Spin-forbidden hydrogen atom transfer reactions in a cobalt biimidazoline system

Abstract

Described here are hydrogen atom transfer (HAT) reactions from high-spin cobalt(II) tris(2,2′-bi-2-imidazoline) (Co^IIIIH₂2bim) to the hydrogen atom acceptors, 2,2,6,6-tetramethyl-1-piperidinyl-oxyl (TEMPO), 2,4,6-tri-tert-butylphenoxyl radical (^tBu₃ArO˙), and benzoquinone (BQ). The cobalt product is the oxidized and deprotonated, low-spin cobalt(III) complex (Co^IIIIIIHbim), and the organic products are TEMPOH, ^tBu₃ArOH, or hydroquinone, respectively. These reactions are formally spin forbidden because the spin state of the reactants is different from that of the products. For instance, quartet Co^IIIIH₂2bim plus doublet RO˙ can have a triplet or quintet ground state, while the Co^IIIIIIHbim + ROH product state is a singlet. Kinetics measured in the forward and reverse directions and thermochemical measurements provide a detailed picture of the reactions. The reactions are quite slow: the reaction of 10 mM Co^IIIIH₂2bim with excess TEMPO requires roughly a day at ambient temperatures to reach equilibrium. This is 3400 times slower than the related reaction of the iron analogue Fe^IIIIH₂2bim, which is 2 kcal mol⁻¹ more uphill. Mechanistic analyses show that the TEMPO reaction occurs by hydrogen atom transfer (HAT), and this is likely for the ^tBu₃ArO˙ and BQ reactions as well. This is an unusually well defined spin-forbidden HAT system, which serves as a model for more complex multi-spin state HAT processes such as those suggested to occur in cytochrome P450 and metal-oxo model systems. In principle, HAT could occur before, after, or concerted with spin change. Computational studies indicate a reaction mechanism involving pre-equilibrium spin state interconversion of quartet ⁴4Co^IIIIH₂2bim to its doublet excited state ²2Co^IIIIH₂2bim, followed by spin-allowed HAT to the organic acceptor. This mechanism is consistent with the available kinetic, thermochemical and spectroscopic measurements. It indicates that the slow rates are due to the large change in geometry between Co^IIIIH₂2bim and Co^IIIIIIHbim, rather than any inherent difficulty in changing spin state. The implications of these results for other spin-forbidden or ‘two-state’ HAT processes are discussed.