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

作者：Gonzalez Emmanuel; Brothers Penelope J^*; Ghosh Abhik

来源：Journal of Physical Chemistry B, 2010, 114(46): 15380-15388.

DOI：10.1021/jp107692m

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 pi-lone pair, unlike hydroxide, alkoxide, and thiolate ligands, which have two. The former therefore engage in a single pi-bonding interaction with one of the Ru d(pi) orbitals, resulting in an S = 0 d(4) electronic configuration. In contrast, the O or S ligands present in compound II engage in pi-bonding with both d(pi) orbitals, resulting in an S = I 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(pi)) -> N-methyleneamido(pi*) backbonding to explain Ru-N-methyleneamido multiple bond character. Instead, we found that the metal-methyleneamido pi-bonding almost exclusively involves N-to-Ru pi-donation and thus is qualitatively very similar to metal-amido pi-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.

• 出版日期2010-11-25