Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

Abstract

The binding of a series of substituted phenols as axial ligands onto a diiron(III)bisporphyrin framework have been investigated. Spectroscopic characterization revealed high-spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin, which only stabilized the pure intermediate-spin (S=3/2) state of the iron centers. The average FeN (porphyrin) and FeO (phenol) distances that were observed with the 2,4,6-trinitrophenolate complex were 1.972(3) Å and 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any Fe(III) porphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π-spin delocalization onto the bound phenolate. Electrochemical data reveals that the E1/2 value for the Fe(III) /Fe(II) couple is positively shifted with increasing acidity of the phenol. However, a plot of the E1/2 values for the Fe(III) /Fe(II) couple versus the pKa values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6-trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6-trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding Fe(III) monoporphyrin only results in stabilization of the high-spin state. The porphyrin macrocycle in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin is the most distorted, whilst the "ruffling" deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6-trinitrophenol, along with hemeheme interactions in the diiron(III)bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate-spin states of the iron centers in the complex.