Abstract
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)L2] 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 MoVI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR3)(NtBu)L2][HB(C6F5)3]. These compounds possess properties highly unusual for molybdenum d0 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(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 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.
Highlights
The chemistry of frustrated Lewis pairs (FLPs) has received large attention in recent years, with a significant spark being the discovery of metal free hydrogen splitting in 2006 by Stephan and co-workers.[1,2] The term FLP refers to Lewis acid– base pairs that contain unquenched acidic and basic centers.[3]
Lewis adduct synthesis Addition of one equivalent of the Lewis acid B(C6F5)3 to the yellow solution of the molybdenum(VI) oxido imido precursor [MoO(NtBu)L2] (1)[35] in pentane led to an immediate color change to deep-red and subsequent formation of the Lewis adduct [Mo{OB(C6F5)3}(NtBu)L2] (2) as a reddish precipitate
Based on the data described above the following steps are suggested for the reaction shown in Scheme 6: i) the Lewis acidic molybdenum cation activates the substrate which allows, under nucleophilic attack of the hydride, insertion into the BÀH bond; ii) in the apolar solvent, the ion pair is in closer contact so that the electrophilic silicon reacts with the benzyloxy borate, leading to the formation of the hydrosilylated product BnOSiEt3 and the Lewis adduct 2
Summary
The chemistry of frustrated Lewis pairs (FLPs) has received large attention in recent years, with a significant spark being the discovery of metal free hydrogen splitting in 2006 by Stephan and co-workers.[1,2] The term FLP refers to Lewis acid– base pairs that contain unquenched acidic and basic centers.[3]. Catalytic hydrosilylation of organic carbonyl functions using B(C6F5) has been intensively investigated.[12,40,41,42] the general mechanism of B(C6F5)3-catalyzed hydrosilylation, elucidated by the work of Piers and Oestreich, is based on the formation of a FLP with the substrate.[11,12,40,41,42,43,44] In addition, some transition metal oxido complexes are known to be active catalysts in hydrosilylation reactions.[45] While a vast number of transition metal compounds are known to react with hydrosilanes,[46] only few examples of well-characterized species resulting from the reaction of hydrosilanes with metal oxido groups have been disclosed by Abu-Omar,[47] Hayton[48,49] and Toste.[50] For this reason, we found hydrosilanes a suitable choice as test reagents to assess the envisioned Lewis acid activation of the molybdenum(VI) oxido group.
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