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

Abstract

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.