Nebebech Belai

Rationalization and improvement of the syntheses of two octadecatungstoarsenates: the novel α-K7[H4AsW18O62]·18H2O and the well known symmetrical α-K6[As2W18O62]·14H2O

Abstract α-[H 4 AsW 18 O 62 ] 7− was synthesized and thoroughly characterized by IR, 183 W NMR and cyclic voltammetry for the first time. Systematic variation of the As–W ratio in an appropriate pH domain, has permitted us to highlight the main parameters in the syntheses of α-[H 4 AsW 18 O 62 ] 7− and α-[As 2 W 18 O 62 ] 6− . Keys to the optimization of their yields are the As–W ratio, the pH of the reaction mixture and the direct precipitation of the solids by KCl. The appropriate pH domain is common for the two complexes. To facilitate discussion, an excess parameter R * is defined as the ratio between the actual As–W ratio and the stoichiometric value of this parameter for each complex. The domain of variation of R * is rather small for the synthesis of α-[H 4 AsW 18 O 62 ] 7− (0.2 R *≤1.0). It turns out that its yield falls to zero before that of α-[As 2 W 18 O 62 ] 6− reaches significant values. Finally, it was shown that the best yields for the two complexes are obtained under rather different conditions: R * is close to its stoichiometric value for the synthesis of α-[H 4 AsW 18 O 62 ] 7− ; larger values of R * are necessary for obtaining α-[As 2 W 18 O 62 ] 6− . However, for this last synthesis, R * can be substantially diminished compared to its values in previous literature, provided the pH of the reaction mixture is properly controlled.

Chelated heteroatoms in polyoxometalates and the topological equivalence of {CoIII(en)} to type II cis-dioxometal centers. Synthesis and structure of [{Co(en)(μ-OH)2Co(en)}{PW10O37Co(en)}2]8− and [K⊂{Co(en)WO4}{WO(H2O)}(PW9O34)2]12−

The first examples of polyoxometalate structures that incorporate embedded chelated heteroatoms point to new possibilities for stereochemical control of applications.

Potassium bis(carbonato-O,O')(ethylenediamine-N,N')cobaltate(III) monohydrate at 173 K.

The title salt, K[Co(C2H8N2)(CO3)2].H2O, consists of a distorted octahedral cobalt complex anion and a seven-coordinate potassium cation. Both metal atoms have crystallographic twofold symmetry, one C2 axis passing through the Co atom and C--C bond, and another along a short K--O (water) bond of 2.600 A (corrected for libration). The carbonate is bidentate to both cobalt and potassium and the water forms a hydrogen bond to a carbonate O atom.

Prospects for Rational Assembly of Composite Polyoxometalates

The directed synthesis of large inorganic polyoxometalates with specific structural and stereochemical features remains an elusive but not unattainable goal. We have shown that opportunities exist for straightforward assembly of stable or metastable polyoxometalate subunits using principles of both coordination chemistry and organic synthesis. Among the many advantages offered by such synthetic approaches is the generation of chiral structures for molecular recognition and catalytic selectivity that we are currently pursuing.