Carboxylate Shift Dynamics in Biomimetic Co₂(μ-OH)₂ Complexes

Carboxylate shift mechanisms provide low-energy pathways to accommodate changes in oxidation state and coordination number required during catalysis in metalloenzyme active sites. These processes are challenging to observe in their native enzymes and molecular models can provide insight into their mechanistic details. We report here the direct observation of a carboxylate shift reaction in biomimetic yet structurally stable dicobalt complexes featuring both monodentate and bridging acetate ligands, as well as intramolecular hydrogen-bonding interactions. Subjecting the series of complexes [Co₂(μ-OH)₂(μ-1,3-OAc)(κ-OAc)₂(py^R)₄]PF₆ ([1^R]PF₆, OAc = acetate, py^R = pyridine with para-R substituents: OMe, H, or CN) to a Lewis acid triggers conversion of a monodentate acetate to a μ-1,3 bridging mode, forming [Co₂(μ-OH)2(μ-1,3-OAc)₂(py^R)₄]²⁺ ([2^R]²⁺). [2^R]²⁺ is susceptible to solvent binding, affording [Co₂(μ-OH)₂(μ-1,3-OAc)(κ-OAc)(MeCN)(py^R)₄]²⁺ ([3^R]²⁺) in MeCN. These reaction products and intermediates were isolated and characterized in the solid state by isotopic labeling and Fourier transform infrared (FTIR) spectroscopy, as well as by X-ray diffraction. The kinetics of the formation and decay of [1^R]⁺, [2^R]²⁺, and [3^R]²⁺ were also examined in situ by ¹H-NMR spectroscopy to provide a kinetic model for the carboxylate shift reaction. The rate constants extracted from global fit analyses of these reactions increase with increasing electron donation from R. Leveraging robust diamagnetic Co^III complexes, these studies provide mechanistic details of carboxylate shift reactivity and highlight the utility of ligand dynamicity in mediating the transient formation of unstable metal complexes.