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The synthesis and characterisation of the S = 1/2 Fe(I) complex [Fe(depe)2]+[BArF4]− ([1]+[BArF4]−), and the facile reversible binding of N2 and H2 in both solution and the solid state to form the adducts [1·N2]+ and [1·H2]+, are reported. Coordination of N2 in THF is thermodynamically favourable under ambient conditions (1 atm; ΔG298 = −4.9(1) kcal mol−1), while heterogenous binding is more favourable for H2 than N2 by a factor of ∼300. [1·H2]+[BArF4]− represents a rare example of a well-defined, open-shell, non-classical dihydrogen complex, as corroborated by ESR spectroscopy. The rapid exchange between N2 and H2 coordination under ambient conditions is unique for a paramagnetic Fe complex.

A series of alkanediyl-spaced bis-bisurea ligands ( L 2 – L 4 ) were synthesized and their anion coordination behavior studied. These ligands form interesting complexes with polymeric and oligomeric dihydrogen phosphate aggregates in the solid state. The ligands L 2 and L 3 coordinate with H 2 PO 4 – anions to form a unique molecular “necklace” with an infinite (H 2 PO 4 – ) n chain and surrounding ligand molecules. Meanwhile, two different dihydrogen phosphate-water oligomers, (H 2 PO 4 ) 6 · (H 2 O) 4 and (H 2 PO 4 ) 4 · (H 2 O) 2 , were observed in the complexes with the ligands L 3 and L 4 . In addition, solution anion binding properties of the ligands were studied by 1 H NMR and UV/Vis spectroscopy.

Transition-metal (TM) atoms are known to form TM-H2 complexes, which are collectively called Kubas dihydrogen complexes. The TM-H2 complexes are formed through the hybridization of the TM d orbitals with the H2 σ and σ* orbitals. The adsorption energy of H2 molecules in the TM-H2 complexes is usually within the range of energy required for reversible H2 storage at room temperature and ambient pressure (−0.4 ~ −0.2 eV/H2). Thus, TM-H2 complexes have been investigated as potential Kubas-type hydrogen-storage materials. Recently, TM-decorated nanomaterials have attracted much attention because of their promising high capacity and reversibility as Kubas-type hydrogen-storage materials. The hydrogen storage capacity of TM-decorated nanomaterials is expected to be as large as ~9 wt%, which is suitable for certain vehicular applications. However, in the TM-decorated nanostructures, the TM atoms prefer to form clusters because of the large cohesive energy (approximately 4 eV), which leads to a significant reduction in the hydrogen-storage capacity. On the other hand, Ca atoms can form complexes with H2 molecules via Kubas-like interactions. Ca atoms attached to nanomaterials have been reported to be able to adsorb as many H2 molecules as TM atoms. Ca atoms tend to cluster less because of the small cohesive energy of bulk Ca (1.83 eV), which is much smaller than those of bulk TMs. These observations suggest thatKubas interactions can occur in d orbital-free elements, thereby making Ca a more suitable element for attracting H2 in hydrogen-storage materials. Recently, Kubas-type TM-based, hydrogen- stor ge materials were experimentally synthesized, and the Kubas-type interactions were measured to be stronger than the van der Waals interactions. In this review, the recent progress of Kubas-type hydrogen- storage materials will be discussed from both theoretical and experimental viewpoints.

A ligand L has been synthesized by the condensation of 4-chlorobenzaldehyde and 2-amino-5-chlorobenzophenone. The complexes 1–5 of Pb+2, Ni+2, Co+2, Cu+2 and Cd+2 with ligand were prepared. The newly synthesized ligand L and its complexes 1–5 were characterized on the basis of elemental analyses, molar conductance, 1H NMR, IR and MS spectral data. The ligand was also characterized by X-ray crystallography. By electronic spectra and magnetic moment data, the square planar geometry for complex 1 and the octahedral geometry for complexes 2–5 were proposed. The conductivity data suggests non-electrolytic nature of the complexes 1–5. The complexes have higher antibacterial activity than the parent ligand L.

Ab initio calculations were used to analyze the interaction of C4B2H6 with HF and LiH molecules at the mp2/6-311++g(2d,2p) computational level. Interaction of C4B2H6 with HF results to H–F···H–C and C–B···H–F, C–C···H–F hydrogen bond as well as B–H···H–F dihydrogen bond complexes. Also interaction of C4B2H6 with LiH results to B–C···LiH, C–C···LiH and B–H···LiH lithium bond as well as C–H···H–Li dihydrogen complexes. In the both cases, complexes involving interaction of HF or LiH with peripheral B–C and C–C bonds of the C4B2H6 backbone have greater stabilities. The structures of complexes have been analyzed using AIM and NBO methodologies.

In the salt 1-methylpiperazine-1,4-diium bis(dihydrogen phosphate), C5H13N22+·2H2PO4−, (I), and the solvated salt 2-(pyridin-2-yl)pyridinium dihydrogen phosphate–orthophosphoric acid (1/1), C10H9N2+·H2PO4−·H3PO4, (II), the formation of O—H...O and N—H...O hydrogen bonds between the dihydrogen phosphate (H2PO4−) anions and the cations constructs a three- and two-dimensional anionic–cationic network, respectively. In (I), the self-assembly of H2PO4−anions forms a two-dimensional pseudo-honeycomb-like supramolecular architecture along the (010) plane. 1-Methylpiperazine-1,4-diium cations are trapped between the (010) anionic layers through three N—H...O hydrogen bonds. In solvated salt (II), the self-assembly of H2PO4−anions forms a two-dimensional supramolecular architecture with open channels projecting along the [001] direction. The 2-(pyridin-2-yl)pyridinium cations are trapped between the open channels by N—H...O and C—H...O hydrogen bonds. From a study of previously reported structures, dihydrogen phosphate anions show a supramolecular flexibility depending on the nature of the cations. The dihydrogen phosphate anion may be suitable for the design of the host lattice for host–guest supramolecular systems.

Thiolate-bridged dinuclear ruthenium and iron complexes are found to work as efficient catalysts toward oxidation of molecular dihydrogen in protic solvents such as water and methanol under ambient reaction conditions. Heterolytic cleavage of the coordinated molecular dihydrogen at the dinuclear complexes and the sequential oxidation of the produced hydride complexes are involved as key steps to promote the present catalytic reaction. The catalytic activity of the dinuclear complexes toward the chemical oxidation of molecular dihydrogen achieves up to 10000 TON (turnover number), and electrooxidation of molecular dihydrogen proceeds quite rapidly. The result of the density functional theory (DFT) calculation on the reaction pathway indicates that a synergistic effect between the two ruthenium atoms plays an important role to realize the catalytic oxidation of molecular dihydrogen efficiently. The present dinuclear ruthenium complex is found to work as an efficient organometallic anode catalyst for the fuel cell. It is noteworthy that the present dinuclear complex worked not only as an effective catalyst toward chemical and electrochemical oxidation of molecular dihydrogen but also as a good anode catalyst for the fuel cell. We consider that the result described in this paper provides useful and valuable information to develop highly efficient and low-cost transition metal complexes as anode catalysts in the fuel cell.

A triad of d10 cobalt dihydrogen complexes was synthesized by utilizing Lewis acidic group 13 metalloligands, M[N((o-C6H4)NCH2PiPr2)3], where M = Al, Ga, and In. These complexes have formal Co(−I) oxidation states, representing the only coordination complexes in which dihydrogen is bound to a subvalent transition metal center. Single-crystal X-ray diffraction and NMR studies support the assignment of these complexes as nonclassical dihydrogen adducts of Co(−I).

Proton transfer promoted by the coordination of protogenic Lewis bases to a Lewis acid is a critical step in catalytic transformations. Although the acidification of water upon coordination to a Lewis acid has been known for decades, no attempts have been made to correlate the Bronsted acidity of the coordinated water molecule with Lewis acid strength. To probe this effect, the pKa's (estimated error of 1.3 pKa units) in acetonitrile of ten protogenic Lewis bases coordinated to seven Lewis acids containing Lewis acidities varying 70 kcal mol−1, were computed. To quantify Lewis acid strength, the ability to transfer a hydride (hydride donor ability) from the respective main group hydride was used. Coordination of a Lewis acid to water increased the acidity of the bound water molecule between 20 and 50 pKa units. A linear correlation exhibiting a 2.6 pKa unit change of the Lewis acid–water adduct per ten kcal mol−1 change in hydride donor ability of the respective main group hydride was obtained. For the ten protogenic Lewis bases studied, the coordinated protogenic Lewis bases were acidified between 10 and 50 pKa units. On average, a ten kcal mol−1 change in hydride donor ability of the respective main group hydride resulted in about a 2.8 pKa unit change in the Bronsted acidity of the Lewis acid–Lewis base adducts. Since attempts to computationally investigate the pKa of main group dihydrogen complexes were unsuccessful, experimental determination of the first reported pKa of a main group dihydrogen complex is described. The pKa of H2-B(C6F5)3 was determined to be 5.8 ± 0.2 in acetonitrile.

Nonclassical and classical osmium polyhydrides containing the diphosphine 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2), coordinated in κ3-mer, κ3-fac, and κ2-P,P fashions, have been isolated during the cyclic formation of H2 by means of the sequential addition of H+ and H– or H– and H+ to the classical trihydride OsH3Cl{xant(PiPr2)2} (1). This complex adds H+ to form the compressed dihydride dihydrogen complex [OsCl(H···H)(η2-H2){xant(PiPr2)2}]+ (2). Under argon, cation 2 loses H2 and the resulting unsaturated fragment dimerizes to give [(Os(H···H){xant(PiPr2)2})2(μ-Cl)2]2+ (3). During the transformation the phosphine changes its coordination mode from mer to fac. The benzofuran counterpart of 1, OsH3Cl{dbf(PiPr2)2} (4; dbf(PiPr2)2 = 4,6-bis(diisopropylphosphino)dibenzofuran), also adds H+ to afford the benzofuran counterpart of 2, [OsCl(H···H)(η2-H2){xant(PiPr2)2}]+ (5), which in contrast to the latter is stable and does not dimerize. Acetonitrile breaks the chloride bridge of 3 to form the dihydrogen [OsCl(η2-H2)(CH3CN){xant(PiPr2)2}]+ (6), regenerating the mer coordination of the diphosphine. The hydride ion also breaks the chloride bridge of 3. The addition of KH to 3 leads to 1, closing a cycle for the formation of H2. Complex 1 reacts with a second hydride ion to give OsH4{xant(PiPr2)2} (7) as consequence of the displacement of the chloride. Similarly to the latter, the oxygen atom of the mer-coordinated diphosphine of 7 has a tendency to be displaced by the hydride ion. Thus, the addition of KH to 7 yields [OsH5{xant(PiPr2)2}]− (8), containing a κ2-P,P-diphosphine. Complex 8 is easily protonated to afford OsH6{xant(PiPr2)2} (9), which releases H2 to regenerate 7, closing a second cycle for the formation of molecular hydrogen., Financial support from the MINECO of Spain (Projects CTQ2014-52799-P and CTQ2014-51912-REDC), Gobierno de Aragon (E35), FEDER, and the European Social Fund is acknowledged.

By using a potential tridentate ligand L ((2-piperazine-1-yl-ethyl)-pyridin-2-yl-methylene-amine), a series of group 12 metal complexes namely, [ZnLHCl2][Zn2LCl5]·2H2O (1), [CdL(SCN)2(CH3OH)]n (2), and [Hg(l-pyCO)Cl2] (3), were synthesized and structurally characterized. In all the complexes the piperazine nitrogen of the ligand takes part in coordination and leads to the complexes of group 12 metal ions having structural diversity. The X-ray diffraction analysis of complex 1 indicates for one Zn(ii) ion a geometry in between trigonal bipyramidal/square pyramidal and for the second a distorted tetrahedral sphere. In the polymeric complex 2 the Cd(ii) ion shows a distorted octahedral environment, while in the mononuclear complex 3, where Hg(ii) exhibits a square-pyramidal geometry, an unexpected condensation between the uncoordinated NH piperazine fragment with 2-pyridinecarboxaldehyde was detected. The M-N bond lengths in all the complexes are in accordance with the metal ionic radius. Continuous shape measures through a DFT approach provide the coordination environment around each metal centre that is comparable with the experimental observations. We have also investigated the importance of hydrogen bonding of methanol in the generation of the polymeric Cd complex 2 along with the rearrangement of the tridentate ligand to generate an octahedral complex. The photoluminescence properties of the complexes as well as of the ligand were investigated in solution at ambient temperature. The low quantum yield of the ligand was ascribed due to a very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated pyridine moiety. Complexation prevents the electron transfer, and consequently an increase in quantum yield was observed in the complexes. Among the three complexes the highest photoluminescence was exhibited by a Zn complex, being lower in Cd and Hg complexes as a consequence of the heavy atom perturbation effect.

The C—H…H dihydrogen-bonded complexes of methane, ethylene, acetylene, and their derivatives with magnesium hydride were systematically investigated at MP2/aug-cc-PVTZ level. The results confirm that the strength of dihydrogen bonding increases in the following order of proton donors: C(sp 3)—H

Aromatic hydrogenation is a challenging transformation typically requiring alkali or transition metal reagents and/or harsh conditions to facilitate the process. In sharp contrast, the aromatic heterocycle 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene is shown to be reduced under 4 atm of H2 to give [3.1.0]bicylo reduction products, with the structure of the major isomer being confirmed by X-ray crystallography. NMR studies show this reaction proceeds via a reversible 1,4-H2 addition to generate an intermediate species, which undergoes an irreversible suprafacial hydride shift concurrent with P-P bond formation to give the isolated products. Further, para-hydrogen experiments confirmed the addition of H2 to triphosphabenzene is a bimolecular process. Density functional theory (DFT) calculations show that facile distortion of the planar triphosphabenzene toward a boat-conformation provides a suprafacial combination of vacant acceptor and donor orbitals that permits this direct and uncatalyzed reduction of the aromatic molecule.

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.

The mechanism of imine hydrogenation catalyzed by thiolate complexes of Rh(III) bearing a hydrotris(3,5-dimethylpyrazolyl)borato ligand has been investigated via the density functional theory calculations. The overall catalytic cycle for heterolytic cleavage of H2 and hydrogenation of N-benzylidenemethylamine by the model catalyst [TpRh(bdt)MeCN)] is presented in detail. The results show that the reaction proceeds via an ionic mechanism through three steps: formation of dihydrogen complex, protonation of imine and the hydride transfer process. Protonation of imine occurs after the formation of Rh(H)-S(H) moiety. For the whole catalytic cycle, the heterolytic splitting of dihydrogen is the step with the highest free energy barrier. © 2014 Wiley Periodicals, Inc.

A new ruthenium pincer complex [RuHCl(CO)(PNP)] (PNP = PhCH2N(CH2CH2PPh2)2) (1) was synthesized and characterized. The reactivity of complex 1 with electrophilic reagents XOTf (X = H, CH3, and Me3Si; OTf = CF3SO3) was studied by variable temperature NMR spectroscopy with an aim to observe and characterize sigma complexes of type [Ru(η2-HX)Cl(CO)(PNP)][OTf] (X = H (2), CH3 (3), Me3Si (4)). Reaction of complex 1 with HOTf resulted in the formation of the dihydrogen complex, [Ru(η2-H2)Cl(CO)(PNP)[OTf] (2). On the other hand, the reaction between complex 1 and MeOTf and Me3SiOTf resulted in the direct elimination of MeCl and Me3SiCl via a SN2 type of reaction without the intermediacy of the respective sigma complexes 3 and 4. This contrasting reactivity behaviour has been rationalized taking into consideration the approach of the relatively bulky electrophiles CH3+ and Me3Si+ onto the hydride moiety of the ruthenium fragment, which is sterically hindered.

Treatment of RuCl2(PPh3)3 with 6-dimethylaminopentafulvene in THF in the presence of water produced (η5-C5H4CHO) RuCl(PPh3)2, which was reduced by NaBH4 to give the Ru-H⋯HO dihydrogen bonded complex (η5-C5H4CH2OH) RuH(PPh3)2. The dihydrogen bonded complex (η5-C5H4CH2OH)RuH(PPh3)2 could also be synthesized by the reduction of complex (η5-C5H4CHO)RuH(PPh3)2, which was obtained by the reaction of RuHCl(PPh3)3 with 6-dimethylaminopentafulvene in the presence of water. The analogous dihydrogen bonded osmium complex (η5-C5H4CH2OH)OsH(PPh3)2 was similarly prepared. Single crystal structures and DFT calculations support the presence of intra-molecular H…H interaction, with separations of around 1.9 to 2.0 A.

Two new Ru(II)-complexes [RuH(Tpms)(PPh3)2] 1 (Tpms = (C3H3N2)3CSO3, tris-(pyrazolyl)methane sulfonate) and [Ru(OTf)(Tpms)(PPh3)2] 2 (OTf = CF3SO3) have been synthesized and characterized wherein Ru–H and Ru–OTf are the key reactive centers. Reaction of 1 with HOTf results in the [Ru(η2-H2)(Tpms)(PPh3)2][OTf] complex 3, whereas reaction of 1 with Me3SiOTf affords the dihydrogen complex 3 and complex 1 through an unobserved σ–silane intermediate. In addition, an attempt to characterize the sigma methane complex via reaction of complex 1 with CH3OTf yields complex 2 and free methane. On the other hand, reaction of [Ru(OTf)(Tpms)(PPh3)2] 2 with H2 and PhMe2SiH at low temperature resulted in σ–H2, 3 and a probable σ–silane complexes, respectively. However, no σ–methane complex was observed for the reaction of complex 2 with methane even at low temperature.

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-substituted PNP-pincer ligands are synthesized by the reduction of the corresponding molybdenum trichloride complexes under 1 atm of molecular dinitrogen. In accordance with a theoretical study, the catalytic activity is enhanced by the introduction of an electron-donating group to the pyridine ring of PNP-pincer ligand, and the complex bearing 4-methoxy-substituted PNP-pincer ligands is found to work as the most effective catalyst, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia based on each molybdenum atom of the catalyst), together with molecular dihydrogen as a side-product. Time profiles for the catalytic reactions indicate that the rates of the formation of ammonia and molecular dihydrogen depend on the nature of the substituent on the PNP-pincer ligand of the complexes. The formation of ammonia and molecular dihydrogen is complementary in the reaction system.

A series of novel Cd(II) complexes incorporating ligand, di(2-pyridinyl)methanone N-(2-pyridinyl)hydrazone (DPMNPH), has been investigated. The ligand, DPMNPH, and its corresponding complexes have been characterized with the help of a number of techniques: microanalysis, FT-IR, 1H and 13C NMR, UV/vis spectroscopy, thermal studies and MS–FAB mass spectrometry. In addition, single crystal X-ray diffraction measurement studies are also employed in one of the complexes showing a distorted trigonal bipyramidal geometry. Furthermore, the existence of NH⋯Npy intramolecular hydrogen bonding interactions in the ligand and its corresponding complexes has also been reported.

A quantum-chemical investigation of the dehydrocoupling reaction of the secondary amine-borane Me2HNBH3 assisted by phosphine chelating [Rh(Ph2P(CH2)n-PPh2)(C6H5F)]+ (n = 3–5) complexes to ultimately afford the cyclic dimer [Me2NBH2]2 is reported. The hypothesis, proposed on the basis of experimental evidence, that the catalytic efficiency of such systems is due to formation of Rh(III) dihydride complexes, which rapidly lose H2 and reform Rh(I) species, has been explored, together with the influence that the structure of the ligand (namely, the chelating phosphine P–Rh–P bite angle) has on the rate of the reaction. Along the pathway that our computational analysis has indicated as the most likely, the first step of the dehydrogenation reaction is the concerted B–H hydride and N–H proton transfer from an additional amine-borane molecule to the rhodium center of the formed [Rh(Ph2P(CH2)n-PPh2)(η2-Me2HNBH3)]+ complexes. The reaction proceeds by formation of dihydrogen complexes, which eliminate molecular hyd...

The promoterless AAD (acceptorless alcohol dehydrogenation) reaction mediated by an iridium catalyst Cp*Ir(bpyO) 1–Ir (Cp* = pentamethylcyclopentadienyl, bpyO = α,α′-bipyridonate) has been theoretically investigated with the density functional theory. The reaction occurs through three steps, including alcohol dehydrogenation, formation of dihydrogen complex, and H2 elimination from the iridium center. In the first two steps, the metal center and the bpyO ligand work cooperatively via the aromatization/dearomatization process of the bpyO ligand. The second step is rate-determining, where the ΔG0≠ and ΔG0 values are 23.9 and 13.9 kcal/mol, respectively. Our calculations demonstrate that the aromatization of the bpyO ligand as well as the charge transfer (CT) from the Cp* ligand to the iridium center plays important roles in stabilizing the transition state of the rate-determining step. We have theoretically and experimentally examined the 4d rhodium analogue Cp*Rh(bpyO) 1–Rh and found that it exhibits simil...

Ternary complexes of NaC3N with HMgH and HCN (HNC) are connected by sodium, hydrogen and dihydrogen bonds. Molecular geometries and interaction energies of dyads and triads are investigated at the MollerPlesset perturbation theory of the second order/aug-cc-pVDZ computational level. Particular attention is paid to parameters, such as cooperative energies and many-body interaction energies. Triads with the HMgH molecule located at the end of the chain show an energetic cooperativity ranging between −2.13 and −10.53 kJ mol−1. When the HMgH molecule is located in the middle, the obtained cluster is diminutive with an energetic effect with values 4.39 and 6.77 kJ mol−1. The electronic properties of the complexes are analysed using parameters derived from the atoms in molecules methodology. Based on the energy decomposition analysis, it can be seen that the stabilities of the complexes are predicted to be attributable mainly to electrostatic effects.

Fifty years ago, when this journal was founded, organometallic chemists could not have imagined that common small molecules such as dinitrogen and especially dihydrogen could function as ligands. Dihydrogen has long been vital in catalytic processes such as hydrogenation and conversions of organic compounds and is now being considered as a future energy storage medium. Dihydrogen is only useful chemically when the two strongly bound H atoms are split apart in a controlled fashion. Although metal hydrides were first well established in 1955, the structure and mechanism by which H 2 binds to and undergoes cleavage on transition metals was not ascertained until even more recently in the history of inorganometallic chemistry, about 20 years after this journal was first published. The activation of dihydrogen is a fascinating saga that has slowly unfolded over the past 80+ years, as will be chronicled in this Perspective. There is a marvelous analogy between the metal-olefin π bonding model first brought to light by Dewar, Chatt, and Duncanson 60 years ago and the bonding model for side-on σ-bond coordination discovered by us 30 years ago. There are two separate pathways for H–H (and X–H σ-bond activation in general) that directly depend on the electronics of the metal σ-ligand bonding. Metal d to σ* X–H backdonation is the key to stabilizing σ-bond coordination and also is crucial to its homolytic cleavage (oxidation addition). For electrophilic complexes, particularly cationic systems with minimal backdonation, heterolytic cleavage of H 2 is common and is a key reaction in industrial and biological catalysis.

A simple equation (pKa(THF) = ∑AL + Ccharge + Cnd + Cd6) can be used to obtain an estimate of the pKa of diamagnetic transition metal hydride and dihydrogen complexes in tetrahydrofuran, and, by use of conversion equations, in other solvents. It involves adding acidity constants AL for each of the ligands in the 5-, 6-, 7-, or 8-coordinate conjugate base complex of the hydride or dihydrogen complex along with a correction for the charge (Ccharge = -15, 0 or 30 for x = +1, 0 or -1 charge, respectively) and the periodic row of the transition metal (Cnd = 0 for 3d or 4d metal, 2 for 5d metal) as well as a correction for d(6) octahedral acids (Cd6 = 6 for d(6) metal ion in the acid, 0 for others) that are not dihydrogen complexes. Constants AL are provided for 13 commonly occurring ligand types; of these, nine neutral ligands are correlated with Lever's electrochemical ligand parameters EL. This method gives good estimates of the over 170 literature pKa values that range from less than zero to 50 with a standard deviation of 3 pKa units for complexes of the metals chromium to nickel, molybdenum, ruthenium to palladium, and tungsten to platinum in the periodic table. This approach allows a quick assessment of the acidity of hydride complexes found in nature (e.g., hydrogenases) and in industry (e.g., catalysis and hydrogen energy applications). The pKa values calculated for acids that have bulky or large bite angle chelating ligands deviate the most from this correlation. The method also provides an estimate of the base strength of the deprotonated form of the complex.

In this paper the synthesis and characterisation of ruthenium dihydrogen complexes bearing rigid aliphatic PNP pincer-type ligands are described. As one result hydride complexes were synthesised in good to high yields by a one-pot direct hydrogenation reaction. As another finding the dihydrogen complex, stabilised with a N-Me group in the ligand frame, can be converted with dimethylamine borane into a rare σ-boron complex [RuH2(BH3)(Me-PNP)] with rapid B-N decoupling. Additionally, we present the first mass spectrometric analysis of the synthesised σ-complexes via liquid injection field desorption/ionisation technique (LIFDI-MS).

Hydrogen activation is a very important industrial process for hydrogenation reactions and ammonia production. The hydrogen splitting and hydride transfer process can be classified as homolytic and heterolytic cleavage of molecular hydrogen on mono- and multinuclear transition metal centers. Hydrogenase enzymes have inspired researchers in the field of organometallic chemistry to develop small molecule structural models of active sites and thus to mimic the biological system to activate molecular hydrogen. Multinuclear cluster complexes, including those containing heavy main group metals, can bind hydrogen molecule under mild conditions in a reversible fashion. This chapter aims at providing introductory review to cover various types of transition metal complexes that can split molecular hydrogen. The interaction between hydrogen molecule and metal centers, which determines the distance between two hydrogen atoms, will affect hydrogen splitting. The mechanism of such interactions will be discussed in details. Hydrogenation reactions catalyzed by transition metal complexes or heterogeneous nanocatalysts derived from metal cluster complexes will also be introduced.

Recent developments in the chemistry of hydride and dihydrogen complexes of iron, cobalt, and nickel are summarized. Applications in homogeneous catalysis are emphasized.

Pincer complexes are widely used in catalysis and activation of small molecules. We present here the synthesis and spectroscopic study of rhodium dihydrogen and dinitrogen complexes with metallocene-based P,C,P pincer ligands. Relative electron donor ability of ferrocene-, ruthenocene- and benzene-derived pincer ligands, as well as H H distance were elucidated based on JHD coupling constants and T1 relaxation time for dihydrogen complexes.

Five new Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) mixed ligand complexes have been synthesized using a Schiff base precursor (obtained by the condensation of N-(4-aminophenyl)acetamide and 4-chlorobenzaldehyde) as main ligand and 1,10-phenanthroline as co-ligand. They have been characterized by microanalytical data, IR, UV–Vis, magnetic moment values, conductivity and electrochemical measurements. The spectral data reveal that all the complexes exhibit octahedral geometry. The high electrical conductance of the complexes supports their electrolytic nature. The monomeric nature of the complexes has been assessed from their magnetic susceptibility values. These complexes are better antimicrobial active agents than the free ligands. DNA (CT) binding properties of these complexes have been explored by UV–Vis., viscosity measurements, cyclic voltammetry, and differential pulse voltammetry measurements. The oxidative cleavage activity of the complexes has been studied using supercoiled pUC19 DNA by gel electrophoresis. The experimental results show that the complexes are good intercalators.

National Natural Science Foundation of China [21174115, 21202054]; National Basic Research Program of China (973 Program) [2012CB821600]; Program for Changjiang Scholars and Innovative Research Team in University

H-H σ-bond activation promoted by Ni[MesB(o-Ph2PC6H4)2] (1(Mes)) was theoretically investigated with the density functional theory method. In 1(Mes), the nickel 3d, 4s, and 4p orbital populations are similar to those of the typical nickel(II) complex. First, one H2 molecule coordinates with the nickel center to form a dihydrogen complex, 2, which induces an increase in the nickel 3d and 4p orbital populations and thus a decrease in the nickel oxidation state. Then, the H-H σ-bond is cleaved under the unusual cooperation of the electron-rich nickel center and the electron-deficient borane ligand in a polarized manner, leading to an unprecedented trans-nickel(II) hydridoborohydrido complex, 3. In the transition state, charge transfer (CT) occuring from the H2 moiety to the 1(Mes) moiety (0.683 e) is much larger than the reverse CT (0.284 e). As a result, cleavage of the H-H σ-bond affords two positively charged hydrogen atoms. In this process, the boron atomic population and the nickel 4p orbital population increase, but the nickel 3d orbital population decreases. After cleavage of the H-H σ-bond, CT from the nickel 4p orbital to these positively charged hydrogen atoms occurs to afford 3, where the oxidation state of the nickel center increases to +2. These electronic processes are different from those of the usual homolytic and heterolytic H-H σ-bond activations. Regeneration of 1(Mes) and the role of the borane ligand in these reactions are also discussed in detail.

A series of dihydrogen complexes trans-[Ru(η2-H2){SC(SR)H}(dppe)2][X][BF4] (R = CH3, X = OTf; R = C6H5CH2, X = BPh4; R = H2C=CHCH2, X = BPh4; dppe = Ph2PCH2CH2PPh2) bearing sulfur-donor ligands has been synthesized by protonation of the (alkyl dithioformate)hydrido complexes trans-[Ru(H){SC(SR)H}(dppe)2][X] by using HBF4·Et2O. Competitive substitution reactions between H2 and SC(SR)H in trans-[Ru(η2-H2){SC(SR)H}(dppe)2][X][BF4] have been studied by treatment with CH3CN, CO, and P(OCH3)3. These resulted in the expulsion of SC(SR)H from the metal center, thus indicating that the alkyl dithioformate ligand is more labile than H2. Bonding of alkyl dithioformate ligands (sulfur-donor ligands) trans to H2 have been studied by comparing the H–H distances and chemical-shift values (1H NMR spectroscopy) of the various dihydrogen complexes bearing different trans ligands. This study qualitatively suggests that the alkyl dithioformate ligands in these trans-dihydrogen complexes show a poor π effect, and it is further supported by density functional theory calculations. The first example of a dihydrogen complex bearing dithioformic acid, trans-[Ru(η2-H2){SC(SH)H}(dppe)2][BF4]2, was obtained by protonation of trans-[Ru(H){SC(S)H}(dppe)2] by using HBF4·Et2O.

Wide bite angle diphosphine ligands were used to prepare [(diphosphine)M(2-(diphenylphosphino)pyridine)] (2+) complexes (M = Pd, Pt). Except for the ligand with the largest bite angle, 2-(diphenylphosphino) pyridine coordinates in a bidentate mode leading to bis-chelate complexes. In the case of Xantphos (9,9-dimethyl-4,5-bis(diphenylphosphino)-xanthene, beta(n) = 111) two types of complexes are formed, in which 2-(diphenylphosphino) pyridine coordinates in a mono-or bidentate fashion, respectively. The crystal structures of three of the Pt complexes were determined. The X-ray crystal structure of [(Xantphos)Pt( 2-(diphenylphosphino) pyridine)] 2+ shows that Xantphos coordinates in a tridentate P,O,P fashion. Under dihydrogen pressure, the pyridyl moiety in the platinum complexes can de-coordinate to provide a vacant coordination site at the metal center. Furthermore it can act as an internal base to assist the heterolytic cleavage of dihydrogen. The reaction yields a platinum hydride with a protonated pyridine moiety in close proximity to one another. The structure as well as the reactivity of the complexes towards dihydrogen is governed by the steric requirements of the diphosphines. The crystal structure of [(dppf)PtH( 2-(diphenylphosphino) pyridinium)](OTf)(2) has been determined. Palladium complexes containing DPEphos or Xantphos decompose under dihydrogen pressure. In the case of dppf slow heterolytic splitting of dihydrogen occurs to form the hydride complex [(dppf) PdH(2-(diphenylphosphino) pyridinium)]( OTf)(2) which contains a protonated 2-(diphenylphosphino) pyridine ligand. In solution, this compound slowly undergoes P-C bond cleavage of the 2-(diphenylphosphino) pyridine ligand to form [(dppf) Pd( PHPh2)(eta(1)-(CH4NH)-H-5)](OTf)(2). When the 6-methyl-2-pyridyldiphenylphosphine ligand is used, the reaction of the palladium complex with dihydrogen is very fast and the hydride complex immediately rearranges to the diphenylphosphino compound resulting from P-C bond cleavage.

Reaction of 1 with B(C6F5)3 under 1 bar of CO2 led to the instantaneous formation of the frustrated Lewis pair (FLP)-type species [ReHBr(NO)(PR3)2(η(2)-O═C═O-B(C6F5)3)] (2, R = iPr a, Cy b) possessing two cis-phosphines and O(CO2)-coordinated B(C6F5)3 groups as verified by NMR spectroscopy and supported by DFT calculations. The attachment of B(C6F5)3 in 2a,b establishes cooperative CO2 activation via the Re-H/B(C6F5)3 Lewis pair, with the Re-H bond playing the role of a Lewis base. The Re(I) η(1)-formato dimer [{Re(μ-Br)(NO)(η(1)-OCH═O-B(C6F5)3)(PiPr3)2}2] (3a) was generated from 2a and represents the first example of a stable rhenium complex bearing two cis-aligned, sterically bulky PiPr3 ligands. Reaction of 3a with H2 cleaved the μ-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH2(NO)(η(1)-OCH═O-B(C6F5)3)(PiPr3)2] (4a) bearing trans-phosphines. Stoichiometric CO2 reduction of 4a with Et3SiH led to heterolytic splitting of H2 along with formation of bis(triethylsilyl)acetal ((Et3SiO)2CH2, 7). Catalytic reduction of CO2 with Et3SiH was also accomplished with the catalysts 1a,b/B(C6F5)3, 3a, and 4a, showing turnover frequencies (TOFs) between 4 and 9 h(-1). The stoichiometric reaction of 4a with the sterically hindered base 2,2,6,6-tetramethylpiperidine (TMP) furnished H2 ligand deprotonation. Hydrogenations of CO2 using 1a,b/B(C6F5)3, 3a, and 4a as catalysts gave in the presence of TMP TOFs of up to 7.5 h(-1), producing [TMPH][formate] (11). The influence of various bases (R2NH, R = iPr, Cy, SiMe3, 2,4,6-tri-tert-butylpyridine, NEt3, PtBu3) was studied in greater detail, pointing to two crucial factors of the CO2 hydrogenations: the steric bulk and the basicity of the base.

Complexes of M(III) & M(II) transition metals with N, N'-ethylene-bis-(2-amino-benzamide) have been prepared and characterized by elemental analyses, molar condcutivity, magnetic susceptibility measurments, thermal studies , IR and electronic spectra. Based on these studies octahedral geometry has been proposed for these complexes.

Biologically active schiff base ligand, N’-2-[(Z)-1-(4-hydroxy-6-methyl-2-oxo-2Hpyranyl)ethylidene]-furancarbohydrazide and its M(III) complexes have been prepared and characterized by elemental analyses, molar conductance, magnetic susceptibility measurement, UV-Vis, IR and NMR spectra. Based on these studies an octahedral geometry has been proposed for these complexes. The ligand and its metal complexes were screened for their antimicrobial activities. The ligand showed higher activity than its corresponding M (III) complexes.

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Bronsted–Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant KaLAC of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKaLAC and pKaTHF, pKaDCM, pKaMeCN for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKaLAC −18) to least acidic (pKaLAC 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid–base features are analyzed for c...

The homobimetallic ruthenium(II) and osmium(II) complexes [{RuR(CO)(PPh(3))(2)}(2)(S(2)COCH(2)C(6)H(4)CH(2)OCS(2))] (R = CH=CHBu(t), CH=CHC(6)H(4)Me-4, C(C[triple bond]CPh)=CHPh, CH=CHCPh(2)OH) and [{Os(CH=CHC(6)H(4)Me-4)(CO)(PPh(3))(2)}(2)(S(2)COCH(2)C(6)H(4)CH(2)OCS(2))] form readily from the reactions of [MRCl(CO)(BTD)(PPh(3))(2)] (M = Ru or Os; BTD = 2,1,3-benzothiadiazole) with the dixanthate KS(2)COCH(2)C(6)H(4)CH(2)OCS(2)K. Addition of KS(2)COCH(2)C(6)H(4)CH(2)OCS(2)K to two equivalents of cis-[RuCl(2)(dppm)(2)] leads to the formation of [{(dppm)(2)Ru}(2)(S(2)COCH(2)C(6)H(4)CH(2)OCS(2))](2+). The benzoate complexes [RuR{O(2)CC(6)H(4)(CH(2)OH)-4}(CO)(PPh(3))(2)] (R = CH=CHBu(t), CH=CHC(6)H(4)Me-4, C(C[triple bond]CPh)=CHPh) are obtained by treatment of [RuRCl(CO)(BTD)(PPh(3))(2)] with 4-(hydroxymethyl)benzoic acid in the presence of base. Reaction of [RuHCl(CO)(PPh(3))(3)] or [RuRCl(CO)(BTD)(PPh(3))(2)] with 4-(hydroxymethyl)benzoic acid in the absence of base leads to formation of the chloride analogue [RuCl{O(2)CC(6)H(4)(CH(2)OH)-4}(CO)(PPh(3))(2)]. The unsymmetrical complex [{Ru(CH=CHC(6)H(4)Me-4)(CO)(PPh(3))(2)}(2)(O(2)CC(6)H(4)CH(2)OCS(2))] forms from the sequential treatment of [Ru(CH=CHC(6)H(4)Me-4){O(2)CC(6)H(4)(CH(2)OH)-4}(CO)(PPh(3))(2)] with base, CS(2) and [Ru(CH=CHC(6)H(4)Me-4)Cl(CO)(BTD)(PPh(3))(2)]. The new mixed-donor xanthate-carboxylate ligand, KO(2)CC(6)H(4)CH(2)OCS(2)K is formed by treatment of 4-(hydroxymethyl)benzoic acid with excess KOH and two equivalents of carbon disulfide. This ligand reacts with two equivalents of [Ru(CH=CHC(6)H(4)Me-4)Cl(BTD)(CO)(PPh(3))(2)] or cis-[RuCl(2)(dppm)(2)] to yield [{(dppm)(2)Ru}(2)(O(2)CC(6)H(4)CH(2)OCS(2))](2+) or [{Ru(CH=CHC(6)H(4)Me-4)(CO)(PPh(3))(2)}(2)(O(2)CC(6)H(4)CH(2)OCS(2))], respectively. Electrochemical experiments are also reported in which communication between the metal centres is investigated.

Photolytic ligand displacement and salt metathesis routes have been exploited to give access to κ(1) σ-alane complexes featuring Al-H bonds bound to [W(CO)(5)] and [Cp'Mn(CO)(2)] fragments, together with a related κ(2) complex of [Cr(CO)(4)]. Spectroscopic, crystallographic, and quantum chemical studies are consistent with the alane ligands acting predominantly as σ-donors, with the resulting binding energies calculated to be marginally greater than those found for related dihydrogen complexes.

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted-Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant Ka(LAC) of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKa(LAC) and pKa(THF), pKa(DCM), pKa(MeCN) for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa(LAC) -18) to least acidic (pKa(LAC) 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid-base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.

Hydrogenation and transfer hydrogenation of imines with cyclohexa-1,4-dienes, as well as with a representative Hantzsch ester dihydrogen surrogate, are reported. Both processes are catalyzed by tethered Ru-S complexes but differ in the activation mode of the dihydrogen source: cooperative activation of the H-H bond at the Ru-S bond leads to the corresponding Ru-H complex and protonation of the sulfur atom, whereas the same cationic Ru-S catalyst abstracts a hydride from a donor-substituted cyclohexa-1,4-diene to form the neutral Ru-H complex and a low-energy Wheland intermediate. A sequence of proton and hydride transfers on the imine substrate then yields an amine. The reaction pathways are analyzed computationally, and the established mechanistic pictures are in agreement with the experimental observations.

The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(II) and -zinc(I) sources ZnMe2 (Me = methyl) and [Zn2Cp*2] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [LaM], yielding [Lb(ZnR)n]. The addition of the dizinc compound [Zn2Cp*2] to coordinatively unsaturated [LaM] by the homolytic cleavage of the Zn–Zn bond can be compared to the classic oxidative addition reaction of H2, forming dihydride complexes [LaM(H)2]. It has also been widely shown that dihydrogen coordinates under preservation of the H–H bond in the case of certain electronic properties of the transition metal fragment. The σ-aromatic triangular clusters [Zn3Cp*3]+ and [Zn2CuCp*3] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn2Cp*2]. With this background in mind the question arises if a series of complexes featuring the Zn2M structural motif can be prepared exhibiting a (more or less) intact Zn–Zn interaction, i.e. di-zinc complexes which are analogous to non-classical dihydrogen complexes of the Kubas type. In order to probe this idea, a series of interrelated organozinc nickel and palladium complexes and clusters were synthesized and characterized as model compounds: [Ni(ZnCp*)(ZnMe)(PMe3)3] (1), [Ni(ZnCp*)2(ZnMe)2(PMe3)2] (2), [{Ni(CNtBu)2(μ2-ZnCp*)(μ2-ZnMe)}2] (3), [Pd(ZnCp*)4(CNtBu)2] (4) and [Pd3Zn6(PCy3)2(Cp*)4] (5). The dependence of Zn⋯Zn interactions as a function of the ligand environments and the metal centers was studied. Experimental X-ray crystallographic structural data and DFT calculations support the analogy between dihydrogen and dizinc transition metal complexes.

Dihydrogen activation by a triruthenium μ3-imido complex, (CpRu)3(μ3-NH)(μ-H)3 (1; Cp = η5-C5H5), was theoretically investigated with the DFT method. Each of the three Ru centers can react with dihydrogen via a transition state, (CpRu)3(μ3-NH)(η2-H2)(μ-H)3 (TS1), with a moderate energy barrier. This TS1 corresponds to a transition state for the approach of a dihydrogen molecule to one of the Ru centers. After TS1, the hydrogen–hydrogen bond cleavage occurs without any other energy barrier to produce the pentahydrido intermediate, (CpRu)3(μ3-NH)(μ-H)3(H)2 (INT1), without formation of a dihydrogen complex as an intermediate. In the hydrogen–hydrogen cleavage, the dπ orbitals of three ruthenium centers overlap with the hydrogen–hydrogen σ* orbital to form the charge transfer from the ruthenium to the hydrogen–hydrogen moiety. Natural population analysis indicates that all three ruthenium centers cooperatively participate in the changes in electronic structure during this process. INT1 further undergoes a nit...

Density functional theory (DFT) calculations have been carried out to explore the catalytic activation of C–H bonds in methane by the iron atom, Fe, and the iron dimer, Fe2. For methane activation on an Fe atom, the calculations suggest that the activation of the first C–H bond is mediated via the triplet excited-state potential energy surface (PES), with initial excitation of Fe to the triplet state being necessary for the reaction to be energetically feasible. Compared with the breaking of the first C–H bond, the cleavage of the second C–H bond is predicted to involve a significantly higher barrier, which could explain experimental observations of the HFeCH3 complex rather than CH2FeH2 in the activation of methane by an Fe atom. For methane activation on an iron dimer, the cleavage of the first C–H bond is quite facile with a barrier only 11.2, 15.8 and 8.4 kcal/mol on the septet state energy surface at the B3LYP/6-311+G(2df,2dp), BPW91/6-311+G(2df,2dp) and M06/B3LYP level, respectively. Cleavage of the second C–H bond from HFe2CH3 involves a barrier calculated respectively as 18.0, 10.7 and 12.4 kcal/mol at the three levels. The results suggest that the elimination of hydrogen from the dihydrogen complex is a rate-determining step. Overall, our results indicate that the iron dimer Fe2 has a stronger catalytic effect on the activation of methane than the iron atom.