Niobium complexes reactions with alkynes

Scheme 56 Reaction of niobium and molybdenum disulfido complexes with alkyne...

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

There is some material of relevance in a review on niobium and tantalum coordination chemistry. 1 The thermal reactions of the ions M" " (M = Ta, Nb, NbO, NbN) with arenes in the gas phase results in cleavage of the arenes to form metallacyclopentadiene ion complexes via retro-alkyne cyclotrimerization mechanisms. ... 

Dinuclear niobium complexes of general formula [ NbCl2L 2( i-Cl)2( i-L)j (L = C HgS) have been prepared from the reaction of diniobium (V)decachloride [NfajClj ,] with magnesium in tetrahydroth-iophene [218]. This complex was successfully used in the catalytic cyclotrimerization of alkynes. In the solid-state structure, the Nb-Nb bond distance found to be 2.6956(9) A that coincides within the range of atomic distances of Nb -Nb double bonds, 2.60-2.86 A [218]. 

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