Metal Dihydrogen Research Articles

Cooperative H2 activation at a nickel(0)-olefin centre.

Catalytic olefin hydrogenation is ubiquitous in organic synthesis. In most proposed homogeneous catalytic cycles, reactive M-H bonds are generated either by oxidative addition of H2 to a metal centre or by deprotonation of a non-classical metal dihydrogen (M-H2) intermediate. Here we provide evidence for an alternative H2-activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H2 molecule to a metal-coordinated olefin. An unusual pincer ligand that features two phosphine ligands and a central olefin supports the formation of a non-classical Ni-H2 complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H2. The usefulness of this cooperative H2-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H2 activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)-(Z) isomerization and catalyst degradation by self-hydrogenation.

How acid can become a dihydrogen complex in water? A DFT study

To accurate know the acidity of a dihydrogen molecule coordinated to a transition metal ion in water medium is an issue of interest in many areas, from electrochemistry to enzymes and catalysis. However, experimental determination of this magnitude is challenging, and very few values have been reported. In this article we describe a computational protocol, based on DFT calculations and employing a discrete-continuum solvent representation, to estimate pKawater of transition metal dihydrogen complexes. In this approach the number of solvent molecules explicitly included in the calculations is determined by the convergence with the solvation Gibbs energy of the proton in the solvent. The approach has been initially validated with experimental data in tetrahydrofuran (THF) solvent. Using (THF)3 clusters a mean absolute deviation from experiments of only 1.4 pKa unit is achieved. In water the convergence is reached with (H2O)10 clusters. Using them in a discrete-continuum model, the pKawater of twelve dihydrogen complexes experimentally characterized in water have been computed. pKawater values span a wide range, from 23 to -4, illustrating how coordination to a transition metal modifies the dihydrogen acidity. Decomposition of the ∆G of the acid-base equilibrium in two contributions, one intrinsic to the complex and another one accounting for solvent effects enables a deeper analysis of the dihydrogen acidities.

Dihydrogen Adduct (Co-H2 ) Complexes Displaying H-Atom and Hydride Transfer.

The prototypical reactivity profiles of transition metal dihydrogen complexes (M-H2 ) are well-characterized with respect to oxidative addition (to afford dihydrides, M(H)2 ) and as acids, heterolytically delivering H+ to a base and H- to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H-atom or hydride transfer from the σ-H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= and an anionic S=0 Co-H2 adduct, both supported by a trisphosphine borane ligand (P3 B ). The thermally stable metalloradical, (P3 B )Co(H2 ), serves as a competent precursor for hydrogen atom transfer to t Bu3 ArO⋅ . What is more, its anionic derivative, the dihydrogen complex [(P3 B )Co(H2 )]1- , is a competent precursor for hydride transfer to BEt3 , establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M-H2 complexes known.

Dihydrogen Adduct (Co–H2) Complexes Displaying H‐Atom and Hydride Transfer

AbstractThe prototypical reactivity profiles of transition metal dihydrogen complexes (M‐H2) are well‐characterized with respect to oxidative addition (to afford dihydrides, M(H)2) and as acids, heterolytically delivering H+ to a base and H− to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H‐atom or hydride transfer from the σ‐H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= and an anionic S=0 Co‐H2 adduct, both supported by a trisphosphine borane ligand (P3B). The thermally stable metalloradical, (P3B)Co(H2), serves as a competent precursor for hydrogen atom transfer to tBu3ArO⋅. What is more, its anionic derivative, the dihydrogen complex [(P3B)Co(H2)]1−, is a competent precursor for hydride transfer to BEt3, establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M‐H2 complexes known.

Advancement toward Polymer Electrolyte Membrane Fuel Cells at Elevated Temperatures.

Elevation of operational temperatures of polymer electrolyte membrane fuel cells (PEMFCs) has been demonstrated with phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. The technical perspective of the technology is simplified construction and operation with possible integration with, e.g., methanol reformers. Toward this target, significant efforts have been made to develop acid-base polymer membranes, inorganic proton conductors, and organic-inorganic composite materials. This report is devoted to updating the recent progress of the development particularly of acid-doped PBI, phosphate-based solid inorganic proton conductors, and their composite electrolytes. Long-term stability of PBI membranes has been well documented, however, at typical temperatures of 160°C. Inorganic proton-conducting materials, e.g., alkali metal dihydrogen phosphates, heteropolyacids, tetravalent metal pyrophosphates, and phosphosilicates, exhibit significant proton conductivity at temperatures of up to 300°C but have so far found limited applications in the form of thin films. Composite membranes of PBI and phosphates, particularly in situ formed phosphosilicates in the polymer matrix, showed exceptionally stable conductivity at temperatures well above 200°C. Fuel cell tests at up to 260°C are reported operational with good tolerance of up to 16% CO in hydrogen, fast kinetics for direct methanol oxidation, and feasibility of nonprecious metal catalysts. The prospect and future exploration of new proton conductors based on phosphate immobilization and fuel cell technologies at temperatures above 200°C are discussed.

ChemInform Abstract: Bronsted‐Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

AbstractReview: 460 refs.

Kubas complexes extended to four centers; a theoretical prediction of novel dihydrogen coordination in bimetallic tungsten and molybdenum compounds

A number of molybdenum and tungsten bimetallic compounds with carbonyl and phosphine ligands are investigated, using theoretical methods, with respect to their ability to bind molecular hydrogen in a Kubas type coordination. Some are found to give novel complexes, containing a four center metal–dihydrogen interaction.

Spin-State Effects on the Thermal Dihydrogen Release from Solid-State [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os) Organometallic Complexes for Hydrogen Storage Applications.

Mössbauer spectroscopy, experimental thermodynamic measurements, and computational studies were performed to investigate the properties of molecular hydrogen binding to the organometallic fragments [MHdppe2]+ (M = Fe, Ru, Os; dppe =1,2-bis(diphenylphosphino)ethane) to form the dihydrogen complex fragments [MH(η2-H2)dppe2]+. Mössbauer spectroscopy showed that the dehydrogenated complex [FeHdppe2]+ adopts a geometry consistent with the triplet spin state, transitioning to a singlet state complex upon addition of the dihydrogen molecule in a manner similar to the previously studied dinitrogen complexes. From simulations, this spin transition behavior was found to be responsible for the strong binding behavior experimentally observed in the iron complex. Spin-singlet to spin-singlet transitions were found to exhibit thermodynamics consistent with the 5d > 3d > 4d binding trend observed for other transition metal dihydrogen complexes. Finally, the method for distinguishing between dihydrogen and dihydride complexes based on partial quadrupole splittings observed in Mössbauer spectra was confirmed, providing a tool for further characterization of these unique species for Mössbauer active compounds.

Transition Metal Complexes Containing Dihydrogen Hypodiphosphate in Eclipsed Conformation as Ligand: Preparation, Crystal Structure, Vibrational Spectra, and Thermal Behavior of K2[M(H2P2O6)2(H2O)2]·H2O (M = Co, Ni, Cu, and Zn)

AbstractThe transition metal dihydrogen hypodiphosphate hydrates K2[Co(H2P2O6)2(H2O)2]·H2O (1), K2[Ni(H2P2O6)2(H2O)2]·H2O (2), K2[Cu(H2P2O6)2(H2O)2]·H2O (3) and K2[Zn(H2P2O6)2(H2O)2]·H2O (4) were synthesized and characterized by single crystal structure determination. The compounds 1–4 crystallize isotypic in the monoclinic space group C2/m (no. 12) with two formula units in the unit cell. The crystal structure is built up by [H2P2O6]2– units in an eclipsed conformation, by the corresponding transition metal, potassium cations, and water molecules. The eclipsed conformation of the [H2P2O6]2– has not been previously observed in none of known hypodiphosphates(IV) analyzed via X‐ray diffraction. However, its proposed based on spectroscopic methods. FT‐IR/FIR and FT‐Raman spectra of the crystalline salts were recorded and the thermal behavior of the compounds was investigated.

AFM-Investigations , Structural Visualization Studies , and Temperature Optimization of Single Metal Cyclotetraphosphates Synthesized at Elevated Temperature

The precursor metal dihydrogen phosphate dihydrate was synthesized by a rapid and simple precipitation method using phosphoric acid, cobalt nitrate at ambient temperature (cobalt (II) was selected as model for metal in these investigations ) . The precursor was fired and sintered at different temperatures (600 ,800 ,1000 and 1100°C ) respectively to optimize best conditions to obtain Co 2 P 4 O 12 crystal form with high purity .The products were monitored by both of XRD ,IR spectra by additional to accurate imaging via scanning electron microscope (SEM) and AFM-microscope to analyze surface topology and microstructural features of the metal cyclotetraphosphate .Structural investigations via XRD proved that the product obtained at 1100°C is the best and fine structure with monoclinic structure phase and C12/C1 space group with lattice parameter a = 11.809(2), b = 8.293(1), c = 9.923(2) A respectively . A visualized investigations were performed to confirm structure validity and stability at temperature of sintering (1100°C ) . Visualization studies indicated that variations of bond distances between Co1,Co2,P1 and P2 and different six oxygen atoms ( O1 ,O2 , O3 ,O4 ,O5 and O6) inside crystal lattice are responsible for increasing lattice flexibility factor ( by controlling in shrinkage and expansion coefficient ) and consequently increase its bonds stability to break .

ChemInform Abstract: Phosphine‐Supported Metal—Dihydrogen Complexes: Elongation of H—H Bond to Reversible Release of H2

AbstractReview: 201 refs.

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.

Rapid Coprecipitation and Non-Isothermal Decomposition Kinetics of New Binary Mn0.5Cu0.5(H2PO4)2·1.5H2O

New binary metal dihydrogen phosphate dihydrate Mn0.5Cu0.5(H2PO4)2·1.5H2O was synthesized by a rapid and simple coprecipitation method using phosphoric acid, manganese metal, and copper oxide at ambient temperature. The thermal stability of Mn0.5Cu0.5(H2PO4)2·1.5H2O was studied by means of the non-isothermal kinetics (Kissinger method). The synthesized Mn0.5Cu0.5(H2PO4)2·1.5H2O shows complex thermal transformations, and its final decomposition product was a binary metal cyclotetraphosphate, MnCuP4O12. The X-ray diffraction (XRD), scanning electron microscopy (SEM), UV−vis−near-IR, and Fourier transform IR (FTIR) results of the synthesized Mn0.5Cu0.5(H2PO4)2·1.5H2O and the decomposed MnCuP4O12 appear to be very similar to those of M(H2PO4)2·2H2O and M2P4O12 (M = Mn and Cu), which indicate the monoclinic phase with space group P21/n and C2/c, respectively. The dominant features of the synthesized Mn0.5Cu0.5(H2PO4)2·1.5H2O and the decomposition product MnCuP4O12 are compared with those of M(H2PO4)2·2H2O and ...

Chemistry in Low‐Temperature Matrices

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Transition metal dihydrogen complexes: isotope effects on reactivity and structure

AbstractComplexation of dihydrogen to transition metal centers was discovered by Kubas and coworkers in 1984. The notion that the simplest molecule in chemistry can act as a ligand to form relatively stable transition metal complexes has led to a paradigm shift in coordination chemistry. Crucial to the exploration of this intriguing new chemistry has been the use of isotope substitution. Several aspects of the coordination chemistry of dihydrogen have revealed fascinating isotope effects on reactivity, spectroscopy and in some cases structure. Complexation of HD has been used to diagnose bond distances from measurements of JHD and isotope effects for D2 versus H2 binding have been evaluated. Examples of quantum mechanical exchange coupling in H2 complexes have been described. These effects disappear when one of the H atoms is replaced by D. In molecules with bound hydrogen adjacent to a hydride ligand, non‐statistical occupancy of hydrogen versus hydride sites by deuterons has been observed. In some cases, isotope‐dependent structures have been established by the study of HD, HT and DT complexes using NMR spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.

Density Functional Study of H−D Coupling Constants in Heavy Metal Dihydrogen and Dihydride Complexes: The Role of Geometry, Spin−Orbit Coupling, and Gradient Corrections in the Exchange-Correlation Kernel

The H-D nuclear spin-spin coupling constants J(H-D) of 14 heavy transition-metal dihydrogen and dihydride complexes were calculated with density functional theory using the "zeroth-order regular approximation" (ZORA) for the one-electron operators. The applied gradient-corrected density functional was able to achieve an average agreement with experimental data that is almost comparable to what has been obtained recently with hybrid functionals [J. Am. Chem. Soc. 2004, 126, 14249]. However, a systematic overestimation of J(H-D) for complexes with short H-D distances was obtained, which could be traced back to problems of the gradient functional to describe the H-D coupling in free dihydrogen well. We implemented gradient corrections for the exchange-correlation (XC) kernel and employed a basis sets with high-exponent 1s function for the coupled hydrogens. The gradient terms in the XC kernel turned out to be very important in order to achieve reasonable agreement with experimental coupling constants. On the other hand, our study reveals that spin-orbit relativistic corrections on the H-D coupling constants are comparatively small and need not to be considered at the accuracy level of currently available "standard" density functionals. The discussion of the results highlights the strong dependence of the coupling constants on the H-D distance and the possibility of large vibrational contributions to them. We also discuss the coupling constant for the hydrogen molecule in detail because of its relevance to the coupling in dihydrogen and dihydride complexes.

Elongated Dihydrogen Complexes: What Remains of the H—H Bond?

AbstractFor Abstract see ChemInform Abstract in Full Text.

Elongated dihydrogen complexes: what remains of the H-H bond?

Of the several hundred examples of transition metal dihydrogen complexes that have been reported to date, the vast majority have H-H distances of less than 1.0 Angstrom. A small number of complexes have been reported with distances in the range of 1.1 to 1.5 Angstrom. These complexes have been termed elongated dihydrogen complexes. In this review, experimental methods for structure determination of such complexes are summarized, along with computational approaches which have proven useful in understanding the structures of these molecules.

The Synthesis and Structure of Bis- and Tris(amino)-Functionalised Cyclopentadienylyttrium Derivatives

Amino-functionalised cyclopentadienyl derivatives have been used to great effect in complexes of the d-block elements.[1,2] As part of our ongoing studies investigating the early transition metal systems for metal dihydrogen bonding we decided, given previous successes, to concentrate on the (N-methylpiperid-4-yl)cyclopentadienyl ligand.[3 6] In this paper we present a number of new yttrium chloride complexes, one of which shows particularly interesting fluxional properties and provides a rare example of stabilisation of a cyclopentadienyl centre without the use of extraneous ligands.

Metal–dihydrogen and σ-bond coordination: the consummate extension of the Dewar–Chatt–Duncanson model for metal–olefin π bonding

There is a marvelous analogy between the metal–olefin π bonding model first brought to light by Dewar 50 years ago and that of σ-bond coordination discovered by us 17 years ago. In some ways it is surprising that 33 years elapsed between the two parallel bonding situations. However this difference pales in comparison to that between the actual synthesis of the first olefin complex, Zeise's salt in 1837, and the first recognized dihydrogen complex nearly 150 years later. This article delineates the principles of σ-bond coordination and activation inspired by the Dewar–Chatt–Duncanson model and illuminates the often-spectacular interplay between theory and experiment in this field. Aside from HH bond coordination and activation towards cleavage, the structure and bonding principles apply to SiH, CH, and virtually any two-electron XH or XY bond. Metal d to σ* XH backdonation is the key to stabilizing σ-bond coordination and is also crucial to homolytic cleavage (oxidation addition). There are some differences in bonding depending on X, and, in the case of BH bond coordination, in metal–borane complexes, backdonation to boron p orbitals occurs. For electrophilic complexes, particularly cationic systems with minimal backdonation, heterolytic cleavage of XH is common and is a key reaction in industrial and biological catalysis. Thus there are two separate pathways for σ-bond activation that directly depend on the electronics of the metal σ-ligand bonding.