Cesium salts of niobo-tungstate isopolyanions with intermediate group V–group VI character. (21st December 2015)

• Record Type: Journal Article
• Authors: Sures, Dylan J.; Molina, Pedro I.; Miró, Pere; Zakharov, Lev N.; Nyman, May
• Is Part Of: New journal of chemistry. Volume 40:Number 2(2016:Feb.)
• Page Start: 928
• Page End: 936
• Publication Date: 2015-12-21
• Subjects: Chemistry -- Periodicals; Chimie -- Périodiques; 540
• DOI: 10.1039/c5nj02914g
• Languages: English
• ISSNs: 1144-0546

Abstract

Abstract : The physical and electronic structures of cesium salts of niobo-tungstate Lindqvist ions vary with Nb content, elucidated experimentally and computationally. Abstract : Alkali metal salts of polyoxometalates (POMs) of the group VI elements (W and Mo) and polycoltanates (Nb and Ta POMs) exhibit opposing trends in their solubility in water and ion-association in solution. Mixed clusters of these two group V metals and tungsten provide an opportunity to probe the reversal in these trends and to understand their origin. A review of a classic study of mixed Nb/W clusters and our own work in Ta/W polyanions have led us to isolate Cs + /Na + salt of [Nb4 W2 O19 ] 6− and two salts, Cs + /Na + and pure Cs +, of [Nb2 W4 O19 ] 4− by using peroxoniobate ([Nb(O2 )4 ] 3− ) instead of hexaniobate ([Nb6 O19 ] 8− ) as the niobium source. Crystallographic analysis shows that Cs + -bonding to clusters increases with Nb-content, following the trend observed in our previous studies of hexaniobate in solution. Fragmentation by ESI-MS suggests that niobium-rich [Nb4 W2 O19 ] 6− is less stable than isostructural [Nb2 W4 O19 ] 4− and this technique, together with FTIR, confirms the predominance of the cis -isomer in the cluster structures. The mixed-metal composition of these isopolyanions is reflected in the crystallographic bond lengths and in the positions of the absorption bands in the UV spectra. DFT calculations reveal that the HOMO–LUMO energy gap widens with increasing Nb content in the cluster framework – an effect ascribed to the overall poorer mixing of Nb4d, versus W5d, atomic orbitals with the corresponding O2p orbitals.