"Long-range" metal-ligand cooperation in H2 activation and ammonia-promoted hydride transfer with a ruthenium-acridine pincer complex

The acridine-based pincer complex 1 exhibits an unprecedented mode of metal-ligand cooperation involving a "long-range" interaction between the distal acridine C9 position and the metal center. Reaction of 1 with H(2)/KOH results in H(2) splitting between the Ru center and C9 with concomitant dearomatization of the acridine moiety. DFT calculations show that this process involves the formation of a Ru dihydride intermediate bearing a bent acridine ligand in which C9 is in close proximity to a hydride ligand followed by through-space hydride transfer. Ammonia induces transfer of a hydride from the Ru center of 1 to C9 of the flexible acridine pincer ligand, forming an unusual dearomatized fac-acridine PNP complex.     

Electron-rich, bulky PNN-type ruthenium complexes: synthesis, characterization and catalysis of alcohol dehydrogenation

Reaction of the electron-rich, bulky tridentate PNN ligand (PNN=2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) with Ru(PPh3)3Cl2 at 65 degrees C resulted in formation of the mononuclear dinitrogen complex (PNN)Ru(Cl)2N2 (minor) and the N2 bridged Ru(II) dinuclear complex [(PNN)Ru(Cl)2]2(micro-N2) (major). These complexes can be interconverted; passing argon through a solution of the mixture resulted in formation of pure . The cationic square-pyramidal [(PNN)Ru(PPh3)Cl]OTf was obtained by the reaction of complex with silver triflate followed by PPh3. Reaction of complex with CO yielded (PNN)Ru(CO)Cl2, which upon reaction with one equiv. of AgBF4 gave the cationic [(PNN)Ru(CO)Cl]BF4. The dicationic [(PNN)Ru(CO)(H2O)(acetone)](BF4)2 was obtained from with 2 equiv. of AgBF4 in acetone solution. Complexes , and were structurally characterized by X-ray crystallography. Complexes and upon addition of an equivalent of base, catalyzed the dehydrogenation of secondary alcohols to the corresponding ketones and primary alcohols to esters in good yields and high selectivity accompanied with the evolution of hydrogen gas.     

C−H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex

The 3d‐metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised C?H bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt–imido bond. It can cleanly cleave strong C?H bonds with a bond dissociation energy of up to 92 kcal mol^?1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds)₂(NH^tBu)]^?. Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt‐mediated catalytic application for substrate dehydrogenation using an organo azide is presented.     

Dynamic Ligand Reactivity in a Rhodium Pincer Complex

Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane‐pyridine‐amide ligand PNN^H2 in combination with Rh^I. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1 , amido species 2 , imine complex 3 and dearomatized compound 4 . Complex 4 bears a dual‐mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible “storage” of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal‐based reactivity, this allows for the mono‐alkylation of o‐toluenesulfonamide with iodomethane. Compounds 1 , 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5 , [Rh(CO)( PNN′′ )], formed by sequential N?H activation of sulfonamide by the dearomatized ligand PNN′ and follow‐up nucleophilic attack of anionic sulfonamide onto the imine fragment.     

Understanding the Deactivation Pathways of Iridium(III) Pyridine-Carboxiamide Catalysts for Formic Acid Dehydrogenation

The degradation pathways of highly active [Cp*Ir(κ²-N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO₃), 2 undergoes partial protonation of the amide moiety, inducing rapid κ²-N,N to κ²-N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ²-N,N key intermediate of FA dehydrogenation ( I_NH ), bearing a N-protonated pica, can easily transform into the κ²-N,O analogue ( I_NH2 ; ΔG^≠≈11 kcal mol⁻¹, ΔG ≈−5 kcal mol⁻¹). Intramolecular hydrogen liberation from I_NH2 is predicted to be rather prohibitive (ΔG^≠≈26 kcal mol⁻¹, ΔG≈23 kcal mol⁻¹), indicating that FA dehydrogenation should involve mostly κ²-N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c . DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.     

Influence of the chelate ligand structure on the amide methanolysis reactivity of mononuclear zinc complexes

Zinc complexes of three new amide-appended ligands have been prepared and isolated. These complexes, [(dpppa)Zn](ClO4)2 (4(ClO4)2; dpppa = N-((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(bdppa)Zn](ClO4)2 (6(ClO4)2; bdppa = N,N-bis((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)amine), and [(epppa)Zn](ClO4)2 (8(ClO4)2; epppa = N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been characterized by X-ray crystallography (4(ClO4)2 and 8(ClO4)2), 1H and 13C NMR, IR, and elemental analysis. Treatment of 4(ClO4)2 or 8(ClO4)2 with 1 equiv of Me4NOH.5H2O in methanol-acetonitrile (5:3) results in amide methanolysis, as determined by the recovery of primary amine-appended forms of the chelate ligand following removal of the zinc ion. These reactions proceed via the initial formation of a deprotonated amide intermediate ([(dpppa-)Zn]ClO4 (5) and [(epppa-)Zn]ClO4 (9)) which in each case has been isolated and characterized (1H and 13C NMR, IR, elemental analysis). Treatment of 6(ClO4)2 with Me4NOH.5H2O in methanol-acetonitrile results in the formation of a deprotonated amide complex, [(bdppa-)Zn]ClO4 (7), which was isolated and characterized. This complex does not undergo amide methanolysis after prolonged heating in a methanol-acetonitrile mixture. Kinetic studies and construction of Eyring plots for the amide methanolysis reactions of 4(ClO4)2 and 8(ClO4)2 yielded thermodynamic parameters that provide a rationale for the relative rates of the amide methanolysis reactions. Overall, we propose that the mechanistic pathway for these amide methanolysis reactions involves reaction of the deprotonated amide complex with methanol to produce a zinc methoxide species, the reactivity of which depends, at least in part, on the steric hindrance imparted by the supporting chelate ligand. Amide methanolysis involving a zinc complex supported by a N2S2 donor chelate ligand (3(ClO4)2) is more complicated, as in addition to the formation of a deprotonated amide intermediate free chelate ligand is present in the reaction mixture.     

Theoretical study of oxidation of cyclohexane diol to adipic anhydride by [Ru(IV)(O)(tpa)(H2O)]2+ complex (tpa ═ tris(2-pyridylmethyl)amine)

The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by Ru(IV)O(tpa) (tpa ═ tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of α-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of α-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by Ru(IV)O(tpa) active species, which is reduced to bis-aqua Ru(II)(tpa) complex. The Ru(II) complex is reactivated using Ce(IV) and water as an oxygen source. There are two different pathways of the first two steps of the conversion depending on whether the direct H-atom abstraction occurs on a C-H bond or on its adjacent oxygen O-H. In the first step, the C-H (O-H) bond dissociation occurs in TS1 (TS2-1) with an activation barrier of 21.4 (21.6) kcal/mol, which is followed by abstraction of another hydrogen with the spin transition in both pathways. The second process also bifurcates into two reaction pathways. TS3 (TS4-1) is leading to dissociation of the C-H (O-H) bond, and the activation barrier of TS3 (TS4-1) is 20.2 (20.7) kcal/mol. In the third step, oxo ligand attack on the carbonyl carbon and hydrogen migration from the water ligand occur via TS5 with an activation barrier of 17.4 kcal/mol leading to a stable tetrahedral intermediate in a triplet state. However, the slightly higher energy singlet state of this tetrahedral intermediate is unstable; therefore, a spin crossover spontaneously transforms the tetrahedral intermediate into a dione complex by a hydrogen rebound and a C-C bond cleavage. Kinetic isotope effects (k(H)/k(D)) for the electronic processes of the C-H bond dissociations calculated to be 4.9-7.4 at 300 K are in good agreement with experiment values of 2.8-9.0.     

Ru(3)(CO)(12) in Acidic Media. Intermediates of the Acid-Cocatalyzed Water-Gas Shift Reaction (WGSR)

The elucidation of the WGSR promoted by ruthenium carbonyls in acidic media started with the detection of the Ru(0), Ru(I), and Ru(II) intermediate complexes, namely Ru(3)(CO)(12), Ru(2)[&mgr;-eta(2)-OC(CF(3))O](2)(CO)(6), and fac-[Ru(CF(3)COO)(3)(CO)(3)](-), which accumulate when CF(3)COOH is employed as an acid cocatalyst. Under catalytic conditions, the three were found to interconvert through elementary steps which produce CO(2) and H(2). In fact, Ru(0) is oxidized by H(+) to Ru(I) and half the hydrogen of the catalytic cycle is supplied by this reaction. On the other hand, Ru(I) disproportionates to Ru(0) and Ru(II), and this latter species undergoes nucleophilic attack by H(2)O. The decomposition of the metallacarboxylic acid intermediate gives back Ru(I), while H(2) and CO(2) are produced in a 1/2 molar ratio. The two alternating pathways for dihydrogen formation, namely Ru(0) oxidation by H(+) and the decomposition of a metallacarboxylic acid intermediate, involve H(2) reductive elimination from the same RuHCF(3)COO(CO)(2)L(2) intermediate (L = H(2)O, ethers). These findings define an acid-cocatalyzed WGSR whose distinctive features are (i) the intervention of a disproportionation reaction to generate a Ru(II) electron poor complex, whose CO ligands can undergo nucleophilic attack by water, (ii) the generation of the hydrido intermediate for dihydrogen production through two distinct reaction patways, and (iii) the reductive elimination of H(2) from the hydrido intermediate without involving H(+) from the medium.     

Redox transformations of bis(2,2'-bipyridine)(1-methyl-1-pyridin-2-yl-ethylamine)ruthenium(II)

The amineruthenium(II) complex Ru(bpy)2(mpea)2+ has been prepared by the direct reaction of 1-methyl-1-pyridin-2-yl-ethylamine (mpea) with Ru(bpy)2Cl2 in ethanol/water and isolated as the hexafluorophosphate salt. Electrochemical analysis of this complex shows that it undergoes sequential one-electron oxidations to an amidoruthenium(III) intermediate (E degrees' = 1.086 V vs NHE) and then to an amidoruthenium(IV) (E degrees' = 0.928 V) or imidoruthenium(IV) (E degrees' = 1.083 V) complex, depending upon the solution pH (pKa = 2.62 for the amidoruthenium(IV) species). At higher potentials ( Epa = 1.5 V in 1.0 M H2SO4), the amido- or imidoruthenium(IV) species is irreversibly oxidized to the corresponding nitrosoruthenium(II) complex. The mechanism for this transformation appears, on the basis of b3lyp/cpcm/cep-31g(d) computations, to proceed through an imidoruthenium(V) intermediate, which is rapidly attacked by water to yield a Ru(II)-bound hydroxylamine radical, which is readily oxidized and deprotonated to produce the nitrosoruthenium(II) complex. The nitrosoruthenium(II) complex is quantitatively reduced to the original [Ru(bpy)2(mpea)]2+ complex at relatively negative potentials ( Epc = -0.2 V in 1.0 M H2SO4).     

Organophotoredox-Mediated Amide Synthesis by Coupling Alcohol and Amine through Aerobic Oxidation of Alcohol

The combination of an organic photocatalyst [4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6 dicyanobenzene) or 5MeOCzBN (2,3,4,5,6-pentakis(3,6-dimethoxy-9 H-carbazol-9-yl)benzonitrile)], quinuclidine, and tetra-n-butylammonium phosphate (hydrogen-bonding catalyst) was employed for amide bond formations. The hydrogen-bonded OH group activated the adjacent C−H bond of alcohols towards hydrogen atom transfer (HAT) by a radical species. The quinuclidinium radical cation, generated through single-electron oxidation of quinuclidine by the photocatalyst, employed to abstract a hydrogen atom from the α-C−H bond of alcohols selectively due to a polarity effect-produced α-hydroxyalkyl radical, which subsequently converted to the corresponding aldehyde under aerobic conditions. Then the coupling of the aldehyde and an amine formed a hemiaminal intermediate that upon photocatalytic oxidation produced the amide.     

Kinetics and mechanism of the oxidation of butane-2,3-diol by alkaline hexacyanoferrate (III), catalyzed by ruthenium trichloride

The kinetics of oxidation of butane-2,3-diol by alkaline hexacyanoferrate (III), catalyzed by ruthenium trichloride has been studied spectrophotometrically. The reaction rate shows a zero-order dependence on oxidant, a first-order dependence on |Ru(III)|_T, a Michaelis-Menten dependence on |diol|, and a variation complicated on |OH⁻|. A reaction mechanism involving the existence of two active especies of catalyst, Ru(OH)₂⁺ and Ru(OH)₃, is proposed. Each one of the active species of catalyst forms an intermediate complex with the substrate, which disproportionates in the rate determining step. The complex disproportionation involves a hydrogen atom transfer from the α C(SINGLE BOND)H of alcohol to the oxygen of hydroxo ligand of ruthenium, to give Ru(II) and an intermediate radical which is then further oxidized. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 1–7, 1997.     

Design and study of Bi[1,8]naphthyridine ligands as potential photooxidation mediators in Ru(II) polypyridyl aquo complexes

A series of 3,3'-polymethylene-bridged bi[1,8]naphthyridine (binap) ligands, 3a-c, are complexed with Ru(II) to afford [Ru(tpy)(3a-c)(H(2)O)](2+) where an uncomplexed nitrogen on 3a-c is situated so it can form a H-bond with the coordinated water. An additional complex involving [Ru(4'-NMe(2)tpy)(3b)(H(2)O)](2+) is also prepared. X-ray analyses of the [Ru(tpy)(3a,c)(H(2)O)](2+) complexes indicate well-organized H-bonds even when the binap is nonplanar. In an attempt to realize photooxidation, the effects of light, varying potential, and pH were examined. A Pourbaix diagram indicated that the oxidation potential decreased by approximately 0.5 V in the pH range of 1.9-11.6. The lowest-energy electronic absorption for the binap complexes involves the metal-to-ligand charge transfer to the binap ligand and is sensitive to ligand planarity. The absorbance shifted to a lower energy as the auxiliary ligand became a better donor (4'-NMe(2)tpy) or as the water was deprotonated. Acetonitrile was found to displace water most easily for the complex of 3c, where the ligand is the least planar. Despite promising features, photooxidation of the bound water was not observed.     

Dual-function of alcohols in gold-mediated selective coupling of amines and alcohols

Oxidative coupling of alcohols (methanol and ethanol) and dimethylamine on atomic-oxygen-activated Au(111) occurs entirely on the surface to form the corresponding amides when the alkoxy of the alcohol and the amide derived from the amine are co-adsorbed. For effective oxygen-assisted coupling the formation of the amide requires excess methanol. Mechanistic studies reveal that molecularly adsorbed methanol removes excess adsorbed atomic oxygen efficiently, precluding either secondary oxidation or oxidative dehydrogenation of dimethylamide to the imine. The adsorbed amide then can react with the aldehyde produced by β-hydride elimination from the alkoxy to form the hemiaminal, the reactive intermediate leading to coupling. The selectivity for formamide production can be increased to nearly 100?% in excess methanol.     

Structural, electronic, and acid/base properties of [Ru(bpy)2(bpy(OH)2)]2+ (bpy = 2,2'-bipyridine, bpy(OH)2 = 4,4'-dihydroxy-2,2'-bipyridine)

We have synthesized the complex [Ru(bpy)(2)(bpy(OH)(2))](2+) (bpy =2,2'-bipyridine, bpy(OH)(2) = 4,4'-dihydroxy-2,2'-bipyridine). Experimental results coupled with computational studies were utilized to investigate the structural and electronic properties of the complex, with particular attention paid toward the effects of deprotonation on these properties. The most distinguishing feature observed in the X-ray structural data is a shortening of the CO bond lengths in the modified ligand upon deprotonation. Similar results are also observed in the computational studies as the CO bond becomes double bond in character after deprotonating the complex. Electrochemically, the hydroxy-modified bipyridyl ligand plays a significant role in the redox properties of the complex. When protonated, the bpy(OH)(2) ligand undergoes irreversible reduction processes; however, when deprotonated, reduction of the substituted ligand is no longer observed, and several new irreversible oxidation processes associated with the modified ligand arise. pH studies indicate [Ru(bpy)(2)(bpy(OH)(2))](2+) has two distinct deprotonations at pK(a1) = 2.7 and pK(a2) = 5.8. The protonated [Ru(bpy)(2)(bpy(OH)(2))](2+) complex has a characteristic UV/Visible absorption spectrum similar to the well-studied complex [Ru(bpy)(3)](2+) with bands arising from Metal-to-Ligand Charge Transfer (MLCT) transitions. When the complex is deprotonated, the absorption spectrum is altered significantly and becomes heavily solvent dependent. Computational methods indicate that the deprotonated bpy(O(-))(2) ligand mixes heavily with the metal d orbitals leading to a new absorption manifold. The transitions in the complex have been assigned as mixed Metal-Ligand to Ligand Charge Transfer (MLLCT).     

Mechanism of asymmetric hydrogenation of ketones catalyzed by BINAP/1,2-diamine-rutheniumII complexes

Asymmetric hydrogenation of acetophenone with trans-RuH(eta(1)-BH(4))[(S)-tolbinap][(S,S)-dpen] (TolBINAP = 2,2'-bis(di-4-tolylphosphino)-1,1'-binaphthyl; DPEN = 1,2-diphenylethylenediamine) in 2-propanol gives (R)-phenylethanol in 82% ee. The reaction proceeds smoothly even at an atmospheric pressure of H(2) at room temperature and is further accelerated by addition of an alkaline base or a strong organic base. Most importantly, the hydrogenation rate is initially increased to a great extent with an increase in base molarity but subsequently decreases. Without a base, the rate is independent of H(2) pressure in the range of 1-16 atm, while in the presence of a base, the reaction is accelerated with increasing H(2) pressure. The extent of enantioselection is unaffected by hydrogen pressure, the presence or absence of base, the kind of base and coexisting metallic or organic cations, the nature of the solvent, or the substrate concentrations. The reaction with H(2)/(CH(3))(2)CHOH proceeds 50 times faster than that with D(2)/(CD(3))(2)CDOD in the absence of base, but the rate differs only by a factor of 2 in the presence of KO-t-C(4)H(9). These findings indicate that dual mechanisms are in operation, both of which are dependent on reaction conditions and involve heterolytic cleavage of H(2) to form a common reactive intermediate. The key [RuH(diphosphine)(diamine)](+) and its solvate complex have been detected by ESI-TOFMS and NMR spectroscopy. The hydrogenation of ketones is proposed to occur via a nonclassical metal-ligand bifunctional mechanism involving a chiral RuH(2)(diphosphine)(diamine), where a hydride on Ru and a proton of the NH(2) ligand are simultaneously transferred to the C=O function via a six-membered pericyclic transition state. The NH(2) unit in the diamine ligand plays a pivotal role in the catalysis. The reaction occurs in the outer coordination sphere of the 18e RuH(2) complex without C=O/metal interaction. The enantiofaces of prochiral aromatic ketones are kinetically differentiated on the molecular surface of the coordinatively saturated chiral RuH(2) intermediate rather than in a coordinatively unsaturated Ru template.     

Dual‐Function of Alcohols in Gold‐Mediated Selective Coupling of Amines and Alcohols

Oxidative coupling of alcohols (methanol and ethanol) and dimethylamine on atomic‐oxygen‐activated Au(111) occurs entirely on the surface to form the corresponding amides when the alkoxy of the alcohol and the amide derived from the amine are co‐adsorbed. For effective oxygen‐assisted coupling the formation of the amide requires excess methanol. Mechanistic studies reveal that molecularly adsorbed methanol removes excess adsorbed atomic oxygen efficiently, precluding either secondary oxidation or oxidative dehydrogenation of dimethylamide to the imine. The adsorbed amide then can react with the aldehyde produced by β‐hydride elimination from the alkoxy to form the hemiaminal, the reactive intermediate leading to coupling. The selectivity for formamide production can be increased to nearly 100 % in excess methanol.     

Reversible (de)protonation-induced valence inversion in mixed-valent diiron(II,III) complexes

The coupling of electron and proton transfers is currently under intense scrutiny. This Communication reports a new kind of proton-coupled electron transfer within a homodinuclear first-row transition-metal complex. The triply-bridged complex [Fe(III)(μ-OPh)(μ(2)-mpdp)Fe(II)(NH(2)Bn)] (1; mpdp(2-) = m-phenylenedipropionate) bearing a terminal aminobenzyl ligand can be reversibly deprotonated to the anilinate complex 2 whose core [Fe(II)(μ-OPh)(μ(2)-mpdp)Fe(III)(NHBn)] features an inversion of the iron valences. This observation is supported by a combination of UV-visible, (1)H NMR, and M?ssbauer spectroscopic studies.     

Direct hydrogenation of amides to alcohols and amines under mild conditions

The selective, direct hydrogenation of amides to the corresponding alcohols and amines with cleavage of the C-N bond was discovered. The expected products of C-O cleavage are not formed (except as traces in the case of anilides). The reaction proceeds under mild pressure and neutral, homogeneous conditions using a dearomatized, bipyridyl-based PNN Ru(II) pincer complex as a catalyst. The postulated mechanism involves metal-ligand cooperation by aromatization-dearomatization of the heteroaromatic pincer core and does not involve hydrolytic cleavage of the amide. The simplicity, generality, and efficiency of this environmentally benign process make it attractive for the direct transformations of amides to alcohols and amines in good to excellent yields.     

Hydrogenation and Transfer Hydrogenation Promoted by Tethered Ru−S Complexes: From Cooperative Dihydrogen Activation to Hydride Abstraction/Proton Release from Dihydrogen Surrogates

Hydrogenation and transfer hydrogenation of imines with cyclohexa‐1,4‐dienes, as well as with a representative Hantzsch ester dihydrogen surrogate, are reported. Both processes are catalyzed by tethered Ru?S complexes but differ in the activation mode of the dihydrogen source: cooperative activation of the H?H bond at the Ru?S bond leads to the corresponding Ru?H complex and protonation of the sulfur atom, whereas the same cationic Ru?S catalyst abstracts a hydride from a donor‐substituted cyclohexa‐1,4‐diene to form the neutral Ru?H complex and a low‐energy Wheland intermediate. A sequence of proton and hydride transfers on the imine substrate then yields an amine. The reaction pathways are analyzed computationally, and the established mechanistic pictures are in agreement with the experimental observations.     

Synthesis, structure and catalytic properties of CNN pincer palladium(II) and ruthenium(II) complexes with N-substituted-2-aminomethyl-6-phenylpyridines

N-substituted-2-aminomethyl-6-phenylpyridines 2a-c have been easily prepared from commercially available 6-bromo-2-picolinaldehyde in two steps. Reaction of 2a-c with PdCl(2) in toluene in the presence of triethylamine gave the CNN pincer Pd(II) complexes 3a-c in 18-28% yields. The CNN pincer Ru(II) complex 5 containing a Ru-NHR functionality could be obtained in a 71% yield by treatment of 2c with a Ru(II) precursor instead of PdCl(2). Additionally, the related CNN pincer Ru(II) complex 7 containing a Ru-NH(2) functionality has been synthesized by the reaction of 2-aminomethyl-6-phenylpyridine with the same Ru(II) precursor in a 68% yield. All the new compounds were characterized by elemental analysis (MS for ligands), (1)H, (13)C NMR, (31)P{(1)H} NMR (for Ru complexes) and IR spectra. Molecular structures of Pd complex 3c as well as Ru complexes 5 and 7 have been determined by X-ray single-crystal diffraction. The obtained Pd complexes 3a-c were effective catalysts for the allylation of aldehydes as well as for three-component allylation of aldehydes, arylamines and allyltributyltin and their activity was found to be much higher than a related NCN Pd(II) pincer in the allylation of aldehyde. On the other hand, the two new CNN pincer Ru(II) complexes 5 and 7 displayed excellent catalytic activity in the transfer hydrogenation of ketones in refluxing 2-propanol with the latter being much more active. The final TOF values were up to 4510 h(-1) with 0.01 mol% of 5 and 220,800 h(-1) with 0.005 mol% of 7, respectively.