1,3-Dimetallacyclobutadiene derivatives of niobium and tantalum supported by carbazole ligation: Reactivity with organic isocyanides

The alkylidyne-bridged substrates [(Me(3)SiCH(2))(2)M(mu-CSiMe(3))(2)M(CH(2)SiMe(3))(2)] (M=Nb, 1a; M=Ta, 1b) react over days at 100 degrees C in hydrocarbon solution with an excess of carbazole reagents (N'H=carbazole (cbH) tetrahydrocarbazole (thcbH), or 3-tert-butylcarbazole (cbH-3Bu(t))) to produce the corresponding derivatives [(N')(2)M(mu-CSiMe(3))(2)M(N')(2)] (N'=cb (2), thcb (3), cbh-3Bu(t) (4)) as dark-red (M=Nb, a) and orange (M=Ta, b) solids. Solutions of the niobium and tantalum complexes in hydrocarbon solvents are deep-blue and deep-red, respectively. Only compound 4b was sufficiently soluble for detection of the bridging alkylidyne carbon resonance at delta 412 ppm in the C-13 NMR spectrum. The solid-state structure of 2a and isomorphous 2b shows the central 1,3-dimetallacyclobutadiene core supported by the four N-donor carbazole ligands with M-M distances of 2.9015(4) Angstrom (M=Nb) and 2.8801(4) Angstrom (M=Ta). Compounds 2a,2b react rapidly with 1 equiv of 2,6-dimethylphenyl isocyanide to form much more soluble organometallic products 5a,5b, respectively. The solid-state structures of 5a and isomorphous 5b show that coupling of the isocyanide molecule has occurred with one of the alkylidyne bridges to produce a new bridging ligand that can be described as an amido-alkyne. The amido nitrogen of this functionality is bonded to one metal center while the alkyne unit is strongly bound to the other metal center in a metallacyclopropene ring. The remaining alkylidyne group is nonsymmetrically bound to the two metal centers with distances of 1.937(8), 2.069(8) Angstrom (M=Nb) and 1.95(1), 2.05(1) Angstrom (M=Ta).