Toppled Molecular-Domino Sets by Self-Assembly of Self-assembly: The π-Polymers

A series of designed binuclear palladium(II)-based self-assemblies, [Pd-2(en)(2)(L)(2)](NO3)(4), 1a; [Pd-2-(tmeda)(2)(L)(2)](NO3)(4), 2a; [Pd-2(bpy)(2)(L)(2)](NO3)(4), 3a; and [Pd-2(phen)(2)(L)(2)](NO3)(4), 4a, are synthesized. These complexes are obtained in good to excellent yields by equimolar combination of the nonchelating bidentate ligand bis(4-pyridylmethyl)piperazine, L, with the corresponding cis-protected palladium(II) component, that is, Pd(N-N)(NO3)(2), under suitable reaction conditions. The cis-protecting N-N units used are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2'-bipyridyl (bpy), and 1,10-phenanthroline (phen). The complexes 1a-4a are well-characterized by H-1 NMR, H-H COSY, and HSQC, and the molecular compositions have been established by ESI-MS. The molecular structures are confirmed for the complexes [Pd-2(tmeda)(2)(L)(2)](ClO4)(4), 2b; [Pd-2(bpy)(2)(L)(2)](ClO4)(4), 3b; and [Pd-2(phen)(2)(L)(2)](NO3)(4), 4a, by the single-crystal X-ray diffraction technique. In the cases of the complexes 3b and 4a, the crystal packing diagrams display a unique one-dimensional predefined organization of molecules by intermolecular pi-pi interactions using the strategically located pi-surfaces of the cis-protecting units. This organization resembles a set of toppled dominos. A given set exhibits further interactions with the neighboring sets in the packing pattern that are influenced by the nature of the N-N moiety, for example, bpy or phen. However, no such arrangement is observed in the case of the complex 2b due to the lack of a pi-surface at the cis-protecting moiety.