Evaluation of Sn(ii) aminoalkoxide precursors for atomic layer deposition of SnO thin films

Abstract

We have successfully prepared and structurally characterized a family of eight tin(II) heteroleptic complexes, [Sn(NR₂)(ON)]_x (NR₂ = NMe₂ (1a–d) or N(SiMe₃)₂ (2a–d); x = 1 or 2) and four homoleptic systems, [Sn(κ²-ON)₂] (3a–d) from a series of aminoalcohols and fluorinated aminoalcohols (H{ON}) having a different number of methyl/trifluoromethyl substituents at the α-carbon atom, [HOC(R¹)(R₂)CH₂NMe₂] (R¹ = R² = H (H{dmae}) (a); R¹ = H, R² = Me (H{dmap}) (b); R¹ = R² = Me (H{dmamp}) (c); R¹ = R² = CF₃ (H{Fdmamp}) (d)). The synthetic route used reactions of either [Sn{N(SiMe₃)₂}₂] or [Sn(NMe₂)₂] with one or two equivalents of the aminoalcohols (a–d) in dry aprotic solvents leading to elimination of amines and formation of the Sn(II) species 1a–d, 2a–d and 3a–d respectively. All complexes were thoroughly characterized by NMR spectroscopy (¹H, ¹³C, ¹⁹F, and ¹¹⁹Sn) as well as single-crystal X-ray diffraction studies. In all case the solid state molecular structures of the complexes have been unambiguously established: the solid state structures 1a–b and 1c are dimeric with central {Sn₂N₂} cores resulting from bridging {μ²-NMe₂} units, in which the Sn(II) atoms are four-coordinate. In contrast, the solid state structures of complexes 1c and 2a–c possess similarly dimeric structures, with four-coordinate Sn(II) atoms, in which the oxygen atoms of the {ON} ligand bridge two Sn(II) centres to form dimers with a central {Sn₂O₂} core. Uniquely in this study, 2d, [Sn(κ²-O,N-OCMe₂CH₂NMe₂){N(SiMe₃)₂}] is monomeric with a three coordinate Sn(II) centre. The homoleptic complexes 3a–d are all isostructural with monomeric four-coordinate structures with disphenoidal geometries. Solution state NMR studies reveal complicated ligand exchange processes in the case of the heteroleptic complexes 1a–d and 2a–d. Contrastingly, the homoleptic systems 3a–d show no such behaviour. While complexes 1a–d and 2a–d displayed either poor thermal stability or multistep thermal decomposition processes, the thermal behaviour of the homoleptic complexes, 3a–d, was investigated in order to determine the effects, if any, of the degree of fluorination and asymmetry of the aminoalkoxide ligands on the suitability of these complexes as ALD precursors for the deposition of SnO thin films.