Keywords: High-spin species / Nonheme / Iron / Oxo ligand / Diamond core / Quantum chemistry 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 non-
A detailed density functional theory study of pseudotetrahedral Fe(III/IV)-imido-phosphine complexes has yielded a host of new insights. The calculations confirm dxy(2)dx(2)-y(2)(2)dz(2)(1) (or dδ(2)dδ'(2)dσ(1)) electronic configurations for Fe(III)-imido complexes of this type, as previously proposed, where the z direction may be identified with the Fe-Nimido vector. However, geometry optimization of a sterically unencumbered model complex indicated a bent (162°) imido linkage, in sharp contrast to the linear imido groups present in the sterically hindered complexes that have been studied experimentally. Under C3v symmetry, the Fe(III)-imido molecular orbital (MO) energy-level diagram indicates the existence of near-degenerate (2)A1 and (2)E states, and accordingly, the bending of the imido group appears to be ascribable to a pseudo-Jahn-Teller distortion. For Fe(IV)-imido complexes, our calculations indicate a dxy(2)dx(2)-y(2)(1)dz(2)(1) (or dδ(2)dδ'(1)dσ(1)) electronic configuration, which is somewhat different from the dxy(1)dx(2)-y(2)(1)dz(2)(2) (or dδ(1)dδ'(1)dσ(2)) configuration proposed in the literature. Not surprisingly, for a sterically unencumbered Fe(IV)-imido complex, the degenerate (3)E state (under C3v symmetry) results in a mild Jahn-Teller distortion and a slightly bent (173°) imido linkage (on relaxing the symmetry constraint). The calculations also shed light on the surprising stability of the dz(2)-based MO, which points directly at the imido nitrogen, relative to the dπ-based MOs. The low-coordinate nature of the complexes [Formula: see text] the absence of equatorial ligands and of a ligand trans with respect to the imido ligand [Formula: see text] plays a key role in stabilizing the dz(2) orbital as well as the complexes as a whole. The electronic configurations of Fe(IV)-imido porphyrins are radically different from that of the pseudotetrahedral complexes studied here, and we have speculated that these differences may well account for the nonobservation so far of Fe(IV)-imido porphyrins.
Density functional theory calculations (PW91/STO-TZP, including basis-set superposition error corrections) have been used to evaluate hydrogen bond energies of five- and six-coordinate heme-NO complexes with phenol and imidazole, chosen as models for distal pocket tyrosine and histidine residues. The calculated interaction energies are approximately 2 kcal/mol for phenol and 3-4 kcal/mol for imidazole, which are 2-4 times smaller than the energies calculated for heme-O(2) complexes hydrogen-bonding with a distal histidine. Interestingly, the hydrogen bond energies are found to be very similar for five- and six-coordinate heme-NO complexes, which may be viewed as contrary to the interpretation of a recent observation on a bacterial H-NOX (Heme-Nitric oxide/OXygen-binding) protein with sequence homology to mammalian-soluble guanylate cyclase.
We have carried out a density functional theory study of the S = 1/2 [FeNO]7 tropocoronand complex, Fe(5,5-TC)NO, as well as of some simplified models of this compound. The calculations accurately reproduce the experimentally observed trigonal-bipyramidal geometry of this complex, featuring a linear NO in an equatorial position and a very short Fe-N(NO) distance. Despite these unique structural features, the qualitative features of the bonding turn out to be rather similar for Fe(5,5-TC)NO and [FeNO]7 porphyrins. Thus, there is a close correspondence between the molecular orbitals (MOs) in the two cases. However, there is a critical, if somewhat subtle, difference in the nature of the singly occupied MOs (SOMOs) between the two. For square-pyramidal heme-NO complexes, the SOMO is primarily Fe d(z)2-based, which favors sigma-bonding interactions with an NO pi orbital, and hence a bent FeNO unit. However, for trigonal-bipyramidal Fe(5,5-TC)(NO), the SOMO is best described as primarily Fe d(x2-z2) in character, with the Fe-N(NO) vector being identified as the z direction. Apparently, such a d orbital is less adept at sigma bonding with NO and, as such, pi bonding dominates the Fe-NO interaction, leading to an essentially linear FeNO unit and a short Fe-N(NO) distance.
A DFT study of cobalt-nitrosyl [n,n]tropocoronand (TC-n,n) complexes has revealed a sharp reduction of singlet-triplet gaps as the structures change from near-square-pyramidal 10 (for n = 3) to trigonal-bipyramidal with an equatorial NO (for n = 5, 6). For n = 6, low-energy triplet states may result in enhanced reactivity, which would account for the failure to isolate [Co(TC-6,6)(NO)] as a stable species. reinvestigation of the molecule. The calculations also revealed a decreasing singlet-triplet gap and low-energy triplet states as the coordination geometry changed from the nearly SQP TC-3,3 complex to the TBP TC-5,5 and TC-6,6 complexes. Thermally accessible triplet states are a distinct possibility for 6,6)(NO)], which would explain its apparent instability. The lowest-energy singlet and triplet states of the [Co(TCn,n)(NO)] complexes (n = 3-6) were optimized with the B3LYP, OLYP, and PW91 functionals and the 6-311G(d,p) basis set. In general, the hybrid functional B3LYP led to a broken-symmetry 45 M S = 0 solution as the ground state, whereas the pure functionals Scheme 1. Dianion of an [n,n]tropocoronand (n,n-TC) ligand.PW91 and OLYP led to closed-shell ground states. Interestingly, 50 depending on the starting point of the optimizations, two distinct triplet states, denoted T 1 and T 2 in the discussion below, could be obtained. The T 1 state may be described as low-spin S = ½ Co(II) ferromagnetically coupled to a NO radical, and the T 2 state may as a high-spin S = 2 Co(III) antiferromagnetically coupled to an S 55 = 1 NO -anion. Table 1
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