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Abstract The interaction of diiodine with quinuclidine (QN) and 4‐dimethylaminopyridine (DMAP) in solutions with 1 : 1 molar ratio of reactants at room temperature produced (in essentially quantitative yields) pure charge‐transfer QN⋅I₂adducts and iodine(I) salt [DMAP‐I‐DMAP]I₃, respectively. In comparison, the quantitative formation of pure iodine (I) salt [QN‐I‐QN]I₅was observed for the room‐temperature reactions of QN with a 50 % excess of I₂, and the charge‐transfer adducts of I₂with DMAP (and other pyridines) were formed when reactions were carried out at low temperatures. Computational analysis related the switch from the formation of charge‐transfer adducts to iodine(I) complexes in these systems to the strength of the halogen bonding of diiodine to the N‐donor bases. It shows that while the halogen‐bonded adducts represent critical intermediates in the formation of iodine(I) complexes, exceedingly strong halogen bonding between diiodine and the base prevents any subsequent transformations. In other words, while halogen bonding usually facilitates electron and halogen transfer, the halogen‐bonded complexes may serve as “black holes” hindering any follow‐up processes if this intermolecular interaction is too strong. 

The role of halogen bonding (HaB) in the reactions of N-chlorosuccinimide (SimCl), a versatile reagent in organic synthesis, was investigated through experimental and computational analyses of its interactions with halides. The reactions of SimCl with Br− or I− resulted in the crystallization of HaB complexes of chloride with N-iodosuccinimide (SimI) or N-bromosuccinimide (SimBr). Computational analysis revealed that halogen rearrangements, which occurred even at −73 °C, were facilitated by halogen bonding. The dissociation of SimCl∙Y− (Y = I or Br) complexes into a Sim− + ClY pair (followed by the rotation and re-binding of the interhalogen molecules) bypassed the formation of the high-energy Sim− + Cl+ pair and drastically (about tenfold) reduced the dissociation energy of the N–Cl bond. Furthermore, while the dissociation energy of individual SimCl is higher (and its HaB is weaker) compared to that of SimI or SimBr, the dissociation of the N-Cl bond in SimCl∙Y− requires less energy than in the complexes of SimBr or SimI. The facile cleavage of such bonds in HaB complexes explains the high reactivity of SimCl and its effectiveness as a halogenating agent. 

The first structures containing bonds between chlorines and tertiary nitrogen atoms and very strong halogen bondsviachlorine (with a substantial contribution of orbital interactions) are reported. 

Anion–π complexes with the electron-deficient alkene, tetracyanoethylene, are similar to that with aromatic and p-benzoquinone π-acceptors, but their persistence is delimited by the 1e-donating strength and nucleophilicity of anions. 

4-Nitroquinoline-N-oxide (NQO) and 4-nitropyridine-N-oxide (NPO) are important precursors for the synthesis of substituted heterocycles while NQO is a popular model mutagen and carcinogen broadly used in cancer research; intermolecular interactions are critical for their reactions or functioning in vivo. Herein, the effects of the coordination of N-oxide’s oxygen atom to Lewis acids on multicenter donor–acceptor bonding were explored via a combination of experimental and computational studies of the complexes of NQO and NPO with a typical π-electron donor, pyrene. Coordination with ZnCl2 increased the positive electrostatic potentials on the surfaces of these π-acceptors and lowered the energy of their LUMO. Analogous effects were observed upon the protonation of the N-oxides’ oxygen or bonding with boron trifluoride. The interaction of ZnCl2, NPO, or NQO and pyrene resulted in the formation of dark co-crystals comprising π-stacked Zn-coordinated N-oxides and pyrene similar to that found with protonated or (reported earlier) BF3-bonded N-oxides. Computational studies indicated that the coordination of N-oxides to zinc(II), BF3, or protonation led to the strengthening of the multicenter bonding of the nitro-heterocycle with pyrene, and this effect was related both to the increased electrostatic attraction and molecular–orbital interactions in their complexes. 

Haloalkanes and amines are common halogen-bond (XB) donors and acceptors as well as typical reagents in nucleophilic substitution reactions. Thus, crystal engineering using these molecules requires an understanding of the interchange between these processes. Indeed, we previously reported that the interaction of quinuclidine (QN) with CHI3 in acetonitrile yielded co-crystals showing a XB network of these two constituents. In the current work, the interactions of QN with C2H5I or 1,4-diazabicyclo[2.2.2]octane (DABCO) with CH2I2 led to nucleophilic substitution producing I− anions and quaternary ammonium (QN-CH2CH3 or DABCO-CH2I+) cations. Moreover, the reaction of QN with CHI3 in dichloromethane afforded co-crystals containing XB networks of CHI3 with either Cl− or I− anions and QN-CH2Cl+ counter-ions. A similar reaction in acetone produced XB networks comprising CHI3, I− and QN-CH2COCH3+. These distinctions were rationalized through a computational analysis of XB complexes and the transition-state energies for the nucleophilic substitution. It indicated that the outcome of the reactions was determined mostly by the relative energies of the products. The co-crystals obtained in this work showed bonding between the cationic (DABCO-CH2I+, QN-CH2Cl+) or neutral (CHI3) XB donors and the anionic (I−, Cl−) or neutral (CHI3) acceptors. Their analysis showed comparable electron and energy densities at the XB bond critical points and similar XB energies regardless of the charges of the interacting species. 

Abstract The studies of the anion‐π interactions advanced during the last two decades from the discussion of the mere existence of this counter‐intuitive bonding to its utilization for anion recognition and transport, catalysis, and other applications. Yet, there are substantial differences in the interpretation of nature and the driving forces of anion‐π bonding. Most surprisingly, there are still different opinions about the meaning of this term (i. e., which associations can be considered anion‐π complexes). After a brief overview of the studies in this area (including early examples of such complexes), we suggested that anion‐π bonding occurs when there is evidence of a net attraction between a (close‐shell) anion and the face of an electrophilic π‐system. This definition encompasses fundamentally similar supramolecular complexes comprising diverse π‐systems and anions and its general acceptance would facilitate a discussion of the nature and distinct driving forces of this fascinating interaction.