Lone-Pair-π Bond Strength Unveiled by a Combined Experimental and Computational Study

A series of naphthalene-diimide (NDI) and perylene-diimide (PDI) connected bis-N-heterocyclic carbene complexes of iridium(III) have been prepared and fully characterized. The analysis of their NMR spectroscopic features, together with their molecular structures show that these species display lone-pair-π interactions between the chloride ligands of the Ir(III) complex and the heterocycles of the NDI/PDI moieties. The detection of this type of interaction in solution is due to the formation of two atropisomers, which are formed as a result of the restricted rotation about the Ir-C_carbene bond imposed by the (Cl)lp⋅⋅⋅π interaction. Variable-temperature ¹H NMR analysis allowed the determination of the strength of this non-covalent interaction, which lies between ΔH=6.6 and 10 kcal/mol. The computational studies performed fully support the experimental findings.