The key role of –CH3 steric hindrance in bis(pyrazolyl) ligand on polyoxometalate-based compounds

Abstract

Through using two kinds of bis(pyrazolyl) ligands, four polyoxometalate (POM)-based compounds were hydrothermally synthesized and structurally characterized, [Ag₃(Hbhpe)₂(H₂O)(H₂PMo₁₂O₄₀)]·H₂O (1), [Ag(H₂bdpm)₂(H₂PW₁₂O₄₀)]·4H₂O (2), [Ag₆(H₂bdpm)₆(HPW^VI₈W^V₄O₄₀)]·2H₂O (3) and [Ag₄(H₂bdpm)₄(H₂P₂W₁₈O₆₂)]·3H₂O (4) (H₂bhpe = 1,2-bis(1-H-pyrazolate)ethane, H₂bdpm = 1,1′-bis(3,5-dimethyl-1H-pyrazolate)methane). In compound 1, the Ag–Hbhpe subunit is a 2D layer containing large penta-membered cycles and small tri-nuclear Ag^I clusters. The Keggin anions covalently float on the large cycles. In compound 2, the Keggin anions are fused by [Ag(H₂bdpm)₂]⁺ subunits to form a 1D chain. Compound 3 contains hexa-membered metal–organic cycles, which are further linked by Keggin anions to build a 1D chain. Adjacent chains share the Ag^I ions to construct a 3D framework of 3. Compound 4 exhibits a wavy double-track chain structure, with the Wells–Dawson anions covalently suspended up and down this chain. The steric hindrance of –CH₃ groups in H₂bdpm leads to the formation of mono-nuclear Ag^I subunits in 2–4. The influence of –CH₃ steric hindrance in bis(pyrazolyl) ligands on the structures of 1–4 is discussed. The electrochemical and photocatalytic properties of the title compounds have been studied.