Toppled molecular-domino sets by self-assembly of self-assembly: The π-polymers

A series of designed binuclear palladium(II)-based self-assemblies, [Pd2(en)2(L)2](NO3)4, 1a; [Pd2(tmeda)2(L)2](NO3)4, 2a; [Pd2(bpy)2(L)2](NO3) 4, 3a; and [Pd2(phen)2(L)2](NO 3)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 1H NMR, H-H COSY, and HSQC, and the molecular compositions have been established by ESI-MS. The molecular structures are confirmed for the complexes [Pd2(tmeda)2(L) 2](ClO4)4, 2b; [Pd2(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 π-π interactions using the strategically located π-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 π-surface at the cis-protecting moiety. © 2013 American Chemical Society.