Order versus Disorder in the Cocrystals of m-Halogenopyridines with m-Halogenobenzoic Acids: The Effects of the I···O Halogen Bond

Abstract

A series of nine cocrystals derived from m-halogenobenzoic acids and m-halogenopyridines (XbzacH–X’py, X, X’ = Cl, Br, I) have been crystallized and their crystal structures are determined. All nine cocrystals comprise XbzacH–X’py molecular complexes with the acid carboxyl group hydrogen bonded to the pyridine nitrogen atom. The four cocrystals in which iodine (the strongest halogen bond donor) is absent are isostructural and crystallize in the monoclinic P 21/c space group with the acid–pyridine complexes disordered over inversion centers, and the halogen atoms (Cl, Br) do not participate in any significant intermolecular interactions. The four cocrystals in which either the acid or the pyridine moiety has an iodine substituent are ordered and comprise chains of XbzacH–X’py molecular complexes interconnected by I···O halogen bonds involving the carbonyl oxygen atom of the XbzacH molecule. However, when both halogen atoms are iodine (IbzacH–Ipy), the iodine atoms no longer participate in I···O halogen bonds but rather form only type II I···I contacts, resulting again in a disordered structure. The dominant interaction between neighboring XbzacH–X’py molecular complexes in the disordered structures are C–H···O hydrogen bonding contacts involving the carboxyl oxygen and aromatic hydrogen atoms. The DFT studies of the electrostatic potential of the hydrogen-bonded XbzacH–X’py complexes have shown the hydrogen atoms of the pyridine atom to be considerably more positive (by ca. 50–90 kJ mol–1 e –1) than those of benzoic acid, making it probable that the C–H···O hydrogen bonding contacts form solely with pyridine hydrogen atoms, and consequently that the structures comprise random stacks of ordered C–H···O bonded layers (in the four structures XbzacH–X’py, X, X’ = Cl, Br) and chains (in IbzacH–Ipy). For a more detailed comparison of the ordered and disordered structures, the cocrystals were also studied by solid-state 13C and 15N CP-MAS NMR spectroscopy and DSC measurements.

Active control of molecular stacking types in a congeneric library of dihalogenated salicylideneaniline crystals and their solid solutions

“…Because of their good compatibility, exchanging Cl and Br atoms in disordered structures often shows little effect on the crystal structure. 38 For 4BC, the large bias in the disorder ratio implies that the type-I interhalogen contact prefers to form between the homologous elements (Fig. S3a †).…”