An Unsaturated Four-Coordinate Dimethyl Dimolybdenum Complex with a Molybdenum-Molybdenum Quadruple Bond

We describe the synthesis and the molecular and electronic structures of the complex [Mo₂ Me₂ {μ-HC(NDipp)₂ }₂ ] (2; Dipp=2,6-iPr₂ C₆ H₃ ), which contains a dimetallic core with an Mo-Mo quadruple bond and features uncommon four-coordinate geometry and has a fourteen-electron count for each molybdenum atom. The coordination polyhedron approaches a square pyramid, with one of the molybdenum atoms nearly co-planar with the basal square plane, in which the trans coordination position with respect to the Mo-Me bond is vacant. The other three sites are occupied by two trans nitrogen atoms of different amidinate ligands and the methyl group. The second Mo atom occupies the apex of the pyramid and forms an Mo-Mo bond of length 2.080(1) Å, consistent with a quadruple bond. Compound 2 reacts with tetrahydrofuran (THF) and trimethylphosphine to yield the mono-adducts [Mo₂ Me(μ-Me){μ-HC(NDipp)₂ }₂ (L)] (3⋅THF and 3⋅PMe₃ , respectively) with one terminal and one bridging methyl group. In contrast, 4-dimethylaminopyridine (dmap) forms the bis-adduct [Mo₂ Me₂ {μ-HC(NDipp)₂ }₂ (dmap)₂ ] (4), with terminally coordinated methyl groups. Hydrogenolysis of complex 2 leads to the bis(hydride) [Mo₂ H₂ {μ-HC(NDipp)₂ }₂ (thf)₂ ] (5⋅THF) with elimination of CH₄ . Computational, kinetic, and mechanistic studies, which included the use of D₂ and of complex 2 labelled with ¹³ C (99 %) at the Mo-CH₃ sites, supported the intermediacy of a methyl-hydride reactive species. A computational DFT analysis of the terminal and bridging coordination of the methyl groups to the Mo≣Mo core is also reported.