Proton-Conducting Cobalt(II) 3D MOFs Incorporating Bis(imidazole) and Polycarboxylate Linkages: Framework Topology and Interpenetration

Metal–organic frameworks are potential proton-conducting electrolytes for proton-exchange membrane fuel cells. Two such new cobalt complexes, namely [Co2(dip)2(obb)2]·2H2O (1) and [Co2(dip)2(btec)(H2O)]·3H2O (2), incorporating an N-donor ligand (dip) and polycarboxylate-based ligands (obb and btec) were prepared from hydrothermal syntheses. As elucidated by X-ray crystallography, their 3D frameworks contain extensive H-bonding networks. Compound 1 possesses a 3-fold interpenetrated cag framework, whereas compound 2 consists of [Co2(O)(COO)2]n cluster chains, which are connected by 2-connected btec and dip ligands. The proton conduction for both compounds has been appraised by AC impedance with relative humidity (33–98%) and temperature variation (25–80 °C). The proton conductivity values of 1 span between 7.01 × 10–7 and 4.28 × 10–9 S cm–1 with 98% RH (relative humidity) at 25–85 °C. In contrast, compound 2 displays a significant proton conductivity of 9.56 × 10–5 S cm–1 at 65 °C under 98% RH.