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

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>

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