A Negative Temperature Effect on the Reversible Conversion between Redox Tautomers of CoIII(OH) and CoIV(O) Complexes Bearing a Redox-Non-Innocent Tetraamido Macrocyclic Ligand

Abstract Elucidating highly reactive intermediates in catalytic reactions relies heavily on cryogenic trapping techniques, where intermediates observed in solution are generally assumed to remain unchanged upon flash-freezing at 77 K. Herein, we demonstrate a striking exception to this long-standing assumption. A cobalt(III)-hydroxo complex bearing a tetraamido macrocyclic ligand radical cation (1: [(TAML•+)CoIII(OH)]−), which is readily observed in solution, is rapidly and quantitatively converted upon freezing to the corresponding cobalt(IV)-oxo complex containing a protonated TAML ligand (2: [(H-TAML)CoIV(O)(HOTf)]−) in the presence of HOTf in acetone. Remarkably, the conversion of 1 to 2 exhibits a negative temperature effect, becoming significantly faster at lower temperatures. Eyring analysis afforded activation parameters of ΔH‡ = −6.0(5) kcal mol–1 and ΔS‡ = −96(7) cal K–1 mol–1. Mechanistic studies reveal that this interconversion proceeds through an acid-assisted proton-coupled electron-transfer (PCET) pathway in which HOTf binding to the redox noninnocent TAML•+ ligand promotes intramolecular electron transfer from the CoIII center to the TAML•+ ligand, coupled with proton transfer from the hydroxo ligand to the TAML framework to produce 2. While the equilibrium constant for the exothermic binding of HOTf increases at lower temperatures, the intrinsic rate constant of the subsequent intramolecular PCET step decreases. Consequently, the overall conversion becomes faster upon cooling because of the interplay between these two processes. This study provides the first example of a negative temperature effect on the reversible conversion between two redox tautomers, demonstrating that cryogenic cooling can drive, rather than simply trap, the interconversion of transient intermediates.