A novel monooxoruthenium(V) complex containing a polydentate pyridyl amine ligand. Syntheses, reactivities, and X-ray crystal structure of [RuIII(N4O)(H2O)](ClO4)2

Abstract

The syntheses and characterization of cis-[RuIII(tepa)Cl2]+, [RuIII(N4O)(H2O)]2+, and [RuV(N4O)O]2+ complexes are described [tepa = tris(2-(2-pyridyl)ethyl)amine, N4OH = bis(2-(2-pyridyl)ethyl)(2-hydroxy-2-(2-pyridyl)ethyl)amine]. The molar magnetic susceptibilities for [RuIII(N4O)(H2O)]2+ and [RuV(N4O)O]2+ complexes are 1.79 and 2.2 μB, respectively. The X-ray structure of [RuIII(N4O)(H2O)](ClO4)2 has been determined: [RuIII(C21H23N4O)(H 2O)](ClO4)2, M = 666.43, monoclinic, space group P2/c (no. 13), a = 11.644 (1) Å, b = 11.937 (3) Å, c = 18.856 (6) Å, β = 105.39 (2)°, V = 2527 (1) Å3, Z = 4, Dx = 1.753, Dc = 1.752 g cm-3, μ(Mo Kα) = 8.82 cm-1. The Ru-O(OH2) and Ru-O(N4O) distances are 2.115 (3) and 1.961 (4) Å, respectively. The [RuV(N4O)O]2+ complex shows an intense Ru=O stretch at 872 cm-1 which is absent in the [RuIII(N4O)(H2O)]2+ complex. The Ru(V) state in [RuV(N4O)O]2+ has also been confirmed by spectrophotometric redox titration in 0.1 M HClO4 by using [RuII(NH3)4(bpy)]2+ (bpy = 2,2′-bipyridine) as the redox titrant. A stoichiometry of 1:3 has been obtained. The cyclic voltammograms of both [RuIIIHN4O)(H2O)]2+ and [RuV(N4O)O]2+ complexs show two couples at 0.35 and 1.02 V vs SCE in aqueous medium at pH = 1, assignable to a Ru(III/II) and a Ru(V/III) couple, respectively. At 5.5 > pH > 3.5, the wave for the Ru(V/III) couple splits into two waves, a pH-independent one-electron wave for the Ru(V/IV) couple and a two-proton-one-electron wave with a slope of -117 mV/pH unit for the Ru(IV/III) couple. The Ru(IV) state is found to be thermodynamically unstable with respect to disproportionation into Ru(III) and Ru(V) at pH < 3.5. The [RuV(N4O)O]2+ complex is found to be an active oxidant, capable of oxidizing both activated C-H bonds and the C-H bond of cyclohexane. Studies on the reactions of [RuV(N4O)O]2+ with organic substrates indicated that the Ru(V)=O has a higher affinity for hydrogen atom/hydride abstraction than oxo-transfer reaction to C=C double bond.