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Chapter 3 Published Results 3.A.2. The 28‐Isopolytungstate Fragment [H2W28O95] 20− Stabilized by two External Lanthanide Ions A. H. Ismail, B. S. Bassil, A. Suchopar, U. Kortz, Eur. J. Inorg. Chem. 2009, 5247 3.A.2.1. Introduction Polyoxometalates (POMs) are a well-known class of anionic metal-oxo clusters of the early (group V and VI) transition metals with molecular and electronic structural versatility. POMs are remarkable due to their reactivity and relevance in fields such as photochemistry, analytical chemistry, clinical chemistry, magnetism, catalysis, biology, medicine and materials science. 1 POMs, especially polyoxotungstates, are effective homogeneous photocatalysts for the mineralization of organic pollutants (i.e. degradation of organic pollutants to CO 2 , H 2 O and the corresponding inorganic anions). 2 The ability of POMs to form peroxo derivatives renders them useful in oxidation reactions. Furthermore, chemical modifications in the composition of POMs allow for fine tuning of their Lewis acidity. 3 Two main types of POMs are known: isopoly- and heteropoly-anions. Isopolyanions are represented with the general formula [M m O y ] n− , where M is the addenda atom in a high oxidation state (e.g. W VI , Mo VI ). Heteropolyanions are represented with the general formula [X x M m O y ] q− (x m) where X is the heteroatom (e.g. P V , As V , Si IV , Ge IV ). 1a So far, lanthanide-containing POMs have been investigated less than their 3d-transitionmetal analogues. The former have also shown interesting properties in the areas of photoluminescence, catalysis, electrochemistry, and magnetism. 4 Most of the lanthanidecontaining POMs reported to date are actually heteropolyanions. 5 Our group has also been working on lanthanide-containing heteropolytungstates. 6 We have reported the ytterbiumcontaining tungstoarsenate [YbAs 2 W 20 O 68 (H 2 O) 3 ] 7− resulting from the interaction of the monolacunary [As 2 W 20 O 68 (H 2 O)] 10− with Yb 3+ ions in acidic aqueous medium. The polyanion consists of two (α-As III W 9 O 33 ) fragments connected by a V-shaped (H 2 O)Yb(OW(H 2 O)) 2 fragment. [6a] The monolanthanide-containing polyanion family [Ln(β 2 -SiW 11 O 39 ) 2 ] 13− (Ln = 75
Chapter 3 Published Results La, Ce, Sm, Eu, Gd, Tb, Yb, Lu) has also been synthesized and structurally characterized. [6c] These polyanions are composed of two chiral (β 2 -SiW 11 O 39 ) units sandwiching the Ln 3+ ion. Recently, we reported the tungstogermanate [Ce 20 Ge 10 W 100 O 376 (OH) 4 (H 2 O) 30 ] 56− containing 20 cerium atoms and 100 tungsten centers. 6d We synthesized this polyanion by the reaction of the trilacunary POM precursor [α-GeW 9 O 34 ] 10− with Ce 3+ ions in acidic aqueous medium. However, to date only two types of lanthanide-containing isopolyanions have been reported. The decatungstate [Ln(W 5 O 18 ) 2 ] n− ({LnW 10 }) sandwich species (Ln 3+ = La, Ce, Pr, Nd, Sm, Ho, Yb and Y, and Ce 4+ ), first reported by Peacock and Weakley, was synthesized by reaction of the lanthanide ions with WO 2- 4 in a solution of pH 6.5−7.5. 5a,7a The structure of the {LnW 10 } family is based on two monolacunary [W 5 O 18 ] 6- Lindqvist fragments encapsulating a central metal ion in a square-antiprismatic fashion. In particular Yamase’s group reported some {LnW 10 } structures with different types of alkali counter cations. 8 Recently our group has reported the synthesis and structure of the yttrium derivative of {LnW 10 } including the use of solution 89 Y NMR spectroscopy. 9 The isopolyanion family [Ln III W 10 O 36 ] 9– , (Ln = Pr, Nd, Sm, Eu, Tb, Dy), has shown high luminescence quantum efficiency. 7b-d On the other hand, the 22-isopolytungstate [Ln 2 (H 2 O) 10 W 22 O 71 (OH) 2 ] 8− ({Ln 2 W 22 }) (Ln 3+ = La, Ce, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y), has been reported very recently by our group. 10 These {Ln 2 W 22 } polyanions were synthesized by the reaction of Na 2 WO 4 and the appropriate lanthanide salts in acidic aqueous medium (pH ~2) in an 11:1 ratio. All the {Ln 2 W 22 } polyanions are isostructural and crystallize as hydrated sodium salts in the triclinic space group P1̅. The structures of the {Ln 2 W 22 } family comprise the isopolyanion {W 22 } coordinated to two {Ln(H 2 O) n } 3+ supporting ions. The {W 22 } structure in turn consists of two undecatungstate units {W 11 } fused by two corner-sharing W-O-W bridges. The {Ln 2 W 22 } family can also be described as a dimeric entity composed of two {LnW 11 } half units related by an inversion center, resulting in the point group C i . These species are the first 76
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