New Coordination Polymers with Unexpected Octahedral copper(II) Geometry: Synthesis, Supramolecular and Theoretical Study

In this work, we report the synthesis of two new copper(II) complexes that preferentially assemble into coordination polymers through potential N,N,N- and N,C,N-type pincer ligands, with the metal centers adopting octahedral geometries. The crystallographic analysis of coordination polymers 2,6-bis((1H-1,2,4-triazol-1-yl)methyl)pyridine-CuCl2 (C1), 3,5-bis(1,2,4-triazol-1-ylmethyl)toluene-CuCl2 (C2), and 2,6-bis((1H-1,2,4-triazol-1-yl)methyl)benzene-CuCl2 (C3) reveals polymeric structures formed through triazole ligands imposing cis conformations and generating comparable unit cells. While in the case of ligands it was observed that 2,6-bis((1H-1,2,4-triazol-1-yl)methyl)pyridine (L1) forms a 3D hydrogen-bonding network via triazoles and pyridine nitrogen, 3,5-bis(1,2,4-triazol-1-ylmethyl)toluene (L2), lacking pyridine, adopts a simpler 1D chain stabilized by C–H···N and C–H···π interactions, with van der Waals forces between chains and with C2 symmetry and 2,6-bis((1H-1,2,4-triazol-1-yl)methyl)benzene (L3) displays both trans and cis forms, creating layered structures and losing symmetry due to directional hydrogen bonds. The computational study established that the formation energy (ΔEf) of the polymer chains C1, C2, and C3 decreases linearly with chain growth, indicating progressive electronic stabilization, with C2 showing the most favorable trend (−2.97 kcal/mol per unit). Similarly, the average stabilization energy per monomer (ΔEavg) becomes progressively more negative, converging at a chain length of about ten units, with C2 again displaying the greatest stability (−2.68 kcal/mol per monomer). In addition, the crystal structure of two azole derivatives, L2 and L3, is presented.