Tungsten(VI) selenide tetrachloride, WSeCl 4 - synthesis, properties, coordination complexes and application of [WSeCl 4 (Se n Bu 2 )] for CVD growth of WSe 2 thin films.

WSeCl 4 was obtained in good yield by heating WCl 6 and Sb 2 Se 3 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 WSeCl 4 . Reaction of WSeCl 4 with ligands L gave [WSeCl 4 (L)] (L = MeCN, DMF, thf, py, OPPh 3 , 2,2'-bipy, SeMe 2 , Se n Bu 2 ), whilst the dimers [(WSeCl 4 ) 2 (μ-L-L)] were formed with L-L = Ph 2 P(O)CH 2 P(O)Ph 2 , 1,4-dioxane and 4,4'-bipyridyl. The complexes were characterised by microanalysis, IR and 1 H NMR spectroscopy, and single crystal X-ray structures determined for [WSeCl 4 (L)] (L = OPPh 3 , MeCN, DMF) and [(WSeCl 4 ) 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 WSe. Alkylphosphines, including PMe 3 and PEt 3 , decompose WSeCl 4 upon contact, forming phosphine selenides (SePR 3 ). In contrast, the selenoether complexes [WSeCl 4 (SeMe 2 )] and [WSeCl 4 (Se n Bu 2 )] were isolated and characterised. The crystal structure of the minor W(VI) by-product, [(WSeCl 4 ) 2 (μ-SeMe 2 )], was determined and using SMe 2 , a few crystals of the W(V) species, [{WCl 3 (SMe 2 )} 2 (μ-Se)(μ-Se 2 )], were obtained and structurally characterised. The isolated W(VI) complexes are compared with those of WOCl 4 and WSCl 4 and the combination of experimental and computational data are consistent with WSeCl 4 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 [WSeCl 4 (Se n Bu 2 )] in the range 660-700 °C (0.1 mmHg) produced highly reflective thin films, which were identified to be WSe 2 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 <00 l > direction.