Niobium(v) and tantalum(v) halide chalcogenoether complexes – towards single source CVD precursors for ME2 thin films

Abstract

A series of pentavalent niobium and tantalum halide complexes with thio-, seleno- and telluro-ether ligands, [MCl₅(EⁿBu₂)] (M = Nb, Ta; E = S, Se, Te), [TaX₅(TeMe₂)] (X = Cl, Br, F) and the dinuclear [(MCl₅)₂{o-C₆H₄(CH₂SEt)₂}] (M = Nb, Ta), has been prepared and characterised by IR, ¹H, ¹³C{¹H}, ⁷⁷Se, ⁹³Nb and ¹²⁵Te NMR spectroscopy, as appropriate, and microanalyses. Confirmation of the tantalum(V)–telluroether coordination follows from the crystal structure of [TaCl₅(TeMe₂)], which represents the highest oxidation state transition metal complex with telluroether coordination structurally authenticated. The Ta(V) monotelluroether complexes are much more stable than the Nb(V) analogues. In the presence of TaCl₅ the ditelluroether, CH₂(CH₂Te^tBu)₂, is decomposed; one of the products is the dealkylated [^tBuTe(CH₂)₃Te][TaCl₆], whose structure was determined crystallographically. Crystal structures of [(MCl₅)₂{o-C₆H₄(CH₂SEt)₂}] (M = Nb, Ta) show ligand-bridged species. The complexes bearing β-hydrogen atoms on the terminal alkyl substituents have also been investigated as single source reagents for the deposition of ME₂ thin films via low pressure chemical vapour deposition. While the tantalum complexes proved to be unsuitable, the [NbCl₅(SⁿBu₂)] and [NbCl₅(SeⁿBu₂)] deposit NbS₂ and NbSe₂ as hexagonal platelets onto SiO₂ substrates at 750 °C and 650 °C, respectively. Grazing incidence and in-plane X-ray diffraction confirm both materials adopt the 3R-polytype (R3mh), and the sulfide shows preferred orientation with the crystallites aligned predominantly with the c axis perpendicular to the substrate. Scanning electron microscopy and Raman spectra are consistent with the X-ray data.