Magnetic Resonance Spectroscopic Investigations of the Electronic Ground and Excited States in Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins

Abstract

A series of axially ligated complexes of iron(III) octamethyltetraphenylporphyrin, (OMTPP)Fe(III), octaethyltetraphenylporphyrin, (OETPP)Fe(III), its perfluorinated phenyl analogue, (F(20)OETPP)Fe(III), and tetra-(beta,beta'-tetramethylene)tetraphenylporphyrin, (TC(6)TPP)Fe(III), have been prepared and characterized by (1)H NMR spectroscopy: chloride, perchlorate, bis-4-(dimethylamino)pyridine, bis-1-methylimidazole, and bis-cyanide. Complete spectral assignments have been made using 1D and 2D techniques. The temperature dependences of the proton resonances of the complexes show significant deviations from simple Curie behavior and evidence of ligand exchange, ligand rotation, and porphyrin ring inversion at ambient temperatures. At temperatures below the point where dynamics effects contribute, the temperature dependences of the proton chemical shifts of the complexes could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in our laboratory that includes consideration of a thermally accessible excited state. The results show that, although the ground state differs for various axial ligand complexes and is usually fully consistent with that observed by EPR spectroscopy at 4.2 K, the excited state often has S = (3)/(2) (or S = (5)/(2) in the cases where the ground state has S = (3)/(2)). The EPR spectra (4.2 K) of bis-4-(dimethylamino)pyridine and bis-1-methylimidazole complexes show "large-g(max)" signals with g(max) = 3.20 and 3.12, respectively, and the latter also shows a normal rhombic EPR signal, indicating the presence of low-spin (LS) (d(xy))(2)(d(xz),d(yz))(3) ground states for both. The bis-cyanide complex also yields a large-g(max) EPR spectrum with g = 3.49 and other features that could suggest that some molecules have the (d(xz),d(yz))(4)(d(xy))(1) ground state. The EPR spectra of all five-coordinate chloride complexes have characteristic features of predominantly S = (5)/(2) ground-state systems with admixture of 1-10% of S = (3)/(2) character.