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Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions
Author(s) -
Bhowmik Susovan,
Dey Soumyajit,
Sahoo Dipankar,
Rath Sankar Prasad
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201301242
Subject(s) - porphyrin , chemistry , ligand (biochemistry) , delocalized electron , crystallography , spin states , coordination complex , heme , stereochemistry , photochemistry , inorganic chemistry , metal , organic chemistry , biochemistry , receptor , enzyme
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 FeN (porphyrin) and FeO (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 E 1/2 value for the Fe III /Fe II couple is positively shifted with increasing acidity of the phenol. However, a plot of the E 1/2 values for the Fe III /Fe II couple versus the p K a 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 hemeheme 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.