Molybdenum(0)–carbonyl complexes supported by mixed benzimidazol-2-ylidene/phosphine ligands: Influence of benzannulation on the donor properties of the NHC groups

• We prepared new molybdenum–carbonyl complexes with benzimidazole-based NHCs.
• Bonding properties of benzimidazol-2-ylidenes are compared to imidazole-based analogues.
• We derived a Mo–CNHC bond description by X-ray analysis, spectroscopy and DFT.
• Benzimidazol- and imidazol-NHCs are pure σ-donor ligands without π-contributions.

A series of molybdenum(0) tetra- and tricarbonyl complexes has been synthesized which are supported by the bi- and tridentate, NHC/phosphine ligands di(1-ethylbenzimidazol-2-ylidene)methane (Benz-CC), 3-(2-diphenylphosphanylethyl)-1-ethylbenzimidazol-2-ylidene (Benz-CP) and 1,3-bis(2-diphenylpho-sphanyl-ethyl)5,6-dimethylbenzimidazol-2-ylidene (DMBenzPCP), respectively. The compounds [Mo(CO)4(Benz-CC)] (1), [Mo(CO)4(Benz-CP)] (2) and fac-[Mo(CO)3(DMBenz-PCP)] (3) are characterized by X-ray structure analysis, vibrational and NMR spectroscopy complemented by DFT calculations. The derived description of the benzimidazol-2-ylidene molybdenum bond is compared to that of the imidazole-based counterparts investigated earlier. The benzimidazole-based ligand systems exhibit a slightly lower activation of the carbonyl ligands, which is mainly explained by a lower σ-donor strength of the benzimidazol-2-ylidene unit. On the other hand there is some evidence for a slightly higher π backbonding capacity of the benzimidazole-based NHC ligands in comparison to their imidazole analogues.

The bonding properties of benzimidazole-derived N-heterocyclic carbene ligands to molybdenum carbonyl complexes are investigated employing a set of mixed NHC–phosphine ligands. The derived description of the benzimidazol-2-ylidene molybdenum bond is compared to that of the imidazole-based counterparts investigated earlier.Figure optionsDownload as PowerPoint slide