Solvent-mediated reduction of carbon dioxide in anionic complexes with silver atoms

Tuning the Electronic Properties of CumAgn Bimetallic Clusters for Enhanced CO2 Activation

The urgent demand for efficient CO2 reduction technologies has driven enormous studies into the enhancement of advanced catalysts. Here, we investigate the electronic properties and CO2 adsorption properties of CumAgn bimetallic clusters, particularly Cu4Ag1, Cu1Ag4, Cu3Ag2, and Cu2Ag3, using generalized gradient approximation (GGA)/density functional theory (DFT). Our results show that the atomic arrangement within these clusters drastically affects their stability, charge transfer, and catalytic performance. The Cu4Ag1 bimetallic cluster emerges as the most stable structure, revealing superior charge transfer and effective chemisorption of CO2, which promotes effective activation of the CO2 molecule. In contrast, the Cu1Ag4 bimetallic cluster, in spite of comparable adsorption energy, indicates insignificant charge transfer, resulting in less pronounced CO2 activation. The Cu3Ag2 and Cu2Ag3 bimetallic clusters also display high adsorption energies with remarkable charge transfer mechanisms, emphasizing the crucial role of metal composition in tuning catalytic characteristics. This thorough examination provides constructive insights into the design of bimetallic clusters for boosted CO2 reduction. These findings could pave the way for the development of cost-effective and efficient catalysts for industrial CO2 reduction, contributing to global efforts in carbon management and climate change mitigation.

Spectroscopic investigation of size-dependent CO2 binding on cationic copper clusters: analysis of the CO2 asymmetric stretch

Photofragmentation spectroscopy, combined with quantum chemical computations, was employed to investigate the position of the asymmetric CO2 stretch in cold, He-tagged Cun[CO2]+ (n = 1-10) and Cun[CO2][H2O]+ (n = 1-7) complexes. A blue shift in the band position was observed compared to the free CO2 molecule for Cun[CO2]+ complexes. Furthermore, this shift was found to exhibit a notable dependence on cluster size, progressively redshifting with increasing cluster size. The computations revealed that the CO2 binding energy is the highest for Cu+ and continuously decreases with increasing cluster size. This dependency could be explained by highlighting the role of polarization in electronic structure, according to energy decomposition analysis. The introduction of water to this complex amplified the redshift of the asymmetric stretch, showing a similar dependency on the cluster size as observed for Cun[CO2]+ complexes.

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.

Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

Observation of inserted oxocarbonyl species in the tantalum cation-mediated activation of carbon dioxide dictated by two-state reactivity

Reductive activation of carbon dioxide (CO2) has drawn increasing attention as an effective and convenient method to unlock this stable molecule, especially via transition metal-catalyzed reactions. Taking the [TaC4O8]+ ion-molecule complex formed in the laser ablation source as a representative, the reactivity of the tantalum metal cation towards CO2 molecules is explored using infrared photodissociation spectroscopy combined with quantum chemical calculations. The strong absorption in the carbonyl stretching region provides solid evidence for the insertion reactions into CO bonds by the tantalum cation. Two inserted oxocarbonyl products are identified based on the great agreement between the experimental results and simulated infrared spectra of energetically low-lying structures in the singlet and triplet states. The pivotal role of two-state reactivity in driving CO2 activation among three different spin states is rationalized by potential energy surface analysis. Our conclusion provides valuable insight into the intrinsic mechanisms of CO2 activation by the tantalum metal cation, highlighting the affinity of tantalum for CO bond insertion in addition to typical "end-on" binding configurations.

Relation Between Bond Angle and Carbon-Oxygen Stretching Frequencies in CO2-Containing Compounds

The symmetric (νs) and antisymmetric (νas) O-C-O stretching modes of CO2-containing compounds encode structural information that can be difficult to decipher, due to the sensitivity of these spectral features to small shifts in charge distribution and structure, as well as the anharmonicities of these two vibrational modes. In this work, we discuss the relation between the frequency of these modes and the geometry of the O-C-O group, showing that the splitting between νs and νas (Δνas-s = νas - νs) can be predicted based only on the O-C-O bond angle obtained from quantum chemical calculations with reasonable accuracy (±46 cm-1, R2 = 0.994). The relationship is shown to hold for the infrared spectra of a variety of CO2-containing molecules measured in vacuo. The origins of this model are discussed in the framework of elementary mode analysis.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Infrared photodissociation spectroscopy of mass-selected [TaO3(CO2)n]+ (n = 2-5) complexes in the gas phase

We report a joint experimental and theoretical study on the structures of gas-phase [TaO₃(CO₂)_n]⁺ (n = 2-5) ion-molecule complexes. Infrared photodissociation spectra of mass-selected [TaO₃(CO₂)_n]⁺ complexes were recorded in the frequency region from 2200 to 2450 cm^-1 and assigned through comparing with the simulated infrared spectra of energetically low-lying structures derived from quantum chemical calculations. With the increasing number of attached CO₂ molecules, the larger clusters show significantly enhanced fragmentation efficiency and a strong band appears at around 2350 cm^-1 near the free CO₂ antisymmetric stretching vibration band, indicating only a small perturbation of CO₂ molecules on the secondary solvation sphere while higher frequency bands corresponding to the core structure remain largely unaffected. A core structure [TaO₃(CO₂)₃]⁺ is identified to which subsequent CO₂ ligands are weakly attached and the most favorable cluster growth path is verified to proceed on the triplet potential energy surface higher in energy than that of ground states. Theoretical exploration reveals a two-state reactivity (TSR) scenario in which the energetically favored triplet transition state crosses over the singlet ground state to form a TaO₃⁺ core ion, providing new information on the cluster formation correlated with the reactivity of tantalum metal oxides towards CO₂.

Tuning the degree of CO2 activation by carbon doping Cun- (n = 3-10) clusters: an IR spectroscopic study

Copper clusters on carbide surfaces have shown a high catalytic activity towards methanol formation. To understand the interaction between CO₂ and the catalytically active sites during this process and the role that carbon atoms could play in this, they are modeled by copper clusters, with carbon atoms incorporated. The formed clusters Cu_nC_m^- (n = 3-10, m = 1-2) are reacted with CO₂ and investigated by IR multiple-photon dissociation (IR-MPD) spectroscopy to probe the degree of CO₂ activation. IR spectra for the reaction products [Cu_nC·CO₂]^-, (n = 6-10), and [Cu_nC₂·CO₂]^-, (n = 3-8) are compared to reference spectra recorded for products formed when reacting the same cluster sizes with CO, and with density functional theory (DFT) calculated spectra. The results reveal a size- and carbon load-dependent activation and dissociation of CO₂. The complexes [Cu_nC·CO₂]^- with n = 6 and 10 show predominantly molecular activation of CO₂, while those with n = 7-9 show only dissociative adsorption. The addition of the second carbon to the cluster leads to the exclusive molecular activation of the CO₂ on all measured cluster sizes, except for Cu₅C₂^- where CO₂ dissociates. Combining these findings with DFT calculations leads us to speculate that at lower carbon-to-metal ratios (CMRs), the C can act as an oxygen anchor facilitating the OCO bond rupture, whereas at higher CMRs the carbon atoms increasingly attract negative charge, reducing the Cu cluster's ability to donate electron density to CO₂, and consequently its ability to activate CO₂.

An infrared study of CO2 activation by holmium ions, Ho+ and HoO. Phys Chem Chem Phys 2022;24:22716-22723. [PMID: 36106954 DOI: 10.1039/d2cp02862j]

We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho⁺ and HoO⁺ centres. Infrared action spectra of Ho(CO₂)_n⁺ and [HoO(CO₂)_n]⁺ ion-molecule complexes have been recorded in the spectral region 1700-2400 cm^-1 and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO₂)_n⁺ complexes with CO₂ binding end-on to the Ho⁺ ion. By contrast, all [HoO(CO₂)_n]⁺ complexes n ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety, . The signature of this structure, a new vibrational band observed around 1840 cm^-1 for n = 3, continues to red-shift monotonically with each successive CO₂ ligand binding with net charge transfer from the ligand rather than the metal centre.

Carbon Dioxide Activation by Alkaline-Earth Metals: Formation and Spectroscopic Characterization of OCMCO3 and MC2O4 (M = Ca, Sr, Ba) in Solid Neon

The reactions of alkaline-earth metal atoms (Ca, Sr, and Ba) with carbon dioxide are investigated using matrix isolation infrared spectroscopy in solid neon. The ground-state metal atoms react with two carbon dioxide molecules to produce the oxalate complexes MC₂O₄ and the carbonate-carbonyl complexes OCMCO₃ (M = Ca, Sr, Ba) spontaneously on annealing. The species are identified by the effects of isotopic substitution on their infrared spectra as well as density functional calculations. Bonding analyses reveal that the attractive forces between M²⁺ and (CO₃)^2- or (C₂O₄)₂^- in the OCMCO₃ and MC₂O₄ complexes come mainly from electrostatic attraction, but covalent orbital interactions also play an important role, which are dominated by the ligand-to-metal donation bonding. The calcium, strontium, and barium metal centers in these complexes use their ns and predominately (n - 1)d atomic orbitals for covalent bonding that mimic transition metals.

Infrared spectroscopy of Be(CO2)4+ in the gas phase: electron transfer and C-C coupling of CO2

Beryllium-carbon dioxide cation complexes Be(CO₂)_n⁺ are produced by a laser vaporization-supersonic expansion ion source in the gas phase. Mass-selected infrared photodissociation spectroscopy supplemented by theoretical calculations confirms that Be(CO₂)₄⁺ is a coordination saturated complex that can be assigned to a mixture of two isomers. The first structure involves a bent CO₂^- ligand that is bound in a monodentate η¹-O coordination mode. Another isomer has a metal oxalate-type C₂O₄^- moiety with a C-C hemibond.

Anionic Activation of CO2 via (Mn-CO2)- Complex on Magic-Numbered Anionic Coinage Metal Clusters Mn- (M = Cu, Ag, Au)

Given the immense challenge of excessive accumulation of carbon dioxide (CO₂) in the earth's atmosphere, an extensive search is under way to convert atmospheric CO₂ to compounds of more utility. With CO₂ being thermodynamically extremely stable, activation of CO₂ is the first and most important step toward its chemical conversion. Building upon our earlier model for the anionic activation of CO₂ with azabenzene and inspired by the work of others on metal atom-CO₂ complexes, we investigated the possibility of anionic activation of CO₂ on small anionic metal clusters, which would have implications for catalytic conversion of CO₂ on metal surfaces with atomic-scale structural irregularities. We carried out theoretical calculations using density functional theory to examine small anionic metal clusters of Cu, Ag, and Au to check whether they form a complex with CO₂, with the sign of CO₂ being chemically activated. We found that a class of anionic metal clusters M_n^- with 1, 2, and 6 atoms consistently produced the activated complex (M_n-CO₂)^- for all three metals. There exists a strong interaction between the CO₂ moiety and M_n^- via a partially covalent M-C bond with a full delocalization of the electronic charge, as a result of electron transfer from the HOMO of M_n^- to the LUMO of CO₂ as in metal-CO₂ π-backbonding. We examined the interaction of frontier orbitals from the viewpoints of the orbital geometry and orbital energetics and found that the above magic numbers are consistent with both aspects.

Spectroscopic Identification of Transition-Metal M[η2-(O,O)C] Species for Highly-Efficient CO2 Activation

The CO₂ activation by transition metals is important in CO₂ utilization but has proven to be challenging for experimental targets. Here we report first synthesis and spectroscopic characterization of transition-metal M[η²-(O,O)C] species with bidentate double oxygen metal-CO₂ coordination in the [ZrO(CO₂)_n≥4]⁺ complexes. The Zr[η²-(O,O)C] species yields a CO₂^- radical ligand, showing a high efficiency in CO₂ activation. We find that two important prerequisites are demanded for certain metals to form this intriguing M[η²-(O,O)C] species. One is that the metal center has high reduction capability, and the other is that the oxidation state of the metal center is lower than its highest one by 1. This study highlights the pivotal roles played by the M[η²-(O,O)C] species in CO₂ activation and also open new avenues toward the development of related single-atom catalysts with isolated transition-metal atoms dispersed on supports.

Probing the Hydrogen Bonding in Microsolvated Clusters of Au1,2-(Solv)n (Solv = C2H5OH, n-C3H7OH; n = 1-3 for Au-; n =1 for Au2-)

The microsolvation of gold anions in different alcohol solvents is demonstrated by the combination of anion photoelectron spectroscopy and quantum chemical calculations on the Au_1,2^-(Solv)_n (Solv = C₂H₅OH, n-C₃H₇OH; n = 1-3 for Au^-; n = 1 for Au₂^-). The microsolvation structures of these clusters and their corresponding neutrals are assigned by comparing calculations with experiments. In terms of overall regularity, the increasing solvation number (n) and carbon chain extension both can increase the stability of the anion. When n ≥ 2, these clusters have low-energy isomers, where conventional hydrogen bonds (HBs) compete with nonconventional HBs (NHBs). NHBs are dominant when n ≤ 2 and when n is increased, vice versa. Interestingly, a variety of theoretical calculations show that after the hydroxy H atom of the ethanol molecule forms a weak ionic HB with Au^-, there are two lowest conformations of ethanol, trans and gauche, which could be coexisting in the molecular beams. Some theoretical methods also suggest that the gauche isomer is more stable than the trans one, which indicates that Au^- may exist as a gold gauche effect similar to fluorine.

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.

Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO2 in [MgCO2 (H2 O)n ].+ , n=0-8

We investigate activation of carbon dioxide by singly charged hydrated magnesium cations Mg ^.+(H₂O)_n, through infrared multiple photon dissociation (IRMPD) spectroscopy combined with quantum chemical calculations. The spectra of [MgCO₂(H₂O)_n]^.+ in the 1250–4000 cm⁻¹ region show a sharp transition from n=2 to n=3 for the position of the CO₂ antisymmetric stretching mode. This is evidence for the activation of CO₂ via charge transfer from Mg ^.+ to CO₂ for n≥3, while smaller clusters feature linear CO₂ coordinated end‐on to the metal center. Starting with n=5, we see a further conformational change, with CO₂^.− coordination to Mg²⁺ gradually shifting from bidentate to monodentate, consistent with preferential hexa‐coordination of Mg²⁺. Our results reveal in detail how hydration promotes CO₂ activation by charge transfer at metal centers.

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.

Reductive Activation of Small Molecules by Anionic Coinage Metal Atoms and Clusters in the Gas Phase

Metal atoms and clusters exhibit chemical properties that are significantly different or totally absent in comparison to their bulk counterparts. Such peculiarity makes them potential building units for the generation of novel catalysts. Investigations of the gas-phase reactions between size- and charge-selected atoms/clusters and small molecules have provided fundamental insights into their intrinsic reactivity, thus leading to a guiding principle for the rational design of the single-atom and cluster-based catalysts. Especially, recent gas-phase studies have elucidated that small molecules such as O₂ , CO₂ , and CH₃ I can be catalytically activated by negatively-charged atoms/clusters via donation of a partial electronic charge. This Minireview showcases typical examples of such "reductive activation" processes promoted by anions of metal atoms and clusters. Here, we focus on anionic atoms/clusters of coinage metals (Cu, Ag, and Au) owing to the simplicity of their electronic structures. The determination of a correlation between their activation modes and the electronic structures might be helpful for the future development of innovative coinage metal catalysts.

Infrared Spectroscopy of Size-Selected Hydrated Carbon Dioxide Radical Anions CO2 .- (H2 O)n (n=2-61) in the C-O Stretch Region

Understanding the intrinsic properties of the hydrated carbon dioxide radical anions CO2 .- (H2 O)n is relevant for electrochemical carbon dioxide functionalization. CO2 .- (H2 O)n (n=2-61) is investigated by using infrared action spectroscopy in the 1150-2220 cm-1 region in an ICR (ion cyclotron resonance) cell cooled to T=80 K. The spectra show an absorption band around 1280 cm-1 , which is assigned to the symmetric C-O stretching vibration νs . It blueshifts with increasing cluster size, reaching the bulk value, within the experimental linewidth, for n=20. The antisymmetric C-O vibration νas is strongly coupled with the water bending mode ν2 , causing a broad feature at approximately 1650 cm-1 . For larger clusters, an additional broad and weak band appears above 1900 cm-1 similar to bulk water, which is assigned to a combination band of water bending and libration modes. Quantum chemical calculations provide insight into the interaction of CO2 .- with the hydrogen-bonding network.

Structures and Infrared Spectra of [M(CO2)7]+ (M = V, Cr, and Mn) Complexes

Gas-phase infrared photodissociation spectra of [V(CO₂) _n]⁺ complexes revealed three new vibrational bands at 1140, 1800, and 3008 cm^-1 at n = 7, the features of which are retained in the larger clusters (Ricks, A. M.; Brathwaite, A. D.; Duncan, M. A. J. Phys. Chem. A 2013, 117, 11490-11498). However, structural assignment of this intriguing feature remains open. Herein, quantum chemical calculations on [V(CO₂)₇]⁺ were carried out to identify the structure of the low-lying isomers and to assign the observed spectral features. The comparison of calculated infrared spectra of [V(CO₂)₇]⁺ with experimental infrared spectra identified the formation of a bent CO₂^- species, suggesting the ligand-induced activation of CO₂ by the vanadium cation. The structures and infrared spectra of [Cr(CO₂)₇]⁺ and [Mn(CO₂)₇]⁺ were also predicted and discussed.

The metallo-formate anions, M(CO2)−, M = Ni, Pd, Pt, formed by electron-induced CO2 activation

The metallo-formate anions, M(CO₂)⁻, M = Ni, Pd, and Pt, were formed by electron-induced CO₂ activation.

Unambiguous hydrogenation of CO2 by coinage-metal hydride anions: an intuitive idea based on in silico experiments

Coinage metal hydride anions, especially AgH⁻, can effectively and deterministically hydrogenate CO₂ to HCO₂⁻.

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.

Carbon-carbon bond formation in the reaction of hydrated carbon dioxide radical anions with 3-butyn-1-ol

Electrochemical activation of carbon dioxide in aqueous solution is a promising way to use carbon dioxide as a C1 building block. Mechanistic studies in the gas phase play an important role to understand the inherent chemical reactivity of the carbon dioxide radical anion. Here, the reactivity of CO2 •-(H2O)n with 3-butyn-1-ol is investigated by Fourier transform ion cyclotron (FT-ICR) mass spectrometry and quantum chemical calculations. Carbon-carbon bond formation takes places, but is associated with a barrier. Therefore, bond formation may require uptake of several butynol molecules. The water molecules slowly evaporate from the cluster due to the absorption of room temperature black-body radiation. When all water molecules are lost, butynol evaporation sets in. In this late stage of the reaction, side reactions occur including H• atom transfer and elimination of HOCO•.

Preparation and characterization of zirconium silico188W-tungstate as a base material for 188W/188Re generator

Zr:W:Si. The optimum gel was prepared using the molar ratio 1:1:7 at pH 8. The ZrSiW gel was investigated by FTIR, XRD, thermal analysis (TGA and DTA), FESEM, XRF, and NAA. Then, XRF and NAA techniques were used to estimate the proportion of the constituents of the gel according to the molecular formula [ZrO₂(Si(WO₄)₂)16H₂O]. ¹⁸⁸W/¹⁸⁸Re generator was prepared based on the selected ZrSi¹⁸⁸W gel, which gives the highest tungsten content (393.3 mg W/g gel) with 75 ± 3% elution yield of ¹⁸⁸Re. Quality control was studied on the ¹⁸⁸Re to make sure its validity for clinical applications.

The activation of carbon dioxide by first row transition metals (Sc-Zn)

The activation of CO2 by chloride-tagged first-row transition metal anions [ClM]- (M = Sc-Zn), was examined by mass spectrometry, quantum chemical calculations, and statistical analysis. The direct formation of [ClM(CO2)]- complexes was demonstrated in the reaction between [ClM]- and neutral CO2. In addition, the reverse reaction was investigated by energy-variable collisionally induced dissociation (CID) of the corresponding [ClM(CO2)]- anions generated in-source. Five different mono- and bi-dentate binding motifs present in the ion/CO2 complexes were identified by quantum chemical calculations and the relative stability of each of these isomers was established and analyzed for all first-row transition metals based on the experimental and theoretical ion/molecule binding energies. It was found that the early first row transition metals form strong covalent bonds with the neutral CO2 molecule, while the late ones and in particular copper and zinc are weakly bonded. Using simple valence bond Lewis diagrams, the different binding motifs and their relative stabilities across the first row were described using multi-configurational self consistent field (MSCSCF) wavefunctions in a quantitative manner based on the electronic structure of the individual metals. This analysis provides an explanation for the change of the most favorite bonding motif of the transition metals with CO2 along the 1st transition metal row. The nature of the activated CO2 complex and the relationship between its stability and other structural and spectral properties was also analyzed by Principal Component Analysis (PCA) and artificial neural networks.

Vibrational Characterization of Radical Ion Adducts between Imidazole and CO2

We address the molecular level origins of the dramatic difference in the catalytic mechanisms of CO₂ activation by the seemingly similar molecules pyridine (Py) and imidazole (Im). This is accomplished by comparing the fundamental interactions of CO₂ radical anions with Py and Im in the isolated, gas phase PyCO₂^- and ImCO₂^- complexes. These species are prepared by condensation of the neutral compounds onto a (CO₂) _n^- cluster ion beam by entrainment in a supersonic jet ion source. The structures of the anionic complexes are determined by theoretical analysis of their vibrational spectra, obtained by IR photodissociation of weakly bound CO₂ molecules in a photofragmentation mass spectrometer. Although the radical PyCO₂^- system adopts a carbamate-like configuration corresponding to formation of an N-C covalent bond, the ImCO₂^- species is revealed to be best described as an ion-molecule complex in which an oxygen atom in the CO₂^- radical anion is H-bonded to the NH group. Species that feature a covalent N-C interaction in ImCO₂^- are calculated to be locally stable structures, but are much higher in energy than the largely electrostatically bound ion-molecule complex. These results support the suggestion from solution phase electrochemical studies (Bocarsly et al. ACS Catal. 2012, 2, 1684-1692) that the N atoms are not directly involved in the catalytic activation of CO₂ by Im.

Infrared Photodissociation Spectra of [Sn(CO2) n]- Cluster Ions

We present infrared spectra and density functional theory calculations of mass selected [Sn(CO₂) _n]^- cluster anions ( n = 2-6). The spectra and structures of these clusters exhibit less structural diversity than those of analogous clusters with first-row transition metals, but are more complex than those for the heavy coinage metals or for the related [Bi(CO₂) _n]^- clusters. The most favorable core ion structure for all cluster sizes can be characterized as a Sn-oxalate complex, Sn[C₂O₄]^-. Higher energy isomers based on a bidentate η²-(C,O) CO₂ ligand tightly bound to the metal atom in SnCO₂^- complexes are also observed, even for the largest cluster sizes studied here. For n = 2, another high energy isomer is found, featuring a CO₂ ligand weakly bound to the metal atom in a SnCO₂^- ion.

Titanium Insertion into CO Bonds in Anionic Ti-CO2 Complexes

We explore the structures of [Ti(CO₂) _y]^- cluster anions using infrared photodissociation spectroscopy and quantum chemistry calculations. The existence of spectral signatures of metal carbonyl CO stretching modes shows that insertion of titanium atoms into C-O bonds represents an important reaction during the formation of these clusters. In addition to carbonyl groups, the infrared spectra show that the titanium center is coordinated to oxalato, carbonato, and oxo ligands, which form along with the metal carbonyls. The presence of a metal oxalato ligand promotes C-O bond insertion in these systems. These results highlight the affinity of titanium for C-O bond insertion processes.

C-C Bond Formation of Mg- and Zn-Activated Carbon Dioxide

Gas-phase activation of CO₂ by chloride tagged metal atoms, [ClM]^- (M=Mg, Zn), has been investigated by mass spectrometry and high-level quantum chemistry. Both metals activate CO₂ with significant bending of the CO₂ moiety to form complexes with the general formula [ClM,CO₂ ]^- . The structure of the metal-CO₂ complex depends on the method of formation, and the energy landscapes and reaction dynamics have been probed by collisional induced dissociation and thermal ion molecule reactions with isotopically labeled species. Having established these structural relationships, the gas-phase reactivity of [ClM(κ² -O₂ C)]^- with acetaldehyde (here considered a carbohydrate mimic) was then studied. Formation of lactate and enolate-pyruvate complexes are observed, showing that CO₂ fixation by C-C bond formation takes place. For M=Zn, even formation of free pyruvate ([C₃ H₃ O₃ ]^- ) is observed. Implications of the observed CO₂ reactivity for the electrochemical conversion of carbon dioxide, and to biochemical and artificial photosynthesis is briefly discussed. Detailed potential energy diagrams obtained by the quantum chemical calculations offer models consistent with experimental observation.

Coordination-induced CO2 fixation into carbonate by metal oxides

Infrared spectroscopic studies reveal how the coordination induces CO₂ fixation into carbonate by a cationic yttrium oxide model catalyst.