Variation of Oxo-Transfer Reactivity of (Nitro)Cobalt Picket Fence Porphyrin with Oxygen-Donating Ligands

Abstract

Derivatives of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands have been prepared in solution and in the solid state. Crystal structures of two of these derivatives, (H2O)CoTpivPP(NO2) and (CH3OH)CoTpivPP(NO2), have been determined. The ethanol complex (C2H5OH)Co(TPP)(NO2) has been obtained and spectrally characterized using sublimed layers methodology. The formation constant and the DeltaH degrees value of the association reaction with ethanol have been determined by FTIR measurements in CCl4 solution. Catalytic oxygen activation and oxo-transfer reactions of these derivatives have been assessed in solution. Correlations between the oxo-transfer reactivity, thermodynamics, and characteristics of the nitro ligand show that although calculated and observed ONO vibrational spectra and bond lengths suggest activation of the NO2 ligand and enhanced oxo-transfer reactions as seen in the analogous five-coordinate complexes, density functional theory calculations support that thermodynamics limits oxo-atom transfer reactions in these six-coordinate systems.

FTIR and optical spectroscopic studies of the reactions of heme models with nitric oxide and other NOx in porous layered solids

“…Since a higher value of ν corresponds a larger ONO angle [25], this is consistent with the ONO angles which are 120-124 • for the Co(III) complexes [25] and are less acute seen for similar nitrito complexes of Fe(III) porphyrins (116-120 • ) [28].…”

“…In addition, as will be discussed below, nitrate can be activated by heme models to produce other highly reactive NO x species [22], and such processes have potential relevance to the chemical biology of the nitrogen oxides. Combinations of redox active metal complexes and nitrogen oxides have also proved to be effective mediators in schemes for catalytic oxidations of various substrates [23][24][25].…”

Crystal structures of manganese- and cobalt-substituted myoglobin in complex with NO and nitrite reveal unusual ligand conformations

“…To date, all crystallographically characterized nitrite adducts of synthetic cobalt porphyrins demonstrate that the nitrite anion is bound to the cobalt centers via the nitro N-binding mode [120][121][122][123]. To the best of our knowledge, there is no exception to this observation for model cobalt porphyrin compounds.…”

“…Thus, the structure of Co III Mb(ONO À ) represents the first report of a cobalt nitrite heme protein, and is the first stable nitrito binding to cobalt porphyrins to be established. Clearly, it is evident that although information from crystal structures of cobalt nitrite compounds of model porphyrins have helped in understanding the mechanisms of action of these compounds as catalysts for chemical transformation such as O-atom transfers in oxidation reactions (generating (por)-Co(NO) intermediates) [123], the structures may not correlate very well with the geometries present in cobaltsubstituted heme proteins. The ability of the distal pocket in Co III Mb to effect the nitrito O-binding preference is significant, and suggests that the hydrogen-bonding provided by the distal His64 residue plays a primary role in stabilizing this binding mode.…”

Mechanistic studies of nitrite reactions with metalloproteins and models relevant to mammalian physiology

“…Particularly relevant to the present review are the reports by Castro et al [137,138] that stoichiometric OAT to various substrates can be accomplished in solutions of 18-C-6 crown ether solubilized nitrite salt [K(18-C-6)]NO 2 and Fe(OEP)Cl (OEP = octaethyl-porphyrinato dianion) in acetic acid/N-methylpyrrolidone (NMP) solutions. A ferric nitro-complex Fe III (OEP)(NO 2 − ) was postulated to be a key reactive intermediate in analogy to OAT reactions from similar cobalt(III) complexes [139,140]. Given the donor solvent used, it may be assumed that Fe III (OEP)(NO 2 − ), if present, is six coordinate, and in the context of the studies by Kurtikyan described above [66], would be the nitro isomer.…”