Direct Measurement of Electron Self-Exchange Rates of Cobalt Porphyrins. 2. Inner-Sphere Exchange

Article abstract of DOI:10.1021/ja00370a022

Inner-sphere electron exchanges between cobalt(III) and cobalt(II) complexes of 5,10,15,20-tetra-p-tolylporphine, TTP, were studied.The Co^IIITTP compounds retain a coordinated anion in the absence of added ligand and the exhanges of the Co(TTP) complexes are fast enough to cause extensive broadening of the 1H NMR spectra.The effect of various bridging ligands was studied, and most rates were determined at the temperatures of coalescence of the p-methyl peaks in several solutions of varying concentrations, yielding rate constants and activation parameters for exchange.For the chloride-bridged exchange in CDCl3, these are k₁₁(298 K) = 2.71 x 10⁴ M^-1 s^-1, ΔH(excit.) = 10.3+/-1.5 kcal/mol, ΔS(excit.) = -3.6+/-4.7 cal.K^-1.mol^-1.The order of bridging efficiencies found is I^- > N3^- ca.SCN^- > Br^- > Cl^-.A solvent dependence study of the chloride-bridged exchange showed relative rates in benzene-d₆, CDCl3, and tetrahydrofuran-d₈ in the ratios of 18:1:0.31.The results indicate a catalysis by 4-6 orders of magnitude through mediation of the anion compared to the corresponding outer-sphere exchange rates.