Theoretical and Crystallographic Study of Lead(IV) Tetrel Bonding Interactions

The ability of tetrahedral lead(IV) to establish noncovalent σ-hole tetrel bonding interactions with electron-rich atoms (ElRs; anions and Lewis bases) has been studied at the PBE0-D3/def2-TZVPD level of theory. An analysis of the Cambridge Crystallographic Database (CSD), which is a convenient storehouse of geometric information, has been performed to investigate the existence of tetrel bonding interactions involving tetrahedral lead(IV) derivatives. Several examples of tetrel bonding interactions that are crucial in crystal packing, ranging from 0D to 2D assemblies, have been found. In addition to the energetic and theoretical study of several XPb(CH₃)₃⋅⋅⋅ElR complexes (X=F, CN, CF₃, and CH₃), Bader's theory of atoms in molecules has also been used to further analyze and characterize the noncovalent interactions described herein.     

Nature of Noncovalent Carbon‐Bonding Interactions Derived from Experimental Charge‐Density Analysis

In an effort to better understand the nature of noncovalent carbon‐bonding interactions, we undertook accurate high‐resolution X‐ray diffraction analysis of single crystals of 1,1,2,2‐tetracyanocyclopropane. We selected this compound to study the fundamental characteristics of carbon‐bonding interactions, because it provides accessible σ holes. The study required extremely accurate experimental diffraction data, because the interaction of interest is weak. The electron‐density distribution around the carbon nuclei, as shown by the experimental maps of the electrophilic bowl defined by a (CN)₂C?C(CN)₂ unit, was assigned as the origin of the interaction. This fact was also evidenced by plotting the Δ²ρ(r) distribution. Taken together, the obtained results clearly indicate that noncovalent carbon bonding can be explained as an interaction between confronted oppositely polarized regions. The interaction is, thus electrophilic–nucleophilic (electrostatic) in nature and unambiguously considered as attractive.     

Design of Lead(II) Metal–Organic Frameworks Based on Covalent and Tetrel Bonding

Three solid materials, [Pb( HL )(SCN)₂] ? CH₃OH ( 1 ), [Pb( HL )(SCN)₂] ( 2 ), and [Pb( L )(SCN)]_n ( 3 ), were obtained from Pb(SCN)₂ and an unsymmetrical bis‐pyridyl hydrazone ligand that can act both as a bridging and as a chelating ligand. In all three the lead center is hemidirectionally coordinated and is thus sterically optimal for participation in tetrel bonding. In the crystal structures of all three compounds, the lead atoms participate in short contacts with thiocyanate sulfur or nitrogen atoms. These contacts are shorter than the sums of the van der Waals radii (3.04–3.47 Å for Pb ??? S and 3.54 Å for Pb ??? N) and interconnect the covalently bonded units (monomers, dimers, and 2D polymers) into supramolecular assemblies (chains and 3D structures). DFT calculations showed these contacts to be tetrel bonds of considerable energy (6.5–10.5 kcal mol^?1 for Pb ??? S and 16.5 kcal mol^?1 for Pb ??? N). A survey of structures in the CSD showed that similar contacts often appear in crystals of Pb^II complexes with regular geometries, which leads to the conclusion that tetrel bonding plays a significant role in the supramolecular chemistry of Pb^II.     

DFT and IsoStar Analyses to Assess the Utility of σ- and π-Hole Interactions for Crystal Engineering

The interpretation of 36 charge neutral ‘contact pairs’ from the IsoStar database was supported by DFT calculations of model molecules 1 – 12 , and bimolecular adducts thereof. The ‘central groups’ are σ-hole donors (H₂O and aromatic C−I), π-hole donors (R−C(O)Me, R−NO₂ and R−C₆F₅) and for comparison R−C₆H₅ (R=any group or atom). The ‘contact groups’ are hydrogen bond donors X−H (X=N, O, S, or R₂C, or R₃C) and lone-pair containing fragments (R₃C−F, R−C≡N and R₂C=O). Nearly all the IsoStar distributions follow expectations based on the electrostatic potential of the ‘central-’ and ‘contact group’. Interaction energies (ΔE^BSSE) are dominated by electrostatics (particularly between two polarized molecules) or dispersion (especially in case of large contact area). Orbital interactions never dominate, but could be significant (∼30 %) and of the n/π→σ*/π* kind. The largest degree of directionality in the IsoStar plots was typically observed for adducts more stable than ΔE^BSSE≈−4 kcal⋅mol⁻¹, which can be seen as a benchmark-value for the utility of an interaction in crystal engineering. This benchmark could be met with all the σ- and π-hole donors studied.     

Halogen‐Bonding Interactions with π Systems: CCSD(T), MP2, and DFT Calculations

Halogen bonding is a noncovalent interaction between a halogen atom and a nucleophilic site. Interactions involving the π electrons of aromatic rings have received, up to now, little attention, despite the large number of systems in which they are present. We report binding energies of the interaction between either NCX or PhX (X=F, Cl, Br, I) and the aromatic benzene system as determined with the coupled cluster with perturbative triple excitations method [CCSD(T)] extrapolated at the complete basis set limit. Results are compared with those obtained by Møller–Plesset perturbation theory to second order (MP2) and density functional theory (DFT) calculations by using some of the most common functionals. Results show the important role of DFT in studying this interaction.     

Crystal Engineering with Multipoint Halogen Bonding: Double Two-Point Donors and Acceptors at Work

The combination of singly or doubly bidentate halogen-bond donors with double bidentate acceptors was investigated as a supramolecular synthon in crystal engineering. The crystal topologies obtained feature novel halogen-bonding motifs like double two-point recognition and infinite chains or networks based on two-point interactions. Induced conformational changes in the double bidentate halogen-bond donors could be exploited to obtain different 1D and 2D networks. All solid-state studies were accompanied by DFT calculations to predict and rationalize the outcome.     

Supramolecular Assembly of Metal Complexes by (Aryl)I⋅⋅⋅d [PtII] Halogen Bonds

The theoretical data for the half-lantern complexes [{Pt( )(μ- )}₂] [ 1 – 3 ; is cyclometalated 2-Ph-benzothiazole; is 2-SH-pyridine ( 1 ), 2-SH-benzoxazole ( 2 ), 2-SH-tetrafluorobenzothiazole ( 3 )] indicate that the Pt ⋅⋅⋅ Pt orbital interaction increases the nucleophilicity of the outer d orbitals to provide assembly with electrophilic species. Complexes 1 – 3 were co-crystallized with bifunctional halogen bonding (XB) donors to give adducts ( 1 – 3 )₂ ⋅ (1,4-diiodotetrafluorobenzene) and infinite polymeric [ 1⋅ 1,1′-diiodoperfluorodiphenyl]_n. X-ray crystallography revealed that the supramolecular assembly is achieved through (Aryl)I ⋅⋅⋅ d [Pt^II] XBs between iodine σ-holes and lone pairs of the positively charged (Pt^II)₂ centers acting as nucleophilic sites. The polymer includes a curved linear chain ⋅⋅⋅ Pt₂ ⋅⋅⋅ I(arene^F)I ⋅⋅⋅ Pt₂ ⋅⋅⋅ involving XB between iodine atoms of the perfluoroarene linkers and (Pt^II)₂ moieties. The ¹⁹⁵Pt NMR, UV/Vis, and CV studies indicate that XB is preserved in CH(D)₂Cl₂ solutions.     

Attractive Dispersion Interactions Versus Steric Repulsion of tert‐Butyl groups in the Crystal Packing of a D3h‐Symmetric Tris(quinoxalinophenanthrophenazine)

The crystalline packing of a π‐extended D_3h‐symmetric triptycene reveals a particular π stacking motif with an almost‐eclipsed arrangement of adjacent π planes despite the steric repulsion of tert‐butyl substituents. Four model compounds were analyzed by using single‐crystal X‐ray diffraction and theoretical calculations to study the influence of dispersion interactions of molecular parts and understand the relationship between the molecular structure and this unique packing motif.     

Theoretical Study on Cooperativity Effects between Anion–π and Halogen‐Bonding Interactions

This article analyzes the interplay between lone pair–π (lp–π) or anion–π interactions and halogen‐bonding interactions. Interesting cooperativity effects are observed when lp/anion–π and halogen‐bonding interactions coexist in the same complex, and they are found even in systems in which the distance between the anion and halogen‐bond donor molecule is longer than 9 Å. These effects are studied theoretically in terms of energetic and geometric features of the complexes, which are computed by ab initio methods. Bader′s theory of “atoms in molecules” is used to characterize the interactions and to analyze their strengthening or weakening depending upon the variation of charge density at critical points. The physical nature of the interactions and cooperativity effects are studied by means of molecular interaction potential with polarization partition scheme. By taking advantage of all aforementioned computational methods, the present study examines how these interactions mutually influence each other. Additionally, experimental evidence for such interactions is obtained from the Cambridge Structural Database (CSD).     

Crystal Engineering Using Polyiodide Halogen and Chalcogen Bonding to Isolate the Phenothiazinium Radical Cation and Its Rare Dimer, 10-(3-Phenothiazinylidene)phenothiazinium

Utilizing facile one-electron oxidation of 10H-phenothiazine by molecular diiodine, the solid-state structure of the 10H-phenothiazinium radical cation was obtained in three cation:iodide ratios, as well as its THF and acetone solvates. Oxidation of 10H-phenothiazine with molecular diiodine in DMSO or DMF provided the structure of the radical coupling product 10-(3-phenothiazinyldene)phenothiazinium, which has not been crystallographically characterized to date. The radical cations were balanced by a mixture (I₇)⁻, (I₅)⁻, (I₃)⁻, and I⁻ anions, where a variety of chalcogen, halogen, and hydrogen bonding interactions stabilize the structures to reveal these interesting cationic species.     

The Nature of Halogen⋅⋅⋅Halogen Interactions: A Model Derived from Experimental Charge‐Density Analysis

Slightly attractive : The attractive and anisotropic nature of the Cl???Cl interaction in C₆Cl₆ is experimentally demonstrated from an expansion of the electron density ρ( r ) around the chlorine nuclei. The interaction is explained in a model in which there is a bonding attraction involving electron‐deficient (see picture, blue) and electron‐rich (red) regions of adjacent Cl atoms.

     

Rational Modification of the Hierarchy of Intermolecular Interactions in Molecular Crystal Structures by Using Tunable Halogen Bonds

A family of 16 isomolecular salts (3‐XpyH)₂[MX′₄] (3‐XpyH=3‐halopyridinium; M=Co, Zn; X=(F), Cl, Br, (I); X′=Cl, Br, I) each containing rigid organic cations and tetrahedral halometallate anions has been prepared and characterized by X‐ray single crystal and/or powder diffraction. Their crystal structures reflect the competition and cooperation between non‐covalent interactions: N? H???X′? M hydrogen bonds, C? X???X′? M halogen bonds and π–π stacking. The latter are essentially unchanged in strength across the series, but both halogen bonds and hydrogen bonds are modified in strength upon changing the halogens involved. Changing the organic halogen (X) from F to I strengthens the C? X???X′? M halogen bonds, whereas an analogous change of the inorganic halogen (X′) weakens both halogen bonds and N? H???X′? M hydrogen bonds. By so tuning the strength of the putative halogen bonds from repulsive to weak to moderately strong attractive interactions, the hierarchy of the interactions has been modified rationally leading to systematic changes in crystal packing. Three classes of crystal structure are obtained. In type A (C? F???X′? M) halogen bonds are absent. The structure is directed by N? H???X′? M hydrogen bonds and π‐stacking interactions. In type B structures, involving small organic halogens (X) and large inorganic halogens (X′), long (weak) C? X???X′? M interactions are observed with type I halogen–halogen interaction geometries (C? X???X′ ≈ X???X′? M ≈155°), but hydrogen bonds still dominate. Thus, minor but quite significant perturbations from the type A structure arise. In type C, involving larger organic halogens (X) and smaller inorganic halogens (X′), stronger halogen bonds are formed with a type II halogen–halogen interaction geometry (C? X???X′ ≈180°; X???X′? M ≈110°) that is electrostatically attractive. The halogen bonds play a major role alongside hydrogen bonds in directing the type C structures, which as a result are quite different from type A and B.     

Exploring Diradical Chemistry: A Carbon‐Centered Radical May Act as either an Anion or Electrophile through an Orbital Isomer

Diradical intermediates, formed by thermolysis of alkynylcyclobutenones, can display radical, anion, or electrophilic character because of the existence of an orbital isomer with zwitterionic and cyclohexatrienone character. Our realization that water, alcohols, and certain substituents can induce the switch provides new opportunities in synthesis. For example, it can be used to shut down radical pathways and to give access to aryl carbonates and tetrasubstituted quinones.     

Enhanced Aerogen–π Interaction by a Cation–π Force

The interaction between a noble gas atom and an aromatic π‐electron system, which mainly originates from the London dispersion force, is very weak and has not attracted enough attention yet. Herein, we reported a type of notably enhanced aerogen–π interaction between cation–π systems and noble gas atoms. The binding strength of a divalent cation–π system with a xenon atom is comparable to a moderate hydrogen bond (up to ca. 7 kcal mol^?1), whereas krypton and argon atoms produce slightly weaker interactions. Energy‐decomposition analysis reveals that the induction interaction is responsible for the stabilization of divalent cation–π?Xe species besides the dispersion interaction. Our results might be helpful to increase the understanding of some unsolved mysteries of aerogens.     

Host–Guest Chemistry: Oxoanion Recognition Based on Combined Charge‐Assisted C−H or Halogen‐Bonding Interactions and Anion⋅⋅⋅Anion Interactions Mediated by Hydrogen Bonds

Several bis‐triazolium‐based receptors have been synthesized and their anion‐recognition capabilities have been studied. The central chiral 1,1′‐bi‐2‐naphthol (BINOL) core features either two aryl or ferrocenyl end‐capped side arms with central halogen‐ or hydrogen‐bonding triazolium receptors. NMR spectroscopic data indicate the simultaneous occurrence of several charge‐assisted aliphatic and heteroaromatic C?H noncovalent interactions and combinations of C?H hydrogen and halogen bonding. The receptors are able to selectively interact with HP₂O₇^3?, H₂PO₄^?, and SO₄^2? anions, and the value of the association constant follows the sequence: HP₂O₇^3?>SO₄^2?>H₂PO₄^?. The ferrocenyl end‐capped 7²⁺?2 BF₄ ^? receptor allows recognition and differentiation of H₂PO₄^? and HP₂O₇^3? anions by using different channels: H₂PO₄^? is selectively detected through absorption and emission methods and HP₂O₇^3? by using electrochemical techniques. Significant structural results are the observation of an anion???anion interaction in the solid state (2:2 complex, 6²⁺? [ H₂P₂O₇ ] ^2? ), and a short C?I???O contact is observed in the structure of the complex [ 8 ²⁺][SO₄]_0.5[BF₄].     

Syntheses and Crystal Structures of Two New Supramolecular Architectures Constructed from the Main Group Metal Chloride and Protonated 2-（3-Pyridyl）benzimidazole

The reactions of SbCl3 and HgCl2 with 2-(3-pyridyl)benzimidazole (PyBIm) in solution acidified with HCl have been investigated. The PyBIm ligands are protonated into 2-(3-pyridinio)benzimidazolium (H2PyBIm) cations and the corresponding metal ions are bonded with chloride atoms into coordination anions, forming two new coordination compounds, namely, (H2PyBIm)(SbCl5) 1 and (H2PyBIm)2(Hg2Cl8) 2. Both compounds were characterized by X-ray crystallography. Crystal data for 1: triclinic, space group P1 with a = 5.7030(7), b = 9.0625(11), c = 16.5929(18) , α = 91.808(7), β = 93.234(6), γ = 99.216(7)o, C12H11N3SbCl5, Mr = 496.24, V = 844.44(17) 3, Z = 2, Dc = 1.952 g/cm3, μ(MoKα) = 2.419 mm-1, F(000) = 480, the final R = 0.0496 and wR = 0.1382 for 3433 observed reflections (I > 2σ(I)). Crystal data for 2: monoclinic, space group P21/c with a = 7.8061(5), b = 15.8127(9), c = 12.2435(9) , β = 91.955(4)o, C24H22N6Hg2Cl8, Mr = 1079.26, V = 1510.40(17) 3, Z = 2, Dc = 2.373 g/cm3, μ(MoKα) = 10.889 mm-1, F(000) = 1008, the final R = 0.0293 and wR = 0.0562 for 2854 observed reflections (I > 2σ(I)). X-ray diffraction analysis reveals that the antimony(III) is five-coordinated, exhibiting a slightly distorted square-pyramidal coordination geometry; while in 2, a dimeric [Hg2Cl8]4-anion consists of two trigonal bipyramids sharing two common edges. The organic cations and coordination anions are connected into a one-dimensional belt and a two-dimensional sheet through N-H···Cl hydrogen bonding interactions in compounds 1 and 2, respectively; both are further aggregated into 3D frameworks by strong π-π contacts.