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]­(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 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 [Pd2­(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.