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-Melm, 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-Melm, 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-Melm)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.