Perspectives on Ligand Properties of N-Heterocyclic Carbenes in Iron Porphyrin Complexes.

There has been considerable research interest in the ligand nature of N-heterocyclic carbenes (NHCs). In this work, two six-coordinate NHC iron porphyrin complexes [Fe II (TTP)(1,3-Me 2 Imd) 2 ] (TTP = tetratolylporphyrin, 1,3-Me 2 Imd = 1,3-dimethylimidazol-2-ylidene) and [Fe III (TDCPP)(1,3-Me 2 Imd) 2 ]ClO 4 (TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) are reported. Single-crystal X-ray characterizations demonstrate that both complexes have strongly ruffled conformations and relatively perpendicular ligand orientations which are forced by the sterically bulky 1,3-Me 2 Imd NHC ligands. Multitemperature (4.2-300 K) and high magnetic field (0-9 T) Mössbauer and low-temperature (4.0 K) EPR spectroscopies definitely confirmed the low-spin states of [Fe II (TTP)(1,3-Me 2 Imd) 2 ] ( S = 0) and [Fe III (TDCPP)(1,3-Me 2 Imd) 2 ]ClO 4 ( S = 1/2). The similarity of 1,3-Me 2 Imd and imidazole, as well as the well-established correlations between the ligand nature and spectroscopic characteristics of [Fe II,III (Porph)(L) 2 ] 0,+ (Porph: porphyrin; L: planar base ligand) species, allowed direct comparisons between the pair of ligands which revealed for the first time that NHC has a stronger π-acceptor ability than imidazoles, in addition to its very strong σ-donation.