Crystalline adducts of some substituted salicylic acids with 4-aminopyridine, including hydrates and solvates: contact and separated ionic complexes with diverse supramolecular synthons

• Published in: CrystEngComm Vol. 14; no. 21; pp. 7466 - 7478
• Main Authors: Montis, Riccardo, Hursthouse, Michael B.
• Format: Journal Article
• Language: English
• Published: 01.01.2012
• Subjects: Construction; Contact; Crystal structure; Hydrates; Monomers; Pyridines; Salicylic acid; Solvents
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/c2ce26008e

Co-crystallizations of some 3, 5 and 6 mono-substituted salicylic acids with 4-aminopyridine, using a variety of solvents, have yielded a number of new complex solid forms, mainly with the 5-halide-substituted acid, including some hydrates and solvates. In all cases, proton transfer occurs from the carboxyl group of the acid to the pyridine nitrogen of the base, with the COO super(-)...H super(+)NPy synthon being found in 12/14 cases. The prime exception is 4-aminopyridinium:5-aminosalicylate:pyridine solvate, where the carboxylate group forms a 2-point synthon with one hydrogen of the 4-amino group on the aminopyridinium supplemented by a C-H...O interaction involving an orthohydrogen. This synthon is also found as one component of a disordered structure of the 4-aminopyridinium:5-chlorosalicylate. The other component adopts the normal pyridinium...carboxylate synthon. The adoption of 2-point or 1-point synthons, and the geometry of the former, is influenced by the presence of other hydrogen-bonding interactions involving hydrate water molecules or the amine of the 4-aminopyridinium group. A detailed packing analysis shows a number of similarities, partly linked to the synthon geometries. The structures generally fall into two groups, one derived from a simple zero-dimensional pyridinium-carboxylate monomer construct, the other from a zero-dimensional pyridinium-carboxylate centrosymmetric dimer construct. The former group contains most of the hydrates and the latter all of the 5-halide and methyl anhydrates, plus the 5-I pyridine solvate. The diversity of structures found confirm the frequent unpredictability in the structures adopted by products of co-crystallizations when ionic forms are produced.