Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z′ = 2 Structure from Pairwise C-H···Metallochelate-π Contacts

Abstract

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 (HL1) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL2), which in turn react with the dinuclear complex of [Rh(η4-cod)(µ-O2CCH3)]2 (cod = cycloocta-1,5-diene) to afford the mononuclear (η4-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ2N,O}rhodium(I), [Rh(η4-cod)(L1)] (1) or (η4-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ2N,O}rhodium(I), [Rh(η4-cod)(L2)] (2) (L1 or L2 = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL2 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). 1H NMR spectra for HL2 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(η4-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 HL2 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.

Polymorphism, mesomorphism and anti-oxidant character of homoleptic bis[(E)-2-(1-((o-ethylphenyl)imino)ethyl)phenolato-κ2-N,O]Ni/Cu(ii) complexes

“…Thermal analysis via differential scanning calorimetry (DSC) was used to probe phase transformation temperatures and the associated enthalpy changes (DH) for 1, microcrystalline 2 isolated from the reaction mixture, and crystallized samples of 2a and 2b. 24,29,[31][32][33] For the nickel complex 1, the DSC heating curve showed an endothermic peak at 246 1C (DH = À36.24 kJ mol À1 ) assigned to melting a crystalline solid to form isotropic liquid (Fig. 6a); upon cooling, an exotherm is evident at 134 1C (DH = +22.35 kJ mol À1 ) assigned to solidification of the isotropic liquid phase.…”

“…32 Similarly, mononuclear Rh(Z 4 -cod) Schiff base complexes of (E)-2-(1-((X-phenyl)imino)methyl)-phenol/-naphthalen-2-ol (X = H, oethyl) are also square planar. 33,34 For these complexes, which each crystallizes in a single morphology, thermal analysis by DSC and PLM reveal a simple phase transformation from crystalline solid to isotropic liquid, with the exception of the Cu(II) species 32 which exhibits mesomorphic properties. In addition, no polymorphism was so far reported from these reported complexes.…”