Quasi-aromatic Möbius Metal Chelates.

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.