Potential of Molecular Catalysts with Electron-Rich Transition Metal Centers for Addressing Long-Standing Chemistry Enigmas

Molecular complexes with electron-rich metal centers are highlighted as potential catalysts for the following five important chemical transformations: selective conversion of methane to methanol, capture and utilization of carbon dioxide, fixation of molecular nitrogen, water splitting, and recycling of perfluorochemicals. Our initial focus lies on negatively charged metal centers and ligands that can stabilize anionic metal atoms. Catalysts with electron-rich metal atoms (CERMAs) can sustain catalytic cycles with a "ping-pong" mechanism, where one or more electrons are transferred from the metal center to the substrate and back. The donated electrons can activate the chemical bonds of the substrate by populating its antibonding orbitals. At the last step of the catalytic cycle, the electrons return to the metal and the product interacts only weakly with the formed anion, which enables the solvent molecules to remove the product fast from the catalytic cycle and prevent subsequent unfavorable reactions. This process resembles electrocatalysis, but the metal serves as both an anode and a cathode (molecular electrocatalysis). We also analyze the usage of CERMAs as the base of Frustrated Lewis pairs proposing a new type of bimetallic catalysts. This Featured Article aspires to initiate systematic experimental and theoretical studies on CERMAs and their reactivity, the potential of which has probably been underestimated in the literature.

Spontaneous Reduction by One Electron on Water Microdroplets Facilitates Direct Carboxylation with CO2

Recent advances in microdroplet chemistry have shown that chemical reactions in water microdroplets can be accelerated by several orders of magnitude compared to the same reactions in bulk water. Among the large plethora of unique properties of microdroplets, an especially intriguing one is the strong reducing power that can be sometimes as high as alkali metals as a result of the spontaneously generated electrons. In this study, we design a catalyst-free strategy that takes advantage of the reducing ability of water microdroplets to reduce a certain molecule, and the reduced form of that molecule can convert CO₂ into value-added products. By spraying the water solution of C₆F₅I into microdroplets, an exotic and fragile radical anion, C₆F₅I^•-, is observed, where the excess electron counter-intuitively locates on the σ* antibonding orbital of the C-I bond as evidenced by anion photoelectron spectroscopy. This electron weakens the C-I bond and causes the formation of C₆F₅^-, and the latter attacks the carbon atom on CO₂, forming the pentafluorobenzoate product, C₆F₅CO₂^-. This study provides a good example of strategically making use of the spontaneous properties of water microdroplets, and we anticipate that microdroplet chemistry will be a green avenue rich in new opportunities in CO₂ utilization.

Molecular-level electrocatalytic CO2 reduction reaction mediated by single platinum atoms

The activation and transformation of H₂O and CO₂ mediated by electrons and single Pt atoms is demonstrated at the molecular level. The reaction mechanism is revealed by the synergy of mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. Specifically, a Pt atom captures an electron and activates H₂O to form a H-Pt-OH^- complex. This complex reacts with CO₂via two different pathways to form formate, where CO₂ is hydrogenated, or to form bicarbonate, where CO₂ is carbonated. The overall formula of this reaction is identical to a typical electrochemical CO₂ reduction reaction on a Pt electrode. Since the reactants are electrons and isolated, single atoms and molecules, we term this reaction a molecular-level electrochemical CO₂ reduction reaction. Mechanistic analysis reveals that the negative charge distribution on the Pt-H and the -OH moieties in H-Pt-OH^- is critical for the hydrogenation and carbonation of CO₂. The realization of the molecular-level CO₂ reduction reaction provides insights into the design of novel catalysts for the electrochemical conversion of CO₂.

Ligated aluminum cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2)

A wide range of low oxidation state aluminum-containing cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2), were produced via reactions between aluminum cluster anions and hexamethyldisilazane (HMDS). These clusters were identified by mass spectrometry, with a few of them (n = 4, 6, and 7) further characterized by a synergy of anion photoelectron spectroscopy and density functional theory (DFT) based calculations. As compared to a previously reported method which reacts anionic aluminum hydrides with ligands, the direct reactions between aluminum cluster anions and ligands promise a more general synthetic scheme for preparing low oxidation state, ligated aluminum clusters over a large size range. Computations revealed structures in which a methyl-group of the ligand migrated onto the surface of the metal cluster, thereby resulting in "two metal-atom" insertion between Si-CH3 bond.

Study of the Reaction of Hydroxylamine with Iridium Atomic and Cluster Anions (n = 1-5)

Elucidating the multifaceted processes of molecular activation and subsequent reactions gives a fundamental view into the development of iridium catalysts as they apply to fuels and propellants, for example, for spacecraft thrusters. Hydroxylamine, a component of the well-known hydroxylammonium nitrate (HAN) ionic liquid, is a safer alternative and mimics the chemistry and performance standards of hydrazine. The activation of hydroxylamine by anionic iridium clusters, Ir_n^- (n = 1-5), depicts a part of the mechanism, where two hydrogen atoms are removed, likely as H₂, and Ir_n(NOH)^- clusters remain. The significant photoelectron spectral differences between these products and the bare clusters illustrate the substantial electronic changes imposed by the hydroxylamine fragment on the iridium clusters. In combination with DFT calculations, a preliminary reaction mechanism is proposed, identifying the possible intermediate steps leading to the formation of Ir(NOH)^-.

CO2 Activation and Hydrogenation by Palladium Hydride Cluster Anions

Mass spectrometric analysis of the anionic products of interaction between palladium hydride anions, PdH^-, and carbon dioxide, CO₂, in a reaction cell shows an efficient generation of the PdHCO₂^- intermediate and isolated formate product. Multiple isomers of the PdHCO₂^- intermediates are identified by a synergy between negative ion photoelectron spectroscopy and quantum-chemical calculations. It is shown that a direct mechanism, in which the H atom in PdH^- directly activates and hydrogenates CO₂, leads to the formation of the formate product. An indirect mechanism, on the other hand, leads to a stable HPdCO₂^- structure, where CO₂ is chemisorbed onto the Pd atom.

Simultaneous Functionalization of Methane and Carbon Dioxide Mediated by Single Platinum Atomic Anions

Mass spectrometric analysis of the anionic products of interaction among Pt^-, methane, and carbon dioxide shows that the methane activation complex, H₃C-Pt-H^-, reacts with CO₂ to form [H₃C-Pt-H(CO₂)]^-. Two hydrogenation and one C-C bond coupling products are identified as isomers of [H₃C-Pt-H(CO₂)]^- by a synergy between anion photoelectron spectroscopy and quantum chemical calculations. Mechanistic study reveals that both CH₄ and CO₂ are activated by the anionic Pt atom and that the successive depletion of the negative charge on Pt drives the CO₂ insertion into the Pt-H and Pt-C bonds of H₃C-Pt-H^-. This study represents the first example of the simultaneous functionalization of CH₄ and CO₂ mediated by single atomic anions.

Using anion photoelectron spectroscopy of cluster models to gain insights into mechanisms of catalyst-mediated H2 production from water

Metal oxide cluster models of catalyst materials offer a powerful platform for probing the molecular-scale features and interactions that govern catalysis. This perspective gives an overview of studies implementing the combination of anion photoelectron (PE) spectroscopy and density functional theory calculations toward exploring cluster models of metal oxides and metal-oxide supported Pt that catalytically drive the hydrogen evolution reaction (HER) or the water-gas shift reaction. The utility in the combination of these experimental and computational techniques lies in our ability to unambiguously determine electronic and molecular structures, which can then connect to results of reactivity studies. In particular, we focus on the activity of oxygen vacancies modeled by suboxide clusters, the critical mechanistic step of forming proximal metal hydride and hydroxide groups as a prerequisite for H2 production, and the structural features that lead to trapped dihydroxide groups. The pronounced asymmetric oxidation found in heterometallic group 6 oxides and near-neighbor group 5/group 6 results in higher activity toward water, while group 7/group 6 oxides form very specific stoichiometries that suggest facile regeneration. Studies on the trans-periodic combination of cerium oxide and platinum as a model for ceria supported Pt atoms and nanoparticles reveal striking negative charge accumulation by Pt, which, combined with the ionic conductivity of ceria, suggests a mechanism for the exceptionally high activity of this system towards the water-gas shift reaction.

Mapping the Electronic Structure of the Uranium(VI) Dinitride Molecule, UN2

A combined anion photoelectron spectroscopic and relativistic coupled-cluster computational study of the electronic structure of the UN₂ molecule is presented. Because the photoelectron spectrum of the uranium dinitride negative ion, UN₂^-, directly reflects the electronic structure of neutral UN₂, we have measured and relied upon the photoelectron spectrum of the UN₂^- anion as a means of mapping the electronic structure of neutral UN₂. In addition to the electron affinity of the UN₂ ground state, energy levels of the UN₂ excited states were well characterized by the close interplay between the experiment and high-level theory. We found that both electron attachment and electronic excitation significantly bend the UN₂ molecule and elongate its U≡N bond. Implications for the activation of UN₂ are discussed.

Ab initio investigation of the ground and excited states of RuO+,0,- and their reaction with water

High-level quantum chemical calculations on RuO0,± elucidate the electronic structure of their low-lying electronic states. For thirty-two states, we report the electronic configurations, bond lengths, vibrational frequencies, spin-orbit splittings, and excitation energies. The electronic states of RuO can be generated from those of RuO+ by adding one electron to the σ non-bonding orbital closely resembling the 5s atomic orbital of Ru. The ground states for RuO and RuO- are clearly identified as 5Δ and 4Δ, but the two states (4Δ and 2Π) compete for RuO+. The difficulty of calculations is revealed by our small binding energies compared to the experimental values. In addition, we studied the reaction of the three species with water in their ground and selected low-lying electronic states. We found a consistent decrease of the activation energy barriers and higher exothermicity as we add electrons to the system. RuO- is found to facilitate the reaction for both kinetic and thermodynamic reasons.

CO2 Hydrogenation to Formate and Formic Acid by Bimetallic Palladium-Copper Hydride Clusters

Mass spectrometric analysis of the anionic products of interaction between bimetallic palladium-copper tetrahydride anions, PdCuH₄^-, and carbon dioxide, CO₂, in a reaction cell shows an efficient generation of the PdCuCO₂H₄^- intermediate and formate/formic acid complexes. Multiple structures of PdCuH₄^- and PdCuCO₂H₄^- are identified by a synergy between anion photoelectron spectroscopy and quantum chemical calculations. The higher energy PdCuH₄^- isomer is shown to drive the catalytic hydrogenation of CO₂, emphasizing the importance of accounting for higher energy isomers for cluster catalytic activity. This study represents the first example of CO₂ hydrogenation by bimetallic hydride clusters.

Characterization of the alkali metal oxalates (MC2O4-) and their formation by CO2 reduction via the alkali metal carbonites (MCO2-)

The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations. The critical step in the reductive process is the coordination of CO2 to an alkali metal anion, forming a metal carbonite MCO2- able to subsequently receive a second CO2 molecule. While the energetic demand for these reactions is generally low, we find that the degree of activation of CO2 in terms of charge transfer and transition state energies is the highest for lithium and systematically decreases down the group (M = Li-Cs). This is correlated to the strength of the binding interaction between the alkali metal and CO2, which can be related to the structure of the oxalate moiety within the product metal complexes evolving from a planar to a staggered conformer with increasing atomic number of the interacting metal. Similar structural changes are observed for crystalline alkali metal oxalates, although the C2O42- moiety is in general more planar in these, a fact that is attributed to the increased number of interacting alkali metal cations compared to the gas-phase ions.

Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2 (H2 O)n - , n=1-10, in the C-O Stretch Region

We investigate anionic [Co,CO2 ,nH2 O]- clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250-2234 cm-1 using an FT-ICR mass spectrometer. We show that both CO2 and H2 O are activated in a significant fraction of the [Co,CO2 ,H2 O]- clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C-O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C-O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 - . However, calculations find Co(HCOO)(OH)- as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590-1730 cm-1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH)- . Upon additional hydration, all species [Co,CO2 ,nH2 O]- , n≥2, undergo IRMPD through loss of H2 O molecules as a relatively weakly bound messenger. The main spectral features are the C-O stretching mode of the CO ligand around 1900 cm-1 , the water bending mode mixed with the antisymmetric C-O stretching mode of the HCOO- ligand around 1580-1730 cm-1 , and the symmetric C-O stretching mode of the HCOO- ligand around 1300 cm-1 . A weak feature above 2000 cm-1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

Dissociative detachment of the fluoroformate anion

3D fragment imaging of the fluoroformate anion (FCO₂⁻) dissociative photodetachment products shows reductive fragmentation, forming FCO + O, as well as a dominant cleavage of the CF bond.

Formation of large clusters of CO2 around anions: DFT study reveals cooperative CO2 adsorption

The cooperative O⋯C secondary interactions compensate for the diminishing effect of primary anion⋯C interactions in anionic clusters of CO₂ molecules.