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 ](NO 3 ) 4, 1a; [Pd 2 (tmeda) 2 ( L ) 2 ](NO 3 ) 4, 2a; [Pd 2 (bpy) 2 ( L ) 2 ](NO 3 ) 4, 3a; and [Pd 2 (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)(NO 3 ) 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 1 H 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 ](ClO 4 ) 4, 2b; [Pd 2 (bpy) 2 ( L ) 2 ](ClO 4 ) 4, 3b; and [Pd 2 (phen) 2 ( L ) 2 ](NO 3 ) 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.