IR Spectroscopy of Nb+(N2)n Complexes: Coordination, Structures, and Spin States

A Coppoborylene Stabilized by Multicenter Covalent Bonding and Its Amphoteric Reactivity to CO

A cationic copper-stabilized coppoborylene was prepared and structurally characterized via infrared photodissociation spectroscopy and density functional theory calculations. This structure exemplifies a new class of borylenes stabilized by three-center-two-electron metal-boron-metal covalent bonding interaction, displaying exceptional σ-acidity and unparalleled π-donor capability for CO activation that outperforms all of the known transition metal cations and is comparable or even superior to the documented base-trapped borylenes. Its neutral form represents a monovalent boron compound with a strongly reactive amphoteric boron center built on transition-metal-boron bonds, which inspires the design and synthesis of new members of the borylene family.

Pentacoordinated pyramidal structures and bonding properties of WN10-/0: anion photoelectron spectroscopy and theoretical calculations

Anion photoelectron spectroscopy and theoretical calculations were used to investigate the structural and bonding properties of WN10-/0. The electron affinity of WN10 is measured to be 1.582 ± 0.030 eV. The frequency of the NN stretch in WN10 is measured to be 2170 ± 80 cm-1, which is red-shifted with respect to that of the dinitrogen molecule indicating that the NN bonds are weakened in WN10. The theoretical adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of WN10- obtained by calculations at the CCSD(T)/CBS level agree well with experimental results. The structures of WN10-/0 are C4v symmetric pentacoordinated pyramidal structures with five end-on dinitrogen ligands. Our experiments show that the peak of WN10- is dominant in the mass spectrum of anionic WNn, whereas the mass peak of WN12+ is dominant in the mass spectrum of cationic WNn, implying that the stabilities of WNn clusters are strongly related to their charge states.

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.

Spectroscopic Identification of the Dinitrogen Fixation and Activation by Metal Carbide Cluster Anions PtCn- (n = 4-6)

Nitrogen fixation is confronted with great challenges in the field of chemistry. Herein, we report that single metal carbides PtC_n^- and PtC_nN₂^- (n = 4-6) are indispensable intermediates in the process of nitrogen fixation by mass spectrometry coupled with anionic photoelectron spectroscopy, quantum chemical calculations, and simulated density-of-state spectra. The most stable isomers of these cluster anions are characterized to have linear chain structures. The fixation and activation of dinitrogen are facilitated by the charge transfer from Pt and C_n to N₂. The significance of π back-donation of the 5d orbital of the Pt atom to the antibonding π orbits of N₂ for dinitrogen fixation and activation is discussed in detail. This study not only provides a theoretical basis at the molecular level for the activation of dinitrogen by mononuclear metal carbide clusters but also provides a new paradigm for dinitrogen fixation.

Reaction of Ta3 - Clusters with Molecular Nitrogen: A Mechanism Investigation

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Ta3 - is an exceptionally small cluster that reacts with N2 at room temperature, leading finally to Ta3N2 -; Ta3N2 - also could react with N2 at room temperature, leading finally to Ta3N4 -, a product of interest in its own right because of its potential as a nitrogen fixation medium. The mechanisms of the Ta3 -- and Ta3N2 --mediated activation of the N≡N triple bond have been investigated. Our extensive computations elucidate mechanisms for the ready reactions, leading to stepwise cleavage of the N≡N bond. Initial isomeric N2/Ta3 - complexes, N≡N elongation, undergo a N≡N split over generally low barriers in a highly exothermic process. The nitrogen-atom or molecular exchange reactions found in the Ta3N2 -/N2 system may be of paramount importance in both applied and fundamental studies.

Recent Progress in Dinitrogen Activation by Gas-Phase Metal Species

Understanding the mechanisms to activate and functionalize dinitrogen (N₂) is of great importance for the rational design of nitrogen-fixation catalysts. Reactions of gas-phase species with N₂ are being actively studied to understand the bond activation and formation processes at a strictly molecular level. This Perspective provides an overview of the recent progress in combined experimental and theoretical studies on the activation and functionalization of N₂ by gas-phase metal species. New mechanistic insights into N₂ molecular adsorption, N≡N cleavage, and N-X (X = C, B, and H) formation have been introduced, in which the new reaction channels of ejecting neutral metal fragments and the coupling reactions of N₂ with other molecules are highlighted. Finally, the current challenges and outlooks of N₂ activation in the gas phase are discussed as well.

Spectroscopic Characterization of the Synergistic Mechanism of Ruthenium-Lithium Hydrides for Dinitrogen Cleavage

Elucidating the role of alkali/alkaline earth metal hydrides in dinitrogen activation remains an important and challenging goal for spectroscopic studies of bulk systems, because their spectral signatures are often masked by the collective effects. Herein, mass-selected photoelectron velocity-map imaging spectroscopic and quantum chemical calculation techniques are utilized to explore the promotion mechanism of LiH in the Ru-based catalysts toward N₂ activation. The RuHN₂^- anion is determined to be a N₂-tagged complex. In contrast, the RuHN₂(LiH)_n^- (n = 1 and 2) anions are characterized to have N≡N bond-cleaved ring structures. These observations indicate that the complexation of LiH to RuH^- significantly facilitates N≡N bond cleavage. Theoretical analyses show that the synergy between Ru and LiH efficiently lowers the energy barrier of N≡N bond cleavage. These findings clarify the pivotal roles played by the LiH species in the transition metal catalysts for N₂ activation and have important practical implications for the prospective design of high-performance catalysts via metal tuning strategies.

Cryo spectroscopy of N2 on cationic iron clusters

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fe_n(N₂)_m]⁺ for n = 8-20 reveal slightly redshifted IR active bands in the region of 2200-2340 cm^-1. These bands mostly relate to stretching vibrations of end-on coordinated N₂ chromophores, a μ_1,end end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe₁₃ ⁺ core structure. The first adsorbate shell closure at (n,m) = (13,12)-as recognized by the accompanying paper on the kinetics of N₂ uptake by cationic iron clusters-comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N₂ adsorbates. DFT modeling predicts spin quenching by N₂ adsorption as evidenced by the shift of the computed spin minima among possible spin states (spin valleys). The IR-PD spectrum of (17,1) surprisingly reveals an absence of any structure but efficient non-resonant fragmentation, which might indicate some weakly bound (roaming) N₂ adsorbate. The multiple and broad bands of (17,m) for all other cases than (17,1) and (17,7) indicate a high degree of variation in N₂ binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N₂ adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120-2200 cm^-1 region, which we associate with a μ_1,side side-on motif. Some additional features in the (18,m) spectra at high N₂ loads indicate a μ_1,tilt tilted end-on adsorption motif.

Photodissociation and Infrared Spectroscopy of Uranium-Nitrogen Cation Complexes

Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U⁺(N₂)_n (n = 1-8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U⁺(N₂)_n indicate that N₂ ligands are intact molecules and that there is no insertion chemistry resulting in UN⁺ or NUN⁺. Fixed frequency photodissociation at 532 and 355 nm indicate that the U⁺-N₂ bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U⁺-N₂ dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N-N stretch vibration via elimination of N₂ molecules. These resonances are observed to be shifted about 130 cm^-1 to the red from the free-N₂ frequency for complexes with n = 3-8. Density functional theory indicates that U⁺ is most stable in the sextet state in these complexes and that N₂ molecules bind in end-on configurations. The fully coordinated complex is predicted to be U⁺(N₂)₈, which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U⁺-N₂ dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.

Infrared Spectroscopy and Bonding of the B(NN)3+ and B2(NN)3,4+ Cation Complexes

The boron-dinitrogen cation complexes B(NN)₃⁺ and B₂(NN)_3,4⁺ are produced in the gas phase and are studied by infrared photodissociation spectroscopy in the N-N stretching vibrational frequency region. The geometric and electronic structures are determined by comparison of the experimental spectra with density functional theory calculations. The B(NN)₃⁺ cation is characterized to have a closed-shell singlet ground state with planar D_3h symmetry. The B₂(NN)₃⁺ cation is determined to have a B═B bonded (NN)₂BBNN structure with C_2v symmetry. Two isomers of the B₂(NN)₄⁺ cation contribute to the experimental spectrum. One is a N₂-tagged complex involving a B₂(NN)₃⁺ core ion. Another one is a B-B bonded B₂(NN)₄⁺ complex with a planar D_2h structure. Bonding analyses reveal that the B-NN interactions in these complexes come mainly from covalent orbital interactions, with the NN → B σ donation being stronger than the B → NN π back-donation.

Identification of octahedral coordinated ZrN12+ cationic clusters by mass spectrometry and structure searches

Cationic zirconium-doped nitrogen clusters were produced by laser ablation of a Zr : BN mixture target and were detected by TOF mass spectrometry. It is found that the mass peak of the ZrN12+ cluster is dominant in the spectrum. The ZrN12+ cluster was further dissociated with 266 nm photons. Extensive structure searches of a cationic ZrN12+ cluster indicate that the ground state structure of ZrN12+ consists of a central Zr atom and six N2 pairs with Oh symmetry. The calculated binding energy of the ZrN12+ cluster is about 0.96 eV, which is in accordance with the result of the photodissociation experiment. The neutral ZrN12 cluster has almost the same geometry, but with D3h symmetry. NBO analysis showed that the molecular orbitals of ZrN12+/0 clusters are mainly composed of Zr 4d and N 2p orbitals. These findings provide rich information for understanding the geometries and the electronic properties of zirconium-doped N clusters, which will offer valuable guidance for the exploration of other metal doped nitrogen clusters.

Side-on-End-on Coordination of Dinitrogen on a Polynuclear Vanadium Nitride Cluster Anion [V5 N5 ]. Chemistry 2019;25:16523-16527. [PMID: 31637740 DOI: 10.1002/chem.201904362]

The side-on-end-on coordination of N₂ can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas-phase species (a polynuclear vanadium nitride cluster anion [V₅ N₅ ]^- ) that can capture N₂ efficiently (12 %), and the quantum chemistry modelling suggests an unusual side-on-end-on coordination. The cluster anions were generated by laser ablation and the reaction with N₂ has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back-donation interactions between the localized d-d bonding orbitals on the low-coordinated dual metal (V) sites and the antibonding π* orbitals of N₂ are the driving forces to adsorb N₂ with a high binding energy (about 2.0 eV).

Octa-coordinated alkaline earth metal-dinitrogen complexes M(N2)8 (M=Ca, Sr, Ba)

We report the isolation and spectroscopic identification of the eight-coordinated alkaline earth metal–dinitrogen complexes M(N₂)₈ (M=Ca, Sr, Ba) possessing cubic (O_h) symmetry in a low-temperature neon matrix. The analysis of the electronic structure reveals that the metal-N₂ bonds are mainly due to [M(d_π)]→(N₂)₈ π backdonation, which explains the observed large red-shift in N-N stretching frequencies. The adducts M(N₂)₈ have a triplet (³A_1g) electronic ground state and exhibit typical bonding features of transition metal complexes obeying the 18-electron rule. We also report the isolation and bonding analysis of the charged dinitrogen complexes [M(N₂)₈]⁺ (M=Ca, Sr).

The study of main group complexes remains important to our fundamental understanding of main group element bonding and properties. Here the authors isolate and spectroscopically characterize a series of 8-coordinated alkaline earth metal–dinitrogen complexes M(N₂)₈ (M=Ca, Sr, Ba) in a low-temperature neon matrix.

Structural evolution of LiNn+ (n = 2, 4, 6, 8, and 10) clusters: mass spectrometry and theoretical calculations

Mixed nitrogen-lithium cluster cations LiN_n⁺ were generated by laser vaporization and analyzed by time-of-flight mass spectrometry. It is found that LiN₈⁺ has the highest ion abundance among the LiN_n⁺ ions in the mass spectrum. Density functional calculations were conducted to search for the stable structures of the Li–N clusters. The theoretical results show that the most stable isomers of LiN_n⁺ clusters are in the form of Li⁺(N₂)_n/2, and the order of their calculated binding energies is consistent with that of Li–N₂ bond lengths. The most stable structures of LiN_n⁺ evolve from one-dimensional linear type (C_∞v, n = 2; D_∞h, n = 4), to two-dimensional branch type (D_3h, n = 6), then to three-dimensional tetrahedral (T_d, n = 8) and square pyramid (C_4v, n = 10) types. Further natural bond orbital analyses show that electrons are transferred from the lone pair on N_α of every N₂ unit to the empty orbitals of lithium atom in LiN_2–8⁺, while in LiN₁₀⁺, electrons are transferred from the bonding orbital of the Li–N_α bonds to the antibonding orbital of the other Li–N_α bonds. In both cases, the N₂ units become dipoles and strongly interact with Li⁺. The average second-order perturbation stabilization energy for LiN₈⁺ is the highest among the observed LiN_n⁺ clusters. For neutral LiN_2–8 clusters, the most stable isomers were also formed by a Li atom and n/2 number of N₂ units, while that of LiN₁₀ is in the form of Li⁺(N₂)₃(η¹-N₄).

LiN_n⁺ clusters were generated by laser ablation and the LiN₈⁺ with tetrahedral Li⁺(N₂)₄ structure has the highest ion abundance.

Infrared photodissociation spectroscopy of M(N2)n(+) (M = Y, La, Ce; n = 7-8) in the gas phase

M(N2)n(+) (M = Y, La, Ce; n = 7-8) complexes have been studied by infrared photodissociation (IRPD) spectroscopy and density functional theory (DFT) calculations. The experimental results indicate that the N-N stretching vibrational frequencies are red-shifted from the gas-phase N2 value. The π back-donation is found to be a main contributor in these systems. IRPD spectra and DFT calculations reveal the coexistence of two isomers in the seven-coordinate M(N2)7(+) and eight-coordinate M(N2)8(+) complexes, respectively. The present studies on these metal-nitrogen complexes shed light on the interactions and coordinations toward N2 with transition and lanthanide metals.

Dinitrogen-supported coordination polymers

Two novel three-dimensional dinitrogen-supported coordination polymers adopting the (5³·7³)₂(5⁴·8²) and (5³)₄(5⁸·6⁴·7⁸·8⁴·9⁴)₂-3,8T16 topologies are reported.

Experimental observation of TiN12+ cluster and theoretical investigation of its stable and metastable isomers

TiN n+ clusters were generated by laser ablation and analyzed experimentally by mass spectrometry. The results showed that the mass peak of the TiN12+ cluster is dominant in the spectrum. The TiN12+ cluster was further investigated by photodissociation experiments with 266, 532 and 1064 nm photons. Density functional calculations were conducted to investigate stable structures of TiN12+ and the corresponding neutral cluster, TiN12. The theoretical calculations found that the most stable structure of TiN12+ is Ti(N2)6+ with Oh symmetry. The calculated binding energy is in good agreement with that obtained from the photodissociation experiments. The most stable structure of neutral TiN12 is Ti(N2)6 with D3d symmetry. The Ti-N bond strengths are greater than 0.94 eV in both Ti(N2)6+ and its neutral counterpart. The interaction between Ti and N2 weakens the N-N bond significantly. For neutral TiN12, the Ti(N3)4 azide, the N5TiN7 sandwich structure and the N6TiN6 structure are much higher in energy than the Ti(N2)6 complex. The DFT calculations predicted that the decomposition of Ti(N3)4, N5TiN7, and N6TiN6 into a Ti atom and six N2 molecules can release energies of about 139, 857, and 978 kJ mol-1 respectively.

Integration of cryogenic ion vibrational predissociation spectroscopy with a mass spectrometric interface to an electrochemical cell

Cryogenic ion vibrational predissociation (CIVP) spectroscopy is used to structurally characterize electrochemically (EC)-generated oxidation products of the benchmark compound reserpine. Ionic products were isolated using EC-electrospray ionization (ESI) coupled to a 25 K ion trap prior to injection into a double-focusing, tandem time-of-flight photofragmentation mass spectrometer. Vibrational predissociation spectroscopy was carried out by photoevaporation of weakly bound N2 adducts over the range 800-3800 cm(-1) in a linear (i.e., single photon) action regime, thus enabling direct comparison of the experimental vibrational pattern with harmonic calculations. The locations of the NH and OH stretching fundamentals are most consistent with formation of 9-hydroxyreserpine, which is a different isomer than considered previously. This approach thus provides a powerful structural dimension for the analysis of electrochemical processes detected with the sensitivity of mass spectrometry.

Diagnostic NH and OH vibrations for oxazolone and diketopiperazine structures: b2 from protonated triglycine

We present infrared multiple photon dissociation (IRMPD) spectra in the hydrogen stretching region of the simplest b fragment, b(2) from protonated triglycine, contrasted to that of protonated cyclo(Gly-Gly). Both spectra confirm the presence of intense, diagnostic vibrations linked to the site of proton attachment. Protonated cyclo(Gly-Gly) serves as a reference spectrum for the diketopiperazine structure, showing a diagnostic O-H(+) stretch of the protonated carbonyl group at 3585 cm(-1). Conversely, b(2) from protonated triglycine exhibits a strong band at 3345 cm(-1), associated with the N-H stretching mode of the protonated oxazolone ring structure. Other weaker N-H stretches can also be discerned, such as the amino NH(2) and amide NH bands. These results demonstrate the usefulness of the hydrogen stretching region, and hence benchtop optical parametric oscillator/amplifier (OPO/A) set-ups, in making structural assignments of product ions in collision-induced dissociation (CID) of peptides.

IR spectroscopy on isolated Co(n)(alcohol)m cluster anions (n=1-4, m=1-3): structures and spin states

Isolated cobalt-alcohol cluster anions containing n=1-4 cobalt and m=1-3 alcohol molecules (alcohol=methanol, ethanol, propanol) are produced in a supersonic beam by using a laser ablation source. By applying IR photodissociation spectroscopy vibrational spectra in the OH stretching region are obtained. Several structures in different spin states are discussed for the (n,m) clusters. In comparison with density functional theory calculations applied to both the Co/alcohol clusters and the naked Co cluster anions, an unambiguous structural assignment is achieved. It turns out that structures are preferred with a maximum number of hydrogen bonds between the OH groups and the Co···Co units. These hydrogen bonds are typical for anionic species leading to an activation of the OH groups which is indicated by large red-shifts of the OH stretching frequencies compared to the naked alcohols. For each (n,m) cluster, the frequency shifts systematically with respect to the different alcohols, but the type of structure is identical for all alcohol ligands. The application of IR spectroscopy turns out to be an ideal tool not only as a probe for structures but also for spin states which significantly influence the predicted OH stretching frequencies.

Vibrational spectroscopy of bare and solvated ionic complexes of biological relevance

The low density of ions in mass spectrometers generally precludes direct infrared (IR) absorption measurements. The IR spectrum of an ion can nonetheless be obtained by inducing photodissociation of the ion using a high-intensity tunable laser. The emergence of free electron lasers (FELs) and recent breakthroughs in bench-top lasers based on nonlinear optics have now made it possible to routinely record IR spectra of gas-phase ions. As the energy of one IR photon is insufficient to cause dissociation of molecules and strongly bound complexes, two main experimental strategies have been developed to effect photodissociation. In infrared multiple-photon dissociation (IR-MPD) many photons are absorbed resonantly and their energy is stored in the bath of vibrational modes, leading to dissociation. In the "messenger" technique a weakly bound van der Waals atom is detached upon absorption of a single photon. Fundamental, historical, and practical aspects of these methods will be presented. Both of these approaches make use of very different methods of ion preparation and manipulation. While in IR-MPD ions are irradiated in trapping mass spectrometers, the "messenger" technique is generally carried out in molecular beam instruments. The main focus of this review is the application of IR spectroscopy to biologically relevant molecular systems (amino acids, peptides, proteins). Particular issues that will be addressed here include gas-phase zwitterions, the (chemical) structures of peptides and their collision-induced dissociation (CID) products, IR spectra of gas-phase proteins, and the chelation of metal-ligand complexes. Another growing area of research is IR spectroscopy on solvated clusters, which offer a bridge between the gas-phase and solution environments. The development of state-of-the-art computational approaches has gone hand-in-hand with advances in experimental techniques. The main advantage of gas-phase cluster research, as opposed to condensed-phase experiments, is that the systems of interest can be understood in detail and structural effects can be studied in isolation. It will be shown that IR spectroscopy of mass-selected (bio)molecular systems is now well-placed to address specific questions on the individual effect of charge carriers (protons and metal ions), as well as solvent molecules on the overall structure.