Computational investigations into hydrogen-atom abstraction from rhodium hydride complexes by methyl radicals in aqueous solution

The controversy in the reported kinetics for the hydrogen-atom abstraction reaction by methyl radicals for cis- and trans-[(Cyclam)Rh^IIIHCl] ⁺ and trans-[(Cyclam)(H₂O)Rh^IIIH]²⁺ has been resolved by studying several feasible mechanistic pathways with density functional theory. The only low-energy reaction mechanism predicted by these calculations involves a single-step radical-propagation mechanism in which the methyl radical simply abstracts the Rh bound H atom from the complex to form methane and the reduced Rh product. Previous experimental work on the chloride and aquo complexes suggested contradictory kinetic isotope effect (KIE) values of 0.66±0.30 and 1.42±0.07 as well as rate constants for the reaction differing by four orders of magnitude. The calculated mechanism predicts a KIE value of 1.08 and a high reaction rate. The alternative mechanisms are described briefly. The controversy in the reported kinetics for the hydrogen-atom abstraction reaction bymethyl radicals for cis- and trans-[(Cyclam)Rh^IIIHCl]⁺ and trans-[(Cyclam)(H ₂O)Rh^IIIH]²⁺ has been resolved by studying several feasible mechanistic pathways with density functional theory.