MULTIMETALLIC RUTHENIUM(II) COMPLEXES BASED ON BIIMIDAZOLE AND BIBENZIMIDAZOLE - EFFECT OF DIANIONIC BRIDGING LIGANDS ON REDOX AND SPECTRAL PROPERTIES

The preparation and properties of ruthenium(II) complexes containing the ligands 2,2′-biimidazole (BiImH2), 2,2′-bibenzimidazole (BiBzImH2), and 2,2′-bipyridine (bpy) are reported. The complexes described are [(Ru(bpy)2)2BiIm]2+and the series [Ru-(bpy)n(BiImH2)3−n]2+, [Ru(bpy)n(BiBzImH2)3−n]2+, and [Ru(bpy)n(BiBzImRu(bpy)2)3− n]2+, n = 0–2. The redox potential for the first Ru3+ / 2+couple shifted negatively from 1.26 to −0.26 V vs SSCE as bpy ligands were replaced by BiImH2, BiBzImH2, and “BiBzImRu(bpy)2” ligands. The BiBzImRu(bpy)2units shifted the redox potential a factor of 3 greater than that for BiBzImH2, which was attributed to greater electrostatic interaction of the dinegatively charged bridging ligands following deprotonation. The magnitude of the shift was such that the tetrametallic species was isolated as the [RuIII, (BiBzImRuII(bpy)2)3)]3+cation. Reductions were found in the −1.5- to −1.6-V range for complexes containing bipyridine ligands, but none were observed out to −2.0 V for [Ru(BilmH2)3]2+or [Ru(BiBzImH2)3]2+. The complexes absorbed energy in the visible and UV regions of the spectrum and emitted radiation, with the exception of the tetrametallic species, in the 600–800-nm region. The emission was weak, ϕr= 10−3−10−4; the excited-state lifetimes ranged from 25 to 161 ns. Estimates of the excited-state redox potentials revealed that the excited-state species were powerful reductants, E1/2(Ru3+ / 2+*) ~ −0.8 V, but were weak oxidants, E,1 / 2(Ru2+*+) ~ 0.2 V. The [RuIII −(BiBzImRu11(bpy)2)3]3+cation absorbed in the near-infrared region at 12.2 × 103cm−1in acetonitrile, but the absorption disappeared upon reduction to [RuII(BiBzImRuII(bpy)2)3]2+. The position of the absorption manifold maximum varied linearly with the static and optical dielectric constant of the solvent. The width of the absorption band was comparable to that of [RuIII(bpy)2BiBzImH2]3+, but its intensity was greater. The band at 12.2 × 103cm−1was attributed to overlapping LMCT and metal to metal charge transfer. © 1990, American Chemical Society. All rights reserved.