Anionic N‐Heterocyclic Carbenes with N , N ′‐Bis(fluoroaryl) and N , N ′‐Bis(perfluoroaryl) Substituents

A series of rhodium complexes, [Rh(cod)(NHC‐F x )(OH 2 )] (cod=1,5‐cyclooctadiene; NHC=N‐heterocyclic carbene), incorporating anionic N‐heterocyclic carbenes with 2‐ tert ‐butylmalonyl backbones and 2,6‐dimethylphenyl ( x =0), 2,6‐difluorophenyl ( x =4), 2,4,6‐trifluorophenyl ( x =6), and pentafluorophenyl ( x =10) N,N′‐substituents, respectively, has been prepared by deprotonation of the corresponding zwitterionic precursors with potassium hexamethyldisilazide, followed by immediate reaction of the resulting potassium salts with [{RhCl(cod)} 2 ]. These complexes could be converted to the related carbonyl derivatives [Rh(CO) 2 (NHC‐F x )(OH 2 )] by displacement of the COD ligand with CO. IR and NMR spectroscopy demonstrated that the degree of fluorination of the N ‐aryl substituents has a considerable influence on the σ‐donating and π‐accepting properties of the carbene ligands and could be effectively used to tune the electronic properties of the metal center. The carbonyl groups on the carbene ligand backbone provided a particularly sensitive probe for the assessment of the metal‐to‐ligand π donation. The ortho ‐fluorine substituents on the N ‐aryl groups in the carbene ligands interacted with the other ligands on rhodium, determining the conformation of the complexes and creating a pocket suitable for the coordination of water to the metal center. Computational studies were used to explain the influence of the fluorinated N‐substituents on the electronic properties of the ligand and evaluate the relative contribution of the σ‐ and π‐interactions to the ligand–metal interaction.