Structural and oxo-transfer reactivity differences of hexacoordinate and pentacoordinate (nitro) (tetraphenylporphinato)cobalt(III) derivatives

The oxo-transfer catalyst (nitro)(pyridyl)cobalt(HI) tetraphenylporphyrin h as been reinvestigated by substitution of the distal pyridine ligand with 4 -N,N-dimethylaminopyridine and 3,5-dichloropyridine. Differences in their s tructures and in the reactivity of the compounds toward catalytic secondary oxo transfer were investigated by FT-IR and UV-visible spectroscopy, cycli c voltammetry, X-ray diffraction, semiempirical calculations, and reactions with alkenes in dichloromethane solution. Very modest differences in the h exacoordinate compounds' structures were predicted and observed, but the se condary oxo-transfer reactivity at the nitro ligand varies markedly with th e basicity of the pyridine ligand and the position of the coordination equi librium. Oxo transfer occurs rapidly through the pentacoordinate species (n itro)cobalt(HI) tetraphenylporphyrin that is generated by dissociation of t he pyridine ligand and therefore is strongly related to the Hammett paramet ers of these nitrogenous bases. The reactive pentacoordinate species CoTPP( NO2) can be generated in solution by addition of lithium perchlorate to (py )CoTPP(NO2) by Lewis acid-base interactions or more simply by using the wea ker Lewis base Cl(2)py instead of py as the distal ligand. In contrast to p entacoordinate (nitro)iron porphyrins, disproportionation reactions of CoTP P(NO2) compound are not evident. This pentacoordinate derivative, CoTPP(NO2 ), is reactive enough to stoichiometrically oxidize allyl bromide in minute s. Preliminary catalytic oxidation reaction studies of alkenes also indicat e the involvement of both radical and nonradical oxo-transfer steps in the mechanism, suggesting formation of a peroxynitro intermediate in the reacti on of the reduced CoTPP(NO) with O-2.