Dihydrogen Interactions Research Articles

Synthesis, structural analysis, and thermal decomposition studies of [(NH3)2BH2]B3H8

Boron–nitrogen containing compounds with high hydrogen contents as represented by ammonia borane (NH3BH3) have recently attracted intense interest for potential hydrogen storage applications. One such compound is [(NH3)2BH2]B3H8 with a capacity of 18.2 wt% H. Two safe and efficient synthetic routes to [(NH3)2BH2]B3H8 have been developed for the first time since it was discovered 50 years ago. The new synthetic routes avoid a dangerous starting chemical, tetraborane (B4H10), and afford a high yield. Single crystal X-ray diffraction analysis reveals N–Hδ+⋯Hδ−–B dihydrogen interactions in the [(NH3)2BH2]B3H8·18-crown-6 adduct. Extended strong dihydrogen bonds were observed in pure [(NH3)2BH2]B3H8 through crystal structure solution based upon powder X-ray analysis. Pyrolysis of [(NH3)2BH2]B3H8 leads to the formation of hydrogen gas together with appreciable amounts of volatile boranes below 160 °C.

Synthesis and Structure of Amido‐ and Imido(pentafluorophenyl)borane Zirconocene and Hafnocene Complexes: NH and BH Activation

Treatment of Me(2)S·B(C(6)F(5))(n) H(3-n) (n=1 or 2) with ammonia yields the corresponding adducts. H(3)N·B(C(6)F(5))H(2) dimerises in the solid state through N-H···H-B dihydrogen interactions. The adducts can be deprotonated to give lithium amidoboranes Li[NH(2)B(C(6)F(5))(n)H(3-n)]. Reaction of the n=2 reagent with [Cp(2)ZrCl(2)] leads to disubstitution, but [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)] is in equilibrium with the product of β-hydride elimination [Cp(2)Zr(H){NH(2)B(C(6)F(5))(2)H}], which proves to be the major isolated solid. The analogous reaction with [Cp(2)HfCl(2)] gives a mixture of [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)] and the N-H activation product [Cp(2)Hf{NHB(C(6)F(5 )(2)H}]. [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)]·PhMe and [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]·4(thf) exhibit β-B-agostic chelate bonding of one of the two amidoborane ligands in the solid state. The agostic hydride is invariably coordinated to the outside of the metallocene wedge. Exceptionally, [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]⋅PhMe has a structure in which the two amidoborane ligands adopt an intermediate coordination mode, in which neither is definitively agostic. [Cp(2)Hf{NHB(C(6)F(5))(2)H}] has a formally dianionic imidoborane ligand chelating through an agostic interaction, but the bond-length distribution suggests a contribution from a zwitterionic amidoborane resonance structure. Treatment of the zwitterions [Cp(2)MMe(μ-Me)B(C(6)F(5))(3)] (M=Zr, Hf) with Li[NH(2)B(C(6)F(5))(n)H(3-n)] (n=2) results in [Cp(2) MMe{NH(2)B(C(6)F(5))(2)H}] complexes, for which the spectroscopic data, particularly (1)J(B,H), again suggest β-B-agostic interactions. The reactions proceed similarly for the structurally encumbered [Cp''(2)ZrMe(μ-Me)B(C(6)F(5))(3)] precursor (Cp''=1,3-C(5)H(3)(SiMe(3))(2) , n=1 or 2) to give [Cp''(2)ZrMe{NH(2)B(C(6)F(5))(n)H(3-n)}], both of which have been structurally characterised and show chelating, agostic amidoborane coordination. In contrast, the analogous hafnium chemistry leads to the recovery of [Cp''(2)HfMe(2)] and the formation of Li[HB(C(6)F(5))(3)] through hydride abstraction.

Electron density shift description of non-bonding intramolecular interactions

A new methodology is described for the study of the electron density shift in intramolecular interactions. The methodology has been tested in an intermolecular complex and compared to the electron density shift obtained as the difference between the complex and the isolated monomers. The molecular fragmentation procedures and its application to hydrogen bonds, chalcogen–chalcogen interactions, nitrogen–boron interactions, dihydrogen interactions and silicon–nitrogen interactions are described. A careful selection of the fragmentation scheme is necessary in order to describe correctly the electron density shift in the intramolecular interactions. For this reason, different orders of fragmentation have been studied and analyzed pointing out the problems and limitations which are inherent to the methodology. It has been found that this methodology is a new tool which provides a good qualitative description of the electron density shift within the interacting region between two or more contacts, in both inter and intramolecular contacts with a reasonable low computational cost.

Boron–nitrogen–hydrogen (BNH) compounds: recent developments in hydrogen storage, applications in hydrogenation and catalysis, and new syntheses

The strong efforts devoted to the exploration of BNH compounds for hydrogen storage have led to impressive advances in the field of boron chemistry. This review summarizes progress in this field from three aspects. It starts with the most recent developments in using BNH compounds for hydrogen storage, covering NH3BH3, B3H8− containing compounds, and CBN compounds. The following section then highlights interesting applications of BNH compounds in hydrogenation and catalysis. The last part is focused on breakthroughs in the syntheses and discovery of new BNH organic analogues. The role of N–Hδ+⋯Hδ−–B dihydrogen interactions in molecule packing, thermal hydrogen evolution, and syntheses is also discussed within the review.

DFT study on the mechanism of water-assisted dihydrogen elimination in group 6 octahedral metal hydride complexes

The reaction of water with octahedral bis-, tris- and tetrakis-(phosphine)tungsten, (phosphine)molybdenum and (phosphine)chromium complexes has been studied using B3LYP/def2-TZVPP level of DFT to elucidate dissociative, associative and hydride migratory insertion mechanisms for hydrogen elimination. In the dissociative mechanism, phosphine dissociation requires 19.3-28.5 kcal mol(-1) of energy. The phosphine-water ligand exchange is endergonic due to poor coordination ability of water to group 6 metals (binding energy 8.8-15.5 kcal mol(-1)). The ligand exchange leads to intermolecular M-H···H(2)O dihydrogen interaction and facilitates dihydrogen elimination (G(act) = 6.8-15.5 kcal mol(-1)). In the associative mechanism, a water molecule in the first solvation shell interacts with the M-H bond through a dihydrogen bond (interaction energy 2.7-4.0 kcal mol(-1)) and leads to the elimination of H(2) by forming a hydroxide complex. Compared to the dissociative mechanism, G(act) of associative mechanisms are ~22 kcal mol(-1) higher. In the hydride migratory insertion mechanism, the hydride ligand shifts to the CO ligand (G(act) = 25.4-30.4 kcal mol(-1)) to afford a formyl complex and subsequently the H-H bond coupling occurs between formyl and water ligand (G(act) = 2.8-4.4 kcal mol(-1)). In many cases, the migratory insertion mechanism can simultaneously operate with the dissociative mechanism as a minor pathway, whereas owing to high G(act) value, the associative mechanism can be described as inactive in the reaction. The general argument that dihydrogen elimination is preceded by the formation of a dihydrogen intermediate is not applicable for the systems studied herein as the most favoured dissociative mechanism does not pass through such an intermediate. On the other hand, irrespective of the mechanisms, dihydrogen elimination invariably occurs with the formation of a dihydrogen bonded transition state. Our results also suggest that group 6 octahedral metal hydride complexes are attractive targets for the design of water splitting reactions.

DFT study of dihydrogen interactions with lithium containing organic complexes C4H4−m Li m and C5H5−m Li m (m = 1, 2)

The interactions of dihydrogen with lithium containing organic complexes C4H4−mLim and C5H5−mLim (m = 1, 2) were studied by means of density functional theory (DFT) calculation. For all the complexes considered, each bonded lithium atom can adsorb up to five H2 molecules with the mean binding energy of 0.59 eV/H2 molecule. The interactions can be attributed to the charge transfer from the H2 bonding orbitals to the Li 2s orbitals. The kinetic stability of these hydrogen-covered organolithium molecules is discussed in terms of the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The results indicate that these organiclithium structures can perhaps be used as building units for potential hydrogen storage materials.

Dihydrogen contacts in alkanes are subtle but not faint

Alkane molecules are held together in the crystal state by purportedly weak homonuclear R-H···H-R dihydrogen interactions. In an apparent contradiction, the high melting points and vaporization enthalpies of polyhedranes in condensed phases require quite strong intermolecular interactions. Two questions arise: 'How strong can a weak C-H···H-C bond be?' and 'How do the size and topology of the carbon skeleton affect these bonding interactions?' A systematic computational study of intermolecular interactions in dimers of n-alkanes and polyhedranes, such as tetrahedrane, cubane, octahedrane or dodecahedrane, showed that attractive C-H···H-C interactions are stronger than usually thought. We identified factors that account for the strength of these interactions, including the tertiary nature of the carbon atoms and their low pyramidality. An alkane with a bowl shape was designed in the search for stronger dihydrogen intermolecular bonding, and a dissociation energy as high as 12 kJ mol⁻¹ is predicted by our calculations.

Accurate Prediction of Hydrogen Adsorption in Metal−Organic Frameworks with Unsaturated Metal Sites via a Combined Density-Functional Theory and Molecular Mechanics Approach

The incorporation of coordinatively unsaturated metal sites in microporous metal−organic frameworks (MOFs) has emerged as an important synthetic strategy for the development of potential room-temperature hydrogen storage materials, because the relatively strong, localized interaction of hydrogen with the metal centers induces an increase of the isosteric heat of hydrogen adsorption. Previous modeling studies have shown that these interactions are not adequately modeled when literature force-field parameters are used. Typical results of grand-canonical Monte Carlo (GCMC) simulations exhibit a pronounced underestimation of the hydrogen uptake at low pressures and low temperatures. In this study, it is shown that this shortcoming can be resolved by deriving a new set of potential parameters to represent the metal−dihydrogen interaction from ab initio calculations for molecular model systems. The approach is computationally efficient and could be applied for any coordination environment of the metal center. T...

Theoretical Investigations on the Formation and Dehydrogenation Reaction Pathways of H(NH2BH2)nH (n = 1−4) Oligomers: Importance of Dihydrogen Interactions

The H(NH(2)BH(2))(n)H oligomers are possible products from dehydrogenation of ammonia borane (NH(3)BH(3)) and ammonium borohydride (NH(4)BH(4)), which belong to a class of boron-nitrogen-hydrogen (BNH(x)) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNH(x)-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wave function theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH(2)BH(2))(n)H (n = 1-4) oligomers, polyaminoborane (PAB), from NH(3)BH(3) monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through computational transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH(3)BH(3) molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH(3)BH(3) molecules. We find that the formation of H(NH(2)BH(2))(n+1)H oligomers occurs first through reactions of the H(NH(2)BH(2))(n)H oligomers with BH(3) followed by reactions with NH(3) and the release of H(2), where the BH(3) and NH(3) intermediates are formed through dissociation of NH(3)BH(3). We also find that the dimerization of the NH(3)BH(3) molecules to form cyclic c-(NH(2)BH(2))(2) is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H(2) molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H(2)), with the largest exothermicity for n = 3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (H(N)) hydrogens interact with hydridic (H(B)) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the R-BH(2)(eta(2)-H(2)) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane.

Molecular simulation of hydrogen adsorption in metal-organic frameworks

The capabilities and limitations of the application of molecular simulation techniques to the adsorption of hydrogen in metal-organic frameworks (MOFs) are explored for selected case studies. Force field based grand-canonical Monte Carlo simulations are employed to investigate the adsorption characteristics of three different isoreticular MOFs, resulting in good agreement with experimental findings. The predictive potential of the method is demonstrated for Zn 4O(mip) 3, a novel system which has not yet been fully characterized experimentally. Further calculations for MOFs with unsaturated metal sites reveal a shortcoming of the simulation technique, as the interaction of hydrogen with these sites is not adequately represented by the potential model. Density functional theory calculations are employed to study the metal–dihydrogen interaction in more detail, making use of a non-periodic model system which is representative for many copper-containing MOFs.

Molecular Tweezers for Hydrogen: Synthesis, Characterization, and Reactivity

The first ansa-aminoborane N-TMPN-CH2C6H4B(C6F5)2 (where TMPNH is 2,2,6,6-tetramethylpiperidinyl) which is able to reversibly activate H2 through an intramolecular mechanism is synthesized. This new substance makes use of the concept of molecular tweezers where the active N and B centers are located close to each other so that one H2 molecule can fit in this void and be activated. Because of the fixed geometry of this ansa-ammonium-borate it forms a short N-H...H-B dihydrogen bond of 1.78 A as determined by X-ray analysis. Therefore, the bound hydrogen can be released above 100 degrees C. In addition, the short H...H contact and the N-H...H (154 degrees) and B-H...H (125 degrees) angles show that the dihydrogen interaction in N-TMPNH-CH2C6H4BH(C6F5)2 is partially covalent in nature. As a basis for discussing the mechanism, quantum chemical calculations are performed and it is found that the energy needed for splitting H2 can arise from the Coulomb attraction between the resulting ionic fragments, or "Coulomb pays for Heitler-London". The air- and moisture-stable N-TMPNH-CH2C6H4BH(C6F5)2 is employed in the catalytic reduction of nonsterically demanding imines and enamines under mild conditions (110 degrees C and 2 atm of H2) to give the corresponding amines in high yields.

First-Principles Study of Single and Multiple Dihydrogen Interaction with Lithium Containing Benzene Molecules

The interactions of molecular hydrogen with Li containing benzene molecules were studied by means of density functional theory (DFT), second-order Moller−Plesset perturbation theory (MP2), and coupled-cluster theory including all single and double excitations in conjunction with a perturbative estimate of triple excitations [CCSD(T)]. At the DFT and MP2 levels the resolution of the identity (RI) approximation was invoked in the framework of the approaches RI-DFT and RI-MP2. The benzene derivatives studied in this work are lithium benzide (C6H5Li), lithium phenoxide (C6H5OLi) and lithium benzoate (C6H5COOLi). By extrapolating the MP2 and CCSD(T) energies to the complete basis set limit, the interaction energy of dihydrogen with these benzene species is found to be 2.54, 2.66 and 2.57 kcal/mol respectively. In all cases, the H2 molecule takes a side-on position on the Li atom. The interactions are larger than typical van der Waals (vdW) forces and can be attributed to dipole···induced dipole electrostatic f...

Nonpolar Dihydrogen Bonds—On a Gliding Scale from Weak Dihydrogen Interaction to Covalent HH in Symmetric Radical Cations [HnE‐H‐H‐EHn]+

The electronic structures and bonding patterns for a new class of radical cations, [HnE-H-H-EHn]+ (EHn=element hydride, E=element of Groups 15-18), have been investigated by applying quantum-chemical methods. All structures investigated give rise to symmetric potential energy minimum structures. We envisage clear periodic trends. The H--H bond length is shorter for elements toward the bottom of the periodic table of elements, and a short H--H bond corresponds to accumulation of electron density in the central H--H region. All [HnE-H-H-EHn]+ of Groups 15-17 are thermodynamically unstable towards loss of either H2 or H. The barriers for these dissociations are rather low. The Group 18 congeners, except E=Xe, appear to be global minima of the respective potential energy surfaces. The findings are discussed in terms of H2 bond activation, and a general mechanistic scheme for the standard reduction process 2H+ + 2e(-) --> H2 is given. Finally, it is proposed that some of the symmetric radical cations are likely to be observed in mass spectrometric or matrix isolation experiments.

Topological analyses and bond characterization of 1,3,5,7-tetra-tert-butyl-s-indacene: a weak Csp3-H...H-Csp2-type dihydrogen interaction.

Topological analyses of experimental and theoretical calculated electron densities of 1,3,5,7-tetra-tert-butyl-s-indacene provide bond characterizations of the chemical bond and of the pi-delocalization. A weak C(sp(3))-H...H-C(sp(2))-type intramolecular dihydrogen interaction (DHI) is identified through this analysis.

Linear hybrid aminoborane/phosphinoborane chains: synthesis, proton-hydride interactions, and thermolysis behavior.

The reaction of the lithiated phosphine-borane adducts Li[PPhR.BH(3)] or Li[CH(2)-PR(2).BH(3)] with Me(2)NH.BH(2)Cl afforded the hybrid linear species Me(2)NH-BH(2)-PPhR-BH(3) (1, R = Ph; 2, R = H) or Me(2)NH-BH(2)-CH(2)-PR(2)-BH(3) (3, R = Ph; 4, R = Me). Single-crystal X-ray diffraction studies on 1 and 3, the first for linear hybrid aminoborane/phosphinoborane adducts, confirmed the expected four-coordinate N-B-P-B and N-B-C-P-B frameworks. In addition, interactions between the protic N-H and hydridic B-H hydrogen atoms resulted in short intermolecular H...H contacts for 1, whereas 3 was found to possess an exceptionally short intramolecular H...H distance of 1.95 A. Solution and solid state infrared studies on 3 and 4 also suggest that these dihydrogen interactions were maintained even in dilute solution. Hydrogen bond strengths in the range of 7.9 to 10.9 kJ mol(-1) indicate the presence of a relatively weak interaction. The thermal and catalytic dehydrocoupling reactivities of 1-4 were also investigated. Chain cleavage reactions were observed for 1 and 2 upon thermolysis at 130 degrees C to afford species such as Me(2)NH.BH(3), [Me(2)N-BH(2)](2), PhPRH.BH(3) (R = Ph, H), PhPRH (R = Ph, H), Ph(2)PH-BH(2)-PPh(2)-BH(3), and also the low molecular weight polyphosphinoborane [PhPH-BH(2)](n) (M(w) approximately 5000). Similar products were observed for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 degrees C). Thermolysis of 3 at 130 degrees C yielded the six-membered ring [BH(2)-CH(2)-PPh(2)](2) (5), which presumably results from the dissociation of Me(2)NH.BH(3) from 3. Thermolysis of 4 at 90 degrees C afforded Me(2)NH.BH(3) and Me(3)P.BH(3), in addition to a product tentatively assigned as [BH(2)-CH(2)-PMe(2)](2) (6).

Do Cooperative Proton−Hydride Interactions Explain the Gas−Solid Structural Difference of BH3NH3?

The solid/gas structural differences in the weak donor−acceptor complex BH3NH3 is theoretically studied using density functional and the topological analysis of the electron density. The analysis shows that the cooperative dihydrogen interactions are not the main organizing factors in the molecular aggregations affecting donor−acceptor bond lengths. These aggregations are primarily controlled by electrostatic dipole−dipole interactions. This fact can be exploited in the development of simple electrostatic models that will allow the prediction of the basic structure of supramolecular architecture.

Density Functional Study of the Interactions between Dihydrogen and Pdn (n = 1−4) Clusters

The dihydrogen interactions with Pdn (n = 1−4) clusters was investigated using hybrid density functional Becke3LYP method and two ECP basis sets. The local minima configurations for a number of H2 molecule approach modes toward Pdn clusters are presented. Some of these states may be interpreted as a physical adsorption and others as dissociative interaction of the H2 molecule with the palladium cluster. Both geometric and energetic characteristics of weakly bonded pre-dissociated complexes on Pd3-4 clusters are very close to those on a bulk Pd (111) surface, while the stable states with dissociated hydrogen molecules show significant differences. In contrast to the bulk surface, 2-fold coordination positions exhibit slightly higher stability of hydrogen bonding in Pd3 and Pd4 clusters than 3-fold ones. The binding energy is significantly higher for small clusters than for the bulk surface. The Pd2 cluster was found to be the most active toward hydrogen capture in accordance with the experimental results.