Homo- and Heteronuclear Ruthenium and Osmium Complexes Containing an Asymmetric Pyrazine-Based Bridging Ligand

Abstract

The synthesis, characterization, and electrochemical, photophysical, and photochemical properties of the compounds [Ru(bpy)(2)(L)](2+) (Ru), [Os(bpy)(2)(L)](2+) (Os), [(L)Os(bpy)(2)Cl](+) (OsCl), [Ru(bpy)(2)(L)Ru(bpy)(2)Cl](3+) (RuRuCl), [Os(bpy)(2)(L)Os(bpy)(2)Cl](3+) (OsOsCl), [Ru(bpy)(2)(L)Os(bpy)(2)Cl](3+) (RuOsCl), and [Os(bpy)(2)(L)Ru(bpy)(2)Cl](3+) (OsRuCl) are reported (bpy = 2,2'-bipyridine, L = 1-methyl-3-(pyrazin-2-yl)-1,2,4-triazole). The Os(bpy)(2) and the Ru(bpy)(2) moieties are coordinated to the pyrazyltriazole ligand in two different ways, i.e. in a bidentate fashion via the triazole ring and N1 of the pyrazine ring and in a monodentate fashion only via N4 of the pyrazine ring. In the homonuclear dimers the monodentate bound metal has an oxidation potential that is approximately 400 mV lower than that of the bidentate bound metal. Spectroelectrochemical investigations suggest the presence of a weak interaction between the metal centers in the dinuclear species. The emission properties of the compounds are indicative of efficient energy transfer in the excited state, leading to emission from only one metal unit. In acetone both RuRuCl and the OsRuCl show photodissociation of the monodentate ruthenium moiety; however, RuOsCl and OsOsCl were found to be photostable.

Ground- and Excited-State Electronic Structure of an Emissive Pyrazine-Bridged Ruthenium(II) Dinuclear Complex

“…The primary interest in such mixed electron acceptor/electron donor ligands is in the ability to tune, independently, ground- and excited-state properties in multinuclear complexes ,−

1 Structure of the ligand H 2 (Metr) 2 pz and complex 1 .

2 Structure of some of the complexes discussed in the text.

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Mono- and Dinuclear Ruthenium(II) and Osmium(II) Polypyridine Complexes Built around Spiro-Bridged Bis(phenanthroline) Ligands: Synthesis, Electrochemistry, and Photophysics

“…Much of this work has been driven by the desire to identify viable sensitizers for the interconversion of light and chemical energy or useful reagents for analytical applications. More recently, attention has focused on polynuclear metal complexes that might exhibit properties different from those of the corresponding monomers, and many such compounds have been prepared and their electrochemical and photophysical properties investigated. − In particular, special emphasis has been given to the investigation of mixed-metal dinuclear complexes containing both Ru(II) and Os(II) polypyridine units where intramolecular triplet energy transfer takes place. − An important restriction within this field, however, is the realization that mechanistic details, such as distinguishing between Dexter-type through-bond or Förster-type through-space interactions, can only be attained if the connecting framework is sterically constrained …”

Excited-State Electronic Structure in Polypyridyl Complexes Containing Unsymmetrical Ligands

“…In photochemical and photophysical studies of polypyridyl complexes, mixed chelates and unsymmetrical polypyridyl ligands are frequently used to fine-tune excited-state properties and build molecular assemblies. − As the symmetry of these complexes is lowered, a number of important questions arise concerning electronic structure in the lowest-lying metal-to-ligand charge transfer (MLCT) excited states: (1) In multiple chelates, which ligand is the ultimate acceptor? (2) In complexes with two or more identical acceptor ligands, is the excited electron localized on one ligand or is it delocalized over both?…”