User:LW14061407/Bis(benzene)chromium

Discovery
In late 1910s, Franz Hein started the investigation of "triphenylchromium" by reacting chromium trichlor ide with a Grignard reagent, phenyl magnesium bromide. Such a reaction gave a mixture of phenyl chromium and Hein suggested that it contained a Cr(I) species, "(C6H5)5CrBr", generated via valence disproportionation.

5C6H5MgBr + 4CrCl3 ⟶ (C6H5)5CrBr + 2MgBr2 + 3MgCl2 + 3CrCl2

This event marked an advance in organochromium chemistry at the time and "(C6H5)5CrBr" was described to have salt-like properties. However, the reported workup procedures for "(C6H5)5CrBr" was challenging and the yield was low. Later scrutinization by Zeiss and Tsutsui revealed that Hein's formulation of the chromium-containing products was flawed.

The actual discovery of bis(benzene)chromium was largely contributed by Ernst Ottot Fischer and Walter Hafner in 1950s. Ernst Otto Fischer postulated that it might be possible to synthesis a neutral chromium(0) complex with two benzene ligands, which has a sandwich structure, similar to that of ferrocene. In 1954, Walter Hafner, a PhD student of Ernst Ottot Fischer at the time, put the idea into practice. A reaction of chromium trichloride, aluminum trichloride, aluminum powder in m-xylene, followed by treatment of sodium dithionite in aqueous sodium hydroxide led to the reduction of Cr(III). The resulting solid was determined to be the target, bis(benzene)chromium.

6C6H6 + 3CrCl3 + 2Al + xAlCl3 ⟶ 3[(C6H6)2Cr][AlCl4]．(x-1)AlCl3

2[(C6H6)2Cr]+ + S2O42− + 4OH− ⟶ 2(C6H6)2Cr + 2SO32− + 2H2O

It was noted that excess aluminum trichloride is needed to solubilize the product.

(Will incorporate my writing here to the current published page)

Properties and Characterization
Bis(benzene)chromium is thermally stable under inert gas atmosphere. As predicted, it is diamagnetic with a dipole moment of zero. In 1956, Fischer and Weiss reported the crystal structure of bis(benzene)chromium to be centrosymmetric and has a cubic symmetry. Electrochemical studies of bis(benzene)chromium suggested that the half-wave potential (E1/2) of +1/0 couple is around -1.10 to -1.25 V versus Fc+/Fc at 298.15K, depending on the experimental conditions.

Bondings and Electronic Structures
Theoretical chemical bondings of bis(benzene)chromium have been investigated since the discovery of this compound. The ground state configuration is (3e2g)4(4a1g)2 (3e2u)0. Analysis of the frontier orbitals suggested that the chromium-benzene interaction is largely contributed by the 𝝅 and/or 𝞭 interactions between the 3d metal orbitals and ligand 𝝅 orbitals. 3e2g (HOMO-1) and 3e1g (HOMO-2) molecular orbitals are 𝞭-bonding interactions between metal 3d𝞭 and ligand 𝝅 orbitals. The highest occupied molecular orbital (HOMO), 4a1g, is the non-bonding metal dz2 orbitals. The lowest unoccupied molecular orbital (LUMO) is 3e2u, which is purely ligand 𝝅 orbital. As for 4e1g (LUMO+1) and 4e2g (LUMO+2), they are composed of anti-bonding interaction between 3d𝝅 and ligand 𝝅 orbitals. In contrast to ferrocene, where 𝝅-interactions dominate the metal-ligand bonds, 𝞭-interactions play a significant role in bis(benzene)chromium. 3d orbitals population of chromium(0) in bis(benzene)chromium was investigated, utilizing NBO analysis. While e2g is largely resulted from electron donation from the metal to the ligand, e1g is mainly composed of the electrons donated from the benzene ligands.

Reactivities
Applications of bis(benzene)chromium are relatively scarce. In late 1990s, Samuel and coworkers revealed that bis(benzene)chromium is an efficient organometallic radical scavenger. In contrast to cobaltocene, which trap radicals (R．) to form 19-valence electron species (η5-C5H5)(η4-C5H5R)Co, bis(benzene)chromium reacts with radicals to form 17-valence electron species  (η6-C6H6)(η5-C6H6R)Cr (R = H, D, isobutyronitrile).

Subsequently, Bis(benzene)chromium was reported to catalyze hydrosilation of alcohols and aldehydes. Unlike late transition metal catlyazed processes involving oxidative addition, the mechanism of this reaction might involve radicals and hydrogen atom abstraction.