Murali Meenu, Bijani Christian, Daran Jean-Claude, Manoury Eric, Poli Rinaldo
CNRS, LCC (Laboratoire de Chimie de Coordination), Université de Toulouse, UPS, INPT 205 Route de Narbonne, BP 44099 F-31077 Toulouse Cedex 4 France
Institut Universitaire de France 1, rue Descartes 75231 Paris Cedex 05 France.
Chem Sci. 2023 Jul 4;14(30):8152-8163. doi: 10.1039/d3sc02204h. eCollection 2023 Aug 2.
The kinetics and mechanism of the acetate ligand exchange with free acetic acid in [ZrO(OH)(OCCH)], used as a molecular model of crosslink migration in [ZrO(OH)(carboxylate)(OH)]-based coordination adaptable networks with vitrimer-like properties, has been thoroughly investigated by dynamic H NMR and DFT calculations. The compound maintains its -symmetric Zr structure in CDCl and CD, while it splits into its Zr subunits in CDOD and DO. In the Zr structure, the topologically different acetates (3 chelating, 6 belt-bridging, 2 intercluster-bridging and 1 inner-face-bridging) of the Zr subunits behave differently in the presence of free CHCOOH: 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 [CHCOOH]. Variable-temperature line broadening studies yielded Δ = 15.0 ± 0.4 kcal mol, Δ = 8 ± 1 cal mol K (-30 to +25 °C range in CDCl) for the chelating acetates and Δ = 22.7 ± 1.6, 22.9 ± 2.1 and 20.6 ± 1.0 kcal mol and Δ = 13 ± 5, 14 ± 6 and 9 ± 3 cal mol K, respectively (+25 to +70 °C range in CD), 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-OH ligands accelerate the exchange processes through H-bonding stabilization of the coordinatively unsaturated intermediate. The lower exchange barrier for the chelated 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.
作为具有类 Vitrimer 性质的基于[ZrO(OH)(羧酸盐)(OH)]的配位适应性网络中交联迁移分子模型的[ZrO(OH)(OCCH)]，其乙酸根配体与游离乙酸的交换动力学和机理已通过动态 H NMR 和 DFT 计算进行了深入研究。该化合物在 CDCl 和 CD 中保持其对称的 Zr 结构，而在 CDOD 和 DO 中则分裂为其 Zr 亚基。在 Zr 结构中，Zr 亚基拓扑结构不同的乙酸根（3 个螯合、6 个带桥连、2 个簇间桥连和 1 个内表面桥连）在游离 CHCOOH 存在下表现不同：螯合乙酸根交换非常快（室温下共振合并），带桥连乙酸根交换较慢（升温时线宽变宽），簇间桥连和内表面桥连乙酸根在高达 65°C 时无明显交换（通过 EXSY NMR）。前两个交换过程的速率对[CHCOOH]呈零级依赖性。变温线宽研究得出，螯合乙酸根在 CDCl 中（-30 至 +25°C 范围）的Δ = 15.0 ± 0.4 kcal mol，Δ = 8 ± 1 cal mol K，对于磁性不等价带桥连乙酸根的三个不同共振，在 CD 中（+25 至 +70°C 范围）的Δ分别为 22.7 ± 1.6、22.9 ± 2.1 和 20.6 ± 1.0 kcal mol 以及Δ = 13 ± 5、14 ± 6 和 9 ± 3 cal mol K。在 DFT 计算的支持下，这些结果表明存在一种操作上的缔合机制，涉及速率决定步骤的部分解离为单齿乙酸根，随后是快速的酸配位和质子转移。簇状μ-OH 配体通过配位不饱和中间体的氢键稳定作用加速交换过程。螯合桥连乙酸根较低的交换势垒与环张力的释放有关。本研究中呈现的结果可能有助于解释其他系统中的羧酸盐交换现象以及新型羧酸盐基材料的设计。
