Sequential Bond Energies of Fe+(CO2)n,n= 1−5, Determined by Threshold Collision-Induced Dissociation and ab Initio Theory†

Covalent Bonding Between Be+ and CO2 in BeOCO+ with a Surprisingly High Antisymmetric OCO Stretching Vibration

The cationic complex BeOCO⁺ is produced in a solid neon matrix. Infrared absorption spectroscopic study shows that it has a very high antisymmetric OCO stretching vibration of 2418.9 cm^-1, which is about 71 cm^-1 blue-shifted from that of free CO₂. The quantum chemical calculations are in very good agreement with the experimental observation. Depending on the theoretical method, a linear or quasi-linear structure is predicted for the cation. The analysis of the electronic structure shows that the bonding of Be⁺ to one oxygen atom induces very little charge migration between the two moieties, but it causes a significant change in the σ-charge distribution that strengthens the terminal C-O bond, leading to the observed blue shift. The bonding analysis reveals that the Be⁺ ← OCO donation results in strong binding due to the interference of the wave function and a charge polarization within the CO₂ fragment and hybridization to Be⁺ but only negligible charge donation.

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.

Reactivity of 4Fe+(CO)n=0-2 + O2: oxidation of CO by O2 at an isolated metal atom

The kinetics of ⁴Fe⁺(CO)_n=0-2 + O₂ are measured under thermal conditions from 300-600 K using a selected-ion flow tube apparatus. Both the bare metal and n = 2 cations are inert to reaction over this temperature range, but ⁴Fe⁺(CO) reacts rapidly (k = 3.2 ± 0.8 × 10^-10 cm³ s^-1 at 300 K, 52% of the collisional rate coefficient) to form FeO⁺ + CO₂. This is an example of the oxidation of CO by O₂ occurring entirely on a single non-noble metal atom. The reaction of the bare metal reaction is known to be endothermic, such that this result is expected; however, the n = 2 reaction has highly exothermic product channels available, such that the lack of reaction is surprising in light of the n = 1 reactivity. Stationary points along all three reaction coordinates are calculated using the TPSSh hybrid functional. These surfaces show that the n = 1 reaction is an example of two-state reactivity; the reaction proceeds initially on the sextet surface over a submerged barrier to a structure with an O-O bond distance longer than that in O₂, but must cross to the quartet surface in order to proceed over a second submerged barrier to rearrange to form CO₂. The n = 2 reaction does not proceed because, on all spin surfaces, the transition state corresponding to O-O separation is at higher energy than the separated reactants. The difference between the n = 1 and n = 2 reactions is not a result of steric effects, but rather because the O₂ is more strongly bound to Fe in the entrance well of the n = 1 case, and that energy is available to overcome the rate-limiting barrier to O-O cleavage. Experimental verification of some of these details are provided by guided ion beam tandem mass spectrometry results. The kinetic energy dependence of the n = 1 reaction shows evidence for a curve crossing and yields relevant thermochemistry for competing reaction channels.

Cationic Noncovalent Interactions: Energetics and Periodic Trends

In this review, noncovalent interactions of ions with neutral molecules are discussed. After defining the scope of the article, which excludes anionic and most protonated systems, methods associated with measuring thermodynamic information for such systems are briefly recounted. An extensive set of tables detailing available thermodynamic information for the noncovalent interactions of metal cations with a host of ligands is provided. Ligands include small molecules (H2, NH3, CO, CS, H2O, CH3CN, and others), organic ligands (O- and N-donors, crown ethers and related molecules, MALDI matrix molecules), π-ligands (alkenes, alkynes, benzene, and substituted benzenes), miscellaneous inorganic ligands, and biological systems (amino acids, peptides, sugars, nucleobases, nucleosides, and nucleotides). Hydration of metalated biological systems is also included along with selected proton-based systems: 18-crown-6 polyether with protonated peptides and base-pairing energies of nucleobases. In all cases, the literature thermochemistry is evaluated and, in many cases, reanchored or adjusted to 0 K bond dissociation energies. Trends in these values are discussed and related to a variety of simple molecular concepts.

IR spectroscopy of gas phase V(CO2)n+ clusters: solvation-induced electron transfer and activation of CO2

Ion-molecule complexes of vanadium and CO2, i.e., V(CO2)n(+), produced by laser vaporization are mass selected and studied with infrared laser photodissociation spectroscopy. Vibrational bands for the smaller clusters (n < 7) are consistent with CO2 ligands bound to the metal cation via electrostatic interactions and/or attaching as inert species in the second coordination sphere. All IR bands for these complexes are consistent with intact CO2 molecules weakly perturbed by cation binding. However, multiple new IR bands occur only in larger complexes (n ≥ 7), indicating the formation of an intracluster reaction product whose nominal mass is the same as that of V(CO2)n(+) complexes. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a solvation-induced metal→ligand electron transfer reaction.

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.

Gas-phase reactivity of selected transition metal cations with CO and CO2 and the formation of metal dications using a sputter ion source

Reaction products and rates were measured for the gas-phase reactions of selected first row transition metal ions (Ti+, V+, Fe+, Co+, Ni+, Cu+, Zn+) and both CO and CO2 using a flowing afterglow instrument. The formation and description of products formed and reaction mechanisms are presented and discussed as well. Ab initio calculations were used to produce potential-energy surface diagrams for selected reactions as a tool to further understand the reaction mechanisms, thermochemistry, and reaction kinetics. Reactions with CO are slow and typically yield complexes of the form M(CO)n+ (n = 1-2), with the second CO molecule appearing to be added faster than the first one. Reactions with CO2 also yield the formation of clusters; however, in the case of Ti+, the reaction produces the oxide TiO+ ion efficiently. An interesting observation was also the formation of metal doubly charged ions. Some dications were easily obtained as the major ion by changing the ionization conditions in the sputter ion source. We are proposing an ionization mechanism for the formation of the dications.