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 <b>M-BPTC</b> (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. <b>Mg-BPTC</b> with the formula {[Mg­(BPTC)_0.5­(H₂O)₃]·5H₂O}_<i>n</i> consists of BPTC^4– 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}_<i>n</i> (<b>Sr-BPTC</b>) 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}_<i>n</i> (<b>Ba-BPTC</b>) shows Ba–O inorganic layer (I²O¹) and 1D channels incorporating large water 14-mers and 18-mers. <b>M-BPTC</b> (M = Mg, Sr) species exhibit excellent water stability and proton conductivity due to their respective appropriate pathways for proton transporting. <b>M-BPTC</b> (M = Sr, Ba) structures are highly thermally stable due to the presence of the inorganic connectivity. The present results suggest that <b>M-BPTC</b> (M = Mg, Sr) are promising materials for proton conduction and provide insight into the hydrogen bonding motif