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

Self-assembled binuclear coordination cages of general
formula
[Pd2(N–N)2(L)2]­(X)4, 1a/b–4a/b are
prepared by the combination of N,N′-bis­(m-pyridyl)­urea, L, with
a variety of cis-protected palladium­(II) components,
Pd­(N–N)­(X)2. The cis-protecting
units “N–N” employed for the synthesis of 1–4 are ethylenediamine (en), tetramethylethylenediamine
(tmeda), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen),
respectively. The term “X” stands for nitrate and perchlorate
for a and b, respectively. The assemblies
are characterized by NMR and electrospray ionization mass spectrometry
(ESI-MS) techniques, and in some cases (i.e., 1a, 2b, 3b, 4a, and 4b) the structures are confirmed by single crystal X-ray diffraction.
The conformations of bound L in the crystal structures
of all the Pd­(II) complexes are found to be syn-syn. The influence of the presence and absence of π cloud at the cis-protecting units on the crystal packing has been studied
in detail. In the packing of [Pd2(phen)2L2]­(NO3)4, 4a, one unit of [Pd2(phen)2L2]4+ 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 [Pd2(en)2(L)2]­(NO3)4, 1a, and [Pd2(tmeda)2(L)2]­(ClO4)4, 2b, such packing is not observed due to the absence of π-cloud
at the strategic locations, instead notable H-bonding interactions
are seen. However, [Pd2(bpy)2(L)2]­(ClO4)4, 3b, displays
a π–π interactions using only two units of [Pd2(bpy)2(L)2]4+(ClO4)−.