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.
Unlike vanadium, the neutral binary carbonyl of niobium is unstable. The salts of the anionionic binary carbonyls, exemplified by [Nb(CO)6]−, can be obtained by reduction of NbCl5 under an atmosphere of CO. Heteroleptic niobium carbonyls supported by Cp ligands can be prepared at various oxidation states of Nb and serve as useful precursors in niobium carbonyl chemistry.
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. The simplest compounds are salts of [Nb(CH3)6]−, which is prepared by alkylation of NbF5 using methyl lithium:
- NbF5 + 6 LiCH3 → Li[Nb(CH3)6] + 5 LiF
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.
The first organoniobium coumpound fully characterized was Cp2NbBr3, 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.
Similar to other d2 transition metals, Nb(III) binds alkynes to produce Nb(V) alkenediyl complexes. The Nb metallocyclopropenes thus formed feature improved thermal stability relative to other organometallics and are more readily characterized and used as synthetic intermediates
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 chemsitry was the use of NbCl3(DME) as a reagent for the reductive coupling of imines with carbonyl compounds to form amino alcohols. This reagent has found further use in other pinacol-type reductive couplings.
Nb(V) alkendiyl complexes have found use as latent dianion equivalents in synthetic reactions. Reduction of alkynes with Nb(III) compounds followed by trapping with suitable electrophiles has allowed the synthesis of various alkenes.
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).
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.
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