Synthesis of a diruthenium µ-η4-α-diimine complex via dehydrogenative coupling of cyclic amines and its role in dehydrogenative oxidation of pyrrolidine.

The reaction of [Cp‡Ru(µ-H)4RuCp‡] (1: Cp‡ = 1,2,4-tri-tert-butylcyclopentadienyl) with cyclic amines at 180 °C afforded a µ-η4-α-diimine complex, [(Cp‡Ru)2(µ-η4-C2nH4n-4N2)] (5a-c: n = 4, 5, 6), via dehydrogenative coupling of two cyclic amine molecules. An intermediate µ-η2-1-pyrroline complex, [{Cp‡Ru(µ-H)}2(µ-η2-C4H7N)] (2a), was synthesized by the photoreaction of 1 with pyrrolidine and 5a was shown to be formed via the disproportionation of 2a upon thermolysis yielding 1 and a µ-imidoyl complex, [(Cp‡Ru)2(µ-η2:η2-C4H6N)(µ-H)] (3a). Complex 3a was transformed into 5avia the incorporation of 1-pyrroline, which was formed by the reaction of 2a with H2. DFT calculations on the model complexes supported by C5H5 groups at the B3LYP level suggested that the µ-η4-α-diimine ligand is formed via the insertion of a terminal cyclic aminocarbene ligand into the Ru-C bond of the µ-imidoyl group followed by the elimination of hydrogen. Although 5a was inert under an Ar atmosphere, it catalyzed the dehydrogenative oxidation of pyrrolidine under an atmosphere of hydrogen to yield γ-butyrolactam. An active species possessing a terminal cyclic aminocarbene ligand was generated via the heterolytic activation of hydrogen at the Ru-N bond followed by C-C bond cleavage.

Formation of an Azaruthenacyclopentadiene Skeleton via Ammonia Activation by an Electron-Deficient Ru3 Cluster.

A dicationic triruthenium complex containing a µ3 -η3 -C3 ring, [(Cp*Ru)3 (µ3 -η3 -C3 MeH2 -)(µ3 -CH)(µ-H)]2+ (1 a, Cp*=η5 -C5 Me5 ), reacted with ammonia to yield a µ-amido complex, [(Cp*Ru)3 (µ3 -η3 -CHCMeCH) (µ3 -CH)(µ-NH2 )]2+ (5), via N-H bond scission. Subsequent treatment with base resulted in C-N bond formation to yield a µ3 -η2 :η2 -1-azabutadien-4-yl complex, [(Cp*Ru)3 (µ3 -CH)(µ3 -η2 :η2 -NH=CH-CMe=CH-)]+ (6 a). The azaruthenacyclopentadiene skeleton was alternatively synthesized by the photolysis of mono-cationic complex [(Cp*Ru)3 (µ3 -η3 -C3 RH2 -)(µ3 -CH)]+ (2 a; R=Me, 2 b; R=H) in the presence of ammonia. The C3 ring skeleton was broken via the electron transfer to the π*(C-C) orbital in the C3 ring, and a transiently generated unsaturated µ3 -allylic species can take up ammonia, resulting in N-H bond scission followed by C-N bond formation.

Four-Electron Reduction of Dioxygen on a Metal Surface: Models of Dissociative and Associative Mechanisms in a Homogeneous System.

Two different four-electron reductions of dioxygen (O2) on a metal surface are reproduced in homogeneous systems. The reaction of the highly unsaturated (56-electron) tetraruthenium tetrahydride complex 1 with O2 readily afforded the bis(µ3-oxo) complex 3 via a dissociative mechanism that includes large electronic and geometric changes, i.e., a four-electron oxidation of the metal centers and an increase of 8 in the number of valence electrons. In contrast, the tetraruthenium hexahydride complex 2 induces a smooth H-atom transfer to the incorporated O2 species, and the O-OH bond is cleaved to afford the mono(µ3-oxo) complex 4 via an associative mechanism. Density functional theory calculations suggest that the higher degree of unsaturation in the tetrahydride system induces a significant interaction between the tetraruthenium core and the O2 moiety, enabling the large changes required for the dissociative mechanism.

Diruthenium complexes having a partially hydrogenated bipyridine ligand: plausible mechanism for the dehydrogenative coupling of pyridines at a diruthenium site.

The reactions of the diruthenium tetrahydrido complex, Cp*Ru(µ-H)4RuCp* (1) (Cp* = η5-C5Me5), with pyridines were investigated in relation to the dehydrogenative coupling of 4-substituted pyridines. Complex 1 reacted with γ-picoline to yield the bis(µ-pyridyl) complex, {Cp*Ru(µ-H)(µ-4-MeC5H3)}2 (2a), with the elimination of dihydrogen. Complex 2a immediately reacted with the liberated dihydrogen to yield µ-η2-dihydrobipyridine (dhbpy) complex 4avia C-C bond formation between the two pyridyl groups, in which one of the pyridine rings underwent partial hydrogenation. The X-ray structure of 4a shows that the dhbpy moiety adopts a µ-η2 coordination mode at the Ru2 site. Complex 4a was reversibly converted to 5avia the elimination of dihydrogen in which the dhbpy moiety adopts a µ-η2:η2 mode. Although 5a was coordinatively saturated, 5a readily reacted with tBuNC to yield 6a. This was owing to the ability of the dhbpy ligand changing its coordination mode between the µ-η2:η2 and µ-η2 modes. This also causes the dehydrogenation from the dhbpy ligand to yield µ-η2:η2-bipyridine complex 7a at 140 °C. However, 7a was not shown to be an intermediate of the catalysis. The reaction of 1 with 1,10-phenanthroline afforded µ-η2-phenanthroline complex 8 containing two hydrides, which can be a model compound for the bipyridine elimination from the Ru2 site. Dynamic NMR studies suggested that 8 was isomerised to an unsaturated µ-N-heterocyclic carbene (NHC) complex. The unsaturated nature of the µ-NHC complex is likely responsible for the uptake of the third pyridine molecule to turn over the catalytic cycle.

Synthesis and characterisation of tetranuclear ruthenium polyhydrido clusters with pseudo-tetrahedral geometry.

Dicationic tetranuclear ruthenium octahydride [(Cp*Ru)4H8]2+ (5) with tetrahedral geometry was obtained by reaction of dinuclear ruthenium tetrahydride (Cp*Ru)2(µ-H)4 (1) with an excess of Brønsted acids, such as HBF4·OEt2, in toluene. Monocationic tetraruthenium heptahydride [(Cp*Ru)4H7]+ (7) was obtained by dropwise addition of a diluted acid to a rigorously stirred solution of 1 at ambient temperature. Dication 5 was converted into monocationic heptahydrido complex 7 in high yield by treatment with sodium methoxide or sodium hydride. The direct conversion of 5 into neutral hexahydrido complex (Cp*Ru)4H6 (8) was achieved in a highly efficient manner by treating 5 with LiAlH4 in tetrahydrofuran (THF). The conversion of 5 into 8 was reversible, and the addition of a Brønsted acid to 8 gave 5via the formation of 7 as an intermediate. Tetranuclear complex 8 was directly obtained from 1 by heating it in THF at 70 °C. Complex 8' and tetraruthenium tetrahydride (CpEtRu)4H4 (10'), where 8' and 10' possessed η5-C5EtMe4 ligands instead of Cp* ligands, were mutually related by the elimination/addition of dihydrogen. The structures of 5, 7, 8, and 10' were determined by X-ray diffraction, and the Ru4 core structure and the coordination mode of hydrido ligands were discussed based on density functional theory (DFT) calculations for model compounds where the methyl groups of Cp* ligands were replaced with hydrogen atoms.

µ3-η(2):η(2):η(2)-Coordination of Primary Silane on a Triruthenium Plane.

A µ3-η(2):η(2):η(2)-silane complex, [(Cp*Ru)3(µ3-η(2):η(2):η(2)-H3SitBu)(µ-H)3] (2 a; Cp* = η(5)-C5Me5), was synthesized from the reaction of [{Cp*Ru(µ-H)}3(µ3-H)2] (1) with tBuSiH3. Complex 2 a is the first example of a silane ligand adopting a µ3-η(2):η(2):η(2) coordination mode. This unprecedented coordination mode was established by NMR and IR spectroscopy as well as X-ray diffraction analysis and supported by a density functional study. Variable-temperature NMR analysis implied that 2 a equilibrates with a tautomeric µ3-silyl complex (3 a). Although 3 a was not isolated, the corresponding µ3-silyl complex, [(Cp*Ru)3(µ3-η(2):η(2)-H2SiPh)(H)(µ-H)3] (3 b), was obtained from the reaction of 1 with PhSiH3. Treatment of 2 a with PhSiH3 resulted in a silane exchange reaction, leading to the formation of 3 b accompanied by the elimination of tBuSiH3. This result indicates that the µ3-silane complex can be regarded as an "arrested" intermediate for the oxidative addition/reductive elimination of a primary silane to a trinuclear site.