Isoprene polymerization with yttrium amidinate catalysts: switching the regio- and stereoselectivity by addition of AlMe3.
Isoprene polymerization with yttrium amidinate catalysts: switching the regio- and stereoselectivity by addition of AlMe3.
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DOI：
10.1002/anie.200705120
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发表时间：
2008-03
影响因子：
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通讯作者：
Lixin Zhang;M. Nishiura;M. Yuki;Yi Luo;Z. Hou
中科院分区：
文献类型：
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作者：
Lixin Zhang;M. Nishiura;M. Yuki;Yi Luo;Z. Hou
通过控制烯烃聚合的区域和立体选择性来制备具有所需微观结构和性能的聚合物是一个重要的研究领域。迄今为止，实现这一目标的方法主要涉及对金属催化剂的辅助配体进行改性。在稀土金属（第3族和镧系元素）基聚合催化剂的开发中使用螯合脒基配体作为取代脒基配体已经受到相当大的关注。尽管迄今为止报道的大多数含脒基的稀土金属配合物含有两个或三个脒基配体，但Hessen及其同事最近的工作表明，在氮原子上具有大体积取代基的苯甲脒基，如N，N ′-双取代基，（2，6-二异丙基苯基）苯甲脒[（NC_6H_4iPr_2 - 2，6）_2]可作为一系列单脒基/二烷基或阳离子单脒基/烷基稀土金属配合物的优良辅助配体。然而，尽管广泛的兴趣，使用脒稀土金属配合物作为聚合催化剂，聚合化学报道，这些配合物的日期已主要限于乙烯和极性单体的聚合;对高级烯烃的聚合的研究仍然很少。特别地，脒基配位的稀土金属催化剂用于共辄二烯如异戊二烯的聚合的用途迄今尚未报道。近年来，我们发现阳离子型烷基稀土配合物可作为各种烯烃聚合和共聚合的优良催化剂。在这些研究中，我们对带有单个脒基配体的阳离子稀土烷基络合物聚合异戊二烯产生了兴趣，并在本文中报道脒基连接的钇络合物[（NCN）Y（o-CH 2C 6 H4 NMe 2）2]（1; NCN = PhC（NC 6 H4 iPr 2 - 2，6）2）是用于异戊二烯聚合的独特催化剂前体。因此，配合物1在1当量的[Ph 3C][B（C6 F5）4]存在下显示出极高的异戊二烯聚合活性和优异的3，4-异专一性。然而，更值得注意的是，该催化剂体系的区域和立体选择性可以简单地通过添加烷基铝化合物如AlMe 3而从3，4-等规性转换为1，4-顺式选择性。虽然异戊二烯在各种催化剂体系下的聚合反应已被广泛研究，但据我们所知，这种区域和立体选择性的戏剧性转换是前所未有的。本文还介绍了从1与AlMe_3反应中分离得到的异三核Y/Al配合物[（NCN）Y{（m-Me）_2AlMe_2}_2]（2）及其在异戊二烯聚合中的性能。将三（氨基苄基）钇络合物[Y（oCH 2C 6 H4 NMe 2）3] [7]用1当量的脒配体N，N '-双（2，6-二异丙基苯基）苯甲脒（NCNH）在THF或甲苯中在室温下处理过夜，得到相应的单（脒基）双（氨基苄基）络合物1，产率为85%（方案1）。在70 ℃ ~ 80 ℃条件下，反应可在3 h内完成。通过1H和13 C NMR光谱、元素分析和X射线晶体学对络合物1进行了充分表征（图1）。1中的NCN单元通过其两个N原子键合到Y中心，如在其他脒基络合物中所观察到的。两个氨基苄基通过N原子和苄基碳原子以螯合方式与Y原子键合。氨基的分子内配位意味着配合物1不具有THF共配体，与含THF的CH 2SiMe 3类似物[（NCN）Y（CH 2SiMe 3）2（thf）]相反。[2d]配合物1在己烷中微溶，但在甲苯和THF中高度可溶。中性配合物1不催化异戊二烯的聚合，但在1当量[Ph 3C][B（C6 F5）4]的存在下，它变得极其活跃，由此它在室温下在2分钟内将750当量的异戊二烯定量转化为聚异戊二烯。该反应以高的3，4-区域选择性（91%）和一定程度的全同立构规整度（mm 50%）进行（表1，条目3）。当聚合在低温（10 8 ℃）下进行时，甚至有更高的区域和立体选择性，
The preparation of polymers with desired microstructures and properties by controlling the regioand stereoselectivity of olefin polymerization is an important research area. Approaches toward this goal have, to date, mainly involved modifying the ancillary ligands of metal catalysts. The use of chelating amidinate ligands as an alternative to cyclopentadienyl ligands in the development of rare-earth-metal (Group 3 and lanthanide) based polymerization catalysts has received considerable attention. Although the majority of amidinate-containing rare-earth-metal complexes reported to date contain two or three amidinate ligands, recent work by Hessen and co-workers has demonstrated that benzamidinates with bulky substituents at the nitrogen atoms, such as N,N’-bis(2,6-diisopropylphenyl)benzamidinate [PhC(NC6H4iPr2-2,6)2] , can serve as excellent ancillary ligands for a series of mono(amidinate)/dialkyl or cationic mono(amidinate)/alkyl rare-earth-metal complexes. However, despite the extensive interest in using amidinate rareearth-metal complexes as polymerization catalysts, the polymerization chemistry reported to date for these complexes has been limited mainly to that of ethylene and polar monomers; studies on the polymerization of higher olefins remain scarce. In particular, the use of an amidinateligated rare-earth-metal catalyst for the polymerization of a conjugated diene, such as isoprene, has not been reported to date. We recently found that cationic rare-earth alkyl complexes can serve as excellent catalysts for the polymerization and copolymerization of various olefins. During these studies, we became interested in the polymerization of isoprene by cationic rare-earth alkyl complexes bearing a single amidinate ligand, and report herein that the amidinateligated yttrium complex [(NCN)Y(o-CH2C6H4NMe2)2] (1; NCN = PhC(NC6H4iPr2-2,6)2) is a unique catalyst precursor for the polymerization of isoprene. Thus, complex 1 shows extremely high activity and excellent 3,4-isospecificity for the polymerization of isoprene in the presence of one equivalent of [Ph3C][B(C6F5)4]. More remarkably, however, the regioand stereoselectivity of this catalyst system can be switched from 3,4-isospecific to 1,4-cis selective simply by adding an alkylaluminum compound, such as AlMe3. Although the polymerization of isoprene by various catalyst systems has been studied extensively, 5] such a dramatic switching of the regioand stereoselectivity is, to our knowledge, unprecedented. Isolation of the heterotrinuclear Y/Al complex [(NCN)Y{(m-Me)2AlMe2}2] (2) from the reaction of 1 with AlMe3 and its performance in the polymerization of isoprene are also described. Treatment of the tris(aminobenzyl)yttrium complex [Y(oCH2C6H4NMe2)3] [7] with one equivalent of the amidine ligand N,N’-bis(2,6-diisopropylphenyl)benzamidine (NCNH) in THF or toluene at room temperature overnight affords the corresponding mono(amidinate) bis(aminobenzyl) complex 1 in 85% yield (Scheme 1). The reaction can be completed in 3 h if it is carried out at 70 8C. Complex 1 was fully characterized by H and C NMR spectroscopy, elemental analysis, and X-ray crystallography (Figure 1). The NCN unit in 1 is bonded to the Y center through its two N atoms, as observed in other amidinate complexes. The two aminobenzyl groups are bonded to the Yatom in a chelating fashion through both the N atom and the benzyl carbon atom. Intramolecular coordination of the amino group means that complex 1 does not possess a THF co-ligand, in contrast with the THF-containing CH2SiMe3 analogue [(NCN )Y(CH2SiMe3)2(thf)]. [2d] Complex 1 is slightly soluble in hexane but highly soluble in toluene and THF. The neutral complex 1 does not catalyze the polymerization of isoprene but it becomes extremely active in the presence of one equivalent of [Ph3C][B(C6F5)4], whereby it converts 750 equivalents of isoprene quantitatively into polyisoprene in 2 min at room temperature. This reaction proceeds with high 3,4-regioselectivity (91%) and some degree of isotacticity (mm 50%) (Table 1, entry 3). When the polymerization is carried out at low temperature ( 10 8C), an even higher regioand stereoselectivity is