Dihydrogen Complexes Research Articles

Doping-Induced Enhancement of Hydrogen Evolution at MoS2 Electrodes.

Rate theory and DFT calculations of hydrogen evolution reaction (HER) on MoS2 with Co, Ni and Pt impurities show the significance of dihydrogen (H2*) complex where both hydrogen atoms are interacting with the surface. Stabilization of such a complex affects the competing Volmer-Heyrovsky (direct H2 release) and Volmer-Tafel (H2* intermediate) pathways. The resulting evolution proceeds with a very small overpotential for all dopants ( =0.1 to 0.2 V) at 25 % edge substitution, significantly reduced from the already low =0.27 V for the undoped edge. At full edge substitution, Co-MoS2 remains highly active ( =0.18 V) while Ni- and Pt-MoS2 are deactivated ( =0.4 to 0.5 V) due to unfavorable interaction with H2*. Instead of the single S-vacancy, the site of intrinsic activity in the basal plane was found to be the undercoordinated central Mo-atom in threefold S-vacancy configurations, enabling hydrogen evolution with =0.52 V via a H2* intermediate. The impurity atoms interact favorably with the intrinsic sulfur vacancies on the basal plane, stabilizing but simultaneously deactivating the triple vacancy configuration. The calculated shifts in overpotential are consistent with reported measurements, and the dependence on doping level may explain variations in experimental observations.

Nondissociative Activated Dihydrogen Binding on CeO2 Revealed by High-Pressure Operando Solid-State NMR Spectroscopy.

Dihydrogen complexes, which retain the H-H bond, have been extensively studied in molecular science and found to be prevalent in homogeneous and enzymatic catalysis. However, their counterparts in heterogeneous catalysis, specifically nondissociative chemisorbed dihydrogen binding on the catalyst surface, are rarely reported experimentally. This scarcity is due to the complexity of typical material surfaces and the lack of effective characterization techniques to prove and distinguish various dihydrogen binding modes. Herein, using high-pressure operando solid-state NMR technology, we report the first unambiguous experimental observation of activated dihydrogen binding on a reduced ceria catalyst through interactions with surface oxygen vacancies. By employing versatile NMR structural and dynamical analysis methods, we establish a proportional relationship between the degree of ceria surface reduction and dihydrogen binding, as evidenced by NMR observations of H-D through-bond coupling (JHD), T1 relaxation, and proton isotropic chemical shifts. In situ NMR analysis further reveals the participation of bound dihydrogen species in a room-temperature ethylene hydrogenation reaction. The remarkable similarities between surface-activated dihydrogen in heterogeneous catalysis and dihydrogen model molecular complexes can provide valuable insights into the hydrogenation mechanism for many other solid catalysts, potentially enhancing hydrogen utilization.

Varied coordination geometries of an Alane/Tris(phosphine) ligand from its Reactions with Pt(0) and Au(I)

Reactions of the tris(2-diisopropylphosphino-N-pyrrolyl)alane ligand 1 (AlP3) furnished an (AlP3)Pt complex 2, which was determined to possess a Pt → Al interaction. Compound 2 reacted with CO to form the corresponding isolable adduct 3. Exposure of 2 to H2 or HD in solution resulted in the observation of equilibrium binding which favors 2 at room temperature, and more strongly favored the adducts 2-H2 and 2-HD at −80 °C. The presence of the Pt–H2/HD moieties is supported by low temperature NMR determination of 1JPPt = 323 Hz (2-H2) and 1JHD=34 Hz (2-HD), ostensibly the first examples of Pt dihydrogen complexes with a trigonal bipyramidal geometry. The reaction of 1 with (tht)AuCl (tht = tetrahydrothiophene) generated compound 4 with loss of tht. An XRD study of 4 revealed the transfer of chloride to Al and a long separation between Al and Au coordinated by the three phosphine arms. Abstraction of chloride from 4 did not lead to a tripodal structure isoelectronic to 2, but (according to solution NMR evidence) instead to the transfer of one of the phosphine arms to Al from Au in compound 5.

Unveiling the mechanism of triphos-Ru catalysed C–O bond disconnections in polymers

Ruthenium complexes with facially coordinating tripodal phosphine ligands are privileged catalysts for a broad range of (de-)hydrogenation-based transformations. Among these, C–O bond hydrogenolysis holds potential for the depolymerisation of both the biopolymer lignin and epoxy resins applied in wind turbine blades, aircrafts and more. However, this methodology is poorly understood in mechanistic terms. Here, we present a detailed investigation on the triphos-Ru catalysed C–O bond scission on a molecular level. A combination of experimental, spectroscopical and theoretical studies elucidates the reactivity of the ruthenium trimethylenemethane precatalyst, revealing the key roles of ruthenium phenolates in both catalyst activation as well as the catalytic cycle itself. Furthermore, a Ru(0)/Ru(II) oxidative addition into the C–O bond is disclosed, with a triphos-Ru(0) dihydrogen complex as entry point. With the molecular nature of the operating triphos-Ru species and the thermodynamics and kinetics of the catalysis unravelled, improvements of established methods as well as design of related transformations may become possible.

Insights into the chemistry of Kubas’ chromium dihydrogen complexes

The structures of the complexes Cr(CO)3(PR3)2, (3) R = cyclohexyl and (4) isopropyl and their labile dihydrogen adducts Cr(η2-H2)(CO)3(PR3)2 (1) R = cyclohexyl and (2) isopropyl are calculated using density functional theory and compared to known structures of 3, Cr(η2-HC-CyPCy2)(CO)3(PCy3) (CSD code JOHNOJ) with an agostic CHCr interaction, and of 2, Cr(η2-H2)(CO)3(PiPr3)2 (CSD LEYCOH) with a very short HH distance. The TPSS functional with Grimme D3 dispersion, with a def2-TZVP basis set on all atoms and with the PCM solvent model for tetrahydrofuran (THF) reproduces fairly accurately the geometries, available vibrational spectra, and enthalpy of dihydrogen coordination in THF solution from literature experiments while other computational settings do not. This is a rare example of the calibration of theory with experiment for the Gibbs energy change in a solution phase reaction involving dihydrogen gas. The agostic form of the molecule is favoured in the crystal by intermolecular CHHC interactions as illustrated using Hirshfeld surface analysis and by the more compact size of this conformation.

Density Functional Theory Investigation of Cu, Ni, and Ag Inclusion in ZSM-5 to Study Dihydrogen Binding for Cryogenic Molecular Sieve Bed Adsorber in Nuclear Fusion Systems

A computational investigation of Cu-, Ni-, and Ag-introduced ZSM-5 as potential hydrogen storage materials for nuclear fusion energy systems is performed. Among the 24 distinct tetrahedral sites of the monoclinic phase of ZSM-5, systematic periodic density functional theory (DFT) computations have been carried out on 15 experimentally identified T sites that show clear Al site preference and stability in high Si ZSM-5. Adsorption energies estimated from DFT studies have revealed that the T sites in the sinusoidal channels T4 and T10 are the most stable for including all three metal ions. Hence, these should also be considered as potential active sites for dihydrogen binding investigations in addition to the common T12 site in the intersection. The average hydrogen binding energies at these representative T sites were −79 to −45 kJ/mol, which correlates well with both the metal-H2 distance and H-H bond elongation distance. The computed hydrogen bond stretching frequency values were in the 3300 to 3755 cm−1 range upon adsorption of H2 onto the Ni, Cu, and Ag, indicating Kubas-type dihydrogen complex formation. The evidence for dihydrogen binding was also obtained from investigating the σ donation and back donation between the metal ion valence orbitals and the H2σ, H2σ* orbitals through projected density of states and natural bond order analysis. Our analysis indicates that Ni is better stabilized in the framework sites and is considered a potential candidate for dihydrogen binding.

Spectroscopic and Computational Evidence of Uranium Dihydrogen Complexes.

Dihydrogen complexation, a phenomenon with robust precedent in the transition metal series, is spectroscopically detected for a uranium(III) complex and thereby extended for the first time to the 5f series. The vacant coordination site and low valence of (C5H4SiMe3)3U prove to be key to the reversible formation of (C5H4SiMe3)3U-H2 (complex 1), and the paramagnetism of the f3 center facilitates the detection of complex 1 by NMR spectroscopy. Density functional theory calculations reveal that the delocalization of the 5f electron density from (C5H4SiMe3)3U onto the side-on dihydrogen ligand is crucial to complex formation, an unusual bonding situation for an actinide acid-base complex. The spectroscopic and computational results are compared to those reported for lanthanide metallocenes to yield insight into the nature of─and future possibilities for─f-element dihydrogen complexation.

Enhanced hydrogen uptake of dihydrogen complex via porous materials support.

This research focuses on enhancing H2 adsorption by using the [Mo(PCy3)2(CO)3] complex supported on porous materials such as silica gel and mesoporous carbon. The study reports a significant increase in hydrogen adsorption capacity, reaching up to 9.3 times that of the bulk complex. This improvement suggests that using mesoporous materials as supports for the [Mo(PCy3)2(CO)3] complex enhances the accessibility of H2 gas to its open-metal sites.

Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation.

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Vibrational spectroscopy of Cu+(H2)4: about anharmonicity and fluxionality.

The vibrational spectra of the copper(I) cation-dihydrogen complexes Cu+(H2)4, Cu+(D2)4 and Cu+(D2)3H2 are studied using cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations. The infrared photodissociation (IRPD) spectra (2500-7300 cm-1) are assigned based on a comparison to IR spectra calculated using vibrational second-order perturbation theory (VPT2). The IRPD spectra exhibit ≈60 cm-1 broad bands that lack rotational resolution, indicative of rather floppy complexes even at an ion trap temperature of 10 K. The observed vibrational features are assigned to the excitations of dihydrogen stretching fundamentals, combination bands of these fundamentals with low energy excitations as well as overtone excitations of a minimum-energy structure with Cs symmetry. The three distinct dihydrogen positions present in the structure can interconvert via pseudorotations with energy barriers less than 10 cm-1, far below the zero-point vibrational energy. Ab initio Born-Oppenheimer molecular dynamics (BOMD) simulations confirm the fluxional behavior of these complexes and yield an upper limit for the timeframe of the pseudorotation on the order of 10 ps. For Cu+(D2)3H2, the H2 and D2 loss channels yield different IRPD spectra indicating non-ergodic behavior.

Molecular Motion in the Interconverting σ-H2, Di-, and Tri-hydride Regimes: .

The transition-metal complex is known to exist in three possible isomeric forms, including a nonclassical, σ-bound dihydrogen complex and two classical dihydride isomers. As such, it has served as a model complex for the energies of conversion between these limiting structural regimes. In the present study, ab initio molecular dynamics computer simulations, combined with enhanced sampling techniques, were utilized to directly assess the degree of motion and isomerization of the dihydrogen/dihydride moieties in this complex. Ligand rotations (for both the H2 unit and the phosphine units) were found to be dominant in the low-temperature (298 K) regime, and the classical thermodynamic distribution showed no probability of thermally accessing dihydride forms, although unrestrained molecular dynamics trajectories showed fleeting configurations outside of the σ-H2 configuration. Simulations at higher temperatures surprisingly revealed new tri-hydride isomers that are energetically competitive with the σ-H2 and cis-/trans-dihydride isomers. Low-energy pathways to hydrogen/hydride transfer and phosphine dissociation were readily accessible, which considerably expands the known isomeric flexibility of this complex.

Antioxidation activity of molecular hydrogen via protoheme catalysis in vivo: an insight from ab initio calculations.

Recently, molecular hydrogen has been found to exhibit antioxidation activity through many clinical experiments, but the mechanism has not been fully understandable at atomic level. In this work, we perform systematic ab initio calculations of protoheme-hydrogen complexes to clarify the antioxidation mechanism of molecular hydrogen. We make molecular modeling of iron-protoporphyrin coordinated by imidazole, FeP(Im), and its hydrogen as well as dihydrogen complexes, together with reactive oxygen/nitrogen species (RONS). We carry out structural optimization and Mulliken charge analysis, revealing the two kinds of bonding characteristics between FeP(Im) and H[Formula: see text]: dihydrogen bonding in the end-on asymmetric configuration and Kubas bonding in the side-on symmetric configuration of H[Formula: see text] molecule. The activation barriers for adsorption and dissociation of H[Formula: see text] on and further desorption of H atom from FeP(Im) are found to be below 2.78 eV at most, which is remarkably lower than the H-H bond breaking energy of 4.64 eV in free H[Formula: see text] molecule. We find that the hydrogen bond dissociation energies of FeP(Im)-H[Formula: see text] and -H complexes are lower than those of RONS-H complexes, indicating the decisive role of protoheme as an effective catalyst in RONS antioxidation by molecular hydrogen in vivo.

Effect of Iodide on the pH-Controlled Hydrogenations of Diphenylacetylene and Cinnamaldehyde Catalyzed by Ru(II)-Sulfonated Triphenylphosphine Complexes in Aqueous–Organic Biphasic Systems

The effect of NaI on hydrogenation of diphenylacetylene catalyzed by the water-soluble [{RuCl(mtppms-Na)2}2(µ-Cl)2] (1) (mtppms-Na = meta-monosulfonated triphenylphosphine sodium salt) is reported. Hydrogenations were performed under mild conditions (P(H2) = 1 bar, T = 50–80 ℃) in aqueous–organic biphasic reaction mixtures wherein the catalyst was dissolved in aqueous phase of various pHs. In acidic solutions, addition of NaI to 1 + mtppms-Na increased the selective conversion of diphenylacetylene to stilbenes from 10% to 90% but did not effect the high Z-selectivity (up to 98%). In contrast, in basic solutions the major product was diphenylethane (up to 70%), and the yield of E-stilbene exceeded that of the Z-isomer. 1H and 31P NMR measurements revealed that depending on the absence or presence of NaI, the catalytically active Ru(II)-hydride species in acidic solutions was [RuHCl(mtppms-Na)3], 2, or [RuHI(mtppms-Na)3], 5, respectively, while in basic solutions, both 2 and 5 were hydrogenated further to yield the same hydride species, cis,fac-[RuH2(H2O)(mtppms-Na)3]. [RuHI(mtppms-Na)3] proved superior to [RuHCl(mtppms-Na)3] as a catalyst for the selective hydrogenation of cinnamaldehyde to dihydrocinamaldehyde. This finding was explained by a facile formation of a (putative) dihydrogen complex [Ru(H2)I2(H2O)(mtppms-Na)2] intermediate, resulting in fast heterolytic activation of H2.

From the Felkin‐Anh Rule to the Grignard Reaction: an Almost Circular 50 Year Adventure in the World of Molecular Structures and Reaction Mechanisms with Computational Chemistry**

AbstractThis rosarium article relates the adventure started 50 years ago of a computational chemist who was interested in molecules; what are they, what are their shape and how do they react. The story describes results, still valid today, obtained with highly simplified models of chemical reality and elementary computational methods and the gain resulting from the use of better models and more elaborate computational methods. It was necessary to select examples. In this presentation, focus is on hydride and dihydrogen complexes as well as on nucleophiles. Nucleophiles were considered as free hydrides in gas phase at the start of the rosarium while the Grignard reaction is treated with ab initio molecular mechanisms at the end of it.

Toggling the Z-type interaction off-on in nickel-boron dihydrogen and anionic hydride complexes.

Completing a series of nickel-group 13 complexes, a coordinatively unsaturated nickel-boron complex and its derivatives with a H2, N2, or hydride ligand were synthesized and characterized. The toggling "on" of a Ni(0)-B(III) inverse-dative bond enabled the stabilization of a nickel-bound anionic hydride with a remarkably low thermodynamic hydricity of kcal mol-1 in THF. The flexible topology of the boron metalloligand confers both favorable hydrogen binding affinity and strong hydride donicity, albeit at the cost of high H2 basicity during deprotonation to form the hydride.

Role of Dihydride and Dihydrogen Complexes in Hydrogen Evolution Reaction on Single-Atom Catalysts.

The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.

Front Cover: Nickel(II) Dihydrogen and Hydride Complexes as the Intermediates of H2 Heterolytic Splitting by Nickel Diazadiphosphacyclooctane Complexes (Eur. J. Inorg. Chem. 41/2021)

Front Cover: Nickel(II) Dihydrogen and Hydride Complexes as the Intermediates of H₂ Heterolytic Splitting by Nickel Diazadiphosphacyclooctane Complexes (Eur. J. Inorg. Chem. 41/2021)

Nickel(II) Dihydrogen and Hydride Complexes as the Intermediates of H2 Heterolytic Splitting by Nickel Diazadiphosphacyclooctane Complexes

AbstractThe interaction of a series of [Ni(PR12NR22)2]2+ complexes 1–4 (where PR12NR22=1,5‐R2‐3,7‐R1‐1,5‐diaza‐3,7‐diphosphacyclooctane; R1=o‐Py, Ph; R2=CHPh2, p‐Tol) with H2 is reversible yielding the products of heterolytic splitting of dihydrogen in the metal coordination sphere ‐ [NiH(PR12NR22)(PR12NR22H)]2+. Experiments conducted with parahydrogen allowed to see the signal at about −4 ppm corresponding to the metal‐bound H2 in the nickel dihydrogen complex [Ni(η2−H2)(PPh2NCHPh22)2]2+ (3‐H2) – the short‐living non‐classical intermediate, which so far has been revealed only computationally.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.

Outer‐sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M−H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square‐planar cobalt(II) hydride complex. Photoactivation results in Co−H bond homolysis. The three‐coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

Dynamics of bound states of dihydrogen at Cu(I) and Cu(II) species coordinated near one and two zeolite framework aluminium atoms: A combined sorption, INS, IR and DFT study

Ambient conditions sorption isotherms of dihydrogen in a series of various levels of Cu-exchanged ZSM-5 zeolites, with two different Si/Al ratios, namely 11.5 and 25, show the presence of different amount of Cu centres able to strongly bind H2. Although the isosteric heats of adsorption derived from these isotherms are rather similar, of the order of 30 kJ/mol H2, Inelastic Neutron Scattering (INS) of adsorbed dihydrogen and Fourier-Transformed Infra-Red (FTIR) spectroscopy measurements of adsorbed CO and NO reveal that copper is encountered in two oxidation states. At least two types of Cu(I) ions are clearly detected as well as some heterogeneity of the Cu(II) species. The number of these Cu species is different in the two investigated ZSM-5 materials and depends on the Cu exchange level. With the aid of DFT model cluster calculations we find that under different coordination environments, determined by the Al distribution, both mono- and divalent Cu ions could bind H2 with a different strength. Unprecedentedly, we found that Cu-ions compensating two Al atoms, i.e. formally Cu(II) species, relatively far apart from each other, may behave very similarly to the monovalent Cu-species or alternatively viewed – as Cu(I) species that compensate for two framework Al-atoms. Such Cu-species also form stable η2 dihydrogen complexes.