Synthesis, Structure, and Dynamics of Molybdenum Imido Alkyne Complexes

The monomeric alkyne complexes (η²-alkyne)Mo(NPh)(o-(Me₃SiN)₂C₆H₄) (3) have been synthesized by the displacement of isobutylene from (η²-isobutylene)Mo(NPh)(o-(Me₃SiN)₂C₆H₄) (2). The alkyne fragment in these complexes is oriented perpendicular to the MoN bond of the cis imido ligand, as confirmed by an X-ray structural analysis of 3e. The deshielded nature of the chemical shifts of the α-carbons and terminal protons of the alkyne fragments in these complexes strongly suggests the participation of the alkyne π_⊥ electrons in the Mo−alkyne interaction. The alkyne fragment in 3 rotates freely about the Mo−alkyne bond, resulting in the fluxional behavior of these complexes at room temperature. An activation barrier of 13.2 kcal/mol for the alkyne rotation was measured using VT NMR spectroscopy. Computational studies using a two-layer ONIOM model, and the B3LYP hybrid functional, provided insight into the Mo−alkyne bonding. The transition state for alkyne rotation has been calculated and is characterized by a parallel orientation of the alkyne fragment to the cis imido ligand. A natural bond orbital (NBO) population analysis reveals that alkyne π_⊥ donation to Mo is more extensive in the transition state than in the ground state. Weaker Mo−N(imido) bonds are also observed in the transition state, because π donation from the alkyne ligand competes with imido π donation.