Square-Planar Cu(II) Diketiminate Complexes in Lactide Polymerization

• Published in: Inorganic chemistry 2013-12, Vol.52 (23), p.13612-13622
• Main Authors: Whitehorne, Todd J. J, Schaper, Frank
• Format: Article
• Language: English
• Subjects: Coordination Complexes - chemistry; Copper - chemistry; Crystallography, X-Ray; Dimerization; Dioxanes - chemistry; Imines - chemistry; Ligands; Models, Molecular; Polymerization
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic402133c

Cu(OiPr)2 was reacted with several β-diketimine ligands, nacnac RH. Sterically undemanding ligands with N-benzyl substituents afforded the dimeric heteroleptic complexes [nacnac BnCu(μ-OiPr)]2 and [3-Cl-nacnac BnCu(μ-OiPr)]2 (Bn = benzyl). With sterically more demanding amines, dimerization was not possible, and the putative nacnacCuOiPr intermediate underwent ligand exchange to the homoleptic bisdiketiminate complexes Cu(nacnac ipp)2 and Cu(nacnac Naph)2 (ipp = 2-isopropylphenyl, Naph = 1-napthyl). Homoleptic complexes were also prepared with N-benzyl ligands to yield Cu(nacnac Bn)2 and Cu(3-succinimido-nacnac Bn)2. All complexes were characterized by single-crystal X-ray diffraction. Even bulkier ligands with N-anthrylmethyl, N-mesitylmethyl, or N-methylbenzyl substituents failed to react with Cu(OiPr)2. In the case of nacnac dippCuOiPr, putative nacnac dippCuOiPr decomposed by β-hydride elimination. Heteroleptic complexes [nacnac BnCu(μ-OiPr)]2 and [3-Cl-nacnac BnCu(μ-OiPr)]2 are very highly active rac-lactide polymerization catalysts, with complete monomer conversion at ambient temperature in solution in 0.5–5 min. In the presence of free alcohol, the homoleptic complexes seem to be in equilibrium with small amounts of the respective heteroleptic complex, which are sufficient to complete polymerization in less than 60 min at room temperature. All catalysts show high control of the polymerization with polydispersities of 1.1 and below. The obtained polymers were essentially atactic, with a slight heterotactic bias at ambient temperature and at −17 °C.