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

A family of novel chiral cobalt(II) complexes with two diketonate and one bis(oxazoline) ligand, [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 distorted 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 1430 cm−1. These values correspond to a ≈ 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 a signal-to-noise (S/N) ratio of ≈ 30 in a single acquisition.