σ-Borane Complexes of Manganese and Rhenium

The late transition metal borane complexes (MeCp)Mn(CO)2(HBR2) (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(HBCy2) (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, Ph3SnH, Ph2MeSiH, 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 ΔH⧧ 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(HBCy2), 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.