47-Electron Organometallic Clusters Derived by Chemical and Electrochemical Oxidation of Trihydrido(alkylidyne)triruthenium and -triosmium Clusters. Ligand Additivity in Metal Clusters

Abstract

Cyclic voltammograms for H3Ru3(μ3-CX)(CO)9 - n L n (X = OMe, SEt, Me, Et, Ph, NMeBz, Br; L = PR3, AsPh3, SbPh3; n = 2, 3), H3Ru3(μ3-COMe)(CO)6(PPh3)2(L) (L = P(OMe)3, CNCH2Ph), and H3Os3(μ3-COMe)(CO)7(PPh3)2 each display a quasi-reversible to reversible, one-electron oxidation followed by an irreversible to quasi-reversible, one-electron oxidation. The ligand and substituent effects upon the oxidation potential are analyzed as an example of ligand additivity in a cluster system; the oxidation potential of the alkylidyne cluster core is as sensitive to π-donor substituent effects as are aromatic π complexes such as CpFe(C5H4X) to substituents on the rings, and the dependence of the oxidation potential of the cluster upon ligand substitution on any given Ru atom is 37% of that expected for the oxidation of a monometallic complex. Reactions with 1 equiv of Ag+ or ferricenium give the 47-electron cations [H3Ru3(CX)(CO)6L3]1+ (X = OMe; L3 = (PPh3)2L‘; L‘ = CO, PPh3, P(OMe)3, CNCH2Ph; X = Ph, NMeBz, SEt, L = PPh3), characterized by EPR spectroscopy; the equilibrium between isomers H3Ru3(COMe)(CO)6(ax-PPh3)2(ax- and eq-PPh3)0/1+ (ax = axial coordination, eq = equatorial coordination) favors the axial coordination for the 48-electron cluster (eq/ax = 0.15) and equatorial coordination for the 47-electron species (eq/ax = 6.4). The rate constant for ax−eq isomerization for the 47-electron species (6.5 s-1) is 4 orders of magnitude greater than that for the 48-electron species (2 × 10-4 s-1). The 47-electron clusters have lifetimes which are correlated with the oxidation potentials of the precursors, ranging from seconds to many hours at room temperature. Slow decomposition of [H3Ru3(COMe)(CO)6L3]1+ in the absence of added CO forms the new 46-electron clusters [H3Ru3(CO)7L3]1+ (L = PPh3, AsPh3). [H3Ru3(COMe)(CO)6(PPh3)3]1+ does not react with CCl4 but does react with Lewis bases such as CO and acetonitrile. Disproportionation occurs with acetonitrile, chloride, and pyridine; the CO products are [HRu3(CO)9(PPh3)3]1+, [MePPh3]1+, and H3Ru3(COMe)(CO)9 - n (PPh3) n (n = 1, 2).

Characterization of Coordination Compounds by Electrochemical Parameters

“…Although typically applied only to hexacoordinate complexes, the model has been recently extended by the current author to square-planar tetracoordinate [28,64] and pentacoordinate [28] Rh I/II complexes; the former has also been studied by others [65] although with a smaller number of compounds. Extensions to particular Ru clusters, [66][67][68] sandwich and half-sandwich complexes, [15,16,69] and to reduction processes centred at ligands [15,16] have also been proposed.…”

Electron-donor/acceptor properties of carbynes, carbenes, vinylidenes, allenylidenes and alkynyls as measured by electrochemical ligand parameters

“…It has also been applied to Ru clusters [38][39][40] and sandwich complexes [20,21,24], and to ligandcentered reduction processes [21].…”