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 -acceptor interactions of the carbene moieties are exactly cancelled by -donor interactions. The sigma-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., N-2) are discussed.