Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Abstract

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1–5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru^II) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru^IIRu^II/Ru^IIRu^III and Ru^IIRu^III/Ru^IIIRu^III, result in large comproportionation constants (K_c) of ≥10¹⁰ for the mixed-valent intermediates. The syn-configurated 3ⁿ exhibits a particularly high K_c of 10¹² for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru–N bond lengths for this Ru^IIRu^III intermediate. The electrogenerated mixed-valent states 1⁺–5⁺ exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304–2.234 and g anisotropies Δg = g₁–g₃ of 0.82–0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1⁺–5⁺ in the NIR region at 1660 nm–1750 nm (ε ≈ 2700 dm³ mol⁻¹ cm⁻¹, Δν_1/2 ≈ 1800 cm⁻¹). DFT calculations of 1⁺ and 3⁺ yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru^2.5)₂. Rather large spin densities of about −0.4 were calculated for the tppz bridges in 1⁺ and 3⁺. The calculated electronic interaction values (V_AB) for 1⁺–5⁺ are about 3000 cm⁻¹, comparable to that for the Creutz–Taube ion at 3185 cm⁻¹. The DFT calculations predict that the Ru^IIIRu^III forms in 1²⁺–5²⁺ prefer a triplet (S = 1) ground state with ΔE (S = 0 − S = 1) ∼5000 cm⁻¹. One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru^II(μ-tppz˙⁻)Ru^II(L)Cl]⁻ (1˙⁻–3˙⁻, 5˙⁻) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4ⁿ is distinguished by lability of the Ru–Cl bonds.