Density Functional Theory Calculations on Ruthenium(IV) Bis(amido) Porphyrins: Search for a Broader Perspective of Heme Protein Compound II Intermediates

Presented herein is a first density functional theory (DFT) (ZORA, STO-TZP) survey of ruthenium(IV) porphyrins with monoanionic nitrogen ligands, modeled after experimentally observed ruthenium porphyrin bis(amido), bis(methyleneamido), and bis(pyrazolato) complexes. Three exchange correlation functionals―PW91, OLYP, and B3LYP, which often behave somewhat differently―provide good, consistent descriptions of the lowest singlet and triplet states. For ruthenium porphyrin bis(amido) and bis(methyleneamido) complexes, the calculations reproduce the experimentally observed S = 0 ground states, with the triplet states only a few tenths of an electron-volt higher in energy. The singlet−triplet energy gaps decrease somewhat along the series PW91 > OLYP > B3LYP. Molecular orbital (MO) analyses also provide a qualitative explanation for the singlet ground states of these complexes, which may be contrasted with the triplet states of heme protein compound II intermediates and their synthetic iron(IV) models. Amido and methyleneamido ligands have a single π-lone pair, unlike hydroxide, alkoxide, and thiolate ligands, which have two. The former therefore engage in a single π-bonding interaction with one of the Ru d_π orbitals, resulting in an S = 0 d⁴ electronic configuration. In contrast, the O or S ligands present in compound II engage in π-bonding with both d_π orbitals, resulting in an S = 1 ground state. For the ruthenium(IV) bis(methyleneamido) complexes, our MO analysis indicates a somewhat different bonding description, relative to that proposed by the experimental researchers, who invoked Ru(d_π) → N_{methyleneamido}(π*) backbonding to explain Ru−N_{methyleneamido} multiple bond character. Instead, we found that the metal−methyleneamido π-bonding almost exclusively involves N-to-Ru π-donation and thus is qualitatively very similar to metal−amido π-bonding. Ruthenium(IV) bis(pyrazolato) complexes provide rare examples of ruthenium(IV) centers with all-nitrogen ligation that are paramagnetic. OLYP successfully captures this “inverse” spin state energetics; PW91 and B3LYP do so less well.