N-donor Stabilized Tin(II) Cations as Efficient ROP Catalysts for Synthesis of Linear and Star-Shaped PLAs via Activated Monomer Mechanism

α-iminopyridine ligands L1 (2-(CH=N(C6H2-2,4,6-Ph3))C5H4N), L2 (2-(CH=N(C6H2-2,4,6-tBu3))C5H4N) and L3 (1,2-(C5H4N-2-CH=N)2CH2CH2) differing by the steric demand of the substituent on the imine CH=N group and by a 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 [L1-3→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 has thus revealed that the reaction is dependent on the type of the ligand. Prepared complexes 4 – 8 together with a previously reported [{2-((CH3)C=N(C6H3-2,6-iPr2))-6-CH3O-C5H3N}SnCl][SnCl3] (1) were tested as catalysts in 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, 4 – 8 together with their ability to interact with the L-lactide monomer.