Mono- and bi-nuclear hydroxamates of bis(2-phenylazopyridine)ruthenium(II)
Abstract
The synthesis and characterisation of chelates of types [Ru(pap)2{ON(R)C(O)C6H4X-p}][ClO4]·H2O, (1), [Ru(pap)2{ONC(O)C6H4X-p}]·H2O, (2), [{Ru(pap)2}2{ON(R)C(O)–C(O)N(R)O}][ClO4]·H2O (3) and [{Ru(pap)2}2{ON(R)C(O)–Y–C(O)N(R)O}][ClO4]2·H2O, (4), where pap = 2-phenylazopyridine [R = H, Me, or Ph; X = H, Me, Cl, OMe, or NO2; Y =p-C6H4 or (CH2)4] are described. The presence of Ru–pap π back bonding and the hydroxamate–hydroximate chelate resonance are demonstrated by i.r. data. All complexes display t2(Ru)→π*(pap) charge-transfer transitions (900–500 nm). Species (1) show the reversible ruthenium(III)–ruthenium(II) couple in acetonitrile [1.03–1.17 V vs. saturated calomel electrode (s.c.e.), cyclic voltammetry]; for (2) the couple shifts to lower potentials by ca. 600 mV. The formal potentials are sensitive to substituents on the ON(R)C(O)C6H4X-p and ONC(O)C6H4X-p functions. Complexes (1; R = H) act as monobasic acids (pK 5.76–7.65) and water–dioxane(60:40). In one case the electroprotic equilibrium [Ru(pap)2L]++ H++ e–⇌[Ru(pap)2(HL)]+[L = ONC(O)Ph](E298⊖= 0.88 V) has been identified with the help of variable-pH cyclic voltammetry. The binuclear species in general display two successive ruthenium(III)–ruthenium(II) couples. All complexes show azo-reductions on the negative side of the s.c.e.