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 reaction with water at 100 degrees C, releases H2 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(II) complex, probably by elimination of hydrogen peroxide, which rapidly disproportionates. Isotopic labeling experiments with H2 17O and H2 18O show unequivocally that the process of oxygen-oxygen bond formation is intramolecular, establishing a previously elusive fundamental step toward dioxygen-generating homogeneous catalysis.

The complex (PNP)IrI(CH2COCH3) 2 (PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine) was prepared by reaction of the dearomatized, electron-rich complex (PNP*)IrI(COE) (1; PNP* = deprotonated PNP, COE = cyclooctene) with acetone. Upon treatment with CO, complex 2 undergoes a surprising elimination of acetone to form the dearomatized species (PNP*)IrI(CO) (4), involving proton migration from the ligand “arm” to the acetonyl moiety. DFT studies reveal that this process occurs via the square-pyramidal intermediate 2+CO, formed upon CO coordination to 2, in which the acetonyl moiety is located at the apical position prior to proton migration. Reaction of 2 with H2 (D2) indicates an equilibrium between complex 2 and the nonaromatic (PNP*)IrIII(H)(CH2COCH3) complex 2b, which is the species that actually activates H2 to exclusively form the trans-dihydride (PNP)IrIII(H)2(CH2COCH3) (5a) and activates D2 to form the trans-hydride−deuteride 5b with benzylic-D incorporation, as also corroborated by DFT studies. Interestingly, benzene C−H activation by complex 2 results in formation of the complex (PNP)IrI(C6H5) (6a) and elimination of acetone. DFT studies show that the benzene C−H bond is actually activated by the dearomatized “bare” (PNP*)IrI intermediate 2c, formed upon acetone elimination from 2.

The anionic dearomatized complex [(PNP*)Rh I Cl]K (2; PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine, PNP* = deprotonated PNP) was prepared by reaction of the aromatic (PNP)Rh I Cl complex 1 with KN(SiMe 3 ) 2 in dry benzene. Spectroscopic characterization and DFT calculations confirm a nonaromatic square-planar structure of complex 2. Under an atmosphere of dry argon, 2 undergoes facile CÀH activation of benzene by cooperation between the metal center and the pincer ligand, with aromatization of the ligand, to form the complex (PNP)Rh I (C 6 H 5 ) (3a). This reaction is inhibited by dinitrogen, which reacts with 2 to form the complex (PNP*)Rh I (N 2 ) (4), indicating higher stabilization of the 14-electron (PNP*)Rh I species 5 by dinitrogen as compared with chloride. Similarly, treatment of 2 with CO results in KCl liberation to form the dearomatized (PNP*)Rh I CO (8). In a protic environment, the dearomatized complex 2 is quickly reprotonated to regenerate the aromatic starting complex 1. Upon treatment with MeI, 2 undergoes oxidative addition to form the nonaromatic (PNP*)Rh III (CH 3 )Cl (10), while the dearomatized ligand remains intact. Complex 2 undergoes facile activation of H 2 to form the monohydride (PNP)Rh I (H) (11a) and with D 2 to form (PNP)Rh I (D) (11b) with benzylic-D incorporation, via metalÀligand cooperation by aromatization of the ligand. The reactivity of 2 with H 2 is significantly higher than that of 4. ' INTRODUCTIONMetalÀligand cooperation plays an important role in homogeneous catalysis by metal complexes, 1À3 such as in the hydrogenation of polar bonds by ruthenium amido complexes. 1b,c Recently, we discovered a new mode of metalÀligand cooperation involving reversible dearomatization of pyridine-and acridine-based pincer complexes. 3À9 MetalÀligand cooperation via reversible dearomatization can play a key role in the catalytic OÀH bond activation of alcohols, 3 exemplified by the direct coupling of alcohols to form esters with the liberation of H 2 , 4a,b the hydrogenation of esters to alcohols under mild pressure, 4c the dehydrogenative coupling of alcohols with amines to produce amides 4d and polyamides, 4e,f the dehydrogenative amidation of esters with amines, 4g and the acylation of alcohols with esters. 4h These reactions are catalyzed by the dearomatized (PNN*)Ru II (H)(CO) complex (PNN = 2-((ditert-butylphosphino)methyl)-6-(diethylaminomethyl)pyridine; PNN* = deprotonated PNN). 4 Furthermore, this powerful complex promotes splitting of water to H 2 and O 2 in consecutive heatand light-induced steps. 5 Very recently, a bipyridine-based PNN analogue of this complex was shown to be effective in the direct hydrogenation of amides to form amines and alcohols. 4i The dearomatized (PNP*)Ru II (H)(CO) analogue (PNP = 2,6-bis-((di-tert-butylphosphino)methyl)pyridine; PNP* = deprotonated PNP) is effective in the NÀH bond activation of amines and ammonia, 6a the coupling of alcohols and amines to form imines and H 2 , 6b and the dehydrogenation of secondary alcohols to ketones....

The activation of strong carbon-hydrogen bonds by transition metals is one of the fundamental fields of current organometallic chemistry. This process occurs by one of several possible pathways that are generally dependant on the electron density at the metal center.[1] For electron-rich, low-valent transition metals the typical pathway for C À H cleavage is oxidative addition, which leads to the corresponding alkyl or aryl hydride complexes and is accompanied by a formal twoelectron oxidation of the metal. Transition metals that lack the electron density necessary for oxidative addition, such as early transition metals or high-valent late transition metals, can activate C À H bonds by alternative routes, namely s-bond metathesis, radical activation, 1,2-addition, and electrophilic substitution.[1] It is widely accepted that both s-bond metathesis and oxidative addition processes take place via scomplexes or agostic intermediates. [2] As far as the oxidative addition of CÀH bonds is concerned, the requirement for high electron density means that strong p-acceptor ligands, such as carbon monoxide, are normally expected to inhibit oxidative addition processes by drawing electron density away from the metal center. Herein, however, we describe an electron-poor cationic Rh I system in which addition of a CO ligand can actually promote oxidative addition of a strong C À H bond. This unique reaction pathway is supported by both experimental and theoretical evidence.The new system was discovered during an attempt to convert the Rh III complex 1 [3] into the CÀH agostic Rh I complex 2 [4] by bubbling excess CO through a chloroform solution of 1 (see Scheme 1 a; crystal structures are shown in Figure 1). After drying the resulting solution and re-dissolving the product in CDCl 3 we found that instead of obtaining 2 as the sole product, [5] a mixture of products was obtained which gave extremely broad 1 H and 31 P NMR signals at room temperature. Cooling the solution to À55 8C provided wellresolved signals and revealed the existence of only two products: complex 2 and the new aryl hydride dicarbonyl Rh III complex 3. Complex 3 can also be obtained directly from pure 2 (rather than 1) simply by treating it with CO.[6] Both complexes 2 and 3 have been fully characterized by solution NMR techniques, which included the use of 13 C-labeled CO. [7] A CDCl 3 solution of complex 2 at À55 8C exhibits a doublet in the 31 P{ 1 H} NMR spectrum at d = 18.95 ppm ( 1 J Rh,P = 99.3 Hz), whereas complex 3 gives rise to a doublet at d = 83.10 ppm ( 1 J Rh,P = 88.7 Hz). The difference in both the chemical shift and Rh,P coupling constant is indicative of a decrease in electron density at the metal center on going from 2 to 3. The difference in the 1 H NMR spectra is also pronounced, with the agostic proton of 2 giving rise to a doublet at d = 4.07 ppm ( 1 J Rh,H = 17.3 Hz), while complex 3 exhibits a hydride signal at d = À8.97 ppm (m, 1 J Rh,H = 14.5 Hz), consistent with a hydride trans to a carbonyl ligand. The 13 C{ 1 H} NMR spec...

The activation of strong carbon-hydrogen bonds by transition metals is one of the fundamental fields of current organometallic chemistry. This process occurs by one of several possible pathways that are generally dependant on the electron density at the metal center.[1] For electron-rich, low-valent transition metals the typical pathway for C À H cleavage is oxidative addition, which leads to the corresponding alkyl or aryl hydride complexes and is accompanied by a formal twoelectron oxidation of the metal. Transition metals that lack the electron density necessary for oxidative addition, such as early transition metals or high-valent late transition metals, can activate C À H bonds by alternative routes, namely s-bond metathesis, radical activation, 1,2-addition, and electrophilic substitution.[1] It is widely accepted that both s-bond metathesis and oxidative addition processes take place via scomplexes or agostic intermediates. [2] As far as the oxidative addition of CÀH bonds is concerned, the requirement for high electron density means that strong p-acceptor ligands, such as carbon monoxide, are normally expected to inhibit oxidative addition processes by drawing electron density away from the metal center. Herein, however, we describe an electron-poor cationic Rh I system in which addition of a CO ligand can actually promote oxidative addition of a strong C À H bond. This unique reaction pathway is supported by both experimental and theoretical evidence.The new system was discovered during an attempt to convert the Rh III complex 1 [3] into the CÀH agostic Rh I complex 2 [4] by bubbling excess CO through a chloroform solution of 1 (see Scheme 1 a; crystal structures are shown in Figure 1). After drying the resulting solution and re-dissolving the product in CDCl 3 we found that instead of obtaining 2 as the sole product, [5] a mixture of products was obtained which gave extremely broad 1 H and 31 P NMR signals at room temperature. Cooling the solution to À55 8C provided wellresolved signals and revealed the existence of only two products: complex 2 and the new aryl hydride dicarbonyl Rh III complex 3. Complex 3 can also be obtained directly from pure 2 (rather than 1) simply by treating it with CO.[6] Both complexes 2 and 3 have been fully characterized by solution NMR techniques, which included the use of 13 C-labeled CO. [7] A CDCl 3 solution of complex 2 at À55 8C exhibits a doublet in the 31 P{ 1 H} NMR spectrum at d = 18.95 ppm ( 1 J Rh,P = 99.3 Hz), whereas complex 3 gives rise to a doublet at d = 83.10 ppm ( 1 J Rh,P = 88.7 Hz). The difference in both the chemical shift and Rh,P coupling constant is indicative of a decrease in electron density at the metal center on going from 2 to 3. The difference in the 1 H NMR spectra is also pronounced, with the agostic proton of 2 giving rise to a doublet at d = 4.07 ppm ( 1 J Rh,H = 17.3 Hz), while complex 3 exhibits a hydride signal at d = À8.97 ppm (m, 1 J Rh,H = 14.5 Hz), consistent with a hydride trans to a carbonyl ligand. The 13 C{ 1 H} NMR spec...

Pincer-type complexes constitute a large family of compounds that have attracted much recent interest. Among these compounds, aryl-anchored, d8 pincer complexes of the type [M II (LCL')] (M = Ni, Pd, Pt; L = neutral ligand such as phosphine, amine, dialkyl sulfide) are a major group that plays important roles in organometallic reactions and mechanisms, catalysis, and in the design of new materials.[1] In contrast, and to our knowledge, no complexes of this type with the metal in the zero oxidation state have been prepared. Such d 10 [M 0 (LCL')] complexes with neutral L ligands and an "anionic" aryl anchor would be anionic, and would be expected to possess distinctly different properties to neutral d 10 (M 0 ) complexes. We chose to utilize bulky bis-chelating pincer-type ligands in this study as they have been shown to be effective in stabilizing reactive species and have led to unusual complexes. [1] We have recently shown that reduction of PCP-type Pd II complexes [Pd(X)C 6 H 3 (CH 2 PiPr 2 ) 2 ] (X= Cl, trifluoroacetate) with sodium metal results in collapse of the pincer system, leading to formation of the diamagnetic binuclear complex [Pd{C 6 H 3 (CH 2 PiPr 2 ) 2 } 2 Pd], which contains a 14-electron linear Pd 0 moiety and a completely nonplanar "butterfly"-type 16-electron Pd II moiety.[2] Oxidation of the binuclear complex, or its reaction with organic halides, regenerates the original mononuclear framework. [2] To avoid collapse of the pincer system upon reduction we decided to use a PCP-Pt II complex with the hope that the more diffuse nature of the Pt orbitals (compared with Pd) might stabilize the reduced metal center.[3] In addition, increasing the steric bulk of the pincer phosphine ligand might protect the reduced metal center against intermolecular reactions and might lead to a rare monometallic Pt Herein we report the preparation, characterization, and computational study of the first thermally stable, monometallic anionic Pt 0 complex. The reactivity of this electron-rich, 16-electron PCP-type anionic Pt 0 complex shows that it is a Brønsted base and an effective electron-transfer reagent that is capable of C À F activation under exceedingly mild conditions.Reduction of the bulky PCP-Pt II complex 1 (Scheme 1) [6] with sodium in dry [D 8 ]THF at room temperature overnight led to a dramatic color change from colorless to dark red. A multinuclear NMR spectroscopy study of the reaction solution revealed that reduction of complex 1 had taken place to give quantitative formation of the diamagnetic 16-electron planar PCP-Pt 0 anion 2 (Scheme 1). Thus, the resonance of 1 at d = 66.7 ppm ( 1 J Pt,P = 2893 Hz) in the 31 P{ 1 H} NMR spectrum is replaced by a signal for the new complex 2 at d = 120.5 ppm ( 1 J Pt,P = 3874 Hz). The 31 P{ 1 H} NMR spectrum of the solution confirmed that this transformation is quantitative. In addition, the 195 Pt{ 1 H} NMR spectrum revealed that the diagnostic triplet of 1 at d = À4105 ppm ( 1 J P,Pt = 2893 Hz) is replaced by a new triplet for 2 at d = À4034 ppm ( 1 J P,...

The six-membered ring NHC complexes Rh(6-NHC)(PPh3)2H (6-NHC = 6-i Pr, 1; 6-Et, 2; 6-Me, 3) have been employed in the catalytic hydrodefluorination (HDF) of C6F5CF3 and 2-C6F4HCF3. Stoichiometric studies showed that 1 reacted with C6F5CF3 at room temperature to afford cis-and trans-phsophine isomers of Rh(6-i Pr)(PPh3)2F (4), which reform 1 upon heating with Et3SiH. Although up to three consecutive HDF steps prove possible with C6F5CF3, the ultimate effectiveness of the catalysts are limited by their propensity to undergo C-H activation of partially fluorinated toluenes to give, for example, Rh(6-i Pr)(PPh3)2(C6F4CF3) (7), which was isolated and structurally characterized.

A series of naphthyl-based PCP Pt(II) complexes was synthesized and characterized. A single-crystal X-ray study of (PCP)PtCl (2) reveals stacking of the aromatic units between each pair of molecules of 2. Chloride abstraction from 2 under nitrogen atmosphere leads to formation of the unsaturated cationic complex [(PCP)Pt] + BF 4 -(3), with the metal center being stabilized by the counteranion (3a) or by the solvent (3b). Abstraction of the chloride ligand from 2 under CO atmosphere leads to formation of the cationic carbonyl complex [(PCP)Pt(CO)] + BF 4 -(4), containing an electrophilic carbonyl ligand. The latter is attacked by nucleophiles (MeOand H -) to give the platinum carbomethoxy complex 5 and a rare platinum formyl complex, 6. Stabilized by the bulky bis-chelating tridentate pincer-type system, the formyl complex 6 was isolated and characterized. Complex 6 is more stable than the previously reported platinum formyls. At room temperature complex 6 is slowly converted (during days) into a hydride complex, 7.