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The role of noncovalent interactions in stabilizing and organizing complex structures throughout nature is indisputable. Of the various classes of noncovalent interactions, those that involve secondary bonding – attractive interactions between σ-hole and nucleophile – are of interest in the design of materials due to their strength and programmability. This report takes the approach of placing nucleophilic imines in close proximity to fused thiophene moieties within naphtho[2,1-b:3,4-b′]dithiophene (α NDT) cores, where an intramolecular N···S interaction is poised to yield rigid chromophores. These types of intramolecular N···S interactions have been observed in the solid-state for several decades, but their solution-state analysis remains rare. Here we detail how crystallography, 1H/13C NMR spectroscopy, and molecular modeling work synergistically to describe the strength and impact of intramolecular N···S interactions on α NDT chromophores α(1)2. The remote substituents on the aryl amines (1) employed as condensation partners have minimal structural impact on the α(1)2 series, but the photophysical properties of strongly electron-deficient (1d and 1dd) or polarizing (1c and 1cc) end-caps are enhanced in comparison to their neutral (1a) and weak (1b and 1bb) counterparts. This design strategy to incorporate intramolecular N···S interactions highlights how NDTs can be incorporated into complex architectures in a programmable manner. 

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. 

To develop synthetic strategies to construct ligands containing secondary sphere acids, we demonstrate that an appended borane of low Lewis acidity (–BPin) can be upgraded to a strong Lewis acid (–BF2). Using a pyridine-pyrazole ligand coordinated to Mo(CO)4, we show that a pendent –BPin group undergoes exhaustive fluorination to –BF3K, a precursor to a highly acidic –BF2 unit (acceptor number ~15x greater than –BPin).