Synthesis,structure, And Properties Of {As₂Mo₁₈} Type Polyoxometalate Molecule Compound

Dawson structure is one of the six classical structures of polyoxometalates（POMs） and has a large number of terminal oxygen and bridging oxygen, which can be as active oxygen atoms to connect with transition elements or organic ligands to form a series of compound with novel structure, superior performance. But it is difficult to segregation due to the instability of the Dawson arsenomolybdate precursor. Only five literature about Dawson arsenomolybdate were reported so far. Therefore, the research on inorganic-organic hybrid materials based on {As₂Mo₁₈} has important significance to enrich the POMs chemistry research. In this paper, eight kinds of compound with novel structure and superior performance have been hydrothermal synthetized by adjusting the p H value, different transition metal, the ratio of raw material, and choosing a relatively simple ligand containing N with a small resistance. Through elemental analysis, X- ray diffraction analysis to determine the chemical formula:（H2btp）3[As2Mo18O62]·6H2O（1）,（H2bimyb）3[As2Mo18O62]（2）,（Himi）6（imi）[As2Mo18 O62]·4H2O（3）,（bib）3[Asâ…¢Asâ…¤2Mo18O62]（4）,（bib）3[Asâ…¢2Asâ…¤2Mo18O62]（5）,（bib）2[Asâ…¢3 Asâ…¤2Moâ…¤3Moâ…¥15O62]（6）, [Cu0.5（imi）2]2[{Cu（imi）2}4{（Asâ…¢0.5）Asâ…¤2Mo18O62}（7） å'Œ [Cu（bzi）2]4[{Na（Asâ…¢Asâ…¤2Moâ…¤2Moâ…¥16O62）}2]（8）（btp = 1,5-bis（triazol） pentane, bimyb = 1,4-bis（imidazol-1-ylmethyl） benze, imi = imidazole, bib = 1,4-Bis（1-imidazoly） benze, bzi = Benzimidazole）.X-ray diffraction showed that the basic building units of compounds 1-8 are {As₂Mo₁₈} anion cluster, in which compounds 1-3 are zero dimensional compounds. In compound 1, {As₂Mo₁₈} cluster is surrounded by three H2 btp organic ligands. Two protonated H2 btp ligands and [As2Mo18O62]6- anion form double-supported structure through two terminal oxygen atoms. The adjacent anion clusters form long chains, 1D chains, 2D layers, and A-B-A-B-type 3D supramolecular structure by supramolecular interaction. In compound 2, each {As₂Mo₁₈} connects with 22 Dawson clusters by fourteen bimyb ligands to form high-connected closely packed supramolecular networkare with（1491）（510）（46） topology. In compound 3, the adjacent {As₂Mo₁₈} clusters form 1D chains through four imidazole ligands, which are further linked to form 2D layers and 3D networks via hydrogen bonding interaction. Compounds 4-6 are zero dimensional Dawson structure with one, two, and three As cap, respectively. Compounds 7-8 are 1D structure with arsenic cap. In compound 7, two As atoms with half occupy are connected with the {As₂Mo₁₈} cluster to form novel {As O3} cap structure through u3-O between equatorial and polar position. Each capped {As₂Mo₁₈} anion connects with two {Cu（imi）2} and two {Cu3（imi）4} tri-nuclear bridge unit to form tetr-supported structure. Adjacent tetr-supported clusters connect each other via form a wireless one dimensional chainrespectively through such Dawson two adjacent three core bridge unit is connected to form a one-dimensional chain tri-nuclear bridge units. The chains are further formed 2D layers and 3D structure through hydrogen bonding interactions. In compound 8, the Dawson-type clusters with single-arsenic cap form infinite 1D chains via bridge of metal sodium along ac plan. On the other hand, each of Dawson clusters connects four {As₂Mo₁₈} via four Cu（bim）2 ligand bridging to form 2D supramolecular layers along the bc pale. The two kinds of sructures further form complex 3D network.Cyclic voltammograms studies have shown that such compounds exhibit very good catalytic effect on the reduction of nitrite and hydrogen peroxide, and against oxidation of the ascorbic acid, which indicats that the Dawson-type POM have bifunctional electrocatalytic activity. In addition, the Dawson-type as catalyst have good catalytic effect on the degradation of dual azo dyesmethylene blue（MB） under the irradiation of ultraviolet light. The degradation rates of them are more than 90%. Thus, compounds 1-8 are a kind of photocatalyst with application prospect.