The effect of host size on binding in host-guest complexes of cyclodextrins and polyoxometalates.

Harnessing flexible host cavities opens opportunities for the design of novel supramolecular architectures that accommodate nanosized guests. This research examines unprecedented gas-phase structures of Keggin-type polyoxometalate PW12O40 3- (WPOM) and cyclodextrins (X-CD, X = α, ß, γ, δ, ε, ζ) including previously unexplored large, flexible CDs. Using ion mobility spectrometry coupled to mass spectrometry (IM-MS) in conjunction with molecular dynamics (MD) simulations, we provide first insights into the binding modes between WPOM and larger CD hosts as isolated structures. Notably, γ-CD forms two distinct structures with WPOM through binding to its primary and secondary faces. We also demonstrate that ε-CD forms a deep inclusion complex, which encapsulates WPOM within its annular inner cavity. In contrast, ζ-CD adopts a saddle-like conformation in its complex with WPOM, which resembles its free form in solution. More intriguingly, the gas-phase CD-WPOM structures are highly correlated with their counterparts in solution as characterized by nuclear magnetic resonance (NMR) spectroscopy. The strong correlation between the gas- and solution phase structures of CD-WPOM complexes highlight the power of gas-phase IM-MS for the structural characterization of supramolecular complexes with nanosized guests, which may be difficult to examine using conventional approaches.

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.

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.

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.

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

We report the development of a new high-flux electrospray ionization-based instrument for soft landing of mass-selected fragment ions onto surfaces. Collision-induced dissociation is performed in a collision cell positioned after the dual electrodynamic ion funnel assembly. The high duty cycle of the instrument enables high-coverage deposition of mass-selected fragment ions onto surfaces at a defined kinetic energy. This capability facilitates the investigation of the reactivity of gaseous fragment ions in the condensed phase. We demonstrate that the observed reactions of deposited fragment ions are dependent on the structure of the ion and the composition of either ionic or neutral species codeposited onto a surface. The newly developed instrument provides access to high-purity ion fragments as building blocks for the preparation of unique ionic layers.

Multiplexing of Electrospray Ionization Sources Using Orthogonal Injection into an Electrodynamic Ion Funnel.

In this contribution, we report an efficient approach to multiplex electrospray ionization (ESI) sources for applications in analytical and preparative mass spectrometry. This is achieved using up to four orthogonal injection inlets implemented on the opposite sides of an electrodynamic ion funnel interface. We demonstrate that both the total ion current transmitted through the mass spectrometer and the signal-to-noise ratio increase by 3.8-fold using four inlets compared to one inlet. The performance of the new multiplexing approach was examined using different classes of analytes covering a broad range of mass and ionic charge. A deposition rate of >10 µg of mass-selected ions per day may be achieved by using the multiplexed sources coupled to preparative mass spectrometry. The almost proportional increase in the ion current with the number of ESI inlets observed experimentally is confirmed using gas flow and ion trajectory simulations. The simulations demonstrate a pronounced effect of gas dynamics on the ion trajectories in the ion funnel, indicating that the efficiency of multiplexing strongly depends on gas velocity field. The study presented herein opens up exciting opportunities for the development of bright ion sources, which will advance both analytical and preparative mass spectrometry applications.