Infrared Spectra and DFT Calculations for the Coinage Metal Hydrides MH, (H2)MH, MH2, M2H, M2H-, and (H2)CuHCu in Solid Argon, Neon, and Hydrogen

Detection and Characterization of Hydride Ligands in Copper Complexes by Hard X-Ray Spectroscopy

Transition metal complexes, particularly copper hydrides, play an important role in various catalytic processes and molecular inorganic chemistry. This study employs synchrotron hard X-ray spectroscopy to gain insights into the geometric and electronic properties of copper hydrides as potential catalysts for CO2 hydrogenation. The potential of high energy resolution X-ray absorption near-edge structure (HERFD-XANES) and valence-to-core X-ray emission (VtC-XES) is demonstrated with measurement on Stryker's reagent (Cu6H6) and [Cu3(μ3-H)(dpmppe)2](PF6)2 (Cu3H), alongside a non-hydride copper compound ICu(dtbppOH) (Cuy-I). The XANES analysis reveals that coordination geometries strongly influence the spectra, providing only indirect details about hydride coordination. The VtC-XES analysis exhibits a distinct signal around 8975 eV, offering a diagnostic tool to identify hydride ligands. Theoretical calculations support and extend these findings by comparing hydride-containing complexes with their hydride-free counterparts.

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

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

Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements

The change in sign of the interaction force constant between element-hydrogen stretching modes of trans-dihydrides of the d block and p block elements is analyzed for the first time. As the transition metal M approaches group 12, the higher energy symmetric trans-H-M-H vibration ν_sym approaches the energy of the antisymmetric vibration ν_asym. Crossing to group 13 elements E, the trans-H-E-H vibration ν_sym increasingly drops below ν_asym. This reversal is attributed to the d orbital that participates in the H-M-H bonding but is nonbonding in the H-E-H compounds. DFT calculations are used to probe the energetics of isoelectronic triatomic [H-M-H]ⁿ⁺ and [H-E-H]^n- to reveal this trend and also to demonstrate that the magnitude of these interactions (ν_gap) increases down groups 11, 12, and 14 but remains fairly constant for group 13. They are also used to show that this reversal is seen in the transition state for hydride transfer to CO₂ from the model compounds trans-NiH₂(porphyrin) and trans-EH₂(porphyrin), E = Si and Ge in their singlet states.

Spectroscopic/Bond Property Relationship in Group 11 Dihydrides via Relativistic Four-Component Methods

Group 11 dihydrides MH2- (M = Cu, Ag, Au, Rg) have been much less studied than the corresponding MH compounds, despite having potentially several interesting applications in chemical research. In this work, their main spectroscopic constants (bond lengths, dissociation energies, and force constants) have been evaluated by means of highly accurate relativistic four-component coupled cluster (4c-CCSD(T)) calculations in combination with large basis sets. Periodic trends have been quantitatively explained by the charge-displacement/natural orbitals for chemical valence (CD-NOCV) analysis based on the four-component relativistic Dirac-Kohn-Sham method, which allows a consistent picture of the nature of the M-H bond to be obtained on going down the periodic table in terms of Dewar-Chatt-Duncanson bonding components. A strong ligand-to-metal donation drives the M-H bond and it is responsible for the heterolytic (HM···H-) dissociation energies to increase monotonically from Cu to Rg, with RgH2- showing the strongest and most covalent M-H bond. The "V"-shaped trend observed for the bond lengths, dissociation energies, and stretching frequencies can be explained in terms of relativistic effects and, in particular, of the relativistically enhanced sd hybridization occurring at the metal, which affects the metal-ligand distances in heavy transition-metal complexes. The sd hybridization is very small for Cu and Ag, whereas it becomes increasingly important for Au and Rg, being responsible for the increasing covalent character of the bond, the sizable contraction of the Au-H and Rg-H bonds, and the observed trend. This work rationalizes the spectroscopic/bond property relationship in group 11 dihydrides within highly accurate relativistic quantum chemistry methods, paving the way for their applications in chemical bond investigations involving heavy and superheavy elements.

Synergistic electroreduction of carbon dioxide to carbon monoxide on bimetallic layered conjugated metal-organic frameworks

Highly effective electrocatalysts promoting CO2 reduction reaction (CO2RR) is extremely desirable to produce value-added chemicals/fuels while addressing current environmental challenges. Herein, we develop a layer-stacked, bimetallic two-dimensional conjugated metal-organic framework (2D c-MOF) with copper-phthalocyanine as ligand (CuN4) and zinc-bis(dihydroxy) complex (ZnO4) as linkage (PcCu-O8-Zn). The PcCu-O8-Zn exhibits high CO selectivity of 88%, turnover frequency of 0.39 s-1 and long-term durability (>10 h), surpassing thus by far reported MOF-based electrocatalysts. The molar H2/CO ratio (1:7 to 4:1) can be tuned by varying metal centers and applied potential, making 2D c-MOFs highly relevant for syngas industry applications. The contrast experiments combined with operando spectroelectrochemistry and theoretical calculation unveil a synergistic catalytic mechanism; ZnO4 complexes act as CO2RR catalytic sites while CuN4 centers promote the protonation of adsorbed CO2 during CO2RR. This work offers a strategy on developing bimetallic MOF electrocatalysts for synergistically catalyzing CO2RR toward syngas synthesis.

First multi-tool exploration of a gas-condensate-pyrolysate system from the environment of burning coal mine heaps: An in situ FTIR and laboratory GC and PXRD study based on Upper Silesian materials

A methodological approach to the complex geochemical analysis of the coal fire in burning coal mine heaps (BCMH) of Upper Silesian Coal Basin has been developed. The other approach used is gas chromatography and indicatory tubes. Powder X-Ray Diffraction is applied for phase analysis to determine the species composition of mineral condensates present within and around gas flues. The gas compositions are proved to be extremely variable, when comparing both different BCMH and flues or flue zones of the same heaps. One outstanding determination concerns GeCl₄, found in most samples often in large quantities. No evident dependence between the gas and mineral condensate compositions is found: N-rich condensates may but do not have to be associated with NH₃-, pyridine-, or NO_x-rich gases. This is also true for S-rich and Cl-rich mineralization in connection with gases of SO₂, H₂S, OCS, CS₂, thiophene, dimethyl sulfide, dimethyl disulfide, HCl, and various halogenated hydrocarbons. Fluorine is rarely present as HF, whereas SiF₄ occurs more frequently and in much larger quantities. AsH₃ is mainly a trace gas but may locally be enriched. Besides the common gases, a number of trace gases is also determined based on residual FTIR spectra. Those with the highest presence chance include cyanogen isocyanate, cyanogen N-oxide, (iso)cyanic acid, c-cyanomethanimine (ethylenediimine), isocyanatomethane, iodocyanoacetylene, acetonitrile, acetaldehyde, m-hydroxybenzonitrile (m-cyanophenol), isonitrosyl chloride, nitrosyl isocyanide, difluorosilane, pentacene, triphenylene, thiazolidine, cyclohexane, and a trinitrenetriazine. The occurrence of some metals and semimetals (e.g., Al, Mg, Ga) as neutral hydroxides, suggested by other authors to occur in natural gases, is possibly confirmed. The presence of trace metal carbonyls, nitrosyls and hydrides is also possible.

Watching Visible Light-Driven CO2 Reduction on a Plasmonic Nanoparticle Catalyst

Photocatalytic reduction of carbon dioxide (CO₂) by visible light has the potential to mimic plant photosynthesis and facilitate the renewable production of storable fuels. Accomplishing desirable efficiency and selectivity in artificial photosynthesis requires an understanding of light-driven pathways on photocatalyst surfaces. Here, we probe with single-nanoparticle spatial resolution the dynamics of a plasmonic silver (Ag) photocatalyst under conditions of visible light-driven CO₂ reduction. In situ surface-enhanced Raman spectroscopy captures discrete adsorbates and products formed dynamically on single photocatalytic nanoparticles, most prominent among which is a surface-adsorbed hydrocarboxyl (HOCO*) intermediate critical to further reduction of CO₂ to carbon monoxide (CO) and formic acid (HCOOH). Density functional theory simulations of the captured adsorbates reveal the mechanism by which plasmonic excitation activates physisorbed CO₂ leading to the formation of HOCO*, indicating close interplay between photoexcited states and adsorbate/metal interactions.

Gold(I) Hydrides as Proton Acceptors in Dihydrogen Bond Formation

Wavefunction and DFT calculations indicate that anionic dihydride complexes of Au^I form strong to moderate directed Au-H⋅⋅⋅H bonds with one or two HF, H₂ O and NH₃ prototype proton donor molecules. The largely electrostatic interaction is influenced by relativistic effects which, however, do not increase the binding energy. Very weak Au⋅⋅⋅H associations-exhibiting a corresponding bond path-occur between neutral AuH and HF units, although ultimately F becomes the preferred donor atom in the most stable structure. Increasing the hydridicity of AuH by attachment of an electron donating NHC ligand effects Au-H⋅⋅⋅H bonding of moderate strength only with HF, whereas competing Au⋅⋅⋅H interactions dominate for H₂ O and NH₃ . Rare η² coordinated and HX (X=F or OH) associated H₂ complexes are produced during interaction with a single ion of stronger acidity, H₂ F⁺ or H₃ O⁺ . Theoretically, reaction of excess [AuH₂ ]^- as proton acceptor with H₃ O⁺ or NH₄⁺ in 3:1 or 4:1 ionic ratios, respectively, affords H⋅⋅⋅H bonded analogues of Eigen-type adducts. Outstanding analytical relationships between selected bonding parameters support the integrity of the results.

Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity

Hydride complexes of copper, silver, and gold encompass a broad array of structures, and their distinctive reactivity has enabled dramatic recent advances in synthesis and catalysis. This Review summarizes the synthesis, characterization, and key stoichiometric reactions of isolable or observable coinage metal hydrides. It discusses catalytic processes in which coinage metal hydrides are known or probable intermediates, and presents mechanistic studies of selected catalytic reactions. The purpose of this Review is to convey how developments in coinage metal hydride chemistry have led to new organic transformations, and how developments in catalysis have in turn inspired the synthesis of reactive new complexes.

The protonation state of thiols in self-assembled monolayers on roughened Ag/Au surfaces and nanoparticles

The protonation state of thiols in self-assembled monolayers (SAMs) on Ag and Au surfaces and nanoparticles (NPs) has been an issue of contestation. It has been recently demonstrated that deuterating the thiol proton produces ostentatious changes in the Raman spectra of thiols and can be used to detect the presence of the thiol functional group. Surface enhanced Raman spectroscopy (SERS) of H/D substituted aliphatic thiols on Ag surfaces clearly shows the presence of S-H vibration between 2150-2200 cm(-1) which shifts by 400 cm(-1) upon deuteration and a simultaneous >20 cm(-1) shift in the C-S vibration of thiol deuteration. Large shifts (>15 cm(-1)) in the C-S vibration are also observed for alkyl thiol SAMs on Au surfaces. Alternatively, neither the S-H vibration nor the H/D isotope effect on the C-S vibration is observed for alkyl thiol SAMs on Ag/Au NPs. XPS data on Ag/Au surfaces bearing aliphatic thiol SAMs show the presence of both protonated and deprotonated thiols while on Ag/Au NPs only deprotonated thiols are detected. These data suggest that aliphatic thiol SAMs on Au/Ag surfaces are partially protonated whereas they are totally deprotonated on Au/Ag NPs. Aromatic PhSH SAMs on Ag/Au surfaces and Ag/Au NPs do not show these vibrations or H/D shifts as well indicating that the thiols are deprotonated at these interfaces.

An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape

Gold, the archetypal "noble metal", used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many "unknowns", and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(iii) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands.

Vibrationally resolved photoelectron imaging of Au3H(-)

We report a combined photoelectron velocity map imaging spectroscopy and density functional theory investigation on the Au3H(-) anion. Transition between the anionic electronic ground state and the neutral electronic ground state is revealed. Vibrationally resolved spectra were recorded at two different photon energies, providing a wealth of spectroscopic information for the electronic ground state of the Au3H. Franck-Condon simulations of the ground-state transition are carried out to assist in the assignment of the vibrationally resolved spectra. The electron affinity and vertical detachment energy of Au3H are measured to be 2.548 ± 0.001 and 2.570 ± 0.001 eV, respectively. Three stretching vibrational modes are determined to be activated upon photodetachment, with the frequencies of 2100 ± 100, 177 ± 10, and 96 ± 10 cm(-1).

The gold-hydrogen bond, Au-H, and the hydrogen bond to gold, Au∙∙∙H-X

In the first part of this review, the characteristics of Au-H bonds in gold hydrides are reviewed including the data of recently prepared stable organometallic complexes with gold(I) and gold(III) centers. In the second part, the reports are summarized where authors have tried to provide evidence for hydrogen bonds to gold of the type Au∙∙∙H-X. Such interactions have been proposed for gold atoms in the Au(-I), Au(0), Au(I), and Au(III) oxidation states as hydrogen bonding acceptors and H-X units with X = O, N, C as donors, based on both experimental and quantum chemistry studies. To complement these findings, the literature was screened for examples with similar molecular geometries, for which such bonding has not yet been considered. In the discussion of the results, the recently issued IUPAC definitions of hydrogen bonding and the currently accepted description of agostic interactions have been used as guidelines to rank the Au∙∙∙H-X interactions in this broad range of weak chemical bonding. From the available data it appears that all the intra- and intermolecular Au∙∙∙H-X contacts are associated with very low binding energies and non-specific directionality. To date, the energetics have not been estimated, because there are no thermochemical and very limited IR/Raman and temperature-dependent NMR data that can be used as reliable references. Where conspicuous structural or spectroscopic effects have been observed, explanations other than hydrogen bonding Au∙∙∙H-X can also be advanced in most cases. Although numerous examples of short Au∙∙∙H-X contacts exist in the literature, it seems, at this stage, that these probably make only very minor contributions to the energy of a given system and have only a marginal influence on molecular conformations which so far have most often attracted researchers to this topic. Further, more dedicated investigations will be necessary before well founded conclusions can be drawn.

Probing the structural and electronic properties of Ag(n)H(-) (n = 1-3) using photoelectron imaging and theoretical calculations

Structural and electronic properties of silver hydride cluster anions (Ag(n)H(-); n = 1-3) have been explored by combining the negative ion photoelectron imaging spectroscopy and theoretical calculations. The photoelectron spectrum of AgH(-) exhibits transitions from AgH(- 2)Σ(+) to AgH (1)Σ(+) and AgH (3)Σ(+), with the electron affinity (EA) 0.57(3) eV. For Ag(2)H(-), the only observed transition is from Ag(2)H(-) (C(∞v)) (1)Σ(+) to Ag(2)H (C(2v)) (2)A(') and the electron affinity is 2.56(5) eV. Two obvious electron bands are observed in photoelectron imaging of Ag(3)H(-), which are assigned to the transitions from Ag(3)H(-) (C(2v)-T, which means C(2v) geometry with top site hydrogen) (2)B(2) to Ag(3)H (C(2v)-T) (1)A(1) and Ag(3)H (C(2v)-T) (3)B(2). The electron affinity is determined to be 1.61(9) eV. The Ag-H stretching modes in the ground states of AgH and Ag(2)H are experimentally resolved and their frequencies are measured to be 1710(80) and 1650(100) cm(-1), respectively. Aside from the above EAs and the vibrational frequencies, the vertical detachment energies to all ground states and some excited states of Ag(n)H (n = 1-3) are also obtained. Theoretical calculations reproduce the experimental energies quite well, and the results are used to assign the geometries and electronic states for all related species.

Mechanisms of chemical generation of volatile hydrides for trace element determination (IUPAC Technical Report)

Aqueous-phase chemical generation of volatile hydrides (CHG) by derivatization with borane complexes is one of the most powerful and widely employed methods for determination and speciation analysis of trace and ultratrace elements (viz. Ge, Sn, Pb, As, Sb, Bi, Se, Te, Hg, Cd, and, more recently, several transition and noble metals) when coupled with atomic and mass spectrometric detection techniques. Analytical CHG is still dominated by erroneous concepts, which have been disseminated and consolidated within the analytical scientific community over the course of many years. The overall approach to CHG has thus remained completely empirical, which hinders possibilities for further development. This report is focused on the rationalization and clarification of fundamental aspects related to CHG: (i) mechanism of hydrolysis of borane complexes; (ii) mechanism of hydrogen transfer from the borane complex to the analytical substrate; (iii) mechanisms through which the different chemical reaction conditions control the CHG process; and (iv) mechanism of action of chemical additives and foreign species. Enhanced comprehension of these different mechanisms and their mutual influence can be achieved in light of the present state of knowledge. This provides the tools to explain the reactivity of a CHG system and contributes to the clarification of several controversial aspects and the elimination of erroneous concepts in CHG.

Gas-phase synthesis and reactivity of binuclear gold hydride cations, (R3PAu)2H+ (R = Me and Ph)

Electrospray ionization of a mixture of the two gold phosphine chlorides, R3PAuCl (R = Ph and Me), silver nitrate and the amino acid N,N-dimethylglycine (DMG) yields a range of gold containing cluster ions including: (R3P)Au(PR'3)+; (R3PAu)(R'3PAu)Cl+ and (R3PAu)(R'3PAu)(DMG-H)+ (where R = R' = Ph; R = R' = Me; R = Me and R' = Ph). Collision induced dissociation (CID) of the (R3PAu)(R'3PAu)(DMG-H)+ precursor ions yielded the hitherto unknown gold hydride dimers (R3PAu)(R'3PAu)H+. The gas-phase chemistry of these dimers was studied using ion-molecule reactions, collision induced dissociation, electronic excitation dissociation (EED) and DFT calculations on the (H3PAu)2H+ model system. A novel phosphine ligand migration was found to occur prior to fragmentation under CID conditions and this was supported by DFT calculations, which revealed a transition state with a bridging phosphine ligand.

The electronic structure of transition metal dihelide dications

Multi-reference configuration interaction (MRCI) calculations have been employed to characterize the low-lying states of first-row transition metal dihelide dications, He(2)TM(2+) (TM = Sc-Cu). The most important state-ordering principles were determined to be the occupation of the 4s orbital and orientation of the occupied 3d orbital. The ground states of all species are predicted to be of D(infinityh) symmetry arising from a 3d(n+1) electronic configuration. For excited states with singly occupied 4s or doubly occupied 3d(sigma) orbitals, bending to C(2v) symmetry typically lowers the energy and shortens the He-TM bond length. Coupled cluster singles and doubles with a perturbative treatment of triple excitations (CCSD(T)) results for ground state spectroscopic properties are in agreement with the MRCI predicted trends.

Infrared Spectra and Structures of the Coinage Metal Dihydroxide Molecules

Laser-ablated Cu, Ag, and Au atoms react with H2O2 and with H2 + O2 molecules during condensation in excess argon to give four new IR absorptions in each system (O-H stretch, M-O-H bend, O-M-O stretch, and M-O-H deformation modes) that are due to the coinage metal M(OH)2 dihydroxide molecules. Isotopic substitution (D2O2, 18O2, 16O18O, D2, and HD) and comparison with frequencies computed by DFT verify these assignments. The calculations converge to 2B(g) ground electronic state structures with C2h symmetry, 111-117 degrees M-O-H bond angles, and substantial covalent character for these new metal dihydroxide molecules, particularly for Au(OH)2. This is probably due to the high electron affinity of gold owing to the effect of relativity.

Electronic structure, reactivity, and spectroscopy of dihydrides of group-IB metals

Atomic pseudopotentials and highly correlated wave functions, including spin-orbit interactions, have been used to evaluate the electronic structure, stability, and spectroscopy of triatomic molecule MH(2), with a metal M belonging to group IB (Cu, Ag, and Au). CuH(2) and AuH(2) have been recently observed by IR spectroscopy in solid hydrogen and bending anharmonic wave numbers have been assigned to these two systems. The AgH(2) molecule has not been detected nor experimentally characterized, despite several theoretical works arguing on its stability. Our results confirm that the MH(2) radicals have a metastable bent ground state separated from the dissociation into [M+H(2)] ground state by barriers which have been evaluated to 1.43, 0.78, and 0.80 eV, for Cu, Ag, and Au compounds, respectively. These barriers are calculated smaller than in previous determinations but still large enough to stabilize the MH(2) systems. Spectroscopic data are calculated for these radicals.