Six-membered N-heterocyclic carbenes with a 1,1′-ferrocenediyl backbone: Bulky ligands with strong electron-donor capacity and unusual non-innocent character

The stable, crystalline N-heterocyclic diaminocarbene fc[N(CH ₂tBu)-C-N(CH₂tBu)] (2d, fc = 1,1′-ferrocenediyl) was prepared by deprotonation of its formamidinium precursor fc[N(CH ₂tBu)-CH-N(CH₂tBu)][BF₄] (1d) and used for the preparation of the 16 valence electron complexes [Mo(2d)-(CO)₄], [RhCl(2d)(cod)] (cod = 1,5-cyclooctadiene) and [RhCl(2d)(CO)₂], 1d, 2d and [RhCl(2d)(cod)] were structurally characterised by single-crystal X-ray diffraction studies. The electrochemical properties of 2d, its 2-adamantyl analogue 2c, its complex [RhCl(2d)(CO)₂] and of the precursors 1d and 1,1′-bis(neopentylamino)ferrocene were investigated by electrochemistry. The carbenes are easily oxidised to the corresponding radical cation, whose persistent nature is unprecedented in the chemistry of N-heterocyclic carbenes. The spin density is located at the Fe atom and the carbene C atom according to the results of EPR spectroscopic studies and DFT calculations.