## Catalysis with Ni/FILs and Ni-NiO/FILs

The catalytic performances of the Ni/FILs and Ni-NiO/FILs NPs systems were investigated for the hydrogenation of 2-cyclohexen-1-one (CH-enon) using the given reaction conditions (130°C, 10 bar H₂, 1 h, 1200 rpm, sub:Ni ratio 100:1) (Scheme 5). Table 5 summarizes the results obtained.

<figure><img src="image_254.png" /><figcaption>**Scheme 5.** Hydrogenation of 2-cyclohexen-1-one (CH-enon) to cyclohexanone (CH-non), 2-cyclohexen-1-ol (CH-enol) and cyclohexanol (CH-ol).</figcaption></figure>

<table><caption>**Table 5.** Hydrogenation of 2-cyclohexen-1-one with Ni/FILs and Ni-NiO/FILs catalysts.ª</caption><thead><tr><th rowspan="2">Catalyst system</th><th rowspan="2">Conversion (%)<sup>b</sup></th><th colspan="3">Selectivity (%)<sup>b</sup></th><th rowspan="2">TOF(h<sup>-1</sup>)</th></tr><tr><th>CH-non</th><th>CH-ol</th><th>CH-enol</th></tr></thead><tbody><tr><td>Ni/H</td><td>&gt;99</td><td>97</td><td>3</td><td>0</td><td>677</td></tr><tr><td>Ni-NiO/H</td><td>&gt;99</td><td>98</td><td>2</td><td>0</td><td></td></tr><tr><td>Ni/MEM</td><td>97</td><td>&gt;99</td><td>0</td><td>0</td><td>814</td></tr><tr><td>Ni-NiO/MEM</td><td>78</td><td>&gt;99</td><td>0</td><td>0</td><td></td></tr><tr><td>Ni/MME</td><td>&gt;99</td><td>&gt;99</td><td>0</td><td>0</td><td>627</td></tr><tr><td>Ni-NiO/MME</td><td>89</td><td>&gt;99</td><td>0</td><td>0</td><td></td></tr><tr><td>Ni/CN</td><td>&gt;99</td><td>&gt;99</td><td>0</td><td>0</td><td>712</td></tr><tr><td>Ni-NiO/CN</td><td>&gt;99</td><td>&gt;99</td><td>0</td><td>0</td><td></td></tr></tbody></table>

As shown in Table 5, both the Ni/FILs and Ni-NiO/FILs systems were active for the catalytic hydrogenation of CH-enon in short reaction time (1 h). A complete conversion of CH-enon selectively towards CH-non was obtained for all Ni/FILs systems. The difference in conversion rate vs. the Ni/FILs systems plots have been provided in Figure 5. The initial turnover frequency (TOF) was calculated for all Ni/FILs systems at 0.25 h. The highest TOF (814 h⁻¹) value was reached by the Ni NPs embedded in the MEM-functionalized ILs. The non-functionalized system and the nitril functionalization yielded high conversion (TOFs of 677 and 712 h⁻¹, respectively), while the had MME methoxy functionalized, being the largest NPs in size showed the lowest (627 h⁻¹). Nevertheless, all the systems showed high activity at the given reaction conditions with the initial order of the activity being Ni/MME < Ni/H < Ni/ CN < Ni/MEM. A comprehensive study with all Ni-NiO/FILs systems was carried out under these reaction conditions. For the Ni-NiO/MEM and Ni-NiO/MME, the activity dropped to 78 and 89% respectively upon oxidation of the NPs surface. Unlike the methoxy systems, it could be noticed that the Ni-NiO/CN and Ni-NiO/H systems did not show inferior activity compared to their non-oxidized analogues. The reason could be a lower oxidation of the NPs surface during air exposure as supported by the XPS data, making these catalysts more effective in the short reaction time applied. No change is selectivity was observed upon oxidation. To assess the potential impact of temperature variation on catalyst selectivity, the hydrogenation of CH-enon was also performed at 150 and 170 °C with the systems, while keeping all other parameters constant. Notably, the hydrogenation of neither the carbonyl group nor the aromatic ring was observed at 150 and 170 °C and full conversion of CH-enon was observed.

<footnote>ª Reaction conditions: 0.01 mmol of Ni, 1 mmol of 2-cyclohexene-1-none (CH-enon), 0.3 mmol of octane (internal standard), 130 °C, 10 bar H₂, 1 h, 1200 rpm. ᵇ Determined by GC using internal standard technique.</footnote>
## Catalysis with Ni/FILs and Ni-NiO/FILs

The catalytic performances of the Ni/FILs and Ni-NiO/FILs NPs systems were investigated for the hydrogenation of 2-cyclohexen-1-one (CH-enon) using the given reaction conditions (130°C, 10 bar H₂, 1 h, 1200 rpm, sub:Ni ratio 100:1) (Scheme 5). Table 5 summarizes the results obtained.

<figure><img src="image_252.png" /><figcaption>**Scheme 5.** Hydrogenation of 2-cyclohexen-1-one (CH-enon) to cyclohexanone (CH-non), 2-cyclohexen-1-ol (CH-enol) and cyclohexanol (CH-ol).</figcaption></figure>

<table><caption>**Table 5.** Hydrogenation of 2-cyclohexen-1-one with Ni/FILs and Ni-NiO/FILs catalysts.ª</caption><thead><tr><th rowspan="2">Catalyst system</th><th rowspan="2">Conversion (%)<sup>b</sup></th><th colspan="3">Selectivity (%)<sup>b</sup></th><th rowspan="2">TOF(h<sup>-1</sup>)</th></tr><tr><th>CH-non</th><th>CH-ol</th><th>CH-enol</th></tr></thead><tbody><tr><td>Ni/H</td><td>&gt;99</td><td>97</td><td>3</td><td>0</td><td>677</td></tr><tr><td>Ni-NiO/H</td><td>&gt;99</td><td>98</td><td>2</td><td>0</td><td></td></tr><tr><td>Ni/MEM</td><td>97</td><td>&gt;99</td><td>0</td><td>0</td><td>814</td></tr><tr><td>Ni-NiO/MEM</td><td>78</td><td>&gt;99</td><td>0</td><td>0</td><td></td></tr><tr><td>Ni/MME</td><td>&gt;99</td><td>&gt;99</td><td>0</td><td>0</td><td>627</td></tr><tr><td>Ni-NiO/MME</td><td>89</td><td>&gt;99</td><td>0</td><td>0</td><td></td></tr><tr><td>Ni/CN</td><td>&gt;99</td><td>&gt;99</td><td>0</td><td>0</td><td>712</td></tr><tr><td>Ni-NiO/CN</td><td>&gt;99</td><td>&gt;99</td><td>0</td><td>0</td><td></td></tr></tbody></table>

As shown in Table 5, both the Ni/FILs and Ni-NiO/FILs systems were active for the catalytic hydrogenation of CH-enon in short reaction time (1 h). A complete conversion of CH-enon selectively towards CH-non was obtained for all Ni/FILs systems. The difference in conversion rate vs. the Ni/FILs systems plots have been provided in Figure 5. The initial turnover frequency (TOF) was calculated for all Ni/FILs systems at 0.25 h. The highest TOF (814 h⁻¹) value was reached by the Ni NPs embedded in the MEM-functionalized ILs. The non-functionalized system and the nitril functionalization yielded high conversion (TOFs of 677 and 712 h⁻¹, respectively), while the had MME methoxy functionalized, being the largest NPs in size showed the lowest (627 h⁻¹). Nevertheless, all the systems showed high activity at the given reaction conditions with the initial order of the activity being Ni/MME < Ni/H < Ni/CN < Ni/MEM. A comprehensive study with all Ni-NiO/FILs systems was carried out under these reaction conditions. For the Ni-NiO/MEM and Ni-NiO/MME, the activity dropped to 78 and 89% respectively upon oxidation of the NPs surface. Unlike the methoxy systems, it could be noticed that the Ni-NiO/CN and Ni-NiO/H systems did not show inferior activity compared to their non-oxidized analogues. The reason could be a lower oxidation of the NPs surface during air exposure as supported by the XPS data, making these catalysts more effective in the short reaction time applied. No change is selectivity was observed upon oxidation. To assess the potential impact of temperature variation on catalyst selectivity, the hydrogenation of CH-enon was also performed at 150 and 170 °C with the systems, while keeping all other parameters constant. Notably, the hydrogenation of neither the carbonyl group nor the aromatic ring was observed at 150 and 170 °C and full conversion of CH-enon was observed.

<footnote>ª Reaction conditions: 0.01 mmol of Ni, 1 mmol of 2-cyclohexene-1-none (CH-enon), 0.3 mmol of octane (internal standard), 130 °C, 10 bar H₂, 1 h, 1200 rpm. ᵇ Determined by GC using internal standard technique.</footnote>
Recyclability of all Ni/FILs NPs catalysts was tested in five successive catalytic runs (Figure 7). The recyclability tests were performed at partial conversions to provide a realistic evaluation of the catalytic performance and to easily identify the leaching of a catalyst, if any. Notably, the CH-enon conversion with the Ni/H NPs system remained constant up to three catalytic runs and started to decrease in the fourth and fifth runs, whereas the three Ni/FILs systems maintained similar CH-enon conversions upon reuse.

<figure><img src="image_255.png" /><figcaption>**Figure 7.** Recyclability tests of the Ni/FILs catalysts in the hydrogenation of 2-cyclohexen-1-one performed at 0.25 h. Reaction conditions: 0.01 mmol of Ni, 1 mmol of CH-enon, 0.3 mmol of octane (internal standard), 10 bar H₂, 130 °C, 1200 rpm.</figcaption></figure>

A series of catalytic systems based on Ni-containing NPs in non-functionalized IL (H) and FILs containing cyano- (CN) and methoxy-groups (MEM and MME) have been synthesized, characterized, and applied for the hydrogenation of substrates containing different functional groups. The Ni/MEM and Ni/MME catalysts were novel systems, whereas the Ni/H and Ni/CN were previously reported and here prepared for comparison purpose. Small Ni NPs were prepared using CN (2.8 nm), compared to the previous literature (7-8 nm) [58]. All the Ni/ILs systems were found to be efficient catalysts for the hydrogenation of 2-cyclohexe-1-none under the applied reaction conditions (substrate/Ni ratio of 100/1, 130°C, 10 bar H₂), providing full conversion of 2-cyclohexe-1-none with full selectivity towards hydrogenation of the olefinic bond in short reaction time (1 h). Compared to the previous literature using Ni NPs synthesized in amine-functionalized ILs [59] (substrate/Ni of 50/1, 90°C, 10 bar H₂, 4 h, water solvent with isolated NPs redispersed in water), this work uses much less Ni metal content but slightly higher temperature to provide complete substrate conversion in much shorter time under neat conditions. Long time air exposure of the catalysts resulted in Ni(OH)₂ layer formation on the Ni/FILs, which led to deactivation and a significant drop-in catalytic activity. The implementation of recyclability tests as well as the characterization of the spent catalysts will allow to complete the evaluation of the catalytic properties of the Ni/FILs. Another perspective could be to deposit the Ni/FILs NPs on a solid support like silica to obtain supported ionic liquid phase (SILP) systems, which can subsequently be used for hydrogenation reactions, including CO₂ reduction. In this context, initial experiments have been performed for grafting of the catalyst system.

The synthesis of RuRe bimetallic NPs in H and MEM ILs was also studied using the organometallic approach. For this purpose, the precursors [Ru(COD)(COT)] and [Re₂(CO)₁₀] were used as they are
Recyclability of all Ni/FILs NPs catalysts was tested in five successive catalytic runs (Figure 7). The recyclability tests were performed at partial conversions to provide a realistic evaluation of the catalytic performance and to easily identify the leaching of a catalyst, if any. Notably, the CH-enon conversion with the Ni/H NPs system remained constant up to three catalytic runs and started to decrease in the fourth and fifth runs, whereas the three Ni/FILs systems maintained similar CH-enon conversions upon reuse.

<figure><img src="image_253.png" /><figcaption>**Figure 7.** Recyclability tests of the Ni/FILs catalysts in the hydrogenation of 2-cyclohexen-1-one performed at 0.25 h. Reaction conditions: 0.01 mmol of Ni, 1 mmol of CH-enon, 0.3 mmol of octane (internal standard), 10 bar H₂, 130 °C, 1200 rpm.</figcaption></figure>

A series of catalytic systems based on Ni-containing NPs in non-functionalized IL (H) and FILs containing cyano- (CN) and methoxy-groups (MEM and MME) have been synthesized, characterized, and applied for the hydrogenation of substrates containing different functional groups. The Ni/MEM and Ni/MME catalysts were novel systems, whereas the Ni/H and Ni/CN were previously reported and here prepared for comparison purpose. Small Ni NPs were prepared using CN (2.8 nm), compared to the previous literature (7-8 nm) [58]. All the Ni/ILs systems were found to be efficient catalysts for the hydrogenation of 2-cyclohexe-1-none under the applied reaction conditions (substrate/Ni ratio of 100/1, 130°C, 10 bar H₂), providing full conversion of 2-cyclohexe-1-none with full selectivity towards hydrogenation of the olefinic bond in short reaction time (1 h). Compared to the previous literature using Ni NPs synthesized in amine-functionalized ILs [59] (substrate/Ni of 50/1, 90°C, 10 bar H₂, 4 h, water solvent with isolated NPs redispersed in water), this work uses much less Ni metal content but slightly higher temperature to provide complete substrate conversion in much shorter time under neat conditions. Long time air exposure of the catalysts resulted in Ni(OH)₂ layer formation on the Ni/FILs, which led to deactivation and a significant drop-in catalytic activity. The implementation of recyclability tests as well as the characterization of the spent catalysts will allow to complete the evaluation of the catalytic properties of the Ni/FILs. Another perspective could be to deposit the Ni/FILs NPs on a solid support like silica to obtain supported ionic liquid phase (SILP) systems, which can subsequently be used for hydrogenation reactions, including CO₂ reduction. In this context, initial experiments have been performed for grafting of the catalyst system.

The synthesis of RuRe bimetallic NPs in H and MEM ILs was also studied using the organometallic approach. For this purpose, the precursors [Ru(COD)(COT)] and [Re₂(CO)₁₀] were used as they are
commercially available and possess ligands that can be eliminated under hydrogenation or heating conditions. However, it must be noted that [Re₂(CO)₁₀] is very stable at room temperature and forced reaction conditions are needed to facilitate its decomposition [60]. The reduction of the Re species may be promoted by hydrogen spillover from the reduced species of the other metal present, as previously reported [61,62]. The [Re₂(C₃H₅)₄] precursor, which decomposes under milder reaction conditions [63] than [Re₂(CO)₁₀], was unfortunately unavailable.

As mentioned above, RuRe NPs were synthesized in earlier work using [Ru(COD)(COT)] and [Re₂(CO)₁₀] as metal precursors employing the ligands PVP and HAD for the stabilization of the RuRe NPs (optimized reaction conditions: 3 bar of H₂, 120 °C, 48 h and anisole as solvent) [64]. Thus, in a first attempt the one-pot synthesis of RuRe/ILs was carried out following the same reaction conditions (Scheme 6). Monometallic Ru/IL and Re/IL were synthesized following the same procedure for comparative purposes.

<figure><img src="image_256.png" /><figcaption>**Scheme 6.** Organometallic synthesis of RuRe/IL from [Ru(COD)(COT)] and [Re₂(CO)₁₀].</figcaption></figure>

TEM analyses were performed of both the bimetallic (Figure 8) and the monometallic samples. The TEM images of RuRe/H and RuRe/ME showed both well-dispersed NPs with average size of 1.6 ± 0.2 nm and 3.3 ± 0.4 nm, respectively, while an average size 2.0 ± 0.2 nm and 2.1 ± 0.2 nm was observed for Ru/H and Ru/MEM, respectively. Regarding the Re samples, the TEM images confirmed the lack of formation of NPs for both Re/H and Re/MEM.

<figure><img src="image_257.png" /><figcaption>**Figure 8.** a) TEM images and b) the corresponding NPs size distributions of synthesized RuRe/ILs NPs (150 °C, 3 bar H₂, anisole) (scale bar = 100 or 50 nm).</figcaption></figure>
commercially available and possess ligands that can be eliminated under hydrogenation or heating conditions. However, it must be noted that [Re₂(CO)₁₀] is very stable at room temperature and forced reaction conditions are needed to facilitate its decomposition [60]. The reduction of the Re species may be promoted by hydrogen spillover from the reduced species of the other metal present, as previously reported [61,62]. The [Re₂(C₃H₅)₄] precursor, which decomposes under milder reaction conditions [63] than [Re₂(CO)₁₀], was unfortunately unavailable.

As mentioned above, RuRe NPs were synthesized in earlier work using [Ru(COD)(COT)] and [Re₂(CO)₁₀] as metal precursors employing the ligands PVP and HAD for the stabilization of the RuRe NPs (optimized reaction conditions: 3 bar of H₂, 120 °C, 48 h and anisole as solvent) [64]. Thus, in a first attempt the one-pot synthesis of RuRe/ILs was carried out following the same reaction conditions (Scheme 6). Monometallic Ru/IL and Re/IL were synthesized following the same procedure for comparative purposes.

<figure><img src="image_254.png" /><figcaption>**Scheme 6.** Organometallic synthesis of RuRe/IL from [Ru(COD)(COT)] and [Re₂(CO)₁₀].</figcaption></figure>

TEM analyses were performed of both the bimetallic (Figure 8) and the monometallic samples. The TEM images of RuRe/H and RuRe/ME showed both well-dispersed NPs with average size of 1.6 ± 0.2 nm and 3.3 ± 0.4 nm, respectively, while an average size 2.0 ± 0.2 nm and 2.1 ± 0.2 nm was observed for Ru/H and Ru/MEM, respectively. Regarding the Re samples, the TEM images confirmed the lack of formation of NPs for both Re/H and Re/MEM.

<figure><img src="image_255.png" /><figcaption>**Figure 8.** a) TEM images and b) the corresponding NPs size distributions of synthesized RuRe/ILs NPs (150 °C, 3 bar H₂, anisole) (scale bar = 100 or 50 nm).</figcaption></figure>
Thus, novel RuRe-containing NPs have been synthesized, using a non-functionalized IL (H) and a methoxy-functionalized one (MEM), and characterized. TEM images show well-dispersed NPs in both H and MEM ILs. Despite the fact that the Re precursor, [Re₂CO₁₀], requires higher decomposition temperature than [Ru(COD(COT))], EDX analysis revealed the formation of bimetallic RuRe NPs with a Re content of ca. 18% in RuRe/H and ca. 34% in RuRe/MEM. Comparing to previous work done at LCC [64], the incorporation of Re metal in the RuRe bimetallic NPs increased in the order: RuRe/H < RuRe/PVP ~ RuRe/HDA < RuRe/MEM. Owing to a incomplete decomposition of [Re₂(CO)₁₀] under the applied conditions, and the difficulty to separate soluble Re species from the formed RuRe NPs, the alternative Re precursor, [Re₂(C₃H₅)₄], is considered for future studies. If successful, the bimetallic RuRe NPs will be supported on silica to obtain SILP systems for their investigation as catalysts for the hydrogenation of amides.

The results reported in the doctoral thesis open interesting perspectives for future research:

1) Ru/FILs systems were found to be highly active and selective to ethylbenzene in the hydrogenation of styrene. These catalysts may be employed in the selective hydrogenation of other substrates than vinylic substrates, such as other olefinic or alkyne compounds, α,β-unsaturated carbonyl compounds, nitriles, or even CO₂. Initial studies on employing the Ru/MEM system in the hydrogenation of 2,3-benzofuran (substrate/Ru ratio 1000/1, 120°C, 10 bar of H₂, 1,4-dioxane solvent) showed promising results of 20% conversion and >99% selectivity towards 2,3-dihydrobenzofuran.

2) The substrate screening of Ni-NiO/MEM showed potential scopes for the application of all the Ni/FILs systems for various hydrogenation reactions, thus opening avenues for their implementation in catalysis.

3) The RuRe/IL bimetallic system would merit to be studied deeper in the hydrogenation of amides. Change in the synthesis protocol, for example by using the organometallic precursor [Re₂(C₃H₅)₄] instead of [Re₂CO₁₀] for the synthesis of the RuRe NPs could help to achieve an active catalyst. Catalytic application of the RuRe/IL system for the hydrogenation of other substrates than amides, for example as those mentioned above, is another perspective.

4) All the metal NP/ILs systems developed can be immobilized onto a support like silica via a simple impregnation process to obtain the corresponding SILP systems with a highly accessible IL film layer. This is a convenient approach to generate materials with improved surface reactivity that facilitates many molecules to simultaneously react, thus leading to enhanced reaction rates and efficiencies when applied in catalysis.

## Reference

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Thus, novel RuRe-containing NPs have been synthesized, using a non-functionalized IL (H) and a methoxy-functionalized one (MEM), and characterized. TEM images show well-dispersed NPs in both H and MEM ILs. Despite the fact that the Re precursor, [Re₂CO₁₀], requires higher decomposition temperature than [Ru(COD(COT))], EDX analysis revealed the formation of bimetallic RuRe NPs with a Re content of ca. 18% in RuRe/H and ca. 34% in RuRe/MEM. Comparing to previous work done at LCC [64], the incorporation of Re metal in the RuRe bimetallic NPs increased in the order: RuRe/H < RuRe/PVP ~ RuRe/HDA < RuRe/MEM. Owing to a incomplete decomposition of [Re₂(CO)₁₀] under the applied conditions, and the difficulty to separate soluble Re species from the formed RuRe NPs, the alternative Re precursor, [Re₂(C₃H₅)₄], is considered for future studies. If successful, the bimetallic RuRe NPs will be supported on silica to obtain SILP systems for their investigation as catalysts for the hydrogenation of amides.

The results reported in the doctoral thesis open interesting perspectives for future research:

1) Ru/FILs systems were found to be highly active and selective to ethylbenzene in the hydrogenation of styrene. These catalysts may be employed in the selective hydrogenation of other substrates than vinylic substrates, such as other olefinic or alkyne compounds, α,β-unsaturated carbonyl compounds, nitriles, or even CO₂. Initial studies on employing the Ru/MEM system in the hydrogenation of 2,3-benzofuran (substrate/Ru ratio 1000/1, 120°C, 10 bar of H₂, 1,4-dioxane solvent) showed promising results of 20% conversion and >99% selectivity towards 2,3-dihydrobenzofuran.

2) The substrate screening of Ni-NiO/MEM showed potential scopes for the application of all the Ni/FILs systems for various hydrogenation reactions, thus opening avenues for their implementation in catalysis.

3) The RuRe/IL bimetallic system would merit to be studied deeper in the hydrogenation of amides. Change in the synthesis protocol, for example by using the organometallic precursor [Re₂(C₃H₅)₄] instead of [Re₂CO₁₀] for the synthesis of the RuRe NPs could help to achieve an active catalyst. Catalytic application of the RuRe/IL system for the hydrogenation of other substrates than amides, for example as those mentioned above, is another perspective.

4) All the metal NP/ILs systems developed can be immobilized onto a support like silica via a simple impregnation process to obtain the corresponding SILP systems with a highly accessible IL film layer. This is a convenient approach to generate materials with improved surface reactivity that facilitates many molecules to simultaneously react, thus leading to enhanced reaction rates and efficiencies when applied in catalysis.

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Max MILEWSKI

ESR12

**Thesis co-directors**

Dr. Anne-Marie Caminade Laboratoire de Chimie de Coordination, Toulouse, France

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

**Thesis defense**

Laboratoire de Chimie de Coordination, 14 December 2023
Max MILEWSKI

ESR12

**Thesis co-directors**

Dr. Anne-Marie Caminade Laboratoire de Chimie de Coordination, Toulouse, France

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

**Thesis defense**

Laboratoire de Chimie de Coordination, 14 December 2023
<figure><img src="image_258.png" /></figure>
<figure><img src="image_256.png" /></figure>
# Carboranylphosphines Meet Dendrimers: Electron-deficient Scaffolds for Ligand Design and Application in Catalysis

## Introduction

This dissertation project aimed at preparing, characterizing, and utilizing a new ligand system for challenging catalytic transformations. The system was built on two chemical building blocks: a PPH dendritic framework, already known for its catalytic application and carboranylphosphine monomeric units, already investigated towards their interesting complexation behavior [1,2] and a few catalytic applications [3]. To combine both constructs, first a synthetic strategy for the suitable functionalization of the B9 position of the carboranylphosphines for anchoring them to supports, namely dendrimers, was developed. The second step was the investigation of different methods of linking them to the dendritic backbone. With respect to the work of CAMINADE and co-workers on PPH dendrimers [4-6], different dendritic phosphorus-containing scaffolds including symmetrical and unsymmetrical PPH dendrimers were chosen as the starting point for an anchoring. Based on previous work of HART and OWEN [7] and the HEY-HAWKINS group [8] on carboranylphosphines and their complexes, 1,2-bis(di-phenylphosphino)-ortho-carborane was chosen as a suitable monomeric unit for further investigations (see Scheme 1).

<figure><img src="image_259.png" /><figcaption>**Scheme 1.** Schematic overview of the major results obtained in the doctoral project. TM = transition metal.</figcaption></figure>

## Preparation of the monomers and dendrimers

The synthesis of different dendritic compounds starting from a P(S)Cl₃ (1-S0) or an N₃P₃Cl₆ core (1-G0) was performed. Several classical dendrimers were obtained in good yields (62-98%) and acceptable purities up to generation 1-G2 or 2-S1, respectively (see Scheme 2). The reiterative synthetic process, which entails a condensation reaction with the phosphorhydrazide H₂NNMeP(S)Cl₂
# Carboranylphosphines Meet Dendrimers: Electron-deficient Scaffolds for Ligand Design and Application in Catalysis

## Introduction

This dissertation project aimed at preparing, characterizing, and utilizing a new ligand system for challenging catalytic transformations. The system was built on two chemical building blocks: a PPH dendritic framework, already known for its catalytic application and carboranylphosphine monomeric units, already investigated towards their interesting complexation behavior [1,2] and a few catalytic applications [3]. To combine both constructs, first a synthetic strategy for the suitable functionalization of the B9 position of the carboranylphosphines for anchoring them to supports, namely dendrimers, was developed. The second step was the investigation of different methods of linking them to the dendritic backbone. With respect to the work of CAMINADE and co-workers on PPH dendrimers [4-6], different dendritic phosphorus-containing scaffolds including symmetrical and unsymmetrical PPH dendrimers were chosen as the starting point for an anchoring. Based on previous work of HART and OWEN [7] and the HEY-HAWKINS group [8] on carboranylphosphines and their complexes, 1,2-bis(di-phenylphosphino)-ortho-carborane was chosen as a suitable monomeric unit for further investigations (see Scheme 1).

<figure><img src="image_257.png" /><figcaption>**Scheme 1.** Schematic overview of the major results obtained in the doctoral project. TM = transition metal.</figcaption></figure>

## Preparation of the monomers and dendrimers

The synthesis of different dendritic compounds starting from a P(S)Cl₃ (1-S0) or an N₃P₃Cl₆ core (1-G0) was performed. Several classical dendrimers were obtained in good yields (62–98%) and acceptable purities up to generation 1-G2 or 2-S1, respectively (see Scheme 2). The reiterative synthetic process, which entails a condensation reaction with the phosphorhydrazide H₂NNMeP(S)Cl₂
and a substitution reaction with 4-hydroxybenzaldehyde, was applied, but it could be shown that the established protocols are vulnerable.

<figure><img src="image_260.png" /><figcaption>**Scheme 2.** Overview of the synthetic steps yielding various classical dendrimers and dendritic phosphorus-containing scaffolds in the doctoral project.</figcaption></figure>

Although the terminal functions of polyphosphorhydrazone (PPH) dendrimers are often similar, having two different types of terminal functions can be advantageous. For this reason, a specific functionalization of the N₃P₃ core was targeted, with at least one function different from the others. Different unsymmetrical AB₅ monomers and dendrimers, with one function distinct from the others up to generation **AB₅-2-G1** (A = 4-(prop-2-yn-1-yloxy)phenol, B = 4-hydroxybenzaldehyde, see Scheme 3) have been prepared. Additionally, to the synthesis of the unsymmetrical AB₅ dendrimers, also a specific functionalization of only half of the terminal P-Cl groups in **1-G0** and **1-G1** was targeted to
and a substitution reaction with 4-hydroxybenzaldehyde, was applied, but it could be shown that the established protocols are vulnerable.

<figure><img src="image_258.png" /><figcaption>**Scheme 2.** Overview of the synthetic steps yielding various classical dendrimers and dendritic phosphorus-containing scaffolds in the doctoral project.</figcaption></figure>

Although the terminal functions of polyphosphorhydrazone (PPH) dendrimers are often similar, having two different types of terminal functions can be advantageous. For this reason, a specific functionalization of the N₃P₃ core was targeted, with at least one function different from the others. Different unsymmetrical AB₅ monomers and dendrimers, with one function distinct from the others up to generation **AB₅-2-G1** (A = 4-(prop-2-yn-1-yloxy)phenol, B = 4-hydroxybenzaldehyde, see Scheme 3) have been prepared. Additionally, to the synthesis of the unsymmetrical AB₅ dendrimers, also a specific functionalization of only half of the terminal P-Cl groups in **1-G0** and **1-G1** was targeted to
obtain further dendritic backbones for the anchoring of different carboranes and carboranyl-
phosphines. Problems were particularly evident during the stochastic monofunctionalization and
purification of typical dendrimers.

<figure><img src="image_261.png" /><figcaption>**Scheme 3.** Overview of the synthetic steps yielding various unsymmetrical AB₅ monomers and dendrimers in the doctoral project.</figcaption></figure>

Starting from 1,2-dicarba-closo-dodecaboranes(12) (ortho isomer, 14) a selection of different B9-substituted carboranes has been prepared. An adaption of the synthetic phosphorylation protocol of ALEXANDER and SCHROEDER [9] that had been used to prepare 1,2-phosphanyl-substituted carborane derivatives in the past was applied and modified yielding novel 1- or 1,2-phosphanyl-substituted carborane derivatives in low to excellent yields (7-95%, see Scheme 4).

While isolated side products revealed the highly stoichiometry-dependent nature of the
phosphorylation step, which in the future could be used to more easily access unsymmetrically
substituted phosphines, these crystalline materials frequently allowed for the elucidation of their solid-
state structures (see Figure 1 and Figure 2). The synthesized carboranes and carboranylphosphines
have C1–C2 (C<sub>Cage</sub>–C<sub>Cage</sub>) bond lengths that are comparable to other known compounds. The electronic
and steric impact of the substituents on the P atoms causes a considerable increase in bond length in
disubstituted compounds. The mean C–P bond lengths and P···P distances of the carboranylphosphines
are consistent with previously described compounds. The obtained results revealed that various
substituents in the B9 position have only a minimal effect on the C1–C2 and C–P bond lengths, whereas
obtain further dendritic backbones for the anchoring of different carboranes and carboranyl-
phosphines. Problems were particularly evident during the stochastic monofunctionalization and
purification of typical dendrimers.

<figure><img src="image_259.png" /><figcaption>**Scheme 3.** Overview of the synthetic steps yielding various unsymmetrical AB₅ monomers and dendrimers in the doctoral project.</figcaption></figure>

Starting from 1,2-dicarba-closo-dodecaboranes(12) (ortho isomer, 14) a selection of different B9-substituted carboranes has been prepared. An adaption of the synthetic phosphorylation protocol of ALEXANDER and SCHROEDER [9] that had been used to prepare 1,2-phosphanyl-substituted carborane derivatives in the past was applied and modified yielding novel 1- or 1,2-phosphanyl-substituted carborane derivatives in low to excellent yields (7-95%, see Scheme 4).

While isolated side products revealed the highly stoichiometry-dependent nature of the
phosphorylation step, which in the future could be used to more easily access unsymmetrically
substituted phosphines, these crystalline materials frequently allowed for the elucidation of their solid-
state structures (see Figure 1 and Figure 2). The synthesized carboranes and carboranylphosphines
have C1–C2 (C<sub>Cage</sub>–C<sub>Cage</sub>) bond lengths that are comparable to other known compounds. The electronic
and steric impact of the substituents on the P atoms causes a considerable increase in bond length in
disubstituted compounds. The mean C–P bond lengths and P···P distances of the carboranylphosphines
are consistent with previously described compounds. The obtained results revealed that various
substituents in the B9 position have only a minimal effect on the C1–C2 and C–P bond lengths, whereas
the substituents at the phosphorus atom have a significant influence. Only a few examples of the functionalized B9-position in carboranylphosphines have previously been documented [2]. This project led to new synthetic protection and deprotection strategies to modify the carborane cage further.

<figure><img src="image_262.png" /><figcaption>**Scheme 4.** Synthesis of B9-substituted carboranylphosphines. (i) n-butyllithium, Et₂O or THF, -78 °C to rt; (ii) PR²Cl, Et₂O or THF, -78 °C / 0 °C to rt. *The NH group in **19** was also substituted by PPh₂.</figcaption></figure>

<figure><img src="image_263.png" /><figcaption>**Figure 1.** Molecular structures of carboranylphosphines **25** and **28–31** in the solid state with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms other than OH in **31** are omitted for clarity.</figcaption></figure>
the substituents at the phosphorus atom have a significant influence. Only a few examples of the functionalized B9-position in carboranylphosphines have previously been documented [2]. This project led to new synthetic protection and deprotection strategies to modify the carborane cage further.

<figure><img src="image_260.png" /><figcaption>**Scheme 4.** Synthesis of B9-substituted carboranylphosphines. (i) n-butyllithium, Et₂O or THF, -78 °C to rt; (ii) PR²Cl, Et₂O or THF, -78 °C / 0 °C to rt. *The NH group in **19** was also substituted by PPh₂.</figcaption></figure>

<figure><img src="image_261.png" /><figcaption>**Figure 1.** Molecular structures of carboranylphosphines **25** and **28–31** in the solid state with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms other than OH in **31** are omitted for clarity.</figcaption></figure>
<figure><img src="image_264.png" /><figcaption>**Figure 2.** Molecular structures of carboranylphosphines **33** and **34** in the solid state with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms other than OH and PH are omitted for clarity.</figcaption></figure>

Coordination behavior

Bearing two phosphorus groups in close proximity, the new carboranylphosphines resemble already known bidentate phosphines and might be promising ligands for the complexation of different transition metals useful for homogeneous catalysis. Especially, the tunability of the steric and electronic properties of the carboranylphosphines allowed the synthesis of new ligands needed for the complexation of different transition metals (Pd, Cu, Au, Rh, Ru). Prompted by the close proximity of both phosphorus atoms when directly attached to the carborane cage (similar to an ethylene or *ortho*-phenylene moiety), the capability to accommodate different transition metals in a 1:1 coordination mode was investigated. According to DFT calculations performed with different B9-substituted carboranylphosphines (29, 31 and 46–49), in most cases the formation of transition metal complexes from these ligands and different complex precursors should be thermodynamically favored (see Table 1 and Figure 3). More stable complexes can be formed after a CH₂ linker is inserted between the carborane cage and the phosphorus atoms. Experiments were carried out to find synthetic proof. Unfortunately, it was not possible to form any complexes with the ligands **47–49** or **53**.

<table><caption>**Table 1.** Computed GIBBS energies [kcal mol⁻¹] of complexes formed with ligands **29**, **31**, **46–49** and different metal complex precursors in THF or CH₂Cl₂ as solvents, relative to the starting materials.</caption><thead><tr><th colspan="2">Carboranylphosphine</th></tr><tr><th colspan="2">Metal complex precursor<br>(solvent for calculation)</th></tr></thead><tbody><tr><td rowspan="6">Y = PPh<sub>2</sub><br>(29, 31, 46)</td><td>[AuCl(tht)] (CH<sub>2</sub>Cl<sub>2</sub>)</td><td>-13.2</td><td>-14.3</td><td>-14.1</td></tr><tr><td>[PdCl<sub>2</sub>(MeCN)<sub>2</sub>] (THF)</td><td>-42.0</td><td>-42.1</td><td>-20.9</td></tr><tr><td>[Cu(MeCN)<sub>4</sub>](OTf) (THF)</td><td>+6.0</td><td>+4.5</td><td>+4.9</td></tr><tr><td>[CuCl(PPh<sub>3</sub>)<sub>3</sub>] (THF)</td><td>-0.2</td><td>+0.4</td><td>+0.1</td></tr><tr><td>[Rh(acac)(CO)<sub>2</sub>] (THF)</td><td>-63.0</td><td>-62.9</td><td>-62.7</td></tr><tr><td>[AuCl(tht)] (CH<sub>2</sub>Cl<sub>2</sub>)</td><td>-24.7</td><td>-23.5</td><td>-25.2</td></tr><tr><td rowspan="6">Y = CH<sub>2</sub>PPh<sub>2</sub><br>(47–49)</td><td>[PdCl<sub>2</sub>(MeCN)<sub>2</sub>] (THF)</td><td>-42.4</td><td>-41.9</td><td>-42.5</td></tr><tr><td>[Cu(MeCN)<sub>4</sub>](OTf) (THF)</td><td>+0.5</td><td>-0.9</td><td>not computed</td></tr><tr><td>[CuCl(PPh<sub>3</sub>)<sub>3</sub>] (THF)</td><td>-10.3</td><td>-9.4</td><td>-9.2</td></tr><tr><td>[Rh(acac)(CO)<sub>2</sub>] (THF)</td><td>-134.2</td><td>-133.3</td><td>-133.5</td></tr></tbody></table>
<figure><img src="image_262.png" /><figcaption>**Figure 2.** Molecular structures of carboranylphosphines **33** and **34** in the solid state with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms other than OH and PH are omitted for clarity.</figcaption></figure>

Coordination behavior

Bearing two phosphorus groups in close proximity, the new carboranylphosphines resemble already known bidentate phosphines and might be promising ligands for the complexation of different transition metals useful for homogeneous catalysis. Especially, the tunability of the steric and electronic properties of the carboranylphosphines allowed the synthesis of new ligands needed for the complexation of different transition metals (Pd, Cu, Au, Rh, Ru). Prompted by the close proximity of both phosphorus atoms when directly attached to the carborane cage (similar to an ethylene or *ortho*-phenylene moiety), the capability to accommodate different transition metals in a 1:1 coordination mode was investigated. According to DFT calculations performed with different B9-substituted carboranylphosphines (29, 31 and 46–49), in most cases the formation of transition metal complexes from these ligands and different complex precursors should be thermodynamically favored (see Table 1 and Figure 3). More stable complexes can be formed after a CH₂ linker is inserted between the carborane cage and the phosphorus atoms. Experiments were carried out to find synthetic proof. Unfortunately, it was not possible to form any complexes with the ligands **47–49** or **53**.

<table><caption>**Table 1.** Computed GIBBS energies [kcal mol⁻¹] of complexes formed with ligands **29**, **31**, **46–49** and different metal complex precursors in THF or CH₂Cl₂ as solvents, relative to the starting materials.</caption><thead><tr><th colspan="2">Carboranylphosphine</th></tr><tr><th colspan="2">Metal complex precursor<br>(solvent for calculation)</th></tr></thead><tbody><tr><td rowspan="6">Y = PPh<sub>2</sub><br>(29, 31, 46)</td><td>[AuCl(tht)] (CH<sub>2</sub>Cl<sub>2</sub>)</td><td>-13.2</td><td>-14.3</td><td>-14.1</td></tr><tr><td>[PdCl<sub>2</sub>(MeCN)<sub>2</sub>] (THF)</td><td>-42.0</td><td>-42.1</td><td>-20.9</td></tr><tr><td>[Cu(MeCN)<sub>4</sub>](OTf) (THF)</td><td>+6.0</td><td>+4.5</td><td>+4.9</td></tr><tr><td>[CuCl(PPh<sub>3</sub>)<sub>3</sub>] (THF)</td><td>-0.2</td><td>+0.4</td><td>+0.1</td></tr><tr><td>[Rh(acac)(CO)<sub>2</sub>] (THF)</td><td>-63.0</td><td>-62.9</td><td>-62.7</td></tr><tr><td>[AuCl(tht)] (CH<sub>2</sub>Cl<sub>2</sub>)</td><td>-24.7</td><td>-23.5</td><td>-25.2</td></tr><tr><td rowspan="6">Y = CH<sub>2</sub>PPh<sub>2</sub><br>(47–49)</td><td>[PdCl<sub>2</sub>(MeCN)<sub>2</sub>] (THF)</td><td>-42.4</td><td>-41.9</td><td>-42.5</td></tr><tr><td>[Cu(MeCN)<sub>4</sub>](OTf) (THF)</td><td>+0.5</td><td>-0.9</td><td>not computed</td></tr><tr><td>[CuCl(PPh<sub>3</sub>)<sub>3</sub>] (THF)</td><td>-10.3</td><td>-9.4</td><td>-9.2</td></tr><tr><td>[Rh(acac)(CO)<sub>2</sub>] (THF)</td><td>-134.2</td><td>-133.3</td><td>-133.5</td></tr></tbody></table>
<figure><img src="image_265.png" /><figcaption>29-[Au]</figcaption></figure>

<figure><img src="image_266.png" /><figcaption>29-[Cu(MeCN)2]</figcaption></figure>

<figure><img src="image_267.png" /><figcaption>29-[CuCl]</figcaption></figure>

<figure><img src="image_268.png" /><figcaption>29-[Pd]</figcaption></figure>

<figure><img src="image_269.png" /><figcaption>29-[Rh]</figcaption></figure>

<figure><img src="image_270.png" /><figcaption>47-[Au]</figcaption></figure>

<figure><img src="image_271.png" /><figcaption>47-[Cu(MeCN)2]</figcaption></figure>

<figure><img src="image_272.png" /><figcaption>47-[CuCl]</figcaption></figure>

<figure><img src="image_273.png" /><figcaption>47-[Pd]</figcaption></figure>

<figure><img src="image_274.png" /><figcaption>47-[Rh]</figcaption></figure>

**Figure 3** Computed complexes formed from 9-(4-MeO-C<sub>6</sub>H<sub>4</sub>-1,2-bis(diphenylphosphino)-29) and 9-(4-MeO-C<sub>6</sub>H<sub>4</sub>-1,2-bis[(diphenylphosphino)methyl]-ortho-carborane (47) and different metal complex precursors. All hydrogen atoms are omitted for clarity. The triflate anion in 29- and 47-[Cu(MeCN)<sub>2</sub>] was optimized separately. The optimized structures obtained for other B9 substituents are not displayed.
<figure><img src="image_263.png" /><figcaption>29-[Au]</figcaption></figure>

<figure><img src="image_264.png" /><figcaption>29-[Cu(MeCN)2]</figcaption></figure>

<figure><img src="image_265.png" /><figcaption>29-[CuCl]</figcaption></figure>

<figure><img src="image_266.png" /><figcaption>29-[Pd]</figcaption></figure>

<figure><img src="image_267.png" /><figcaption>29-[Rh]</figcaption></figure>

<figure><img src="image_268.png" /><figcaption>47-[Au]</figcaption></figure>

<figure><img src="image_269.png" /><figcaption>47-[Cu(MeCN)2]</figcaption></figure>

<figure><img src="image_270.png" /><figcaption>47-[CuCl]</figcaption></figure>

<figure><img src="image_271.png" /><figcaption>47-[Pd]</figcaption></figure>

<figure><img src="image_272.png" /><figcaption>47-[Rh]</figcaption></figure>

**Figure 3** Computed complexes formed from 9-(4-MeO-C<sub>6</sub>H<sub>4</sub>-1,2-bis(diphenylphosphino)-29) and 9-(4-MeO-C<sub>6</sub>H<sub>4</sub>-1,2-bis[(diphenylphosphino)methyl]-ortho-carborane (47) and different metal complex precursors. All hydrogen atoms are omitted for clarity. The triflate anion in 29- and 47-[Cu(MeCN)<sub>2</sub>] was optimized separately. The optimized structures obtained for other B9 substituents are not displayed.
For this reason, only the monomeric carboranylphosphine derivatives 9-(TBDMS)O-C<sub>6</sub>H<sub>4</sub>-1,2-(PPh<sub>2</sub>)<sub>2</sub>-C<sub>2</sub>B<sub>10</sub>H<sub>9</sub> (28) and 9-(4-MeO-C<sub>6</sub>H<sub>4</sub>-1,2-(PPh<sub>2</sub>)<sub>2</sub>-C<sub>2</sub>B<sub>10</sub>H<sub>9</sub> (29) have been used for the first complexation experiments with a Pd<sup>II</sup>, two Cu<sup>I</sup>, an Au<sup>I</sup>, a Rh<sup>I</sup>, and a Ru<sup>0</sup> complex precursor. The silyl ether protected carboranylphosphine 28 was the most available of all compounds produced throughout the practical work of the dissertation. The methoxyphenyl substituent in 29 mimics the phenoxy linker needed in immobilization reactions. It can be considered as the monomeric equivalent of the carboranyl-substituted dendrimers. Rhodium(I) complex 28-[Rh], gold(I) complex 29-[Au], and copper(I) complex 29-[CuCl] could be synthesized and characterized by NMR spectroscopy and mass spectrometry. Additionally, it was possible to proof the formation of Pd<sup>II</sup> complex 29-[Pd], but due to its high reactivity, an isolation was not feasible. Instead the dimeric hydrolysis product, Pd<sup>II</sup> complex 56, was isolated (see Scheme 5).

<figure><img src="image_275.png" /><figcaption>**Scheme 5.** Overview of the synthesis of different carboranylphosphine transition metal complexes.</figcaption></figure>

## Preliminary catalytic investigations

Using an automated parallel library screening system at BASF SE, preliminary catalytic studies for the hydrogenation and hydroformylation of oct-1-ene and cyclohexene were conducted with various rhodium and ruthenium complex precursors and 9-(4-TBDMSO-C<sub>6</sub>H<sub>4</sub>-1,2-(PPh<sub>2</sub>)<sub>2</sub>-C<sub>2</sub>B<sub>10</sub>H<sub>9</sub> (28). The most promising systems were then transferred into an autoclave to test the reactions under reproducible conditions. Ligand 28 showed the best performance for the hydrogenation of oct-1-ene with [Rh(acac)(CO)<sub>2</sub>] performing better than already known systems. It further showed a better conversion in the hydrogenation of cyclohexene with [Ru<sub>3</sub>(CO)<sub>12</sub>] than the non-substituted carboranylphosphine derivative 1,2-bis(diphenylphosphino)-*ortho*-carborane (dppc, 73), but worked
For this reason, only the monomeric carboranylphosphine derivatives 9-(TBDMS)O-C₆H₄-1,2-(PPh₂)₂-C₂B₁₀H₉ (28) and 9-(4-MeO-C₆H₄)-1,2-(PPh₂)₂-C₂B₁₀H₉ (29) have been used for the first complexation experiments with a Pd¹¹, two Cu¹, an Au¹, a Rh¹, and a Ru⁰ complex precursor. The silyl ether protected carboranylphosphine 28 was the most available of all compounds produced throughout the practical work of the dissertation. The methoxyphenyl substituent in 29 mimics the phenoxy linker needed in immobilization reactions. It can be considered as the monomeric equivalent of the carboranyl-substituted dendrimers. Rhodium(I) complex 28-[Rh], gold(I) complex 29-[Au], and copper(I) complex 29-[CuCl] could be synthesized and characterized by NMR spectroscopy and mass spectrometry. Additionally, it was possible to proof the formation of Pd¹¹ complex 29-[Pd], but due to its high reactivity, an isolation was not feasible. Instead the dimeric hydrolysis product, Pd¹¹ complex 56, was isolated (see Scheme 5).

<figure><img src="image_273.png" /><figcaption>**Scheme 5.** Overview of the synthesis of different carboranylphosphine transition metal complexes.</figcaption></figure>

## Preliminary catalytic investigations

Using an automated parallel library screening system at BASF SE, preliminary catalytic studies for the hydrogenation and hydroformylation of oct-1-ene and cyclohexene were conducted with various rhodium and ruthenium complex precursors and 9-(4-TBDMSO-C₆H₄)-1,2-(PPh₂)₂-C₂B₁₀H₉ (28). The most promising systems were then transferred into an autoclave to test the reactions under reproducible conditions. Ligand 28 showed the best performance for the hydrogenation of oct-1-ene with [Rh(acac)(CO)₂] performing better than already known systems. It further showed a better conversion in the hydrogenation of cyclohexene with [Ru₃(CO)₁₂] than the non-substituted carboranylphosphine derivative 1,2-bis(diphenylphosphino)-ortho-carborane (dppc, 73), but worked
not as good as a simple bisphosphine, namely 1,2-bis(diphenylphosphino)-benzene (dppb, **67**). Despite the fact that several of the reactions demonstrated good conversion, the selectivity is not superior to other known systems; hence these systems were not subjected to additional testing in an industrial setting. However, to get more significant results, further runs would have been required. Investigation of the catalytic species produced *in situ* from the metal complexes and ligands would have been required as well. It would be essential to repeat several catalytic studies, look into the scope and limitations of the performed hydrogenation and hydroformylation, and further develop better catalytic systems in order to better comprehend the results.

## Anchoring of carboranes and polyphosphorhydrazone dendrimers

Various dendritic compounds were evaluated for their suitability of being grafted to various new phenol-linked carboranes and carboranylphosphines. DFT calculations have proven that the reaction proceeds similar to an $S_N$2 reaction, with the nucleophilic phenoxide **17*** approaching the electrophilic phosphorus center of the $N_3P_3Cl_6$ core opposite to the leaving group $Cl^-$. Products of multiple substitution reactions with **17*** were also optimized to computationally check the stability of such products. It could be shown that the six-fold substitution of **1-G0** forms a product with greater stability by 194.5 kcal mol$^{-1}$ relative to the starting material (see Scheme 6).

<figure><img src="image_276.png" /><figcaption>**Scheme 6.** Computed GIBBS energy values in THF as solvent (in kcal mol$^{-1}$) for the substitution of **1-G0** with *n* equivalents of **17*** relative to the starting material.</figcaption></figure>

As calculations support the possibility of multiple substitution of chloride centers in $N_3P_3Cl_6$ by carboranyl groups, this hypothesis was experimentally checked (see Scheme 7). A grafting to the model molecule **35**, to different phosphorus-containing compounds **1-G0** and **1-S0** as well as to dendrimer **1-G1** was tested. Despite the lack of a full substitution with a carboranylphosphine, novel carborane-substituted dendritic compounds were synthesized. Further, the selective grafting of a single carborane per $P(S)Cl_2$ function was investigated yielding a monofunctionalized dendrimer **43** (see Figure 4). Yet, the separation process of differently carborane-substituted dendrimers is still a focus of investigation. Also, the selective grafting of a single carborane per $P(S)Cl_2$ function should be investigated for other generations of dendrimers because it will open the way to the grafting of
not as good as a simple bisphosphine, namely 1,2-bis(diphenylphosphino)-benzene (dppb, **67**). Despite the fact that several of the reactions demonstrated good conversion, the selectivity is not superior to other known systems; hence these systems were not subjected to additional testing in an industrial setting. However, to get more significant results, further runs would have been required. Investigation of the catalytic species produced *in situ* from the metal complexes and ligands would have been required as well. It would be essential to repeat several catalytic studies, look into the scope and limitations of the performed hydrogenation and hydroformylation, and further develop better catalytic systems in order to better comprehend the results.

## Anchoring of carboranes and polyphosphorhydrazone dendrimers

Various dendritic compounds were evaluated for their suitability of being grafted to various new phenol-linked carboranes and carboranylphosphines. DFT calculations have proven that the reaction proceeds similar to an $S_N$2 reaction, with the nucleophilic phenoxide **17*** approaching the electrophilic phosphorus center of the $N_3P_3Cl_6$ core opposite to the leaving group $Cl^-$. Products of multiple substitution reactions with **17*** were also optimized to computationally check the stability of such products. It could be shown that the six-fold substitution of **1-G0** forms a product with greater stability by 194.5 kcal mol$^{-1}$ relative to the starting material (see Scheme 6).

<figure><img src="image_274.png" /><figcaption>**Scheme 6.** Computed GIBBS energy values in THF as solvent (in kcal mol$^{-1}$) for the substitution of **1-G0** with *n* equivalents of **17*** relative to the starting material.</figcaption></figure>

As calculations support the possibility of multiple substitution of chloride centers in $N_3P_3Cl_6$ by carboranyl groups, this hypothesis was experimentally checked (see Scheme 7). A grafting to the model molecule **35**, to different phosphorus-containing compounds **1-G0** and **1-S0** as well as to dendrimer **1-G1** was tested. Despite the lack of a full substitution with a carboranylphosphine, novel carborane-substituted dendritic compounds were synthesized. Further, the selective grafting of a single carborane per $P(S)Cl_2$ function was investigated yielding a monofunctionalized dendrimer **43** (see Figure 4). Yet, the separation process of differently carborane-substituted dendrimers is still a focus of investigation. Also, the selective grafting of a single carborane per $P(S)Cl_2$ function should be investigated for other generations of dendrimers because it will open the way to the grafting of
additional functionalities to replace the remaining Cl, for instance with water-solubilizing functions, in view of biological experiments [10].

<figure><img src="image_277.png" /><figcaption>**Scheme 7.** Reaction of different equivalents of **17** with the N<sub>3</sub>P<sub>3</sub>Cl<sub>6</sub> core **1-G0**, P(S)Cl<sub>3</sub> **1-S0** and model molecule **35**. Conditions: (i) **1-G0**, K<sub>2</sub>CO<sub>3</sub>, acetone, 40 °C for 120 h, then rt for 72 h; (ii) **1-G0**, C<sub>2</sub>CO<sub>3</sub>, THF, rt, 25 d; (iii) **1-G0**, K<sub>2</sub>CO<sub>3</sub>, acetone, rt, 72 h; (iv) **1-S0**, Cs<sub>2</sub>CO<sub>3</sub>, THF, 40 °C, 96 h; (v) **35**, DBU, THF, rt, 72 h; (vi) **1-G1**, Cs<sub>2</sub>CO<sub>3</sub>, THF, rt for 72 h, then 40 °C for 23 h. All compounds were analyzed by NMR spectroscopy and ESI-MS. Only one isomer of the incompletely substituted dendrimers (A<sub>3</sub>B<sub>3</sub>, A<sub>2</sub>B<sub>4</sub>) is displayed.</figcaption></figure>
additional functionalities to replace the remaining Cl, for instance with water-solubilizing functions, in view of biological experiments [10].

<figure><img src="image_275.png" /><figcaption>**Scheme 7.** Reaction of different equivalents of **17** with the N<sub>3</sub>P<sub>3</sub>Cl<sub>6</sub> core **1-G0**, P(S)Cl<sub>3</sub> **1-S0** and model molecule **35**. Conditions: (i) **1-G0**, K<sub>2</sub>CO<sub>3</sub>, acetone, 40 °C for 120 h, then rt for 72 h; (ii) **1-G0**, C<sub>2</sub>CO<sub>3</sub>, THF, rt, 25 d; (iii) **1-G0**, K<sub>2</sub>CO<sub>3</sub>, acetone, rt, 72 h; (iv) **1-S0**, Cs<sub>2</sub>CO<sub>3</sub>, THF, 40 °C, 96 h; (v) **35**, DBU, THF, rt, 72 h; (vi) **1-G1**, Cs<sub>2</sub>CO<sub>3</sub>, THF, rt for 72 h, then 40 °C for 23 h. All compounds were analyzed by NMR spectroscopy and ESI-MS. Only one isomer of the incompletely substituted dendrimers (A<sub>3</sub>B<sub>3</sub>, A<sub>2</sub>B<sub>4</sub>) is displayed.</figcaption></figure>
<figure><img src="image_278.png" /><figcaption>**Figure 4.** Full structure of dendrimer 43.</figcaption></figure>

## In a nutshell

Several *ortho*-carboranes bearing a phenoxy or a phenylamino group in the B9 position were prepared employing various protection and deprotection strategies. Furthermore, the synthetic access to novel 1- or 1,2-phosphanyl-substituted carborane derivatives was developed in this dissertation project. The resulting diethyl-, diisopropyl-, di-*tert*-butyl-, diphenyl- or diethoxyphosphines bearing a tunable *ortho*-carborane moiety are intriguing ligands for future applications in homogeneous catalysis or the medicinal sector. The ligand framework has shown a distinctive coordination behavior to different transition metals in a 1:1 coordination mode by the chelating carboranylbisphosphines. A catalyst generated *in situ* from a mixture of Rh or Ru complex precursors and carboranylbisphosphine derivative **28** could be used for the hydrogenation and hydroformylation of unsaturated olefins.

Following established protocols, dendritic compounds were synthesized from a hexachlorocyclotriphosphazene or thiophosphoryl chloride core, and possible anchoring options for the B9-substituted *ortho*-carboranes were investigated experimentally and theoretically (DFT). In spite of the synthetic challenges, different carborane-containing dendrimers could be obtained and the results should open the door to the synthesis of carboranylphosphine-containing dendrimers in order to improve the catalytic performance and exploring new reaction pathways. Following studies on differently substituted carboranylphosphines as well as other catalytic applications would be highly interesting. So far, only electron-deficient C–P carboranylphosphines were utilized for the anchoring. Changing the system to electron-rich B–P carboranylphosphines (similar to the work of SPOKOYNY and co-workers) [11] linking the carboranes *via* their $C_{\text{Cage}}$ atoms to the dendrimers could be an interesting switch. It is obvious that the research in this field is not exhausted, and depending on the approach chosen, a lot of interesting results can certainly be obtained.
<figure><img src="image_276.png" /><figcaption>**Figure 4.** Full structure of dendrimer 43.</figcaption></figure>

In a nutshell

Several *ortho*-carboranes bearing a phenoxy or a phenylamino group in the B9 position were prepared employing various protection and deprotection strategies. Furthermore, the synthetic access to novel 1- or 1,2-phosphanyl-substituted carborane derivatives was developed in this dissertation project. The resulting diethyl-, diisopropyl-, di-*tert*-butyl-, diphenyl- or diethoxyphosphines bearing a tunable *ortho*-carborane moiety are intriguing ligands for future applications in homogeneous catalysis or the medicinal sector. The ligand framework has shown a distinctive coordination behavior to different transition metals in a 1:1 coordination mode by the chelating carboranylbisphosphines. A catalyst generated *in situ* from a mixture of Rh or Ru complex precursors and carboranylbisphosphine derivative **28** could be used for the hydrogenation and hydroformylation of unsaturated olefins.

Following established protocols, dendritic compounds were synthesized from a hexachlorocyclotriphosphazene or thiophosphoryl chloride core, and possible anchoring options for the B9-substituted *ortho*-carboranes were investigated experimentally and theoretically (DFT). In spite of the synthetic challenges, different carborane-containing dendrimers could be obtained and the results should open the door to the synthesis of carboranylphosphine-containing dendrimers in order to improve the catalytic performance and exploring new reaction pathways. Following studies on differently substituted carboranylphosphines as well as other catalytic applications would be highly interesting. So far, only electron-deficient C–P carboranylphosphines were utilized for the anchoring. Changing the system to electron-rich B–P carboranylphosphines (similar to the work of SPOKOYNY and co-workers) [11] linking the carboranes *via* their $C_{	ext{Cage}}$ atoms to the dendrimers could be an interesting switch. It is obvious that the research in this field is not exhausted, and depending on the approach chosen, a lot of interesting results can certainly be obtained.
References

[1] I. B. Sivaev, M. Y. Stogniy, V. I. Bregadze, Coord. Chem. Rev. **2021**, *436*, 213795. DOI: **10.1016/j.ccr.2021.213795**.

[2] E. V. Oleshkevich, E. G. Rys, V. V. Bashilov, P. V. Petrovskii, V. A. Ol'shevskaya, S. K. Moiseev, A. B. Ponomaryov, V. N. Kalinin, *Russ. J. Gen. Chem.* **2017**, *87*, 2589–2595. DOI: **10.1134/S1070363217110123**.

[3] S. Bauer, E. Hey-Hawkins, *Phosphorus-Substituted Carboranes in Catalysis*, in *Boron Science* (N. S. Hosmane, Editor), CRC Press, **2011**, pp. 529–578. DOI: **10.1201/b11199-30**.

[4] N. Launay, A. M. Caminade, R. Lahana, J. P. Majoral, *Angew. Chem. Int. Ed. Engl.* **1994**, *33*, 1589–1592. DOI: **10.1002/anie.199415891**.

[5] N. Launay, A. M. Caminade, J. P. Majoral, *J. Organomet. Chem.* **1997**, *529*, 51–58. DOI: **10.1016/S0022-328X(96)06293-6**.

[6] E. Cavero, M. Zablocka, A. M. Caminade, J. P. Majoral, *Eur. J. Org. Chem.* **2010**, 2759–2767. DOI: **10.1002/ejoc.200901291**.

[7] F. A. Hart, D. W. Owen, *Inorg. Chim. Acta* **1985**, *103*, L1–L2. DOI: **10.1016/S0020-1693(00)85196-8**.

[8] P. Coburger, G. Kahraman, A. Straube, E. Hey-Hawkins, *Dalton Trans.* **2019**, *48*, 9625–9630. DOI: **10.1039/c9dt00628a**.

[9] R. P. Alexander, H. Schroeder, *Inorg. Chem.* **1963**, *2*, 1107–1110. DOI: **10.1021/ic50010a006**.

[10] A. M. Caminade, M. Milewski, E. Hey-Hawkins, *Pharmaceutics* **2023**, *15*, 2117. DOI: **10.3390/pharmaceutics15082117**.

[11] A. M. Spokoyny, C. D. Lewis, G. Teverovskiy, S. L. Buchwald, *Organometallics* **2012**, *31*, 8478–8481. DOI: **10.1021/om301116**.
References

[1] I. B. Sivaev, M. Y. Stogniy, V. I. Bregadze, Coord. Chem. Rev. **2021**, *436*, 213795. DOI: **10.1016/j.ccr.2021.213795**.

[2] E. V. Oleshkevich, E. G. Rys, V. V. Bashilov, P. V. Petrovskii, V. A. Ol'shevskaya, S. K. Moiseev, A. B. Ponomaryov, V. N. Kalinin, *Russ. J. Gen. Chem.* **2017**, *87*, 2589–2595. DOI: **10.1134/S1070363217110123**.

[3] S. Bauer, E. Hey-Hawkins, *Phosphorus-Substituted Carboranes in Catalysis*, in *Boron Science* (N. S. Hosmane, Editor), CRC Press, **2011**, pp. 529–578. DOI: **10.1201/b11199-30**.

[4] N. Launay, A. M. Caminade, R. Lahana, J. P. Majoral, *Angew. Chem. Int. Ed. Engl.* **1994**, *33*, 1589–1592. DOI: **10.1002/anie.199415891**.

[5] N. Launay, A. M. Caminade, J. P. Majoral, *J. Organomet. Chem.* **1997**, *529*, 51–58. DOI: **10.1016/S0022-328X(96)06293-6**.

[6] E. Cavero, M. Zablocka, A. M. Caminade, J. P. Majoral, *Eur. J. Org. Chem.* **2010**, 2759–2767. DOI: **10.1002/ejoc.200901291**.

[7] F. A. Hart, D. W. Owen, *Inorg. Chim. Acta* **1985**, *103*, L1–L2. DOI: **10.1016/S0020-1693(00)85196-8**.

[8] P. Coburger, G. Kahraman, A. Straube, E. Hey-Hawkins, *Dalton Trans.* **2019**, *48*, 9625–9630. DOI: **10.1039/c9dt00628a**.

[9] R. P. Alexander, H. Schroeder, *Inorg. Chem.* **1963**, *2*, 1107–1110. DOI: **10.1021/ic50010a006**.

[10] A. M. Caminade, M. Milewski, E. Hey-Hawkins, *Pharmaceutics* **2023**, *15*, 2117. DOI: **10.3390/pharmaceutics15082117**.

[11] A. M. Spokoyny, C. D. Lewis, G. Teverovskiy, S. L. Buchwald, *Organometallics* **2012**, *31*, 8478–8481. DOI: **10.1021/om301116**.
Massimo PETRICCONE

ESR13

**Thesis co-directors**

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

Dr. Anne-Marie Caminade Laboratoire de Chimie de Coordination, Toulouse, France

**Thesis defense**

Department of Chemistry, Universitat Autònoma de Barcelona, 1 March 2024
Massimo PETRICCONE

ESR13

**Thesis co-directors**

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

Dr. Anne-Marie Caminade Laboratoire de Chimie de Coordination, Toulouse, France

**Thesis defense**

Department of Chemistry, Universitat Autònoma de Barcelona, 1 March 2024
<figure><img src="image_279.png" /></figure>
<figure><img src="image_277.png" /></figure>
# Specifically Functionalized Dendrimers for Catalysis in Special Media

## Introduction

Dendrimers, inspired by dendritic patterns in nature, are molecular structures widely utilized in chemistry and materials science [1,2]. With a tree-like, they offer precise control over size, shape, and surface chemistry [3]. They are synthesized through divergent [4] or convergent approaches (Scheme 1) [5].

### Divergent synthesis

<figure><img src="image_280.png" /></figure>

### Convergent synthesis

<figure><img src="image_281.png" /><figcaption>**Scheme 1.** Synthesis of dendrimers according to divergent and convergent methods.</figcaption></figure>

Dendrimers come in various structural variations and chemical compositions, including PAMAM [6], PPI [7], and more (Figure 1). Phosphorhydrazone (PPH) dendrimers, a versatile subclass, find applications in bioconjugation and metal chelation for catalysis [8-10].

<figure><img src="image_282.png" /><figcaption>**Figure 1.** Representations of 1) PAMAM dendrimers, 2) PPI dendrimers and 3) PPH dendrimers.</figcaption></figure>
# Specifically Functionalized Dendrimers for Catalysis in Special Media

## Introduction

Dendrimers, inspired by dendritic patterns in nature, are molecular structures widely utilized in chemistry and materials science [1,2]. With a tree-like, they offer precise control over size, shape, and surface chemistry [3]. They are synthesized through divergent [4] or convergent approaches (Scheme 1) [5].

### Divergent synthesis

<figure><img src="image_278.png" /></figure>

### Convergent synthesis

<figure><img src="image_279.png" /><figcaption>**Scheme 1.** Synthesis of dendrimers according to divergent and convergent methods.</figcaption></figure>

Dendrimers come in various structural variations and chemical compositions, including PAMAM [6], PPI [7], and more (Figure 1). Phosphorhydrazone (PPH) dendrimers, a versatile subclass, find applications in bioconjugation and metal chelation for catalysis [8-10].

<figure><img src="image_280.png" /><figcaption>**Figure 1.** Representations of 1) PAMAM dendrimers, 2) PPI dendrimers and 3) PPH dendrimers.</figcaption></figure>
Surface functionalization of dendrimers is a common and powerful strategy to tailor their properties. Examples include the addition of polyethylene glycol (PEG) chains to enhance water solubility [11], particularly valuable for biological applications. Another illustrative case involves the addition of perfluorinated polyether chains to improve solubility in environmentally friendly media (Figure 2) [12]. Phosphorhydrazone dendrimers, with specific functionalization, have demonstrated solubility in fluorinated media [13], showcasing their potential in unconventional solvents like supercritical CO₂ and thermo-responsive biphasic fluorous/organic systems.

<figure><img src="image_283.png" /><figcaption>**Figure 2.** A) G4 PPI dendrimer functionalized with perfluorinated polyether chains; B) G2 phosphorhydrazone dendrimer functionalized with phenols containing alkyl perfluorinated chains.</figcaption></figure>

Bifunctionalization of dendrimers represents a versatile strategy in nanotechnology, enabling the incorporation of two distinct types of terminal functions for diverse applications [14]. While terminal functions can be uniformly modified to introduce new properties, the desire for dual functionalities arises in cases where unique characteristics are required for specific applications. For instance, one terminal function might contribute to solubility, while the other imparts catalytic or biological properties. Alternatively, one function could be dedicated to grafting onto materials, while the other provides additional desirable properties (Figure 3).

<figure><img src="image_284.png" /><figcaption>**Figure 3.** (A) Schematization of stochastically substituted dendrimers mixture; (B) schematization of precisely substituted dendrimers mixture.</figcaption></figure>

However, the traditional stochastic approach to bifunctionalization, where two or more functions are introduced onto the dendrimer surface without precise control, contradicts the goal of achieving reproducibility in dendrimer synthesis. The stochastic method introduces variability in the type and location of substitutions, potentially impacting the dendrimer's activity [15]. To address this challenge, a preferable alternative involves the precise synthesis of bifunctionalized dendrimers, ensuring a consistent and controlled structure [16]. Three distinct pathways for synthesizing
Surface functionalization of dendrimers is a common and powerful strategy to tailor their properties. Examples include the addition of polyethylene glycol (PEG) chains to enhance water solubility [11], particularly valuable for biological applications. Another illustrative case involves the addition of perfluorinated polyether chains to improve solubility in environmentally friendly media (Figure 2) [12]. Phosphorhydrazone dendrimers, with specific functionalization, have demonstrated solubility in fluorinated media [13], showcasing their potential in unconventional solvents like supercritical CO₂ and thermo-responsive biphasic fluorous/organic systems.

<figure><img src="image_281.png" /><figcaption>**Figure 2.** A) G4 PPI dendrimer functionalized with perfluorinated polyether chains; B) G2 phosphorhydrazone dendrimer functionalized with phenols containing alkyl perfluorinated chains.</figcaption></figure>

Bifunctionalization of dendrimers represents a versatile strategy in nanotechnology, enabling the incorporation of two distinct types of terminal functions for diverse applications [14]. While terminal functions can be uniformly modified to introduce new properties, the desire for dual functionalities arises in cases where unique characteristics are required for specific applications. For instance, one terminal function might contribute to solubility, while the other imparts catalytic or biological properties. Alternatively, one function could be dedicated to grafting onto materials, while the other provides additional desirable properties (Figure 3).

<figure><img src="image_282.png" /><figcaption>**Figure 3.** (A) Schematization of stochastically substituted dendrimers mixture; (B) schematization of precisely substituted dendrimers mixture.</figcaption></figure>

However, the traditional stochastic approach to bifunctionalization, where two or more functions are introduced onto the dendrimer surface without precise control, contradicts the goal of achieving reproducibility in dendrimer synthesis. The stochastic method introduces variability in the type and location of substitutions, potentially impacting the dendrimer's activity [15]. To address this challenge, a preferable alternative involves the precise synthesis of bifunctionalized dendrimers, ensuring a consistent and controlled structure [16]. Three distinct pathways for synthesizing
bifunctionalized dendrimers in a controlled manner include attaching a compound containing both desired functions to each terminal function, modifying an existing function on the dendrimer's surface to generate two functions, or sequentially grafting two different functions onto the dendrimer's surface (Scheme 2).

<figure><img src="image_285.png" /><figcaption>**Scheme 2.** Schematic representation of different pathways to synthesize controlled bifunctionalized dendrimers: (A) grafting of a bifunctional monomer on the surface; (B) modification of an existing function on the surface; (C) sequential substitution with two different functions.</figcaption></figure>

Notably, phosphorhydrazone (PPH) dendrimers have demonstrated versatility in bifunctionalization due to their terminal groups, such as aldehyde or P(S)Cl₂ functions, which can be easily functionalized through replacement with different moieties. Bifunctional monomer grafting onto the surface of PPH dendrimers has found extensive applications in biology and catalysis. The integration of phosphine complexes as terminal functions in catalysis, especially in mitigating palladium leaching in Suzuki cross-coupling reactions, exemplifies the versatility of these dendrimers (Figure 4) [17].

<figure><img src="image_286.png" /><figcaption>**Figure 4.** PPH dendrimer functionalized with a bifunctional substituent: a chiral iminophosphine.</figcaption></figure>

Moreover, modifying a function on the surface of PPH dendrimers, enables the introduction of a second substituent. This approach, extended up to the fourth generation of dendrimers, results in dendrimers endowed with stereogenic functions on the surface, equipped with phosphines capable of coordinating with metals for various catalytic applications (Scheme 3) [18].
bifunctionalized dendrimers in a controlled manner include attaching a compound containing both desired functions to each terminal function, modifying an existing function on the dendrimer's surface to generate two functions, or sequentially grafting two different functions onto the dendrimer's surface (Scheme 2).

<figure><img src="image_283.png" /><figcaption>**Scheme 2.** Schematic representation of different pathways to synthesize controlled bifunctionalized dendrimers: (A) grafting of a bifunctional monomer on the surface; (B) modification of an existing function on the surface; (C) sequential substitution with two different functions.</figcaption></figure>

Notably, phosphorhydrazone (PPH) dendrimers have demonstrated versatility in bifunctionalization due to their terminal groups, such as aldehyde or P(S)Cl₂ functions, which can be easily functionalized through replacement with different moieties. Bifunctional monomer grafting onto the surface of PPH dendrimers has found extensive applications in biology and catalysis. The integration of phosphine complexes as terminal functions in catalysis, especially in mitigating palladium leaching in Suzuki cross-coupling reactions, exemplifies the versatility of these dendrimers (Figure 4) [17].

<figure><img src="image_284.png" /><figcaption>**Figure 4.** PPH dendrimer functionalized with a bifunctional substituent: a chiral iminophosphine.</figcaption></figure>

Moreover, modifying a function on the surface of PPH dendrimers, enables the introduction of a second substituent. This approach, extended up to the fourth generation of dendrimers, results in dendrimers endowed with stereogenic functions on the surface, equipped with phosphines capable of coordinating with metals for various catalytic applications (Scheme 3) [18].
<figure><img src="image_287.png" /><figcaption>**Scheme 3.** Synthesis of PPH dendrimer functionalized with a stereogenic moiety and subsequently modified with the addition of a phosphine [18].</figcaption></figure>

The sequential grafting of two different substituents on the surface of PPH dendrimers is a less common approach to bifunctionalization. It involves sequential substitution with two different functional groups, such as amines or phenols. While there are limited examples of sequential grafting with two phenols, a specific case involving the grafting of a phenol derivative of ethacrynic acid followed by the grafting of either 4-hydroxybenzaldehyde or the phenol of an azabisphosphonate demonstrates the potential in this approach (Scheme 4) [19].

<figure><img src="image_288.png" /><figcaption>**Scheme 4.** Sequential grafting of two different phenolic substituents on the P(S)Cl₂ terminal functions of PPH dendrimers.</figcaption></figure>
<figure><img src="image_285.png" /><figcaption>**Scheme 3.** Synthesis of PPH dendrimer functionalized with a stereogenic moiety and subsequently modified with the addition of a phosphine [18].</figcaption></figure>

The sequential grafting of two different substituents on the surface of PPH dendrimers is a less common approach to bifunctionalization. It involves sequential substitution with two different functional groups, such as amines or phenols. While there are limited examples of sequential grafting with two phenols, a specific case involving the grafting of a phenol derivative of ethacrynic acid followed by the grafting of either 4-hydroxybenzaldehyde or the phenol of an azabisphosphonate demonstrates the potential in this approach (Scheme 4) [19].

<figure><img src="image_286.png" /><figcaption>**Scheme 4.** Sequential grafting of two different phenolic substituents on the P(S)Cl₂ terminal functions of PPH dendrimers.</figcaption></figure>
In recent decades, the focus on green chemistry and atom economy has driven research towards optimizing catalytic processes, especially in organic solvents. Dendrimers, with their precisely defined structures, substantial surface area, solubility in organic solvents, and tunable chemical functionalities, have emerged as promising candidates for catalytic applications. Metalloendrimers, in particular, exhibit various modes of metal interaction, contributing to structural diversity and influencing reactivity [20].

The dendrimer recovery in catalysis can be accomplished through various methods, each tailored to specific properties and applications. These methods include nanofiltration, phase separation, precipitation, and magnetism [21,22].

The dendrimer effect, manifested as the multivalency dendritic effect and the generation dendritic effect, plays a significant role in influencing the properties and behaviour of functional moieties attached to dendrimers. This non-linear phenomenon can have both positive and negative effects, depending on the specific application. In the context of catalysis, the dendrimer effect can enhance reaction rates, yields, or selectivity and even play a role in improving the stability of metal complexes, reducing metal leaching, and aligning with the principles of green and sustainable chemistry [23-25].

Phosphorhydrazone dendrimers, with their solubility in various organic solvents, have gained prominence in homogeneous catalysis. The use of dendrimers with a metal core or surface-functionalized with ligands provides control over coordination chemistry, leading to enhanced catalytic efficiency and selectivity. Phosphorhydrazone dendrimers functionalized with various ligands able to complex different metals were used to perform a great variety of coupling reactions (Figure 5) [26].

<figure><img src="image_289.png" /><figcaption>**Figure 5.** Phosphorhydrazone dendrimers with SP and $N_3P_3$ cores, functionalized on the surface with different metal complexes.</figcaption></figure>

Exploring unconventional solvents in homogeneous catalysis, such as supercritical carbon dioxide ($scCO_2$), represents a significant stride towards sustainable practices. Dendrimers have been employed in catalysis in $scCO_2$, albeit limited examples. For instance, carbosilane dendrimers with $PdCl_2$ complexed with phosphorus at the core exhibited activity in the Heck cross-coupling reaction in $scCO_2$. Additionally, the control of a PPI dendrimer's solubility in $scCO_2$ through surface
In recent decades, the focus on green chemistry and atom economy has driven research towards optimizing catalytic processes, especially in organic solvents. Dendrimers, with their precisely defined structures, substantial surface area, solubility in organic solvents, and tunable chemical functionalities, have emerged as promising candidates for catalytic applications. Metalloendrimers, in particular, exhibit various modes of metal interaction, contributing to structural diversity and influencing reactivity [20].

The dendrimer recovery in catalysis can be accomplished through various methods, each tailored to specific properties and applications. These methods include nanofiltration, phase separation, precipitation, and magnetism [21,22].

The dendrimer effect, manifested as the multivalency dendritic effect and the generation dendritic effect, plays a significant role in influencing the properties and behaviour of functional moieties attached to dendrimers. This non-linear phenomenon can have both positive and negative effects, depending on the specific application. In the context of catalysis, the dendrimer effect can enhance reaction rates, yields, or selectivity and even play a role in improving the stability of metal complexes, reducing metal leaching, and aligning with the principles of green and sustainable chemistry [23-25].

Phosphorhydrazone dendrimers, with their solubility in various organic solvents, have gained prominence in homogeneous catalysis. The use of dendrimers with a metal core or surface-functionalized with ligands provides control over coordination chemistry, leading to enhanced catalytic efficiency and selectivity. Phosphorhydrazone dendrimers functionalized with various ligands able to complex different metals were used to perform a great variety of coupling reactions (Figure 5) [26].

<figure><img src="image_287.png" /><figcaption>**Figure 5.** Phosphorhydrazone dendrimers with SP and $N_3P_3$ cores, functionalized on the surface with different metal complexes.</figcaption></figure>

Exploring unconventional solvents in homogeneous catalysis, such as supercritical carbon dioxide ($scCO_2$), represents a significant stride towards sustainable practices. Dendrimers have been employed in catalysis in $scCO_2$, albeit limited examples. For instance, carbosilane dendrimers with $PdCl_2$ complexed with phosphorus at the core exhibited activity in the Heck cross-coupling reaction in $scCO_2$. Additionally, the control of a PPI dendrimer's solubility in $scCO_2$ through surface
functionalization with perfluorinated polyethers facilitated the Heck cross-coupling reaction with high selectivity (Scheme 5) [28,29].

<figure><img src="image_290.png" /><figcaption>**Scheme 5.** PPI dendrimer functionalize on the surface with perfluorinated polyethers chains, encapsulating Pd nanoparticles to perform Heck cross-coupling in scCO₂.</figcaption></figure>

Homogeneous catalysis in fluorous/organic biphasic systems, particularly those with thermoreversible characteristics, offers a unique approach to catalytic methodologies (Figure 6) [30]. The integration of dendrimers into such biphasic systems is a relatively unexplored area. However, a notable example involves PPI dendrimers surface-functionalized with perfluorinated polyethers and encapsulated with Pd nanoparticles, demonstrating potential in the Heck cross-coupling reaction (Scheme 6) [31].

<figure><img src="image_291.png" /><figcaption>**Figure 6.** Representation of the principle of fluorous/organic biphasic systems in homogenous catalysis, in termomorphic behaviour.</figcaption></figure>
functionalization with perfluorinated polyethers facilitated the Heck cross-coupling reaction with high selectivity (Scheme 5) [28,29].

<figure><img src="image_288.png" /><figcaption>**Scheme 5.** PPI dendrimer functionalize on the surface with perfluorinated polyethers chains, encapsulating Pd nanoparticles to perform Heck cross-coupling in scCO₂.</figcaption></figure>

Homogeneous catalysis in fluorous/organic biphasic systems, particularly those with thermoreversible characteristics, offers a unique approach to catalytic methodologies (Figure 6) [30]. The integration of dendrimers into such biphasic systems is a relatively unexplored area. However, a notable example involves PPI dendrimers surface-functionalized with perfluorinated polyethers and encapsulated with Pd nanoparticles, demonstrating potential in the Heck cross-coupling reaction (Scheme 6) [31].

<figure><img src="image_289.png" /><figcaption>**Figure 6.** Representation of the principle of fluorous/organic biphasic systems in homogenous catalysis, in termomorphic behaviour.</figcaption></figure>
### Heck cross-coupling:

<figure><img src="image_292.png" /><figcaption>**Scheme 6.** Heck cross-coupling performed in a fluorous biphasic system using as catalyst a PPI dendrimer encapsulating Pd nanoparticles.</figcaption></figure>

In conclusion, the multifaceted applications of dendrimers in bifunctionalization, homogeneous catalysis, and unconventional solvent systems underscore their versatility in nanotechnology and green chemistry. The controlled synthesis of bifunctionalized dendrimers provides reproducibility, while the dendrimer effect enhances catalytic efficiency. The exploration of unconventional solvents and biphasic systems further expands the potential applications of dendrimers in sustainable and efficient catalytic processes.

## Synthesis of dendrimeric catalysts

The research embarked on a nuanced exploration into the intricate realm of phosphorhydrazone (PPH) dendrimers, shedding light on their meticulous synthesis and detailed functionalization through para-substituted phenols [19]. This journey is marked by a meticulous sequence of reactions initiated from the dendrimer core, ultimately leading to the incorporation of carefully chosen functional groups, with a specific focus on creating precisely asymmetrically disubstituted dendrimeric catalysts. The multifaceted nature of this study unfolds in various stages, each contributing to a comprehensive understanding of the synthesized dendrimers and their potential catalytic applications.

In the early sections of the research, the stage is set by elucidating the meticulous preparation of para-substituted phenols. These phenols serve as crucial precursors for the subsequent functionalization of PPH dendrimers. Two distinct sets of para-substituted phenols are synthesized, with one set designed to act as ligands for future metal introduction [32], and the other set endowed with moieties tailored to impart the desired solubility in non-conventional media. This strategic approach underscores the intent to create versatile dendrimeric catalysts with specific attributes for enhanced functionality.

The study introduces two metal ligands, compounds **2** and **3**, subjected to detailed exploration for their catalytic potential. The catalysts are evaluated in Stille cross-coupling reactions in organic solvents, highlighting their utility and paving the way for broader catalytic applications [32,33]. The careful selection and subsequent synthesis of these ligands underscore the nuanced considerations involved in developing effective dendrimeric catalysts (Figure 7).

An essential facet of the research involves addressing challenges associated with the solubility of dendrimers in non-conventional media. This is achieved through the incorporation of perfluorinated ligands, with a particular emphasis on creating phenols with alkylpolyfluorinated chains. The synthesis journey explores different routes, optimizing reaction conditions to achieve the desired products (Figure 7). This pursuit not only underscores the meticulous nature of the research but also emphasizes the importance of solubility in non-conventional solvents for the envisioned catalytic applications.
### Heck cross-coupling:

<figure><img src="image_290.png" /><figcaption>**Scheme 6.** Heck cross-coupling performed in a fluorous biphasic system using as catalyst a PPI dendrimer encapsulating Pd nanoparticles.</figcaption></figure>

In conclusion, the multifaceted applications of dendrimers in bifunctionalization, homogeneous catalysis, and unconventional solvent systems underscore their versatility in nanotechnology and green chemistry. The controlled synthesis of bifunctionalized dendrimers provides reproducibility, while the dendrimer effect enhances catalytic efficiency. The exploration of unconventional solvents and biphasic systems further expands the potential applications of dendrimers in sustainable and efficient catalytic processes.

## Synthesis of dendrimeric catalysts

The research embarked on a nuanced exploration into the intricate realm of phosphorhydrazone (PPH) dendrimers, shedding light on their meticulous synthesis and detailed functionalization through para-substituted phenols [19]. This journey is marked by a meticulous sequence of reactions initiated from the dendrimer core, ultimately leading to the incorporation of carefully chosen functional groups, with a specific focus on creating precisely asymmetrically disubstituted dendrimeric catalysts. The multifaceted nature of this study unfolds in various stages, each contributing to a comprehensive understanding of the synthesized dendrimers and their potential catalytic applications.

In the early sections of the research, the stage is set by elucidating the meticulous preparation of para-substituted phenols. These phenols serve as crucial precursors for the subsequent functionalization of PPH dendrimers. Two distinct sets of para-substituted phenols are synthesized, with one set designed to act as ligands for future metal introduction [32], and the other set endowed with moieties tailored to impart the desired solubility in non-conventional media. This strategic approach underscores the intent to create versatile dendrimeric catalysts with specific attributes for enhanced functionality.

The study introduces two metal ligands, compounds **2** and **3**, subjected to detailed exploration for their catalytic potential. The catalysts are evaluated in Stille cross-coupling reactions in organic solvents, highlighting their utility and paving the way for broader catalytic applications [32,33]. The careful selection and subsequent synthesis of these ligands underscore the nuanced considerations involved in developing effective dendrimeric catalysts (Figure 7).

An essential facet of the research involves addressing challenges associated with the solubility of dendrimers in non-conventional media. This is achieved through the incorporation of perfluorinated ligands, with a particular emphasis on creating phenols with alkylpolyfluorinated chains. The synthesis journey explores different routes, optimizing reaction conditions to achieve the desired products (Figure 7). This pursuit not only underscores the meticulous nature of the research but also emphasizes the importance of solubility in non-conventional solvents for the envisioned catalytic applications.
<figure><img src="image_293.png" /><figcaption>**Figure 7.** Representation of phenolic moieties with bidentate ligand (red), phenolic moieties tailored to tune the solubility (blue).</figcaption></figure>

Moving forward, the study delves into the synthesis of PPH dendrimers, meticulously progressing from the first generation to subsequent generations. Each generation is synthesized through a two-step process, with nucleophilic substitutions and condensation reactions carefully orchestrated to yield dendrimers with distinct structures. The synthesis is tracked through ³¹P-NMR spectra, providing a visual representation of dendrimer growth and surface modifications (Figure 8) [19].

The crux of the research lies in the precise functionalization of PPH dendrimers. The endeavour to achieve asymmetric substitution while minimizing di-symmetric substitution is a key challenge addressed in the synthesis. A detailed exploration of reaction conditions, base selection, and sequential substitution strategies results in the triumphant synthesis of precisely asymmetrically disubstituted dendrimers, such as **prc-G1** and **prc-M**. The meticulous efforts to control substitution patterns underscore the fine-tuned nature of dendrimer synthesis (Figure 9) [34].

<figure><img src="image_294.png" /><figcaption>**Figure 8.** Representation of model molecule (M) and PPH dendrimers (Gc₁, Gc₂ and Gc₃) synthesized.</figcaption></figure>
<figure><img src="image_291.png" /><figcaption>**Figure 7.** Representation of phenolic moieties with bidentate ligand (red), phenolic moieties tailored to tune the solubility (blue).</figcaption></figure>

Moving forward, the study delves into the synthesis of PPH dendrimers, meticulously progressing from the first generation to subsequent generations. Each generation is synthesized through a two-step process, with nucleophilic substitutions and condensation reactions carefully orchestrated to yield dendrimers with distinct structures. The synthesis is tracked through ³¹P-NMR spectra, providing a visual representation of dendrimer growth and surface modifications (Figure 8) [19].

The crux of the research lies in the precise functionalization of PPH dendrimers. The endeavour to achieve asymmetric substitution while minimizing di-symmetric substitution is a key challenge addressed in the synthesis. A detailed exploration of reaction conditions, base selection, and sequential substitution strategies results in the triumphant synthesis of precisely asymmetrically disubstituted dendrimers, such as **prc-G1** and **prc-M**. The meticulous efforts to control substitution patterns underscore the fine-tuned nature of dendrimer synthesis (Figure 9) [34].

<figure><img src="image_292.png" /><figcaption>**Figure 8.** Representation of model molecule (M) and PPH dendrimers (Gc₁, Gc₂ and Gc₃) synthesized.</figcaption></figure>
<figure><img src="image_295.png" /><figcaption>**Figure 9.** Representation of precise substituted products A) prc-M and B) prc-G1.</figcaption></figure>

Metal complexation, particularly with palladium, emerges as a critical step to enhance stability and reactivity of the synthesized dendrimers. The successful complexation is a testament to the careful design and execution of synthetic methodologies. The resulting metal-complexed dendrimers undergo thorough characterization, encompassing various spectroscopic techniques, ensuring a comprehensive understanding of their structural attributes. Using this methodology where synthesized different precisely substituted PPH dendrimers up to the third generation (Figure 10).

A comparative turn in the research introduces stochastically substituted dendrimers, characterized by a more random distribution of substituents. The stochastic substitution process is detailed, and resultant mixtures undergo metal complexation, mirroring the methodology applied to precisely substituted counterparts. This comparative analysis sets the stage for exploring potential differences in activities between precisely substituted and stochastically substituted dendrimers (Figure 11).
<figure><img src="image_293.png" /><figcaption>**Figure 9.** Representation of precise substituted products A) prc-M and B) prc-G1.</figcaption></figure>

Metal complexation, particularly with palladium, emerges as a critical step to enhance stability and reactivity of the synthesized dendrimers. The successful complexation is a testament to the careful design and execution of synthetic methodologies. The resulting metal-complexed dendrimers undergo thorough characterization, encompassing various spectroscopic techniques, ensuring a comprehensive understanding of their structural attributes. Using this methodology where synthesized different precisely substituted PPH dendrimers up to the third generation (Figure 10).

A comparative turn in the research introduces stochastically substituted dendrimers, characterized by a more random distribution of substituents. The stochastic substitution process is detailed, and resultant mixtures undergo metal complexation, mirroring the methodology applied to precisely substituted counterparts. This comparative analysis sets the stage for exploring potential differences in activities between precisely substituted and stochastically substituted dendrimers (Figure 11).
<figure><img src="image_296.png" /><figcaption>**Figure 10.** Representation of precise substituted products A) prc-M-[Pd], B) prc-G1-[Pd], C) prc-G2-[Pd], D) prc-G3-[Pd], E) 2prc-G1-[Pd] and F) 3prc-G1.</figcaption></figure>
<figure><img src="image_294.png" /><figcaption>**Figure 10.** Representation of precise substituted products A) prc-M-[Pd], B) prc-G1-[Pd], C) prc-G2-[Pd], D) prc-G3-[Pd], E) 2prc-G1-[Pd] and F) 3prc-G1.</figcaption></figure>
<figure><img src="image_297.png" /><figcaption>**Figure 11.** Representation of precise substituted products A) rdm-G1-[Pd] and B) 3rdm-G1.</figcaption></figure>

As the research narrative unfolds, the emphasis extends to the successful scaling up of synthesis protocols to gram-scale production. This not only speaks to the reproducibility of the methods but also positions the dendrimers for practical applications in larger quantities. The comprehensive characterization of the synthesized compounds encompasses a range of analytical techniques, including ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ³¹P-NMR, IR, and melting point determination, ensuring a robust understanding of their chemical attributes.

## Study of catalytic activity

We have used **prc-Gn-[Pd]** for different C-C coupling reactions in organic solvents to demonstrate the activity of the catalyst [32].

* Catalysts **prc-Gn-[Pd]** demonstrated a good activity for the Stille cross-coupling in THF, showing a positive multivalence and generation dendritic effect. We also tried the recycling of the catalyst **prc-G1-[Pd]**, obtaining a 50% drop of the conversion from the second entry, we can attribute this loss of activity to the leaching of palladium and the formation of palladium nanoparticles that probably are less active than the complexed palladium.

* The catalyst **prc-G1-[Pd]** was showing activity for the Heck cross-coupling giving specific stereoselectivity for the trans isomer [31]. The catalyst **prc-G1-[Pd]** was showing a positive multivalence dendritic effect, while the catalyst **prc-G2-[Pd]** showed a negative generation dendritic effect (Scheme 8).
<figure><img src="image_295.png" /><figcaption>**Figure 11.** Representation of precise substituted products A) rdm-G1-[Pd] and B) 3rdm-G1.</figcaption></figure>

As the research narrative unfolds, the emphasis extends to the successful scaling up of synthesis protocols to gram-scale production. This not only speaks to the reproducibility of the methods but also positions the dendrimers for practical applications in larger quantities. The comprehensive characterization of the synthesized compounds encompasses a range of analytical techniques, including ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ³¹P-NMR, IR, and melting point determination, ensuring a robust understanding of their chemical attributes.

## Study of catalytic activity

We have used **prc-Gn-[Pd]** for different C-C coupling reactions in organic solvents to demonstrate the activity of the catalyst [32].

* Catalysts **prc-Gn-[Pd]** demonstrated a good activity for the Stille cross-coupling in THF, showing a positive multivalence and generation dendritic effect. We also tried the recycling of the catalyst **prc-G1-[Pd]**, obtaining a 50% drop of the conversion from the second entry, we can attribute this loss of activity to the leaching of palladium and the formation of palladium nanoparticles that probably are less active than the complexed palladium.

* The catalyst **prc-G1-[Pd]** was showing activity for the Heck cross-coupling giving specific stereoselectivity for the trans isomer [31]. The catalyst **prc-G1-[Pd]** was showing a positive multivalence dendritic effect, while the catalyst **prc-G2-[Pd]** showed a negative generation dendritic effect (Scheme 8).
<figure><img src="image_298.png" /><figcaption>**Scheme 7.** A) Scheme of the Stille cross-coupling reaction; B) Graph of the conversion in the Stille cross-coupling under stirring with different catalysts.</figcaption></figure>

<figure><img src="image_299.png" /><figcaption>**Scheme 8.** A) Scheme of the Heck cross-coupling reaction; B) Graph of the conversion in the Heck cross-coupling with different catalysts.</figcaption></figure>

* The model molecule **prc-M-[Pd]** was used to perform the Suzuki coupling showing activity for this reaction using an inorganic base (Cs₂CO₃) [35]. In order to translate the reaction in non-conventional solvents we needed to perform the reaction using organic bases, in these
<figure><img src="image_296.png" /><figcaption>**Scheme 7.** A) Scheme of the Stille cross-coupling reaction; B) Graph of the conversion in the Stille cross-coupling under stirring with different catalysts.</figcaption></figure>

<figure><img src="image_297.png" /><figcaption>**Scheme 8.** A) Scheme of the Heck cross-coupling reaction; B) Graph of the conversion in the Heck cross-coupling with different catalysts.</figcaption></figure>

* The model molecule **prc-M-[Pd]** was used to perform the Suzuki coupling showing activity for this reaction using an inorganic base (Cs₂CO₃) [35]. In order to translate the reaction in non-conventional solvents we needed to perform the reaction using organic bases, in these
conditions **prc-M-[Pd]** and **prc-G1-[Pd]** showed a very low activity for this reaction (Scheme 9).

<figure><img src="image_300.png" /><figcaption>**Scheme 9.** A) Scheme of the Suzuki coupling reaction tested in this chapter section; B) graph of the conversion for the Suzuki coupling using different catalysts.</figcaption></figure>

After verifying the activity of the catalyst in organic solvents we have moved to verify the solubility and the activity of our catalysts in non-conventional solvents starting from scCO₂.

* Surprisingly despite the presence of alkylpolyfluorinated chains on the surface of our dendrimeric catalysts, they were not showing solubility in scCO₂.

* We have decided to perform catalysis in heterogeneous conditions, optimizing reaction conditions for the Stille cross-coupling reaction in scCO₂ in a little reactor (6 mL) without the stirring. We obtained that the optimal amount of catalyst was 5%, and that the optimal pressure was 100 bars (Table 1).

<table><caption>**Table 1.** Table showing the conversion of the Stille cross-coupling reaction in 6 ml scCO₂ reactor following conditions in Scheme 4.6.</caption><thead><tr><th>Cat.</th><th>Pd mol%</th><th>Time (h)</th><th>Conv. (%)<sup>(a)</sup></th></tr></thead><tbody><tr><td rowspan="2">prc-M-[Pd]</td><td>5</td><td>4</td><td>0</td></tr><tr><td>5</td><td>22</td><td>0</td></tr><tr><td rowspan="4">prc-G1-[Pd]</td><td>5</td><td>4</td><td>43</td></tr><tr><td>5</td><td>22</td><td>43</td></tr><tr><td>10</td><td>4</td><td>39</td></tr><tr><td>10</td><td>22</td><td>39</td></tr></tbody></table>

<footnote>(a) Determined by ¹H-NMR (CDCl₃, 400 MHz).</footnote>
conditions **prc-M-[Pd]** and **prc-G1-[Pd]** showed a very low activity for this reaction (Scheme 9).

<figure><img src="image_298.png" /><figcaption>**Scheme 9.** A) Scheme of the Suzuki coupling reaction tested in this chapter section; B) graph of the conversion for the Suzuki coupling using different catalysts.</figcaption></figure>

After verifying the activity of the catalyst in organic solvents we have moved to verify the solubility and the activity of our catalysts in non-conventional solvents starting from scCO₂.

* Surprisingly despite the presence of alkylpolyfluorinated chains on the surface of our dendrimeric catalysts, they were not showing solubility in scCO₂.

* We have decided to perform catalysis in heterogeneous conditions, optimizing reaction conditions for the Stille cross-coupling reaction in scCO₂ in a little reactor (6 mL) without the stirring. We obtained that the optimal amount of catalyst was 5%, and that the optimal pressure was 100 bars (Table 1).

<table><caption>**Table 1.** Table showing the conversion of the Stille cross-coupling reaction in 6 ml scCO₂ reactor following conditions in Scheme 4.6.</caption><thead><tr><th>Cat.</th><th>Pd mol%</th><th>Time (h)</th><th>Conv. (%)<sup>(a)</sup></th></tr></thead><tbody><tr><td rowspan="2">prc-M-[Pd]</td><td>5</td><td>4</td><td>0</td></tr><tr><td>5</td><td>22</td><td>0</td></tr><tr><td rowspan="4">prc-G1-[Pd]</td><td>5</td><td>4</td><td>43</td></tr><tr><td>5</td><td>22</td><td>43</td></tr><tr><td>10</td><td>4</td><td>39</td></tr><tr><td>10</td><td>22</td><td>39</td></tr></tbody></table>

<footnote>(a) Determined by ¹H-NMR (CDCl₃, 400 MHz).</footnote>
* We moved to perform the reaction in a big reactor (29 mL) with the mechanical stirring, where we performed the Stille cross-coupling reaction using optimal conditions previously found. We have observed a good activity of catalysts **prc-Gn-[Pd]** in this reaction, showing a positive multivalence and generation dendritic effect from the first to the third generation (Table 2).

* We have tried to recover the catalyst **prc-G1-[Pd]**, but for the geometry of the reactor used it was not possible to perform the purification of the catalyst *in situ*, while after the washing and purification we have experienced a partial recovery of the catalyst.

* We have also performed the Heck cross-coupling in scCO₂ using as catalyst **prc-G1-[Pd]** obtaining a low conversion.

<table><caption>**Table 2.** Table showing conversion of the Stille cross-coupling reaction in 29 ml scCO₂ reactor following conditions in Scheme 4.6.</caption><thead><tr><th>Cat.</th><th>Pd mol%</th><th>Time (h)</th><th>Conv. (%)<sup>(a)</sup></th></tr></thead><tbody><tr><td rowspan="4">prc-G1-[Pd]</td><td>5</td><td>4</td><td>39</td></tr><tr><td>5</td><td>22</td><td>82</td></tr><tr><td>1</td><td>4</td><td>15</td></tr><tr><td>1</td><td>22</td><td>36</td></tr><tr><td>prc-G1-[Pd]<sup>(b)</sup></td><td>5</td><td>22</td><td>51</td></tr><tr><td rowspan="2">2prc-G1-[Pd]</td><td>5</td><td>4</td><td>18</td></tr><tr><td>5</td><td>22</td><td>25</td></tr><tr><td>prc-G2-[Pd]</td><td>5</td><td>4</td><td>66</td></tr><tr><td>prc-G3-[Pd]</td><td>5</td><td>4</td><td>68</td></tr></tbody></table>

We have also verified the solubility of our catalyst **prc-G1-[Pd]** in perfluorinated solvents in order to perform catalysis in biphasic fluorous/organic systems.

* We have tried to solubilize **prc-G1-[Pd]** in two perfluorinated solvents (perfluorodecalin and perfluoromethylcyclohexane), our catalyst resulted to be slightly soluble in perfluorodecaline and partially soluble in perfluoromethylcyclohexane.

* Despite the partial solubility of **prc-G1-[Pd]** in perfluoromethylcyclohexane we have decided to perform the Heck cross-coupling in heterogeneous conditions, obtaining a similar result obtained in organic solvents, but it was not possible to recycle and to proceed with the reuse of the catalyst.

## Precise versus stochastically functionalized PPH dendrimers in catalysis

In the last part of this work we have focused our attention on the comparison of the activity between precisely (**prc-Gn-[Pd]**)) and stochastically substituted (**rdm-Gn-[Pd]**)) phosphorus dendrimeric catalysts.

* Catalysts **prc-Gn-[Pd]** and **rdm-Gn-[Pd]** were used for the Stille cross-coupling reaction in organic solvent. In stochastically substituted catalysts (**rdm-Gn-[Pd]**)) we observed a positive

<footnote>(a) Determined by ¹H-NMR (CDCl₃, 400 MHz). (b) Reaction performed using 1/10 of concentrations of all reagents.</footnote>
* We moved to perform the reaction in a big reactor (29 mL) with the mechanical stirring, where we performed the Stille cross-coupling reaction using optimal conditions previously found. We have observed a good activity of catalysts **prc-Gn-[Pd]** in this reaction, showing a positive multivalence and generation dendritic effect from the first to the third generation (Table 2).

* We have tried to recover the catalyst **prc-G1-[Pd]**, but for the geometry of the reactor used it was not possible to perform the purification of the catalyst *in situ*, while after the washing and purification we have experienced a partial recovery of the catalyst.

* We have also performed the Heck cross-coupling in scCO₂ using as catalyst **prc-G1-[Pd]** obtaining a low conversion.

<table><caption>**Table 2.** Table showing conversion of the Stille cross-coupling reaction in 29 ml scCO₂ reactor following conditions in Scheme 4.6.</caption><thead><tr><th>Cat.</th><th>Pd mol%</th><th>Time (h)</th><th>Conv. (%)<sup>(a)</sup></th></tr></thead><tbody><tr><td rowspan="4">prc-G1-[Pd]</td><td>5</td><td>4</td><td>39</td></tr><tr><td>5</td><td>22</td><td>82</td></tr><tr><td>1</td><td>4</td><td>15</td></tr><tr><td>1</td><td>22</td><td>36</td></tr><tr><td>prc-G1-[Pd]<sup>(b)</sup></td><td>5</td><td>22</td><td>51</td></tr><tr><td rowspan="2">2prc-G1-[Pd]</td><td>5</td><td>4</td><td>18</td></tr><tr><td>5</td><td>22</td><td>25</td></tr><tr><td>prc-G2-[Pd]</td><td>5</td><td>4</td><td>66</td></tr><tr><td>prc-G3-[Pd]</td><td>5</td><td>4</td><td>68</td></tr></tbody></table>

We have also verified the solubility of our catalyst **prc-G1-[Pd]** in perfluorinated solvents in order to perform catalysis in biphasic fluorous/organic systems.

* We have tried to solubilize **prc-G1-[Pd]** in two perfluorinated solvents (perfluorodecalin and perfluoromethylcyclohexane), our catalyst resulted to be slightly soluble in perfluorodecaline and partially soluble in perfluoromethylcyclohexane.

* Despite the partial solubility of **prc-G1-[Pd]** in perfluoromethylcyclohexane we have decided to perform the Heck cross-coupling in heterogeneous conditions, obtaining a similar result obtained in organic solvents, but it was not possible to recycle and to proceed with the reuse of the catalyst.

## Precise versus stochastically functionalized PPH dendrimers in catalysis

In the last part of this work we have focused our attention on the comparison of the activity between precisely (**prc-Gn-[Pd]**)) and stochastically substituted (**rdm-Gn-[Pd]**)) phosphorus dendrimeric catalysts.

* Catalysts **prc-Gn-[Pd]** and **rdm-Gn-[Pd]** were used for the Stille cross-coupling reaction in organic solvent. In stochastically substituted catalysts (**rdm-Gn-[Pd]**)) we observed a positive

<footnote>(a) Determined by ¹H-NMR (CDCl₃, 400 MHz). (b) Reaction performed using 1/10 of concentrations of all reagents.</footnote>
generation dendrimeric effect, going from the first to the third generation, as we observed for precisely substituted catalysts (prc-Gn-[Pd]). While we observed for all generations a higher activity of prc-Gn-[Pd] compared to the activity of rdm-Gn-[Pd] (Scheme 10).

<figure><img src="image_301.png" /><figcaption>**Scheme 10.** A) Scheme of the Stille cross-coupling reaction; B) Graph representing results of the Stille cross-coupling using different catalysts.</figcaption></figure>

* Catalysts **3prc-G1** and **3rdm-G1** were complexed *in situ* with Cul and used for the C-N coupling [33]. Both showed a higher activity compared to the Cul alone. For this reaction the stochastically substituted catalyst (**3rdm-G1**) showed a higher activity compared to the precisely substituted catalyst (**3prc-G1**) (Scheme 11). We speculate that this behaviour may be explained by two catalytical sites in the vicinity working in a cooperative way, but more research is required to confirm this.

<figure><img src="image_302.png" /></figure>

<table><caption>**Scheme 11.** Scheme of the C-N coupling reaction and table of results of the C-N coupling in biphasic fluorous/organic system. <sup>(a)</sup> Determined by ¹H-NMR (CDCl₃, 400 MHz).</caption><thead><tr><th>Cat./Ligand</th><th>Time (h)</th><th>Conv. (%)<sup>(a)</sup></th></tr></thead><tbody><tr><td>Cul</td><td>20</td><td>33</td></tr><tr><td>3prc-G1</td><td>20</td><td>46</td></tr><tr><td>3rdm-G1</td><td>20</td><td>59</td></tr></tbody></table>
generation dendrimeric effect, going from the first to the third generation, as we observed for precisely substituted catalysts (prc-Gn-[Pd]). While we observed for all generations a higher activity of prc-Gn-[Pd] compared to the activity of rdm-Gn-[Pd] (Scheme 10).

<figure><img src="image_299.png" /><figcaption>**Scheme 10.** A) Scheme of the Stille cross-coupling reaction; B) Graph representing results of the Stille cross-coupling using different catalysts.</figcaption></figure>

* Catalysts **3prc-G1** and **3rdm-G1** were complexed *in situ* with Cul and used for the C-N coupling [33]. Both showed a higher activity compared to the Cul alone. For this reaction the stochastically substituted catalyst (**3rdm-G1**) showed a higher activity compared to the precisely substituted catalyst (**3prc-G1**) (Scheme 11). We speculate that this behaviour may be explained by two catalytical sites in the vicinity working in a cooperative way, but more research is required to confirm this.

<table><caption>**Scheme 11.** Scheme of the C-N coupling reaction and table of results of the C-N coupling in biphasic fluorous/organic system. <sup>(a)</sup> Determined by ¹H-NMR (CDCl₃, 400 MHz).</caption><thead><tr><th>Cat./Ligand</th><th>Time (h)</th><th>Conv. (%)<sup>(a)</sup></th></tr></thead><tbody><tr><td>Cul</td><td>20</td><td>33</td></tr><tr><td>3prc-G1</td><td>20</td><td>46</td></tr><tr><td>3rdm-G1</td><td>20</td><td>59</td></tr></tbody></table>
The main objectives of this study were to create new dendrimer catalysts by integrating surface functionalization with two surface functions, one capable of metal complexation and the other of rendering them soluble in non-traditional solvents. We succeeded in synthesizing several types of bifunctionalized dendrimers and studying their catalytic properties. Such work provides valuable insights for the future and suggests avenues for refining sustainable catalytic processes.

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[12] A. I. Cooper, J. D. Londono, G. Wignall, J. B. McClain, E. T. Samulski, J. S. Lin, A. Dobrynin, M. Rubinstein, A. L. C. Burke, J. M. J. Fréchet, J. M. DeSimone, *Nature*, **1997**, *389*, 368–371. DOI: 10.1038/38706.

[13] C. O. Turrin, V. Maraval, J. Leclair, E. Dantras, C. Lacabanne, A. M. Caminade, J. P. Majoral, *Tetrahedron*, **2003**, *59*, 3965–3973. DOI: 10.1016/S0040-4020(03)00465-4.

[14] A. M. Caminade, J. P. Majoral, *Molecules*, **2016**, *21*, 538. DOI: 10.3390/molecules21040538.

[15] S. M. Mignani, N. El Brahmi, S. El Kazzouli, R. Laurent, S. Ladeira, A. M. Caminade, E. Pedziwiatr-Werbicka, E. M. Szewczyk, M. Bryszewska, M. M. Bousmina, T. Cresteil, J. P. Majoral, *Mol. Pharmaceutics*, **2017**, *14*, 4087–4097. DOI: 10.1021/acs.molpharmaceut.7b00771.

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[22] P. Neumann, H. Dib, A. M. Caminade, E. Hey-Hawkins, *Angew. Chem. Int. Ed.*, **2015**, *54*, 311–314. DOI: 10.1002/anie.201408314.
The main objectives of this study were to create new dendrimer catalysts by integrating surface functionalization with two surface functions, one capable of metal complexation and the other of rendering them soluble in non-traditional solvents. We succeeded in synthesizing several types of bifunctionalized dendrimers and studying their catalytic properties. Such work provides valuable insights for the future and suggests avenues for refining sustainable catalytic processes.

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Aswin CHANDRAN

ESR14

**Thesis co-directors**

Dr. Mary Grellier Laboratoire de Chimie de Coordination, Toulouse, France

Prof. Jason Lynam University of York, York, United Kingdom

Dr. John Slattery

**Thesis defense**

Laboratoire de Chimie de Coordination, 13 December 2023
Aswin CHANDRAN

ESR14

**Thesis co-directors**

Dr. Mary Grellier Laboratoire de Chimie de Coordination, Toulouse, France

Prof. Jason Lynam University of York, York, United Kingdom

Dr. John Slattery

**Thesis defense**

Laboratoire de Chimie de Coordination, 13 December 2023
<figure><img src="image_303.png" /></figure>
<figure><img src="image_300.png" /></figure>
# Coordination of Si–H and Ge–H Bonds to Ruthenium Metal Centres: Structure, Reactivity and Catalytic Applications

## Introduction

This thesis discusses the coordination chemistry and reactivity of ruthenium complexes with silicon and germanium hydrides. Gaining fundamental insights into new reactivities of E-H bonds (E = Si, Ge) to transition metals and coordination modes of these hydrides with transition metals is crucial for developing new selective catalytic systems. This thesis focuses on elucidating the mechanism governing the activation of Si–H and Ge–H bonds at the ruthenium metal centre.

The first part provides a brief introduction to the importance of investigating the coordination of silicon and germanium with transition metals. It focuses on the different coordination modes of silicon and germanium at transition metal centres, including α-complexes and oxidative addition adducts.

Among all the elements in the periodic table, transition metals play the most significant role in homogenous catalysis due to their unique characteristics. The ability to change their formal oxidation state and coordinate a wide range of substrates leading to various transition metal coordination complexes, providing an alternate reaction pathway with lower activation energy makes them the best candidates for very efficient and selective catalysts for numerous chemical reactions [1]. These catalytic processes enabled the synthesis of many chemical compounds, materials, and polymers.

Silicon and germanium compounds are an important class of organic compounds with a wide range of applications in synthesis leading to products ranging from things we handle in a day-to-day life at household to spacecrafts in the lower earth orbitals [2-6].

Organosilicon compounds have found widespread applications in materials science, synthetic chemistry, and pharmaceutical synthesis and some of the important applications include the following. Organosilanes and siloxanes are commonly used to modulate the hydrophobicity and surface properties of materials [7,8]. Side-chain liquid-crystalline siloxanes are utilized in the manufacturing of electronic device displays [9]. Organofunctionalized silicates can be used to immobilize peptides and as supports for metal complex catalysts [10,11]. Organosilanes serve as coupling agents between organic polymers and silicate materials, leading to applications in dental composites [12] and adhesives [13]. They are also used as precursors for silicon carbide ceramics [14]. In synthetic chemistry, organosilicon reagents enable mild, selective, and functional group-tolerant transformations. They are widely used as protective groups in organic synthesis [15]. Additionally, they are also used in stereospecific oxidations and carbon-carbon bond formations like cross-couplings, olefinations, and allylations [16-18].

While organogermanium chemistry and applications have been less explored compared to silicon [5,19,20], germanium compounds have found utility in medicinal and catalytic roles. Certain germanium compounds exhibit promising bioactivities or medicinal properties. [21] Germylated steroids have displayed potential as antitumor and antiviral agents [22]. The organogermanium compounds, Asaigermanium and propagermanium have been reported as pharmaceutical drugs for use in Japan [21]. Beyond medicine, organogermanes also hold promise as catalysts in the emerging trend of main group based catalysts. Germanium compounds have been studied as catalysts for the polymerization of lactide monomers [23]. Additionally, heavier Ge(II) species can serve as catalysts [24] for cyanoisilylation [25] and hydroboration of carbonyl compounds [26], hydroboration of CO₂ [27],
# Coordination of Si–H and Ge–H Bonds to Ruthenium Metal Centres: Structure, Reactivity and Catalytic Applications

## Introduction

This thesis discusses the coordination chemistry and reactivity of ruthenium complexes with silicon and germanium hydrides. Gaining fundamental insights into new reactivities of E-H bonds (E = Si, Ge) to transition metals and coordination modes of these hydrides with transition metals is crucial for developing new selective catalytic systems. This thesis focuses on elucidating the mechanism governing the activation of Si–H and Ge–H bonds at the ruthenium metal centre.

The first part provides a brief introduction to the importance of investigating the coordination of silicon and germanium with transition metals. It focuses on the different coordination modes of silicon and germanium at transition metal centres, including α-complexes and oxidative addition adducts.

Among all the elements in the periodic table, transition metals play the most significant role in homogenous catalysis due to their unique characteristics. The ability to change their formal oxidation state and coordinate a wide range of substrates leading to various transition metal coordination complexes, providing an alternate reaction pathway with lower activation energy makes them the best candidates for very efficient and selective catalysts for numerous chemical reactions [1]. These catalytic processes enabled the synthesis of many chemical compounds, materials, and polymers.

Silicon and germanium compounds are an important class of organic compounds with a wide range of applications in synthesis leading to products ranging from things we handle in a day-to-day life at household to spacecrafts in the lower earth orbitals [2-6].

Organosilicon compounds have found widespread applications in materials science, synthetic chemistry, and pharmaceutical synthesis and some of the important applications include the following. Organosilanes and siloxanes are commonly used to modulate the hydrophobicity and surface properties of materials [7,8]. Side-chain liquid-crystalline siloxanes are utilized in the manufacturing of electronic device displays [9]. Organofunctionalized silicates can be used to immobilize peptides and as supports for metal complex catalysts [10,11]. Organosilanes serve as coupling agents between organic polymers and silicate materials, leading to applications in dental composites [12] and adhesives [13]. They are also used as precursors for silicon carbide ceramics [14]. In synthetic chemistry, organosilicon reagents enable mild, selective, and functional group-tolerant transformations. They are widely used as protective groups in organic synthesis [15]. Additionally, they are also used in stereospecific oxidations and carbon-carbon bond formations like cross-couplings, olefinations, and allylations [16-18].

While organogermanium chemistry and applications have been less explored compared to silicon [5,19,20], germanium compounds have found utility in medicinal and catalytic roles. Certain germanium compounds exhibit promising bioactivities or medicinal properties. [21] Germylated steroids have displayed potential as antitumor and antiviral agents [22]. The organogermanium compounds, Asaigermanium and propagermanium have been reported as pharmaceutical drugs for use in Japan [21]. Beyond medicine, organogermanes also hold promise as catalysts in the emerging trend of main group based catalysts. Germanium compounds have been studied as catalysts for the polymerization of lactide monomers [23]. Additionally, heavier Ge(II) species can serve as catalysts [24] for cyanosilylation [25] and hydroboration of carbonyl compounds [26], hydroboration of CO₂ [27],
oxidative coupling catalysts for silane functionalization using aldehydes [28]. They are also good ligands to various transition metals [29].

An efficient transition metal-based catalyst for synthesising useful silicon and germanium compounds at ambient reaction conditions is highly desirable. A transition metal based catalyst can selectively activate targeted bonds of the substrates and form new bonds between them leading to desired products with very high efficiency. Hydrometallation reaction in which a hydrogen-silicon/germanium bond is added to C-X multiple bonds such as alkynes, alkenes, and nitriles is one of the best methods to synthesise these organosilicon/germanium compounds using simpler hydrosilanes and germanes [30]. The selective monohydrometallation reaction of hydrosilanes or hydrogermanes with more than one hydrogen can lead to the synthesis of organosilicon/germanium compounds of a higher value that still have at least one Si-H or Ge-H bond, which can be further used as building blocks in synthetic chemistry. Understanding the bonding between the E-H bond of R₂EH₂ (E = Si, Ge, R = alkyl, aryl) and the transition metal centre is essential for developing a better homogenous catalyst for hydrometallation reactions.

The second part details an in-depth spectroscopic and structural analysis of the coordination and activation of Si-H bonds of diphenylsilane at a ruthenium bis-dihydrogen dihydride complex, [RuH₂(H₂)₂(PCy₃)₂] (1). A new reactivity leads to the formation of new ruthenium σ-silane complexes such as [Ru(H)₂(η²-H₂)(η²-HSiHPh₂)(PCy₃)₂] (2), followed by [Ru(H)₂(η³-H₂SiPh₂)(PCy₃)₂] (3) by the sequential displacement of the H₂ ligands of the precursor complex is discussed. This reveals unique bonding interactions and provides greater clarity on secondary silane coordination to the ruthenium metal centre.

Hydrosilanes, compounds having a silicon-hydrogen bond, can react with transition metals leading to the formation of different transition metal-silicon products [31]. The interaction of the Si-H bond with a coordinately unsaturated transition metal can vary from weak interaction to complete oxidative addition. The weak interaction or σ-Si-H coordination can be represented as a three-centred two-electron bond between the transition metal, silicon, and hydrogen atoms. The complete oxidative addition of the Si-H bond to the metal centre can lead the transition metal to classical bonding interactions with the hydrogen and the silicon. The type of coordination can be based on the degree of activation of the Si-H bond at the transition metal centre. The Si-H bond activation can be represented as a continuum between the σ-Si-H complex and the oxidative addition product [32]. Hence the formation of σ-Si-H complex is considered as a result of incomplete Si-H bond activation by an electrophilic metal centre [33]. They are usually represented as shown in Scheme 68.

<figure><img src="image_304.png" /><figcaption>**Scheme 68.** E-H bond activation at transition metal centre; a continuum between the σ-complex and the oxidative addition product.</figcaption></figure>

Compared to the most widely explored σ-dihydrogen ligand, σ-silane complexes exhibit an asymmetry in electronegativity, size, and coordination mode, which makes the Si-H bond more basic than the H-H bond. The silane σ-coordination can be explained by the σ-donor property of the Si-H bond and, at the same time, its better ability to accept π electrons to the σ*(Si-H) orbital. Hence the
oxidative coupling catalysts for silane functionalization using aldehydes [28]. They are also good ligands to various transition metals [29].

An efficient transition metal-based catalyst for synthesising useful silicon and germanium compounds at ambient reaction conditions is highly desirable. A transition metal based catalyst can selectively activate targeted bonds of the substrates and form new bonds between them leading to desired products with very high efficiency. Hydrometallation reaction in which a hydrogen-silicon/germanium bond is added to C-X multiple bonds such as alkynes, alkenes, and nitriles is one of the best methods to synthesise these organosilicon/germanium compounds using simpler hydrosilanes and germanes [30]. The selective monohydrometallation reaction of hydrosilanes or hydrogermanes with more than one hydrogen can lead to the synthesis of organosilicon/germanium compounds of a higher value that still have at least one Si-H or Ge-H bond, which can be further used as building blocks in synthetic chemistry. Understanding the bonding between the E-H bond of R₂EH₂ (E = Si, Ge, R = alkyl, aryl) and the transition metal centre is essential for developing a better homogenous catalyst for hydrometallation reactions.

The second part details an in-depth spectroscopic and structural analysis of the coordination and activation of Si-H bonds of diphenylsilane at a ruthenium bis-dihydrogen dihydride complex, [RuH₂(H₂)₂(PCy₃)₂] (1). A new reactivity leads to the formation of new ruthenium σ-silane complexes such as [Ru(H)₂(η²-H₂)(η²-HSiHPh₂)(PCy₃)₂] (2), followed by [Ru(H)₂(η³-H₂SiPh₂)(PCy₃)₂] (3) by the sequential displacement of the H₂ ligands of the precursor complex is discussed. This reveals unique bonding interactions and provides greater clarity on secondary silane coordination to the ruthenium metal centre.

Hydrosilanes, compounds having a silicon-hydrogen bond, can react with transition metals leading to the formation of different transition metal-silicon products [31]. The interaction of the Si-H bond with a coordinately unsaturated transition metal can vary from weak interaction to complete oxidative addition. The weak interaction or σ-Si-H coordination can be represented as a three-centred two-electron bond between the transition metal, silicon, and hydrogen atoms. The complete oxidative addition of the Si-H bond to the metal centre can lead the transition metal to classical bonding interactions with the hydrogen and the silicon. The type of coordination can be based on the degree of activation of the Si-H bond at the transition metal centre. The Si-H bond activation can be represented as a continuum between the σ-Si-H complex and the oxidative addition product [32]. Hence the formation of σ-Si-H complex is considered as a result of incomplete Si-H bond activation by an electrophilic metal centre [33]. They are usually represented as shown in Scheme 68.

<figure><img src="image_301.png" /><figcaption>**Scheme 68.** E-H bond activation at transition metal centre; a continuum between the σ-complex and the oxidative addition product.</figcaption></figure>

Compared to the most widely explored σ-dihydrogen ligand, σ-silane complexes exhibit an asymmetry in electronegativity, size, and coordination mode, which makes the Si-H bond more basic than the H-H bond. The silane σ-coordination can be explained by the σ-donor property of the Si-H bond and, at the same time, its better ability to accept π electrons to the σ*(Si-H) orbital. Hence the
silane σ-coordination is more favoured when compared to the σ-dihydrogen analogue [33]. This has enabled the synthesis of various σ-silane complexes from dihydrogen complexes [34,35].

The coordination of silicon-hydrogen bonds to ruthenium metal centre is interesting because of the silicon coordinated complexes has already proved its potential to steer various catalytic hydrosilylation reactions. The ruthenium poly hydride complex [RuH₂(H₂)₂(PCy₃)₂] (1) being a good candidate complex that has marked its remarkable ability to activate various S-H bonds at ambient conditions, was chosen as a candidate to study the coordination of secondary silanes to ruthenium metal centre. Previous studies on the coordination of secondary and tertiary silanes led to the redistribution reaction of silane and the in-depth knowledge on the coordination of silicon compounds with more than one hydride to ruthenium complex 1 have been lacking up to now. In this study, an investigation of the reaction between diphenylsilane and the ruthenium complex [RuH₂(H₂)₂(PCy₃)₂] under controlled, low temperature conditions has been carried out. The reaction of the ruthenium bis-dihydrogen complex 1 with diphenylsilane was investigated using NMR spectroscopic monitoring, X-ray crystallography and DFT calculations. The step-by-step pathway for silane coordination and activation has been studied using variable temperature NMR. The reaction was found to coordinate silane Si-H bonds to the ruthenium metal centre via σ-bonds. It led to the formation of new ruthenium σ-silane complexes such as [Ru(H)₂(η²-H₂)(η²-HSiHPh₂)(PCy₃)₂] (2), followed by [Ru(H)₂(η³-H₂SiPh₂)(PCy₃)₂] (3) by the sequential displacement of the H₂ ligands of the precursor complex.

The addition of diphenylsilane to a THF solution of 1 at 233 K, followed by slow warming to room temperature, resulted in the formation of σ-Si-H complexes [Ru(H)₂(η²-H₂)(η²-HSiHPh₂)(PCy₃)₂], 2, and [Ru(H)₂(η³-H₂SiPh₂)(PCy₃)₂], 3 (Scheme 69).

<figure><img src="image_305.png" /><figcaption>**Scheme 69.** Reversible coordination of Ph₂SiH₂ to complex 1 to form complexes 2 and 3.</figcaption></figure>

The molecular structures of 2 and 3 were established by detailed NMR spectroscopy experiments. Complex 3 was subsequently isolated and its structure was also confirmed by single crystal X-ray diffraction. Crystals of 3 suitable for study by single crystal X-ray diffraction were obtained by the slow cooling of a pentane-THF solution of 3 to 233 K. The ORTEP view of the molecular structure obtained by X-ray diffraction analysis of 3 is shown in Figure 1.

DFT calculations assisted to confirm the spectroscopic and X-ray elucidated molecular structures. The new reactivities and description of binding modes observed have expanded current knowledge of secondary silane interactions with ruthenium. It is important to note that the stepwise dehydrogenation of the dihydrogen ligands with the coordination of each of the Si-H bond can be utilized for selective catalytic reactions. The fundamental insights gained from this reactivity and coordination study can guide future development of silane coordinated transition metal systems for the development of selective catalysts.
silane σ-coordination is more favoured when compared to the σ-dihydrogen analogue [33]. This has enabled the synthesis of various σ-silane complexes from dihydrogen complexes [34,35].

The coordination of silicon-hydrogen bonds to ruthenium metal centre is interesting because of the silicon coordinated complexes has already proved its potential to steer various catalytic hydrosilylation reactions. The ruthenium poly hydride complex [RuH₂(H₂)₂(PCy₃)₂] (1) being a good candidate complex that has marked its remarkable ability to activate various S-H bonds at ambient conditions, was chosen as a candidate to study the coordination of secondary silanes to ruthenium metal centre. Previous studies on the coordination of secondary and tertiary silanes led to the redistribution reaction of silane and the in-depth knowledge on the coordination of silicon compounds with more than one hydride to ruthenium complex 1 have been lacking up to now. In this study, an investigation of the reaction between diphenylsilane and the ruthenium complex [RuH₂(H₂)₂(PCy₃)₂] under controlled, low temperature conditions has been carried out. The reaction of the ruthenium bis-dihydrogen complex 1 with diphenylsilane was investigated using NMR spectroscopic monitoring, X-ray crystallography and DFT calculations. The step-by-step pathway for silane coordination and activation has been studied using variable temperature NMR. The reaction was found to coordinate silane Si-H bonds to the ruthenium metal centre via σ-bonds. It led to the formation of new ruthenium σ-silane complexes such as [Ru(H)₂(η²-H₂)(η²-HSiHPh₂)(PCy₃)₂] (2), followed by [Ru(H)₂(η³-H₂SiPh₂)(PCy₃)₂] (3) by the sequential displacement of the H₂ ligands of the precursor complex.

The addition of diphenylsilane to a THF solution of 1 at 233 K, followed by slow warming to room temperature, resulted in the formation of σ-Si-H complexes [Ru(H)₂(η²-H₂)(η²-HSiHPh₂)(PCy₃)₂], 2, and [Ru(H)₂(η³-H₂SiPh₂)(PCy₃)₂], 3 (Scheme 69).

<figure><img src="image_302.png" /><figcaption>**Scheme 69.** Reversible coordination of Ph₂SiH₂ to complex 1 to form complexes 2 and 3.</figcaption></figure>

The molecular structures of 2 and 3 were established by detailed NMR spectroscopy experiments. Complex 3 was subsequently isolated and its structure was also confirmed by single crystal X-ray diffraction. Crystals of 3 suitable for study by single crystal X-ray diffraction were obtained by the slow cooling of a pentane-THF solution of 3 to 233 K. The ORTEP view of the molecular structure obtained by X-ray diffraction analysis of 3 is shown in Figure 1.

DFT calculations assisted to confirm the spectroscopic and X-ray elucidated molecular structures. The new reactivities and description of binding modes observed have expanded current knowledge of secondary silane interactions with ruthenium. It is important to note that the stepwise dehydrogenation of the dihydrogen ligands with the coordination of each of the Si-H bond can be utilized for selective catalytic reactions. The fundamental insights gained from this reactivity and coordination study can guide future development of silane coordinated transition metal systems for the development of selective catalysts.
<figure><img src="image_306.png" /><figcaption>**Figure 30.** X-ray molecular structure of complex 3, [RuH₂(Ru²-H₂SiPh₂)(PCy₃)₂] drawn at 50% probability as ellipsoids (PCy₃ and phenyl groups are shown in wireframe for clarity).</figcaption></figure>

The third part demonstrates the efficacy of the ruthenium *bis*-dihydrogen dihydride complex for selective monohydrosilylation of nitriles to *N*-silylimines. A range of nitriles and silanes are used in the catalytic scope study, and a close look at reactivities between silane, nitrile, and ruthenium centres is discussed. The identification and structural characterisation of a new ruthenium *σ*-(*N*-silylimine complex (4), along with a possible new mechanism involving this complex, is described. Mechanistic investigations provide critical insights into the activation of the Si–H and C≡N bonds.

The mechanism and role of ruthenium *σ*-complexes in catalytic hydrosilylation of nitriles were evaluated, leading to the identification of complex **1** as a highly efficient catalyst for monohydrosilylation of nitriles using secondary silanes. This reaction exhibited exclusive selectivity for the formation of *N*-silylimines. A comprehensive understanding of the interactions between the nitrile group and the silicon and hydrogen atoms of the silane was developed.

Catalytic hydrosilylation of nitriles with a series of secondary silanes and nitriles was observed with 2.5 mol% of **1** (Table 9). The results showed that the catalyst was found to be highly active and efficient, achieving near-complete conversion and excellent yield for the desired reaction at room temperature. The complete conversion was typically observed in 15 minutes with a maximum turnover frequency (TOF) of 400 and an impressive 99% yield (Table 9, entry 3), highlighting the extraordinary effectiveness of **1** in catalytic hydrosilylation reaction.

The detailed mechanistic and spectroscopic data show how the initial addition of the nitrile to the complex occurs at silicon rather than ruthenium and provide pathways for C–H and N–Si bond formation, leading to the selective formation of *N*-silylimines. The direct detection of *N*-silylimine bound to the ruthenium also demonstrates the synergy between the transition metal and main centres in the formation of the new Si–N and C–H bonds. To gain a deeper understanding of the reaction mechanism, selected stoichiometric reactions were conducted under controlled temperature conditions, and key ruthenium *σ*-complex involved in the catalytic reaction was identified.
<figure><img src="image_303.png" /><figcaption>**Figure 30.** X-ray molecular structure of complex 3, [RuH₂(Ru²-H₂SiPh₂)(PCy₃)₂] drawn at 50% probability as ellipsoids (PCy₃ and phenyl groups are shown in wireframe for clarity).</figcaption></figure>

The third part demonstrates the efficacy of the ruthenium *bis*-dihydrogen dihydride complex for selective monohydrosilylation of nitriles to *N*-silylimines. A range of nitriles and silanes are used in the catalytic scope study, and a close look at reactivities between silane, nitrile, and ruthenium centres is discussed. The identification and structural characterisation of a new ruthenium *σ*-(*N*-silylimine complex (4), along with a possible new mechanism involving this complex, is described. Mechanistic investigations provide critical insights into the activation of the Si–H and C≡N bonds.

The mechanism and role of ruthenium *σ*-complexes in catalytic hydrosilylation of nitriles were evaluated, leading to the identification of complex 1 as a highly efficient catalyst for monohydrosilylation of nitriles using secondary silanes. This reaction exhibited exclusive selectivity for the formation of *N*-silylimines. A comprehensive understanding of the interactions between the nitrile group and the silicon and hydrogen atoms of the silane was developed.

Catalytic hydrosilylation of nitriles with a series of secondary silanes and nitriles was observed with 2.5 mol% of 1 (Table 9). The results showed that the catalyst was found to be highly active and efficient, achieving near-complete conversion and excellent yield for the desired reaction at room temperature. The complete conversion was typically observed in 15 minutes with a maximum turnover frequency (TOF) of 400 and an impressive 99% yield (Table 9, entry 3), highlighting the extraordinary effectiveness of 1 in catalytic hydrosilylation reaction.

The detailed mechanistic and spectroscopic data show how the initial addition of the nitrile to the complex occurs at silicon rather than ruthenium and provide pathways for C–H and N–Si bond formation, leading to the selective formation of *N*-silylimines. The direct detection of *N*-silylimine bound to the ruthenium also demonstrates the synergy between the transition metal and main centres in the formation of the new Si–N and C–H bonds. To gain a deeper understanding of the reaction mechanism, selected stoichiometric reactions were conducted under controlled temperature conditions, and key ruthenium *σ*-complex involved in the catalytic reaction was identified.
<figure><img src="image_307.png" /><figcaption>**Table 9.** Catalytic hydrosilylation of nitriles with secondary silanes.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Nitrile</th><th>Silane</th><th>TON</th><th>TOF [h<sup>-1</sup>]<sup>[b]</sup></th><th>Conversion<sup>[c]</sup><br>(min)</th><th>Yield<sup>[d]</sup><br>(%)</th><th>Product</th></tr></thead><tbody><tr><td>1</td><td>PhCN</td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>185<br>(100%)</td><td>&gt;99%<br>(13)</td><td>96%</td><td></td></tr><tr><td>2</td><td>PhCN</td><td>Et<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>171<br>(100%)</td><td>&gt;99%<br>(14)</td><td>97%</td><td></td></tr><tr><td>3</td><td>PhCN</td><td>PhMeSiH<sub>2</sub></td><td>40</td><td>400 (100%)</td><td>&gt;99%<br>(6)</td><td>87%</td><td></td></tr><tr><td>4</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>240 (100%)</td><td>&gt;99%<br>(10)</td><td>89%</td><td></td></tr><tr><td>5</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>240 (100%)</td><td>&gt;99%<br>(10)</td><td>96%</td><td></td></tr><tr><td>6</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>38</td><td>53<br>(44.5%)</td><td>&gt;99%<br>(150)</td><td>85%</td><td></td></tr><tr><td>7</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>200 (100%)</td><td>&gt;99%<br>(12)</td><td>86%</td><td></td></tr><tr><td>8</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>160 (100%)</td><td>&gt;99%<br>(15)</td><td>99%</td><td></td></tr><tr><td>9</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>160 (100%)</td><td>&gt;99%<br>(15)</td><td>89%</td><td></td></tr><tr><td>10</td><td>PhCN</td><td>2 Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>160<br>(100%)</td><td>&gt;99%<br>(15)</td><td>100%</td><td>No further reaction after<br>heating 60 °C for 1 day.</td></tr><tr><td>11</td><td>CH<sub>3</sub>CN</td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>0</td><td>0</td><td>0</td><td>0</td><td>No catalytic reaction</td></tr></tbody></table>

[a] General procedure – 2.5 mol % of [RuH<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>)<sub>2</sub>(PCy<sub>3</sub>)<sub>2</sub>] was added to a solution of nitrile and silane in C<sub>6</sub>D<sub>6</sub>.

[b] Calculated at the conversion shown in parentheses. [c] Calculated from <sup>1</sup>H NMR data. [d] Yield is determined by <sup>1</sup>H NMR.
<figure><img src="image_304.png" /><figcaption>**Table 9.** Catalytic hydrosilylation of nitriles with secondary silanes.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Nitrile</th><th>Silane</th><th>TON</th><th>TOF [h<sup>-1</sup>]<sup>[b]</sup></th><th>Conversion<sup>[c]</sup><br>(min)</th><th>Yield<sup>[d]</sup><br>(%)</th><th>Product</th></tr></thead><tbody><tr><td>1</td><td>PhCN</td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>185<br>(100%)</td><td>&gt;99%<br>(13)</td><td>96%</td><td></td></tr><tr><td>2</td><td>PhCN</td><td>Et<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>171<br>(100%)</td><td>&gt;99%<br>(14)</td><td>97%</td><td></td></tr><tr><td>3</td><td>PhCN</td><td>PhMeSiH<sub>2</sub></td><td>40</td><td>400 (100%)</td><td>&gt;99%<br>(6)</td><td>87%</td><td></td></tr><tr><td>4</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>240 (100%)</td><td>&gt;99%<br>(10)</td><td>89%</td><td></td></tr><tr><td>5</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>240 (100%)</td><td>&gt;99%<br>(10)</td><td>96%</td><td></td></tr><tr><td>6</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>38</td><td>53<br>(44.5%)</td><td>&gt;99%<br>(150)</td><td>85%</td><td></td></tr><tr><td>7</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>200 (100%)</td><td>&gt;99%<br>(12)</td><td>86%</td><td></td></tr><tr><td>8</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>160 (100%)</td><td>&gt;99%<br>(15)</td><td>99%</td><td></td></tr><tr><td>9</td><td></td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>160 (100%)</td><td>&gt;99%<br>(15)</td><td>89%</td><td></td></tr><tr><td>10</td><td>PhCN</td><td>2 Ph<sub>2</sub>SiH<sub>2</sub></td><td>40</td><td>160<br>(100%)</td><td>&gt;99%<br>(15)</td><td>100%</td><td>No further reaction after<br>heating 60 °C for 1 day.</td></tr><tr><td>11</td><td>CH<sub>3</sub>CN</td><td>Ph<sub>2</sub>SiH<sub>2</sub></td><td>0</td><td>0</td><td>0</td><td>0</td><td>No catalytic reaction</td></tr></tbody></table>

[a] General procedure – 2.5 mol % of [RuH<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>)<sub>2</sub>(PCy<sub>3</sub>)<sub>2</sub>] was added to a solution of nitrile and silane in C<sub>6</sub>D<sub>6</sub>.

[b] Calculated at the conversion shown in parentheses. [c] Calculated from <sup>1</sup>H NMR data. [d] Yield is determined by <sup>1</sup>H NMR.
The reaction of the ruthenium bis-σ-Si-H complex 3 with benzonitrile gives a quantitative
formation of a ruthenium σ-N-silylimine complex 4. Complex 4 stands as the first example for the direct
evidence of N-silylimine coordinating to ruthenium metal centre. The molecular structure of 4 were
established by detailed NMR spectroscopy experiments, and the compound was isolated and
characterised by X-ray diffraction.

The stoichiometric reaction of ruthenium *bis* σ-Si–H complex **3** with an equivalent of benzonitrile was thoroughly examined, with a specific emphasis on the potential for the addition of the activated Si–H bond to the benzonitrile. This reaction was of great interest due to the prospect of new Si–N and C–H bond formation. It was found that the reaction led to the formation of a new ruthenium complex, designated as **4**, which contains an *N*-silylimine ligand resulting from the selective addition of only one Si–H bond of diphenylsilane to the nitrile group (Scheme 70).

<figure><img src="image_308.png" /><figcaption>**Scheme 70.** Reactivity of complex **3** with benzonitrile.</figcaption></figure>

The molecular structure of **4** in solid state was determined by single crystal X-ray diffraction
analysis (Figure 31), which demonstrated the unique coordination mode of *N*-silylimine to the
ruthenium metal centre. Specifically, the *N*-silylimine moiety coordinates to the ruthenium atom via
an η²-Si-H bond and the nitrogen atom, resulting in a pseudo-octahedral environment and is located
in a plane along with Si, N, and hydrogen atoms.

<figure><img src="image_309.png" /><figcaption>**Figure 31.** X-ray molecular structure of complex 4 [RuH₂(η²-HSiPh₂NCHPh(PCy₃)₂] drawn at 50% probability as ellipsoids (PCy₃ and phenyl groups are shown in wireframe for clarity).</figcaption></figure>

Complex **4** features a unique σ-Si-H interaction alongside the nitrogen atom of the imine
coordination to the ruthenium centre. The successful isolation of complexes **3** and **4** and the
demonstration of their active participation in catalysis helped to understand the mechanism of the
catalytic reaction better. Two possible mechanisms were studied using DFT calculations, and along
with the experimental pieces of evidence, a favourable mechanism developed using DFT is proposed.
The ruthenium-based system of σ-Si-H complexes has proved its competence for the challenging
The reaction of the ruthenium bis-σ-Si-H complex 3 with benzonitrile gives a quantitative formation of a ruthenium σ-N-silylimine complex 4. Complex 4 stands as the first example for the direct evidence of N-silylimine coordinating to ruthenium metal centre. The molecular structure of 4 were established by detailed NMR spectroscopy experiments, and the compound was isolated and characterised by X-ray diffraction.

The stoichiometric reaction of ruthenium bis σ-Si-H complex 3 with an equivalent of benzonitrile was thoroughly examined, with a specific emphasis on the potential for the addition of the activated Si-H bond to the benzonitrile. This reaction was of great interest due to the prospect of new Si-N and C-H bond formation. It was found that the reaction led to the formation of a new ruthenium complex, designated as 4, which contains an N-silylimine ligand resulting from the selective addition of only one Si-H bond of diphenylsilane to the nitrile group (Scheme 70).

<figure><img src="image_305.png" /><figcaption>**Scheme 70.** Reactivity of complex 3 with benzonitrile.</figcaption></figure>

The molecular structure of 4 in solid state was determined by single crystal X-ray diffraction analysis (Figure 31), which demonstrated the unique coordination mode of N-silylimine to the ruthenium metal centre. Specifically, the N-silylimine moiety coordinates to the ruthenium atom via an η²-Si-H bond and the nitrogen atom, resulting in a pseudo-octahedral environment and is located in a plane along with Si, N, and hydrogen atoms.

<figure><img src="image_306.png" /><figcaption>**Figure 31.** X-ray molecular structure of complex 4 [RuH₂(η²-HSiPh₂NCHPh(PCy₃)₂] drawn at 50% probability as ellipsoids (PCy₃ and phenyl groups are shown in wireframe for clarity).</figcaption></figure>

Complex 4 features a unique σ-Si-H interaction alongside the nitrogen atom of the imine coordination to the ruthenium centre. The successful isolation of complexes 3 and 4 and the demonstration of their active participation in catalysis helped to understand the mechanism of the catalytic reaction better. Two possible mechanisms were studied using DFT calculations, and along with the experimental pieces of evidence, a favourable mechanism developed using DFT is proposed. The ruthenium-based system of σ-Si-H complexes has proved its competence for the challenging
transformation of nitriles to an imine level selectively but also enables low-energy pathways for Si–H bond cleavage and C–H bond formation.

<figure><img src="image_310.png" /><figcaption>**Scheme 71.** DFT calculated the steps of the catalytic hydrosilylation process at 298K, involving the nitrile coordination to the silicon centre.</figcaption></figure>

Based on the experimental and DFT observations, we propose an overall representation of the pathway of the catalytic reaction, which involves the coordination of the diphenylsilane before the coordination of the nitrile and, thus, the catalytic cycle (Scheme 72).

<figure><img src="image_311.png" /><figcaption>**Scheme 72.** Representation of the overall steps of the catalytic hydrosilylation process.</figcaption></figure>
transformation of nitriles to an imine level selectively but also enables low-energy pathways for Si–H bond cleavage and C–H bond formation.

<figure><img src="image_307.png" /><figcaption>**Scheme 71.** DFT calculated the steps of the catalytic hydrosilylation process at 298K, involving the nitrile coordination to the silicon centre.</figcaption></figure>

Based on the experimental and DFT observations, we propose an overall representation of the pathway of the catalytic reaction, which involves the coordination of the diphenylsilane before the coordination of the nitrile and, thus, the catalytic cycle (Scheme 72).

<figure><img src="image_308.png" /><figcaption>**Scheme 72.** Representation of the overall steps of the catalytic hydrosilylation process.</figcaption></figure>
The fourth part examines the reactivity of the ruthenium bis-σ-silane complex (3) with amines and alcohols. The new reactivity study of complex 3 with benzylamine and benzylalcohol disclosed new ruthenium amino (5) and alkoxy (6) functionalised silane complexes. The role of these reactivity studies that led to a highly efficient ruthenium-catalysed dehydrogenative coupling reaction of silanes with amine and alcohol is explained.

In summary, this work has conducted an in-depth investigation into the reactivity of the silane coordinated complex 3 with benzylamine and benzylalcohol under controlled reaction conditions. Through extensive NMR spectroscopic analysis at low temperature conditions and structural elucidation, the coordination of these substrates to the silicon centre has been explained. This has provided a better mechanistic insight into the dehydrogenative coupling reaction of silane with amine or alcohol. The molecular structures of new ruthenium complexes 5 and 6 formed after the reaction with amine and alcohol have been characterized using multinuclear 1D and 2D NMR spectroscopic techniques.

<figure><img src="image_312.png" /><figcaption>**Scheme 73.** Reactivity of benzylamine with $[Ru(H)_2(\eta^3-H_2SiPh_2)(PCy_3)_2]$ to form new aminosilane coordinated ruthenium complex.</figcaption></figure>

<figure><img src="image_313.png" /><figcaption>**Scheme 74.** Reactivity of benzylalcohol with $[Ru(H)_2(\eta^3-H_2SiPh_2)(PCy_3)_2]$ to form new alkoxysilane coordinated ruthenium complex.</figcaption></figure>

Leveraging the understanding of this new fundamental reactivity, the $[Ru(H)_2(H_2)(PCy_3)_2]$ complex has been proved to be an efficient pre-catalyst for the dehydrogenative coupling of silanes with amines and alcohols. The catalytic products have been thoroughly characterized by NMR spectroscopy, confirming the formation of new Si–N and Si–O bonds. By systematically investigating the stepwise reactivity of silanes, amines, and alcohols at the ruthenium centre, more insights into the mechanism of the catalytic dehydrogenative coupling reaction was developed which helped to propose a mechanism for this catalytic reaction. Overall, this study has opened a new strategy and methodology to synthesise new organosilicon compounds with Si–N and Si–O bonds efficiently at room temperature conditions.

<figure><img src="image_314.png" /><figcaption>**Scheme 75.** Catalytic dehydrogenative coupling reaction of benzylamine and $Ph_2SiH_2$.</figcaption></figure>
The fourth part examines the reactivity of the ruthenium bis-σ-silane complex (3) with amines and alcohols. The new reactivity study of complex 3 with benzylamine and benzylalcohol disclosed new ruthenium amino (5) and alkoxy (6) functionalised silane complexes. The role of these reactivity studies that led to a highly efficient ruthenium-catalysed dehydrogenative coupling reaction of silanes with amine and alcohol is explained.

In summary, this work has conducted an in-depth investigation into the reactivity of the silane coordinated complex 3 with benzylamine and benzylalcohol under controlled reaction conditions. Through extensive NMR spectroscopic analysis at low temperature conditions and structural elucidation, the coordination of these substrates to the silicon centre has been explained. This has provided a better mechanistic insight into the dehydrogenative coupling reaction of silane with amine or alcohol. The molecular structures of new ruthenium complexes 5 and 6 formed after the reaction with amine and alcohol have been characterized using multinuclear 1D and 2D NMR spectroscopic techniques.

<figure><img src="image_309.png" /><figcaption>**Scheme 73.** Reactivity of benzylamine with $[Ru(H)_2(\eta^3-H_2SiPh_2)(PCy_3)_2]$ to form new aminosilane coordinated ruthenium complex.</figcaption></figure>

<figure><img src="image_310.png" /><figcaption>**Scheme 74.** Reactivity of benzylalcohol with $[Ru(H)_2(\eta^3-H_2SiPh_2)(PCy_3)_2]$ to form new alkoxysilane coordinated ruthenium complex.</figcaption></figure>

Leveraging the understanding of this new fundamental reactivity, the $[Ru(H)_2(H_2)(PCy_3)_2]$ complex has been proved to be an efficient pre-catalyst for the dehydrogenative coupling of silanes with amines and alcohols. The catalytic products have been thoroughly characterized by NMR spectroscopy, confirming the formation of new Si–N and Si–O bonds. By systematically investigating the stepwise reactivity of silanes, amines, and alcohols at the ruthenium centre, more insights into the mechanism of the catalytic dehydrogenative coupling reaction was developed which helped to propose a mechanism for this catalytic reaction. Overall, this study has opened a new strategy and methodology to synthesise new organosilicon compounds with Si–N and Si–O bonds efficiently at room temperature conditions.

<figure><img src="image_311.png" /><figcaption>**Scheme 75.** Catalytic dehydrogenative coupling reaction of benzylamine and $Ph_2SiH_2$.</figcaption></figure>
<figure><img src="image_315.png" /><figcaption>**Scheme 76.** Catalytic dehydrogenative coupling reaction of benzylalcohol and Ph₂SiH₂.</figcaption></figure>

These stoichiometric reactions with benzylamine and alcohol demonstrated the electrophilic nature of silicon in the ruthenium bis-σ-Si-H complex **3**. The similar results and spectroscopic signals across the series of these new ruthenium complexes **5** and **6** implies they have analogous molecular structure and probably similar mechanism for catalysis. Based on the above discussed reactivity studies between the ruthenium precursor, silane, amine/alcohol substrates, a mechanistic pathway for the dehydrogenative coupling catalysis is proposed as shown below in Scheme 77.

<figure><img src="image_316.png" /><figcaption>**Scheme 77.** Proposed mechanism for catalytic dehydrogenative coupling of silanes with amines and alcohols using [Ru(H₂)(H₂)₂(PCy₃)₂] complex.</figcaption></figure>

The fifth part details the isolation and reactivity studies of a ruthenium germylene complex [Ru(=GePh₂)(H)₂(H₂)(PCy₃)₂] (8). The germylene complex **8** is shown to undergo a substitution reaction with amines and alcohols, elucidating new amino and alkoxy germylene complexes (9-13). The structure of new functionalised germylene complexes is elucidated with the help of NMR spectroscopy, X-ray diffraction and DFT calculation. This study expands the coordination chemistry of ruthenium-germylene species.

This work has undertaken an extensive investigation into the reactivity study of the ruthenium germylene complex [Ru(=GePh₂)(H)₂(H₂)(PCy₃)₂] (8) formed from [RuH₂(H₂)₂(PCy₃)₂] and diphenylgermane. Through a scaled-up synthesis and isolation of this key intermediate complex, new reactivity studies of this complex with benzylamine and benzylalcohol have been conducted. The reaction was closely monitored using low-temperature NMR spectroscopic techniques. The new ruthenium germylene complexes (9-13) have been characterized by NMR spectroscopy. The molecular structure of aminogermylene complex **9** has been obtained by X-ray analysis.
<figure><img src="image_312.png" /><figcaption>**Scheme 76.** Catalytic dehydrogenative coupling reaction of benzylalcohol and Ph₂SiH₂.</figcaption></figure>

These stoichiometric reactions with benzylamine and alcohol demonstrated the electrophilic nature of silicon in the ruthenium bis-σ-Si-H complex **3**. The similar results and spectroscopic signals across the series of these new ruthenium complexes **5** and **6** implies they have analogous molecular structure and probably similar mechanism for catalysis. Based on the above discussed reactivity studies between the ruthenium precursor, silane, amine/alcohol substrates, a mechanistic pathway for the dehydrogenative coupling catalysis is proposed as shown below in Scheme 77.

<figure><img src="image_313.png" /><figcaption>**Scheme 77.** Proposed mechanism for catalytic dehydrogenative coupling of silanes with amines and alcohols using [Ru(H₂)(H₂)₂(PCy₃)₂] complex.</figcaption></figure>

The fifth part details the isolation and reactivity studies of a ruthenium germylene complex [Ru(=GePh₂)(H)₂(H₂)(PCy₃)₂] (8). The germylene complex **8** is shown to undergo a substitution reaction with amines and alcohols, elucidating new amino and alkoxy germylene complexes (9-13). The structure of new functionalised germylene complexes is elucidated with the help of NMR spectroscopy, X-ray diffraction and DFT calculation. This study expands the coordination chemistry of ruthenium-germylene species.

This work has undertaken an extensive investigation into the reactivity study of the ruthenium germylene complex [Ru(=GePh₂)(H)₂(H₂)(PCy₃)₂] (8) formed from [RuH₂(H₂)₂(PCy₃)₂] and diphenylgermane. Through a scaled-up synthesis and isolation of this key intermediate complex, new reactivity studies of this complex with benzylamine and benzylalcohol have been conducted. The reaction was closely monitored using low-temperature NMR spectroscopic techniques. The new ruthenium germylene complexes (9-13) have been characterized by NMR spectroscopy. The molecular structure of aminogermylene complex **9** has been obtained by X-ray analysis.
<figure><img src="image_317.png" /><figcaption>**Scheme 78.** Synthesis of ruthenium aminogermylene complex.</figcaption></figure>

<figure><img src="image_318.png" /><figcaption>**Figure.32.** X-ray structure of aminogermylene complex 9.</figcaption></figure>

The Ru–Ge bond length of 2.347 Å along with DFT calculations confirmed the Ru–Ge double bond character, a ruthenium germylene type of coordination. Low $T_1$ relaxation time values confirmed the presence of dihydrogen ligand in this complex. DFT calculations were performed to figure out the possible intermediates and transition states formed during the reaction between benzylamine and the ruthenium germylene complex. A possible mechanism was proposed based on the calculations and the NMR spectroscopic evidence. The reaction was found to proceed through a complex **10** in which benzylamine nitrogen atom is coordinated to the germanium centre.

<figure><img src="image_319.png" /><figcaption>**Scheme 79.** Proposed structure for intermediate complex **10** formed in reaction before aminogermylene 9.</figcaption></figure>
<figure><img src="image_314.png" /><figcaption>**Scheme 78.** Synthesis of ruthenium aminogermylene complex.</figcaption></figure>

<figure><img src="image_315.png" /><figcaption>**Figure.32.** X-ray structure of aminogermylene complex 9.</figcaption></figure>

The Ru–Ge bond length of 2.347 Å along with DFT calculations confirmed the Ru–Ge double bond character, a ruthenium germylene type of coordination. Low $T_1$ relaxation time values confirmed the presence of dihydrogen ligand in this complex. DFT calculations were performed to figure out the possible intermediates and transition states formed during the reaction between benzylamine and the ruthenium germylene complex. A possible mechanism was proposed based on the calculations and the NMR spectroscopic evidence. The reaction was found to proceed through a complex **10** in which benzylamine nitrogen atom is coordinated to the germanium centre.

<figure><img src="image_316.png" /><figcaption>**Scheme 79.** Proposed structure for intermediate complex 10 formed in reaction before aminogermylene 9.</figcaption></figure>
<figure><img src="image_320.png" /><figcaption>**Scheme 80.** Proposed energy profile for the reaction of benzylamine with ruthenium germylene complex. B3PW91 functional, 6-31G\*\* basis set. Pseudo potential SDDALL for Ru and Ge.</figcaption></figure>

It was found that the aminogermylene complex **9** further reacted with an additional equivalent of benzylamine to form the diaminogermylene complex **11**. It was unfortunate that decomposition of the reaction occurred in parallel due to heating reaction conditions, due to an existing competition between the diaminogermylene formation and decomposition reaction. Even though isolation of the product from the reaction mixture containing the liberated byproduct phosphine was not achieved, the diaminogermylene complex **11** was fully characterized using various NMR spectroscopic techniques. This study has successfully demonstrated the stepwise reactivity of the electrophilic germanium centre in **8** with benzylamine to synthesise new amino functionalised ruthenium germylene complexes.

<figure><img src="image_321.png" /><figcaption>**Scheme 81.** Reaction with the second equivalent of benzylamine</figcaption></figure>

The reaction of benzylalcohol with diphenylgermylene was also investigated and resulted in the formation of the new alkoxygermylene complex **12**. Similarly to the amine reactivity, the addition of the second equivalent of benzylalcohol resulted in the complete transformation to the dialkoxygermylene complex **13**. These new ruthenium alkoxy germylene complexes were also characterised by multinuclear NMR spectroscopy. This reactivity further supported the electrophilic nature of the germanium centre in the ruthenium germylene complex.
<figure><img src="image_317.png" /><figcaption>**Scheme 80.** Proposed energy profile for the reaction of benzylamine with ruthenium germylene complex. B3PW91 functional, 6-31G\*\* basis set. Pseudo potential SDDALL for Ru and Ge.</figcaption></figure>

It was found that the aminogermylene complex **9** further reacted with an additional equivalent of benzylamine to form the diaminogermylene complex **11**. It was unfortunate that decomposition of the reaction occurred in parallel due to heating reaction conditions, due to an existing competition between the diaminogermylene formation and decomposition reaction. Even though isolation of the product from the reaction mixture containing the liberated byproduct phosphine was not achieved, the diaminogermylene complex **11** was fully characterized using various NMR spectroscopic techniques. This study has successfully demonstrated the stepwise reactivity of the electrophilic germanium centre in **8** with benzylamine to synthesise new amino functionalised ruthenium germylene complexes.

<figure><img src="image_318.png" /><figcaption>**Scheme 81.** Reaction with the second equivalent of benzylamine</figcaption></figure>

The reaction of benzylalcohol with diphenylgermylene was also investigated and resulted in the formation of the new alkoxygermylene complex **12**. Similarly to the amine reactivity, the addition of the second equivalent of benzylalcohol resulted in the complete transformation to the dialkoxygermylene complex **13**. These new ruthenium alkoxy germylene complexes were also characterised by multinuclear NMR spectroscopy. This reactivity further supported the electrophilic nature of the germanium centre in the ruthenium germylene complex.
<figure><img src="image_322.png" /><figcaption>**Scheme 82.** The reaction of diphenylgermylene with benzylalcohol</figcaption></figure>

<figure><img src="image_323.png" /><figcaption>**Scheme 83.** The reaction of alkoxy-germylene with the second equivalent of benzylalcohol</figcaption></figure>

Overall, this work has significantly expanded the chemistry of ruthenium germanium coordinated complexes which paves new paths for applications of these new germylene complexes in future.

Finally, the sixth part explores the reactivity of cationic ruthenium half-sandwich diphosphine complexes with diphenylgermane. This reveals initial Ge–H bond activation at various coordinatively unsaturated ruthenium complexes, leading to new ruthenium germanium complexes (24, 26-29). A stoichiometric Markonikov hydrogermylation of phenylacetylene was achieved, and the involvement of the new ruthenium germanium complexes and ruthenium vinylidene complexes in the course of the hydrogermylation reaction is described.

This study has successfully investigated the reactivity of GePh₂H₂ with a number of half-sandwich ruthenium diphosphine complexes. To examine the interaction of Ge–H bonds to the activated alkyne, ruthenium vinylidene complexes were synthesised and characterized. Extensive investigation into the reactivity of these complexes with diphenylgermane has revealed new insights into the interaction between Ge–H bonds and the metal centre. Further detailed spectroscopic and structural analysis enabled the pathways between diphenyl germane and cationic complexes [CpRuP₂OTf], P₂ = (PPh₃)₂, dppe leading to the instantaneous formation of a ruthenium dihydride complex 23 or Ge–H sigma complex 26 to be established. This complex was later found to transform to a germanium-coordinated complex 24.

<figure><img src="image_324.png" /><figcaption>**Scheme 84.** Overall reaction of bis-triphenylphosphine ruthenium complex with TMSOTf and diphenyl germane.</figcaption></figure>