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

Abstract

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.

Anionic N-heterocyclic carbenes derived from sydnone imines such as molsidomine. Trapping reactions with selenium, palladium, and gold

“…This is a legacy of their origin, as they are deprotonated conjugated heterocyclic mesomeric betaines. By contrast, the pyrimidine derivative 4 [36][37][38][39] was generated from a cross-conjugated mesomeric betaine. Consequently, the negative charge is exclusively delocalized in the 3-oxobut-1-en-1-olate partial structure without mesomeric charge distribution into the diaminocarbene moiety (c.f.…”

Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent

“…Reactions of these anionic NHCs with transition metals afford corresponding zwitterionic complexes. Anionic, six-membered ring NHCs based on a malonate unit (maloNHC), introduced by Cesar, Lavigne and co-workers [30], are particularly interesting as they are attractive ligands to stabilize late transition metal ion complexes such as those involving Ag I , Au I , Rh I , Cu I [30][31][32][33][34][35], as well as Fe II [36].…”

Anionic N‐Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety

“…[17] Thus, the imidazolin-2-ylidene-4-olate 6 as well as the malonateand imidato-type six-membered carbenes 7 and 8 were presented. [17][18][19][20] Although the coordination of 6-8 (and of related fluorinated species) [21] to transition metals in most cases does not involve metal interactions with the potentially reactive backbone moieties in the solid state, the possibility of such interactions is likely to affect the catalytic performance of homogeneous transition-metal catalysts that contain these carbene ligands. [19] Therefore, to minimize the accessibility of an additional basic site during catalytic processes, the introduction of a non-nucleophilic and chemically robust anionic functionality is required.…”