Elsevier

Polyhedron

Volume 59, 1 August 2013, Pages 85-90
Polyhedron

Synthesis of heterobimetallic tungsten acetylacetonate/alkoxide complexes and their application as molecular precursors to metal tungstates

https://doi.org/10.1016/j.poly.2013.04.031Get rights and content

Abstract

A series of M2W2(O)2(acac)2(OMe)10 [M = Co (1), Ni (2), Mg (3), Zn (4)] have been synthesised and structurally characterised by X-ray crystallography. All the compounds share common structural features which are identical to the analogous molybdenum species, with four edge-sharing octahedra situated in the same plane, and incorporating two μ3-bridging and four μ2-bridging methoxide groups. The divalent metal M and tungsten have both octahedral coordination environments with six oxygen atoms. The structure of Na[Zn(acac)3] (6), a by-product of a failed synthesis of 4, is also reported, and is a linear polymer in which [Zn(acac)3]− anions use all six oxygen atoms to link sodium cations either side of the anion.

Thermal decomposition of representative samples (1, 3) shows them to be single-source precursors for metal tungstates MWO4. When the decomposition is carried out under an autogenerated pressure (RAPET) at 700 °C 1 forms CoWO4 nanoparticles, along with WOx-filled carbon nanotubes and amorphous carbon, while 3 forms MgWO4 nanoparticles and rods, though the latter lack a carbon shell and are much shorter in length.

Graphical abstract

The structures of M2W2(O)2(acac)2(OMe)10 [M = Co (1), Ni (2), Mg (3), Zn (4)] have been determined and 1, 3 have been shown to act as single-source precursors for MWO4 materials.

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Introduction

Tungsten oxide thin films have a number of applications e.g. electrochromic, photochromic displays [1], photocatalysts [2], [3], [4], gas sensors [5], solar control coatings [6], particularly when in thin film format. Such films are usually produced either by hydrolysis of a suitable precursor in a sol–gel protocol [7], or by chemical vapour deposition (CVD) from a volatile precursor [8], [9], [10]. The properties of these films can be modified by the inclusion of a variety of additional metals [11], [12], [13], which from a technological perspective require co-hydrolysis of two metal moieties (sol–gel) or a dual-source CVD approach. One variation which removes the practical difficulties of matching hydrolysis/decomposition rates is the use of a single-source precursor (SSP) which incorporates both metals in the same molecule. In this paper we explore the synthesis of W–O–M type compounds and their thermal decomposition to mixed metal oxides.

This preparation of heterometallic, heteroleptic acetylacetonate/alkoxide complexes of type MaMb(acac)x(OR)y has been widely used as a way of controlling the properties of the molecule by virtue of the distinct ligand types therein [14]. MAl2(acac)4(OPri)4 (M = Co, Ni, Mg) were first synthesised by Mehrotra et al. by modification of mixtures of the homoleptic alkoxides with acetylacetone [15]. Since then, other related compounds such as Co2Zr2(acac)2(OPrn)10 [16], Mo2M2(O)2(acac)2(OMe)10 (M = Co, Ni) [17], Co2Ti2(acac)2(OPri)10 [18] and Co2Al2(acac)4(OPri)6 [19] have been described. In particular, the interaction of M′(acac)2 (M′ = Co, Ni, Zn or Mg) with M(OMe)5 (M = Ta, Nb) in a hydrocarbon solvent was found to provide M′2M2(acac)2(OMe)12 and M(acac)(OMe)4 [20]. Of more direct relevance, mixed Mo–O–M species Mo2M2(O)2(acac)2(OR)10 have been reported by others [17], and herein we extend this family to cover the tungsten analogues.

Section snippets

General procedures

Elemental analyses were performed using an Exeter Analytical CE 440 analyser. 1H and 13C{1H} NMR spectra were recorded on a Bruker Advance 300 MHz FT–NMR spectrometer, as saturated solutions at room temperature, unless stated otherwise; chemical shifts are in ppm with respect to Me4Si, coupling constants are in Hz.

All manipulations were carried out under a dry nitrogen or argon atmosphere using a standard Schlenk techniques and standard glove-box. All dry solvents were purified by an Innovative

Results and discussion

Following the methodology used to prepare the analogous molybdenum compounds [20], heterobimetallic tungsten acetylacetonate alkoxides M2W2(O)2(acac)2(OMe)10 [M = Co (1), Ni (2), Mg (3), Zn (4)] have been synthesised first by reacting two equivalents of WO(OMe)4 and one equivalent of M(acac)2 in refluxing toluene solvent (Eq. (1)); yields were in the range 31–34%.M(acac)2+2W(O)(OMe)4⟶toluene1/2M2W2(acac)2(OMe)10M=Co(1),Ni(2),Mg(3),Zn(4)+W(O)(OMe)3(acac)However, this approach requires the use of

Conclusions

The bimetallic precursors W2M2(O)2(OMe)10(acac)2 (M = Co, Ni, Mg, Zn) have been prepared in an efficient manner from the reaction of W(O)(OMe)4 and M(OMe)(acac). The molecular structures of all four compounds are similar and replicate those of their molybdenum analogues. Representative examples (M = Co, Mg) show these compounds are single-source precursors for MWO4 materials.

Acknowledgement

We thank the EPSRC for a studentship (to H.C).

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