Dibenzenechromium metalation

The transfer of charge from the metal to the ligand caused by back-donation can also be seen from a comparison of the ionization potentials of chromium in different complexes. The ionization potential of complex compounds is higher than that of the uncomplexed coordination center (6.76 eV), e.g. for dibenzenechromium ) 7.07 eV, and for hexacarbonyl-chromium 8.03 eV 43). It may be expected that a decrease in the net positive charge at the coordination center will give rise to an increase in ti-EPD properties. Hence the Fe-C distance will be shorter in [Fe(CO) 4] than in Fe(CO) 5. Likewise iron is more strongly coordinated in ferrocene than in the ferrocinium ion. 

Although dibenzenechromium is thermally quite stable, it is less so than ferrocene and melts with decomposition at 285° to give benzene and metallic chromium. Furthermore, it appears to lack the aromatic character of either benzene or ferrocene as judged by the fact that it is destroyed by reagents used for electrophilic substitution reactions. 

C. Elschenbroich, Metalation of Dibenzenechromium by V,V,V ,V -Tetramethy-lethylenediamine Complexes of n-Butyllithium and Phenyllithium, J. Organomet. Chem. 14, 157-163 (1968). 

The correct identification of the sandwich structure of ferrocene led Fischer to consider the possibility of arenes acting as hexahapto 6VE ligands. By simple arithmetic, a neutral bis(arene) sandwich complex of a zerovalent Group 6 element, e.g. chromium (Figure 6.71), was anticipated, a line of reasoning which led Fischer to develop the synthesis of dibenzenechromium. His approach resulted in the comparatively general Fischer-Hafner synthesis (1955), which is applicable to many metals and arenes (devoid of Lewis-basic substituents, Figure 7.34). 

Dibenzenechromium, which forms dark brown crystals, is much more sensitive to air than is ferrocene, with which it is isoelectronic it does not survive the reaction conditions of aromatic substitution. Structural studies on arene complexes show that the C—C bond lengths are usually equivalent, or nearly so. Many other transition metals, including some lanthanides and uranium form arene complexes, and... 

For ligands having all carbons bonded to a metal, sometimes the superscript is omitted. Ferrocene may therefore be written (n-C5H5)2Fe and dibenzenechromium (T -C6H6)2Cr. Similarly, it with no superscript may occasionally be used to designate that all atoms in the tt system are bonded to the metal (for example, (Tr-C5H5)2Fe),... 

It was later reported that the yields and nature e.g., density, molecular weight and terminal vinyl content) could be altered by varying hydrogen pressure in the oxygenated dibenzenechromium catalyst. This type of catalysis is not considered to be catalysis at the atomic level although dibenzenechromium arene 7r-complex was employed. The seemingly essential role of freshly prepared zero-valent transition metal is not apparent. 

Dibenzenechromium is lithiated more readily than PhH polylithiation can result o . Arene-Cr(CO)3 compounds, h -XC6H5Cr(CO)3 (X = H, OMe, F, Qyio-712 are metallated by n-BuLi, e.g. ... 

Benzene and its derivatives are among the better known of the many r 6-arene complexes. The best known of these is dibenzenechromium, (C6H6)2Cr. Like other compounds of formula (C6H6)2M, dibenzenechromium can be prepared by the Fischer-Hafner synthesis, using a transition metal halide, aluminum as a reducing agent, and the Lewis acid A1C13 ... 

Dibenzenechromium, like ferrocene, exhibits eclipsed rings in its most stable conformation. The metal-ligand bonding in this compound may be interpreted using the group orbital approach applied to ferrocene earlier in Chapter 5. 

In the valence shell electron count for the complex (Ti -C3Ph3)NiCl(py)2 (py = pyridine) (Fig. 4.1), the cyclopropenyl hgand C3Ph3 supplies three electrons, the chlorine atom one electron, and each pyridine ligand supplies two electrons. Added to the 10 electrons in the valence shell of the nickel atom, this generates a total of 18 electrons. In the case of the sandwich compound dibenzenechromium, (q -C/H jiCr (Fig. 4.2),six electrons from each benzene ligand may be added to the six electrons in the metal valence shell to generate the total of 18 electrons. 

Elschenbroich (38) studied the metalation of di(benzene)chromium, using a 5 1 molar ratio of BuLi-TMEDA to this ir-arene complex. The extent and orientation of metalation were studied by mass spectrometry of the products after quenching with D20. A lithium substituent on dibenzenechromium strongly activates the molecule toward further meta-... 

Preparation of ferrocene was reported at about the same time by two research groups, and a sandwich structure was proposed, based on ferrocene s physical properties (Kauffman, pp. 185-186). The sandwich structnre was confirmed by x-ray diffraction studies. Since then, other metallocenes composed of other metals and other carbon ring molecules, such as dibenzenechromium (see Figure 3) and uranocene (see Figure 4), have been prepared. 

The prismatic cyclopentadienyl and benzene complexes of transition metals (see, e.g., Ref. [3-9]) are reminiscent of the polycyclic hydrocarbon prismanes. Figure 3-35 shows ferrocene, (C5Hj)2Fe, for which both the barrier to rotation and the free energy difference between the prismatic (eclipsed) and antiprismatic (staggered) conformations are very small [3-51]. Figure 3-35 presents also a prismatic model with symmetry for dibenzenechromium, (C,H,)2Cr. 

A variety of organometallic compounds can be prepared by metal atom reactions. Atoms of a metal are vaporized from a very hot metal sample into an evacuated chamber. They are condensed onto a surface cooled with liquid nitrogen (-196°C) along with the desired ligand. Molecules which are too unstable to be prepared by more conventional techniques can be made in this way as well as stable molecules such as ferrocene and dibenzenechromium. Although solid chromium does not react with either liquid or gaseous CsHs, reaction (15), gaseous chromium atoms react even at low temperatures. 

Dibenzenevanadium( — 1) and dibenzenechromium( 1 —) may be obtained by reduction of V(PhH)2 by metallic potassium in aprotic chelating solvents such as 1,2-di-methoxyethane or hexamethylphosphoramide. The compound Ru2Cl4(PhH)2 reacts with sodium hydroxide to give [Ru2(ju-OH)3 (PhH)2]Cl, while with sodium carbonate it furnishes the tetranuclear complex [Ru4(OH)4 (PhH)4](S04)2 I2H2O (Figure 10.4c). Many similar reactions in which the arene ligands are not involved may also be carried out. 

Both arene and cyclopentadienyl 7i-complexes can undergo redistribution reactions. These reactions involve the exchange of ligands between two species possessing the same or different metal. For example, the reaction of dibenzenechromium with chromium hexacarbonyl provides a method for the preparation of benzenechromium tricarbonyl (2-6). A second example of a... 

Dibenzenechromium(O) has an octahedral configuration consistent with d sp hybridization of the chromium atomic orbitals, (Fig. 3-2). If we accept this formulation of the electronic configuration of dibenzene chromium(O), this TT-complex can be classified as a coordination compound. The electronic configuration of most metal 7i-complexes thus can be accounted for by VBT. Therefore, metal r-complexes can be regarded as a new class of coordination compounds. 

Dibenzenechromium(O) can be metalated with sodium and, in turn, converted to a methyl ester or a ketone, equation (6-36). Reaction of ditoluene chromium(O) with mercuric compounds does not give any mercurated derivatives but rather metallic mercury and an oxidized ditoluene chro-mium(I) cation. Ditoluenechromium undergoes metalation with butyl-lithium to give (LiC6H4 CH3)2Cr, which reacts further with carbon dioxide to yield (CH3 C6H4-COOLi)2Cr. 

---