Chelate complex Ni(S2C2H2)2 as a molecular model of the hydrodesulfurization active center

Electronic states of Ni atom in a square-planar complex Ni(S₂C₂H₂)₂ and its molecular adduct with H₂S were studied by means ofab initio molecular orbital calculations. H₂S adsorption stabilizes the Ni(IV) state (d⁶) in the complex with the Ni atom shifted from the plane by 0.35 ?.     

Oxidative addition of dihydrogen to the bimetallic sulfide catalysts: evidence by X-ray photoelectron spectroscopy

Reversible transformations Ni(IV)↔Ni(II) in alumina and Sibunit supported (Ni,Mo) sulfide catalysts were observed after in situ thermal treatment of catalysts in an X-ray photoelectron spectrometer chamber. The phenomenon is interpreted as a reductive elimination of occluded hydrogen under low pressure and high temperature, and oxidative addition of hydrogen after catalyst treatment with an (H₂+H₂S) mixture.     

Oxidative addition of dihydrogen to Ni(II) complexes.ab initio MO/MP4 calculations of the electronic structure of NiH2Cl2(PH3)2 complex

Electronic state d⁶ Ni(IV) in the complex [NiH₂Cl₂(PH₃)₂] was studied by means ofab initio MO/MP4 calculations.     

Sulfur‐33 Isotope Tracing of the Hydrodesulfurization Process: Insights into the Reaction Mechanism,Catalyst Characterization and Improvement

The novel approach based on ³³S isotope tracing is proposed for the elucidation of hydrodesulfurization (HDS) mechanisms and characterization of molybdenum sulfide catalysts. The technique involves sulfidation of the catalyst with ³³S‐isotope‐labeled dihydrogen sulfide, followed by monitoring the fate of the ³³S isotope in the course of the hydrodesulfurization reaction by online mass spectrometry and characterization of the catalyst after the reaction by temperature‐programmed oxidation with mass spectrometry (TPO‐MS). The results point to different pathways of thiophene transformation over Co or Ni‐promoted and unpromoted molybdenum sulfide catalysts, provide information on the role of promoter and give a key for the design of new efficient HDS catalysts.     

Sulfur‐33 Isotope Tracing of the Hydrodesulfurization Process: Insights into the Reaction Mechanism,Catalyst Characterization and Improvement

The novel approach based on ³³S isotope tracing is proposed for the elucidation of hydrodesulfurization (HDS) mechanisms and characterization of molybdenum sulfide catalysts. The technique involves sulfidation of the catalyst with ³³S‐isotope‐labeled dihydrogen sulfide, followed by monitoring the fate of the ³³S isotope in the course of the hydrodesulfurization reaction by online mass spectrometry and characterization of the catalyst after the reaction by temperature‐programmed oxidation with mass spectrometry (TPO‐MS). The results point to different pathways of thiophene transformation over Co or Ni‐promoted and unpromoted molybdenum sulfide catalysts, provide information on the role of promoter and give a key for the design of new efficient HDS catalysts.     

A theoretical study of the photochemical reductive elimination and thermal oxidative addition of molecular hydrogen from and to the Ir-complex

The electronic mechanisms of the cyclic processes of photochemical reductive elimination of H₂ from [IrClH₂(PH₃)₃] and thermal oxidative addition of H₂ to [IrCl(PH₃)₃] are investigated theoretically. The geometries of the ground and excited states are optimized using the Hartree-Fock and single excitation configuration interaction methods, respectively, and higher level calculations for the ground and excited states are carried out by the symmetry adapted cluster (SAC)/SAC–configuration interaction method. The present calculation shows that the reductive elimination of H₂ from [IrClH₂(PH₃)₃] dose not occur thermally but photochemically through diabatic conversion from the lowest A′ excited state to the ground state (A′), while the oxidative addition of H₂ to [IrCl(PH₃)₃] easily proceeds thermally. The lowest ¹A′ excited state involves the nature of the Ir-H₂ antibonding. Received: 10 December 1997 / Accepted: 16 January 1998 / Published online: 17 June 1998     

Modeling Thioredoxin Reductase-Like Activity with Cyclic Selenenyl Sulfides: Participation of an NH⋅⋅⋅Se Hydrogen Bond through Stabilization of the Mixed Se−S Intermediate

At the redox-active center of thioredoxin reductase (TrxR), a selenenyl sulfide (Se−S) bond is formed between Cys497 and Sec498, which is activated into the thiolselenolate state ([SH,Se⁻]) by reacting with a nearby dithiol motif ([SH^Cys59,SH^Cys64]) present in the other subunit. This process is achieved through two reversible steps: an attack of a cysteinyl thiol of Cys59 at the Se atom of the Se−S bond and a subsequent attack of a remaining thiol at the S atom of the generated mixed Se−S intermediate. However, it is not clear how the kinetically unfavorable second step progresses smoothly in the catalytic cycle. A model study that used synthetic selenenyl sulfides, which mimic the active site structure of human TrxR comprising Cys497, Sec498, and His472, suggested that His472 can play a key role by forming a hydrogen bond with the Se atom of the mixed Se−S intermediate to facilitate the second step. In addition, the selenenyl sulfides exhibited a defensive ability against H₂O₂-induced oxidative stress in cultured cells, which suggests the possibility for medicinal applications to control the redox balance in cells.     

Anilinium dihydrogen phosphate

The triclinic structure of the title compound, C₆H₈N⁺·H₂PO₄⁻, with three symmetry‐independent structural units (Z′ = 3), is formed of separate organic and inorganic layers alternating along the b axis. The building blocks of the inorganic layer are deformed H₂PO₄ tetrahedra assembled into infinite ladders by short and hence strong hydrogen bonds. The anilinium cations forming the organic layer are not hydrogen bonded to one another, but they are anchored by four N—H...O crosslinks between the dihydrogen phosphate chains of adjacent ladders. Two H atoms of each –NH₃ group then form one normal and one bifurcated N—H...O hydrogen bond to the P=O oxygens of two tetrahedra of one chain, while the third H atom is hydrogen bonded to the nearest O atom of an adjacent chain belonging to another dihydrogen phosphate ladder.     

Elucidation of hydrodesulfurization mechanism on molybdenum‐based catalysts using 35S radioisotope pulse tracer methods

Elucidation of the hydrodesulfurization (HDS) mechanism on molybdenum‐based catalysts using radioisotope tracer methods and reaction kinetics is reviewed. Firstly, to investigate the sulfidation state in Mo/Al₂O₃ and Co–Mo/Al₂O₃ catalysts, presulfiding of these catalysts has been performed using a ³⁵S pulse tracer method. Secondly, HDS of radioactive ³⁵S‐labeled dibenzothiophene was carried out over a series of sulfided molybdena–alumina catalysts and cobalt‐promoted molybdena–alumina catalysts in a pressurized flow reactor to estimate the behavior of sulfur on the working catalysts. Finally, sulfur exchange of a ³⁵S‐labeled catalyst with hydrogen sulfide was performed to estimate the relationship between the amount of labile sulfur and catalytically active sites.     

Hydrogen‐bonded networks in crystals of 1‐(diaminomethylene)thiouron‐1‐ium perchlorate,hydrogen sulfate,dihydrogen phosphate and dihydrogen arsenate

Crystals of the title compounds, namely 1‐(diaminomethylene)thiouron‐1‐ium perchlorate, C₂H₇N₄S⁺·ClO₄⁻, 1‐(diaminomethylene)thiouron‐1‐ium hydrogen sulfate, C₂H₇N₄S⁺·HSO₄⁻, 1‐(diaminomethylene)thiouron‐1‐ium dihydrogen phosphate, C₂H₇N₄S⁺·H₂PO₄⁻, and its isomorphic relative 1‐(diaminomethylene)thiouron‐1‐ium dihydrogen arsenate, C₂H₇N₄S⁺·H₂AsO₄⁻, are built up from a nonplanar 1‐(diaminomethylene)thiouron‐1‐ium cation and the respective anion linked together via N—H...O hydrogen bonds. Both arms of the cation are planar, but they are twisted with respect to one another around the central N atom. Ionic and extensive hydrogen‐bonding interactions join oppositely charged units into layers in the perchlorate, double layers in the hydrogen sulfate, and a three‐dimensional network in the dihydrogen phosphate and dihydrogen arsenate salts. This work demonstrates the usefulness of 1‐(diaminomethylene)thiourea in crystal engineering for the formation of supramolecular networks with acids.     

Density functional complete study of hydrogen bonding between the water molecule and the hydroxyl radical (H2O · HO)

The hydrogen bonding complexes formed between the H₂O and OH radical have been completely investigated for the first time in this study using density functional theory (DFT). A larger basis set 6‐311++G(2d,2p) has been employed in conjunction with a hybrid density functional method, namely, UB3LYP/6‐311++G(2d,2p). The two degenerate components of the OH radical ²Π ground electronic state give rise to independent states upon interaction with the water molecule, with hydrogen bonding occurring between the oxygen atom of H₂O and the hydrogen atom of the OH radical. Another hydrogen bond occurs between one of the H atoms of H₂O and the O atom of the OH radical. The extensive calculation reveals that there is still more hydrogen bonding form found first in this investigation, in which two or three hydrogen bonds occur at the same time. The optimized geometry parameter and interaction energy for various isomers at the present level of theory was estimated. The infrared (IR) spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. The estimates of the H₂O · OH complex's vibrational modes and predicted IR spectra for these structures are also made. It should be noted that a total of 10 stationary points have been confirmed to be genuine minima and transition states on the potential energy hypersurface of the H₂O · HO system. Among them, four genuine minima were located. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Chloro­bis­(tri­phenyl­phosphine)­nickel(I) tetra­hydro­furan solvate and an unsolvated trigonal phase of chloro­tris­(tri­phenyl­phosphine)­nickel(I)

In [NiCl(C₁₈H₁₅P)₂]·C₄H₈O, the Ni atom is coordinated by three ligands in a distorted trigonal–planar configuration, with a P—Ni—P angle of 111.52 (2)°. In [NiCl(C₁₈H₁₅P)₃], there are three independent mol­ecules in the asymmetric unit, with each Ni—Cl bond on a crystallographic threefold rotation axis. Each Ni centre is tetrahedrally bound. The Ni atoms in both compounds have a d⁹ configuration and a formal oxidation state of Ni^I. A comparison is made between the form of [NiCl(PPh₃)₃] presented here and that of a known toluene solvate.     

An experimental and theoretical method for determination of standard electrode potential for the redox couple diphenyl sulfone/diphenyl sulfide

DFT calculations were performed for diphenyl sulfide and diphenyl sulfone. The electrochemistry of diphenyl sulfide on the gold electrode was investigated by cyclic voltammety and the results show that standard electrode potential for redox couple diphenyl sulfone/diphenyl sulfide is 1.058 V, which is consistent with that of 1.057 calculated at B3LYP/6-31++G(d,p)-IEFPCM level. The front orbit theory and Mulliken charges of molecular explain well on the oxidation of diphenyl sulfide in oxidative desulfurization. According to equilibrium theory the experimental equilibrium constant in the oxidative desulfurization of H₂O₂, is 1.17 × 10⁴⁸, which is consistent with the theoretical equilibrium constant is 2.18 × 10⁴⁸ at B3LYP/6-31++G(d,p)-IEFPCM level.     

An ab initio molecular orbital study of the structures and energies of neutral and charged bimolecular complexes of NH3 with the hydrides AHn (A = N,O, F,P, S,and Cl)

Hartree-Fock 6-31G(d) structures for the neutral, positive ion, and negative ion bimolecular complexes of NH₃ with the first- and second-row hydrides AH_n (AH_n = NH₃, OH₂, FH, PH₃, SH₂, and ClH) have been determined. All of the stable neutral complexes except (NH₃)₂, the positive ion complexes with NH₃ as the proton acceptor, and the negative ion complexes containing first-row anions exhibit conventional hydrogen bonded structures with essentially linear hydrogen bonds and directed lone pairs of electrons. The positive ion complex NH₄⁺ …? OH₂ has the dipole moment vector of H₂O instead of a lone pair directed along the intermolecular line, while the complexes of NH₄⁺ with SH₂, FH, and ClH have structures intermediate between the lone-pair directed and dipole directed forms. The negative ion complexes containing second-row anions have nonlinear hydrogen bonds. The addition of diffuse functions on nonhydrogen atoms to the valence double-split plus polarization 6-31G(d,p) basis set usually decreases the computed stabilization energies of these complexes. Splitting d polarization functions usually destabilizes these complexes, whereas splitting p polarization functions either has no effect or leads to stabilization. The overall effect of augmenting the 6-31G(d,p) basis set with diffuse functions on nonhydrogen atoms and two sets of polarization functions is to lower computed stabilization energies. Electron correlation stabilizes all of these complexes. The second-order Møller–Plesset correlation term is the largest term and always has a stabilizing effect, whereas the third and fourth-order terms are smaller and often of opposite sign. The recommended level of theory for computing the stabilization energies of these complexes is MP2/6-31+G(2d,2p), although MP2/6-31+G(d,p) is appropriate for the negative ion complexes.     

Ab initio quantum chemical calculation of the electronic structure of the chelate complex Ni(S2C2H2)2

The electronic structures and equilibrium geometries of the square-planar Ni(S₂C₂H₂)₂ complex and its dianion are calculated at the MP2 level of theory by the ab initio SCF MO LCAO method in the split-valence Gaussian basis set. The calculated two-electron affinity is 57 kJ/mole. In the ground state, the complex has a d⁸ configuration of Ni(II). The square-pyramidal structure of the complex is also considered. The transformation of the complex structure from square-planar to square-pyramidal involves the two-electron d-d transition. Based on the calculated electronic structure of the complex and on the experimental data on its stable dimers, we assumed an unusual valent state of nickel, Ni(IV), in the complex with the d⁶ electronic configuration. Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 231–236, March–April, 1996. Translated by I. Izvekova     

Rapid Dihydrogen Cleavage by Persistent Nitroxide Radicals under Frustrated Lewis Pair Conditions

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C₆F₅)₃ boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H₂ splitting reaction path.     

Ultra Deep Hydrodesulfurization of Gas Oils Over Sulfide and/or Noble Metal Catalysts

Deep hydrodesulfurization (HDS) of sterically hindered sulfur compounds in gas oils will require enhanced hydrogenation activity to hydrogenate the aromatic rings of the sulfur compounds. Although H₂S is known to inhibit the direct HDS route for most of the sulfided catalysts, its promotion to the hydrogenation and subsequent HDS was newly observed for unsupported MoS₂. This promotion suggests that ultra deep HDS over sulfide catalysts can be achieved along with high metal loading, minimal support-metal interactions and optimal dependence on the Ni species. On the other hand, the strong hydrogenation activity of sulfur-tolerant noble metal catalysts suggests that ultra deep HDS as well as deep aromatics saturation can be achieved. This paper discusses recent catalytic approaches for ultra deep HDS using conventional sulfide catalysts and/or noble metal catalysts, such as the newly developed Pd-Pt/Yb-USY zeolite catalyst.     

Why Does the Intramolecular Trimethylene‐Bridged Frustrated Lewis Pair Mes2PCH2CH2CH2B(C6F5)2 Not Activate Dihydrogen?

The methyl labelled C₃‐bridged frustrated phosphane borane Lewis pair (P/B FLP) 2 b was prepared by treatment of Mes₂PCl with a methallyl Grignard reagent followed by anti‐Markovnikov hydroboration with Piers’ borane [HB(C₆F₅)₂)]. The FLP 2 b is inactive toward dihydrogen under typical ambient conditions, in contrast to the C₂‐ and C₄‐bridged FLP analogues. Dynamic NMR spectroscopy showed that this was not due to kinetically hindered P???B dissociation of 2 b . DFT calculations showed that the hydrogen‐splitting reaction of the parent compound 2 a is markedly endergonic. The PH⁺/BH^? H₂‐splitting product of 2 b was indirectly synthesized by a sequence of H⁺/H^? addition. It lost H₂ at ambient conditions and confirmed the result of the DFT analysis.     

mer‐Di­aqua­bis(1H‐imidazole‐κN3)(orotato‐κ2N3,O4)­nickel(II)

The title mononuclear complex, [Ni(C₅H₂N₂O₄)(C₃H₄N₂)₂(H₂O)₂] or [Ni(HOr)(im)₂(H₂O)₂] (im is imidazole and H₃Or is orotic acid, or 2,6‐dioxo‐1,2,3,6‐tetra­hydro­pyrimidine‐4‐carboxylic acid), has been synthesized and the crystal structure determination is reported. The Ni^II ion in the complex has a distorted octahedral coordination geometry comprised of one deprotonated pyrimidine N atom and the adjacent carboxyl­ate O atom of the orotate ligand, two tertiary imidazole N atoms and two aqua ligands. An extensive three‐dimensional network of OW—H⋯O and N—H⋯O hydrogen bonds, and π–π and π–ring interactions are responsible for crystal stabilization.     

Nickel(I)‐Komplexe mit 1,1′‐Bis(phosphino)ferrocenen als Liganden

Nickel(I) Complexes with 1,1′‐Bis(phosphino)ferrocenes as Ligands The thermically stable monomeric Nickel(I) complexes [(d^tbpf)Ni(acac)] ( 1 ) and [(dⁱppf)NiCl] ( 2 ) were synthesized and characterized by elemental analyses, EPR spectroscopy, and by X‐ray crystal structure analyses of single crystals (d^tbpf: 1,1′‐bis(di‐tertbutylphosphino)ferrocene; dⁱppf: 1,1′‐bis(diisopropylphosphino)ferrocene). 1 is formed by reduction of Ni(acac)₂ with triethylaluminium in the presence of d^tbpf, together with the nickel(0) complex [(d^tbpf)Ni(C₂H₄)]. 1 contains a Ni^I atom surrounded of two O‐ and two P donor atoms in a distorted tetrahedral coordination. 2 was obtained by reduction of [(dⁱppf)NiCl₂] with NaBH₄. In 2 the nickel(I) atom adopts trigonal planar coordination.