Record Dissymmetry in Vibrational Optical Activity: Combining Vibronic Coupling and Aggregation in Cobalt(II) Camphorate Complexes

A family of cobalt(II) bis-trifluoroacetylcamphorate complexes (1-3), along with the diamagnetic zinc(II) analogue 4, has been synthesized and structurally characterized. The cobalt complexes exhibit solvent-driven interconversion between cis- and trans-octahedral geometries, coupled to labile solvent coordination. Their chiroptical response spans the UV, visible, NIR and IR ranges and is highly sensitive to coordination geometry and solvent. In dilute solutions, vibrational circular dichroism (VCD) dissymmetry factors up to |g| ≈ 4×10-3 reflect vibronic coupling with low-lying electronic states, a hallmark of high-spin CoII centres. At concentrations above 30 mM in acetonitrile, spontaneous microcrystallization yields suspensions with an exceptional g value of approximately 0.2 at 1600 cm-1, that is the largest dissymmetry factor ever reported by VCD. This record value arises from the unprecedented concurrence of vibronic and aggregation-induced enhancement in the same species, as supported by crystallographic, spectroscopic and computational evidence.