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User:LW14061407/Bis(benzene)chromium

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Synthesis and Characterization

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Discovery

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In late 1910s, Franz Hein started the investigation of "triphenylchromium" by reacting chromium trichloride 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.[1][2]

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. [1][2] Later scrutinization by Zeiss and Tsutsui revealed that Hein's formulation of the chromium-containing products was flawed.[1]

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.[3][4]

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.[3]

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

Properties and Characterization

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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.[5] 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. [6][7][8]

Bondings and Electronic Structures

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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. [9][10] 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.[9][10]

 
Molecular orbitals of bis(benzene)chromium, visualized by IboView. (Top to bottom: LUMO+2, LUMO+1, LUMO, HOMO, HOMO-1, HOMO-2)
Charge Distribution (%) of Ground State Bis(benzene)chromium [10]
Cr C H
Molecular Orbital No. of Electrons s p d s p𝝈 p𝝅 s
4e1g 0 73 2 2 10
3e2u 0 73
4a1g 2 1 82 1 1 1
3e2g 4 50 2 21
3e1g 4 16 1 52
4e1u 4 4 49
2e2u 4 60 17
2e2g 4 1 60 18
4a2u 2 4 61
3a1g 2 8 53

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.[11]

3d orbital population of Cr in Bis(benzene)chromium[11]
Molecular Orbitals NBO X-ray
4a1g 1.896/36% 1.62(1)/35%
3e2g 2.412/45% 1.953(7)/42%
3e1g 1.026/19% 1.112(7)/24%

Reactivities

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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).[12]

 
Spin trapping by bis(benzene)chromium (R = H, D, isobutyronitrile)[12]

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. [13]

 
Hydrosilation of ketones catalyzed by bis(benzene)chromium[13]

yReferences

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  1. ^ a b c Seyferth, Dietmar (2002-04-01). "Bis(benzene)chromium. 1. Franz Hein at the University of Leipzig and Harold Zeiss and Minoru Tsutsui at Yale". Organometallics. 21 (8): 1520–1530. doi:10.1021/om0201056. ISSN 0276-7333.
  2. ^ a b Hein, Franz (1921-09-17). "Chromorganische Verbindungen, I. Mitteilung: Pentaphenyl‐chromhydroxyd". Berichte der deutschen chemischen Gesellschaft (A and B Series). 54 (8): 1905–1938. doi:10.1002/cber.19210540821. ISSN 0365-9488.
  3. ^ a b Seyferth, Dietmar (2002-07-01). "Bis(benzene)chromium. 2. Its Discovery by E. O. Fischer and W. Hafner and Subsequent Work by the Research Groups of E. O. Fischer, H. H. Zeiss, F. Hein, C. Elschenbroich, and Others". Organometallics. 21 (14): 2800–2820. doi:10.1021/om020362a. ISSN 0276-7333.
  4. ^ FISCHER, HAFNER, Von E. O., W. (1955). "Di-benzol-chrom Über Aromatenkomplexe von Metallen I" (PDF). Z. Naturforschg. 10b: 665.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Weiss, E.; Fischer, E. O. (1956-07). [http://dx.doi.org/10.1002/zaac.19562860305 "�ber Aromatenkomplexe von Metallen. II. Zur Kristallstruktur und Molekelgestalt des Di-benzol-chrom(0)"]. Zeitschrift f�r anorganische und allgemeine Chemie. 286 (3–4): 142–145. doi:10.1002/zaac.19562860305. ISSN 0044-2313. {{cite journal}}: Check date values in: |date= (help); replacement character in |journal= at position 14 (help); replacement character in |title= at position 1 (help)
  6. ^ Tsierkezos, Nikos G. (2007-08). "Investigation of electron-transfer kinetics for bis(benzene) chromium(1+/0) redox couple in acetonitrile/dichloromethane binary mixtures at 298.15K". Inorganica Chimica Acta. 360 (11): 3626–3632. doi:10.1016/j.ica.2007.05.008. ISSN 0020-1693. {{cite journal}}: Check date values in: |date= (help)
  7. ^ Gritzner, G.; Kuta, J. (1984-01-01). "Recommendations on reporting electrode potentials in nonaqueous solvents (Recommendations 1983)". Pure and Applied Chemistry. 56 (4): 461–466. doi:10.1351/pac198456040461. ISSN 1365-3075.
  8. ^ Treichel, P.M; Essenmacher, G.P; Efner, H.F; Klabunde, K.J (1981-01). "An electrochemical study of chromium(0) complexes of arenes which have electronegative substituent groups". Inorganica Chimica Acta. 48: 41–44. doi:10.1016/s0020-1693(00)90063-x. ISSN 0020-1693. {{cite journal}}: Check date values in: |date= (help)
  9. ^ a b Rayón, Víctor M.; Frenking, Gernot (2003-07-04). "Bis(benzene)chromium Is a δ-Bonded Molecule and Ferrocene Is a π-Bonded Molecule". Organometallics. 22 (16): 3304–3308. doi:10.1021/om020968z. ISSN 0276-7333.
  10. ^ a b c Weber, Jaques; Kundig, E. Peter; Goursot, Annick; Penigault, Edouard (1985-07-01). "The electronic structures of bis(η 6 -benzene)- and bis(η 6 -naphthalene)chromium(0)". Canadian Journal of Chemistry. 63 (7): 1734–1740. doi:10.1139/v85-291. ISSN 0008-4042.
  11. ^ a b Lyssenko, Konstantin A.; Korlyukov, Alexander A.; Golovanov, Denis G.; Ketkov, Sergey Yu.; Antipin, Mikhail Yu. (2006-05-01). "Estimation of the Barrier to Rotation of Benzene in the (η 6 -C 6 H 6 ) 2 Cr Crystal via Topological Analysis of the Electron Density Distribution Function". The Journal of Physical Chemistry A. 110 (20): 6545–6551. doi:10.1021/jp057516v. ISSN 1089-5639.
  12. ^ a b Samuel, Caurant, Gourier, Elschenbroich, Agbaria, E., D., D., Ch., K. (1998). "Bis(benzene)chromium. A Sandwich Complex Spin Trap As Revealed by ENDOR Spectroscopy". Journal of the American Chemical Society (120): 8088–8092.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ a b Bideau, Franck Le; Henique, Josette; Samuel, Edmond; Elschenbroich, Ch. (1999). "Bis(benzene)chromium: a pre-catalyst for the hydrosilation of ketones and aldehydes, and for the dehydrocoupling of triphenylsilane with primary alcohols†". Chemical Communications (15): 1397–1398. doi:10.1039/a901234f. ISSN 1359-7345.
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