pH and Ligand Dependent Assembly of Well–Dawson Arsenomolybdate Capped Architectures

Five Well–Dawson-type arsenomolybdates, formulated as [Cu­(2,2′-bpy)₂]­[{Cu­(2,2′-bpy)}₃­{As₂^VMo₂^VMo₁₆^VIO₆₂}]·4H₂O (1), [H₂(4,4′-bpy)]_2.5­[As^III(As₂^VMo₂^VMo₁₆^VIO₆₂)]·5H₂O (2), (pyr)­(imi)­(Himi)₃­[As₂^III(As₂^VMo₃^VMo₁₅^VIO₆₂)]·3H₂O (3), [As₃^III(As₂^VMo₃^V­Mo₁₅^VIO₆₂)]·4H₂O (4), and (H₂btp)₃[As₂^V­Mo₁₈^VIO₆₂]·6H₂O (5) (bpy = bipyridine, pyr = pyrazine, imi = imidazole, btp = 1,5-bis­(triazol)­pentane), have been hydrothermally synthesized and structurally characterized by the elemental analysis, TG, IR, UV–vis–NIR, XPS, XRD, and single-crystal X-ray diffraction. The structural analysis indicates that compounds 1–4 contain rare reduced Dawson {As₂Mo₁₈O₆₂} (abbreviated as {As₂Mo₁₈}) anions as parent cluster unit, which are capped by a certain number of Cu­(II) or As­(III) species on different coordination positions via altering pH values and organic ligand of the reaction system. Compounds 1 and 2 are asymmetric tricopper and monoarsenate­(III) capped assemble by three {Cu­(bpy)}²⁺ and a {AsO₃} fragments, respectively. Compounds 3 and 4 are symmetric biarsenate­(III) and triarsenate­(III) capped cluster by four and six half occupancy {AsO₃} units, respectively. Compound 5 is uncapped {As₂Mo₁₈} structures. Compounds 1–4 represent infrequent Dawson arsenomolybdate capped architectures, especially 2–4, as arsenate­(III) capped Dawson-type assemblies are observed for the first time. Compounds 1–5 display good electrocatalytic activity on reduction of nitrite. Compounds 1, 2, 3, and 5 exhibit fluorescent properties in the solid state at room temperature. In addition, magnetic properties of 1–4 have been investigated in detail.