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

Abstract

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(μ-Cl)(cod)]2 in ...