Atomic Carbon as a Terminal Ligand: Studies of a Carbidomolybdenum Anion Featuring Solid-State ^(13)C NMR Data and Proton-Transfer Self-Exchange Kinetics

Abstract

Anion [CMo(N[R]Ar)_3]- (R = C(CD3)_2CH_3 or tBu, Ar = 3,5-C_6H_3Me_2) containing one-coordinate carbon as a terminal substituent and related molecules have been studied by single-crystal X-ray crystallography, solution and solid-state ^(13)C NMR spectroscopy, and density functional theory (DFT) calculations. Chemical reactivity patterns for [CMo(N[R]Ar)_3]- have been investigated, including the kinetics of proton-transfer self-exchange involving HCMo(N[R]Ar)_3, the carbidomolybdenum anion's conjugate acid. While the Mo⋮C bond lengths in [K(benzo-15-crown-5)_2][CMo(N[R]Ar)_3] and the parent methylidyne, HCMo(N[R]Ar)_3, are statistically identical, the carbide chemical shift of δ 501 ppm is much larger than the δ 282 ppm shift for the methylidyne. Solid-state ^(13)C NMR studies show the carbide to have a much larger chemical shift anisotropy (CSA, 806 ppm) and smaller 95Mo−13C coupling constant (60 Hz) than the methylidyne (CSA = 447 ppm, 1J_MoC = 130 Hz). DFT calculations on model compounds indicate also that there is an increasing MoC overlap population on going from the methylidyne to the terminal carbide. The pKa of methylidyne HCMo(N[R]Ar)_3 is approximately 30 in THF solution. Methylidyne HCMo(N[R]Ar)3 and carbide [CMo(N[R]Ar)3]- undergo extremely rapid proton-transfer self-exchange reactions in THF, with k = 7 × 10^6 M^(-1) s^(-1). Besides being a strong reducing agent, carbide [CMo(N[R]Ar)_3]- reacts as a nucleophile with elemental chalcogens to form carbon−chalcogen bonds and likewise reacts with PCl_3 to furnish a carbon−phosphorus bond.

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