Hydrogen chromocenes

A chromocene catalyst supported on silica has been studied (94), with ethylene adding to a Cr—H bond. It is remarkable that the chromium atom, as well as the migrating hydrogen atom, appears to be essentially neutral. For a discussion, see the original article (94). 

Study of these new catalysts is intensive. Small molecular-weight distribution was demonstrated by Petrova (112) and by Baulin et al. (113). In addition, polymer substrates have been used (114-116) in order to increase lifetime and activity. As shown by Suzuki (36), stabilization is caused by inhibition of reduction by polymeric ligands. Karol (117, 118) described the reaction of chromocene with silica to form highly active catalysts sensitive to hydrogen. An unknown role is played by the structure mt—CH2—CH2—mt which is formed with ethylene and reduced forms of titanium (119). For soluble systems, it has been shown that the mt—CH2—CH2—mt structure is formed in a biomolecular reaction with /3-hydrogen transfer (120). It was considered that this slow, but unavoidable, reaction is the reason for changes in activity during reaction and that the only way to avoid it is to prevent bimolecular reaction of two alkylated species. 

Chromocene catalyst has excellent hydrogen response for molecular weight control. 

Chain termination occurs primarily by j8-elimination with hydride transfer to chromium and by /3-elimination with hydride transfer to monomer. These terminations are analogous to those previously shown for Ziegler-Natta polymerizations (see eq 3.8 and 3.9 in Chapter 3). In some cases, supported chromium catalysts, e.g., chromocene on SiO and Cr on AlPO, are responsive to hydrogen... 

Similarly, reduction of chromocene with hydrogen in the presence of carbon monoxide also aifords a w-cyclopentenyl complex (27, 2S). 

Examination of Figure 3.22 shows that the chromocene-based catalyst has extremely high hydrogen response resulting in a highly-saturated polymer chain with almost exclusively methyl end groups, while the catalyst... 

Figure 3.21 Chain transfer reactions that are used to control polyethylene molecular weight. The chromocene-based catalyst exhibits very high hydrogen response. Cp ligand on Cr not shown.

The indenyl-based and fluorenyl-based experimental catalysts each required approximately three times as much hydrogen to produce a similar polymer molecular weight as the chromocene-based catalyst, while... 

The sandwich bis-7r-cyclopentadienyl complexes are thermally rather stable, and many melt without decomposition at about 173X. They are stable to hydrolysis, and the C5H5 rings resist catalytic hydrogenation. Their stability to oxidation, however, varies greatly with the nature of the metal. At room temperature the 18-electron complex ferrocene is inert to molecular oxygen, whereas chromocene (which possesses a 16-electron configuration) is pyrophoric in air. 

A new catalyst in which chromocene was supported on silica gel was developed by Union Carbide" for use in gas-phase polymerization reactions. A typical catalyst contained about 2.5% by weight of chromium, and was activated by the addition of small amounts of alkylaluminium alkoxides, such as diethyl-aluminium ethoxide. Use of such a catdyst formulation gave rise to a polymer with much narrower molecular weight distributions. The average molecular weight could be controlled by the addition of hydrogen, which caused termination of the growing chain. 

Cyclopentadiene iron tricarbonyl has been prepared and decomposes thermally to the binuclear carbonyl [a -C5H5Fe(CO)2]2 [26o]. The binuclear iron complex may further react with cyclopentadiene or thermally decompose ( 200°) [27, 28] to give ferrocene. Monosubstituted ferrocenes may be prepared by the former reaction [27]. Chromium hexacarbonyl and cyclopentadiene at 280-350° react to give chromocene [29] the reaction is reversible since treatment of chromocene with carbon monoxide under pressure affords chromium hexacarbonyl, together with intermediate products such as [jr-CpCr(CO)3]2, [jr-Cp2Cr][3r-CpCr(CO)3] and, when hydrogen is also present, the cyclopentenyl complex jr-CsH5CrC5H7(CO)2, 4.1, is formed [30, 31, 32]. 

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