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

Reactions are described that interconvert vanadium(IV) oxo−hydroxo complexes [VIVO(OH)(R2bpy)2]BF4 (1a−c) and vanadium(V) dioxo complexes [VVO2(R2bpy)2]BF4 (2a−c) [R2bpy = 4,4′-di-tert-butyl-2,2′-bipyridine (tBu2bpy), a; 4,4′-dimethyl-2,2′-bipyridine (Me2bpy), 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 VIV-tBu2bpy derivative 1a has a useful 1H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H• from organic substrates with weak O−H and C−H bonds, converting 2,6-tBu2-4-MeO-C6H2OH (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 ⇌ 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-tBu3C6H2O•. All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b ⇄ 2a + 1b, using the tBu- and Me-bpy substituents as labels, also occurs slowly, with kse = 1.3 × 10−2 M−1 s−1 at 298 K, ΔH⧧ = 15 ± 2 kcal mol−1, and ΔS⧧ = 16 ± 5 cal mol−1 K. Using this kse and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 102 of the observed values. The vanadium self-exchange reaction is ca. 106 slower than that for the related RuIVO(py)(bpy)22+/RuIIIOH(py)(bpy)22+ 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.