Structure of Amido Pyridinium Betaines: Persistent Intermolecular C−H⋅⋅⋅N Hydrogen Bonding in Solution

A hydrogen bond of the type C?H???X (X=O or N) is known to influence the structure and function of chemical and biological systems in solution. C?H???O hydrogen bonding in solution has been extensively studied, both experimentally and computationally, whereas the equivalent thermodynamic parameters have not been enumerated experimentally for C?H???N hydrogen bonds. This is, in part, due to the lack of systems that exhibit persistent C?H???N hydrogen bonds in solution. Herein, a class of molecule based on a biologically active norharman motif that exhibits unsupported intermolecular C?H???N hydrogen bonds in solution has been described. A pairwise interaction leads to dimerisation to give bond strengths of about 7 kJ mol^?1 per hydrogen bond, which is similar to chemically and biologically relevant C?H???O hydrogen bonding. The experimental data is supported by computational work, which provides additional insight into the hydrogen bonding by consideration of electrostatic and orbital interactions and allowed a comparison between calculated and extrapolated NMR chemical shifts.     

Sugar–Edge Interactions in a DNA–RNA G‐Quadruplex: Evidence of Sequential C−H⋅⋅⋅O Hydrogen Bonds Contributing to RNA Quadruplex Folding

DNA G‐quadruplexes were systematically modified by single riboguanosine (rG) substitutions at anti‐dG positions. Circular dichroism and NMR experiments confirmed the conservation of the native quadruplex topology for most of the DNA–RNA hybrid structures. Changes in the C8 NMR chemical shift of guanosines following rG substitution at their 3′‐side within the quadruplex core strongly suggest the presence of C8?H???O hydrogen‐bonding interactions with the O2′ position of the C2′‐endo ribonucleotide. A geometric analysis of reported high‐resolution structures indicates that such interactions are a more general feature in RNA quadruplexes and may contribute to the observed preference for parallel topologies.     

Self‐Assembly and (Hydro)gelation Triggered by Cooperative π–π and Unconventional CH⋅⋅⋅X Hydrogen Bonding Interactions

Weak C? H???X hydrogen bonds are important stabilizing forces in crystal engineering and anion recognition in solution. In contrast, their quantitative influence on the stabilization of supramolecular polymers or gels has thus far remained unexplored. Herein, we report an oligophenyleneethynylene (OPE)‐based amphiphilic Pt^II complex that forms supramolecular polymeric structures in aqueous and polar media driven by π–π and different weak C‐H???X (X=Cl, O) interactions involving chlorine atoms attached to the Pt^II centers as well as oxygen atoms and polarized methylene groups belonging to the peripheral glycol chains. A collection of experimental techniques (UV/Vis, 1D and 2D NMR, DLS, AFM, SEM, and X‐Ray diffraction) demonstrate that the interplay between different weak noncovalent interactions leads to the cooperative formation of self‐assembled structures of high aspect ratio and gels in which the molecular arrangement is maintained in the crystalline state.     

OH⋅⋅⋅N and CH⋅⋅⋅O Hydrogen Bonds Control Hydration of Pivotal Tropane Alkaloids: Tropinone⋅⋅⋅H2O Complex

The effect of monohydration in equatorial/axial isomerism of the common motif of tropane alkaloids is investigated in a supersonic expansion by using Fourier‐transform microwave spectroscopy. The rotational spectrum reveals the equatorial isomer as the dominant species in the tropinone???H₂O complex. The monohydrated complex is stabilized primarily by a moderate O?H???N hydrogen bond. In addition, two C?H???O weak hydrogen bonds also support this structure, blocking the water molecule and avoiding any molecular dynamics in the complex. The water molecule acts as proton donor and chooses the ternary amine group over the carbonyl group as a proton acceptor. The experimental work is supported by theoretical calculations; the accuracy of the B3LYP, M06‐2X, and MP2 methods is also discussed.     

Cation–Cation Pairing by NCH⋅⋅⋅O Hydrogen Bonds

The pairing of ions of opposite charge is a fundamental principle in chemistry, and is widely applied in synthesis and catalysis. In contrast, cation–cation association remains an elusive concept, lacking in supporting experimental evidence. While studying the structure and properties of 4‐oxopiperidinium salts [OC₅H₈NH₂]X for a series of anions X^? of decreasing basicity, we observed a gradual self‐association of the cations, concluding in the formation of an isolated dicationic pair. In 4‐oxopiperidinium bis(trifluoromethylsulfonyl)amide, the cations are linked by N? H???O?C hydrogen bonds to form chains, flanked by hydrogen bonds to the anions. In the tetra(perfluoro‐tert‐butoxy)aluminate salt, the anions are fully separated from the cations, and the cations associate pairwise by N? C? H???O?C hydrogen bonds. The compounds represent the first genuine examples of self‐association of simple organic cations based merely on hydrogen bonding as evidenced by X‐ray structure analysis, and provide a paradigm for an extension of this class of compounds.     

Probing the CH⋅⋅⋅π Weak Hydrogen Bond in Anesthetic Binding: The Sevoflurane–Benzene Cluster

Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi‐isotopic broadband rotational spectra. This heterodimer is held together by a primary C? H???π hydrogen bond, assisted by multiple weak C? H???F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C? H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum.     

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.     

Supramolecular Cl⋅⋅⋅H and O⋅⋅⋅H Interactions in Self‐Assembled 1,5‐Dichloroanthraquinone Layers on Au(111)

The role of halogen bonds in self‐assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5‐dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl???H and O???H hydrogen bonds with marginal contributions from Cl‐related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br‐ and I‐related halogen bonds, Cl‐related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.     

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 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).     

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

A study of the strong N?X???^?O?N⁺ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N‐halosaccharins and pyridine N‐oxides (PyNO). DFT calculations were used to investigate the X???O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X???O XBs. The XB interaction energies vary from ?47.5 to ?120.3 kJ mol^?1; the strongest N?I???^?O?N⁺ XBs approaching those of 3‐center‐4‐electron [N?I?N]⁺ halogen‐bonded systems (ca. 160 kJ mol^?1). ¹H NMR association constants (K_XB) determined in CDCl₃ and [D₆]acetone vary from 2.0×10⁰ to >10⁸ m ^?1 and correlate well with the calculated donor×acceptor complexation enthalpies found between ?38.4 and ?77.5 kJ mol^?1. In X‐ray crystal structures, the N‐iodosaccharin‐PyNO complexes manifest short interaction ratios (R_XB) between 0.65–0.67 for the N?I???^?O?N⁺ halogen bond.     

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.     

Interplay between Metal⋅⋅⋅π Interactions and Hydrogen Bonds: Some Unusual Synergetic Effects of Coinage Metals and Substituents

The ternary systems of C₂H₄ (C₂H₂ or C₆H₆)‐MCN‐HF (M=Cu, Ag, Au) and the respective binary systems were investigated to study the interplay between metal???π interactions and hydrogen bonds. The metal???π interactions in C₂H₄‐MCN become stronger with the irregular order Ag<Cu<Au, while the hydrogen bonds in MCN‐HF become weaker following the same order. The metal???π interactions are weakened as the H atoms in the π system are replaced with electron‐withdrawing groups and enhanced by electron‐donating groups. Type 1 of these ternary systems, in which MCN acts as Lewis base and acid simultaneously, is more stable than type 2, in which C₂H₄ acts as a double Lewis base. Negative cooperativity is present in type 2 ternary systems with a weakening of the metal???π interactions and the hydrogen bonds. Positive cooperativity is found in type 1 ternary systems with an enhancement of the metal???π interactions and the hydrogen bonds, except for C₂(CN)₄‐AuCN‐HF‐1. The weaker metal???π interaction in C₆H₆‐AuCN has a greater enhancing effect on the hydrogen bond in AuCN‐HF than those in C₂H₄‐AuCN and C₂H₂‐AuCN. These synergetic effects were analyzed with the natural bond orbital and energy decomposition.     

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.     

A Platinum(II) Terpyridine Metallogel with an L‐Valine‐Modified Alkynyl Ligand: Interplay of Pt⋅⋅⋅Pt, π–π and Hydrogen‐Bonding Interactions

A series of platinum(II) terpyridine complexes with L ‐valine‐modified alkynyl ligands has been synthesized. A complex with an unsubstituted terpyridine and one valine unit on the alkynyl is shown to be capable of gel formation, which is in sharp contrast to the gelation properties of the corresponding organic counterparts. Upon sol–gel transition, a drastic color change from yellow to red is observed, which is indicative of the involvement of Pt ??? Pt interactions. Through the concentration‐ and temperature‐dependent UV/Vis absorption, emission, circular dichroism, and ¹H NMR studies, the contribution of hydrogen bonding, Pt ??? Pt and π–π stacking interactions as driving forces for gelation have been established, and the importance of maintaining a delicate balance between different intermolecular forces has also been illustrated.     

Luminescent Benzoquinolate‐Isocyanide Platinum(II) Complexes: Effect of Pt⋅⋅⋅Pt and π⋅⋅⋅π Interactions on their Photophysical Properties

The neutral compounds [Pt(bzq)(CN)(CNR)] (R=tBu ( 1 ), Xyl ( 2 ), 2‐Np ( 3 ); bzq= benzoquinolate, Xyl=2,6‐dimethylphenyl, 2‐Np=2‐napthyl) were isolated as the pure isomers with a trans‐C_bzq,CNR configuration, as confirmed by ¹³C{¹H} NMR spectroscopy in the isotopically marked [Pt(bzq)(¹³CN)(CNR)] (R=tBu ( 1′ ), Xyl ( 2′ ), 2‐Np ( 3′ )) derivatives (δ¹³C_CN≈110 ppm; ¹J(Pt,¹³C)≈1425 Hz]. By contrast, complex [Pt(bzq)(C≡CPh)(CNXyl)] ( 4 ) with a trans‐N_bzq,CNR configuration, has been selectively isolated from [Pt(bzq)Cl(CNXyl)] (trans‐N_bzq,CNR) using Sonogashira conditions. X‐ray diffraction studies reveal that while 1 adopts a columnar‐stacked chain structure with Pt–Pt distances of 3.371(1) Å and significant π???π interactions (3.262 Å), complex 2 forms dimers supported only by short Pt???Pt (3.370(1) Å) interactions. In complex 4 the packing is directed by weak bzq???Xyl and bzq???C≡E (C, N) interactions. In solid state at room temperature, compounds 1 and 2 both show a bright red emission (?=42.1 % 1 , 57.6 % 2 ). Luminescence properties in the solid state at 77 K and concentration‐dependent emission studies in CH₂Cl₂ at 298 K and at 77 K are also reported for 1 , 1·CHCl3 , 2 , 2' , 2·CHCl3 , 3 , 4 .     

Self‐Complementary Dimers of Oxalamide‐Functionalized Resorcinarene Tetrabenzoxazines

Self‐complementarity is a useful concept in supramolecular chemistry, molecular biology and polymeric systems. Two resorcinarene tetrabenzoxazines decorated with four oxalamide groups were synthesized and characterized. The oxalamide groups possessed self‐complementary hydrogen bonding sites between the carbonyls and amide groups. The self‐complementary nature of the oxalamide groups resulted in self‐included dimeric assemblies. The hydrogen bonding interactions within the tetrabenzoxazines gave rise to the formation of dimers, which were confirmed by single‐crystal X‐ray diffractions analysis and supported by NMR spectroscopy and mass spectrometry. The self‐included dimers were connected by numerous and strong intermolecular N?H???O and C?H???O hydrogen bonds supplemented with C?H???π interactions, forming one‐dimensional polymers, which were then further linked into three‐dimensional networks.