Heterolytic Si-H Bond Cleavage at a Molybdenum-Oxido-Based Lewis Pair

The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C6F5)(3) gave access to the Lewis adduct [Mo{OB(C6F5)(3)}(NtBu)L-2] featuring reversible B-O bonding in solution. The resulting frustrated Lewis pair (FLP)-like reactivity is reflected by the compound's ability to heterolytically cleave Si-H bonds, leading to a clean formation of the novel cationic Mo-VI species 3a (R=Et) and 3b (R=Ph) of the general formula [Mo(OSiR3)(NtBu)L-2][HB(C6F5)(3)]. These compounds possess properties highly unusual for molybdenum d(0) species such as an intensive, charge-transfer-based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single-crystal X-ray diffraction analyses of 2 and 4b, a derivative of 3b featuring the [FB(C6F5)(3)](-) anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3a and 3b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B-H bond of the borohydride anion.