Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N3,C2 ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies

Abstract

Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with 0.5 equiv. of [Ir(cod)(μ-Cl)]₂ yielded two geometrical isomers of a doubly C2,N3-bridged dinuclear complex [Ir(cod){μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (3), 3_H–H, a head-to-head (H–H) isomer of C_S symmetry, and 3_H–T, the thermodynamically preferred head-to-tail (H–T) isomer of C₂ symmetry. The metallated carbon of the 4 electron donor anionic bridging ligands has some carbene character, reminiscent of the situation in N-metallated protic NHC complexes. Displacement of cod ligands from 3_H–H and 3_H–T afforded the tetracarbonyl complexes [Ir(CO)₂{μ-C₃H₂N₂(Mes)-κC2,κN3}]₂4_H–H and 4_H–T, respectively. The reaction with PMe₃, which gave only one complex, [Ir(CO)(PMe₃){μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (5), demonstrates that the isomerization of the central core Ir[μ-C₃H₂N₂(Mes)-κC2,κN3]₂Ir from H–H to H–T on going from 4_H–H to 5 is readily triggered by phosphine substitution under mild conditions. Oxidative-addition of MeI to 5 afforded the formally metal–metal bonded d⁷–d⁷ complex [Ir₂(CO)₂(PMe₃)₂(Me)I{μ-C₃H₂N₂(Mes)-κC2,κN3}₂] (6). The blue [Ir(C₂H₄)₂{μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (7) and purple [Rh(C₂H₄)₂{μ-C₃H₂N₂(Dipp)-κC2,κN3}]₂ (9) tetraethylene complexes were also obtained with only a H–T arrangement of the bridging ligands. Although only modestly efficient in alkane dehydrogenation, complex 7 was found to be a more active pre-catalyst than 3_H–T, 4_H–T and 5, probably because of the favorable lability of the ethylene ligands. From cyclic voltammetry, exhaustive coulometry and spectroelectrochemistry studies, it was concluded that 3_H–T undergoes a metal-based one electron oxidation to generate the mixed-valent Ir(I)/Ir(II) system. The energy of the intervalence band for the orange dirhodium complex [Rh(cod){μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (8) is shifted toward lower energies in comparison with 3_H–T, reflecting the decrease of the energy with the intermetallic distance. It was concluded from the EPR study that the Ir and Rh centres contribute substantially to the experimental magnetic anisotropy and thus to the singly occupied molecular orbital (SOMO) in the mixed-valent Ir(I)/Ir(II) and Rh(I)/Rh(II) systems. The molecular structures of 3_H–H, 3_H–T, 8 and 9 have been determined by X-ray diffraction.