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

The rather exotic and uncommon five-membered metallacyclocumulenes (1-metallacyclopenta-2,3,4-trienes) Cp‘ 2 M(η 4 -RC 4 R) (Cp‘ = substituted η 5 -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, Cp 2 M(η 4 -RC 4 R) (M = Zr, R = t -Bu; M = Ti, R = t -Bu, R = Ph), Cp* 2 M(η 4 -RC 4 R‘) (M = Zr, R = R‘ = Me 3 Si, Me, Ph, t -BuMe 2 Si; R = t -Bu, R‘ = C⋮C- t -Bu), (η 6 -C 5 H 5 BX) 2 Zr(η 4 -RC 4 R) (X = i- Pr 2 N, R = Ph; X = Ph, R = Ph, Et), (η 5:η 1 -C 5 Me 4 SiMe 2 N- t -Bu)Ti(η 4 -RC 4 R) (R = t -Bu, SiMe 3 ), and [Cp 2 Zr] 3 [1,3,5-(η 4 - t -BuC 4 ) 3 C 6 H 3 ], 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‘ 2 M(η 4 -R 2 C 4 R 2 ) (M = Zr: Cp‘ = Cp, R 2 = t- Bu/H, Me 3 Si/H, H 2; Cp‘ = t- BuC 5 H 4, R 2 = H 2; M = Ti: Cp‘ = Cp, R 2 = H 2 ) 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.