Synthesis and crystal structure of novel biheterometal and triheterometal alkoxide clusters - Highly active catalysts for the polymerization of μ-caprolactone

摘要

The effect of the alkali metal on the synthesis, crystal structure, and catalytic reactivity of lanthanide-alkali metal alkoxide clusters is reported. Anhydrous LnCl_3 reacts with 6.5 equiv. of KOCH_2CH _2N(CH_3)_2 and 1.5 equiv. of KOH in tetrahydrofuran (THF) to give the corresponding lanthanide-potassium biheterometal alkoxide clusters [Ln_4K_(20)(OCH _2CH_2NMe_2)_(26)(OH)_6] [Ln = Nd (1), Pr (2), Yb (3)] in high yield. Anhydrous YbCl_3 reacts with KOCH_2CH_2N(CH_3)_2 and NaOH with different molar ratios of 1:9:3 and 1:9:4 to afford the lanthanide-potassium- sodium triheterometal alkoxide clusters [Yb_2K_(10)Na _6(OCH_2CH_2NMe_2)_(18)(OH) _4] (4) and [Yb_2K_8Na_8(OCH _2CH_2NMe_2)_(18)(OH)_4] (5), respectively. These clusters were fully characterized by elemental analysis, IR, 1H NMR, and single-crystal structural analysis. The heterometal alkoxide clusters 1-5 exhibited good catalytic activity for the ring-opening polymerization of μ-caprolactone (μ-CL). It is interesting to note that the catalytic activity of these heterometal alkoxide clusters increases with the increase of the molar ratio of alkali metal to lanthanide metal. For the same molar ratio of alkali metal to lanthanide metal, however, the catalytic activity of the heterometal clusters is highly dependent on the type and molar ratio of the alkali metal centers. The higher the molar ratio of potassium to sodium, the higher the catalytic activity. Five lanthanide-potassium and lanthanide-potassium-sodium heterometal alkoxide clusters were synthesized in highyield. These clusters exhibited good catalytic activity for the ring-opening polymerization of μ-caprolactone. The catalytic activity was found to depends on the ratio of alkali metal to lanthanide metal as well as the ratio of potassium to sodium.