The synthesis of W–O–M (M = Al, Ti, Ni, Zn) μ–oxo clusters by hydrolysis of tungsten aminoalkoxides and their structural characterisation
Graphical abstract
The structures of four novel heterobimetallic W–O–M oxo-clusters W2Ti2(O)2(OPri)2(μ2–O)4(OPri)6(μ2–bdmap)2, W2Zn2(O)2(μ2–O)3(acac)2(μ2–bdmap)2 W2Al2(O)2(OPri)2(μ2–O)4(μ2–bdmap)4 (3) and W4Ni4(O)4(OMe)4(μ2–OMe)4(μ2–O)4(dmap)8 have been determined.
Introduction
Compounds incorporating donor-functionalised alkoxide ligands play a key role in contemporary materials chemistry, specifically in the design of precursors for the formation of metal oxides by either chemical vapour deposition (CVD) [1] or sol–gel routes [2]. The additional donor groups, commonly NR2 or OR, serve several purposes. In the case of CVD precursors where volatility is a key issue, the additional donor sites on the ligand aid coordination saturation at the metal and hence minimise unwanted intermolecular interactions [3]. In this respect, we have recently demonstrated the ability of the amino-alkoxides Hdmae, Hbdmap and Htdmap to control the nuclearity of organozinc- [4] and organocadmium alkoxides [5], (RML)n [R = Me, Et; M = Zn, Cd; L = dmae, bdmap, tdmap; n = 4, 3, 2].
In addition, the additional donors can also reinforce any μ2– or μ3–oxygen bridging interactions in which the alkoxides participates, which can be extremely useful in maintaining the integrity of hetero-metallic alkoxides with respect to dissociation at elevated temperatures e.g. Sr[Ta2(OEt)10(dmae)2] and Sr[Ta2(OEt)10(bdmap)2] for the deposition of SrTa2O6 and SrBi2Ta2O9, respectively [6], [7].
This reinforcement of molecular integrity also applies to the hydrolysis of precursors in a sol–gel protocol, where a bi-metallic precursor has the advantage over separate metal sources in obviating the need to match hydrolysis rates. Moreover, the coordination saturation which is important in CVD chemistry also serves to control the rate of hydrolysis in sol–gel chemistry, which can lead to more uniform final materials.
We have previously reported on the controlled hydrolysis of the tungsten aminoalcohols W(O)(OPri)3(L) (L = dmae, bdmap, tdmap) to afford the novel aggregates W4O4(μ–O)6(dmae)4, W4O4(μ–O)4(OPri)4(bdmap)4, W6O6(μ–O)9(tdmap)6 and W4O4(μ–O)6(tdmap)6 [8]. In this paper we report results on the controlled hydrolysis of the donor-functionalised tungsten alkoxide W(O)(OPri)3(bdmap) in the presence of a second metal species (alkoxide or μ-diketonate) to afford novel W–O–M (M = Ti, Al, Zn) oxo-clusters, along with a related W–O–Ni species formed by adventitious co-hydrolysis of W(O)(OMe)4 and Ni(dmap)2.
Section snippets
Experimental
General procedures: Elemental analyses were performed using an Exeter Analytical CE 440 analyser. 1H and 13C NMR spectra were recorded on a Bruker Advance 300 or 500 MHz FT-NMR spectrometers as indicated, as saturated solutions at room temperature, unless stated otherwise; chemical shifts are in ppm with respect to Me4Si, coupling constants are in Hz.
All reactions were carried out under an inert atmosphere using standard Schlenk techniques. Solvents were dried and degassed under an argon
Crystallography
Experimental details relating to the single-crystal X-ray crystallographic studies are summarised in Table 1. Data were collected on a Nonius Kappa CCD diffractometer at 150(2) K using Mo Kα radiation (λ = 0.71073 Å). For all structures a symmetry-related (multi-scan) absorption correction has been applied. Structure solution, followed by full-matrix least squares refinement was performed using the wingx-1.80 suite of programs throughout [11].
Specific details: In 1 there is disorder in the position
Results and discussion
Mixed W–O–M [M = Ti (1), Zn (2), Al (3), Ni (4)] oxo-clusters have been prepared by the slow hydrolysis of mixtures of W(O)(OR)4 and appropriate metal alkoxide or μ–diketonate, according to the following Eqs. (1–4):
In the case of (1) the controlled hydrolysis afforded a material that could be fully characterised analytically, while 2 was sufficiently pure to give clear NMR spectra but accurate microanalysis data were not achievable. 3 was prepared in low yield and analytically impure, but
Conclusions
Four novel W–O–M clusters (M = Ti, Al, Zn, Ni) have been prepared by the controlled hydrolysis of W(O)(OPri)3(bdmap) with Ti(OPri)4, Zn(acac)2(H2O)2, Al(OPri)3 (as appropriate), or from W(O)(OMe)4 and Ni(dmap)2. Clusters containing Ti or Zn are based on an adamantane-like W2M2On core (n = 6, M = Ti; n = 5, M = Zn), while that containing Al adopts an isomeric W2Al2O6 structure based on a cube with two sides missing. The Ni derivative forms a square of four oxo-bridged W–O–Ni units.
All four clusters have a
Acknowledgement
We thank the EPSRC for the award of a studentship (to H.C.).
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