N-Donor stabilized tin(ii) cations as efficient ROP catalysts for the synthesis of linear and star-shaped PLAs via the activated monomer mechanism

Abstract

α-Iminopyridine ligands L¹ (2-(CHN(C₆H₂-2,4,6-Ph₃))C₅H₄N), L² (2-(CHN(C₆H₂-2,4,6-tBu₃))C₅H₄N) and L³ (1,2-(C₅H₄N-2-CHN)₂CH₂CH₂) differing by the steric demand of the substituent on the imine CHN group and by the number of donating nitrogen atoms were utilized to initiate a Lewis base mediated ionization of SnCl₂ in an effort to prepare ionic tin(II) species [L^1–3 → SnCl][SnCl₃]. The reaction of L¹ and L² with SnCl₂ led to the formation of neutral adducts [L¹ → SnCl₂] (2) and [L² → SnCl₂] (3). The preparation of the desired ionic compounds was achieved by subsequent reactions of 2 and 3 with an equivalent of SnCl₂ or GaCl₃. In contrast, ligand L³ containing four donor nitrogen atoms showed the ability to ionize SnCl₂ and also Sn(OTf)₂, yielding [L³ → SnCl][SnCl₃] (7) and [L³ → Sn(H₂O)][OTf]₂ (8). The study thus revealed that the reaction is dependent on the type of the ligand. The prepared complexes 4–8 together with the previously reported [{2-((CH₃)CN(C₆H₃-2,6-iPr₂))-6-CH₃O-C₅H₃N}SnCl][SnCl₃] (1) were tested as catalysts for the ROP of L-lactide, which could operate via an activated monomer mechanism. Finally, a DFT computational study was performed to evaluate the steric and electronic properties of the ionic tin(II) species 1 and 4–8 together with their ability to interact with the L-lactide monomer.