Heterolytic Hydrogen Splitting Research Articles

Three-body interaction effects in heterolytic hydrogen splitting by frustrated Lewis pairs.

The reaction of heterolytic dihydrogen splitting by frustrated Lewis pairs P(R)3 and B(C6F5)3 (where R = t-butyl and 1-adamantene) is driven by strong three-body contributions which originate from the induction and charge transfer effects. The three-body effect increases dramatically as a function of inter-hydrogen distance. As predicted by the symmetry adapted perturbation theory, the "frustration" of Lewis pairs originates from the dual role of the exchange effects. First, the exchange manifests itself in the first-order Pauli repulsion by keeping the pairs away. Second, and equally important, the second-order exchange-induction almost completely cancels the effects of the second-order induction. This suppression of induction effects eases up upon the interaction of the frustrated pairs with H2. The activation of induction in this instance constitutes the three-body effect.

Hydrogen Activation by Frustrated and Not So Frustrated Lewis Pairs Based on Pyramidal Lewis Acid 9-Boratriptycene: A Computational Study.

Structural features and reactivity of frustrated Lewis pairs (FLPs) formed by pyramidal group 13 Lewis acids based on 9-bora and 9-alatriptycene and bulky phosphines P t Bu3, PPh3, and PCy3 are considered at the M06-2X/def2-TZVP level of theory. Classic FLP is formed only in the B(C6Me4)3CH/P t Bu3 system, while both FLP and donor-acceptor (DA) complex are observed in the B(C6F4)3CF/P t Bu3 system. Formation of DA complexes was observed in other systems; the B(C6H4)3CH·P t Bu3 complex features an elongated DA bond and can be considered a "latent" FLP. Transition states and reaction pathways for molecular hydrogen activation have been obtained. Processes of heterolytic hydrogen splitting are energetically more favored in solution compared to the gas phase, while activation energies in the gas phase and in solution are close. The alternative processes of hydrogenation of B-C or Al-C bonds in the source pyramidal Lewis acids in the absence of a Lewis base are exergonic but have larger activation energies than those for heterolytic hydrogen splitting. The tuning of Lewis acidity of 9-boratriptycene by changing the substituents allows one to control its reactivity with respect to hydrogen activation. Interestingly, the most promising system from the practical point of view is the DA complex B(C6H4)3CH·P t Bu3, which is predicted to provide both low activation energy and thermodynamic reversibility of the heterolytic hydrogen splitting process. It appears that such "not so frustrated" or "latent" FLPs are the best candidates for reversible heterolytic hydrogen splitting.

Solid micellar Ru single-atom catalysts for the water-free hydrogenation of CO2 to formic acid

The catalytic hydrogenation of CO2 to formic acid is one of the most promising pathways towards a renewable hydrogen-storage system. The reaction is usually performed in aqueous phase in the presence of basic molecules over homogeneous or heterogeneous catalysts, generating relatively dilute formate solutions (<1 M).The newly designed solid micellar Ru single-atom catalyst enables efficient and stable water-free CO2 hydrogenation to formate under mild reaction conditions. Concentrated formate solutions (up to 4 M) are produced directly from the hydrogenation of carbon dioxide in water-free tertiary amine. In the catalyst, Ru(III) single sites are incorporated into the walls of MCM-41 during hydrolysis creating a solid micelle structure. The presence of the CTA+ surfactant in the pores of MCM-41 stabilizes the Ru sites and prevents catalyst deactivation. DFT modelling suggests that the reaction proceeds via heterolytic hydrogen splitting, forming a Ru-H species and subsequent hydride transfer to CO2.

Formation of macrocyclic ring systems by carbonylation of trifunctional P/B/B frustrated Lewis pairs.

The trifunctional P/B/B frustrated Lewis pairs 11a-c featuring bulky aryl groups at phosphorus [Dmesp (a), Tipp (b), Mes* (c)] react with H2 by heterolytic hydrogen splitting followed by cleavage of HB(C6F5)2 to give the zwitterionic six-membered heterocyclic PH phosphonium/borate products 14a-c. Compounds 11a,b react with carbon monoxide by means of a Lewis acid induced CO insertion/rearrangement sequence that eventually results in the formation of the macrocyclic dimers 17a,b. The respective carbonylation reaction of the Mes*P/B/B FLP gives the macrocyclic trimer 18c. The new products were characterized spectroscopically and by X-ray diffraction and the reaction mechanism was analyzed by DFT calculations.

Complexes of adamantane-based group 13 Lewis acids and superacids: Bonding analysis and thermodynamics of hydrogen splitting.

The electronic structure and chemical bonding in donor-acceptor complexes formed by group 13 element adamantane and perfluorinated adamantane derivatives EC9 R'15 (E = B, Al; R' = H, F) with Lewis bases XR3 and XC9 H15 (X = N, P; R= H, CH3 ) have been studied using energy decomposition analysis at the BP86/TZ2P level of theory. Larger stability of complexes with perfluorinated adamantane derivatives is mainly due to better electrostatic and orbital interactions. Deformation energies of the fragments and Pauli repulsion are of less importance, with exception for the boron-phosphorus complexes. The MO analysis reveals that LUMO energies of EC9 R'15 significantly decrease upon fluorination (by 4.7 and 3.6 eV for E = B and Al, respectively) which results in an increase of orbital interaction energies by 27-38 (B) and 15-26 (Al) kcalmol(-1) . HOMO energies of XR3 increase in order PH3 < NH3 < PMe3 < PC9 H15 < NMe3 < NC9 H15 . For the studied complexes, there is a linear correlation between the dissociation energy of the complex and the energy difference between HOMO of the donor and LUMO of the acceptor. The fluorination of the Lewis acid significantly reduces standard enthalpies of the heterolytic hydrogen splitting H2 + D + A = [HD](+) + [HA](-) . Analysis of several types of the [HD](+) ···[HA](-) ion pair formation in the gas phase reveals that structures with additional H···F interactions are energetically favorable. Taking into account the ion pair formation, hydrogen splitting is predicted to be highly exothermic in case of the perfluorinated derivatives both in the gas phase and in solution. Thus, fluorinated adamantane-based Lewis superacids are attractive synthetic targets for the construction of the donor-acceptor cryptands. © 2016 Wiley Periodicals, Inc.

Ruthenium nanoparticles supported on magnesium oxide: A versatile and recyclable dual-site catalyst for hydrogenation of mono- and poly-cyclic arenes, N-heteroaromatics, and S-heteroaromatics

The development of catalysts capable of promoting hydrogenation of aromatics while being resistant to poisoning by nitrogen- and sulfur-containing species is of much interest in connection with hydrotreating of fossil fuels. We report a catalyst composed of ruthenium nanoparticles supported on magnesia, designed to promote heterolytic hydrogen splitting and surface ionic hydrogenation pathways. The catalyst, prepared through a one-pot procedure, promotes the hydrogenation of mono- and poly-cyclic arenes, as well as N- and S-heteroaromatics representative of fossil fuels components. Of particular significance are the superior activity and wider substrate scope of the catalyst, in relation to other known supported noble metals, and the excellent recyclability and long catalyst lifetime. Based on our experimental data, a dual-site catalyst structure and an associated dual-pathway mechanism are proposed, which may have interesting implications for the development of new poison-tolerant noble metal catalytic systems.

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Frustrated Lewis pairs are compounds containing both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and have been shown to be capable of heterolysis of molecular hydrogen, a property that has led to their use in hydrogenation reactions of polarized multiple bonds. Here, we describe a general approach to the hydrogenation of alkynes to cis-alkenes under mild conditions using the unique ansa-aminohydroborane as a catalyst. Our approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies.

2.2]Paracyclophane derived bisphosphines for the activation of hydrogen by FLPs: application in domino hydrosilylation/hydrogenation of enones

The heterolytic splitting of hydrogen by two types of [2.2]paracyclophane derived bisphosphines (1, 2a and 2b) in combination with tris(pentafluorophenyl)borane (3) at room temperature is described. The corresponding frustrated Lewis pairs (FLPs) exhibit different behavior in the activation of hydrogen. This results from diverse steric and electronic properties of the bisphosphines. The reactivity of the frustrated Lewis pairs was exploited in the first diastereoselective domino hydrosilylation/hydrogenation reaction catalyzed by FLPs.

Novel group 13 Lewis superacids and 13–15 donor–acceptor cryptands for hydrogen activation: a theoretical study

Structures of group 13 Lewis acids ER(3) and group 15 Lewis bases YR'(3) as well as respective hydrogen splitting products [HER(3)](-) and [HYR'(3)](+) (E = B, Al, Ga; Y = N, P, As; R(3) = (C(6)F(5))(3), (C(6)H(2)F(2))(3)CF; R' = H, C(6)H(5), 2,6-Me(2)C(6)H(3)) have been fully optimized at B3LYP/pVDZ and B3LYP/def2-TZVPP levels of theory. Energetic characteristics of the hydrogen-splitting process ER(3) + YR'(3) + H(2) = [HYR'(3)](+) + [HER(3)](-) have been obtained. Energetics for the ion formation does not depend on the nature of group 13 element E. However, the Lewis acid environment (pyramidal vs. planar) and nature of Lewis base play a significant role in the energetics of heterocyclic hydrogen splitting. The endothermicity of the heterolytic hydrogen splitting process is reduced by about 80 kJ mol(-1) by using pyramidal Lewis acids E(C(6)H(2)F(2))(3)CF, and by 40 kJ mol(-1) by using Lewis bases with 2,6-Me(2)C(6)H(3) substituents. Thus, the overall process of the heterolytic hydrogen splitting is predicted to be endothermic by only 130 kJ mol(-1) for the E(C(6)H(2)F(2))(3)CF-P(2,6-Me(2)C(6)H(3))(3) combinations (compared to 450 kJ mol(-1) for E(C(6)F(5))(3)-PH(3)). It is shown that H(2) splitting in the gas phase by nitrogen-containing DA cryptands, featuring group 13 element acceptor centers with a pyramidalized environment, is highly exothermic.

Ammonia formation by metal–ligand cooperative hydrogenolysis of a nitrido ligand

Bioinspired hydrogenation of N(2) to ammonia at ambient conditions by stepwise nitrogen protonation/reduction with metal complexes in solution has experienced remarkable progress. In contrast, the highly desirable direct hydrogenation with H(2) remains difficult. In analogy to the heterogeneously catalysed Haber-Bosch process, such a reaction is conceivable via metal-centred N(2) splitting and unprecedented hydrogenolysis of the nitrido ligands to ammonia. We report the synthesis of a ruthenium(IV) nitrido complex. The high nucleophilicity of the nitrido ligand is demonstrated by unusual N-C coupling with π-acidic CO. Furthermore, the terminal nitrido ligand undergoes facile hydrogenolysis with H(2) at ambient conditions to produce ammonia in high yield. Kinetic and quantum chemical examinations of this reaction suggest cooperative behaviour of a phosphorus-nitrogen-phosphorus pincer ligand in rate-determining heterolytic hydrogen splitting.

Rationalizing the Reactivity of Frustrated Lewis Pairs: Thermodynamics of H2 Activation and the Role of Acid−Base Properties

The acid-base strengths of recently reported frustrated Lewis pairs and their relation with the thermodynamic feasibility of heterolytic hydrogen splitting reactions are analyzed in terms of quantum chemical calculations. Reaction free energies of hydrogenation processes are computed, and an energy partitioning scheme is introduced, which involves quantitative measures of the acidity and basicity of the reacting Lewis centers. Additional terms are also included that account for possible dative bond formation between the active sites and for stabilizing electrostatic interactions occurring in the product species. For intermolecular combinations of donor-acceptor components, the calculated reaction free energies are found to correlate well with the cumulative acid-base strengths. Product stabilization for these systems represents a notable contribution to the overall energetics; however, it generally shows only a slight variation for the investigated series. The reactivity of linked donor-acceptor pairs is primarily governed by acid-base properties as well, but the magnitude of stabilizing effects arising from acid-base cooperativity of active sites is also of significant importance in determining the thermodynamic feasibility of the reactions.

On the Mechanism of B(C6F5)3-Catalyzed Direct Hydrogenation of Imines: Inherent and Thermally Induced Frustration

The reaction mechanism for the transition metal free direct hydrogenation of bulky imines catalyzed by the Lewis acid B(C6F5)3 is investigated in detail by quantum chemical calculations. A recently introduced mechanistic model of heterolytic hydrogen splitting that is based on noncovalent association of bulky Lewis acid-base pairs is shown to account for the reactivity of imine-borane as well as amine-borane systems. Possible catalytic cycles are examined, and the results provide solid support for the imine reduction pathway proposed from experimental observations. In addition, the feasibility of an autocatalytic route initiated by amine-borane hydrogen cleavage is demonstrated. Conceptual issues regarding the notion of frustration are also discussed. The observed reactivity is interpreted in terms of thermally induced frustration, which refers to thermal activation of strained dative adducts of bulky Lewis donor-acceptor pairs to populate their reactive frustrated complex forms.

Mechanism of hydrogen activation by frustrated Lewis pairs: A molecular orbital approach

AbstractA detailed molecular orbital treatment of the heterolytic hydrogen splitting by bulky Lewis acid‐base pairs is presented. The frontier molecular orbitals of the proposed reactive intermediate are shown to be preorganized but otherwise practically identical to those of the free acid and base molecules. The concerted interaction of the Lewis centers with hydrogen leading to the polarization and, ultimately, to the cleavage of the HH bond is examined, and the bridge role of hydrogen molecule in the electron transfer is pointed out. The formation of the new covalent bonds is monitored by bond order and natural localized molecular orbital calculations, and found to be synchronous. The stability of the product is interpreted on the basis of favorable orbital interactions. A comparison of various hydrogen activation mechanisms emphasizes the common donation/back‐donation motifs and the different ways of making them feasible. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Heterolytic Splitting of Hydrogen with Rhodium(I) Amides

A split decision: Efficient heterolytic hydrogen splitting is achieved with novel tetracoordinate rhodium(I) amides. This reaction is reversible, and DFT calculations imply that the cleavage is a one-step process with a relatively low activation barrier. Ketones and imines are hydrogenated by the amide complexes without the need for further additives.

Homogeneous catalysis by Pt(II)–Sn(II) chloride complex. Part 1.—Kinetics and mechanisms of the hydrogenations of acetylene and ethylene

Detailed kinetics of the hydrogenation of acetylene and ethylene catalyzed by methanolic solutions of Pt(II)–Sn(II) chloride complexes have been studied within a temperature range 0 to 20°C and at pressures up to 100 Torr. Initial rates of hydrogenation exhibit maxima in terms of Sn to Pt atomic ratio or catalyst concentration, expressed by ν=kKhcPhcPh/(1 +KhcPhc) for fixed Sn/Pt = 10, where Ph and Phc are pressures of hydrogen and hydrocarbon respectively, and k and Khc are constants. Acetylene is consecutively hydrogenated to ethane via ethylene as an intermediate. In the reaction of C2H2 or C2H4 with D2 in CH3OH, hydrogen atoms in the hydrocarbon are hardly substituted with deuterium, and C2H4 or C2H6 is produced predominantly. Acetylene-d2 also hardly exchanges its D-atoms in the reaction with H2 in CH3OH and cis-C2H2D2 is formed preferentially, while the exchange proceeds to a considerable extent in the case of C2D4. These results are interpreted in terms of heterolytic splitting of hydrogen, competitive coordination of acetylene and ethylene, and successive reactions with hydride and protonic hydrogens.

Effect of Fluoride Ion on the Activation of Molecular Hydrogen by Silver(I)

ACCORDING to Halpern1 the effect of complex forming ligands on the activation of molecular hydrogen by metal ions can be explained by the following scheme: Thus the heterolytic splitting of hydrogen is the consequence of the concerted action of the central metal ion and the ligand bond. Great activation effect can be expected from the ligands which, although weak acids, form unstable complexes with the metal ion, that is, which do not screen the positive charge partly responsible for the heterolytic splitting. These requirements are contradictory in general, being a parallelism between the proton affinity and the complex forming tendency of the ligands. In the case of silver(I), the reduction of which by hydrogen was thoroughly investigated by Halpern et al.2–5, the fluoride ion fulfils the requirements mentioned: although hydrogen fluoride is a rather weak acid (pK = 3.4), the fluoride ion forms an unstable monofluorosilver(I) complex6,7.