Molecular Recombination Phenomena in Palladium(II)-Based Self-Assembled Complexes

A series of self-assembled binuclear complexes (1–4) of Pd₂L′₂L₂ composition are prepared by combination of selected cis-protected palladium­(II) component cis-PdL′ with equimolar amount of the nonchelating bidentate ligand 1,3-bis­((pyridine-4-yl)­methyl)­urea, L. The cis-protecting unit L′ crafted in the complexes [Pd₂(L′)₂(L)₂]­(NO₃)₄, 1–4, are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen), respectively. Complexation of palladium­(II) with the ligand L, however, resulted in the self-assembled trinuclear complex [Pd₃(L)₆]­(NO₃)₆, 5, of Pd₃L₆ composition. A few other mononuclear complexes relevant to this work are the homoleptic (6–9) and the heteroleptic (10–12) of PdL′₂ composition, where the term L′ in the formula stands for en, tmeda, bpy, and phen. The two units of L′ in the complexes 6–12 are en–en, tmeda–tmeda, bpy–bpy, phen–phen, en–tmeda, en–bpy, and en–phen, respectively. Keeping in mind the recombination of fragments in biomolecular systems, it was considered to study the recombination of fragments among assorted self-assembled coordination complexes. In this context, the complexes 1–12 are employed. The recombination was planned on the basis of information already coded in the complexes that we realized from our experience. Designated complexes from this list are subjected to deliberate circumstances, whereupon remarkable “molecular recombination” processes ensued. For instance, combination of [Pd₂(en)₂(L)₂]­(NO₃)₄, 1 with [Pd­(tmeda)₂]­(NO₃)₂, 7 at 1:2 molar ratio produced [Pd₂(tmeda)₂(L)₂]­(NO₃)₄, 2 and concomitant [Pd­(en)­(tmeda)]­(NO₃)₂, 10 at 1:2 molar ratio. A number of reorganization reactions are studied through selective combinations of two types of complexes from the pool of 1–12. The detailed finding is described in the article so as to conceptualize molecular recombination and other related dynamic processes in self-assembled coordination complexes of a chosen family. Crystal structures of a few complexes (2, 4, and 5) and intermolecular interactions in the crystal packing with particular reference to π–π interactions in 4 are also discussed.