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

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-(R²R³PO)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 L1-6 were reacted with SnCl₂ and Sn(OTf)₂ with the aim of studying the influence of different R²R³PO functional groups on the Lewis base mediated ionization of SnCl₂ and Sn(OTf)₂. While all ligands L1-6 provided the corresponding ionic tin(II) complexes [L1-6 → SnCl]⁺[SnCl₃]^- (1-6), only ligands L1, L4 and L6 were able to stabilize tin(II) dications [L1,4,6 → Sn(H₂O)][OTf]₂ (7-9). The auto-ionized compounds [L3-6 → 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 PO → 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 L1-6 as well as the nature of the bonds in the resulting products, with a particular focus on complexes 10-13.