Experimental and theoretical study of the structure and reactivity of Fe1-2O< or =6- clusters with CO

Synergistic studies employing experiments in the gas phase and theoretical first principles calculations have been carried out to investigate the structure, stability, and reactivity toward CO of iron oxide cluster anions, Fe(x)O(y)- (x = 1-2, y < or = 6). Collision-induced dissociation studies of iron oxide species, employing xenon collision gas, show that FeO3- and FeO2- are the stable building blocks of the larger iron oxide clusters. Theoretical calculations show that the fragmentation patterns leading to the production of O or FeO(n) fragments are governed both by the energetics of the overall process as well as the number of intermediate states and the changes in spin multiplicity. Mass-selected experiments identified oxygen atom transfer to CO as the dominant reaction pathway for most anionic iron oxide clusters. A theoretical analysis of the molecular level pathways has been carried out to highlight the role of energetics as well as the spin states of the intermediates on the oxidation reaction.     

A kinetic study of the reactions FeO+ + O, Fe+.N2 + O, Fe+.O2 + O and FeO+ + CO: implications for sporadic E layers in the upper atmosphere

These gas-phase reactions were studied by pulsed laser ablation of an iron target to produce Fe(+) in a fast flow tube, with detection of the ions by quadrupole mass spectrometry. Fe(+).N(2) and Fe(+).O(2) were produced by injecting N(2) and O(2), respectively, into the flow tube. FeO(+) was produced from Fe(+) by addition of N(2)O, or by ligand-switching from Fe(+).N(2) following the addition of atomic O. The following rate coefficients were measured: k(FeO(+) + O --> Fe(+) + O(2), 186-294 K) = (3.2 +/- 1.5) x 10(-11); k(Fe(+).N(2) + O --> FeO(+)+ N(2), 294 K) = (4.6 +/- 2.5) x 10(-10); k(Fe(+).O(2) + O --> FeO(+) + O(2), 294 K) = (6.3 +/- 2.7) x 10(-11); and k(FeO(+) + CO --> Fe(+) + CO(2), 294 K) = (1.59 +/- 0.34) x 10(-10) cm(3) molecule(-1) s(-1), where the quoted uncertainties are a combination of the 1sigma standard errors in the kinetic data and the systematic experimental errors. The surprisingly slow reaction between FeO(+) and O is examined using ab initio quantum calculations of the relevant potential energy surfaces. The importance of this reaction for controlling the lifetime of sporadic E layers is then demonstrated using a model of the upper mesosphere and lower thermosphere.     

Experimental and theoretical study of the reactions between neutral vanadium oxide clusters and ethane, ethylene, and acetylene

Reactions of neutral vanadium oxide clusters with small hydrocarbons, namely C2H6, C2H4, and C2H2, are investigated by experiment and density functional theory (DFT) calculations. Single photon ionization through extreme ultraviolet (EUV, 46.9 nm, 26.5 eV) and vacuum ultraviolet (VUV, 118 nm, 10.5 eV) lasers is used to detect neutral cluster distributions and reaction products. The most stable vanadium oxide clusters VO2, V2O5, V3O7, V4O10, etc. tend to associate with C2H4 generating products V(m)O(n)C2H4. Oxygen-rich clusters VO3(V2O5)(n=0,1,2...), (e.g., VO3, V3O8, and V5O13) react with C2H4 molecules to cause a cleavage of the C=C bond of C2H4 to produce (V2O5)(n)VO2CH2 clusters. For the reactions of vanadium oxide clusters (V(m)O(n)) with C2H2 molecules, V(m)O(n)C2H2 are assigned as the major products of the association reactions. Additionally, a dehydration reaction for VO3 + C2H2 to produce VO2C2 is also identified. C2H6 molecules are quite stable toward reaction with neutral vanadium oxide clusters. Density functional theory calculations are employed to investigate association reactions for V2O5 + C2H(x). The observed relative reactivity of C2 hydrocarbons toward neutral vanadium oxide clusters is well interpreted by using the DFT calculated binding energies. DFT calculations of the pathways for VO3+C2H4 and VO3+C2H2 reaction systems indicate that the reactions VO3+C2H4 --> VO2CH2 + H2CO and VO3+C2H2 --> VO2C2 + H2O are thermodynamically favorable and overall barrierless at room temperature, in good agreement with the experimental observations.     

Comparison between the geometric and electronic structures and reactivities of [FeNO]7 and [FeO2]8 complexes: a density functional theory study

In a previous study, we analyzed the electronic structure of S = 3/2 [FeNO](7) model complexes [Brown et al. J. Am. Chem. Soc. 1995, 117, 715-732]. The combined spectroscopic data and SCF-X alpha-SW electronic structure calculations are best described in terms of Fe(III) (S = 5/2) antiferromagnetically coupled to NO(-) (S = 1). Many nitrosyl derivatives of non-heme iron enzymes have spectroscopic properties similar to those of these model complexes. These NO derivatives can serve as stable analogues of highly labile oxygen intermediates. It is thus essential to establish a reliable density functional theory (DFT) methodology for the geometry and energetics of [FeNO](7) complexes, based on detailed experimental data. This methodology can then be extended to the study of [FeO(2)](8) complexes, followed by investigations into the reaction mechanisms of non-heme iron enzymes. Here, we have used the model complex Fe(Me(3)TACN)(NO)(N(3))(2) as an experimental marker and determined that a pure density functional BP86 with 10% hybrid character and a mixed triple-zeta/double-zeta basis set lead to agreement between experimental and computational data. This methodology is then applied to optimize the hypothetical Fe(Me(3)TACN)(O(2))(N(3))(2) complex, where the NO moiety is replaced by O(2). The main geometric differences are an elongated Fe[bond]O(2) and a steeper Fe[bond]O[bond]O angle in the [FeO(2)](8) complex. The electronic structure of [FeO(2)](8) corresponds to Fe(III) (S = 5/2) antiferromagnetically coupled to O(2)(-) (S = 1/2), and, consistent with the extended bond length, the [FeO(2)](8) unit has only one Fe(III)-O(2)(-) bonding interaction, while the [FeNO](7) unit has both sigma and pi type Fe(III)-NO(-) bonds. This is in agreement with experiment as NO forms a more stable Fe(III)-NO(-) adduct relative to O(2)(-). Although NO is, in fact, harder to reduce, the resultant NO(-) species forms a more stable bond to Fe(III) relative to O(2)(-) due to the different bonding interactions.     

The dramatic effect of NH3 co-ligation on the Fe+-assisted activation of carbon dioxide in the gas phase: from bare metal ions to complexes

The catalytic efficiency of Fe(+) ion over the CO(2) decomposition in the gas phase has been extensively investigated with the help of electronic structure calculation methods. Potential-energy profiles for the activation process Fe(+) + CO(2) --> CO + FeO(+) along two rival potential reaction paths, namely the insertion and addition pathways, originating from the end-on kappa(1)-O and kappa(2)-O,O coordination modes of CO(2) with the metal ion, respectively, have been explored by DFT calculations. For each pathway the potential energy surfaces of the high-spin sextet (S = 5/2) and the intermediate-spin quartet (S = 3/2) spin-states have been explored. The complete energy reaction profile calculated by a combination of ab initio and density functional theory (DFT) computational techniques reveals a two-state reactivity, involving two spin inversions, for the decomposition process and accounts well for the experimentally observed inertness of bare Fe(+) ions towards CO(2) activation. Furthermore, the coordination of up to three extra ancillary NH(3) ligands with the Fe(+) metal ion has been explored and the geometric and energetic reaction profiles of the CO(2) activation processes Fe(+) + n x NH(3) + CO(2) --> [Fe(NH(3))(n)(CO(2))](+) --> [Fe(NH(3))(n)(O)(CO)](+) --> CO + [Fe(O)(NH(3))(n)](+) (n = 1, 2 or 3) have thoroughly been scrutinized for both the insertion and the addition mechanisms. Inter alia, the geometries and energies of the various states of the [Fe(NH(3))(n)(CO(2))](+) and [Fe(NH(3))(n)(O)(CO)](+) complexes are explored and compared. Finally, a detailed analysis of the coordination modes of CO(2) in the cationic [Fe(NH(3))(n)(CO(2))](+) (n = 0, 1, 2 and 3) complexes is presented.     

A two-state reactivity rationale for counterintuitive axial ligand effects on the C-H activation reactivity of nonheme FeIV=O oxidants

This paper addresses the observation of counterintuitive reactivity patterns of iron-oxo reagents, TMC(L)FeO(2+,1+); L=CH(3)CN, CF(3)CO(2) (-), N(3) (-), and SR(-), in O-transfer to phosphines versus H-abstraction from, for example, 1,4-cyclohexadiene. Experiments show that O-transfer reactivity correlates with the electrophilicity of the oxidant, but H-abstraction reactivity follows an opposite trend. DFT/B3 LYP calculations reveal that two-state reactivity (TSR) serves as a compelling rationale for these trends, whereby all reactions involve two adjacent spin-states of the iron(IV)-oxo species, triplet and quintet. The ground state triplet surface has high barriers, whereas the excited state quintet surface features lower ones. The barriers, on any single surface, are found to increase as the electrophilicity of TMC(L)FeO(2+,1+) decreases. Thus, the counterintuitive behavior of the H-abstraction reactions cannot be explained by considering the reactivity of only a single spin state but can be rationalized by a TSR model in which the reactions proceed on the two surfaces. Two TSR models are outlined: one is traditional involving a variable transmission coefficient for crossover from triplet to quintet, followed by quintet-state reactions; the other considers the net barrier as a blend of the triplet and quintet barriers. The blending coefficient (x), which estimates the triplet participation, increases as the quintet-triplet energy gap of the TMC(L)FeO(2+,1+) reagent increases, in the following order of L: CH(3)CN > CF(3)CO(2) (-) > N(3) (-) > SR(-). The calculated barriers predict the dichotomic experimental trends and the counterintuitive behavior of the H-abstraction series. The TSR approaches make a variety of testable predictions.     

Gas-phase IR spectroscopy of anionic iron carbonyl clusters

The first gas-phase vibrational spectra are presented for several anionic iron carbonyl clusters, ranging in size from Fe(CO)4- to Fe5(CO)14- in the CO-stretching region (1600-2100 cm-1). The experimental spectra provide some immediate structural information about the clusters in the form of low-wavenumber (1750-1850 cm-1) bands marking the presence of bridging carbonyl ligands (mu2-COs). Supporting DFT calculations are presented for the smaller clusters (<3 Fe atoms) and give good agreement with the experimental data, allowing structural assignments for these cases. The Fe2(CO)7- spectrum suggests a structure lacking bridging carbonyl ligands, in agreement with the DFT results. For the case of Fe2(CO)8-, there are two possible structures based on the calculations, both with and without bridging carbonyls. The presence of a low-frequency band ( approximately 1770 cm-1) in the experimental spectrum conclusively demonstrates the existence of the bridged form. The ramifications of these data for metal-metal bonding in the clusters are also considered.     

CO bond cleavage on supported nano-gold during low temperature oxidation

The oxidation of CO by Au/Fe(2)O(3) and Au/ZnO catalysts is compared in the very early stages of the reaction using a temporal analysis of products (TAP) reactor. For Au/Fe(2)O(3) pre-dosing the catalyst with (18)O labelled water gives an unexpected evolution order for the labelled CO(2) product with the C(18)O(2) emerging first, whereas no temporal differentiation is found for Au/ZnO. High pressure XPS experiments are then used to show that CO bond cleavage does occur for model catalysts consisting of Au particles deposited on iron oxide films but not when deposited on ZnO films. DFT calculations, show that this observation requires carbon monoxide to dissociate in such a way that cleavage of the CO bond occurs along with dynamically co-adsorbed oxygen so that the overall process of Au oxidation and CO dissociation is energetically favourable. Our results show that for Au/Fe(2)O(3) there is a pathway for CO oxidation that involves atomic C and O surface species which operates along side the bicarbonate mechanism that is widely discussed in the literature. However, this minor pathway is absent for Au/ZnO.     

The subtle effects of iron-containing metal surfaces on the reductive carbonylation of RuCl3

The use of iron-containing metal surfaces, Fe, Fe-Cr-alloy and stainless steel, for the synthesis of mixed metal Ru-Fe compounds has been studied. The studied process was reductive carbonylation of RuCl3 in the presence of a metal surface. Reactions were carried out in ethanol solutions under 10-50 bar carbon monoxide pressure at 125 degrees C using an autoclave. During the reaction the metal surface was oxidized, releasing iron into the solution and acting as a sacrificial source of iron. Under these conditions the corrosion of the metal surface was facile and produced a series of iron-containing species. In addition to the formation of most obvious iron(II) products, such as [Fe(H2O)6]2+ or [FeCl2(H2O)4] the use of the metal surface also provided a route to novel labile trinuclear [Ru2Cl2(mu-Cl)4(CO)6FeL2] (L = H2O, EtOH) complexes. The stability and reactivity of the [Ru2Cl2(mu-Cl)4(CO)6FeL2] complexes were further studied using computational DFT methods. Based on the computational results a reaction route has been suggested for the formation and decomposition of [Ru2Cl2(mu-Cl)4(CO)6FeL2].     

Tuning cluster reactivity by charge state and composition: experimental and theoretical investigation of CO binding energies to Ag(n)Au(m)(+/-) (n + m = 3)

Temperature-dependent gas-phase reaction kinetics measurements and equilibrium thermodynamics under multicollision conditions in conjunction with ab initio DFT calculations were employed to determine the binding energies of carbon monoxide to triatomic silver-gold binary cluster cations and anions. The binding energies of the first CO molecule to the trimer clusters increase with increasing gold content and with changing charge from negative to positive. Thus, the reactivity of the binary clusters can be sensitively tuned by varying charge state and composition. Also, multiple CO adsorption on the clusters was investigated. The maximum number of adsorbed CO molecules was found to strongly depend on cluster charge and composition as well. Most interestingly, the cationic carbonyl complex Au(3)(CO)(4)(+) is formed at cryogenic temperature, whereas for the anion, only two CO molecules are adsorbed, leading to Au(3)(CO)(2)(-). All other trimer clusters adsorb three CO molecules in the case of the cations and are completely inert to CO in our experiment in the case of the anions.     

Intermediates of CO oxidation on iron oxides: an experimental and theoretical study

Reactions of laser-ablated iron oxides with CO in excess argon are investigated by infrared adsorption spectroscopy and density functional theoretical calculations. The carbonyl iron oxides OFe(CO)(n) (n = 1-3) and O(2)Fe(CO)(m) (m = 1, 2) are generated during sample deposition or annealing, whereas CO(2) is greatly produced at the expense of these carbonyl iron oxides upon UV irradiation, showing the formation of intermediate carbonyl iron oxides in the oxidation of carbon monoxide to carbon dioxide. These intermediate carbonyl iron oxides are characterized on the basis of isotopic substitution, stepwise annealing, change of CO concentration and laser energy, and comparison with theoretical calculations. The overall agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these complexes from the matrix infrared spectra. The reaction pathways for the formation of the products are proposed based on the experimental and theoretical results presented.     

A theoretical study on the complete catalytic cycle of the hetero-Pauson-Khand-type [2+2+1] cycloaddition reaction of ketimines,carbon monoxide and ethylene catalyzed by iron carbonyl complexes

The [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin-2-one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6-311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)(3)] complex of square-pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square-pyramidal complex. This reaction yields an intermediate I(4) in which ethylene is coordinated to the iron centre and a new C-C bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal-carbon bond between ethylene and iron produces the key intermediate I(7). The reaction proceeds by metal-assisted formation of a lactam P. The catalytic cycle is closed by a ligand-exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)(3)] fragment.     

Partial oxidation of propylene catalyzed by VO3 clusters: a density functional theory study

Density functional theory (DFT) calculations are carried out to investigate partial oxidation of propylene over neutral VO 3 clusters. C=C bond cleavage products CH 3CHO + VO 2CH 2 and HCHO + VO 2CHCH 3 can be formed overall barrierlessly from the reaction of propylene with VO 3 at room temperature. Formation of hydrogen transfer products H 2O + VO 2C 3H 4, CH 2=CHCHO + VO 2H 2, CH 3CH 2CHO + VO 2, and (CH 3) 2CO + VO 2 is subject to tiny (0.01 eV) or small (0.06 eV, 0.19 eV) overall free energy barriers, although their formation is thermodynamically more favorable than the formation of C=C bond cleavage products. These DFT results are in agreement with recent experimental observations. VO 3 regeneration processes at room temperature are also investigated through reaction of O 2 with the CC bond cleavage products VO 2CH 2 and VO 2CHCH 3. The following barrierless reaction channels are identified: VO 2CH 2 + O 2 --> VO 3 + CH 2O; VO 2CH 2 + O 2 --> VO 3C + H 2O, VO 3C + O 2 --> VO 3 + CO 2; VO 2CHCH 3 + O 2 --> VO 3 + CH 3CHO; and VO 2CHCH 3 + O 2 --> VO 3C + CH 3OH, VO 3C + O 2 --> VO 3 + CO 2. The kinetically most favorable reaction products are CH 3CHO, H 2O, and CO 2 in the gas phase model catalytic cycles. The results parallel similar behavior in the selective oxidation of propylene over condensed phase V 2O 5/SiO 2 catalysts.     

Influence of stoichiometry and charge state on the structure and reactivity of cobalt oxide clusters with CO

Cationic and anionic cobalt oxide clusters, generated by laser vaporization, were studied using guided-ion-beam mass spectrometry to obtain insight into their structure and reactivity with carbon monoxide. Anionic clusters having the stoichiometries Co2O3(-), Co2O5(-), Co3O5(-) and Co3O6(-) were found to exhibit dominant products corresponding to the transfer of a single oxygen atom to CO, indicating the formation of CO 2. Cationic clusters, in contrast, displayed products resulting from the adsorption of CO onto the cluster accompanied by the loss of either molecular O 2 or cobalt oxide units. In addition, collision induced dissociation experiments were conducted with N 2 and inert xenon gas for the anionic clusters, and xenon gas for the cationic clusters. It was found that cationic clusters fragment preferentially through the loss of molecular O 2 whereas anionic clusters tend to lose both atomic oxygen and cobalt oxide units. To further analyze how stoichiometry and ionic charge state influence the structure of cobalt oxide clusters and their reactivity with CO, first principles theoretical electronic structure studies within the density functional theory framework were performed. The calculations show that the enhanced reactivity of specific anionic cobalt oxides with CO is due to their relatively low atomic oxygen dissociation energy which makes the oxidation of CO energetically favorable. For cationic cobalt oxide clusters, in contrast, the oxygen dissociation energies are calculated to be even lower than for the anionic species. However, in the cationic clusters, oxygen is calculated to bind preferentially in a less activated molecular O 2 form. Furthermore, the CO adsorption energy is calculated to be larger for cationic clusters than for anionic species. Therefore, the experimentally observed displacement of weakly bound O 2 units through the exothermic adsorption of CO onto positively charged cobalt oxides is energetically favorable. Our joint experimental and theoretical findings indicate that positively charged sites in bulk-phase cobalt oxides may serve to bind CO to the catalyst surface and specific negatively charged sites provide the activated oxygen which leads to the formation of CO 2. These results provide molecular level insight into how size, stoichiometry, and ionic charge state influence the oxidation of CO in the presence of cobalt oxides, an important reaction for environmental pollution abatement.     

Communication: CO oxidation by silver and gold cluster cations: identification of different active oxygen species

The oxidation of carbon monoxide with nitrous oxide on mass-selected Au(3)(+) and Ag(3)(+) clusters has been investigated under multicollision conditions in an octopole ion trap experiment. The comparative study reveals that for both gold and silver cations carbon dioxide is formed on the clusters. However, whereas in the case of Au(3)(+) the cluster itself acts as reactive species that facilitates the formation of CO(2) from N(2)O and CO, for silver the oxidized clusters Ag(3)O(x)(+) (n=1-3) are identified as active in the CO oxidation reaction. Thus, in the case of the silver cluster cations N(2)O is dissociated and one oxygen atom is suggested to directly react with CO, whereas a second kind of oxygen strongly bound to silver is acting as a substrate for the reaction.     

Experimental and theoretical studies of reactions of neutral vanadium and tantalum oxide clusters with NO and NH3

Reactions of neutral vanadium and tantalum oxide clusters with NO, NH(3), and an NO/NH(3) mixture in a fast flow reactor are investigated by time of flight mass spectrometry and density functional theory (DFT) calculations. Single photon ionization through a 46.9 nm (26.5 eV) extreme ultraviolet (EUV) laser is employed to detect both neutral cluster distributions and reaction products. Association products VO(3)NO and V(2)O(5)NO are detected for V(m)O(n) clusters reacting with pure NO, and reaction products, TaO(3,4)(NO)(1,2), Ta(2)O(5)NO, Ta(2)O(6)(NO)(1-3), and Ta(3)O(8)(NO)(1,2) are generated for Ta(m)O(n) clusters reacting with NO. In both instances, oxygen-rich clusters are the active metal oxide species for the reaction M(m)O(n)+NO→M(m)O(n)(NO)(x). Both V(m)O(n) and Ta(m)O(n) cluster systems are very active with NH(3). The main products of the reactions with NH(3) result from the adsorption of one or two NH(3) molecules on the respective clusters. A gas mixture of NO:NH(3) (9:1) is also added into the fast flow reactor: the V(m)O(n) cluster system forms stable, observable clusters with only NH(3) and no V(m)O(n)(NO)(x)(NH(3))(y) species are detected; the Ta(m)O(n) cluster system forms stable, observable mixed clusters, Ta(m)O(n)(NO)(x)(NH(3))(y), as well as Ta(m)O(n)(NO)(x) and Ta(m)O(n)(NH(3))(y) individual clusters, under similar conditions. The mechanisms for the reactions of neutral V(m)O(n) and Ta(m)O(n) clusters with NO/NH(3) are explored via DFT calculations. Ta(m)O(n) clusters form stable complexes based on the coadsorption of NO and NH(3). V(m)O(n) clusters form weakly bound complexes following the reaction pathway toward end products N(2)+H(2)O without barrier. The calculations give an interpretation of the experimental data that is consistent with the condensed phase reactivity of V(m)O(n) catalyst and suggest the formation of intermediates in the catalytic chemistry.     

Structure of Bromotrinitrosyl Iron, [Fe(NO)3Br], and DFT Calculations of the Structures of [Fe(NO)3X] (X = Cl,Br, I)

Bromotrinitrosyl iron was prepared by passing a stream of nitrogen monoxide over a mixture of iron dibromide and iron powder at elevated temperatures. It readily loses NO to give [(ON)₂Fe(μ‐Br)Fe(CO)₂]. The structure of freshly obtained [Fe(NO)₃Br] was determined by X‐ray diffraction at 200 K and shows (distorted) tetrahedral coordination with N–Fe–N and N–Fe–Br angles of 107.9(2)° and 111.0(2)° and bent Fe–N–O groups (162.5(6)°). The DFT calculations in the series [Fe(NO)₃X] (X = Cl, Br, I) reproduce well the experimental structural parameters and vibrational frequencies.     

Probing the structural and electronic properties of small vanadium monoxide clusters

The structural evolution and bonding of a series of early transition-metal oxide clusters, V(n)O(q) (n = 3-9, q = 0,-1), have been investigated with the aid of previous photoelectron spectroscopy (PES) and theoretical calculations. For each vanadium monoxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium monoxide clusters obtained from the DFT calculations. The results demonstrate that the combination of photoelectron spectroscopy experiments and DFT calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers. The second difference energies and adsorption energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The adsorption energies of one O atom on the anionic (6.64 → 8.16 eV) and neutral (6.41 → 8.13 eV) host vanadium clusters are shown to be surprisingly high, suggesting strong capabilities to activate O by structural defects in vanadium oxides.     

Closed Shell Iron(IV) Oxo Complex with an Fe–O Triple Bond: Computational Design,Synthesis, and Reactivity

Iron(IV)-oxo intermediates in nature contain two unpaired electrons in the Fe–O antibonding orbitals, which are thought to contribute to their high reactivity. To challenge this hypothesis, we designed and synthesized closed-shell singlet iron(IV) oxo complex [(quinisox)Fe(O)]⁺ ( 1⁺ ; quinisox-H =(N-(2-(2-isoxazoline-3-yl)phenyl)quinoline-8-carboxamide). We identified the quinisox ligand by DFT computational screening out of over 450 candidates. After the ligand synthesis, we detected 1⁺ in the gas phase and confirmed its spin state by visible and infrared photodissociation spectroscopy (IRPD). The Fe–O stretching frequency in 1⁺ is 960.5 cm⁻¹, consistent with an Fe–O triple bond, which was also confirmed by multireference calculations. The unprecedented bond strength is accompanied by high gas-phase reactivity of 1⁺ in oxygen atom transfer (OAT) and in proton-coupled electron transfer reactions. This challenges the current view of the spin-state driven reactivity of the Fe–O complexes.     

Enhancement of ammonia dehydrogenation by introduction of oxygen onto cobalt and iron cluster cations

Reactions of oxygen-chemisorbed cobalt and iron cluster cations (Co(n)O(m)(+) and Fe(n)O(m)(+); n = 3-6, m = 1-3) with an NH(3) molecule have been investigated in comparison with their bare metal cluster cations at a collision energy of 0.2 eV by use of a guided ion beam tandem mass spectrometer. We have observed three kinds of reaction products, which come from NH(3) chemisorption with and without release of a metal atom from the cluster and dehydrogenation of the chemisorbed NH(3). Reaction cross sections and branching fractions are strongly influenced by the number of oxygen atoms introduced onto the metal clusters. Oxygen-chemisorbed metal clusters with particular compositions such as Co(4)O(+), Co(5)O(2)(+), and Fe(5)O(2)(+) are extremely reactive for NH(3) dehydrogenation, whereas Co(4)O(2)(+) and Fe(4)O(2)(+) exhibit high reactivity for NH(3) chemisorption with metal release. The enhancement of dehydrogenation for specific compositions can be interpreted in terms of competition between O-H and neighboring Co-H (or Fe-H) formation.