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

摘要

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 ~(II)Ru ~(II)/Ru ~(II)Ru ~(III) and Ru ~(II)Ru ~(III)/Ru ~(III)Ru ~(III), result in large comproportionation constants (K _c) of ≥10 ~(10) for the mixed-valent intermediates. The syn-configurated 3 ~n exhibits a particularly high K _c of 10 ~(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru ~(II)Ru ~(III) intermediate. The electrogenerated mixed-valent states 1 ~+-5 ~+ exhibit anisotropic EPR spectra at 110 K with average values of 2.304-2.234 and g anisotropies Δg = g _1-g _3 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 dm ~3 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 ~(III)Ru ~(III) forms in 1 ~(2+)-5 ~(2+) 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 4 ~n is distinguished by lability of the Ru-Cl bonds.