Electronic Structure and Ligand Vibrations in FeNO, CoNO, and FeOO Porphyrin Adducts

Abstract

The gaseous ligands, CO, NO and O2 interact with the Fe ion in heme proteins largely via backbonding of Fe electrons to the π* orbitals of the XO (X = C, N, O) ligands. In these FeXO adducts, the Fe–X stretching frequency varies inversely with the X–O stretching frequency, since increased backbonding strengthens the Fe–X bond while weakening the X–O bond. Inverse frequency correlations have been observed for all three ligands, despite differing electronic and geometric structures, and despite variable composition of the `FeX' vibrational mode, in which Fe–X stretching and Fe–X–O coordinates are mixed for bent FeXO adducts. We report experimental data for 5-coordinate CoII(NO) porphyrin adducts (isoelectronic with FeII(OO) adducts), and the results of DFT modeling for 5-coordinate FeII(NO), CoII(NO) and FeII(OO) adducts. Inverse ν(MX)/ν(XO) correlations are obtained computationally, using model porphyrins with graded electron-donating and -withdrawing substituents to modulate the backbonding. Computed slopes agree satisfactorily with experiment, provided non-hybrid functionals are used, which avoid overemphasizing high-spin states. The BP86 functional gives correct ground states, a closed-shell singlet for CoII(NO) and an open-shell singlet for the isoelectronic FeII(OO), as corroborated by structural data for CoII(NO), and the ν(MX)/ν(XO) slope agreement with experiment for both adducts. However, for FeII(OO) adducts, the computed inverse ν(MX)/ν(XO) correlation applies only to porphyrins with electron-donating and withdrawing substituents of moderate strength. For substituents more donating than –CH3, a direct correlation is obtained, the Fe–O and O–O bonds weakening in concert. This effect is ascribed to the dominance of σ bonding via the in-plane dxz(+dz2)-π* orbital, when electron-donating substituents raise the d orbital energies sufficiently to render backbonding (dyz-π*) unimportant.

Role of Spin States in Nitric Oxide Binding to Cobalt(II) and Manganese(II) Porphyrins. Is Tighter Binding Always Stronger?

“…For more elaborate discussions of vibrational frequencies of CoP(NO) and structural parameters of both complexes, the reader is referred to the literature. 9,23 , formally assigned as the NO + complex, is slightly longer than the analogous distance in CoP(NO), formally assigned as the NO − complex. This holds true at least according to DFT:BP86 calculations, whereas the crystal structure data are less conclusive (the mentioned difference in bond distance is small and thus easily affected by crystal packing effects), although they also indicate that the (Co)N−O distance is about the same as the (Mn)N−O one, if not slightly shorter.…”

“…In fact, it is already a significant challenge to describe the complicated electronic structures of the resulting oxy-and nitrosyl-complexes, and to depict the metal−ligand bonding mechanism in strict chemical terms. 9,14−16 Another challenge for computational methods is to reproduce the metal−ligand bond dissociation energy with chemical accuracy. 11,12,17 In this regard, the recent studies of NO binding to Fe(II) and Fe(III) porphyrins revealed significant errors of common density functional theory (DFT) methods, some of which (hybrid functionals) tend to underestimate while others (pure functionals) tend to overestimate the Fe−NO bond energies.…”

CO, NO and O2 as vibrational probes of heme protein interactions

“…DFT modeling of 6-coordinate Fe(II)O 2 adducts with imidazole as axial ligand shows the same pattern as the 5-coordinate adducts [49] (Fig. 20).…”

“…Despite the longer Fe-O bonds, the νFeO frequencies shift to higher values, reflecting a shift in mode composition induced by the axial ligation [38, 49], as also seen for Fe(II)NO adducts ( Section 5.1).…”

“…Evidence that BP86 gives the correct ground state can be seen in comparison of computed and experimental structures[44-48] for Co(II)NO; (5-coordinate Fe(II)O 2 experimental structures are unavailable) and frequencies (see below). [49] For Fe(II)O 2 , when the closed-shell singlet state is used in the frequency calculation, the νFeO/νOO correlation is in poor agreement with experiment.…”