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

Alpha-Iminopyridine ligands L-1(2-(CH=N(C6H2-2,4,6-Ph-3))C5H4N), L-2 (2-(CH=N(C6H2-2,4,6-tBu(3)))C5H4N) and L-3 (1,2-(C5H4N-2-CH=N)(2)CH2CH2) differing by the steric demand of the substituent on the imine CH=N 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 [L1-3 -> SnCl][SnCl3]. The reaction of L-1 and L-2 with SnCl2 led to the formation of neutral adducts [L-1 -> SnCl2] (2) and [L-2 -> 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 L-3 containing four donor nitrogen atoms showed the ability to ionize SnCl2 and also Sn(OTf)(2), yielding [L-3 -> SnCl][SnCl3] (7) and [L-3 -> 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)C=N(C6H3-2,6-iPr(2)))-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.