A systematic evaluation of the interplay of weak and strong supramolecular interactions in a series of Co(II) and Zn(II) complexes tuned by ligand modification

A systematic investigation on a designed series of 21 transition metal complexes has been carried out with the intention to explore and assess the relative strength and the way in which intermolecular interactions, namely, weak and strong hydrogen-bonding and π-π interactions, cooperate and direct molecular association during crystallization. The complexes were prepared using the general M ^II/X ^-/L or HL′ (M ^II = Co ^II, Zn ^II; X ^- = Cl ^-, Br ^-, I ^-, NO ₃ ^-, NO ₂ ^-, ClO ₄ ^-; L = 1-methyl-4,5-diphenylimidazole; and HL′ = 4,5-diphenylimidazole) reaction system and were characterized by single-crystal X-ray crystallography. Although the two ligands are structurally similar, the crystal packing organization of their complexes is markedly different. In structures with L, the 3D assembly is based only on weak C-H•••X, C-H•••π, and intramolecular π•••π stacking interactions, whereas in those with HL′, it is the recurring N-H•••X motifs that clearly dominate and guide the molecular self-assembly. The formation of such synthons has been activated by choosing appropriate anions X, acting as terminal ligands or counterions. In parallel, the conformational flexibility of the two ligands serves a dual purpose: (i) L contributes to the stabilization of complexes via intramolecular π•••π stacking interactions, and (ii) HL′ facilitates the synthon formation by adopting appropriate conformations, even at the expenses of the stabilizing intramolecular π•••π stacking.