Reductive Approach to Mixed Valency (n = 1−) in the Pyrazine Ligand-Bridged [(acac)2Ru(μ-L2–)Ru(acac)2]n (L2– = 2,5-Pyrazine-dicarboxylate) through Experiment and Theory

The diruthenium(III) complex [(acac)2Ru(μ-L2–)Ru(acac)2] (1) with acac– = acetylacetonato = 2,4-pentanedionato and a 2,5-pyrazine-dicarboxylato bridge, L2–, has been obtained and structurally characterized as the rac (ΔΔ,ΛΛ) diastereomer. The RuIIIRuIII configuration in 1 (dRu–Ru = 6.799 Å) results in a triplet ground state (μ = 2.82 μB at 300 K) with a density functional theory (DFT) calculated triplet-singlet gap of 10840 cm–1 and the metal ions as the primary spin-bearing centers (Mulliken spin densities: Ru, 1.711; L, 0.105; acac, 0.184). The paramagnetic 1 exhibits broad, upfield shifted 1H NMR signals with δ values ranging from −10 to −65 ppm and an anisotropic electron paramagnetic resonance (EPR) spectrum (⟨g⟩ = 2.133, g1 – g3 = Δg = 0.512), accompanied by a weak half-field signal at g = 4.420 in glassy frozen acetonitrile at 4 K. Compound 1 displays two closely spaced oxidation steps to yield labile cations. In contrast, two well separated reversible reduction steps of 1 signify appreciable electrochemical metal–metal interaction in the RuIIRuIII mixed-valent state 1– (Kc ≈ 107). The intermediate 1– shows a weak, broad RuII→RuIII intervalence charge transfer (IVCT) band at about 1040 nm (ε = 380 M–1 cm–1); the DFT approach for 1– yielded Mulliken spin densities of 0.460 and 0.685 for the two metal centers. The monitoring of the νCO frequencies of the uncoordinated CO groups of L2– in 1n by IR spectroelectrochemistry suggests valence averaging (Ru2.5Ru2.5) in 1– on the vibrational time scale. The mixed-valent 1– displays a rhombic EPR signal (⟨g⟩ = 2.239 and Δg = 0.32) which reveals non-negligible contributions from the bridging ligand, reflecting a partial hole-transfer mechanism and being confirmed by the DFT-calculated spin distribution (Mulliken spin density of −0.241 for L in 1–). The major low energy electronic transitions in 1n (n = 0,–,2−) have been assigned as charge transfer processes with the support of TD-DFT analysis.