SYNTHESIS, X-RAY STRUCTURES, AND SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES OF (MU-OXO)BIS(MU-CARBOXYLATO)DIRUTHENIUM COMPLEXES HAVING 6 IMIDAZOLE BASES AS TERMINAL LIGANDS

Complexes [Ru2O(O2CR)(2)(1-MeIm)(6)](ClO4)(2) (la-c), [Ru2O(O2CR)(2)(I mH)(6)](ClO4)(2) (2a,b), and [Ru2O(O2CR)(2)(4-MeImH)(6)](ClO4)(2) (3a, b) with a (mu-oxo)bis(mu-carboxylato)diruthenium(III) core have been p repared by reacting Ru2Cl(O2CR)(4) with the corresponding imidazole ba se, viz. 1-methylimidazole (1-MeIm), imidazole (ImH), and 4-methylimid azole (4-MeImH) in methanol. followed by treatment with NaClO4 in wate r (R: Me, a; C6H4-p-OMe, b; C6H4-p-Me, c). Diruthenium(III,IV) complex es [Ru2O(O2CR)(2)(1-MeIm)(6)](ClO4)(3) (R: Me, 4a; C6H4-p-OMe, 4b; C6H 4-p-Me, 4c) have been prepared by one-electron oxidation of 1 in MeCN with K2S2O8 in water. Complexes la, 2a . 3H(2)O, and 4a . 1.5H(2)O hav e been structurally characterized. Crystal data for the complexes are as follows: la, orthorhombic, P2(1)2(1)2(1), a = 7.659(3) Angstrom, b = 22.366(3) Angstrom, c = 23.688(2) Angstrom, V = 4058(2) Angstrom(3), Z = 4, R = 0.0475, and R-w = 0.0467 for 2669 reflections with F-o > 2 sigma(F-o); 2a . 3H(2)O, triclinic, <P(1)over bar>, a = 13.735(3) Ang strom, b = 14.428(4) Angstrom, c = 20.515(8) Angstrom, alpha = 87.13(3 )degrees, beta = 87.61(3)degrees, gamma = 63.92(2)degrees, V = 3646(2) Angstrom(3), Z = 4, R = 0.0485 and R-w = 0.0583 for 10 594 reflection s with F-o > 6 sigma(F-o); 4a . 1.5H(2)O triclinic, <P(1)over bar>, a = 11.969(3) Angstrom, b = 12.090(6) Angstrom, c = 17.421(3) Angstrom, alpha = 108.93(2)degrees, beta = 84.42(2)degrees, gamma = 105.97(2)deg rees, V = 2292(1) Angstrom(3), Z = 2, R = 0.0567, and R-w = 0.0705 for 6775 reflections with F-o > 6 sigma(F-o). The complexes have a diruth enium unit held by an oxo and two carboxylate ligands, and the imidazo le ligands occupy the terminal sites of the core. The Ru-Ru distance a nd the Ru-O-oxo-Ru angle in la and 2a . 3H(2)O are 3.266(1), 3.272(1) Angstrom and 122.4(4), 120.5(2)degrees, while in 4a . 1.5H(2)O these v alues are 3.327(1) Angstrom and 133.6(2)degrees. The diruthenium(III) complexes 1-3 are blue in color and they exhibit an intense visible ba nd in the range 560-575 nm. The absorption is charge transfer in natur e involving the Ru(III)-d pi and O-oxo-p pi orbitals. The diruthenium( III,IV) complexes are red in color and show an intense band near 500 n m. 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 me thanol yields the [Ru-2(mu-OH)(mu-O2CR)(2)](3+) core. The hydroxo spec ies shows a visible band al 550 nm. The pK(a) value for la is 2.45. Th e 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)](C lO4)(3) has been structurally characterized. The diruthenium(III) comp lexes are essentially diamagnetic and show characteristic H-1 NMR spec tra 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 th e one-electron reversible Ru-2(III)/(RuRuIV)-Ru-III, one-electron quas ireversible (RuRuIV)-Ru-III/Ru-2(IV), and two-electron quasireversible Ru-2(III)/Ru-2(II) 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 voltamme tric studies. Complexes 2 and 3 exhibit only reversible Ru-2(III)/(RuR uIV)-Ru-III and the quasireversible (RuRuIV)-Ru-III/Ru-2(IV) couples n ear 0.4 and 1.6 V vs SCE, respectively, The observation of a quasireve rsible one-step two-electron transfer reduction process in 1 is signif icant considering its relevance to the rapid and reversible Fe-2(III)/ Fe-2(II) redox process known for the tribridged diiron core in the oxy and deoxy forms of hemerythrin.