Chromium complexes ethylene

Figure 16 shows the octahedral geometry of two chromium complexes with N204 coordination. The first with the Schiff base [iV,./V-ethylene-bis(salicylideneimine)] + two water molecules, [Cr(salen) (H202)2]+ 29 the second with two molecules of [2,2 -bipyridine-6-phenolate], [Cr 6-(2-C6H40)bipy 2]+.3°... 

Fio. 18. Spectra of a series of chromium complex salts with successive replacement of ethylene diamine by the SON ion. 

Leelasubcharoen S (2001) Pentamethylcyclopentadienyl Chromium Complexes in Ethylene Polymerization and Oxidation Reactions. PhD Thesis, University of Delaware... 

The following procedure can be extended to prepare l-oxacyclopent-2-ylidene chromium complexes substituted in the 3 or 5 position.4 Thus the reaction of the anion of pentacarbonyl(l-methoxypropylidene)chromium(0) with ethylene oxide gives pentacarbonyl [dihydro-3-methyl-2(3//)-furylidene] chromium(O) [pentacarbonyl(3-methyl-l-oxacyclopent-2-ylidene)chromium(P)], and the reaction of the anion of pentacarbonyl(l-methoxyethylidene)chromium(0) with propylene oxide gives pentacarbonyl(dihydro-5-methyl-2(3//)-furanylidene)-chromium(O) [pentacarbonyl(5-methyl-l-oxacyclopent-2-ylidene)chromium(0)]. 

The chromium complex cyclizes to a 5-membered complex (vi), which loses ethylene and the metal (vii), to give methoxycyclopentadienone. Subsequent reduction (viii) under the reaction conditions by Cr(0)-H2O affords the methoxycyclopentenone. The correspond-... 

Diethyl(ethylene)tellurourea formed complexes with chromium, molybdenum, tungsten, and manganese carbonyls, in which the tellurium is coordinated to the transition metal1. The solid complexes are moderately stable in air. They do not decompose when stored in the dark at 20° under an inert atmosphere. A toluene solution of the chromium complex at 20° deposited tellurium forming the chromium-carbene complex1. 

The idea that complex formation may be important in the catalytic process can be carried further. It has been found, for example, that bis-arene chromium complexes supported on silica-alumina are active for ethylene polymerization. These catalysts are prepared by activating the support alone in the usual manner and then impregnating with a hydrocarbon solution of the bis-arene compound at room temperature in the absence of air or other oxidizing agent. 

Noncyclopentadienyl chromium complexes have also figured in polyolefin catalysis. The /3-diketiminato chromium complexes like (16) polymerize ethylene and copolymerize ethylene and o -olefins in the presence of MAO. The 1,3,5-triazacyclohexane complexes of CrCb (18) polymerize ethylene in the presence of MAO or [HNMe2Ph][B(C6F5)4]/Al-i-Bu3. Some trimers are formed, resulting in butyl branches in the chain higher a-olefins are trimerized. 

Bisbenzene chromium has been found to be a catalyst for the polymerization of ethylene at 200°-250°C. Chromium metal was postulated as the active catalyst in the system 410, 411). The polymerization of ethylene by bisarene chromium(I) salts in the presence of (i-Bu)3Al has also been studied (406). The catalytic activity was found to be a function of both the arene and the anion present. When bisarene chromium complexes are air oxidized in water, hydrogen peroxide is produced ... 

Trimerization of Ethylene into 1-Hexene Catalyzed by Chromium Complexes... 

Arene chromium tricarbonyl complex Ethylene derivatives from 1,3-dienes Preferential hydrogenation... 

The mechanism of polymerization was initially believed to be as follows. Cr-O-Si bonds are present on the catalyst surface. Contact with ethylene results in an oxidation-reduction reaction and formation of an ethylene-chromium complex. The initiations are accompanied by gains of hydride ions and formations of terminal methyl groups. Polymer growth was pictured according to the reactions shown in Fig. 3.6 ... 

Supported metal complex catalysts for alkene polymerization. Supported chromium complexes on silica have been used for many years in the Phillips process for ethylene polymerization, and promoters are not required. Like these supported complexes, the classical TiClj Ziegler polymerization catalysts have also long been viewed as presenting surface catalytic sites that are well described as molecular analogues. 

The selective trimerization and tetramerization of ethylene to form 1-hexene and 1-octene has become an important process to generate monomers for the synthesis of LLDPE. 1-Hexene was detected as a byproduct in the polymerization of ethylene catalyzed by homogeneous chromium complexes. Chromium complexes have now been identified that catalyze this oligomerization to form 1-hexene with remarkably high selectivity. Phillips patented the combination of 2,5-dimethylpyrrole, triethylalu-minum, and diethylaluminum chloride with a chromium(lll) salt, and researchers at Union Carbide patented a catalyst generated from chromium(lll) 2-ethylhexanoate and hydrolyzed triisobutylaluminum. - Researchers at BP have described a particularly active and selective catalyst based on a chromium(lll) precursor, a bis(diphosphino)amine ligand, (o-MeO-C H ),PN(CH,)P(o-MeOC H ) and methylaluminoxane (MAO). " ... 

Improvement in the preparation of the trans-diamminebis-(ethylene-diamine)cobalt(III) ion has allowed a quantitative spectral evaluation of the products of the ammonatlon(acid ammonolysis). For example, one product of an inversion reaction has been found to consist of about 65% trans-diammine, 22%L -cis-diammine, and 13%D -cis-diammine. Extension of these studies to chromium complexes in ammonia has given no indication of comparable inversions. 

The consumption of ethylene clearly shows an induction period, which may be associated with the formation of the first reactant-chromium complexes, followed by a deactivation process, which leads to an almost stationary rate of comsumption of ethylene. It is clear from... 

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