Schiff Base in Ketoamine Form and Rh(η-cod)-Schiff Base Complex with Z' = 2 Structure from Pairwise C-H···Metallochelate-π Contacts.

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of ()-2-(((2-ethylphenyl)imino)methyl)phenol (HL) or ()-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL), which in turn react with the dinuclear complex of [Rh(η-cod)(µ-OCCH)] (cod = cycloocta-1,5-diene) to afford the mononuclear (η-cod){()-2-(((2-ethylphenyl)imino)methyl)phenolato-κN,O}rhodium(I), [Rh(η-cod)(L)] () or (η-cod){()-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κN,O}rhodium(I), [Rh(η-cod)(L)] () (L or L = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H···O(phenol) form (ketoamine form). H NMR spectra for HL in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for and showed that the deprotonated Schiff base ligand coordinates to the Rh(η-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in in a structure with Z' = 2. The supramolecular packing in HL was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in and . Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in . This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in . Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.