We report herein the observation that electronic and steric factors of cobalt dioxygen complexes with systematically derivatized pentadentate Schiff-base ligands affect their ability to catalyze the oxidation of substituted phenols and alter the rate of catalyst decomposition. The electronic and steric factors can be separated sufficiently to indicate that the activity of the cobalt dioxygen catalyst roughly parallels the trends in basicity of the bound 02 suggested by EPR cobalt hyperfine coupling constants of the adducts and predicted from the electronic effects of substituents.
The reactivity of [ 1,4,7,10,13-pentaazacyclohexadecane-14,16-dionato(2-)] nickel(II), NinL, toward H3CSCH2CH2SO3-, methyl-coenzyme M, possesses striking similarities to the active site chemistry of methyl-coenzyme M reductase, which contains the nickel tetrapyrrole species F430. The initial rate of substrate conversion for a series of substrate analogues of the formula RX(CH2)"Y" is reported, where the R and X groups have the greatest influence on reactivity. When R = CH3 and X = S, little change in reactivity is observed when = 2, 3, or 6 or when Y" = S03", C02", or P03". These results are consistent with the two-site model for substrate binding at F430, the proposed enzyme active site.
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