σ-Borane Complexes of Manganese and Rhenium

The late transition metal borane complexes (MeCp)Mn(CO) 2 (HBR 2 ) (R = alkyl, alkoxy) and Cp*Re(CO) 2 (HBpin) (HBpin = pinacolborane, Cp* = pentamethylcyclopentadienyl) have been synthesized and isolated. All complexes were characterized spectroscopically, and X-ray crystal structure analyses were performed for (MeCp)Mn(CO) 2 (HBpin), (MeCp)Mn(CO) 2 (HBcat) (HBcat = catecholborane) and (MeCp)Mn(CO) 2 (HBCy 2 ) (Cy = cyclohexyl). These data show that the complexes contain a borane ligand with a weakened but intact B−H bond. The borane ligand in (MeCp)Mn(CO) 2 (HBcat) is replaced by PhCCPh, Ph 3 SnH, Ph 2 MeSiH, CO, and excess HBpin in ligand substitution reactions. The mechanism of substitution by PhCCPh was investigated. Kinetic studies showed that the reaction is first-order in complex and that borane and PhCCPh react competitively with the 16-electron intermediate, (MeCp)Mn(CO) 2 . The Δ H ⧧ value for borane dissociation was 24 ± 3 kcal/mol for (MeCp)Mn(CO) 2 (HBcat) and 21 ± 1 kcal/mol for (MeCp)Mn(CO) 2 (HBCy 2 ), which provided an upper limit on the metal−borane binding energies. Dynamic NMR spectroscopy of (MeCp)Mn(CO) 2 (HBcat) revealed two types of rotations of the borane ligand.