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.