Bridging the opposite chemistries of tantalum and tungsten polyoxometalates

Abstract

The disparate solubility, redox activity, and pH stability of the group V and group VI polyoxometalates (POMs) confer very different functionality on these species, and tailoring cluster properties by varying the ratio of group V to group VI metals poses both an opportunity and a synthetic challenge. A classic series of studies reported over 40 years ago provided some insight into W/Nb POMs, from which researchers have built on to date. However, the analogous W/Ta series has never been addressed in a systematic manner. Three members of this W/Ta series are presented here, synthesized from simple oxo- and peroxocoltanate precursors. [Ta3W3O19](5-) displays the Lindqvist-type structure, while [TaW9O32](5-) and [Ta2W8O32](6-) are isostructural with decatungstate ([W10O32](4-)). Additionally, the use of peroxoniobate instead of hexaniobate as the starting material drives the formation of the decatungstate-type structure [NbW9O32](5-) instead of the Lindqvist ion that was established to be the foundational cluster geometry in prior work. The electronic structure of the Nb/Ta substituted decatungstates is directly related to the degree of substitution inasmuch as the HOMO-LUMO energy gap (Egap) slightly increases as more Nb/Ta atoms are incorporated into the structure. The poor mixing of the d-orbitals of Nb/Ta and W is responsible for the observed trends in the UV spectra and cyclic voltammetry. Moreover, the stability of the molecular frameworks in the gas phase is also related to the extent of substitution as revealed by electrospray mass-spectrometry (ESI-MS).

Crystallizing Elusive Chromium Polycations

“…These data show 1) The Cr:Al ratios of the observed compositions correlate well with the bulk composition determined from structure, electron-microprobe, and theory, and 2) Clusters with higher Cr content are more extensively fragmented by the ionization process, providing evidence for their lower kinetic stability. (Miras et al, 2009;Molina et al, 2015) In contrast to the reaction solutions, clusters are readily detected in solutions prepared by dissolution of Zn(CrAl)12 crystals. These data confirmed our SAXS observations: clusters are not present in the reaction solutions at the point of saturation and crystallization of Zn(CrAl)12.…”

Trapping of NbV by {XW9O33}9– (X = As, Sb): Formation of New Sandwich‐Type POM Complexes and Their Solution Behavior

“…They were introduced and explored by Klemperer, [1] Finke, [2][3][4] Hill, [5][6][7][8][9][10] and Niu [11,12] groups, and include classical Lindqvist-type [13][14][15] Keggin-type [XW 9 Nb 3 O 40 ] n-(X = Si IV , Ge IV , n = 7; X = P V , As V , n = 6), [2,3,16,17] Dawson-type [P 2 W 12 (NbO 2 ) 6 O 56 ] 12-, [18] [P 2 W 15 Nb 3 O 62 ] 9-, [7,19] [P 2 W 17 (NbO 2 )-O 61 ] 7-, [6] as well as more complicated structures based on the condensation of simpler building blocks via Nb-O-Nb bridges. [21,22] Such complexes demonstrate photocatalytic activity in oxidation [5] and reduction [11] processes, and can be used as precursors for hydrodesulfurization/isomerization catalysts [14] or as active catalysts in the preparation of linearlyextended polyalkylenepolyamines. [21,22] Such complexes demonstrate photocatalytic activity in oxidation [5] and reduction [11] processes, and can be used as precursors for hydrodesulfurization/isomerization catalysts [14] or as active catalysts in the preparation of linearlyextended polyalkylenepolyamines.…”

“…Reactions of [NbO(C 2 O 4 ) 2 (H 2 O) 2 ]with [α-B-XW 9 O 33 ] 9-(X = As, Sb) result in formation of sandwich-type anions. [21,22] Such complexes demonstrate photocatalytic activity in oxidation [5] and reduction [11] processes, and can be used as precursors for hydrodesulfurization/isomerization catalysts [14] or as active catalysts in the preparation of linearlyextended polyalkylenepolyamines. They were introduced and explored by Klemperer, [1] Finke, [2][3][4] Hill, [5][6][7][8][9][10] and Niu [11,12] groups, and include classical Lindqvist-type [13][14][15] Keggin-type [XW 9 Nb 3 O 40 ] n-(X = Si IV , Ge IV , n = 7; X = P V , As V , n = 6), [2,3,16,17] Dawson-type [P 2 W 12 (NbO 2 ) 6 O 56 ] 12-, [18] [P 2 W 15 Nb 3 O 62 ] 9-, [7,19] [P 2 W 17 (NbO 2 )-O 61 ] 7-, [6] as well as more complicated structures based on the condensation of simpler building blocks via Nb-O-Nb bridges.…”

Controlling Fast Nucleation and Crystallization of Two New Polyoxoniobates

“…High nuclearity polyoxometalates (POMs) with cluster shells of different shape, charge, and functionality form an outstanding class of materials with enormous structural varieties and applications in various fields like medicine and catalysis. − While high-nuclearity POM clusters based on Mo or W crystallize in a wide pH range between approximately 0 and 8.5, − the product formation among group V POMs is more sensitive to changes of pH value. − In contrast to molybdates and tungstates, where a great variety of precursors like, e.g., [MoO 4 ] 2– , [WO 4 ] 2– , {Mo 7 O 24 }, {PW 12 O 40 }, or {P 2 W 18 O 64 } is available, the number of suitable starting materials containing prebuilt cluster shells in the field of polyoxovanadates (POVs) and polyoxoniobates (PONbs) is limited. The usage of heteroatom containing POVs as synthons was recently identified as a promising approach for the generation of new POVs, , but such preformed compounds do not exist in the field of PONbs. ,, …”