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 Fe-IV 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 (FeO)-O-IV intermediates was also undertaken. A key finding is that the Fe-O distance and O spin population are quite similar across all mononuclear (FeO)-O-IV species studied, regardless of the heme versus non-heme environment and of the S = 1 versus 2 spin state, reflecting the essential similarity of the Fe(d(pi))-O(p(pi)) orbital interactions in all the species studied. However, the spin density profiles of high-spin (FeO)-O-IV 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 Fe-IV complex. Our calculations further suggest that with the help of sterically hindered ligands such as 6-me(3)-tpa, it might be possible to generate synthetic high-spin (FeO)-O-IV models of the unique TauD intermediate. Finally, our calculations confirm the aptness of describing the [(6-me(3)-tpa)Fe-III(mu-O)(2)Fe-IV(6-me(3)-tpa)](3+) cation as a flexible diamond core and indicate the presence of a fairly discrete high-spin (FeO)-O-IV unit within the dinuclear core. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.