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

Abstract

The effect of the alkali metal on the synthesis, crystal structure, and catalytic reactivity of lanthanide–alkali metal alkoxide clusters is reported. Anhydrous LnCl3 reacts with 6.5 equiv. of KOCH2CH2N(CH3)2 and 1.5 equiv. of KOH in tetrahydrofuran (THF) to give the corresponding lanthanide–potassium biheterometal alkoxide clusters [Ln4K20(OCH2CH2NMe2)26(OH)6] [Ln = Nd (1), Pr (2), Yb (3)] in high yield. Anhydrous YbCl3 reacts with KOCH2CH2N(CH3)2 and NaOH with different molar ratios of 1:9:3 and 1:9:4 to afford the lanthanide–potassium–sodium triheterometal alkoxide clusters [Yb2K10Na6(OCH2CH2NMe2)18(OH)4] (4) and [Yb2K8Na8(OCH2CH2NMe2)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 ϵ‐caprolactone (ϵ‐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.

Remarkable effect of alkali metal on polymerization of cyclic esters catalyzed by samarium–alkali metal multinuclear alkoxide clusters

“…As reported in our previous work, the addition of definite amount of NaOH or KOH into the reaction solution results in the rational preparation of lanthanide-alkali metal alkoxide clusters containing hydroxide groups. [35][36][37] To understand the effect of alkali metal on catalytic reactivity, here we report a detailed investigation of the catalytic behavior of a series of samarium clusters, by changing the kind of alkali metal and the molar ratio of alkali metal to samarium metal. [Sm 2 Na 8 (OCH 2 CH 2 NMe 2 ) 12 -(OH) 2 ] (1) was prepared in high yield according to the previous literature.…”

“…2, the cluster 3 contains two samarium atoms, twelve potassium atoms, four sodium atoms, eighteen In addition, the coordination environment around the alkali metal atoms and the average bond lengths of M (K or Na)-OCH 2 CH 2 NMe 2 for 2 and 3 are also normal, and comparable each other. 35,37 Catalytic activity of clusters 1-3 for ring-opening polymerization of ε-caprolactone (ε-CL)…”

“…[32][33][34][35] In addition, we have noticed that the catalytic activity of these clusters is influenced by the alkali metal. 37 Whether we can adjust the reactivity through changing the kind of alkali metal and the mole ratio of alkali metal to central metal? With this opinion in mind, we were prompted to further study the effect of the alkali metal for the catalysis activity for polymerization of ε-CL.…”

“…[21][22][23][24][25][26][27][28][29][30][31] Recently, we have reported that the lanthanide-alkali metal multinuclear alkoxide clusters can be synthesized in high yields and showed superior polymerization activity relative to previously reported homometallic catalysts due to the cooperative effect between lanthanide and alkali metals. [32][33][34][35][36][37] Can we adjust the reactivity by changing the kind of alkali metal and the mole ratio of alkali metal to central metal? If our goal can be achieved, we will find a simple and efficient method to improve the catalyst activity.…”

Alkaline and alkaline earth metals

“…76 Anhydrous LnCl 3 [Ln = Nd, Pr, Yb] has been found to react with KOCH 2 CH 2 N(CH 3 ) 2 and KOH to give the corresponding lanthanide-potassium alkoxide clusters [Ln 4 K 20 (OCH 2 CH 2 NMe 2 ) 26 (OH) 6 ]. 77 Treatment of [In(18-crown-6)]-[O 3 SCF 3 ] with several potassium salts has resulted in the release of the indium(I) cation from the crown ether and the production of salts of [K(18-crown-6)] + and uncomplexed [In] + . 78 A fluoro-metallocrown has been found to bind caesium, extracting it from aqueous solutions into an organic layer.…”