Electronic structure and spectra of nitrosyl complexes with cobalt and manganese porphyrins

Theoretical Study on Electronic Structural Properties of Catalytically Reactive Metalloporphyrin Intermediates

Metalloporphyrins have attracted great attention in the potential application of biomimetic catalysis. Especially, they were widely investigated as green catalysts in the chemical oxidation of various hydrocarbons through the catalytic activation of molecular oxygen. The structural properties of active central metal ions were reported to play a decisive role in catalytic activity. However, those delicate structural changes are difficult to be experimentally captured or elucidated in detail. Herein, we explored the electronic structural properties of metalloporphyrins (metal porphyrin (PMII, PMIIICl)) and their corresponding catalytically active intermediates (metal(III)-peroxo(PMIII-O2), metal(III)-hydroperoxo(PMIII-OH), and metal(IV)-oxo(PMIV=O), (M=Fe, Mn, and Co)) through the density functional theory method. The ground states of these intermediates were determined based on the assessment of relative energy and the corresponding geometric structures of ground states also further confirmed the stability of energy. Furthermore, our analyses of Mulliken charges and frontier molecular orbitals revealed the potential catalytic behavior of reactive metalloporphyrin intermediates.

A unified understanding of the direct coordination of NO to first-transition-row metal centers in metal-ligand complexes

The binding of nitric oxide (NO) to heme-proteins is an important biochemical process involved in a variety of physiological functions. Here, using hybrid density-functional calculations, we systematically investigate the adsorption of NO to first-transition-row metal centers in metal-ligand complexes. Through the comparative study for different transition metal (TM) centers, we provide a unified understanding of the microscopic interactions of NO with the TM centers and related chemical trends. We found that as the atomic number of the TM center increases, the binding strength of NO is largely reduced from 207 kJ mol^-1 to near zero due to the low d-orbital energies for late TM centers. The intermolecular spin coupling between the localized spins at the TM center and the NO molecule is generally antiferromagnetic, except for the case of Sc. The spin-spin coupling is determined in such a way to avoid the energy penalty associated with the electron occupation in the antibonding states of the NO-bound complex. The adsorption strength of NO is generally larger than of CO because the unpaired electron of NO occupies the associated bonding state.

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

Binding of nitric oxide (NO) to metalloporphyrins and heme groups is important in biochemistry while challenging to describe accurately by density functional theory (DFT) calculations. Here, the structural and thermochemical aspect of NO binding to Co(II) and Mn(II) porphyrins is investigated by DFT and DFT-D (dispersion-corrected) calculations, supported by reliable coupled-cluster methodology (CCSD(T)), and critically correlated with the experimental data. It is argued that whereas the bonding of NO to Co(II) porphyrin is a simple radical recombination, the bonding of NO to Mn(II) porphyrin is accompanied by a crossing of spin states. For this reason, the spin-state conversion energy contributes to the Mn-NO bond energy, and the paradigmatic correlation between bond length and bond energy is violated for the considered nitrosyl complexes: the Mn-NO bond is (structurally) shorter by ∼0.2 Å, albeit (energetically) weaker by a few kcal/mol, compared with the Co-NO bond. Moreover, none of the many tested DFT methods can reproduce the Co-NO and Mn-NO bond energies simultaneously, except for calculations with B3LYP*-D3, TPSSh-D3, and M06-D3 methods supplemented with the proposed spin-state energy correction (to compensate for an error on the calculated spin-state conversion energy). The results of this study are important to appreciate the role of spin-state changes in ligand binding properties of heme-related models. They also highlight the need for accurate calculations for correct interpretation of experimental data, including the qualitative structure-energy relationship.

Electronic structure and ligand vibrations in FeNO, CoNO, and FeOO porphyrin adducts

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 Co(II)(NO) porphyrin adducts (isoelectronic with Fe(II)(OO) adducts), and the results of density functional theory (DFT) modeling for 5-coordinate Fe(II)(NO), Co(II)(NO), and Fe(II)(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 nonhybrid functionals are used, which avoid overemphasizing high-spin states. The BP86 functional gives correct ground states, a closed-shell singlet for Co(II)(NO) and an open-shell singlet for the isoelectronic Fe(II)(OO), as corroborated by structural data for Co(II)(NO), and the ν(MX)/ν(XO) slope agreement with experiment for both adducts. However, for Fe(II)(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(+dz(2))-π* orbital, when electron-donating substituents raise the d orbital energies sufficiently to render backbonding (dyz-π*) unimportant.