Pseudohalides regulated diverse helicity in copper(II) coordination polymers derived from a bis(aminoethoxy) ligand

Abstract

This paper reports the syntheses and structural characterization of three new Cu(II) coordination polymers, namely [Cu(BAEE)(N3)2]∝ (1), [Cu(BAEE)(NCO)2]∝ (2) and [Cu(BAEE)(NCS)2]∝ (3), where BAEE is 1,2-bis(2-aminoethoxy)ethane. Compound 1 is a one-dimensional (1D) single-stranded normal helical structure, while compounds 2 and 3 are extraordinary single-stranded meso-helical polymers. The influence of pseudohalides in these systems is observed as the azide ions are cis coordinated in 1 and the thiocyanate ions are trans coordinated in 3, while alternate metal centers are either cis or trans coordinated with cyanate ions in 2. These geometrical preferences play a crucial role in the diverse helicity in these chain structures, where the cis arrangement (in 1) favors a uniform helicity in the structure and the trans arrangement (in 2 and 3) of the pseudohalide ligands around the copper(II) center brings meso-helicity in the chains. The energetic difference of the cis/trans isomers and the formation energies of the different assemblies using DFT calculations suggest that the different H-bond acceptor ability of the thiocyanate ion determines the different architecture observed in 3. Our results highlight how the geometrical preferences of the pseudohalides around the copper(II) center influence the helical diversity in these systems and thereby provide a clue toward the development of uncommon coordination polymers with amazing meso-helical character.