75778-52-8Relevant academic research and scientific papers
Structural and oxo-transfer reactivity differences of hexacoordinate and pentacoordinate (nitro) (tetraphenylporphinato)cobalt(III) derivatives
Goodwin,Bailey,Pennington,Rasberry,Green,Shasho,Yongsavanh,Echevarria,Tiedeken,Brown,Fromm,Lyerly,Watson,Long,De Nitto
, p. 4217 - 4225 (2008/10/08)
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.
Observations regarding the mechanisms of O atom transfer from metal nitro ligands to oxidizable substrates
Andrews, Mark A.,Chang, Tony C.-T.,Cheng, Chi-Wen F.
, p. 268 - 274 (2008/10/08)
New experimental evidence is presented related to the mechanism of O atom transfer from metal nitro complexes to alkenes and carbon monoxide. Oxidation of alkenes by the nitro complex (py)CoTPPNO2 in the presence of Pd(CH3CN)2Cl2 as a cocatalyst is found to give product distributions that do not exactly match those expected for the simple intermolecular bimetallic O atom transfer originally proposed. Pyridine, nitro, and nitrosyl ligand exchange between the cobalt and palladium are found to be facile. In one plausible revised mechanism, the bimetallic alkene oxidation could therefore proceed via nitro group transfer to palladium, oxidation via the same metallacycles as for the monometallic Pd(CH3CN)2ClNO2 catalyst, and transfer of the resulting nitrosyl ligand back to cobalt. Several other complex mechanisms cannot be ruled out. Other systems were also investigated, but no evidence could be found for a bimetallic open-chain intermediate formed by intermolecular addition of a metal nitro complex to a coordinated alkene. A literature claim for intermolecular O atom transfer from metal nitro groups to carbon monoxide based on isotopic double-labeling experiments is incomplete due to the absence of suitable control experiments. Attempts to carry out the requisite controls were hampered by experimental limitations, but the results obtained again show that there is no compelling evidence for intermolecular O atom transfer from a metal nitro group.
Activation of Cobalt-Nitro Complexes by Lewis Acids: Catalytic Oxidation of Alcohols by Molecular Oxygen
Tovrog,Benjamin S.,Diamond, Steven E.,Mares, Frank,Szalkiewicz, Andrew
, p. 3522 - 3526 (2007/10/02)
Lewis acid dramatically enhance the oxidation power of cobalt-nitro complexes.Thus, in the presence of BF3*Et2O or LiPF6, cobalt-nitro complexes such as pyCo(saloph)NO2 or pyCo(TPP)NO2 oxidize primary alcohols to aldehydes and secondary alcohols to ketones.No reaction is observed in the absence of Lewis acid.The effect of Lewis acids is attributed to their association with the nitro ligands, thereby increasing its electrophilicity.The results strongly suggest that the oxidation proceeds via an "ester-like" intermediate, which in a nonradical pathway collapses to the carbonyl product, water, and the corresponding nitrosyl complex.Importantly, it has been found that the reoxidation of the nitrosyl complexes by molecular oxygen is facile in the presence of Lewis acids.This finding facilitated the conversion of the stoichiometric oxidation of alcohols into a catalytic system using molecular oxygen as the oxidant.Initial oxidation rates are rapid.However, the rates decline as the byproduct, water, accumulates in the reaction mixture.
