Organotantalum chemistry
{{Short description|Chemistry of compounds containing a carbon-to-tantalum bond}}
Organotantalum chemistry is the chemistry of chemical compounds containing a carbon-to-tantalum chemical bond. A wide variety of compound have been reported, initially with cyclopentadienyl and CO ligands. Oxidation states vary from −1 to +5.
Classes of organotantalum compounds
=Alkyl and aryl complexes=
Pentamethyltantalum was reported by Richard Schrock in 1974.
Salts of [Ta(CH3)6]− are prepared by alkylation of TaF5 using methyl lithium:{{cite journal|author1=Kleinhenz, S. |author2=Pfennig, V. |author3=Seppelt, K. |journal=Chem. Eur. J.|year=1998|volume=4|page= 1687|doi=10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R|title=Preparation and Structures of [W(CH3)6], [Re(CH3)6], [Nb(CH3)6]−, and [Ta(CH3)6]−|issue=9}}
:TaF5 + 6 LiCH3 → Li[Ta(CH3)6] + 5 LiF
=Alkylidene complexes=
Tantalum alkylidene complexes arise by treating trialkyltantalum dichloride with alkyl lithium reagents. This reaction initially forms a thermally unstable tetraalkyl-monochloro-tantalum complex, which undergoes α-hydrogen elimination, followed by alkylation of the remaining chloride.{{Cite journal|last=Schrock|first=Richard R.|date=1979-03-01|title=Alkylidene complexes of niobium and tantalum|journal=Accounts of Chemical Research|volume=12|issue=3|pages=98–104|doi=10.1021/ar50135a004|issn=0001-4842}}
File:Synthesis of tantalum alkylidenes 2.tif
Tantalum alkylidene complexes are nucleophilic. They effect a number of reactions including: olefinations, olefin metathesis, hydroaminoalkylation of olefins, and conjugate allylation of enones.
File:Tantalum alkylidene promoted olefination.tif
Ethylene, propylene, and styrene react with tantalum alkylidene complexes to yield olefin metathesis products.{{Cite journal|last1=McLain|first1=S. J.|last2=Wood|first2=C. D.|last3=Schrock|first3=R. R.|date=1977-05-01|title=Multiple metal-carbon bonds. 6. The reaction of niobium and tantalum neopentylidene complexes with simple olefins: a route to metallocyclopentanes|journal=Journal of the American Chemical Society|volume=99|issue=10|pages=3519–3520|doi=10.1021/ja00452a064|issn=0002-7863}}
=Cyclopentadienyl complexes=
Some of the first reported organotantalum complexes were cyclopentadienyl derivatives. These arise from the salt metathesis reactions of sodium cyclopentadienide and tantalum pentachloride. An example of this is the first transition metal trihydride, Cp2TaH3. More soluble and better developed are derivatives of pentamethylcyclopentadiene such as Cp*TaCl4, Cp*2TaCl2, and Cp*2TaH3.{{cite book|journal=Inorg. Synth.|volume=36|author=Endy Y.-J. Min |author2=John E. Bercaw |chapter=Bis(η 5 -Pentamethylcyclopentadienyl) Complexes of Niobium and Tantalum |title=Inorganic Syntheses: Volume 36|pages=52–57|year=2014|doi=10.1002/9781118744994.ch11|isbn=978-1-118-74499-4}}
=Tantalum carbonyls and isocyanides=
Reduction of TaCl5 under an atmosphere of CO gives the salts of [Ta(CO)6]−.{{cite book|title=Tris[Bis(2-Methoxyethyl)Ether]Potassium and Tetraphenylarsonium Hexacarbonylmetallates(1–) of Niobium and Tantalum|volume=16|author=J. E. Ellis |author2=A. Davison |chapter=Tris[Bis(2-Methoxyethyl)Ether]Potassium and Tetraphenylarsonium Hexacarbonylmetallates(1-) of Niobium and Tantalum|pages=68–73|doi=10.1002/9780470132470.ch21|year=1976|series=Inorganic Syntheses|isbn=978-0-470-13247-0}} These same anions can be obtained by carbonylation of tantalum arene complexes.
A number of tantalum isocyanide complexes are also known.{{cite journal |doi=10.1107/S2053229619000834|title=Tantalum isocyanide complexes: TaI(CNDipp)6 (Dipp is 2,6-diisopropylphenyl) and ionic [Ta(CNDipp)7][Ta(CNDipp)6], a formal disproportionation product of the 17-electron Ta0 metalloradical Ta(CNDipp)6|year=2019|last1=Brennessel|first1=William W.|last2=Romanenkov|first2=Alexander|last3=Young|first3=Victor G.|last4=Ellis|first4=John E.|journal=Acta Crystallographica Section C: Structural Chemistry|volume=75|issue=2|pages=135–140|pmid=30720451|s2cid=73450348 }}
= Tantalum arenes and alkyne complexes =
Treatment of tantalum pentachloride with hexamethylbenzene (C6Me6), aluminium, and aluminium trichloride gives [M(η6-C6Me6)AlCl4]2.{{cite journal|author=Pampaloni, G. |title=Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis(η6-arene) complexes|journal=Coordination Chemistry Reviews|year=2010|volume=254|issue=5–6|pages=402–419|doi=10.1016/j.ccr.2009.05.014}}
Tantalum-alkyne complexes{{Cite journal|last1=Labinger|first1=Jay A.|last2=Schwartz|first2=Jeffrey|last3=Townsend|first3=John M.|date=1974-06-01|title=Iodo- and hydridotantalum(III) complexes of dialkylacetylenes|journal=Journal of the American Chemical Society|volume=96|issue=12|pages=4009–4011|doi=10.1021/ja00819a047|issn=0002-7863}} catalyze cyclotrimerizations.{{Cite journal|last1=Cotton|first1=F. Albert|last2=Hall|first2=William T.|date=1979-08-01|title=Reactions of tantalum(III) with alkynes and nitriles|journal=Journal of the American Chemical Society|volume=101|issue=17|pages=5094–5095|doi=10.1021/ja00511a064|issn=0002-7863}}{{Cite journal|last1=Bruck|first1=M. A.|last2=Copenhaver|first2=A. S.|last3=Wigley|first3=D. E.|date=1987-10-01|title=Alkyne cyclizations at reduced tantalum centers: synthesis and molecular structure of (.eta.6-C6Me6)Ta(O-2,6-i-Pr2C6H3)2Cl|journal=Journal of the American Chemical Society|volume=109|issue=21|pages=6525–6527|doi=10.1021/ja00255a056|issn=0002-7863}} Some tantalum-alkyne complexes are precursors to allylic alcohols.{{Cite journal|last1=Takai|first1=Kazuhiko|last2=Kataoka|first2=Y.|last3=Utimoto|first3=K.|date=1990-03-01|title=Tantalum-alkyne complexes as synthetic intermediates. Stereoselective preparation of trisubstituted allylic alcohols from acetylenes and aldehydes|journal=The Journal of Organic Chemistry|volume=55|issue=6|pages=1707–1708|doi=10.1021/jo00293a008|issn=0022-3263}} Tantalacyclopropenes are invoked as intermediates.
=Tantalum-amido complexes =
Organotantalum compounds are invoked as intermediates in C-alkylation of secondary amines with 1-alkenes using Ta(NMe2)5.{{Cite journal|last1=Clerici|first1=Mario G.|last2=Maspero|first2=Federico|date=1980-01-01|title=Catalytic C-Alkylation of Secondary Amines with Alkenes|journal=Synthesis|language=en|volume=1980|issue=4|pages=305–306|doi=10.1055/s-1980-29002|s2cid=94579838 |issn=0039-7881}} The chemistry developed by Maspero was later brought to fruition when Hartwig and Herzon reported the hydroaminoalkylation of olefins to form alkylamines:{{Cite journal|last1=Herzon|first1=Seth B.|last2=Hartwig|first2=John F.|date=2007-05-01|title=Direct, Catalytic Hydroaminoalkylation of Unactivated Olefins with N-Alkyl Arylamines|journal=Journal of the American Chemical Society|volume=129|issue=21|pages=6690–6691|doi=10.1021/ja0718366|issn=0002-7863|pmc=2590937|pmid=17474747}}
File:Hydroaminoalkylation hartwig correction.tif
The catalytic cycle may proceed by β-hydrogen abstraction of the bisamide, which forms the metallaaziridine. Subsequent olefin insertion, protonolysis of the tantalum-carbon bond, and β-hydrogen abstraction affords the alkylamine product.{{Cite journal|last1=Eisenberger|first1=Patrick|last2=Ayinla|first2=Rashidat O.|last3=Lauzon|first3=Jean Michel P.|last4=Schafer|first4=Laurel L.|date=2009-10-19|title=Tantalum–Amidate Complexes for the Hydroaminoalkylation of Secondary Amines: Enhanced Substrate Scope and Enantioselective Chiral Amine Synthesis|journal=Angewandte Chemie International Edition|language=en|volume=48|issue=44|pages=8361–8365|doi=10.1002/anie.200903656|pmid=19787670|issn=1521-3773}}{{Cite journal|last1=Dörfler|first1=Jaika|last2=Doye|first2=Sven|date=2014-05-01|title=A Commercially Available Tantalum Catalyst for the Highly Regioselective Intermolecular Hydroaminoalkylation of Styrenes|journal=European Journal of Organic Chemistry|language=en|volume=2014|issue=13|pages=2790–2797|doi=10.1002/ejoc.201400082|issn=1099-0690}}{{Cite journal|last1=Payne|first1=Philippa R.|last2=Garcia|first2=Pierre|last3=Eisenberger|first3=Patrick|last4=Yim|first4=Jacky C.-H.|last5=Schafer|first5=Laurel L.|date=2013-05-03|title=Tantalum Catalyzed Hydroaminoalkylation for the Synthesis of α- and β-Substituted N-Heterocycles|journal=Organic Letters|volume=15|issue=9|pages=2182–2185|doi=10.1021/ol400729v|pmid=23600625|issn=1523-7060}}
= Transmetalation =
Organotantalum reagents arise via transmetalation of organotin compounds with tantalum(V) chloride.{{Cite journal|last1=Shibata|first1=Ikuya|last2=Kano|first2=Takeyoshi|last3=Kanazawa|first3=Nobuaki|last4=Fukuoka|first4=Shoji|last5=Baba|first5=Akio|date=2002-04-15|title=Generation of Organotantalum Reagents and Conjugate Addition to Enones|journal=Angewandte Chemie|language=en|volume=114|issue=8|pages=1447–1450|doi=10.1002/1521-3773(20020415)41:8<1389::AID-ANIE1389>3.0.CO;2-D|pmid=19750774|issn=1521-3757}} These organotantalum reagents promote the conjugate allylation of enones. Although the direct allylation of carbonyl groups is prevalent throughout the literature, little has been reported on the conjugate allylation of enones.{{Cite journal|last1=Yamamoto|first1=Yoshinori|last2=Asao|first2=Naoki|date=1993-09-01|title=Selective reactions using allylic metals|journal=Chemical Reviews|volume=93|issue=6|pages=2207–2293|doi=10.1021/cr00022a010|issn=0009-2665}}
Applications
Organotantalum compounds are of academic interest, but few or no commercial applications have been described.
References
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