The Rotational Orientation Angles of Axial Ligands in Low-Spin Fe(III) Heme Complexes As Determined from Frozen-Solution Samples

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Hyperfine (hf) splittings due to the ^<57>Fe nucleus (I=1/2) have been observed in frozen-solution ESR measurements on a series of low-spin Fe(III) heme complexes, Fe(TPP)L_1L_2, where TPP =tetraphenylporphine dianion, L_1 = RS^-, L_2 = MeOH, DMF, and RSH, and R = Et, n-Pr, i-Pr, n-Bu, s-Bu, and t-Bu. The hf splitting (A_<(i)>) values of the g_i absorption peaks (i = X, Y, Z) are analyzed within the framework of a low-symmetry (C_2) crystal field model to determine the rotational orientation angle (phi) of the axes of rhombic component field and that (phi_l) of the axial thiolato ligand plane (Fe-S-C plane) with the approximation of phi_l = phi. It is found that phi_l varies from 6 to 37ﾟ depending upon the thiolato ligand (RS^-), the 6th axial ligand (L_2), and the solvent system (CH_2Cl_2 - MeOH, DMF-MeOH, or CH_2Cl_2-RSH), but with no clear correlation between phi_l and RS^-. However, phi_l correlates well with the energy splittings in the three t_2 orbitals: phi_l increases as the tetragonal splitting decreases or as the rhombic splitting and crystal field rhombicly increase. These results suggest that two major factors, namely, steric and electronic effects, influence the axial ligand orientation. The observed angles phi_l = 6-37ﾟ are the results of the two effects.