Discriminating Halogen-Bonding from Other Noncovalent Interactions by a Combined NOE NMR/DFT Approach

Iodine Molecules within Triethylenediamine-Based Metal-Organic Frameworks for Hydrolysis/Alkylation Tandem Reactions

The integration of radionuclide iodine molecules in metal-organic frameworks (MOFs) for organic synthesis is attracting considerable research attention due to their specific catalytic performance. However, understanding the comprehensive catalytic behaviors of different types of molecular iodine encapsulated in MOFs for a sequential organic transformation is a great challenge. To address this issue, we have designed two triethylenediamine-functionalized MOFs assembled from 1,3,5-tricarboxyphenyl-2-(triethylenediaminemethyl)benzene-linker and {Cd(COO)3N} or {Cu4(u3-OH)2(COO)6N} clusters. Both MOFs show good stability and adsorption of I2 in the solution and vapor phases. Catalysts obtained after treatment with ethyl acetate present efficient catalytic activity in hydrolysis/alkylation tandem reactions in water. The mechanistic investigations disclose a sequential catalytic process comprising a "hidden" Brønsted acid catalytic hydrolysis of acetals to aldehydes followed by the I2-bonding Lewis acid catalytic alkylation of aldehydes to 3,3'-disubstituted 1H-indoles.

Gauging the Strength of the Molecular Halogen Bond via Experimental Electron Density and Spectroscopy

Strong and weak halogen bonds (XBs) in discrete aggregates involving the same acceptor are addressed by experiments in solution and in the solid state. Unsubstituted and perfluorinated iodobenzenes act as halogen donors of tunable strength; in all cases, quinuclidine represents the acceptor. NMR titrations reliably identify the strong intermolecular interactions in solution, with experimental binding energies of approx. 7 kJ/mol. Interaction of the σ hole at the halogen donor iodine leads to a redshift in the symmetric C-I stretching vibration; this shift reflects the interaction energy in the halogen-bonded adducts and may be assessed by Raman spectroscopy in condensed phase even for weak XBs. An experimental picture of the electronic density for the XBs is achieved by high-resolution X-ray diffraction on suitable crystals. Quantum theory of atoms in molecules (QTAIM) analysis affords the electron densities and energy densities in the bond critical points of the halogen bonds and confirms stronger interaction for the shorter contacts. For the first time, the experimental electron density shows a significant effect on the atomic volumes and Bader charges of the quinuclidine N atoms, the halogen-bond acceptor: strong and weak XBs are reflected in the nature of their acceptor atom. Our experimental findings at the acceptor atom match the discussed effects of halogen bonding and thus the proposed concepts in XB activated organocatalysis.

Terpene dispersion energy donor ligands in borane complexes

Structural characterization of the complex [B(β-pinane)₃] (1) reveals non-covalent H⋯H contacts that are consistent with the generation of London dispersion energies involving the β-pinane ligand frameworks. The homolytic fragmentations of 1, and camphane and sabinane analogues ([B(camphane)₃] (2) and [B(sabinane)₃] (3)) were studied computationally. Isodesmic exchange results showed that London dispersion interactions are highly dependent on the terpene's stereochemistry, with the β-pinane framework providing the greatest dispersion free energy (ΔG = -7.9 kcal mol^-1) with Grimme's dispersion correction (D3BJ) employed. PMe₃ was used to coordinate to [B(β-pinane)₃], giving the complex [Me₃P-B(β-pinane)₃] (4), which displayed a dynamic coordination equilibrium in solution. The association process was found to be slightly endergonic at 302 K (ΔG = +0.29 kcal mol^-1).

Monolayer-Protected Gold Nanoparticles Functionalized with Halogen Bonding Capability─An Avenue for Molecular Detection Schemes

The use of functionalized nanoparticles (NPs) and their aggregation in the presence of a targeted analyte is a well-established molecular detection strategy predicated on harnessing specific molecular interactions to the NP periphery. Molecules able to specifically interact with the functionalized NPs alter the unique optical and electrochemical properties of the NPs as a function of interparticle spacing. While many intermolecular interactions have been successfully exploited in this manner in conjunction with aqueous NP systems, the use of non-aqueous NPs in the same capacity is significantly less explored. A fundamental interaction that has not been previously investigated in NP schemes is halogen bonding (XB). XB is an orthogonal, electrostatic interaction between a region of positive electrostatic potential (δ+) on a halogen atom (i.e., XB donor) and a negative (δ-) Lewis base (XB acceptor) molecule. To couple XB with NP systems, ligands featuring a molecular structure that promotes XB interactions need to be identified, optimized, and synthesized for subsequent attachment to NPs. Herein, density functional theory (DFT) and NMR techniques are used to identify a strong XB-donor moiety (-C₆F₄I) and a synthetic scheme for a thiolate ligand featuring that functionality is devised and executed with high purity/yield (78%). Ligand-exchange reactions allow functionalization of non-aqueous alkanethiolate-protected gold NPs or monolayer-protected clusters (MPCs) with the XB-donor ligands. Functionalized MPCs (f-MPCs), within both assembled films and in solution, are shown to engage in XB interactions with target XB-acceptor molecules. Molecular recognition events, including induced aggregation of the f-MPCs, are characterized with UV-vis spectroscopy, cyclic voltammetry, TEM imaging, and diffusion-ordered spectroscopy NMR with limits of detection of 50-100 nM for strong XB acceptors. While fundamental exploration of XB interactions is ongoing, this study represents a step toward utilizing XB within molecular detection schemes, an application with implications for supramolecular chemistry, forensic, and environmental chemical sensing.

Assessing the effects of covalent, dative and halogen bonds on the electronic structure of selenoamides

The C–NMe2 bond rotation of a selenoamide is proposed as an experimental probe to compare different chemical interactions.

Evaluating Halogen-Bond Strength as a Function of Molecular Structure Using Nuclear Magnetic Resonance Spectroscopy and Computational Analysis

Halogen bonding (XB) is a highly directional, non-covalent intermolecular interaction between a molecule (XB donor) presenting a halogen with an electron-deficient region or sigma hole (σ-hole) and an electron-rich or Lewis-base molecule (XB acceptor). A systematic, experimental, and theoretical study of solution-phase XB strength as a function of the molecular structure for both XB donor and acceptor molecules is presented. The impact of specific structural features is assessed using ¹⁹F and ¹H nuclear magnetic resonance (NMR) titrations to determine association constants, density functional theory calculations for interaction energies and bond lengths, as well as ¹⁹F-¹H HOESY NMR measurements of intermolecular cross-relaxation between the interacting XB donor-acceptor adducts. For XB donor molecules (perfluoro-halogenated benzenes), results indicate the critical importance of iodine coupled with electron-withdrawing entities. Prominent structural components of XB acceptor molecules include a central atom working in conjunction with a Lewis-base atom to present high electron density directed at the σ-hole (e.g., tributylphosphine oxide). Additionally, larger surrounding aliphatic R groups (e.g., butyl and octyl) were found to significantly stabilize strong XB, particularly in solvents that promote the interaction. With a more thorough understanding of structure-optimized XB, one can envision harnessing XB interactions more strategically for specific design of optimal materials and chemical applications.

Weak yet Decisive: Molecular Halogen Bond and Competing Weak Interactions of Iodobenzene and Quinuclidine

The halogen bonded adduct between the commonly used constituents quinuclidine and iodobenzene is based on a single weak nitrogen-iodine contact, and the isolation of this adduct was initially unexpected. Iodobenzene does not contain any electron-withdrawing group and therefore represents an unconventional halogen bond donor. Based on excellent diffraction data of high resolution, an electron density study was successfully accomplished and confirmed one of the longest N···I molecular halogen bonds with a distance of 2.9301(4) Å. The topological analysis identified the XB as a directional but weak σ hole interaction and revealed secondary contacts between peripheral regions of opposite charge. These additional contacts and their competition with a nitrogen-based interaction were confirmed by NOESY experiments in solution. Integration enabled us to determine the relative NOE ratios and provided insight regarding the existing interactions.

A Comparative Study on the Thermodynamics of Halogen Bonding of Group 10 Pincer Fluoride Complexes

The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5‐R₂‐^tBuPOCOP^tBu)MF] (3,5‐R₂‐^tBuPOCOP^tBu=κ³‐C₆HR₂‐2,6‐(OPtBu₂)₂ with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1‐tBu (R=tBu, M=Ni), 2‐H (R=H, M=Pd), 2‐tBu (R=tBu, M=Pd), 2‐COOMe (R=COOMe, M=Pd) and 3‐tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2‐tBu with 1,4‐diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.

Binary Donor-Acceptor Adducts of Tetrathiafulvalene Donors with Cyclic Trimetallic Monovalent Coinage Metal Acceptors

Reactions between the π-acidic cyclic trimetallic coinage metal(I) complexes {[Cu(μ-3,5-(CF₃)₂pz)]₃, {[Ag(μ-3,5-(CF₃)₂pz)]₃, and {[Au(μ-3,5-(CF₃)₂pz)]₃ with TTF, DBTTF and BEDT-TTF give rise to a series of coinage metal(I)-based new binary donor-acceptor adducts {[Cu(μ-3,5-(CF₃)₂pz)]₃DBTTF} (1), {[Ag(μ-3,5-(CF₃)₂pz)]₃DBTTF} (2), {[Au(μ-3,5-(CF₃)₂pz)]₃DBTTF} (3), {[Cu(μ-3,5-(CF₃)₂pz)]₃TTF} (4), {[Ag(μ-3,5-(CF₃)₂pz)]₃TTF} (5), {[Au(μ-3,5-(CF₃)₂pz)]₃TTF} (6), {[Cu(μ-3,5-(CF₃)₂pz)]₃BEDT-TTF} (7), {[Ag(μ-3,5-(CF₃)₂pz)]₃BEDT-TTF} (8), and {[Au(μ-3,5-(CF₃)₂pz)]₃BEDT-TTF} (9), where pz = pyrazolate, TTF = tetrathiafulvalene, DBTTF = dibenzotetrathiafulvalene, and BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene. This series of binary donor-acceptor adducts has been found to exhibit remarkable supramolecular structures in both the solid state and solution, whereby they exhibit supramolecular stacked chains and oligomers, respectively. The supramolecular solid-state and solution binary donor-acceptor adducts also exhibit superior shelf stability under ambient laboratory storage conditions. Structural and other electronic properties of solids and solutions of these adducts have been characterized by single-crystal X-ray diffraction (XRD) structural analysis, ¹H and ¹⁹F NMR, UV-vis-near-IR spectroscopy, Fourier transform infrared, and computational investigations. The combined results of XRD structural data analysis, spectroscopic measurements, and theoretical studies suggest sustenance of the donor-acceptor stacked structure and electronic communication in both the solid state and solution. These properties are discussed in terms of potential applications for this new class of supramolecular binary donor-acceptor adducts in molecular electronic devices, including solar cells, magnetic switching devices, and field-effect transistors.

Relaxation Dispersion NMR to Reveal Fast Dynamics in Brønsted Acid Catalysis: Influence of Sterics and H-Bond Strength on Conformations and Substrate Hopping

NMR provides both structural and dynamic information, which is key to connecting intermediates and to understanding reaction pathways. However, fast exchanging catalytic intermediates are often inaccessible by conventional NMR due its limited time resolution. Here, we show the combined application of the 1H off-resonance R1ρ NMR method and low temperature (185-175 K) to resolve intermediates exchanging on a μs time scale (ns at room temperature). The potential of the approach is demonstrated on chiral phosphoric acid (CPA) catalysts in their complexes with imines. The otherwise inaccessible exchange kinetics of the E-I ⇌ E-II imine conformations and thermodynamic E-I:E-II imine ratios inside the catalyst pocket are experimentally determined and corroborated by calculations. The E-I ⇌ E-II exchange rate constants (kex185 K) for different catalyst-substrate binary complexes varied between 2500 and 19 000 s-1 (τex = 500-50 μs). Theoretical analysis of these exchange rate constants revealed the involvement of an intermediary tilted conformation E-III, which structurally resembles the hydride transfer transition state. The main E-I and E-II exchange pathway is a hydrogen bond strength dependent tilting-switching-tilting mechanism via a bifurcated hydrogen bond as a transition state. The reduction in the sterics of the catalyst showed an accelerated switching process by at least an order of magnitude and enabled an additional rotational pathway. Hence, the exchange process is mainly a function of the intrinsic properties of the 3,3'-substituents of the catalyst. Overall, we believe that the present study opens a new dimension in catalysis via experimental access to structures, populations, and kinetics of catalyst-substrate complexes on the μs time scale by the 1H off-resonance R1ρ method.

Motional Dynamics of Halogen-Bonded Complexes Probed by Low-Field NMR Relaxometry and Overhauser Dynamic Nuclear Polarization

Halogen bonding is a subject of considerable interest owing to wide-ranging chemical, materials and biological applications. The motional dynamics of halogen-bonded complexes play a pivotal role in comprehending the nature of the halogen-bonding interaction. However, not many attempts appear to have been made to shed light on the dynamical characteristics of halogen-bonded species. For the first time, we demonstrate here that the combination of low-field NMR relaxometry and Overhauser dynamic nuclear polarization (ODNP) makes it possible to obtain a cogent picture of the motional dynamics of halogen-bonded species. We discuss here the advantages of this combined approach. Low-field relaxometry allows us to infer the hydrodynamic radius and rotational correlation time, whereas ODNP probes the molecular translational correlation times (involving the substrate as well as the organic radical) with high sensitivity at low field.

Benchmarking of Halogen Bond Strength in Solution with Nickel Fluorides: Bromine versus Iodine and Perfluoroaryl versus Perfluoroalkyl Donors

The energetics of halogen bond formation in solution have been investigated for a series of nickel fluoride halogen bond acceptors; trans-[NiF(2-C5 NF4 )(PEt3 )2 ] (A1), trans-[NiF{2-C5 NF3 (4-H)}(PEt3 )2 ] (A2), trans-[NiF{2-C5 NF3 (4-NMe2 )}(PEt3 )2 ] (A3) and trans-[NiF{2-C5 NF2 H(4-CF3 )}(PCy3 )2 ] (A4) with neutral organic halogen bond donors, iodopentafluorobenzene (D1), 1-iodononafluorobutane (D2) and bromopentafluorobenzene (D3), in order to establish the significance of changes from perfluoroaryl to perfluoroalkyl donors and from iodine to bromine donors. 19 F NMR titration experiments have been employed to obtain the association constants, enthalpy, and entropy for the halogen bond formed between these donor-acceptor partners in protiotoluene. For A2-A4, association constants of the halogen bonds formed with iodoperfluoroalkane (D2) are consistently larger than those obtained for analogous complexes with the iodoperfluoroarene (D1). For complexes formed with A2-A4, the strength of the halogen bond is significantly lowered upon modification of the halogen donor atom from I (in D1) to Br (in D3) (for D1: 5≤K285 ≤12 m-1 , for D3: 1.0≤K193 ≤1.6 m-1 ). The presence of the electron donating NMe2 substituent on the pyridyl ring of acceptor A3 led to an increase in -ΔH, and the association constants of the halogen bond complexes formed with D1-D3, compared to those formed by A1, A2 and A4 with the same donors.

Dicarboxylic Acid Separation by Dynamic and Size-Matched Recognition in Solution and in the Solid State

Bis(trimethylammonium) alkane diiodides dynamically encapsulate dicarboxylic acids through intermolecular hydrogen bonds between the I^- anions of the hosts and the carboxylic OH groups of the guests. A selective recognition is realized when the size of the I^- ⋅⋅⋅HOOC(CH₂ /CF₂ )_n COOH⋅⋅⋅I^- superanion matches the dication alkyl chain length. Dynamic recognition is also demonstrated in solution, where the presence of the size-matching organic salt boosts the acid solubility profile, thus allowing efficient mixture separation.

Cooperativity between hydrogen- and halogen bonds: the case of selenourea

A combined experimental/theoretical investigation on the cooperativity between hydrogen- and halogen bonds is presented. In this study, it is shown that selenourea can interact at the same time with a benzoate anion and a polarized iodine, with the two interactions influencing and enhancing each other.

The halogen bond: Nature and applications

The halogen bond, corresponding to an attractive interaction between an electrophilic region in a halogen (X) and a nucleophile (B) yielding a R−X⋯B contact, found applications in many fields such as supramolecular chemistry, crystal engineering, medicinal chemistry, and chemical biology. Their large range of applications also led to an increased interest in their study using computational methods aiming not only at understanding the phenomena at a fundamental level, but also to help in the interpretation of results and guide the experimental work. Herein, a succinct overview of the recent theoretical and experimental developments is given starting by discussing the nature of the halogen bond and the latest theoretical insights on this topic. Then, the effects of the surrounding environment on halogen bonds are presented followed by a presentation of the available method benchmarks. Finally, recent experimental applications where the contribution of computational chemistry was fundamental are discussed, thus highlighting the synergy between the lab and modeling techniques.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Disclosing the multi-faceted world of weakly interacting inorganic systems by means of NMR spectroscopy

The potential of NMR spectroscopy to investigate inorganic systems whose structure and reactivity is affected by non-covalent interactions is described; supramolecular assemblies based on relatively unusual intermolecular forces or on more classical ones, still rather unexplored in solution, are considered.

A PGSE NMR approach to the characterization of single and multi-site halogen-bonded adducts in solution

The diffusional PGSE NMR technique allows a detailed characterization of single- and multi-site halogen-bonded systems.

Charge-displacement analysis as a tool to study chalcogen bonded adducts and predict their association constants in solution

The secondary interaction between an atom of tellurium and different Lewis bases has been studied by charge displacement analysis, providing a detailed description of the interaction and a computational insight into experimental data.