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


Natalia Curado, Mario Carrasco, Jesús Campos, Celia Maya, Amor Rodríguez, Eliseo Ruiz, Santiago Álvarez, Ernesto Carmona Chemistry: A European Journal 2017, Vol. 23, 194–205

We describe the synthesis, molecular, and electronic structure of the complex [Mo2Me2{μ-HC(NDipp)2}2] (2), that contains a dimetallic core with a Mo-Mo quadruple bond and features uncommon four-coordinate geometry and fourteen-electron count at each molybdenum atom (Dipp = 2,6-iPr2C6H3). The coordination polyhedron approaches a square pyramid with one of the molybdenum atoms nearly co-planar with the basal square plane in which the coordination position trans with respect to the Mo-Me bond is empty. The other three sites contain two trans nitrogen atoms of different amidinate ligands and the methyl group. The second Mo atom occupies the apex of the pyramid and forms a 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 [Mo2Me(μ-Me){μ-HC(NDipp)2}2(L)] (3·THF and 3·PMe3, respectively) with one terminal and one bridging methyl groups. In contrast, 4-dimethylaminopyridine (dmap) forms the bis-adduct [Mo2Me2{μ-HC(NDipp)2}2(dmap)2] (4), with terminally coordinated methyl groups. Hydrogenolysis of complex 2 leads to the bis(hydride) [Mo2H2{μ-HC(NDipp)2}2(thf)2] (5·THF) with elimination of CH4. Computational, kinetic and mechanistic studies, that include the use of D2, and of complex 2 labelled with 13C (99%) at the Mo-CH3 sites, support the intermediacy of a methyl-hydride reactive species. A computational DFT analysis of the terminal and bridging coordination of the methyl group to the core is also reported.