Chromocene

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Dimesitylimidazolium chloride with chromocene gives the carbene 32 (R = C1) (990M529). With phenylmagnesium chloride, 32 (R = C1) gives 32 (R = Ph), the product of substitution of the chloride ligand by phenyl radical. In chloroform, 32 (R = C1) gives the chromium(III) species 33. In contrast,... 

In the case of the catalyst chromocene + Si(V Karol et at. (9) do not favor the interpretation of the initiation reaction as the insertion of a monomer into the Cr—C5H5 bond either. 

Chromocene and nido-5,6-B2BgH,2 form the bimetallic chromium complex ... 

Chisholm, M. H. et al., Synth. React. Inorg. Metal-Org. Chem., 1977, 7, 283 This adduct of chromocene with hexafluoro-2-butyne decomposes after a few mins, at ambient temperature, either under nitrogen or vacuum. The process is autocat-alytic and violently exothermic, several explosions having occurred. 

For reasons given earlier the putative dl and d2 complexes are excluded from consideration, and for the d4 system, chromocene, (and as yet for other d4 species) further speculation is inappropriate in the absence of an esr signal. (For Cr(Cp)2 however Krieger and Voitldnder (62) have given an extensive analysis of the theoretical possibilities.)... 

The cyclopentadienyl rings of chromocene, as for vanadocene, assume a staggered conformation. 

In agreement with the presence of an additional electron in the bonding orbital, compared to vanadocene (chromocene 16 valence electrons, terminal electronic configuration e a ), the metal-carbon distance is shorter (2.15 A compared to 2.27 A of vanadocene). 

As illustrated in Figure 44, chromocene exhibits both a Crn/Crm oxidation and a Cr /Cr1 reduction.78... 

Like ferrocene and vanadocene, chromocene can be permethylated to decamethylchromocene (Cr-Cp (centroid)= 1-78 A79). 

The electronic effects of the methyl and phenyl substituents have been already discussed for the corresponding chromocenes. 

Loosely bound fj -cyclopentadienyl anions can also serve as the base to deproto-nate imidazolium salts. When chromocene is reacted with an imidazolium chloride in THF the metal precursor loses one molecule of cyclopentadiene to form the 14-electron complex [( 7 -C5H5)Cr(NHC)Cl] [Eq. (13)]. This complex can be further oxidized by CHCI3 to give [( 7 -C5H5)Cr(NHC)Cl2]. This route also works with nickelocene to generate the corresponding [( -C5H5)Ni(NHC)Cl] complex. ... 

Scheme 7. Reaction of chromocene with imidazolin-2-ylidenes and imidazolium salts.

In certain cases it is also possible to replace anionic ligands by neutral NHC ligands. Nickelocene and chromocene are capable of coordinating one imidazolin-2-ylidene ligand by shifting one of the cyclopentadienyl anions from to... 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

Magnetic data for the low-spin, air-stable chromium(II) metallocarborane sandwich118 compound [NEt fCrfCjBioHi ] have been compared with those for chromocene. 

Many alkoxides have been prepared from chromocene in THF or hydrocarbons (equation 14). Only partial replacement of cyclopent dienyl by R occurs with Me3COH and silanols (equation (15)). 

Chromocene can also be used as a starting material (Scheme 22).164... 

The reaction of bis(trimethylsilyl)diazene with chromocene or CpCrCl2 affords the dark violet crystalline imide with structure (297). The imide shows distinct trimethylsilyl signals in the NMR spectrum, and in methanol forms [CpCr(NH)(NSiMe3)]2.1425... 

It was later found that stable organometallic compounds of transition metals exhibiting very low polymerization activity could be transformed into high-activity catalysts when deposited on silica, alumina, or silica-alumina.287-289 Interaction of surface hydroxyl groups with the organometallic compounds such as chromocenes, benzyl, and Tt-allyl complexes results in the formation of surface-bound organometallic complexes (41-43) 289-291... 

---