Oxalic acid/phenols and oxalic acid/cholesterol co-crystals:
a solid state 13C CPMAS NMR study
Concepción López,* a Rosa M. Claramunt,a and José Elguerob
a Departamento de Química Orgánica y Bio-Orgánica, Universidad Nacional de Educación a
Distancia (UNED), Facultad de Ciencias, Senda del Rey 9, 28040 Madrid, Spain
b Instituto de Química Médica (CSIC), Juan de la Cierva 3, 28006, Madrid, Spain
E-mail:
[email protected]
Dedicated to Professor Irina Beletskaya on the occasion of her 75th anniversary
Abstract
The structures of the crystals obtained by mixing oxalic acid (1) and five hydroxy derivatives, phenol (2a), p-cresol (2b), hydroquinone (2c), ß-naphthol (2d), and cholesterol (2e) have been studied by 13C CPMAS NMR. It has been proved that they are co-crystals of defined stoichiometry (oxalic/hydroxy derivative) 1:2 (1/2a, 1/2b, 1/2e), 2:2 (1/2c), and 1:4 (1/2d).
Keywords: Co-crystals, NMR, CPMAS, oxalic acid, phenols, cholesterol
Introduction
In 1916 Antonio Madinaveitia published two papers in the Spanish journal Anales de Química (Productos de adición del ácido oxálico and Sobre la separación de la colesterina y la isocolesterina.) where he discussed the structure of the compounds that are formed when oxalic acid (1) is mixed with phenols or cholesterol (C-hydroxy compounds 2).1,2 These papers were really in advance to Madinaveitia's time since they involve two fundamental concepts: hydrogen bonds and co-crystals. The concept of hydrogen bonding was introduced by Huggins and by Latimer and Rodebush in 1919.3 The notion of co-crystals is much more difficult to date because this term includes a variety of situations, considering any system in which the molecule of interest can be crystallized with some other molecular species in order to yield a unique structure. There are a number of terms that describe this class of structures: solvate, salt, clathrate, host–guest compound, inclusion compound, 1:1 addition compound, binary or ternary crystal, hydrate, and coordination complex. All of these can be classified under the single term ‘co-crystal’ because all of them describe ways of creating unique crystal structures that contain a selected species plus some other molecular component.4 Nevertheless, co-crystals and crystal
