Alkaline Earth Metal (Mg, Sr, Ba)–Organic Frameworks Based on 2,2′,6,6′-Tetracarboxybiphenyl for Proton Conduction

Three new alkaline earth metal based metal–organic frameworks (MOFs), namely M-BPTC (M = Mg, Sr, Ba), have been synthesized by using BPTC (2,2′,6,6′-tetracarboxy­biphenyl) as ligand under hydrothermal conditions. These MOFs exhibit interesting structural diversity, variable chemical and thermal stability, as well as proton conductivity. Mg-BPTC with the formula {[Mg­(BPTC)0.5­(H2O)3]·5H2O}n consists of BPTC4– extended metal layers, and novel highly ordered infinite tape-like structures of cyclic water octamers reside interlayer. Three-dimensional porous {[Sr2(BPTC)­(H2O)6]·H2O}n (Sr-BPTC) features inorganic Sr–O chains (I1O2) and open hydrophilic channels where water heptamers and carboxyl oxygen atoms conspire to form H-bond networks, whereas 3D {[Ba6(BPTC)3­(H2O)6]·11H2O}n (Ba-BPTC) shows Ba–O inorganic layer (I2O1) and 1D channels incorporating large water 14-mers and 18-mers. M-BPTC (M = Mg, Sr) species exhibit excellent water stability and proton conductivity due to their respective appropriate pathways for proton transporting. M-BPTC (M = Sr, Ba) structures are highly thermally stable due to the presence of the inorganic connectivity. The present results suggest that M-BPTC (M = Mg, Sr) are promising materials for proton conduction and provide insight into the hydrogen bonding motif.

本研究通过水热合成法，以BPTC（2,2′,6,6′-四羧基联苯）为配体，成功构筑了三款新型碱土基金属有机框架（metal–organic frameworks, MOFs），即M-BPTC（M = Mg、Sr、Ba）。此类MOFs展现出引人关注的结构多样性、各异的化学与热稳定性，以及质子传导性能。
化学式为{[Mg(BPTC)₀.₅(H₂O)₃]·5H₂O}ₙ的Mg-BPTC，由BPTC⁴⁻拓展形成的金属层构成，层间存在新颖的高度有序无限带状结构的环状八聚水团簇。
三维多孔结构的Sr-BPTC化学式为{[Sr₂(BPTC)(H₂O)₆]·H₂O}ₙ，其结构特征为无机Sr-O链（I1O2）与开放亲水通道，通道内七聚水簇与羧基氧原子协同形成氢键网络；而化学式为{[Ba₆(BPTC)₃(H₂O)₆]·11H₂O}ₙ的Ba-BPTC，则展现出Ba-O无机层（I2O1）与一维通道，通道内封装了大型14聚水簇与18聚水簇。
M-BPTC（M = Mg、Sr）凭借各自适宜的质子传输路径，展现出优异的水稳定性与质子传导性能；而M-BPTC（M = Sr、Ba）因存在无机连接结构，具备极高的热稳定性。
本研究结果表明，M-BPTC（M = Mg、Sr）是极具应用前景的质子传导材料，同时为氢键基元的相关研究提供了重要见解。