Ion‐Pairing of Phosphonium Salts in Solution: C?H⋅⋅⋅Halogen and C?H⋅⋅⋅π Hydrogen Bonds

The ¹H NMR chemical shifts of the C(α)? H protons of arylmethyl triphenylphosphonium ions in CD₂Cl₂ solution strongly depend on the counteranions X^?. The values for the benzhydryl derivatives Ph₂CH? PPh₃⁺ X^?, for example, range from δ_H=8.25 (X^?=Cl^?) over 6.23 (X^?=BF₄^?) to 5.72 ppm (X^?=BPh₄^?). Similar, albeit weaker, counterion‐induced shifts are observed for the ortho‐protons of all aryl groups. Concentration‐dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl^? > Br^? ? BF₄^? > SbF₆^?. For the less bulky derivatives PhCH₂? PPh₃⁺ X^?, we also find C? H???Ph interactions between C(α)? H and a phenyl group of the BPh₄^? anion, which result in upfield NMR chemical shifts of the C(α)? H protons. These interactions could also be observed in crystals of (p‐CF₃‐C₆H₄)CH₂? PPh₃⁺ BPh₄^?. However, the dominant effects causing the counterion‐induced shifts in the NMR spectra are the C? H???X^? hydrogen bonds between the phosphonium ion and anions, in particular Cl^? or Br^?. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh₄^? anions. The concentration dependence of the ¹H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD₂Cl₂ solution. The cation–anion interactions increase with the acidity of the C(α)? H protons and the basicity of the anion. The existence of C? H???X^? hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X‐ray analyses of the crystals. The IR spectra of the Cl^? and Br^? salts in CD₂Cl₂ solution show strong red‐shifts of the C? H stretch bands. The C? H stretch bands of the tetrafluoroborate salt PhCH₂? PPh₃⁺ BF₄^? in CD₂Cl₂, however, show a blue‐shift compared to the corresponding BPh₄^? salt.     

The Intermolecular S?H⋅⋅⋅Y (Y=S,O) Hydrogen Bond in the H2S Dimer and the H2S–MeOH Complex

The nature of the S? H???S hydrogen‐bonding interaction in the H₂S dimer and its structure has been the focus of several theoretical studies. This is partly due to its structural similarity and close relationship with the well‐studied water dimer and partly because it represents the simplest prototypical example of hydrogen bonding involving a sulfur atom. Although there is some IR data on the H₂S dimer and higher homomers from cold matrix experiments, there are no IR spectroscopic reports on S? H???S hydrogen bonding in the gas phase to‐date. We present experimental evidence using VUV ionization‐detected IR‐predissociation spectroscopy (VUV‐ID‐IRPDS) for this weak hydrogen‐bonding interaction in the H₂S dimer. The proton‐donating S? H bond is found to be red‐shifted by 31 cm^?1. We were also able to observe and assign the symmetric (ν₁) stretch of the acceptor and an unresolved feature owing to the free S? H of the donor and the antisymmetric (ν₃) SH stretch of the acceptor. In addition we show that the heteromolecular H₂S–MeOH complex, for which both S? H???O and O? H???S interactions are possible, is S‐H???O bound.     

Cooperative Catalysis of Metal and O?H⋅⋅⋅O/sp3‐C?H⋅⋅⋅O Two‐Point Hydrogen Bonds in Alcoholic Solvents: Cu‐Catalyzed Enantioselective Direct Alkynylation of Aldehydes with Terminal Alkynes

Catalyst–substrate hydrogen bonds in artificial catalysts usually occur in aprotic solvents, but not in protic solvents, in contrast to enzymatic catalysis. We report a case in which ligand–substrate hydrogen‐bonding interactions cooperate with a transition‐metal center in alcoholic solvents for enantioselective catalysis. Copper(I) complexes with prolinol‐based hydroxy amino phosphane chiral ligands catalytically promoted the direct alkynylation of aldehydes with terminal alkynes in alcoholic solvents to afford nonracemic secondary propargylic alcohols with high enantioselectivities. Quantum‐mechanical calculations of enantiodiscriminating transition states show the occurrence of a nonclassical sp³‐C? H???O hydrogen bond as a secondary interaction between the ligand and substrate, which results in highly directional catalyst–substrate two‐point hydrogen bonding.     

Spin–Spin Coupling Constants Transmitted through Ir?H⋅⋅⋅H?N Dihydrogen Bonds

Relativity matters: Calculations of NMR shielding tensors and spin–spin coupling constants transmitted through Ir? H???H? N dihydrogen bonds are presented. The picture shows one of the simplified models employed. It is shown that the spin–orbit relativistic effects influence the NMR shielding constants far more than the spin–spin coupling constants.

     

Hydrogen‐Bond Dynamics of C?H⋅⋅⋅O Interactions: The Chloroform⋅⋅⋅Acetone Case

Spectroscopic evidence for C? H ??? O hydrogen bonding in chloroform ??? acetone [Cl₃CH ??? O?C(CH₃)₂] mixtures was obtained from vibrational inelastic neutron scattering (INS) spectra. Comparison between the INS spectra of pure samples and their binary mixtures reveals the presence of new bands at about 82, 130 and 170 cm^?1. Assignment of the 82 cm^?1 band to the νO ??? H anti‐translational mode is considered and discussed. In addition, the βC? H mode of CHCl₃ at 1242 cm^?1 is split in the spectra of the mixtures, and the high‐wavenumber component is assigned to the hydrogen‐bonded complex. The plot of the integrated intensity of this component shows a maximum for x=0.5, in agreement with the 1:1 stoichiometry of the chloroform ??? acetone complex, with a calculated complexation constant of 0.15 dm³ mol^?1. Results also show that the complex behaves as an independent entity, that is, despite being weak, such interactions play a key role in supramolecular chemistry.     

Self‐Assembly of Halogenated Cobaltacarborane Compounds: Boron‐Assisted C?H⋅⋅⋅X?B Hydrogen Bonds?

Full structural characterisation and complete synthetic procedures for three monohalogenated cobaltacarborane compounds closo-[3-Co(eta5-C5H5)-8-X-1,2-C2B9H10] (X=Cl (1), Br (2), I (3)) and the dibromo derivative closo-[3-Co(eta5-C5H5)-8,9-Br2-1,2-C2B9H9] (4) are reported. The supramolecular structures of 1, 3, and 4 reveal the existence of intermolecular C--HX--B interactions. The role of these interactions has been investigated through a CSD search and subsequent analysis of the reported crystalline compounds. The results show that halogens become reasonably good hydrogen-bond acceptors when bonded to boron and, in this respect, are comparable in strength to metal-bound halogens.     

An Interplay of Cooperativity between Cation⋅⋅⋅π, Anion⋅⋅⋅π and C?H⋅⋅⋅Anion Interactions

Mixed cation (Li⁺, Na⁺ and K⁺) and anion (F^?, Cl^?, Br^?) complexes of the aromatic π‐surfaces (top and bottom) are studied by using dispersion‐corrected density functional theory. The selectivity of the aromatic surface to interact with a cation or an anion can be tuned and even reversed by the electron‐donating/electron‐accepting nature of the side groups. The presence of a methyl group in the ? OCH₃, ? SCH₃, ? OC₂H₅ in the side groups of the aromatic ring leads to further cooperative stabilization of the otherwise unstable/weakly stable anion???π complexes by bending of the side groups towards the anion to facilitate C? H???anion interactions. The cooperativity among the interactions is found to be as large as 100 kcal mol^?1 quantified by dissection of the three individual forces from the total interaction energy. The crystal structures of the fluoride binding tripodal and hexapodal ligands provide experimental evidence for such cooperative interactions.     

Molecular Pair Analysis: C?H⋅⋅⋅F Interactions in the Crystal Structure of Fluorobenzene? And Related Matters

The crystal structure of fluorobenzene is compared with isomorphous crystal structures of molecules of roughly similar shape. The lowest-energy fluorobenzene dimers are identified by theoretical calculations. Molecular pair analysis of the crystal structure of fluorobenzene and of an isomorphous virtual low-energy polymorph of benzene suggests that the important intermolecular interactions in the two structures are closely similar. In particular, the intermolecular C-H...F interactions in the fluorobenzene crystal have approximately the same structure-directing ability and influence on the intermolecular energy as the corresponding C-H...H interactions in benzene. Molecular pair analysis of the isomorphous crystal structures of benzonitrile, alloxan, and cyclopentene-1,2,3-trione indicates that essentially the same crystal structure can be adopted with quite different patterns of pair energies and atom-atom interactions. The question as to whether the packing radius of organic fluorine is larger or smaller than that of hydrogen, is addressed, but not answered.     

Influence of Intermolecular N?H⋅⋅⋅π Interactions on Molecular Packing and Field‐Effect Performance of Organic Semiconductors

Charge transport in organic semiconductors is strongly dependent on their molecular packing modes in the solid state. Therefore, understanding the relationship between molecular packing and charge transport is imperative, both experimentally and theoretically. However, so far, the fundamental effects of solid‐state packing and molecular interactions (e.g. N? H ??? π) on charge transport need further elucidation. Herein, indolo[3,2‐b]carbazole (ICZ) and a derivative thereof are used as examples to approach this scientific target. An interesting insight obtained thereby is that N? H ??? π interactions among ICZ molecules facilitate charge transport for higher mobility. Subtle changes in the of N? H ??? π interactions can significantly influence both the molecular packing and the charge‐transport properties. Therefore, a method for exploiting intermolecular N? H ??? π interactions would yield novel molecular systems with designable characteristics.     

[Pd(Fmes)2(tmeda)]: A Case of Intermittent C?H⋅⋅⋅F?C Hydrogen‐Bond Interaction in Solution

The X‐ray structure of the title compound [Pd(Fmes)₂(tmeda)] (Fmes=2,4,6‐tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′‐tetramethylethylenediamine) shows the existence of uncommon C? H???F? C hydrogen‐bond interactions between methyl groups of the TMEDA ligand and ortho‐CF₃ groups of the Fmes ligand. The ¹⁹F NMR spectra in CD₂Cl₂ at very low temperature (157 K) detect restricted rotation for the two ortho‐CF₃ groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen‐bond energy shows that it is too weak (about 7 kJ mol^?1) to account for the rotational barrier observed (ΔH^≠=26.8 kJ mol^?1), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.