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

在冷冻溶液中对一系列低自旋铁（III）血红素配合物Fe(TPP)L_1L_2进行了ESR测量，观察到^<57>Fe核（I=1/2）引起的超细（hf）分裂，其中TPP =四苯基卟啉二离子，L_1 = RS^-, L_2 = MeOH， DMF和RSH， R = Et, n-Pr, I - pr, n-Bu， s-Bu和t-Bu。在低对称（C_2）晶体场模型的框架内，分析了g_i吸收峰（i = X， Y， Z）的hf分裂（A_<(i)>）值，确定了菱形分量场轴线的旋转取向角（phi）和轴向硫原子配体平面（Fe-S-C平面）的旋转取向角（phi_l = phi）。发现ph_l随硫代配体（RS^-）、第6轴配体（L_2）和溶剂体系（CH_2Cl_2 -MeOH、DMF-MeOH或CH_2Cl_2- rsh）的不同而变化在6 ~ 37之间，但ph_l与RS^-之间没有明显的相关性。然而，phi_l与三个t_2轨道的能量分裂有很好的相关性：phi_l随着四方分裂的减少或随着菱形分裂和晶体场菱形分裂的增加而增加。这些结果表明两个主要因素，即空间效应和电子效应，影响轴向配体的取向。观察到的角度phi_l = 6 ~ 37是两种作用的结果。

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.