Reversible carbon-boron bond formation at platinum centers through σ-BH complexes.

A reversible carbon-boron bond formation has been observed in the reaction of the coordinatively unsaturated, cyclometalated, Pt(ii) complex [Pt(I t BuiPr')(I t BuiPr)][BArF], 1, with tricoordinated boranes HBR2. X-ray diffraction studies provided structural snapshots of the sequence of reactions involved in the process. At low temperature, we observed the initial formation of the unprecedented σ-BH complexes [Pt(HBR2)(I t BuiPr')(I t BuiPr)][BArF], one of which has been isolated. From -15 to +10 °C, the σ-BH species undergo a carbon-boron coupling process leading to the platinum hydride derivative [Pt(H)(I t BuiPr-BR2)(I t BuiPr)][BArF], 4. Surprisingly, these compounds are thermally unstable undergoing carbon-boron bond cleavage at room temperature that results in the 14-electron Pt(ii) boryl species [Pt(BR2)(I t BuiPr)2][BArF], 2. This unusual reaction process has been corroborated by computational methods, which indicate that the carbon-boron coupling products 4 are formed under kinetic control whereas the platinum boryl species 2, arising from competitive C-H bond coupling, are thermodynamically more stable. These findings provide valuable information about the factors governing productive carbon-boron coupling reactions at transition metal centers.