Titanium bimetallic complexes

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

Chae, H. K. Payne, D. A. Xu, Z. Ma, L. 1994. Molecular structure of a new lead titanium bimetallic alkoxide complex [PbTi2( t4-0)(00CCH3)(0CH2CH3)7]2 Evolution of structure on heat treatment and the formation of thin-layer dielectrics. Chem. Mat. 6 1589-1592. 

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. 

The activation energy of the insertion of coordinated ethylene estimated by the ab initio method was found to be 15 kcal/mol Despite the application of a more advanced calculation technique these results are less compatible with the experimental data on solid titanium chloride-based catalysts, when the activation energy of the propagation step is 3-6 kcal/mol (Table 10). Probably, this incompatability is due to the model used in ref. which describes the AC as a bimetallic complex CljTiCHj with A1(CH3)3. However, it is important to note that the calculations performed by means of the nonempirical method confirm the concept implying that in the active center the alkyl group occupies an intermediate position between the octahedral sites and that in olefin coordination the AC structure is reconstructed. 

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). 

The metals that are more frequently found as components of Ziegler-Natta catalysts are some light elements of groups I-III of the periodic table (e.g., Li, Be, Mg, Al), present as organometaUic compounds and halides, or other derivatives of transition metals of groups IV-Vm (e.g., Ti, V, Cr, Mo, Rh, Rn, Co, and Ni). A typical example is the product(s) of the reaction between triethylaluminum and titanium tetrachloride. The composition of the product is not well defined but is believed to be either an alkylated metal halide (monometallic I) or a bimetallic complex involving a bridge between the two metals (II). 

In a fifth contribution, Pierre Le Gendre et al. review the catalytic performances of early-late bimetallic complexes. After an introduction dealing with bifunctional and cooperative dual catalysis in which the cooperative effects arise from two catalytic functions present in the same or two different molecules, this paper focuses on an inventory of early-late heterobimetallic complexes in catalysis. It includes those where the two metal centers belong to the same complex, with bridging ligands as in the following example in which the chiral information is provided by the titanium metal center ... 

The reaction of (/ )-81 with Ti(0 Pr)4 in the presence of molecular sieves produces a polymeric titanium complex (/ )-93 (Scheme 39) [73). The monomeric binaphthyl titanium complex (R)-94 is also prepared from the reaction of (/ )-8 with Ti(0 Pr)4. Unlike the polyaluminum complex (/ )-87, although (R)-94 can catalyze the reaction of 88 and 89 at room temperature, no reaction is observed when the polytitanium complex (/ )-93 is used even in a refluxing methylene chloride solution. It has been proposed that when R)-94 is used in the catalysis of organic reactions, the real catalytically active species is a bimetallic complex [73b,c]. Our observation here supports this assumption. In (/ )-93, all of the titanium centers are isolated in a rigid polymer chain so that the bimetallic catalyst cannot be generated, which probably causes the loss of the catalytic activity. 

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. 

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

Titanium species having different oxidation states, single and double-bonded to the surface of silica as well as strongly chemically and physically absorbed, can be formed. The role of the ester is to moderate the alkylating divalent titanium species. The bimetallic complex may be formed in the preparation of Si02 supported catalysts by impregnation [5] with TiCU-AlEtj. 

In the context of bimetallic catalysts, an unusual cationic Ti-Rh bimetallic complex 5 exhibits an interesting cooperative effect in catalyzing the hydrosilation of acetophenone with Ph2SiH2. A weak bonding interaction between rhodium and a Cl on titanium apparently stabilizes a lower-valent Rh intermediate such that the bimetallic complex delivers a much higher hydrosilation yield than the corresponding Rh(COD) monometallic complex based on l,2-bis(diphenylphosphino)benzene. 

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