Synthesis and Characterization of Heterobimetallic Oxo-Bridged Aluminum–Rare Earth Metal Complexes

Reaction of the aluminum hydroxide LAl(OH)[C(Ph)CH(Ph)] (1, L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3) with Y(CH2SiMe3)3(THF)2 yielded the oxo-bridged heterobimetallic yttrium dialkyl complex LAl[C(Ph)CH(Ph)](μ–O)Y(CH2SiMe3)2(THF)2 (2). Alkane elimination reaction of 2 with 2-(imino)pyrrole [NN]H ([NN]H = 2-(ArNCH)-5-tBuC4H2NH) afforded the yttrium monoalkyl complex LAl[C(Ph)CH(Ph)] (μ–O)Y(CH2SiMe3)[NN](THF)2 (5). Alternatively, 5 can be prepared in high yield by reaction of 1 with [NN]Y(CH2SiMe3)2(THF)2 (3). The analogous samarium alkyl complex LAl[C(Ph)CH(Ph)](μ–O)Sm(CH2SiMe3)[NN](THF)2 (6) was prepared similarly. Reactions of 5 and 6 with 1 equiv of iPrOH yielded the corresponding alkoxyl complexes 7 and 8, respectively. The molecular structures of 3, 6, and 8 have been determined by X-ray single-crystal analysis. Complexes 2, 3, 5, 7, and 8 have been investigated as lactide polymerization initiators. The heterobimetallic alkoxyl 8 is highly active to yield high molecular weight (Mn = 6.91 × 104) polylactides with over 91% conversion at the lactide-to-initiator molar ratio of 2000.