Structural Studies of Tin Compounds in Acid Media

Abstract

<p>Reactions of stannane or methylstannanes (CH₃)₄_nSnHn (n = 1-4) in fluorosulphuric acid at low temperatures (-85°C) produce [(CH₃)₃_nSnHn]⁺ (where n = 0→3) and ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopy have been used in their characterization. Hydrogen evolution, ¹¹⁹Sn NMR and Mössbauer spectroscopic evidence is presented to show that at higher temperatures these species decompose to Sn²⁺, Sn²⁺ plus (CH₃)₂Sn²⁺ and (CH₃)₂Sn²⁺ depending on the starting hydride. Tin-119 NMR spectra of dialkyltin cationic species in strong acid solutions are field dependent. Relaxation time measurements at three different magnetic fields have established that the dominant spin-lattice relaxation mechanism for these species at higher magnetic field is shielding anisotropy. A comparison of spin-lattice and spin-spin relaxation rates indicates that at ambient temperature a rapid chemical exchange process is occuring. In the case of the (CH₃)₂Sn(SO₃F)₂-HSO₃F system, variable temperature ¹¹⁹Sn NMR spectra reveals the presence of three tin species which are involved in this exchange process.</p> <p>Tin-119 NMR data for a series of tin(lI) and tin(IV) acid derivatives have been determined. Tin(II) compounds exhibit a wide range of chemical shifts varying from -617 ppm to -1628 ppm, whereas tin(IV) derivatives cover a narrow range around -800 ppm. Stoichiometric mixtures of tin(II) and tin(lV) derivatives of the same acid have been analysed by ¹¹⁹Sn NMR and Mössbauer spectroscopy. Reaction occurs only between Sn(OCOCF₃)₂ and Sn(OCOCF₃)₄ to form a mixed valence tin(II), tin(lV) compound. Partial oxidation of Sn(OCOCF₃)₂ results in the formation of a mixed valence tin compound whose structure has been determined by X-ray crystallography to be [Sn(II)₄Sn(IV)O₂(O₂CCF₃)₈]. This structure consist of discrete noncentrosymmetric units with point group S₄, the central feature of which is a Sn(11)₄Sn(lV)O₂ unit containing two μ₃ oxygen atoms which each form a bridge between a Sn(lV) atom and two symmetry related Sn(lI) atoms. Among the two pairs of Sn(ll) atoms, each pair is bridged by two trifluoroacetates.</p> <p>A systematic route has been established in the preparation of mixed valence tin compounds by the reaction of SnF₂ with tin(lV) carboxylates. These reactions have been followed by ¹⁹F and ¹¹⁹Sn NMR spectroscopy. In one case an X-ray crystal structure determination showed the compound to be [Sn(lI)₂Sn(lV)₂F₄(O₂CCF₃)₈2CF₃CO₂H]. This consists of eight membered rings with alternating Sn(II) and Sn(IV) atoms bridged by fluorine and trifluoroacetate groups.</p>