Development of Coordination Polymer Nanocrystals

Abstract

Coordination polymers (CPs), in particular macroscale non-porous CPs constructed from flexible bridging ligands, have rarely been utilised in practical applications owing to their insolubility in most solvents. Accordingly, the focus of this study is to investigate and optimise methods to produce CPs at the nanoscale with high dispersibility with the ultimate goal of exploiting the large effective surface area inherent to nanoscale materials to expand the scope of their utility. In this respect, two series of carboxylate and disulfide multidentate ligands with varying alkyl chain lengths (CnCOOH, n = 3-5, 7, 10, 11) and (CnSSCn, n = 6, 8, 10, 12) were synthesised. Solution phase studies showed a 1 : 1 coordination stoichiometry between the ligands and M(ClO4)2.6H2O (M = Zn or Cu) in all cases. However, only the complexes of CnCOOH could be crystallised to produce crystals of sufficient quality for X-ray structure determination. The coordination geometries and complex topologies were found to be dependent on the alkyl chain length of the ligand CnCOOH. The ZnII- and CuII- complexes of the short alkyl chain ligands (n ≤ 5) exhibit 1D coordination polymeric structures with somewhat different conformations, whereas the ligands with longer alkyl chains (n ≥ 7) participate in forming Zn2L2 metallomacrocyclic structures. As a result, the complexes of CnCOOH were further investigated for preparation as nanocrystal coordination polymers (NCPs). A heptane-based microemulsion system containing reverse micelles was developed as an approach for the synthesis of the ZnII- and CuII-NCPs. This method provides control over size and morphology of the NCPs which were found to be readily dispersible in heptane. By applying this method to alter both size and morphology, the NCPs have been observed to form two distinctive morphological structures: i) single elongated crystals of relatively uniform size and shape that are 28 ± 8 nm in length and 13 ± 4 nm in width, and ii) ultra-small seed crystals in the range of ~ 4-6 nm, which are clustered together giving rise to a raspberry-like morphology. The NCPs were fully characterised and it was confirmed that the NCPs have the same crystal structure to that of their macroscale counterparts. Both ZnII- and CuII-NCPs demonstrate high catalytic activity in the ring opening reaction of cyclohexene oxide with aniline (yields = 94–96 %). In addition, reduced Cu-NCPs can be employed as an efficient catalyst for the azide-alkyne cycloaddition “click” reaction of benzyl azide and phenylacetylene in heptane (yield = 98 %).