Synthesis and structural studies of lithium and sodium complexes with OOO-tridentate bis(phenolate) ligands: effective catalysts for the ring-opening polymerization of L-lactide.
A series of lithium and sodium complexes with OOO-tridentate bis(phenolate) ligands have been synthesized and fully characterized. The reaction of 2,2'-dihydroxy-3,3',5,5'-tetrakis[(1-methyl-1-phenyl)ethyl]dibenzyl ether (L(1)-H(2)) with different ratios of (n)BuLi in toluene or tetrahydrofuran (THF) gave [Li(2)(L(1)-H)(2)] (1), [Li(4)L(1)(2)] (2), and [Li(2)L(1)(THF)(3)] (3), respectively. Similarly, [Na(L(1)-H)(THF)] (4), [Na(2)(L(1)-H)](2) (5), and [Na(4)L(1)(2)] (6) were prepared by the reaction of L(1)-H(2) and NaN[Si(CH(3))(3)](2) or sodium metal. In addition, the reaction of 2,2'-dihydroxy-3,3',5,5'-tetra-tert-butyldibenzyl ether (L(2)-H(2)) with (n)BuLi in toluene or THF yields Li(2)(L(2)-H)(2)] (7) and [Li(2)(L(2)-H)(2)(THF)(2)] (8), respectively. Further treatment of 7 with 2 mol equiv of benzyl alcohol provides [Li(2)(L(2)-H)(2)(BnOH)(2)] (9). Complexes 1-4 and 6-9 have been structurally characterized by single-crystal X-ray analysis. The dinuclear nature of complexes 1 and 3 was confirmed from their molecular structure. Complexes 2 and 6 illustrate tetranuclear species; however, complex 4 shows a mononuclear feature. A p-π interaction exists from the phenyl ring of the 2-(methyl-1-phenylethyl) groups to the central metal in complexes 2, 4, and 6, which could effectively stabilize the metal center. Among them, complexes 1, 2, and 5-9 displayed efficient catalytic behavior for the ring-opening polymerization of L-lactide in the presence of benzyl alcohol. Experimental results indicate that among these alkali-metal complexes, the sodium compound 6 displays a rapid catalytic polymerization of L-lactide in "living" fashion, yielding poly(L-lactide) with a controlled molecular weight and narrow polydispersity indices for a wide range of monomer-to-initiator ratios.
Huang
Yong
Y
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, Republic of China.
Tsai
Yueh-Hsuan
YH
Hung
Wen-Chou
WC
Lin
Chieh-Shen
CS
Wang
Wei
W
Huang
Jui-Hsien
JH
Dutta
Saikat
S
Lin
Chu-Chieh
CC
D019905:Benzyl Alcohol / Q000737:chemistry; D002384:Catalysis; D018360:Crystallography, X-Ray; D004146:Dioxanes / Q000737:chemistry*; D008024:Ligands; D008094:Lithium / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D058105:Polymerization; D012964:Sodium / Q000737:chemistry*
D004146:Dioxanes; D008024:Ligands; D009942:Organometallic Compounds; D010636:Phenols; C091880:dilactide; D008094:Lithium; D012964:Sodium; D019905:Benzyl Alcohol