Benzenechromium

Bailey, M. F. Dahl, L. F. (1965) "Three-dimensional crystal structure of benzenechromium tricarbonyl with further comments on the dibenzenechromium structure, Inorg. Chem. 4, 1314-1319. 

These compounds provide interesting subjects for study, since there arises the possibility of two quite different types of product molecule, in addition to the target compound the carbonyl and the pure diarene. These possibilities are well illustrated in the study by Baumgartner and Zahn of benzenechromium tricarbonyl, as is shown in Table 11. 

Table 11. Retention and yields of Cr from benzenechromium tricarbonyl (Baumgartner and Zahn )...

The first such compound to be studied was benzenechromium tricarbonyl (14), which gave the results shown in Table VI. The striking thing about... 

Thus, although the initial X-ray study of Cotton, Dollase and Wood (21) was interpreted (22) in terms of a uniform C—C distance and an overall D6h symmetry, Jellinek (23, 24) concluded from his crystallographic data that the benzenoid rings showed alternating C—C bond lengths, with an effective symmetry no higher thanZ)3d. Finally however more accurate low temperature crystal data due to Keulen and Jellinek (25), and electron diffraction measurements by Haoland (26) showed conclusively that bis-benzenechromium has in fact sixfold, D6h, symmetry in both crystal and the gas phase. 

The best known bis-arene system, bis-benzenechromium, Cr(Bz)2, has been studied by a number of authors (94, 95, 96), but the results of Feltham, in cyclohexane solution, are typical a broad band with a maximum (e = 25) at 15.6 kK. is the only feature at lower energies, a stronger peak (e = 8000) being found at 31.25 kK. with a rising absorption... 

Benzenechromium tricarbonyl 371 is deprotonated by BuLi in EtiO-THF at —40 °C in a reaction that needs carefnl control for good yields . The prodnct 372 can be sily-lated to give 373 in 60% yield (Scheme 158). Toluenechrominm tricarbonyl lithiates non-regioselectively on the ring (bnt at the benzylic position with Na or K bases). Excess base can lead to polylithiation . 

I. NATURE OF ELECTRONIC ABSORPTION IN FERROCENE, Aw-BENZENECHROMIUM(I) AND DERIVATIVES... 

Electronic absorption spectra of fc-benzenechromium (I) iodide (5) and its derivatives have been recorded and discussed by Yamada et al.26,26 They noted a loss of the benzene rings aromaticity in these compounds with the n electrons being drawn toward the metal and thereby conferring more a cyclopentadienyl radical character on the rings. 

When Baizani56 photolyzed the te-benzenechromium(I) hydroxide two characterizable products were chromium(III) and benzene (Eq. 11). 

Razuvaev found that his-benzenechromium itself could be photolyzed67 in the presence of an alkyl halide to obtain the halide of to-benzenechromium(I) plus hydrocarbons derived most likely from the free radical corresponding to the original alkyl group. Most likely the decomposition of ferrocene19 also affords a variety of hydrocarbons, but this has apparently not been investigated. 

Among the compounds that form complexes with silver and other metals are benzene47 (represented as in 6) and cyclooctatetraene. When the metal involved has a coordination number greater than 1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these complexes are called carbonyl groups. Thus, benzenechromium tricarbonyl (7) is a stable compound.4 Three arrows are... 

In a related compound, benzenechromium tricarbonyl, C6H6Cr(CO)3377, internal rotation was seen to be nearly unhindered. 

Ietraeyanoathylene forms a rr-type complex with ferrocene (I). Another metallocene is bis-benzenechromium iodide (II) a sandwitch type complex. 

These, to some extent, are related to redistribution reactions but, of course, do not lie within the scope of this review. Probably one of the first redistribution reactions of ir-bonded compounds is the exchange of carbonyl groups with 7r-bonded benzene (85) on chromium leading to the mixed compound, benzenechromium tricarbonyl. 

It is well known that certain transition metal organometallic complexes, such as ferrocene, cymantrene (cyclopentadienylmanganese tricarbonyl) and butadieneiron tricarbonyl, and to a lesser extent, benzenechromium tricarbonyl, form organic derivatives and have an extensive derivative chemistry of their own. The parent complexes have already been discussed there follows a summary of the main features observed in the mass spectra of their organic derivatives (Table II, at the end of this section). 

Selective 1,4-hydrogenation of 1,3-dienes to (Z)-alkenes is possible with benzenechromium tricarbonyl as a catalyst precursor. Only those conjugated dienes that can adopt a cisoid conformation, and hence chelate to the metal, are hydrogenated. Methyl sorbate (68) is reduced to methyl (Z)-3-hexenoate (70) [46,47]. The reaction... 

Benzenechromium tricarbonyl (Fluorobenzene)chromium tricarbonyl Cyclopentadienyl)rhenium tricarbonyl (1,3-Cyclohexadiene)iron tricarbonyl (1,3-Cyclohexadienyl)manganese tricarbonyl (T rimethylenemethane)iron tricarbonyl (Cyclopentadienyl)manganese tricarbonyl (1,3-Butadiene)iron tricarbonyl... 

Figure 1. Molecular structure of benzenechromium tricarbonyl complex with thiourea as determined by x-ray analysis. There are three thiourea molecules per organometallic unit.

Figure 2. View of molecular packing arrangement in benzenechromium tricarbonyl-(thiourea)3 complex. The view is down the crystal c-axis, looking down the honeycomb formed by the thiourea lattice.

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