Elusive Free Bisimino-N-heterocyclic Carbene and Its Rearrangement by C-C Coupling. Characterization of Relevant Iridium(I) and Chromium(II) Complexes

The potentially pincer-type N-heterocyclic (NHC) precursor salt 1,3-bis((E)-1-((2,6-diisopropylphenyl)imino)ethyl)-1H-imidazol-3-ium chloride, 3(Cl-), was prepared by reaction of N-(2,6-diisopropylphenyl)acetimidoyl chloride (1) with N-(1-(1H-imidazol-1-yl)ethylidene)-2,6-diisopropylaniline (2). Attempts to crystallize 3(Cl-) at different temperatures afforded single crystals of 3(Cl-). MeCN from MeCN and the mixed salt 4 from toluene, in which 2H(+)(Cl-) and 3(Cl-) have cocrystallized. Deprotonation of 3(Cl-) yielded 1,3-bis[1-(2,6-diisopropylphenylimino)ethyl]imidazol-2-ylidene (5), the first bis(imino)-N-heterocyclic free carbene, and a novel product, 6, resulting formally from the insertion of the carbene carbon atom of 5 into the C-H bond of the N=CHN moiety of 2 and formation of a new C-C bond. The structures and NMR spectra of 4 and 6 indicate that loss of one N-substituent from the bis(imino) NHC core by exocyclic imidazole N-C bond cleavage is relatively easy. Reaction of 5 with [Ir(mu-Cl)(cod)](2) in pentane afforded in good yield the mononuclear, 16e Ir(I) NHC complex, [IrCl(cod)(5)] (7). The reaction of 6 with [CrCl2(THF)(2)] led to the isolation of the paramagnetic complex trans-[CrCl2(6)(2)] (8), in which the coordination geometry at the chromium is slightly distorted square planar. This complex is the first structurally characterized Cr(II) imidazole complex. The compounds 3(Cl-), 4, 6, 7, and 8 were identified by X-ray diffraction methods.