Turning on Red and Near-Infrared Phosphorescence in Octahedral Complexes with Metalated Quinones

We report the synthesis of π-bonded ruthenium,
rhodium, and
iridium o-benzoquinones [Cp*M­(o-C6H4O2)]n [M
= Ru (2), n = 1–; Rh (3), n = 0; Ir (4), n = 0] following a novel synthetic procedure. Compounds 2–4 were fully characterized by spectroscopic
methods and used as chelating organometallic linkers, “OM-linkers”, toward luminophore bricks such as Ru­(bpy)22+, Rh­(ppy)2+, and Ir­(ppy)2+ (bpy = 2,2′-bipyridine; ppy = 2-phenylpyridine)
for the design of a novel family of octahedral bimetallic complexes
of the general formula [(L-L)2M­(OM-linkers)]­[X]m (X = counteranion; m = 0, 1, 2)
whose luminescent properties depend on the choice of the OM-linker
and the luminophore brick. Thus, dinuclear assemblies such as [(bpy)2Ru­(2)]­[OTf] (5-OTf), [(bpy)2Ru­(2)]­[Δ-TRISPHAT] (5-ΔT)
{TRISPHAT = tris­[tetrachlorobenzene-1,2-bis­(olato)]­phosphate}, [(bpy)2Ru­(3)]­[OTf]2 (6-OTf),
[(bpy)2Ru­(4)]­[OTf]2 (7-OTf), [(bpy)2Ru­(4)]­[Δ-TRISPHAT]2 (7-ΔT), [(ppy)2Rh­(2)] (8), [(ppy)2Rh­(3)]­[OTf] (9-OTf), [(ppy)2Rh­(4)]­[OTf] (10-OTf), [(ppy)2Rh­(4)]­[Δ-TRISPHAT]
(10-ΔT), [(ppy)2Ir­(2)]
(11), [(ppy)2Ir­(3)]­[OTf] (12-OTf), [(ppy)2Ir­(4)]­[OTf] (13-OTf), and [(ppy)2Ir­(4)]­[Δ-TRISPHAT]
(13-ΔT) were prepared and fully characterized.
The X-ray molecular structures of three of them, i.e., 5-OTf, 8, and 11, were determined. The structures
displayed a main feature: for instance, the two oxygen centers of
the OM-linker [Cp*Ru­(o-C6H4O2)]− (2) chelate the octahedral
chromophore metal center, whether it be ruthenium, rhodium, or iridium.
Further, the carbocycle of the OM-linker 2 adopts a η4-quinone form but with some catecholate contribution due to
metal coordination. All of these binuclear assemblies showed a wide
absorption window that tailed into the near-IR (NIR) region, in particular
in the case of the binuclear ruthenium complex 5-OTf
with the anionic OM-linker 2. The latter feature is no
doubt related to the effect of the OM-linker, which lights up the
luminescence in these homo- and heterobinuclear compounds, while no
effect has been observed on the UV–visible and emission properties
because of the counteranion, whether it be triflate (OTf) or Δ-TRISPHAT.
At low temperature, all of these compounds become luminescent; remarkably,
the o-quinonoid linkers [Cp*M­(o-C6H4O2)]n (2–4) turn on red and NIR phosphorescence
in the binuclear octahedral species 5–7. This trend was even more observable when the ruthenium OM-linker 2 was employed. These assemblies hold promise as NIR luminescent
materials, in contrast to those made from organic 1,2-dioxolene ligands
that conversely are not emissive.