Chromocene sensitivity

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. 

The preparation of pentadienylsodium was being studied at about the same time as ferrocene was discovered. It was not until 1968, however, that the first binary pentadienyl complex of a transition element, bis(pentadienyl)-chromium, was obtained from PINa and CrCl2 (121). This compound forms green, air-sensitive crystals, and like chromocene it has two unpaired electrons (Ht = 2.74 BM). This discovery was shortly followed by that of the curious complex, bis(pentadienyl)dinickel (12) which was prepared from NiCl2 and triethylaluminium in 1,4-pentadiene (122). The pentadienyl ligands in 12... 

Chromocene catalysts are not very sensitive to the choice of support used. They tend to produce polymers having the same narrow MW distribution. All these characteristics are different from those of the other organochromium catalysts and of chromium oxide catalysts. They are attributed to the influence of the remaining Cp ligand, which probably provides a more crowded and electron-rich environment than is formed on the other catalysts. 

Metal cyclopentadienyl complexes can also be used as cocatalysts, with the intent of creating chromocene-like structures on the surface of the catalyst, as shown in Scheme 46. Chromocene catalysts, which contain mono-attached chromium species incorporating one cyclopentadienyl ligand, are noted for their sensitivity to H2. It is believed that Cr/silica catalysts can be modified to make this species by the addition of metal cyclopentadienyls to the reactor, such as LiCp or MgCp2 [695],or by use of a combination of cyclopentadiene or indene with an aluminum alkyl cocatalyst [696]. When these modified catalysts are allowed to polymerize ethylene in the presence of a remarkable broadening of the polymer MW distribution is observed, mainly as a result of a shift of the low-MW part of the MW distribution. The chromocene surface species is known for its ability to incorporate H2 (thus lowering the polymer MW) and also to reject 1-hexene. Thus, these unusual cocatalysts have the potential to reverse the normal branch profile of polymers made with Cr/silica catalysts (i.e., to put more branches into the longer chains). 

Because Cr/silica does not respond to H2 in this way, this response is attributed to the formation of chromocene-like structures, which are highly sensitive to H2. Chromocene is also known for rejecting 1-hexene comonomer, so that such structures can produce low-MW polymer... 

H2 is often added to the reactor to decrease the polymer MW. The MW reduction is thought to occur by simple hydrogenolysis, as shown in Scheme 14. Chromium oxide on silica is not as sensitive to H2 as some other catalysts, such as Ziegler or chromocene catalysts. However, its H2 sensitivity is also not unusual, as many Ballard (zirconium or titanium) catalysts fall into the same category [297,376]. The sensitivity of chromium oxide catalysts can vary considerably, depending on the support, suggesting that various sites respond quiet differently. 

The dianion of 112 yields the chromocene 117 in 43% yield as a dark red and air sensitive solid, by reacting it with CrCl2 (Fig. 25) [102a]. This alternate... 

Ofele and Herberhold explored the coordination chemistry of NHC with chromium tetra-carbonyl- and penta-carbonyl-chromium complexes more than 40 years ago [5,73]. Since then, several complexes of the general formula (NHC)Cr(CO)5 or (NHC)2Cr(CO)4 have been structurally characterized [74-78]. A variety of monodentate Cr-NHC adducts (with various oxidation states for the chromium center) have also been synthesized and characterized. For instance, the reactivity of NHCs with chromium (II) metallocene complexes has been studied. The reaction of chromocene with imidazolium chloride was reported to afford 14-electron Cr (II) complexes of the type CpCr(NHC)Cl 50 via a CpH elimination reaction (Scheme 14.26), isolated as highly air sensitive species [79,80]. Oxidation of the latter with PbCl2 (when NHC = PPr) or with CHCI3 (when NHC = IMes) produced the corresponding Cr (III) NHC dichloro... 

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