Complex bimetallic titanium-aluminum

Angelescu and coworkers propose the following mechanism for the dimerization of ethylene on the bimetallic titanium-aluminum complex [67,68] ... 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

XX is generally considered as the active species formed from titanium chloride and alkyla-luminum components. The in the structure represents an unoccupied (vacant) site of the octahedral titanium complex. XX represents an active titanium site at the surface of a TiCls crystal after modification by reaction with the alkylaluminum component. The titanium atom shares four chloride ligands with its neighboring titanium atoms and has an alkyl ligand (incorporated via exchange of alkyl from the alkylaluminum for chloride) and a vacant orbital. There are molecular mechanics calculations that indicate dimeric Ti2Cl6 may be the active species instead of monomeric TiCls [Monaco et al., 2000]. Other proposals for the active species include bimetallic species that contain both titanium and aluminum [Liu et al., 2002]. To simplify matters, our discussions will center on the monomeric and monometallic titanium species, especially since the mechanistic details of stereoselectivity and activity are essentially the same for both monomeric and dimeric titanium species as well as titanium-aluminum species. 

Figure 19-1. Structures of Ziegler catalysts based on titanium/aluminum. X, Anion O, vacant ligand site. As shown, the complexes responsible for the monometallic mechanism may be monometallic or bimetallic, (a) Known structure in a soluble Ziegler catalyst. Also shown are proposed structures in heterogeneous Ziegler catalysts for (b) a bimetallic mechanism (c) or (d) a monometallic mechanism.

According to the bimetallic mechanism of monomer polymerization on the catalytic titanium-aluminum system, it is assumed that at the first stage a complex between the n bond of monomer and the titanium atom is formed, which is connected with the aluminum atom by alkyl groups. It is followed by polarization of the transition metal-alkyl group bond and the formation in the transition state of a hexacomponent ring, incorporating the monomer molecule ... 

Evidence which was interpreted as support for the necessary participation of the aluminum center in a bimetallic complex has been cited by Natta and coworkers who isolated bimetallic complexes from the reaction of aluminum alkyls and bis-(eyelopentadienyl)-titanium dichloride which polymerized ethylene at low pressure 11). 

Ternary and quaternary a-hydroxy-phosphonates, an important class of biologically active compounds, are commonly obtained by addition of dialkylphosphites onto aldehydes or ketones [30]. Well-defined mono- or bimetallic complexes of rare-earth metals, titanium, or aluminum have emerged over the past two decades as effective catalysts for this so-called hydrophosphonylation of aldehydes [31] and, with more difficulty, that of ketones [31c,d, 32], which are far less reactive because of their lower electrophilicity. In some cases, good enantioselectivities could be achieved thanks to the use of chiral metal-based precatalysts [31, 32], Despite their several similarities with rare-earth elements, we were surprised to see that discrete complexes of the large Ae metals had never been utilized to catalyze hydrophosphonylation reactions. 

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