Structurally Characterized Lead(II) Alkoxides as Potent Ring-Opening Polymerization Catalysts.
Résumé
The lead(II) alkoxides [Pb(μ2-OiPr)2]∞ (2), [Pb(μ2-OiPr)(N(SiMe3)2)]2 (3), and [LiPb(μ2-OtBu)2(μ3-OtBu)]2 (5) were synthesized and fully characterized, including by x-ray diffraction methods and 207Pb soln. NMR spectroscopy. Original and varying coordination patterns about the metal atoms are illustrated in these complexes, which represent valuable addns. to the limited set of lead(II) alkoxides that have already been structurally characterized. The heteroleptic complex 3 is accessed via simple alcoholysis of Pb[N(SiMe3)2]2, and this constitutes a convenient alternative to Sita's heterocumulene route. Plumbylenes 2, 3, and 5 act as competent (pre)catalysts for the living ring-opening polymn. of lactides. With 2, ≥50 ppm of metal are required to yield poly(lactide) in a controlled fashion during reactions performed under immortal conditions in the presence of excess alc. as a co-initiator. Across the homogeneous series of tetrelenes [M(μ2-OiPr)2]n, the ROP catalytic activity increases very substantially according to M = Ge(II) « Sn(II) \textless Pb(II).