pH-Controlled assembly of [ZnW12O40]6--based hybrids from a 0D dimer to a 2D network: synthesis, crystal structure, and photocatalytic performance in transformation of toluene into benzaldehyde.

Polyoxometalate-based organic-inorganic hybrids have attracted considerable attention due to their fascinating structures and wide application prospects. In this work, using the same building blocks, ligands and metal ions (ZnW12O406-(ZnW12), 2,2'-bipy = 2,2'-bipyridine (2,2'-bipy), Cu2+), we synthesized three new POM-based hybrids by controlling the pH values of the reaction systems. These three compounds {(Zn0.6(H2)0.4W12O40)[Cu(2,2'-bipy)(H2O)][Cu(2,2'-bipy)(H2O)2][Cu(2,2'-bipy)(H2O)3]}2·6H2O (1), (Me4N)2{ZnW12O40[Cu(2,2'-bipy)(H2O)][Cu(2,2'-bipy)(H2O)3]}·5H2O (2), and {(Zn0.5(H2)0.5W12O40)[Cu(2,2'-bipy)][Cu(2,2'-bipy)(H2O)][Cu(2,2'-bipy)(H2O)2]}·5H2O (3) have been structurally characterized by single-crystal X-ray diffraction. Compound 1 appears as a dimeric cluster structure, while compounds 2 and 3 are 1D chain structure and 2D network, respectively. The semiconducting properties of compounds 1-3 are different, which was demonstrated by band gap (Eg) and photocurrent response measurements. Compound 3 can efficiently catalyze the photooxidation of toluene to benzaldehyde with high selectivity using molecular oxygen as oxidant component. Moreover, compound 3 was recycled and reused for three times, without significant degradation in conversion and selectivity. In addition, the mechanism of the photocatalystic reaction was also investigated.