Hydrates of N-((10-Chloroanthracen-9-yl)methyl)-3-(1H-imidazol-1-yl)propan-1-ammonium Cobalt(II), Copper(II), and Zinc(II) 2,6-Pyridinedicarboxylate: Reversible Crystallization

Abstract

In a quest to explore interconvertible assemblies of hydrates of cobalt(II), copper(II), and zinc(II) 2,6-pyridinedicarboxylate (26-pdc), complexes having cation of a chloro-substituted analogue N-{(10-chloroanthracen-9-yl)methyl}-3-(1H-imidazol-1-yl)propan-1-amine were investigated. In the case of cobalt and copper complexes, a crystallized stable hydrate and a less stable methanol hydrate were guided by concentration-dependent crystallizations. The unit-cells of the crystals of the methanol hydrates of the two cobalt and copper complexes each belong to the P1̅ space group but have different stoichiometries as well as large differences in packing. These hydrates could be reversibly crystallized in a predictable manner. The unit-cell volumes of the methanol hydrate of the cobalt complex were four-times smaller than that of the respective stable form (C2/c space group), whereas similar hydrates of the copper complex had a two-times smaller unit-cell volume than that of the stable form. The cations of the stable forms assembled together and formed zigzag ladder-like chains. The spaces present in between the assembled chains were filled with clusters of face to face stacked anions. The transformation to stable form required a bottom-up building process of the unit-cell starting from a smaller unit-cell of the less stable hydrates. Fluorescence spectroscopic studies showed the possibility of two forms of assemblies of the zinc-complex in solution, but crystallization had yielded only the stable form.

Figure 1

(a)–(c) The stacking among metal 2,6-pyridinedicarboxylates (Drawn with Chem Draw Ultra 3D). (d) The precursor (Clanth) of the cationic counterparts of the complexes.

Figure 2

(a) The crystal structure of Clanth and (b) the angle between the planes containing the chloro-anthracenyl and imidazole parts of Clanth.

Figure 3

Hydrates of the cobalt, copper, and zinc-26-pdc complexes having H₂Clanth cation.

Figure 4

(a) Self-assembly of Co–H₂Clanth showing two rings of the anions eclipsing each other to have stacking interaction. (b) The crystal structure of Co–H₂Clanth-Int. (c) The hydrogen bonded assemblies of Co–H₂Clanth-Int showing hydrogen bond between the ions.

Figure 5

(a) The hydrogen bonded assembly of Cu–H₂Clanth-Int. Stacking among the rings in self-assembly of (b) Cu–H₂Clanth-Int, (c) Co–H₂Clanth-Int.

Figure 6

FESEM image of Co–H₂Clanth-Int.

Figure 7

Unit cells of crystals of the hydrates of the (a) cobalt and (b) copper complexes, respectively, that accompany incorporation of water and release of methanol molecule(s) in their formation by crystallization.

Figure 8

Fluorescence titration of a 2 mL solution taken from a bulk methanol solution containing zinc(II) acetate dihydrate (0.02 mM), H₂26pdc (0.04 mM), and Clanth (0.02 mM) by adding 0–50% water. Inset (a) and (b): change in fluorescence intensity at 425 and 431 nm after systematic addition of water during titration (excitation at 300 nm).