Schiff Base in Ketoamine Form and Rh(η 4 -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 ( E )-2-(((2-ethylphenyl)imino)methyl)phenol (HL 1 ) or ( E )-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL 2 ), which in turn react with the dinuclear complex of [Rh(η 4 -cod)(µ-O 2 CCH 3 )] 2 (cod = cycloocta-1,5-diene) to afford the mononuclear (η 4 -cod){( E )-2-(((2-ethylphenyl)imino)methyl)phenolato-κ 2 N,O}rhodium(I), [Rh(η 4 -cod)(L 1 )] ( 1 ) or (η 4 -cod){( E )-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ 2 N,O}rhodium(I), [Rh(η 4 -cod)(L 2 )] ( 2 ) (L 1 or L 2 = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL 2 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). 1 H NMR spectra for HL 2 in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η 4 -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 1 in a structure with Z' = 2. The supramolecular packing in HL 2 was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2 . Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1 . This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1 . 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.