Synthesis, solution dynamics and chemical vapour deposition of heteroleptic zinc complexes via ethyl and amide zinc thioureides

Abstract

Ethyl and amide zinc thioureides [L¹ZnEt]₂ (1), [L¹*ZnEt]₂ (2) and [L¹Zn(N(SiMe₃)₂)]₂ (3) have been synthesised from the equimolar reaction of thiourea ligands (HL¹ = ⁱPrN(H)CSNMe₂ and HL¹* = PhN(H)CSNMe₂) with diethyl zinc and zinc bis[bis(trimethylsilyl)amide] respectively. New routes towards heteroleptic complexes have been investigated through reactions of 1, 2 and 3 with β-ketoiminates (HL² = [(Me)CN(H){ⁱPr}–CHC(Me)O]), bulky aryl substituted β-diiminates (HL³ = [(Me)CN(H){Dipp}–CHC(Me)N{Dipp}] (Dipp = diisopropylphenyl) and HL³* = [(Me)CN(H){Dep}–CHC(Me)N{Dep}] (Dep = diethylphenyl)) and donor-functionalised alcohols (HL⁴ = Et₂N(CH₂)₃OH and HL⁴* = Me₂N(CH₂)₃OH) and have led to the formation of the heteroleptic complexes [L¹*ZnL³*] (5), [L¹ZnL⁴]₂ (6), [L¹ZnL⁴*]₂ (7), [L¹*ZnL⁴] (8) and [L¹*ZnL⁴*] (9). All complexes have been characterised by ¹H and ¹³C NMR, elemental analysis, and the X-ray structures of HL¹*, 1, 2, 6 and 7 have been determined via single crystal X-ray diffraction. Variable temperature ¹H, COSY and NOESY NMR experiments investigating the dynamic behaviour of 5, 6 and 7 have shown these molecules to be fluxional. On the basis of solution state fluxionality and thermogravimetric analysis (TGA), alkoxyzinc thioureides 6 and 7 were investigated as single-source precursors for the deposition of the ternary material zinc oxysulfide, Zn(O,S), a buffer layer used in thin film photovoltaic devices. The aerosol-assisted chemical vapour deposition (AACVD) reaction of 7 at 400 °C led to the deposition of the heterodichalcogenide material Zn(O,S), which was confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX), with optical properties investigated using UV/vis spectroscopy, and surface morphology and film thickness examined using scanning electron microscopy (SEM).