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We report here the first example of concentration-triggered helicate-to-tetrahedron transformation in supramolecular lanthanide edifices, along with their highly efficient and selective luminescence sensing properties toward PA at the ppb level.

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)2 L 2](NO3)4, 4a, one unit of [Pd2(phen)2 L 2]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)−.

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Complexation of the bridging bidentate ligand N,N'-(pyridine-2,6-diyl)dinicotinamide, L with palladium(II) resulted in a single discrete M(2)L(4) self-assembly, 1, in a quantitative manner. The entropically-controlled assembly of 1 resulted in a rare saturated, quadruple-stranded metallo-helicate, in which both the left-handed (M) and right-handed (P) helicates exist in the crystal structure.

A series of designed binuclear palladium(II)based self-assemblies, [Pd 2 (en) 2 (L) 2 ](NO 3 ) 4 , 1a; [Pd 2 -(tmeda) 2 (L) 2 ](NO 3 ) 4 , 2a; [Pd 2 (bpy) 2 (L) 2 ](NO 3 ) 4 , 3a; and [Pd 2 (phen) 2 (L) 2 ](NO 3 ) 4 , 4a, are synthesized. These complexes are obtained in good to excellent yields by equimolar combination of the nonchelating bidentate ligand bis(4pyridylmethyl)piperazine, L, with the corresponding cis-protected palladium(II) component, that is, Pd(N-N)(NO 3 ) 2 , under suitable reaction conditions. The cis-protecting N-N units used are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridyl (bpy), and 1,10-phenanthroline (phen). The complexes 1a−4a are well-characterized by 1 H NMR, H−H COSY, and HSQC, and the molecular compositions have been established by ESI-MS. The molecular structures are confirmed for the complexes [Pd 2 (tmeda) 2 (L) 2 ](ClO 4 ) 4 , 2b; [Pd 2 (bpy) 2 (L) 2 ](ClO 4 ) 4 , 3b; and [Pd 2 (phen) 2 (L) 2 ](NO 3 ) 4 , 4a, by the single-crystal X-ray diffraction technique. In the cases of the complexes 3b and 4a, the crystal packing diagrams display a unique onedimensional predefined organization of molecules by intermolecular π−π interactions using the strategically located π-surfaces of the cis-protecting units. This organization resembles a set of toppled dominos. A given set exhibits further interactions with the neighboring sets in the packing pattern that are influenced by the nature of the N-N moiety, for example, bpy or phen. However, no such arrangement is observed in the case of the complex 2b due to the lack of a π-surface at the cis-protecting moiety.

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