NMR and EPR Studies of Low-Spin Fe(III) Complexes of meso-Tetra-(2,6-Disubstituted Phenyl)Porphyrinates Complexed to Imidazoles and Pyridines of Widely Differing Basicities

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Abstract

A series of bis-axially ligated complexes of iron(III) tetramesitylporphyrin, TMPFe(III), tetra-(2,6-dibromophenyl)porphyrin, (2,6-Br2)4TPPFe(III), tetra-(2,6-dichlorophenyl)porphyrin, (2,6-Cl2)4TPPFe(III), tetra-(2,6-difluorophenyl)porphyrin, (2,6-F2)4TPPFe(III), and tetra-(2,6-dimethoxyphenyl)porphyrin, (2,6-(OMe)2)4TPPFe(III), where the axial ligands are 1-methylimidazole, 2-methylimidazole, and a series of nine substituted pyridines ranging in basicity from 4-(dimethylamino)pyridine (pK(a)(PyH(+)) = 9.70) to 3- and 4-cyanopyridine (pKa(PyH+) = 1.45 and 1.1, respectively), have been prepared and characterized by EPR and 1H NMR spectroscopy. The EPR spectra, recorded at 4.2 K, show "large g(max)", rhombic, or axial signals, depending on the iron porphyrinate and axial ligand, with the g(max) value decreasing as the basicity of the pyridine decreases, thus indicating a change in electron configuration from (d(xy))2(d(xz),d(yz)3 to (d(xz),d(yz))4(d(xy))1 through each series at this low temperature. Over the temperature range of the NMR investigations (183-313 K), most of the high-basicity pyridine complexes of all five iron(III) porphyrinates exhibit simple Curie temperature dependence of their pyrrole-H paramagnetic shifts and beta-pyrrole spin densities, rho(C) approximately 0.015-0.017, that are indicative of the S = 1/2 (d(xy))(2)(d(xz),d(yz))(3) electron configuration, while the temperature dependences of the pyrrole-H resonances of the lower-basicity pyridine complexes (pK(a)(PyH(+)) < 6.00) show significant deviations from simple Curie behavior which could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in this laboratory that includes consideration of a thermally accessible excited state. In most cases, the ground state of the lower-basicity pyridine complexes is an S = 1/2 state with a mixed (d(xy))2(d(xz),d(yz))3/(d(xz),d(yz))4(d(xy))1 electron configuration, indicating that these two are so close in energy that they cannot be separated by analysis of the NMR shifts; however, for the TMPFe(III) complexes with 3- and 4-CNPy, the ground states were found to be fairly pure (d(xz),d(yz))4(d(xy))1 electron configurations. In all but one case of the intermediate- to low-basicity pyridine complexes of the five iron(III) porphyrinates, the excited state is found to be S = 3/2, with a (d(xz),d(yz))3(d(xy))1(d(z)2)1 electron configuration, lying some 120-680 cm(-1) higher in energy, depending on the particular porphyrinate and axial ligand. Full analysis of the paramagnetic shifts to allow separation of the contact and pseudocontact contributions could be achieved only for the [TMPFe(L)2]+ series of complexes.

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