The Amine Group as Halogen Bond Acceptor in Cocrystals of Aromatic Diamines and Perfluorinated Iodobenzenes

Erik Uran,Dominik Cinčić,Vladimir Stilinović,Luka Fotović,Nikola Bedeković

doi:10.3390/cryst11050529

Abstract

In order to study the proclivity of primary amine groups to act as halogen bond acceptors, three aromatic diamines (p-phenylenediamine (pphda), benzidine (bnzd) and o-tolidine (otol)) were cocrystallised with three perfluorinated iodobenzenes (1,4-tetrafluorodiiodobenzene (14tfib), 1,3-tetrafluorodiiodobenzene (13tfib) and 1,3,5-trifluorotriiodobenzene (135tfib)) as halogen bond donors. Five cocrystals were obtained: (pphda)(14tfib), (bnzd)(13tfib)2, (bnzd)(135tfib)4, (otol)(14tfib) and (otol)(135tfib)2. In spite of the variability of both stoichiometries and structures of the cocrystals, in all the prepared cocrystals the amine groups form exclusively I···N halogen bonds, while the amine hydrogen atoms participate mostly in N–H⋯F contacts. The preference of the amine nitrogen atom toward the halogen bond, as opposed to the hydrogen bond (with amine as a donor), is rationalised by means of computed hydrogen and halogen bond energies, indicating that the halogen bond energy between a simple primary amine (methylamine) and a perfluorinated iodobenzene (pentafluoroiodobenze ne) is ca. 15 kJ mol−1 higher than the energy of the (H)NH∙∙∙NH2 hydrogen bond between two amine molecules.

Highlights

In the structure of the cocrystal, each diamine molecule is connected to two neighbouring 14tfib molecules by a pair of I···NH2 halogen bonds (d(I1···N1) = 2.958(6) Å, ∠ (C4–I1···N1) = 179.0(2)◦ ) so that both amine nitrogen atoms act as halogen bond acceptors
This leads to the formation of halogen bonded chains (Figure 1) in which the amine hydrogen atoms remain free to form hydrogen bonds with fluorine atoms of the 14tfib molecules from neighbouring chains, interconnecting the chains through N–H· · · F contacts (d(N1···F2) = 3.375(6) Å) into layers (Figure 1)
N–H⋯N hydrogen bonds, leaving the amine hydrogen atoms participating only in of N–H· · · N hydrogen bonds, leaving the amine hydrogen atoms participating only in weakly bonding contacts. This is in line with the difference in the corresponding calcuweakly bonding contacts. This is in line with the difference in the corresponding calculated lated bond energies with methylamine as the halogen/hydrogen bond acceptor

Summary

Introduction

From the beginning of the intensive research into halogen bonding at the turn of the millennium [1,2,3,4,5], one of the main areas of interest (apart from the fundamental studies of the nature and properties of the halogen bond) has been to utilize the halogen bond as a reliable non-covalent molecular interaction in supramolecular chemistry in general [6,7,8,9], and in crystal engineering [10,11,12,13,14], as a means of the deliberate design of multi-component organic [15,16] and metal-organic [17,18,19,20] materials, both comprising ionic species (salts) [21,22,23,24,25,26] and neutral molecules (cocrystals) [27,28,29,30,31,32,33,34,35,36].The most commonly employed neutral halogen bond donors in crystal engineering to date have been perfluorinated iodobenzenes, namely, 1,2-tetrafluorodiiodobenzene (12tfib), 1,4-tetrafluorodiiodobenzene (14tfib), 1,3-tetrafluorodiiodobenzene (13tfib) and1,3,5-trifluorotriiodobenzene (135tfib) [37,38,39,40,41,42]. The presence of the electron-withdrawing fluorine atoms in the molecule increases the positive electrostatic potential of the σ-holes of the iodine atoms, making them reliable halogen bond donors for a wide range of organic and metal-organic Lewis bases [43,44]. Their reliability as halogen bond donors can be severely reduced by the formation of competing hydrogen bonds. The result of a supramolecular synthesis in systems where there is competition

Methods

Results

Conclusion