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

Cu­(OiPr)2 was reacted with several
β-diketimine ligands, nacnacRH.
Sterically undemanding ligands with N-benzyl substituents
afforded the dimeric heteroleptic complexes [nacnacBnCu­(μ-OiPr)]2 and [3-Cl-nacnacBnCu­(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(nacnacipp)2 and Cu­(nacnacNaph)2 (ipp = 2-isopropylphenyl, Naph = 1-napthyl). Homoleptic complexes
were also prepared with N-benzyl ligands to yield
Cu­(nacnacBn)2 and Cu­(3-succinimido-nacnacBn)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 nacnacdippCuOiPr, putative nacnacdippCuOiPr decomposed
by β-hydride elimination. Heteroleptic complexes [nacnacBnCu­(μ-OiPr)]2 and [3-Cl-nacnacBnCu­(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.