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

WSeCl4 was obtained in good yield by heating WCl6 and Sb2Se3 in vacuo. Green crystals grown by sublimation were shown by single crystal X-ray structure analysis to contain square pyramidal monomers with apical W = Se, 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, (SeBu2)-Bu-n), whilst the dimers [(WSeCl4)(2)(mu-L-L)] were formed with L-L = Ph2P(O)CH2P(O)Ph-2, 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)(mu-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 WvSe. Alkylphosphines, including PMe3 and PEt3, decompose WSeCl4 upon contact, forming phosphine selenides (SePR3). In contrast, the selenoether complexes [WSeCl4(SeMe2)] and [WSeCl4(SenBu(2))] were isolated and characterised. The crystal structure of the minor W(VI) by-product, [(WSeCl4))(2)(mu-SeMe2)], was determined and using SMe2, a few crystals of the W(V) species, [{WCl3(SMe2)}(2)(mu-Se)(mu-Se-2)], 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((SeBu2)-Bu-n)] in the range 660-700 degrees 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.

Automated summary

In this study, WSeCl4 was synthesized by heating WCl6 and Sb2Se3 in vacuo and characterized. The reaction of WSeCl4 with various ligands was investigated, and the properties of the resulting complexes were analyzed. Additionally, highly reflective WSe2 thin films were obtained through low-pressure chemical vapor deposition.