2,2'-dipyridylketone (dpk) as ancillary acceptor and reporter ligand in complexes [(dpk)(Cl)Ru(μ -tppz)Ru(Cl)(dpk)]ⁿ⁺ where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine

Abstract

The tppz-bridged diruthenium(II) complex [(dpk)(Cl)Ru^II(μ -tppz)Ru^II(Cl)(dpk)](ClO₄)₂, [2](ClO₄)₂, and mononuclear [(dpk)(Cl)Ru^II(tppz)](ClO₄), [1](ClO₄) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, dpk = 2,2'-dipyridylketone], have been synthesized. The 260 mV separation between successive one-electron oxidation couples in [2]²⁺ translates to a relatively small comproportionation constant, K_c, of 2.5 × 10⁴ for the intermediate. It is shown how electrochemical data (E_ox, E_red, K_c) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(μ -tppz)RuCl(L)]ⁿ, particularly the competition between L and tppz for electron density from the metal. According to EPR (g₁ = 2.470, g₂ = 2.195, and g₃ = 1.873 at 4 κ) the intermediate [2]³⁺ is a mixed-valent Ru^IIRu^III species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (ε = 1500 M^-1 cm^-1). The width at half-height (Δν_½ ) of 700 cm^-1 of the IVCT band is much smaller than the calculated value of 3584 cm^-1, obtained by using the Hush formula Δν_½ = (2310E_op)_½ (E_op = 5556 cm^-1, energy of the IVCT transition) which would be applicable to localized (Class II) mixed-valent Ru^IIRu^III systems. Valence delocalization in [2]³⁺ is supported by the uniform shift of the ν_C=O band of the N,N'-coordinated dpk ligands from 1676 cm^-1 in the RuIIRuII precursor to 1690 cm^-1 in the Ru^2.5Ru^2.5 form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but π-conjugated organic carbonyl group. The apparent contradiction between the moderate value of K_c and the narrow IVCT band is being discussed considering "borderline" or "hybrid" "Class II-III" concepts of mixed-valency, as well as coordination aspects, i.e., the bis-tridentate nature of the π-acceptor bridging ligand. Altogether, the complex ions [1]⁺ and [2]²⁺ display four and five successive reduction processes, respectively, involving both tppz^- and dpk-based unoccupied π orbitals. The one-electron reduced form [2]⁺ has been assigned as a tppz^·- radical-anion-containing species which exhibits a free-radical-type EPR signal at 4κ (g_|| = 2.002, g_⊥ = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (ε = 1100 M^-1 cm^-1).