Synthesis and Crystal Structure of Novel Biheterometal and Triheterometal Alkoxide Clusters – Highly Active Catalysts for the Polymerization of ϵ-Caprolactone

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 2 CH 2 N(CH 3 ) 2 and 1.5 equiv. of KOH in tetrahydrofuran (THF) to give the corresponding lanthanide-potassium biheterometal alkoxide clusters [Ln 4 K 20 -(OCH 2 CH 2 NMe 2 ) 26 (OH) 6 ] [Ln = Nd (1), Pr (2), Yb (3)] in high yield. Anhydrous YbCl 3 reacts with KOCH 2 CH 2 N(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 2 K 10 Na 6 (OCH 2 CH 2 NMe 2 ) 18 (OH) 4 ] (4) and [Yb 2 -K 8 Na 8 (OCH 2 CH 2 NMe 2 ) 18 (OH) 4 ] (5), respectively. These clusters were fully characterized by elemental analysis, IR, 1 H NMR, and single-crystal structural analysis. The heterometal alkoxide clusters 1-5 exhibited good catalytic activity for the ring-opening polymerization of e-caprolactone (e-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.