Influence of Nitrogen Base Ligation and Hydrogen Bonding on the Rate Constants for Oxygen Transfer from Percarboxylic Acids and Alkyl Hydroperoxides to (meso-Tetraphenylporphinato>manganese(III) Chloride

Article abstract of DOI:10.1021/ja00267a039

Equilibrium constants for axial ligation of imidazole (ImH), N-methylimidazole (N-MeIm), 4'-(imidazo-1-yl)-acetophenone (NAcPhIm), 2,6-lutidine (2,6-Py), and 3,4-lutidine (3,4-Py) with (meso-tetraphenylporphinato)manganese chloride ((TPP)Mn^IIICl) have been determined.The rates of oxygen atom transfer from percarboxylic acids and alkyl hydroperoxides (YOOH) to the manganese(III) porphyrin in the presence of varying concentrations of the nitrogen bases were determined.For this purpose, 2,2-diphenyl-1-picrylhydrazine (DPPH) was employed as a trap for the generated higher valent oxo-manganese porphyrin species.From the equilibrium and kinetic data, there was then calculated the second-order rate constants for oxygen atom transfer from YOOH compounds to the species (TPP)Mn^IIICl, TPP(Cl)Mn^IIIN, and TPP(Cl)Mn^IIIN2 (where N = ImH, N-MeIm, and 3,4-Py).Only the percarboxylic acids exhibit measurable rate constants for oxygen transfer to (TPP)Mn^IIICl, whereas alkyl hydroperoxides and percarboxylic acids transfer oxygen to the TPP(Cl)Mn^IIIN species.Of the species TPP(Cl)Mn^IIIN2, reaction with YOOH compounds is seen only when N is imidazole.This is attributed to an equilibrium of the unreactive bis axially ligated TPP(Cl)Mn^III(ImH)2 with the reactive isomeric mono axial-ligated complex Cl(1-)...H-Im...H-Im...Mn^IIITPP.Nitrogen base ligation of (TPP)Mn^IIICl provides minimally a 1E3 increase in the rate constants for oxygen transfer in methylene chloride.Linear free-energy plots of the log of the second-order rate constants for oxygen transfer from YOOH vs. the pK_a of YOH establish that β_1g for oxygen transfer in which heterolytic O-O bond scission is rate-determining is large and negative.The value of β_1g when oxygen transfer involves rate-determining homolytic O-O bond scission is small and negative.