Local Oxidation States in {FeNO}6–8 Porphyrins: Insights from DMRG/CASSCF–CASPT2 Calculations

A first DMRG/CASSCF–CASPT2 study of a series of paradigmatic {FeNO}6, {FeNO}7, and {FeNO}8 heme–nitrosyl complexes has led to substantial new insight as well as uncovered key shortcomings of the DFT approach. By virtue of its balanced treatment of static and dynamic correlation, the calculations have provided some of the most authoritative information available to date on the energetics of low- versus high-spin states of different classes of heme–nitrosyl complexes. Thus, the calculations indicate low doublet–quartet gaps of 1–4 kcal/mol for {FeNO}7 complexes and high singlet–triplet gaps of ≳20 kcal/mol for both {FeNO}6 and {FeNO}8 complexes. In contrast, DFT calculations yield widely divergent spin state gaps as a function of the exchange–correlation functional. DMRG–CASSCF calculations also help calibrate DFT spin densities for {FeNO}7 complexes, pointing to those obtained from classic pure functionals as the most accurate. The general picture appears to be that nearly all the spin density of Fe[P](NO) is localized on the Fe, while the axial ligand imidazole (ImH) in Fe[P](NO)(ImH) pushes a part of the spin density onto the NO moiety. An analysis of the DMRG–CASSCF wave function in terms of localized orbitals and of the resulting configuration state functions in terms of resonance forms with varying NO(π*) occupancies has allowed us to address the longstanding question of local oxidation states in heme–nitrosyl complexes. The analysis indicates NO(neutral) resonance forms [i.e., Fe(II)–NO0 and Fe(III)–NO0] as the major contributors to both {FeNO}6 and {FeNO}7 complexes. This finding is at variance with the common formulation of {FeNO}6 hemes as Fe(II)–NO+ species but is consonant with an Fe L-edge XAS analysis by Solomon and co-workers. For the {FeNO}8 complex {Fe[P](NO)}−, our analysis suggests a resonance hybrid description: Fe(I)–NO0 ↔ Fe(II)–NO–, in agreement with earlier DFT studies. Vibrational analyses of the compounds studied indicate an imperfect but fair correlation between the NO stretching frequency and NO(π*) occupancy, highlighting the usefulness of vibrational data as a preliminary indicator of the NO oxidation state.