Monosubstituted Borane Ruthenium Complexes RuH₂(η²:η²‑H₂BR)(PR′₃)₂: A General Approach to the Geminal Bis(σ-B–H) Coordination Mode

A series of borane bis­(σ-B–H) ruthenium complexes RuH₂(η²:η²-H₂BR)­(PR′₃)₂ (R = alkyl, aryl; R′ = Cy, Cyp, ^<i>i</i>Pr) has been prepared by using two synthetic strategies. The first one is based on a simple substitution reaction by adding the corresponding monosubstituted H₂BR borane to the bis­(dihydrogen) ruthenium complex RuH₂(η²-H₂)₂(PCy₃)₂. The second one, more general, results from the reaction of the chloro complex RuHCl­(H₂)­(PR′₃)₂ (R′ = Cy, Cyp, ^<i>i</i>Pr) with the corresponding lithium monosubstituted borohydrides RBH₃Li (R = Mes, ^<i>t</i>Bu, Me, C₄H₃S, Ph). All the complexes have been characterized by multinuclear NMR, IR, and X-ray diffraction studies. DFT calculations have been used to better define the bonding mode of the borane ligand to the metal center as well as to establish the thermodynamic cycle that delineates the coordination process. The ^<i>t</i>Bu species displays a dynamic behavior evidencing an equilibrium between a borohydride and a σ-borane formulation. The thienyl case illustrates the competition between sulfur coordination and a bis­(σ-B–H) coordination mode

Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1<i>H</i>‑2,1-Benzazaboroles) at Ruthenium

We had recently disclosed a catalyzed transformation toward the synthesis of BN molecules under an H₂ atmosphere under mild conditions. We now report an in-depth mechanistic study to understand how a substrate featuring two different functional groups, CN and B–H, namely the 2-cyanophenyl­(amino)­borane HB­(N^<i>i</i>Pr₂)­C₆H₄(CN) (<b>2</b>), can be transformed into the BN heterocycle 1<i>H</i>-2,1-benzazaborole (<b>3</b>). Such a complex transformation has direct links with three key important processes: hydrogenation of nitriles, hydroboration of polar bonds, and B–N bond formation. A combination of in situ monitoring of the catalytic reaction, stoichiometric experiments, and variable-temperature multinuclear NMR and DFT studies allowed us to decipher the catalytic cycle. We show that the catalyst precursor [RuH₂(η²-H₂)₂(PCy₃)₂] (<b>1</b>) is regenerated at the end of the transformation. We intercepted the transformation of the starting substrate <b>2</b>, in the form of a 1<i>H</i>-2,1-benzazaborolyl ligand coordinated to the metal center by the formed BN cycle. The corresponding benzazaboryl complex [Ru­{(η⁵-C­(H)­N­(H)­B­(NⁱPr₂)­(C₆H₄)}­{(η³-C₆H₈)­PCy₂}] (<b>9</b>) was independently prepared and fully characterized by X-ray diffraction and multinuclear NMR. We also showed that complex <b>9</b> undergoes stepwise hydrogenation, followed by haptotropic rearrangement before release of the final product <b>3</b> and regeneration of the catalyst precursor <b>1</b>. We were able to provide a fairly good view of the activation of the CN and B–H bonds. So far, it appears that nitrile hydroboration with metal hydrides starts with nitrile reduction but subsequent steps are highly dependent on the system. In our case, after the first hydrogen transfer to the nitrile, a boron–nitrogen interaction is highly favored, B–H bond cleavage occurring at a later stage. This field needs further investigation for promising developments of BN molecules. Prospects on reactions involving at least two different intramolecular reactive functions should be encouraged for future development in catalysis

Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1<i>H</i>‑2,1-Benzazaboroles) at Ruthenium

We had recently disclosed a catalyzed transformation toward the synthesis of BN molecules under an H₂ atmosphere under mild conditions. We now report an in-depth mechanistic study to understand how a substrate featuring two different functional groups, CN and B–H, namely the 2-cyanophenyl­(amino)­borane HB­(N^<i>i</i>Pr₂)­C₆H₄(CN) (<b>2</b>), can be transformed into the BN heterocycle 1<i>H</i>-2,1-benzazaborole (<b>3</b>). Such a complex transformation has direct links with three key important processes: hydrogenation of nitriles, hydroboration of polar bonds, and B–N bond formation. A combination of in situ monitoring of the catalytic reaction, stoichiometric experiments, and variable-temperature multinuclear NMR and DFT studies allowed us to decipher the catalytic cycle. We show that the catalyst precursor [RuH₂(η²-H₂)₂(PCy₃)₂] (<b>1</b>) is regenerated at the end of the transformation. We intercepted the transformation of the starting substrate <b>2</b>, in the form of a 1<i>H</i>-2,1-benzazaborolyl ligand coordinated to the metal center by the formed BN cycle. The corresponding benzazaboryl complex [Ru­{(η⁵-C­(H)­N­(H)­B­(NⁱPr₂)­(C₆H₄)}­{(η³-C₆H₈)­PCy₂}] (<b>9</b>) was independently prepared and fully characterized by X-ray diffraction and multinuclear NMR. We also showed that complex <b>9</b> undergoes stepwise hydrogenation, followed by haptotropic rearrangement before release of the final product <b>3</b> and regeneration of the catalyst precursor <b>1</b>. We were able to provide a fairly good view of the activation of the CN and B–H bonds. So far, it appears that nitrile hydroboration with metal hydrides starts with nitrile reduction but subsequent steps are highly dependent on the system. In our case, after the first hydrogen transfer to the nitrile, a boron–nitrogen interaction is highly favored, B–H bond cleavage occurring at a later stage. This field needs further investigation for promising developments of BN molecules. Prospects on reactions involving at least two different intramolecular reactive functions should be encouraged for future development in catalysis

Monosubstituted Borane Ruthenium Complexes RuH₂(η²:η²‑H₂BR)(PR′₃)₂: A General Approach to the Geminal Bis(σ-B–H) Coordination Mode

A series of borane bis­(σ-B–H) ruthenium complexes RuH₂(η²:η²-H₂BR)­(PR′₃)₂ (R = alkyl, aryl; R′ = Cy, Cyp, ^<i>i</i>Pr) has been prepared by using two synthetic strategies. The first one is based on a simple substitution reaction by adding the corresponding monosubstituted H₂BR borane to the bis­(dihydrogen) ruthenium complex RuH₂(η²-H₂)₂(PCy₃)₂. The second one, more general, results from the reaction of the chloro complex RuHCl­(H₂)­(PR′₃)₂ (R′ = Cy, Cyp, ^<i>i</i>Pr) with the corresponding lithium monosubstituted borohydrides RBH₃Li (R = Mes, ^<i>t</i>Bu, Me, C₄H₃S, Ph). All the complexes have been characterized by multinuclear NMR, IR, and X-ray diffraction studies. DFT calculations have been used to better define the bonding mode of the borane ligand to the metal center as well as to establish the thermodynamic cycle that delineates the coordination process. The ^<i>t</i>Bu species displays a dynamic behavior evidencing an equilibrium between a borohydride and a σ-borane formulation. The thienyl case illustrates the competition between sulfur coordination and a bis­(σ-B–H) coordination mode

Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1<i>H</i>‑2,1-Benzazaboroles) at Ruthenium

We had recently disclosed a catalyzed transformation toward the synthesis of BN molecules under an H₂ atmosphere under mild conditions. We now report an in-depth mechanistic study to understand how a substrate featuring two different functional groups, CN and B–H, namely the 2-cyanophenyl­(amino)­borane HB­(N^<i>i</i>Pr₂)­C₆H₄(CN) (<b>2</b>), can be transformed into the BN heterocycle 1<i>H</i>-2,1-benzazaborole (<b>3</b>). Such a complex transformation has direct links with three key important processes: hydrogenation of nitriles, hydroboration of polar bonds, and B–N bond formation. A combination of in situ monitoring of the catalytic reaction, stoichiometric experiments, and variable-temperature multinuclear NMR and DFT studies allowed us to decipher the catalytic cycle. We show that the catalyst precursor [RuH₂(η²-H₂)₂(PCy₃)₂] (<b>1</b>) is regenerated at the end of the transformation. We intercepted the transformation of the starting substrate <b>2</b>, in the form of a 1<i>H</i>-2,1-benzazaborolyl ligand coordinated to the metal center by the formed BN cycle. The corresponding benzazaboryl complex [Ru­{(η⁵-C­(H)­N­(H)­B­(NⁱPr₂)­(C₆H₄)}­{(η³-C₆H₈)­PCy₂}] (<b>9</b>) was independently prepared and fully characterized by X-ray diffraction and multinuclear NMR. We also showed that complex <b>9</b> undergoes stepwise hydrogenation, followed by haptotropic rearrangement before release of the final product <b>3</b> and regeneration of the catalyst precursor <b>1</b>. We were able to provide a fairly good view of the activation of the CN and B–H bonds. So far, it appears that nitrile hydroboration with metal hydrides starts with nitrile reduction but subsequent steps are highly dependent on the system. In our case, after the first hydrogen transfer to the nitrile, a boron–nitrogen interaction is highly favored, B–H bond cleavage occurring at a later stage. This field needs further investigation for promising developments of BN molecules. Prospects on reactions involving at least two different intramolecular reactive functions should be encouraged for future development in catalysis

B–C Bond Cleavage and Ru–C Bond Formation from a Phosphinoborane: Synthesis of a Bis‑σ Borane Aryl-Ruthenium Complex

Compared with the reactivity of <i>o</i>-Ph₂P­(C₆H₄–CH₂)­BH­(N^<i>i</i>Pr₂) with [RuH₂(η²-H₂)₂­(PCy₃)₂], the behavior of the phosphinoborane Ph₂P­(CH₂–C₆H₄)­BH­(N^<i>i</i>Pr₂) is radically different. No agostic σ-B–H complex could be observed, the reaction leading to the isolation of a new bis-σ borane aryl-ruthenium complex via B–C bond cleavage and Ru–C bond formation. Reactivity studies of this complex with dihydrogen and/or HBAr^F₄ as a proton source enable the formation of a tethered chiral-at-Ru piano-stool cationic complex