Kinetics,mechanism, and catalyst selection criteria in the hydrogenation of compounds with >C=O and -C≡N bonds

Data on the kinetics and mechanism of reactions in the hydrogenation of carbon monoxide, carbon dioxide, and aliphatic carbonyl and nitrile compounds over transition metals have been summarized and correlated. The ranking of specific catalytic activity and selectivity with respect to the addition of hydrogen to CO, CO₂, and the > C=O and -CN groups is determined mainly by the energies of the bonds between the metal and the compound being hydrogenated, and between the metal and hydrogen. In turn, the bond energies depend on the chemical nature of the metal and also on the degree of dispersity of the metal. The role of the dimensional factor, which influences the concentration of active centers, has also been investigated. On the basis of the relationships that have been found between the physicochemical and catalytic properties of monotypical substances, criteria have been formulated for the selection of catalysts for the hydrogenation of these compounds.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 5, pp. 395–417, September–October, 1993.     

Reaction of N≡W(O(t)Bu)3 with σ3λ5-phosphoranes. The [2 + 2] cycloaddition across the W≡N triple bond results in the first representative of an inorganic four-membered metallacycle with conjugated endo- and exocyclic double bonds

The reactions of N≡W(O(t)Bu)(3) with the low-coordinate phosphorus compounds (Me(3)Si)(2)NP(NSiMe(3))(2) (I) and (Me(3)Si)(2)NPS(N(t)Bu) (II) were studied. Quantum chemical calculations were used to determine why Mo- and W-containing compounds with the same composition have different molecular structures.     

Nucleophilic addition of alcohols to the C-N multiple bonds of the nitrilium substituent in the anion [2-B10H9(N≡CMe)]−

The reactions of salts of the anion [2-B₁₀H₉(N≡CMe)]⁻ with aliphatic alcohols ROH (R = C _n H_2n+1 (n = 1–6) CH₂CH₂(OEt), Prⁱ, Buⁱ, Bu^t, i-C₅H₁₁) are studied. These reactions result in hydrolytically stable imidates [2-B₁₀H₉{NH=C(OR)Me}]⁻. Their structures were confirmed by the data from mass spectrometry, IR, ¹H, ¹¹B, and ¹³C NMR spectroscopy. The molecular geometry of [2(Z)-B₁₀H₉{NH=C(OBu)Me}]⁻, which formed in nucleophilic addition reaction of n-butyl alcohol to [2-B₁₀H₉(N≡CMe)]⁻, was established by X-ray diffraction analysis.     

Strength from weakness: opportunistic CH···O hydrogen bonds differentially dictate the conformational fate in solid and solution states

We show that the CH···O hydrogen bond can be opportunistic to differentially dictate the conformation of cyclitols in solution and solid states. While several intermolecular CH···O stabilize the chair-conformation in the solid, single intramolecular CH···O stabilizes an otherwise unfavorable boat-conformation in solutions analogous to the boat-conformation of cis-1,4-cyclohexanediols by OH···O bonds.     

A molecular orbital explanation for the B?N bond shortening in H3BNH3 on going from the gaseous to the solid state

A simple molecular orbital model has been applied to explanation of the B? N bond shortening in H₃BNH₃ on going from the gaseous to the solid state. In this model, the shortening is attributed to the bond order increase that is caused by the fact that each atom in the crystal experiences different external electrostatic potential to each other and thus the orbital energy level of each atom is changed. To illustrate this model, Effective Fragment Potential (EFP) method has been applied to the system consisting of a H₃BNH₃ molecule and 30 dipole moments whose magnitudes are determined by Lorentz's local field theory. This EFP computation has brought significant B? N bond shortening (1.668 → 1.623 Å), which is about 50% of the actual shortening. The factor of the remaining discrepancy has been analyzed by Morokuma decomposition under EFP and localized orbital analysis. These analyses have revealed that the remaining discrepancy is almost compensated by incorporating the dihydrogen bonds (B? H···H? N) that are formed by the orbital interaction between the bonding orbital of the B? H and the antibonding orbital of the N? H. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Communication: The insertion of silylene in C?H bonds; rate constant limits and the energy barrier

The technique of laser flash photolysis has been used to set limits on the rate constants for the bimolecular reactions of SiH₂ with methane (CH₄) and tetramethylsilane (SiMe₄) at both ambient and elevated temperatures (ca 600 K). These limits show that the energy barriers to insertion reactions of SiH₂ in the C H bonds of CH₄ are at least 45(±6) kJ mol⁻¹ and in the C H and/or Si C bonds of SiMe₄ are at least 23(±6) kJ mol⁻¹. The best thermochemical estimate of the activation energy for SiH₂+CH₄ is 59(±12) kJ mol⁻¹. Reasons for the greatly diminished reactivity of SiH₂ with C H as compared with Si H bonds are discussed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 393–395, 1999     

Potassium Natural Asphalt Sulfonate (K-NAS): Synthesis and characterization as a new recyclable solid basic nanocatalyst and its application in the formation of carbon–carbon bonds

In this research, we synthesized and characterized a new heterogeneous basic nanocatalyst and its catalytic application was studied in the Claisen-Schmidt and Knoevenagel condensations. In order to prepare this nanocatalyst, first, the Iranian natural asphalt was sulfonated with the concentrated sulfuric acid and then, converted to the potassium natural asphalt sulfonate (K-NAS). In order to characterization of the nanocatalyst, used of FT-IR spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), inductively coupled plasma (ICP) and thermogravimetric analysis (TGA) techniques. This new basic heterogeneous nanocatalyst have advantages such as being eco-friendly, huge specific surface area, high reactivity and recyclability .     

A reliable solid phase microextraction-gas chromatography–triple quadrupole mass spectrometry method for the assay of selenomethionine and selenomethylselenocysteine in aqueous extracts: Difference between selenized and not-enriched selenium potatoes

A new analytical approach is exploited in the assay of selenium speciation in selenized and not selenium enriched potatoes based on the widely available solid-phase microextraction (SPME) coupled to-GC–triple quadrupole mass spectrometry (SPME-GC–QqQ MS) method.     

Exchange coupling mediated by N-H···Cl hydrogen bonds: experimental and theoretical study of the frustrated magnetic system in bis(o-phenylenediamine)nickel(II) chloride

The title compound crystallizes in the monoclinic P2(1)/c space group with a = 11.2470(3) ?, b = 5.9034(2) ?, c = 12.0886(3) ?, β = 115.143(1)°, and V = 726.58(4) ?(3) and consists of discrete monomeric NiCl(2)(o-phenylendiamine)(2) molecules. Each o-phen ligand coordinates in a bidentate mode with the chloride ions occupying trans positions in the resulting tetragonally distorted octahedral coordination sphere. Two discrete sets of N-H···Cl hydrogen bonds link the octahedral molecules into a two-dimensional network, with type 1 interactions linking adjacent monomers along the c axis and type 2 interactions linking monomers along the diagonals in the bc plane. Analysis of the magnetic data reveals the existence of weak antiferromagnetic coupling within the layers via these hydrogen bonds, in addition to the presence of zero field splitting, with the best fit obtained for a 1d antiferromagnetic model with g = 2.0917(7), J/k = -2.11(4) K [J = -1.47(3) cm(-1)], and D = 1.05(3) cm(-1) [β = D/|J| = 0.72(6)] for the model with D > 0 and g = 2.0911(6), J/k = -2.26(1) K [J = -1.57(1) cm(-1)], and D = -0.86(1) cm(-1) [β = D/|J| = 0.55(6)] for the model with D < 0. Theoretical calculations of the exchange coupling confirm the experimental results, yielding values of J(1) = -1.39 cm(-1) for the type 1 hydrogen bonds and J(2)/k = -0.56 cm(-1) for the type 2 hydrogen bonds.     

Ab initio and density functional studies on bonding nature of the N?N bonds in 1,2,5‐trinitroimidazole and 1,2,4,5‐tetranitroimidazole

The bonding nature of the N N bonds in 1,2,5‐trinitroimidazole ( I ) and 1,2,4,5‐tetranitroimidazole ( II ) was examined with various levels of ab initio and density functional (DF) theories. The second‐order Møller–Plesset perturbation method (MP2) with the 6‐31G** basis set has predicted significantly long N N bond lengths in I and II , that is, 1.737 and 1.824 Å, respectively. Two DF theories, BLYP/6‐31G** and BP86/6‐31G**, provided similar results to those of MP2/6‐31G**. On the other hand, Hartree–Fock (HF) calculation with the 6‐311++G** basis set evaluated these bond lengths of I and II to be 1.443 and 1.414 Å, respectively. Bond properties including the bond critical density are strongly dependent on the equilibrium bond length. Thus, accurate prediction of geometric parameters is of particular importance to derive reliable bond properties. Especially, a substantial difference in bonding properties is observed when the electron correlation effect is included. According to our analyses with bonding natures and CHELPG charges at the MP2 level, (1) the N N bonds of I and II appear to have a significant ionic nature, and (2) the 1‐nitro group bears a considerable positive charge and has attractive electrostatic interactions with O atoms of adjacent nitro groups. Although all the theories utilized in this study predict that both I and II are stable in their potential‐energy surfaces, significantly long N N bond lengths calculated with MP2 and DF theories imply a strong hyperconjugation effect, which may explain a tendency to form a salt in these compounds easily. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 145–154, 1999     

Photoinduced rearrangement of aromatic N-chloroamides to chloroaromatic amides in the solid state: inverted Π(N)-Σ(N) occupational stability of amidyl radicals

We report a solid-state photochemical rearrangement reaction by which aromatic N-chloroamides exposed to UV light or sunlight are rapidly and efficiently converted to chloroaromatic amides. The course, the intermediate (nascent chlorine vs dichlorine) and the outcome of the reaction depend on the excitation (exposure time, wavelength, and intensity) and on inherent structural factors (the directing role of the substituents and, as demonstrated by the different reactivity of two polymorphs of N-chlorobenzanilide, the supramolecular structure). The photolysis of the chloroamides provides facile photochemical access to arylamidyl radicals as intermediates, which in the absence of strong hydrogen bond donors are stabilized in the reactant crystals by C-H/N-Cl···π interactions, thus, providing insight into their structure and chemistry. Thorough theoretical modeling of the factors determinant to the stability and the nature of the spin-hosting orbital evidenced that although the trans-Π(||) state (Np spin) of the amidyls is normally preferred over the trans-Σ(⊥) configuration (Nsp(2) spin), stabilization by aromatic conjugation, steric and geometry factors, as well as by electronic effects from the substituents can decrease the Π-Σ gap in these intermediates significantly, resulting in similar and, in the case of the orthogonal amide-phenyl disposition, even reversed population of the unpaired electron in the two orbitals. Quantitative correlation established that the inverted occupational spin stability and the Π(N)-Σ(N) crossover are collectively facilitated by the conformation, valence angle, and disposition of the amide group relative to the aromatic system. The stabilization and detection of a trans-Σ(⊥) radical was experimentally accomplished by steric locking of the orthogonal trans-amide conformation with double ortho-tert-butyl substitution at the phenyl ring. The effects of the single para-phenyl substituents on the relative occupational stability of the arylamidyl radical states point out to non-Hammett behavior. By including cumulative electronic effects from multiple substitutions, four distinct families of the aromatic amidyl radicals were identified. The Π(∥) state is the most stable structure of the N-phenylacetamidyl radical and of most of the substituted arylamidyls, although the Σ(⊥) and Π(⊥) states can also be stabilized by introducing tert-butyl and nitro groups, respectively.     

A nucleic acid base derivative tethered to a ruthenium carbene complex: hydrogen bonded dimers in both the solid state and solution?

A ruthenium carbene bearing a uracil (Ur) substituent has been prepared and has a dimeric structure in the solid state-the dimer being held together by hydrogen bonds between two uracil groups on neighbouring molecules: evidence for the persistence of this interaction in solution has been obtained.     

Synthesis of a uranium(VI)-carbene: reductive formation of uranyl(V)-methanides, oxidative preparation of a [R2C═U═O]2+ analogue of the [O═U═O]2+ uranyl ion (R = Ph2PNSiMe3), and comparison of the nature of U(IV)═C, U(V)═C, and U(VI)═C double bonds

We report attempts to prepare uranyl(VI)- and uranium(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl(VI)-methanide complex [(BIPMH)UO(2)Cl(THF)] [1, BIPMH = HC(PPh(2)NSiMe(3))(2)] with benzyl-sodium did not afford a uranyl(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO(2)(BIPMH)(μ-Cl)UO(μ-O){BIPMH}] (2) and [UO(μ-O)(BIPMH)(μ(3)-Cl){UO(μ-O)(BIPMH)}(2)] (3), respectively, with concomitant elimination of dibenzyl, was observed. Complexes 2 and 3 represent the first examples of organometallic uranyl(V), and 3 is notable for exhibiting rare cation-cation interactions between uranyl(VI) and uranyl(V) groups. In contrast, two-electron oxidation of the uranium(IV)-carbene [(BIPM)UCl(3)Li(THF)(2)] (4) by 4-morpholine N-oxide afforded the first uranium(VI)-carbene [(BIPM)UOCl(2)] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R(2)C═U═O](2+) analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me(3)NO, C(5)H(5)NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium(V)-carbene [(BIPM)UCl(2)I] (5), and 6 reveal polarized covalent U═C double bonds in each case whose nature is significantly affected by the oxidation state of uranium. Natural Bond Order analyses indicate that upon oxidation from uranium(IV) to (V) to (VI) the uranium contribution to the U═C σ-bond can increase from ca. 18 to 32% and within this component the orbital composition is dominated by 5f character. For the corresponding U═C π-components, the uranium contribution increases from ca. 18 to 26% but then decreases to ca. 24% and is again dominated by 5f contributions. The calculations suggest that as a function of increasing oxidation state of uranium the radial contraction of the valence 5f and 6d orbitals of uranium may outweigh the increased polarizing power of uranium in 6 compared to 5.     

DFT study on activation of carbon dioxide by (tBuArN)3M≡N (M=Nb,V,Ta): the electronic structure and activity

The reaction mechanism for the reduction of CO(2) gas activated by (tBuArN)(3)M≡N was studied by the means of density functional theory (DFT) calculations. The calculations indicated that this reaction has a two step reaction mechanism. From our calculations, we found that (tBuArN)(3)Ta≡N held the best activity among the three (tBuArN)(3)M≡N complexes studied. Our results also indicated that the reaction of (tBuArN)(3)M≡N with CO(2) occurred under orbital control involving the HOMO-3 orbital of (tBuArN)(3)M≡N, which could give higher overlapping with the LUMO of the CO(2) molecule. The substitutions on the amino donor ligands studied here took larger effect on the HOMO structure of the (tBuArN)(3)M≡N molecules. The electronic structure of the (tBuArN)(3)M≡N complexes also showed their ability for activating CO(2) molecules, in the order of M = V < Nb < Ta.     

Z/E isomerism in Nα-Nα-disubstituted hydrazides and the amidoxy bond: application to the conformational analysis of pseudopeptides built of hydrazinoacids and α-aminoxyacids

We have investigated the Z/E isomerism of the hydrazide link (CO-NH-N) and amidoxy link (CO-NH-O). The study was first focused on small molecular models using NMR and X-ray diffraction. It allowed determination of simple NMR criterions to differentiate easily the Z and E forms, which were then applied to investigate the behavior of these links inside the corresponding oligomers. Our results concerning the hydrazide link corroborate pioneering work that had been done in the 1970s except in the case were it is located inside aza-β(3)-cyclopeptides, where the old empirical rules failed to predict the right geometry of the link. The geometrical preference of the amidoxy bond is also unambiguously established and differs clearly from recent theoretical calculations. Our findings help rationalize the close self-organization ability of aza-β(3)-peptides and α-aminoxypeptides, two recently described foldamers.     

Crystal structure of the biclotymol–<Emphasis Type=Italic>N,N</Emphasis>-dimethylacetamide solvate 1:1: experimental and theoretical conformational analysis of biclotymol molecule

The crystal structure of the N,N-dimethylacetamide–biclotymol 1:1 solvate has been determined at room temperature by X-ray single-crystal diffraction. The molecular geometry of the biclotymol molecule is compared to the geometries observed in the N,N-dimethylformamide and dimethylsulfoxide solvates, and in the unsolvated form Phase-I. Rotational energy profile of the isolated molecule was obtained by semi-empirical AM1 calculations with respect to two selected torsional angles, which were rotated from 0° to 360° by steps of 10°. Not only, we find the two conformations experimentally observed, but also the conformation proposed by simple force fields calculations 30 years ago. Moreover, we propose three additional possible conformations that may offer potential candidates for the structure biclotymol Phase-II, which remains to be solved.     

Reaction site diversity in the reactions of titanium hydrazides with organic nitriles, isonitriles and isocyanates: Ti=N(α) cycloaddition, Ti=N(α) insertion and N(α) -N(β) bond cleavage

We report a range of new transformations of the diamide–amine supported Ti?NNPh₂ functional group with a variety of unsaturated substrates, along with DFT studies of the key mechanisms. Reaction of [Ti(N₂N^py)(NNPh₂)(py)] ( 4 , N₂N^py=(2‐NC₅H₄)CMe(CH₂NSiMe₃)₂; py=pyridine) with MeCN gave the dimeric species [Ti₂(N₂N^py)₂{μ‐NC(Me)(NNPh₂)}₂] through a [2+2] cycloaddition process. Reaction of 4 or [Ti(N₂N^Me)(NNPh₂)(py)] ( 5 , N₂N^Me=MeN(CH₂CH₂NSiMe₃)₂) with fluorinated benzonitriles gave the terminal hydrazonamide complexes [Ti(N₂N^R){NC(Ar)NNPh₂}(py)] (R=py or Me; Ar=2,6‐C₆H₃F₂ or C₆F₅). DFT studies showed that this proceeds through an overall [2+2] cycloaddition–reverse cycloaddition, resulting in net insertion of ArCN into the Ti?N_α bonds of the respective hydrazides. Reaction of 4 with a mixture of MeCN and PhCCMe gave the metallacycle [Ti(N₂N^py){NC(Me)C(Ph)C(Me)NNPh₂}] by sequential coupling of Ti?NNPh₂ with PhCCMe and then MeCN. A related product, [Ti(N₂N^py){NC(Me)C(Ar^F)C(H)NNPh₂}], was formed by insertion of MeCN into the Ti? C bond of the isolated azatitanacyclobutene [Ti(N₂N^py){N(NPh₂)C(H)C(Ar^F)}] (Ar^F=3‐C₆H₄F). Reaction of 4 with two equivalents of B(Ar)₃ (Ar=C₆F₅) formed the zwitterionic borate [Ti(N₂N^py){η²‐N(NPh₂)B(Ar)₃}] by electrophilic attack at N_α. Compounds 4 and 5 reacted with tBuNC and/or XylNC (Xyl=2,6‐C₆H₃Me₂) to give the N_α? N_β bond cleavage products, [Ti(N₂N^R)(NCNR′)(NPh₂)] (R=py or Me; R′=tBu or Xyl), containing metallated carbodiimide ligands. DFT studies of these reactions found an initial addition of RNC across Ti?N_α followed by N_β coordination, and finally complete N_α transfer from the NNPh₂ to the RNC fragment. Reaction of 5 with Ar′NCE (E=O, S, Se; Ar′=2,6‐C₆H₃iPr₂) gave the [2+2] cycloaddition products [Ti(N₂N^Me){N(NPh₂)C(NAr′)O}(py)] and [Ti(N₂N^Me){N(NPh₂)C(NAr′)E}] (E=S or Se), which did not undergo further transformation of the Ti? N? NPh₂ moiety.