N-Donor stabilized tin(ii) cations as efficient ROP catalysts for the synthesis of linear and star-shaped PLAs via the activated monomer mechanism. Issue 44 (15th October 2021) - Novák, Miroslav
Abstract : N-donor stabilized tin(ii ) cations were synthesized by a chloride abstraction from neutral complexes or by an auto-ionization concept. Prepared tin(ii ) cations are universal catalysts for the production of linear and star-shaped PLAs via ROP. Abstract : α-Iminopyridine ligands L 1 (2-(CHN(C6 H2 -2, 4, 6-Ph3 ))C5 H4 N), L 2 (2-(CHN(C6 H2 -2, 4, 6- t Bu3 ))C5 H4 N) and L 3 (1, 2-(C5 H4 N-2-CHN)2 CH2 CH2 ) 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 [L 1–3 → 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(H2 O)][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(C6 H3 -2, 6-iPr2 ))-6-CH3 O-C5 H3 N}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.