13C NMR Studies of the Electronic Structure of Low-Spin Iron(III) Tetraphenylchlorin Complexes

A series of low-spin six-coordinate (tetraphenylchlorinato)iron(III) complexes [Fe(TPC)(L)2]± (L = 1-MeIm, CN-, 4-CNPy, and tBuNC) have been prepared, and their 13C NMR spectra have been examined to reveal the electronic structure. These complexes exist as the mixture of the two isomers with the (d xy )2(d xz , d yz )3 and (d xz , d yz )4(d xy )1 ground states. Contribution of the (d xz , d yz )4(d xy )1 isomer has increased as the axial ligand changes from 1-MeIm, to CN- (in CD2Cl2 solution), CN- (in CD3OD solution), and 4-CNPy, and then to tBuNC as revealed by the meso and pyrroline carbon chemical shifts; the meso carbon signals at 146 and −19 ppm in [Fe(TPC)(1-MeIm)2]+ shifted to 763 and 700 ppm in [Fe(TPC)(tBuNC)2]+. In the case of the CN- complex, the population of the (d xz , d yz )4(d xy )1 isomer has increased to a great extent when the solvent is changed from CD2Cl2 to CD3OD. The result is ascribed to the stabilization of the d xz and d yz orbitals of iron(III) caused by the hydrogen bonding between methanol and the coordinated cyanide ligand. Comparison of the 13C NMR data of the TPC complexes with those of the TPP, OEP, and OEC complexes has revealed that the populations of the (d xz , d yz )4(d xy )1 isomer in TPC complexes are much larger than those in the corresponding TPP, OEC, and OEP complexes carrying the same axial ligands.