Subvalent Group 14 metal compounds. Part 16. Synthesis, crystal structure and characterisation of some β-functionalised-alkyltin(II) complexes, SnR(X){R = C5H4N[C(SiMe3)2]-2; X = R, Cl or N(SiMe3)2}

Abstract

Crystalline monomeric alkyltin(II) compounds were prepared using the β-N-functionalised ligand 2-pyridylbis(trimethylsilyl)methyl, C₅H₄N[C(SiMe₃)₂]-2 (R): (a) SnR₂1 from (LiR)₂ and SnCl₂ or Sn(OC₆H₃Bu^t₂-2,6)₂(b) SnR(Cl)2 from SnCl₂ and SnR₂ or ½(LiR)₂; and (c) SnR[N(SiMe₃)₂]3 from Sn[N(SiMe₃)₂]₂ and ½(LiR)₂. Single-crystal X-ray diffraction data showed that in each of 1–3 R^– behaves as a chelating, C,N-centred ligand, and that the lone pair on tin is stereochemically active. The mean Sn–C [and Sn–NC₅H₄] bond lengths (Å) are 2.35(2)[2.42(2)] in 1, 2.32(2)[2.27(3)] in 2, and 2.356(8)[2.299(5)] in 3. Variable-temperature multinuclear (¹H, ¹³C, ²⁹Si and ¹¹⁹Sn) NMR spectral data were obtained for 1–3 in [²H₈]toluene. These showed that dynamic processes were operative, believed to be Sn–N(C₅H₄) dissociations, with ΔG^‡_Tc= 43.5 ± 0.8 for 2 or 42.7 ± 0.8 kJ mol^–1 for 3(the coalescence temperature T_c being 220 ± 2 for 2 and 214 ± 2 K for 3); for 3 a further fluxion with ΔG^‡_Tc= 76.1 ± 0.8 kJ mol^–1(T_c= 368 ± 2 K) is attributed to the barrier to rotation about the tin–amide nitrogen bond. Attempts were made to reduce 2[evidence for (SnR)_n], and to prepare analogues of Ge^II and Pb^II of 1 and 2. A further β-functionalised complex, SnCl(OCBu^t₂CH₂PMe₂)4, was obtained from SnCl₂ and ½2[Li(OCBu^t₂CH₂PMe₂)]₂; there was no evidence for ³¹P–¹¹⁹Sn coupling, and a plausible structure is [Sn(µ-Cl)(OCBu^t₂CH₂PMe₂)]₂. A similar metathetical reaction between SnCl₂ and Sn(OC₆H₃Bu^t₂-2,6)₂ in tetrahydrofuran (thf) gave 1//_m[SnCl(OC₆H₃Bu^t₂–2,6)(thf)_n]_m, whereas SnCl₂+ SnR′₂[R′= CH(SiMe₃)₂] yielded SnR′₂Cl₂+ Sn.