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

Abstract

Rare earth metal complexes incorporating non-Cp type of ligands have attracted much attention since their interesting structural features and applications in molecular catalysis such as alkene and lactide polymerization reactions and intramolecular hydroamination of alkenes. 1 A number of bulky organic ligands have been employed for the stabilization of well-defined rare earth metal alkyls, amides, and alkoxides. In polymerization reactions, aluminum alkyls and aluminoxanes are frequently used as cocatalysts. 2 Consequently, the studies on the reactions of rare earth metal complexes with aluminum complexes have been extensively performed for the understanding polymerization mechanism. 3 An alternative approach for the investigation of oxo-bridged aluminumÀgroup 4 metal complexes indicated that these compounds are not only excellent for the polymerization reactions with low Al/Zr ratios but also display interesting structural features as indicated by the large AlÀOÀM angles and short MÀO bond lengths. 4 Inspired by these successful results, we are interested in the synthesis, characterization, and lactide polymerization behaviors of oxo-bridged aluminumÀrare earth metal (AlÀOÀLn) complexes. The known oxo-bridged AlÀOÀLn complexes are very limited and only confined to those containing Cp 2 Ln and β-diketiminato-Ln fragments. 5 Bimetallic AlÀOÀLn complexes possessing a reactive LnÀE σ-bond (E = CR 3 , NR 2 , OR), to the best our knowledge, have not been reported so far.Ring-opening polymerization (ROP) of lactide with welldefined metal complexes has been one of the recent focuses for the preparation of biodegradable polylactides. 6 Both rare-earth metal and aluminum complexes have been investigated as catalysts for the ROP. By incorporating suitable ancillary ligands, aluminum alkoxides exhibit relatively low rate but well-controlled stereochemistry over the polymerization reactions. 7 In contrast, rare-earth metal complexes generally display remarkable high activity. 8 It has been recently shown that several lanthanide complexes initiate the polymerization of rac-lactide in stereospecific manners. 9 However, oxo-bridged AlÀLn complexes have not been explored as ROP catalysts. Recent studies have revealed that aluminum hydroxides can function as building blocks for the synthesis of oxo-bridged heterometallic complexes. 4,5,10 We and others have previously reported several routes to prepare organoaluminum hydroxides that are stabilized by the bulky β-diketiminato ligand L (HC[(CMe)(NAr)] 2 , Ar = 2,6-iPr 2 C 6 H 3 ). 4a,11 Herein, we report on the synthesis and characterization of novel heterobimetallic AlÀOÀLn alkyls and aloxides, prepared by alkane elimination reactions of LAl(OH)(CPhCHPh) (1) with Ln (Ln = Y, Sm) alkyls. In addition, lactide polymerization reactions with these complexes have also been investigated.

Advances in heterometallic ring-opening (co)polymerisation catalysis

“…Ln-based heterometallic ROP complexes have also been extended beyond alkali metals to Al 58 , Mg 59 and Ni 60 . Heterometallic Ln/Al (Ln = Y, Sm) complexes 18a-d (Fig.…”

“…Heterometallic Ln/Al (Ln = Y, Sm) complexes 18a-d (Fig. 4 ) were studied in rac -LA ROP 58 . While 18a showed activity and control enhancements compared to the mono-Al and Y counterparts, converting 123 equiv.…”

“…Combining Lewis acidic metals with more electronegative metals with weaker M-OR bonds may accelerate coordination and nucleophilic attack. Heterometallic complexes based on Al/Zn 46 , K/Zn 31 , La/Mg 59 , Li/In 38 , Li/Mg and Li/Zn 32 , Li/Sm 55 , Mg/Al 40 , Na/Sm 53 , Na/Zn 31 , Sm/Al 58 and Ti/Zn 47 have all displayed superior activities compared to the homometallic analogues. Most of these complexes feature a M-O-M′ framework, enabling intermetallic electronic communication and/or “ate”-type activation (vide supra), which can lead to enhanced nucleophilicity of the M- R bond (e.g.…”

“…Most of these complexes feature a M-O-M′ framework, enabling intermetallic electronic communication and/or “ate”-type activation (vide supra), which can lead to enhanced nucleophilicity of the M- R bond (e.g. R = alkoxide) 38 , 47 , 48 , 58 . Despite these promising results, future ROP studies should explore the solution-state structure of heterometallic catalysts to confirm that the enhanced activities can be accredited to heterometallic cooperativity.…”

“…Epoxide activation is a key ROCOP step, and Ln metals have displayed good monomer coordination in ROP 53 , 55 , 58 , 59 . A range of Ln/Zn complexes have also been developed for CHO/CO 2 ROCOP, including complexes 29a-b (Ln = Nd, Y) 96 , 30b-c (Ln = Y, Nd or Sm) 97 and 31a-j (Ln = Y, Lu, Dy, Sm or La) 98 (Fig.…”