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 [Pd-2(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 pi cloud at the cis-protecting units on the crystal packing has been studied in detail. In the packing of [Pd-2(phen)(2)L-2](NO3)(4), 4a, one unit of [Pd-2(phen)(2)L-2](4+) is associated with two other units by pi-pi stacking interactions thus giving a one-dimensional growth as envisioned on the basis of a design principle. In the case of [Pd-2(en)(2)(L)(2)](NO3)(4), 1a, and [Pd-2(tmeda)(2)(L)(2)](ClO4)(4), 2b, such packing is not observed due to the absence of pi-cloud at the strategic locations, instead notable H-bonding interactions are seen. However, [Pd-2(bpy)(2)(L)(2)](ClO4)(4), 3b, displays a pi-pi interactions using only two units of [Pd-2(bpy)(2)(L)(2)](4+)(ClO4)(-).