SINGLE-CRYSTAL SPECTROSCOPIC STUDIES OF FE(SC6H4PH-2)4(2-) ELECTRONIC-STRUCTURE OF THE FERROUS SITE IN RUBREDOXIN

Low-temperature single-crystal magnetic circular dichroism (MCD) and polarized absorption studies were performed on an axial ferrous rubredoxin structural model complex, [Et4N]2[Fe(SR)4] (R = 2-(Ph)C6H4). This complex was found to have a d(x2-y2) ground state with the d(z2) orbital at 1400 cm-1. A ligand field analysis of the observed and assigned E5 --> T5(2) and E5 --> GAMMA-3 transitions gives Dq = -350 cm-1, C = 2800 cm-1, and B = 620 cm-1. In contrast to ferric thiolate complexes which exhibit much larger reductions, the 70% reduction in ferrous electron repulsion parameters from the free ion values is accounted for based on standard spin restricted ligand field theory. This indicates that the inverted bonding description found for ferric complexes is not present in the ferrous complexes. Thus a large electronic relaxation takes place upon reduction which should affect redox properties of iron thiolate complexes. A calculation of the ground-state zero-field splitting based on spin-orbit coupling to the T5(2) and GAMMA-3 ligand field excited states gives D(calc) = -8.7 cm-1 which is in excellent agreement with D(exp) = -8.7 +/- 0.7 cm-1 as determined from the MCD temperature dependence. The splitting of the ferrous 3d orbitals is found to depend on the interactions with the S-Fe-sigma bonding orbital as determined by the alpha-C orientation. This effect accounts for the ground-state differences between the model complex and ferrous rubredoxin and reveals a strong dependence of the ground state on the alpha-C orientation.