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)(OH2)] (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)(OH2)] 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 sigma-donating and pi-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 pi 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 sigma- and pi-interactions to the ligand-metal interaction.