Long-Range Intramolecular Electronic Communication in a Trinuclear Ruthenium Tropolonate Complex

Abstract

Dinuclear and trinuclear ruthenium complexes, [Ru(trop)(Ctrop)Ru(dppe)Cp] [2b; trop = tropolonato, Ctrop = ethynyltropolonato, dppe = 1,2-bis(diphenylphosphino)ethane] and [Ru(trop){(Ctrop)Ru(dppe)Cp}] (3), were synthesized, and their electronic and electrochemical properties were investigated in comparison with our previously reported complex [Ru(acac)(Ctrop)Ru(dppe)Cp] (2a). The electron-donating Ru(dppe)Cp unit and electron-accepting RuO unit are connected by Ctrop in these complexes. 2a incorporates acetylacetonate as an ancillary ligand, while 2b and 3 incorporate tropolonate as an ancillary ligand. Every complex, 2a, 2b, and 3, exhibits similar UV-vis-near-IR (NIR) absorption spectra, demonstrating the lack of explicit intramolecular electronic communication between the units at least in the neutral state. The weak NIR absorption in 2a further diminished upon electrochemical oxidation, indicating almost no electronic communication between the units. In contrast, 2b and 3 exhibit broad NIR absorptions upon oxidation. Additionally, 3 exhibits four stepwise redox couples in the electrochemical study, which are formally attributed to [Ru(trop)]/[Ru(trop)], two [Ru(dppe)Cp]/[Ru(dppe)Cp], and [Ru(trop)]/[Ru(trop)] couples. Clear separation of the redox couples attributed to the two terminal [Ru(dppe)Cp] units demonstrates the thermodynamic stability of the intermediate oxidation states with respect to disproportionation. Further electrochemical studies using an electrolyte including perfluorinated weakly coordinating anions and density functional theory/time-dependent density functional theory calculations confirmed the effect of ancillary ligands, acetylacetonate and tropolonate. In the case of 2a, electronic delocalization over the whole complex, especially over the [Ru(acac)(trop)] unit, appears to be small. In contrast, the electronic communication between [Ru(dppe)Cp] and [Ru(trop)] units in 3 seems to be enhanced upon oxidation, resulting in the long-range intramolecular electronic communication.

Synthesis, Structure, Electrochemical, and Spectroscopic Properties of Hetero-Bimetallic Ru(II)/Fe(II)-Alkynyl Organometallic Complexes

“…Bimetallic or polymetallic inorganic and organometallic complexes bridged by π-conjugated spacers have received remarkable interest not only for their fascinating mixed valence chemistry but also for their emerging potential applications in molecular electronics, electrochromism, information storage, ion sensing, molecular magnetism, and dye sensitized solar cells . The pioneering discovery of Creutz–Taube ion, [(NH 3 ) 5 Ru-pyrazine-Ru­(NH 3 ) 5 ] 5+ , led the scientists to design and develop binuclear complexes to examine the extent of charge delocalization in their mixed-valence states and its fascinating spectroscopic properties .…”

Design, Synthesis, and Characterization of Organometallic BODIPY-Ru(II) Dyads: Redox and Photophysical Properties with Singlet Oxygen Generation Capability

“…In these complexes, the exchange of unpaired electrons or holes between the two metal termini occurs through the molecular bridging ligand, resulting in charge interconversion. A significant number of NIR absorbing mixed-valence metal complexes − by varying the redox-active metallic termini and by modifying the π-conjugated spacer units linked between metal centers to modulate intramolecular electronic communication along the molecular backbone have been intensively investigated since the first synthesis of binuclear Ru 2 II,III complex [(NH 3 ) 5 Ru-pyrazine-Ru­(NH 3 ) 5 ] 5+ by Creutz and Taube. , In this regard, BODIPY-based π-conjugated congeners have emerged recently as excellent building units to construct NIR-absorbing organometallic mixed-valence species. − Several research groups have reported long-range metal–metal coupling in ferrocenyl-functionalized BODIPYs or aza-BODIPYs, in which ferrocenyl moieties are connected directly to the macrocyclic core . Moreover, a conceptually reverse system bearing hybrid organic–inorganic redox-active centers has also been developed to investigate the electronic delocalization in this type of metal–organic electrophores. − The electronic interplay between the organic electrophore and the metallic center often guides this type of dyad to NIR absorption.…”