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Paper of the month

Lithium Di- and Trimethyl Dimolybdenum(II) Complexes with Mo−Mo Quadruple Bonds and Bridging Methyl Groups


N. Curado, M. Carrasco, E. Álvarez, C. Maya, R. Peloso, A. Rodríguez, J. López-Serrano, E. Carmona J. Am. Chem. Soc. 2015, Vol. 137, 12378-12387

New dimolybdenum complexes of composition [Mo2{μ-Me}2Li(S)}(μ-X)(μ-N^N)2] (3a−3c), where S = THF or Et2O and N^N represents a bidentate aminopyridinate or amidinate ligand that bridges the quadruply bonded molybdenum atoms, were prepared from the reaction of the appropriate [Mo2{μ-O2CMe}2(μ-N^N)2] precursors and LiMe. For complex 3a, X = MeCO2, while in 3b and 3c, X = Me. Solution NMR studies in C6D6 solvent support formulation of the complexes as contact ion pairs with weak agostic Mo−CH3···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of 3a and 3b. Samples of 3c enriched in 13C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C6D6 and THF-d8 solvents. Crystallization of 3c from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li(THF)4] [Mo2(Me)2(μ-Me){μ-HC(NDipp)2}2] (4c), where Dipp stands for 2,6-iPr2C6H3. A computational analysis of the Mo2(μ-Me)2Li core of complexes 3a and 3b has been developed, which is consistent with a small but non-negligible electron-density sharing between the C and Li atoms of the mainly ionic CH3···Li interactions.