Synthesis, X-ray structures, and spectroscopic and electrochemical properties of (μ-oxo)bis(μ-carboxylato)diruthenium complexes having six imidazole bases as terminal ligands

Abstract

Complexes [Ru₂O(O₂CR)₂(1-MeIm)₆](ClO₄)₂ (la-c), [Ru₂O(O₂CR)₂(ImH)₆](ClO₄)₂ (2a,b), and [Ru₂O(O₂CR)₂(4-MeImH)₆](ClO₄)₂ (3a,b) with a (μ-oxo)bis(μ-carboxylato)diruthenium(III) core have been prepared by reacting Ru₂Cl(O₂CR)₄ with the corresponding imidazole base, viz. 1-methylimidazole (1-MeIm), imidazole (ImH), and 4-methylimidazole (4-MeImH) in methanol, followed by treatment with NaClO₄ in water (R: Me, a; C₆H₄-p-OMe, b; C₆H₄-p-Me, c). Diruthenium(III,IV) complexes [Ru₂O(O₂CR)₂(1-MeIm)₆](ClO₄)₃ (R: Me, 4a; C₆H₄-p-OMe, 4b; C₆H₄-p-Me, 4c) have been prepared by one-electron oxidation of 1 in MeCN with K₂S₂O₈ in water. Complexes la, 2a·3H₂O, and 4a·1.5H₂O have been structurally characterized. Crystal data for the complexes are as follows: la, orthorhombic, P2₁2₁2₁, a = 7.659(3) Å, b = 22.366(3) Å, c = 23.688(2) Å, V = 4058(2)ų, Z = 4, R = 0.0475, and R_w=0.0467 for 2669 reflections with F_o>2σ(F_o); 2a·3H₂O, triclinic, P1̅, a = 13.735(3) Å, b = 14.428(4)Å, c = 20.515(8)Å, α= 87.13(3)°, β= 87.61(3)°, γ= 63.92(2)°, V = 3646(2) Å(3), Z = 4, R = 0.0485 and R_w = 0.0583 for 10 594 reflections with F_o> 6 σ(F_o); 4a·1.5H₂O triclinic, ‹P1;, a = 11.969(3) Å, b = 12.090(6) Å, c = 17.421(3) Å, α= 108.93(2)°, β= 84.42(2)°, γ= 105.97(2)°, V = 2292(1) Å(3), Z = 2, R = 0.0567, and R_w = 0.0705 for 6775 reflections with F_o > 6 σ(F_o). The complexes have a diruthenium unit held by an oxo and two carboxylate ligands, and the imidazole ligands occupy the terminal sites of the core. The Ru-Ru distance and the Ru-O_oxo-Ru angle in 1a and 2a·3H₂O are 3.266(1), 3.272(1) Å and 122.4(4), 120.5(2)°, while in 4a·1.5H₂O these values are 3.327(1)Å and 133.6(2)°. The diruthenium(III) complexes 1-3 are blue in color and they exhibit an intense visible band in the range 560-575 nm. The absorption is charge transfer in nature involving the Ru(III)-dπ and O_oxo-pπ orbitals. The diruthenium(III,IV) complexes are red in color and show an intense band near 500 nm. The diruthenium(III) core readily gets oxidized with K₂S₂O₈ forming quantitatively the diruthenium(III,IV) complex. The visible spectral record of the conversion shows an isosbestic point at 545 nm for 1 and at 535 nm for 2 and 3. Protonation of the oxide bridge by HClO₄ in methanol yields the [Ru₂(μ-OH)(μ-O₂CR)²]³⁺ core. The hydroxo species shows a visible band al 550 nm. The pK_a value for 1a is 2.45. The protonated species are unstable. The 1-MeIm species converts to the diruthenium(III,IV) core, while the imidazole complex converts to [Ru(ImH)⁶)]³⁺ and some uncharacterized products. Complex [Ru(ImH)₆](ClO₄)₃has been structurally characterized. The diruthenium(III) complexes are essentially diamagnetic and show characteristic ¹H NMR spectra indicating the presence of the dimeric structure in solution. The diruthenium(III,IV) complexes are paramagnetic and display rhombic EPR spectral features. Complexes 1-3 are redox active. Complex 1 shows the one-electron reversible Ru^III₂/(Ru^IIIRu^IV), one-electron quasireversible (Ru^IIIRu^IV)/Ru^IV₂, and two-electron quasireversible Ru^III₂/Ru^II₂ couples near 0.4, 1.5, and -1.0 V vs SCE In MeCN-0.1 M TBAP, respectively, in the cyclic and differential pulse voltammetric studies. Complexes 2 and 3 exhibit only reversible Ru^III₂/(Ru^IIIRu^IV and the quasireversible (Ru^IIIRu^IV)/Ru^IV₂ couples near 0.4 and 1.6 V vs SCE, respectively, The observation of a quasireversible one-step two-electron transfer reduction process in 1 is significant considering its relevance to the rapid and reversible Fe^III₂/Fe^II₂ redox process known for the tribridged diiron core in the oxy and deoxy forms of hemerythrin.