σ-Hole, lone-pair-hole, and π-hole site-based interactions in aerogen-comprising complexes: a comparative study

Herein, the potential of ZO3 and ZF2 aerogen-comprising molecules (where Z = Ar, Kr, and Xe) to engage in σ-, lp-, and π-hole site-based interactions was comparatively studied using various ab initio computations. For the first time, a premier in-depth elucidation of the external electric field (EEF) influence on the strength of the σ-, lp-, and π-hole site-based interactions within the ZO3/ZF2⋯NH3 and ⋯NCH complexes was addressed using oriented EEF with disparate magnitude. Upon the energetic features, σ-hole site-based interactions were noticed with the most prominent preferability in comparison to lp- and π-hole analogs. This finding was ensured by the negative interaction energy values of -11.65, -3.50, and -2.74 kcal mol-1 in the case of σ-, lp-, and π-hole site-based interactions within the XeO3⋯ and XeF2⋯NH3 complexes, respectively. Detailedly, the strength of the σ- and lp-hole site-based interactions directly correlated with the atomic size of the aerogen atoms and the magnitude of the positively oriented EEF. Unexpectedly, an irregular correlation was noticed for the interaction energies of the π-hole site-based interactions with the size of the π-hole. Interestingly, the π-hole site-based interactions within Kr-comprising complexes exhibited higher negative interaction energies than the Ar- and Xe-comprising counterparts. Notwithstanding, a direct proportion between the interaction energies of the π-hole site-based interactions and π-hole size was obtained by employing EEF along the positive orientation with high strength. The present outcomes would be a fundamental basis for forthcoming progress in studying the σ-, lp-, and π-hole site-based interactions within aerogen-comprising complexes and their pertinent applications in materials science and crystal engineering.

σ-Hole Site-Based Interactions within Hypervalent Pnicogen, Halogen, and Aerogen-Bearing Molecules with Lewis Bases: A Comparative Study

σ-Hole site-based interactions in the trigonal bipyramidal geometrical structure of hypervalent pnicogen, halogen, and aerogen-bearing molecules with pyridine and NCH Lewis bases (LBs) were comparatively examined. In this respect, the ZF5···, XF3O2···, and AeF2O3···LB complexes (where Z = As, Sb; X = Br, I; Ae = Kr, Xe; and LB = pyridine and NCH) were investigated. The electrostatic potential (EP) analysis affirmations outlined the occurrence of σ-holes on the systems under consideration with disparate magnitudes that increased according to the following order: AeF2O3 < XF3O2 < ZF5. In line with EP outcomes, the proficiency of σ-hole site-based interactions increased as the atomic size of the central atom increased with a higher favorability for the pyridine-based complexes over NCH-based ones. The interaction energy showed the most favorable negative values of -35.97, -44.53, and -56.06 kcal/mol for the XeF2O3···, IF3O2···, and SbF5···pyridine complexes, respectively. The preferentiality pattern of the studied interactions could be explained as a consequence of (i) the dramatic rearrangement of ZF5 molecules from the trigonal bipyramid geometry to the square pyramidal one, (ii) the significant and tiny deformation energy in the case of the interaction of XF3O2 molecules with pyridine and NCH, respectively, and (iii) the absence of geometrical deformation within the AeF2O3···pyridine and ···NCH complexes other than the XeF2O3···pyridine one. Quantum theory of atoms in molecules and noncovalent interaction index findings reveal the partially covalent nature of most of the investigated interactions. Symmetry-adapted perturbation theory affirmations declared that the electrostatic component was the driving force beyond the occurrence of the considered interactions. The obtained findings will help in improving our understanding of the effect of geometrical deformation on intermolecular interactions.

Hole interactions of aerogen oxides with Lewis bases: an insight into σ-hole and lone-pair-hole interactions

σ-Hole and lone-pair (lp)-hole interactions of aerogen oxides with Lewis bases (LB) were comparatively inspected in terms of quantum mechanics calculations. The ZOn ⋯ LB complexes (where Z = Kr and Xe, n = 1, 2, 3 and 4, and LB = NH3 and NCH) showed favourable negative interaction energies. The complexation features were explained in light of σ-hole and lp-hole interactions within optimum distances lower than the sum of the respective van der Waals radii. The emerging findings outlined that σ-hole interaction energies generally enhanced according to the following order: KrO4 ⋯ < KrO⋯ < KrO3⋯ < KrO2⋯LB and XeO4⋯ < XeO⋯ < XeO2⋯ < XeO3⋯LB complexes with values ranging from -2.23 to -12.84 kcal mol-1. Lp-hole interactions with values up to -5.91 kcal mol-1 were shown. Symmetry-adapted perturbation theory findings revealed the significant contributions of electrostatic forces accounting for 50-65% of the total attractive forces within most of the ZOn⋯LB complexes. The obtained observations would be useful for the understanding of hole interactions, particularly for the aerogen oxides, with application in supramolecular chemistry and crystal engineering.

Influence of Internal Angular Arrangement on Pnicogen Bond Strength

The three Z-X covalent bonds of a ZX3 unit (Z = P, As, Sb, Bi) are normally arranged in a pyramidal structure. Quantum chemical calculations show that pnicogen bonds (ZBs) to the central Z are weakened if ZX3 is flattened, as in the opening of an umbrella. The partial closing of the umbrella has the opposite effect of substantially strengthening these ZBs, even amounting to a 2- or 3-fold magnification in certain cases. The strongest such bonds, wherein Sb and Bi are in a strained configuration within a ZO3CH model system, have interaction energies of 20 kcal/mol with an NH3 base. Most of these systems, whether flattened or more pyramidal, are capable of engaging in three ZBs simultaneously, despite a certain amount of negative cooperativity.

Transition between the Noncovalency and Covalency of σ-Hole Bonds

The properties of the bond between a N-ligand and a Lewis acid containing a σ-hole are studied by quantum chemical methods. Interactions considered include pnicogen bonds involving SbX5, PX5, and PX3, where X represents any of the halogen atoms F, Cl, Br, or I. Also studied are the tetrel bonds of PbX4 and SiX4, as well as the chalcogen bond involving TeOX4. Both NH3 and NCH are applied as two possible bases of differing potency. Some of the bonds are very strong with interaction energies easily exceeding 25 kcal/mol and with AIM bond critical point densities much higher than 0.04 au, suggesting their classification as coordinate covalent bonds. The pentavalent SbX5 and PX5 fall into this category when combined with NH3, as does TeOX4. Although the tetrel bonds involving PbX4 are only slightly weaker, they are probably better viewed as a strong noncovalent bond on the cusp of covalency. Changing the internal bonding of hypervalent SbX5 to the more conventional SbX3 weakens the interaction to a classical noncovalent pnicogen bond. Reducing the base nucleophilicity from NH3 to NCH weakens the bonds so that they are clearly noncovalent.

Relation between Halogen Bond Strength and IR and NMR Spectroscopic Markers

The relationship between the strength of a halogen bond (XB) and various IR and NMR spectroscopic quantities is assessed through DFT calculations. Three different Lewis acids place a Br or I atom on a phenyl ring; each is paired with a collection of N and O bases of varying electron donor power. The weakest of the XBs display a C-X bond contraction coupled with a blue shift in the associated frequency, whereas the reverse trends occur for the stronger bonds. The best correlations with the XB interaction energy are observed with the NMR shielding of the C atom directly bonded to X and the coupling constants involving the C-X bond and the C-H/F bond that lies ortho to the X substituent, but these correlations are not accurate enough for the quantitative assessment of energy. These correlations tend to improve as the Lewis acid becomes more potent, which makes for a wider range of XB strengths.

Enzymatic reversion of Pt(II) nucleophilicity through charge dumping: the case of Pt(CN)42

Combining computations and X-ray structure analysis we have demonstrated that [Pt(CN4)]2- can behave as a Lewis acid inside an enzyme's cavity. The nature of a counterintuitive contact found between a catalytically active GLN residue belonging to a mitochondrial synthase and the Pt(II) center was investigated by combining molecular dynamics and quantum mechanics calculations. Results confirm the electron acceptor role of [Pt(CN4)]2-, serving as an inspiration for the design of biomolecular cages able to tweak the nucleophilic/electrophilic character of an organometallic compound.

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.

Fabrication and application of Zn5 functionalized copolymer monolithic column for pipette tip micro-solid phase extraction of 4 polycyclic aromatic hydrocarbons in edible oil

The contamination of polycyclic aromatic hydrocarbons (PAHs) in edible oil is a health threat. Thus, trace analysis of PAHs is of high necessity. Based on the efficient adsorption of PAHs on Zn₅ metal cluster, a Zn₅ functionalized copolymer monolithic column was rationally designed for pipette tip micro-solid phase extraction (μ-SPE). The modified Zn₅ improved the adsorption selectivity and capacity of the monolith for naphthalene, phenanthrene, fluoranthene and pyrene. Chemical doping and copolymerization stabilized the monolith with a long life. Under optimal extraction conditions, a μ-SPE-high performance liquid chromatography with ultraviolet detector method was established for the detection of 4 PAHs in edible oils. The method yielded detection ranges of 0.15-250 μg L^-1 (R² > 0.997), detection limits of 0.050-1.5 μg L^-1, satisfactory recoveries (86.3-101.5 %), and high precisions (RSDs < 7.9 %). The results indicated that the Zn₅ functionalized copolymer monolithic column was an ideal separation medium for the detection of PAHs residues in edible oils.

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.

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.

Involvement of Arsenic Atom of AsF3 in Five Pnicogen Bonds: Differences between X-ray Structure and Theoretical Models

Bonding within the AsF₃ crystal is analyzed via quantum chemical methods so as to identify and quantify the pnicogen bonds that are present. The structure of a finite crystal segment containing nine molecules is compared with that of a fully optimized cluster of the same size. The geometries are qualitatively different, with a much larger binding energy within the optimized nonamer. Although the total interaction energy of a central unit with the remaining peripheral molecules is comparable for the two structures, the binding of the peripherals with one another is far larger in the optimized cluster. This distinction of much stronger total binding within the optimized cluster is not limited to the nonamer but repeats itself for smaller aggregates as well. The average binding energy of the cluster rises quickly with size, asymptotically approaching a value nearly triple that of the dimer.

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

Open and cyclic chains from two to eight units of ICl and IF are constructed and examined by density functional theory (DFT) calculations. These chains contain either I···I or I···X halogen bonds (XBs) where X refers to Cl or F. The closed rings are more stable than the open chains due to the presence of an additional XB and enhanced cooperativity. This pattern is true even for most trimers where there is sizable geometric distortion in the rings. I···F rings are generally more stable than the corresponding I···I cycles as the I···F bond is stronger than I···I even in the simple dimer. However, I···I rings are comparable in energy to I···Cl. It is possible to construct I···I rings of at least as large as eight units, which are held together exclusively by XBs. On the other hand, the maximum possible size of I···X rings is 6. Red shifts are observed in the I-X stretching frequency bands, which magnify as the chain, both cyclic and open, grows longer. The NMR chemical shielding of the I atoms increases for I···I chains but diminishes when I···Cl bonds are present.

Various Sorts of Chalcogen Bonds Formed by an Aromatic System

The chalcogen Y atom in the aromatic ring of thiophene and its derivatives YC₄H₄ (Y = S, Se, Te) can engage in a number of different interactions with another such unit within the homodimer. Quantum calculations show that the two rings can be oriented perpendicular to one another in a T-shaped dimer in which the Y atom accepts electron density from the π-system of the other unit in a Y···π chalcogen bond (ChB). This geometry best takes advantage of attractions between the electrostatic potentials surrounding the two monomers. There are two other geometries in which the two Y atoms engage in a ChB with one another. However, instead of a simple interaction between a σ-hole on one Y and the lone pair of its neighbor, the interaction is better described as a pair of symmetrically equivalent Y···Y interactions, in which charge is transferred in both directions simultaneously, thereby effectively doubling the strength of the bond. These geometries differ from what might be expected based simply on the juxtaposition of the electrostatic potentials of the two monomers.

Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2

The potential reaction of the nitrate radical (NO₃), the main nighttime atmospheric oxidant, with five alkyl halides, halons (CH₂ClBr, CH₂ICl, CH₂BrI, CHCl₂Br, and CHClBr₂) has been studied theoretically. The most favorable reaction corresponds to a hydrogen atom transfer. The stationary points on the potential energy surfaces of these reactions have been characterized. The reactions can be classified into two groups based on the number of hydrogen atoms in the halon molecules (1 or 2). The reactions with halons with only one hydrogen atom show more exothermic profiles than those with two hydrogen atoms. In addition, the kinetics of the reaction of NO₃ + CH₂BrI was studied in much higher detail using a multi-well Master Equation solver as a representative example of the nitrate radical reactivity against these halocarbons. These results indicate that the chemical lifetime of the alkyl halides would not be substantially affected by nitrate radical reactions, even in the case of NO₃-polluted atmospheric conditions.

Competition between Intra and Intermolecular Pnicogen Bonds. Complexes between Naphthalene Derivatives and Neutral or Anionic Bases

The PnF 2 (Pn=P,As,Sb,Bi) on a naphthalene scaffold can engage in an internal pnicogen Pn···N bond (PnB) with a NH 2 group placed close to it on the adjoining ring. An approaching neutral NH 3 molecule can engage in a second PnB with the central Pn, which tends to weaken the intramolecular bond. The presence of the latter in turn weakens the intermolecular PnB with respect to that formed in its absence. Replacement of the external NH 3 by a CN - anion causes a fundamental change in the bonding pattern, producing a fourth covalent bond with Pn, which rearranges into a trigonal bipyramidal motif. This addition disrupts the internal Pn···N pnicogen bond, recasting the PnF 2 ···NH 2 interaction into a NH···F H-bond.

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.

Stand up for Electrostatics: The Disiloxane Case

The basicity of the simplest silicone, disiloxane (H 3 Si-O-SiH 3 ), is strongly affected by the Si-O-Si angle (α). We use high-level ab initio MP2/aug'-cc-pVTZ calculations and the molecular electrostatic potential (MEP) to analyze the relationship between the increase in basicity and the reduction of α. Our results clearly point out that this increase can be explained through the MEP, as the interactions between oxygen from disiloxane and the acceptors are mostly electrostatic. Furthermore, the effect of α on the tetrel bond between disiloxane and several Lewis bases can again be rationalized using the MEP. Finally, we explore the cooperativity throughout α for ternary complexes where disiloxane simultaneously interacts with a Lewis acid and a Lewis base. Both non-covalent interactions remain cooperative for all α values, although the largest cooperativity effects are not always those maximizing the binding energy in the binary complexes. Overall, the MEP remains a powerful predictor for noncovalent interactions.

R•-hole interactions of group IV-VII radical-containing molecules: A comparative study

For the first time, the potentiality of the sp²-hybridized group IV-VII radical (R^•)-containing molecules to participate in R^•-hole interactions was comparatively assessed using ^•SiF_3,^•POF₂, ^•SO₂F, and ^•ClO₃ models in the trigonal pyramidal geometry. In that spirit, a plethora of quantum mechanical calculations was performed at the MP2/aug-cc-pVTZ level of theory. According to the results, all the investigated R^•-containing molecules exhibited potent versatility to engage in R^•-hole … Lewis base interactions with significant negative binding energies for the NCH-based complexes. The strength of R^•-hole interactions was perceived to obey the ^•ClO₃ … > ^•SO₂F … > ^•POF₂ … > ^•SiF₃ … Lewis base order, outlining an inverse correlation between the binding energy and the atomic size of the R^•-hole donor. Benchmarking of the binding energy at the CCSD/CBS(T) computational level was executed for all the explored interactions and addressed an obvious similarity between the MP2 and CCSD energetic findings. QTAIM analysis critically unveiled the closed-shell nature of the explored R^•-hole interactions. SAPT-EDA proclaimed the reciprocal contributions of electrostatic and dispersion forces to the total binding energy. These observations demonstrate in better detail the nature of R^•-hole interactions, leading to a convincing amelioration for versatile fields relevant to materials science and drug design.

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.

Chalcogen versus Dative Bonding in [SF3]+ Lewis Acid-Base Adducts: [SF3(NCCH3)2]+, [SF3(NC5H5)2]+, and [SF3(phen)]+ (phen = 1,10-phenanthroline)

The Lewis-acid behavior of [SF₃][MF₆] (M = Sb, As) salts toward mono- and bidentate nitrogen bases was explored. Reactions of [SF₃][MF₆] with excesses of CH₃CN and C₅H₅N yielded [SF₃(L)₂]⁺ (L = CH₃CN, C₅H₅N) salts, whereas the reaction of [SF₃][SbF₆] with equimolar 1,10-phenanthroline (phen) in CH₃CN afforded [SF₃(phen)][SbF₆]·2CH₃CN. Salts of these cations were characterized by low-temperature X-ray crystallography and Raman spectroscopy in the solid state as well as by ¹⁹F NMR spectroscopy in solution. In the solid state, the geometries of [SF₃(NC₅H₅)₂]⁺ and [SF₃(phen)]⁺ are square pyramids with negligible cation-anion contacts, whereas the coordination of CH₃CN and [SbF₆]^- to [SF₃]⁺ in [SF₃(NCCH₃)₂][SbF₆] results in a distorted octahedral coordination sphere with a minimal perturbation of the trigonal-pyramidal SF₃ moiety. ¹⁹F NMR spectroscopy revealed that [SF₃(L)₂]⁺ is fluxional in excess L at -30 °C, whereas [SF₃(phen)]⁺ is rigid in CH₂Cl₂ at -40 °C. Density functional theory (DFT-B3LYP) calculations suggest that the S-N bonds in [SF₃(NC₅H₅)₂]⁺ and [SF₃(phen)]⁺ possess substantial covalent character and result in a regular AX₅E VSEPR geometry, whereas those in [SF₃(NCCH₃)₂]⁺ are best described as S···N chalcogen-bonding interactions via σ-holes on [SF₃]⁺, which is consistent with the crystallographic data.

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.

External electric field effects on the σ-hole and lone-pair hole interactions of group V elements: a comparative investigation

σ-hole and lone-pair (lp) hole interactions of trivalent pnicogen-bearing (ZF3) compounds were comparatively scrutinized, for the first time, under field-free and external electric field (EEF) conditions. Conspicuously, the sizes of the σ-hole and lp-hole were increased by applying an EEF along the positive direction, while the sizes of both holes decreased through the reverse EEF direction. The MP2 energetic calculations of ZF3⋯FH/NCH complexes revealed that σ-holes exhibited more impressive interaction energies compared to the lp-holes. Remarkably, the strengths of σ-hole and lp-hole interactions evolved with the increment of the positive value of the considered EEF; i.e., the interaction energy increased as the utilized EEF value increased. Unexpectedly, under field-free conditions, nitrogen-bearing complexes showed superior strength for their lp-hole interactions than phosphorus-bearing complexes. However, the reverse picture was exhibited for the interaction energies of nitrogen- and phosphorus-bearing complexes interacting within lp-holes by applying the high values of a positively directed EEF. These results significantly demonstrate the crucial influence of EEF on the strength of σ-hole and lp-hole interactions, which in turn leads to an omnipresent enhancement for variable fields, including biological simulations and material science.

Experimental and Theoretical Studies of Dimers Stabilized by Two Chalcogen Bonds in the Presence of a N···N Pnicogen Bond

The structure of the 5,6-dichloro-2,1,3-benzoselenadiazole homodimer, obtained by adding the ligand, 4,5-dichloro-o-phenylenediamine, to the methanolic solution of SeCl₄, was determined by X-ray crystallography, augmented by Fourier transform infrared, Raman, and NMR spectroscopy. The binding motif involves a pair of Se···N chalcogen bonds, with a supplementary N···N pnicogen bond. Quantum calculations provide assessments of the strengths of the individual interactions as well as their contributing factors. All together, these three bonds compose a total interaction energy between 5.4 and 16.8 kcal/mol, with the larger chalcogen atom associated with the strongest interactions. Replacement of the Se atoms by S and Te analogues allows analysis of the dependence of these forces on the nature of the chalcogen atom. Calculations also measure the importance to the binding of the presence of a second N atom on each diazole unit as well as the substituted phenyl ring to which it is fused.

Reliable Comparison of Pnicogen, Chalcogen, and Halogen Bonds in Complexes of 6-OXF2-Fulvene (X = As, Sb, Se, Te, Be, I) With Three Electron Donors

The pnicogen, chalcogen, and halogen bonds between 6-OXF₂-fulvene (X = As, Sb, Se, Te, Br, and I) and three nitrogen-containing bases (FCN, HCN, and NH₃) are compared. For each nitrogen base, the halogen bond is strongest, followed by the pnicogen bond, and the chalcogen bond is weakest. For each type of bond, the binding increases in the FCN < HCN < NH₃ pattern. Both FCN and HCN engage in a bond with comparable strengths and the interaction energies of most bonds are < -6 kcal/mol. However, the strongest base NH₃ forms a much more stable complex, particularly for the halogen bond with the interaction energy going up to -18 kcal/mol. For the same type of interaction, its strength increases as the mass of the central X atom increases. These bonds are different in strength, but all of them are dominated by the electrostatic interaction, with the polarization contribution important for the stronger interaction. The presence of these bonds changes the geometries of 6-OXF₂-fulvene, particularly for the halogen bond formed by NH₃, where the F-X-F arrangement is almost vertical to the fulvene ring.

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.

Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations

This study marks chemical space available for pnictogen-bonding catalysis, and demonstrates that reactivity accessible in this space is unique.

Pnictogen-bond donors are attractive for use in catalysis because of deep σ holes, high multivalency, rich hypervalency, and chiral binding pockets. We here report natural product inspired epoxide-opening polyether cyclizations catalyzed by fluoroarylated Sb(v) > Sb(iii) > Bi > Sn > Ge. The distinctive characteristic found for pnictogen-bonding catalysis is the breaking of the Baldwin rules, that is selective endo cyclization into the trans-fused ladder oligomers known from the brevetoxins. Moreover, tris(3,4,5-trifluorophenyl)stibines and their hypervalent stiborane catecholates afford different anti-Baldwin stereoselectivity. Lewis (SbCl₃), Brønsted (AcOH) and π acids fail to provide similar access to these forbidden rings. Like hydrogen-bonding catalysis differs from Brønsted acid catalysis, pnictogen-bonding catalysis thus emerges as the supramolecular counterpart of covalent Lewis acid catalysis.

Self-assembly of reversed bilayer vesicles through pnictogen bonding: water-stable supramolecular nanocontainers for organic solvents

A new air and moisture stable antimony thiolate compound has been prepared that spontaneously forms stable hollow vesicles. Structural data reveals that pnictogen bonding drives the self-assembly of these molecules into a reversed bilayer. The ability to make these hollow, spherical, and chemically and temporally stable vesicles that can be broken and reformed by sonication allows these systems to be used for encapsulation and compartmentalisation in organic media. This was demonstrated through the encapsulation and characterization of several small organic reporter molecules.

Regium Bonds between Silver(I) Pyrazolates Dinuclear Complexes and Lewis Bases (N2, OH2, NCH, SH2, NH3, PH3, CO and CNH)

A theoretical study and Cambridge Structural Database (CSD) search of dinuclear Ag(I) pyrazolates interactions with Lewis bases were carried out and the effect of the substituents and ligands on the structure and on the aromaticity were analyzed. A relationship between the intramolecular Ag–Ag distance and stability was found in the unsubstituted system, which indicates a destabilization at longer distances compensated by ligands upon complexation. It was also observed that the asymmetrical interaction with phosphines as ligands increases the Ag–Ag distance. This increase is dramatically higher when two simultaneous PH3 ligands are taken into account. The calculated 109Ag chemical shielding shows variation up to 1200 ppm due to the complexation. Calculations showed that six-membered rings possessed non-aromatic character while pyrazole rings do not change their aromatic character significantly upon complexation.

On Transannulation in Azaphosphatranes: Synthesis and Theoretical Analysis

A combined synthetic-theoretical study has been undertaken to determine the factors that influence transannulation in azaphosphatranes. The commonly used proazaphosphatrane P(i-BuNCH₂CH₂)₃N and several of its oxidized congeners are used as model systems. The haloazaphosphatranes of P(i-BuNCH₂CH₂)₃N were synthesized, including a rare fluoroazaphopshatrane, and used as references for computational investigations. Comparisons of the experimental and theoretical observations highlight the flexibility observed in transannulated atranes and the potential for multiple local energetic minima depending on the identity of the equatorial substituents for a given azaphosphatrane. Theoretical calculations also identify the role of the ethylene linker in azaphosphatrane bonding, the influence of transannulation on P-electrophile interactions, and the contribution of electrostatic interactions to transannulation.

121/123Sb Nuclear Quadrupole Resonance Spectroscopy: Characterization of Non-Covalent Pnictogen Bonds and NQR Crystallography

Pnictogen (or pnicogen) bonding is an attractive interaction between the electrophilic region of group 15 elements (N, P, As, Sb, Bi) and a nucleophile. This interaction for which unique applications in catalysis have recently been uncovered continues to gain popularity. Here, we investigate a series of pnictogen-bonded cocrystals based on SbF₃ and SbCl₃, prepared via mechanochemical ball milling, with ^121/123Sb ( I = ⁵/₂ and ⁷/₂, respectively) nuclear quadrupole resonance (NQR) spectroscopy. Observed NQR frequency shifts upon cocrystallization are on the order of 0.1 to 10 MHz and are clearly diagnostic of the formation of pnictogen bonds to antimony. Further evidence for pnictogen bonding is obtained by complementary ¹³C cross-polarization magic-angle spinning solid-state NMR experiments. DFT calculations of NMR parameters as well as natural localized molecular orbital analyses support the experimental findings and elucidate the electronic origins of the experimental NQR frequency shifts. This work provides insights into the changes in the antimony quadrupolar coupling constant upon pnictogen bonding: strikingly, the decreases noted here parallel those known for hydrogen bonds, but contrast with the increases reported for halogen bonds. The utility of the observed antimony nuclear quadrupolar coupling constants in constraining structural models of cocrystals for which diffraction-based structures are unavailable, i.e., a rudimentary implementation of NQR crystallography, is established. Overall, this work offers a new approach to understand emerging classes of electrophilic interactions and to contextualize them in the broader landscape of established chemical bonding paradigms.

Differential Binding of Tetrel-Bonding Bipodal Receptors to Monatomic and Polyatomic Anions

Previous work has demonstrated that a bidentate receptor containing a pair of Sn atoms can engage in very strong interactions with halide ions via tetrel bonds. The question that is addressed here concerns the possibility that a receptor of this type might be designed that would preferentially bind a polyatomic over a monatomic anion since the former might better span the distance between the two Sn atoms. The binding of Cl⁻ was thus compared to that of HCOO⁻, HSO₄⁻, and H₂PO₄⁻ with a wide variety of bidentate receptors. A pair of SnFH₂ groups, as strong tetrel-binding agents, were first added to a phenyl ring in ortho, meta, and para arrangements. These same groups were also added in 1,3 and 1,4 positions of an aliphatic cyclohexyl ring. The tetrel-bonding groups were placed at the termini of (-C≡C-)_n (n = 1,2) extending arms so as to further separate the two Sn atoms. Finally, the Sn atoms were incorporated directly into an eight-membered ring, rather than as appendages. The ordering of the binding energetics follows the HCO₂⁻ > Cl⁻ > H₂PO₄⁻ > HSO₄⁻ general pattern, with some variations in selected systems. The tetrel bonding is strong enough that in most cases, it engenders internal deformations within the receptors that allow them to engage in bidentate bonding, even for the monatomic chloride, which mutes any effects of a long Sn···Sn distance within the receptor.

Competition between tetrel bond and pnicogen bond in complexes of TX3-ZX2 and NH3

The complexes formed between TX₃-ZX₂ (T = C, Si, Ge; Z = P, As, Sb; X = F, Cl) and NH₃ were studied at the MP2/aug-cc-pVTZ(PP) level. For each TX₃-ZX₂, two types of complex were obtained. For CX₃-ZX₂, NH₃ is inclined to approach the σ-hole on the Z atom, forming a pnicogen bond. For TX₃-ZX₂ (T = Si and Ge), however, the base favors engaging in a tetrel bond with the σ-hole on the T atom although the corresponding pnicogen-bonded complex is also stable. When NH₃ approaches the CX₃ terminal of CX₃-ZX₂, weak interactions are observed that may be classified as van der Waals interactions. The relative stability of both types of complexes is not affected by the substituent X. The tetrel bond is very strong and the largest interaction energy is up to -144 kJ mol^-1. Dispersion is dominant in the weak van der Waals complexes, while tetrel- and pnicogen-bonded complexes are dominated by electrostatic interactions, with comparable contributions from polarization.

Comparative Strengths of Tetrel, Pnicogen, Chalcogen, and Halogen Bonds and Contributing Factors

Ab initio calculations are employed to assess the relative strengths of various noncovalent bonds. Tetrel, pnicogen, chalcogen, and halogen atoms are represented by third-row atoms Ge, As, Se, and Br, respectively. Each atom was placed in a series of molecular bonding situations, beginning with all H atoms, then progressing to methyl substitutions, and F substituents placed in various locations around the central atom. Each Lewis acid was allowed to engage in a complex with NH₃ as a common nucleophile, and the strength and other aspects of the dimer were assessed. In the context of fully hydrogenated acids, the strengths of the various bonds varied in the pattern of chalcogen > halogen > pnicogen ≈ tetrel. Methyl substitution weakened all bonds, but not in a uniform manner, resulting in a greatly weakened halogen bond. Fluorosubstitution strengthened the interactions, increasing its effect as the number of F atoms rises. The effect was strongest when the F atom lay directly opposite the base, resulting in a halogen > chalcogen > pnicogen > tetrel order of bond strength. Replacing third-row atoms by their second-row counterparts weakened the bonds, but not uniformly. Tetrel bonds were weakest for the fully hydrogenated acids and surpassed pnicogen bonds when F had been added to the acid.

Comparison between Tetrel Bonded Complexes Stabilized by σ and π Hole Interactions

The σ-hole tetrel bonds formed by a tetravalent molecule are compared with those involving a π-hole above the tetrel atom in a trivalent bonding situation. The former are modeled by TH₄, TH₃F, and TH₂F₂ (T = Si, Ge, Sn) and the latter by TH₂=CH₂, THF=CH₂, and TF₂=CH₂, all paired with NH₃ as Lewis base. The latter π-bonded complexes are considerably more strongly bound, despite the near equivalence of the σ and π-hole intensities. The larger binding energies of the π-dimers are attributed to greater electrostatic attraction and orbital interaction. Each progressive replacement of H by F increases the strength of the tetrel bond, whether σ or π. The magnitudes of the maxima of the molecular electrostatic potential in the two types of systems are not good indicators of either the interaction energy or even the full Coulombic energy. The geometry of the Lewis acid is significantly distorted by the formation of the dimer, more so in the case of the σ-bonded complexes, and this deformation intensifies the σ and π holes.

Tetrel Bonding as a Vehicle for Strong and Selective Anion Binding

Tetrel atoms T (T = Si, Ge, Sn, and Pb) can engage in very strong noncovalent interactions with nucleophiles, which are commonly referred to as tetrel bonds. The ability of such bonds to bind various anions is assessed with a goal of designing an optimal receptor. The Sn atom seems to form the strongest bonds within the tetrel family. It is most effective in the context of a -SnF₃ group and a further enhancement is observed when a positive charge is placed on the receptor. Connection of the -SnF₃ group to either an imidazolium or triazolium provides a strong halide receptor, which can be improved if its point of attachment is changed from the C to an N atom of either ring. Aromaticity of the ring offers no advantage nor is a cyclic system superior to a simple alkyl amine of any chain length. Placing a pair of -SnF₃ groups on a single molecule to form a bipodal dicationic receptor with two tetrel bonds enhances the binding, but falls short of a simple doubling. These two tetrel groups can be placed on opposite ends of an alkyl diamine chain of any length although SnF₃⁺NH₂(CH₂)nNH₂SnF₃⁺ with n between 2 and 4 seems to offer the strongest halide binding. Of the various anions tested, OH− binds most strongly: OH− > F− > Cl− > Br− > I−. The binding energy of the larger NO₃− and HCO₃− anions is more dependent upon the charge of the receptor. This pattern translates into very strong selectivity of binding one anion over another. The tetrel-bonding receptors bind far more strongly to each anion than an equivalent number of K⁺ counterions, which leads to equilibrium ratios in favor of the former of many orders of magnitude.

Modulation of in:out and out:out conformations in [X.X'.X''] phosphatranes by Lewis acids

A theoretical study of [X.X'.X'']phosphatrane:Lewis acid complexes has been carried out in order to analyze how the in:out and out:out conformations can be modulated by the interaction with Lewis acids (LA). It has been found that in:out structures are more stable in larger systems i.e. in [4.4.3]:LA and [4.4.4]:LA than in [3.3.3]:LA and [4.3.3]:LA. The results obtained for the relative energies in conjunction with electron density properties showed that upon complexation, in:out conformers become more stable with the increasing acidity of the corresponding Lewis acid. In fact, the binding energies found for in:out complexes are larger than those obtained for out:out complexes. The complexes with the largest relative energy favoring the in:out structure correspond to those with charged Lewis acids, followed by the complexes with ClF. In all cases, the complexes are cooperative, reaching a maximum value of 168.5 kJ mol^-1 for the [4.3.3]:F⁺ complex.

Implications of monomer deformation for tetrel and pnicogen bonds

Monomer rearrangement raises the interaction energy by up to 20 kcal mol⁻¹and intensifies its σ-hole by a factor of 1.5–2.9.

21st century developments in the understanding and control of molecular solids

This highlight article surveys some of the key recent advances in crystallization techniques, polymorphism, co-crystals, amorphous materials and crystal engineering.

The effect of anions on noncovalent interactions in model clusters of chalcogen-containing (CH3)2X (X = O, S, Se) molecules

A computational study of F⁻⋯(CH₃)₂O⋯CH₃F with F⁻ bound to the protons of the two methyl groups, found significant enhancement of the O⋯C interaction relative to the neutral (CH₃)₂O⋯CH₃F dyad.