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-C 6 H 4 )ethyl-2-hydroxy-1-naphthaldimine {X = H [( R or S )-HL1], p -CH 3 O [( 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-C 6 H 4 )ethyl-2-oxo-1-naphthaldiminato-κ 2 N , O ]-Λ/Δ-cobalt(II) {X = H [Λ/Δ-Co-( R or S )-L1], p -CH 3 O [Λ/Δ-Co-( R or S )-L2], and p -Br [Λ/Δ-Co-( R or S )-L3]} ( 1 - 3 ), respectively. Induced Λ and Δ 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 Co 2+ -centered low-lying 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 Co 2+ core.