Relevant and unprecedented C–H/σ supramolecular interactions involving σ-aromatic M2X2 cores

Abstract

A novel type of C–H/σ supramolecular interaction involving σ-aromatic M₂X₂ (M = Cu, Hg; X = Cl, Br, I, S) cores is reported for the first time. Three new polymeric coordination copper complexes, {[Cu(μ-Cl)(Cl)(μ-L)]₂}_n (1), {[Cu(μ-I)(μ-L)]₂}_n (2) and [Cu(Br)₂(μ-L)(CH₃CN)₂]₂ (3), have been synthesized with the organic ligand α,ω-bis(benzotriazoloxy)propane system (L) and halides as counterions. A very interesting C–H/σ supramolecular interaction has been observed in the solid state structure of compound 2, similar to a C–H⋯π interaction, which has been confirmed by Bader's “atoms-in-molecules” AIM analysis. The Nucleus Independent Chemical Shift (NICs) method was used to evaluate the aromatic character of the different cores in this study. The influence of the nature of the metal ions, bridging atoms, oxidation states, and coordination environments around the metal centers on the strength of the aromaticity of the M₂X₂ cores was theoretically analyzed and explained. The binding ability of the 1-alkoxy-1,2,3-benzotriazole ring to establish π–π and C–H/π interactions and how its coordination to Cu(I) and Cu(II) ions affects the strength of the aforementioned interactions have been discussed. The electron deficient triazole ring and its π-acidity increases upon coordination of the Cu ion, leading to the formation of a lone pair (lp)–π interaction involving the five-membered ring of the ligand, which has also been analyzed. We have also analyzed the C–H/σ interactions of previously reported X-ray crystal structures of different coordination polymers based on a binuclear copper(I) complex and 2,3-dimethylpyrazine, dithioethers, and benzotriazol-1-yl-based pyridyl units as ligands using I⁻ as a counter-ion. Complex 1 shows antiferromagnetic behavior with a magnetic coupling constant of J = −7.9 cm⁻¹. Moreover, photoluminescence and TGA studies of the complexes were also carried out.