Tin(ii) cations stabilized by non-symmetric N,N′,O-chelating ligands: synthesis and stability

Abstract

A series of novel non-symmetric neutral N,N′,O-chelating ligands derived from the α-iminopyridine 2-(C(R¹)N(C₆H₃-2,6-iPr₂))-6-(R²R³PO)C₅H₃N (L¹: R¹ = H, R² = R³ = Ph; L²: R¹ = Me, R² = R³ = Ph; L³: R¹ = H; R² = Ph, R³ = EtO; L⁴: R¹ = Me, R² = Ph, R³ = EtO; L⁵: R¹ = H, R² = R³ = iPrO; L⁶: R1 = Me, R² = R³ = iPrO) were synthesized. Ligands L^1–6 were reacted with SnCl₂ and Sn(OTf)₂ with the aim of studying the influence of different R²R³PO functional groups on the Lewis base mediated ionization of SnCl₂ and Sn(OTf)₂. While all ligands L^1–6 provided the corresponding ionic tin(II) complexes [L^1–6 → SnCl]⁺[SnCl₃]⁻ (1–6), only ligands L¹, L⁴ and L⁶ were able to stabilize tin(II) dications [L^1,4,6 → Sn(H₂O)][OTf]₂ (7–9). The auto-ionized compounds [L^3–6 → SnCl]⁺[SnCl₃]⁻ possessing ethylphenyl phosphinate and diisopropylphosphite substituents undergo elimination of EtCl and iPrCl, respectively, yielding compounds 10–13. These can either be interpreted as neutral tin(II)phosphinate chloride (10, 11) and tin(II)phosphonate chloride (12, 13), respectively, containing Sn–O and Sn–Cl bonds, and a PO → SnCl₂ interaction, or as zwitterionic compounds, where the positive charge of the central tin atom is compensated by an [OSnCl₂]⁻ anion. Finally, DFT studies were performed to better understand the steric and electronic properties of the ligands L^1–6 as well as the nature of the bonds in the resulting products, with a particular focus on complexes 10–13.