Four supramolecular isomers of dichloridobis(1,10-phenanthroline)cobalt(II): Synthesis, structure characterization and isomerization

Crystal engineering can be described as the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding to design new solids with desired physical and chemical properties. Free-energy differences between supramolecular isomers are generally small and minor changes in the crystallization conditions may result in the occurrence of new isomers. The study of supramolecular isomerism will help us to understand the mechanism of crystallization, a very central concept of crystal engineering. Two supramolecular isomers of dichloridobis(1,10-phenanthroline-κ² N,N')cobalt(II), [CoCl₂(C₁₂H₈N₂)₂], i.e. (IA) (orthorhombic) and (IB) (monoclinic), and two supramolecular isomers of dichloridobis(1,10-phenanthroline-κ² N,N')cobalt(II) N,N-dimethylformamide monosolvate, [CoCl₂(C₁₂H_8N2)₂]·C₃H₇NO, i.e. (IIA) (orthorhombic) and (IIB) (monoclinic), were synthesized in dimethylformamide (DMF) and structurally characterized. Of these, (IA) and (IIA) have been prepared and structurally characterized previously [Li et al. (2007). Acta Cryst. E63, m1880-m1880; Cai et al. (2008). Acta Cryst. E64, m1328-m1329]. We found that the heating rate is a key factor for the crystallization of (IA) or (IB), while the temperature difference is responsible for the crystallization of (IIA) or (IIB). Based on the crystallization conditions, isomerization behaviour, the KPI (Kitajgorodskij packing index) values and the density data, (IB) and (IIA) are assigned as the thermodynamic and stable kinetic isomers, respectively, while (IA) and (IIB) are assigned as the metastable kinetic products. The 1,10-phenanthroline (phen) ligands interact with each other through offset face-to-face (OFF) π-π stacking in (IB) and (IIB), but by edge-to-face (EF) C-H⋯π interactions in (IA) and (IIA). Meanwhile, the DMF molecules in (IIB) connect to neighbouring [CoCl2(phen)2] units through two C-H⋯Cl hydrogen bonds, whereas there are no obvious interactions between DMF molecules and [CoCl2(phen)2] units in (IIA). Since OFF π-π stacking is generally stronger than EF C-H⋯π interactions for transition-metal complexes with nitrogen-containing aromatic ligands, (IIA) is among the uncommon examples that are stable and densely packed but that do not following Etter's intermolecular interaction hierarchy.