Role of Aggregation in the Synthesis and Polymerization Activity of SalBinap Indium Alkoxide Complexes

The reaction of racemic SalBinap ligand, (+/-)center dot H-2(ONN*O-Me), with InCI3 and excess Na0Et generated a mixture of two dinudear compounds [(mu-x(2)-ONN*O-Me)In(mu-OEt)](2)(1) and [k(4)-ONN*Ome)In(mu-OEt)](2) (1b), which were isolated and fully characterized. Polymerization of racemic lactide with la and 1b was slow in refluxing THF and showed only modest stereoselectivity. Catalyst 1b displayed better control than la, with the experimental molecular weights of the resulting poly(lactic acid) in agreement with the expected values. The higher-than-expected molecular weights observed in polymers formed by la were due to partial initiation of the catalyst. The reaction of (+/-)center dot H-2(ONN*O-tBa) with InCl3 yielded (e-ONN*O-tBu)InCl (2); however, further reactivity of the compound formed a mixture of products. An attempt to prevent aggregation by reacting (+/-)center dot H-2(ONN*O-Me) with InCl3 and excess (NaOPr)-Pr-j yielded an intractable mixture, including OH) (3). The thermal stabilities of compounds la and lb under polymerization conditions were investigated. Examination of the polymerization behavior of complexes la and lb and the reaction equilibrium between the two illustrates the importance of aggregation in indium salen complexes compared to their aluminum counterparts.