Phosphinoborane-induced fragmentation of the unsaturated hydride H2Re2(CO)8: X-ray structure of HRe(CO)4(κB,P-Ph2PCH2CH2BR2) (where BR2 = 9-borabicyclo[3.3.1]nonanyl) and DFT Evaluation of hydride versus CO coordination by the ancillary borane

The reactivity of the unsaturated dimer H2Re2(CO)8 (1) with the ambiphilic phosphinoborane Ph2PCH2CH2BR2 (2; where BR2 = 9-borabicyclo[3.3.1]nonanyl) has been explored. Coordination of the ambiphilic ligand to 1 is rapid at room temperature, leading to the fragmentation of 1 and formation of HRe(CO)4(κB,P-Ph2PCH2CH2BR2) (3) in high yield. 3 has been characterized in solution by IR and NMR spectroscopy, and the molecular structure established by X-ray diffraction analysis. The solid-state structure confirms the presence of a chelated phosphinoborane ligand through coordination of the phosphine to the rhenium center and the formation of a three-center, two-electron (3c,2e) Re–H–B interaction. The nature of the Re–H–B interaction in 3 has been investigated by DFT, and the energetics for borane dissociation and hydride abstraction by the coordinated Lewis acid in 3 have also been computationally evaluated. The coordination of an oxygen in one of the ancillary CO groups by the pendant borane in HRe(CO)4(κP-Ph2PCH2CH2BR2) is thermodynamically unfavorable relative to the formation of the 3c,2e Re–H–B bond.