Strong CH⋅⋅⋅Halide Hydrogen Bonds from 1,2,3‐Triazoles Quantified Using Pre‐Organized and Shape‐Persistent Triazolophanes

The structures associated with halide (F^?, Cl^?, Br^?) complexation inside CH hydrogen‐bonding macrocyclic receptors, called triazolophanes, are characterized using density functional theory (DFT). The associated binding energies in the gas and solution phases are evaluated. The ruffles in the empty triazolophane become smoothed‐out upon Cl^?‐ and Br^?‐ion binding directly into the middle of the cavity. The largely pre‐organized cavity morphs into an elliptical shape to facilitate shorter hydrogen bonds in the north and south regions and longer ones west and east. The smaller F^? ion sits in, and flattens‐out, only the north (or south) region. The 1,2,3‐triazoles show shorter CH???Cl^? contacts than for the phenylenes. Both Cl^? and Br^? show the same binding geometries but Cl^? has a larger binding energy consistent with its stronger Lewis basicity. Model triads were used to decompose the overall binding energy into those of its components. In the course of this triad analysis, anion polarization was identified and its contribution to the triad???Cl^? binding energy estimated. Consequently, the binding energies for the individual aryl units within the comparatively non‐polarized triazolophanes were estimated. The 1,2,3‐triazoles are twice as strong as the phenylenes thus contributing most of the interaction energy to Cl^?‐ion binding. Therefore, the 1,2,3‐triazoles appear to approach the hydrogen bond strengths of the NH donors of pyrrole units.     

Cooperativity between the Halogen Bond and the Hydrogen Bond in H3N⋅⋅⋅XY⋅⋅⋅HF Complexes (X,Y=F,Cl, Br)

Ab initio calculations are used to provide information on H₃N???XY???HF triads (X, Y=F, Cl, Br) each having a halogen bond and a hydrogen bond. The investigated triads include H₃N???Br₂‐HF, H₃N???Cl₂???HF, H₃N???BrCI???HF, H₃N???BrF???HF, and H₃N???ClF???HF. To understand the properties of the systems better, the corresponding dyads are also investigated. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads are studied at the MP2 level of theory with the 6‐311++G(d,p) basis set. Because the primary aim of this study is to examine cooperative effects, particular attention is given to parameters such as cooperative energies, many‐body interaction energies, and cooperativity factors. The cooperative energy ranges from ?1.45 to ?4.64 kcal mol^?1, the three‐body interaction energy from ?2.17 to ?6.71 kcal mol^?1, and the cooperativity factor from 1.27 to 4.35. These results indicate significant cooperativity between the halogen and hydrogen bonds in these complexes. This cooperativity is much greater than that between hydrogen bonds. The effect of a halogen bond on a hydrogen bond is more pronounced than that of a hydrogen bond on a halogen bond.     

The Role of Molecular Electrostatic Potentials in the Formation of a Halogen Bond in Furan⋅⋅⋅XY and Thiophene⋅⋅⋅XY Complexes

The halogen bonding of furan???XY and thiophene???XY (X=Cl, Br; Y=F, Cl, Br), involving σ‐ and π‐type interactions, was studied by using MP2 calculations and quantum theory of “atoms in molecules” (QTAIM) studies. The negative electrostatic potentials of furan and thiophene, as well as the most positive electrostatic potential (V_S,max) on the surface of the interacting X atom determined the geometries of the complexes. Linear relationships were found between interaction energy and V_S,max of the X atom, indicating that electrostatic interactions play an important role in these halogen‐bonding interactions. The halogen‐bonding interactions in furan???XY and thiophene???XY are weak, “closed‐shell” noncovalent interactions. The linear relationship of topological properties, energy properties, and the integration of interatomic surfaces versus V_S,max of atom X demonstrate the importance of the positive σ hole, as reflected by the computed V_S,max of atom X, in determining the topological properties of the halogen bonds.     

Torsional and Electronic Factors Control the C−H⋅⋅⋅O Interaction

The precise role of non‐conventional hydrogen bonds such as the C?H???O interaction in influencing the conformation of small molecules remains unresolved. Here we survey a series of β‐turn mimetics using X‐ray crystallography and NMR spectroscopy in conjunction with quantum calculation, and conclude that favourable torsional and electronic effects are important for the population of states with conformationally influential C?H???O interactions. Our results also highlight the challenge in attempting to deconvolute a myriad of interdependent noncovalent interactions in order to focus on the contribution of a single one. Within a small molecule that is designed to resemble the complexity of the environment within peptides and proteins, the interplay of different steric burdens, hydrogen‐acceptor/‐donor properties and rotational profiles illustrate why unambiguous conclusions based solely on NMR chemical shift data are extremely challenging to rationalize.     

Comparison of Hydrogen and Gold Bonding in [XHX]−, [XAuX]−, and Isoelectronic [NgHNg]+, [NgAuNg]+ (X=Halogen,Ng=Noble Gas)

Quantum chemical calculations at the MP2/aug‐cc‐pVTZ and CCSD(T)/aug‐cc‐pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D_∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX]^? and [XAuX]^? decrease from [FHF]^? to [IHI]^? and from [FAuF]^? to [IAuI]^?. The BDEs of the noble gas compounds [NgHNg]⁺ and [NgAuNg]⁺ become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI]^?, in which case the gold atom has a small negative charge of ?0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg]⁺ and to the halogen atoms in [XHX]^?. The covalent character of the bonding in the gold systems [NgAuNg]⁺ and [XAuX]^? is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X^?→H⁺←X^? or Ng→H⁺←Ng while the heavier homologues exhibit electron sharing through two‐electron, three‐center bonds. Dative bonds X^?→Au⁺←X^? and Ng→Au⁺←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron‐shared bonds.     

Interactions between temozolomide and guanine and its S and Se‐substituted analogues

Temozolomide was paired with guanine, 6‐selenoguanine, and 6‐thioguanine, as well as the SH tautomer of the latter. The potential energy surface of each heterodimer was searched for all minima, using Dispersion‐Corrected Density Functional Theory and MP2 methods. Among the dozens of minima, three categories were observed. Stacked geometries place the aromatic systems of the two molecules parallel to one another, while the two systems are roughly perpendicular to one another in a second category. Also found are coplanar structures held together by H‐bonds. Dispersion proves to be a dominating attractive force for the stacked structures, less so for perpendicular, and smallest for the coplanar dimers. Geometries and energetics are relatively insensitive to S and Se substitution, but tautomerization reverses relative stabilities of different geometries.     

CH⋅⋅⋅N Hydrogen‐Bonding Interaction in 7‐Azaindole:CHX3 (X=F,Cl) Complexes

The C?H???Y (Y=hydrogen‐bond acceptor) interactions are somewhat unconventional in the context of hydrogen‐bonding interactions. Typical C?H stretching frequency shifts in the hydrogen‐bond donor C?H group are not only small, that is, of the order of a few tens of cm^?1, but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen‐bond acceptor. In this work we examine the C?H???N interaction in complexes of 7‐azaindole with CHCl₃ and CHF₃ that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra‐red (FDIR) method. Although the hydrogen‐bond acceptor, 7‐azaindole, has multiple sites of interaction, it is found that the C?H???N hydrogen‐bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S₁ state than in the S₀ state. The C?H stretching frequency is found to be red shifted by 82 cm^?1 in the CHCl₃ complex, which is the largest redshift reported so far in gas‐phase investigations of 1:1 haloform complexes with various substrates. In the CHF₃ complex the observed C?H frequency is blue shifted by 4 cm^?1. This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm^?1. This discrepancy is analogous to that reported for the pyridine‐CHF₃ complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A­ 2005 , 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C?H bending and stretching modes. The dissociation energies, (D₀), of the CHCl₃ and CHF₃ complexes are computed (MP2/aug‐cc‐pVDZ level) as 6.46 and 5.06 kcal mol^?1, respectively.     

Influence of the Li⋅⋅⋅π Interaction on the H/X⋅⋅⋅π Interactions in HOLi⋅⋅⋅C6H6⋅⋅⋅HOX/XOH (X=F,Cl, Br,I) Complexes

The influences of the Li???π interaction of C₆H₆???LiOH on the H???π interaction of C₆H₆???HOX (X=F, Cl, Br, I) and the X???π interaction of C₆H₆???XOH (X=Cl, Br, I) are investigated by means of full electronic second‐order Møller–Plesset perturbation theory calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The binding energies, binding distances, infrared vibrational frequencies, and electron densities at the bond critical points (BCPs) of the hydrogen bonds and halogen bonds prove that the addition of the Li???π interaction to benzene weakens the H???π and X???π interactions. The influences of the Li???π interaction on H???π interactions are greater than those on X???π interactions; the influences of the H???π interactions on the Li???π interaction are greater than X???π interactions on Li???π interaction. The greater the influence of Li???π interaction on H/X???π interactions, the greater the influences of H/X???π interactions on Li???π interaction. QTAIM studies show that the intermolecular interactions of C₆H₆???HOX and C₆H₆???XOH are mainly of the π type. The electron densities at the BCPs of hydrogen bonds and halogen bonds decrease on going from bimolecular complexes to termolecular complexes, and the π‐electron densities at the BCPs show the same pattern. Natural bond orbital analyses show that the Li???π interaction reduces electron transfer from C₆H₆ to HOX and XOH.     

Influence of the Homopolar Dihydrogen Bonding CH⋅⋅⋅HC on Coordination Geometry: Experimental and Theoretical Studies

The reaction of the N‐thiophosphorylated thiourea (HOCH₂)(Me)₂CNHC(S)NHP(S)(OiPr)₂ (HL), deprotonated by the thiophosphorylamide group, with NiCl₂ leads to green needles of the pseudotetrahedral complex [Ni(L‐1,5‐S,S′)₂] ? 0.5 (n‐C₆H₁₄) or pale green blocks of the trans square‐planar complex trans‐[Ni(L‐1,5‐S,S′)₂]. The former complex is stabilized by homopolar dihydrogen C?H???H?C interactions formed by n‐hexane solvent molecules with the [Ni(L‐1,5‐S,S′)₂] unit. Furthermore, the dispersion‐dominated C?H??? H?C interactions are, together with other noncovalent interactions (C?H???N, C?H???Ni, C?H???S), responsible for pseudotetrahedral coordination around the Ni^II center in [Ni(L ‐1,5‐S,S′)₂] ? 0.5 (n‐C₆H₁₄).     

Theoretical Investigation on the Effect of Different Nitrogen Donors on Intramolecular Se⋅⋅⋅N Interactions

The effect of different donor nitrogen atoms on the strength and nature of intramolecular Se ??? N interactions is evaluated for organoselenium compounds having N,N‐dimethylaminomethyl (dime), oxazoline (oxa) and pyridyl (py) substituents. Quantum chemical calculations on three series of compounds [2‐(dime)C₆H₄SeX ( 1 a – g ), 2‐(oxa)C₆H₄SeX ( 2 a – g ), 2‐(py)C₆H₄SeX ( 3 a – g ); X=Cl, Br, OH, CN, SPh, SePh, CH₃] at the B3LYP/6‐31G(d) level show that the stability of different conformers depends on the strength of intramolecular nonbonded Se ??? N interactions. Natural bond orbital (NBO), NBO deletion and atoms in molecules (AIM) analyses suggest that the nature of the Se ??? N interaction is predominantly covalent and involves nN→σ*_{Se? X} orbital interaction. In the three series of compounds, the strength of the Se ??? N interaction decreases in the order 3 > 2 > 1 for a particular X, and it decreases in the order Cl>Br>OH>SPh≈CN≈SePh>CH₃ for all the three series 1 – 3 . However, further analyses suggest that the differences in strength of Se ??? N interaction in 1 – 3 is predominantly determined by the distance between the Se and N atoms, which in turn is an outcome of specific structures of 1 , 2 and 3 , and the nature of the donor nitrogen atoms involved has very little effect on the strength of Se ??? N interaction. It is also observed that Se ??? N interaction becomes stronger in polar solvents such as CHCl₃, as indicated by the shorter r_{Se ??? N} and higher E_{Se ??? N} values in CHCl₃ compared to those observed in the gas phase.     

Walking Metals in d8⋅⋅⋅d8 Hetero‐bimetallic Complexes: An Original Dynamic Phenomenon

In the course of our investigations on polymetallic complexes derived from 1,3‐bis(thiophosphinoyl)indene (Ind(Ph₂P?S)₂), we observed original fluxional behavior and report herein a joint experimental/computational study of this dynamic process. Starting from the indenylidene chloropalladate species [Pd{Ind(Ph₂P?S)₂}Cl]^? ( 1 ), the new Pd^II???Rh^I hetero‐bimetallic pincer complex [PdCl{Ind(Ph₂P?S)₂}Rh(nbd)] ( 2 ; nbd=2,5‐norbornadiene) was prepared. X‐ray crystallography and DFT calculations substantiate the presence of a d⁸???d⁸ interaction. According to multinuclear variable‐temperature NMR spectroscopic experiments, the pendant {Rh(nbd)} fragment of 2 readily shifts in solution at room temperature between the two edges of the SCS tridentate ligand. To assess the role of the pincer‐based polymetallic structure on this fluxional behavior, the related monometallic Rh complex [Rh{IndH(Ph₂P?S)₂}(nbd)] ( 3 ) was prepared. No evidence for a metal shift was observed in that case, even at high temperature, thus indicating that inplane pincer coordination to the Pd center plays a crucial role. The previously described Pd^II???Ir^I bimetallic complex 4 exhibited fluxional behavior in solution, but with a significantly higher activation barrier than 2 . This finding demonstrates the generality of this metal‐shift process and the strong influence of the involved metal centers on the associated activation barrier. DFT calculations were performed to shed light onto the mechanism of such metal‐shift processes and to identify the factors that influence the associated activation barriers. Significantly different pathways were found for bimetallic complexes 2 and 4 on one hand and the monometallic complex 3 on the other hand. The corresponding activation barriers predicted computationally are in very good agreement with the experimental observations.     

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].     

The Cage Structure of IndanCHF3 is Based on the Cooperative Effects of CH⋅⋅⋅π and CH⋅⋅⋅F Weak Hydrogen Bonds

The structural and energetic features of the C?H???π interaction and the internal dynamics of the CHF₃ group change drastically in going from benzene?CHF₃ to indan?CHF₃, according to the analysis of the rotational spectrum of the latter complex generated in a supersonic expansion.     

Spectroscopic and Ab Initio Investigation of C−H⋅⋅⋅N Hydrogen‐Bonded Complexes of Fluorophenylacetylenes: Frequency Shifts and Correlations

The C?H???N hydrogen‐bonded complexes of several fluorophenyacetylenes with ammonia and methylamine were characterized by a redshift in the acetylenic C?H stretching vibration of the phenylacetylene moiety. These redshifts were linearly correlated with the stabilization energies calculated at the CCSD(T)/CBS//MP2‐aug‐cc‐pVDZ level. Analysis of various components of the interaction energy indicated that the observed redshifts were weakly correlated with the electrostatic component. The weaker linear correlation between the frequency shifts and the electrostatic component between two data sets can perhaps be attributed to the marginal differences in the Stark tuning rate and zero‐field shifts. The induction and exchange‐repulsion components were linearly correlated. However, the dispersion component depends on the nature of the hydrogen‐bond acceptor and shows a quantum jump when the hydrogen‐bond acceptor is changed from ammonia to methylamine. The observed linear correlation between the redshifts in the C?H stretching frequencies and the total stabilization energies is due to mutual cancellation of deviations from linearity between various components.     

Behavior of Halogen Bonds of the Y−X⋅⋅⋅π Type (X,Y=F,Cl, Br,I) in the Benzene π System,Elucidated by Using a Quantum Theory of Atoms in Molecules Dual‐Functional Analysis

The nature of halogen bonds of the Y?X‐?‐π(C₆H₆) type (X, Y=F, Cl, Br, and I) have been elucidated by using the quantum theory of atoms in molecules (QTAIM) dual‐functional analysis (QTAIM‐DFA), which we proposed recently. Asterisks (?) emphasize the presence of bond‐critical points (BCPs) in the interactions in question. Total electron energy densities, H_b( r _c), are plotted versus H_b( r _c)?V_b( r _c)/2 [=(?²/8m)?²ρ_b( r _c)] for the interactions in QTAIM‐DFA, in which V_b( r _c) are potential energy densities at the BCPs. Data for perturbed structures around fully optimized structures were used for the plots, in addition to those of the fully optimized ones. The plots were analyzed by using the polar (R, θ) coordinate for the data of fully optimized structures with (θ_p, κ_p) for those that contained the perturbed structures; θ_p corresponds to the tangent line of the plot and κ_p is the curvature. Whereas (R, θ) corresponds to the static nature, (θ_p, κ_p) represents the dynamic nature of the interactions. All interactions in Y?X‐?‐π(C₆H₆) are classified by pure closed‐shell interactions and characterized to have vdW nature, except for Y?I‐?‐π(C₆H₆) (Y=F, Cl, Br) and F?Br‐?‐π(C₆H₆), which have typical hydrogen‐bond nature without covalency. I?I‐?‐π(C₆H₆) has a borderline nature between the two. Y?F‐?‐π(C₆H₆) (Y=Br, I) were optimized as bent forms, in which Y‐?‐π interactions were detected. The Y‐?‐π interactions in the bent forms are predicted to be substantially weaker than those in the linear F?Y‐?‐π(C₆H₆) forms.