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Electronic Structure of High‐Spin Iron( IV ) Complexes
Author(s) -
Ghosh Abhik,
Tangen Espen,
Ryeng Hege,
Taylor Peter R.
Publication year - 2004
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200400362
Subject(s) - chemistry , spin states , density functional theory , ground state , ligand (biochemistry) , spin (aerodynamics) , ab initio , population , oxidation state , electronic structure , crystallography , computational chemistry , ab initio quantum chemistry methods , catalysis , molecule , atomic physics , inorganic chemistry , organic chemistry , thermodynamics , physics , biochemistry , receptor , demography , sociology
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 Fe IV O intermediates was also undertaken. A key finding is that the Fe−O distance and O spin population are quite similar across all mononuclear Fe IV O 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(d π )−O(p π ) orbital interactions in all the species studied. However, the spin density profiles of high‐spin Fe IV O 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 Fe IV O models of the unique TauD intermediate. Finally, our calculations confirm the aptness of describing the [(6‐me 3 ‐tpa)Fe III (μ‐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 Fe IV O unit within the dinuclear core. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)