Mononuclear and Dinuclear Complexes of Dibenzoeilatin: Synthesis, Structure, and Electrochemical and Photophysical Properties

This work describes a study of Ru(II) and Os(II) polypyridyl complexes of the symmetrical, fused-aromatic bridging
ligand dibenzoeilatin (1). The synthesis, purification, and structural characterization by NMR of the mononuclear
complexes [Ru(bpy)2(dbneil)]2+ (2), [Ru(tmbpy)2(dbneil)]2+ (3), and [Os(bpy)2(dbneil)]2+ (4), the homodinuclear
complexes [{Ru(bpy)2}2{μ-dbneil}]4+ (5), [{Ru(tmbpy)2}2{μ-dbneil}]4+ (6), and [{Os(bpy)2}2{μ-dbneil}]4+ (7), and
the heterodinuclear complex [{Ru(bpy)2}{μ-dbneil}{Os(bpy)2}]4+ (8) are described, along with the crystal structures
of 4, 6, and 7. Absorption spectra of the mononuclear complexes feature a low-lying MLCT band around 600 nm.
The coordination of a second metal fragment results in a dramatic red shift of the MLCT band to beyond 700 nm.
Cyclic and square wave voltammograms of the mononuclear complexes exhibit one reversible metal-based oxidation,
as well as several ligand-based reduction waves. The first two reductions, attributed to reduction of the dibenzoeilatin
ligand, are substantially anodically shifted compared to [M(bpy)3]2+ (M = Ru, Os), consistent with the low-lying π*
orbital of dibenzoeilatin. The dinuclear complexes exhibit two reversible, well-resolved, metal-centered oxidation
waves, despite the chemical equivalence of the two metal centers, indicating a significant metal−metal interaction
mediated by the conjugated dibenzoeilatin ligand. Luminescence spectra, quantum yield, and lifetime measurements
at room temperature in argon-purged acetonitrile have shown that the complexes exhibit 3MLCT emission, which
occurs in the IR-region between 950 and 1300 nm. The heterodinuclear complex 8 exhibits luminescence only
from the Ru-based fragment, the intensity of which is less than 1% of that observed in the corresponding
homodinuclear complex 5; no emission from the Os-based unit is observed, and an intramolecular quenching
constant of kq ≥ 3 ×109 s-1 is evaluated. The nature of the quenching process is briefly discussed.