C−H vs C−C Bond Activation of Acetonitrile and Benzonitrile via Oxidative Addition: Rhodium vs Nickel and Cp* vs Tp′ (Tp′ = Hydrotris(3,5-dimethylpyrazol-1-yl)borate, Cp* = η⁵-Pentamethylcyclopentadienyl)

The photochemical reaction of (C5Me5)Rh(PMe3)H2 (1) in neat acetonitrile leads to formation of the C−H activation product, (C5Me5)Rh(PMe3)(CH2CN)H (2). Thermolysis of this product in acetonitrile or benzene leads to thermal rearrangement to the C−C activation product, (C5Me5)Rh(PMe3)(CH3)(CN) (4). Similar results were observed for the reaction of 1 with benzonitrile. The photolysis of 1 in neat benzonitrile results in C−H activation at the ortho, meta, and para positions. Thermolysis of the mixture in neat benzonitrile results in clean conversion to the C−C activation product, (C5Me5)Rh(PMe3)(C6H5)(CN) (5). DFT calculations on the acetonitrile system show the barrier to C−H activation to be 4.3 kcal mol−1 lower than the barrier to C−C activation. A high-energy intermediate was also located and found to connect the transition states leading to C−H and C−C activation. This intermediate has an agostic hydrogen interaction with the rhodium center. Reactions of acetonitrile and benzonitrile with the fragment [Tp′Rh(CNneopentyl)] show only C−H and no C−C activation. These reactions with rhodium are compared and contrasted to related reactions with [Ni(dippe)H]2, which show only C−CN bond cleavage.