Chiroptical switching behavior of heteroleptic ruthenium complexes bearing acetylacetonato and tropolonato ligands.

Four types of tris-chelate ruthenium complexes bearing acetylacetonato (acac) and tropolonato (trop) ligands were synthesized and optically resolved into Δ and Λ isomers: [Ru(acac)3] (Ru-0), [Ru(acac)2(trop)] (Ru-1), [Ru(acac)(trop)2] (Ru-2), and [Ru(trop)3] (Ru-3). Chiral HPLC chromatograms, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) of the four ruthenium complexes were systematically investigated. As a result, the absolute configurations of the newly prepared enantiomeric complexes Ru-2 and Ru-3 were determined. For the case of Ru-2, its absolute configuration was also confirmed by single crystal X-ray diffraction analysis. The ECD changes upon chemical oxidation were further investigated for the four complexes. An ECD change in enantiomeric Ru-1 was observed upon oxidation, but the oxidized species soon returned to the neutral state within a few minutes. Enantiomers of Ru-3 also showed explicit ECD changes upon oxidation. Further, the lifetime of the oxidized state was the longest among the four investigated complexes, whereas they racemized in solution at room temperature. In contrast, the enantiomers of heteroleptic complexes (Ru-1 and Ru-2) concurrently exhibited ECD changes, relatively long lifetime of the oxidized state, and nil or quite slow racemization behavior. The coexistence of acac and trop ligands was key to making the competing factors compatible in the resultant ruthenium complexes.

Hexagonal ordering of racemic Ni(II) complexes in the interlayer space of a clay mineral.

The molecular arrangement of racemic [Ni(phen)3]2+ complexes in the interlayer space of montmorillonite was investigated using X-ray diffraction measurements. When the amount of the adsorbed complexes exceeded the cation exchange capacity, they were found to form a double-molecular layer with the 2D hexagonal arrangement in the interlayer space.

Tris(tropolonato) ruthenium as a hub for connecting π-conjugated systems.

In this study, the intramolecular electronic communication between π-conjugated moieties bridged by the tris-chelate [Ru(trop)3] (trop = tropolonate) framework has been investigated and compared with [Ru(acac)3] (acac = acetylacetonate) derivatives. Two types of π-conjugated groups, -C2SiMe3 and -C2Ph, which were each introduced at the 5-position of tropolonate, were found to behave almost independently in the resultant ruthenium complexes. In contrast, the cyclic voltammetry study of [Ru(trop)3] derivatives showed a clear decrease in ΔE, the difference in the potentials of the two redox couples, with an increase in the number of π-conjugated groups introduced into the [Ru(trop)3] framework. The lower ΔE values observed in [Ru(trop)3] derivatives compared to those in [Ru(acac)3] derivatives illustrated the non-innocent behavior of the tropolonate ligand, i.e., the mixing between the metal- and ligand-based frontier orbitals. This orbital mixing was exemplified in the oxidized [Ru(trop)3] derivatives, which showed a broad near-infrared (NIR) absorption. The increase in the red shift of the NIR absorption with the increasing number of terminal groups indicated intramolecular electronic communication between the terminal π-conjugated moieties. In contrast, no NIR absorption was observed upon oxidation of [Ru(acac)3] derivatives with -C2Ph groups. The lifetimes of the in situ formed [Ru(trop)3]+ and [Ru(acac)3]+ in acetonitrile solution were found to be several hours and minutes, respectively. Density functional theory calculations for [Ru(trop)3] and [Ru(acac)3] with terminal -C2Ph groups demonstrated that the spin densities in their mono-oxidized states were evenly distributed over the entire molecular framework and localized mainly on one ligand, respectively. These results confirmed that the mono-oxidized [Ru(trop)3]+ framework can behave as a hub and induce moderate intramolecular electronic communication between terminal π-conjugated groups.