Electronic Structure of High-Spin Iron(IV) Complexes

High‐spin (S = 2) iron(IV) species are rare but increasingly recognized as reactive intermediates in the catalytic cycles of several nonheme iron enzymes. A question of some interest, therefore, concerns how much higher in energy the low‐spin (S = 1) state is for these species. With the use of density functional theory (DFT) and high‐level ab initio calculations [CASPT2 and CCSD(T)], we have attempted to answer this question for the so‐called Collins complex, a square‐pyramidal FeIV complex with a tetraamido‐N equatorial ligand set, a chloride axial ligand, and an S = 2 ground state. The calculations suggest that relative to the ground state, the low‐spin state is higher in energy by at least 0.3 eV and possibly as much as 0.7 eV. Using DFT calculations, a broad quantum chemical survey of high‐spin FeIVO intermediates was also undertaken. A key finding is that the Fe−O distance and O spin population are quite similar across all mononuclear FeIVO species studied, regardless of the heme versus nonheme environment and of the S = 1 versus 2 spin state, reflecting the essential similarity of the Fe(dπ)−O(pπ) orbital interactions in all the species studied. However, the spin density profiles of high‐spin FeIVO species, currently believed to be known only as a nonheme iron enzyme (TauD) intermediate, are predicted to be very different from that of Collins’ high‐spin FeIV complex. Our calculations further suggest that with the help of sterically hindered ligands such as 6‐me3‐tpa, it might be possible to generate synthetic high‐spin FeIVO models of the unique TauD intermediate. Finally, our calculations confirm the aptness of describing the [(6‐me3‐tpa)FeIII(μ‐O)2FeIV(6‐me3‐tpa)]3+ cation as a flexible diamond core and indicate the presence of a fairly discrete high‐spin FeIVO unit within the dinuclear core. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)