Hydride Complexes Research Articles

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex.

Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]- which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]- fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3- fragment, which undergoes subsequent deoxygenation.

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex

AbstractAddition of CO to a tetrametallic magnesium hydride cluster results in both carbon‐carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]− which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]− fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3− fragment, which undergoes subsequent deoxygenation.

Advances in highly hydrided palladium

Palladium is a prototypical hydride-forming metal, which can accommodate a large volume of hydrogen through the formation of either interstitial or complex hydrides. Interstitial palladium hydrides, especially those with exceptionally high hydrogen loadings, have attracted considerable interest from the low-energy nuclear reaction (LENR) community, as they have been invoked to explain the anomalous nuclear effects related to the known but controversial Pons-Fleischmann experiment. Complex palladium hydrides also constitute a class of solid-state hydrides that present stoichiometric PdH2, PdH3, or PdH4 units within the crystal structure, but remain unexplored as far as the unusual H/Pd ratio is concerned. This minireview gives a brief introduction to these two types of solid-state palladium hydrides, with the hope of providing some information for materials development relevant to LENR research.

Exploring Spin-Orbit Effects in a [Cu6Tl]+ Nanocluster Featuring an Uncommon Tl-H Interaction.

Reaction of [CuH(PPh3)]6 with 1 equiv. of Tl(OTf) results in formation of [Cu6TlH6(PPh3)6][OTf] ([1]OTf]), which can be isolated in good yields. Variable-temperature 1H NMR spectroscopy, in combination with density functional theory (DFT) calculations, confirms the presence of a rare Tl-H orbital interaction. According to DFT, the 1H chemical shift of the Tl-adjacent hydride ligands of [1]+ includes 7.7 ppm of deshielding due to spin-orbit effects from the heavy Tl atom. This study provides valuable new insights into a rare class of metal hydrides, given that [1][OTf] is only the third isolable species reported to contain a Tl-H interaction.

Mechanistic study of alkene hydrogenation catalyzed by cationic Ni(II)-hydride PNHP complexes – Ni(II/IV) vs. Ni(II) cycle

The hydrogenation mechanisms of alkenes catalyzed by square-planar cationic Ni(II)-PNHP hydride complexes have been computed (M06L-GD3-SCRF). It is found that the reaction prefers a non-bifunctional inner-sphere mechanism, the Ni(II)/Ni(IV) cycle involving H2 oxidative addition forming unusual Ni(IV) oxidation state and reductive elimination is accessible and has been proven to be superior or competitive with the widely accepted Ni(II) cycle undergoing concerted H2 σ-bond metathesis in formal Ni(II) oxidation state. Systematic investigations into the electronic and geometric effects of phosphine ligand substituents uncover their significant influence on the Gibbs free energy barrier of elementary steps, leading to differences in catalytic activity. Analysis into these effects demonstrates a strong correlation between the apparent free energy barrier and frontier orbital energy, natural charge of Ni-center and buried volume ratio of the catalyst. These findings offer valuable insights for the design of future catalysts.

Monomeric gold hydrides for carbon dioxide reduction: ligand effect on the reactivity.

We analyzed the ligand electronic effect in the reaction between a [LAu(I)H]0/- hydride species and CO2, leading to a coordinated formate [LAu(HCOO)]0/-. We explored 20 different ligands, such as carbenes, phosphines and others, carefully selected to cover a wide range of electron-donor and -acceptor properties. We included in the study the only ligand, an NHC-coordinated diphosphene, that, thus far, experimentally demonstrated facile and reversible reaction between the monomeric gold(I) hydride and carbon dioxide. We elucidated the previously unknown reaction mechanism, which resulted to be concerted and common to all the ligands: the gold-hydrogen bond attacks the carbon atom of CO2 with one oxygen atom coordinating to the gold center. A correlation between the ligand σ donor ability, which affects the electron density at the reactive site, and the kinetic activation barriers of the reaction has been found. This systematic study offers useful guidelines for the rational design of new ligands for this reaction, while suggesting a few promising and experimentally accessible potential candidates for the stoichiometric or catalytic CO2 activation.

Effect of Ligand Substituents on Spectroscopic and Catalytic Properties of Water-Compatible Cp*Ir-(pyridinylmethyl)sulfonamide-Based Transfer Hydrogenation Catalysts.

Transition-metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups were synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable-temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereoinversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable-temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step in the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.

Theoretical prediction of closed-shell paramagnetism for scandium and yttrium hydride.

Following chemical intuition, one would expect that all closed-shell molecules are diamagnetic. However, it is known that this is not the case for some second-row hydrides with low-lying unoccupied orbitals due to an unquenching of the total angular momentum in the presence of an external magnetic field. In this article, the transition-metal hydrides ScH and YH are investigated, assuming a similar unquenching effect involving low-lying unoccupied and orbitals formed from the metal orbitals rather than the orbitals. We are comparing results obtained with various quantum-chemical methods (HF, CCSD, CCSD(T), CCSDT) and basis sets. The obtained positive values for the magnetizabilities clearly indicate paramagnetic behavior. Vibrational effects on the magnetizability tensor are also considered, but these effects are small and do not change the overall conclusion that both ScH and YH are further examples for closed-shell paramagnetism.

Isolation of an Osmium(III) Hydrido Complex

AbstractSynthesis and characterization of [OsH(PNP)(bipy)]+ (PNP=N(CHCHPtBu2)2) are reported, as the second osmium(III) hydrido complex next to [OsH(en)2(py)]2+. While the osmium(II) precursor, [OsH(PNP)(bipy)], exhibits two reversible oxidations in the cyclic voltammogram, the product from 2‐electron oxidation could not be isolated due to rapid decay. Thermochemical examinations attribute the stability of the osmium(III) hydride to both thermodynamic and kinetic reasons. In contrast, the high estimated acidity of [OsH(PNP)(bipy)]2+ presumably leads to rapid decay.

Synthesis, Structure, and Reactivity of Molybdenum- and Tungsten-Indane Complexes with Tris(pyrazolyl)borate Ligand.

The reaction of molybdenum complexes with a tris(pyrazolyl)borate ligand (Et4N[TpMo(CO)3] and Et4N[Tp*Mo(CO)3] (Tp = hydridotris(pyrazolyl)borate, Tp* = hydridotris(3,5-dimethylpyrazolyl)borate)) and InBr3 at a 1:1 molar ratio afforded molybdenum-indane complexes (Et4N[TpMo(CO)3(InBr3)] 1 and Et4N[Tp*Mo(CO)3(InBr3)] 2). In addition, tungsten-indane complexes, Et4N[TpW(CO)3(InBr3)] 3 and Et4N[Tp*W(CO)3(InBr3)] 4, were obtained by the reaction of corresponding tungsten complexes. Complex 4 reacted with H2O to form the hydrido complex Tp*W(CO)3H, in which the W-In bond was cleaved. On the other hand, 4 reacted with three equiv. of AgNO3 to form Et4N[Tp*W(CO)3{In(ONO2)}] 5, in which three substituents on the In were exchanged while retaining the W-In dative bond. Complexes 1-5 were fully characterized using NMR measurements and elemental analyses, and the structures of 1-5 and Et4N[Tp*W(CO)3] were determined via X-ray crystallography. These are the first examples of mononuclear molybdenum- and tungsten-indane complexes with Mo-In and W-In dative bonds.

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst.

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Insertion of Molecular Oxygen into a Gold(III)-Hydride Bond.

The use of molecular oxygen as an oxidant in chemical synthesis has significant environmental and economic benefits, and it is widely used as such in large-scale industrial processes. However, its adoption in highly selective homogeneous catalytic transformations, particularly to produce oxygenated organics, has been hindered by our limited understanding of the mechanisms by which O2 reacts with transition metals. Of particular relevance are the mechanisms of the reactions of oxygen with late transition metal hydrides as these metal centers are better poised to release oxygenated products. Homogeneous catalysis with gold complexes has markedly increased, and herein we report the synthesis and full characterization of a rare AuIII-H, supported by a diphosphine pincer ligand (tBuPCP = 2,6-bis(di-tert-butylphosphinomethyl)benzene). [(tBuPCP)AuIII-H]+ was found to cleanly react with molecular oxygen to yield a stable AuIII-OOH complex that was also fully characterized. Extensive kinetic studies on the reaction via variable temperature NMR spectroscopy have been completed, and the results are consistent with an autoaccelerating radical chain mechanism. The observed kinetic behavior exhibits similarities to that of previously reported PdII-H and PtIV-H reactions with O2 but is not fully consistent with any known O2 insertion mechanism. As such, this study contributes to the nascent fundamental understanding of the mechanisms of aerobic oxidation of late metal hydrides.

Thermodynamic and kinetic study of the thermal dehydrogenation of Sr(AlH4)2 taking into account the by-products NaCl and LiCl

The presented work sets out to investigate the influence of LiCl and NaCl, the typical by-products of the mechanochemical synthesis of complex metal hydrides such as alanates and boranates, on the kinetics and thermodynamics of the dehydrogenation of Sr(AlH4)2. We found no effect of NaCl on the decomposition pathway of Sr(AlH4)2. Although LiCl does not affect the first dehydrogenation to SrAlH5, it does alter the further decomposition to the final products. Thermodynamic properties such as the heat capacity functions, enthalpies of formation and absolute entropies were determined for Sr(AlH4)2 itself and its decomposition product SrAlH5 within this study.

Stabilization of NH- Group Adjacent to Naked Silicon(II) Atom in Base Stabilized Aminosilylenes.

Aminosilylene, comprising reactive NH- and Si(II) sites next to each other, is an intriguing class of compounds due to its ability to show diverse reactivity. However, stabilizing the reactive NH- group next to the free Si(II) atom is challenging and has not yet been achieved. Herein, we report the first examples of base stabilized free aminosilylenes Ar*NHSi(PhC(Nt Bu)2 ) (1 a) and Mes*NHSi(PhC(Nt Bu)2 ) (1 b) (Ar*=2,6-dibenzhydryl-4-methylphenyl and Mes*=2,4,6-tri-tert-butylphenyl), tolerating a NH- group next to the naked Si(II) atom. Remarkably, 1 a and 1 b exhibited interesting differences in their reactivity upon heating. With 1 a, an intramolecular C(sp3 )-H activation of one of the benzhydryl methine hydrogen atoms to the Si(II) atom produced the five-membered cyclic silazane 2. However, with 1 b, a rare 1,2-hydrogen shift to the Si(II) atom afforded a silanimine 3, with a hydride ligand attached to an unsaturated silicon atom. Further, the coordination capabilities of 1 a were also tested with Ru(II) and Fe(0) precursors. Treatments of 1 a with [Ru(η6 -p-cymene)Cl2 ]2 led to the isolation of a η6 -arene tethered complex [RuCl2 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si-η6 -arene}] (4), whereas with the Fe(CO)5 precursor a Fe(0) complex [Fe(CO)4 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si}] (5) was obtained. Density functional theory (DFT) calculations were conducted to shed light on the structural, bonding, and energetic aspects in 1-5.

Stabilization of NH− Group Adjacent to Naked Silicon(II) Atom in Base Stabilized Aminosilylenes

AbstractAminosilylene, comprising reactive NH− and Si(II) sites next to each other, is an intriguing class of compounds due to its ability to show diverse reactivity. However, stabilizing the reactive NH− group next to the free Si(II) atom is challenging and has not yet been achieved. Herein, we report the first examples of base stabilized free aminosilylenes Ar*NHSi(PhC(NtBu)2) (1 a) and Mes*NHSi(PhC(NtBu)2) (1 b) (Ar*=2,6‐dibenzhydryl‐4‐methylphenyl and Mes*=2,4,6‐tri‐tert‐butylphenyl), tolerating a NH− group next to the naked Si(II) atom. Remarkably, 1 a and 1 b exhibited interesting differences in their reactivity upon heating. With 1 a, an intramolecular C(sp3)−H activation of one of the benzhydryl methine hydrogen atoms to the Si(II) atom produced the five‐membered cyclic silazane 2. However, with 1 b, a rare 1,2‐hydrogen shift to the Si(II) atom afforded a silanimine 3, with a hydride ligand attached to an unsaturated silicon atom. Further, the coordination capabilities of 1 a were also tested with Ru(II) and Fe(0) precursors. Treatments of 1 a with [Ru(η6‐p‐cymene)Cl2]2 led to the isolation of a η6‐arene tethered complex [RuCl2{Ar*NHSi(PhC(tBuN)2)‐κ1‐Si‐η6‐arene}] (4), whereas with the Fe(CO)5 precursor a Fe(0) complex [Fe(CO)4{Ar*NHSi(PhC(tBuN)2)‐κ1‐Si}] (5) was obtained. Density functional theory (DFT) calculations were conducted to shed light on the structural, bonding, and energetic aspects in 1–5.

Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer.

Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation. We describe the solution characterization of an elusive cobalt(III) fluoride complex, which surprisingly is not the species that reacts with silane under catalytic conditions; rather, a cobalt(III) aquo complex is more active. Accordingly, the addition of water (0.15 M) speeds the catalytic reaction, and kinetic studies show that water addition enables catalytic product formation in 2 h at -50 °C in acetone. Under these conditions, cobalt(III) resting states can be observed by UV-vis spectrophotometry, including a cobalt(III)-alkyl complex. It comes from a transient cobalt(III) hydride complex that is formed in the turnover-limiting step of the catalytic cycle. This hydride readily degrades but not to H2; it releases H+ through a bimetallic pathway that explains the [Co]2 dependence of the off-cycle reaction. In contrast, the rate of the catalytic reaction follows the power law kobs[Co]1[silane]1. Because of the different [Co] dependence of the catalytic reaction and the degradation reaction, lower catalyst loading improves the yield of the catalytic reaction by reducing the relative rate of unproductive silane/oxidant consumption. These studies illuminate mechanistic details of oxidative MHAT hydrofunctionalization of alkenes and lay the groundwork for understanding other catalytic reactions mediated by cobalt hydride and cobalt alkyl complexes.

Organoplatinum Chemistry Related to Alkane Oxidation: The Effect of a Nitro Substituent in Ligands Having an Appended Phenol Group

The organoplatinum chemistry of the ligands 2-C5H4N-CH2-NH-C6H3-2-OH-5-X (L1, X = H; L3, X = NO2) and 2-C5H4N-CH=N-C6H3-2-OH-5-X (L2, X = H; L4, X = NO2), which contain an appended phenol substituent, is described. Comparisons are made between the ligands with amine or imine groups (L1, L3 vs. L2, L4) and ligands with X = H or NO2 (L1, L2 vs. L3, L4), and major differences are observed. Thus, on reaction with the cycloneophylplatinum(II) complex [{Pt(CH2CMe2C6H4)(μ-SMe2)}2], ligands L1, L2 and L4 give the corresponding platinum(II) complexes [Pt(CH2CMe2C6H4)(κ2-N,N′-L)], containing a Pt··HO hydrogen bond, whereas L3 gives a mixture of isomeric platinum(IV) hydride complexes [PtH(CH2CMe2C6H4)(κ3-N,N′,O-L3-H)], which are formed by oxidative addition of the phenol O-H bond and which react further with oxygen to give [Pt(OH)(CH2CMe2C6H4)(κ3-N,N′,O-L3-H)]. The differences in reactivity are proposed to be due to the greater acidity of the nitro-substituted phenol groups in L3 and L4 and to the greater ability of the deprotonated amine ligand L3 over L4 to stabilize platinum(IV) by adopting the fac-κ3-N,N′,O-L3-H coordination mode.

RETRACTED: Nanoscale silicon porous materials for efficient hydrogen storage application

The increasing energy demand and the worldwide energy crisis must be met in large part via the sustainable growth of hydrogen energy, since this economy is relied upon to provide clean and carbon-free energy carriers. It is anticipated that the growing demand for light and heavy fuel cell cars would stimulate the development of onboard solid-state hydrogen technology. The research here concentrates on the importance of various permeable substances such as polymers in general, metallic substances, and complex metal hydrides, even if Si nanostructures (SiNSs) have become known as the obvious contender for solid-state hydrogen storage systems. SiNSs have gained prominence as a leading candidate for solid-state hydrogen storage systems. Renowned for their high storage capacity, SiNSs, including silicon nanowires and quantum dots, exhibit promising potential in addressing challenges associated with traditional hydrogen storage methods, positioning them as a key player in advancing clean and efficient energy storage technologies. SiNSs play a crucial role in advancing solid-state hydrogen storage technology. SiNSs, including silicon nanowires and quantum dots, exhibit high storage capacity. Despite challenges like surface oxidation, SiNS holds promise for efficient hydrogen storage, contributing to the development of sustainable energy solutions and mitigating the environmental impact associated with conventional automotive technologies. We focus on the processes that result in permeable silicon, nanowires made of porous silicon, and Si quantum dots. This investigation elucidates the characteristics and patterns of hydrogen's aid, the value of hydrogen power in automobiles for reducing global warming occurrences, and the potential of using SiNSs for hydrogen storage in tandem with other forms of transition and alkali earth materials to meet these difficulties. It demonstrates how catalysts are critical to fixing the current reversibility and desorption problems with hydrogen energy storage. As a result of the analysis, energy suppliers and Si-based fuel cells may be better able to tailor their services to individual customers' needs, which might boost the growth of the hydrogen energy industry.

Accurate measurement of hydrogen concentration in transition metal hydrides utilizing electronic excitations by MeV ions

This study focuses on enhancing the accuracy of hydrogen content verification in hydrogen-rich ultrathin materials relevant for sustainable energy applications. Ion beams are used for distinctive real-space detection leveraging elastic and inelastic interactions with hydrogen atoms. However, the lack of experimental reference data on electronic interactions poses a challenge to the accuracy of analytical techniques. We investigate the effect of absorbed hydrogen on the electronic energy deposition of 15N-ions in amorphous transition metal compounds, specifically V and Zr, covering concentrations >1H/M. Employing resonant nuclear reactions and Rutherford backscattering, the energy loss is found to increase considerably with hydrogen content, in line with Bragg's additivity. The electronic energy loss cross section for 15N-ions at 6.5 MeV measured (64.55±3.38) eV cm2/1015 atoms. Results are compared to semi-empirical and theoretical models. The findings improve hydrogen profiling accuracy using 15N-nuclear reaction analysis and enable unprecedented methods for hydrogen quantification by other, commonly available ion beams.

Understanding electronic structure tunability by metal dopants for promoting MgB2 hydrogenation

Hydrogen is a promising energy carrier, but its onboard application is limited by the need for compact, low-pressure storage solutions. Solid-state complex metal hydride systems, such as MgB2/Mg(BH4)2, offer high storage capacities but suffer from sluggish kinetics and poor reversibility. One avenue for improving reactivity is to introduce metal dopants to alter electronic and atomic properties, but the role of these chemical additives remains poorly understood, particularly for the hydrogenation reaction. In this work, we used density functional theory calculations on model MgB2 systems to rationalize the potential role of metal dopants in destabilizing B–B bonding within the MgB2 lattice. We carried out detailed electronic structure analyses for 28 different metal dopant adatoms to identify properties that contribute to a dopant’s efficacy. Based on the simulation results, we propose that an intermediate ionic and covalent character of the bonds between adatoms and B atoms is desirable for facilitating charge redistribution, disrupting the B–B bond network, and promoting H2 dissociation and H atom chemisorption on MgB2.