Charge transfer between a metal-bound halide and a quinone through π-hole interactions leads to bulk conductivity

π-Hole interactions between a metal-bound halide and a quinoid ring are described in four novel isostructural co-crystals with the formula [Cu(terpy)ClX]·X'4Q (terpy = 2,2':6',2''-terpyridine; Q = quinone; X = Br, I; X' = Cl, Br). An unusually strong π-hole interaction between Cu-X and the quinoid ring is noted. Periodic DFT computations estimate the energy of the X⋯quinone interaction to be -20.79 kcal mol-1, indicating a very strong non-covalent interaction attributed to a higher degree of polarization along the bonding path. The black colour of the crystals originates from a cooperative intermolecular charge transfer between the [Cu(terpy)ClX] complex and the quinone π-system, with iodine playing a dominant role in this process by facilitating the π-hole interaction that enhances the charge transfer mechanism. All the compounds are considered to be weak semiconductors with the σDC magnitude ranging between 10-11 and 10-9 S cm-1. It is anticipated that by a smart choice of electron donors and electron acceptors, one can substantially enhance the effect and engineer more efficient conductive materials.

Halogen Bond Strength in Solids Quantified via Zeeman-Perturbed Nuclear Quadrupole Resonance Spectroscopy

Proton NMR is a ubiquitous and valuable probe of hydrogen bonds. Conversely, 127I NMR of strong halogen bond (XB) donors is hopeless due to quadrupolar coupling constants (CQ) on the order of GHz. We report here an innovative implementation of Zeeman-perturbed nuclear quadrupole resonance (Zp-NQR) spectroscopy, employing adjustable magnetic fields on the order of mT, which renders possible the acquisition and analysis of spectra of 127I and 79Br nuclei subject to quadrupolar couplings of up to 2.3 GHz in solid powders. This approach is demonstrated on three series of halogen-bonded cocrystals based on so-called "iconic" strong XB donors p-diiodotetrafluorobenzene, sym-trifluorotriiodobenzene, and p-dibromotetrafluorobenzene (27 compounds). Analysis of the spectra using a diagonalization of the Zeeman-quadrupolar Hamiltonian provides CQ values and quadrupolar asymmetry parameters, thereby overcoming various limitations encountered in pure NMR and pure NQR. Inspection of the data reveals strong correlations with geometrical and structural features of the halogen bond, including its length. Dispersion-corrected zeroth-order regular approximation relativistic DFT computations of the interaction energies of the XB donor are strongly correlated with experimental and computed values of CQ(127I) and CQ(79Br). It is concluded that the electric field gradient at the XB donor site is a useful metric for quantifying XB strength in solids. The XB interaction energies range from ∼5 to 10 kcal mol-1 for the systems studied herein. The Zp-NQR approach is amenable to widespread application to diverse problems in the chemical and materials sciences related to energy materials, crystal engineering, and many systems comprising strongly quadrupolar isotopes.

Applications of Antimony in Catalysis

Antimony is a fifth-period element in the nitrogen family, a silver-white metalloid with weak conductivity and thermal conductivity. It is stable at room temperature and does not react easily with oxygen and water in the air. Natural minerals are found in the form of sulfides. Current research and applications are mostly concentrated on material modification, utilizing the properties of antimony in traditional chemical industries, helping alloys improve their flame retardancy, stability, increasing semiconductor performance, etc. For example, to enhance the electronic conductivity, after coating or embedding antimony or its derivatives in thin layers in photonic nanomaterials, the performance of the original material in photoelectrochemical catalysis can be effectively increased, thereby expanding the efficiency of oxidation-reduction reactions accounting for the degradation of organic matter in wastewater. However, the catalytic reaction between the derivatives of antimony and organic compounds beside the material is less studied, and the mechanism of the studies in organic synthesis is relatively unclear. The reported organic synthesis related to antimony is mainly in the form of Lewis acid catalysts or dual-metal catalytic systems combined with other metals. This Review will focus on the application of antimony in photocatalysis, electrocatalysis, and other organic syntheses in the past 10 years.

Pnictogen Bond-Mediated Coassemblies for Noncovalent Molecular Glass

Pnictogen bond (PnB) occurring on the group-15 elements is recognized as σ- or π-hole-based interaction that has garnered attention in the fields of anion recognition and organocatalysis. Due to the polyvalent feature of pnictogens and high directionality, PnB possesses potential in the design of convergent coassembled materials with acceptors containing lone pair electrons or anions, which however is rarely explored so far. Herein, we unveil the role of antimony (Sb)-based PnB donors in producing self-assembled chiroptical materials with lone pair electron containing acceptors. Steric effect and electronic properties determined the exposure and strength of σ-holes that direct the complexation between components. PnB complexation leads to profound property and self-assembly behavior evolutions compared to the pristine assembly, including crystallinity, photophysical, morphological, and chiroptical properties. The PnB complexes exhibited an accelerated photoisomerization. Ascribed to the multiple σ-holes in Sb donors, amorphous structures were generated, enabling the formation of glassy materials.

Are spodium bonds directional?

Statistical analyses and computational studies have been performed on spodium bonds with four- and five-coordinated spodium complexes. Though statistical analyses without geometry correction suggest that the most populous geometries are far from linearity, geometry-corrected analyses suggest that these bonds are directional and linear. Computational studies suggest that the bond strength can be moderately strong.

Fluxional halogen bonds in linear complexes of tetrafluorodiiodobenzene with dinitrobenzene

The fluxional nature of halogen bonds (XBs) in small molecular clusters, supramolecules, and molecular crystals has received considerable attention in recent years. In this work, based on extensive density-functional theory calculations and detailed electrostatic potential (ESP), natural bonding orbital (NBO), non-covalent interactions-reduced density gradient (NCI-RDG), and quantum theory of atoms in molecules (QTAIM) analyses, we unveil the existence of fluxional halogen bonds (FXBs) in a series of linear (IC6F4I)m(OONC6H4NOO)n (m + n = 2-5) complexes of tetrafluorodiiodobenzene with dinitrobenzene which appear to be similar to the previously reported fluxional hydrogen bonds (FHBs) in small water clusters (H2O)n (n = 2-6). The obtained GS ⇌ TS ⇌ GS ' fluxional mechanisms involve one FXB in the systems which fluctuates reversibly between two linear CI···O XBs in the ground states (GS and GS') via a bifurcated CI  O2N van der Waals interaction in the transition state (TS). The cohesive energies (Ecoh) of these complexes with up to four XBs exhibit an almost perfect linear relationship with the numbers of XBs in the systems, with the average calculated halogen bond energy of Ecoh/XB = 3.48 kcal·mol-1 in the ground states which appears to be about 55% of the average calculated hydrogen bond energy (Ecoh/HB = 6.28 kcal·mol-1) in small water clusters.

An orbital-overlap complement to σ-hole electrostatic potentials

A σ-hole is an electron-deficient region of positive electrostatic potential (ESP) opposite from a half-filled p orbital involved in forming a covalent bond. The σ-hole concept helps rationalize directional noncovalent interactions, known as σ-hole bonds, between covalently bonded group V-VII atoms and electron-pair donors. The magnitude and orientation of σ-holes are correlated with the strength and geometry of halogen bonds. However, ESP computed for isolated σ-holes are not always predictive of interaction energies. For example, the σ-holes of isolated CHFBr2 and isolated CH2FI have identical ESP on the molecule surface, but halogen bonds to these molecules generally have different strengths. Here we show that the compact/diffuse nature of the orbitals involved plays an important role. Our orbital overlap distance quantifies the compact/diffuse nature of the "test orbital" that best overlaps with a systems orbitals at each point. The overlap distance captures the response properties of σ-holes: diffuse σ-holes with large overlap distance are typically "softer" and more polarizable. This aids visualization and interpretation. A linear fit to overlap distance and ESP is predictive of the halogen bond strengths of CH3X and CF3X (X = Cl, Br and I). We suggest that the overlap distance will be a useful partner to ESP for characterizing σ-holes.

Unconventional Radical and Radical-Hole Site-Based Interactions in Halogen-Bearing Dimers and Trimers: A Comparative Study

Radical (R•) and R•-hole site-based interactions are comparatively studied, for the first time, using ab initio methods. In this regard, R•-bearing molecules •XO3 (where X = Cl, Br, and I) were subjected to direct interaction with NH3 within dimeric and trimeric forms in the form of NH3···•XO3/•XO3···NH3 and NH3···•XO3···NH3 complexes, respectively. As confirmed by electrostatic potential analysis, the studied R•-bearing molecules •XO3 had the outstanding potentiality to interact as Lewis acid centers via two positive sites dubbed as R• and R•-hole sites. Such an observation proposed the potentiality of the considered •XO3 molecules to engage in unconventional R• and well-established R•-hole site-based interactions with Lewis bases. This was confirmed by negative interaction (E int) energies, ranging from -4.93 to -19.89 kcal/mol, with higher favorability for R• site-based interactions over the R•-hole site-based ones. MP2 energetic features furnished higher preferability for the R• site-based interactions than the R•-hole site-based ones in the case of chlorine- and bromine-bearing complexes, and the reverse was true for the iodine-bearing complexes. Moreover, elevated E int values were recorded for the NH3···•XO3···NH3 trimers over the NH3···•XO3 and •XO3···NH3 dimers, outlining the higher preference of the •XO3 molecules to engage in R• and R•-hole site-based interactions in the trimeric form over the dimeric one. These results might be considered a requisite linchpin for numerous forthcoming supramolecular chemistry and crystal engineering studies.

Halide Complexes of 5,6-Dicyano-2,1,3-Benzoselenadiazole with 1 : 4 Stoichiometry: Cooperativity between Chalcogen and Hydrogen Bonding

The [M4 -Hal]- (M=the title compound; Hal=Cl, Br, and I) complexes were isolated in the form of salts of [Et4 N]+ cation and characterized by XRD, NMR, UV-Vis, DFT, QTAIM, EDD, and EDA. Their stoichiometry is caused by a cooperative interplay of σ-hole-driven chalcogen (ChB) and hydrogen (HB) bondings. In the crystal, [M4 -Hal]- are connected by the π-hole-driven ChB; overall, each [Hal]- is six-coordinated. In the ChB, the electrostatic interaction dominates over orbital and dispersion interactions. In UV-Vis spectra of the M+[Hal]- solutions, ChB-typical and [Hal]- -dependent charge-transfer bands are present; they reflect orbital interactions and allow identification of the individual [Hal]- . However, the structural situation in the solutions is not entirely clear. Particularly, the UV-Vis spectra of the solutions are different from the solid-state spectra of the [Et4 N]+ [M4 -Hal]- ; very tentatively, species in the solutions are assigned [M-Hal]- . It is supposed that the formation of the [M4 -Hal]- proceeds during the crystallization of the [Et4 N]+ [M4 -Hal]- . Overall, M can be considered as a chromogenic receptor and prototype sensor of [Hal]- . The findings are also useful for crystal engineering and supramolecular chemistry.

Sigma-Hole and Lone-Pair-Hole Site-Based Interactions of Seesaw Tetravalent Chalcogen-Bearing Molecules with Lewis Bases

For the first time, sigma (σ)- and lone-pair (lp)-hole site-based interactions of SF4 and SeF4 molecules in seesaw geometry with NH3 and FH Lewis bases were herein comparatively investigated. The obtained findings from the electrostatic potential analysis outlined the emergence of sundry holes on the molecular entity of the SF4 and SeF4 molecules, dubbed the σ- and lp-holes. The energetic viewpoint announced splendid negative binding energy values for σ-hole site-based interactions succeeded by lp-hole analogues, which were found to be -9.21 and -0.50 kcal/mol, respectively, for SeF4···NH3 complex as a case study. Conspicuously, a proper concurrence between the strength of chalcogen σ-hole site-based interactions and the chalcogen's atomic size was obtained, whereas a reverse pattern was proclaimed for the lp-hole counterparts. Further, a higher preference for the YF4···NH3 complexes with elevated negative binding energy was promulgated over the YF4···FH ones, indicating the eminent role of Lewis basicity. The indications of the quantum theory of atoms in molecules generally asserted the closed-shell nature of all the considered interactions. The observation of symmetry-adapted perturbation theory revealed the substantial contributing role of the electrostatic forces beyond the occurrence of σ-hole site-based interactions. In comparison, the dispersion forces were specified to govern the lp-hole counterparts. Such emerging findings would be a gate for the fruitful forthcoming applications of chalcogen bonding interactions in crystal engineering and biological systems.

Intermolecular Covalent Interactions: Nature and Directionality

Quantum chemical methods were employed to analyze the nature and the origin of the directionality of pnictogen (PnB), chalcogen (ChB), and halogen bonds (XB) in archetypal F_m Z⋅⋅⋅F^- complexes (Z=Pn, Ch, X), using relativistic density functional theory (DFT) at ZORA-M06/QZ4P. Quantitative Kohn-Sham MO and energy decomposition analyses (EDA) show that all these intermolecular interactions have in common that covalence, that is, HOMO-LUMO interactions, provide a crucial contribution to the bond energy, besides electrostatic attraction. Strikingly, all these bonds are directional (i.e., F-Z⋅⋅⋅F^- is approximately linear) despite, and not because of, the electrostatic interactions which, in fact, favor bending. This constitutes a breakdown of the σ-hole model. It was shown how the σ-hole model fails by neglecting both, the essential physics behind the electrostatic interaction and that behind the directionality of electron-rich intermolecular interactions. Our findings are general and extend to the neutral, weaker ClI⋅⋅⋅NH₃ , HClTe⋅⋅⋅NH₃ , and H₂ ClSb⋅⋅⋅NH₃ complexes.

The nature of π-hole interactions between iodide anions and quinoid rings in the crystalline state

The investigated co-crystal of 3-chloro-N-methylpyridinium iodide with tetrabromoquinone (3-Cl-N-MePy·I·Br4Q) reveals a π-hole interaction between an iodide anion and a quinoid ring involving an n → π* charge transfer. The quinoid ring has a partial negative charge (estimated to be in the range 0.08-0.11e) and a partial radical character, which is related to the black colour of the crystals (crystals of neutral tetrabromoquinone are yellow). A detailed X-ray charge density study revealed two symmetry-independent bond critical points between the iodide anions and carbon atoms of the ring. Their maximum electron density of 0.065 e Å-3 was reproduced by quantum chemical modelling. The energy of the interaction is estimated to be -11.16 kcal mol-1, which is comparable to the strength of moderate hydrogen bonding (about -10 kcal mol-1); it is dominantly electrostatic in nature, with a considerable dispersion component.

Yet another perspective on hole interactions, part II: lp-hole vs. lp-hole interactions

Most of the experimental and theoretical work in hole interactions (HIs) is mainly focused on exploiting the nature and characteristics of σ and π-holes. In this perspective, we focus our attention on understanding the origin and properties of lone-pair holes. These holes are present on an atom opposite to its lone-pair region. Utilizing some new and old examples, such as X₃N/P⋯F^- (X = F/Cl/Br/I), F-Cl/Br/I⋯H₃P⋯NCH and H₃B-NBr₃ along with other molecular systems, we explored to what extent these lp-holes participate in lp-hole interactions, if they participate at all.

Shedding Light on the Vibrational Signatures in Halogen-Bonded Graphitic Carbon Nitride Building Blocks

The relative contributions of halogen and hydrogen bonding to the interaction between graphitic carbon nitride monomers and halogen bond (XB) donors containing C-X and C≡C bonds were evaluated using computational vibrational spectroscopy. Conventional probes into select vibrational stretching frequencies can often lead to disconnected results. To elucidate this behavior, local mode analyses were performed on the XB donors and complexes identified previously at the M06-2X/aVDZ-PP level of theory. Due to coupling between low and high energy C-X vibrations, the C≡C stretch is deemed a better candidate when analyzing XB complex properties or detecting XB formation. The local force constants support this conclusion, as the C≡C values correlate much better with the σ-hole magnitude than their C-X counterparts. The intermolecular local stretching force constants were also assessed, and it was found that attractive forces other than halogen bonding play a supporting role in complex formation.

Halogen bonding and chalcogen bonding mediated sensing

Sigma-hole interactions, in particular halogen bonding (XB) and chalcogen bonding (ChB), have become indispensable tools in supramolecular chemistry, with wide-ranging applications in crystal engineering, catalysis and materials chemistry as well as anion recognition, transport and sensing. The latter has very rapidly developed in recent years and is becoming a mature research area in its own right. This can be attributed to the numerous advantages sigma-hole interactions imbue in sensor design, in particular high degrees of selectivity, sensitivity and the capability for sensing in aqueous media. Herein, we provide the first detailed overview of all developments in the field of XB and ChB mediated sensing, in particular the detection of anions but also neutral (gaseous) Lewis bases. This includes a wide range of optical colorimetric and luminescent sensors as well as an array of electrochemical sensors, most notably redox-active host systems. In addition, we discuss a range of other sensor designs, including capacitive sensors and chemiresistors, and provide a detailed overview and outlook for future fundamental developments in the field. Importantly the sensing concepts and methodologies described herein for the XB and ChB mediated sensing of anions, are generically applicable for the development of supramolecular receptors and sensors in general, including those for cations and neutral molecules employing a wide array of non-covalent interactions. As such we believe this review to be a useful guide to both the supramolecular and general chemistry community with interests in the fields of host-guest recognition and small molecule sensing. Moreover, we also highlight the need for a broader integration of supramolecular chemistry, analytical chemistry, synthetic chemistry and materials science in the development of the next generation of potent sensors.

Revision of the Crystal Structure of the Orthorhombic Polymorph of Oxyma: On the Importance of π–Hole Interactions and Their Interplay with H–Bonds

In this work the crystal structure of the previously described orthorhombic polymorph of the coupling reagent Oxyma has been revised, corrected now as centrosymmetric and analyzed by means of DFT calculations. In the solid state the structure forms a network of H-bonds and self–assembled dimers that are held together by the formation of N···C π–hole interactions involving the C-atom of the imino group. The H-bonding and π–hole interactions observed in the solid state were rationalized using molecular electrostatic potential (MEP) surfaces, focusing on the H-bond donor-acceptor groups and the π-hole observed above and below the molecular plane. The interactions and their interplay have been characterized by using two methodologies based on the topology of the electron density, which are the quantum theory of “atom-in-molecules” (QTAIM) and the noncovalent interaction plot (NCIplot).

Origin of iodine preferential attack at sulfur in phosphorothioate and subsequent P-O or P-S bond dissociation

Iodine-induced cleavage at phosphorothioate DNA (PT-DNA) is characterized by extremely high sensitivity (∼1 phosphorothioate link per 106 nucleotides), which has been used for detecting and sequencing PT-DNA in bacteria. Despite its foreseeable potential for wide applications, the cleavage mechanism at the PT-modified site has not been well established, and it remains unknown as to whether or not cleavage of the bridging P-O occurs at every PT-modified site. In this work, we conducted accurate ωB97X-D calculations and high-performance liquid chromatography-mass spectrometry to investigate the process of PT-DNA cleavage at the atomic and molecular levels. We have found that iodine chemoselectively binds to the sulfur atom of the phosphorothioate link via a strong halogen-chalcogen interaction (a type of halogen bond, with binding affinity as high as 14.9 kcal/mol) and thus triggers P-O bond cleavage via phosphotriester-like hydrolysis. Additionally, aside from cleavage of the bridging P-O bond, the downstream hydrolyses lead to unwanted P-S/P-O conversions and a loss of the phosphorothioate handle. The mechanism we outline helps to explain specific selectivity at the PT-modified site but also predicts the dynamic stoichiometry of P-S and P-O bond breaking. For instance, Tris is involved in the cascade derivation of S-iodo-phosphorothioate to S-amino-phosphorothioate, suppressing the S-iodo-phosphorothioate hydrolysis to a phosphate diester. However, hydrolysis of one-third of the Tris-O-grafting phosphotriester results in unwanted P-S/P-O conversions. Our study suggests that bacterial DNA phosphorothioation may more frequently occur than previous bioinformatic estimations have predicted from iodine-induced deep sequencing data.

σ-Hole Bonds and the VSEPR Model—From the Tetrahedral Structure to the Trigonal Bipyramid

Complexes linked by various interactions are analysed in this study. They are characterized by the tetrahedral configuration of the Lewis acid centre. Interactions, being a subject of this study, are classified as σ-hole bonds, such as the halogen, chalcogen, pnicogen, and tetrel bonds. In the case of strong interactions, the tetrahedral configuration of the Lewis acid centre changes into the trigonal bipyramid configuration. This change is in line with the Valence-Shell Electron-Pair Repulsion model, VSEPR, and this is supported here by the results of high-level ab initio calculations. The theoretical results concerning the geometries are supported mainly by the Natural Bond Orbital, NBO, method.

Reassessing the Role of σ Holes in Noncovalent Interactions: It is Pauli Repulsion that Counts

A number of prototypical weak electron donor-electron acceptor complexes are investigated by the Symmetry Adapted Perturbation Theory, some of which belong to novel classes of weak bonds such as halogen and chalcogen bonds. Also included are complexes involving strong Lewis acids such as BeO and AuF. The common view in the literature is to associate these novel bonds with a variety of "holes", σ, π, δ, or positive areas in their electrostatic potential maps. The presumption is that these positive areas of the electrostatic potential are indicative of the electrostatic nature of these noncovalent bonds. The electrostatic view extends to the explanations of the directionality of approaches between the subsystems forming these bonds. This work demonstrates that one common feature of these electrostatic potential "holes" is the local depletion of electron density of which the best detector is the first-order Pauli repulsion. The minimization of this repulsion determines the bond directionality and its relative angular rigidity. In relatively strong complexes of BeO with rare gases, where BeO shows a clear cavity in electron density-an ultimate "σ hole"-the electrostatic effect does not control the bending potential-the exchange repulsion does. In halogen bonds, the halogen atom is nonspherical, displaying an axial "σ hole" in its electrostatic potential. However, in no examined case, from rare gas acting as an electron donor to a polar donor to an anionic donor, is the electrostatic energy responsible for the directionality of the halogen bond. In fact, it is not even maximized in the direction of the σ hole in N2-ClF and NH3-ClF. Yet, in all the cases, the exchange repulsion is minimized in the direction of the σ hole. The minimized exchange repulsion associated with the subtle and less subtle depletions of the electron density occur on the nodal planes or on the intersections thereof in the highest occupied molecular orbitals of Lewis acids, provided that the systems are closed-shell. The role of nodal planes in covalent and coordinate covalent bonds is well recognized. This work points to their similarly equal importance in certain types of donor-acceptor noncovalent interactions.

From weak to strong interactions: structural and electron topology analysis of the continuum from the supramolecular chalcogen bonding to covalent bonds

The relationship between covalent and supramolecular bonding, and the criteria of the assignments of different interactions were explored via the review of selenium and tellurium containing structures in the Cambridge Structural Database and their computational analysis using Quantum Theory of Atoms in Molecules (QTAIM). This combined study revealed continuums of the interatomic Se⋯Br and Te⋯I distances, d_Ch⋯X, in the series of associations from the sums of the van der Waals radii of these atoms (r_Ch + r_X) to their covalent bond lengths. The electron densities, ρ(r), at Bond Critical Points (BCPs) along the chalcogen bond paths increased gradually from about 0.01 a.u. common for the non-covalent interactions to about 0.1 a.u. typical for the covalent bonds. The log ρ(r) values fell on the same linear trend line when plotted against normalized interatomic distances, R_XY = d_Ch⋯X/(r_Ch + r_X). The transition from the positive to negative values of the energy densities, H(r), at the BCPs (related to a changeover of essentially non-covalent into partially covalent interactions) were observed at R_XY ≈ 0.80. Synchronous changes of bonding characteristics with R_XY (similar to that found earlier in the halogen-bonded systems) designated normalized interatomic separation as a critical factor determining the nature of these bondings. The uninterrupted continuums of Te⋯I and Se⋯Br bond lengths and BCPs' characteristics signified an intrinsic link between limiting types of bonding involving chalcogen atoms and between covalent and supramolecular bonding in general.

σ-Hole and LP-Hole Interactions of Pnicogen···Pnicogen Homodimers under the External Electric Field Effect: A Quantum Mechanical Study

σ-Hole and lone-pair (lp)-hole interactions within σ-hole···σ-hole, σ-hole···lp-hole, and lp-hole···lp-hole configurations were comparatively investigated on the pnicogen···pnicogen homodimers (PCl3)2, for the first time, under field-free conditions and the influence of the external electric field (EEF). The electrostatic potential calculations emphasized the impressive versatility of the examined PCl3 monomers to participate in σ-hole and lp-hole pnicogen interactions. Crucially, the sizes of σ-hole and lp-hole were enlarged under the influence of the positively directed EEF and decreased in the case of reverse direction. Interestingly, the energetic quantities unveiled more favorability of the σ-hole···lp-hole configuration of the pnicogen···pnicogen homodimers, with significant negative interaction energies, than σ-hole···σ-hole and lp-hole···lp-hole configurations. Quantum theory of atoms in molecules and noncovalent interaction index analyses were adopted to elucidate the nature and origin of the considered interactions, ensuring their closed shell nature and the occurrence of attractive forces within the studied homodimers. Symmetry-adapted perturbation theory-based energy decomposition analysis alluded to the dispersion force as the main physical component beyond the occurrence of the examined interactions. The obtained findings would be considered as a fundamental underpinning for forthcoming studies pertinent to chemistry, materials science, and crystal engineering.

π-Hole Tetrel Bonds—Lewis Acid Properties of Metallylenes

The MP2/aug-cc-pVTZ calculations were performed on the dihalometallylenes to indicate their Lewis acid and Lewis base sites. The results of the Cambridge Structural Database search show corresponding and related crystal structures where the tetrel center often possesses the configuration of a trigonal bipyramid or octahedron. The calculations were also carried out on dimers of dichlorogermylene and dibromogermylene and on complexes of these germylenes with one and two 1,4-dioxide molecules. The Ge⋯Cl, Ge⋯Br, and Ge⋯O interactions are analyzed. The Ge⋯O interactions in the above mentioned germylene complexes may be classified as the π-hole tetrel bonds. The MP2 calculations are supported by the results of the Quantum Theory of Atoms in Molecules (QTAIM) and the Natural Bond Orbital (NBO) approaches.

Anatomy of π-hole bonds: Linear systems

The list of σ-hole bonds is long and growing, encompassing both H-bonds and its closely related halogen, chalcogen, etc., sisters. These bonds rely on the asymmetric distribution of electron density, whose depletion along the extension of a covalent bond leaves a positive region of electrostatic potential from which these bonds derive their name. However, the density distributions of other molecules contain analogous positive regions that lie out of the molecular plane known as π-holes, which are likewise capable of engaging in noncovalent bonds. Quantum calculations are applied to study such π-hole bonds that involve linear molecules, whose positive region is a circular belt surrounding the molecule, rather than the more restricted area of a σ-hole. These bonds are examined in terms of their most fundamental elements arising from the spatial dispositions of their relevant molecular orbitals and the π-holes in both the total electron density and the electrostatic potential to which they lead. Systems examined comprise tetrel, chalcogen, aerogen, and triel bonds, as well as those involving group II elements, with atoms drawn from various rows of the Periodic Table. The π-hole bonds established by linear molecules tend to be weaker than those of comparable planar systems.

Harnessing Bismuth Coordination Chemistry to Achieve Bright, Long-Lived Organic Phosphorescence

A new bismuth(III)-organic compound, Hphen[Bi₂(HPDC)₂(PDC)₂(NO₃)]·4H₂O (Bi-1; PDC = 2,6-pyridinedicarboxylate and phen = 1,10-phenanthroline), was synthesized, and the structure was determined by single-crystal X-ray diffraction. The compound was found to display bright-blue-green phosphorescence in the solid state under UV irradiation, with a luminescent lifetime of 1.776 ms at room temperature. The room temperature and low-temperature (77 K) emission spectra exhibited the vibronic structure characteristic of Hphen phosphorescence. Time-dependent density functional theory studies showed that the excitation pathway arises from an energy transfer from the dimeric structural unit to Hphen, with participation from a nine-coordinate Bi center. The triplet state of Hphen is believed to be stabilized via supramolecular interactions, which, when coupled with the heavy-atom effect induced by Bi, leads to the observed long-lived luminescence. The compound displayed a solid-state quantum yield of over 27%. To the best of our knowledge, this is the first such compound to exhibit phenanthrolinium phosphorescence with such long-lived, room temperature lifetimes in the solid state. To further elucidate the energy-transfer mechanism, Ln³⁺ (Ln = Eu, Tb, Sm) ions were successfully doped into the parent compound, and the resulting materials exhibited dual emission from Hphen and Ln, promoting tunability of the emission color.

Are Heavy Pnictogen-π Interactions Really "π Interactions"?

The noncovalent interactions of heavy pnictogens with π-arenes play a fundamental role in fields like crystal engineering or catalysis. The strength of such bonds is based on an interplay between dispersion and donor/acceptor interactions, and is generally attributed to the presence of π-arenes. Computational studies of the interaction between the heavy pnictogens As, Sb and Bi and cyclohexane, in comparison with previous studies on the interaction between heavy pnictogens and benzene, show that this concept probably has to be revised. A thorough analysis of all the different energetic components that play a role in these systems, carried out with state-of-the-art computational methods, sheds light on how they influence one another and the effect that their interplay has on the overall system. Furthermore, the analysis of such interactions leads us to the unexpected finding that the presence of the pnictogen compounds strongly affects the conformational equilibrium of cyclohexane, reversing the relative stability of the chair and boat-twist conformers, and thus suggesting a possible application of tuneable dispersion energy donors to stabilise the desired conformation.

Yet another perspective on hole interactions

Hole interactions are known by different names depending on the key atom of the bond (halogen bond, chalcogen bond, hydrogen bond, etc.), and the geometry of the interaction (σ if in line, π if perpendicular to the Lewis acid plane). However, its origin starts with the creation of a Lewis acid by an underlying covalent bond, which forms an electrostatic depletion and a virtual antibonding orbital, which can create non-covalent interactions with Lewis bases. In this (maybe subjective) perspective, we will claim that hole interactions must be defined via the molecular orbital origin of the molecule. Under this premise we can better explore the richness of such bonding patterns. For that, we will study old, recent and new systems, trying to pinpoint some misinterpretations that are often associated with them. We will use as exemplars the triel bonds, a couple of metal complexes, a discussion on convergent σ-holes, and many cases of anti-electrostatic hole interactions.

Natural Syringyl Mediators Accelerate Laccase-Catalyzed β-O-4 Cleavage and Cα-Oxidation of a Guaiacyl Model Substrate via an Aggregation Mechanism

Laccase-mediator systems (LMSs) have been intensively investigated in lignin degradation. Although only natural metabolites are available for fungal lignin degradation, mediator molecules from metabolites have received substantially less attention than artificial organic-synthetic compounds. It remains unclear which metabolites can accelerate laccase-catalyzed reactions and how those natural mediators influence lignin degradation. In this work, we evaluated Trametes versicolor laccase-catalyzed reaction kinetics on a lignin guaiacyl subunit model (guaiacylglycerol-β-guaiacyl ether, G-β-GE) in the presence of a group of lignin syringyl subunit molecules: syringaldehyde, acetosyringone, and methyl syringate. We then compare their performance to a well-known synthetic mediator ABTS, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid). Time-resolved UPLC-TOF-MS revealed that the syringyl mediators were more effective in accelerating the β-O-4 cleavage and Cα-oxidation of G-β-GE than ABTS under laccase-catalysis, despite the syringyl compounds possessing slower individual oxidation rates. In addition, the product profile of polymerization was also promoted dramatically, compared to that of the ABTS/laccase system. The LMS kinetic modeling suggested that mediator-substrate aggregation played a critical role in the laccase-mediator system; in which, the lignin syringyl and guaiacyl subunits likely form a π-π stacking van der Waals complex that can be oxidized faster than the syringyl or guaiacyl monomers by themselves. This syringyl-guaiacyl aggregation hypothesis postulates that the weak interactions in lignin biopolymers are able to accelerate the laccase-catalyzed biodegradation.

The Effects of Electronegativity of X and Hybridization of C on the X-C⋅⋅⋅O Interactions: A Statistical Analysis on Tetrel Bonding

Cone and distance-cone corrected statistical analyses have been performed on X-C⋅⋅⋅O (X=H, B, C, N, O and F; the C atom is sp² and sp³ hybridized) tetrel bonds. The sp³ -C and sp² -C prefer to form the interactions through σ-hole (∠XCO≈180°) and π-hole (∠XCO≈90°), respectively. With the increase in electronegativity of X, the preference for the particular angles of the respective geometries increases and the C⋅⋅⋅O distance becomes shorter. The angular preference is found to be more prominent in the cases of π-hole interactions than that in the σ-hole interactions. A similar distance-cone corrected statistical analysis on O=C⋅⋅⋅O interaction also suggests that the preferred ∠OCO angle is ∼90° and the preferred C⋅⋅⋅O distance is around the sum of van der Waals radii (3.22 Å) of the C and O atoms. However, a cone-corrected statistical analysis on X-Si⋅⋅⋅O interactions suggests that the preference for linearity in this case is much higher than that for the X-C⋅⋅⋅O σ-hole interactions.

Acid-Base and Anion Binding Properties of Tetrafluorinated 1,3-Benzodiazole, 1,2,3-Benzotriazole and 2,1,3-Benzoselenadiazole

The influence of fluorination on the acid-base properties and the capacity of structurally related 6-5 bicyclic compounds - 1,3-benzodiazole 1, 1,2,3-benzotriazole 2 and 2,1,3-benzoselenadiazole 3 to σ-hole interactions, i. e. hydrogen (1 and 2) and chalcogen (3) bondings, is studied experimentally and computationally. The tetrafluorination increases the Brønsted acidity of the diazole and triazole scaffolds and the Lewis acidity of selenadiazole scaffold decreases the basicity. Increased Brønsted acidity facilitates anion binding via the formation of hydrogen bonds; particularly, tetrafluorinated derivative of 1 (compound 4) binds Cl^- . Increased Lewis acidity of tetrafluorinated derivative of 3 (compound 10), however, is not enough for binding with Cl^- and F^- via chalcogen bonds in contrast to previously studied Te analog of 10. It is suggested that the maximum positive values of molecular electrostatic potential at the σ-holes, V_S,max , can be a reasonable metric for design and synthesis of new anion receptors with selenadiazole-diazole/triazole hybrids as a special target. Related chlorinated compounds are also discussed.

Classification of So-Called Non-Covalent Interactions Based on VSEPR Model

The variety of interactions have been analyzed in numerous studies. They are often compared with the hydrogen bond that is crucial in numerous chemical and biological processes. One can mention such interactions as the halogen bond, pnicogen bond, and others that may be classified as σ-hole bonds. However, not only σ-holes may act as Lewis acid centers. Numerous species are characterized by the occurrence of π-holes, which also may play a role of the electron acceptor. The situation is complicated since numerous interactions, such as the pnicogen bond or the chalcogen bond, for example, may be classified as a σ-hole bond or π-hole bond; it ultimately depends on the configuration at the Lewis acid centre. The disadvantage of classifications of interactions is also connected with their names, derived from the names of groups such as halogen and tetrel bonds or from single elements such as hydrogen and carbon bonds. The chaos is aggravated by the properties of elements. For example, a hydrogen atom can act as the Lewis acid or as the Lewis base site if it is positively or negatively charged, respectively. Hence names of the corresponding interactions occur in literature, namely hydrogen bonds and hydride bonds. There are other numerous disadvantages connected with classifications and names of interactions; these are discussed in this study. Several studies show that the majority of interactions are ruled by the same mechanisms related to the electron charge shifts, and that the occurrence of numerous interactions leads to specific changes in geometries of interacting species. These changes follow the rules of the valence-shell electron-pair repulsion model (VSEPR). That is why the simple classification of interactions based on VSEPR is proposed here. This classification is still open since numerous processes and interactions not discussed in this study may be included within it.

Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

Accompanying the rapidly growing list of σ-hole bonds has come the acknowledgment of parallel sorts of noncovalent bonds which owe their stability in large part to a deficiency of electron density in the area above the molecular plane, known as a π-hole. The origins of these π-holes are probed for a wide series of molecules, comprising halogen, chalcogen, pnicogen, tetrel, aerogen, and spodium bonds. Much like in the case of their σ-hole counterparts, formation of the internal covalent π-bond in the Lewis acid molecule pulls density toward the bond midpoint and away from its extremities. This depletion of density above the central atom is amplified by an electron-withdrawing substituent. At the same time, the amplitude of the π*-orbital is enhanced in the region of the density-depleted π-hole, facilitating a better overlap with the nucleophile's lone pair orbital and a stabilizing n → π* charge transfer. The presence of lone pairs on the central atom acts to attenuate the π-hole and shift its position somewhat, resulting in an overall weakening of the π-hole bond. There is a tendency for π-hole bonds to include a higher fraction of induction energy than σ-bonds with proportionately smaller electrostatic and dispersion components, but this distinction is less a product of the σ- or π-character and more a function of the overall bond strength.

Asymmetric Pnictogen-Bonding Catalysis: Transfer Hydrogenation by a Chiral Antimony(V) Cation/Anion Pair

Pnictogen-bonding catalysis based on σ-hole interactions has recently attracted the attention of synthetic chemists. As a proof-of-concept for asymmetric pnictogen-bonding catalysis, we report herein an enantioselective transfer hydrogenation of benzoxazines catalyzed by a novel chiral antimony cation/anion pair. The chiral pnictogen catalyst library could be rapidly accessed from triarylstibine with readily available mandelic acid analogues, and the catalyst displays remarkable efficiency and enantiocontrol potency even at 0.05 mol % loading. Moreover, the properties of the catalyst and the mechanistic insights have been investigated by nonlinear effect studies, ¹H NMR, LC-MS, and control experiments.

Noncovalent Bonds through Sigma and Pi-Hole Located on the Same Molecule. Guiding Principles and Comparisons

Over the last years, scientific interest in noncovalent interactions based on the presence of electron-depleted regions called σ-holes or π-holes has markedly accelerated. Their high directionality and strength, comparable to hydrogen bonds, has been documented in many fields of modern chemistry. The current review gathers and digests recent results concerning these bonds, with a focus on those systems where both σ and π-holes are present on the same molecule. The underlying principles guiding the bonding in both sorts of interactions are discussed, and the trends that emerge from recent work offer a guide as to how one might design systems that allow multiple noncovalent bonds to occur simultaneously, or that prefer one bond type over another.

Comparison of ±σ-hole and ±R˙-hole interactions formed by tetrel-containing complexes: a computational study

For the first time, unconventional ±R˙-hole interactions were unveiled in tetrel-containing complexes. The nature and characteristics of ±R˙-hole interactions were explored relative to their ±σ-hole counterparts for ˙TF3⋯ and W-T-F3⋯B/R˙/A complexes (where T = C, Si, and Ge, W = H and F, B = Lewis bases, R˙ = free radicals, and A = Lewis acids). In an effort to thoroughly investigate such interactions, a plethora of quantum mechanical calculations, including molecular electrostatic potential (MEP), maximum positive electrostatic potential (V s,max), point-of-charge (PoC), interaction energy, symmetry adapted perturbation theory (SAPT), and reduced density gradient-noncovalent interaction (RDG-NCI) calculations, were applied. The most notable findings to emerge from this study are that (i) from the electrostatic perspective, the molecular stabilization energies of ˙TF3 and W-T-F3 monomers became more negative as the Lewis basicity increased, (ii) the most stable complexes were observed for the ones containing Lewis bases, forming -σ-hole and -R˙-hole interactions, and the interaction energies systematically increased in the order H-T-F3⋯B < ˙TF3⋯B < F-T-F3⋯B, (iii) contrariwise, the +σ-hole and +R˙-hole interactions with Lewis acids are more energetically favorable in the order F-T-F3⋯A < ˙TF3⋯A < H-T-F3⋯A, and (iv) generally, the dispersion force plays a key role in stabilizing the tetrel-containing complexes, jointly with the electrostatic and induction forces for the interactions with Lewis bases and acids, respectively. Concretely, the findings presented in this paper add to our understanding of the characteristics and nature of such intriguing interactions.

Novel co-crystals with π-hole interactions between iodide anions and quinoid rings involving charge transfer

Six novel co-crystals of tetrabromoquinone with iodide salts of organic cations displaying short contacts between iodide anions and the quinoid rings have been structurally characterised.

The HOX⋯SO2 (X=F, Cl, Br, I) Binary Complexes: Implications for Atmospheric Chemistry

Sulfur dioxide and hypohalous acids (HOX, X=F, Cl, Br, I) are ubiquitous molecules in the atmosphere that are central to important processes like seasonal ozone depletion, acid rain, and cloud nucleation. We present the first theoretical examination of the HOX⋯SO₂ binary complexes and the associated trends due to halogen substitution. Reliable geometries were optimized at the CCSD(T)/aug-cc-pV(T+d)Z level of theory for HOF and HOCl complexes. The HOBr and HOI complexes were optimized at the CCSD(T)/aug-cc-pV(D+d)Z level of theory with the exception of the Br and I atoms which were modeled with an aug-cc-pwCVDZ-PP pseudopotential. 27 HOX⋯SO₂ complexes were characterized and the focal point method was employed to produce CCSDT(Q)/CBS interaction energies. Natural Bond Orbital analysis and Symmetry Adapted Perturbation Theory were used to classify the nature of each principle interaction. The interaction energies of all HOX⋯SO₂ complexes in this study ranged from 1.35 to 3.81 kcal mol^-1 . The single-interaction hydrogen bonded complexes spanned a range of 2.62 to 3.07 kcal mol^-1 , while the single-interaction halogen bonded complexes were far more sensitive to halogen substitution ranging from 1.35 to 3.06 kcal mol^-1 , indicating that the two types of interactions are extremely competitive for heavier halogens. Our results provide insight into the interactions between HOX and SO₂ which may guide further research of related systems.

Intermolecular Interactions Involving Heavy Alkenes H2Si=TH2 (T = C, Si, Ge, Sn, Pb) with H2O and HCl: Tetrel Bond and Hydrogen Bond

The intermolecular interactions between the heavy alkenes H₂Si=TH₂ (T = C, Si, Ge, Sn, Pb) and H₂O or HCl have been explored at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level. The various hydrogen bond (HB) and tetrel bond (TB) complexes can be located on the basis of molecular electrostatic potential maps of the isolated monomers. The competition between TB and HB interactions has been investigated through the relaxed potential energy surface scan. The results indicate that the HB complexes become more and more unstable relative to the TB complexes with the increase of the T atomic number, and cannot even retain as a minimum in some cases, for H₂Si=TH2···H₂O systems. In contrast, the HB complexes are generally more stable than TB complexes, and the TB complexes exhibit rather weak binding strength, for H₂Si=TH₂···HCl systems. The majority of the TB complexes formed between H₂Si=TH₂ and H₂O possesses very strong binding strength with covalent characteristics. The noncovalent TB complexes can be divided into two types on the basis of the orbital interactions: π-hole complexes, with binding angles ranging from 91 to 111°, and hybrid σ/π-hole complexes, with binding angles ranging from 130 to 165°. The interplay between different molecular interactions has been explored, and an interesting result is that the covalent TB interaction is significantly abated and becomes noncovalent because of the competitive effect.

Can modified DNA base pairs with chalcogen bonding expand the genetic alphabet? A combined quantum chemical and molecular dynamics simulation study

A comprehensive (DFT and MD) computational study is presented with the goal to design and analyze model chalcogen-bonded modified nucleobase pairs that replace one (i.e., A_XY:T, G:C_XY, G_XY:C) or two (G_XY:C_X'Y', X/X' = S, Se and Y/Y' = F, Cl, Br) Watson-Crick (WC) hydrogen bonds of the canonical A:T or G:C pair with chalcogen bond(s). DFT calculations on 18 base pair combinations that replace one WC hydrogen bond with a chalcogen bond reveal that the bases favorably interact in the gas phase (binding strengths up to -140 kJ mol^-1) and water (up to -85 kJ mol^-1). Although the remaining hydrogen bond(s) exhibits similar characteristics to those in the canonical base pairs, the structural features of the (Y-XO) chalcogen bond(s) change significantly with the identity of X and Y. The 36 doubly-substituted (G_XY:C_X'Y') base pairs have structural deviations from canonical G:C similar to those of the singly-substituted modifications (G:C_XY or G_XY:C). Furthermore, despite the replacement of two strong hydrogen bonds with chalcogen bonds, some G_XY:C_X'Y' pairs possess comparable binding energies (up to -132 kJ mol^-1 in the gas phase and up to -92 kJ mol^-1 in water) to the most stable G:C_XY or G_XY:C pairs, as well as canonical G:C. More importantly, G:C-modified pairs containing X = Se (high polarizability) and Y = F (high electronegativity) are the most stable, with comparable or slightly larger (by up to 13 kJ mol^-1) binding energies than G:C. Further characterization of the chalcogen bonding in all modified base pairs (AIM, NBO and NCI analyses) reveals that the differences in the binding energies of modified base pairs are mainly dictated by the differences in the strengths of their chalcogen bonds. Finally, MD simulations on DNA oligonucleotides containing the most stable chalcogen-bonded base pair from each of the four classifications (A_XY:T, G:C_XY, G_XY:C and G_XY:C_X'Y') reveal that the singly-modified G:C pairs best retain the local helical structure and pairing stability to a greater extent than the modified A:T pair. Overall, our study identifies two (G:C_SeF and G_SeF:C) promising pairs that retain chalcogen bonding in DNA and should be synthesized and further explored in terms of their potential to expand the genetic alphabet.

Physical organic studies and dynamic covalent chemistry of picolyl heterocyclic amino aminals

A dynamic covalent system of the picolyl heterocyclic amino aminals has been studied. The aminals are characterized as a metastable species and easily switch to other forms via external stimuli. The solvent, temperature, acid-base and substituent effects have been examined to evaluate the dynamic covalent system. The results reveal that a more polar solvent, a lower temperature, basic conditions and an electron-withdrawing moiety contribute to the stabilities of aminals. The existence of the n → π* interaction between acetonitrile and the C[double bond, length as m-dash]N moiety makes the N-pyrimidyl imine (4c and 4d) yield higher in CD3CN. In a similar fashion, all aminals tend to convert to the corresponding hemiaminal ethers in a methanol environment. According to these findings, we successfully synthesized the following species: (a) N-2-picolylpyrimidin-2-amine 6c obtained by reduction using acetonitrile as the specific solvent; (b) a picolyl aromatic amino aminal 3e prepared from 2-pyridinecarboxaldehyde and the electron withdrawing 2-methoxy-5-nitroaniline.

Resonance-assisted/impaired anion-π interaction: towards the design of novel anion receptors

Substituents alter the electron density distribution in benzene in various ways, depending on their electron withdrawing and donating capabilities, as summarized by the empirical Hammett equation. The change of the π electron density distribution subsequently impacts the interaction of substituted benzenes or other cyclic conjugated rings with anions. Currently the design and synthesis of conjugated cyclic receptors capable of binding anions is an active field due to their applications in the sensing and removal of environmental contaminants and molecular recognition. By using the block-localized wavefunction (BLW) method, which is a variant of ab initio valence bond (VB) theory and can derive the reference resonance-free state self-consistently, we quantified the resonance-assisted (RA) or resonance-impaired (RI) phenomena in anion–π interactions from both structural and energetic perspectives. The frozen interaction, in which the electrostatic attraction is involved, has been shown to be the governing factor for the RA or RI interactions with anions. Energy analyses based on the empirical point charge (EPC) model indicated that the anion–π interactions can be simplified as the attraction between a negative point charge (anion) and a group of local dipoles, affected by the enriched or diminished π-cloud due to the resonance between the substituents and the conjugated ring. Hence, two strategies for the design of novel anion receptors can be envisioned. One is the enhancement of the magnitudes and/or numbers of local dipoles (polarized σ bonds), and the other is the reduction of π electron density in conjugated rings. For cases with the RI characteristics, “curved” aromatic molecules are preferred to be anion receptors. Indeed, extremely strong binding was found in complexes formed with fluorinated corannulene (F-CDD) and fluorinated [5]cycloparaphenylene (F-[5]CPP). Inspired by the RA phenomenon, complexes of p-, o- and m-benzoquinones with halides were revisited.

Substituents alter the electron density distribution in benzene in various ways, depending on their electron withdrawing and donating capabilities, as summarized by the empirical Hammett equation.

σ-Hole and Lone-Pair Hole Interactions in Chalcogen-Containing Complexes: A Comparative Study

The potentiality of sp3-hybridized chalcogen-containing molecules to participate in lone-pair (lp) hole interactions was reported for the first time. lp hole interactions were characterized and compared to σ-hole ones for OF2 and SF2 molecules as a case study. Various quantum mechanical calculations, including molecular electrostatic potential (MEP), maximum positive electrostatic potential (V s,max), point of charge (PoC), symmetry-adapted perturbation theory (SAPT), quantum theory of atoms in molecule (QTAIM), and reduced density gradient-noncovalent interaction (RDG-NCI) calculations, were carried out. The more significant findings to emerge from this study are the following: (i) the V s,max calculation was proved to be an unreliable method to determine the precise σ-hole and lp hole locations. (ii) The maximum positive electrostatic potential of the σ hole and lp hole was found to be at the F-Chal···PoC angle (θ) of 180° and at the centroid of XYlp plane, respectively. (iii) Lewis basicity has a significant effect on the strength of σ-hole and lp hole interactions. (iv) The studied molecules more favorably interact with Lewis bases via the σ hole compared to the lp hole, and (v) stabilization of the σ-hole and lp hole interactions stems from the electrostatic and dispersion forces, respectively.

π covalency in the halogen bond

Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the σ-symmetric component of halogen bonding. The possibility of π-covalency in a halogen bond is not contemplated in any known models. Here we present evidence of π-covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways.

Halogen Bonding Interactions of Polychlorinated Biphenyls and the Potential for Thyroid Disruption

Polychlorinated biphenyl (PCB) flame retardants are persistent pollutants and inhibit neurodevelopment, particularly in the early stages of life. Halogen bonding (XB) to the iodothyronine deiodinases (Dio) that modulate thyroid hormones (THs) is a potential mechanism for endocrine disruption. Cl⋅⋅⋅Se XB interactions of PCBs with SeMe- , a small model of the Dio active site selenocysteine, are compared with previous results on polybrominated diphenylethers (PBDEs) and THs using density functional theory. PCBs generally display weaker XB interactions compared to PBDEs and THs, consistent with the dependence of XB strength on the size of the halogen (I>Br>Cl). PCBs also do not meet a proposed energy threshold for substrates to undergo dehalogenation, suggesting they may behave as competitive inhibitors of Dio in addition to other mechanisms of endocrine disruption. XB interactions in PCBs are position-dependent, with ortho interactions slightly more favorable than meta and para interactions, suggesting that PCBs may have a greater effect on certain classes of Dio. Flexibility of PCBs around the biphenyl C-C bond is limited by ortho substitutions relative to the biphenyl linkage, which may contribute to the ability to inhibit Dio and other TH-related proteins.

Visible Light-Induced Homolytic Cleavage of Perfluoroalkyl Iodides Mediated by Phosphines

In an effort to explain the experimentally observed variation of the photocatalytic activity of t Bu 3 P, n Bu 3 P and (MeO) 3 P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine- and phosphite-IC 4 F 9 adducts by means of relativistic density functional theory and multireference configuration interaction methods. Based on the results of these computational and complementary experimental studies, we offer an explanation for the broad tailing of the absorption of t Bu 3 P-IC 4 F 9 and (MeO) 3 P-IC 4 F 9 into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in n Bu 3 P-IC 4 F 9 , in particular with regard to the P-I-C bending angle, reduce the Franck-Condon factors and thus the absorption probability compared to t Bu 3 P-IC 4 F 9 . Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO) 3 P may be the result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed mechanism for the phosphine-catalyzed homolytic C-I cleavage of perfluorobutane iodide involves S₁ ← S₀ absorption of the adduct followed by intersystem crossing to the photochemically active T 1 state.

Application of Halogen Bonding to Organocatalysis: A Theoretical Perspective

The strong, specific, and directional halogen bond (XB) is an ideal supramolecular synthon in crystal engineering, as well as rational catalyst and drug design. These attributes attracted strong growing interest in halogen bonding in the past decade and led to a wide range of applications in materials, biological, and catalysis applications. Recently, various research groups exploited the XB mode of activation in designing halogen-based Lewis acids in effecting organic transformation, and there is continual growth in this promising area. In addition to the rapid advancements in methodology development, computational investigations are well suited for mechanistic understanding, rational XB catalyst design, and the study of intermediates that are unstable when observed experimentally. In this review, we highlight recent computational studies of XB organocatalytic reactions, which provide valuable insights into the XB mode of activation, competing reaction pathways, effects of solvent and counterions, and design of novel XB catalysts.

Partitioning of interaction-induced nonlinear optical properties of molecular complexes. II. Halogen-bonded systems

Following our study on hydrogen-bonded (HB) complexes [Phys. Chem. Chem. Phys., 2018, 20, 19841], the physical nature of interaction-induced (non)linear optical properties of another important class of molecular complexes, namely halogen-bonded (XB) systems, was analyzed in this study. The excess electronic and nuclear relaxation (hyper)polarizabilities of nine representative XB complexes covering a wide range of halogen-bond strengths were computed. The partitioning of the excess properties into individual interaction-energy components (electrostatic, exchange, induction, dispersion) was performed by using the variational-perturbational energy decomposition scheme at the MP2/aug-cc-pVTZ level of theory and further supported by calculations with the SCS-MP2 method. In the case of the electronic interaction-induced properties, the physical composition of Δαel and Δγel was found to be very similar for the two types of bonding, despite the different nature of the binding. For Δβel, the XB complexes exhibit a more systematic interplay of interaction-energy contributions compared to the HB systems studied in the previous work. Our analysis revealed that the patterns of interaction-energy contributions to the interaction-induced nuclear-relaxation contributions to the linear polarizability and the first hyperpolarizability are very similar. For both properties the exchange repulsion term is canceled out by the electrostatic and delocalization terms. The physical composition of these contributions is analogous to those observed for the HB complexes.

Frontiers in Halogen and Chalcogen-Bond Donor Organocatalysis

Non-covalent molecular interactions on the basis of halogen and chalcogen bonding represent a promising, powerful catalytic activation mode. However, these "unusual" non-covalent interactions are typically employed in the solid state and scarcely exploited in catalysis. In recent years, an increased interest in halogen and chalcogen bonding has been awaken, as they provide profound characteristics that make them an appealing alternative to the well-explored hydrogen bonding. Being particularly relevant in the binding of "soft" substrates, the similar strength to hydrogen bonding interactions and its higher directionality allows for solution-phase applications with halogen and chalcogen bonding as the key interaction. In this mini-review, the special features, state-of-the-art and key examples of these so-called σ-hole interactions in the field of organocatalysis are presented.