Molecular complexes of homologous alkanedicarboxylic acids with isonicotinamide: X-ray crystal structures, hydrogen bond synthons, and melting point alternation

Crystallization of alpha,omega-alkanedicarboxylic acids (HOOC-(CH2)(n-2)-COOH, n = 2-6) with isonicotinamide (IN) is carried out in 1:2 and 1:1 stoichiometry. Five cocrystals of (diacid).(IN)2 composition (diacid = oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) are characterized by X-ray diffraction at 153(2) K. Tapes of acid-pyridine O-H...N and amide-amide N-H...O hydrogen bond synthons stabilize these five crystal structures as predicted by the hierarchic model: the best donor (COOH) and best acceptor group (pyridine N) hydrogen bond as acid-pyridine and the second best donor-acceptor group (CONH2) aggregates as an amide dimer. Glutaric acid and adipic acid cocrystallize in 1:1 stoichiometry also, (diacid).(IN), with acid-pyridine and acid-amide hydrogen bonds. Synthon energy calculations (DeltaE(synthon), RHF/6-31G**) explain the observed hydrogen bond preferences in 1:2 (five examples) and 1:1 (two examples) cocrystals. The acid-pyridine hydrogen bond is favored over the acid-amide dimer for strong carboxylic acids because the difference between DeltaE(acid-pyridine) and DeltaE(acid-amide) (-2.21 kcal mol(-1)) is greater than the difference for weak acids (-0.77 kcal mol(-1)), which cocrystallize with both of these hydrogen bond synthons. We suggest DeltaE(synthon) as a semiquantitative parameter to rank hydrogen bond preferences and better understand supramolecular organization in the multifunctional acid-IN system. Melting point alternation in five homologous (diacid).(IN)(2) cocrystals is correlated with changes in crystal density and packing fraction.