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)]ClO4 (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) Mossbauer 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)]ClO4 (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,Fe-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 pi-acceptor ability than imidazoles, in addition to its very strong sigma-donation.