Lone Pair–π Interactions Modulated by Coordination Geometry in NDI-di-NHC Metal Chloride Complexes

Two dimetallic complexes of Au(I) and Ru(II), featuring a naphthalene-diimide (NDI)-linked bis-N-heterocyclic carbene (NHC) ligand, have been synthesized and fully characterized. Spectroscopic analyses, combined with structural data, reveal the presence of lone pair–π (lp–π) interactions between the chloride ligands coordinated to the metal centers and the central NDI unit. The nature of this interaction varies with the metal's coordination geometry: in the di-Au(I) complex, the lp–π interaction is intermolecular, whereas in the di-Ru(II) complex it is intramolecular, leading to the formation of two atropisomers that are distinguishable by conventional spectroscopic techniques. NMR studies of both complexes provided key insights into the lp–π interaction. For the di-Ru(II) complex in particular, these studies enabled the determination of the kinetic and thermodynamic parameters governing the equilibrium between the atropisomers, revealing a barrier to interconversion (ΔH≠) of 10.4 kcal mol^–1, indicative of a strong covalent character in the lp–π interaction.