C−H Oxidative Addition of Bisimidazolium Salts to Iridium and Rhodium Complexes, and N-Heterocyclic Carbene Generation. A Combined Experimental and Theoretical Study

Abstract

A series of bis-N-heterocyclic carbenes of rhodium and iridium have been obtained and characterized. The formation of the M-C bond has been studied according to experimental and theoretical data, showing that two different mechanisms are operative for the first (single proton deprotonation of the bisimidazolium salt, or oxidative addition followed by deprotonation of the metal hydride) and second (oxidative addition of the second bisimidazolium C-H bond, yielding a NHC-Ir III -H species) metalation processes. In the case of complexes with long linkers between the imidazolium rings, reductive elimination of HCl affords bisimidazolylidene complexes of Ir I . According to the theoretical studies we concluded that thermodynamic parameters would determine the formation of the NHC-Ir III -H species, while Ir I -NHC species would be kinetically favored in the case of complexes with long linkers between the azole rings. The crystal structures of a series of Ir-bis(NHC) complexes are described.

Modular Synthesis of a New Type of Chiral Bis(carbene) Ligand from L‐Valinol and Iridium(I) and Rhodium(I) Complexes Thereof

“…The M-C carbene bond lengths are between 2.015 and 2.068 Å, as expected for (NHC)rhodium(I) and -iridium(I) compounds. [33,34,36,37] The seven-membered rings formed by the ligands and the metal atoms adopt boat-like conformations, as known for cycloheptane, with the backbone isopropyl group in either the endo (Figure 1) or exo position (Figure 2). A comparison of the distances between the metal atom and the backbone carbon atoms of both stereoisomers shows that these carbon atoms change their positions relative to the metal atom.…”

“…Whereas the iridium complexes were prepared by stirring the reaction mixtures overnight at ambient temperature, the reactions with rhodium were conducted at reflux temperature for 2 h and then with stirring at ambient temperature for 3 h. All complexes were purified by gradient column chromatography on silica gel, with dichloromethane/acetone as eluent and KPF 6 for anion exchange, to give the rhodium(I) (6a,b) and iridium(I) (7a,b) complexes in 58-68 % yield. [33,34] An additional aspect of complexation was the formation of two diastereomers, which were not separable by column chromatography. The reaction between [IrCl(cod)] 2 , Ag 2 O and 5a or 5b gave the exo isomer as the major product, where the backbone isopropyl group lies out of the coordination plane, as depicted in Figure 2 (vide infra).…”

“…15 ppm for the vinylic cod signals. [34] The existence of two stereoisomers is indicated by a double set of signals with intensities that correspond to the exo/endo ratios. The 1 H NMR shifts of the backbone protons are characteristic for the exo and endo isomers.…”

Platinum(ii) N-heterocyclic carbene complexes: coordination and cyclometallation.

“…Other interesting methods include the utilisation of an electron-reservoir complex, together with air, to generate NHCs, 9 or the use of imidazolium-2-carboxylates which lose CO 2 to generate NHCs, 10 though, increasingly, direct reaction of the imidazolium salt with the metal has found favour. [11][12][13][14] More recently, NHC ligands have been shown not to be innocent spectators, but reactive intermediates themselves, with a number of new products being formed. 15,16 Whilst most the studies on catalytically active systems have their emphasis on palladium based chemistry, 17 platinum complexes are also of interest.…”

Unconventional Reactivity of Imidazolylidene Pyridylidene Ligands in Iridium(III) and Rhodium(III) Complexes

“…This hypothesis is supported by our previous experimental and theoretical studies on the mechanisms of the formation of bis(NHC) complexes of Cp*M (M = Rh and Ir). [13,16] The formation of complex 5 may occur through the reductive elimination of diiodine from the reaction intermediate A (Scheme 3), and subsequent oxidative addition of the pyridinium ring to form a Rh III hydride with a pyridylidene imidazolylidene ligand. The deprotonation of the Cp* ligand may induce a nucleophilic attack of the fulvene onto the pyridylidene carbon atom, thus affording the C À C coupling between the Cp* and the pyridinium rings.…”