Discrete polyoxovanadate cluster into an organic free metal-oxide-based material: syntheses, crystal structures, and magnetic properties of a new series of lanthanide linked-POV compounds [{Ln(H2O)6}2As8V14O42(SO3)] x 8 H2O (Ln = La3+, Sm3+, and Ce3+).
This article describes the linking propensity of the sulfite encapsulated polyoxovanadate (POV) anion, [As(8)V(14)O(42)(SO(3))](6-), with aqua-lanthanide complex cations [Ln(H(2)O)(6)](3+) in a controlled wet synthesis resulting in a series of organic free metal-oxide-based materials [{Ln(H(2)O)(6)}(2)As(8)V(14)O(42)(SO(3))] x 8 H(2)O, Ln = La(3+) (1), Sm(3+) (2), and Ce(3+) (3). The title compounds have been characterized by elemental analyses, IR, diffuse reflectance, electron paramagnetic resonance, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and single-crystal X-ray diffraction studies. All three compounds crystallize in the monoclinic space group P2(1)/n. Crystal data for 1: a = 13.4839(7), b = 12.3388(6), c = 18.3572(10) A, beta = 108.2570 (10) degrees, V = 2900.4(3) A(3). Crystal data for 2: a = 13.4156(3), b = 12.2588(3), c = 18.2501(4) A, beta = 108.049(3) degrees, V = 2853.8(10) A(3). Crystal data for 3: a = 13.4934(3), b = 12.3983(3), c = 18.3992(4) A, beta = 108.025(3), V = 2927.0(10) A(3). Crystal structure shows that each cluster is surrounded by six [Ln(H(2)O)(6)](3+) complex cations, and each [Ln(H(2)O)(6)](3+) cation is coordinated to three surrounding POV cluster anions. The electron spin resonance spectra of compounds 1-3 show a typical single line (g = 1.9671 for 1, g = 1.9669 for 2, and g = 1.9704 for 3), characteristic for a V(4+) (d(4)) ion; in addition, a supplementary signal appears for compound 2 at g = 5.9238 due to the presence of the Sm(3+) (f(5)) ion. All vanadium atoms exit in +4 oxidation states that have been confirmed by bond valence sum calculations. Variable-temperature magnetic studies for all three compounds 1-3 are performed and are discussed in terms of antiferromagnetic coupling interactions, giving importance to linking/assembling the {V(14)} cluster anions. TGA/mass analyses of compounds 1-3 (linked system) have been compared with that of the starting precursor [NH(4)](6)[As(8)V(14)O(42)(SO(3))] (discrete building unit). Interestingly, the evolution of SO(2) gas takes place for the discrete cluster compound [NH(4)](6)[As(8)V(14)O(42)(SO(3))] in a temperature range of 480-520 degrees C with the decomposition of the POV cluster anion, whereas the same evolution occurs at 520-580 degrees C for compounds 1-3. These comparative TGA/mass studies help to understand how the organic free linker elevates the thermal stability of the sulfite encapsulated POV cluster anion in going from a discrete cluster anion to the linked system (molecule to material). It has also been demonstrated that the stability of the sulfite anion increases to a greater extent when it is included in the cluster cage. The powder XRD studies of compounds 1-3 confirm that these are isostructural materials and provide information about the phase purity.
Arumuganathan
T
T
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India.
Das
Samar K
SK