Abstract
Equilibrium constants for mono- and bisligation of imidazole (ImH) with (meso-tetrakis(2,6-dimethylphenyl)porphinato)manganese(III) chloride ((Me/sub 8/TPP)Mn/sup III/Cl) have been determined so that the concentrations (dry CH/sub 2/Cl/sub 2/) of the three species (Me/sub 8/TPP)Mn/sup III/Cl, ((Me/sub 8/TPP)Mn/sup III/(ImH))Cl, and ((Me/sub 8/TPP)Mn/sup III/(ImH)/sub 2/)Cl may be calculated at different ImH concentrations. The equilibrium constants for ligation of the one and two imidazoles are K/sub 1/ = 245 M/sup -1/ and ..beta../sub 2/ = 1.80 x 10/sup 5/ M/sup -1/. The reaction of p-cyano-N,N-dimethylaniline N-oxide (NO) with the manganese(III)porphyrin (under the pseudo-first-order conditions of (NO)/sub i/ >> ((Me/sub 8/TPP)Mn/sup III/Cl)/sub i/ and in the presence and absence of ImH) is first order in both NO and manganese(III) porphyrin, and the rate-controlling step involves oxygen transfer with formation of higher valent manganese-oxo porphyrin species plus p-cyano-N,N-dimethylaniline (DA). From the dependence of the pseudo-first-order rate constants (k/sub obsd/) upon (ImH)/sub i/ and a knowledge of the equilibrium constants for imidazole ligation there has been calculated the second-order rate constants for the kinetic terms k/sub 1/(NO)((Me/sub 8/TPP)Mn/sup III/Cl), k/sub 2/(NO)((Me/sub 8/TPP)Mn/sup III/(ImH))Cl, and k/sub 3/(NO)((Me/sub 8/TPP)Mn/sup III/(ImH)/sub 2/)Cl). Comparison of the second-order rate constants (k/sub 1/ = 3.3 x 10/sup -2/ M/sup -1/ s/sup -1/, k/sub 2/more » = 5.53 M/sup -1/ s/sup -1/, and k/sub 4/ = 7.32 x 10/sup -2/ M/sup -1/ s/sup -1/) establishes that ligation by one imidazole increases the rate of reaction of the manganese(III) porphyrin with NO by approx. 166-fold.« less
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