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

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.