Small-Molecule Activation with Molybdenum(0) Complexes Supported by Mixed Imidazol-2-Ylidene/Phosphanyl Hybrid Ligands – Electronic and Structural Consequences of Substituting a Phosphane by a Carbene Group



By employing a series of mixed bi- and tridentate N-heterocyclic carbene (NHC)/phosphane ligands, the new molybdenum(0) carbonyl complexes [Mo(CO)4(CP)] [1, CP = 3-(2-diphenylphosphanylethyl)-1-ethylimidazol-2-ylidene], fac-[Mo(CO)3(PCP)] [2, PCP = 1,3-bis(2-diphenylphosphanylethyl)imidazol-2-ylidene], and fac-[Mo(CO)3(CPC)] [3, CPC = bis(1-ethyl-3-ethyleneimidazol-2-ylidene)phenylphosphane] have been synthesized. The compounds were investigated by X-ray structure analysis and vibrational and NMR spectroscopy in conjunction with DFT calculations. For comparison, the complexes [Mo(CO)4(CC)] [4, CC = di(1-ethylimidazol-2-ylidene)methane], fac-[Mo(CO)3(dpepp)] [5, dpepp = bis(2-diphenylphosphanylethyl)phenylphosphane], and [Mo(CO)4(dppp)] [6, dppp = 1,3-bis(diphenylphosphanyl)propane] have also been prepared and investigated. In contrast to the phosphane donors, which exhibiting metal–ligand backbonding interactions, the π-acceptor interactions of the carbene moieties are exactly cancelled by π-donor interactions. The σ-donor strengths of phosphane and carbene groups, on the other hand, are found to be comparable. The implications of this result with respect to the activation of small molecules (e.g., N2) are discussed.