Mixed Cobaltocenium−Ferrocene Heterobimetallic Complexes and Their Binding Interactions with β-Cyclodextrin. A Three-State, Host−Guest System under Redox Control

A new organometallic compound (2) containing a cobaltocenium and a ferrocene residue
was synthesized, and its structure was determined by X-ray diffraction methods. The two
metallocene subunits in 2 are linked by a connector featuring a positively charged quaternary
ammonium nitrogen atom, which increases the aqueous solubility of 2 in every oxidation
state, as compared to the previously reported heterobimetallic cobaltocenium−ferrocene
complex 1, which has an uncharged tether. The electrochemical behavior of 1 and 2 in 0.1
M NaCl is characterized by the reversible one-electron oxidation of the ferrocene residue
and by the one-electron reduction of the cobaltocenium subunit. The latter electrochemical
process is reversible only in 2. In compound 1, the voltammetric reduction wave is distorted
by precipitation of the fully reduced form on the electrode surface. Both compounds exhibit
three accessible oxidation states that can be represented as Fc+-Cob+, Fc-Cob+, and Fc-Cob.
Cyclic voltammetric experiments in the presence of the host β-cyclodextrin (β-CD) reveal
that the fully oxidized form, Fc+-Cob+, is not bound, the intermediate oxidation state Fc-Cob+ forms a stable complex by inclusion of its ferrocene site, and the fully reduced form,
Fc-Cob, presents two binding sites for the CD host. The voltammetric behavior was analyzed
using digital simulation techniques in order to obtain the thermodynamic parameters for
the binding equilibria.