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^IVFe^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.