Noncovalent interactions under extreme conditions:: High-pressure and low-temperature diffraction studies of the isostructural metal-organic networks (4-chloropyridinium)2[CoX4] (X = CI, Br)

The crystal structures of the two isostructural metal-organic salts, (4-chloropyridinium)(2)[COX4], X = CI (1), Br (2), have each been determined at nine temperatures from 30 to 300 K and at nine pressures from atmospheric pressure to 4.2 GPa. A 5% reduction in unit cell volume is observed upon temperature reduction, whereas an 18-19% reduction is observed upon increasing the pressure over the ranges studied. The structures adopt a tape arrangement propagated by bifurcated N-H center dot center dot center dot X2Co hydrogen bonds and by Co-X center dot center dot center dot CI-C halogen bonds. Intertape interactions include type I Co-X center dot center dot center dot CI-C and C-CI center dot center dot center dot CI-C halogen-halogen interactions as well as offset T-stacking between the aromatic rings of the cations. Although little anisotropy in compression is seen upon temperature reduction, marked anisotropy in compression is observed upon pressure increase. Compression between tapes far exceeds compression along the tape, consistent with the strong attractive nature of the intratape N-H center dot center dot center dot X2Co and Co-X center dot center dot center dot CI-C interactions and the weaker dispersion dominated Co-X center dot center dot center dot CI-C and C-CI center dot center dot center dot CI-C halogen-halogen interactions and pi-stacking interactions. Increased distortion of the [CoX4](2-) anions from idealized tetrahedral geometry arises upon pressure increase, consistent with local changes in the electric field that result from compression of the pairs of pi-stacked cations. The study of the isostructural pair of compounds permits a rare opportunity for quantitative evaluation of the "internal" or "chemical" pressure exerted by changing the [CoBr4](2-) anion for the smaller [CoCl4](2-) anion. Thus, crystal structures of 1 and 2 with equivalent unit cell volumes require an additional pressure of ca. 1 GPa exerted upon the structure containing the larger [CoBr4](2-) anion (2). This internal pressure increases to ca. 1.9 GPa at the highest pressures used in this study. Most significant is that examination of the isovolumetric pairs of structures shows that the structures containing the [CoCl4](2-) anion are contracted along the < 1 01 > vector, the direction of tape propagation, by ca. 1.2% and correspondingly expanded in other directions, relative to those containing the [CoBr4](2-) anion. Since the effect of difference in anion size has been removed by application of pressure, this anisotropy in dimensions clearly indicates that the N-H center dot center dot center dot X2Co hydrogen bonds and Co-X center dot center dot center dot CI-C halogen bonds are more strongly attractive for X = CI rather than X = Br. Use of internal pressure thereby provides unique insight into the relative strength of intermolecular interactions.