Triaminoalkenes Derived from Cyclic (Alkyl)(amino)carbenes with a 1,1′-Ferrocenylene Backbone and N-Heterocyclic Carbenes: fcCAAC–NHC Heterodimers

Abstract

The study addresses triaminoalkenes derived from [3]ferrocenophane-type cyclic (alkyl)(amino)carbenes (fcCAACs) fc(CPh2-C-NR) (fc = 1,1 '-ferrocenylene) and N-heterocyclic carbenes (NHCs). Stable target compounds are obtained in good yields as crystalline solids by the combination of [fc(CPh2-CH=NMe)][BF4] with N,N '-dimethylimidazolin-2-ylidene and of [fc(CPh2-CH=N-p-C6H4-tBu)](OTf) with 1,3,4,5-tetramethylimidazolin-2-ylidene, respectively, followed by treatment of the resulting addition product with KN(SiMe3)2. Due to the presence of a triaminoethene unit and a ferrocene moiety, four redox states are expected for such fcCAAC-NHC heterodimers, viz., electroneutral, mono-, di-, and tricationic. An investigation of their redox behavior by electrochemical methods (cyclic voltammetry and differential pulse voltammetry) has revealed that these compounds undergo two consecutive one-electron oxidations, with the poor stability of the dicationic species in solution preventing an observation of the tricationic redox state. A density functional theory (DFT) study shows that the highest occupied molecular orbital (HOMO) is localized on the C=CN2 atom, which, in agreement with electron paramagnetic resonance results, is the site of the first oxidation. The second oxidation mainly involves the Fe atom, where the HOMO-1 is localized, resulting in a species with a triplet ground state composed, to a first approximation, of a carbon-centered and an iron-centered radical.