Organoniobium chemistry is the chemistry of compounds containingniobium-carbon (Nb-C) bonds. Compared to the other group 5 transition metalorganometallics, the chemistry of organoniobium compounds most closely resembles that oforganotantalum 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]
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 ofNbCl5 under an atmosphere of CO.
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 ofNbF5 usingmethyl lithium:[3]
The first organoniobium compound fully characterized wasCp2NbBr3,[4] however the paramagnetic Nb(IV) metallocenes such asniobocene dichloride are more prevalent. Complexes are typically prepared by treatment ofNbCl5 withNaCp to form the bis(cyclopentadienyl) complex followed by further functionalization. Derivatives ofpentamethylcyclopentadiene 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]
Along with the relatedorganotantalum species, niobium alkylidenes were among the firstScrock carbenes studied. The first syntheses of these complexes involved addition oforganolithium 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]
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]
No commercial applications of organoniobium compounds have been reported. They have found limited use in organic synthesis.
A prominent early synthetic application of organoniobium chemistry was the use ofdimethoxyethane niobium trichloride, NbCl3(DME), as a reagent for the reductive coupling ofimines with carbonyl compounds to formamino alcohols.[9] This reagent has found further use in otherpinacol-type reductive couplings.[10][8]
A number of formal [2+2+2]cycloadditions have been realized under Nb catalysis, includingalkyne 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]
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]
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