Organoniobium chemistry

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Organoniobium chemistry is the chemistry of compounds containing niobium-carbon (Nb-C) bonds. Compared to the other group 5 transition metal organometallics, the chemistry of organoniobium compounds most closely resembles that of 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. [1]

Contents

Compound classes

Structure of [Li(OEt2)3] [NbMe6] . POZJAP.png
Structure of [Li(OEt2)3] [NbMe6] .

Carbonyls

Unlike vanadium, which forms the neutral hexacarbonyl, niobium does not easily form an analogous complex. The salts of the anionic binary carbonyl, [Nb(CO)6]−, are however well characterized. They are obtained by reduction of 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. [2] The simplest compounds are salts of [Nb(CH3)6]−, which is prepared by alkylation of NbF5 using methyl lithium: [3]

NbF5 + 6 LiCH3 → Li[Nb(CH3)6] + 5 LiF

Cyclopentadienyl derivatives

Niobocene dichloride ( (C5H5)2NbCl2). Cp2NbCl2.png
Niobocene dichloride ((C5H5)2NbCl2).

The first organoniobium compound fully characterized was Cp2NbBr3, [4] however the paramagnetic Nb(IV) metallocenes such as niobocene dichloride are more prevalent. Complexes are typically prepared by treatment of NbCl5 with NaCp to form the bis(cyclopentadienyl) complex followed by further functionalization. Derivatives of pentamethylcyclopentadiene are also known, such as (C5Me5)2NbH3. [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. [5]

Alkylidenes

Along with the related organotantalum species, niobium alkylidenes were among the first Scrock carbenes studied. The first syntheses of these complexes involved addition of organolithium reagents 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. [6]

Alkyne complexes

The adducts from Nb(III) and alkynes are often described as metallacyclopropenes. NbIII alkyne complex.png
The adducts from Nb(III) and alkynes are often described as metallacyclopropenes.

Similar to other d2 transition metals, Nb(III) produce adducts with alkynes. These derivatives are sometimes called Nb(V) alkenediyls metallacyclopropenes. [7] These alkendiyl complexes function as latent dianion equivalents. They react with electrophiles to give alkene derivatives. [7] [8]

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 dimethoxyethane niobium trichloride, NbCl3(DME), as a reagent for the reductive coupling of imines with carbonyl compounds to form amino alcohols. [9] This reagent has found further use in other pinacol-type reductive couplings. [10] [8]

NbCl3(DME) mediated reductive coupling. Hydrolysis step is shown with implicit water. NbIII Azapinacol.png
NbCl3(DME) mediated reductive coupling. Hydrolysis step is shown with implicit water.

Catalytic reactions

A number of formal [2+2+2] cycloadditions 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). [8]

Nb(III) catalyzed formal [2+2+2] cycloadditions NbIII 222s.png
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. [11]

Nb(III) catalyzed alkyne semihydrogenation NbIII Semihydrogenation catalytic cycle.png
Nb(III) catalyzed alkyne semihydrogenation

Related Research Articles

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McMurry reaction

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Titanocene dichloride Chemical compound

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

Organozinc compound

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Organocopper compound Compound with carbon to copper bonds

Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry.

Organotitanium compound

Organotitanium compounds in organometallic chemistry contain carbon-titanium chemical bonds. Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry and are involved in major industrial processes.

Titanocene dicarbonyl Chemical compound

Dicarbonylbis(cyclopentadienyl)titanium is the chemical compound with the formula (η5-C5H5)2Ti(CO)2, abbreviated Cp2Ti(CO)2. This maroon-coloured, air-sensitive species is soluble in aliphatic and aromatic solvents. It has been used for the deoxygenation of sulfoxides, reductive coupling of aromatic aldehydes and reduction of aldehydes.

Niobocene dichloride is the organometallic compound with the formula (C5H5)2NbCl2, abbreviated Cp2NbCl2. This paramagnetic brown solid is a starting reagent for the synthesis of other organoniobium compounds. The compound adopts a pseudotetrahedral structure with two cyclopentadienyl and two chloride substituents attached to the metal. A variety of similar compounds are known, including Cp2TiCl2.

Organonickel chemistry

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Organozirconium chemistry

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Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. While iron adopts oxidation states from Fe(−II) through to Fe(VII), Fe(IV) is the highest established oxidation state for organoiron species. Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.

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Cyclopentadienyliron dicarbonyl dimer Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

Transition metal alkyl complexes Coordination complex

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Organotantalum chemistry Chemistry of compounds containing a carbon-to-tantalum bond

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Bis(cyclopentadienyl)titanium(III) chloride Chemical compound

Bis(cyclopentadienyl)titanium(III) chloride, also known as the Nugent–RajanBabu reagent, is the organotitanium compound which exists as a dimer with the formula [(C5H5)2TiCl]2. It is an air sensitive green solid. The complex finds specialized use in synthetic organic chemistry as a single electron reductant.

Pentamethyltantalum Chemical compound

Pentamethyltantalum is a homoleptic organotantalum compound. It has a propensity to explode when it is melted. Its discovery was part of a sequence that lead to Richard R. Schrock's Nobel Prize discovery in olefin metathesis.

Niobium(III) chloride also known as niobium trichloride is a compound of niobium and chlorine. The binary phase NbCl3 is not well characterized but many adducts are known.

In chemistry, alpha elimination refers to particular types of elimination reactions. The definition of alpha elimination differs for organometallic and organic chemistry.

References

  1. ↑ Waters, T.; Wedd, A. G.; Ziolek, M.; Nowak, I. (2004). Niobium and Tantalum. Comprehensive Coordination Chemistry. Vol. 4 (2nd ed.). Oxford: Elsevier. pp. 241–312. ISBN   978-0-08-0443263.
  2. 1 2 Labinger, J. A. (1982). Niobium and Tantalum. Comprehensive Organometallic Chemistry. Vol. 3 (1st ed.). Oxford: Pergamon. pp. 705–782. ISBN   978-0-08-025269-8.
  3. ↑ Kleinhenz, S.; Pfennig, V.; Seppelt, K. (1998). "Preparation and Structures of [W(CH3)6], [Re(CH3)6], [Nb(CH3)6]−, and [Ta(CH3)6]−". Chem. Eur. J. 4 (9): 1687. doi:10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R.
  4. ↑ Wilkinson, G.; Birmingham, J. M. (1954). "Bis-cyclopentadienyl Compounds of Ti, Zr, V, Nb and Ta". Journal of the American Chemical Society. 76 (17): 4281–4284. doi:10.1021/ja01646a008. ISSN   0002-7863.
  5. ↑ Wigley, D. E.; Gray, S. D. (1995). Niobium and Tantalum. Comprehensive Organometallic Chemistry. Vol. 5 (2nd ed.). Oxford: Pergamon. pp. 57–154. ISBN   978-0-08-042312-8.
  6. ↑ Schrock, Richard R. (1979). "Alkylidene complexes of niobium and tantalum". Accounts of Chemical Research. 12 (3): 98–104. doi:10.1021/ar50135a004. ISSN   0001-4842.
  7. 1 2 Parker, Kyle D. J.; Fryzuk, Michael D. (2015-06-08). "Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne Complexes". Organometallics. 34 (11): 2037–2047. doi:10.1021/om5010385. ISSN   0276-7333.
  8. 1 2 3 Satoh, Yasushi; Obora, Yasushi (2015). "Niobium Complexes in Organic Transformations: From Stoichiometric Reactions to Catalytic [2+2+2] Cycloaddition Reactions: Niobium Complex Organic Chemistry". European Journal of Organic Chemistry. 2015 (23): 5041–5054. doi:10.1002/ejoc.201500358.
  9. ↑ Roskamp, Eric J.; Pedersen, Steven F. (1987). "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 of the American Chemical Society. 109 (21): 6551–6553. doi:10.1021/ja00255a073. ISSN   0002-7863.
  10. ↑ Kammermeier, Bernhard; Beck, Gerhard; Holla, Wolfgang; Jacobi, Detlev; Napierski, Bernd; Jendralla, Heiner (1996). "Vanadium(II)- and Niobium(III)-Induced, Diastereoselective Pinacol Coupling of Peptide Aldehydes to Give aC2-Symmetrical HIV Protease Inhibitor". Chemistry - A European Journal. 2 (3): 307–315. doi:10.1002/chem.19960020312.
  11. ↑ Beaumier, Evan P.; Pearce, Adam J.; See, Xin Yi; Tonks, Ian A. (2019). "Modern applications of low-valent early transition metals in synthesis and catalysis". Nature Reviews Chemistry. 3 (1): 15–34. doi:10.1038/s41570-018-0059-x. ISSN   2397-3358. PMC   6462221 . PMID   30989127.
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