Unsymmetrical N-heterocyclic carbenes with a 1,1'-ferrocenediyl backbone

This paper focuses on ferrocene-based expanded-ring N-heterocyclic carbenes (NHCs) of the type [Fe(C5H4-NR-C-NR'-C5H4)] (), which contain two different N-substituents. Three combinations were addressed, with R = neopentyl (Np) in each case and R' being either 2-adamantyl (Ad), phenyl (Ph) or 9-anthracenylmethyl (Acm). The NHCs were generated by reaction of the corresponding formamidinium tetrafluoroborates [H-][BF4] with lithium diisopropylamide (LDA). While only was sufficiently stable for isolation, and could be efficiently trapped in situ by complexation reactions. Two series of Rh(I) complexes were prepared, viz. [RhCl(cod)()] (cod = 1,5-cyclooctadiene) by reacting [{Rh(μ-Cl)(cod)}2] with and cis-[RhCl(CO)2()] by reacting [RhCl(cod)(')] with CO. All complexes exhibit pronounced anagostic α-CHRh interactions, both in solution and in the solid state, in accord with a strong influence of the N-substituents on the steric ligand properties, as is chemically illustrated by the huge reactivity difference of [RhCl(cod)()] (R = R' = Ad) and [RhCl(cod)()] towards CO, the former complex being inert. Tolman electronic parameter (TEP) values are 2050 ± 1 cm(-1) for the unsymmetrical NHCs studied, indicating only a weak influence of the N-substituents on the electronic ligand properties.