Spectroscopic and oxidation studies of meso-tetraphenyltetrabenzoporphyrin carbonyl complexes of ruthenium(II): CO as the probe to elucidate the bonding characteristics of porphyrins

The carbonyl complex of (meso-tetraphenyltetrabenzoporphyrinato)ruthenium(II), Ru(TPTBP)(CO), has been synthesized and characterized by FABMS, UV/vis, H-1 NMR, and IR spectroscopy. Six-coordinate complexes Ru(TPTBP)(CO)(L) with different pi-bonding-capability ligands (L = NEt3, pip, 1-MeIm, py, PBu3) coordinated trans to CO have been studied. The shifts in nu(CO) for this series of complexes are consistent with the existence of M --> CO pi-back-bonding. In contrast to what would be expected by nitrogen basicity, nu(CO) values for Ru(TPTBP)(CO), Ru(TPP)(CO), and Ru(OEP)(CO) are 1959, 1930, and 1917 cm(-1), respectively. This result suggests that TPTBP should be both a better sigma-donor and a better pi-acceptor than normal porphyrin systems (P). Oxidation studies of Ru(TPTBP)(CO), Ru(TPTBP)(CO)(py), and Ru(TPTBP)(py)(2) have been carried out both electrochemically and chemically. H-1 NMR, ESR, and electronic spectroscopic studies suggest that there are two different types of oxidation products. The sites of oxidation should both be on the porphyrin ring to give two different types of ruthenium(II) porphyrin pi-cation radicals [Ru-II(TPTBP)(.+)(L)(L')]X of A(1u) and A(2u) character, respectively. In marked contrast to other ruthenium porphyrins reported in the literature, extraplanar ligands in the Ru(TPTBP) system do not affect the site of oxidation (metal vs ring) but only mediate the level of oxidation on the ring (a(1u) vs a(2u)). These results can be ascribed to the extended pi-system and the ring deformation of the TPTBP porphyrin macrocycle and are also consistent with the fact that TPTBP is a stronger pi-acceptor than other porphyrin systems.