We report the design as well as structural and spectroscopic characterizations of two new coordination compounds obtained from Cd(NO)·4HO and polydentate ligands, benzilbis(pyridin-2-yl)methylidenehydrazone (L) and benzilbis(acetylpyridin-2-yl)methylidenehydrazone (L), in a mixture with two equivalents of NHNCS in MeOH, namely [Cd(SCN)(NCS)(L)(MeOH)] (1) and [Cd(NCS)(L)(MeOH)] (2). Both L and L are bound via two pyridyl-imine units yielding a tetradentate coordination mode giving rise to the 12 π electron chelate ring. It has been determined for the first time (qualitatively and quantitatively), using the EDDB electron population-based method, the HOMA index, and the ETS-NOCV charge and energy decomposition scheme, that the chelate ring containing Cd can be classified as a quasi-aromatic Möbius motif. Notably, using the methyl-containing ligand L controls the exclusive presence of the NCS connected with the Cd atom (structure 2), while applying L allows us to simultaneously coordinate NCS and SCN ligands (structure 1). Both systems are stabilized mostly by hydrogen bonding, C-H···π interactions, aromatic π···π stacking, and dihydrogen C-H···H-C bonds. The optical properties have been investigated by diffused reflectance spectroscopy as well as molecular and periodic DFT/TD-DFT calculations. The DFT-based ETS-NOCV analysis as well as periodic calculations led us to conclude that the monomers which constitute the obtained chelates are extremely strongly bonded to each other, and the calculated interaction energies are found to be in the regime of strong covalent connections. Intramolecular van der Waals dispersion forces, due to the large size of L and L, appeared to significantly stabilize these systems as well as amplify the aromaticity phenomenon.

We present here two different coordination polyhedra of pincer type N 2 O hydrazone based ligands supplemented with thiocyanate ions. The compounds namely [Hg(SCN) 2 (HL 1 )] (1) and[Hg(SCN) 2 (HL 2 )] (2) have a common isonicotinohydrazone fragment and have been prepared by using a coordination driven self-assembly of the Hg(SCN) 2 with two different ligands including 2-benzoylpyridine-isonicotinoylhydrazone (HL 1 ), and 2-acetylpyridineisonicotinoylhydrazone (HL 2 ). In compound 1 the ligand coordinated to the mercury center in the keto form (=N-NH=C=O) whereas, in compound 2, the proton at the hydrazine group have been shifted to the uncoordinated pyridine ring and the ligand acted as a zwitterion. The structures provide a complementary system for proton transfer within the ligand molecule involving the keto-enol tautomerization of amide group and 4-pyridyl N protonation. As a result, the relative location of orbitals and ligands in the complexes are different as well as the bonding strength and the coordination polyhedra. We have also studied electrostatically enhanced π···π (either conventional or involving the chelate ring) interactions observed in the solid state of both compounds and analyzed them using DFT calculations, MolecularElectrostatic Potential surface and the Bader's theory of atoms in molecules.

In this manuscript we report the synthesis and X-ray characterization of four ZnIJII) complexes and three CdIJII) complexes with an asymmetrical hydrazone-pyridine based ligand {HL = 2-acetyl-pyridylisonicotinoylhydrazone (HAPIH)} 7). The Schiff base acts as a tridentate N 2 Odonor ligand through the oxygen, the imine and pyridine nitrogen atoms in all the complexes. In most complexes, the ligand is observed to coordinate as a zwitterion since the proton in the hydrazine group (N-NH-CO) shifted to the uncoordinated pyridine ring, except in 7. On the other hand, in 5 the ligand acts as a negatively charged species and is bound to the cadmium center in the enolic form (N-NC-O − ). In complexes 2-4, 6 and 7, the coordination geometry around each metal center is distorted trigonal bipyramidal, with the coordination sphere of the metal completed by two halide or NCS anions. On the other hand, in homoleptic complexes 1 and 5, the metal, chelated by two tridentate Schiff base ligands, exhibits an octahedral geometry. In the crystal packing of all compounds, the pyridine rings favour π-π interactions among the symmetry related complexes. The noncovalent interactions among the complexes have been analyzed using Hirshfeld surface analysis and DFT calculations using Grimme's D3 dispersion correction to properly describe the π-π interactions.

We report seven Zn(ii)/Cd(ii) complexes with picolinoyl/isonicotinoyl hydrazone based ligands exhibiting relevant chelate–π interactions.