Article pubs.acs.org/IC
A Comparative Study on Dinuclear and Mononuclear Aluminum Methyl Complexes Bearing Piperidyl−Phenolato Ligands in ROP of Epoxides Wenyi Li,†,‡ Hao Ouyang,† Lijuan Chen,† Dan Yuan,*,† Yong Zhang,† and Yingming Yao*,† †
Key Laboratory of Organic Synthesis of Jiangsu Province and Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, College of Chemistry, Chemical Engineering & Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China ‡ Department of Chemistry and Biological Engineering, Hunan University of Science and Engineering, Yongzhou 425199, Hunan, People’s Republic of China S Supporting Information *
ABSTRACT: Dinuclear aluminum methyl complexes stabilized by piperidyl−phenolato ligands were prepared and characterized. The ringopening polymerizations of cyclohexene oxide (CHO) and propylene oxide (PO) initiated by dinuclear complexes and mononuclear analogues were investigated and compared. Enhanced activity of dinuclear complexes compared to that of mononuclear analogues in both the ring-opening polymerization of CHO and PO proves the synergistic interaction of two Al centers in the former. End-group analysis of oligomers by MALDI-TOF mass spectrometry confirms the role of methyl groups as initiating groups. A bimetallic mechanism is proposed, in which the cooperation of two Al centers are involved in polymerization processes.
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INTRODUCTION Aluminum is the most abundant metal in the earth crust, and development of aluminum complexes as catalysts is attracting considerable interest. Organoaluminum compounds have found widespread application in mediating organic transformations1 and in initiating polymerization of monomers, such as cyclic esters (e.g., lactide and ε-caprolactone)2 and cyclic ethers [e.g., cyclohexene oxide (CHO)3 and propylene oxide (PO)].4−6 Aluminum complexes stabilized by various ligands, including porphyrin,4b,5p,6a,b Salen-type,5d,h,l,m,o and (poly)phenolato ligands,3a,e,4a,d,e,5c,e−g,6c have been developed as initiators for the ring-opening polymerization (ROP) of epoxides.7 Although these complexes are mainly mononuclear, bimetallic mechanisms involving the cooperation of two metal centers have been generally accepted.4e,6,8 One metal center is responsible for activating the monomer through complexation, while initiating groups from another complex causes ring-opening. Despite the important synergistic interaction between two Al centers, synthesis and application of well-defined dinuclear aluminum complexes in ROP of epoxides remain rare, and need to be further investigated.3e,4a,d,5c,f,g,i,o Aluminum alkyls (AlEt3 and AliBu3) were reported to show low activity (40% yield after two weeks of reaction at room temperature) in initiating the ROP of PO.9 Sterically demanding aluminum alkyl complexes have been reported to function as monomer activators, instead of initiators, in promoting the ROP.6b,c,10 Although some other aluminum © XXXX American Chemical Society
alkyl complexes have been applied in the ROP of epoxides,3b−e,4d,e,5e−g,i,k,n no study has ever unambiguously confirmed the role of alkyl groups in the polymerization process. Our group has recently reported bimetallic aluminum alkyl complexes supported by piperazine-bridged bis(phenolato) ligands, which efficiently initiated the ROP of ε-caprolactone.11 More importantly, the dinuclear system showed enhanced activities compared to their mononuclear analogues, which proves the important role of the cooperative effect between two Al centers. To further explore the synthesis and application of dinuclear aluminum complexes, a series of aluminum complexes with easily available and structurally versatile phenolato ligands were prepared. Mono- and dinuclear complexes stabilized by the same ligand are accessible through simple control of the ratio of ligand-to-metal precursors. The performances of mono- and dinuclear aluminum complexes as initiators in the ROP of CHO and PO are studied and compared.
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RESULTS AND DISCUSSION Synthesis of Bimetallic Aluminum Complexes 1−3. Dinuclear aluminum alkyl complexes Al2LMe5 [L = [NO]1 (1), [NO]2 (2), [NO]3 (3)] were synthesized through alkane Received: March 14, 2016
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DOI: 10.1021/acs.inorgchem.6b00639 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
2.053 (3) Å. The distance between Al1 and Al2 is found to be 3.3608(18) Å. Polymerization Studies. Activities of complexes 1−6 and AlMe3 in initiating the polymerization of CHO were investigated and compared (Table 1). The ROP initiated by
elimination reactions by reacting three different piperidyl− phenols H[NO]n (n = 1, 2, 3) with 2 equiv of AlMe3 in moderate to good yields of 59−76% (Scheme 1). In contrast, treatment of H[NO]n (n = 1, 2, 3) with 1 equiv of AlMe3 led to the formation of the monometallic aluminum complex AlLMe2 [L = [NO]1 (4), [NO]2 (5), [NO]3 (6)].11
Table 1. Polymerization of CHO Initiated by Complexes 1− 4 and AlMe3a
Scheme 1. Synthesis of Aluminum Complexes 1−6
entry 1 2d 3e 4f 5g 6 7 8 9
Complexes 1−3 are all sensitive to air and moisture. They are all soluble in n-hexane, toluene, and tetrahydrofuran, and their solubility in n-hexane decreases in the order of 1, 2, and 3 as a result of the stepwise replacement of tBu with Me group. Complexes 1−3 were characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction analysis on single crystals. In their 1H NMR spectra, the absence of signals for the phenolic protons in ligand precursors supports successful deprotonation. Characteristic signals corresponding to methyl groups bound to Al centers are found in the highfield region (
