Ferrocenyl Cumulenes

The subject of this research proposal is a systematic study of the chemistry of cumulenes (Fc)2 Cn (Fc)2 (n= 0, 1, 2, 3, 4,...; Fc = ferrocenyl) substituted with up to four ferrocenyl groups. The synthesis, characterization, reactivity, and physical properties of this class of compounds will be investigated in detail. In comparison to purely organic cumulenes, the combination of a cumulenic backbone with directly attached ferrocenyl substituents will significantly alter the usually observed reactivity and stability, respectively. The terminal pairs of metallocenyl substituents serve as redox-active electrophores, inductive donors, and sterically encumbered groups, effecting: i. electronic coupling through the cumulenic bridging 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. Different reactivity patterns, stereochemistry, and structural properties dependent on the number of cumulated carbons or cumulated double bonds, respectively, will be observed by comparing homologous even and odd numbered cumulenes. In a broader context, this proposed research is related to the currently actively investigated chemistry of metalla-cumulenes, conjugatively linked multi-metallic compounds, functionally substituted fullerene compounds, and organometallic NLO materials. Based on the results obtained already on the shorter cumulenes and cumulenium salts the proposed goal of synthesizing and investigating new and longer metallocenyl cumulene compounds will be achieved.

Conjugated linearly bonded cumulated carbon chains with a length of up to 12 x 10-10 m or 10 carbon atoms, respectively, containing electroactive metallocenyl termini were synthesized and evaluated as "molecular wires" for potential applications in molecular electronics. In todays electronics a "scale-down" principle is used to improve the efficiancy and miniaturization of logic elements. However, this design principle will soon reach its technologically feasible limit. To circumvent this limit and to increase computing power by a magnitude of orders, molecular electronics consists in a "bottom-up" design principle which uses molecular compounds with electronic functions to get ultimately access - probably within the next few decades - to molecular computing on a nano-sized scale. To reach this goal, defined molecular building blocks with useful electronic properties are necessary. In this research project, molecular conducting wires consisting of all carbon chains connected in a perfectly linear manner with the maximum degree of unsaturation (so-called cumulenes) were synthesized and investigated. The "proof of concept" is experimentally corroberated by the model compounds prepared which show a detectable electronic communication through the cumulene carbon chain of up to a length of 12 Å. However, for real life applications the limited stability of these systems presents an as yet unsolved problem which might be overcome by supramolecular chemistry; isolation of the cumulene carbon chain by wrapping it with protecting molecular cylinders might be accomplished in the future.