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'-diferroce nyl imidazol(in)ium salts as precursors of imidazol(in)-2-ylidenes. The nec essary starting material for this chemistry is aminoferrocene, which was pr epared by an improved and large-scale synthesis by the sequence solid lithi oferrocene, iodoferrocene, N-ferrocenylphthalimide, aminoferrocene. The pre paration of N,N'-diferrocenyl heterocycles involves condensation of aminofe rrocene with glyoxal to afford N,N'-diferrocenyldiazabutadiene [Fc-DAB], re duction, condensation with formaldehyde, and oxidation with trityl salts to yield N,N'-diferrocenylimidazol(in)ium salts. In situ deprotonation and tr apping with electrophiles yielded the expected metal complexes and derivati ves in some cases [Ag+ or S-8], but attempted reaction with other transitio n metals [e.g., Pd(II)] failed to give the corresponding complexes, due to (i) steric hindrance by the two N-ferrocenyl substituents, (ii) reduced aci dity of the imidazol(in)ium precursors, and (iii) inaccessibility of the fr ee carbenes. Spectroscopic [IR, Raman, UV-vis, MS, NMR (H-1, C-13, Ag-109)] , structural [X-ray], and electrochemical [CV] properties are reported and compared to those of other N-heterocyclic carbene derivatives.