Date Published: April 01, 2019
Publisher: International Union of Crystallography
Author(s): Anna Vráblová, Juraj Černák, Larry R. Falvello, Milagros Tomás.
Reactions of Co(OH)2 with the Schiff base bis(2-hydroxy-3-methoxybenzylidene)ethylenediamine (o-van-en) yielded previously reported [CoII(o-van-en)(H2O)] under anaerobic conditions and two novel polymorphs of [CoIII2(o-van-en)3]·4CH3CN in the presence of air. Structural data were used in a knowledge-based approach to elucidate the origin of the polymorphism.
Polymorphism in the crystalline state – ‘the ability of a compound to crystallize in more than one crystal structure’ (Cruz-Cabeza et al., 2015 ▸) – is important from a scientific, as well as from an industrial, point of view as sometimes subtle differences in the crystal structures of the polymorphs may lead to substantially different properties. Such behaviour has been observed in the case of nonlinear optical materials (Munshi et al., 2008 ▸), single molecule magnets (Pavlov et al., 2016 ▸), materials with spin-crossover (Tao et al., 2012 ▸) or gas-absorption properties (Pal et al., 2016 ▸), or the properties of pharmaceutically active materials (Covaci et al., 2017 ▸; Rodríguez-Spong et al., 2004 ▸; Potticary et al., 2016 ▸), to mention a few examples. Recently, progress in the prediction of the crystal structures of polymorphs using solid-state density functional theory (DFT) simulations has been reported (Hasnip et al., 2014 ▸).
From the system Co(OH)2 + H2(o-van-en) under different experimental conditions, two cobalt complexes were isolated in a total of three solid forms – under anaerobic conditions, the already structurally characterized CoII complex 1, and in the presence of air, two novel CoIII-containing solids 2 and 3. The new complexes were chemically and spectroscopically characterized. Products 2 and 3 are monoclinic and triclinic polymorphs, respectively, and both are formed by centrosymmetric dinuclear [Co2(o-van-en)3] complex molecules in which two tetradentate o-van-en ligands chelate the two hexacoordinated CoIII atoms, while the remaining o-van-en ligand bridges the two CoIII atoms in a bis-chelate fashion. The composition of both polymorphs is completed by two MeCN solvent molecules. The two polymorphs differ in the packing of the dinuclear [Co2(o-van-en)3] complex molecules and conformational differences in the complex molecules were also observed. The Hirshfeld surfaces reflect the observed disorder for both polymorphs and suggest possible reasons for it; they also confirm the presence of contacts represented by weak hydrogen-bonding interactions, and they further indicate that the MeCN molecules play a role in the packing as they fill the hollows formed between the packed complex molecules. The FIMs of both polymorphs show that the regions of intermolecular interactions are occupied by congeners of the complex, leaving unrequited hydrogen-bonding capability and suggesting an explanation for the polymorphism. Furthermore, acetonitrile solvent molecules as rich electron donors do not enter the acceptor regions of the complex in either of the two polymorphs. These observations corroborate the observed low stability of both polymorphs with respect to the loss of their solvent molecules.