NMR and EPR Studies of Chloroiron(III) Tetraphenylchlorin and Its Complexes with Imidazoles and Pyridines of Widely Differing Basicities

摘要

The NMR and EPR spectra of two bis-imidazole and three bis-pyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]⁺ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py and 4-CNPy) have been investigated. The full resonance assignments of the [(TPC)Fe(L)2]⁺ complexes of this study have been made from COSY and NOESY experiments and ADF calculations. Unlike the [(OEC)Fe(L)2]⁺ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. >2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]⁺ complexes of this study indicate that the ground state is S = ½ for each bis-ligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]⁺ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the π system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the non-hindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH⁺ pKa = 9.7, H2Im⁺ pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bis-ligand complexes studied over the temperature range of the NMR investigations shows that most of them have mixed (dxy)²(dxz,d_yz)³/(d_xzd_yz)⁴(d_xy)¹ electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicates that it has a pure (d_xzd_yz)⁴(d_xy)¹ ground state and that it is ruffled, because ruffling mixes the a_2u(π)-like orbital of the chlorin into the SOMO. This mixing accounts for the negative chemical shift of the pyrroline-H (−6.5 ppm at −40 °C), and thus the negative spin density at the pyrroline-α-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)₂]⁺, whose pyrroline-H chemical shift is −36 ppm at 25 °C (Simon-neaux, G.; Kobeissi, M. Dalton Trans. >2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend upon chemical exchange between this complex and its bis-4-Me₂NPy adduct.