Chemistry of low and medium-valent niobium complexes supported by anionic amides.

Abstract

Reaction of [NbCl3(TMEDA)]2 with LDA in a stoichiometric ratio of 1:3 Nb/Lithium amide, resulted in a new diamagnetic niobium (II) cluster (TMEDA)3Nb2Cl5Li (2.1) with a very short Nb≡Nb bond. Similarly, reaction of NbCl4(THF) 2 with 2 equiv. of Cy2NLi caused the reduction of the metal center to produce a diamagnetic and tetranuclear cluster, [Nb4Cl 12Li2(THF)8] (2.2). Further reaction of [NbCl3(TMEDA)]2 or (TMEDA)3Nb2Cl 5Li (2.1) with (3,5-Me2Ph)(Ad)NLi in THF yielded the tetravalent, dinuclear and diamagnetic {[(3,5-Me2Ph)(Ad)N]Nb(3,5Me 2Ph)}2(mu-NAd)2.2(ether) (2.3). In the reaction of Nb2Cl6(TMEDA)2 with azobenzene cleavage of the N=N bond results in the formation of the niobium imido complex, NbCl3(C6H5N)(TMEDA) (2.4). The reaction of Nb2Cl6(TMEDA)2 with the potassium salt of tripyrrolylmethane in a 1:1 ratio afforded two products which have been isolated and characterized. The first compound is the dinuclear {[H-C(C4H3N)3]Nb(THF)}2 •2THF (3.1) with two trivalent and diamagnetic metals bridged by two tripyrrolyl trianions. The second product of the reaction is {([H-C(C 4H3N)3]2NbK)2}{Nb4 Cl11[K(THF)2]2}•2THF (3.2 ) which displays some unusual features. The complex is ionic with the cationic {Nb4Cl11[K (THF)2]2} 2+ unit containing the metal in a mixed valence state and the cationic moiety are balanced by two identical anionic {([H-C(C4H3N) 3]2NbK)2}- units each containing Nb in the tetravalent state. A similar reaction carried out with the lithium salt of the tripyrrolide anion led instead to a monomeric tetravalent {[H-C(C 4H3N)3]2Nb}{Li (THF)4} 2 (3.3) complex. Reaction of Nb2Cl5Li(TMEDA)3 with Ph 2NK in THF afforded a complex reaction from which three complex were isolated and characterized. The first compound Cl2Nb 2(mu-Cl)3(TMEDA)2 (4.1) was obtained in analytically pure form as dark purple, paramagnetic and moderately air-sensitive crystals. The second compound is a trivalent and dinuclear {[(Ph2N) 2Nb]2[mu-NPh(mu-eta1:eta2-C 6H4)](mu-H)}{Li(TMEDA)2}.toluene (4.2 ) which was formed via oxidative addition of the two Nb(II) centers to the C--H sigma-bond of one aromatic ring. This species thermally rearranges into the dinuclear and diamagnetic (Ph2N) 2Nb{[mu-NPh(eta6-C6H5)] 2Nb}.ether (4.3). (Abstract shortened by UMI.)