Cationic and neutral cumulene sp-carbon chains with ferrocenyl termini

The synthesis, physical and chemical properties of cationic (Fc)(2)C=C-n=C(Fc)(+) and neutral (Fc)(2)C=C-n=(Fc)(2) cumulenes with ferrocenyl substituents and with up to ten cumulated carbons is summarized. In comparison to metallacumulenes and to purely organic cumulenes, the combination of a cumulenic backbone with directly attached ferrocenyl substituents significantly alters the usually observed reactivity and/or stability. The terminal metallocenyl groups serve as redox-active centers, inductive donors, and sterically encumbered groups, effecting (i) electronic coupling through the bridging cumulene ligand, (ii) stabilization of adjacent electron-deficient carbenium centers, (iii) increased nucleophilic reactivity of the cumulene moiety, and (iv) steric hindrance with accompanying reduced reactivity for shorter cumulenes and enhanced stability for longer cumulenes. The chemical reactivity of sterically unhindered neutral cumulenes (Fc)(z)C=C-n=C(Fc)(2) includes complex formation with late transition metals, ferrocenophane formation with zero-valent Ni, and cycloadditions with olefins, alkynes, and fullerenes. (Fc)(2)C=C-n=C(Fc)(+) salts are novel organometallic non-linear optical materials which show significant second-order non-linear optical properties. (C) 2000 Elsevier Science S.A. All rights reserved.