title: 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)(2)Ru(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 (ΔΔ,ΛΛ) diastereomer. The Ru(III)Ru(III) configuration in 1 (d(Ru-Ru) = 6.799 Å) results in a triplet ground state (μ = 2.82 μ(B) 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 (1)H NMR signals with δ values ranging from -10 to -65 ppm and an anisotropic electron paramagnetic resonance (EPR) spectrum (<linear span>g<linear span> = 2.133, g(1) - g(3) = 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 Ru(II)Ru(III) mixed-valent state 1(-) (K(c) ≈ 10(7)). 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 1(n) by IR spectroelectrochemistry suggests valence averaging (Ru(2.5)Ru(2.5)) in 1(-) on the vibrational time scale. The mixed-valent 1(-) displays a rhombic EPR signal (<linear span>g<linear span> = 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 1(n) (n = 0,-,2-) have been assigned as charge transfer processes with the support of TD-DFT analysis.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India.
authors.forename: Amit
authors.initials: A
authors.lastname: Das
authors.forename: Thomas
authors.initials: T
authors.lastname: Scherer
authors.forename: Somnath
authors.initials: S
authors.lastname: Maji
authors.forename: Tapan Kumar
authors.initials: TK
authors.lastname: Mondal
authors.forename: Shaikh M
authors.initials: SM
authors.lastname: Mobin
authors.forename: Francisco A
authors.initials: FA
authors.lastname: Urbanos
authors.forename: Reyes
authors.initials: R
authors.lastname: Jiménez-Aparicio
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Kaim
authors.forename: Goutam Kumar
authors.initials: GK
authors.lastname: Lahiri