Bidentate salicylaldiminato tin(II) complexes and their use as lactide polymerisation initiators

A study of the reactions of several salicylaldimines (ortho-iminophenols) with Sn(NMe2)2 reveals that the sterics and electronics of the N-substituent play a principal role in determining the product mixture. Thus mono(chelate) tin(II) amides have only been isolated with bulky N-anilido substituents, including [2,4-X2-6-{CHN-2,6-iPr2C6H3}-C6H2O]Sn(-NMe2)]2, X = Cl, 1; X = I, 2 and [2,4-Cl2-6-{CHN-2,4,6-tBu3C6H2}-C6H2O]Sn(NMe2), 3. With smaller N-aryl and N-alkyl substituted salicylaldimines, mixtures of mono- and bis- chelate complexes are formed, even if the phenoxide ring bears large tert-butyl substituents. Further, when the anilido group bears electron-withdrawing substituents, the imino carbon is activated towards nucleophilic attack (as demonstrated by the formation of [2,4-tBu2-6-{CH(NMe2)N-2,4,6-Br3C6H2}-C6H2O]Sn, 4); no such reactivity has been observed when the halo substituents are located on the phenolic ring. The abilities of complexes 1-4 to initiate the ring-opening polymerisation of rac-lactide have also been studied. Complexes 1, 2 and 4 possess comparitively similar activities, but propagation with 3 is at least one order of magnitude slower, an observation rationalised in terms of steric congestion.