Slow Hydrogen Atom Transfer Reactions of Oxo- and Hydroxo-Vanadium Compounds: The Importance of Intrinsic Barriers

Abstract

Abstract:Reactions are described that interconvert vanadium(IV) oxo-hydroxo complexes [V IV O(OH)(R 2 bpy) 2 ]BF 4 (1a-c) and vanadium(V) dioxo complexes [V V O 2 (R 2 bpy) 2 ]BF 4 (2a-c) [R 2 bpy ) 4,4′-di-tert-butyl-2,2′-bipyridine ( t Bu 2 bpy), a; 4,4′-dimethyl-2,2′-bipyridine (Me 2 bpy), b; 2,2′-bipyridine (bpy), c]. These are rare examples of pairs of isolated, sterically unencumbered, first-row metal-oxo/hydroxo complexes that differ by a hydrogen atom (H + + e -). The V IV -t Bu 2 bpy derivative 1a has a useful 1 H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H • from organic substrates with weak O-H and C-H bonds, converting 2,6-t Bu 2 -4-MeO-C 6 H 2 OH (ArOH) and 2,2,6,6-tetramethyl-N-hydroxypiperidine (TEMPOH) to their corresponding radicals ArO • and TEMPO, hydroquinone to benzoquinone, and dihydroanthracene to anthracene. The equilibrium constant for 2a + ArOH h 1a + ArO • is (4 ( 2) × 10 -3 , implying that the VO-H bond dissociation free energy (BDFE) is 70.6 ( 1.2 kcal mol -1 . Consistent with this value, 1a is oxidized by 2,4,6-t Bu 3 C 6 H 2 O • . All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b a 2a + 1b, using the t Bu-and Me-bpy substituents as labels, also occurs slowly, with k se ) 1.3 × 10 -2 M -1 s -1 at 298 K, ∆H q ) 15 ( 2 kcal mol -1 , and ∆S q ) 16 ( 5 cal mol -1 K. Using this k se and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 10 2 of the observed values. The vanadium self-exchange reaction is ca. 10 6 slower than that for the related Ru IV O(py)(bpy) 2 2+ /Ru III OH(py)(bpy) 2 2+ self-exchange. The origin of this dramatic difference has been probed with DFT calculations on the self-exchange reactions of 1c + 2c and on monocationic ruthenium complexes with pyrrolate or fluoride in place of the py ligands. The calculations reproduce the difference in barrier heights and show that transfer of a hydrogen atom involves more structural reorganization for vanadium than the Ru analogues. The vanadium complexes have larger changes in the metal-oxo and metal-hydroxo bond lengths, which is traced to the difference in d-orbital occupancy in the two systems. This study thus highlights the importance of intrinsic barriers in the transfer of a hydrogen atom, in addition to the thermochemical (bond strength) factors that have been previously emphasized.

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

“…This is because X and X′ are very similar and because it is essentially isoergic [K eq ¼ 1.45 AE 0.13 in C 6 H 6 (30)]. In such cases (9,11,31), k XH∕X• is taken as the geometric mean of the forward and reverse rate constants k XH∕X 0 • and k X 0 H∕X• (Eq. 8).…”

“…2 (7), a corollary of the Marcus theory of electron transfer, as the basis for a general model for HAT rates (8)(9)(10)(11)(12). This use of the CR grew out of recognizing HAT as one class of a larger set of reactions in which one electron and one proton are transferred (H…”

Catalysis Involving the H• Transfer Reactions of First‐Row Transition Metals

“…The transfer requires a great deal of structural reorganization, as the V -O distance increases by 0.264 Å when V = O is converted to V -OH. The barrier to transfer of an H • from V -OH to V = O (in Marcus theory, the " intrinsic barrier " to selfexchange) is thus large, and its size is refl ected in the size of the barrier to the C → O H • transfer in Equation 1.13 [46] .…”

Proton-coupled electron transfer reactions: kinetic studies on the oxidation of dihydroxybenzenes by heteropoly 10-tungstodivanadophosphate in aqueous acidic medium

“…Oxidation of hydroquinone and catechol can proceed through ET-PT, PT-ET, s-CPET or HAT mechanisms, depending on the nature of oxidant, solvent, pH of the reaction medium, etc. Extensive kinetic studies on the oxidation of hydroquinone and catechol by a variety of transition metal complexes have been reported [8][9][10][11][12][13][14][15][16][17][18][19], including [Fe 2 (bby) 2 [17] proceeds through an ET-PT mechanism. With macrocyclic Ni(III) complexes, an s-CPET mechanism in which water acts as H ?…”

“…Oxidation of hydroquinone, by dehaloperoxidasehemoglobin-A, proceeds through an s-CPET mechanism [20]. With transition metal oxidants containing oxoanions, a HAT mechanism is operative for the oxidation of hydroquinone and catechol [15,16,18,19].…”