Intermolecular n→π* Interactions in Supramolecular Chemistry and Catalysis

The n→π* interactions describing attractive force between lone pairs (lps) of nucleophile and carbonyl or polarized unsaturated bonds have recently attracted growing attentions in various disciplines. So far, such non-covalent driving force are mainly concentrated to intramolecular systems. Intermolecular n→π* interactions in principle could produce fascinated supramolecular systems or facilitate organic reactions, however, they remain largely underexplored due to the very weak energy of individual interaction. This review attempts to give an overview of the challenging intermolecular n→π* interactions, much efforts emphasize the supramolecular systems, catalytic processes and spectroscopic measurements.

Allosteric Regulation of Aptamer Affinity through Mechano-Chemical Coupling

The capacity to precisely modulate aptamer affinity is important for a wide variety of applications. However, most such engineering strategies entail laborious trial-and-error testing or require prior knowledge of an aptamer's structure and ligand-binding domain. We describe here a simple and generalizable strategy for allosteric modulation of aptamer affinity by employing a double-stranded molecular clamp that destabilizes aptamer secondary structure through mechanical tension. We demonstrate the effectiveness of the approach with a thrombin-binding aptamer and show that we can alter its affinity by as much as 65-fold. We also show that this modulation can be rendered reversible by introducing a restriction enzyme cleavage site into the molecular clamp domain and describe a design strategy for achieving even more finely-tuned affinity modulation. This strategy requires no prior knowledge of the aptamer's structure and binding mechanism and should thus be generalizable across aptamers.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Hydrogen Bonding versus Halogen Bonding: Spectroscopic Investigation of Gas-Phase Complexes Involving Bromide and Chloromethanes

Hydrogen bonding and halogen bonding are important non-covalent interactions that are known to occur in large molecular systems, such as in proteins and crystal structures. Although these interactions are important on a large scale, studying hydrogen and halogen bonding in small, gas-phase chemical species allows for the binding strengths to be determined and compared at a fundamental level. In this study, anion photoelectron spectra are presented for the gas-phase complexes involving bromide and the four chloromethanes, CH₃ Cl, CH₂ Cl₂ , CHCl₃ , and CCl₄ . The stabilisation energy and electron binding energy associated with each complex are determined experimentally, and the spectra are rationalised by high-level CCSD(T) calculations to determine the non-covalent interactions binding the complexes. These calculations involve nucleophilic bromide and electrophilic bromine interactions with chloromethanes, where the binding motifs, dissociation energies and vertical detachment energies are compared in terms of hydrogen bonding and halogen bonding.

Water Networks Repopulate Protein-Ligand Interfaces with Temperature

High-resolution crystal structures highlight the importance of water networks in protein-ligand interactions. However, as these are typically determined at cryogenic temperature, resulting insights may be structurally precise but not biologically accurate. By collecting 10 matched room-temperature and cryogenic datasets of the biomedical target Hsp90α, we identified changes in water networks that impact protein conformations at the ligand binding interface. Water repositioning with temperature repopulates protein ensembles and ligand interactions. We introduce Flipper conformational barcodes to identify temperature-sensitive regions in electron density maps. This revealed that temperature-responsive states coincide with ligand-responsive regions and capture unique binding signatures that disappear upon cryo-cooling. Our results have implications for discovering Hsp90 selective ligands, and, more generally, for the utility of hidden protein and water conformations in drug discovery.

Anthracene Walled Acyclic CB[n] Receptors: In Vitro and In Vivo Binding Properties Toward Drugs of Abuse

We report studies of the interaction of six acyclic CB[n]-type receptors toward a panel of drugs of abuse by a combination of isothermal titration calorimetry and 1H NMR spectroscopy.  Anthracene walled acyclic CB[n] host (M3) displays highest binding affinity toward methamphetamine (Kd = 15 nM) and fentanyl (Kd = 4 nM).  Host M3 is well tolerated by Hep G2 and HEK 293 cells up to 100 mM according to MTS metabolic and adenylate kinase release assays.  An in vivomaximum tolerated dose study with Swiss Webster mice showed no adverse effects at the highest dose studied (44.7 mg kg-1). Host M3 is not mutagenic based on the Ames fluctuation test and does not inhibit the hERG ion channel.  In vivoefficacy studies showed that pretreatment of mice with M3 significantly reduces the hyperlocomotion after treatment with methamphetamine, but M3 does not function similarly when administered 30 seconds after methamphetamine.

Host-assisted delivery of a model drug to genomic DNA: Key information from ultrafast spectroscopy and in silico study

Intended drug delivery to a target without adverse effect is one of the major criteria for its acceptance in real use. Herein, we have made an attempt to explore the delivery efficacy of SDS surfactant in a monomer and micellar stage during the delivery of model drug, Toluidine Blue (TB) from micellar cavity to DNA. Molecular recognition of pre-micellar SDS encapsulated TB with DNA occurs at a rate constant (k1~652 s-1). On the contrary, no significant release of encapsulated TB at micellar concentration was observed within the experimental time frame. This originated from the higher binding affinity of TB towards the nano cavity of SDS at micellar concentration which doesn't allow the delivery of TB from the nano cavity of SDS micelle to DNA. Thus, molecular recognition controls the extent of DNA recognition by TB which in turn modulates the rate of delivery of TB from SDS in a concentration dependent morphology.

In Vitro and In Vivo Sequestration of Methamphetamine by a Sulfated Acyclic CB[n]-Type Receptor

We report the synthesis of two new acyclic sulfated acyclic CB[n]-type receptors (TriM0 and Me4 TetM0) and investigations of their binding properties toward a panel of drugs of abuse (1-13) by a combination of 1 H NMR spectroscopy and isothermal titration calorimetry. TetM0 is the most potent receptor with Ka ≥106  M-1 toward methamphetamine, fentanyl, MDMA and mephedrone. TetM0 is not cytotoxic toward HepG2 and HEK 293 cells below 100 μM according to MTS metabolic and adenylate kinase release assays and is well tolerated in vivo when dosed at 46 mg kg-1 . TetM0 does not inhibit the hERG ion channel and is not mutagenic based on the Ames fluctuation test. Finally, in vivo efficacy studies show that the hyperlocomotion of mice treated with methamphetamine can be greatly reduced by treatment with TetM0 up to 5 minutes later. TetM0 has potential as a broad spectrum in vivo sequestrant for drugs of abuse.

A Conformationally Stable Acyclic β-Hairpin Scaffold Tolerating the Incorporation of Poorly β-Sheet-Prone Amino Acids

The β-hairpin is a structural element of native proteins, but it is also a useful artificial scaffold for finding lead compounds to convert into peptidomimetics or non-peptide structures for drug discovery. Since linear peptides are synthetically more easily accessible than cyclic ones, but are structurally less well-defined, we propose XWXWXpPXK(/R)X(R) as an acyclic but still rigid β-hairpin scaffold that is robust enough to accommodate different types of side chains, regardless of the secondary-structure propensity of the X residues. The high conformational stability of the scaffold results from tight contacts between cross-strand cationic and aromatic side chains, combined with the strong tendency of the d-Pro-l-Pro dipeptide to induce a type II' β-turn. To demonstrate the robustness of the scaffold, we elucidated the NMR structures and performed molecular dynamics (MD) simulations of a series of peptides displaying mainly non-β-branched, poorly β-sheet-prone residues at the X positions. Both the NMR and MD data confirm that our acyclic β-hairpin scaffold is highly versatile as regards the amino-acid composition of the β-sheet face opposite to the cationic-aromatic one.

Biomimetic Recognition of Quinones in Water by an Endo-Functionalized Cavity with Anthracene Sidewalls

Selective molecular recognition in water is the foundation of numerous biological functions but is a challenge for most synthetic hosts. We employ the concept of endo-functionalized cavity and the strategy of simultaneous construction to address this issue. The concept and the strategy were demonstrated in the construction of a biomimetic host for selectively recognizing quinones in water. The host was synthesized by joining two pieces of bent anthracene dimer through amide bond formation, affording a deep hydrophobic cavity and inward-directing hydrogen bonding sites. The host can recognize quinones over their close analogues in water, and its association affinity to p-benzoquinone is the highest among all the known hosts and is even comparable to that of the bioreceptor. The binding with an anthraquinone reaches nanomolar affinity. Shielded hydrogen bonding, C-H⋅⋅⋅π, and charge transfer interactions, and the hydrophobic effect are responsible for the high binding affinity and selectivity.

"Anti-electrostatic" Halogen Bonding between Ions of Like Charge

Halogen bonding occurs between molecules featuring Lewis acidic halogen substituents and Lewis bases. It is often rationalized as a predominantly electrostatic interaction and thus interactions between ions of like charge (e. g., of anionic halogen bond donors with halides) seem counter-intuitive. Herein, we provide an overview on such complexes. First, theoretical studies are described and their findings are compared. Next, experimental evidences are presented in the form of crystal structure database analyses, recent examples of strong "anti-electrostatic" halogen bonding in crystals, and the observation of such interactions also in solution. We then compare these complexes to select examples of "counter-intuitive" adducts formed by other interactions, like hydrogen bonding. Finally, we comment on key differences between charge-transfer and electrostatic polarization.

Enantioselective Construction of Modular and Asymmetric Baskets

The precise positioning of functional groups about the inner space of abiotic hosts is a challenging task and of interest for developing more effective receptors and catalysts akin to those found in nature. To address it, we herein report a synthetic methodology for preparing basket-like cavitands comprised of three different aromatics as side arms with orthogonal esters at the rim for further functionalization. First, enantioenriched A (borochloronorbornene), B (iodobromonorbornene), and C (boronorbornene) building blocks were obtained by stereoselective syntheses. Second, consecutive A-to-B and then AB-to-C Suzuki-Miyaura (SM) couplings were optimized to give enantioenriched ABC cavitand as the principal product. The robust synthetic protocol allowed us to prepare (a) an enantioenriched basket with three benzene sides and each holding either tBu, Et, or Me esters, (b) both enantiomers of a so-called "spiral staircase" basket with benzene, naphthalene, and anthracene groups surrounding the inner space, and (c) a photo-responsive basket bearing one anthracene and two benzene arms.

Crystalline-Phase-Recognition-Induced Domino Phase Transition and Luminescence Switching for Advanced Information Encryption

Herein, a new mechanism, namely, crystalline phase recognition (CPR), is proposed for the single-crystal-to-single-crystal (SCSC) transition of metal halides. Chiral β-[Bmmim]₂ SbCl₅ (Bmmim=1-butyl-2,3-methylimidazolium) can recognize achiral α-[Bmmim]₂ SbCl₅ on the basis of a key-lock feature through intercontact of their single crystals, resulting in a domino phase transition (DPT). The concomitant photoluminescence (PL) switching enables observation of the DPT in situ. The liquid eutectic interface, stress-strain transfer, and feasible thermodynamics are key issues for the CPR. DFT calculations and PL measurements revealed that the optical absorption and emission of the isomers mainly originate from [SbCl₅ ]^2- anions. The structural effects (e.g., supramolecular interactions and [SbCl₅ ]^2- distortion) on the optical emission are clarified. As a novel type of stimuli response, the CPR-induced DPT and luminescence switching exhibit potential for application in advanced time-resolved information encryption.

Easy Production of "Difficult Peptides" Using Cell-Free Protein Synthesis and a New Methionine Analogue as a Latent Peptide Cleavage Site

Aliphatic γ-chloro-α-amino acids incorporated in place of their canonical analogues through cell-free protein synthesis act as heat-labile linkers, offering a useful strategy for the straightforward production of target peptides as fusion proteins, from which the targets are readily released. Until now, the natural abundance of aliphatic amino acids in peptides has limited the scope of the method, as it leads to undesired cleavage sites in synthesized products, but here the authors report the development of a new cleavable chloro amino acid that incorporates in place of the relatively rare amino acid methionine, thus greatly expanding the scope of producible targets. This new strategy is employed for simplified peptide synthesis with a methionine-free fusion partner, allowing single-site incorporation of the cleavable linker for clean release and easy purification of the target peptide. Its utility is demonstrated through the straightforward preparation of two peptides reported to be challenging targets and not accessible through standard solid-phase chemical methodologies, as well as analogues.

Molecular Recognition of Nerve Agents and Their Organophosphorus Surrogates: Toward Supramolecular Scavengers and Catalysts

Nerve agents are tetrahedral organophosphorus compounds (OPs) that were developed in the last century to irreversibly inhibit acetylcholinesterase (AChE) and therefore impede neurological signaling in living organisms. Exposure to OPs leads to a rapid development of symptoms from excessive salivation, nasal congestion and chest pain to convulsion and asphyxiation which if left untreated may lead to death. These potent toxins are prepared on a large scale from inexpensive staring materials, making it feasible for terrorist groups or states to use them against military and civilians. The existing antidotes provide limited protection and are difficult to apply to a large number of affected individuals. While new prophylactics are currently being developed, there is still need for therapeutics capable of both preventing and reversing the effects of OP poisoning. In this review, we describe how the science of molecular recognition can expand the pallet of tools for rapid and safe sequestration of nerve agents.

What We Learned in 25 Years of Interactive Molecular Design Sessions

We retrace Prof. François Diederich's consultancy work for Roche and its impact over the years he worked with us. François Diederich uniquely shaped our approach to molecular design, and interactions with him and his research group at ETH Zurich have created deep insights into molecular recognition. Herein we share how his style and approach continue to inspire us.

Radically Enhanced Dual Recognition

Complexation between a viologen radical cation (V^.+ ) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT^2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V^.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV^.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)^2(.+) , i.e., CBPQN^2(.+) . Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN^2(.+) suitable for binding two MV^.+ with relatively short (3.05-3.25 Å) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV^.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex - namely, [(MV)₂ ⊂CBPQN]^{4( .+)} - in MeCN was confirmed by VT ¹ H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)₂ ⊂CBPQN]^{4( .+)} reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V^.+ , suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV^.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.

Hydrogen Bond Acceptor Propensity of Different Fluorine Atom Types: An Analysis of Experimentally and Computationally Derived Parameters

The propensity of organic fluorine acting as a weak hydrogen bond acceptor (HBA) in intermolecular and intramolecular interactions has been the subject of many experimental and theoretical studies often reaching different conclusions. Over the last few years, new and stronger evidences have emerged for the direct involvement of fluorine in weak hydrogen bond (HB) formation. However, not all the fluorine atom types can act as weak HBA. In this work, the differential HBA propensity of various types of fluorine atoms was analyzed with a particular emphasis for the different types of alkyl fluorides. This was carried out by evaluating ab initio computed parameters, experimental ¹⁹ F NMR chemical shifts and small molecule crystallographic structures (extracted from the CSD database). According to this analysis, shielded (with reference to the ¹⁹ F NMR chemical shift) alkyl mono-fluorinated motifs display the highest HBA propensity in agreement with solution studies. Although much weaker than other well-characterized HB complexes, the fragile HBs formed by these fluorinated motifs have important implications for the chemical-physical and structural properties of the molecules, chemical reactions, and protein-ligand recognition.

François Diederich (1952-2020): 40 Years of Organic Chemistry

François Diederich, Professor of Organic Chemistry and long-time Chair of the Editorial Board of Angewandte Chemie, sadly passed away on September 23, 2020. He will be remembered for his groundbreaking research in the chemistry of fullerenes and carbon-rich molecules, in supramolecular and medicinal chemistry, as an engaging teacher, and as a generous and fascinating human being.

Making Waxy Salts in Water: Synthetic Control of Hydrophobicity for Anion-Induced and Aggregation-Enhanced Light Emission

We show that multipodal polycationic receptors function as anion-responsive light-emitters in water. Prevailing paradigms utilize rigid holes and cavities for ion recognition. We instead built open amphiphilic scaffolds that trigger polar-to-nonpolar environment transitions around cationic fluorophores upon anion complexation. This ion-pairing and aggregation event produces a dramatic enhancement in the emission intensity, as demonstrated by perchlorate as a non-spherical hydrophobic anion model. A synergetic interplay of C-H⋅⋅⋅anion hydrogen bonding and tight anion-π⁺ contacts underpins this supramolecular phenomenon. By changing the aliphatic chain length, we demonstrate that the response profile and threshold of this signaling event can be controlled at the molecular level. With appropriate molecular design, inherently weak, ill-defined, and non-directional van der Waals interaction enables selective, sensitive, and tunable recognition in water.

Highly Selective Radical Monoreduction of Dihalides Confined to a Dynamic Supramolecular Host

Reduction of alkyl dihalide guests (2-5 and 7) with trialkylsilanes (R₃ SiH) was performed in water-soluble host 1 to investigate the effects of confinement on fast radical reactions (k≥10³  m^-1  s^-1 ). High selectivity (>95 %) for mono-reduced products was observed for primary and secondary dihalide guests under mild conditions. The results highlight the importance of host-guest complexation rates to modulate the product selectivity in radical reactions.

London Dispersion and Hydrogen-Bonding Interactions in Bulky Molecules: The Case of Diadamantyl Ether Complexes

Diadamantyl ether (DAE, C20 H30 O) represents a good model to study the interplay between London dispersion and hydrogen-bond interactions. By using broadband rotational spectroscopy, an accurate experimental structure of the diadamantyl ether monomer is obtained and its aggregates with water and a variety of aliphatic alcohols of increasing size are analyzed. In the monomer, C-H⋅⋅⋅H-C London dispersion attractions between the two adamantyl subunits further stabilize its structure. Water and the alcohol partners bind to diadamantyl ether through hydrogen bonding and non-covalent Owater/alcohol ⋅⋅⋅H-CDAE and C-Halcohol ⋅⋅⋅H-CDAE interactions. Electrostatic contributions drive the stabilization of all the complexes, whereas London dispersion interactions become more pronounced with increasing size of the alcohol. Complexes with dominant dispersion contributions are significantly higher in energy and were not observed in the experiment. The results presented herein shed light on the first steps of microsolvation and aggregation of molecular complexes with London dispersion energy donor (DED) groups and the kind of interactions that control them.

Kinetic Stabilities and Exchange Dynamics of Water-Soluble Bis-Formamide Caviplexes Studied Using Diffusion-Ordered NMR Spectroscopy (DOSY)

A deep cavitand binds long-chain trans,trans- and trans,cis-bis-formamide isomers in water solution giving a pair of caviplexes in a ca. 60:40 ratio. Both caviplexes display in/out guest exchange dynamics that are slow on the ¹ H NMR chemical shift timescale, but fast on the EXSY timescale. We apply diffusion-ordered NMR spectroscopy (DOSY) to characterize the caviplexes. On the diffusion timescale, the guest in/out exchange processes feature intermediate dynamics allowing the assessment of their kinetic stabilities. We found that the trans,cis-bis-formamide isomers form kinetically more stable caviplexes than the trans,trans-counterparts. We also show that the kinetic stabilities of the bis-formamide caviplexes relate well with their relative thermodynamic stabilities. Fortunately, the tuning of the DOSY parameters allowed the observation of the exchange dynamics as slow processes on the experiment timescale.

"Anti-Electrostatic" Halogen Bonding

Halogen bonding is often described as being driven predominantly by electrostatics, and thus adducts between anionic halogen bond (XB) donors (halogen-based Lewis acids) and anions seem counterintuitive. Such "anti-electrostatic" XBs have been predicted theoretically but for organic XB donors, there are currently no experimental examples except for a few cases of self-association. Reported herein is the synthesis of two negatively charged organoiodine derivatives that form anti-electrostatic XBs with anions. Even though the electrostatic potential is universally negative across the surface of both compounds, DFT calculations indicate kinetic stabilization of their halide complexes in the gas phase and particularly in solution. Experimentally, self-association of the anionic XB donors was observed in solid-state structures, resulting in dimers, trimers, and infinite chains. In addition, co-crystals with halides were obtained, representing the first cases of halogen bonding between an organic anionic XB donor and a different anion. The bond lengths of all observed interactions are 14-21 % shorter than the sum of the van der Waals radii.

Is There a Single Ideal Parameter for Halogen-Bonding-Based Lewis Acidity?

Halogen-bond donors (halogen-based Lewis acids) have now found various applications in diverse fields of chemistry. The goal of this study was to identify a parameter obtainable from a single DFT calculation that reliably describes halogen-bonding strength (Lewis acidity). First, several DFT methods were benchmarked against the CCSD(T) CBS binding data of complexes of 17 carbon-based halogen-bond donors with chloride and ammonia as representative Lewis bases, which revealed M05-2X with a partially augmented def2-TZVP(D) basis set as the best model chemistry. The best single parameter to predict halogen-bonding strengths was the static σ-hole depth, but it still provided inaccurate predictions for a series of compounds. Thus, a more reliable parameter, Ωσ* , has been developed through the linear combination of the σ-hole depth and the σ*(C-I) energy, which was further validated against neutral, cationic, halogen- and nitrogen-based halogen-bond donors with very good performance.

On the Interplay between Charge-Shift Bonding and Halogen Bonding

The nature of halogen-bond interactions has been analysed from the perspective of the astatine element, which is potentially the strongest halogen-bond donor. Relativistic quantum calculations on complexes formed between halide anions and a series of Y₃ C-X (Y=F to X, X=I, At) halogen-bond donors disclosed unexpected trends, e. g., At₃ C-At revealing a weaker donating ability than I₃ C-I despite a stronger polarizability. All the observed peculiarities have their origin in a specific component of C-Y bonds: the charge-shift bonding. Descriptors of the Quantum Chemical Topology show that the halogen-bond strength can be quantitatively anticipated from the magnitude of charge-shift bonding operating in Y₃ C-X. The charge-shift mechanism weakens the ability of the halogen atom X to engage in halogen bonds. This outcome provides rationales for outlier halogen-bond complexes, which are at variance with the consensus that the halogen-bond strength scales with the polarizability of the halogen atom.

Host-Guest Recognition and Fluorescence of a Tetraphenylethene-Based Octacationic Cage

We report the synthesis and characterization of a three-dimensional tetraphenylethene-based octacationic cage that shows host-guest recognition of polycyclic aromatic hydrocarbons (e.g. coronene) in organic media and water-soluble dyes (e.g. sulforhodamine 101) in aqueous media through CH⋅⋅⋅π, π-π, and/or electrostatic interactions. The cage⊃coronene exhibits a cuboid internal cavity with a size of approximately 17.2×11.0×6.96 ų and a "hamburger"-type host-guest complex, which is hierarchically stacked into 1D nanotubes and a 3D supramolecular framework. The free cage possesses a similar cavity in the crystalline state. Furthermore, a host-guest complex formed between the octacationic cage and sulforhodamine 101 had a higher absolute quantum yield (Φ_F =28.5 %), larger excitation-emission gap (Δλ_ex-em =211 nm), and longer emission lifetime (τ=7.0 ns) as compared to the guest (Φ_F =10.5 %; Δλ_ex-em =11 nm; τ=4.9 ns), and purer emission (Δλ_FWHM =38 nm) as compared to the host (Δλ_FWHM =111 nm).

Bridge-Clamp Bis(tetrazine)s with [N]8 π-Stacking Interactions and Azido-s-Aryl Tetrazines: Two Classes of Doubly Clickable Tetrazines

Click chemistry at a tetrazine core is useful for bioorthogonal labeling and crosslinking. Introduced here are two new classes of doubly clickable s-aryl tetrazines synthesized by Cu-catalyzed cross-coupling. Homocoupling of o-brominated s-aryl tetrazines leads to bis(tetrazine)s structurally characterized by tetrazine cores arranged face-to-face. [N]₈ π-stacking interactions are essential to the conformation. Upon inverse electron demand Diels-Alder (iEDDA) cycloaddition, the bis(tetrazine)s produce a unique staple structure. The o-azidation of s-aryl tetrazines introduces a second proximal intermolecular clickable function that leads to double click chemistry opportunities. The stepwise introduction of fluorophores and then iEDDA cycloaddition, including bioconjugation to antibodies, was achieved on this class of tetrazines. This method extends to (thio)etherification, phosphination, trifluoromethylation and the introduction of various bioactive nitrogen-based heterocycles.

A Modular Phosphorylated Glycoluril-Derived Molecular Tweezer for Potent Binding of Aliphatic Diamines

A molecular tweezer based on a glycoluril-derived framework bearing four phosphate groups was synthesized and shown to be capable of binding organic amines in aqueous solution. This work reports the K_a values for 30 complexes of this molecular tweezer and amine guests, determined by means of ¹ H NMR titrations. Both the hydrophobic cavity and the phosphate groups contribute to the binding. Bulkier molecules and molecules bearing negatively charged groups like carboxylates in amino acids bind less tightly due to a steric clash and coulombic repulsion. The narrow cavity and the strong ionic interactions of the phosphate groups with ammonium guests favor binding of aliphatic diamines. These binding properties clearly distinguish this system from structurally related molecular clips and tweezers.

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.