Tuning Intrinsic and Extrinsic Proton Conduction in Metal–Organic Frameworks by the Lanthanide Contraction

Seven isomorphous lanthanide metal–organic frameworks in the PCMOF-5 family, [Ln­(H5L)­(H2O)n]­(H2O) (L = 1,2,4,5-tetrakis­(phosphonomethyl)­benzene, Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) have been synthesized and characterized. This family contains 1-D water-filled channels lined with free hydrogen phosphonate groups and gives a very low activation energy pathway for proton transfer. The lanthanide contraction was employed to systematically vary the unit cell dimensions and tune the proton conducting pathways. LeBail fitting of the crystalline series shows that the crystallographic a-axis, along the channel, can be varied in increments less than 0.02 Å correspondingly shortening the proton transfer pathway. The proton conductivities for the La and Pr complexes were roughly an order of magnitude higher than other members of the series (10–3 S cm–1 versus 10–4 S cm–1). Single crystal structures of the high and low conducting members of the series (La, Pr for high and Ce for low) affirm the structural similarities extend beyond the unit cell parameters to positions of free acid groups and included water molecules. Scanning electron microscopy reveals marked differences in particle size of the different members of the Ln series owing to lattice strain effects induced by changing the lanthanide. Notably, the high conducting La and Pr complexes have the largest particle sizes. This result contradicts any notion that degradation of the MOF at grain boundaries is enabling the observed conductivity as proton conduction dominated by extrinsic pathways would be enabled by small particles (i.e., the La and Pr complexes would be the worst conductors). Proton conductivity measurements of a ball milled sample of the La complex corroborate this result.本研究合成并表征了PCMOF-5家族中的7个同构镧系金属有机框架（metal–organic frameworks, MOFs），其化学式为[Ln(H₅L)(H₂O)ₙ](H₂O)，其中L为1,2,4,5-四(膦酰甲基)苯，Ln分别为镧(La)、铈(Ce)、镨(Pr)、钕(Nd)、钐(Sm)、铕(Eu)及钆(Gd)。该系列框架拥有一维含水通道，通道内壁修饰有游离氢膦酸酯基团，可为质子转移提供极低活化能的传输通路。研究借助镧系收缩效应，系统调控该系列框架的晶胞参数，以此精准调节质子传导通路。对该晶态系列的LeBail精修结果显示，沿通道方向的晶胞a轴可在小于0.02 Å的步长范围内调控，进而相应缩短质子传输路径。镧(La)与镨(Pr)配合物的质子电导率约比该系列其他成员高一个数量级，分别为10⁻³ S·cm⁻¹与10⁻⁴ S·cm⁻¹。对该系列高电导率（镧、镨）与低电导率（铈）成员的单晶结构解析证实，二者的结构相似性不仅局限于晶胞参数，还延伸至游离酸性基团与客体水分子的排布位置。扫描电子显微镜（Scanning Electron Microscopy, SEM）表征结果显示，因更换镧系元素引发的晶格应变效应，该镧系系列不同成员的晶粒尺寸存在显著差异。值得注意的是，高电导率的镧与镨配合物拥有最大的晶粒尺寸。该结果与“晶界处金属有机框架的降解是引发观测到质子传导的原因”这一论断相悖：若质子传导由外禀通路主导，则小晶粒更利于质子传输，换言之镧与镨配合物本应表现为导电性最差的成员，而实际情况恰好相反。对球磨处理后的镧配合物样品开展的质子电导率测试结果，进一步佐证了这一结论。