Four Keggin-type polyoxometalate-based complexes derived from bis(pyrazine)–bis(amide) ligands for electrochemical sensing of multiple analytes and adsorbing dye molecules

Abstract

Four new Keggin-type polyoxometalate (POM)-based metal–organic complexes constructed from two flexible bis(pyrazine)–bis(amide) ligands N,N′-bis(2-pyrazinecarboxamide)-1,2-ethane (bpae) and N,N′-bis(5-methyl-2-pyrazinecarboxamide)-1,2-ethane (bmpae), namely, [Cu₂(bpae)₂(H₂O)₂(PMo^VI₉Mo^V₃O₃₉)]·5.5H₂O (1), [Cu₂(bpae)₂(SiMo₁₂O₄₀)]·3H₂O (2), {[Co₂(bmpae)₂(H₂O)₄](SiW₁₂O₄₀)}·1.5H₂O (3), and [Cu₂(bmpae)₂(SiW₁₂O₄₀)]·8H₂O (4), have been hydrothermally synthesized and structurally characterized. Complex 1 is a [PMo^VI₉Mo^V₃O₃₉]⁴⁻ (PMo₁₂)-based 3D coordination framework, which is constructed by three kinds of 1D chains: [Cu(bpae)]_n²ⁿ⁺, [PMo₁₂]_n⁴ⁿ⁻ and [Cu(PMo₁₂)]_n²ⁿ⁻ chains. Complex 2 shows a 2D structure, which is composed of tridentate [SiMo₁₂O₄₀]⁴⁻ anions (SiMo₁₂) and [Cu(bpae)]_n²ⁿ⁺ chains. Complex 3 is a 0D structure, which is constructed from [Co₂(bmpae)₂]⁴⁺ binuclear rings and discrete [SiW₁₂O₄₀]⁴⁻ (SiW₁₂) anions. In 4, the 1D [Cu(bmpae)]_n²ⁿ⁺ chains are connected by the SiW₁₂ anions to produce a 2D structure. The diverse structures of 1–4 demonstrate that the different metal ions and POMs have pivotal influences on the formation of the final coordination frameworks. In this work, the adsorption performances of 1–4 for the organic dyes gentian violet (GV) and methylene blue (MB) were examined, and 1 possesses good reusability. In addition, 1–4 can be employed as amperometric sensors to identify multiple analytes (BrO₃⁻, NO₂⁻, Cr(VI) and Fe(III) ions) and possess good selectivity in the presence of some other ions.