Mechanisms of electron transfer and hydrogen activation in selected triruthenium clusters with multiple bridging ligands. Part I. The attenuation of radical reactivity in triruthenium trihydrido(alkylidyne) clusters. Electrochemistry and kinetics of decomposition of the 47-electron cluster radical, via a disproportionation path. Part II. The activation of molecular hydrogen at transition metal centers via kinetics and mechanism of reversible oxidative addition of hydrogen across the metal-metal bond of (mu-hydrogen)(2)Ruthenium(3)(Carbon monoxide)(8)(mu-phosphorus(t-BU)(2))(2).

摘要

Part I. The redox chemistry of the organometallic clusters H3Ru3(μ3-CX)(CO)9−nL n (X = OMe; L = PPh3; n = 0–3: X = OMe, SEt; L 2 = dppm; L = PPh3; n = 3: X = SEt, NMeBz; L = PR3, SbPh3; n = 2, 3) has been studied both chemically and electrochemically. The chemically oxidized 47-electron species have been characterized by EPR spectroscopy. Kinetic studies indicate that the radical cation clusters decompose by way of a disproportionation path. The clusters display electrochemical oxidation potentials which become more negative as the degree of ligand substitution increases and as the pi donor ability of the methylidyne substituent (CX) increases. For the series X = OMe, L = PPh3, n = 0–3, a dramatic shift in relative potentials of the first and second oxidations occurs. When n = 2 or 3, two sequential one-electron oxidations are observed, in which the first oxidation is essentially reversible, as the rate of disproportionation is too slow to be observed electrochemically. However, when n = 1 or 0 the oxidation potential of the second ET is, respectively, equivalent to, or easier than the first ET, allowing disproportionation of the 47-electron species to occur very rapidly. The driving force for disproportionation appears to be the follow-up reactivity of the dication species. That is, the second oxidation forms a dication product which appears to be chemically, but not electrochemically, reversible with the starting 48-electron cluster. It is speculated that a structural change is occurring within the 46-electron dication.;Part II. The kinetics of the reaction (μ-H) 2Ru3(CO)8(μ-P(t-Bu)2)2 + H2 &rlarr2; (μ-H)2Ru3(CO)8(H)2(μ-P(t-Bu) 2)2 have been studied. The reaction of (μ-H)2Ru 3(CO)8(μ-P(t-Bu)2)2 with H 2 has a rate law which is first-order in cluster concentration and in hydrogen pressure and inverse order in CO pressure. On the basis of the rate law, activation parameters, and deuterium kinetic isotope effect, hydrogen addition is proposed to involve rapid, reversible dissociation of a carbonyl ligand, followed by rate-determining oxidative addition of hydrogen through a three-center transition state at a single metal atom. Loss of hydrogen from (μ-H)2Ru3(CO)8(H)2(μ-P(t-Bu) 2)2 also involves reversible loss of a carbonyl, followed by rate-determining reductive elimination of molecular hydrogen. The reaction is highly sensitive to the steric bulk of the phosphido substituents, as (μ-H) 2Ru3(CO)8(μ-P(R)2)2, R = cyclohexyl and phenyl, do not react with hydrogen. In addition, the rate of exchange with 13CO is much faster for R = t-Bu than for R = cyclohexyl. Based upon the temperature dependence of the equilibrium constant for hydrogenation, the energy for the unbridged Ru-Ru bond of (μ-H) 2Ru3(CO)8(μ-P(t-Bu)2)2 is estimated to be 47–59 kJ/mol. The low value is attributed to steric stain.