Toppled Molecular-Domino Sets by Self-Assembly of
Self-assembly: The π‑Polymers
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Abstract
A series
of designed binuclear palladium(II)-based self-assemblies,
[Pd<sub>2</sub>(en)<sub>2</sub>(<b>L</b>)<sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>1a</b>; [Pd<sub>2</sub>(tmeda)<sub>2</sub>(<b>L</b>)<sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>2a</b>; [Pd<sub>2</sub>(bpy)<sub>2</sub>(<b>L</b>)<sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>3a</b>; and [Pd<sub>2</sub>(phen)<sub>2</sub>(<b>L</b>)<sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>4a</b>, are synthesized. These complexes are obtained
in good to excellent yields by equimolar combination of the nonchelating
bidentate ligand bis(4-pyridylmethyl)piperazine, <b>L</b>, with the corresponding <i>cis</i>-protected palladium(II)
component, that is, Pd(N-N)(NO<sub>3</sub>)<sub>2</sub>, under suitable reaction conditions. The <i>cis</i>-protecting
N-N units used are ethylenediamine (en), tetramethylethylenediamine
(tmeda), 2,2′-bipyridyl (bpy), and 1,10-phenanthroline (phen).
The complexes <b>1a</b>–<b>4a</b> are well-characterized
by <sup>1</sup>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<sub>2</sub>(tmeda)<sub>2</sub>(<b>L</b>)<sub>2</sub>](ClO<sub>4</sub>)<sub>4</sub>, <b>2b</b>; [Pd<sub>2</sub>(bpy)<sub>2</sub>(<b>L</b>)<sub>2</sub>](ClO<sub>4</sub>)<sub>4</sub>, <b>3b</b>; and [Pd<sub>2</sub>(phen)<sub>2</sub>(<b>L</b>)<sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>4a</b>, by the single-crystal X-ray diffraction technique. In the cases
of the complexes <b>3b</b> and <b>4a</b>, 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 <i>cis</i>-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 <b>2b</b> due to the lack
of a π-surface at the <i>cis</i>-protecting moiety
