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.