A Family of Co(II) Complexes Exhibiting Record Dissymmetry Factors in Vibrational Circular Dichroism

A family of novel chiral cobalt(II) complexes with one bis(oxazoline) and two diketonate ligands, [Co (dkt)2(BOX)], is reported. These complexes exhibit octahedral coordination geometry and feature a chiroptical superspectrum extending from UV to mid-IR regions. Vibrational circular dichroism (VCD) enhancement occurs thanks to the coupling between ligand vibrations and low-lying electronic transitions allied to the octahedral high-spin CoII d7 core, generating VCD bands of unprecedented intensity in the mid-IR region with dissymmetry factors up to 3.6×10–2 for a VCD peak at 1453 cm–1 and > 5×10–2 for its shoulder at 1420 cm–1. These values correspond to ≈ 100-fold enhancement with respect to the diamagnetic ZnII analog and set a record among non-aggregated species in solution. The extraordinarily enhanced intensity allows recording a VCD spectrum with S/N ratio ≈ 30 in a single acquisition.