Acetate exchange mechanism on a Zr12 oxo hydroxo cluster: relevance for reshaping Zr-carboxylate coordination adaptable networks

Abstract

The kinetics and mechanism of the acetate ligand exchange with free acetic acid in [Zr₆O₄(OH)₄(O₂CCH₃)₁₂]₂, used as a molecular model of crosslink migration in [Zr₆O₄(OH)₄(carboxylate)_12-n(OH)_n]-based coordination adaptable networks with vitrimer-like properties, has been thoroughly investigated by dynamic ¹H NMR and DFT calculations. The compound maintains its C_2h-symmetric Zr₁₂ structure in CD₂Cl₂ and C₆D₆, while it splits into its Zr₆ subunits in CD₃OD and D₂O. In the Zr₁₂ structure, the topologically different acetates (3 chelating, 6 belt-bridging, 2 intercluster-bridging and 1 inner-face-bridging) of the Zr₆ subunits behave differently in the presence of free CH₃COOH: very fast exchange for the chelating (coalesced resonance at room temperature), slower exchange for the belt-bridging (line broadening upon warming), no observable exchange up to 65 °C (by EXSY NMR) for the intercluster- and inner-face-bridging. The rates of the first two exchange processes have zero-order dependence on [CH₃COOH]. Variable-temperature line broadening studies yielded ΔH^‡ = 15.0 ± 0.4 kcal mol^-1, ΔS^‡ = 8 ± 1 cal mol^-1 K^-1 (-30 to +25 °C range in CD₂Cl₂) for the chelating acetates and ΔH^‡ = 22.7 ± 1.6, 22.9 ± 2.1 and 20.6 ± 1.0 kcal mol^-1 and ΔS^‡ = 13 ± 5, 14 ± 6 and 9 ± 3 cal mol^-1 K^-1, respectively (+25 to +70 °C range in C₆D₆), for three distinct resonances of magnetically inequivalent belt-bridging acetates. With support of DFT calculations, these results point to an operationally associative mechanism involving a rate-determining partial dissociation to monodentate acetate, followed by rapid acid coordination and proton transfer. The cluster μ₃-OH ligands accelerate the exchange processes through H-bonding stabilization of the coordinatively unsaturated intermediate. The lower exchange barrier for the chelated vs. bridging acetates is associated to the release of ring strain. The results presented in this investigation may help the interpretation of carboxylate exchange phenomena in other systems and the design of new carboxylate-based materials.