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(C(6)F(5))(3) gave access to the Lewis adduct [Mo{OB(C(6)F(5))(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 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR(3))(NtBu)L(2)][HB(C(6)F(5))(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 4 b, a derivative of 3 b featuring the [FB(C(6)F(5))(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 3 a and 3 b 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.