On the Interaction between N-Heterocyclic Carbenes and Organic Acids: Structural Authentication of the First N−H⋅⋅⋅C Hydrogen Bond and Remarkably Short C−H⋅⋅⋅O Interactions

Two unprecedented hydrogen bonds were observed in the adducts obtained by the reactions of an N-heterocyclic carbene with a bulky phenol and with diphenylamine. In the former case, proton transfer yields an imidazolium aryloxide (1) containing the shortest C−H⋅⋅⋅O interaction seen to date and in the latter, the hydrogen atom remains bound to the organic acid but forms a unique N−H⋅⋅⋅C hydrogen bond (2).