Halogen Bond to Experimentally Significant N-Heterocyclic Carbenes (I, IMe(2), I(i)Pr(2), I(t)Bu(2), IPh(2), IMes(2), IDipp(2), IAd(2); I = Imidazol-2-ylidene)

The subjects of the article are halogen bonds between either XCN or XCCH (X = Cl, Br, I) and the carbene carbon atom in imidazol-2-ylidene (I) or its derivatives (IR [Formula: see text]) with experimentally significant and systematically increased R substituents at both nitrogen atoms: methyl = Me, iso-propyl = [Formula: see text] Pr, tert-butyl = [Formula: see text] Bu, phenyl = Ph, mesityl = Mes, 2,6-diisopropylphenyl = Dipp, 1-adamantyl = Ad. It is shown that the halogen bond strength increases in the order Cl < Br < I and the XCN molecule forms stronger complexes than XCCH. Of all the carbenes considered, IMes [Formula: see text] forms the strongest and also the shortest halogen bonds with an apogee for complex IMes [Formula: see text] ICN for which [Formula: see text] = 18.71 kcal/mol and [Formula: see text] = 2.541 Å. In many cases, IDipp [Formula: see text] forms as strong halogen bonds as IMes [Formula: see text]. Quite the opposite, although characterized by the greatest nucleophilicity, I [Formula: see text] Bu [Formula: see text] forms the weakest complexes (and the longest halogen bonds) if X ≠ Cl. While this finding can easily be attributed to the steric hindrance exerted by the highly branched tert-butyl groups, it appears that the presence of the four C-H⋯X hydrogen bonds may also be of importance here. Similar situation occurs in the case of complexes with IAd [Formula: see text].