Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation

Abstract

The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) 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 k(3H) = 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 (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/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(II)imH(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.

Figure 1

(a) Plot of [Ru^IIimH][TEMPO^•]/[Ru^IIIim] vs. [TEMPO-H], with the slope 1/K_3H = (5.5 ± 0.6) × 10⁻⁴ at 298 K and (b) van’t Hoff plot for Ru^IIimH(D) + TEMPO^• ⇌Ru^IIIim + TEMPO-H(D) (K_3H = ●, K_3D = ■) in MeCN.

Figure 2

(a) Overlay of UV-vis spectra for the reaction of Ru^IIimH (0.053 mM) with TEMPO^• (0.53 mM) in MeCN at 298 K over 10 s. (b) Absorbance at 568 nm showing the raw data (○) and first order A → B fit using SPECFIT (—).

Figure 3

(a) Pseudo first order plot of k_obs vs. [TEMPO^•] at 298 K and (b) Eyring plot for the reactions of Ru^IIimH(D) with TEMPO^• in MeCN [k_3H = ● (no CH₃OH), ▲ (25 mM CH₃OH); k_3D = ■ (25 mM CD₃OD)]. The Eyring plot also shows the calculated k_3H,calc and k_3D,calc values in dashed lines (---) using the Marcus cross relation (see below).

Figure 4

Partial ¹H NMR spectra of Ru^IIimH (2.0 mM, top spectrum) in CD₃CN at 298 K, showing line broadening with increasing concentrations of Ru^IIIim (0.092–0.73 mM).

Figure 5

(a) Plot of π(Δfwhm) vs. [Ru^IIIim] at 298 K and (b) Eyring plot for the self-exchange reactions of Ru^IIimH(D) with Ru^IIIim in CD₃CN [k_4H = ● (no CH₃OH), ▲ (250 mM CH₃OH); k_4D = ■ (250 mM CD₃OD)].

Chart 1

Ground state free energy changes for possible initial steps in reaction 3.