Acetate exchange mechanism on a Zr 12 oxo hydroxo cluster: relevance for reshaping Zr-carboxylate coordination adaptable networks.

The kinetics and mechanism of the acetate ligand exchange with free acetic acid in [Zr ₆ O ₄ (OH) ₄ (O ₂ CCH ₃ ) ₁₂ ] ₂ , used as a molecular model of crosslink migration in [Zr ₆ O ₄ (OH) ₄ (carboxylate) _{12- n} (OH) _n ]-based coordination adaptable networks with vitrimer-like properties, has been thoroughly investigated by dynamic ¹ H NMR and DFT calculations. The compound maintains its C _2h -symmetric Zr ₁₂ structure in CD ₂ Cl ₂ and C ₆ D ₆ , while it splits into its Zr ₆ subunits in CD ₃ OD and D ₂ O. In the Zr ₁₂ structure, the topologically different acetates (3 chelating, 6 belt-bridging, 2 intercluster-bridging and 1 inner-face-bridging) of the Zr ₆ subunits behave differently in the presence of free CH ₃ COOH: 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 [CH ₃ COOH]. Variable-temperature line broadening studies yielded Δ H ^‡ = 15.0 ± 0.4 kcal mol ^-1 , Δ S ^‡ = 8 ± 1 cal mol ^-1 K ^-1 (-30 to +25 °C range in CD ₂ Cl ₂ ) for the chelating acetates and Δ H ^‡ = 22.7 ± 1.6, 22.9 ± 2.1 and 20.6 ± 1.0 kcal mol ^-1 and Δ S ^‡ = 13 ± 5, 14 ± 6 and 9 ± 3 cal mol ^-1 K ^-1 , respectively (+25 to +70 °C range in C ₆ D ₆ ), 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 μ ₃ -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.
Competing Interests: There are no conflicts to declare.
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