Zirconium ansa metallocenes

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Constrained-geometry catalysts for C2H4 polymerization 88 that are counterparts of well-known ansa-metallocene systems have been prepared and shown to be active, in combination with MAO, toward polymerization of ethylene the product is almost entirely polyethylene, with ca. 1% of 1-octene obtained. The titanium complex was found to be four times as active as the zirconium species.1... 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

Figure 3.14 Additionally a-substituted ansa-metallocene precursors, rac.-dimethyls-ilylenebis[l-(2-methylindenyl)]zirconium dichloride [rac.-Me2Si(MeInd)2ZrCl2] and rac.-dimethylsilylenebis[l-(2-methyl-4-naphthylindenyl)]zirconium dichloride [rac.-Me2 Si(Me,NphInd)2 ZrCl2], for obtaining high-molecular weight polypropylene. Front side view. Reproduced by permission from Ref. 263. Copyright 1994 American Chemical Society...

Jordan and coworkers have recently prepared several ansa-bis(indenyl) compounds (AlMe2L)indenyl 2SiMe2 and 1,2- (AlMe2L)indenyl 2C2H4 (L = THE, Et20, 1,4-dioxane) by a salt metathesis route. These were reacted with M(NMc2)4 (M = Zr, Hf) to eliminate [Me2AlNMc2]2 and form ansa-metallocene complexes of bis(dimethylamido)zirconium and bis(dimethylamido)hafmum. A simple tris(indenyl)aluminum compound has been prepared by reaction of bis(indenyl)mer-cury with aluminum metal. ... 

Monoanionic boratabenzene ligands are isoelectronic to cyclopentadienide anion and can be nsed to synthesize Zr equivalent metallocenes (eqnation 29). A more congested complex of 9-phenyl-9-borata-anthracene (C 1-synunetry) was obtained by a similar way. Mono and bis boratabenzene complexes can also be prepared by reaction of a nentral boratabenzene-PMes adduct with tetrabenzyl zirconium or hafnium. Boratabenzene ansa metallocenes with CH2-CH2 or Si(CH3)2 bridges as well as Cp-boratabenzene mixed complexes or CG complexes were synthesized. 

A new type of enantioselective diene polymerization is found with cyclopolymerization of 1,5-hexadiene which leads to polymers with a saturated chiral main chain28,58>109. As catalyst, (—)-(7 )-[l,T-ethylenebis(4,5,6,7-tetrahydro-l-indenyl)]zirconium (/ )-binaphtholate is used in the presence of methylalumoxane to give optically active poly(methylene-1,3-cyclopentane) (3) with 68% trans configuration in the five-membered ring (diisotacticity). If the (S)-enantiomer of the ansa-metallocene with (ft)-binaphthol is used as catalyst then the opposite rotation of the polymer is observed58. 

Diamond, G M. Chemega, A. N. Mountford, P Green, M. L. H. New mono- and bi-nuclear ansa-metallocenes of zirconium and hafnium as catalysts for the polymerization of ethene and propene. J. Chem. Soc., Dalton Trans. 1996, 6, 921-938. 

Alt, H. G Foettinger, K. Milius, W. Synthesis, characterization and polymerization potential of ansa-metallocene dichloride complexes of titanium, zirconium and hafnium containing a Si-N-Si bridging... 

The first isospecific propene polymerization with an ethanediyl-bridged bis (indenyl)-titanium complex demonstrated in 1983 the capability of ansa-metaUocenes to control the tacticity of a growing polypropylene chain [4, 48], thus creating a strong interest in exploring the potential use of ansa-metallocenes for industrial polypropylene production. A first step towards industrially suitable systems was the use of much more stable zirconium complexes, which when activated by the methylaluminoxane co-catalyst [3] allowed higher polymerization temperatures and thus higher activities [5]. 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

Besides the bis(indenyl) ansa compounds, C2 symmetric bridged bis(cyclo-pentadienyl) metallocenes of zirconium and hafnium were found to be able to produce isotactic polypropene (Table 8) [103]. The key for high isotaeticity are substituents in positions 2,4,3 and 5 generating a surrounding of the transition metal similar to the one in bis(indenyl) metallocenes. 

Cationic metallocene-alkyl complexes are attracting much interest because of their catalytic activity for a-olefin polymerization [47,48]. The most convenient method for generating the ion pairs is the reaction of metallocene dialkyl complexes of zirconium and hafnium with B(C6F5)3, forming cationic complexes [Cp2MR] [RB(C6F5)3]" (M = Zr, Hf) [49,50]. ansa-TypQ cationic compounds have been applied in the stereospecific polymerization of a-olefins [51]. 

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