Consequence of Presence
and Absence of π‑Clouds
at Strategic Locations of Designed Binuclear Pd(II) Complexes on Packing:
Self-Assembly of Self-Assembly by Intermolecular Locking and Packing
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Abstract
Self-assembled binuclear coordination cages of general
formula
[Pd<sub>2</sub>(N–N)<sub>2</sub>(<b>L</b>)<sub>2</sub>](X)<sub>4</sub>, <b>1a/b</b>–<b>4a/b</b> are
prepared by the combination of <i>N</i>,<i>N</i>′-bis(<i>m</i>-pyridyl)urea, <b>L</b>, with
a variety of <i>cis</i>-protected palladium(II) components,
Pd(N–N)(X)<sub>2</sub>. The <i>cis</i>-protecting
units “N–N” employed for the synthesis of <b>1</b>–<b>4</b> are ethylenediamine (en), tetramethylethylenediamine
(tmeda), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen),
respectively. The term “X” stands for nitrate and perchlorate
for <b>a</b> and <b>b</b>, respectively. The assemblies
are characterized by NMR and electrospray ionization mass spectrometry
(ESI-MS) techniques, and in some cases (i.e., <b>1a</b>,<b> 2b</b>,<b> 3b</b>,<b> 4a</b>, and <b>4b</b>) the structures are confirmed by single crystal X-ray diffraction.
The conformations of bound <b>L</b> in the crystal structures
of all the Pd(II) complexes are found to be <i>syn-syn</i>. The influence of the presence and absence of π cloud at the <i>cis-</i>protecting units on the crystal packing has been studied
in detail. In the packing of [Pd<sub>2</sub>(phen)<sub>2</sub><b>L</b><sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>4a</b>, one unit of [Pd<sub>2</sub>(phen)<sub>2</sub><b>L</b><sub>2</sub>]<sup>4+</sup> is associated with two other units by π–π
stacking interactions thus giving a one-dimensional growth as envisioned
on the basis of a design principle. In the case of [Pd<sub>2</sub>(en)<sub>2</sub>(<b>L)</b><sub>2</sub>](NO<sub>3</sub>)<sub>4</sub>, <b>1a</b>, and [Pd<sub>2</sub>(tmeda)<sub>2</sub>(<b>L</b>)<sub>2</sub>](ClO<sub>4</sub>)<sub>4</sub>, <b>2b</b>, such packing is not observed due to the absence of π-cloud
at the strategic locations, instead notable H-bonding interactions
are seen. However, [Pd<sub>2</sub>(bpy)<sub>2</sub>(<b>L</b>)<sub>2</sub>](ClO<sub>4</sub>)<sub>4</sub>, <b>3b</b>, displays
a π–π interactions using only two units of [Pd<sub>2</sub>(bpy)<sub>2</sub>(<b>L</b>)<sub>2</sub>]<sup>4+</sup>(ClO<sub>4</sub>)<sup>−</sup>
