issued by the reaction of PMe₃ (1 equiv.) and Py-F₅ (1 equiv.) in C₆D₆ at 20 °C.</figcaption></figure>

This outcome strongly suggests that the C–F bond activation event observed in the reaction of 6 with Py-F₅ was initiated by dissociated PMe₃ reacting with the heteroaromatic to give 8 and 9, which subsequently reacted with 6 to provide 7. This is consistent with the dissociation energy values of the bonds involved: the Zr–F bond (BDE = 623 ± 63 kJ/mol) is indeed stronger than the P–F (BDE ≤ 405 kJ/mol) and C–F bonds (BDE = 513 kJ/mol [28]). Considering the nucleophilic attack of the phosphine on the fluoroheteroaromatic as a crucial mechanistic step in the reaction outlined in Scheme 37, it was expected that the use of a less electron-poor pyridine (e.g. with a reduced degree of fluorination) could circumvent the undesired reactivity of PMe₃ with the fluoroheteroarene. Therefore, Py-F₄, 2,4,6-trifluoropyridine (Py-F₃) and 2,6-difluoropyridine (Py-F₂) were identified as suitable candidates for this investigation.
When **6** was reacted with an equimolar amount of **Py-F₄**, complete consumption of the reagents was observed after 48 hours at 50 °C in C₆D₆. The main product was identified as the C–H activation product **10**. Multinuclear and 2D-NMR analyses further proved the formation of **7**.

<figure><img src="image_128.png" /><figcaption>**Scheme 39.** Synthesis of the C–H activation product **10** by reaction of **6** (1 equiv.) and **Py-F₄** (1 equiv.) in C₆D₆ at 50 °C for 48 hours.</figcaption></figure>

Complex **10** is believed to result from 1,2-C–H addition over the Zr–(η²-C≡C) bond in **6**, a hypothesis further supported by the absence of deuterium incorporation when the reaction was conducted in deuterated benzene. Further evidence for C–H activation was collected through NMR analyses of the reaction mixture after treatment with iodinolysis.

In the case of **Py-F₃** and **Py-F₂** the targeted 1,3-C–F bond addition over the Zr-η²-cycloalkyne motif took place, affording **11** and **12**, respectively (Scheme 40). Various solvents (THF, chlorobenzene, toluene, benzene, diethyl ether, cyclohexane) and temperatures were tested to find the optimal conditions for these reactions. The system exhibited the highest yield (71% isolated yield of **11**) and selectivity when cyclohexane was used as solvent at 40 °C.

<figure><img src="image_129.png" /><figcaption>**Scheme 40.** Synthesis of **11** and **12** by reaction of **6** (1 equiv.) with **Py-F₃** or **Py-F₂** (3–5 equiv.) in C₆D₁₂ at 40 °C.</figcaption></figure>

The addition product **11** is characterised by a singlet at δ(^19F) = –35.4 ppm, corresponding to the Zr–F bond, a doublet at δ(^19F) = –60.7 ppm (⁴J<sub>FF</sub> = 24.0 Hz) and a doublet of doublets of doublets at δ(^19F) = –94.8 ppm (⁴J<sub>FF</sub> = 24.0, ³J<sub>FH</sub> = 9.5 Hz, ³J<sub>FH</sub> = 7.5 Hz), associated with the *ortho*- and *para*-fluorine of the pyridine, respectively (Figure 15). Additionally, the formation of **11** was evidenced by the generation of 2-(2-iodocyclohexen-1-yl)-4,6-difluoropyridine, characterised by GC(EI)-MS and NMR analyses, after iodinolysis of the complex. Crystals suitable for a single crystal X-ray diffraction (sc-XRD) analysis were grown after vapour diffusion of tetramethylsilane, as an antisolvent, into a concentrated toluene solution of **11**. Its molecular structure is shown in Figure 16.

The Zr atom lies in a distorted trigonal bipyramid environment, with apical sites occupied by the nitrogen of the pyridine ligand and the fluoride. The former belongs to a 1-zirconia-2-aza-cyclopentadiene motif that connects it to the alkenyl carbon found in the equatorial plane, forcing the F-Zr-N array to deviate significantly from linearity [F1-Zr1-N1 148.46(6)°] and imposing a tilt angle for the Cp-Zr-Cp moiety of 135.8°. Similar deviations from the ideal trigonal bipyramidal geometry are found in other crystallographically characterised five-coordinated, Cp-supported 1-zirconia-2-aza-cyclopentadienes [29-34]. Compared to structurally related species [29-35], the angles measured
When **6** was reacted with an equimolar amount of **Py-F₄**, complete consumption of the reagents was observed after 48 hours at 50 °C in C₆D₆. The main product was identified as the C–H activation product **10**. Multinuclear and 2D-NMR analyses further proved the formation of **7**.

<figure><img src="image_127.png" /><figcaption>**Scheme 39.** Synthesis of the C–H activation product **10** by reaction of **6** (1 equiv.) and **Py-F₄** (1 equiv.) in C₆D₆ at 50 °C for 48 hours.</figcaption></figure>

Complex **10** is believed to result from 1,2-C–H addition over the Zr–(η²-C≡C) bond in **6**, a hypothesis further supported by the absence of deuterium incorporation when the reaction was conducted in deuterated benzene. Further evidence for C–H activation was collected through NMR analyses of the reaction mixture after treatment with iodinolysis.

In the case of **Py-F₃** and **Py-F₂** the targeted 1,3-C–F bond addition over the Zr-η²-cycloalkyne motif took place, affording **11** and **12**, respectively (Scheme 40). Various solvents (THF, chlorobenzene, toluene, benzene, diethyl ether, cyclohexane) and temperatures were tested to find the optimal conditions for these reactions. The system exhibited the highest yield (71% isolated yield of **11**) and selectivity when cyclohexane was used as solvent at 40 °C.

<figure><img src="image_128.png" /><figcaption>**Scheme 40.** Synthesis of **11** and **12** by reaction of **6** (1 equiv.) with **Py-F₃** or **Py-F₂** (3–5 equiv.) in C₆D₁₂ at 40 °C.</figcaption></figure>

The addition product **11** is characterised by a singlet at δ(^19F) = –35.4 ppm, corresponding to the Zr–F bond, a doublet at δ(^19F) = –60.7 ppm (⁴J<sub>FF</sub> = 24.0 Hz) and a doublet of doublets of doublets at δ(^19F) = –94.8 ppm (⁴J<sub>FF</sub> = 24.0, ³J<sub>FH</sub> = 9.5 Hz, ³J<sub>FH</sub> = 7.5 Hz), associated with the *ortho*- and *para*-fluorine of the pyridine, respectively (Figure 15). Additionally, the formation of **11** was evidenced by the generation of 2-(2-iodocyclohexen-1-yl)-4,6-difluoropyridine, characterised by GC(EI)-MS and NMR analyses, after iodinolysis of the complex. Crystals suitable for a single crystal X-ray diffraction (sc-XRD) analysis were grown after vapour diffusion of tetramethylsilane, as an antisolvent, into a concentrated toluene solution of **11**. Its molecular structure is shown in Figure 16.

The Zr atom lies in a distorted trigonal bipyramid environment, with apical sites occupied by the nitrogen of the pyridine ligand and the fluoride. The former belongs to a 1-zirconia-2-aza-cyclopentadiene motif that connects it to the alkenyl carbon found in the equatorial plane, forcing the F-Zr-N array to deviate significantly from linearity [F1-Zr1-N1 148.46(6)°] and imposing a tilt angle for the Cp-Zr-Cp moiety of 135.8°. Similar deviations from the ideal trigonal bipyramidal geometry are found in other crystallographically characterised five-coordinated, Cp-supported 1-zirconia-2-aza-cyclopentadienes [29-34]. Compared to structurally related species [29-35], the angles measured
within the five-membered ring as well as the bond lengths of the conjugated part show no significant irregularity. However, the Zr–C bond is rather short and the Zr–N, rather long. In the considered family of compounds selected for comparison, the former varies slightly in length, while the Zr–N distance fluctuates depending on the L or X character of the nitrogen ligand and the N-substituent. In the present case, we explain the long Zr–N bond by the presence of the fluoride in *trans* relationship, coupled with the electron-poor properties of the difluoropyridine ring. The elongation of the Zr–N bond mechanically impacts the length of the Zr–C bond due to the rigidity of the 1-zirconia-2-azacyclopentadiene core. An additional observation is that the N1-C11-F3 angle is quite acute [114.7(2)°], presumably as a result of a long-range interaction of the fluorine atom with the electropositive Zr atom.

<figure><img src="image_130.png" /><figcaption>**Figure 15.** <sup>19</sup>F NMR (377 MHz, C<sub>6</sub>D<sub>6</sub>) spectrum of the crude reaction mixture of the reaction between the zirconocene-PMe<sub>3</sub> adduct **6** (1 equiv.) with Py-F<sub>3</sub> (5 equiv.) in cyclohexane at 40 °C for 42 hours, dried and dissolved in C<sub>6</sub>D<sub>6</sub>. Magnifications of the signals of complex **11** are given.</figcaption></figure>

<figure><img src="image_131.png" /><figcaption>**Figure 16.** Molecular structure of **11** in the solid. Ellipsoids drawn at the 25% probability level.</figcaption></figure>
within the five-membered ring as well as the bond lengths of the conjugated part show no significant irregularity. However, the Zr–C bond is rather short and the Zr–N, rather long. In the considered family of compounds selected for comparison, the former varies slightly in length, while the Zr–N distance fluctuates depending on the L or X character of the nitrogen ligand and the N-substituent. In the present case, we explain the long Zr–N bond by the presence of the fluoride in *trans* relationship, coupled with the electron-poor properties of the difluoropyridine ring. The elongation of the Zr–N bond mechanically impacts the length of the Zr–C bond due to the rigidity of the 1-zirconia-2-azacyclopentadiene core. An additional observation is that the N1-C11-F3 angle is quite acute [114.7(2)°], presumably as a result of a long-range interaction of the fluorine atom with the electropositive Zr atom.

<figure><img src="image_129.png" /><figcaption>**Figure 15.** <sup>19</sup>F NMR (377 MHz, C<sub>6</sub>D<sub>6</sub>) spectrum of the crude reaction mixture of the reaction between the zirconocene-PMe<sub>3</sub> adduct **6** (1 equiv.) with Py-F<sub>3</sub> (5 equiv.) in cyclohexane at 40 °C for 42 hours, dried and dissolved in C<sub>6</sub>D<sub>6</sub>. Magnifications of the signals of complex **11** are given.</figcaption></figure>

<figure><img src="image_130.png" /><figcaption>**Figure 16.** Molecular structure of **11** in the solid. Ellipsoids drawn at the 25% probability level.</figcaption></figure>
Complex **12**, obtained from treating the zirconocene-PMe₃ adduct **6** with **Py-F₂** (five equivalents) in cyclohexane at 40 °C for 3 days (Scheme 40), exhibits similar NMR features to **11**. The ¹⁹F NMR spectrum displays a singlet at δ = 34.7 ppm, corresponding to the Zr–F bond, and a doublet at δ = –64.3 ppm (d, *J* = 7.6 Hz. It is remarkable to note that the system was highly selective and, despite the presence of a proton in the *para*-position, the C–H bond activation product was not detected.

Several hypotheses may be given to explain the differences in reactivity of the four polyfluoropyridines towards **6**. The absence of a 1,3-C–F addition product after treatment with **Py–F₅** can reasonably be attributed to the fast reaction of the latter with dissociated PMe₃. The low Lewis basicity of **Py–F₅** may also lead to a loosely bound adduct with the transient phosphine-free cyclohexyne complex **5**, hampering a C–F bond activation step to occur before the substrate reacts with the phosphine. Additionally, the structure of **11** (Figure 16) reveals a 1,3-allylic (¹,³A) strain between the cycles’ backbones (carbons C5 to C8), along the ring junction. This destabilising interaction would be exacerbated if the heteroaromatic ring was fully fluorinated and may exist before the C–C bond formation event, imparting a kinetic penalty for a C–F bond activation pathway.

The observation of the C–H activation product in the reaction with **Py–F₄** may be the result of two factors: on the one hand, the high level of fluorination of the heterocycle makes it, akin to **Py–F₅**, a poor Lewis base that is not likely to strongly bind the Zr centre. Besides, a potential C–F/CH₂ repulsion, resulting in the above-spotted ¹,³A strain in the cyclometalated product (Figure 16), is likely to arise from **Py–F₄** coordination. Moreover, Perutz and co-workers conducted an extensive study on the competition between metal-assisted C–H and C–F bond activation processes and identified several factors, such as the nature of the metal and its ligands, as well as the fluorinated compound and its fluorine substitution pattern, both playing crucial roles in the selectivity of the system [25,36,37]. An important parameter is the presence of *ortho*-fluorine substituents in the generated metal-fluororanyl complex, which makes C–H bond activation more energetically favourable. The presence of fluorine atoms in *ortho* to the M–C bond increases the ionic character of this bond, resulting in a negative charge at the *ipso*-carbon of the M-aryl bond. This charge is stabilised by *ortho*-fluorine substituents through hyperconjugation to the C–F antibonding σ* orbitals and due to the electron-withdrawing character of fluorine [36]. In our system, the metalation of the acidic C–H provides a metal-4-pyridyl compound with highly stabilising two-fold *ortho*-fluorine substitution.

In the case of **Py–F₃** and **Py–F₂**, higher Lewis basicity may allow the heteroarene to better interact with the Zr atom after PMe₃ dissociation and thus bringing its *ortho* C–F bonds closer to the reactive cyclohexyne ligand. This sets the stage for a regioselective C–F bond activation to take place [22]. The formation of complex **7** was not observed in these systems as the fluorinated pyridines were not sufficiently electrophilic to react with the dissociated PMe₃. Remarkably, in the case of **Py–F₂** the 4-position is not fluorinated, yet complete chemoselectivity for C–F over C–H activation was observed. This is likely the result of a stronger Zr···N interaction favouring C–F activation as well as the absence of fluorine in the 3- and 5-positions that would stabilise the C–H activation product.

## Stoichiometric reactivity between trialkylphosphines and Py–F₅

The stoichiometric reactivity between PMe₃ and **Py–F₅** was further investigated to gain a better understanding of the system. The objective was to assess whether changing the system conditions, including the phosphine, solvent, temperature and reagent ratio, could limit or suppress this reactivity.

The preparation of an equimolar solution of PMe₃ and **Py–F₅** in C₆D₆ triggered the formation of **Py–F₄**, **8** and **9**, along with a fourth minor species **13** not unambiguously identified and unreacted PMe₃,
Complex **12**, obtained from treating the zirconocene-PMe₃ adduct **6** with **Py-F₂** (five equivalents) in cyclohexane at 40 °C for 3 days (Scheme 40), exhibits similar NMR features to **11**. The ¹⁹F NMR spectrum displays a singlet at δ = 34.7 ppm, corresponding to the Zr–F bond, and a doublet at δ = –64.3 ppm (d, *J* = 7.6 Hz. It is remarkable to note that the system was highly selective and, despite the presence of a proton in the *para*-position, the C–H bond activation product was not detected.

Several hypotheses may be given to explain the differences in reactivity of the four polyfluoropyridines towards **6**. The absence of a 1,3-C–F addition product after treatment with **Py–F₅** can reasonably be attributed to the fast reaction of the latter with dissociated PMe₃. The low Lewis basicity of **Py–F₅** may also lead to a loosely bound adduct with the transient phosphine-free cyclohexyne complex **5**, hampering a C–F bond activation step to occur before the substrate reacts with the phosphine. Additionally, the structure of **11** (Figure 16) reveals a 1,3-allylic (¹,³A) strain between the cycles’ backbones (carbons C5 to C8), along the ring junction. This destabilising interaction would be exacerbated if the heteroaromatic ring was fully fluorinated and may exist before the C–C bond formation event, imparting a kinetic penalty for a C–F bond activation pathway.

The observation of the C–H activation product in the reaction with **Py–F₄** may be the result of two factors: on the one hand, the high level of fluorination of the heterocycle makes it, akin to **Py–F₅**, a poor Lewis base that is not likely to strongly bind the Zr centre. Besides, a potential C–F/CH₂ repulsion, resulting in the above-spotted ¹,³A strain in the cyclometalated product (Figure 16), is likely to arise from **Py–F₄** coordination. Moreover, Perutz and co-workers conducted an extensive study on the competition between metal-assisted C–H and C–F bond activation processes and identified several factors, such as the nature of the metal and its ligands, as well as the fluorinated compound and its fluorine substitution pattern, both playing crucial roles in the selectivity of the system [25,36,37]. An important parameter is the presence of *ortho*-fluorine substituents in the generated metal-fluororanyl complex, which makes C–H bond activation more energetically favourable. The presence of fluorine atoms in *ortho* to the M–C bond increases the ionic character of this bond, resulting in a negative charge at the *ipso*-carbon of the M–aryl bond. This charge is stabilised by *ortho*-fluorine substituents through hyperconjugation to the C–F antibonding σ* orbitals and due to the electron-withdrawing character of fluorine [36]. In our system, the metalation of the acidic C–H provides a metal-4-pyridyl compound with highly stabilising two-fold *ortho*-fluorine substitution.

In the case of **Py–F₃** and **Py–F₂**, higher Lewis basicity may allow the heteroarene to better interact with the Zr atom after PMe₃ dissociation and thus bringing its *ortho* C–F bonds closer to the reactive cyclohexyne ligand. This sets the stage for a regioselective C–F bond activation to take place [22]. The formation of complex **7** was not observed in these systems as the fluorinated pyridines were not sufficiently electrophilic to react with the dissociated PMe₃. Remarkably, in the case of **Py–F₂** the 4-position is not fluorinated, yet complete chemoselectivity for C–F over C–H activation was observed. This is likely the result of a stronger Zr···N interaction favouring C–F activation as well as the absence of fluorine in the 3- and 5-positions that would stabilise the C–H activation product.

## Stoichiometric reactivity between trialkylphosphines and Py–F₅

The stoichiometric reactivity between PMe₃ and **Py–F₅** was further investigated to gain a better understanding of the system. The objective was to assess whether changing the system conditions, including the phosphine, solvent, temperature and reagent ratio, could limit or suppress this reactivity.

The preparation of an equimolar solution of PMe₃ and **Py–F₅** in C₆D₆ triggered the formation of **Py–F₄**, **8** and **9**, along with a fourth minor species **13** not unambiguously identified and unreacted PMe₃,
according to NMR and GC(EI)-MS analysis. Different parameters, including temperature, reagent ratio, phosphine and solvent were screened at an NMR scale. Using a 3:1 **Py-F<sub>5</sub>/PMe<sub>3</sub>** ratio, the phosphine was completely consumed, but some **Py-F<sub>5</sub>** remained. The phosphine scope study revealed that the reactivity observed between PMe<sub>3</sub> and **Py-F<sub>5</sub>** extends to other more sterically hindered phosphines (e.g. PEt<sub>3</sub>, PMe<sub>2</sub>Ph, P<sup>i</sup>Pr<sub>3</sub>, PCy<sub>3</sub>) while still maintaining high reactivity and that for most solvents (excluding CD<sub>3</sub>CN) there is no solvent effect on the reaction outcome. The conditions for the reaction require the phosphine to have a proton in the α-carbon to the phosphorus and not excessively bulky R groups. The kinetic limit of the system was reached when P(<sup>i</sup>Bu)<sub>2</sub>Cy was employed. In this case an alternative reactivity pathway was favoured, resulting in the formation of the simple difluorophosphorane and 4,4'-octafluorobipyridine (15) (Scheme 41). The same outcome was observed in the absence of a proton in the α-carbon in the phosphine, such as with P<sup>i</sup>Bu<sub>3</sub>. Furthermore, the investigation of the solvent effect suggested that this second reaction pathway was promoted, even with less sterically hindered phosphines, by the use of a more polar solvents such as acetonitrile.

<figure><img src="image_132.png" /><figcaption>**Scheme 41.** Reactivity of P(<sup>i</sup>Bu)<sub>2</sub>Cy and P<sup>i</sup>Bu<sub>3</sub> with **Py-F<sub>5</sub>** in C<sub>6</sub>D<sub>6</sub> at 40°C for 4 hours and 7 days, respectively.</figcaption></figure>

In parallel to other strategies reported in the literature for the synthesis of these species, this approach offers a useful and versatile route for preparing F<sub>2</sub>PR<sub>3</sub> compounds under simple and mild reaction conditions. The potential advantage of this method lies in the generally low cost of phosphines and **Py-F<sub>5</sub>**. Moreover, it could also be used as a pathway for the synthesis of compound **15**.

The presence of the formal HDF product **Py-F<sub>4</sub>** in the reaction of Scheme 37 is reminiscent of the recent work from the group of García on the PEt<sub>3</sub>-promoted HDF of fluoroaromatics [38,39]. The methodology they reported differs from the above-mentioned experiments by the reaction conditions, in their case traces of water were critical to observe the desired outcome. Strict exclusion of moisture in our case rules out the involvement of García's mechanism to explain the formation of **Py-F<sub>4</sub>**, although the nucleophilic attack of the phosphine onto the 4-position of **Py-F<sub>5</sub>** must be a common initial elementary step in both reactions. Considering the reaction between PMe<sub>3</sub> and **Py-F<sub>5</sub>** and given that the formation of **9** cannot be explained by other means than the occurrence of a PMe<sub>3</sub>-derived ylide, it is proposed that the observed outcome of the reaction of **Py-F<sub>5</sub>** and PMe<sub>3</sub> stems from the mechanism depicted in Scheme 42. After the nucleophilic attack of PMe<sub>3</sub> onto **Py-F<sub>5</sub>**, the zwitterionic Meisenheimer complex I would evolve into the fluorophosphorane II. In the latter, the acidity of the Me groups may be high enough to be deprotonated by PMe<sub>3</sub> (pK<sub>a</sub> varying from ~9 in THF up to 15.5 in CH<sub>3</sub>CN), followed by proton transfer from [HPR<sub>3</sub>]<sup>+</sup> to the tetrafluoropyridyl carbon and formation of **Py-F<sub>4</sub>** [40-43]. Alternatively, a tetrafluoropyridyl anion, obtained by dissociation from fluorophosphorane II, may act as the base, deprotonating another molecule of intermediate II. This would result in ion pair III, with a hydro- or fluorophosphonium as counterion, that would undergo P-bond protiolysis to release PMe<sub>3</sub>, **Py-F<sub>4</sub>** and the P-fluorinated ylide IV [44]. The latter could then attack another equivalent of **Py-F<sub>5</sub>** to lead to complex **9** via another Meisenheimer complex V, followed by C-to-P fluoride transfer sequence. Overall, this rationale explains well the observed ~1:1 **Py-F<sub>4</sub>:9** ratio. A
according to NMR and GC(EI)-MS analysis. Different parameters, including temperature, reagent ratio, phosphine and solvent were screened at an NMR scale. Using a 3:1 **Py-F<sub>5</sub>/PMe<sub>3</sub>** ratio, the phosphine was completely consumed, but some **Py-F<sub>5</sub>** remained. The phosphine scope study revealed that the reactivity observed between PMe<sub>3</sub> and **Py-F<sub>5</sub>** extends to other more sterically hindered phosphines (e.g. PEt<sub>3</sub>, PMe<sub>2</sub>Ph, P<sup>i</sup>Pr<sub>3</sub>, PCy<sub>3</sub>) while still maintaining high reactivity and that for most solvents (excluding CD<sub>3</sub>CN) there is no solvent effect on the reaction outcome. The conditions for the reaction require the phosphine to have a proton in the α-carbon to the phosphorus and not excessively bulky R groups. The kinetic limit of the system was reached when P(<sup>i</sup>Bu)<sub>2</sub>Cy was employed. In this case an alternative reactivity pathway was favoured, resulting in the formation of the simple difluorophosphorane and 4,4'-octafluorobipyridine (15) (Scheme 41). The same outcome was observed in the absence of a proton in the α-carbon in the phosphine, such as with P<sup>i</sup>Bu<sub>3</sub>. Furthermore, the investigation of the solvent effect suggested that this second reaction pathway was promoted, even with less sterically hindered phosphines, by the use of a more polar solvents such as acetonitrile.

<figure><img src="image_131.png" /><figcaption>**Scheme 41.** Reactivity of P(<sup>i</sup>Bu)<sub>2</sub>Cy and P<sup>i</sup>Bu<sub>3</sub> with **Py-F<sub>5</sub>** in C<sub>6</sub>D<sub>6</sub> at 40°C for 4 hours and 7 days, respectively.</figcaption></figure>

In parallel to other strategies reported in the literature for the synthesis of these species, this approach offers a useful and versatile route for preparing F<sub>2</sub>PR<sub>3</sub> compounds under simple and mild reaction conditions. The potential advantage of this method lies in the generally low cost of phosphines and **Py-F<sub>5</sub>**. Moreover, it could also be used as a pathway for the synthesis of compound **15**.

The presence of the formal HDF product **Py-F<sub>4</sub>** in the reaction of Scheme 37 is reminiscent of the recent work from the group of García on the PEt<sub>3</sub>-promoted HDF of fluoroaromatics [38,39]. The methodology they reported differs from the above-mentioned experiments by the reaction conditions, in their case traces of water were critical to observe the desired outcome. Strict exclusion of moisture in our case rules out the involvement of García's mechanism to explain the formation of **Py-F<sub>4</sub>**, although the nucleophilic attack of the phosphine onto the 4-position of **Py-F<sub>5</sub>** must be a common initial elementary step in both reactions. Considering the reaction between PMe<sub>3</sub> and **Py-F<sub>5</sub>** and given that the formation of **9** cannot be explained by other means than the occurrence of a PMe<sub>3</sub>-derived ylide, it is proposed that the observed outcome of the reaction of **Py-F<sub>5</sub>** and PMe<sub>3</sub> stems from the mechanism depicted in Scheme 42. After the nucleophilic attack of PMe<sub>3</sub> onto **Py-F<sub>5</sub>**, the zwitterionic Meisenheimer complex I would evolve into the fluorophosphorane II. In the latter, the acidity of the Me groups may be high enough to be deprotonated by PMe<sub>3</sub> (pK<sub>a</sub> varying from ~9 in THF up to 15.5 in CH<sub>3</sub>CN), followed by proton transfer from [HPR<sub>3</sub>]<sup>+</sup> to the tetrafluoropyridyl carbon and formation of **Py-F<sub>4</sub>** [40-43]. Alternatively, a tetrafluoropyridyl anion, obtained by dissociation from fluorophosphorane II, may act as the base, deprotonating another molecule of intermediate II. This would result in ion pair III, with a hydro- or fluorophosphonium as counterion, that would undergo P-bond protiolysis to release PMe<sub>3</sub>, **Py-F<sub>4</sub>** and the P-fluorinated ylide IV [44]. The latter could then attack another equivalent of **Py-F<sub>5</sub>** to lead to complex **9** via another Meisenheimer complex V, followed by C-to-P fluoride transfer sequence. Overall, this rationale explains well the observed ~1:1 **Py-F<sub>4</sub>:9** ratio. A
light-activated mechanism was excluded, as a test experiment conducted in the absence of light yielded the same outcome.

<figure><img src="image_133.png" /><figcaption>**Scheme 42.** Proposed reaction mechanism of Scheme 37.</figcaption></figure>

When the reaction between trialkylphosphines and **Py-F₅** was conducted in CD₃CN, or when bulky phosphines (such as P(tBu)₂Cy) or phosphines lacking a proton in the Cα were employed (e.g. P(tBu₃), the formation of the simple difluorophosphorane and the reductive coupling product **15** took place instead. The proposed mechanism for this reaction is reported in Scheme 43. Similar to the previous mechanisms, the Meisenheimer complex **I** formation and its conversion into fluorophosphorane **II** take place. The 2,3,5,6-tetrafluoropyridyl group could be expected to be a better leaving group than other Ar- fragments owing to the delocalisation of the negative charge due to the high degree of fluorination of [**Py-F₄**]⁻, the presence of polar solvents and the steric repulsion arising from bulky R groups on the phosphine. Furthermore, the absence of any evidence of compound **II** in both our systems and the existing literature [38,39] suggests that **II** is highly reactive. The generation of difluorophosphorane **X** and phosphorane **XI** could occur through two distinct pathways:

- *via* pyridyl attack on the phosphorus centre, leading to the release of fluoride, which then migrates to the P⁺ of **VIII** (Scheme 43, blue box on the left);

- through the attack of **VIII** which acts as a Lewis acid by abstracting a fluoride from **II**, followed by the interaction of the resulting P⁺ with the 2,3,5,6-tetrafluoropyridyl anion **IX** (Scheme 43, blue box on the right).

Finally, the ligand-ligand coupling in species **XI** would form **15**, as previously reported in similar systems [45-47], along with the liberation of free PR₃.

A comparable solvent effect was observed in the context of the PCy₃-catalysed intermolecular acylfluorination of alkynes in toluene [48].
light-activated mechanism was excluded, as a test experiment conducted in the absence of light yielded the same outcome.

<figure><img src="image_132.png" /><figcaption>**Scheme 42.** Proposed reaction mechanism of Scheme 37.</figcaption></figure>

When the reaction between trialkylphosphines and **Py-F<sub>5</sub>** was conducted in CD<sub>3</sub>CN, or when bulky phosphines (such as P(<sup>t</sup>Bu)<sub>2</sub>Cy) or phosphines lacking a proton in the C<sub>α</sub> were employed (e.g. P<sup>t</sup>Bu<sub>3</sub>), the formation of the simple difluorophosphorane and the reductive coupling product **15** took place instead. The proposed mechanism for this reaction is reported in Scheme 43. Similar to the previous mechanisms, the Meisenheimer complex **I** formation and its conversion into fluorophosphorane **II** take place. The 2,3,5,6-tetrafluoropyridyl group could be expected to be a better leaving group than other Ar- fragments owing to the delocalisation of the negative charge due to the high degree of fluorination of [<b>Py-F<sub>4</sub></b>]<sup>-</sup>, the presence of polar solvents and the steric repulsion arising from bulky R groups on the phosphine. Furthermore, the absence of any evidence of compound **II** in both our systems and the existing literature [38,39] suggests that **II** is highly reactive. The generation of difluorophosphorane **X** and phosphorane **XI** could occur through two distinct pathways:

- *via* pyridyl attack on the phosphorus centre, leading to the release of fluoride, which then migrates to the P<sup>+</sup> of **VIII** (Scheme 43, blue box on the left);

- through the attack of **VIII** which acts as a Lewis acid by abstracting a fluoride from **II**, followed by the interaction of the resulting P<sup>+</sup> with the 2,3,5,6-tetrafluoropyridyl anion **IX** (Scheme 43, blue box on the right).

Finally, the ligand-ligand coupling in species **XI** would form **15**, as previously reported in similar systems [45-47], along with the liberation of free PR<sub>3</sub>.

A comparable solvent effect was observed in the context of the PCy<sub>3</sub>-catalysed intermolecular acylfluorination of alkynes in toluene [48].
<figure><img src="image_134.png" /><figcaption>**Scheme 43.** Proposed mechanism for the formation of 15.</figcaption></figure>

Tobisu *et al.* proposed the participation of a fluorophosphorane intermediate in the catalytic cycle which, after ligand coupling and release of the acylfluorination product, regenerates the P¹¹¹ catalyst. In polar solvents such as acetonitrile, the reaction yields the simple difluorophosphorane F₂PCy₃ and the hydroacylated product, with the acylfluorination product detected only in minimal amounts. The authors proposed that the fluorophosphorane is in equilibrium with the corresponding phosphonium salt, which is more stable in acetonitrile. The phosphonium species is expected to decompose more readily than the fluorophosphorane and the protonation of the system would lead to the formation of F₂PCy₃ and the hydroacylated product. Traces of water (*via* nucleophilic attack onto phosphonium ion) or cyclohexyl groups (through β-hydrogen elimination) could act as proton sources in this system.

### PnBu₃-catalysed hydro- and aminodefluorination of polyfluoroaromatics

In recent years, there has been a significant interest in the activation and functionalisation of small molecules and strong bonds by main-group element compounds that mimic the reactivity of TMs. An essential concept driving this research is metallomimetic behaviour, where main-group elements can be induced to exhibit reactivity similar to that of transition metals [49]. One of the peculiar reactivity of TMs is to undergo reversible two-electron redox processes, such as oxidative addition and reductive elimination. Nevertheless, redox cycles with main-group elements are relatively rare. Among these elements, the pnictogens stand out, as they often provide access to stable species in various oxidation states [50]. A crucial structural feature that seems to enable the observation of redox cycling in Pn-containing systems involves the introduction of geometric constraints around pnictogen atoms [51]. Recent studies in this field described how geometrically constrained pnictogens can effectively hydrodefluorinate perfluorinated aromatics, both stoichiometrically and catalytically
<figure><img src="image_133.png" /><figcaption>**Scheme 43.** Proposed mechanism for the formation of 15.</figcaption></figure>

Tobisu *et al.* proposed the participation of a fluorophosphorane intermediate in the catalytic cycle which, after ligand coupling and release of the acylfluorination product, regenerates the P¹¹¹ catalyst. In polar solvents such as acetonitrile, the reaction yields the simple difluorophosphorane F₂PCy₃ and the hydroacylated product, with the acylfluorination product detected only in minimal amounts. The authors proposed that the fluorophosphorane is in equilibrium with the corresponding phosphonium salt, which is more stable in acetonitrile. The phosphonium species is expected to decompose more readily than the fluorophosphorane and the protonation of the system would lead to the formation of F₂PCy₃ and the hydroacylated product. Traces of water (*via* nucleophilic attack onto phosphonium ion) or cyclohexyl groups (through β-hydrogen elimination) could act as proton sources in this system.

### PnBu₃-catalysed hydro- and aminodefluorination of polyfluoroaromatics

In recent years, there has been a significant interest in the activation and functionalisation of small molecules and strong bonds by main-group element compounds that mimic the reactivity of TMs. An essential concept driving this research is metallomimetic behaviour, where main-group elements can be induced to exhibit reactivity similar to that of transition metals [49]. One of the peculiar reactivity of TMs is to undergo reversible two-electron redox processes, such as oxidative addition and reductive elimination. Nevertheless, redox cycles with main-group elements are relatively rare. Among these elements, the pnictogens stand out, as they often provide access to stable species in various oxidation states [50]. A crucial structural feature that seems to enable the observation of redox cycling in Pn-containing systems involves the introduction of geometric constraints around pnictogen atoms [51]. Recent studies in this field described how geometrically constrained pnictogens can effectively hydrodefluorinate perfluorinated aromatics, both stoichiometrically and catalytically
[52-54]. These works highlight the crucial role of this structural design in accessing the Pn<sup>III</sup>/Pn<sup>V</sup> redox cycle, enabling the specific reactivity desired for this transformation.

The new results reported in this study demonstrated that complex molecular architectures are not required for P<sup>III</sup> systems to act as catalysts for HDF or aminodefluorination of highly fluorinated arenes and heterocycles. Simple trialkylphosphines were found to be fast and effective catalysts for these reactions with a range of substrates, but especially for highly electron-poor ones (Scheme 44). This is an important observation, as some trialkylphosphines, particularly the P<sup>n</sup>Bu<sub>3</sub> used here, are cheap, readily available from commercial suppliers and easy to handle.

<figure><img src="image_135.png" /><figcaption>**Scheme 44.** Representative scheme of our system for the catalytic reaction of HDF and hydroamination.</figcaption></figure>

The reaction optimisation showed that the combination of P<sup>n</sup>Bu<sub>3</sub> and Ph<sub>2</sub>SiH<sub>2</sub> as hydride source in acetonitrile provided the best reaction conditions allowing selective HDF of **Py-F<sub>5</sub>** to give **Py-F<sub>4</sub>** in excellent yield (93%), after only 20 minutes at 20 °C. This performance stands among the best reported for catalytic aromatic HDF by a main-group-element catalyst [52-56]. Moreover, this phosphine-silane combination proved to be most economical and readily available among those tested.

The substrate scope for HDF was investigated, extending the studies to various fluoroarenes and heterocycles (Figure 17). In addition, it was possible to explore preliminary aminodefluorination reactions (products **24** to **27**) to allow C–F functionalisation by amines.

Exploration of the degree of fluorination of fluoropyridines and the regioselectivity of the HDF reaction gave insights into the reaction mechanism and synthetic possibilities. With **Py-F<sub>4</sub>** as a substrate, it was possible to observe the formation of **17**, but in low yield and requiring much more forcing conditions (13% after 5 days at 70 °C). This explains the selectivity of the HDF in Scheme 44, as a second HDF at the less activated *ortho*-position of the pyridine is substantially slower than the initial reaction at the *para*-position of **Py-F<sub>5</sub>**. The presence of other tri- and difluoropyridines suggests that the rate of HDF at the *ortho*- and *meta*-positions is similar to each other and slower than the rate of HDF at the *para*-position. Reducing the degree of fluorination of the substrate, but maintaining a C–F bond at the most reactive *para*-position also allowed HDF and formation of **Py-F<sub>2</sub>** (79% yield), but the reaction was relatively slow and a small amount of unreacted **Py-F<sub>3</sub>** (5%) was present, along with some 2,4-difluoropyridine (14%), which results from *ortho*-HDF, after 6 days. It is clear that more electron-poor heteroaromatics lead to faster reactions, which is consistent with the mechanistic proposal. The catalytic HDF reaction is not limited to pyridines. Reaction of other electron-poor aromatics, such as pentafluorobenzonitrile, perfluorotoluene, hexafluorobenzene and perfluorobiphenyl led to the formation of the mono or diHDF product in good yields (84–93%). Extending the catalytic methodology to other C–F functionalisation reactions was also explored. Preliminary studies investigated C–F amination by silylamides in the presence of catalytic P<sup>n</sup>Bu<sub>3</sub>. Thus, when **Py-F<sub>5</sub>** was reacted with Ph<sub>2</sub>Si(Cl)(NEt<sub>2</sub>) with 10 mol% of P<sup>n</sup>Bu<sub>3</sub> at 60 °C for 18 h a good yield (75%) of the aminated product **24** was seen, alongside a small amount of unreacted **Py-F<sub>5</sub>** (10%).
[52-54]. These works highlight the crucial role of this structural design in accessing the Pn$^{III}$/Pn$^V$ redox cycle, enabling the specific reactivity desired for this transformation.

The new results reported in this study demonstrated that complex molecular architectures are not required for P$^{III}$ systems to act as catalysts for HDF or aminodefluorination of highly fluorinated arenes and heterocycles. Simple trialkylphosphines were found to be fast and effective catalysts for these reactions with a range of substrates, but especially for highly electron-poor ones (Scheme 44). This is an important observation, as some trialkylphosphines, particularly the P$^n$Bu$_3$ used here, are cheap, readily available from commercial suppliers and easy to handle.

<figure><img src="image_134.png" /><figcaption>**Scheme 44.** Representative scheme of our system for the catalytic reaction of HDF and hydroamination.</figcaption></figure>

The reaction optimisation showed that the combination of P$^n$Bu$_3$ and Ph$_2$SiH$_2$ as hydride source in acetonitrile provided the best reaction conditions allowing selective HDF of **Py-F$_5$** to give **Py-F$_4$** in excellent yield (93%), after only 20 minutes at 20 °C. This performance stands among the best reported for catalytic aromatic HDF by a main-group-element catalyst [52-56]. Moreover, this phosphine-silane combination proved to be most economical and readily available among those tested.

The substrate scope for HDF was investigated, extending the studies to various fluoroarenes and heterocycles (Figure 17). In addition, it was possible to explore preliminary aminodefluorination reactions (products **24** to **27**) to allow C–F functionalisation by amines.

Exploration of the degree of fluorination of fluoropyridines and the regioselectivity of the HDF reaction gave insights into the reaction mechanism and synthetic possibilities. With **Py-F$_4$** as a substrate, it was possible to observe the formation of **17**, but in low yield and requiring much more forcing conditions (13% after 5 days at 70 °C). This explains the selectivity of the HDF in Scheme 44, as a second HDF at the less activated *ortho*-position of the pyridine is substantially slower than the initial reaction at the *para*-position of **Py-F$_5$**. The presence of other tri- and difluoropyridines suggests that the rate of HDF at the *ortho*- and *meta*-positions is similar to each other and slower than the rate of HDF at the *para*-position. Reducing the degree of fluorination of the substrate, but maintaining a C–F bond at the most reactive *para*-position also allowed HDF and formation of **Py-F$_2$** (79% yield), but the reaction was relatively slow and a small amount of unreacted **Py-F$_3$** (5%) was present, along with some 2,4-difluoropyridine (14%), which results from *ortho*-HDF, after 6 days. It is clear that more electron-poor heteroaromatics lead to faster reactions, which is consistent with the mechanistic proposal. The catalytic HDF reaction is not limited to pyridines. Reaction of other electron-poor aromatics, such as pentafluorobenzonitrile, perfluorotoluene, hexafluorobenzene and perfluorobiphenyl led to the formation of the mono or diHDF product in good yields (84–93%). Extending the catalytic methodology to other C–F functionalisation reactions was also explored. Preliminary studies investigated C–F amination by silylamides in the presence of catalytic P$^n$Bu$_3$. Thus, when **Py-F$_5$** was reacted with Ph$_2$Si(Cl)(NEt$_2$) with 10 mol% of P$^n$Bu$_3$ at 60 °C for 18 h a good yield (75%) of the aminated product **24** was seen, alongside a small amount of unreacted **Py-F$_5$** (10%).
<figure><img src="image_136.png" /><figcaption>**Figure 17.** Results of substrate scope studies. Ph₂SiH₂ was used for all HDF reactions and Ph₂Si(Cl)(NR₂) were used for aminodefluorination reactions. <sup>a</sup> Isolated yield after flash column chromatography shown in parentheses. <sup>b</sup> Reactions were stirred at 60 °C for 4 days, followed by the remaining period at 80 °C.</figcaption></figure>

Extending this reaction to more complex amines also proved possible. Reaction of **Py-F₅** with Ph₂Si(Cl)(pro) (where pro is L-proline methyl ester) gave a very good yield of **25** (88%) alongside a small amount of unreacted **Py-F₅**. This demonstrated that potentially sensitive functional groups are tolerated by this catalytic aminodefluorination reaction and that it may find application in the preparation of highly functionalised, fluorinated amines, e.g. in pharmaceutical or agrochemical synthesis. Extending the fluoroarene substrates beyond **Py-F₅** also proved possible, with both pentafluorobenzonitrile and perfluorotoluene also reacting with Ph₂Si(Cl)(NEt₂) in the presence of 10 mol% PⁿBu₃ to give **26** and **27** in 82% and 50% yield respectively. However, these substrates proved more sluggish under the reaction conditions optimised for HDF and further optimisation is desirable in future studies to extend the aminodefluorination reactions beyond these preliminary results.

Experimental and computational mechanistic studies were undertaken to probe the pathways underpinning the HDF reaction. Under the optimised reaction conditions for the HDF of **Py-F₅** the reaction occurred too quickly to observe any intermediates. However, for slower reactions it was possible to gain insights through the observation of intermediate phosphorus-containing species. When **Py-F₅** was reacted with PhSiH₃, with 10 mol% PⁿBu₃ in MeCN at 20 °C, after 1 h the ³¹P NMR spectrum showed very clean conversion of PⁿBu₃ into a new species with an apparent septet resonance at δ = 38.1 ppm (*J* = *ca*. 5 Hz). This was accompanied by new complex multiplet signals in the ¹⁹F NMR at δ = -88.5 and -130.3 ppm. The only other resonances present in the ¹⁹F NMR were from unreacted **Py-F₅**, the HDF product **Py-F₄** and the fluorosilane produced by H/F exchange. The resonance at δ = 38.1 ppm in the ³¹P NMR spectrum is characteristic of a phosphonium cation and was therefore assigned to the [(C₅F₄N)PⁿBu₃]⁺ ion, [16]⁺. This was confirmed by independent preparation of [(C₅F₄N)PⁿBu₃]Br ([16]Br) by reaction of PⁿBu₃ with 4-bromo-2,3,5,6-tetrafluoropyridine (Scheme 45).
<figure><img src="image_135.png" /><figcaption>**Figure 17.** Results of substrate scope studies. Ph₂SiH₂ was used for all HDF reactions and Ph₂Si(Cl)(NR₂) were used for aminodefluorination reactions. <sup>a</sup> Isolated yield after flash column chromatography shown in parentheses. <sup>b</sup> Reactions were stirred at 60 °C for 4 days, followed by the remaining period at 80 °C.</figcaption></figure>

Extending this reaction to more complex amines also proved possible. Reaction of **Py-F₅** with Ph₂Si(Cl)(pro) (where pro is L-proline methyl ester) gave a very good yield of **25** (88%) alongside a small amount of unreacted **Py-F₅**. This demonstrated that potentially sensitive functional groups are tolerated by this catalytic aminodefluorination reaction and that it may find application in the preparation of highly functionalised, fluorinated amines, e.g. in pharmaceutical or agrochemical synthesis. Extending the fluoroarene substrates beyond **Py-F₅** also proved possible, with both pentafluorobenzonitrile and perfluorotoluene also reacting with Ph₂Si(Cl)(NEt₂) in the presence of 10 mol% PⁿBu₃ to give **26** and **27** in 82% and 50% yield respectively. However, these substrates proved more sluggish under the reaction conditions optimised for HDF and further optimisation is desirable in future studies to extend the aminodefluorination reactions beyond these preliminary results.

Experimental and computational mechanistic studies were undertaken to probe the pathways underpinning the HDF reaction. Under the optimised reaction conditions for the HDF of **Py-F₅** the reaction occurred too quickly to observe any intermediates. However, for slower reactions it was possible to gain insights through the observation of intermediate phosphorus-containing species. When **Py-F₅** was reacted with PhSiH₃, with 10 mol% PⁿBu₃ in MeCN at 20 °C, after 1 h the ³¹P NMR spectrum showed very clean conversion of PⁿBu₃ into a new species with an apparent septet resonance at δ = 38.1 ppm (*J* = *ca*. 5 Hz). This was accompanied by new complex multiplet signals in the ¹⁹F NMR at δ = -88.5 and -130.3 ppm. The only other resonances present in the ¹⁹F NMR were from unreacted **Py-F₅**, the HDF product **Py-F₄** and the fluorosilane produced by H/F exchange. The resonance at δ = 38.1 ppm in the ³¹P NMR spectrum is characteristic of a phosphonium cation and was therefore assigned to the [(C₅F₄N)PⁿBu₃]⁺ ion, [16]⁺. This was confirmed by independent preparation of [(C₅F₄N)PⁿBu₃]Br ([16]Br) by reaction of PⁿBu₃ with 4-bromo-2,3,5,6-tetrafluoropyridine (Scheme 45).
At the end of the catalytic reaction, when Py-F₅ was fully consumed, the signal for [16]⁺ disappeared and free phosphine was observed in the reaction mixture (³¹P δ = -32.1 ppm), suggesting its role as a resting state.

<figure><img src="image_137.png" /><figcaption>**Scheme 45.** HDF reaction of Py-F₅ (1 equiv.) with PhSiH₃ (1 equiv.) and PⁿBu₃ (10 mol%) in CD₃CN at 20 °C.</figcaption></figure>

To investigate whether [16]⁺ was a relevant catalytic intermediate, Ph₂SiH₂ was added to the independently prepared sample of [16]Br in MeCN. After 10 minutes at 20 °C there was no change in the ³¹P or ¹⁹F NMR spectra. The mixture was heated to 40 °C overnight and similarly showed no reaction between [16]Br and the silane. However, on addition of one equivalent of anhydrous [Me₄N]F to the system at 20 °C, Py-F₄ was seen to form after only 10 minutes, alongside the formation of fluorosilane Ph₂Si(F)ₙH₂-ₙ and some unreacted [16]⁺. Over time, the amount of Py-F₄ increased as [16]⁺ decreased. This suggested that [16]⁺ is an intermediate in the catalytic cycle, but that it does not react directly with the silane to form the HDF product. We propose that addition of fluoride to the silane to form a silicate anion, [Ph₂Si(F)H₂]⁻, is necessary to promote H⁺ transfer and liberation of the hydrodefluorinated product. Looking at the catalytic reaction as a whole, the experimental mechanistic studies suggested that the phosphine catalyst adds to the fluoroarene/heteroarene to initially form a fluorophosphorane, e.g. P(F)(ⁿBu)₃(C₅F₄N), but that the silane then acts as a Lewis acid to abstract a fluoride ion from the phosphorane to form a phosphonium salt, e.g. [(C₅F₄N)PⁿBu₃][Ph₂Si(F)H₂]. The hydrosilicate anion then transfers a hydride to the phosphonium ion to form another phosphorane that eliminates the hydrodefluorinated product and re-generates the phosphine (Scheme 46).

<figure><img src="image_138.png" /><figcaption>**Scheme 46.** Proposed catalytic cycle and computed potential energy surface (PES) for R = Me. All energies at the PBE0/def2-TZVP//BP86/SV(P) level in MeCN. Relative Gibbs energies (in kJ mol⁻¹ at 298 K) shown outside brackets and relative enthalpies (in kJ mol⁻¹ at 298 K) shown in brackets.</figcaption></figure>
At the end of the catalytic reaction, when Py-F₅ was fully consumed, the signal for [16]⁺ disappeared and free phosphine was observed in the reaction mixture (³¹P δ = -32.1 ppm), suggesting its role as a resting state.

<figure><img src="image_136.png" /><figcaption>**Scheme 45.** HDF reaction of Py-F₅ (1 equiv.) with PhSiH₃ (1 equiv.) and PⁿBu₃ (10 mol%) in CD₃CN at 20 °C.</figcaption></figure>

To investigate whether [16]⁺ was a relevant catalytic intermediate, Ph₂SiH₂ was added to the independently prepared sample of [16]Br in MeCN. After 10 minutes at 20 °C there was no change in the ³¹P or ¹⁹F NMR spectra. The mixture was heated to 40 °C overnight and similarly showed no reaction between [16]Br and the silane. However, on addition of one equivalent of anhydrous [Me₄N]F to the system at 20 °C, Py-F₄ was seen to form after only 10 minutes, alongside the formation of fluorosilane Ph₂Si(F)ₙH₂-ₙ and some unreacted [16]⁺. Over time, the amount of Py-F₄ increased as [16]⁺ decreased. This suggested that [16]⁺ is an intermediate in the catalytic cycle, but that it does not react directly with the silane to form the HDF product. We propose that addition of fluoride to the silane to form a silicate anion, [Ph₂Si(F)H₂]⁻, is necessary to promote H⁺ transfer and liberation of the hydrodefluorinated product. Looking at the catalytic reaction as a whole, the experimental mechanistic studies suggested that the phosphine catalyst adds to the fluoroarene/heteroarene to initially form a fluorophosphorane, e.g. P(F)(ⁿBu)₃(C₅F₄N), but that the silane then acts as a Lewis acid to abstract a fluoride ion from the phosphorane to form a phosphonium salt, e.g. [(C₅F₄N)PⁿBu₃][Ph₂Si(F)H₂]. The hydrosilicate anion then transfers a hydride to the phosphonium ion to form another phosphorane that eliminates the hydrodefluorinated product and re-generates the phosphine (Scheme 46).

<figure><img src="image_137.png" /><figcaption>**Scheme 46.** Proposed catalytic cycle and computed potential energy surface (PES) for R = Me. All energies at the PBE0/def2-TZVP//BP86/SV(P) level in MeCN. Relative Gibbs energies (in kJ mol⁻¹ at 298 K) shown outside brackets and relative enthalpies (in kJ mol⁻¹ at 298 K) shown in brackets.</figcaption></figure>
This H/F exchange between P and Si is driven by differences in the fluoride ion and hydride ion affinities of the phosphorane and silane, which have been discussed and computed for related species [36]. The reactivity of non-fluorinated pyridylphosphonium salts with nucleophiles to form substituted pyridines has also been reported and, like the processes observed here, is proposed to occur *via* phosphorane intermediates [57]. Underpinning all of this reactivity is the facile redox cycling between P<sup>III</sup> and P<sup>V</sup> oxidation states that is remarkable to see in simple trialkylphosphines.

DFT calculations were performed to support the experimental mechanistic studies. These showed that initial reaction of the phosphine with pentafluoropyridine takes place through a Meisenheimer-like transition state (Scheme 46, **TS**<sub>12</sub>) to form a fluorophosphorane (**M2**) in a similar manner to that proposed by García and coworkers for reaction of **Py**-**F**<sub>5</sub> with PEt<sub>3</sub> [38]. This is associated with a low barrier of 55 kJ mol<sup>-1</sup>. The structure of **TS**<sub>12</sub> (Figure 18) shows the early nature of this transition state, where P-C bond formation and C-F bond elongation precedes fluorine transfer to phosphorus to form the phosphorane. This step is effectively initiated by a nucleophilic addition of PR<sub>3</sub> to the pentafluoropyridine and explains the preference for electron-poor arenes/heteroarenes in this reaction, where this will be promoted. No additional intermediates or transition states for fluorine transfer to phosphorus were identified. The addition of **Py**-**F**<sub>5</sub> to the phosphine leads to a formal oxidation state change from P<sup>III</sup> to P<sup>V</sup> and so can be characterised as an oxidative addition process, albeit one that is highly asynchronous in terms of C-F bond cleavage and reminiscent of concerted S<sub>N</sub>Ar mechanisms. The key structural parameters of **TS**<sub>12</sub> are almost identical to those calculated by Dobrovetsky and coworkers for addition of **Py**-**F**<sub>5</sub> to a geometrically constrained σ<sup>3</sup>-P compound, suggesting a similar activation process, despite the very different structural frameworks involved [54].

<figure><img src="image_139.png" /><figcaption>**Figure 18.** Transition states for addition of PMe<sub>3</sub> to **Py**-**F**<sub>5</sub> (**TS**<sub>12</sub>) and elimination of **Py**-**F**<sub>4</sub> from phosphorane **M6** (**TS**<sub>61</sub>). Hydrogen is shown in white, carbon in grey, phosphorus in orange, nitrogen in blue and fluorine in green. Selected distances (in Å) are shown.</figcaption></figure>

The fluorophosphorane that is initially formed (**M2**) can undergo isomerisation to a lower energy isomer (**M3**) with fluorine *trans* to the tetrafluoropyridyl group, both in the apical positions. Fluoride transfer to Ph<sub>2</sub>SiH<sub>2</sub> results in the formation of the observed phosphonium ion [16]<sup>+</sup>, in this case as the salt **M4**. Moving between neutral and ionic manifolds in this way will be strongly influenced by solvation effects. This leads to a small mismatch between the computed energies and the experimental
This H/F exchange between P and Si is driven by differences in the fluoride ion and hydride ion affinities of the phosphorane and silane, which have been discussed and computed for related species [36]. The reactivity of non-fluorinated pyridylphosphonium salts with nucleophiles to form substituted pyridines has also been reported and, like the processes observed here, is proposed to occur *via* phosphorane intermediates [57]. Underpinning all of this reactivity is the facile redox cycling between P<sup>III</sup> and P<sup>V</sup> oxidation states that is remarkable to see in simple trialkylphosphines.

DFT calculations were performed to support the experimental mechanistic studies. These showed that initial reaction of the phosphine with pentafluoropyridine takes place through a Meisenheimer-like transition state (Scheme 46, **TS**<sub>12</sub>) to form a fluorophosphorane (**M**<sub>2</sub>) in a similar manner to that proposed by García and coworkers for reaction of **Py**-**F**<sub>5</sub> with PEt<sub>3</sub> [38]. This is associated with a low barrier of 55 kJ mol<sup>-1</sup>. The structure of **TS**<sub>12</sub> (Figure 18) shows the early nature of this transition state, where P-C bond formation and C–F bond elongation precedes fluorine transfer to phosphorus to form the phosphorane. This step is effectively initiated by a nucleophilic addition of PR<sub>3</sub> to the pentafluoropyridine and explains the preference for electron-poor arenes/heteroarenes in this reaction, where this will be promoted. No additional intermediates or transition states for fluorine transfer to phosphorus were identified. The addition of **Py**-**F**<sub>5</sub> to the phosphine leads to a formal oxidation state change from P<sup>III</sup> to P<sup>V</sup> and so can be characterised as an oxidative addition process, albeit one that is highly asynchronous in terms of C–F bond cleavage and reminiscent of concerted S<sub>N</sub>Ar mechanisms. The key structural parameters of **TS**<sub>12</sub> are almost identical to those calculated by Dobrovetsky and coworkers for addition of **Py**-**F**<sub>5</sub> to a geometrically constrained σ<sup>3</sup>-P compound, suggesting a similar activation process, despite the very different structural frameworks involved [54].

<figure><img src="image_138.png" /><figcaption>**Figure 18.** Transition states for addition of PMe<sub>3</sub> to **Py**-**F**<sub>5</sub> (**TS**<sub>12</sub>) and elimination of **Py**-**F**<sub>4</sub> from phosphorane **M**<sub>6</sub> (**TS**<sub>61</sub>). Hydrogen is shown in white, carbon in grey, phosphorus in orange, nitrogen in blue and fluorine in green. Selected distances (in Å) are shown.</figcaption></figure>

The fluorophosphorane that is initially formed (**M**<sub>2</sub>) can undergo isomerisation to a lower energy isomer (**M**<sub>3</sub>) with fluorine *trans* to the tetrafluoropyridyl group, both in the apical positions. Fluoride transfer to Ph<sub>2</sub>SiH<sub>2</sub> results in the formation of the observed phosphonium ion [16]<sup>+</sup>, in this case as the salt **M**<sub>4</sub>. Moving between neutral and ionic manifolds in this way will be strongly influenced by solvation effects. This leads to a small mismatch between the computed energies and the experimental
observations, where **M4** is higher in energy than the phosphoranes, although phosphonium ions and not the phosphoranes are observed experimentally. This is likely due to limitations in using a dielectric continuum solvation model, which under-solvates the ions and raises their energies relative to neutral species. The neutral/ionic manifold switch may help to explain the solvent effect seen in this system, where moving from o-C<sub>6</sub>H<sub>4</sub>F<sub>2</sub> (ε 13.4 = at 25 °C) to the more polar MeCN (ε = 35.9 at 25 °C) led to an increase in the catalytic rate. It is well known that more polar solvents like MeCN promote the formation of ionic species from phosphoranes and this would facilitate the formation of **M4**.

Hydride transfer from the silicate anion then leads to phosphorane **M5**, which can isomerise to form **M6**. This is very different from the mechanistic proposal of Dobrovetsky for HDF by geometrically constrained σ³-P systems, where it was suggested that PhSiH₃ reacts directly with the fluorophosphorane through a transition state that involves concerted hydride transfer to P and fluoride transfer to Si. The experiments described above showed that for simple trialkylphosphines, phosphonium ions are intermediates and these do not react directly with the neutral silanes. It may be that the positive charge on Dobrovetsky's constrained σ³-P systems disfavours this pathway and leads to this divergence in mechanistic behaviour.

The final step involves reductive elimination of **Py-F**<sub>4</sub> from **M6** via **TS**<sub>61</sub> (Figure 17). This transition state is more concerted than **TS**<sub>12</sub>, although still somewhat asynchronous, presumably because direct concerted reductive elimination from the axial and equatorial positions of a phosphorane is symmetry forbidden [58]. **TS**<sub>61</sub> is again Meisenheimer-like, although less so than in the constrained σ³-P systems of Radosevich [53], where the C–H bond length of a related reductive elimination TS is 1.33 Å, and Dobrovetsky where it is 1.53 Å. It seems as though the substituents and geometric environment around phosphorus have a significant impact on the reductive elimination process. These final steps give rise to the energetic span for the reaction, which is defined by the turnover-determining intermediate (TDI) **M5** and transition state (TDTS) **TS**<sub>61</sub>, and give an overall barrier for the catalytic reaction of 79 kJ mol<sup>-1</sup>. This low barrier is consistent with the observed fast reaction between **Py-F**<sub>5</sub> and Ph<sub>2</sub>SiH<sub>2</sub> when a sterically relatively small phosphine like P<sup>n</sup>Bu<sub>3</sub> is used as the catalyst (full conversion in 20 minutes at 20 °C in MeCN). The calculated energetic span for HDF of **Py-F**<sub>5</sub> by a constrained σ³-P system was significantly larger (140 kJ mol<sup>-1</sup>), which is consistent with the slower reactions observed in that study [54].

## Conclusions

Over the past two decades, the functionalisation of C–F bonds of perfluorinated compounds has attracted significant attention due to the numerous industrial applications of fluorinated organic compounds and the inherent challenges associated with these transformations, including the high strength of the C–F bond and the difficulty in the control of the reaction's selectivity. Furthermore, as fluorocarbons tend to be inert upon decomposition, they are persistent in the environment and the exploration of alternative methods for defluorination is of great interest.

This work presents novel protocols for the regioselective C–F bond functionalisation of polyfluorinated aromatics which are promoted by zirconium and catalysed by phosphines. Mechanistic investigations supported by experimental and, in selected examples, computational studies, complement the synthetic findings.

The reactivity of a zirconocene complex featuring a highly strained cyclohexyne ligand toward differently fluorinated pyridines was investigated. This complex, isolated by coordination of a PMe₃ ligand, exhibited different reactivity pathways depending on the fluorination pattern of the
observations, where **M4** is higher in energy than the phosphoranes, although phosphonium ions and not the phosphoranes are observed experimentally. This is likely due to limitations in using a dielectric continuum solvation model, which under-solvates the ions and raises their energies relative to neutral species. The neutral/ionic manifold switch may help to explain the solvent effect seen in this system, where moving from o-C<sub>6</sub>H<sub>4</sub>F<sub>2</sub> (ε 13.4 = at 25 °C) to the more polar MeCN (ε = 35.9 at 25 °C) led to an increase in the catalytic rate. It is well known that more polar solvents like MeCN promote the formation of ionic species from phosphoranes and this would facilitate the formation of **M4**.

Hydride transfer from the silicate anion then leads to phosphorane **M5**, which can isomerise to form **M6**. This is very different from the mechanistic proposal of Dobrovetsky for HDF by geometrically constrained σ³-P systems, where it was suggested that PhSiH₃ reacts directly with the fluorophosphorane through a transition state that involves concerted hydride transfer to P and fluoride transfer to Si. The experiments described above showed that for simple trialkylphosphines, phosphonium ions are intermediates and these do not react directly with the neutral silanes. It may be that the positive charge on Dobrovetsky's constrained σ³-P systems disfavours this pathway and leads to this divergence in mechanistic behaviour.

The final step involves reductive elimination of **Py-F**<sub>4</sub> from **M6** via **TS**<sub>61</sub> (Figure 17). This transition state is more concerted than **TS**<sub>12</sub>, although still somewhat asynchronous, presumably because direct concerted reductive elimination from the axial and equatorial positions of a phosphorane is symmetry forbidden [58]. **TS**<sub>61</sub> is again Meisenheimer-like, although less so than in the constrained σ³-P systems of Radosevich [53], where the C–H bond length of a related reductive elimination TS is 1.33 Å, and Dobrovetsky where it is 1.53 Å. It seems as though the substituents and geometric environment around phosphorus have a significant impact on the reductive elimination process. These final steps give rise to the energetic span for the reaction, which is defined by the turnover-determining intermediate (TDI) **M5** and transition state (TDTS) **TS**<sub>61</sub>, and give an overall barrier for the catalytic reaction of 79 kJ mol<sup>-1</sup>. This low barrier is consistent with the observed fast reaction between **Py-F**<sub>5</sub> and Ph<sub>2</sub>SiH<sub>2</sub> when a sterically relatively small phosphine like P<sup>n</sup>Bu<sub>3</sub> is used as the catalyst (full conversion in 20 minutes at 20 °C in MeCN). The calculated energetic span for HDF of **Py-F**<sub>5</sub> by a constrained σ³-P system was significantly larger (140 kJ mol<sup>-1</sup>), which is consistent with the slower reactions observed in that study [54].

## Conclusions

Over the past two decades, the functionalisation of C–F bonds of perfluorinated compounds has attracted significant attention due to the numerous industrial applications of fluorinated organic compounds and the inherent challenges associated with these transformations, including the high strength of the C–F bond and the difficulty in the control of the reaction's selectivity. Furthermore, as fluorocarbons tend to be inert upon decomposition, they are persistent in the environment and the exploration of alternative methods for defluorination is of great interest.

This work presents novel protocols for the regioselective C–F bond functionalisation of polyfluorinated aromatics which are promoted by zirconium and catalysed by phosphines. Mechanistic investigations supported by experimental and, in selected examples, computational studies, complement the synthetic findings.

The reactivity of a zirconocene complex featuring a highly strained cyclohexyne ligand toward differently fluorinated pyridines was investigated. This complex, isolated by coordination of a PMe₃ ligand, exhibited different reactivity pathways depending on the fluorination pattern of the
heteroaromatic. Remarkably, the reactive metal-alkyne bond could insert into the *ortho*-C–F bonds of 2,4,6-trifluoropyridine and 2,6-difluoropyridine, resulting in the net alkenylation of the polyfluorinated heterocycle. The reaction of the complex with 2,3,5,6-tetrafluoropyridine led instead to the 1,2-C–H addition over the Zr-(η²-C≡C). Conversely, the reaction with pentafluoropyridine mainly involved a parasitic S<sub>N</sub>Ar reaction of the dissociated PMe₃ ligand with the substrate, generating a series of difluorophosphoranes that triggered decomposition of the Zr complex.

These results led to the investigation into the stoichiometric reactivity between simple phosphines and pentafluoropyridine. Various trialkylphosphines exhibited the ability to activate the *para*-F of the heteroaromatic, leading to the formation of the products arising from hydrodefluorination or reductive homocoupling, along with methylenetetrafluoropyridyl-substituted or simple difluorophosphoranes, depending on the reaction conditions.

Because metal-free C–F bond activation is highly topical, these findings set the stage for the development of a metallomimetic catalytic system for the hydrodefluorination and aminodefluorination of polyfluorinated aromatics in the presence of silanes or silylamides. Remarkably, the catalyst employed for these transformations is the simple and readily available PⁿBu₃. It undergoes facile redox cycling between the P¹¹ and P⁵ oxidation states, enabling oxidative addition of the substrate, pseudotransmetalation and reductive elimination of the product throughout the catalytic cycle – typical steps observed in classical transition-metal-catalysis. The key features of this catalytic system are simplicity and cost. As all reagents are commercially available and easy to handle, there is no barrier to entry to these catalytic reactions, broadening their potential applicability in a range of settings.

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heteroaromatic. Remarkably, the reactive metal-alkyne bond could insert into the *ortho*-C–F bonds of 2,4,6-trifluoropyridine and 2,6-difluoropyridine, resulting in the net alkenylation of the polyfluorinated heterocycle. The reaction of the complex with 2,3,5,6-tetrafluoropyridine led instead to the 1,2-C–H addition over the Zr-(η²-C≡C). Conversely, the reaction with pentafluoropyridine mainly involved a parasitic S<sub>N</sub>Ar reaction of the dissociated PMe₃ ligand with the substrate, generating a series of difluorophosphoranes that triggered decomposition of the Zr complex.

These results led to the investigation into the stoichiometric reactivity between simple phosphines and pentafluoropyridine. Various trialkylphosphines exhibited the ability to activate the *para*-F of the heteroaromatic, leading to the formation of the products arising from hydrodefluorination or reductive homocoupling, along with methylenetetrafluoropyridyl-substituted or simple difluorophosphoranes, depending on the reaction conditions.

Because metal-free C–F bond activation is highly topical, these findings set the stage for the development of a metallomimetic catalytic system for the hydrodefluorination and aminodefluorination of polyfluorinated aromatics in the presence of silanes or silylamides. Remarkably, the catalyst employed for these transformations is the simple and readily available PⁿBu₃. It undergoes facile redox cycling between the P¹¹ and P⁷ oxidation states, enabling oxidative addition of the substrate, pseudotransmetalation and reductive elimination of the product throughout the catalytic cycle – typical steps observed in classical transition-metal-catalysis. The key features of this catalytic system are simplicity and cost. As all reagents are commercially available and easy to handle, there is no barrier to entry to these catalytic reactions, broadening their potential applicability in a range of settings.

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Wimonsiri HUADSAI

ESR6

**Thesis co-directors**

Prof. Matthias Westerhausen Friedrich-Schiller-Universität, Jena, Germany

Dr. Sébastien Bontemps Laboratoire de Chimie de Coordination, Toulouse, France

**Thesis defense**

Department of Chemistry, Friedrich-Schiller-Universität, 19 March 2024
Wimonsiri HUADSAI

ESR6

**Thesis co-directors**

Prof. Matthias Westerhausen Friedrich-Schiller-Universität, Jena, Germany

Dr. Sébastien Bontemps Laboratoire de Chimie de Coordination, Toulouse, France

**Thesis defense**

Department of Chemistry, Friedrich-Schiller-Universität, 19 March 2024
<figure><img src="image_140.png" /></figure>
<figure><img src="image_139.png" /></figure>
# Activation of Carbon Dioxide with Highly Lewis Acidic Compounds

## Introduction

Carbon dioxide is easily available and an important $C_1$ building block in synthetic chemistry [1]. Therefore, its properties in coordination chemistry and its use in stoichiometric and catalytic reduction processes is of utmost importance. Global abundance and non-toxicity of the alkaline-earth metals Mg and Ca elicit a vivid development of their reagents for stoichiometric and catalytic applications [2]. To suppress redistribution reactions of the ligands (Schlenk-type equilibria) and precipitation of insoluble $AeH_2$, the metal center can be shielded by very bulky ligands to stabilize heteroleptic molecules [3]. This strategy hinders far-reaching aggregation of hydrido alkaline-earth metal complexes despite the fact that Mg- and Ca-bound hydrido ligands avoid terminal binding modes and are commonly observed as bridging ligands. Therefore, bulky β-diketiminato and amidinato ligands were chosen to shield the reactive $Ae-H$ moieties. For probing the reactivity of $Ae-H$ functionalities towards $CO_2$, we started our investigations with the well-known dinuclear β-diketiminato alkaline-earth metal hydrides of magnesium (A1) [4] and calcium (A2) [5].

## Reactivity of [Ae]-based hydride complexes towards $CO_2$

### Reactivity of β-diketiminate Mg hydride with $CO_2$

The first section discusses the reduction of $CO_2$ by employing [Ae]-based hydride complexes. Overall, even in the absence of the Lewis acid $B(C_6F_5)_3$, the Mg hydride (A1) underwent a rapid reaction (<10 min) under one atmosphere of $CO_2$ at room temperature in THF-$d_8$, resulting in the formation of different

<figure><img src="image_141.png" /></figure>

<figure><img src="image_142.png" /></figure>

<figure><img src="image_143.png" /></figure>

(

Scheme 47). The ¹H DOSY NMR spectra indicated the presence of a mixture of compounds with different nuclearity in solution. These were assigned to the tetrameric and trimeric structures.

<figure><img src="image_144.png" /></figure>

<figure><img src="image_145.png" /></figure>

<figure><img src="image_146.png" /><figcaption>**Scheme 47.** The reaction of Mg hydride A1 with $CO_2$ at room temperature.</figcaption></figure>
# Activation of Carbon Dioxide with Highly Lewis Acidic Compounds

## Introduction

Carbon dioxide is easily available and an important $C_1$ building block in synthetic chemistry [1]. Therefore, its properties in coordination chemistry and its use in stoichiometric and catalytic reduction processes is of utmost importance. Global abundance and non-toxicity of the alkaline-earth metals Mg and Ca elicit a vivid development of their reagents for stoichiometric and catalytic applications [2]. To suppress redistribution reactions of the ligands (Schlenk-type equilibria) and precipitation of insoluble $AeH_2$, the metal center can be shielded by very bulky ligands to stabilize heteroleptic molecules [3]. This strategy hinders far-reaching aggregation of hydrido alkaline-earth metal complexes despite the fact that Mg- and Ca-bound hydrido ligands avoid terminal binding modes and are commonly observed as bridging ligands. Therefore, bulky $\beta$-diketiminato and amidinato ligands were chosen to shield the reactive $Ae-H$ moieties. For probing the reactivity of $Ae-H$ functionalities towards $CO_2$, we started our investigations with the well-known dinuclear $\beta$-diketiminato alkaline-earth metal hydrides of magnesium (A1) [4] and calcium (A2) [5].

## Reactivity of [Ae]-based hydride complexes towards $CO_2$

### Reactivity of $\beta$-diketiminate Mg hydride with $CO_2$

The first section discusses the reduction of $CO_2$ by employing [Ae]-based hydride complexes. Overall, even in the absence of the Lewis acid $B(C_6F_5)_3$, the Mg hydride (A1) underwent a rapid reaction (<10 min) under one atmosphere of $CO_2$ at room temperature in THF-$d_8$, resulting in the formation of different

<figure><img src="image_140.png" /></figure>

<figure><img src="image_141.png" /></figure>

<figure><img src="image_142.png" /></figure>

Scheme 47. The $^1H$ DOSY NMR spectra indicated the presence of a mixture of compounds with different nuclearity in solution. These were assigned to the tetrameric and trimeric structures.

<figure><img src="image_143.png" /></figure>

<figure><img src="image_144.png" /><figcaption>**Scheme 47.** The reaction of Mg hydride A1 with $CO_2$ at room temperature.</figcaption></figure>
In detail, the analysis of the resulting ¹H and ¹³C{¹H} NMR spectra indicated a rapid reaction of **A1** with CO₂, as evidenced by the disappearance of the hydride signal of the Mg-H moieties within 10 minutes at room temperature and the appearance of mixtures of compounds with new formate resonances. Two different sets of signals were observed. The most prominent proton NMR signal of these formate groups appeared as a sharp doublet at δ(¹H) = 8.13 ppm (¹J<sub>CH</sub> = 200 Hz, coupling with carbon of ¹³C-labeled CO₂), denoted as * in Figure 19. This proton signal was correlated with the carbon peak at δ(¹³C) = 169.4 ppm in the ¹H-¹³C{¹H} HSQC NMR spectrum. In addition, other broadening signals presumably from different species, were observed near or underneath these sharp peaks (marked as # in Figure 19).

Moreover, another CO₂ molecule was found to react at the central methine unit of the β-diketiminate ligand, leading to the formation of the newly formed C–C bonds (ii) (Scheme 47). This reactivity was characterized by the appearance of doublet signals in the ¹H NMR spectrum, originating from the CH backbone of the ligand. These signals, coupled to the carbon atoms of the newly formed carboxylato groups (–OCO) derived from ¹³C-labeled CO₂, were detected in the range of 4.69 to 4.85 ppm with a ²J<sub>CH</sub> of about 8 Hz (* in Figure 19). Adjacent to these sharp peaks, broader signals, assumed to be from other species, were also observed (marked as # in Figure 19).

<figure><img src="image_147.png" /><figcaption>**Figure 19.** Mixture of compounds (1)₃,₄ derived from ¹³CO₂ and ¹²CO₂. Bottom: sections of the ¹H NMR spectrum in the formate region and the central methine center of the ligand backbone derived from the reaction with ¹²CO₂. Top: compound mixture derived from ¹³CO₂. Signals with a * label are assigned to the trimer and those with a # label are assigned to the tetramer.</figcaption></figure>

Furthermore, the details of the composition and nuclearity of the species present in the solution mixture of (1)₃,₄ were determined using diffusion-ordered NMR spectroscopy (DOSY) at 298 K. This DOSY analysis revealed two distinct diffusion coefficients (D) and hydrodynamic radius (R<sub>H</sub>) for the system in THF-d<sub>8</sub> (conc. 18 mM). These D values are 3.6 × 10<sup>-10</sup> m² s<sup>-1</sup> with a calculated R<sub>H</sub> of 12.0 Å and D of 4.2 × 10<sup>-10</sup> m² s<sup>-1</sup> with a calculated R<sub>H</sub> = 10.5 Å along the self-diffusion dimension (Figure 20). The former diffusion constant, which is approximately twice the radius of the starting dimeric Mg (A1) (R<sub>H</sub> = 6.8 Å), suggests the presence of a tetrameric structure in the solution. On the other hand, the
In detail, the analysis of the resulting ¹H and ¹³C{¹H} NMR spectra indicated a rapid reaction of **A1** with CO₂, as evidenced by the disappearance of the hydride signal of the Mg-H moieties within 10 minutes at room temperature and the appearance of mixtures of compounds with new formate resonances. Two different sets of signals were observed. The most prominent proton NMR signal of these formate groups appeared as a sharp doublet at δ(¹H) = 8.13 ppm (¹J<sub>CH</sub> = 200 Hz, coupling with carbon of ¹³C-labeled CO₂), denoted as * in Figure 19. This proton signal was correlated with the carbon peak at δ(¹³C) = 169.4 ppm in the ¹H-¹³C{¹H} HSQC NMR spectrum. In addition, other broadening signals presumably from different species, were observed near or underneath these sharp peaks (marked as # in Figure 19).

Moreover, another CO₂ molecule was found to react at the central methine unit of the β-diketiminate ligand, leading to the formation of the newly formed C–C bonds (ii) (Scheme 47). This reactivity was characterized by the appearance of doublet signals in the ¹H NMR spectrum, originating from the CH backbone of the ligand. These signals, coupled to the carbon atoms of the newly formed carboxylato groups (–OCO) derived from ¹³C-labeled CO₂, were detected in the range of 4.69 to 4.85 ppm with a ²J<sub>CH</sub> of about 8 Hz (* in Figure 19). Adjacent to these sharp peaks, broader signals, assumed to be from other species, were also observed (marked as # in Figure 19).

<figure><img src="image_145.png" /><figcaption>**Figure 19.** Mixture of compounds (1)₃,₄ derived from ¹³CO₂ and ¹²CO₂. Bottom: sections of the ¹H NMR spectrum in the formate region and the central methine center of the ligand backbone derived from the reaction with ¹²CO₂. Top: compound mixture derived from ¹³CO₂. Signals with a * label are assigned to the trimer and those with a # label are assigned to the tetramer.</figcaption></figure>

Furthermore, the details of the composition and nuclearity of the species present in the solution mixture of (1)₃,₄ were determined using diffusion-ordered NMR spectroscopy (DOSY) at 298 K. This DOSY analysis revealed two distinct diffusion coefficients (D) and hydrodynamic radius (R<sub>H</sub>) for the system in THF-d<sub>8</sub> (conc. 18 mM). These D values are 3.6 × 10<sup>-10</sup> m² s<sup>-1</sup> with a calculated R<sub>H</sub> of 12.0 Å and D of 4.2 × 10<sup>-10</sup> m² s<sup>-1</sup> with a calculated R<sub>H</sub> = 10.5 Å along the self-diffusion dimension (Figure 20). The former diffusion constant, which is approximately twice the radius of the starting dimeric Mg (A1) (R<sub>H</sub> = 6.8 Å), suggests the presence of a tetrameric structure in the solution. On the other hand, the
latter diffusion coefficient, with a slightly smaller solution volume, indicates a more compact structure in solution, implying a trimeric character.

<figure><img src="image_148.png" /><figcaption>**Figure 20.** ¹H DOSY NMR of the mixture (1)₃,₄ (600 MHz, 298 K, THF-d₈). The horizontal scale shows the ¹H chemical shifts (ppm), whereas the vertical dimension shows the diffusion scale (x 10⁻¹⁰ m²s⁻¹). The visualization of the spectrum was processed using the exponential processing method. The self-diffusion coefficients (D) were obtained by fitting the peak areas at the formate signal (8.13 ppm) to the Stejskal-Tanner equation from the T1/T2 analysis in TopSpin 4.1.4 software.</figcaption></figure>

Subsequent heating of a mixture of compounds (1)₃,₄ (maintained under one atmosphere of CO₂) at 60 °C for 4 days resulted in the transformation of these compounds to the compound (2-THF)₆ as a hexamer (as depicted in Scheme 48). This transformation involved the decoordination of the two nitrogen donors in the ligand of compounds (1)₃,₄ during the heating process. Simultaneously, the proton shifted from CH-backbone of the ligand, where the carbon atoms of the Nacnac ligands were modified through transannular bonding of CO₂ (ii) in mixture (1)₃,₄, to the nitrogen atom. This shift resulted in the generation of free amines (-NH moieties) within the Nacnac ligand. These processes facilitated the coordination of the carboxylate groups, [(DippNacnac)-OCO], to Mg metal centers. Ultimately, the reaction underwent hexamerization, resulting in the formation of (2-THF)₆.

<figure><img src="image_149.png" /><figcaption>**Scheme 48.** The synthesis of compound (2-THF)₆.</figcaption></figure>
latter diffusion coefficient, with a slightly smaller solution volume, indicates a more compact structure in solution, implying a trimeric character.

<figure><img src="image_146.png" /><figcaption>**Figure 20.** ¹H DOSY NMR of the mixture (1)₃,₄ (600 MHz, 298 K, THF-d₈). The horizontal scale shows the ¹H chemical shifts (ppm), whereas the vertical dimension shows the diffusion scale (x 10⁻¹⁰ m²s⁻¹). The visualization of the spectrum was processed using the exponential processing method. The self-diffusion coefficients (D) were obtained by fitting the peak areas at the formate signal (8.13 ppm) to the Stejskal-Tanner equation from the T1/T2 analysis in TopSpin 4.1.4 software.</figcaption></figure>

Subsequent heating of a mixture of compounds (1)₃,₄ (maintained under one atmosphere of CO₂) at 60 °C for 4 days resulted in the transformation of these compounds to the compound (2-THF)₆ as a hexamer (as depicted in Scheme 48). This transformation involved the decoordination of the two nitrogen donors in the ligand of compounds (1)₃,₄ during the heating process. Simultaneously, the proton shifted from CH-backbone of the ligand, where the carbon atoms of the Nacnac ligands were modified through transannular bonding of CO₂ (ii) in mixture (1)₃,₄, to the nitrogen atom. This shift resulted in the generation of free amines (-NH moieties) within the Nacnac ligand. These processes facilitated the coordination of the carboxylate groups, [(DippNacnac)-OCO], to Mg metal centers. Ultimately, the reaction underwent hexamerization, resulting in the formation of (2-THF)₆.

<figure><img src="image_147.png" /><figcaption>**Scheme 48.** The synthesis of compound (2-THF)₆.</figcaption></figure>
Over the course of the 4-day reaction at 60 °C, block-shaped colorless crystals of compound (2-THF)₆ gradually grew and accumulated under one atmosphere of CO₂ in THF-d₈. These crystals were obtained in a good yield (from 77% to 81%) and were analyzed by X-ray diffraction analysis.

The molecular structure of (2-THF)₆ is depicted in Figure 21 (a), while Figure 21 (b) represents its monomeric unit along with selected bond distances. Within its molecular structure, the hexamer has a crystallographic center of inversion, rendering the six hexagonal sides of the hexamer identical. Each Mg center adopts an octahedral geometry, with coordination by two carboxylate ligands and three formate groups, as well as an additionally coordinated THF molecule. The ligand derived from the β-diketiminate system exhibits a terminal κ²O,O' carboxylato ligand, while those arising from the formate moieties (–OCHO) bridge the two Mg centers *via* a symmetrical mode, μ-formato-κO:κO'. The first Mg center is bridged to the second Mg center by the Mg–O(1) bond distance of 2.067(5) Å, which is similar to the Mg–O(2) distance in the molecule. Interestingly, the oxygen atoms O(2) are also bridged toward the third Mg center, displaying a comparable distance of 2.095(5) Å to the other two in the molecule. Additionally, the decoordination of the two nitrogen donor ligands at the Mg center is also demonstrated. The C–N bonds within the ligand clearly consist of both single bonds and double bonds, with C(1)–N(1) and C(9)–N(2) bond distances of 1.335(8) and 1.293(9), respectively. This indicates the formation of a free bidentate Nacnac ligand with free NH moieties within the structure of complex (2-THF)₆.

<figure><img src="image_150.png" /><figcaption>(b) Monomeric unit of (2-THF)₆</figcaption></figure>

<figure><img src="image_151.png" /></figure>

**Figure 21.** ORTEP representations of molecular structure of hexameric Mg-formate (b) and monomer unit (a) of (2-THF)₆ with selective atom labelling. Displacement ellipsoids are set at 30% probability; hydrogen atoms except H(1) and H(281) are omitted for clarity. The 2,6-iPr₂C₆H₃ groups are shown as wireframes for simplicity. Selected bond distances (Å): Mg1–O(1), 2.087(5); Mg1–O(2), 2.103(6); Mg1–O(4), 2.005(5); Mg1–O(5), 2.160(6); O2–C(28), 1.262(8); O1–C(28), 1.231(8); O4–C(4), 1.242(8); O3–C(4), 1.270(8); N1–C(2), 1.442(8); N1–C(1), 1.336(8); N1–H(1), 0.863(19); N2–C(9), 1.293(9); C28–H(281), 0.983.
Over the course of the 4-day reaction at 60 °C, block-shaped colorless crystals of compound (2-THF)₆ gradually grew and accumulated under one atmosphere of CO₂ in THF-d₈. These crystals were obtained in a good yield (from 77% to 81%) and were analyzed by X-ray diffraction analysis.

The molecular structure of (2-THF)₆ is depicted in Figure 21 (a), while Figure 21 (b) represents its monomeric unit along with selected bond distances. Within its molecular structure, the hexamer has a crystallographic center of inversion, rendering the six hexagonal sides of the hexamer identical. Each Mg center adopts an octahedral geometry, with coordination by two carboxylate ligands and three formate groups, as well as an additionally coordinated THF molecule. The ligand derived from the β-diketiminate system exhibits a terminal κ²O,O' carboxylato ligand, while those arising from the formate moieties (–OCHO) bridge the two Mg centers *via* a symmetrical mode, μ-formato-κO:κO'. The first Mg center is bridged to the second Mg center by the Mg–O(1) bond distance of 2.067(5) Å, which is similar to the Mg–O(2) distance in the molecule. Interestingly, the oxygen atoms O(2) are also bridged toward the third Mg center, displaying a comparable distance of 2.095(5) Å to the other two in the molecule. Additionally, the decoordination of the two nitrogen donor ligands at the Mg center is also demonstrated. The C–N bonds within the ligand clearly consist of both single bonds and double bonds, with C(1)–N(1) and C(9)–N(2) bond distances of 1.335(8) and 1.293(9), respectively. This indicates the formation of a free bidentate Nacnac ligand with free NH moieties within the structure of complex (2-THF)₆.

<figure><img src="image_148.png" /><figcaption>(b) Monomeric unit of (2-THF)₆</figcaption></figure>

<figure><img src="image_149.png" /></figure>

**Figure 21.** ORTEP representations of molecular structure of hexameric Mg-formate (b) and monomer unit (a) of (2-THF)₆ with selective atom labelling. Displacement ellipsoids are set at 30% probability; hydrogen atoms except H(1) and H(281) are omitted for clarity. The 2,6-iPr₂C₆H₃ groups are shown as wireframes for simplicity. Selected bond distances (Å): Mg1–O(1), 2.087(5); Mg1–O(2), 2.103(6); Mg1–O(4), 2.005(5); Mg1–O(5), 2.160(6); O2–C(28), 1.262(8); O1–C(28), 1.231(8); O4–C(4), 1.242(8); O3–C(4), 1.270(8); N1–C(2), 1.442(8); N1–C(1), 1.336(8); N1–H(1), 0.863(19); N2–C(9), 1.293(9); C28–H(281), 0.983.
### Reactivity of β-diketiminato Ca hydride with CO₂

Further studies involving the reactions of Ca hydride (A2) with CO₂ (1 atm) showed a reactivity similar to that of Mg (A1), with the observed NMR spectra showing resonances corresponding to a different mixture of formate species with different nuclearity depending on the reaction conditions (as depicted in Scheme 49). Several attempts of crystallization under different conditions failed, often resulting in gel-like materials in concentrated solutions.

<figure><img src="image_152.png" /><figcaption>**Scheme 49.** The reaction of calcium hydride A2 with CO₂ in different reaction conditions and the proposed products.</figcaption></figure>

### Synthesis and reactivity of amidinate Mg hydride (6)₂

In contrast to β-diketiminate ligands, the two-coordinate amidinate ligand systems exhibit a distinct profile with larger coordination sites. These features could possibly modify the reactivity of the complex. Accordingly, the preparation of the amidinato Mg hydride complex (6)₂ was conducted. Through the hydrosilane metathesis reaction, Mg complex (6)₂ was obtained in 80% yield (Scheme 50). This Mg hydride was crystallographically characterized, and the molecular structure of (6)₂ is depicted in Figure 22.

In the molecular structure of (6)₂, the complex forms a hydride-bridged dimer, with the metal centers N,N'-chelated by amidinate ligands. The coordination mode is similar to that of the closely related amidinato-Mg hydride complex with the κ²-N,N' coordination mode [6]. A comparison of the structures of (6)₂ and A1 reveals that the N–Mg–N angles in (6)₆ are 64.55(8)° and 63.34(8)° for Mg(1) and Mg(2), respectively, which is smaller than the angle of the β-diketiminate Mg hydride A1 of 93.14(9)° [7]. This observation suggests that the Mg center in (6)₆ is less shielded, and the accessibility of the substrates to the complex is more facilitated than in A1.

With the amidinate Mg hydride complex (6)₂ in hand, an investigation on its reactivity with CO₂ was conducted. It was observed that Mg hydride (6)₂ cleanly reacted with 1 atm of CO₂ at room temperature in THF-d₈ to yield a single new species (7)₂ within 16 h (Scheme 51). Compound (7)₂ was isolated from concentrated THF at ambient temperature with a 72% yield. This observation was further confirmed through a single crystal X-ray diffraction analysis (Figure 23). Compound (7)₂ is a centrosymmetric amidinato magnesium formate dimer, where two Mg centers are bridged by formate units in a μ₂-fashion, originating from the reduction of CO₂ through hydride transfer.
### Reactivity of β-diketiminato Ca hydride with CO₂

Further studies involving the reactions of Ca hydride (A2) with CO₂ (1 atm) showed a reactivity similar to that of Mg (A1), with the observed NMR spectra showing resonances corresponding to a different mixture of formate species with different nuclearity depending on the reaction conditions (as depicted in Scheme 49). Several attempts of crystallization under different conditions failed, often resulting in gel-like materials in concentrated solutions.

<figure><img src="image_150.png" /><figcaption>**Scheme 49.** The reaction of calcium hydride A2 with CO₂ in different reaction conditions and the proposed products.</figcaption></figure>

### Synthesis and reactivity of amidinate Mg hydride (6)₂

In contrast to β-diketiminate ligands, the two-coordinate amidinate ligand systems exhibit a distinct profile with larger coordination sites. These features could possibly modify the reactivity of the complex. Accordingly, the preparation of the amidinato Mg hydride complex (6)₂ was conducted. Through the hydrosilane metathesis reaction, Mg complex (6)₂ was obtained in 80% yield (Scheme 50). This Mg hydride was crystallographically characterized, and the molecular structure of (6)₂ is depicted in Figure 22.

In the molecular structure of (6)₂, the complex forms a hydride-bridged dimer, with the metal centers N,N'-chelated by amidinate ligands. The coordination mode is similar to that of the closely related amidinato-Mg hydride complex with the κ²-N,N' coordination mode [6]. A comparison of the structures of (6)₂ and A1 reveals that the N-Mg-N angles in (6)₆ are 64.55(8)° and 63.34(8)° for Mg(1) and Mg(2), respectively, which is smaller than the angle of the β-diketiminate Mg hydride A1 of 93.14(9)° [7]. This observation suggests that the Mg center in (6)₆ is less shielded, and the accessibility of the substrates to the complex is more facilitated than in A1.

With the amidinate Mg hydride complex (6)₂ in hand, an investigation on its reactivity with CO₂ was conducted. It was observed that Mg hydride (6)₂ cleanly reacted with 1 atm of CO₂ at room temperature in THF-d₈ to yield a single new species (7)₂ within 16 h (Scheme 51). Compound (7)₂ was isolated from concentrated THF at ambient temperature with a 72% yield. This observation was further confirmed through a single crystal X-ray diffraction analysis (Figure 23). Compound (7)₂ is a centrosymmetric amidinato magnesium formate dimer, where two Mg centers are bridged by formate units in a μ₂-fashion, originating from the reduction of CO₂ through hydride transfer.
<figure><img src="image_153.png" /><figcaption>**Scheme 50.** Synthetic procedure for the formation of magnesium compound (6)₂ via hydrosilane metathesis.</figcaption></figure>

<figure><img src="image_154.png" /><figcaption>**Figure 22.** ORTEP representation of the molecular structure of (6)₂ with selective atom labeling. Displacement ellipsoids are set at 50% probability; hydrogen atoms except for the hydride protons H(1) and H(2) are omitted for clarity. The 2,6-iPr₂C₆H₃ and 2,6-bis(diphenylmethyl)-4-methylphenyl moieties, adamantly and THF coordinated parts, have been set as wireframes for clarity reasons. Selected bond lengths (Å): Mg1–Mg2, 2.8533(12); Mg1–N(1), 2.046(2); Mg1–N(2), 2.087(2); Mg1–H(1), 1.8454(9); Mg1–H(2), 1.8842(8); Mg2–N(3), 2.049(2); Mg2–N(4), 2.142(2); Mg2–O(1), 2.0801(19); Mg2–H(1), 2.1816(8); Mg2–H(2), 1.9119(9). Selected bond angles (°): H(1)–Mg1–H(2), 91.38(4); H(1)–Mg2–H(2), 81.09(3); N(1)–Mg1–N(2), 64.55(8); N(3)–Mg2–N(4), 63.34(8).</figcaption></figure>

<figure><img src="image_155.png" /><figcaption>**Scheme 51.** Reaction of magnesium hydride (6)₂ with CO₂ to give dimeric compound (7)₂.</figcaption></figure>

<figure><img src="image_156.png" /><figcaption>**Figure 23.** ORTEP representation of the molecular structure of (7)₂ with selective atomic labeling. Displacement ellipsoids are set at 50% probability; hydrogen atoms except for the formate protons are omitted for clarity. The 2,6-iPr₂C₆H₃ and 2,6-bis(diphenylmethyl)-4-methylphenyl moieties, adamantyl and THF-coordinated parts, have been set as wireframes for clarity. Selected bond lengths (Å): Mg1–N(1), 2.096(2); Mg1–N(2), 2.162(2); Mg1–O(1), 1.961(2); Mg1–O(2), 1.9404(19); Mg1–O(3), 2.1349(18); O1–C(8), 1.245(3); O2–C(8), 1.242(3). Selected bond angles (°): N(1)–Mg–N(2), 61.88(7); O(1)–Mg–O(2), 112.44(8); O(2)–Mg–N(2), 101.59(8); O(1)–Mg–N(2), 103.79(8); N(1)–Mg–O(3), 95.27(8); O(1)–Mg–O(3), 88.59(8); O(1)–Mg–N(1), 113.24(8). Symmetry operations: (i) $-x+2, -y+1, -z+1$.</figcaption></figure>
<figure><img src="image_151.png" /><figcaption>**Scheme 50.** Synthetic procedure for the formation of magnesium compound (6)₂ via hydrosilane metathesis.</figcaption></figure>

<figure><img src="image_152.png" /><figcaption>**Figure 22.** ORTEP representation of the molecular structure of (6)₂ with selective atom labeling. Displacement ellipsoids are set at 50% probability; hydrogen atoms except for the hydride protons H(1) and H(2) are omitted for clarity. The 2,6-iPr₂C₆H₃ and 2,6-bis(diphenylmethyl)-4-methylphenyl moieties, adamantly and THF coordinated parts, have been set as wireframes for clarity reasons. Selected bond lengths (Å): Mg1–Mg2, 2.8533(12); Mg1–N(1), 2.046(2); Mg1–N(2), 2.087(2); Mg1–H(1), 1.8454(9); Mg1–H(2), 1.8842(8); Mg2–N(3), 2.049(2); Mg2–N(4), 2.142(2); Mg2–O(1), 2.0801(19); Mg2–H(1), 2.1816(8); Mg2–H(2), 1.9119(9). Selected bond angles (°): H(1)–Mg1–H(2), 91.38(4); H(1)–Mg2–H(2), 81.09(3); N(1)–Mg1–N(2), 64.55(8); N(3)–Mg2–N(4), 63.34(8).</figcaption></figure>

<figure><img src="image_153.png" /><figcaption>**Scheme 51.** Reaction of magnesium hydride (6)₂ with CO₂ to give dimeric compound (7)₂.</figcaption></figure>

<figure><img src="image_154.png" /><figcaption>**Figure 23.** ORTEP representation of the molecular structure of (7)₂ with selective atomic labeling. Displacement ellipsoids are set at 50% probability; hydrogen atoms except for the formate protons are omitted for clarity. The 2,6-iPr₂C₆H₃ and 2,6-bis(diphenylmethyl)-4-methylphenyl moieties, adamantyl and THF-coordinated parts, have been set as wireframes for clarity. Selected bond lengths (Å): Mg1–N(1), 2.096(2); Mg1–N(2), 2.162(2); Mg1–O(1), 1.961(2); Mg1–O(2), 1.9404(19); Mg1–O(3), 2.1349(18); O1–C(8), 1.245(3); O2–C(8), 1.242(3). Selected bond angles (°): N(1)–Mg–N(2), 61.88(7); O(1)–Mg–O(2), 112.44(8); O(2)–Mg–N(2), 101.59(8); O(1)–Mg–N(2), 103.79(8); N(1)–Mg–O(3), 95.27(8); O(1)–Mg–O(3), 88.59(8); O(1)–Mg–N(1), 113.24(8). Symmetry operations: (i) $-x+2, -y+1, -z+1$.</figcaption></figure>
### Hydroboration of CO₂ mediated by [Ae]-based catalysts and C-S bond formation [8].

In this following section, we have demonstrated the efficient activity of [Ae]-based hydride complexes in the selective hydroboration of CO₂ to produce a 4e⁻ reduction of CO₂ yielding bis(boryl)acetal (BBA). In this process, two different hydroboranes, namely borabicyclo[3.3.1]nonane (9-BBN) and dicyclohexylborane (HBCy₂), were employed as reducing agents. Subsequently, the *in situ* generated BBAs were used as a source of C₁ units to produce compounds containing C–S bonds. Interestingly, this approach allowed the synthesis of new borylated hemithioacetal compounds (R'S–CH₂–OBR₂) under mild and neutral reaction conditions. These hemithioacetals were then transformed into dithioacetals by the addition of a second thiol equivalent in the presence of acid H₂SO₄ or to hemithioaminals through reactions with secondary amines (Scheme 52).

**Our strategy**

<figure><img src="image_157.png" /><figcaption>**Scheme 52.** Selective alkaline earth catalyzed 4e⁻ Reduction of CO₂ to give BBA as an intermediate to generate compounds containing S–CH₂ bonds.</figcaption></figure>

### Catalytic hydroboration of CO₂

Initially, the catalytic activity of Mg (A1) and Ca (A2) hydride complexes with 9-BBN was evaluated. In this regard, these experiments utilized 1 mol% catalytic loading of either complex Mg (A1) or Ca (A2) together with 0.13 mmol of 9-BBN in THF-d₈ in a pressurized NMR tube. For internal reference in the NMR conversion, hexamethylbenzene was introduced. After degassing the sample, it was exposed to isotopically ¹³C-enriched CO₂ (1 atm), and the reaction was monitored by ¹H and ¹³C{¹H} NMR analyses at room temperature.

In terms of catalytic activity, the Mg (A1) and Ca (A2) complexes proved to be efficient catalysts with high selectivity in driving the 4e⁻ reduction of CO₂ at room temperature. When used at 1 mol% catalytic loading, BBA<sup>BBN</sup> achieved remarkable *in situ* yields of 79% and 87% for Mg hydride (A1) and Ca hydride (A2), respectively, under 1 atm of CO₂ pressure (Table 6, entries 1 and 2). These reactions were completed within 7 h 25 min for Mg hydride (A1) and 1 h 40 min for Ca hydride (A2), resulting in TONs of 79 and 87 and TOFs of 11 and 52 h⁻¹ for Mg and Ca compounds, respectively. The speciation's kinetic plots were also constructed to illustrate the changes in product yield and 9-BBN conversion over time (in minutes) (Figure 24).

Furthermore, the catalytic reactivity of our newly developed amidinato-Mg hydride complex (6)₂ towards the reduction of CO₂ with 9-BBN was explored. An analogous CO₂ hydroboration with 9-BBN, using catalytic amounts of Mg complex (6)₂ (1 mol%) in THF-d₈, was conducted. Surprisingly, the TOF decreased to 8 h⁻¹ (requiring 16 h for complete borane conversion), accompanied by the formation of a mixture consisting of BBA (76%) and methoxyborane (22%).
### Hydroboration of CO₂ mediated by [Ae]-based catalysts and C-S bond formation [8].

In this following section, we have demonstrated the efficient activity of [Ae]-based hydride complexes in the selective hydroboration of CO₂ to produce a 4e⁻ reduction of CO₂ yielding bis(boryl)acetal (BBA). In this process, two different hydroboranes, namely borabicyclo[3.3.1]nonane (9-BBN) and dicyclohexylborane (HBCy₂), were employed as reducing agents. Subsequently, the *in situ* generated BBAs were used as a source of C₁ units to produce compounds containing C–S bonds. Interestingly, this approach allowed the synthesis of new borylated hemithioacetal compounds (R'S–CH₂–OBR₂) under mild and neutral reaction conditions. These hemithioacetals were then transformed into dithioacetals by the addition of a second thiol equivalent in the presence of acid H₂SO₄ or to hemithioaminals through reactions with secondary amines (Scheme 52).

**Our strategy**

<figure><img src="image_155.png" /><figcaption>**Scheme 52.** Selective alkaline earth catalyzed 4e⁻ Reduction of CO₂ to give BBA as an intermediate to generate compounds containing S–CH₂ bonds.</figcaption></figure>

### Catalytic hydroboration of CO₂

Initially, the catalytic activity of Mg (A1) and Ca (A2) hydride complexes with 9-BBN was evaluated. In this regard, these experiments utilized 1 mol% catalytic loading of either complex Mg (A1) or Ca (A2) together with 0.13 mmol of 9-BBN in THF-d₈ in a pressurized NMR tube. For internal reference in the NMR conversion, hexamethylbenzene was introduced. After degassing the sample, it was exposed to isotopically ¹³C-enriched CO₂ (1 atm), and the reaction was monitored by ¹H and ¹³C{¹H} NMR analyses at room temperature.

In terms of catalytic activity, the Mg (A1) and Ca (A2) complexes proved to be efficient catalysts with high selectivity in driving the 4e⁻ reduction of CO₂ at room temperature. When used at 1 mol% catalytic loading, BBA<sup>BBN</sup> achieved remarkable *in situ* yields of 79% and 87% for Mg hydride (A1) and Ca hydride (A2), respectively, under 1 atm of CO₂ pressure (Table 6, entries 1 and 2). These reactions were completed within 7 h 25 min for Mg hydride (A1) and 1 h 40 min for Ca hydride (A2), resulting in TONs of 79 and 87 and TOFs of 11 and 52 h⁻¹ for Mg and Ca compounds, respectively. The speciation's kinetic plots were also constructed to illustrate the changes in product yield and 9-BBN conversion over time (in minutes) (Figure 24).

Furthermore, the catalytic reactivity of our newly developed amidinato-Mg hydride complex (6)<sub>2</sub> towards the reduction of CO₂ with 9-BBN was explored. An analogous CO₂ hydroboration with 9-BBN, using catalytic amounts of Mg complex (6)<sub>2</sub> (1 mol%) in THF-d₈, was conducted. Surprisingly, the TOF decreased to 8 h⁻¹ (requiring 16 h for complete borane conversion), accompanied by the formation of a mixture consisting of BBA (76%) and methoxyborane (22%).
<figure><img src="image_158.png" /><figcaption>**Table 6.** Hydroborations of CO₂ to BBA<sup>BBN</sup> and BBA<sup>Cy</sup> are catalyzed by β-diketiminate Mg (A4), Ca (A32), complexes, or amidine Mg (6)₂.</figcaption></figure>

[a] Reaction conditions: 1 atm CO₂, 0.13 mmol HBR₂, 1 mol% catalyst, 650 µl THF-d₈, 25 °C. [b] Yields were calculated in situ using hexamethylbenzene as an internal standard. [c] The TON numbers were calculated from the BBA yields.

<figure><img src="image_159.png" /><figcaption>**Figure 24.** Plot of the evolution of reactant and products versus time in the hydroboration of CO₂ with 9-BBN with 1 mol% of amidinato Mg hydride A1 (left) and Ca hydride A2 (right).</figcaption></figure>

In addition, the catalytic reactions of CO₂ (1 atm) with 1 mol% of catalyst Mg hydride (A1) or Ca hydride (A2) with HBCy₂ in THF-d₈ were performed. These reactions led to the selective generation of BBA<sup>Cy</sup>, with yields of 95% and 90% within only 18 and 19 min, respectively, with the calculated TOF of 317 and 284 h⁻¹ measured for Mg hydride A1 and Ca hydride A2, respectively (Table 6, entries 4-5). The observed TOF values are particularly high for CO₂ hydroboration reactions catalyzed by [Ae]-based complexes and also for BBA synthesis from CO₂. Nevertheless, the use of the amidinate-Mg catalyst (6)₂ provided mixtures of BBA<sup>Cy</sup> and methoxyborane, with only 56% of BBA<sup>Cy</sup> conversion observed after 1 h 25 minutes.

Overall, among the [Ae]-based catalysts employed, Ca hydride (A2) demonstrated superior effectiveness and selectivity, resulting in high TOFs and BBA yields. Conversely, the Mg amidinate
<figure><img src="image_156.png" /><figcaption>**Table 6.** Hydroborations of CO₂ to BBA<sup>BBN</sup> and BBA<sup>Cy</sup> are catalyzed by β-diketiminate Mg (A4), Ca (A32), complexes, or amidine Mg (6)₂.</figcaption></figure>

[a] Reaction conditions: 1 atm CO₂, 0.13 mmol HBR₂, 1 mol% catalyst, 650 µl THF-d₈, 25 °C. [b] Yields were calculated in situ using hexamethylbenzene as an internal standard. [c] The TON numbers were calculated from the BBA yields.

<figure><img src="image_157.png" /><figcaption>**Figure 24.** Plot of the evolution of reactant and products versus time in the hydroboration of CO₂ with 9-BBN with 1 mol% of amidinate Mg hydride A1 (left) and Ca hydride A2 (right).</figcaption></figure>

In addition, the catalytic reactions of CO₂ (1 atm) with 1 mol% of catalyst Mg hydride (A1) or Ca hydride (A2) with HBCy₂ in THF-d₈ were performed. These reactions led to the selective generation of BBA<sup>Cy</sup>, with yields of 95% and 90% within only 18 and 19 min, respectively, with the calculated TOF of 317 and 284 h⁻¹ measured for Mg hydride A1 and Ca hydride A2, respectively (Table 6, entries 4-5). The observed TOF values are particularly high for CO₂ hydroboration reactions catalyzed by [Ae]-based complexes and also for BBA synthesis from CO₂. Nevertheless, the use of the amidinate-Mg catalyst (6)₂ provided mixtures of BBA<sup>Cy</sup> and methoxyborane, with only 56% of BBA<sup>Cy</sup> conversion observed after 1 h 25 minutes.

Overall, among the [Ae]-based catalysts employed, Ca hydride (A2) demonstrated superior effectiveness and selectivity, resulting in high TOFs and BBA yields. Conversely, the Mg amidinate
catalyst exhibited lower efficiency and selectivity in CO₂ reduction with hydroboranes to BBA. The observed reactivity trend can be summarized as **A2>A1>(6)₂**, which is consistent with the trend in hydricity of the metal center, suggesting that CO₂ insertion into the metal hydride bond is a rate-determining step. Nonetheless, the poor catalytic performance of the amidinato Mg complex **1** in comparison to β-diketiminate Mg and Ca complexes remains unclear. It is plausible that the amidinato Mg compound (6)₂ underwent deactivation during catalysis, and the actual active species responsible for catalyzing CO₂ hydroboration may differ from those initially present.

### C-S bond formation from BBA

After the successful preparation of BBA compounds by selective double CO₂ hydroboration, their reactivity with thiol compounds was investigated. Thereby, the Ca-based catalyst (**A2**) was selected to selectively generate the BBA compound. First, the reactivity of BBA<sup>BBN</sup> was investigated. In this study, <sup>13</sup>C-labeled CO₂ was employed for the *in-situ* generation of BBA<sup>BBN</sup>, which allows us to closely monitor the fate of the carbon atom arising from CO₂, particularly the methylene moiety (<sup>13</sup>CH₂).

<figure><img src="image_160.png" /><figcaption>**Scheme 53.** Reactivity of BBA<sup>BBN</sup> toward thiophenol, 1,3-propanedithiol, and 2-aminothiophenol.</figcaption></figure>

As a result, the combination of the *in situ* generated <sup>13</sup>C-BBA<sup>BBN</sup> with one equivalent of thiophenol (pathway **A**) resulted in the formation of a novel borylated hemithioacetal, identified as compound **8**, with a 57% NMR yield at room temperature within 16 hours (Scheme 53). During the reaction, the formation of adduct **8** was clearly evident in the NMR spectra, as indicated by the disappearance of the methylene signal of <sup>13</sup>C-BBA<sup>BBN</sup> at δ(<sup>1</sup>H) 5.54 (d, <sup>1</sup>J<sub>CH</sub> = 165.1 Hz) and δ(<sup>13</sup>C{<sup>1</sup>H}) 87.0 ppm in THF-d<sub>8</sub>. Simultaneously, a new methylene signal corresponding to compound **8** appeared at δ(<sup>1</sup>H) 5.38 (d, <sup>1</sup>J<sub>CH</sub> = 159.5 Hz) and δ(<sup>13</sup>C{<sup>1</sup>H}) 70.6 ppm.

In addition, when two equivalents of thiophenol were introduced under similar reaction conditions, the expected compound **9** was not formed, even at elevated temperatures. Instead, the characteristic methylene resonance of intermediate **8** was detected in the NMR spectra. However, when sulfuric acid (H₂SO₄) was added to the reaction solution, it resulted in the formation of the preliminary protonated **8** species at room temperature. In the process, the OBR₂ fragment within **8**
catalyst exhibited lower efficiency and selectivity in CO₂ reduction with hydroboranes to BBA. The observed reactivity trend can be summarized as A2>A1>(6)₂, which is consistent with the trend in hydricity of the metal center, suggesting that CO₂ insertion into the metal hydride bond is a rate-determining step. Nonetheless, the poor catalytic performance of the amidinato Mg complex 1 in comparison to β-diketiminate Mg and Ca complexes remains unclear. It is plausible that the amidinato Mg compound (6)₂ underwent deactivation during catalysis, and the actual active species responsible for catalyzing CO₂ hydroboration may differ from those initially present.

### C-S bond formation from BBA

After the successful preparation of BBA compounds by selective double CO₂ hydroboration, their reactivity with thiol compounds was investigated. Thereby, the Ca-based catalyst (A2) was selected to selectively generate the BBA compound. First, the reactivity of BBA<sup>BBN</sup> was investigated. In this study, ¹³C-labeled CO₂ was employed for the *in-situ* generation of BBA<sup>BBN</sup>, which allows us to closely monitor the fate of the carbon atom arising from CO₂, particularly the methylene moiety (¹³CH₂).

<figure><img src="image_158.png" /><figcaption>**Scheme 53.** Reactivity of BBA<sup>BBN</sup> toward thiophenol, 1,3-propanedithiol, and 2-aminothiophenol.</figcaption></figure>

As a result, the combination of the *in situ* generated ¹³C-BBA<sup>BBN</sup> with one equivalent of thiophenol (pathway A) resulted in the formation of a novel borylated hemithioacetal, identified as compound 8, with a 57% NMR yield at room temperature within 16 hours (Scheme 53). During the reaction, the formation of adduct 8 was clearly evident in the NMR spectra, as indicated by the disappearance of the methylene signal of ¹³C-BBA<sup>BBN</sup> at δ(¹H) 5.54 (d, ¹J<sub>CH</sub> = 165.1 Hz) and δ(¹³C{¹H}) 87.0 ppm in THF-d₈. Simultaneously, a new methylene signal corresponding to compound 8 appeared at δ(¹H) 5.38 (d, ¹J<sub>CH</sub> = 159.5 Hz) and δ(¹³C{¹H}) 70.6 ppm.

In addition, when two equivalents of thiophenol were introduced under similar reaction conditions, the expected compound 9 was not formed, even at elevated temperatures. Instead, the characteristic methylene resonance of intermediate 8 was detected in the NMR spectra. However, when sulfuric acid (H₂SO₄) was added to the reaction solution, it resulted in the formation of the preliminary protonated 8 species at room temperature. In the process, the OBR₂ fragment within 8
was protonated, facilitating the interaction with the added second equivalent of thiophenol. As a result, **compound 9** was generated within 3 h at 80 °C with a yield of 55% on BBA basis (Scheme 53).

In addition to thiophenol, an acyclic dithiol was utilized to react with the BBA<sup>BBN</sup> compound, resulting in the formation of hemithioacetals under neutral conditions at room temperature (pathway **B**, Scheme 53). When one equivalent of 1,3-propanedithiol was used, a mixture of hemiacetal compounds was generated. A major species, identified as hemithioacetal compound **10**, was observed, yielding 72% *in situ* based on BBA. Its signals in the ¹H NMR and ¹³C{¹H} NMR spectra were assigned at 5.07 ppm and 68.7 ppm, respectively. Another species, identified as a minor product, was the diborylated hemithioacetal **11**, with a very similar NMR signature to that of adduct **10**, obtained with an 18% NMR yield. Notably, the introduction of sulfuric acid into the solution containing compounds **10** and **11** led to the rapid formation of dithiane **12** with a yield of 57% within 10 minutes. On the contrary, the addition of 0.4 equivalents of 1,3-propanedithiol to BBA<sup>BBN</sup> favored the formation of compound **11** with an increase of the NMR yield to 78% (Scheme 53).

In addition, the reaction of the *in situ* generated BBA<sup>BBN</sup> with 2-aminothiophenol led to the formation of an unidentified mixture at room temperature, which completely converted the BBA substrate (pathway **C**, Scheme 53). Under acidic conditions, the generation of the 2,3-dihydrobenzothiazole compound **13** was detected, which was obtained in 89% yield based on the BBA reactant (Scheme 53).

To broaden the range of thiol compounds and to explore the reactivity of BBA<sup>BBN</sup> with various thiols, experiments were conducted with 1-butanethiol, benzylmercaptan, and 2-mercaptobenzothiazole reacting with BBA<sup>BBN</sup> (Scheme 54). These reactions proceeded smoothly at room temperature, yielding hemithioacetal compounds (**14-16**) with favorable NMR yields of 63%, 73%, and 53%, respectively, within 16 hours. Analysis of the ¹H NMR spectra of compounds **8** and **14-16** revealed variations in the methylene moieties at δ(CH₂) 5.38, 5.06, 4.94, and 6.32 ppm, respectively. These signals fell within a narrow range (67.1 < δ(CH₂) < 70.9) in the ¹³C{¹H} spectra.

<figure><img src="image_161.png" /><figcaption>**Scheme 54.** Reactivity of BBA<sup>BBN</sup> toward various thiol reactants. Yield based on the quantity BBA<sup>BBN</sup>.</figcaption></figure>

In addition to BBA<sup>BBN</sup>, the interaction of BBA<sup>cy</sup> with thiol was also investigated. The reaction between 0.5 equivalents of 1,3-dipropanethiol and *in situ* generated BBA<sup>cy</sup> resulted in the formation of the di-borylated compound **11**, albeit with a slightly lower yield of 60% based on BBA compared to the BBA<sup>BBN</sup> substrate under the same conditions. Interestingly, in this reaction unidentified by-products were detected in addition to the desired compound.

Previously, it was emphasized that the intermediates proposed in the mechanism involving the condensation reaction of BBA with E-H functionality have never been experimentally characterized or isolated. In our case, the hemithioacetal (R'S-CH₂-OBR₂) was identified as an intermediate like those reported in the existing literature (see Figure 25). Consequently, the identification of this hemithioacetal intermediate suggests a sequential mechanism, which led us to study the reactions of
was protonated, facilitating the interaction with the added second equivalent of thiophenol. As a result, compound **9** was generated within 3 h at 80 °C with a yield of 55% on BBA basis (Scheme 53).

In addition to thiophenol, an acyclic dithiol was utilized to react with the BBA<sup>BBN</sup> compound, resulting in the formation of hemithioacetals under neutral conditions at room temperature (pathway B, Scheme 53). When one equivalent of 1,3-propanedithiol was used, a mixture of hemiacetal compounds was generated. A major species, identified as hemithioacetal compound **10**, was observed, yielding 72% *in situ* based on BBA. Its signals in the ¹H NMR and ¹³C{¹H} NMR spectra were assigned at 5.07 ppm and 68.7 ppm, respectively. Another species, identified as a minor product, was the diborylated hemithioacetal **11**, with a very similar NMR signature to that of adduct **10**, obtained with an 18% NMR yield. Notably, the introduction of sulfuric acid into the solution containing compounds **10** and **11** led to the rapid formation of dithiane **12** with a yield of 57% within 10 minutes. On the contrary, the addition of 0.4 equivalents of 1,3-propanedithiol to BBA<sup>BBN</sup> favored the formation of compound **11** with an increase of the NMR yield to 78% (Scheme 53).

In addition, the reaction of the *in situ* generated BBA<sup>BBN</sup> with 2-aminothiophenol led to the formation of an unidentified mixture at room temperature, which completely converted the BBA substrate (pathway C, Scheme 53). Under acidic conditions, the generation of the 2,3-dihydrobenzothiazole compound **13** was detected, which was obtained in 89% yield based on the BBA reactant (Scheme 53).

To broaden the range of thiol compounds and to explore the reactivity of BBA<sup>BBN</sup> with various thiols, experiments were conducted with 1-butanethiol, benzylmercaptan, and 2-mercaptobenzothiazole reacting with BBA<sup>BBN</sup> (Scheme 54). These reactions proceeded smoothly at room temperature, yielding hemithioacetal compounds (**14-16**) with favorable NMR yields of 63%, 73%, and 53%, respectively, within 16 hours. Analysis of the ¹H NMR spectra of compounds **8** and **14-16** revealed variations in the methylene moieties at δ(CH₂) 5.38, 5.06, 4.94, and 6.32 ppm, respectively. These signals fell within a narrow range (67.1 < δ(CH₂) < 70.9) in the ¹³C{¹H} spectra.

<figure><img src="image_159.png" /><figcaption>**Scheme 54.** Reactivity of BBA<sup>BBN</sup> toward various thiol reactants. Yield based on the quantity BBA<sup>BBN</sup>.</figcaption></figure>

In addition to BBA<sup>BBN</sup>, the interaction of BBA<sup>CV</sup> with thiol was also investigated. The reaction between 0.5 equivalents of 1,3-dipropanethiol and *in situ* generated BBA<sup>CV</sup> resulted in the formation of the di-borylated compound **11**, albeit with a slightly lower yield of 60% based on BBA compared to the BBA<sup>BBN</sup> substrate under the same conditions. Interestingly, in this reaction unidentified by-products were detected in addition to the desired compound.

Previously, it was emphasized that the intermediates proposed in the mechanism involving the condensation reaction of BBA with E-H functionality have never been experimentally characterized or isolated. In our case, the hemithioacetal (R'S-CH₂-OBR₂) was identified as an intermediate like those reported in the existing literature (see Figure 25). Consequently, the identification of this hemithioacetal intermediate suggests a sequential mechanism, which led us to study the reactions of
hemithioacetal **8** with different nucleophilic reagents to gain insight into the possible mechanism (see Scheme 55).

<figure><img src="image_162.png" /><figcaption>**Figure 25.** Proposed O-borylated hemiacetal intermediates.</figcaption></figure>

<figure><img src="image_163.png" /><figcaption>**Scheme 55.** Reaction of compound **8** toward phenol, thiophenol and amine.</figcaption></figure>

Accordingly, when hemithioacetal **8** was subjected to a reaction with phenol, the expected products were not formed (Scheme 55). Previously, similar results were observed with an excess of thiophenol (2 equivalents), where the reaction proceeded with only one equivalent of thiol substrate, producing hemithioacetal even at higher temperatures (Scheme 53). However, in the presence of sulfuric acid, hemithioacetal **8** reacted with a second equivalent of thiophenol to give dithioacetal **9**. Consequently, these reactants did not appear to be nucleophilic enough to undergo successive nucleophilic attacks on the electrophilic carbon of hemithioacetal **8**, preventing the release of the O-boryl moiety.

Using strong nucleophilic agents such as diphenylamine and diethylamine, we successfully synthesized hemithioaminals **17-18** through the reaction of hemithioacetal **8** under neutral conditions (Scheme 55). It is noteworthy that this reaction did not proceed at room temperature, even with an extended reaction time of up to 3 days. Interestingly, the reaction of compound **8** with linear diethylamine produced the related hemithioaminal compound **18** in 86% yield in less than 10 minutes at room temperature. Therefore, this observation emphasizes the importance of the nucleophilicity of the reactant.

## Tandem hydrosilylation of CO₂ mediated by [Ae]-based hydride complexes.

In this section, we present the tandem hydrosilylation of CO₂ by various hydrosilanes using [Ae]-based hydride catalysts in combination with Lewis-acid B(C₆F₅)₃ as a co-catalyst, which allowed us to extend previous studies on the first and only example of Mg-catalyzed hydrosilylation of CO₂.
hemithioacetal **8** with different nucleophilic reagents to gain insight into the possible mechanism (see Scheme 55).

<figure><img src="image_160.png" /><figcaption>**Figure 25.** Proposed O-borylated hemiacetal intermediates.</figcaption></figure>

<figure><img src="image_161.png" /><figcaption>**Scheme 55.** Reaction of compound **8** toward phenol, thiophenol and amine.</figcaption></figure>

Accordingly, when hemithioacetal **8** was subjected to a reaction with phenol, the expected products were not formed (Scheme 55). Previously, similar results were observed with an excess of thiophenol (2 equivalents), where the reaction proceeded with only one equivalent of thiol substrate, producing hemithioacetal even at higher temperatures (Scheme 53). However, in the presence of sulfuric acid, hemithioacetal **8** reacted with a second equivalent of thiophenol to give dithioacetal **9**. Consequently, these reactants did not appear to be nucleophilic enough to undergo successive nucleophilic attacks on the electrophilic carbon of hemithioacetal **8**, preventing the release of the O-boryl moiety.

Using strong nucleophilic agents such as diphenylamine and diethylamine, we successfully synthesized hemithioaminals **17-18** through the reaction of hemithioacetal **8** under neutral conditions (Scheme 55). It is noteworthy that this reaction did not proceed at room temperature, even with an extended reaction time of up to 3 days. Interestingly, the reaction of compound **8** with linear diethylamine produced the related hemithioaminal compound **18** in 86% yield in less than 10 minutes at room temperature. Therefore, this observation emphasizes the importance of the nucleophilicity of the reactant.

## Tandem hydrosilylation of CO₂ mediated by [Ae]-based hydride complexes.

In this section, we present the tandem hydrosilylation of CO₂ by various hydrosilanes using [Ae]-based hydride catalysts in combination with Lewis-acid B(C₆F₅)₃ as a co-catalyst, which allowed us to extend previous studies on the first and only example of Mg-catalyzed hydrosilylation of CO₂.
reported by Parkin *et al.* in 2017 and 2019 [9,10]. In our study, we have notably provided the first example of [Ca]-catalyzed CO₂ hydrosilylation by hydrosilanes.

The reactivity and selectivity of Ae-H containing compounds for the hydrosilylation of CO₂ reduction processes are unique for each catalytic system and depend on the steric bulk surrounding the Si-H bonds [11]. Therefore, the use of different hydrosilanes, including mono-, di-, and tri-phenylsilanes (PhSiH₃, Ph₂SiH₂, and Ph₃SiH) has been investigated. The neutral hydride compounds of β-diketiminate [(DippNacnac)Mg(μ-H)₂] (A1), its Ca analog (A2), or amidinate-Mg hydride (6)₂ were employed as the precatalysts in combination with B(C₆F₅)₃ for CO₂ reduction with hydrosilanes. As a result, the reduction of CO₂ with PhSiH₃ and Ph₂SiH₂ in the presence of excess B(C₆F₅)₃ were found to be effective in reducing CO₂ to the CH₄ level and Ph₃SiH to the bis(silyl)acetal, BSA, level. The [Ae]/B(C₆F₅)₃ catalyst systems are sensitive to steric hindrance around the silane substrate that effect the rate of the reaction, for example, Ph₂SiH₂ reacts slower than PhSiH₃ (

Table 7, entries 1-6), while the bulkier Ph₃SiH requires a prolonged reaction time to yield BSA (Table 7, Entries 7-9) [12].

<figure><img src="image_164.png" /><figcaption>**Table 7:** CO₂ hydrosilylation by [Ae]-based catalysts with different hydrosilanes.</figcaption></figure>

<table><thead><tr><th>Hydrosilanes</th><th>catalysts</th><th>t (h)</th><th>Si product (NMR% yield)<sup>c</sup></th><th>TON<sup>d</sup><br>(TOF/h<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="3">PhSiH<sub>3</sub></td><td>A1</td><td>20 min</td><td>CH<sub>4</sub> (58) (PhSiO<sub>1.5</sub>)<sub>n</sub> (25%)</td><td>117(353.9)</td></tr><tr><td>A2</td><td>7 h</td><td>CH<sub>4</sub> (20) (PhSiO<sub>1.5</sub>)<sub>n</sub> (23%)</td><td>131(18.7)</td></tr><tr><td>(6)<sub>2</sub></td><td>2 h 40 min</td><td>CH<sub>4</sub> (26) (PhSiO<sub>1.5</sub>)<sub>n</sub> (25%)</td><td>144(53.9)</td></tr><tr><td rowspan="3">Ph<sub>2</sub>SiH<sub>2</sub></td><td>A1</td><td>4 h</td><td>CH<sub>4</sub> (13) (HPh<sub>2</sub>Si)<sub>2</sub>O (44%)</td><td>121(24.1)</td></tr><tr><td>A2</td><td>1 h 45 min</td><td>CH<sub>4</sub> (28) (HPh<sub>2</sub>Si)<sub>2</sub>O (41%)</td><td>115(72.9)</td></tr><tr><td>(6)<sub>2</sub></td><td>5 h 40 min</td><td>CH<sub>4</sub> (12) (HPh<sub>2</sub>Si)<sub>2</sub>O (44%)</td><td>105(18.6)</td></tr><tr><td rowspan="3">Ph<sub>3</sub>SiH</td><td>A1</td><td>74 h</td><td>BSA (97%)</td><td>46(0.6)</td></tr><tr><td>A2</td><td>30 h</td><td>BSA (98%)</td><td>49(1.5)</td></tr><tr><td>(6)<sub>2</sub></td><td>220 h</td><td>BSA (97%)</td><td>50(0.3)</td></tr></tbody></table>

<footnote><sup>a</sup>Reaction conditions: toluene-d<sub>8</sub> (0.1 ml, 298 K), [Si-H]<sub>0</sub> = 0.14 mmol, [cat]<sub>0</sub> = 0.0028 mmol (2 mol% per Si-H bond), [B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] = 0.014 mmol (10 mol% per Si-H), [M]/[B] = 1/5 ratio, n(CO<sub>2</sub>)<sub>0</sub> = 1.3 x 10<sup>-4</sup> mol (1 atm, 298 K), internal standard (mesitylene): 0.14 mmol. <sup>b</sup>The conversion of hydrosilane is determined by the integration of the <sup>1</sup>H NMR resonances vs. those of the standard, mesitylene. <sup>c</sup>Yield of the Si product is determined by the integration of the corresponding <sup>1</sup>H NMR peak vs. those of the standard, mesitylene. <sup>d</sup>TON Number is calculated on the basis of Si-H bonds consumed per mmol of catalyst. TOF is calculated as TON h<sup>-1</sup>. CH<sub>4</sub> yield is calculated as [mmol of reacted silane]/(mmol of silane) x 100. Si by product calculated in relation to the CH<sub>4</sub>.</footnote>
reported by Parkin *et al.* in 2017 and 2019 [9,10]. In our study, we have notably provided the first example of [Ca]-catalyzed CO₂ hydrosilylation by hydrosilanes.

The reactivity and selectivity of Ae-H containing compounds for the hydrosilylation of CO₂ reduction processes are unique for each catalytic system and depend on the steric bulk surrounding the Si-H bonds [11]. Therefore, the use of different hydrosilanes, including mono-, di-, and tri-phenylsilanes (PhSiH₃, Ph₂SiH₂, and Ph₃SiH) has been investigated. The neutral hydride compounds of β-diketiminate [(DippNacnac)Mg(μ-H)₂] (A1), its Ca analog (A2), or amidinate-Mg hydride (6)₂ were employed as the precatalysts in combination with B(C₆F₅)₃ for CO₂ reduction with hydrosilanes. As a result, the reduction of CO₂ with PhSiH₃ and Ph₂SiH₂ in the presence of excess B(C₆F₅)₃ were found to be effective in reducing CO₂ to the CH₄ level and Ph₃SiH to the bis(silyl)acetal, BSA, level. The [Ae]/B(C₆F₅)₃ catalyst systems are sensitive to steric hindrance around the silane substrate that effect the rate of the reaction, for example, Ph₂SiH₂ reacts slower than PhSiH₃ (Table 7, entries 1-6), while the bulkier Ph₃SiH requires a prolonged reaction time to yield BSA (Table 7, Entries 7-9) [12].

<figure><img src="image_162.png" /><figcaption>**Table 7:** CO₂ hydrosilylation by [Ae]-based catalysts with different hydrosilanes.</figcaption></figure>

<table><thead><tr><th>Hydrosilanes</th><th>catalysts</th><th>t (h)</th><th>Si product (NMR% yield)<sup>c</sup></th><th>TON<sup>d</sup><br>(TOF/h<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="3">PhSiH<sub>3</sub></td><td>A1</td><td>20 min</td><td>CH<sub>4</sub> (58) (PhSiO<sub>1.5</sub>)<sub>n</sub> (25%)</td><td>117(353.9)</td></tr><tr><td>A2</td><td>7 h</td><td>CH<sub>4</sub> (20) (PhSiO<sub>1.5</sub>)<sub>n</sub> (23%)</td><td>131(18.7)</td></tr><tr><td>(6)<sub>2</sub></td><td>2 h 40 min</td><td>CH<sub>4</sub> (26) (PhSiO<sub>1.5</sub>)<sub>n</sub> (25%)</td><td>144(53.9)</td></tr><tr><td rowspan="3">Ph<sub>2</sub>SiH<sub>2</sub></td><td>A1</td><td>4 h</td><td>CH<sub>4</sub> (13) (HPh<sub>2</sub>Si)<sub>2</sub>O (44%)</td><td>121(24.1)</td></tr><tr><td>A2</td><td>1 h 45 min</td><td>CH<sub>4</sub> (28) (HPh<sub>2</sub>Si)<sub>2</sub>O (41%)</td><td>115(72.9)</td></tr><tr><td>(6)<sub>2</sub></td><td>5 h 40 min</td><td>CH<sub>4</sub> (12) (HPh<sub>2</sub>Si)<sub>2</sub>O (44%)</td><td>105(18.6)</td></tr><tr><td rowspan="3">Ph<sub>3</sub>SiH</td><td>A1</td><td>74 h</td><td>BSA (97%)</td><td>46(0.6)</td></tr><tr><td>A2</td><td>30 h</td><td>BSA (98%)</td><td>49(1.5)</td></tr><tr><td>(6)<sub>2</sub></td><td>220 h</td><td>BSA (97%)</td><td>50(0.3)</td></tr></tbody></table>

<footnote><sup>a</sup>Reaction conditions: toluene-d<sub>8</sub> (0.1 ml, 298 K), [Si-H]<sub>0</sub> = 0.14 mmol, [cat]<sub>0</sub> = 0.0028 mmol (2 mol% per Si-H bond), [B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] = 0.014 mmol (10 mol% per Si-H), [M]/[B] = 1/5 ratio, n(CO<sub>2</sub>)<sub>0</sub> = 1.3 x 10<sup>-4</sup> mol (1 atm, 298 K), internal standard (mesitylene): 0.14 mmol. <sup>b</sup>The conversion of hydrosilane is determined by the integration of the <sup>1</sup>H NMR resonances vs. those of the standard, mesitylene. <sup>c</sup>Yield of the Si product is determined by the integration of the corresponding <sup>1</sup>H NMR peak vs. those of the standard, mesitylene. <sup>d</sup>TON Number is calculated on the basis of Si-H bonds consumed per mmol of catalyst. TOF is calculated as TON h<sup>-1</sup>. CH<sub>4</sub> yield is calculated as [mmol of reacted silane/4/(mmol of silane)]x 100. Si by product calculated in relation to the CH<sub>4</sub>.</footnote>
In addition, the nature of the [Ae] catalysts play a significant role in dictating the overall rate of the reaction as measured in terms of the TOF values. Among them, the combination of [Mg-A1]/B(C₆F₅)₃ showed the best performance within the series by using PhSiH₃ as a reducing agent, providing a fast kinetic rate. On the other hand, in Ph₂SiH₂ and Ph₃SiH systems, the [Ca-A2]/B(C₆F₅)₃ catalyst exhibits high activity towards the reduction of CO₂ to the CH₄ and BSA, respectively.

The kinetics of each catalytic reaction have been studied. The differences in the overall reaction order were observed in varied [Ae]/B(C₆F₅)₃ catalyst systems, indicating different mechanisms and rate determining steps. Specifically, the zeroth-order reaction may involve a sophisticated reaction mechanism in which each reduction step may be catalyzed by a different active species. For instance, the [Ca]-catalyzed hydrosilylation of CO₂, which has the zeroth-reaction order, was postulated to proceed through a hypervalent six-coordinate silicon intermediate, where the Ca catalyst (A2) does not react with CO₂ but with PhSiH₃ to give hypervalent species in the first step, instead of a common FLP-type hydrosilylation mechanism.

Further studies on Eyring and Arrhenius analyses provided an activation parameter for the reduction of CO₂ by Ph₃SiH using the [Ca]/B(C₆F₅)₃ catalyst. The reactions were carried out at four temperatures between 298 K and 353 K under the standard condition of 2 mol% of Ca hydride (A2) catalyst with 10 mol% of B(C₆F₅)₃. The resulting silane conversion vs. time at different temperatures is depicted in Figure 26. Extraction of the observed rate constants forms plots of [Ph₃SiH] or [BSA] vs. time, which allows the construction of the Arrhenius and Eyring plots.

<figure><img src="image_165.png" /><figcaption>**Figure 26.** Plot of [Ph₃SiH]/[Ph₃SiH]t=0 vs. time for variable temperatures using 2 mol% of A2 and 10 mol% [B] as a catalyst, toluene-d₈, and 1 atm of ¹³CO₂ pressure.</figcaption></figure>

The activation energy value (Ea) could be obtained from the slope of the best-fit line from the data plot of ln(Kobs) vs. 1/T. The activation energy of the process was found to be 50.63 kJ mol⁻¹. The enthalpy of activation (ΔH‡) can then be obtained from the slope of the linear regression from the data plot of ln(Kobs/T) vs 1/T, which was found to be 44.21 kJ mol⁻¹ (Figure 27). The entropy of activation (ΔS‡) can be obtained from the intercept of the y-axis, giving a value for ΔS‡ of -169 J mol⁻¹K⁻¹. The corresponding Gibbs free energies (ΔG‡) at 298 K were found to be 94.6 kJ mol⁻¹ [13,14].
In addition, the nature of the [Ae] catalysts play a significant role in dictating the overall rate of the reaction as measured in terms of the TOF values. Among them, the combination of [Mg-A1]/B(C6F5)3 showed the best performance within the series by using PhSiH3 as a reducing agent, providing a fast kinetic rate. On the other hand, in Ph2SiH2 and Ph3SiH systems, the [Ca-A2]/B(C6F5)3 catalyst exhibits high activity towards the reduction of CO2 to the CH4 and BSA, respectively.

The kinetics of each catalytic reaction have been studied. The differences in the overall reaction order were observed in varied [Ae]/B(C6F5)3 catalyst systems, indicating different mechanisms and rate determining steps. Specifically, the zeroth-order reaction may involve a sophisticated reaction mechanism in which each reduction step may be catalyzed by a different active species. For instance, the [Ca]-catalyzed hydrosilylation of CO2, which has the zeroth-reaction order, was postulated to proceed through a hypervalent six-coordinate silicon intermediate, where the Ca catalyst (A2) does not react with CO2 but with PhSiH3 to give hypervalent species in the first step, instead of a common FLP-type hydrosilylation mechanism.

Further studies on Eyring and Arrhenius analyses provided an activation parameter for the reduction of CO2 by Ph3SiH using the [Ca]/B(C6F5)3 catalyst. The reactions were carried out at four temperatures between 298 K and 353 K under the standard condition of 2 mol% of Ca hydride (A2) catalyst with 10 mol% of B(C6F5)3. The resulting silane conversion vs. time at different temperatures is depicted in Figure 26. Extraction of the observed rate constants forms plots of [Ph3SiH] or [BSA] vs. time, which allows the construction of the Arrhenius and Eyring plots.

<figure><img src="image_163.png" /><figcaption>**Figure 26.** Plot of [Ph3SiH]/[Ph3SiH]t=0 vs. time for variable temperatures using 2 mol% of A2 and 10 mol% [B] as a catalyst, toluene-d8, and 1 atm of ¹³CO₂ pressure.</figcaption></figure>

The activation energy value (Ea) could be obtained from the slope of the best-fit line from the data plot of ln(Kobs) vs. 1/T. The activation energy of the process was found to be 50.63 kJ mol⁻¹. The enthalpy of activation (ΔH‡) can then be obtained from the slope of the linear regression from the data plot of ln(Kobs/T) vs 1/T, which was found to be 44.21 kJ mol⁻¹ (Figure 27). The entropy of activation (ΔS‡) can be obtained from the intercept of the y-axis, giving a value for ΔS‡ of -169 J mol⁻¹K⁻¹. The corresponding Gibbs free energies (ΔG‡) at 298 K were found to be 94.6 kJ mol⁻¹ [13,14].
<figure><img src="image_166.png" /><figcaption>**Figure 27:** Eyring plot to determine the activation parameters for [Ca]/[B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] catalyst in CO<sub>2</sub> reduction by Ph<sub>3</sub>SiH.</figcaption></figure>

Comparing the ΔH<sup>‡</sup> and ΔS<sup>‡</sup> values for CO<sub>2</sub> reduction by Ph<sub>3</sub>SiH using [Ca]/[B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] catalyst, the reaction appears to be governed by entropic rather than enthalpic contributions, resulting in a significantly negative activation entropy of ca. -170 J K<sup>-1</sup> mol<sup>-1</sup>. This is indicative of the associative formation of activated complexes through the assembly of two or more reaction components and must be accounted for in any mechanistic rationale. In other words, it suggests the assembly of a highly organized rate-determining transition state.

Collectively, by considering the observed data, the [Ae] metal plays a significant role in enabling a nucleophilic attack upon CO<sub>2</sub> in the first step of the reaction, whereas B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> activates the silane to mediate the subsequent steps of catalytic turnover. The nature of the [Ae]-based hydride catalysts is important for the overall rate and order of the reaction.

## Conclusion

In conclusion, we have shown that the reactivity of Mg and Ca hydride complexes supported by β-diketiminate and amidinate ligands for CO<sub>2</sub> reduction allows the investigation of the precedented [Ae]-catalyzed CO<sub>2</sub> hydroboration to yield selective BBA and subsequent C–S bond formation. Furthermore, it has been demonstrated that these [Ae]-based complexes, in combination with B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> efficiently reduce CO<sub>2</sub> with hydrosilanes to various silylated compounds, depending on the silane used. In our perspective, further mechanistic possibilities and kinetic differences of these Mg and Ca-based catalytic systems will be explored. Additionally, the scope of this research could be extended to investigate the reactivity of group 2 metal hydride complexes with CO via CO homologation.

## References

[1] (a) M. Aresta, A. Dibenedetto, *Dalton Trans.* **2007**, 2975–2992. DOI: [10.1039/b700658f](https://doi.org/10.1039/b700658f); (b) M. Aresta, A. Dibenedetto, A. Angelini, *Chem. Rev.* **2014**, *114*, 1709–1742. DOI: [10.1021/cr4002758](https://doi.org/10.1021/cr4002758).
<figure><img src="image_164.png" /><figcaption>**Figure 27:** Eyring plot to determine the activation parameters for [Ca]/[B(C₆F₅)₃] catalyst in CO₂ reduction by Ph₃SiH.</figcaption></figure>

Comparing the ΔH‡ and ΔS‡ values for CO₂ reduction by Ph₃SiH using [Ca]/[B(C₆F₅)₃] catalyst, the reaction appears to be governed by entropic rather than enthalpic contributions, resulting in a significantly negative activation entropy of ca. -170 J K⁻¹ mol⁻¹. This is indicative of the associative formation of activated complexes through the assembly of two or more reaction components and must be accounted for in any mechanistic rationale. In other words, it suggests the assembly of a highly organized rate-determining transition state.

Collectively, by considering the observed data, the [Ae] metal plays a significant role in enabling a nucleophilic attack upon CO₂ in the first step of the reaction, whereas B(C₆F₅)₃ activates the silane to mediate the subsequent steps of catalytic turnover. The nature of the [Ae]-based hydride catalysts is important for the overall rate and order of the reaction.

## Conclusion

In conclusion, we have shown that the reactivity of Mg and Ca hydride complexes supported by β-diketiminate and amidinate ligands for CO₂ reduction allows the investigation of the precedented [Ae]-catalyzed CO₂ hydroboration to yield selective BBA and subsequent C–S bond formation. Furthermore, it has been demonstrated that these [Ae]-based complexes, in combination with B(C₆F₅)₃ efficiently reduce CO₂ with hydrosilanes to various silylated compounds, depending on the silane used. In our perspective, further mechanistic possibilities and kinetic differences of these Mg and Ca-based catalytic systems will be explored. Additionally, the scope of this research could be extended to investigate the reactivity of group 2 metal hydride complexes with CO via CO homologation.

## References

[1] (a) M. Aresta, A. Dibenedetto, *Dalton Trans.* **2007**, 2975–2992. DOI: [10.1039/b700658f](https://doi.org/10.1039/b700658f); (b) M. Aresta, A. Dibenedetto, A. Angelini, *Chem. Rev.* **2014**, *114*, 1709–1742. DOI: [10.1021/cr4002758](https://doi.org/10.1021/cr4002758).
[2] S. Harder, *Early Main Group Metal Catalysis Concepts and Reactions*, Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469, Weinheim, Germany, **2020**. DOI: **10.1002/9783527818020**.

[3] A. G. Avent, M. R. Crimmin, M. S. Hill, P. B. Hitchcock, *Dalton Trans.* **2005**, 278–284. DOI: **10.1039/b415468a**.

[4] S. P. Green, C. Jones, A. Stasch, *Angew. Chem. Int. Ed.* **2008**, *47*, 9079–9083. DOI: **10.1002/anie.200803960**.

[5] M. H. Chisholm, J. C. Gallucci, K. Phomphrai, *Inorg. Chem.* **2004**, *43*, 6717–6725. DOI: **10.1021/ic0490730**.

[6] C. N. de Bruin-Dickason, T. Sutcliffe, C. Alvarez Lamsfus, G. B. Deacon, L. Maron, C. Jones, *Chem. Commun.* **2018**, *54*, 786–789. DOI: **10.1039/C7CC09362D**.

[7] S. P. Green, C. Jones, A. Stasch, *Angew. Chem. Int. Ed.* **2008**, *47*, 9079–9083. DOI: **10.1002/anie.200803960**.

[8] W. Huadsai, M. Westerhausen, S. Bontemps: "Alkaline Earth Catalyzed CO2 Hydroboration into Acetal Derivatives Leading to C-S Bond Formation", *Organometallics* **2023** *42*, 2921–2926, DOI:10.1021/acs.organomet.3c00335.

[9] M. Rauch, G. Parkin, *J. Am. Chem. Soc.* **2017**, *139*, 18162–18165. DOI: **10.1021/jacs.7b10776**.

[10] M. Rauch, Z. Strater, G. Parkin, *J. Am. Chem. Soc.* **2019**, *141*, 17754–17762. DOI: **10.1021/jacs.9b08342**.

[11] (a) F. J. Fernández-Alvarez, L. A. Oro, *ChemCatChem* **2018**, *10*, 4783–4796. DOI: **10.1002/cctc.201800699**;
(b) J. Chen, M. McGraw, E. Y. X. Chen, *ChemSusChem* **2019**, *12*, 4543–4569. DOI: **10.1002/cssc.201901764**.

[12] T. Matsuo, H. Kawaguchi, *J. Am. Chem. Soc.* **2006**, *128*, 12362–12363. DOI: **10.1021/ja0647250**.

[13] W. Huang, T. Roisnel, V. Dorcet, C. Orione, E. Kirillov, *Organometallics* **2020**, *39*, 698–710. DOI: **10.1021/acs.organomet.9b00853**.

[14] M. North, M. Omedes-Pujol, *Beilstein J. Org. Chem.* **2010**, *6*, 1043–1055. DOI: **10.3762/bjoc.6.119**.
[2] S. Harder, *Early Main Group Metal Catalysis Concepts and Reactions*, Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469, Weinheim, Germany, **2020**. DOI: 10.1002/9783527818020.

[3] A. G. Avent, M. R. Crimmin, M. S. Hill, P. B. Hitchcock, *Dalton Trans.* **2005**, 278–284. DOI: 10.1039/b415468a.

[4] S. P. Green, C. Jones, A. Stasch, *Angew. Chem. Int. Ed.* **2008**, *47*, 9079–9083. DOI: 10.1002/anie.200803960.

[5] M. H. Chisholm, J. C. Gallucci, K. Phomphrai, *Inorg. Chem.* **2004**, *43*, 6717–6725. DOI: 10.1021/ic0490730.

[6] C. N. de Bruin-Dickason, T. Sutcliffe, C. Alvarez Lamsfus, G. B. Deacon, L. Maron, C. Jones, *Chem. Commun.* **2018**, *54*, 786–789. DOI: 10.1039/C7CC09362D.

[7] S. P. Green, C. Jones, A. Stasch, *Angew. Chem. Int. Ed.* **2008**, *47*, 9079–9083. DOI: 10.1002/anie.200803960.

[8] W. Huadsai, M. Westerhausen, S. Bontemps: "Alkaline Earth Catalyzed CO2 Hydroboration into Acetal Derivatives Leading to C-S Bond Formation", *Organometallics* **2023** *42*, 2921–2926. DOI:10.1021/acs.organomet.3c00335.

[9] M. Rauch, G. Parkin, *J. Am. Chem. Soc.* **2017**, *139*, 18162–18165. DOI: 10.1021/jacs.7b10776.

[10] M. Rauch, Z. Strater, G. Parkin, *J. Am. Chem. Soc.* **2019**, *141*, 17754–17762. DOI: 10.1021/jacs.9b08342.

[11] (a) F. J. Fernández-Alvarez, L. A. Oro, *ChemCatChem* **2018**, *10*, 4783–4796. DOI: 10.1002/cctc.201800699;
(b) J. Chen, M. McGraw, E. Y. X. Chen, *ChemSusChem* **2019**, *12*, 4543–4569. DOI: 10.1002/cssc.201901764.

[12] T. Matsuo, H. Kawaguchi, *J. Am. Chem. Soc.* **2006**, *128*, 12362–12363. DOI: 10.1021/ja0647250.

[13] W. Huang, T. Roisnel, V. Dorcet, C. Orione, E. Kirillov, *Organometallics* **2020**, *39*, 698–710. DOI: 10.1021/acs.organomet.9b00853.

[14] M. North, M. Omedes-Pujol, *Beilstein J. Org. Chem.* **2010**, *6*, 1043–1055. DOI: 10.3762/bjoc.6.119.
Anastasiia SHERSTIUK

ESR7

**Thesis co-directors**

Prof. Evamarie Hey-Hawkins Universität Leipzig, Leipzig, Germany

Prof. Rosa Maria Sebastián Universitat Autònoma de Barcelona, Bellaterra, Spain

**Thesis defense**

Faculty of Chemistry and Mineralogy, University of Leipzig, 9 February 2024
Anastasiia SHERSTIUK

ESR7

**Thesis co-directors**

Prof. Evamarie Hey-Hawkins Universität Leipzig, Leipzig, Germany

Prof. Rosa Maria Sebastián Universitat Autònoma de Barcelona, Bellaterra, Spain

**Thesis defense**

Faculty of Chemistry and Mineralogy, University of Leipzig, 9 February 2024
<figure><img src="image_167.png" /></figure>
<figure><img src="image_165.png" /></figure>
# Photoswitchable phosphines for light-modulated metal catalysts

## Introduction and Objectives

The last century has witnessed enormous progress in the field of homogeneous catalysis, which has led to the development of a large variety of catalytic systems to enhance the reactivity and selectivity of chemical reactions. Switchable catalysis is a new emerging topic in this area that paves the way to reach even higher levels of control of catalytic reactions. Inspired by nature, where enzymatic processes in cells involve sophisticated feedback loops and trigger-induced effects for spatial and temporal control [1], switchable catalysis aims to provide a bio-like level of control over chemical transformations through the response to tunable external stimuli, such as light, pH, metal coordination, redox switching or mechanical forces [2-4].

Among all these external stimuli to accomplish switchable catalysis, light stands out because of a series of advantageous properties: it is non-invasive, cheap, ubiquitous, environmentally friendly, easily tunable in terms of wavelength, power and polarization, and even more importantly, provides precise temporal and spatial control – i.e., light doses can be administered locally (using lenses) and for very short lapses of time (using pulsed illumination sources) with high degree of precision. Photoswitchable catalysis (Figure 1) enables the reversible control of a catalytic system between distinct states through light irradiation. This approach is valuable for applications requiring high spatial or temporal precision in chemical reactions, such as bioconjugation or polymer synthesis. To ensure the modulation in catalytic properties photoswitchable moieties undergoing light-induced isomerization are introduced into the catalyst structure. Within homogeneous catalysis, transition metal complexes hold a unique position due to their versatility and extensive capacity to facilitate various chemical transformations. Consequently, the prospect of further developing them into photoswitchable systems is advantageous, resulting in a reversible modulation of catalytic activity and precise control over chemical reactions. The modulation of catalytic activity in metal catalysts is possible through alterations in geometrical and/or electronic effects of the catalytic center.

<figure><img src="image_168.png" /><figcaption>**Figure 1.** The general concept of photoswitchable catalysis, where a catalyst presents two different states with distinct catalytic properties (A and B) that can be mutually interconverted through light absorption.</figcaption></figure>

The success of the molecular design of a photoswitchable metal catalyst depends on two main structural features: the photoisomerizable scaffold and the binding group chosen to complex the metal. The choice of the former is essentially determined by the change in the metal properties required to accomplish catalysis modulation – i.e., whether it will be achieved by altering the electron density on the metal center or the geometry around it. In many instances, both of these parameters are important, and the final preference should be based on taking into account all the electronic and
# Photoswitchable phosphines for light-modulated metal catalysts

## Introduction and Objectives

The last century has witnessed enormous progress in the field of homogeneous catalysis, which has led to the development of a large variety of catalytic systems to enhance the reactivity and selectivity of chemical reactions. Switchable catalysis is a new emerging topic in this area that paves the way to reach even higher levels of control of catalytic reactions. Inspired by nature, where enzymatic processes in cells involve sophisticated feedback loops and trigger-induced effects for spatial and temporal control [1], switchable catalysis aims to provide a bio-like level of control over chemical transformations through the response to tunable external stimuli, such as light, pH, metal coordination, redox switching or mechanical forces [2-4].

Among all these external stimuli to accomplish switchable catalysis, light stands out because of a series of advantageous properties: it is non-invasive, cheap, ubiquitous, environmentally friendly, easily tunable in terms of wavelength, power and polarization, and even more importantly, provides precise temporal and spatial control – i.e., light doses can be administered locally (using lenses) and for very short lapses of time (using pulsed illumination sources) with high degree of precision. Photoswitchable catalysis (Figure 1) enables the reversible control of a catalytic system between distinct states through light irradiation. This approach is valuable for applications requiring high spatial or temporal precision in chemical reactions, such as bioconjugation or polymer synthesis. To ensure the modulation in catalytic properties photoswitchable moieties undergoing light-induced isomerization are introduced into the catalyst structure. Within homogeneous catalysis, transition metal complexes hold a unique position due to their versatility and extensive capacity to facilitate various chemical transformations. Consequently, the prospect of further developing them into photoswitchable systems is advantageous, resulting in a reversible modulation of catalytic activity and precise control over chemical reactions. The modulation of catalytic activity in metal catalysts is possible through alterations in geometrical and/or electronic effects of the catalytic center.

<figure><img src="image_166.png" /><figcaption>**Figure 1.** The general concept of photoswitchable catalysis, where a catalyst presents two different states with distinct catalytic properties (A and B) that can be mutually interconverted through light absorption.</figcaption></figure>

The success of the molecular design of a photoswitchable metal catalyst depends on two main structural features: the photoisomerizable scaffold and the binding group chosen to complex the metal. The choice of the former is essentially determined by the change in the metal properties required to accomplish catalysis modulation – i.e., whether it will be achieved by altering the electron density on the metal center or the geometry around it. In many instances, both of these parameters are important, and the final preference should be based on taking into account all the electronic and
steric factors that can affect the catalytic properties of the metal center. As for the selection of the binding group, it is mainly dependent on the nature of the metal that would be active in the catalysis. Existing examples include not only NHCs and crown ethers but also pyridine [5,6], salen [7], oxazoline [8,9], cyano [10] and phosphine [11] groups.

Phosphines play a crucial role in the advancement of modern catalysis due to their particular electronic and steric properties, which makes them very versatile ligands for metal complexation (Figure 2a) [12]. Thus, phosphines are known for their capacity to bind a variety of metals in both high and low oxidation states thanks to their adjustable electronic features [13]. When acting as σ-donors, they bind metals through electronic density donation from the lone pair of electrons of their phosphorous atom to the empty orbitals of the metal. At the same time, these ligands can act as π-acceptors, receiving electron density from filled metal d orbitals into phosphorous antibonding σ* orbitals [14]. The contribution of these two types of electronic interactions and, therefore, the nature of the metal-phosphine bond is influenced by the substituents incorporated at the phosphorus atom [15]. Phosphines containing electron-withdrawing groups exhibit reduced electron density on phosphorus, resulting in a diminished σ-donor capacity; this effect is accompanied by the lowering of phosphorous σ* orbital energy, thus increasing their tendency to act as π-acceptors. In contrast, the introduction of electron-donating groups to phosphines enhances their σ-donor character at the expense of their π-acceptor properties. Therefore, the modulation of phosphine electronic features could significantly change the nature of its bonding to a metal center and, eventually, affect its catalytic activity.

<figure><img src="image_169.png" /><figcaption>**Figure 2.** a) Electronic and structural properties of tertiary phosphine ligands in metal complexes. b) Example of monodentate triphenylphosphine ligand 1 and bidentate 1,2 bis(diphenylphosphino)ethane 2.</figcaption></figure>

Steric effects imparted by tertiary phosphines also play a very significant role in the catalytic activity of coordinated metals [12,16]. To rationalize this behavior, phosphine ligands can be described through Tolman cone angle and percent buried volume. Tolman defined the cone angle as the "apex angle of a cylindrical cone, centered at 2.28 Å from the center of the P atom, which touches the van der Waals radii of the outmost atoms of the model" [17,18]. As for the buried volume [19], it is defined as the percentage of a sphere ($r = 3.5 Å$) around the metal center that is occupied by a given ligand. These steric descriptors of phosphines should be considered during the design of photoswitchable
steric factors that can affect the catalytic properties of the metal center. As for the selection of the binding group, it is mainly dependent on the nature of the metal that would be active in the catalysis. Existing examples include not only NHCs and crown ethers but also pyridine [5,6], salen [7], oxazoline [8,9], cyano [10] and phosphine [11] groups.

Phosphines play a crucial role in the advancement of modern catalysis due to their particular electronic and steric properties, which makes them very versatile ligands for metal complexation (Figure 2a) [12]. Thus, phosphines are known for their capacity to bind a variety of metals in both high and low oxidation states thanks to their adjustable electronic features [13]. When acting as σ-donors, they bind metals through electronic density donation from the lone pair of electrons of their phosphorous atom to the empty orbitals of the metal. At the same time, these ligands can act as π-acceptors, receiving electron density from filled metal d orbitals into phosphorous antibonding σ* orbitals [14]. The contribution of these two types of electronic interactions and, therefore, the nature of the metal-phosphine bond is influenced by the substituents incorporated at the phosphorus atom [15]. Phosphines containing electron-withdrawing groups exhibit reduced electron density on phosphorus, resulting in a diminished σ-donor capacity; this effect is accompanied by the lowering of phosphorous σ* orbital energy, thus increasing their tendency to act as π-acceptors. In contrast, the introduction of electron-donating groups to phosphines enhances their σ-donor character at the expense of their π-acceptor properties. Therefore, the modulation of phosphine electronic features could significantly change the nature of its bonding to a metal center and, eventually, affect its catalytic activity.

<figure><img src="image_167.png" /><figcaption>**Figure 2.** a) Electronic and structural properties of tertiary phosphine ligands in metal complexes. b) Example of monodentate triphenylphosphine ligand 1 and bidentate 1,2 bis(diphenylphosphino)ethane 2.</figcaption></figure>

Steric effects imparted by tertiary phosphines also play a very significant role in the catalytic activity of coordinated metals [12,16]. To rationalize this behavior, phosphine ligands can be described through Tolman cone angle and percent buried volume. Tolman defined the cone angle as the "apex angle of a cylindrical cone, centered at 2.28 Å from the center of the P atom, which touches the van der Waals radii of the outmost atoms of the model" [17,18]. As for the buried volume [19], it is defined as the percentage of a sphere ($r = 3.5 Å$) around the metal center that is occupied by a given ligand. These steric descriptors of phosphines should be considered during the design of photoswitchable
catalysts, as the size of the ligand affects the reactivity of the attached metal center [20]. In addition, ligands containing more than one phosphine group in their structure can act as bidentate or even tridentate ligands (Figure 2b), thus opening a different way to alter their influence on catalytic metal centers by varying their coordination mode or the steric and electronic properties of the linker.

The light-responsive scaffolds used in photoswitchable catalysis also exhibit a variety of tunable characteristic features [21]. Generally, existing molecular photoswitches can be divided into two main groups depending on the type of isomerization process induced by light. The first group is formed by the photoswitches that undergo ring-cyclization and ring-opening reactions; for example, the spiropyran in Scheme 1. The second group of photoswitches are characterized by a light-triggered E/Z isomerization around a double bond. This is the case of the 1,1'-biindane in Scheme 1, an illustrative example of the stiff stilbene photoswitches used for the preparation of molecular rotors and for which Ben Feringa was awarded with the Nobel Prize in Chemistry in 2016 [22]. Additionally, photoswitches are also divided depending on the stimuli required for their back-isomerization. Those that are thermally unstable in their photoinduced state and isomerize back to their most stable configuration with time in the dark are called thermally reversible type or T-type (e.g. spiopyrans). On the other hand, photoswitches that only undergo back-isomerization under light irradiation are classified as photochemically reversible type or P-type (e.g. 1,1'-biindane).

<figure><img src="image_170.png" /><figcaption>**Scheme 1.** Examples of different types of photoswitches. Left: a spiropyran photoswitch undergoing ring-closing and ring-opening reactions upon light irradiation; in this case, reversed back-isomerization is also possible through thermal relaxation in the dark (T-type). Right: 1,1'-biindane photoswitch undergoing E/Z photoisomerization only upon light irradiation (P-type).</figcaption></figure>

Various parameters are utilized to assess the light-induced efficacy of molecular photoswitches. Firstly, photoisomerization quantum yields ($\Phi_i$) serve as a metric for the efficiency of the process in the excited state - i.e., they quantify the number of molecules undergoing the isomerization reaction per photon absorbed, excluding those that revert back to the initial ground state without isomerizing through alternative processes such as luminescence emission or internal conversion [23]. Secondly, the composition of the photostationary states (PSS) [24] defines the extent of the photoreaction - i.e., the composition of the final equilibrium mixtures of the two interconverting isomers that are achieved under continuous irradiation when the rates of the photoisomerization process and the back-reaction become equal. The composition of PSSs depends on factors such as the absorbance of the two isomers at the irradiation wavelength, their respective photoisomerization quantum yields, and, in the case of T-type photochromes, the thermal lifetime of the photoinduced isomer along with the irradiation intensity. Thirdly, fatigue resistance [25] characterizes the decline in performance of the molecular photoswitch over repetitive photoisomerization cycles, a phenomenon attributed to photo-degradation.
catalysts, as the size of the ligand affects the reactivity of the attached metal center [20]. In addition, ligands containing more than one phosphine group in their structure can act as bidentate or even tridentate ligands (Figure 2b), thus opening a different way to alter their influence on catalytic metal centers by varying their coordination mode or the steric and electronic properties of the linker.

The light-responsive scaffolds used in photoswitchable catalysis also exhibit a variety of tunable characteristic features [21]. Generally, existing molecular photoswitches can be divided into two main groups depending on the type of isomerization process induced by light. The first group is formed by the photoswitches that undergo ring-cyclization and ring-opening reactions; for example, the spiropyran in Scheme 1. The second group of photoswitches are characterized by a light-triggered E/Z isomerization around a double bond. This is the case of the 1,1'-biindane in Scheme 1, an illustrative example of the stiff stilbene photoswitches used for the preparation of molecular rotors and for which Ben Feringa was awarded with the Nobel Prize in Chemistry in 2016 [22]. Additionally, photoswitches are also divided depending on the stimuli required for their back-isomerization. Those that are thermally unstable in their photoinduced state and isomerize back to their most stable configuration with time in the dark are called thermally reversible type or T-type (e.g. spiopyrans). On the other hand, photoswitches that only undergo back-isomerization under light irradiation are classified as photochemically reversible type or P-type (e.g. 1,1'-biindane).

<figure><img src="image_168.png" /><figcaption>**Scheme 1.** Examples of different types of photoswitches. Left: a spiropyran photoswitch undergoing ring-closing and ring-opening reactions upon light irradiation; in this case, reversed back-isomerization is also possible through thermal relaxation in the dark (T-type). Right: 1,1'-biindane photoswitch undergoing E/Z photoisomerization only upon light irradiation (P-type).</figcaption></figure>

Various parameters are utilized to assess the light-induced efficacy of molecular photoswitches. Firstly, photoisomerization quantum yields ($\Phi_i$) serve as a metric for the efficiency of the process in the excited state - i.e., they quantify the number of molecules undergoing the isomerization reaction per photon absorbed, excluding those that revert back to the initial ground state without isomerizing through alternative processes such as luminescence emission or internal conversion [23]. Secondly, the composition of the photostationary states (PSS) [24] defines the extent of the photoreaction - i.e., the composition of the final equilibrium mixtures of the two interconverting isomers that are achieved under continuous irradiation when the rates of the photoisomerization process and the back-reaction become equal. The composition of PSSs depends on factors such as the absorbance of the two isomers at the irradiation wavelength, their respective photoisomerization quantum yields, and, in the case of T-type photochromes, the thermal lifetime of the photoinduced isomer along with the irradiation intensity. Thirdly, fatigue resistance [25] characterizes the decline in performance of the molecular photoswitch over repetitive photoisomerization cycles, a phenomenon attributed to photo-degradation.
Typically, photoisomerization is accompanied by alteration in the geometrical and electronic properties of the photoswitch, which in turn leads to ample variation of their physicochemical properties (e.g. polarity, light absorbance and redox potentials). The extent of these changes depends on the specific scaffold, providing a high degree of flexibility in selecting desired outcomes.

Given the scarcity of examples utilizing photoswitchable phosphine ligands in transition metal catalysis [11], this doctoral project focused on development of the novel photoswitchable phosphine ligands for their incorporation into transition metal complexes, eventually aiming to enable light-induced modulation of their catalytic properties. Two different approaches were explored toward this goal, which involved the utilization of two distinct photoswitchable scaffolds: dithienylethenes (DTEs) and diazocines.

To ensure electron density modulation, dithienylethene (DTE)-based phophine ligands were invisioned. DTEs are a well-known class of molecular photoswitches which, in their more thermodynamically stable open isomer, consist of two thiophene moieties connected through a Z-ethene bridge (Figure 3a) [26]. This is a particular case of the broader family of DAE photoswitches, wherein the thiophene rings may be substituted with other aryl groups. Due to the occurrence of free rotation between the ethene linker and the thiophene rings, the structure of the open-state DTE is non-planar. Consequently, electronic communication between the two aryl groups is absent, resulting in strong absorption primarily in the UV, and occasionally violet, regions (Figure 3b) [27].

<figure><img src="image_171.png" /><figcaption>**Figure 3.** a) Ring-closing and ring-opening photochemical reactions of DTEs. b) Typical UV-vis absorption spectra of the open and closed forms of DTEs ($R_1 = H$, $R_2 = R_3 = COCF_3$).</figcaption></figure>

Upon exposure to UV light, the open state of DTEs undergoes a 6π electrocyclization reaction to produce the corresponding closed isomer, which displays an absorption band in the visible region due to its extended conjugation path (Figure 3b). The closed state of DTEs typically exhibits thermal stability, precluding spontaneous back-isomerization in the absence of external stimuli over time in the dark. The recovery of the open isomer therefore necessitates exposure to visible light, inducing an electrocyclic ring-opening reaction [27].

Thus, introduction of a diphenylphosphanyl group in one thiophene ring and an electron-withdrawing substituent in the other was expected to result in two distinct states of the phosphine. In the open state where the two thiophene rings are fully isolated, no electronic communication should occur between these two groups and, consequently, the phosphine ligand must have a strong σ-donor character. However, conjugation between the phosphanyl and EWG groups should occur upon photocyclization, thus decreasing the σ-donating ability of the phosphine. To further amplify this effect, two DTE moieties were devised to be introduced to the same phosphorus atom, whose
Typically, photoisomerization is accompanied by alteration in the geometrical and electronic properties of the photoswitch, which in turn leads to ample variation of their physicochemical properties (e.g. polarity, light absorbance and redox potentials). The extent of these changes depends on the specific scaffold, providing a high degree of flexibility in selecting desired outcomes.

Given the scarcity of examples utilizing photoswitchable phosphine ligands in transition metal catalysis [11], this doctoral project focused on development of the novel photoswitchable phosphine ligands for their incorporation into transition metal complexes, eventually aiming to enable light-induced modulation of their catalytic properties. Two different approaches were explored toward this goal, which involved the utilization of two distinct photoswitchable scaffolds: dithienylethenes (DTEs) and diazocines.

To ensure electron density modulation, dithienylethene (DTE)-based phophine ligands were invisioned. DTEs are a well-known class of molecular photoswitches which, in their more thermodynamically stable open isomer, consist of two thiophene moieties connected through a Z-ethene bridge (Figure 3a) [26]. This is a particular case of the broader family of DAE photoswitches, wherein the thiophene rings may be substituted with other aryl groups. Due to the occurrence of free rotation between the ethene linker and the thiophene rings, the structure of the open-state DTE is non-planar. Consequently, electronic communication between the two aryl groups is absent, resulting in strong absorption primarily in the UV, and occasionally violet, regions (Figure 3b) [27].

<figure><img src="image_169.png" /><figcaption>**Figure 3.** a) Ring-closing and ring-opening photochemical reactions of DTEs. b) Typical UV-vis absorption spectra of the open and closed forms of DTEs ($R_1 = H$, $R_2 = R_3 = COCF_3$).</figcaption></figure>

Upon exposure to UV light, the open state of DTEs undergoes a 6π electrocyclization reaction to produce the corresponding closed isomer, which displays an absorption band in the visible region due to its extended conjugation path (Figure 3b). The closed state of DTEs typically exhibits thermal stability, precluding spontaneous back-isomerization in the absence of external stimuli over time in the dark. The recovery of the open isomer therefore necessitates exposure to visible light, inducing an electrocyclic ring-opening reaction [27].

Thus, introduction of a diphenylphosphanyl group in one thiophene ring and an electron-withdrawing substituent in the other was expected to result in two distinct states of the phosphine. In the open state where the two thiophene rings are fully isolated, no electronic communication should occur between these two groups and, consequently, the phosphine ligand must have a strong σ-donor character. However, conjugation between the phosphanyl and EWG groups should occur upon photocyclization, thus decreasing the σ-donating ability of the phosphine. To further amplify this effect, two DTE moieties were devised to be introduced to the same phosphorus atom, whose
electronic properties could then be modulated by electronic communication to two EWGs simultaneously.

To explore the potential of the geometry alterations on catalytic activity, the goal of modulating the coordination mode of bisphosphine ligand upon the photoisomerization was set. Diazocine scaffold (Figure 4a) was sought of due to its specific properties. First, due to ring strain, the most stable configuration of diazocines is the Z isomer. Second, because of the non-planar structure of the Z and E diazocine isomers deriving from the short C₂ bridge, they exhibit stronger n→π* absorption bands in the visible region of the spectrum that are fairly separated, which can therefore be used to promote all-visible reversible photoisomerization (Figure 4b). Thus, Z-to-E photoisomerization takes place by irradiation with violet-blue light (370-400 nm) with high efficiencies [28].

<figure><img src="image_172.png" /><figcaption>**Figure 4.** a) Z/E isomerization of diazocines. b) Typical UV-vis absorption spectra of the Z and E diazocine isomers.</figcaption></figure>

While two phosphine groups can be in close proximity in the Z geometry, they should be rather far away in the E state, thus ensuring the switching of their coordination mode between mono- and bidentate. However, this scaffold is rather underrepresented in the literature and the challenge involves finding a suitable synthetic route for the preparation of diazocine-based phosphines. For instance, the electrophilicity of the N=N bond precludes the usage of many organometallic reagents [29], while the high ring-strain of diazocines can easily result in degradation at harsh reaction conditions. Consequently, despite reported synthesis of brominated diazocines, there are limited examples of their further functionalization [5,29,30,31].

## Dithienylethene-based photoswitchable phosphines

In a first step, the photomodulation of metal catalysts was pursued by exploring the electronic effects achieved on phosphine ligands through the photoisomerization of connected DTE photoswitches.

To validate this concept, the preparation of DTE-based phosphine ligands containing electron-withdrawing groups (EWGs): trifluoroacetyl – **DTE-COCF**₃ and perfluorophenyl – **DTE-C₆F₅**, and of an additional compound containing an electronically neutral substituent (phenyl group, **DTE-Ph**) was first undertaken (Figure 5). Ligands' synthesis involved the derivatization of a common dichlorinated DTE substrate promoted through sequential chlorine/lithium exchange reactions. Introduction of the phosphanyl and EWG groups was accomplished by quenching the resulting organolithiated compounds with the required electrophile. As for phenyl ring installation, it was accomplished through subsequent borylation and Suzuki coupling reactions.
electronic properties could then be modulated by electronic communication to two EWGs simultaneously.

To explore the potential of the geometry alterations on catalytic activity, the goal of modulating the coordination mode of bisphosphine ligand upon the photoisomerization was set. Diazocine scaffold (Figure 4a) was sought of due to its specific properties. First, due to ring strain, the most stable configuration of diazocines is the Z isomer. Second, because of the non-planar structure of the Z and E diazocine isomers deriving from the short C₂ bridge, they exhibit stronger n→π* absorption bands in the visible region of the spectrum that are fairly separated, which can therefore be used to promote all-visible reversible photoisomerization (Figure 4b). Thus, Z-to-E photoisomerization takes place by irradiation with violet-blue light (370-400 nm) with high efficiencies [28].

<figure><img src="image_170.png" /><figcaption>**Figure 4.** a) Z/E isomerization of diazocines. b) Typical UV-vis absorption spectra of the Z and E diazocine isomers.</figcaption></figure>

While two phosphine groups can be in close proximity in the Z geometry, they should be rather far away in the E state, thus ensuring the switching of their coordination mode between mono- and bidentate. However, this scaffold is rather underrepresented in the literature and the challenge involves finding a suitable synthetic route for the preparation of diazocine-based phosphines. For instance, the electrophilicity of the N=N bond precludes the usage of many organometallic reagents [29], while the high ring-strain of diazocines can easily result in degradation at harsh reaction conditions. Consequently, despite reported synthesis of brominated diazocines, there are limited examples of their further functionalization [5,29,30,31].

## Dithienylethene-based photoswitchable phosphines

In a first step, the photomodulation of metal catalysts was pursued by exploring the electronic effects achieved on phosphine ligands through the photoisomerization of connected DTE photoswitches.

To validate this concept, the preparation of DTE-based phosphine ligands containing electron-withdrawing groups (EWGs): trifluoroacetyl – **DTE-COCF**₃ and perfluorophenyl – **DTE-C₆F₅**, and of an additional compound containing an electronically neutral substituent (phenyl group, **DTE-Ph**) was first undertaken (Figure 5). Ligands' synthesis involved the derivatization of a common dichlorinated DTE substrate promoted through sequential chlorine/lithium exchange reactions. Introduction of the phosphanyl and EWG groups was accomplished by quenching the resulting organolithiated compounds with the required electrophile. As for phenyl ring installation, it was accomplished through subsequent borylation and Suzuki coupling reactions.
<figure><img src="image_173.png" /><figcaption>**Figure 5.** Synthesis of DTE-based photoswitchable phosphines.</figcaption></figure>

Upon light irradiation in cyclohexane solution, the obtained DTE-based ligands underwent successful reversible photoisomerization under UV (for ring-closing) and visible light irradiation (for ring-opening). In addition, TD-DFT calculations at the CAM-B3LYP-D3/6,31G(d,p) level were performed to further investigate the electronic excitations of these compounds, for which ground state geometries were first computed. For ligands, only their photocyclizable antiparallel conformation was considered in the calculations. All open state phosphine ligands were characterized by a strong absorption in the UV region with maxima around $\lambda_{abs} \approx 290$ nm (Figure 6), which was reproduced in computations and could be attributed to $\pi-\pi^*$ transitions (HOMO - LUMO+1 or HOMO - LUMO) of their core. In the case of DTE-COCF$_3$, a shallower absorption band ranging up to $\lambda_{abs} \approx 400$ nm was detected due to the lowering of the energy of the LUMO caused by the introduction of a strong EWG. For open state ligands, a broad, red-shifted absorption band emerged in the visible part of the spectrum upon UV illumination of their cyclohexane solutions, which changed from colorless to deep blue ($\lambda_{abs}$= 598 nm for DTE$^c$-COCF$_3$) or deep pink ($\lambda_{abs}$ $\approx$ 518 nm for DTE$^c$-C$_6$F$_5$ and DTE$^c$-Ph) (Figure 6).

<figure><img src="image_174.png" /><figcaption>**Figure 6.** Variation of the absorption spectrum of free ligands DTE$^o$-COCF$_3$ ($c$ = 3.0·10$^{-5}$ M), DTE$^o$-C$_6$F$_5$ ($c$ = 1.0·10$^{-5}$ M) and DTE-Ph ($c$ = 5.2·10$^{-5}$ M) in cyclohexane upon sequential irradiation with UV ($\lambda_{exc}$ = 355 or 312 nm) and green light ($\lambda_{exc}$ = 520 nm) until the corresponding ring-closing and ring-opening photostationary states (PSSs) are obtained, respectively.</figcaption></figure>

The equilibrium between the open (o) and closed (c) isomers achieved upon ring-closure was defined as a photostationary state (PSS), while the cycloreversion proceeds quantitatively. The best photoswitching performance was observed for the ligand containing a trifluoroacetyl group (DTE-COCF$_3$) (Table 1), exhibiting high ring-cyclization quantum yield and conversion (91% c at the PSS@365