Probing Reactivity of Gold Atoms with Acetylene and Ethylene with VUV Photoionization Mass Spectrometry and Ab Initio Studies

Co+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy and Theory

Co+(C2H2)n (n = 1-6) complexes produced with laser vaporization in a supersonic molecular beam are studied with infrared photodissociation spectroscopy and computational chemistry. Infrared spectra are measured in the C-H stretching region using the method of tagging with argon atoms to enhance the photodissociation yields. C-H stretch vibrations for all clusters studied are shifted to lower frequencies than those of the well-known acetylene vibrations from ligand → metal charge transfer interactions. The magnitude of the red shifts decreases in the larger clusters as the interaction is distributed over more ligands. Computational studies identify various unreacted complexes with individual acetylene ligands in cation-π bonding configurations as well as reacted isomers in which ligand coupling reactions have taken place. Infrared spectra are consistent only with unreacted structures, even though the formation of reacted structures such as the metal ion-benzene complex is highly exothermic. Large activation barriers are predicted by theory along the reaction coordinates for the n = 2 and 3 complexes, which inhibit reactions in these smaller clusters, and this situation is presumed to persist in larger clusters.

Pt+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy

Platinum cation complexes with multiple acetylene molecules are studied with mass spectrometry and infrared laser spectroscopy. Complexes of the form Pt⁺(C₂H₂)_n are produced in a molecular beam by laser vaporization, analyzed with a time-of-flight mass spectrometer, and selected by mass for studies of their vibrational spectroscopy. Photodissociation action spectra in the C-H stretching region are compared to the spectra predicted for different structural isomers using density functional theory. The comparison between experiment and theory demonstrates that platinum forms cation-π complexes with up to three acetylene molecules, producing an unanticipated asymmetric structure for the three-ligand complex. Additional acetylenes form solvation structures around this three-ligand core. Reacted structures that couple acetylene molecules (e.g., to form benzene) are found by theory to be energetically favorable, but their formation is inhibited under the conditions of these experiments by large activation barriers.

UV-vis spectroscopy of gas-phase ions

Photodissociation action spectroscopy has made a great progress in expanding investigations of gas-phase ion structures. This review deals with aspects of gas-phase ion electronic excitations that result in wavelength-dependent dissociation and light emission via fluorescence, chiefly covering the ultraviolet and visible regions of the spectrum. The principles are briefly outlined and a few examples of instrumentation are presented. The main thrust of the review is to collect and selectively present applications of UV-vis action spectroscopy to studies of stable gas-phase ion structures and combinations of spectroscopy with ion mobility, collision-induced dissociation, and ion-ion reactions leading to the generation of reactive intermediates and electronic energy transfer.

Dicoordinate Au(I)-Ethylene Complexes as Hydroamination Catalysts

A series of gold(I)-ethylene π-complexes containing a family of bulky phosphine ligands has been prepared. The use of these sterically congested ligands is crucial to stabilize the gold(I)-ethylene bond and prevent decomposition, boosting up their catalytic performance in the highly underexplored hydroamination of ethylene. The precatalysts bearing the most sterically demanding phosphines showed the best results reaching full conversion to the hydroaminated products under notably mild conditions (1 bar of ethylene pressure at 60 °C). Kinetic analysis together with density functional theory calculations revealed that the assistance of a second molecule of the nucleophile as a proton shuttle is preferred even when using an extremely congested cavity-shaped Au(I) complex. In addition, we have measured a strong primary kinetic isotopic effect that is consistent with the involvement of X-H bond-breaking events in the protodeauration turnover-limiting step.

A dicoordinate gold(I)-ethylene complex

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold. Besides, the bonding situation of this species has been investigated by means of state-of-the-art Density Functional Theory (DFT) calculations indicating that π-backdonation plays a minor role compared with tricoordinate analogues.

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold.

Probing Cluster-π Interactions between Cun- and C2H2/C2H4 for Gas Separation

Copper-related materials are used for separation of ethylene and acetylene gases in chemistry; however, the precise mechanism regarding selectivity is elusive to be fully understood. Here, we have conducted a joint experimental and theoretical study of the Cu_n^- (n = 7-30) clusters in reacting with C₂H₄ and C₂H₂. It is found that all of the Cu_n^- clusters readily react with C₂H₂, giving rise to C₂H₂-addition products; however, Cu₁₈^- and Cu₁₉^- do not react with C₂H₄. We illustrate the superatomic stability of Cu₁₈^- and advocate its availability to separate C₂H₄ from C₂H₂. Further, we demonstrate the atomically precise mechanism regarding selectivity by fully unveiling the size-dependent cluster-π interactions.

C/C Exchange in Activation/Coupling Reaction of Acetylene and Methane Mediated by Os+: A Comparison with Ir+, Pt+, and Au. J Phys Chem Lett 2020;11:8346-8351. [PMID: 32885973 DOI: 10.1021/acs.jpclett.0c02068]

The activation and coupling reactions of methane and acetylene mediated by M⁺ (M = Os, Ir, Pt, and Au) have been comparatively studied at room temperature by the techniques of mass spectrometry in conjunction with theoretical calculations. Studies have shown that Os⁺ and Ir⁺ can mediate the activation/coupling reaction of CH₄ and C₂H₂, while Pt⁺ and Au⁺ cannot, which could be explained by the number of empty valence orbitals in the metal atom. In addition, there are different competition channels for the reaction mediated by Os⁺ and Ir⁺: an expected dehydrogenation and an unexpected C/C exchange. We find that if the rare C/C exchange reaction takes place, there are symmetric carbon atoms in the reaction intermediate and the C/C exchange reaction is favored kinetically. The C/C exchange reaction must be considered, which will affect the yield of the products in the primary reaction. This study shows the molecular-level mechanisms which include the C/C exchange reaction in the activation and coupling reaction of organic compounds mediated by different metals.

Methane Adducts of Gold Dimer Cations: Thermochemistry and Structure from Collision-Induced Dissociation and Association Kinetics

The bond dissociation energies at 0 K (BDE) of Au₂⁺-CH₄ and Au₂CH₄⁺-CH₄ have been determined using two separate experimental methods. Analyses of collision-induced dissociation cross sections for Au₂CH₄⁺ + Xe and Au₂(CH₄)₂⁺ + Xe measured using a guided ion beam tandem mass spectrometer (GIBMS) yield BDEs of 0.71 ± 0.05 and 0.57 ± 0.07 eV, respectively. Statistical modeling of association kinetics of Au₂(CH₄)_0-2⁺ + CH₄ + He measured from 200 to 400 K and at 0.3-0.9 Torr using a selected-ion flow tube (SIFT) apparatus yields slightly higher values of 0.81 ± 0.21 and 0.75 ± 0.25 eV. The SIFT data also place a lower limit on the BDE of Au₂C₂H₈⁺-CH₄ of 0.35 eV, likely an activated isomer, not Au₂(CH₄)₂⁺-CH₄. Particular emphasis is placed on determining the uncertainty in the derivation from association kinetics measurements, including uncertainties in collisional energy transfer, calculated harmonic frequencies, and possible contribution of isomerization of the association complexes. This evaluation indicates that an uncertainty of ±0.2 eV should be expected and that an uncertainty of better than ±0.1 eV is unlikely to be reasonable.

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

Synchrotron radiation provides insight into spectroscopy and dynamics in clusters and nanoparticles.

Au2+ cannot catalyze conversion of methane to ethene at low temperature

The previously reported conversion of methane to ethene catalyzed by Au₂⁺ at thermal energies is investigated through a combination of experiment and theory. The conversion is found not to occur, in-line with well-established thermodynamics.