Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation

摘要

The reaction of Ru^II(acac)2(py-imH) (>Ru^IIimH) with TEMPO^• (2,2,6,6-tetramethyl-piperidine-1-oxyl radical) in MeCN quantitatively gives Ru^III(acac)2(py-im) (>Ru^IIIim) and the hydroxylamine TEMPO-H by transfer of H^• (H⁺ + e⁻) (acac = 2,4-pentanedionato, py-imH = 2-(2′-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k3H = 1400 ± 100 M⁻¹ s⁻¹ at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (ΔG°). Deuterium transfer from Ru^II(acac)2(py-imD) (>Ru^IIimD) to TEMPO^• is surprisingly much slower at k3D = 60 ± 7 M⁻¹ s⁻¹, with k3H/k3D = 23 ± 3 at 298 K. Temperature dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, Ea3D − Ea3H = 1.9 ± 0.8 kcal mol⁻¹. The large k3H/k3D and ΔEa values appear to be greater than the semi-classical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between >Ru^IIimH and >Ru^IIIim, measured by ¹H NMR line broadening, occurs with k4H = (3.2 ± 0.3) × 10⁵ M⁻¹ s⁻¹ at 298 K and k4H/k4D = 1.5 ± 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A4H/A_4D) = −0.5 ± 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for >Ru^IIimH(D) + TEMPO^• that are greater than those observed: k_3H,calc/k_3H = 31 ± 4 and k_3D,calc/k_3D = 140 ± 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.