Capped Keggin Type Polyoxometalate-Based Inorganic-Organic Hybrids Involving In Situ Ligand Transformation as Supercapacitors and Efficient Electrochemical Sensors for Detecting Cr(VI).

To construct polyoxometalate-based complexes as electrode materials for supercapacitors and electrochemical sensors, we intentionally used in situ ligand transformation during the reaction. Two complexes based on polyoxometalates capped by zinc ions, H{Zn4(DIBA)4[(DIBA)(HPO2)]2(α-PMoVI8MoV4O40Zn2)} (1) and [ε-PMoV8MoVI4O37(OH)3Zn4(HDBIBA)2]·6H2O (2) [DIBA = 3,5-di(1H-imidazol-1-yl)benzoic acid, and DBIBA = 3,5-bis(1H-benzoimidazol-1-yl)benzoic acid], have been prepared successfully. The DIBA and DBIBA ligands were generated in situ from initial materials 3,5-di(1H-imidazol-1-yl)benzonitrile and 3,5-di(1H-benzoimidazol-1-yl)benzonitrile. The three-dimensional structure of 1 consisted of two-dimensional interpenetrating layers and polyoxometalate-based chains composed of bicapped α-PMo12Zn2 polyoxoanions and phosphite-modified DIBA ligands. In 2, a kind of tetracapped ε-PMo12Zn4 polyoxoanion exists, which was further linked by DBIBA ligands into a one-dimensional chain. Two complexes could be employed as not only electrode materials for supercapacitors with specific capacitances of 171.17 F g-1 for 1 and 146.77 F g-1 for 2 at 0.5 A g-1 but also efficient electrochemical sensors for detecting Cr(VI) with excellent limits of detection of 0.026 μM for 1 and 0.035 μM for 2, which represents a hopeful approach for exploiting polyoxometalate-based complexes as supercapacitor and electrochemical sensor materials.