Reductive approach to mixed valency (n = 1-) in the pyrazine ligand-bridged [(acac)₂Ru(μ-L^2-)Ru(acac)₂]ⁿ (L^2- = 2,5-pyrazine-dicarboxylate) through experiment and theory

Abstract

The diruthenium(III) complex [(acac)₂Ru(μ-L^2-)Ru(acac)₂] (1) with acac- = acetylacetonato = 2,4-pentanedionato and a 2,5-pyrazine-dicarboxylato bridge, L^2-, has been obtained and structurally characterized as the rac (Δ Δ ,Λ Λ ) diastereomer. The Ru^IIIRu^III configuration in 1 (dRu-Ru = 6.799 Å ) results in a triplet ground state (μ = 2.82 μ_B at 300 κ) 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 κ . 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 Ru^IIRu^III mixed-valent state 1^- (K_c ≈ 10⁷). The intermediate 1^- shows a weak, broad Ru^II→Ru^III 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 ^ν C-O frequencies of the uncoordinated C-O groups of L^2- in 1ⁿ by IR spectroelectrochemistry suggests valence averaging (Ru^2.5Ru^2.5) 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.