Tungsten(vi) selenide tetrachloride, WSeCl4 – synthesis, properties, coordination complexes and application of [WSeCl4(SenBu2)] for CVD growth of WSe2 thin films

Abstract

WSeCl₄ was obtained in good yield by heating WCl₆ and Sb₂Se₃in vacuo. Green crystals grown by sublimation were shown by single crystal X-ray structure analysis to contain square pyramidal monomers with apical WSe, and powder X-ray diffraction (PXRD) analysis confirmed this to be the only form present in the bulk sample. Density functional theory (DFT) calculations using the B3LYP-D3 functional replicated the structure, identified the key bonding orbitals, and were used to aid assignment of the IR spectrum of WSeCl₄. Reaction of WSeCl₄ with ligands L gave [WSeCl₄(L)] (L = MeCN, DMF, thf, py, OPPh₃, 2,2′-bipy, SeMe₂, SeⁿBu₂), whilst the dimers [(WSeCl₄)₂(μ-L–L)] were formed with L–L = Ph₂P(O)CH₂P(O)Ph₂, 1,4-dioxane and 4,4′-bipyridyl. The complexes were characterised by microanalysis, IR and ¹H NMR spectroscopy, and single crystal X-ray structures determined for [WSeCl₄(L)] (L = OPPh₃, MeCN, DMF) and [(WSeCl₄)₂(μ-L–L)] (L–L = 1,4-dioxane, 4,4′-bipyridyl). All except the 2,2′-bipy complex, which is probably seven-coordinate, contain six-coordinate tungsten with the neutral donor trans to WSe. Alkylphosphines, including PMe₃ and PEt₃, decompose WSeCl₄ upon contact, forming phosphine selenides (SePR₃). In contrast, the selenoether complexes [WSeCl₄(SeMe₂)] and [WSeCl₄(SeⁿBu₂)] were isolated and characterised. The crystal structure of the minor W(VI) by-product, [(WSeCl₄)₂(μ-SeMe₂)], was determined and using SMe₂, a few crystals of the W(V) species, [{WCl₃(SMe₂)}₂(μ-Se)(μ-Se₂)], were obtained and structurally characterised. The isolated W(VI) complexes are compared with those of WOCl₄ and WSCl₄ and the combination of experimental and computational data are consistent with WSeCl₄ being a weaker Lewis acid and its complexes significantly less stable than those of the lighter analogues, especially in solution. Low pressure chemical vapour deposition (LPCVD) using [WSeCl₄(SeⁿBu₂)] in the range 660–700 °C (0.1 mmHg) produced highly reflective thin films, which were identified to be WSe₂ by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XRD analysis of the thinner films revealed them to be highly oriented in the <00l> direction.