| Summary: | 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 CHI<sub>3</sub> in acetonitrile yielded co-crystals showing a XB network of these two constituents. In the current work, the interactions of QN with C<sub>2</sub>H<sub>5</sub>I or 1,4-diazabicyclo[2.2.2]octane (DABCO) with CH<sub>2</sub>I<sub>2</sub> led to nucleophilic substitution producing I<sup>−</sup> anions and quaternary ammonium (QN-CH<sub>2</sub>CH<sub>3</sub> or DABCO-CH<sub>2</sub>I<sup>+</sup>) cations. Moreover, the reaction of QN with CHI<sub>3</sub> in dichloromethane afforded co-crystals containing XB networks of CHI<sub>3</sub> with either Cl<sup>−</sup> or I<sup>−</sup> anions and QN-CH<sub>2</sub>Cl<sup>+</sup> counter-ions. A similar reaction in acetone produced XB networks comprising CHI<sub>3</sub>, I<sup>−</sup> and QN-CH<sub>2</sub>COCH<sub>3</sub><sup>+</sup>. 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-CH<sub>2</sub>I<sup>+</sup>, QN-CH<sub>2</sub>Cl<sup>+</sup>) or neutral (CHI<sub>3</sub>) XB donors and the anionic (I<sup>−</sup>, Cl<sup>−</sup>) or neutral (CHI<sub>3</sub>) 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.
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