Transition Metal Dihydrogen Complexes Research Articles

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.

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.

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

AbstractReview: 201 refs.

Phosphine supported metal-dihydrogen complexes: Elongation of H−H bond to reversible release of H2

Abstract Monodentate and bidentate phosphine supported transition metal-dihydrogen complexes were studied extensively in last three decades by using spectroscopic (IR, NMR) and structural (X-ray and neutron single crystals diffraction) characterization techniques. A major concern was to investigate various changes in physical and chemical properties of H 2 when bound to a metal center. This article emphasizes the role of the electron rich phosphin ligands in forming molecular hydrogen complexes for a large number of transition metal ions and how the bonding of the metal with the molecular hydrogen (H 2 ) affects the ultimate fate of the H−H bond. Important features of the H 2 complexes such as elongation of the H−H bond, rapid dynamics of the site exchange of H atoms in hydride/dihydrogen complexes, binding of H 2 to the coordinately unsaturated metal centers are discussed in great detail. In many cases, the results of spectroscopic and structural studies were supported by theoretical calculations. Besides these, the focus of the present article is on the phenomena of reversible release of H 2 observed in the case of certain recently reported metal-H 2 complexes which has relevance in H 2 production. Various features of the elongated H 2 complexes and reversible binding of H 2 to a metal center discussed in this article will be of great interest to inorganic and organometallic chemists in general and to inorganic-material chemists working in the area of catalysis and storage materials using H 2 . Discussions on certain new directions of metal-dihydrogen chemistry such as photochemical generation of H 2 complexes, ionic H 2 activiation, and H 2 reductive elimination from polyhydride complexes have been included (202 references).

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.

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.

Determination of the H−H Distance in Transition-Metal Dihydrogen Complexes: Effects of High Magnetic Fields

Determination of the H−H Distance in Transition-Metal Dihydrogen Complexes: Effects of High Magnetic Fields

Periodic Trends and Ligand Effects in Transition Metal Dihydrogen Complexes: A Quasi-relativistic Density Functional Study

A nonlocal, quasi-relativistic density functional method has been applied in a study of dihydrogen and dihydride isomers of M(CO)3(PH3)2H2 (M = Cr, Mo, W) and Mo(CO)n(PH3)5-nH2 (n = 1, 3, 5). The optimized structures and calculated energetics are in good agreement with available experimental data. The H2 dissociation energies in the dihydrogen complexes M(CO)3(PH3)2(H2) (M = Cr, Mo, W) exhibit a typical V-like trend along the triad, as in other transition metal carbonyls and π-complexes. An extended transition state energy decomposition scheme revealed that both H2 to metal donation and metal to H2 back-donation increase down the triad. The back-donation, as well as the degree of H2 activation, decreases with the number of strong π-accepting ligands. However, the stability of the dihydrogen complex increases compared to its dihydride isomer with the number of strong π-acceptor ligands.

Structure and Bonding of Transition Metal Dihydrogen Complexes [M(CO)5(H2)] (M = Cr, Mo, W)

Important intermediates in catalytic hydrogenation, η2‐dihydrogen complexes are also appealing subjects for theoretical studies. The MH2 binding enthalpies for the complexes [M(CO)5(H2)] (M = Cr, Mo, W) were calculated at the CCSD(T)/MP2 level as 15.9 (M = Cr), 12.8 (M = Mo), and 16.3 kcal mol−1 (M = W). Charge decomposition analysis suggests that H2 is a significant σ donor and a moderate π acceptor. The topological analysis of the electron density distribution indicates a cyclic structure for the MH2 moiety.

Isolation of an Extremely Labile Dihydrogen Complex, Cr(CO)3(PPri3)2(H2), Containing the Shortest Ligated H-H Bond

First-row members of the group 6 series of transition metal dihydrogen complexes Cr(CO){sub 3}(PR{sub 3}){sub 2}({eta}{sup 2}-H{sub 2})(R = Cy, Pr{sup i}) have been synthesized and examined by X-ray crystallography and NMR, infrared, and inelastic neutron scattering spectroscopies. The complexes completely dissociate H{sub 2} in solution to give Cr(CO){sub 3}(PR{sub 3}){sub 2}, which contains an agostic C-H interaction. An X-ray structure of Cr(CO){sub 3}(PPr{sup i}{sub 3}){sub 2}(H{sub 2}) showed a side-bonded dihydrogen ligand with H-H = 0.67(5) {angstrom}, and proton NMR of the HD isotopomer showed a J(HD) value of 35 Hz. Solid-state H NMR spectroscopy of the PCy{sub 3} complex indicated that the H-H separation is 0.85(1) {angstrom}, which is believed to be a more precise and accurate measure of this key parameter. The barrier to rotation of the {eta}{sup 2}-H{sub 2} has been recalculated to be 1.17(10) kcal/mol, as compared with 1.32(10) and 1.9(1) kcal/mol for the Mo and W analogues, based on H-H distances from solid-state NMR. Correlations of properties of the group 6 species indicate that the H{sub 2} {yields} M {sigma}-bonding strength appears to increase as Mo < Cr {approximately} W but that d{pi} {yields} H{sub 2} {sigma}* back-bonding increases down the group. It mightmore » be generally concluded that back-bonding is more of a factor than {sigma}-bonding in influencing stability, H-H distance, and possibly overall bond strengths. Crystallographic data: space group P2{sub 1}/c, a = 13.457-(3) {angstrom}, b = 11.493(3) {angstrom}, c = 16.639(4) {angstrom}, {beta} = 102.25(2){sup o}, Z = 4, R = 3.32%.« less

Reactions of transition metal dihydrogen complexes

A. Introduction (i) The scope of the review (ii) The preparation of dihydrogen complexes (a) Preparation from dihydrogen gas (b) Protonation of a hydride complex (c) Other methods of preparation B. Homolytic splitting of coordinated dihydrogen (i) Introduction (ii) Observing the dihydrogen-dihydride equilibrium (iii) Factors influencing the dihydrogen-dihydride equilibrium (iv) Thermodynamics of the dihydrogen-dihydride equilibrium (v) Kinetics of the dihydrogen-dihydride equilibrium (vi) Rapid dihydrogen-dihydride equilibrium or elongated H ... H ligand? ..... (vii) Other dihydrogen to dihydride reactions (viii) Intermolecular homolytic cleavage of a dihydrogen ligand C. Exchange of H atoms between dihydrogen and other hydrogen-donor ligands .... (i) Introduction (ii) Intramolecular H atom exchange for a three-hydrogen system M(H,)H ..... (a) Mechanisms of H atom exchange (b) Observing H atom exchange for a three-hydrogen system M(H,)H .... (c) Complete line-shape analysis (d) Activation parameters from complete line-shape studies (e) Slow exchange with incomplete T, averaging (f) Line-shape coalescence (g) No line-shape decoalescence (h) Intramolecular exchange of isotopes (iii) Intramolecular H atom exchange for dihydrogenpolyhydride systems M(H,)H, (a) Mechanisms (b) Line-shape coalescence (c) No line-shape decoalescence (d) Intramolecular exchange of isotopes (iv) H/D exchange in the reaction of M(H,)L, with D, (v) Intramolecular H atom exchange in bimetallic systems D. Heterolytic cleavage of the dihydrogen ligand 159 160 161 162 162 163 163 163 164 170 174 178 182 185 187 188 188 189 189 193 194 197 198 201 203 203 206 206 207 209 211 212 214 215