Kinetics and Mechanism of Hydrogen‐Atom Abstraction from Rhodium Hydrides by Alkyl Radicals in Aqueous Solutions

The kinetics of the reaction of benzyl radicals with [L 1 (H 2 O)RhH{D}] 2+ (L 1 =1,4,8,11‐tetraazacyclotetradecane) were studied directly by laser‐flash photolysis. The rate constants for the two isotopologues, k =(9.3±0.6) × 10 7   M −1  s −1 (H) and (6.2±0.3) × 10 7   M −1  s −1 (D), lead to a kinetic isotope effect k H / k D =1.5±0.1. The same value was obtained from the relative yields of PhCH 3 and PhCH 2 D in a reaction of benzyl radicals with a mixture of rhodium hydride and deuteride. Similarly, the reaction of methyl radicals with {[L 1 (H 2 O)RhH] 2+ + [L 1 (H 2 O)RhD] 2+ } produced a mixture of CH 4 and CH 3 D that yielded k H / k D =1.42±0.07. The observed small normal isotope effects in both reactions are consistent with reduced sensitivity to isotopic substitution in very fast hydrogen‐atom abstraction reactions. These data disprove a literature report claiming much slower kinetics and an inverse kinetic isotope effect for the reaction of methyl radicals with hydrides of L 1 Rh.