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

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.