Five-Membered Titana- and Zirconacyclocumulenes: Stable 1-Metallacyclopenta-2,3,4-trienes

Abstract

The rather exotic and uncommon five-membered metallacyclocumulenes (1-metallacyclopenta-2,3,4-trienes) Cp‘2M(η4-RC4R) (Cp‘ = substituted η5-cyclopentadienyl) are formed by “cis” complexation of titanocene or zirconocene fragments with 1,3-butadiynes or by coupling of two σ-acetylide groups in the coordination sphere of the metals. Examples of these complexes, Cp2M(η4-RC4R) (M = Zr, R = t-Bu; M = Ti, R = t-Bu, R = Ph), Cp*2M(η4-RC4R‘) (M = Zr, R = R‘ = Me3Si, Me, Ph, t-BuMe2Si; R = t-Bu, R‘ = C⋮C-t-Bu), (η6-C5H5BX)2Zr(η4-RC4R) (X = i-Pr2N, R = Ph; X = Ph, R = Ph, Et), (η5:η1-C5Me4SiMe2N-t-Bu)Ti(η4-RC4R) (R = t-Bu, SiMe3), and [Cp2Zr]3[1,3,5-(η4-t-BuC4)3C6H3], are discussed with regard to their genesis, synthesis, dynamics, structures, and bonding. These compounds are compared with the also uncommon and very similar five-membered metallacyclopentynes (1-metallacyclopent-3-ynes) Cp‘2M(η4-R2C4R2) (M = Zr: Cp‘ = Cp, R2 = t-Bu/H, Me3Si/H, H2; Cp‘ = t-BuC5H4, R2 = H2; M = Ti: Cp‘ = Cp, R2 = H2) formed by analogous “cis” complexation of 1,2,3-butatrienes, in terms of their chemistry, structures, and bonding. Furthermore, reactions in which metallacyclocumulenes were assumed to be intermediates are described, together with the reactivity of metallacyclocumulenes toward different complexes and substrates: e.g. metal complexes, carbon dioxide, diisobutylaluminum hydride, and tris(pentafluorophenyl)borane. Upon complexation with a second transition-metal complex fragment either bridging formal μ-“cis” or μ-“trans” complexes of 1,3-butadiynes and 1,2,3-butatrienes were formed.