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Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into dioxygen and dihydrogen is a major goal of renewable energy research. We describe a solution-phase reaction scheme that leads to the stoichiometric liberation of dihydrogen and dioxygen in consecutive thermal- and light-driven steps mediated by mononuclear, well-defined ruthenium complexes. The initial reaction of water at 25[degrees]C with a dearomatized ruthenium (II) [Ru(II)] pincer complex yields a monomeric aromatic Ru(II) hydrido-hydroxo complex that, on further reacfion with water at 100[degrees]C, releases [H.sub.2] and forms a cis dihydroxo complex. Irradiation of this complex in the 320-to-420--nanometer range liberates oxygen and regenerates the starting hydrido-hydroxo Ru(Il) complex, probably by elimination of hydrogen peroxide, which rapidly disproportionates. Isotopic labeling experiments with [H.sub.2][sup.17]O and [H.sub.2][sup.18] O show unequivocally that the process of oxygen--oxygen bond formation is intramolecular, establishing a previously elusive fundamental step toward dioxygen-generating homogeneous catalysis.

Terminal oxo complexes of transition metals have critical roles in various biological and chemical processes (1,2). For example, the catalytic oxidation of organic molecules (3,4), some oxidative enzymatic transformations (5,7), [...]

New platinasiloxanes were synthesized and their reactivity was studied. Oxidative addition of [(HSi[Me.sub.2]).sub.2]O to Pt[(P[Et.sub.3]).sub.3] led to dehydrogenation and quantitative formation of the four-membered platinacyclosiloxane [[(P[Et.sub.3]).sub.2]Pt([(Si[Me.sub.2]).sub.2]O)] which was characterized by X-ray diffraction. This complex was stable towards non-polar substrates and aerobic conditions, but reacted readily with electrophiles and Lewis acids. Reaction with MeI resulted in ring opening, while MeOTf gave an oxonium complex, with retention of the platinacycle structure. A complex resulting from B[F.sub.3] coordination to the ring oxygen atom was also obtained. An unprecedented open-chain diplatinasiloxane containing the Pt-Si-O-Si-O-Si-Pt fragment was formed by oxidative addition of H-Si[Me.sub.2]-O-Si[Me.sub.2]-O-Si[Me.sub.2]-H to Pt(P[Et.sub.3]).sub.3] and it was characterized by X-ray diffraction. Key words: cyclosiloxane, metallacycle, platinum silyl, Lewis base, oxidative addition. On a realise la synthese de nouveaux platinasiloxanes et on en a etudie la reactivite. L'addition oxydante du [(HSi[Me.sub.2]).sub.2]O] a du Pt[(P[Et.sub.3]).sub.3] conduit a une deshydrogenation et a la formation quantitative du platinacyclosiloxane a quatre chainons [[(P[Et.sub.3]).sub.2]Pt([(Si[Me.sub.2]).sub.2]O)] qui a ete caracterise par diffraction des rayons X. Ce complexe est stable vis-a-vis des substrats non polaires et des conditions aerobiques, mais il reagit facilement avec les electrophiles et les acides de Lewis. Sa reaction avec du MeI conduit a une ouverture de cycle, alors que sa reaction avec du MeOTf conduit a la formation d'un complexe d'oxonium, avec retention de la structure platinacyclique. Un complexe resultant de la coordination du B[F.sub.3] a l'atome d'oxygene du cycle a aussi ete obtenu. L'addition oxydante de H-Si[Me.sub.2]-O-Si[Me.sub.2]-O-Si[Me.sub.2]H a du Pt[(P[Et.sub.3]).sub.3] conduit d'un diplatinasiloxane a chaine ouverte contenant le fragment Pt-Si-O-Si-O-Si-Pt inconnu jusqu' a maintenant et qui a ete caracterise par diffraction des rayons X. Mots cles : cyclosiloxane, metallacyclique, silyle platine, base de Lewis, addition oxydante. [Traduit par la Redaction]

The synthesis of new ligand systems incorporating two and three 4,4'-bipyridine units are described. These complexes were fully characterized by X-ray structural analysis.

Research was conducted to examine the formation of eta-2 C-H rhodium arene complexes and their importance to electrophilic bond activation. The X-ray crystal structure and NMR data revealed strong interaction between the agostic C-H bond and the metal center which leads to the high acidity of the agostic proton. Results indicate that the deprotonation of eta-2 aromatic C-H agostic complexes can be proposed as an alternative route to electrophilic metalation of aromatic compounds.

Experiments showed that the reaction of [RhCl(C8H14)2]2 (C8H14 = cyclooctene) with 2 equiv of the aryl methyl ether phosphine 1 in C6D6 resulted to an unprecedented metal insertion into the strong sp2-sp3-aryl-oxygen bond. The extraordinary transfer of an alkoxy group from carbon to silicon was suggestive of the functionalization of an unstrained aryl-oxygen single bond by a silane. Moreover, the findings imply that metal complexes can be used to selectively activate and functionalize unstrained C-O single bonds under mild homogenous conditions.

A study was conducted to examine the reactivity of a quinone methide complex to strong electrophiles and nucleophiles. Rare examples of stable quinone methides and xylylenes that were stabilized by metal complexation and can be transformed into methylene arenium complexes were developed. The methylene arenium compounds can be described as a discrete resonance of a benzyl cation with a ring-localized charge stabilized through metal complexation.

The coordination behavior before C-M bond formation of the chelating aromatic PCP substrate DPPMH (3; DPPMH=1,3-bis((diphenylphosphino)methylene)mesitylene) was investigated to determine the factors which regulate the complex formation of such ligands. Reacting 3 with (RCN)2MCl2 (R=Me,Ph; M=Pd,Pt) and (COD)PtX2 (X=Cl,Me; COD=1,5-cyclooctadiene) produced several 8- and 16-membered mono- and binuclear palladium(II) macrocycles. An unusual change in the coordination behavior of Pt(II) was observed when the neutral or anionic ligands were varied.

The selective, direct hydrogenation of amides to the corresponding alcohols and amines with cleavage of the C-N bong is described. The reaction has proceeded under mild pressure and neutral, homogeneous conditions by using a dearomatized, bipyridyl-based PNN Ru(II) pincer complex as a catalyst.

The acridine-based pincer complex has displayed an unprecedented mode of metal-ligand cooperation involving a ' long-range' interaction between the distal acridine C9 position and the metal center. Ammonia has induced transfer of a hydride from the Ru center of the acridine-based pincer complex to C9 of the flexible acridine pincer ligand, which has formed an unusual dearomatized fac-acridine PNP complex.

Metal-organic networks (MONs) are created by a stepwise solution deposition approach from vinylpyridine-based building blocks and Pd[Cl.sub.2]. The X-ray photoelectron spectroscopy (XPS) elemental composition has shown the formation of a palladium coordination-based network.

A nonoxidative addition pathway is described for the activation of NH bonds of ammonia and amines by a Ru(II) complex, which has cleaved N-H bonds via metal-ligand cooperation involving aromatization of the pincer ligand. Electron-rich N-H bond substrates have led to reversible activation, while electron-poor substrates have resulted in stable activation products.

The article discusses the synthesis of a new series of the cationic, neutral and anionic [Pd.sup.II] and [Pt.sup.II] complexes. The thermodynamic properties and the reactions of these compounds are also explained.

Isolation and structural characterization of an O-O bond cleaved product, [Ru][(O).sub.2] with formation of two Ru-O-Se moieties obtained from intramolecular reaction between a ruthenium(II) bipyridine complex and [O.sub.2] in a protic slightly acidic solvent, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is described. [Ru][(O).sub.2] is observed to be a potent oxygen transfer species of both [O.sub.2]-derived atoms to triphenylphosphine and a nucleophilic alkene such as 2,3-dimethyl-2-butene in both HFIP and acetonitrile.

The crystallographically characterized ruthenium complex bearing a nonplanar acridine moiety has catalyzed in a neutral medium the transformation of primary alcohols to the corresponding acetals with the liberation of [H.sub.2]. This complex has catalyzed the dehydrogenative coupling of alcohols to form esters and acetal formation has involved hemiacetal dehydration to form enol ether followed by alcohol addition to the double bond.

Selective and quantitative reductive elimination (RE) of C[H.sub.3]I or C[H.sub.3]CN is seen at ambient temperature from the complex [(PNP)Rh(CN)(C[H.sub.3])] upon reaction in aprotic or protic solvents. The reductively eliminated C[H.sub.3]I have undergone selective electrophilic attack on the cyano ligand of the Rh complex in an aprotic solvent to give the corresponding alkyl isonitrole complex.

Pincer-type PSiP silanol Pt(II) complexes were prepared to generate a silanone intramolecularly bound to platinum. The intramolecular binding to transition metals in pincer-type complexes provide new pathways for the construction of more stable silanones and related transients by using additional protection against nucleophilic attack.

The exclusive activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) in the presence of aryl-I containing substrates is described. The reaction has obeyed first-order kinetics in the [[eta].sup.2]-coordination complexes with identical activation parameters for Br and I derivatives and the ring-walking process is shown to be kinetically accessible.

The NMR spectroscopy was used to study the fluxional behavior of two analogous platinum complexes and demonstrate their reaction mechanism and determine the activation parameters. The results from NMR spectroscopy showed that the migration of the metal center between the C=C bonds involves an intramolecular process.

Paramagnetic monomeric Rh(II) PNP-type complexes were synthesized by oxidation of Rh(I) complexes with silver salts. The structure and reactivity was studied using x-ray diffraction, EPR, cyclic voltammetry and dipole moment measurement.

The first [sigma]-bonded metal complexes of widely utilized organic dyes, perylene tetracarboxylic acid dimides (PDIs) which were synthesized by C-Br oxidative addition of 1,7-dibromo-N,N-dicyclohexyl PDI ([Br.sub.2]PDI) to Pd(0) phosphine complexes is reported.

A number of unusual reactions in which the PNP ligand takes an active role in the activation of [H.sub.2] and benzene, via facile aromatization/dearomatization processes of the ligand are reported. Results reveal that the new, dearomatized electron-rich PNP-based IR(I) complex 2 activates benzene to form an Ir(I)-Ph complex, which upon treatment with CO undergoes an oxidation process to form the Ir(III) complex involving proton migration from the arm to the metal with consistent dearomatization.

A selective chemical transformation in which metal coordination to a stilbazole double bond is followed by a ring-walking process prior to intramolecular aryl-halide oxidative addition to a low-valent metal center is demonstrated. The metal center walks over the pi-conjugated ring system until it reaches and activates the aryl-halide bond.

A PNP based cationic Iridium(I) system exhibits high activity and selectivity in aromatic C-H activation. A stable square pyramidal Ir-hydridophenyl complex is crystallographically characterized.

The oxidative addition of water to novel Ir(I) complexes stabilized by dimethyl sulfoxide ligands is detailed. The Ir(I)DMSO complexes contain no other dative ligands and these complexes can be obtained from olefin precursors and can easily activate water at room temperature to give isolated novel hydrido hydroxo complexes in good yield.

The synthesis and structure of the first stable metallaquinone is reported.

The catalytic generation of the silicon-oxygen double bond from secondary silanols using a transition metal complex is described. The reaction, which is an unprecedented form of catalysis, involves platinum-catalyzed silanol dehydrogenation and proceeds in solution under very mild conditions. Its mechanism, including isolation and structural characterization of a key intermediate by spectroscopy and X-ray diffraction, is reported. It is formally analogous to the catalytic dehydrogenation of alcohols that leads to ketones.