Synthesis, Spectroelectrochemical Behavior, and Chiroptical Switching of Tris(β-diketonato) Complexes of Ruthenium(III), Chromium(III), and Cobalt(III)

Five tris(β-diketonato) complexes of ruthenium(III), chromium(III), and cobalt(III) [Ru(Buacac)₃ (1), Ru(Oacac)₃ (2), Cr(Buacac)₃ (3), Cr(Oacac)₃ (4), and Co(Buacac)₃ (5), where Buacac = 3-butylpentane-2,4-dionato and Oacac = 3-octylpentane-2,4-dionato] with a chiral propeller-like structure have been prepared. Ligands and complexes syntheses are presented together with characterization of the compounds by ¹H and ¹³C NMR spectroscopy, mass spectrometry, IR, UV-vis, electronic circular dichroism (ECD) spectroscopy, electrochemistry studies, and first-principles calculations. The crystal structures of 1 and 5 have also been obtained and analyzed. A comparison of the ¹H NMR spectra of diamagnetic (ligands and 5) and paramagnetic (1 and 2) species is presented. Optical resolution of the five complexes has been achieved for the first time by supercritical fluid chromatography using a chiral column, giving rise to very high purity grades in all cases. ECD measurements and calculations have led to the assignment of the absolute configuration, δ or λ, of each enantiomer for 1-5. Spectroelectrochemical UV-vis and ECD studies have been performed on ruthenium λ-2 and chromium λ-4 complexes, revealing their redox-triggered chiroptical switching confirming the noninnocence character of the β-diketonate ligands.