# Seven-Coordinate Group 6 Metal Hydrides Obtained by H2 Activation at B(C6F5)3 Adducts of N2 Complexes: Frustrated Lewis Pair-Type Reactivity of The B–N Linkage

Marie-Christine Boegli, Anaïs Coffinet, Christian Bijani, Antoine Simonneau

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Marie-Christine Boegli, Anaïs Coffinet, Christian Bijani, Antoine Simonneau. Seven-Coordinate Group 6 Metal Hydrides Obtained by H2 Activation at B(C6F5)3 Adducts of N2 Complexes: Frustrated Lewis Pair-Type Reactivity of The B–N Linkage. Chemistry - An Asian Journal, 2024, 19 (12), pp.e202400451. 10.1002/asia.202400451. hal-04633325

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# Seven-Coordinate Group 6 Metal Hydrides Obtained by H₂ Activation at B(C₆F₅)₃ Adducts of N₂ Complexes: Frustrated Lewis Pair-Type Reactivity of The B–N Linkage

Marie-Christine Boegli,[a] Anaïs Coffinet,[a] Christian Bijani,[a] and Antoine Simonneau*[a]

The adducts 2<sup>M,R</sup> of general formula *trans*-[((L)M{R<sub>2</sub>P-(CH<sub>2</sub>)<sub>2</sub>PR<sub>2</sub>}<sub>2</sub>{N<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>}] (L=ø or N<sub>2</sub>, M=Mo or W, R=Et or Ph), formed from Lewis acid-base pairing of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to a dinitrogen ligand of zero-valent group 6 bis(phosphine) complexes *trans*[M{R<sub>2</sub>P(CH<sub>2</sub>)<sub>2</sub>PR<sub>2</sub>}<sub>2</sub>(N<sub>2</sub>)<sub>2</sub>] are shown to react with dihydrogen to afford hepta-coordinated bis(hydride) complexes [M(H)<sub>2</sub>{R<sub>2</sub>P-(CH<sub>2</sub>)<sub>2</sub>PR<sub>2</sub>}<sub>2</sub>{N<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>}] 3<sup>M,R</sup> which feature the rare ability to activate both dinitrogen and dihydrogen at a single metal center, except in the case where M=Mo and R=Ph for which fast precipitation of insoluble [Mo(H)<sub>4</sub>{dppe}<sub>2</sub>] (dppe=1,2-bis(diphenylphosphino)ethane) occurs. The frustrated Lewis pair (FLP)-related reactivity of the B–N linkage in compounds 3<sup>W,R</sup>

was explored and led to distal N functionalization without involvement of the hydride ligands. It is shown in one example that the resulting bis(hydride) diazenido compounds may also be obtained through a sequence involving first FLP-type N-functionalization followed by oxidative addition of H₂. Those oily compounds were found to have limited stability in solution or in their isolated states. Finally, treatment of 3<sup>W,Et</sup> with the Lewis base N,N-dimethylaminopyridine (DMAP) affords the simple but unknown bis(hydride)-dinitrogen species [W-(H<sub>2</sub> (depe)<sub>2</sub> (N<sub>2</sub>)) 11<sup>Et</sup> (depe=1,2-bis(diethylphosphino)ethane) which direct, selective formation from *trans*-[W(N<sub>2</sub>)<sub>2</sub> (depe)<sub>2</sub>] is not possible.

## Introduction

Nitrogen is an element essential for life, yet its principal source on Earth, dinitrogen, is basically unreactive. Consequently, it cannot be metabolized by most of the living organisms, except those that express the nitrogenase enzymes.[1,2] At the advent of the 20th century, mankind had to devise a way to manufacture large quantities of nitrogen fertilizers to sustain a growing population. Haber and Bosch developed a process that catalytically reduces N₂ into ammonia in the presence of H₂ over a heterogeneous Fe catalyst, which remains nowadays one of the largest processes in the chemical industry.[3] However, the high energy demand for its functioning, coupled with its important contribution to CO₂ emissions, has prompted chemists to search for more sustainable alternatives.[4] Approaching the mild conditions under which the organometallic active site of the nitrogenase operates by designing soluble metal complexes that can activate dinitrogen remains a compelling strategy.[5-10] Some of them eventually achieved catalytic ammonia synthesis at ambient temperature and pressure.[11-14] Akin to nitrogenases, sequential supply of protons and electrons to the catalyst, with minimal recombination thereof, is key to a successful dinitrogen

reduction. Reactions of N₂ complexes or N₂-derived nitrides[15] with hydrogen that lead to N–H bond formation and eventually the stoichiometric release of ammonia were also discovered.[16,17] The necessity to access low oxidation states for N₂ binding and activation as well as the lability of the N₂ ligand are factors that generally drive the equilibrated or exothermic formation of off-cycle hydrides when N₂ complexes are exposed to an H₂ atmosphere,[18–21] making the use of H₂ for homogeneous ammonia synthesis challenging. Our group has previously reported that group VI Chatt-Hidai N₂ complexes can combine with strong boron Lewis acids to effect N₂ functionalization[22–24] that is reminiscent of the frustrated Lewis pair (FLP) chemistry (Scheme 1).[25–31] We have continued the exploration of the reactivity of these combinations with hydrogen, hoping to observe a new way to form N–H bonds under homogeneous conditions.[32] In a recent report,[33] we have shown on a tungsten complex that the expected FLP-type heterolytic splitting of H₂[34] by the B–N linkage did not occur. Dihydrogen favored reacting with the metal center to lead to seven-coordinate complexes exhibiting the rare feature of having N₂ and H₂ activated at a single metal center.[21,35–37] Herein, we expand the characterization of this family of complexes and examine their reactivity. Inspired by recent observations that functionalization of the distal nitrogen in N₂ complexes may trigger insertion of the N₂ unit into a metal-ligand bond,[38-40] we have focused in the present study on the FLP-type functionalization of the distal nitrogen.

<footnote>[a] Dr. M.-C. Boegli, Dr. A. Coffinet, Dr. C. Bijani, Dr. A. Simonneau
LCC-CNRS, Université de Toulouse, CNRS, UPS
205 route de Narbonne, BP44099, F-31077 Toulouse cedex 4, France
E-mail: antoine.simonneau@lcc-tdoulouse.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202400451</footnote>

<footnote>© 2024 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Overview of the reactivity of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adducts of group 6 N<sub>2</sub>-complexes with hydrosilanes, hydroboranes and dihydrogen.</figcaption></figure>

## Results and Discussion

We have previously reported the syntheses and characterizations of the B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adducts of complexes 1<sup>M,R</sup> to give adducts 2<sup>M,R</sup>.<sup>[22,41]</sup> We have already shown that, in deuterated benzene, 2<sup>W,Et</sup> reacts rapidly with H<sub>2</sub> to afford the seven-coordinate bis-hydride N<sub>2</sub> complex 3<sup>W,Et</sup> after the loss of one N<sub>2</sub> ligand and isomerization of a diphosphine.<sup>[33]</sup> Surprisingly, under the same conditions, 2<sup>Mo,Ph</sup> converts into a mixture of [MoH<sub>4</sub>(dppe)<sub>2</sub>] (4) and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>. The fast precipitation of crystalline 4, even in the more polar o-dichlorobenzene, presumably drives the reaction to this outcome. Tetrahydride 4 is a known compound that can be obtained from the reaction of 1<sup>Mo,Ph</sup> with dihydrogen.<sup>[19,42]</sup> Contrastingly, 2<sup>Mo,Et</sup> and 2<sup>W,Ph</sup> reacted the same way as 2<sup>W,Et</sup> did, furnishing the seven-coordinate bis-hydride N<sub>2</sub> complexes 3<sup>Mo,Et</sup> and 3<sup>W,Ph</sup> (Scheme 2). These compounds are characterized by the appearance of centrosymmetric hydride multiplets in ¹H NMR spectroscopy (δ = -2.38, -0.38 and -1.74 ppm for 3<sup>Mo,Et</sup>, 3<sup>W,Ph</sup> and 3<sup>W,Et</sup>, respectively) and the splitting of the phosphorus resonances of 2<sup>M,R</sup> into three multiplets due to the loss in symmetry. Notably,

<figure><img src="image_5.png" /><figcaption>**Scheme 2.** Reactivity of metal-dinitrogen/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> Lewis pair with dihydrogen.</figcaption></figure>

the phosphorus nuclei in trans relationship to the N<sub>2</sub> unit are particularly shielded, and exhibit a low ³¹P-¹⁸³W coupling constant (for 3<sup>W,Et</sup>, J<sub>PW</sub> = 145 Hz vs. 183 Hz and 181 Hz for the equatorial nuclei). This decrease when compared to 1<sup>W,Et</sup> (310.7 Hz) point to a significant trans influence<sup>[43]</sup> of the boranyl-diazo moiety and indicate substantial negative polarization on the trans phosphorus atom.<sup>[44-46]</sup> The stability of the complexes in solution varies in the order 3<sup>Mo,Et</sup> ≪ 3<sup>W,Ph</sup> < 3<sup>W,Et</sup>. In the case of 3<sup>Mo,Et</sup>, its lability precluded isolation in pure form and further reactivity study. The latter and 3<sup>W,Ph</sup> tend to rapidly evolve into the cationic pentahydride [MH<sub>5</sub>(R<sub>2</sub>P(CH<sub>2</sub>)<sub>2</sub>PR<sub>3</sub>)]<sup>+</sup>.<sup>[47-52]</sup>

Having previously established that adducts 2<sup>M,Ph</sup> can activate Si–H and B–H bonds,<sup>[22]</sup> we have examined whether the tungsten hydride complexes 3<sup>W,R</sup> would display similar behavior (Scheme 3). Its treatment with triethylsilane results in the generation of the [HB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]⁻ anion, as evidenced by ¹¹B NMR spectroscopy. This observation, along with a shift in phosphorus resonances (see Figures S19–23) in the ³¹P NMR spectrum and the disappearance of the N≡N bond stretching band in infrared (IR) spectroscopy, was indicative of productive N–Si bond formation. These reactions yield new diazenido compounds 5<sup>R</sup>, with conversion rates ranging from 88% to 95% based on ³¹P NMR. Furthermore, the presence of shielded signals in the ¹H NMR spectrum integrating for two protons in the 0.0–2.5 ppm range (depending on the phosphine ligand) ruled out an implication of the hydrides in the process. Complex 3<sup>W,Ph</sup> reacted slowly with triphenylsilane, requiring two days to reach complete consumption. However, it led to a mixture of compounds, among which the silyldiazenido bis(hydride) complex 6<sup>Ph</sup> was detected (ca. 40% according to ³¹P NMR spectroscopy). In contrast, no reaction was observed in the case of 3<sup>W,Et</sup>. The reactivity of adducts 3<sup>M,R</sup> with silanes was more sluggish than that of 2<sup>W,Ph</sup>. We believe this is the result of a stronger interaction between B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> and the N<sub>2</sub> ligand in 3<sup>W,R</sup>. This can be judged by a lower energy N≡N bond stretching band in IR than in 2<sup>M,Et</sup> (ν = 1712 cm<sup>-1</sup> for 3<sup>W,Et</sup> vs. 1767 cm<sup>-1</sup> for 2<sup>W,Et</sup>) indicating a higher back-donation from the metal to the N<sub>2</sub> unit. Since it is likely that the Si–H bond activation implies dissociation of the Lewis pair to form a transient Si–H···B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adduct,<sup>[53-56]</sup> a more electron-rich N<sub>2</sub> ligand results in the dissociation equilibrium being further displaced towards the Lewis pair. In addition, the isomerization of the first ligand sphere in 3<sup>M,R</sup> also decreases steric repulsion between the borane's substituents and the phosphines' ones, improving the cohesion of the edifice. Of note, partial decomposition of 3<sup>W,Ph</sup> into [WH<sub>4</sub>(dppe)<sub>2</sub>] and [WH<sub>5</sub>(dppe)<sub>2</sub>]<sup>+</sup> and an unidentified complex was noticed upon conversion of the former. All silyldiazenido compounds were obtained as oils and were found to be very reluctant to precipitate under powder or crystalline form, precluding definitive confirmation of their structure by single crystal X-ray diffraction (sc-XRD). They also showed limited stability either in solution or in their isolated form, restricting their characterization to NMR spectroscopy only.

We next turned our attention towards the activation of B–H bonds. The tungsten complexes 3<sup>W,R</sup> were reacted with 9-borabicyclononane (9-BBN) and Piers’ borane (HB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>)
<figure><img src="image_6.png" /><figcaption>**Scheme 3.** Formation of main-group diazenido-bis(hydride) complexes via FLP-type splitting of Si-H and B-H bonds (3<sup>W,R</sup>→5<sup>R</sup>, 6<sup>Ph</sup>, 7<sup>Et</sup>, 8<sup>R</sup> or 9<sup>Et</sup>) or oxidative addition of H<sub>2</sub> on a N<sub>2</sub>-derived boryldiazenido complex (10→8<sup>Et</sup>). Conversions are calculated on the basis of 31P NMR spectroscopy integration.</figcaption></figure>

(Scheme 3). If the reaction of 3<sup>W,Ph</sup> with 9-BBN led to a mixture of compounds, in which it was difficult to establish unambiguously the formation of the boryldiazenido bis(hydride) complex 7<sup>Ph</sup>, 3<sup>W,Et</sup> afforded selectively the depe analog 7<sup>Et</sup> after 18 h at 60 °C, on the basis of multinuclear NMR analysis. Akin to the silyldiazenido compounds, the oily nature of these species, when isolated from the reaction mixture, precluded their characterization by sc-XRD. They also demonstrated evident signs of decomposition upon storage in solution or as oils. Complexes 5<sup>Et</sup> and 7<sup>Et</sup> show similar 31P NMR spectra, exhibiting a shielded resonance for the phosphorus nuclei in the trans positions (δ=10.5 and 9.1 ppm, respectively) when compared to the precursor 3<sup>W,Et</sup> (15.6 ppm). This again reflects a more significant trans influence of the silyl- or boryl-diazenido ligand than the boranyl-diazo unit found in 3<sup>W,Et</sup>. When Piers' borane[58] (HB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>) was employed, 3<sup>W,Ph</sup> underwent a rapid transformation to yield 8<sup>Ph</sup>, while the reaction proceeded more slowly with 3<sup>W,Et</sup>. Following the previous observations, the trans phosphorus nucleus is shielded (δ=5.3 ppm) in 8<sup>Et</sup> compared to the starting bis(hydride) compound. Finally, to complete the range of borane our system is able to activate, we treated 3<sup>W,Et</sup> with catechol borane. This furnished a supplementary example of N<sub>2</sub> borylation through B-H bond splitting, as it selectively converted (97%) to the corresponding boryldiazenido complex 9<sup>Et</sup> under conditions similar to those with Piers' borane. However, the dppe analogue 3<sup>W,Ph</sup> reacted in a nonselective manner. Like 8<sup>Et</sup>, 9<sup>Et</sup> also features a shielded trans phosphorus nucleus (δ=5.4 ppm) and deshielded hydride resonances (δ=3.04 ppm). Across the whole series of depe-supported complexes, dramatic variations of the 31P<sub>trans</sub>-183W coupling constants are noticed, from 145.2 Hz for 3<sup>W,Et</sup> down to 71.6 Hz for 9<sup>Et</sup>. This trend was found less spectacular when considering the 31P<sub>cis</sub>-183W ones

(see the Supporting Information for details). This reflects an increasingly stronger trans influence[43] exerted on the W-P bond by the opposing NNE (E=B or Si) ligand.[44-46]

This sequence of reactions spurred us to investigate whether formally divalent group VI boryldiazenido compounds, derived from Chatt-Hidai N<sub>2</sub> complexes,[22-24,59-62] would react with dihydrogen in an analogous manner to complexes 2<sup>M,R</sup> of lower valency. This would lead to compounds of the type 7–8<sup>R</sup> as described above. To verify this, we subjected an N<sub>2</sub>-derived boryldiazenido tungsten(II) complex we have previously described[23] (10, Scheme 3) with H<sub>2</sub>. The resulting product, 8<sup>Et</sup>, emerged as the main component of a mixture, differing solely from 8<sup>Et</sup> by the nature of its anion. Thus, the formal +II oxidation state of the metal center (compared to 0 in complexes 2<sup>M,R</sup>) is not detrimental for H<sub>2</sub> activation to occur. The reaction of 10 with 2H<sub>2</sub> furnishes [2H]-8<sup>Et</sup> rather selectively. The disappearance of a pseudo doublet of doublet of doublet (ddd) at δ=2.40 ppm allowed unambiguous assignment of this signal to the hydride resonances. Upon broadband phosphorus decoupling, this signal transforms into two overlapping singlets, suggesting the hydride resonance is actually a set of two doublet of doublet (dd). After a night, the solution of [2H]-8<sup>Et</sup> in C<sub>6</sub>D<sub>6</sub> shows significant 1H-2H exchange between the tungsten center and the hydridoborate anion. The 15N isotopologue of 8<sup>Et</sup>, [15N]-8<sup>Et</sup>, was also prepared. The two resonances for the N<sub>2</sub> ligand are observed at δ=-48.9 and -89.1 ppm (Figure S59). The deshielded one was tentatively assigned to the proximal nitrogen nucleus (N<sub>α</sub>), due to its strong correlation with the hydride signals in 15N-1H HMQC 2D-NMR experiment, and appears as a pseudo doublet of doublet of quadruplet due to scalar coupling with the two hydrides and the three phosphorus nuclei (Figure S60). This is also reflected in the 1H
and ³¹P[{¹H} spectra of [¹⁵N]-8⁴Et. A splitting of the hydride resonances into a pseudo doublet of doublet of doublet of (ddd) due to a small 2.3 Hz coupling with ¹⁵N is observed in the former (see Figure S57), while in the latter all phosphorus resonances exhibit higher multiplicity. Expectedly, the trans ¹⁵N-³¹P coupling constant is found higher than the cis one (ca. 22 Hz vs. ca. 6 Hz, respectively, see Figure S58).

Compound 3<sup>W,Et</sup> also reacted with dimethylaminopyridine (DMAP) to give the Lewis acid-base adduct DMAP·B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> and [W(H)<sub>2</sub>(N<sub>2</sub>)(depe)<sub>2</sub>] (11<sup>Et</sup>) (Scheme 4). The ³¹P{¹H} NMR spectrum of 11<sup>Et</sup> (Figure S64) displayed a single phosphorous resonance, contrasting with the one of 3<sup>W,Et</sup> (Figure S12) and suggesting higher symmetry with all phosphorus nuclei located in the equatorial plane. Although the isolation of clean 11<sup>Et</sup> proved impossible, complicating further characterization beyond NMR spectroscopy, we confirmed its nature by the observation of a high-energy N≡N bond stretching band (1924 cm<sup>-1</sup>) from a THF solution (Figure S65). Furthermore, treatment of 11<sup>Et</sup> with B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> regenerated 3<sup>W,Et</sup>. Despite being simple in terms of structure, 11<sup>Et</sup> is an unknown compound. It may occur as an intermediate in the reaction of 1<sup>W,Et</sup> with H<sub>2</sub> to form [W(H)<sub>4</sub>(depe)<sub>2</sub>]<sup>[19,42]</sup> or the reverse conversion thereof into 1<sup>W,Et</sup>.<sup>[63]</sup> It is remarkable that the Lewis acidic borane B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, through complexation to one N<sub>2</sub> ligand of a bis(dinitrogen) W complex, drives selectively the substitution of just one N<sub>2</sub> ligand by H<sub>2</sub>. This is not achievable in the case of the bare complex as full substitution of the two N<sub>2</sub> ligands occurs to afford polyhydric species, which coordination spheres are devoid of N<sub>2</sub>. Mixtures of sily- or boryl-diazenido complexes 5–9 and DMAP did not lead, on the other hand, to detectable reactivity.

Conclusions

We have reported herein the syntheses of new bis(hydride) N₂ complexes. The latter were obtained by oxidative addition of H₂ to the B(C₆F₅)₃ adducts of formally zero-valent Chatt-Hidai type bis(phosphine), bis(dinitrogen) group 6 complexes. Remarkably, in these species, elemental forms of the ingredients of ammonia are activated at the same metal center. The present study focused on the FLP-type reactivity of the N→B linkage, hoping that functionalization of the distal nitrogen would trigger hydride migration onto the proximal nitrogen atom, pertaining to recent demonstrations of related rearrangement of the first coordination sphere of iron-N₂ complexes.[³⁸,³⁹,⁶⁴] In our case, no N–H bond formation occurred, and recent work from our group

has shown that two-fold coordination of a bis(borane) to the
distal N is equally unsuccessful in triggering a metal-to-nitrogen
hydride transfer. Nonetheless, our study further exemplifies the
use of an N₂-ligand in FLP-related chemistry as a strategy to
functionalize this diatomic molecule. We also show that the
B(C₆F₅)₃ adduct of a dinitrogen bis(hydride) complex 3W,Et reacts
with a Lewis base to give a simple, yet unknown, heptacoordi-
nated dinitrogen bis(hydride) complex 11Et. This complex is
likely to be an intermediate in the conversion of trans-
[W(N₂)₂(depe)₂] (or related complexes) into the tetrakis(hydride)
species [WH₄(depe)₂] (ibid.). Therefore, borane complexation to
a N₂ ligand may appear as a means to "maintain" it while
conducting reactions that would otherwise labilize it. Current
work in our laboratory is aimed at exploring the reactivity of
these bis(hydride)-dinitrogen complexes with small molecules,
as well as their redox properties.

**Supporting Information**

Experimental methods. Key spectroscopic and analytical data.
The authors have cited additional references within the
Supporting Information.[64–70]

Acknowledgements

M.-C. B. and A. S. are indebted to the European Research Council for funding (ERC Starting Grant n° 757501). A.C. is grateful to the French Ministry of National and Superior Education and Research (MENESR) for a Ph.D. fellowship. We are grateful to the CNRS (Centre National de la Recherche Scientifique) for providing access to facilities.

A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions.

<figure><img src="image_7.png" /></figure>

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available
from the corresponding author upon reasonable request.

Keywords: dinitrogen · dihydrogen · group 6 metals · silanes · boranes

<figure><img src="image_8.png" /><figcaption>**Scheme 4.** Reaction of 3<sup>W,Et</sup> with a Lewis base leads to the formation of the bis-hydrido dinitrogen complex 11<sup>Et</sup>.</figcaption></figure>
[1] L. C. Seefeldt, Z.-Y. Yang, D. A. Lukoyanov, D. F. Harris, D. R. Dean, S. Raugei, B. M. Hoffman, *Chem. Rev.* **2020**, *120*, 5082–5106.

[2] B. M. Hoffman, D. Lukoyanov, Z.-Y. Yang, D. R. Dean, L. C. Seefeldt, *Chem. Rev.* **2014**, *114*, 4041–4062.

[3] M. Appl, in *Ullmanns Encyclopedia of Industrial Chemistry*, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, **2011**, pp. 139–225.

[4] J. G. Chen, R. M. Crooks, L. C. Seefeldt, K. L. Bren, R. M. Bullock, M. Y. Darensbourg, P. L. Holland, B. Hoffman, M. J. Janik, A. K. Jones, M. G. Kanatzidis, P. King, K. M. Lancaster, S. V. Lymar, P. Pfromm, W. F. Schneider, R. R. Schrock, *Science* **2018**, *360*, eaar6611.

[5] F. Masero, M. A. Perrin, S. Dey, V. Mougel, *Chem. Eur. J.* **2021**, *27*, 3892–3928.

[6] K. Tanifuji, Y. Ohki, *Chem. Rev.* **2020**, *120*, 5194–5251.

[7] D. Singh, W. R. Buratto, J. F. Torres, L. J. Murray, *Chem. Rev.* **2020**, *120*, 5517–5581.

[8] Y. Nishibayashi, Ed., *Transition Metal-Dinitrogen Complexes: Preparation and Reactivity*, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, **2019**.

[9] R. J. Burford, M. D. Fryzuk, *Nat. Chem. Rev.* **2017**, *1*, 1–13.

[10] N. Khoenkhoen, B. de Bruin, J. N. H. Reek, W. I. Dzik, *Eur. J. Inorg. Chem.* **2015**, *2015*, 567–598.

[11] Y. Tanabe, Y. Nishibayashi, *Chem. Soc. Rev.* **2021**, *50*, 5201–5242.

[12] M. J. Chalkley, M. W. Drover, J. C. Peters, *Chem. Rev.* **2020**, *120*, 5582–5636.

[13] N. Stucke, B. M. Flöser, T. Weyrich, F. Tuczek, *Eur. J. Inorg. Chem.* **2018**, *2018*, 1337–1355.

[14] Y. Roux, C. Duboc, M. Gennari, *ChemPhysChem* **2017**, *18*, 2606–2617.

[15] S. J. K. Forrest, B. Schluschaß, E. Y. Yuzik-Klimova, S. Schneider, *Chem. Rev.* **2021**, *121*, 6522–6587.

[16] C. Sivasankar, P. K. Madarasi, M. Tamizmani, *Eur. J. Inorg. Chem.* **2020**, *2020*, 1383–1395.

[17] T. Shima, Z. Hou, *Top. Organomet. Chem.* **2017**, *60*, 23–43.

[18] A. Sacco, M. Rossi, *Inorg. Chim. Acta* **1968**, *2*, 127–132.

[19] L. J. Archer, T. A. George, *Inorg. Chem.* **1979**, *18*, 2079–2082.

[20] D. G. H. Hetterscheid, B. S. Hanna, R. R. Schrock, *Inorg. Chem.* **2009**, *48*, 8569–8577.

[21] H. Fong, M.-E. Moret, Y. Lee, J. C. Peters, *Organometallics* **2013**, *32*, 3053–3062.

[22] A. Simonneau, R. Turrel, L. Vendier, M. Etienne, *Angew. Chem. Int. Ed.* **2017**, *56*, 12268–12272.

[23] A. Coffinet, D. Specklin, L. Vendier, M. Etienne, A. Simonneau, *Chem. Eur. J.* **2019**, *25*, 14300–14303.

[24] A. Coffinet, D. Zhang, L. Vendier, S. Bontemps, A. Simonneau, *Dalton Trans.* **2021**, *50*, 5582–5589.

[25] J. C. Slootweg, A. R. Jupp, *Frustrated Lewis Pairs*, Springer Nature, **2020**.

[26] J. Paradies, *Coord. Chem. Rev.* **2019**, *380*, 170–183.

[27] D. W. Stephan, *Science* **2016**, *354*, aaf7229.

[28] G. Erker, D. W. Stephan, Eds., *Frustrated Lewis Pairs II: Expanding the Scope*, Springer Berlin Heidelberg, **2013**.

[29] G. Erker, D. W. Stephan, Eds., *Frustrated Lewis Pairs I: Uncovering and Understanding*, Springer Berlin Heidelberg, **2013**.

[30] D. W. Stephan, G. Erker, *Angew. Chem. Int. Ed.* **2010**, *49*, 46–76.

[31] D. W. Stephan, G. Erker, *Angew. Chem. Int. Ed.* **2015**, *54*, 6400–6441.

[32] M. Tamizmani, C. Sivasankar, *Eur. J. Inorg. Chem.* **2017**, *2017*, 4239–4245.

[33] D. Specklin, M.-C. Boegli, A. Coffinet, L. Escomel, L. Vendier, M. Grellier, A. Simonneau, *Chem. Sci.* **2023**, *14*, 14262–14270.

[34] D. W. Stephan, *J. Am. Chem. Soc.* **2021**, *143*, 20002–20014.

[35] W. H. Bernskoetter, E. Lobkovsky, P. J. Chirik, *Organometallics* **2005**, *24*, 6250–6259.

[36] R. P. Yu, J. M. Darmon, S. P. Semproni, Z. R. Turner, P. J. Chirik, *Organometallics* **2017**, *36*, 4341–4343.

[37] S. M. Rummelt, H. Zhong, N. G. Léonard, S. P. Semproni, P. J. Chirik, *Organometallics* **2019**, *38*, 1081–1090.

[38] M. M. Deegan, J. C. Peters, *Chem. Sci.* **2018**, *9*, 6264–6270.

[39] S. F. McWilliams, D. L. J. Broere, C. J. V. Halliday, S. M. Bhutto, B. Q. Mercado, P. L. Holland, *Nature* **2020**, *584*, 221–226.

[40] S. M. Bhutto, R. X. Hooper, B. Q. Mercado, P. L. Holland, *J. Am. Chem. Soc.* **2023**, *145*, 4626–4637.

[41] D. Specklin, A. Coffinet, L. Vendier, I. del Rosal, C. Dinoi, A. Simonneau, *Inorg. Chem.* **2021**, *60*, 5545–5562.

[42] L. J. Archer, T. Adrian George, *J. Organomet. Chem.* **1973**, *54*, C25–C26.

[43] T. G. Appleton, H. C. Clark, L. E. Manzer, *Coord. Chem. Rev.* **1973**, *10*, 335–422.

[44] A. Pidcock, R. E. Richards, L. M. Venanzi, *J. Chem. Soc. Inorg. Phys. Theor.* **1966**, *0*, 1707–1710.

[45] F. H. Allen, A. Pidcock, *J. Chem. Soc. Inorg. Phys. Theor.* **1968**, *0*, 2700–2704.

[46] G. G. Mather, A. Pidcock, *J. Chem. Soc. Inorg. Phys. Theor.* **1970**, *1226*, 1229.

[47] E. Carmona-Guzman, G. Wilkinson, *J. Chem. Soc. Dalton Trans.* **1977**, *1716–1721*.

[48] A. P. Borisov, V. D. Makhaev, G. N. Boiko, K. N. Semenenko, *Koord. Khim.* **1978**, *4*, 1274.

[49] V. D. Makhaev, A. P. Borisov, G. N. Boiko, K. N. Semenenko, *Koord. Khim.* **1982**, *8*, 963–969.

[50] T. Ito, A. Takahashi, S. Tamura, *Bull. Chem. Soc. Jpn.* **1986**, *59*, 3489–3494.

[51] S. Kim, Y. Park, J. Kim, T. P. Pabst, P. J. Chirik, *Nat. Synth.* **2022**, *1*, 297–303.

[52] N. Queyriaux, N. Durvin, D. Leon, M.-C. Boegli, L. Vendier, A. Simonneau, *Eur. J. Inorg. Chem.* **2023**, *n/a*, e202300426.

[53] D. J. Parks, W. E. Piers, *J. Am. Chem. Soc.* **1996**, *118*, 9440–9441.

[54] D. J. Parks, J. M. Blackwell, W. E. Piers, *J. Org. Chem.* **2000**, *65*, 3090–3098.

[55] S. Rendler, M. Oestreich, *Angew. Chem. Int. Ed.* **2008**, *47*, 5997–6000.

[56] A. Y. Houghton, J. Hurmalainen, A. Mansikkamäki, W. E. Piers, H. M. Tuononen, *Nat. Chem.* **2014**, *6*, 983–988.

[57] C. C. Carter, B. R. Allen, D. V. Bui, D. A. Daley, J. M. Goncalves, C. M. Holinej, S. Lira, D. E. Muniz, A. I. Santiago, N. Sukran, T. R. Cundari, M. Yousufuddin, *Inorg. Chim. Acta* **2020**, *508*, 119638.

[58] E. A. Patrick, W. E. Piers, *Chem. Commun.* **2020**, *56*, 841–853.

[59] D. J. Parks, *J. Org. Chem.* **1998**, *27*, 677–678.

[60] A. Rempel, S. K. Mellerup, F. Fantuzzi, A. Herzog, A. Deißenberger, R. Bertermann, B. Engels, H. Braunschweig, *Chem. Eur. J.* **2020**, *26*, 16019–16027.

[61] A. Bouammali, A. Coffinet, L. Vendier, A. Simonneau, *Dalton Trans.* **2022**, *51*, 10697–10701.

[62] L. C. Haufe, M. Arrowsmith, M. Dietz, A. Gärtner, R. Bertermann, H. Braunschweig, *Dalton Trans.* **2022**, *51*, 12786–12790.

[63] R. Pierantozzi, G. L. Geoffroy, *Inorg. Chem.* **1980**, *19*, 1821–1822.

[64] A. M. Borys, *Organometallics* **2023**, *42*, 182–196.

[65] J. R. Dilworth, R. L. Richards, *Inorg. Synth.* **1990**, *28*, 33–43.

[66] A. C. Filippou, G. Schnakenburg, A. I. Philippopoulos, N. Weidemann, *Angew. Chem. Int. Ed.* **2005**, *44*, 5979–5985.

[67] T. Ueno, I. Katsumi, N. Yamamoto, H. Mitsui, *Producing Process of Fluoroaryl Magnesium Derivative and Producing Process of (Fluoroaryl)Borane Compound*, **2003**, EP0825195B1.

[68] D. J. Parks, R. E. von H Spence, W. E. Piers, *Angew. Chem. Int. Ed.* **1995**, *34*, 809–811.

[69] R. K. Harris, E. D. Becker, S. M. Cabral de Menezes, R. Goodfellow, P. Granger, *Pure Appl. Chem.* **2001**, *73*, 1795–1818.

[70] R. K. Harris, E. D. Becker, S. M. Cabral de Menezes, P. Granger, R. E. Hoffman, K. W. Zilm, *Pure Appl. Chem.* **2008**, *80*, 59–84.

<footnote>Manuscript received: April 23, 2024</footnote>

<footnote>Revised manuscript received: June 7, 2024</footnote>

<footnote>Accepted manuscript online: June 12, 2024</footnote>

<footnote>Version of record online: July 25, 2024</footnote>