Carbon-Bridged Bis(phenolato)lanthanide Alkoxides:  Syntheses, Structures, and Their Application in the Controlled Polymerization of ε-Caprolactone

Abstract

A convenient method for the synthesis of lanthanide alkoxo complexes supported by a carbon-bridged bis(phenolate) ligand 2,2'-methylenebis(6-tert-butyl-4-methylphenoxo) (MBMP2-) is described. The reaction of (C5H5)3Nd with MBMPH2 in a 1:1 molar ratio in THF gave the bis(phenolato)lanthanide complex (C5H5)Nd(MBMP)(THF)2 (1) in a nearly quantitative yield. Complex 1 further reacted with 1 equiv of 2-propanol in THF to yield the bis(phenolato)lanthanide isopropoxide [(MBMP)2Nd(mu-OPr(i))(THF)2]2 (2) in high yield. Complex 2 can also be synthesized by the direct reaction of (C5H5)3Nd with MBMPH2 in a 1:1 molar ratio and then with 1 equiv of 2-propanol in situ in THF. Thus, the analogue bis(phenolato)lanthanide alkoxides [(MBMP)2Ln(mu-OR)(THF)2]2 [R = Pr(i), Ln = Yb (3); R = Me, Ln = Nd (4), Yb (5); R = CH2Ph, Ln = Nd (6), Yb (7)] were obtained by the reactions of (C5H5)3Ln (Ln = Nd, Yb) with MBMPH2 and then with 2-propanol, methanol, or benzyl alcohol, respectively. The ytterbium complex {[(MBMP)2Yb(THF)2]2(mu-OCH2Ph)(mu-OH)} (8) was also isolated as a byproduct. The single-crystal structural analyses of complexes 1-3 and 8 revealed that the coordination geometry around lanthanide metal can be best described as a distorted tetrahedron in complex 1 and as a distorted octahedron in complexes 2, 3, and 8. A O-H...Yb agostic interaction was observed in complex 8. Complexes 2-7 were shown to be efficient catalysts for the controlled polymerization of epsilon-caprolactone.

Synthesis and Structure of Lanthanide Aryloxo Complexes and Their Polymerization Behavior for ϵ‐Caprolactone

“…(C 5 H 5 )Ln(MBMP)(THF) n (Ln = La, Nd) was prepared according to the literature procedures. [22,23] Lanthanide analyses were performed by EDTA titration with an xylenol orange indicator and a hexamine buffer. [24] Carbon and hydrogen analyses were performed by direct combustion with a Carlo-Erba EA-1110 instrument.…”

“…[21,22] This method has the advantage for avoiding formation of "ate" lanthanide derivatives. Therefore, we intend to expand the scope of this method, and synthesize bis(phenolate) lanthanide aryloxo complexes.…”

“…The Nd(1)-O[bis(phenolate)] bond lengths are 2.169(2) and 2.217(2) Å, respectively, giving the average of 2.193(2) Å, which is comparable with the corresponding bond lengths in complexes 1 and 3 when the difference in ionic radii between neodymium and lanthanum is considered, and the bond length in [(MBMP)Nd(μ-OiPr)(THF) 2 ] 2 . [22] The tert-butylphenoxo group is asymmetrically coordinated to the neodymium ions. The Nd (1) [22] but apparently longer than the La(1)-O(Ar) bond length in complexes 1 and 3, although the ionic radius of lanthanum ion is larger than that of neodymium ion.…”

“…[11,12,14] Recently, we also found that the lanthanide alkoxo and amido complexes stabilized by carbon bridged bis(phenolate) groups are efficient initiators for the ring-opening polymerization of ε-caprolactone, and the polymerization controllability is greatly influenced by the property of the initiating group. [20][21][22] To further understand the influence of the initiating group on the catalytic property of carbon bridged bis(phenolate) lanthanide complexes, some lanthanide aryloxo complexes stabilized by 2,2Ј-methylene-bis(6-tert-butyl-4-methyl-phenolate) (MBMP 2-) group were synthesized by protic exchange reaction, and their catalytic behavior for the polymerization of ε-caprolactone was explored. Herein we report these results.…”

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

“…The activity sequence of Yb Ͻ Pr ≈ Nd observed here is consistent with the increase in the ionic radius, which is similar to those found in polymerization systems with neutral lanthanide phenoxide and lanthanocene-based catalysts. [28,29] To further understand the effect of the alkali metal on the catalytic activity, we compared the activity of [Yb 4 -K 20 (OCH 2 CH 2 NMe 2 ) 26 (6), which all contain the same central metal, Yb. These clusters all showed an unexpectedly high activity.…”