Ligand-to-Metal Charge Transfer Resulting in Metalloaromaticity of [R,R-Cyclohexyl-1,2-bis(2-Oxidonaphthylideneiminato-N,N',O,O')]Cu(II): A Scrutinized Structural Investigation

Molecular and crystal structure analysis of a tetradentate ligand-to-metal charge transfer (LMCT) complex, [R,R-cyclohexyl-1,2-bis(2-oxidonaphthylideneiminato-N,N',O,O')]Cu(II), are described and characterized by X-ray crystallography and FTIR spectroscopy. The Cu(II) centers are coordinated by four atoms of the donor set [N2O2] in a distorted square-planar fashion, which can be attributed to an active electronic delocalization within metallacycles and polychelating property of the pro-ligand. Bond valences of copper centers in four crystallographically independent monomers are slightly distinguished from each other, which is associated with differences in charge delocalization levels of the pro-ligands. Total valence of the copper center is increased with increasing pi-electron donation from naphthalene fragments to metallacycles. This charge transfer leads to pi center dot center dot center dot pi interactions between metallacycles including imine bridges. In addition, decreases in the centroid-centroid distances of pi center dot center dot center dot pi interactions are associated with increased aromatic character of metallacycles. Molecules of the complex are stacked as dimers in the crystal structure formed by pi center dot center dot center dot pi interactions and aggregations of these dimeric formations along a-axis are strengthened by C-H center dot center dot center dot O type H-bonds and C-H center dot center dot center dot pi interactions.