Octadecanuclear Gear Wheels by Self-Assembly of Self-Assembled "Double Saddle"-Type Coordination Entities: Molecular "Rangoli"

A series of self-assembled "double saddle"-type trinuclear complexes of [Pd3L3'L-2] formulation have been synthesized by complexation of a series of cis-protected palladium(II) components with a slightly divergent "E-shaped" non-chelating tridentate ligand, 1,1'-(pyridine-3,5-diyl)bis(3-(pyridin-3-yl)urea (L). The cis-protecting agents L' employed in the study are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen), for 1, 2, 3, and 4, respectively. The crystal structures of [Pd-3(tmeda)(3)(L)(2)](NO3)(6) (2), [Pd-3(bpy)(3)(L)(2)](NO3)(6) (3), and [Pd-3(phen)(3)(L)(2)](NO3)(6) (4) unequivocally support the new architecture. Two of the "double saddle"-type complexes (3 and 4) are suitably crafted with pi surfaces at the strategically located cis-protecting sites to facilitate intermolecular pi-pi interactions in the solid state. As a consequence, six units of the 3 (or 4) are assembled, by means of six-pairs of pi-pi stacking interactions, in a circular geometry to form an octadecanuclear molecular ring of [(Pd3L3'L-2)(6)] composition. The overall arrangement of the rings in the crystal packing is equated with the traditional Indian art form rangoli.