Evidence for an intrinsic binding force between dodecaborate dianions and receptors with hydrophobic binding pockets

Probing the Electronic Structure of [B10H10]2- Dianion Encapsulated by an Octamethylcalix[4]pyrrole Molecule

Despite being an important closo-borate in condensed phase boron chemistry, isolated [B10H10]2- is electronically unstable and has never been detected in the gas phase. Herein, we report a successful capture of this fleeting species through binding with an octamethylcalix[4]pyrrole (omC4P) molecule to form a stable gaseous omC4P·[B10H10]2- complex and its characterizations utilizing negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum, contributed by both omC4P and [B10H10]2-, is deconvoluted by subtracting the omC4P contribution to yield a [B10H10]2- spectrum. The obtained [B10H10]2- spectrum consists of four major bands spanning the electron binding energy (EBE) range from 1 to 5 eV, with the EBE gaps matching excellently with the energy intervals of computed high-lying occupied molecular orbitals of the [B10H10]2- dianion. This study showcases a generic method to utilize omC4P to capture unstable multiply charged anions in the gas phase for experimental determination of their electronic structures.

Observation of a super-tetrahedral cluster of acetonitrile-solvated dodecaborate dianion via dihydrogen bonding

We launched a combined negative ion photoelectron spectroscopy and multiscale theoretical investigation on the geometric and electronic structures of a series of acetonitrile-solvated dodecaborate clusters, i.e., B12H122-·nCH3CN (n = 1-4). The electron binding energies of B12H122-·nCH3CN are observed to increase with cluster size, suggesting their enhanced electronic stability. B3LYP-D3(BJ)/ma-def2-TZVP geometry optimizations indicate each acetonitrile molecule binds to B12H122- via a threefold dihydrogen bond (DHB) B3-H3 ⁝⁝⁝ H3C-CN unit, in which three adjacent nucleophilic H atoms in B12H122- interact with the three methyl hydrogens of acetonitrile. The structural evolution from n = 1 to 4 can be rationalized by the surface charge redistributions through the restrained electrostatic potential analysis. Notably, a super-tetrahedral cluster of B12H122- solvated by four acetonitrile molecules with 12 DHBs is observed. The post-Hartree-Fock domain-based local pair natural orbital- coupled cluster singles, doubles, and perturbative triples [DLPNO-CCSD(T)] calculated vertical detachment energies agree well with the experimental measurements, confirming the identified isomers as the most stable ones. Furthermore, the nature and strength of the intermolecular interactions between B12H122- and CH3CN are revealed by the quantum theory of atoms-in-molecules and the energy decomposition analysis. Ab initio molecular dynamics simulations are conducted at various temperatures to reveal the great kinetic and thermodynamic stabilities of the selected B12H122-·CH3CN cluster. The binding motif in B12H122-·CH3CN is largely retained for the whole halogenated series B12X122-·CH3CN (X = F-I). This study provides a molecular-level understanding of structural evolution for acetonitrile-solvated dodecaborate clusters and a fresh view by examining acetonitrile as a real hydrogen bond (HB) donor to form strong HB interactions.

Large anion binding in water

Large water-soluble anions with chaotropic character display surprisingly strong supramolecular interactions in water, for example, with macrocyclic receptors, polymers, biomembranes, and other hydrophobic cavities and interfaces. The high affinity is traced back to a hitherto underestimated driving force, the chaotropic effect, which is orthogonal to the common hydrophobic effect. This review focuses on the binding of large anions with water-soluble macrocyclic hosts, including cyclodextrins, cucurbiturils, bambusurils, biotinurils, and other organic receptors. The high affinity of large anions to molecular receptors has been implemented in several lines of new applications, which are highlighted herein.

Highly Structured Water Networks in Microhydrated Dodecaborate Clusters

We report a combined photoelectron spectroscopy and theoretical investigation of a series of size-selected hydrated closo-dodecaborate clusters B₁₂X₁₂^2-·nH₂O (X = H, F, or I; n = 1-6). Distinct structural arrangements of water clusters from monomer to hexamer can be achieved by using different B₁₂X₁₂^2- bases, illustrating the evident solute specificity. Because B-H···H-O dihydrogen bonds are stronger than O···H-O hydrogen bonds in water, the added water molecules are arranged in a unified binding mode by forming highly structured water networks manipulated by B₁₂H₁₂^2-. As a comparison, the hydrated B₁₂F₁₂^2- clusters display similar water evolution for n values of 1 and 2 but different binding modes for larger clusters, while water networks in B₁₂I₁₂^2- share similarities with the free water clusters. This finding provides a consistent picture of the structural diversity of hydrogen bonding networks in microhydrated dodecaborates and a molecular-level understanding of microsolvation dynamics in aqueous borate chemistry.

Encapsulation of closo-dodecaiodododecaborate in 2-hydroxypropyl-γ-cyclodextrin prevents hemolysis

Na₂B₁₂I₁₂ has many of the properties desired by an X-ray contrast agent but is lethal at the concentrations needed for medical imaging. We demonstrate here that PBS solutions with >50 mM Na₂B₁₂I₁₂ induce hemolysis, consistent with the previously reported superchaotropic nature of the anion. The presence of <1 equiv. of 2-hydroxypropyl-γ-cyclodextrin prevents hemolysis and suggests a strategy for exploiting B₁₂I₁₂^2- as an X-ray contrast agent.

Ex Vivo and In Vivo Evaluation of Dodecaborate-Based Clusters Encapsulated in Ferumoxytol Nanoparticles

Host-guest interactions represent a growing research area with recent work demonstrating the ability to chemically manipulate both host molecules as well as guest molecules to vary the type and strength of bonding. Much less is known about the interactions of the guest molecules and hybrid materials containing similar chemical features to typical macrocyclic hosts. This work uses in vitro and in vivo kinetic analyses to investigate the interaction of closo-dodecahydrododecaborate derivatives with ferumoxytol, an iron oxide nanoparticle with a carboxylated dextran coating. We find that several boron cluster derivatives can become encapsulated into ferumoxytol, and the lack of pH dependence in these interactions suggests that ion pairing, hydrophobic/hydrophilic interaction, and hydrogen bonding are not the driving force for encapsulation in this system. Biodistribution experiments in BALB/c mice show that this system is nontoxic at the reported dosage and demonstrate that encapsulation of dodecaborate-based clusters in ferumoxytol can alter the biodistribution of the guest molecules.

Measuring Electronic Structure of Multiply Charged Anions to Understand Their Chemistry: A Case Study on Gaseous Polyhedral closo-Borate Dianions

Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the past decades, mainly to understand fundamental molecular physics phenomena, for example, intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs' chemistry appears often vague. Here, we discuss how insights into the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision-induced dissociation can provide a fundamental understanding of the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified in our studies on polyhedral closo-borate dianions [B_nX_n]^2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are tuned to either bind noble gases effectively or activate small molecules like CO and N₂. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [B_nX_n]^2-, and gas phase NIPES provides insights into the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions that cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are discussed.

Gaseous cyclodextrin-closo-dodecaborate complexes χCD·B12X122- (χ = α, β, and γ; X = F, Cl, Br, and I): electronic structures and intramolecular interactions

A fundamental understanding of cyclodextrin-closo-dodecaborate inclusion complexes is of great interest in supramolecular chemistry. Herein, we report a systematic investigation on the electronic structures and intramolecular interactions of perhalogenated closo-dodecaborate dianions B12X122- (X = F, Cl, Br and I) binding to α-, β-, and γ-cyclodextrins (CDs) in the gas phase using combined negative ion photoelectron spectroscopy (NIPES) and density functional theory (DFT) calculations. The vertical detachment energy (VDE) of each complex and electronic stabilization of each dianion due to the CD binding (ΔVDE, relative to the corresponding isolated B12X122-) are determined from the experiments along α-, β- and γ-CD in the form of VDE (ΔVDE): 4.00 (2.10), 4.33 (2.43), and 4.30 (2.40) eV in X = F; 4.09 (1.14), 4.64 (1.69), and 4.69 (1.74) eV in X = Cl; 4.11 (0.91), 4.58 (1.38), and 4.70 (1.50) eV in X = Br; and 3.54 (0.74), 3.88 (1.08), and 4.05 (1.25) eV in X = I, respectively. All complexes have significantly higher VDEs than the corresponding isolated dodecaborate dianions with ΔVDE spanning from 0.74 eV at (α, I) to 2.43 eV at (β, F), sensitive to both host CD size and guest substituent X. DFT-optimized complex structures indicate that all B12X122- prefer binding to the wide openings of CDs with the insertion depth and binding motif strongly dependent on the CD size and halogen X. Dodecaborate anions with heavy halogens, i.e., X = Cl, Br, and I, are found outside of α-CD, while B12F122- is completely wrapped by γ-CD. Partial embedment of B12X122- into CDs is observed for the other complexes via multipronged B-XH-O/C interlocking patterns. The simulated spectra based on the density of states agree well with those of the experiments and the calculated VDEs well reproduce the experimental trends. Molecular orbital analyses suggest that the spectral features at low binding energies originated from electrons detached from the dodecaborate dianion, while those at higher binding energies are derived from electron detachment from CDs. Energy decomposition analyses reveal that the electrostatic interaction plays a dominating role in contributing to the host-guest interactions for the X = F series partially due to the formation of a O/C-HX-B hydrogen bonding network, and the dispersion forces gradually become important with the increase of halogen size.

Fused 1,2-Diboraoxazoles Based on closo-Decaborate Anion-Novel Members of Diboroheterocycle Class

The novel members of the 1,2-diboraoxazoles family have been obtained. In the present work, we have carried out the intramolecular ring-closure reaction of borylated iminols of general type [B₁₀H₉N=C(OH)R]⁻ (R = Me, Et, ⁿPr, ⁱPr, ^tBu, Ph, 4-Cl-Ph). This process is conducted in mild conditions with 83–87% yields. The solid-state structures of two salts of 1,2-diboraoxazoles were additionally investigated by X-ray crystallography. In addition, the phenomena of bonding interactions in the 1,2-diboraoxazole cycles have been theoretically studied by the Quantum Theory of Atoms in Molecules analysis. Several local and integral topological properties of the electron density involved in these interactions have been computed.

Synthesis, Electronic Properties and Reactivity of [B12 X11 (NO2 )]2- (X=F-I) Dianions

Nitro‐functionalized undecahalogenated closo‐dodecaborates [B₁₂X₁₁(NO₂)]²⁻ were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X‐diffraction, mass spectrometry, and gas‐phase ion vibrational spectroscopy. The NO₂ substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B₁₂X₁₂]²⁻ (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum‐chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO^.). The homolytic NO^. elimination from the dianion under very soft collisional excitation in gas‐phase ion experiments results in the formation of the radical [B₁₂X₁₁O]^2−.. Theoretical investigations suggest that the loss of NO^. proceeds via the rearrangement product [B₁₂X₁₁(ONO)]²⁻. The O‐bonded nitrosooxy structure is thermodynamically more stable than the N‐bonded nitro structure and its formation by radical recombination of [B₁₂X₁₁O]^2−. and NO^. is demonstrated.

Properties of gaseous closo-[B6X6]2- dianions (X = Cl, Br, I)

Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B₆X₆]^2- (X = Cl-I) are investigated in direct comparison with their closo-[B₁₂X₁₂]^2- analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B₆X₆]^2- dianions are electronically significantly less stable than the corresponding [B₁₂X₁₂]^2- species. Although [B₆Cl₆]^2- is slightly electronically unstable, [B₆Br₆]^2- and [B₆I₆]^2- are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e^- loss) is observed in CID of [B₆X₆]^2- (X = Cl, Br) but not for [B₆I₆]^2-. Halogenide loss (X^- loss) is common for [B₆X₆]^2- (X = Br, I) and [B₁₂X₁₂]^2- (X = Cl, Br, I). Meanwhile, X˙ loss is only observed for [B₁₂X₁₂]^2- (X = Br, I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e^-, X^- and X˙ loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e^- and X^- losses. A significantly lower RCB for X^- loss than for e^- loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X^- losses are significantly lower for [B₆X₆]^2- than for [B₁₂X₁₂]^2-, while enthalpies for X˙ losses are higher. These observations are consistent with a difference in bond character of the B-X bonds in [B₆X₆]^2- and [B₁₂X₁₂]^2-. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B-X bonds in [B₁₂X₁₂]^2- have a stronger covalent character than in [B₆X₆]^2-, in which X has a stronger halide character.

Photoelectron spectroscopy and computational investigations of the electronic structures and noncovalent interactions of cyclodextrin-closo-dodecaborate anion complexes χ-CD·B12X122− (χ = α, β, γ; X = H, F)

We report a joint negative ion photoelectron spectroscopy and computational study on the electronic structures and noncovalent interactions of a series of cyclodextrin-closo-dodecaborate dianion complexes, χ-CD·B₁₂X₁₂²⁻ (χ = α, β, γ; X = H, F).

Gas-Phase Fragmentation of Host-Guest Complexes of Cyclodextrins and Polyoxometalates

Gas-phase fragmentation pathways of host-guest complexes of cyclodextrins (CDs) and polyoxometalates (POMs) were examined using collision-induced dissociation (CID). The host-guest complexes studied here were composed of two different classes of POMs-Keggin (PW₁₂O₄₀^3-) and Lindqvist (M₆O₁₉^2-, M = Mo, W)-and three types of CDs (α-, β-, and γ-CD) differing in the diameter of the inner cavity. The CD-POM complexes were generated either by mixing methanol solutions of POM and CD or through a one-step acidic condensation of tetraoxometalates MO₄^2- (M = Mo, W) with CDs for complexes with Keggin and Lindqvist anions, respectively, and introduced into the gas phase using electrospray ionization (ESI). We observe distinct differences in fragmentation pathways of the complexes of Keggin and Lindqvist POMs under high- and low-energy CID conditions. Specifically, direct dissociation and proton transfer from CD to POM accompanied by the separation of fragments is observed in CID of Keggin CD-POM complexes. In contrast, dissociation of CD complexes with Lindqvist POMs is dominated by the simultaneous loss of multiple water molecules. This unusual fragmentation channel is attributed to dissociation of the POM cluster inside the CD cavity accompanied by covalent bond formation between the fragments and CD and elimination of multiple water molecules. The observed covalent coupling of metal oxide clusters opens up opportunities for derivatization of macrocyclic host molecules using collisional excitation of gaseous non-covalent complexes.

Properties of perhalogenated {closo-B10} and {closo-B11} multiply charged anions and a critical comparison with {closo-B12} in the gas and the condensed phase

Dependence of electronic properties and reactivity of closo-borates with size and halogen substituent was investigated.

Synthesis and Structural Characterization of Amidine, Amide, Urea and Isocyanate Derivatives of the Amino-closo-dodecaborate Anion [B12H11NH₃]. Molecules 2018;23:molecules23123137. [PMID: 30501105 PMCID: PMC6321512 DOI: 10.3390/molecules23123137]

The synthesis and structural characterization of new derivatives of [B₁₂H₁₂]²⁻ is of fundamental interest and is expected to allow for extended applications. Herein we report on the synthesis of a series of amidine, amide, urea and isocyanate derivatives based on the amino-closo-dodecaborate anion [B₁₂H₁₁NH₃]⁻. Their structures have been confirmed by spectroscopic methods, and nine crystal structures are presented.

The Chaotropic Effect as an Assembly Motif in Chemistry

Following up on scattered reports on interactions of conventional chaotropic ions (for example, I- , SCN- , ClO4- ) with macrocyclic host molecules, biomolecules, and hydrophobic neutral surfaces in aqueous solution, the chaotropic effect has recently emerged as a generic driving force for supramolecular assembly, orthogonal to the hydrophobic effect. The chaotropic effect becomes most effective for very large ions that extend beyond the classical Hofmeister scale and that can be referred to as superchaotropic ions (for example, borate clusters and polyoxometalates). In this Minireview, we present a continuous scale of water-solute interactions that includes the solvation of kosmotropic, chaotropic, and hydrophobic solutes, as well as the creation of void space (cavitation). Recent examples for the association of chaotropic anions to hydrophobic synthetic and biological binding sites, lipid bilayers, and surfaces are discussed.

Endo or Exo? Structures of Gas-Phase Alkali Metal Cation/Aromatic Half-Belt Complexes

1,1,9,9-Tetramethyl[9](2,11)teropyrenophane (TM9TP), a belt-shaped molecule, has a sizable cavity that molecules or ions could occupy. In this study, the question of whether TM9TP forms gas-phase ion-molecule complexes with metal cations (K⁺ , Rb⁺ , Cs⁺ ) situated inside or outside the TM9TP cavity was addressed using both experimental and computational methods. Complexes were trapped in a Fourier transform ion cyclotron resonance mass spectrometer and their structures were explored by some novel physical chemistry/mass spectrometry methods. Blackbody infrared radiative dissociation kinetics reveal two populations of ions, a fast dissociating fraction and a persistent fraction. Infrared multiphoton dissociation spectra (vibrational spectra) provide very strong evidence that the most abundant population is a complex where the metal cation is inside the TM9TP cavity, endo-TM9TP. Red-shifted C-H stretching bands present in the gas-phase vibrational spectra of these ionic complexes show that there is an interaction between the metal cation and bridge C-H bonds due to the cation sitting inside the cavity of TM9TP. B3LYP/6-31+G(d,p) calculations showed the endo complexes to be the lowest in energy; about 60 kJ mol^-1 more thermodynamically stable and more than 120 kJ mol^-1 kinetically more stable than the exo complex.

The closo-Dodecaborate Dianion Fused with Oxazoles Provides 3D Diboraheterocycles with Selective Antimicrobial Activity

The synthesis and application of icosahedral boron cluster compounds has been studied extensively since their discovery several decades ago; however, two aspects of their chemistry have received little attention: The possibility to form inorganic/organic fused boraheterocycles and their potential to act as antimicrobial agents. This work comprises the preparation of a class of 3D diborabenzoxazole analogues with the closo-dodecaborate in place of the benzene moiety. The presented synthetic procedures provide access to a wide range of diboraheterocycles under mild conditions. These 3D heterocycles exhibit strong and selective antimicrobial activity against Neisseria gonorrhoeae, a widespread bacterial pathogen that has shown increasing incidences of multidrug resistance and for which the development of new antimicrobial compounds is urgently needed.

Self-organizing layers from complex molecular anions

The formation of traditional ionic materials occurs principally via joint accumulation of both anions and cations. Herein, we describe a previously unreported phenomenon by which macroscopic liquid-like thin layers with tunable self-organization properties form through accumulation of stable complex ions of one polarity on surfaces. Using a series of highly stable molecular anions we demonstrate a strong influence of the internal charge distribution of the molecular ions, which is usually shielded by counterions, on the properties of the layers. Detailed characterization reveals that the intrinsically unstable layers of anions on surfaces are stabilized by simultaneous accumulation of neutral molecules from the background environment. Different phases, self-organization mechanisms and optical properties are observed depending on the molecular properties of the deposited anions, the underlying surface and the coadsorbed neutral molecules. This demonstrates rational control of the macroscopic properties (morphology and size of the formed structures) of the newly discovered anion-based layers.

High-affinity host-guest chemistry of large-ring cyclodextrins

The host-guest chemistry of large-ring cyclodextrins (LRCDs) has been largely unexplored due to the lack of suitable guest molecules that bind with significant affinities to enable potential applications. Herein, we report their complexation with dodecaborate anions (B12X12(2-)), a novel class of guest molecules. The binding constants of the inorganic guests (10(4)-10(6) M(-1)) allow their classification as the first tight binders for LRCDs.

Interaction of B12F122- with All-cis 1,2,3,4,5,6 Hexafluorocyclohexane in the Gas Phase

Clusters of all-cis 1,2,3,4,5,6-hexafluorocyclohexane and the dodecafluorododecaboron dianion, [C₆F₆H₆]_n[B₁₂F₁₂]^2- (n = 0-4), are investigated in a combined experimental and computational study. DFT calculations and IRMPD spectra in the region of 800-2000 cm^-1 indicate that C₆H₆F₆ binds to open trigonal faces of B₁₂F₁₂^2- via a three-point interlocking binding motif. Calculated binding interactions reveal substantial contributions from C-H···F hydrogen bonding and binding energies that are among the strongest observed for a neutral-anion system.

Binary twinned-icosahedral [B21H18]− interacts with cyclodextrins as a precedent for its complexation with other organic motifs

A macropolyhedral boron hydride anion with two counterions can form stable complexes with β- and γ-cyclodextrin in the gas phase.

Hierarchical host–guest assemblies formed on dodecaborate-coated gold nanoparticles

Undecahydro-mercapto-closo-dodecaborate (BSH), a purely inorganic cluster anion, serves as an unconventional stabilizing ligand for the preparation of gold nanoparticles (AuNPs).