Symmetry-Dependent Vibrational Circular Dichroism Enhancement in Co(II) Salicylaldiminato Complexes.

Chiral coordination compounds of Co(II) and other metal ions with low-lying excited states (LLES) display enhanced vibrational circular dichroism (VCD) spectra which can be used for sensing purposes. In addition to the enhancement, a series of Co(II) salicylaldiminato complexes exhibit unique VCD spectra which are almost monosignate, instead of showing the usual alternation of positive and negative bands. The two phenomena of VCD enhancement and monosignate appearance are strictly related and symmetry-dependent. In fact, the normal modes of B symmetry of the C2-symmetric pseudo-tetrahedral species are selectively affected, thanks to their combination with one or more LLES having the same B symmetry. The symmetry dependence of VCD enhancement is here demonstrated for the first time.