Synthesis And Characterization Of Bis（2,4-Bu₂- Pentadienyl） Divalent Lanthanide Complexes And Their Catalytic Activity For The Polymerization Of ε-Caprolactone

By reviewing the divalence lanthanide organometallic chemistry, Pdl(pentadienyl) ligand arouse lots of our interest while various kinds of Pdl had utilized in organometallics(“openmetallocenes”). Besides bonding to metal in a variety of η1, η3 and η5 modes, it was found that the usual η5-Pdl is a more sterically demanding, and even more strongly bonding to center metal than the seemingly similar Cp(cyclopentadienyl). Herein, we report the synthesis of the novel bis(2,4-tBu2-C5H5)Sm(II) and Yb(II) complexes by the reactions between 2,4-di-butyl-pentadienyl potassium(KPdl’) and lanthanide halides.The complexes(η5-Pdl’)2Yb(THF)(1) and(η5-Pdl’)2Sm(DME)(2) were successfully prepared in high yield by direct salt metathesis between LnI2 and 2 equivalents of K(Pdl’). Complexes 1 and 2 could also be obtained smoothly from the reaction of LnCl3 and 3 equivalents of K(Pdl’) through oxidation-reduction reaction. These two compounds were well characterized and the X-Ray single crystal analysis revealed a predominantly η5-pentadienyl-metal bonding. The carbon atoms of the pentadienyl fragments lie in a plane, and the C-C distances show a distinct short-long-long-short pattern. The Yb-C distances of complex 1 are the same as the previous analog(2,4-Me2-C5H5)2Yb(DME). The conformation angles χ of these two complexes are all approximately 180 o. Both two compounds could initiate ring-opening polymerization of ε-caprolactone rapidly at room temperature and led to narrow polydispersities with high activity. It can be assumed that in the ε-caprolactone polymerization procedure, a η5-η3-η1 allyl transformation of the pentadienyl could be taken place readily, thus, accelerated the coordination and insertion of the monomer, and elevated the activity of polymerization. A plausible mechanism of monoelectronic transfer-oxidative polymerization process was proposed, in which one chain is growing per two Sm, which can be attributed to polymerization induced by the oxidation of SmII, then involving two samarium centres. The MALDI-TOF-MS result was in agreement with our mechanistic hypothesis.