Tuning the Dimensionality of Polyoxometalate-Based Materials by Using a Mixture of Ligands

Abstract

Five molecular one-, two-, or three-dimensional (1D, 2D, or 3D) organic–inorganic hybrid polyoxometalates (POMs) based on the {ε-PMoV8MoVI4O40Zn4} (εZn) Keggin unit have been synthesized under hydrothermal conditions using a mixture of O- and N-donor ligands. (TBA)6[PMoV8MoVI4O37(OH)3Zn4]2(C14H8O4)3·6H2O (ε2(biphen)3) is a 3D material with two interpenetrated networks built from dimeric (εZn)2 POMs linked by 4,4′-biphenyldicarboxylate (biphen) ligands. (TBA)2[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)3(C14H8O4)1/2·H2O (ε(bim)3(biphen)1/2), (TBA)3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)2(C8H4O4)·6H2O (ε(bim)2(isop)), (TBA)7/3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)8/3(C8H4O4)2/3 (ε(bim)8/3(bdc)2/3), and (TBA)3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)2(C9H3O6)2/3·6H2O (ε(bim)2(trim)2/3) all consist of monomeric εZn units bound to two types of organic ligands: benzimidazole (bim) and one of the following carboxylate ligands: biphen, 1,3-benzenedicarboxylate (isop), 1,4-benzenedicarboxylate (bdc), or 1,3,5-benzenetricarboxylate (trim) ligands. While ε(bim)3(biphen)1/2 is a molecular complex, ε(bim)2(isop) and ε(bim)8/3(bdc)2/3 adopt a chain arrangement, and ε(bim)2(trim)2/3 is a 2D compound. In these materials, the limitation of the dimensionality is a direct consequence of the protonation of the nitrogen atom of the bim ligands. The electrocatalytic activity for the hydrogen evolution reaction (HER) of these five new POM-based coordination polymers has been studied, showing that their performance depends mainly on the presence of the εZn Keggin units but also on their structure. Modified electrodes fabricated with ε(bim)2(trim)2/3 entrapped in a carbon paste revealed that this hybrid is the most efficient electrocatalyst of the series, being stable and catalyzing the HER in the pH 1–5 range.