1D Polyoxometalate-Based Composite Compounds Derived from the Wells−Dawson Subunit:  Synthesis and Crystal Structure of [{Ce(DMF)₄(H₂O)₃}{Ce(DMF)₄(H₂O)₄}(P₂W₁₈O₆₂)]·H₂O and [{La(DMF)₆(H₂O)}{La(DMF)4.5(H₂O)2.5}(P₂W₁₈O₆₂)]

Two composite compounds, [{Ce(DMF) 4 (H 2 O) 3 }{Ce(DMF) 4 (H 2 O) 4 }(P 2 W 18 O 62 )]·H 2 O ( 1 ) (DMF = N, N -dimethylformamide) and [{La(DMF) 6 (H 2 O)}{La(DMF) 4.5 (H 2 O) 2.5 }·(P 2 W 18 O 62 )] ( 2 ), have been constructed at room temperature and characterized by single crystal X-ray structural analyses, ESR spectroscopy, and thermogravimetric (TG) and differential thermal analysis (DTA) measurements. Single crystal X-ray structural analyses reveal that the adjacent polyanions are polymerized to form a 1D infinitely linear chain by the bridging RE coordination cations and exhibit an -ACACAC- type of structure, and that two types of terminal oxygen atoms of the Wells−Dawson polyanion located in the “cap” and “belt” sites are involved in coordination with the RE 3+ ions, respectively. ESR spectra indicate that after irradiation with sunlight, compound 2 forms a mixed-valence heteropoly compound with unshared electrons. From TG-DTA analyses, we surmise that the frameworks of polyanions of this kind of compound decompose at ca. 620 °C.