Reductive approach to mixed valency (n = 1-) in the pyrazine ligand-bridged [(acac)<SUB>2</SUB>Ru(μ-L<SUP>2-</SUP>)Ru(acac)<SUB>2</SUB>]<SUP>n</SUP> (L<SUP>2-</SUP> = 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 (Δ Δ ,Λ Λ ) diastereomer. The RuIIIRuIII configuration in 1 (dRu-Ru = 6.799 Å ) results in a triplet ground state (μ = 2.82 μB 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 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 ν C-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