N,N‘-Diferrocenyl-N-heterocyclic Carbenes and Their Derivatives

In continuation of our work on Wanzlick/Arduengo carbenes containing redox-active ferrocenyl substituents we report on the synthesis of N,N‘-diferrocenyl imidazol(in)ium salts as precursors of imidazol(in)-2-ylidenes. The necessary starting material for this chemistry is aminoferrocene, which was prepared by an improved and large-scale synthesis by the sequence solid lithioferrocene, iodoferrocene, N-ferrocenylphthalimide, aminoferrocene. The preparation of N,N‘-diferrocenyl heterocycles involves condensation of aminoferrocene with glyoxal to afford N,N‘-diferrocenyldiazabutadiene [Fc-DAB], reduction, condensation with formaldehyde, and oxidation with trityl salts to yield N,N‘-diferrocenylimidazol(in)ium salts. In situ deprotonation and trapping with electrophiles yielded the expected metal complexes and derivatives in some cases [Ag⁺ or S₈], but attempted reaction with other transition metals [e.g., Pd(II)] failed to give the corresponding complexes, due to (i) steric hindrance by the two N-ferrocenyl substituents, (ii) reduced acidity of the imidazol(in)ium precursors, and (iii) inaccessibility of the free carbenes. Spectroscopic [IR, Raman, UV−vis, MS, NMR (¹H, ¹³C, ¹⁰⁹Ag)], structural [X-ray], and electrochemical [CV] properties are reported and compared to those of other N-heterocyclic carbene derivatives.