Supramolecular polymers self-assembled from trans-bis(pyridine) dichloropalladium(II) and platinum(II) complexes

Two structurally similar trans-bis(pyridine) dichloropalladium(II)- and platinum(II)-type complexes were synthesized and characterized. They both self-assemble in n-hexane to form viscous fluids at lower concentrations, but form metallogels at sufficient concentrations. The viscous solutions were studied by capillary viscosity measurements and UV/Vis absorption spectra monitored during the disassembly process indicated that a metallophilic interaction was involved in the supramolecular polymerization process. For the two supramolecular assemblies, uncommon continuous porous networks were observed by using SEM and TEM revealed that they were built from nanofibers that fused and crosslinked with the increase of concentration. The xerogels of the palladium and platinum complexes were carefully studied by using synchrotron radiation WAXD and EXAFS. The WAXD data show close stacking distances driven by π-π and metal-metal interactions and an evident dimer structure for the platinum complex was found. The coordination bond lengths were extracted from fitting of the EXAFS data. Moreover, close Pt(II) -Pt(II) (Pd(II) -Pd(II) ) and PtCl (PdCl) interactions proposed from DFT calculations in the reported oligo(phenylene ethynylene) (OPE)-based palladium(II) pyridyl supramolecular polymers were also confirmed by using EXAFS. The Pt(II) -Pt(II) interaction is more feasible for supramolecular interaction than the Pd(II) -Pd(II) interaction in our simple case.