Three new high-nuclear transition-metal-substituted heteropolytungstates: syntheses, crystal structures, magnetic studies and NLO properties

Abstract

Three new nickel-cluster-substituted polyoxometalates (POMs), [Ni₆(H₂O)₉(μ₃-OH)₃(HSiW₉O₃₄)]₂·12H₂O (1), [Ni(en)(H₂O)₃][Ni₆(H₂O)₃(en)₂L(μ₃-OH)₃(HSiW₉O₃₄)]₂·9H₂O (2) (en = ethylenediamine; L = C₇H₅O₂ = benzoic acid) and [Ni₆(L′)₆(CH₃COO)(H₂O)₃(μ₃-OH)₃(HPW₉O₃₄)]₂·9H₂O (3) (L′ = C₅NH₅ = pyridine) were successfully isolated under hydro-/solvothermal conditions. 1–3 were structurally characterized by single-crystal XRD, elemental analyses, PXRD, IR, and TGA. Compound 1 mainly consists of a pair of {Ni₆SiW₉} fragments and some water molecules. Interestingly, the whole structure can be regarded as the connection of {Ni₆SiW₉} and another unit rotated 180° through Ni–O–W bonds, forming a dimeric structure {Ni₆SiW₉}₂. Compounds 2 and 3 also have an {Ni₆XW₉}₂ (X = Si, P) dimer, but there is a big difference in the connection between {Ni₆SiW₉} units. On this basis, mono-dentate conjugated organic ligands (benzoic acid (L) and pyridine (L′)) were successfully introduced. It is noteworthy that pyridine molecules were first integrated into {Ni₆SiW₉}-based clusters and 3 features the highest number of organic ligands (six pyridines per Ni₆) in the reported {Ni₆XW₉}-based clusters. The introduction of organic ligands to compounds 2 and 3 can bring about better third-order nonlinear optical properties. Magnetic research indicated the existence of ferromagnetic interactions in 2–3.