Study on Syntheses of Cationic Borane a Complexes and Heterolytic Cleavage of a Boron-Hydrogen Bond

New borane complexes were synthesized by the coordination of borane-Lewis base adducts，which have extremely low p-accepting ability，to positively charged metal centers.In the complexes obtained，the bonding between the central metal and coordinated BH hydrogen atom has proved to be more stable than in the neutral borane complexes.Furthermore，it was found that the borane ligand was polarized through the coordination to the cationic center and thereby chemically activated.Cationic manganese and molybdenum borane complexes，[Mn(CO)_4(PR_3)(η^1-BH_3·PMe_3)]^(1)and[Cp^*Mo(CO)_3(η^1-BH_3·PMe_3)]^(2)，were obtained by protonation of the corresponding boryl complexes.A ruthenium derivative，[CpRu(PMe_3_)2(η^1-BH_3·PMe_3)]^(3)，was prepared by the borane coordination to the coordinatively unsaturated species generated by the chloride abstruction of[CpRu(PMe_3)_2Cl]。Compounds1and2decompose at room temperature through BH cleavage leaving a hydride onto the metal center，rather than through borane dissociation。This is a novel type of reaction，in which a BH bond is heterolytically activated on metal.It was difficult to isolate compounds1and2because their BH heterolytic cleavage rapidly proceeds.In contrast，by the use of the cationic yet inherently electron-rich metal fragment，the ruthenium complex 3 has been very stable that it has not undergone the BH cleavage under vacuum.This is probably due to the metal-borane linkage stabilized by the positive charge on the metal atom and modest polarization of the borane ligand caused by the electron-rich nature of the metal fragment。However，the presence of trace water promoted the BH heterolytic cleavage of 3。

New borane complexes were synthesized by the coordination of borane-Lewis base adducts, which have extremely low p-accepting ability, to positively charged metal centers. In the complexes obtained, the bonding between the central metal and coordinated BH hydrogen atom has proved to be more stable than in the neutral borane complexes. Furthermore, it was found that the borane ligand was polarized through the coordination to the cationic center and thereby chemically activated.Cationic manganese and molybdenum borane complexes, [Mn(CO)_4(PR_3)(η^1-BH_3・PMe_3)]^+ (1) and [Cp^*Mo(CO)_3(η^1-BH_3・PMe_3)]^+ (2), were obtained by protonation of the corresponding boryl complexes. A ruthenium derivative, [CpRu(PMe_3_)2(η^1-BH_3・PMe_3)]^+(3), was prepared by the borane coordination to the coordinatively unsaturated species generated by the chloride abstruction of [CpRu(PMe_3)_2Cl].Compounds 1 and 2 decompose at room temperature through BH cleavage leaving a hydride onto the metal center, rather than through borane dissociation. This is a novel type of reaction, in which a BH bond is heterolytically activated on metal.It was difficult to isolate compounds 1 and 2 because their BH heterolytic cleavage rapidly proceeds. In contrast, by the use of the cationic yet inherently electron-rich metal fragment, the ruthenium complex 3 has been very stable that it has not undergone the BH cleavage under vacuum. This is probably due to the metal-borane linkage stabilized by the positive charge on the metal atom and modest polarization of the borane ligand caused by the electron-rich nature of the metal fragment. However, the presence of trace water promoted the BH heterolytic cleavage of 3.