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 2 t Bu)‐C‐N(CH 2 t Bu)] ( 2d , fc = 1,1′‐ferrocenediyl) was prepared by deprotonation of its formamidinium precursor fc[N(CH 2 t Bu)‐CH‐N(CH 2 t Bu)][BF 4 ] ( 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)