The reverse water-gas shift reaction CO(2) + H(2) --> H(2)O + CO has been investigated using a set of homogeneous catalyst models L'M(I) (L' = beta-diketiminate, C(3)N(2)H(5)(-); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). The thermodynamics of prototypical reaction pathways were simulated at two levels of theory: B3LYP/6-311+G(d) and B3LYP/aug-cc-pVTZ. The modeled catalytic reaction has been considered in the following steps: coordination of CO(2) by the catalyst to generate a carbon dioxide complex, L'M(CO(2)); scission of L'M(CO(2)) to yield L'M(CO) and L'M(O); L'M(O) hydrogenation to form L'M(H(2)O). The final products, H(2)O and CO, were obtained from the dissociation of L'M(H(2)O) and L'M(CO). All of the reactants, intermediates, and products were modeled, where different possible conformers and multiplicities were identified and considered as potential minima. The reaction enthalpy DeltaH, of all steps for each catalyst as a function of transition metal have been determined. The Mn and Fe catalysts show more thermodynamically accessible pathways than the other catalyst models studied. The overall reaction enthalpy has been determined not only by B3LYP/6-311+G(d) and B3LYP/aug-cc-pVTZ but also via a more rigorous ab initio electron-correlation-based approach, the correlation consistent Composite Approach (ccCA).
The (dippe)palladium(0) fragment generated from [(dippe)Pd(μ-H)] 2 (1) has been shown to form an intermediate η 2 -nitrile complex with acetonitrile (dippe)Pd(η 2 -C,N-CH 3 CN-BEt 3 ) (2a) in the presence of BEt 3 [(dippe = bisdiisopropylphosphino)ethane)]. On introducing a solution of 2a to 1 equiv of BPh 3 , rapid formation of (dippe)Pd(η 2 -C,N-CH 3 CNBPh 3 ) (2a′) is observed. Heating 2a′ at 100°C in THF-d 8 results in the C−CN activation product 3a′, (dippe)Pd(CH 3 )(CNBPh 3 ). Reaction of 1 with benzonitrile in the presence of BEt 3 gives the C−CN activation product (dippe)Pd(Ph)(CN-BEt 3 ) (3b) exclusively. The complexes 2a, 2a′, 3a′, and 3b were characterized by 1 H, 31 P{ 1 H}, and 13 C{ 1 H} NMR spectroscopy, elemental analysis, IR spectroscopy, and X-ray diffraction.I nterest in the carbon−carbon bond activation of alkyl, allyl, and aryl nitriles has risen due to potential industrial 1 and organic synthesis applications. 2 Recently, the complexes [(dippe)Ni(μ-H)] 2 and [(dippe)PtH] 2 (dippe = bisdiisopropylphosphino)ethane) have been employed in stoichiometric activation of nitriles in our group in collaboration with Garcı́a. 3,4 These complexes furnish a reactive 14-electron (dippe)M 0 (M = Ni, Pt) fragment that can cleave the C−CN bond in the nitriles. Garcı́a and co-workers have since expanded [(dippe)Ni(μ-H)] 2 studies by providing the first catalytic example of low-valent nickel additions of alcohols to aryl, alkyl, and heteroatomic nitriles. 5 As an extension to the studies with group 10 hydride dimers in our group, we wanted to investigate the palladium hydride dimer, [(dippe)Pd(μ-H)] 2 (1), in the activation of the C−CN bond in nitriles. Complex 1 has been shown to be effective in cleaving the carbon−sulfur bonds in thiophenes and thioethers. 6 Herein, we report Lewis acid assisted C−CN cleavage to form (dippe)palladium(II) intermediate complexes that are crucial in cross-coupling, hydrodecyanation, and cycloaddition reactions. 7 Complex 1, [(dippe)Pd(μ-H)] 2 , 6,8 is generated in situ when a (dippe)PdCl 2 suspension in THF is treated with 2 equiv of potassium triethylborohydride with the loss of KCl/BEt 3 (Scheme 1). After filtration to remove the KCl salt, the filtrate is treated with an excess of acetonitrile, causing a color change from intense dark red to light transparent orange. A pair of doublet resonances at δ 75.63 (d, 2 J P-P = 25.0 Hz) and 61.77 (d, 2 J P-P = 25.0 Hz) is observed in the 31 P{ 1 H} NMR spectrum, consistent with asymmetrically η 2 -coordinated (dippe)Pd(0) complexes. 9 The 13 C{ 1 H} NMR spectrum of 2a in THF shows a distinct downfield shift of the CN to δ 149.70 compared to δ 117 in free acetonitrile. The 11 B{ 1 H} NMR spectrum shows a resonance at δ −13.2, the region for BEt 3 complexes. 10 Attempts to isolate 2a as a solid by removing solvent in vacuo results in 2a decomposing to [(μ-dippe)Pd] 2 , 11 which has the characteristic signal at δ 33 in the 31 P{ 1 H} NMR spectrum. However, X-ray quality crystals grew from the THF solution of 2a at room temperature ove...
A new series of molybdenum and tungsten tricarbonyl pincer complexes, bearing pyridine-based PONOPtype pincer ligands, have been synthesized and fully characterized. Addition of HBF 4 •Et 2 O to these tricarbonyl complexes generated seven-coordinate molybdenum and tungsten hydride complexes, and these compounds have been isolated in good yields. These metal hydrides show fluxional behavior in solution.The hydride ligands on these metal complexes are acidic in nature and are readily deprotonated by bases. The molybdenum hydride complex is shown to catalyze isomerization of 1-hexene to internal isomers under mild conditions.
Carbon–sulfur activation of phenyl p-tolyl sulfide by a mixture of [Pd(dippe)(μ-H)]2 (1a) and dinuclear Pd(0), [(μ-dippe)Pd]2 (1b) (dippe = 1,2-bis(diisopropylphosphino)ethane), to yield four carbon–sulfur activation products, (dippe)Pd(p-tolyl)(SPh) (3a), (dippe)Pd(Ph)(S-p-tolyl) (3b), (dippe)Pd(SPh)(Ph)(3c), and (dippe)Pd(p-tolyl)(S-p-tolyl) (3d), was investigated. The carbon–sulfur complexes 3a–3d were completely characterized by 1H, 31P, and 13C NMR spectroscopy, elemental analysis, and X-ray diffraction. Exchange interactions between arylthiolate groups in (dippe)Pd(Ar)(SAr′) (3a–3d) were investigated, leading to understanding the mechanism of interconversions among the complexes.
Supporting Information Table of Contents ________________________________________________________________________________________Experimental details for the structure determination of 9. S-72Experimental details for the structure determination of 10. S-93Experimental details for the structure determination of 14. S-106Experimental details for the structure determination of 14 and 15. S-126Experimental details for the structure determination of (dippe)Pd(Cl) 2 . S-143
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