Proton-Conductive Keggin-Type Clusters Decorated by the Complex Moieties of Cu(II) 2,2′-Bipyridine-4,4′-dicarboxylate/Diethyl Analogues

Three proton-conductive decorated Keggin-type clusters, {[Cu­(debpdc)­(H2O)3]­[Cu­(debpdc)­(H2O)­Cl]­[PMo12O40]} · 2CH3OH·1.5CH3CN·3H2O (1), {[Cu­(H2bpdc)­(H2O)2Cl0.5]2[PW12O40]}·10H2O (2), and {[Cu­(H2bpdc)­(H2O)2.5]2[SiW12O40]}·10H2O (3) (where debpdc is diethyl 2,2′-bipyridine-4,4′-dicarboxylate and H2bpdc is 2,2′-bipyridine-4,4′-dicarboxylic acid), were synthesized through electrostatic and coordination interactions between Keggin-type anions and Cu­(II) H2bpdc/debpdc complex moieties. Interestingly, in the three complexes, both the H2bpdc/debpdc and the Keggin anion are covalently linked to the Cu2+ ions as polydentate organic and inorganic ligands, respectively. Notably, complexes 2 and 3 are the first examples of the functionalization of a Keggin-type cluster with Cu­(II)-H2bpdc complex moieties, thereby providing a pathway to design and synthesize multifunctional hybrid materials with cluster structures based on two building units. In them, the free COOH groups of the H2bpdc ligand can act as both hydrogen bond acceptors and proton carriers. 1 has debpdc ligands with ethoxycarbonyl groups, while 2 and 3 have the H2bpdc ligands with free COOH groups; thus, the three complexes help us to understand the influence of the different substituents on the proton conductivity. The measurement results reveal that 2 and 3 have a high conductivity value of over 10–3 S cm–1 at 100 °C under 98% relative humidity, which is 2 orders of magnitude higher than that of 1 under the same conditions.