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1.
Low-valent metals traditionally dominate the domain of catalytic hydrogenation. However, metal-ligand cooperating (MLC) catalytic systems, operating through heterolytic H−H bond splitting by a Lewis acidic metal and a basic ligand site, do not require an electron-rich metal. On the contrary, high-valent metals that induce weaker back donation facilitate heterolytic bond activation. Here we report, for the first time, the efficient hydrogenation of carbonyl and carboxyl compounds under molecular hydrogen catalyzed by a structurally well-defined RuIV catalyst bearing a bifunctional PCP pincer ligand. The catalyst exhibits reactivity toward molecular hydrogen superior to that of the low-valent analog and allows hydrogen activation even at room temperature.  相似文献   

2.
The oxidative addition of BF3 to a platinum(0) bis(phosphine) complex [Pt(PMe3)2] ( 1 ) was investigated by density functional calculations. Both the cis and trans pathways for the oxidative addition of BF3 to 1 are endergonic (ΔG°=26.8 and 35.7 kcal mol?1, respectively) and require large Gibbs activation energies (ΔG°=56.3 and 38.9 kcal mol?1, respectively). A second borane plays crucial roles in accelerating the activation; the trans oxidative addition of BF3 to 1 in the presence of a second BF3 molecule occurs with ΔG° and ΔG° values of 10.1 and ?4.7 kcal mol?1, respectively. ΔG° becomes very small and ΔG° becomes negative. A charge transfer (CT), F→BF3, occurs from the dissociating fluoride to the second non‐coordinated BF3. This CT interaction stabilizes both the transition state and the product. The B?F σ‐bond cleavage of BF2ArF (ArF=3,5‐bis(trifluoromethyl)phenyl) and the B?Cl σ‐bond cleavage of BCl3 by 1 are accelerated by the participation of the second borane. The calculations predict that trans oxidative addition of SiF4 to 1 easily occurs in the presence of a second SiF4 molecule via the formation of a hypervalent Si species.  相似文献   

3.
We newly designed and prepared a novel molybdenum complex bearing a 4-[3,5-bis(trifluoromethyl)phenyl]pyridine-based PNP-type pincer ligand, based on the bond dissociation free energies (BDFEs) of the N−H bonds in molybdenum-imide complexes bearing various substituted pyridine-based PNP-type pincer ligands. The complex worked as an excellent catalyst toward ammonia formation from the reaction of an atmospheric pressure of dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions, where up to 3580 equivalents of ammonia were formed based on the molybdenum atom of the catalyst. The catalytic activity was significantly improved by one order of magnitude larger than that observed when using the complex before modification.  相似文献   

4.
Metal–ligand cooperation (MLC) plays an important role in catalysis. Systems reported so far are generally based on a single mode of MLC. We report here a system with potential for MLC by both amine–amide and aromatization–dearomatization ligand transformations, based on a new class of phosphino–pyridyl ruthenium pincer complexes, bearing sec‐amine coordination. These pincer complexes are effective catalysts under unprecedented mild conditions for acceptorless dehydrogenative coupling of alcohols to esters at 35 °C and hydrogenation of esters at room temperature and 5 atm H2. The likely actual catalyst, a novel, crystallographically characterized monoanionic de‐aromatized enamido–RuII complex, was obtained by deprotonation of both the N?H and the methylene proton of the N‐arm of the pincer ligand.  相似文献   

5.
Density functional theory calculations modelling selective exo-H/D exchange observed in the Rh σ-alkane complex [(Cy2PCH2CH2PCy2)Rh(η22-endo-NBA)][BArF4], [1-NBA][BArF 4 ] , are reported, where ArF=3,5-C6H3(CF3)2 and NBA=norbornane, C7H12. Two models were considered 1) an isolated molecular cation, [1-NBA]+ and 2) a full model in which [1-NBA][BArF 4 ] is treated in the solid state through periodic DFT. After an initial endo-exo rearrangement, both models predict H/D exchange to proceed through D2 addition and oxidative cleavage followed by a rate-limiting C−H activation of the norbornane through a σ-CAM step to form a [1-Rh(D)(η2-HD)(norbornyl)]+ intermediate. HD rotation followed by a σ-CAM C−D bond formation, HD reductive coupling and HD loss then complete the H/D exchange process. exo-H/D exchange is facilitated by a supporting agostic interaction and is consistently more accessible kinetically than the potentially competing endo-H/D exchange (isolated cation: ΔGexo=+15.9 kcal/mol, ΔGendo=+18.4 kcal/mol; solid state: ΔGexo=+22.1 kcal/mol, ΔGendo=+25.1 kcal/mol). The solid-state environment has a significant impact on the computed energetics, with barriers increasing by ca. 7 kcal/mol, while only the solid-state model correctly predicts the endo-bound NBA complex to be the resting state of the system. These outcomes reflect solid-state confinement effects within the pocket occupied by the [1-NBA]+ cation and defined by the pseudo-octahedral array of neighbouring [BArF4] anions. The asymmetry of the solid-state environment also requires a second H/D exchange pathway to be defined to account for reaction at all four exo-C−H bonds. These entail slightly higher barriers (ΔGexo= +24.8 kcal/mol, ΔGendo=+27.5 kcal/mol) but retain a distinct preference for exo- over endo-H/D exchange.  相似文献   

6.
Quantum-chemical calculations in terms of the density functional theory showed that cyclopolyenyl isocyanides RNC are considerably less stable than the corresponding cyanides and that they are capable of undergoing RNC → RCN isomerization according to both 1,2-shift mechanism (cyclopropenyl and cyclopentadienyl isocyanides; ΔE = 35.0 and 37.5 kcal/mol, respectively) and previously unknown 2,5-sigmatropic shift mechanism (cycloheptatrienyl isocyanide, ΔE = 26.4 kcal/mol). Migration of cyano group in the cyclopentadiene and cycloheptatriene systems follows the 1,5-sigmatropic shift pattern. The activation barrier to 1,5-shift of cyano group around the cycloheptatriene ring was estimated by dynamic NMR in deuterated nitrobenzene (ΔG 190°C = 26.5 kcal/mol).  相似文献   

7.
A family of seven cationic gold complexes that contain both an alkyl substituted π‐allene ligand and an electron‐rich, sterically hindered supporting ligand was isolated in >90 % yield and characterized by spectroscopy and, in three cases, by X‐ray crystallography. Solution‐phase and solid‐state analysis of these complexes established preferential binding of gold to the less substituted C?C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C?C bond. Kinetic analysis of intermolecular allene exchange established two‐term rate laws of the form rate=k1[complex]+k2[complex][allene] consistent with allene‐independent and allene‐dependent exchange pathways with energy barriers of ΔG1=17.4–18.8 and ΔG2=15.2–17.6 kcal mol?1, respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG=8.9–11.4 kcal mol?1) intramolecular exchange of the allene π faces through η1‐allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)2o‐binaphthyl}Au(η2‐4,5‐nonadiene) ]+SbF6? ( 5 ), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG298K=17.4 kcal mol?1), which ruled out the participation of a η1‐allylic cation species in the low‐energy π‐face exchange process for this complex.  相似文献   

8.
A quantum chemical study of the mechanism and determination of the activation barriers of intramolecular η66-inner-ring haptotropic rearrangements (IHR), consisting in moving a chromium tricarbonyl group Cr(CO)3 from one six-membered aromatic ring to another, are carried out using the density functional theory (DFT) for the respective η6-complexes of coronene I and kekulene II. The stationary states on the potential energy surface, determining the mechanism of η66-IHR, have a lower hapticity, which is of interest for catalysis because of the possibility of coordinating an additional substrate and reagent around the transition metal during the rearrangement. The processes of η66-IHR complexes I and II occur with similar energy barriers (ΔG ≈ 20–25 kcal/mol) that are lower than the barriers (ΔG ≈ 30 kcal/mol) of similar transformations previously calculated or measured for naphthalene complexes and a number of small polyaromatic hydrocarbons.  相似文献   

9.
We report that 2,6‐lutidine?trichloroborane (Lut?BCl3) reacts with H2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl2. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl3 was calculated to react with H2 and form the ion pair, [LutH+][HBCl3?], with a barrier of ΔH=24.7 kcal mol?1G=29.8 kcal mol?1). Metathesis with a second molecule of Lut?BCl3 produced Lut?BHCl2 and [LutH+][BCl4?]. The overall reaction is exothermic by 6.0 kcal mol?1rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl2+) and the four‐membered boracycle [(CH2{NC5H3Me})BCl2]. Barriers for addition of H2 across the Lut/LutBCl2+ pair and the boracycle B?C bond are substantially higher (ΔG=42.1 and 49.4 kcal mol?1, respectively), such that these pathways are excluded. The barrier for addition of H2 to the boracycle B?N bond is comparable (ΔH=28.5 and ΔG=32 kcal mol?1). Conversion of the intermediate 2‐(BHCl2CH2)‐6‐Me(C5H3NH) to Lut?BHCl2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl3. Intramolecular protodeboronation, which could form Lut?BHCl2 directly, is prohibited by a high barrier (ΔH=52, ΔG=51 kcal mol?1).  相似文献   

10.
Activation barriers for fast 1,3-N,N' migrations of phenylmercury groups in the corresponding derivatives of N,N'-di(p-tolyl)form(benz)amidines have been calculated by density functional theory B3LYP/Gen, 6-311++G(d,p)/SDD to be ΔE ZPE = 4.5 and 3.0 kcal/mol. The results correspond to the data of dynamic NMR, which have shown the upper limit of activation barriers of these rearrangements (ΔG) to be below 8 kcal/mol. The calculations have shown that the most stable is the E-syn form of N-phenylmercury-N,N'-di(p-tolyl)form(benz)amidines stabilized by supplementary intramolecular coordination of mercury atom with imine nitrogen atom of the amidine triad.  相似文献   

11.
The mechanisms of the hydrogenation of phenylacetylene and styrene to ethylbenzene on the Pd{111} surface, which are secondary reactions of the selective hydrogenation of phenylacetylene, were studied by the DFT-PBE method. The position of the Ph group of the styrene molecule with respect to the Pd surface is shown to exert a noticeable effect on the mechanism of the process. If the Ph group is adsorbed on the Pd surface, then the addition of the first H atom to the terminal C atom of the styrene molecule is most probable. If Ph does not contact the surface, then the most substituted C atom of the styrene molecule is first hydrogenated. On the whole, the interaction of the Ph group with the Pd{111} surface results in stable adsorption structures, the hydrogenation of which on the Pd{111} surface is strongly hindered. The Gibbs activation energy (ΔG298) is 22.9 and 27.1 kcal mol–1 in the most probable reaction routes of styrene hydrogenation to ethylbenzene and direct hydrogenation of phenylacetylene to ethylbenzene, respectively.  相似文献   

12.
A unique Ir complex (tBuNCCP)Ir with the pyridine–phosphine pincer as the sole ligand, featuring a dual agostic interaction between the Ir and two σ C−H bonds from a t Bu substituent, has been prepared. This complex exhibits exceptionally high activity and excellent regio‐ and stereoselectivity for monoisomerization of 1‐alkenes to trans ‐2‐alkenes with wide functional‐group tolerance. Reactions can be performed in neat reactant on a more than 100 gram scale using 0.005 mol % catalyst loadings with turnover numbers up to 19000.  相似文献   

13.
The rational optimization of homogeneous catalysts requires ligand platforms that are easily tailored to improve catalytic performance. Here, it is demonstrated that pyridylidene amides (PYAs) provide such a platform to custom-shape transfer hydrogenation catalysts with exceptional activity. Specifically, a series of meta-PYA pincer ligands with differently substituted PYA units has been synthezised and coordinated to ruthenium(II) centres to form bench-stable tris-acetonitrile complexes [Ru(R-PYA-pincer)(MeCN)3](PF6)2 (R=OMe, Me, H, Cl, CF3). Analytic studies including 1H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography reveal a direct influence of the substituents on the electronic properties of the ruthenium center. The complexes are active in the catalytic transfer hydrogenation of ketones, with activities directly encoded by the PYA substitution pattern. Their perfomance improves further upon exchange of an ancillary MeCN ligand with PPh3. While complexes [Ru(R-PYA-pincer)(PPh3)(MeCN)2](PF6)2 were only isolated for R=H, Me, an in situ protocol was developed to generate these complexes in situ for R=OMe, Cl, CF3 by using a 1:2 ratio of the complexes and PPh3. This in situ protocol together with a short catalyst pre-activation provided highly active catalytic systems. The most active pre-catalyst featured the methoxy-substituted PYA ligand and reached turnover frenquencies of 210 000 h−1 under an exceptionally low catalyst loading of 25 ppm for the benchmark substrate benzophenone, representing one of the most active transfer hydrogenation systems known to date.  相似文献   

14.
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 ( INH ), bearing a N-protonated pica, can easily transform into the κ2-N,O analogue ( INH2 ; ΔG≈11 kcal mol−1, ΔG ≈−5 kcal mol−1). Intramolecular hydrogen liberation from INH2 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.  相似文献   

15.
Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] ( 2 – 7 ; Josiphos=1‐[1‐(dicyclohexylphosphano)ethyl]‐2‐(diarylphosphano)ferrocene) have been prepared by treating [RuCl2(PPh3)3] with (S,R)‐Josiphos diphosphanes and 1‐substituted‐1‐(6‐arylpyridin‐2‐yl)methanamines (HCNN; substituent=H ( 1 a ), Me ( 1 b ), and tBu ( 1 c )) with NEt3. By using 1 b and 1 c as a racemic mixture, complexes 4 – 7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY), achieving enantioselectivities of up to 99 %. In 2‐propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol % and afforded a turnover frequency (TOF) of 105–106 h?1 (60 and 82 °C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H2 (5 atm) at 0.01 mol % relative to the complex with a TOF of ≈104 h?1 at 40 °C.  相似文献   

16.
Quantum chemical calculations by the density functional theory method at the B3LYP/6-311++G** level have shown that 5-nitro-5-methyl-1,2,3,4-tetramethoxycarbonylcyclopentadiene (1) and 5-nitro-2-methyl- 1,3,4,5-tetramethoxycarbonylcyclopentadiene (2) undergo interconversion by consecutive 1,5-sigmatropic shifts via the formation of an unstable isomer, 5-nitro-1-methyl-2,3,4,5- tetramethoxycarbonylcyclopentadiene (3), rather than through the NMR-detected 1,3-shift of the nitro group over the cyclopentadiene ring perimeter. According to calculations in the gas phase, isomer 3 is by ΔE ZPE of 3.6 kcal/mol less stable than isomer 1, while the activation barrier of the stepwise 1 → 2 process is 24.5 kcal/mol, which agrees well with NMR data (ΔG25C, chlorobenzene, 26.5 kcal/mol).  相似文献   

17.
A new efficient metal-based frustrated Lewis pair constructed by (PtBu3)2Pt and B(C6F5)3 was designed through density functional theory calculations for the catalytic dehydrogenation of ammonia borane (AB). The reaction was composed by the successive dehydrogenation of AB and H2 liberation, which occurs through the cooperative functions of the Pt(0) center and the B(C6F5)3 moiety. Two equivalents of H2 were predicted to be liberated from each AB molecule. The generation of the second H2 is the rate-determining step, with a Gibbs energy barrier and reaction energy of 27.4 and 12.8 kcal/mol, respectively.  相似文献   

18.
《Thermochimica Acta》1987,122(2):289-294
The standard enthalpy of formation of potassium metasilicate (K2SiO3), determined by hydrofluoric acid solution calorimetry, was found to be ΔHof,298 = −363.866±0.421 kcal mol−1 (−1522.415±1.762 kj mol−1). The standard enthalpy of formation from the oxides was found to beΔHo298 = −64.786±0.559 kcal mol−1 (−271.065±2.339 kJ mol−1).These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the literature data. The standard enthalpies of formation and Gibbs energies of formation are given as functions of temperature. The standard Gibbs energy of formation is ΔGf,298.150 = −341.705 kcal mol−1 (−1429.694 kJ mol−1).  相似文献   

19.
Thermodynamic knowledge of the metal–ligand (M−L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd−X (X=OR and NHAr) bond heterolysis energies (ΔGhet(Pd−X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N′,N′-tetramethylethylenediamine) as the model complexes. The ΔGhet(Pd−X) scales span a range of 2.6–9.0 kcal mol−1 for ΔGhet(Pd−O) values and of 14.5–19.5 kcal mol−1 for ΔGhet(Pd−N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X−H bond activation reveal that the M−X bond metathesis is dominated by differences of the X−H and Pd−X bond strengths, the former being more influential. The ΔGhet(Pd−X) and pKa(X−H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd−X complexes.  相似文献   

20.
A detailed mechanism for alkyne alkoxycarbonylation mediated by a palladium catalyst has been characterised at the B3PW91‐D3/PCM level of density functional theory (including bulk solvation and dispersion corrections). This transformation, investigated via the methoxycarbonylation of propyne, involves a uniquely dual role for the P,N hemilabile ligand acting co‐catalytically as both an in situ base and proton relay coupled with a Pd0 centre, allowing for surmountable barriers (highest ΔG of 22.9 kcal mol?1 for alcoholysis). This proton‐shuffle between methanol and coordinated propyne accounts for experimental requirements (high acid concentration) and reproduces observed regioselectivities as a function of ligand structure. A simple ligand modification is proposed, which is predicted to improve catalytic turnover by three orders of magnitude.  相似文献   

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