On the Versatility of BH2 X (X=F, Cl, Br, and I) Compounds as Halogen-, Hydrogen-, and Triel-Bond Donors: An Ab Initio Study

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.

Can H2 be Superacidic? A Computational Study of Triel-Bonded Brønsted Acids

The abundance of XIII group element compounds in science and industry together with their electron-deficient character gives rise to their influence on properties of the systems they interact with. This paper is an attempt to assess the strength, nature, and effect of formation of a triel bond on acidity. A wide set of Brønsted acids among others comprising hydrocarbons, halogen hydrides, and amines bonded with B, Al, and Ga trifluorides forming HX/TF3 was selected for the research. Various computational approaches (e.g., MP2, GFN2-xTB, SAPT2 + 3(CCD)δMP2, quantum theory of atoms in molecules analysis, and density overlap regions indicator) are used to describe the triel-bonded systems. Among other things, it was found that the electrostatics may not be the dominant contribution to the triel binding in some cases. Additionally, it was established that even weak Brønsted acids such as C2H2 or H2 may be superacidic if bonded to a Lewis acid (TF3) that is strong enough. The calculations indicate a significant covalent character of some of the studied HX/TF3 triel-bonded systems. Moreover, the effect of solvation of HX with TF3 as well as that of the reverse process on the acidity of the resulting system is thoroughly described.

Triel Bond Formed by Malondialdehyde and Its Influence on the Intramolecular H-Bond and Proton Transfer

Malondialdehyde (MDA) engages in a triel bond (TrB) with TrX₃ (Tr = B and Al; X = H, F, Cl, and Br) in three modes, in which the hydroxyl O, carbonyl O, and central carbon atoms of MDA act as the electron donors, respectively. A H···X secondary interaction coexists with the TrB in the former two types of complexes. The carbonyl O forms a stronger TrB than the hydroxyl O, and both of them are better electron donors than the central carbon atom. The TrB formed by the hydroxyl O enhances the intramolecular H-bond in MDA and thus promotes proton transfer in MDA-BX₃ (X = Cl and Br) and MDA-AlX₃ (X = halogen), while a weakening H-bond and the inhibition of proton transfer are caused by the TrB formed by the carbonyl O. The TrB formed by the central carbon atom imposes little influence on the H-bond. The BH₂ substitution on the central C-H bond can also realise the proton transfer in the triel-bonded complexes between the hydroxyl O and TrH₃ (Tr = B and Al).

Triel bonds within anionanion complexes

The ability of two anions to interact with one another is tested in the context of pairs of TrX₄^- homodimers, where Tr represents any of the triel atoms B, Al, Ga, In, or Tl, and X refers to a halogen substituent F, Cl, or Br. None of these pairs engage in a stable complex in the gas phase, but the situation reverses in water where the two monomers are held together by Tr⋯X triel bonds, complemented by stabilizing interactions between X atoms. Some of these bonds are quite strong, notably those involving TrF₄^-, with interaction energies surpassing 30 kcal mol^-1. Others are very much weaker, with scarcely exothermic binding energies. The highly repulsive electrostatic interactions are counteracted by large polarization energies.

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.

Influence of halogen atom substitution and neutral HCN/anion CN- Lewis base on the triel-bonding interactions

Triel-bonding interactions composed of Lewis acid TrOHH₂/TrOH₂X/TrOHX₂ (Tr = B, Al, Ga; X = F, Cl, Br) molecule and Lewis base neutral HCN or anionic CN^- molecule are of research significance in bond properties, which has been investigated at MP2/aug-cc-pVTZ theory level. It is also feasible to study the halogen atom substituent effect and influence of different Lewis bases on the formation of triel bond. AIM analyses reveal that Tr (Tr = B, Al, Ga)···N bond critical point (BCP) exists in all studied triel bond. In the formation of triel bonding, compared with Lewis base HCN molecule, Lewis base anionic CN^- can participate in a stronger triel bond. Specifically, the structural change, deformation energy, and charge transfer of CN^- complexes are all larger than that of HCN complexes. In addition, halogen atom substitution effect is also discussed. MEP value and binding energy of HCN and CN^- complexes all increase after replacing one or two hydrogen atoms by halogen atoms (F, Cl, Br) in Lewis acid. Especially, replacing two hydrogen atoms by halogen atoms in Lewis acid has more remarkable enhancement in MEP value and binding energy than that of replacing only one hydrogen atom. After replacement, binding energy can be increased by 21.77 kcal/mol. The neutral and anionic triel-bonded complexes composed by Lewis acid TrOHH₂/TrOH₂X/TrOHX₂ (Tr = B, Al, Ga; X = F, Cl, Br) with Lewis base HCN and CN^- are systematically investigated at MP2/aug-cc-pVTZ level. The neutral (HCN) triel bonding is weaker than the anionic (CN^-) triel bonding due to the smaller MEP value of the neutral HCN molecule. The replacement of hydrogens (-H) in Lewis acid by electron-withdrawing groups (-F, -Cl, -Br) has a prominent enhancement effect on the MEP value of π-hole and triel-bonding strength.

Dominance of unique Pπ phosphorus bonding with π donors: evidence using matrix isolation infrared spectroscopy and computational methodology

Albeit the first account of hypervalentπ interactions has been reported with halogenπ interactions, the feasibility of their extension to other hypervalent atoms as possible Lewis acids is still open. In this work, the role of phosphorus as an acceptor from the π electron cloud (Pπ pnicogen or phosphorus bonding) in PCl3-C2H2 and PCl3-C2H4 heterodimers is explored, by combining matrix isolation infrared spectroscopy with ab initio and DFT computational methodologies. The respective potential energy surfaces of the PCl3-C2H2 and PCl3-C2H4 heterodimers reveal unique minima stabilized by a concert of reasonably strong to weak interactions, of which Pπ phosphorus bonding was energetically dominant. Heterodimers, trimers and tetramers bound primarily by this unique phosphorus bond were generated at low temperatures. The dominance of phosphorus bonding in the PCl3-C2H2 and PCl3-C2H4 heterodimers over other interactions (such as Hπ, HCl, HP, Clπ and lone pair-π interactions) was confirmed and substantiated using extended quantum theory of atoms in molecules, natural bond orbital, electrostatic potential mapping and energy decomposition analyses. The following inferences in correlation with results from non-covalent-interaction analysis offer a complete understanding of the nature of the Pπ phosphorus bonding interactions. The significance of electrostatic forces kinetically favoring the formation of phosphorus bonded heterodimers, in addition to thermodynamic stabilization, is demonstrated experimentally.

Not Only Hydrogen Bonds: Other Noncovalent Interactions

In this review, we provide a consistent description of noncovalent interactions, covering most groups of the Periodic Table. Different types of bonds are discussed using their trivial names. Moreover, the new name “Spodium bonds” is proposed for group 12 since noncovalent interactions involving this group of elements as electron acceptors have not yet been named. Excluding hydrogen bonds, the following noncovalent interactions will be discussed: alkali, alkaline earth, regium, spodium, triel, tetrel, pnictogen, chalcogen, halogen, and aerogen, which almost covers the Periodic Table entirely. Other interactions, such as orthogonal interactions and π-π stacking, will also be considered. Research and applications of σ-hole and π-hole interactions involving the p-block element is growing exponentially. The important applications include supramolecular chemistry, crystal engineering, catalysis, enzymatic chemistry molecular machines, membrane ion transport, etc. Despite the fact that this review is not intended to be comprehensive, a number of representative works for each type of interaction is provided. The possibility of modeling the dissociation energies of the complexes using different models (HSAB, ECW, Alkorta-Legon) was analyzed. Finally, the extension of Cahn-Ingold-Prelog priority rules to noncovalent is proposed.

Xe⋯chalcogen aerogen bond. Effect of substituents and size of chalcogen atom

We have studied the effect of substituent and size of chalcogen atom on the aerogen bond between F₂XeO and R₁YR₂.

Triel bonds in RZH2···NH3: hybridization, solvation, and substitution

The influence of hybridization, substitution, and solvation on the triel bond has been investigated in the complexes of RZH₂···NH₃ (Z = B and Al). The magnitude of the π-hole on the triel atom is related to the nature of the Z atom and the hybridization of R. CH₃BH₂ has the largest π-hole among RBH₂, while for RAlH₂ the largest π-hole is found in CH≡CAlH₂. The interaction energy is partly inconsistent with the magnitude of the π-hole on the triel atom and the orbital interaction from the N lone pair of NH₃ into the empty p orbital of the triel atom. The strongest B···N triel bond is found in CH≡CBH₂···NH₃, while the weakest Al···N triel bond is in CH₃AlH₂···NH₃. The strength of the triel bond is increased in solvents, and its enhancement is prominent with the increase of solvent polarity. Solvents also change the nature of the Al···N triel bond from an electrostatic interaction to a partially covalent one. The F substituent in the triel donor strengthens the triel bond, depending on the substitution position and number. Graphical Abstract The π-hole triel bonded complexes between RZH2 (Z =B and Al) and NH3 have been investigated. We focused on the effects of hybridization, solvent, and substitution on the strength and nature of π-hole triel bond.

Square-Planar Pt(II) and Ir(I) Complexes as the Lewis Bases: Donor-Acceptor Adducts with Group 13 Trihalides and Trihydrides

The stability and properties of donor-acceptor adducts of square-planar Pt(II) and Ir(I) complexes (designated as PtX, IrX, or generally MX complexes) with trihydrides and trihalides of group 13 elements of general formula YZ₃ (Y = B, Al, Ga; Z = H, F, Cl, Br) were studied theoretically using DFT methodology in the gas phase. MX complexes were represented by wide range of the ligand environment which included model complexes [Ir(NH₃)₃X]⁰ and cis-[Pt(NH₃)₂X₂]⁰ (X = H, CH₃, F, Cl, Br) and isoelectronic complexes [Ir(NNN)(CH₃)]⁰ and [Pt(NCN)(CH₃)]⁰ with tridentate NNN and NCN pincer ligands. MX complexes acted as the Lewis bases donating electron density from the doubly occupied 5d _z² atomic orbital of the metal M atom to the empty valence p _z orbital of Y whose evidence was clearly provided by the natural atomic orbital (NAO) analysis. This charge transfer led to the formation of pentacoordinated square pyramidal MX·(YZ₃) adducts with M·Y dative bond. Binding energies were -44.7 and -75.2 kcal/mol for interaction of GaF₃ as the strongest acid with PtNCN and IrNNN pincer ligands complexes. Only M·B bonds had covalent character although MX·BZ₃ adducts were the least stable due to large values of Pauli repulsion and deformation energies. The highest degree of covalent character was found for adducts of BH₃ in all series of structures studied. Al and Ga adducts showed remarkably similar behavior with respect to geometry and binding energies.

Quantitative Assessment of Tetrel Bonding Utilizing Vibrational Spectroscopy

A set of 35 representative neutral and charged tetrel complexes was investigated with the objective of finding the factors that influence the strength of tetrel bonding involving single bonded C, Si, and Ge donors and double bonded C or Si donors. For the first time, we introduced an intrinsic bond strength measure for tetrel bonding, derived from calculated vibrational spectroscopy data obtained at the CCSD(T)/aug-cc-pVTZ level of theory and used this measure to rationalize and order the tetrel bonds. Our study revealed that the strength of tetrel bonds is affected by several factors, such as the magnitude of the σ-hole in the tetrel atom, the negative electrostatic potential at the lone pair of the tetrel-acceptor, the positive charge at the peripheral hydrogen of the tetrel-donor, the exchange-repulsion between the lone pair orbitals of the peripheral atoms of the tetrel-donor and the heteroatom of the tetrel-acceptor, and the stabilization brought about by electron delocalization. Thus, focusing on just one or two of these factors, in particular, the σ-hole description can only lead to an incomplete picture. Tetrel bonding covers a range of -1.4 to -26 kcal/mol, which can be strengthened by substituting the peripheral ligands with electron-withdrawing substituents and by positively charged tetrel-donors or negatively charged tetrel-acceptors.

Dual function of the boron center of BH(CO)2/BH(N2)2 in halogen- and triel-bonded complexes with hypervalent halogens

The complexes between BH(CO)₂/BH(N₂)₂ and XF₃/XF₅ are stabilized by a halogen bond and a triel bond. The MEP analyses of BH(CO)₂/BH(N₂)₂ indicate that there are both a region with negative MEPs on the B atom in the vertical direction of the molecular plane and a σ-hole at the B-H bond end. Therefore, the boron atom in BH(CO)₂/BH(N₂)₂ plays a dual role of a Lewis base and an acid in the halogen bond and triel bond, respectively. The halogen and triel bonds are stronger in order of IF₃< BrF₃< ClF₃, IF₅< BrF₅< ClF₅, and BH(CO)₂< BH(N₂)₂ in most complexes. These complexes have large stability since the interaction energy varies from -5 to -115 kcal/mol. The halogen bond belongs to a covalent interaction or a partially covalent interaction in most complexes. The subsystems in these complexes have prominent deformation, accompanied with big charge transfer and large polarization.

Concurrent aerogen bonding and lone pair/anion-π interactions in the stability of organoxenon derivatives: a combined CSD and ab initio study

In this manuscript the ability of organoxenon fluorides to establish concurrent π-hole aerogen bonding and lone pair/anion-π interactions has been studied at the RI-MP2/def2-TZVP level of theory. The presence of both an aromatic system (benzene, trifluorobenzene and pentafluorobenzene) and a xenon atom makes these molecules suitable for simultaneously establishing both interactions. In this regard, we have used CH₃CN, NH₃, O(CH₃)₂, Cl^-, CN^- and BF₄^- as neutral and charged electron donors, respectively. Moreover, the NBO analysis showed that orbital effects contribute to the global stabilization of the complexes studied. Furthermore, we have used Bader's theory of "atoms in molecules" to analyse and characterize the noncovalent interactions described herein from a charge-density perspective. Finally, several examples retrieved from the CSD are also included, highlighting the impact of these interactions in the solid state chemistry of Xe.

Intermolecular interactions in molecular crystals: what’s in a name?

Structure–property relationships are the key to modern crystal engineering, and for molecular crystals this requires both a thorough understanding of intermolecular interactions, and the subsequent use of this to create solids with desired properties. There has been a rapid increase in publications aimed at furthering this understanding, especially the importance of non-canonical interactions such as halogen, chalcogen, pnicogen, and tetrel bonds. Here we show how all of these interactions – and hydrogen bonds – can be readily understood through their common origin in the redistribution of electron density that results from chemical bonding. This redistribution is directly linked to the molecular electrostatic potential, to qualitative concepts such as electrostatic complementarity, and to the calculation of quantitative intermolecular interaction energies. Visualization of these energies, along with their electrostatic and dispersion components, sheds light on the architecture of molecular crystals, in turn providing a link to actual crystal properties.