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{ON=C(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 ON=C(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 = ON=C(O)Ph](E₂₉₈θ= 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.