Design and Performance of a Soft-Landing Instrument for Fragment Ion Deposition

Probing fragment ion reactivity towards functional groups on coordination polymer surfaces

Functionalization of surface-grown coordination polymer layers by ion soft-landing of highly reactive molecular fragment ions is demonstrated. The ions form covalent bonds to terminal functional groups of the polymer at the vacuum interface, opening new perspectives for controlled bond formation using reactive ions.

Spontaneous ligand loss by soft landed [Ni(bpy)3]2+ ions on perfluorinated self-assembled monolayer surfaces

Transition metal (TM) complexes are widely used in catalysis, photochemical energy conversion, and sensing. Understanding factors that affect ligand loss from TM complexes at interfaces is important both for generating catalytically-active undercoordinated TM complexes and for controlling the degradation pathways of photosensitizers and photoredox catalysts. Herein, we demonstrate that well-defined TM complexes prepared on surfaces using ion soft landing undergo substantial structural rearrangements resulting in ligand loss and formation of both stable and reactive undercoordinated species. We employ nickel bipyridine (Ni-bpy) cations as a model system and explore their structural reorganization on surfaces using a combination of experimental and computational approaches. The controlled preparation of surface layers by mass-selected deposition of [Ni(bpy)3]2+ cations provides insights into the chemical reactivity of these species on surfaces. Both surface characterization using mass spectrometry and electronic structure calculations using density functional theory (DFT) indicate that [Ni(bpy)3]2+ undergoes a substantial geometry distortion on surfaces in comparison with its gas-phase structure. This distortion reduces the ligand binding energy and facilitates the formation of the undercoordinated [Ni(bpy)2]2+. Additionally, charge reduction by the soft landed [Ni(bpy)3]2+ facilitates ligand loss. We observe that ligand loss is inhibited by co-depositing [Ni(bpy)3]2+ with a stable anion such as closo-dodecaborate dianion, [B12F12]2-. The strong electrostatic interaction between [Ni(bpy)3]2+ and [B12F12]2- diminishes the distortion of the cation due to interactions with the surface. This interaction stabilizes the soft landed cation by reducing the extent of charge reduction and its structural reorganization. Overall, this study shows the intricate interplay of charge state, ion surface interactions, and stabilization by counterions on the structure and reactivity of metal complexes on surfaces. The combined experimental and computational approach used in this study offers detailed insights into factors that affect the integrity and stability of active species relevant to energy production and catalysis.

Generation and reactivity of the fragment ion [B12I8S(CN)]- in the gas phase and on surfaces

Gaseous fragment ions generated in mass spectrometers may be employed as "building blocks" for the synthesis of novel molecules on surfaces using ion soft-landing. A fundamental understanding of the reactivity of the fragment ions is required to control bond formation of deposited fragments in surface layers. The fragment ion [B12X11]- (X = halogen) is formed by collision-induced dissociation (CID) from the precursor [B12X12]2- dianion. [B12X11]- is highly reactive and ion soft-landing experiments have shown that this ion binds to the alkyl chains of organic molecules on surfaces. In this work we investigate whether specific modifications of the precursor ion affect the chemical properties of the fragment ions to such an extent that attachment to functional groups of organic molecules on surfaces occurs and binding of alkyl chains is prevented. Therefore, a halogen substituent was replaced by a thiocyanate substituent. CID of the precursor [B12I11(SCN)]2- ion preferentially yields the fragment ion [B12I8S(CN)]-, which shows significantly altered reactivity compared to the fragment ions of [B12I12]2-. [B12I8S(CN)]- has a previously unknown structural element, wherein a sulfur atom bridges three boron atoms. Gas-phase reactions with different neutral reactants (cyclohexane, dimethyl sulfide, and dimethyl amine) accompanied by theoretical studies indicate that [B12I8S(CN)]- binds with higher selectivity to functional groups of organic molecules than fragment ions of [B12I12]2- (e.g., [B12I11]- and [B12I9]-). These findings were further confirmed by ion soft-landing experiments, which showed that [B12I8S(CN)]- ions attacked ester groups of adipates and phthalates, whereas [B12I11]- ions only bound to alkyl chains of the same reagents.

Oxovanadium electronics for in-memory, neuromorphic, and quantum computing applications

Vanadium is a critical raw material. In the nearby future, it may, however, become one of the key elements of computer devices based on two-dimensional arrays of spin qubits for quantum information processing or charge- and resistance-based data memory cells for non-volatile in-memory and neuromorphic computing. The research and development (R&D) of vanadium-containing electronic materials and methods for their responsible fabrication underpins the transition to innovative hybrid semiconductors for energy- and resource-efficient memory and information processing technologies. The combination of standard and emerging solid-state semiconductors with stimuli-responsive oxo complexes of vanadium(IV,V) is envisioned to result in electronics with a new room-temperature device nanophysics, and the ability to modulate and control it at the sub-nanometer level. The development of exponential (Boolean) logics based on the oxovanadium-comprising circuitry and crossbar arrays of individual memristive cells for in-memory computing, the implementation of basic synaptic functions via dynamic electrical pulses for neuromorphic computing, and the readout and control of spin networks and interfaces for quantum computing are strategically important future areas of molecular chemistry and applied physics of vanadium.

Enhancing Energy Storage Capacity of 3D Carbon Electrodes Using Soft Landing of Molecular Redox Mediators

The incorporation of redox-active species into the electric double layer is a powerful strategy for enhancing the energy density of supercapacitors. Polyoxometalates (POM) are a class of stable, redox-active species with multielectron activity, which is often used to tailor the properties of electrochemical interfaces. Traditional synthetic methods often result in interfaces containing a mixture of POM anions, unreactive counter ions, and neutral species. This leads to degradation in electrochemical performance due to aggregation and increased interfacial resistance. Another significant challenge is achieving the uniform and stable anchoring of POM anions on substrates to ensure the long-term stability of the electrochemical interface. These challenges are addressed by developing a mass spectrometry-based subambient deposition strategy for the selective deposition of POM anions onto engineered 3D porous carbon electrodes. Furthermore, positively charged functional groups are introduced on the electrode surface for efficient trapping of POM anions. This approach enables the deposition of purified POM anions uniformly through the pores of the 3D porous carbon electrode, resulting in unprecedented increase in the energy storage capacity of the electrodes. The study highlights the critical role of well-defined electrochemical interfaces in energy storage applications and offers a powerful method to achieve this through selective ion deposition.

Ion Mobility Mass Spectrometry for Large Synthetic Molecules: Expanding the Analytical Toolbox

Understanding the composition, structure and stability of larger synthetic molecules is crucial for their design, yet currently the analytical tools commonly used do not always provide this information. In this perspective, we show how ion mobility mass spectrometry (IM-MS), in combination with tandem mass spectrometry, complementary techniques and computational methods, can be used to structurally characterize synthetic molecules, make and predict new complexes, monitor disassembly processes and determine stability. Using IM-MS, we present an experimental and computational framework for the analysis and design of complex molecular architectures such as (metallo)supramolecular cages, nanoclusters, interlocked molecules, rotaxanes, dendrimers, polymers and host-guest complexes.

Control of Intermediates and Products by Combining Droplet Reactions and Ion Soft-Landing

Despite being recognized primarily as an analytical technique, mass spectrometry also has a large potential as a synthetic tool, enabling access to advanced synthetic routes by reactions in charged microdroplets or ionic thin layers. Such reactions are special and proceed primarily at surfaces of droplets and thin layers. Partial solvation of the reactants is usually considered to play an important role for reducing the activation barrier, but many mechanistic details still need to be clarified. In our study, we showcase the synergy between two sequentially applied "preparative mass spectrometry" methods: initiating accelerated reactions within microdroplets during electrospray ionization to generate gaseous ionic intermediates in high abundance, which are subsequently mass-selected and soft-landed to react with a provided reagent on a substrate. This allows the generation of products at a nanomolar scale, amenable to further characterization. In this proof-of-concept study, the contrasting reaction pathways between intrinsically neutral and pre-charged reagents, respectively, both in microdroplets and in layers generated by ion soft-landing are investigated. This provides new insights into the role of partially solvated reagents at microdroplet surfaces for increased reaction rates. Additionally, further insights into reactions of ions of the same polarity under various conditions is obtained.

Binding Properties of Small Electrophilic Anions [B6 X5 ]- and [B10 X9 ]- (X=Cl, Br, I): Activation of Small Molecules Based on π-Backbonding

Superelectrophilic anions constitute a special class of molecular anions that show strong binding of weak nucleophiles despite their negative charge. In this study, the binding characteristics of smaller gaseous electrophilic anions of the types [B6 X5 ]- and [B10 X9 ]- (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B12 X11 ]- . The positive charge of vacant boron increases from [B6 X5 ]- via [B10 X9 ]- to [B12 X11 ]- , as evidenced by increasing attachment enthalpies towards typical σ-donor molecules (noble gases, H2 O). However, this behavior is reversed for σ-donor-π-acceptor molecules. [B6 Cl5 ]- binds most strongly to N2 and CO, even more strongly than to H2 O. Energy decomposition analysis confirms that the orbital interaction is responsible for this opposite trend. The extended transition state natural orbitals for chemical valence method shows that the π-backdonation order is [B6 X5 ]- >[B10 X9 ]- >[B12 X11 ]- . This predicted order explains the experimentally observed red shifts of the CO and N2 stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B6 Cl5 N2 ]- : 222 cm-1 . Therefore, strong activation of unreactive σ-donor-π-acceptor molecules (commonly observed for cationic transition metal complexes) is achieved with metal-free molecular anions.

Chemical Synthesis with Gaseous Molecular Ions: Harvesting [B12 Br11 N2 ]- from a Mass Spectrometer

Mass spectrometry frequently reveals the existence of transient gas phase ions that have not been synthesized in solution or in bulk. These elusive ions are, therefore, often considered to be primarily of analytical value in fundamental gas phase studies. Here, we provide proof-of-concept that the products of ion-molecule reactions in mass spectrometers may be collected on surfaces to generate condensed matter and thus serve as building blocks to synthesize new compounds. The highly reactive fragment anion [B12 Br11 ]- was generated in a mass spectrometer and converted to [B12 Br11 N2 ]- in the presence of molecular nitrogen followed by its mass-selection and soft-landing on surfaces. The molecular structure of [B12 Br11 N2 ]- , which has not been synthetically obtained before, was confirmed by conventional methods of molecular analysis, including nuclear magnetic resonance and infrared spectroscopy. The [B12 Br11 N2 ]- ion is stable on surfaces and in solution at room temperature, but thermal annealing induces elimination of N2 and provides access to the highly reactive intermediate [B12 Br11 ]- in the condensed phase, which can be further used as a reagent, for example, for electrophilic aromatic substitutions. Thus, isolation of [B12 Br11 N2 ]- expands the repertoire of the available diazo ions that can be employed as versatile intermediates in various chemical transformations.

Controlled Formation of Fused Metal Chalcogenide Nanoclusters Using Soft Landing of Gaseous Fragment Ions

The complete ligation of nanoclusters significantly reduces their chemical reactivity, catalytic activity, and charge transfer properties. Therefore, in applications, nanoclusters are activated through partial ligand removal to take advantage of their full potential. However, the precise control of ligand removal in the condensed phase is challenging. In this study, we examine the reactivity of well-defined activated nanoclusters on surfaces prepared through controlled ligand removal in the gas phase. To accomplish this, we utilized a specially designed ion soft-landing instrument equipped with a collision cell to prepare mass-selected fragment ions, which were then deposited onto self-assembled monolayer (SAM) surfaces. Specifically, we generated fragment ions by selectively removing one or two ligands from a series of atomically precise ligated metal sulfide clusters, Co5MS8(L1)6+ (M = Co, Mn, Fe, or Ni, L1 = PEt3). Removal of one ligand from Co5MS8(L1)6+ (M = Co, Mn, Ni) generates Co5MS8(L1)5+ species, which undergo selective dimerization on SAMs. Meanwhile, Co5FeS8(L1)5+ is unreactive and remains intact when it is deposited onto a SAM surface. In contrast, fragments formed by the removal of two ligands, Co5MS8(L1)4+, undergo several nonselective reactions and generate larger fused clusters. We found that the reactivity of the Co5MS8(L1)5+ fragment ions is correlated with the gas phase stability of the corresponding precursor ion toward ligand loss. Specifically, the relatively unstable precursor ion, Co5FeS8(L1)6+, generates the least reactive fragment. Meanwhile, the more stable precursor ions generate more reactive Co5MS8(L1)5+ fragments that dimerize on surfaces. This observation was also confirmed by co-deposition of fragment ions with two different ligands, Co5MS8(L1)5+ and Co5MS8(L2)5+ (L1 = PEt3 and L2 = PEt2Ph), where fragments generated from more stable precursor ions tend to dimerize and generate dimers with mixed ligands. This study unveils the previously unrecognized potential of fragment ions in generating compounds that are difficult to synthesize using conventional methods. Additionally, it provides a mechanistic understanding of the observed reactivity. Mass-selected deposition of well-defined fragment ions emerges as a powerful approach for designing materials by precisely activating and depositing undercoordinated ligated nanoclusters onto surfaces.

Gas Phase Reactivity of [Mo6X14]2- Dianions (X = Cl - I)

We investigate collision-induced dissociation (CID) of [Mo₆X₁₄]^2- (X = Cl, Br, I) and the reactivity of fragment ions of these precursors with background gases. Ion mobility measurements and theoretical calculations provide structural information for some of the observed ions. Sequential losses of MoX₂ units dominate the dissociation pathways of [Mo₆Cl₁₄]^2-. Meanwhile, loss of X radicals is the main channel for X = Br and I. Ion mobility measurements and computational investigations indicate minor structural changes in the octahedral Mo₆ unit for [Mo₆I_m]^- (m = 6-13) fragments. We observe that mass spectra obtained using CID substantially vary among mass spectrometers: Specifically, ions with molecular formula [Mo₆X_m(O₂)_n]^- (X = Br and I) are observed as dominant species produced through reactions with O₂ in several mass spectrometers, but also adduct free fragment ions were observed in other instruments, depending on the background conditions. Ion-trap fragmentation combined with theoretical investigations indicates that spontaneous losses of X radicals occur upon binding of O₂ to [Mo₆I_m]^- fragments (m ≤ 12). Theoretical investigations indicate that both oxygen atoms are bound to the vacant sites of the Mo₆ units. This study opens up a new vista to generate and study a large variety of hexanuclear Mo₆X_m(O₂)_n anions.

A Preparative Mass Spectrometer to Deposit Intact Large Native Protein Complexes

Electrospray ion-beam deposition (ES-IBD) is a versatile tool to study the structure and reactivity of molecules from small metal clusters to large protein assemblies. It brings molecules gently into the gas phase, where they can be accurately manipulated and purified, followed by controlled deposition onto various substrates. In combination with imaging techniques, direct structural information on well-defined molecules can be obtained, which is essential to test and interpret results from indirect mass spectrometry techniques. To date, ion-beam deposition experiments are limited to a small number of custom instruments worldwide, and there are no commercial alternatives. Here we present a module that adds ion-beam deposition capabilities to a popular commercial MS platform (Thermo Scientific Q Exactive UHMR mass spectrometer). This combination significantly reduces the overhead associated with custom instruments, while benefiting from established high performance and reliability. We present current performance characteristics including beam intensity, landing-energy control, and deposition spot size for a broad range of molecules. In combination with atomic force microscopy (AFM) and transmission electron microscopy (TEM), we distinguish near-native from unfolded proteins and show retention of the native shape of protein assemblies after dehydration and deposition. Further, we use an enzymatic assay to quantify the activity of a noncovalent protein complex after deposition on a dry surface. Together, these results not only indicate a great potential of ES-IBD for applications in structural biology, but also outline the challenges that need to be solved for it to reach its full potential.

Navigate Flying Molecular Elephants Safely to the Ground: Mass-Selective Soft Landing up to the Mega-Dalton Range by Electrospray Controlled Ion-Beam Deposition

The prototype of a highly versatile and efficient preparative mass spectrometry system used for the deposition of molecules in ultrahigh vacuum (UHV) is presented, along with encouraging performance data obtained using four model species that are thermolabile or not sublimable. The test panel comprises two small organic compounds, a small and very large protein, and a large DNA species covering a 4-log mass range up to 1.7 MDa as part of a broad spectrum of analyte species evaluated to date. Three designs of innovative ion guides, a novel digital mass-selective quadrupole (dQMF), and a standard electrospray ionization (ESI) source are combined to an integrated device, abbreviated electrospray controlled ion-beam deposition (ES-CIBD). Full control is achieved by (i) the square-wave-driven radiofrequency (RF) ion guides with steadily tunable frequencies, including a dQMF allowing for investigation, purification, and deposition of a virtually unlimited m/z range, (ii) the adjustable landing energy of ions down to ∼2 eV/z enabling integrity-preserving soft landing, (iii) the deposition in UHV with high ion beam intensity (up to 3 nA) limiting contaminations and deposition time, and (iv) direct coverage control via the deposited charge. The maximum resolution of R = 650 and overall efficiency up to T_total = 4.4% calculated from the solution to UHV deposition are advantageous, whereby the latter can be further enhanced by optimizing ionization performance. In the setup presented, a scanning tunneling microscope (STM) is attached for in situ UHV investigations of deposited species, demonstrating a selective, structure-preserving process and atomically clean layers.

Binding of Saturated and Unsaturated C6-Hydrocarbons to the Electrophilic Anion [B12Br11]−: A Systematic Mechanistic Study

The highly reactive gaseous ion [B12Br11]– is a metal-free closed-shell anion which spontaneously forms covalent bonds with hydrocarbon molecules, including alkanes. Herein, we systematically investigate the reaction mechanism for binding...