View source for Chemistry:Organoniobium chemistry

'''Organoniobium chemistry''' is the chemistry of compounds containing [[Chemistry:Niobium|niobium]]-[[Chemistry:Carbon|carbon]] (Nb-C) bonds. Compared to the other group 5 transition metal [[Chemistry:Organometallic chemistry|organometallics]], the chemistry of organoniobium compounds most closely resembles that of [[Chemistry:Organotantalum chemistry|organotantalum]] compounds. Organoniobium compounds of oxidation states +5, +4, +3, +2, +1, 0, -1, and -3 have been prepared, with the +5 oxidation state being the most common.<ref name=":1">{{Cite book|title=Niobium and Tantalum|last=Waters|first=T.|last2=Wedd|first2=A. G.|last3=Ziolek|first3=M.|last4=Nowak|first4=I.|work=Comprehensive Coordination Chemistry|publisher=Elsevier|year=2004|isbn=978-0-08-0443263|edition=2nd|volume=4|location=Oxford|pages=241–312}}</ref>
==Compound classes==
[[File:POZJAP.png|thumb|Structure of [Li(OEt<sub>2</sub>)<sub>3</sub><nowiki>]</nowiki><sup>+</sup>[NbMe<sub>6</sub><nowiki>]</nowiki><sup>−</sup>.]]
=== Carbonyls ===
Unlike vanadium, which forms the neutral hexacarbonyl, niobium does not easily form an analogous complex. The salts of the anionic binary carbonyl, {{chem2|[Nb(CO)6]–}}, are however well characterized.  They are obtained by reduction of {{chem2|[[Chemistry:Niobium(V) chloride|NbCl5]]}} under an atmosphere of CO. 

=== Alkyl ===
A wide variety of alkyl Nb compounds have been prepared. Low coordination number complexes require the absence of any β-hydrogen to prevent rapid β-hydride elimination.<ref name=":2">{{Cite book|title=Niobium and Tantalum|last=Labinger|first=J. A.|work=Comprehensive Organometallic Chemistry|publisher=Pergamon|year=1982|isbn=978-0-08-025269-8|edition=1st|volume=3|location=Oxford|pages=705–782}}</ref> The simplest compounds are salts of {{chem2|[Nb(CH3)6]−}}, which is prepared by alkylation of {{chem2|NbF5}} using methyl lithium:<ref name="Sepp2">{{cite journal|author1=Kleinhenz, S.|author2=Pfennig, V.|author3=Seppelt, K.|year=1998|title=Preparation and Structures of [W(CH<sub>3</sub>)<sub>6</sub>], [Re(CH<sub>3</sub>)<sub>6</sub>], [Nb(CH<sub>3</sub>)<sub>6</sub>]<sup>−</sup>, and [Ta(CH<sub>3</sub>)<sub>6</sub>]<sup>−</sup>|journal=Chem. Eur. J.|volume=4|issue=9|page=1687|doi=10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R}}</ref>

: {{chem2|NbF5 + 6 LiCH3  →  Li[Nb(CH3)6] +  5 LiF}}

=== Cyclopentadienyl derivatives ===
[[Image:Cp2NbCl2.png|thumb|right|122px|[[Chemistry:Niobocene dichloride|Niobocene dichloride]] ({{chem2|(C5H5)2NbCl2}}).]]
The first organoniobium compound fully characterized was {{chem2|Cp2NbBr3}},<ref>{{Cite journal|last=Wilkinson|first=G.|last2=Birmingham|first2=J. M.|date=1954|title=Bis-cyclopentadienyl Compounds of Ti, Zr, V, Nb and Ta|journal=Journal of the American Chemical Society|language=en|volume=76|issue=17|pages=4281–4284|doi=10.1021/ja01646a008|issn=0002-7863}}</ref> however the paramagnetic Nb(IV) metallocenes such as [[Chemistry:Niobocene dichloride|niobocene dichloride]] are more prevalent. Complexes are typically prepared by treatment of {{chem2|[[Chemistry:Niobium(V) chloride|NbCl5]]}} with [[Chemistry:Sodium cyclopentadienide|NaCp]] to form the bis(cyclopentadienyl) complex followed by further functionalization. Derivatives of [[Chemistry:Pentamethylcyclopentadiene|pentamethylcyclopentadiene]] are also known, such as {{chem2|(C5Me5)2NbH3}}.<ref name=":2" />

Niobium carbonyls supported by Cp ligands can be prepared at various oxidation states of Nb and serve as useful precursors in niobium carbonyl chemistry.<ref name=":0">{{Cite book|title=Niobium and Tantalum|last=Wigley|first=D. E.|last2=Gray|first2=S. D.|work=Comprehensive Organometallic Chemistry|publisher=Pergamon|year=1995|isbn=978-0-08-042312-8|edition=2nd|volume=5|location=Oxford|pages=57–154}}</ref>

=== Alkylidenes ===
Along with the related [[Chemistry:Organotantalum chemistry|organotantalum]] species, niobium alkylidenes were among the first [[Chemistry:Transition metal carbene complex|Scrock carbenes]] studied. The first syntheses of these complexes involved addition of [[Chemistry:Organolithium reagent|organolithium reagent]]s lacking β-hydrogens into hindered Nb(V) complexes followed by α-proton elimination. As compared to tantalum alkylidenes, niobium alkylidenes are less thermally and hydrolytically stable.<ref>{{Cite journal|last=Schrock|first=Richard R.|date=1979|title=Alkylidene complexes of niobium and tantalum|journal=Accounts of Chemical Research|language=en|volume=12|issue=3|pages=98–104|doi=10.1021/ar50135a004|issn=0001-4842}}</ref>

=== Alkyne complexes ===
[[File:NbIII_alkyne_complex.png|center|thumb|219x219px|The adducts from Nb(III) and alkynes are often described as metallacyclopropenes.]]
Similar to other d<sup>2</sup> transition metals, Nb(III) produce adducts with alkynes. These derivatives are sometimes called Nb(V) alkenediyls metallacyclopropenes.<ref name=":3">{{Cite journal|last=Parker|first=Kyle D. J.|last2=Fryzuk|first2=Michael D.|date=2015-06-08|title=Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne Complexes|journal=Organometallics|language=en|volume=34|issue=11|pages=2037–2047|doi=10.1021/om5010385|issn=0276-7333}}</ref>
These alkendiyl complexes function as latent dianion equivalents. They react with electrophiles to give alkene derivatives.<ref name=":3" /><ref name=":4" />

== Applications ==
No commercial applications of organoniobium compounds have been reported.  They have found limited use in organic synthesis.

=== Stoicheometric niobium reagents ===
A prominent early synthetic application of organoniobium chemistry was the use of [[Chemistry:Dimethoxyethane|dimethoxyethane]] niobium trichloride, NbCl<sub>3</sub>(DME), as a reagent for the reductive coupling of [[Chemistry:Imine|imine]]s with carbonyl compounds to form amino alcohols.<ref>{{Cite journal|last=Roskamp|first=Eric J.|last2=Pedersen|first2=Steven F.|date=1987|title=The first practical niobium(III) reagent in organic synthesis. A convenient route to 2-amino alcohols via the coupling of imines with aldehydes or ketones promoted by NbCl3(DME)|journal=Journal of the American Chemical Society|language=en|volume=109|issue=21|pages=6551–6553|doi=10.1021/ja00255a073|issn=0002-7863}}</ref> This reagent has found further use in other [[Chemistry:Pinacol coupling reaction|pinacol]]-type reductive couplings.<ref>{{Cite journal|last=Kammermeier|first=Bernhard|last2=Beck|first2=Gerhard|last3=Holla|first3=Wolfgang|last4=Jacobi|first4=Detlev|last5=Napierski|first5=Bernd|last6=Jendralla|first6=Heiner|date=1996|title=Vanadium(II)- and Niobium(III)-Induced, Diastereoselective Pinacol Coupling of Peptide Aldehydes to Give aC2-Symmetrical HIV Protease Inhibitor|journal=Chemistry - A European Journal|volume=2|issue=3|pages=307–315|doi=10.1002/chem.19960020312}}</ref><ref name=":4">{{Cite journal|last=Satoh|first=Yasushi|last2=Obora|first2=Yasushi|date=2015|title=Niobium Complexes in Organic Transformations: From Stoichiometric Reactions to Catalytic [2+2+2] Cycloaddition Reactions: Niobium Complex Organic Chemistry|journal=European Journal of Organic Chemistry|language=en|volume=2015|issue=23|pages=5041–5054|doi=10.1002/ejoc.201500358}}</ref>
[[File:NbIII Azapinacol.png|alt=|center|thumb|387x387px|NbCl<sub>3</sub>(DME) mediated reductive coupling.  Hydrolysis step is  shown with implicit water.]]

=== Catalytic reactions ===
A number of formal [2+2+2] [[Chemistry:Cycloaddition|cycloaddition]]s have been realized under Nb catalysis, including alkyne trimerizations and couplings of alkynes with alkenes or nitriles to form cyclohexadienes or pyridines, respectively. Typically a Nb(III) catalyst will form a Nb(V) metallocyclopropene with a terminal alkyne component and then engage in sequential migratory insertions and reductive elimination to furnish the six membered ring and regenerate the Nb(III).<ref name=":4" />
[[File:NbIII_222s.png|center|thumb|355x355px|Nb(III) catalyzed formal [2+2+2] cycloadditions]]
An organoniobium catalyst has also been developed for (Z)-selective semihydrogenation of alkynes. The mechanistic pathway for this reaction is distinct from other transition metal catalyzed hydrogenations, proceeding through the Nb(V) metallocyclopropene which engages with hydrogen either through direct sigma-bond metathesis or outer sphere 1,2-addition.<ref>{{Cite journal|last=Beaumier|first=Evan P.|last2=Pearce|first2=Adam J.|last3=See|first3=Xin Yi|last4=Tonks|first4=Ian A.|date=2019|title=Modern applications of low-valent early transition metals in synthesis and catalysis|journal=Nature Reviews Chemistry|language=en|volume=3|issue=1|pages=15–34|doi=10.1038/s41570-018-0059-x|issn=2397-3358|pmc=6462221|pmid=30989127}}</ref>
[[File:NbIII_Semihydrogenation_catalytic_cycle.png|alt=|center|thumb|678x678px|Nb(III) catalyzed alkyne semihydrogenation]]

==References==
<references />{{ChemicalBondsToCarbon}}
{{niobium compounds}}

[[Category:Organoniobium compounds| ]]
[[Category:Organometallic chemistry]]

{{Sourceattribution|Organoniobium chemistry}}