Structural and Oxo-Transfer Reactivity Differences of Hexacoordinate and Pentacoordinate (Nitro)(tetraphenylporphinato)cobalt(III) Derivatives

The oxo-transfer catalyst (nitro)(pyridyl)cobalt(III) tetraphenylporphyrin has been reinvestigated by substitution
of the distal pyridine ligand with 4-N,N-dimethylaminopyridine and 3,5-dichloropyridine. Differences in their
structures and in the reactivity of the compounds toward catalytic secondary oxo transfer were investigated by
FT-IR and UV−visible spectroscopy, cyclic voltammetry, X-ray diffraction, semiempirical calculations, and
reactions with alkenes in dichloromethane solution. Very modest differences in the hexacoordinate compounds'
structures were predicted and observed, but the secondary oxo-transfer reactivity at the nitro ligand varies markedly
with the basicity of the pyridine ligand and the position of the coordination equilibrium. Oxo transfer occurs
rapidly through the pentacoordinate species (nitro)cobalt(III) tetraphenylporphyrin that is generated by dissociation
of the pyridine ligand and therefore is strongly related to the Hammett parameters 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 weaker Lewis base Cl2py
instead of py as the distal ligand. In contrast to pentacoordinate (nitro)iron porphyrins, disproportionation reactions
of CoTPP(NO2) compound are not evident. This pentacoordinate derivative, CoTPP(NO2), is reactive enough to
stoichiometrically oxidize allyl bromide in minutes. Preliminary catalytic oxidation reaction studies of alkenes
also indicate the involvement of both radical and nonradical oxo-transfer steps in the mechanism, suggesting
formation of a peroxynitro intermediate in the reaction of the reduced CoTPP(NO) with O2.