Niobium alkynes

Scheme 51 Formation of niobium-alkyne and tantalum-alkyne complexes.

Migration of more complex alkyl groups was recently reported [70]. Reversible migratory insertion/(3-carbon elimination occurs between the coordinated alkyne and the bound alkyl group of alkyl-niobium(alkyne) complex 52. 

The a-agostic ethyl and n-propyl niobium alkyne complexes of the type [Tp Nb(Cl)(p-H-CHR)(PhCCR )] (Tp = hydridotris(3,5-dimethylpyrazolyl)borate R = Me, Et R = Me, Et, Pr") were reported 29 to undergo a thermolytic exchange of niobium and alkyne-bound alkyl groups to give [Tp Nb(Cl)(R )(PhC CH2R)]. First order kinetic parameters were obtained when R = R = Me. 

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]. 

Carbon-carbon bonds can be generated between a nitrile carbon and an alkyne while they are coordinated to niobium.178 Protonation of the -nitrile of 115 gives intermediate 116 from which metallocycle 117 is generated (Scheme 49). 

Bis(cyclooctene)—iridium(I) complexes, preparation, 7, 316 Bis(cyclopentadienyl) alkenes, with tantalum, 5, 157 Bis(cyclopentadienyl) alkyne niobium complexes, characteristics, 5, 81... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

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