Synthesis and Characterization of Bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]-Λ/Δ-cobalt(II) (X = H, P-Ch3o, P-Br) with Symmetry- and Distance-Dependent Vibrational Circular Dichroism Enhancement and Sign Inversion.

The enantiopure Schiff bases (R or S)-N-1-(X-C6H4)ethyl-2-hydroxy-1-naphthaldimine {X = H [(R or S)-HL1], p-CH3O [(R or S)-HL2], and p-Br [(R- or S)-HL3]} react with cobalt(II) acetate to give bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-kappa N-2,O]-Lambda/Delta-cobalt(II) {X = H [Lambda/Delta-Co-(R or S)-L1], p-CH3O [Lambda/Delta-Co-(R or S)-L2], and p-Br [Lambda/Delta-Co-(R or S)-L3]} (1-3), respectively. Induced Lambda and Delta chirality originates at the metal center of the C-2-symmetric molecule in pseudotetrahedral geometry. Differential scanning calorimetry analyses explored the thermal stability of the complexes, which undergo reversible phase transformation from crystalline solid to isotropic liquid phase for 1 and 3 but irreversible phase transformation for 2. Like other cobalt(II) complexes, compounds 1-3 exhibit a continuous ensemble of absorption and circular dichroism bands, which span from the UV to IR region and can be collected into a superspectrum. Infrared vibrational circular dichroism (IR-VCD) spectra witness the coupling between Co2+-centered lowlying electronic states and ligand-centered vibrations. The coupling produces enhanced and almost monosignate VCD spectra, with both effects being mode-dependent in terms of the A or B symmetry (in the C-2 point group) and distance from the Co2+ core.