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

Abstract

Crystallization of α,ω-alkanedicarboxylic acids (HOOC-(CH 2)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 (ΔEsynthon, 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 ΔE acid - pyridine and ΔEacid - 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 ΔEsynthon 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.