Charge Distribution in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: A Combined X-ray, XAS, XES, DFT, Mossbauer, and Catalysis Approach

The charge delocalization between the N-heterocyclic carbene (NHC) and the metal in cationic molybdenum imido alkylidene NHC mono(nonafluoro-tert-butoxide) complexes has been studied for different NHCs, i.e., 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene (IMesCl(2)), 1,3-dimesityl-4,5-dimethylimidazol-2-ylidene (IMesMe(2)), and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH(2)). The binding situation in the corresponding cationic complexes Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(NHC)(OC(CF3)(3))(+) B(Ar-F)(4) - (NHC = IMes (1), IMesCl(2) (2), IMesMe(2) (3), and IMesH(2) (4) was compared to that of the analogous neutral Schrock catalyst Mo(N-2,6-Me2C6H3)(CHCMe2Ph)((OC(CF3)(3)))(2) (5). Single-crystal X-ray data were used as a starting point for the optimization of the geometries of the catalysts at the PBE0-D3BJ/def2-SVP level of theory; the obtained data were compared to those obtained from X-ray absorption (XAS) and emission spectroscopy (XES). The very similar X-ray spectroscopic signatures of the XANES (X-ray absorption near-edge structure) and K beta-XES of catalysts 1, 2, and 5 suggest that a similar oxidation state and charge are present at the Mo center in all three cases. However, charge delocalization is more pronounced in 1 and 2 compared to 5. This is supported by quantum chemical (QC) calculations, which reveal that all NHCs compensate to a very similar extent for the cationic charge at molybdenum, leading to charge model 5 (CM5) partial charges at Mo between +1.292 and +1.298. Accordingly, the partial charge in the NHCs was in the range of +0.486 to +0.515. This strong delocalization of the positive charge in cationic molybdenum imido alkylidene NHC (nonafluoro-tert-butoxide) complexes is also illustrated by the finding that the analogous neutral Schrock catalyst 5 has a more positive charge at molybdenum (+1.435) despite being a neutral 14-electron complex. Complementarily, charge analysis on complexes 1 and 2 and the acetonitrile-containing derivatives 1 center dot MeCN and 2 center dot MeCN revealed that a small partial positive charge of about +0.1 was found on acetonitrile, accompanied by an increase in positive charge on Mo. Accordingly, the partial charges at the imido, the alkoxide, and NHC ligands decreased slightly. Finally, the catalytic activity of complexes 1-4 was determined for a number of purely hydrocarbon-based substrates in a set of olefin metathesis reactions. A correlation of the Tolman electronic parameter (TEP) with catalyst activity, expressed as the turnover frequency after 3 min, TOF3min, was found for complexes 1-3 based on imidazol-2-ylidenes. Fe-57-Mossbauer measurements on Mo(N-2,6-Me2C6H3)(CH-ferrocenyl)(NHC)(OTf)(2) and Mo(N-2,6-Me2C6H3)(CH-ferrocenyl)(NHC)(OTf)(+) B(Ar-F)(4)(-) (NHC = IMes (6, 8) and IMesH(2) (7, 9)) revealed significant changes in the quadrupole splitting of these complexes. These suggest a significantly more efficient charge distribution between the cationic molybdenum center and an imidazol-2-ylidene-based NHC compared to the same catalysts containing the IMesH(2) ligand.