H₃O₂ Bridging Ligand in a Metal–Organic Framework. Insight into the Aqua-Hydroxo↔Hydroxyl Equilibrium: A Combined Experimental and Theoretical Study

A metal–organic framework (MOF) bearing the aqua-hydroxo species (O2H3)− in the framework, as well as the processes that govern the equilibrium aqua-hydroxo (O2H3)−↔hydroxyl (OH) in Sc-MOFs, are studied experimentally and theoretically. Computational studies were employed to determine the relative energies for the two compounds that coexist under certain hydrothermal conditions at pH < 2.8. The thermodynamically more stable [Sc3(3,5-DSB)2(μ-O2H3)­(μ-OH)2(H2O)2] (from now on, (O2H3)­Sc-MOF; 3,5-DSB = 3,5-disulfobenzoic acid) was obtained as a pure and stable phase. It was impossible to isolate [Sc3(3,5-DSB)2(μ-OH)3(H2O)4] as a pure phase, as it turned out to be the precursor of (O2H3)­Sc-MOF. Additionally, a third compound that appears at pH between 3.5 and 4, [Sc3(3,5-DSB)­(μ-OH)6(H2O)] and a fourth, [Sc­(3,5-DSB)­(Phen)­(H2O)]­(H2O), in whose formula neither OH groups nor H3O2– anions appear, are reported for comparative purposes. A study of the (O2H3)­Sc-MOF electronic structure, and some heterogeneous catalytic tests in cyanosilylation of aldehydes reactions, are also reported.本研究通过实验与理论结合的系统手段，对框架中载有羟水合阴离子（O₂H₃）⁻的金属有机框架（metal–organic framework, MOF），以及钪基金属有机框架（Sc-MOF）中调控(O₂H₃)⁻与羟基（OH）之间动态平衡的过程展开了研究。本研究通过计算模拟确定了两种在pH<2.8的特定水热条件下共存的化合物的相对能量。热力学稳定性更优的[Sc₃(3,5-DSB)₂(μ-O₂H₃)(μ-OH)₂(H₂O)₂]（下文简称(O₂H₃)⁻-Sc-MOF；3,5-DSB=3,5-二磺酸苯甲酸）可被分离为纯相且具有优异稳定性。无法将[Sc₃(3,5-DSB)₂(μ-OH)₃(H₂O)₄]分离为纯相，因为该化合物实为(O₂H₃)⁻-Sc-MOF的前驱体。此外，本研究还报道了另外两种化合物：其一为pH处于3.5~4区间时生成的[Sc₃(3,5-DSB)(μ-OH)₆(H₂O)]；其二为[Sc(3,5-DSB)(邻菲啰啉(Phen))(H₂O)](H₂O)，其化学式中既不含羟基基团也不含H₃O₂⁻阴离子，相关结果用于对比分析。本研究同时开展了对(O₂H₃)⁻-Sc-MOF电子结构的表征分析，以及其在醛类氰硅化反应中的多相催化性能测试。