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(CPh2CNR) (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(CPh2CHNMe)][BF4] with N,N′‐dimethylimidazolin‐2‐ylidene and of [fc(CPh2CHN‐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 CCN2 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.