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Bis(benzene)chromium
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<h2 id="historical-background">Historical background</h2> <p>In the late 1910s, <a href="/facts/Franz_Hein/IVssKuAT">Franz Hein</a> started the investigation of "triphenylchromium" by reacting <a href="/facts/Chromium(III)_chloride/1GwXxIFm">chromium trichloride</a> with a <a href="/facts/Grignard_reagent/0vkLtPo1">Grignard reagent</a>, phenyl magnesium bromide. Such a reaction gave a mixture of phenyl chromium and Hein suggested that it contained a Cr(VI) species, "(C6H5)5CrBr", generated via valence <a href="/facts/Disproportionation/67QMkXrV">disproportionation</a>.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a><a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> </p> 5 C6H5MgBr + 4 CrCl3 → (C6H5)5CrBr + 2 MgBr2 + 3 MgCl2 + 3 CrCl2 <p>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.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Later scrutinization by Zeiss and Tsutsui revealed that Hein's formulation of the chromium-containing products was flawed.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h2 id="preparation">Preparation</h2> <p>The actual discovery of bis(benzene)chromium was largely contributed by <a href="/facts/Ernst_Otto_Fischer/v8zCR6Ke">Ernst Otto Fischer</a> and Walter Hafner in the 1950s. Ernst Otto Fischer postulated that it might be possible to synthesize a neutral chromium(0) complex with two benzene ligands, which has a sandwich structure, similar to that of <a href="/facts/Ferrocene/jKtu9Cp8">ferrocene</a>. In 1954, Walter Hafner, a PhD student of Ernst Otto Fischer at the time, put the idea into practice. A reaction of <a href="/facts/Chromium(III)_chloride/1GwXxIFm">chromium trichloride</a>, <a href="/facts/Aluminium_chloride/xrhvSxDr">aluminium trichloride</a>, <a href="/facts/Aluminium/7wmR8jYE">aluminium</a> powder in <a href="/facts/M-Xylene/R18IDFIS">m-xylene</a> resulted in the formation of yellow [Cr(C6H6)2]+, which was then reduced by <a href="/facts/Sodium_dithionite/wNoKv995">sodium dithionite</a> in aqueous <a href="/facts/Sodium_hydroxide/xjtdYtq7">sodium hydroxide</a>. The resulting solid was determined to be the target, bis(benzene)chromium.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> 6 C6H6 + 3 CrCl3 + 2 Al + <i>x</i> AlCl3 → 3 [(C6H6)2Cr][AlCl4]·(<i>x</i>−1)AlCl3 2 [(C6H6)2Cr]+ + S2O2−4 + 4 OH− → 2 (C6H6)2Cr + 2 SO2−3 + 2 H2O <p>It was noted that excess aluminium trichloride is needed to solubilize the product.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> The substance is air sensitive and its synthesis requires <a href="/facts/Air-free_technique/pEW9wY6e">air-free techniques</a>. The reaction, utilizing Al and AlCl3, is so-called the reductive Friedel-Crafts method pioneered by <a href="/facts/Ernst_Otto_Fischer/v8zCR6Ke">Fischer</a> and his students.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>Fischer and Seus soon prepared Hein's [Cr(C6H5−C6H5)2]+ by an unambiguous route, thus confirming that Hein had unknowingly discovered sandwich complexes, a half-century ahead of the work on <a href="/facts/Ferrocene/jKtu9Cp8">ferrocene</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a><a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> Illustrating the rapid pace of this research, the same issue of <a href="/facts/Chemische_Berichte/u7yEqybf"><i>Chem. Ber.</i></a> also describes the Mo(0) complex.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p><p>Using the technique of <a href="/facts/Metal_vapor_synthesis/dGzRfp2Q">metal vapor synthesis</a>, bis(benzene)chromium and many analogous compounds can be prepared by co-condensation of Cr vapor and arenes. In this way, the <a href="/facts/Phosphabenzene/YSwlmt30">phosphabenzene</a> complex [Cr(C5H5P)2] can be prepared.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h2 id="properties-and-characterization">Properties and characterization</h2> <p>Bis(benzene)chromium is thermally stable under an <a href="/facts/Inert_gas/3EkBEoYh">inert gas</a> atmosphere. As predicted, it is <a href="/facts/Diamagnetism/dgFavn9K">diamagnetic</a> with a <a href="/facts/Magnetic_dipole_moment/VpAQKJtr">dipole moment</a> of zero. In 1956, Fischer and Weiss reported the crystal structure of bis(benzene)chromium to be <a href="/facts/Centrosymmetry/nIFJqujU">centrosymmetric</a> and has a cubic symmetry.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> Electrochemical studies of bis(benzene)chromium suggested that the half-wave potential (E1/2) of the +1/0 couple is around -1.10 to -1.25 V versus Fc+/Fc at 298.15K, depending on the experimental conditions.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p> <h2 id="bonding-and-electronic-structure">Bonding and electronic structure</h2> <p>Theoretical chemical bonding 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 <a href="/facts/Frontier_orbital/XyofpH0p">frontier orbitals</a> suggested that the chromium-benzene interaction is largely contributed by the <a href="/facts/Pi_bond/3Q6LAGYK">𝝅</a> and/or <a href="/facts/Delta_bond/aQeulDez">𝞭 interactions</a> between the 3d metal orbitals and ligand 𝝅 orbitals.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> 3e2g (HOMO-1) and 3e1g (HOMO-2) molecular orbitals are 𝞭-bonding interactions between metal 3d𝞭 and ligand 𝝅 orbitals. The <a href="/facts/HOMO_and_LUMO/XyofpH0p">highest occupied molecular orbital</a> (HOMO), 4a1g, is the non-bonding metal dz2 orbital. The <a href="/facts/HOMO_and_LUMO/XyofpH0p">lowest unoccupied molecular orbital</a> (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. </p> Charge Distribution (%) of Ground State Bis(benzene)chromium<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><table><tbody><tr><th></th><th></th><th colspan="3">Cr</th><th colspan="3">C</th><th>H</th></tr><tr><th>Molecular Orbital</th><th>No. of Electrons</th><th>s</th><th>p</th><th>d</th><th>s</th><th>p𝝈</th><th>p𝝅</th><th>s</th></tr><tr><td>4e1g</td><td>0</td><td></td><td></td><td>73</td><td>2</td><td>2</td><td>10</td><td></td></tr><tr><td>3e2u</td><td>0</td><td></td><td></td><td></td><td></td><td></td><td>73</td><td></td></tr><tr><td>4a1g</td><td>2</td><td>1</td><td></td><td>82</td><td></td><td>1</td><td>1</td><td>1</td></tr><tr><td>3e2g</td><td>4</td><td></td><td></td><td>50</td><td></td><td>2</td><td>21</td><td></td></tr><tr><td>3e1g</td><td>4</td><td></td><td></td><td>16</td><td></td><td>1</td><td>52</td><td></td></tr><tr><td>4e1u</td><td>4</td><td></td><td>4</td><td></td><td></td><td></td><td>49</td><td></td></tr><tr><td>2e2u</td><td>4</td><td></td><td></td><td></td><td></td><td>60</td><td></td><td>17</td></tr><tr><td>2e2g</td><td>4</td><td></td><td></td><td>1</td><td></td><td>60</td><td></td><td>18</td></tr><tr><td>4a2u</td><td>2</td><td></td><td>4</td><td></td><td></td><td></td><td>61</td><td></td></tr><tr><td>3a1g</td><td>2</td><td>8</td><td></td><td></td><td></td><td></td><td>53</td><td></td></tr></tbody></table> <p>3d orbitals population of chromium(0) in bis(benzene)chromium was investigated, utilizing <a href="/facts/Natural_bond_orbital/fmU1d69B">NBO</a> analysis. While e2g largely results from electron donation from the metal to the ligand, e1g is mainly composed of the electrons donated from the benzene ligands.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> </p> 3d orbital population of Cr in Bis(benzene)chromium<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a><table><tbody><tr><th>Molecular Orbitals</th><th>NBO</th><th>X-ray</th></tr><tr><td>4a1g</td><td>1.896/36%</td><td>1.62(1)/35%</td></tr><tr><td>3e2g</td><td>2.412/45%</td><td>1.953(7)/42%</td></tr><tr><td>3e1g</td><td>1.026/19%</td><td>1.112(7)/24%</td></tr></tbody></table> <p>In contrast to ferrocene, where 𝝅-interactions dominate the metal-ligand bonds, 𝞭-interactions play a significant role in bis(benzene)chromium.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a><a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p> Energy Decomposition Analysis at BP86/TZP of Ferrocene and Bisbenzenechromium<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><table><tbody><tr><th colspan="2"></th><th>Ferrocene</th><th>Bis(benzene)chromium</th></tr><tr><td colspan="2">Electrostatic Interaction %</td><td>51.1</td><td>37.9</td></tr><tr><td colspan="2">Covalent Interaction %</td><td>48.9</td><td>62.1</td></tr><tr><td rowspan="2">Contribution to Covalent Interaction %</td><td>𝝅-interactions (e1g)</td><td>64.7</td><td>14.7</td></tr><tr><td>𝞭-interactions (e2g)</td><td>8.3</td><td>73.4</td></tr></tbody></table> <h2 id="reactivity">Reactivity</h2> <p>The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as <a href="/facts/Chromium(II)_acetate/vUCPTqW1">chromium(II) acetate</a>. Oxidation affords [Cr(C6H6)2]+. Carbonylation gives <a href="/facts/(Benzene)chromium_tricarbonyl/PG0n6F7r">(benzene)chromium tricarbonyl</a>. </p><p>In late 1990s, Samuel and coworkers revealed that bis(benzene)chromium is an efficient organometallic <a href="/facts/Scavenger_(chemistry)/DqKCmI8Q">radical scavenger</a>. In contrast to <a href="/facts/Cobaltocene/2aELFlSc">cobaltocene</a>, which traps 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, <a href="/facts/Isobutyronitrile/WKlgsI7U">isobutyronitrile</a>).<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p> <p>Subsequently, Bis(benzene)chromium was reported to catalyze hydrosilation of alcohols and aldehydes. Unlike late transition metal catalyzed processes involving <a href="/facts/Oxidative_addition/kPNIb4LB">oxidative addition</a>, the mechanism of this reaction might involve radicals and <a href="/facts/Hydrogen_atom_abstraction/mqaVtjJA">hydrogen atom abstraction</a>.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> </p> <p>The compound finds limited use in <a href="/facts/Organic_synthesis/aprl6Iq7">organic synthesis</a>.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>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. <a href="https://doi.org/10.1021%2Fom0201056" target="_blank">https://doi.org/10.1021%2Fom0201056</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>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. <a href="https://dx.doi.org/10.1002/cber.19210540821" target="_blank">https://dx.doi.org/10.1002/cber.19210540821</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>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. <a href="https://doi.org/10.1021%2Fom0201056" target="_blank">https://doi.org/10.1021%2Fom0201056</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>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. <a href="https://dx.doi.org/10.1002/cber.19210540821" target="_blank">https://dx.doi.org/10.1002/cber.19210540821</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>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. <a href="https://doi.org/10.1021%2Fom0201056" target="_blank">https://doi.org/10.1021%2Fom0201056</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>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. <a href="https://doi.org/10.1021%2Fom020362a" target="_blank">https://doi.org/10.1021%2Fom020362a</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>FISCHER, HAFNER, Von E. O., W. (1955). "Di-benzol-chrom" (PDF). Zeitschrift für Naturforschung B. 10 (12): 665–668. doi:10.1515/znb-1955-1201. S2CID 209642269.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="https://zfn.mpdl.mpg.de/data/Reihe_B/10/ZNB-1955-10b-0665.pdf" target="_blank">https://zfn.mpdl.mpg.de/data/Reihe_B/10/ZNB-1955-10b-0665.pdf</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>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. <a href="https://doi.org/10.1021%2Fom020362a" target="_blank">https://doi.org/10.1021%2Fom020362a</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>King, R. B. Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN 0-444-42607-8 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Elschenbroich, C.; Salzer, A. "Organometallics : A Concise Introduction" (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN 3-527-28165-7 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Fischer, E. O; Seus, D. (1956). "Zur Frage der Struktur der Chrom-phenyl-Verbindungen. Über Aromatenkomplexe von Metallen VI". Chemische Berichte. 89 (8): 1809–1815. doi:10.1002/cber.19560890803. <a href="/wiki/Chemische_Berichte" target="_blank">/wiki/Chemische_Berichte</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Hein, F. (1956). "Zur Frage der Struktur der Chrom-phenyl-Verbindungen. Bemerkungen zur Abhandlung von E. O. Fischer und D. Seus". Chemische Berichte. 89 (8): 1816–1821. doi:10.1002/cber.19560890804. <a href="/wiki/Chemische_Berichte" target="_blank">/wiki/Chemische_Berichte</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Fischer, E. O.; Stahl, H.-O. (1956). "Di-benzol-molybdän (O). Über Aromatenkomplexe von Metallen V". Chemische Berichte. 89 (8): 1805–1808. doi:10.1002/cber.19560890802. <a href="/wiki/Chemische_Berichte" target="_blank">/wiki/Chemische_Berichte</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>E. Schmidt, K. J. Klabunde, A. Ponce, A. Smetana, D. Heroux "Metal Vapor Synthesis of Transition Metal Compounds" Encyclopedia of Inorganic Chemistry 2006, John Wiley & Sons. doi:10.1002/0470862106.ia137 <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Weiss, E.; Fischer, E. O. (July 1956). "Über Aromatenkomplexe von Metallen. II. Zur Kristallstruktur und Molekelgestalt des Di-benzol-chrom(0)". Zeitschrift für anorganische und allgemeine Chemie. 286 (3–4): 142–145. doi:10.1002/zaac.19562860305. ISSN 0044-2313. <a href="https://dx.doi.org/10.1002/zaac.19562860305" target="_blank">https://dx.doi.org/10.1002/zaac.19562860305</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Tsierkezos, Nikos G. (August 2007). "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. <a href="https://dx.doi.org/10.1016/j.ica.2007.05.008" target="_blank">https://dx.doi.org/10.1016/j.ica.2007.05.008</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>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. S2CID 907422. <a href="https://doi.org/10.1351%2Fpac198456040461" target="_blank">https://doi.org/10.1351%2Fpac198456040461</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Treichel, P.M; Essenmacher, G.P; Efner, H.F; Klabunde, K.J (January 1981). "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. <a href="https://dx.doi.org/10.1016/s0020-1693(00)90063-x" target="_blank">https://dx.doi.org/10.1016/s0020-1693(00)90063-x</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>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. 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