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Issue 39, 2015
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Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N3,C2 ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies

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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), 3H–H, a head-to-head (H–H) isomer of CS symmetry, and 3H–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 3H–H and 3H–T afforded the tetracarbonyl complexes [Ir(CO)2{μ-C3H2N2(Mes)-κC2,κN3}]24H–H and 4H–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 4H–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 3H–T, 4H–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 3H–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 3H–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 3H–H, 3H–T, 8 and 9 have been determined by X-ray diffraction.

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

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Supplementary files

Article information


Submitted
24 Jun 2015
Accepted
23 Jul 2015
First published
23 Jul 2015

Dalton Trans., 2015,44, 17030-17044
Article type
Paper
Author version available

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

F. He, L. Ruhlmann, J. Gisselbrecht, S. Choua, M. Orio, M. Wesolek, A. A. Danopoulos and P. Braunstein, Dalton Trans., 2015, 44, 17030
DOI: 10.1039/C5DT02403J

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