Half sandwich compound

{{Short description|Class of coordination compounds}} '''Half sandwich compounds''', also known as '''piano stool complexes''', are organometallic complexes that feature a cyclic polyhapto ligand bound to an ML_n center, where L is a unidentate ligand. Thousands of such complexes are known.<ref>{{Cite book|last=Elschenbroich|first=Christoph|url=https://books.google.com/books?id=yUJCAQAAIAAJ|title=Organometallics|date=2006-03-10|publisher=Wiley|isbn=978-3-527-29390-2|language=en|oclc=1004583759}}</ref>{{page needed|date=October 2019}} Well-known examples include cyclobutadieneiron tricarbonyl and (C₅H₅)TiCl₃. Commercially useful examples include (C₅H₅)Co(CO)₂, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.

<gallery> MMT-2D-skeletal.png|MMT is a commercially useful antiknock compound. Cpco(CO)2.png|CpCo(CO)₂ is a catalyst for the synthesis of pyridines. Cyclobutadiene-iron-tricarbonyl-from-xtal-3D-balls.png|(C₄H₄)Fe(CO)₃. Cp2Fe(CO)2I-2D-skeletal.png|CpFe(CO)₂I is an example of a piano stool complex with two different monodentate ligands. RuCymCl2.png|The diruthenium of cymene is readily cleaved by ligands to give monoRu half-sandwich derivatives. CHTMo(CO)3.png|Cycloheptatriene molybdenum tricarbonyl CPPCDV01.png|Cp₂V₂(CO)₅ featuring a pair of semi-bridging CO ligands.<ref>{{cite journal|last1=Huffman |first1=J. C. |last2=Lewis |first2=L. N. |last3=Caulton |first3=K. G.|title=A donor semibridge? Molecular structures of dicyclopentadienyldivanadiumtetracarbonyltriphenylphosphine and dicyclopentadienyldivanadiumpentacarbonyl|journal=Inorganic Chemistry|year=1980|volume=19|issue=9|pages=2755–2762|doi=10.1021/ic50211a052}}</ref> </gallery>

==(''η''⁵-C₅H₅) piano stool compounds== Half sandwich complexes containing cyclopentadienyl ligands are common. Well studied examples include (''η''⁵-C₅H₅)V(CO)₄, (''η''⁵-C₅H₅)Cr(CO)₃H, (''η''⁵-CH₃C₅H₄)Mn(CO)₃, (''η''⁵-C₅H₅)Cr(CO)₃H, [(''η''⁵-C₅H₅)Fe(CO)₃]⁺, (''η''⁵-C₅H₅)V(CO)₄I, and (''η''⁵-C₅H₅)Ru(NCMe){{su|b=3|p=+}}. (''η''⁵-C₅H₅)Co(CO)₂ is a two-legged piano stool complex. Bulky cyclopentadienyl ligands such as 1,2,4-C₅H₂(''tert''-Bu)₃⁻ form unusual half-sandwich complexes.<ref name=Walter>{{cite book|doi=10.1002/9781119477822.ch8|title=Inorganic Syntheses|year=2018|last1=Reiners|first1=Matthias|last2=Ehrlich|first2=Nico|last3=Walter|first3=Marc D.|chapter=Synthesis of Selected Transition Metal and Main Group Compounds with Synthetic Applications |volume=37|page=199|isbn=978-1-119-47782-2 |s2cid=105376454}}</ref>

==(''η''⁶-C₆H₆) piano stool compounds== thumb|200px|right In organometallic chemistry, (''η''⁶-C₆H₆) piano stool compounds are half-sandwich compounds with (''η''⁶-C₆H₆)ML₃ structure (M = Cr, Mo, W, Mn(I), Re(I) and L = typically CO). (''η''⁶-C₆H₆) piano stool complexes are stable 18-electron coordination compounds with a variety of chemical and material applications. Early studies on (''η''⁶-C₆H₆)Cr(CO)₃ were carried out by Natta, Ercoli and Calderazzo,<ref name="Synthesis of bzCr(CO)3">{{cite journal|title=(''η''-C₆H₆)Cr(CO)₃ |last1=Natta|first1=G.|first2=R. |last2=Ercoli |first3=Calderazzo |last3=F. |journal=Chimica e Industria|year=1958|volume=40|pages=1003}}</ref> and Fischer and Ofele,<ref name="Fischer Ofele paper">{{cite journal|last1=Fischer|first1=E. O.|first2=K. |last2=Ofele |first3=H. |last3=Essler |first4=W. |last4=Frohlich |first5=J. P. |last5=Mortensen |first6=W. |last6=Semmlinger |journal=Chemische Berichte|year=1958|volume=91|issue=12|pages=2763–2772| doi = 10.1002/cber.19580911231 |title=Über Aromatenkomplexe von Metallen. XXIV. Über gemischte Tricarbonylkomplexe des Chroms, Molybdäns und Wolframs mit Benzol und seinen Derivaten |trans-title=On aromatic complexes of metals. 24. On mixed tricarbonyl complexes of chromium, molybdenum and tungsten with benzene and its derivatives}}</ref><ref name="Fischer paper">{{cite journal|last1=Fischer|first1=E. O.|last2=Ofele|first2=K.|title=Über Aromatenkomplexe von Metallen. XIII. Benzol-Chrom-Tricarbonyl |trans-title=On aromatic complexes of metals. 13. Benzene chromium tricarbonyl|journal=Chemische Berichte|year=1957|volume=90|issue=11|pages=2532–2535|doi=10.1002/cber.19570901117}}</ref> and the crystal structure was determined by Corradini and Allegra in 1959.<ref name="xstal structure of benzenechromiumtricarbonyl">{{cite journal|last1=Corradini|first1=P.|first2=G. |last2=Allegra|title=X-ray determination of the structure of tricarbonylchromium-benzene|journal=Journal of the American Chemical Society|year=1959|volume=81|issue=9|pages=2271–2272|doi=10.1021/ja01518a065}}</ref> The X-ray data indicate that the plane of the benzene ring is nearly parallel to the plane defined by the oxygen atoms of the carbonyl ligands, and so the structure resembles a benzene seat mounted on three carbonyl legs tethered by the metal atom.

===Cr and Mn(I) (''η''⁶-C₆H₆) piano stool complexes=== Piano stool complexes of the type (''η''⁶-C₆H₆)M(CO)₃ are typically synthesized by heating the appropriate metal carbonyl compound with benzene. Alternately, the same compounds can be obtained by carbonylation of the bis(arene) sandwich compounds, such as (''η''⁶-C₆H₆)₂M compound with the metal carbonyl compound. This second approach may be more appropriate for arene ligands containing thermally fragile substituents.<ref name="Organotransition metal chemistry">{{cite book|last=Hartwig|first=John | authorlink = John F. Hartwig |title=Organotransition Metal Chemistry|year=2010|publisher=University Science Books|location=Sausalito|isbn=978-1-891389-53-5|pages=443}}</ref>

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====Reactivity of (''η''⁶-C₆H₆)Cr(CO)₃==== The benzene ligand in (''η''⁶-C₆H₆)Cr(CO)₃ is prone to deprotonation.<ref name="Organometallic chem of trans metals">{{cite book|last=Crabtree|first=R.|title=The Organometallic Chemistry of Transition Metals |edition=5th|year=2009|publisher=John Wiley & Sons|location=Hoboken, NJ|isbn=978-0-470-25762-3|pages=145}}</ref> For example, Organolithium compounds form adducts featuring cyclohexadienyl ligands. Subsequent oxidation of the complex results in the release of a substituted benzene.<ref name=Astruc>{{cite book|last=A.|first=Didier|title=Organometallic Chemistry and Catalysis|year=2007|publisher=Springer-Verlag|location=Berlin|isbn=978-3-540-46128-9|pages=243–246}}</ref><ref>{{cite encyclopedia|last1=Herndon |first1=J. W. |last2=Laurent |first2=S. E. |title=(''η''⁶-Benzene)tricarbonylchromium |encyclopedia=Encyclopedia of Reagents for Organic Synthesis |publisher=John Wiley & Sons |location=Chichester |date=2008 |doi=10.1002/047084289X.rb025.pub2|isbn=978-0471936237 }}</ref> Oxidation of the chromium atom by I₂ and other iodine reagents has been shown to promote exchange of arene ligands, but the intermediate chromium iodide species has not been characterized.<ref name="Iodine-catalyzed arene exchange of (arene)tricarbonyl chromium(0) complexes">{{cite journal|last=Harrison|first=J. J.|title=Iodine-catalyzed arene exchange of (arene)tricarbonyl chromium(0) complexes|journal=Journal of the American Chemical Society|year=1984|volume=106|issue=5|pages=1487–1489|doi=10.1021/ja00317a052}}</ref>

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(''η''⁶-C₆H₆)Cr(CO)₃ complexes exhibit "''cine''" and "''tele''" nucleophilic aromatic addition.<ref name="Nucleophilic aromatic substitutions: hydrodealkoxylation, hydrodehalogenation, and hydrodeamination of alkoxy, halogeno, and amino (eta6-arene)tricarbonylchromium complexes">{{cite journal|last1=Djukic|first1=J.-P. |first2=F. |last2=Rose-Munch |first3=E. |last3=Rose |first4=F. |last4=Simon |first5=Y. |last5=Dromzee|title=Nucleophilic aromatic substitutions: hydrodealkoxylation, hydrodehalogenation, and hydrodeamination of alkoxy, halogeno, and amino (''η''⁶-arene)tricarbonylchromium complexes|journal=Organometallics|year=1995|volume=14|issue=4 |pages=2027–2038|doi=10.1021/om00004a065}}</ref> Processes of this type involve reaction of (''η''⁶-C₆H₆)Cr(CO)₃ with an alkyl lithium reagent. Subsequent treatment with an acid results in the addition of a nucleophile to the benzene ring at a site ''ortho'' ("''cine''"), ''meta'' or ''para'' ("''tele''") to the ''ipso'' carbon (see Arene substitution patterns).

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Reflecting its increased acidity, the benzene ligand can be lithiated with ''n''-butyllithium. The resulting organolithium compound serves as a nucleophile in various reactions, for example, with trimethylsilyl chloride:{{citation needed|date=October 2019}}

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(''η''⁶-C₆H₆)Cr(CO)₃ is a useful catalyst for the hydrogenation of 1,3-dienes. The product alkene results from 1,4-addition of hydrogen. The complex does not hydrogenate isolated double bonds.{{citation needed|date=October 2019}}

A variety of arenes ligands have been installed aside from benzene.<ref name="Photochemistry of (eta6-arene)Cr(CO)3">{{cite journal|last1=Clark|first1=I. P. |first2=M. W. |last2=George |first3=G. M. |last3=Greetham |first4=E. C. |last4=Harvey |first5=C. |last5=Long |first6=J. C. |last6=Manton |first7=M. T. |last7=Pryce|title=Photochemistry of (''η''⁶-arene)Cr(CO)₃ (arene = methylbenzoate, naphthalene, or phenanthreen) in ''n''-heptane solution: Population of two excited states following 400 nm excitation as detected by picosecond time-resolved infrared spectroscopy|journal= Journal of Physical Chemistry A|year=2011|volume=115|issue=14 |pages=2985–2993|doi=10.1021/jp112168u|pmid=21413775 |bibcode=2011JPCA..115.2985C}}</ref> Weakly coordinating ligands may be employed to improve ligand exchange and thus the turnover rates for (''η''⁶-C₆H₆)M(CO)₃ complexes.<ref name="Organotransition metal chemistry"/>{{rp|248}}(''η''⁶-C₆H₆)M(CO)₃ complexes have been incorporated into high surface area porous materials.<ref name="Piano Stool porous materials">{{cite journal|last1=Kamegawa|first1=T. |first2=M. |last2=Saito |first3=T. |last3=Sakai |first4=M. |last4=Matsuoka |first5=M. |last5=Anpo|title=Characterization of phenylene-bridged hybrid mesoporous materials incorporating arenetricarbonyl complexes (-C₆H₄Me(CO)₃-; Me = Cr, Mo) and their catalytic activities|journal=Catalysis Today|year=2012|volume=181|issue=1|pages=14–19|doi=10.1016/j.cattod.2011.10.019}}</ref>

(''η''⁶-C₆H₆)M(CO)₃ complexes serve as models for the interaction of metal carbonyls with graphene and carbon nanotubes.<ref name="TM benzene complexes">{{cite journal|last=Duncan|first=M. A.|title=Structures, energetics and spectroscopy of gas phase transition metal ion-benzene complexes|journal=International Journal of Mass Spectrometry|year=2008|volume=272|issue=2–3|pages=99–118|doi=10.1016/j.ijms.2008.01.010|bibcode=2008IJMSp.272...99D}}</ref> The presence of M(CO)₃ on extended π-network materials has been shown to improve electrical conductivity across the material.<ref name="Piano stools on SWNTs">{{cite journal|last1=Kalinina|first1=Irina|first2=E. |last2=Bekyarova |first3=S. |last3=Sarkar |first4=F. |last4=Wang |first5=M. |last5=Itkis |first6=X. |last6=Tian |first7=S. |last7=Niyogi |first8=N. |last8=Jha |first9=R. C. |last9=Haddon |title=Hexahapto-metal carbonyl complexes of single walled carbon nanotubes|journal=Macromolecular Chemistry and Physics|year=2012|volume=213|issue=3–4|pages=1001–1019|doi=10.1016/j.ccr.2008.04.014}}</ref>

====Reactivity of [(''η''⁶-C₆H₆)Mn(CO)₃]⁺==== Typical arene tricarbonyl piano stool complexes of Mn(I) and Re(I) are cationic and thus exhibit enhanced reactivity toward nucleophiles. Subsequent to nucleophilic addition, the modified arene can be recovered from the metal.<ref name="Patterns of nucleophilic attack">{{cite journal|last1=Walker|first1=P. J. C.|first2=R. J. |last2=Mawby|title=Patterns of nucleophilic attack on tricarbonyl pi-arene complexes of manganese(I)|journal=Inorganica Chimica Acta|year=1973|volume=7|pages=621–625|doi=10.1016/s0020-1693(00)94897-7}}</ref><ref name="lithium dimethyl cuprate with C6H6Mn(CO)3">{{cite journal|last1=Brookhart|first1=M. |first2=A. R. |last2=Pinhas |first3=A. |last3=Lukacs|title=Reaction of lithium dimethyl cuprate with C₆H₆Mn(CO)₃. Observation of methyl group migration from manganese to arene ring in C₆H₆(CO)₂MnMe|journal=Organometallics|year=1982|volume=1|issue=12|pages=1730–1731|doi=10.1021/om00072a040}}</ref>

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===(''η''⁶-C₆H₆)Ru complexes=== Half-sandwich compounds employing Ru(II), such as (cymene)ruthenium dichloride dimer, have been mainly investigated as catalysts for transfer hydrogenation.<ref>{{cite journal|first1=T. |last1=Ikariya |first2=A. J. |last2=Blacker |title=Asymmetric Transfer Hydrogenation of Ketones with Bifunctional Transition Metal-Based Molecular Catalysts |journal=Accounts of Chemical Research |date=2007 |volume=40 |issue=12 |pages=1300–1308 |doi=10.1021/ar700134q |pmid=17960897}}</ref> These complexes feature three coordination sites that are susceptible to substitution, while the arene ligand is tightly bonded and protects the metal against oxidation to Ru(III). They are prepared by reaction of RuCl₃·''x''(H₂O) with 1,3-cyclohexadienes.<ref>{{cite book|last1=Bennett |first1=M. A. |last2=Huang |first2=T. N. |last3=Matheson |first3=T. W. |last4=Smith |first4=A. K. |title=(''η''⁶-Hexamethylbenzene)ruthenium Complexes |chapter=16. (η6 -Hexamethylbenzene)Ruthenium Complexes |series=Inorganic Syntheses |date=1982 |volume=21 |pages=74–78 |doi=10.1002/9780470132524.ch16|isbn=9780470132524 }}</ref> Work is also conducted on their potential as anticancer drugs.<ref name="Controlling platinum, ruthenium, and osmium reactivity for anticancer drug design">{{cite journal|last1=Bruijnincx|first1=P. C. A.|first2=P. J.|last2=Sadler|title=Controlling platinum, ruthenium, and osmium reactivity for anticancer drug design|journal=Advances in Inorganic Chemistry|year=2009|volume=61|pages=1–62|doi=10.1016/S0898-8838(09)00201-3|pmid=21258628|pmc=3024542|isbn=9780123750334}}</ref> :500px

(''η''⁶-C₆H₆)RuCl₂ readily undergoes ligand exchange via cleavage of the chloride bridges, making this complex a versatile precursor to Ru(II) piano stool derivatives.<ref name="Functionalised η6-arene ruthenium complexes">{{cite journal|last=Therrien|first=B.|title=Functionalised ''η''⁶-arene ruthenium complexes|journal=Coordination Chemistry Reviews|year=2009|volume=253|issue=3–4|pages=493–519|doi=10.1016/j.ccr.2008.04.014}}</ref>

==References== {{reflist}} {{organometallics}}

Category:Half sandwich compounds Category:Organic compounds Category:Organometallic chemistry Category:Coordination chemistry