SYNTHESIS AND REACTIVITY OF TANTALUM ALKYLIDENE COMPLEXES CONTAINING THE C,N,N'-CHELATING ARYLDIAMINE LIGAND [C6H4(CH2N(ME)CH2CH2NME2)-2](-) (CNN) - X-RAY STRUCTURES OF [TACL2(=CH-T-BU)(CNN)], [TA((CH2)(3)-1,3) (CNN)(O-T-BU)(2)], AND [TA(CNN)(O-T-BU)(2)(H2C=CH2)]

The potentially C,N,N'-chelating anionic aryldiamine ligand [C6H4(CH2N (Me)CH2CH2-nMe(2))-2](-) (CNN) has been employed in the preparation of six-coordinate Ta(V) alkylidene complexes. The new dichloro alkyliden e complex [TaCl2(=CH-t-Bu)(CNN)], 2, prepared from [TaCl3(=CH-t-Bu)(TH F)(2)] and [Li(CNN)](2), 1, has been obtained as a mixture of three di astereoisomers (2a-c) in 51% yield. The molecular structure of the maj or isomer 2a, determined by X-ray methods, shows this to be a tetragon al bipyramidal complex in which the metal coordination sphere is compr ised of the C,N,N' facially bonded CNN, two cis-positioned chlorides i n the equatorial plane, and a neopentylidene group and the NMe2 nitrog en donor mutually trans-positioned at the apices. The dialkoxide compl exes [Ta(=CHR)(CNN)(o-t-Bu)(2)] (R = t-Bu (3a), CMe2Ph(3b)) have been obtained by transmetallation of [TaCl(=CHR)(O-t-Bu)(2)(PMe3)] with 1, The neopentylidene complex 3a was isolated as an analytically pure ora nge solid in 47% yield, whereas the neophylidene complex 3b was isolat ed as an orange oil of +/-90% purity. Complex 3b reacts in pentane wit h ethene at room temperature to afford the tantalacyclobutane complex [Ta{(CH2)(3)-1,3}(CNN)(O-t-Bu)(2)], 4, which was isolated as a white s olid in 81% yield, An X-ray diffraction study of4 shows it to be a hep tacoordinate pentagonal bipyramidal tantalum species in which two mutu ally trans alkoxide groups are in the apical positions; the meridional ligation is comprised of the C,N,N'-coordination of CNN and the sigma -bonded carbon atoms of the metallacyle. Reaction of 3b in hexane at 6 9 degrees C with ethene gives a mixture of the new tantalum alkene com plex [Ta(CNN)(O-t-Bu)(2)(H2C=CH2)], 5, and 4 in a 5:2 ratio. Complex 5 can be isolated from this mixture as a yellow solid by recrystallizat ion from diethyl ether at -30 degrees C. The X-ray molecular structure of 5 shows it to be a pentagonal bipyramidal complex in which the arr ay of atoms directly bonded Do tantalum is like that in 4. However, in 5, the meridional ligation is comprised of the C,N,N'-coordination of CNN and the two C atoms of the ethene molecule. Complexes 3b and 4 in itiate ring-opening metathesis polymerization with strained cyclic ole fins; dicyclopentadiene produces cross-linked polymers, whereas norbor nene produces polycyclopentamers with approximately 90% trans-vinylene bonds. In contrast, none of the isomers of alkylidene complex 2 show any reactivity toward either ethene or norbornene.