Synthesis and characterization of nickel dithiocarbamate complexes bearing ferrocenyl subunits

Syntheses of a unique molecule, nickel(II) dithiocarbamate bearing two ferrocenyl groups (3), and its oxidized product, nickel(IV) dithiocarbamate bearing three ferrocenyl groups (4⁺), are reported. Spectroelectrochemical investigations have shown that the complex 4⁺ undergoes a three-electron oxidation process, according to two quasireversible steps ([Ni(IV)(Fe(II))₃]⁺ ⇆ [Ni(IV)(Fe(II))2Fe(III)]²⁺+e^-, [Ni(IV)(Fe(II))₂- Fe(III)]²⁺ ⇆ [Ni(IV)(Fe(III))₃]⁴⁺+2e^-), whose redox potentials are separated by ΔE = 250 mV. The value ΔE is related to the comproportionation equilibrium ([Ni(IV)(Fe(II))₃]⁺+[Ni(IV)(Fe(III))₂Fe(II)]³⁺ ⇆ 2[Ni(IV)Fe(III)(Fe(II))₂]²⁺) and results from the combination of a statistical contribution and a term which reflects the electrostatic repulsive interaction between the metal centers. In spite of the chemical equivalence of the three ferrocenyl groups, the mixed-valence state [Ni(IV)Fe(III)(Fe(II))₂]²⁺ (4²⁺) substantially persists in solution. Model studies with ethylenebridged bis(ferrocenylimine) (6) have revealed that the electrostatic term is much lower in the absence of the nickel(IV) center. Preliminary force-field simulations on 4⁺ have shown that enhanced electrostatic repulsion caused by the oxidation of the ferrocenyl subunits affects the conformation of the molecule, which results in a significant dimensional increment. Stereochemical features of the molecules are related to the electrostatic interaction and ΔG⁰ associated with the comproportionation process is affected by such strong deformation that a decrease in electrostatic repulsion results.