Noncovalent Interactions under Extreme Conditions: High-Pressure and Low-Temperature Diffraction Studies of the Isostructural Metal−Organic Networks (4-Chloropyridinium)₂[CoX₄] (X = Cl, Br)

Abstract

The crystal structures of the two isostructural metal-organic salts, (4-chloropyridinium) 2[CoX 4], X = Cl ( 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...X 2Co hydrogen bonds and by Co-X...Cl-C halogen bonds. Intertape interactions include type I Co-X...Cl-C and C-Cl...Cl-C halogen-halogen interactions as well as offset pi-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...X 2Co and Co-X...Cl-C interactions and the weaker dispersion dominated Co-X...Cl-C and C-Cl...Cl-C halogen-halogen interactions and pi-stacking interactions. Increased distortion of the [CoX 4] (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 [CoBr 4] (2-) anion for the smaller [CoCl 4] (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 [CoBr 4] (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 [CoCl 4] (2-) anion are contracted along the<101> vector, the direction of tape propagation, by ca. 1.2% and correspondingly expanded in other directions, relative to those containing the [CoBr 4] (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...X 2Co hydrogen bonds and Co-X...Cl-C halogen bonds are more strongly attractive for X = Cl rather than X = Br. Use of internal pressure thereby provides unique insight into the relative strength of intermolecular interactions.

Organizing Radical Species in the Solid State with Halogen Bonding

“…The ability of halometallates [38][39][40] (as well as cyano [41,42] and thiocyanato [19] analogs) to act as halogen bond acceptors has been extensively explored [43], particularly by Brammer et al, and several series of salts with halo-pyridinium cations as halogen bond donors were reported. Neutral halopyridines (Fig.…”

Supramolecular Interactions and Smart Materials: C–X…X′–M Halogen Bonds and Gas Sorption in Molecular Solids

“…The formation of such interactions was clearly demonstrated in the preparation of neutral [47] and ionic crystalline materials [17,[48][49][50] using as halogen bond…”

“…This structural study was combined with DFT calculations of electrostatic potentials, which gave theoretical confirmation of the trends in C-X· · ·X -M halogen bond geometry observed across the series [52]. Two members of the previous series of isostructural compounds, (4-ClpyH) 2 [CoCl 4 ] and (4-ClpyH) 2 [CoBr 4 ], were further investigated at extreme conditions [50]. Specifically, structural changes were studied by single crystal X-ray diffraction at nine temperatures (from 300 to 30 K) and nine pressures (from atmospheric pressure to about 4 GPa) in order to gather information on the compressibility of the different types of noncovalent interactions.…”

A pressure-induced displacive phase transition in Tris(ethylenediamine) Nickel(II) nitrate

“…Metal-ligand bonds in transition metal complexes have been found to be more sensitive to pressure than covalent bonds in organic materials [9], leading to changes in coordination number [10][11][12], bond angles [13] and conformation [14]. This makes high pressure a very valuable tool for exploring the relationship between structure and physical properties in coordination compounds including magnetism [15,16], colour [17,18], spin state [19,20] and absorption characteristics of metal-organic frameworks [21][22][23].…”