Cm Ho

Tuning the reactivities of ruthenium–oxo complexes with robust ligands. A ruthenium(IV)–oxo complex of 6,6′-dichloro-2,2′-bipyridine as an active oxidant for stoichiometric and catalytic organic oxidation

The synthesis and characterization of [RuII(terpy)(dcbipy)(H2O)]2+(terpy = 2,2′:6′,2″-terpyridine; dcbipy = 6,6′-dichloro-2,2′-bipyridine) are described. Oxidation of this complex with CeIV gave [RuIV(terpy)(dcbipy)O]2+, isolated as a greenish yellow perchlorate salt. The RuIVO complex is paramagnetic (µeff= 2.83) and exhibits a RuO stretch at 780 cm–1. It is a powerful oxidant with E°(RuIV–RuIII)= 1.13 V vs. saturated calomel electrode at pH 1.0 and can selectively oxidize the tertiary C–H bond of adamantane. The mechanism of alkene oxidation by RuIVO has been investigated and discussed. The complex [RuII(terpy)(dcbipy)(H2O)]2+ is a robust catalyst for the oxidation of saturated alkanes by tert-butyl hydroperoxide.

Kinetics of C–H bond and alkene oxidation by trans-dioxoruthenium(VI) porphyrins

A. Series of [RuVILO2] complexes (H2L =Para-substituted tetraphenylporphyrins) have been synthesised and characterized, and the kinetics and mechanism of oxidation of the C–H bond and alkenes investigated. The complexes were selective towards tertiary C–H bonds in saturated alkanes but were almost inactive towards secondary C–H bonds. However, they were reactive towards aromatic hydrocarbons and the second-order rate constants (k2) for the oxidation of ethylbenzene and cumene by [Ru(tpp)O2](tpp = 5, 10, 15, 20-tetraphenylporphyrinate) were 2.21 × 10–4 and 3.16 × 10–4dm3 mol–1 s–1 respectively. A kinetic isotope effect (KH/KD) of 11.7 was found for the allylic oxidation of cyclohexene by [Ru(tpp)O2]. The major organic products of the oxidation of alkenes in CH2Cl2–MeOH mixtures were epoxides and [Ru(tpp)O2] gave a monomeric product formulated as [RuIV(tpp)O]·EtOH. or [RuIV(tpp)(OH)2·]EtOH. Similar reactions with [RuVI(oep)O2](oep = 2,3,7,8,12,13,17,18-octaethylporphyrinate) gave [{RuVI(oep)(OH)}2O] in non-co-ordinating solvents. The observed rate law for alkene oxidation was rate =K2[RuVI][alkene]. There exists an almost linerar free-energy relationship between log k2 and E½(one-electron oxidation potentials of alkenes) with slope =–1.1 V–1 for the [Ru(tpp)O2] system. Activation parameters have been determined for the oxidation of styrene, norbornene and cyclooctene by [RuVILO2]. Non-linear and U-shaped Hammett plots were observed for the oxidation of substituted styrenes. The mechanism of alkene oxidation is proposed to involve a continum of transition states, the structures of which may change and be stabilized by defferent substituents.

Mechanism of C-H bond oxidation by a monooxoruthenium(V) complex

The complex [RuvL(O)]2+{HL =[2-hydroxy-2-(2-pyridyl)ethyl]bis[2-(2-pyridyl)ethyl]amine} was found to be a very active stoichiometric oxidant towards various hydrocarbons. Oxidation of alcohols was studied either in 0.1 mol dm–3 HClO4 or CH3CN, the observed rate law being rate =k[Ruv][alcohol]. The second-order rate constants for various alcohols follow the order benzylic > secondary > primary alcohols. Kinetic isotope effects (kH/kD) of 5.9 and 5.3 were found for benzyl alcohol and PriOH respectively. Similar kinetic studies for the oxidation of tetrahydrofuran gave kH/kD= 6.8. Stoichiometric oxidation of saturated alkanes in acetonitrile gave tertiary alcohols exclusively or ketones when only methylene groups are present. The measured kH/kD for the oxidation of cyclohexane determined from competitive oxidation experiments was 5.3. No cyclohexyl chloride was found when the oxidation of cyclohexane was carried out in the presence of CCl4. The mechanism of C–H bond oxidation by Ruv=O is discussed.

A ruthenium(IV) oxo complex that contains a tertiary diamine ligand

The complex [Ru(terpy)(tmen)(OH2)]2+(terpy = 2,2′:6′2″-terpyridine, tmen =N,N,N′,N′-tetra-methylethylenediamine) and its corresponding oxo complex have been prepared. Their spectral properties and redox properties in aqueous solution were investigated. Evidence for two one-electron redox processes has been obtained for the RuIV–RuIII and RuIII–RuII couples.The reactions of the new ruthenium(IV) oxo species with olefins have been found to proceed with second-order kinetics, rate =k2[RuIV][olefin] where k2=(3.3 ± 0.3)× l0–3 and (2.0 ± 0.2)× 10–2 dm3 mol–1 s–1 at 25 °C for norbornene and styrene respectively.