Impact of isomeric ligands on molecular stacking and semiconducting behavior in square planar Ni(ii) complexes†

This study investigates the impact of isomeric ligands on the structural and electronic properties of square-planar nickel(II) complexes with salen-type Schiff base ligands. Two isomeric complexes, [Ni(L¹)]·H₂O (1) and [Ni(L²)]·H₂O (2), were synthesized using trans- and cis-N,N′-bis(3-methoxy-5-methylsalicylidene)-cyclohexane-1,2-diamine ligands. Single-crystal X-ray diffraction analysis of their structures reveals distinct packing arrangements resulting from differences in ligand geometry. Complex 1 features extended π-stacking interactions along the crystallographic b-axis, whereas complex 2 forms discrete π-stacked dimers. SEM and TEM images revealed rod-shaped and plate-shaped morphologies for complexes 1 and 2, respectively. Additionally, high-resolution transmission electron microscopy (HRTEM) images demonstrated that complex 2 exhibits a higher degree of crystallinity compared to complex 1. Optical band gap analyses show values of 4.35 eV for 1 and 3.68 eV for 2, reflecting the electronic influence of ligand isomerism. The semiconducting behavior of these complexes was further investigated through current–voltage measurements and theoretical DFT studies, highlighting a stronger electrical conductivity and charge transport in 2. Furthermore, theoretical and Hirshfeld surface analysis provided valuable insights into various intermolecular interactions. These findings demonstrate the potential of isomeric ligands in tuning the properties of nickel-based semiconducting materials for advanced electronic applications.