Synthesis and Reactivity of Tantalum Alkylidene Complexes Containing the <i>C,N,N</i>‘-Chelating Aryldiamine Ligand [C₆H₄(CH₂N(Me)CH₂CH₂NMe₂)-2]^- (CNN). X-ray Structures of [TaCl₂(CH-<i>t</i>-Bu)(CNN)], [Ta{(CH₂)₃-1,3}(CNN)(O-<i>t</i>-Bu)₂], and [Ta(CNN)(O-<i>t</i>-Bu)₂(H₂CCH₂)]

Abstract

The potentially C,N,N‘-chelating anionic aryldiamine ligand [C6H4(CH2N(Me)CH2CH2NMe2)-2]- (CNN) has been employed in the preparation of six-coordinate Ta(V) alkylidene complexes. The new dichloro alkylidene complex [TaCl2(CH-t-Bu)(CNN)], 2, prepared from [TaCl3(CH-t-Bu)(THF)2] and [Li(CNN)]2, 1, has been obtained as a mixture of three diastereoisomers (2a−c) in 51% yield. The molecular structure of the major isomer 2a, determined by X-ray methods, shows this to be a tetragonal bipyramidal complex in which the metal coordination sphere is comprised of the C,N,N‘ facially bonded CNN, two cis-positioned chlorides in the equatorial plane, and a neopentylidene group and the NMe2 nitrogen donor mutually trans-positioned at the apices. The dialkoxide complexes [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 orange solid in 47% yield, whereas the neophylidene complex 3b was isolated as an orange oil of ±90% purity. Complex 3b reacts in pentane with 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 solid in 81% yield. An X-ray diffraction study of 4 shows it to be a heptacoordinate pentagonal bipyramidal tantalum species in which two mutually trans alkoxide groups are in the apical positions; the meridional ligation is comprised of the C,N,N‘-coordination of CNN and the σ-bonded carbon atoms of the metallacyle. Reaction of 3b in hexane at 69 °C with ethene gives a mixture of the new tantalum alkene complex [Ta(CNN)(O-t-Bu)2(H2CCH2)], 5, and 4 in a 5:2 ratio. Complex 5 can be isolated from this mixture as a yellow solid by recrystallization from diethyl ether at −30 °C. The X-ray molecular structure of 5 shows it to be a pentagonal bipyramidal complex in which the array of atoms directly bonded to 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 initiate ring-opening metathesis polymerization with strained cyclic olefins; dicyclopentadiene produces cross-linked polymers, whereas norbornene 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.