Synthesis and Structural Characterization of Redox‐Active Dodecamethoxoheptaoxohexavanadium Clusters

Abstract

The chemical behavior of alkoxo-oxovanadium clusters lies between that of metal oxides and metal alkoxides. [1,2] These compounds are important as precursors for the synthesis of new polyoxometalates and as reagents for sol-gel processes and chemical vapor deposition (CVD) methods for the generation of metal oxides.[3] The interest in this substance class for basic research as well as for technical applications is attributed to vanadium's facile and often reversible change in its oxidation state.[4] By varying the alkoxo ligands and the V IV /V V ratio in such clusters, it is possible to control the properties and the composition of the metal oxides obtained by their thermal decomposition, which is of particular importance, for example, for the development of heterogeneous catalysts based on vanadium oxides.It [2b] or by complementary ligands such as squarate, [7] oxalate, acetylacetonate, or phenylphosphonate [2b] as well as metal [8] and organometallic [9] complexes. Recently we synthesized a series of dodecamethoxohep-which, while differing in their V IV /V V ratios (n = 3, 4, 5, 6), remain identical in their structural composition. Noteworthy is the fact that these compounds were not only observed as intermediates by electrochemical and spectroscopical methods, but have also been isolated and characterized by X-ray structural analysis.Besides vanadium oxides and vanadates, [1] polyoxovanadates containing large organic cations, [10] and thus soluble in organic solvents, as well as orthovanadic acid esters VO(OR) 3 of primary alcohols have also proven to be suitable precursors for the synthesis of alkoxo-oxovanadium compounds. [7,11] The uncharged coumpound of mixed valency [V 8À (M = Nb, Ta), [12] yet unknown for vanadium, by replacing the 12 m 2 -bridging oxygen atoms with methoxo groups. In this cluster, four vanadium atoms are present in the oxidation state + IV and the remaining two in the oxidation state + v. As could be expected from the strong molar mass peak in the mass spectrum (see Experimental Section), 1 can be sublimed under reduced pressure without noticeable decomposition. This makes 1 an interesting candidate for CVD experiments. During the thermal decomposition of the compound in the absence of oxygen, continuous mass loss occurs beginning at 200 8C and ending with the formation of V 2 O 3 at approximately 380 8C (mass loss (%): found: 43.0; calcd: 43.1).The cyclic voltammogram [13] of 1 (Figure 2) turns out to be a challenge for synthetic chemists because it reveals a series of electronic transitions assigned to hexametalate clusters(4Àn) of identical composition and structure, yet having different V IV /V V ratios. Starting from 1 (n = 4) and using suitable reducing and oxidizing agents, it was indeed possible to synthesize the singly (n = 5) and doubly (n = 6) reduced clusters in compounds 3 b, 3 a, 2 a, and 2 b respectively, as well as the singly [14] For the preparation of the singly reduced cluster in 3 b tetrabutylammonium tetrabutylborate was used, which acts as a mild reducing a...

Intramolecular and Lattice Dynamics in V6-nIVVnVO7(OCH3)12Crystal

“…12 compound belongs to the P 2 1 /n space group with four magnetically non-equivalent cluster-molecules in the unit cell [10,11]. In each mixed valent cluster V vanadium nuclei all equally bind one terminal oxygen atom and four µ 2 -bridging methoxo ligands, respectively (see Table and • .…”

“…We shall begin the discussion of the V Table) and corresponds to the valence-sum calculations [11]. The spin Hamiltonian can thus be written as…”

Hetero and lacunary polyoxovanadate chemistry: Synthesis, reactivity and structural aspects

“…This serves to diminish the charge, and a reducing state of V IV is preferred by the introduction of alkoxide ligands [83]. The substitution of all bridging oxygens with CH 3 O - [84,85] [98]. This straightforward synthesis of alkoxides provides a facile route for further development of polyoxovandate synthesis.…”

Ferromagnetic Coupling in a Mixed‐Valence Hexavanadate Core: Quantum‐Chemical Forecast

“…(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) Hartl et al [6,7] In these complexes, the hexavanadate core contains an OV 6 unit in which the vanadium ions are arranged in an octahedral fashion; six oxo groups act as terminal ligands and twelve alkoxo groups act as bridging ligands. The chemical synthesis and crystal structures of such alkoxo-polyoxovanadium compounds differing in their V IV content (n = 2, 3, 4, 5, 6) have been described (see Figure 1).…”

“…[1][2][3][4] It has been found that unstable POM architectures can be stabilized by the introduction of alkoxo ligands because of the charge compensation associated with formal substitution of peripheral oxo ligands. [5] Among many other alkoxo-polyoxometalates, the fully alkylated polyoxovanadate derivatives [V IV n V V 6-n O 7 (OR) 12 ] (4-n) (R = CH 3 , C 2 H 5 ) of the classical (Lindqvist) POM structure [M 6 O 19 ] n-, which as an entity is not known for vanadium, have been reported by Such interactions are very rare in oxovanadium complexes and are a consequence of a specific core geometry imposed by the polyoxometalate architecture. It has been shown that the earlier proposed intramolecular electron transfer cannot change the ground spin-state multiplicity irrespective of the sign of the exchange parameters.…”