Helical Metallacyclophanes DOI: 10.1002/anie.201306674 A Ball-Joint-Type Host–Guest System that Consists of Conglomerate Helical Metallacyclophanes** Haeri Lee, Tae Hwan Noh, and Ok-Sang Jung* If suitable molecular units could be advantageously arranged, what would the resultant properties of their molecular aggregates be? This sort has inspired molecular engineers for the past decade. [1–7] The assembly and the related proper- ties of novel supramolecular aggregates are a hot topic and, not entirely coincidentally, a great challenge in the field of molecular chemistry. [6–10] A full understanding of the driving forces behind those aggregations is a prerequisite for the design and construction of molecular arrays. Cyclophanes are attractive models for understanding weak transannular inter- actions as well as distinct p···p systems. [11–13] They can also serve as basic examples for explaining molecular recognition, chemosensing, molecular electronics, and electrical, mag- netic, and capsule effects that are introduced by significant donor–acceptor and electron-transfer processes, or by through-space effects. [14–23] Metallacyclophanes have been synthesized by the coordination-directed self-assembly of metals and multidentate ligands. [14, 24–29] These structures are of particular interest owing to the many and various advantages inherent to organic cyclophanes: easily accessible modularity for dimensions and topology, [14] and straightfor- ward incorporation of functions, such as catalytic effects, [30, 31] luminescence, [32] redox-activity, [33] electron transfer, [34] and helicity. [35, 36] The elucidation of the thermodynamics and kinetics of such metallacyclophane systems would certainly provide important insights into the evolution and emergence of delicate supramolecular systems. [37, 38] To that end, three important synthetic approaches have been introduced: angu- lar directional bonding, symmetry interaction, and weak interaction. [16] Helical motifs are ubiquitous in nature and have provided an important impetus for the generation of helical molecules. Indeed, discrete or polymeric helicates the synthesis of which is driven by coordination is intriguing, because it enables various applications, including asymmetric catalysis, chiral chemistry, nonlinear optical materials, template precursors, memory devices, biomimetics, DNA, structural biology, specific ion sensors, and molecular reaction vessels. [35, 36, 39–43] Helicity can be induced on purpose by means of conforma- tional restrictions that are due to the coordination with metal ions. [42, 43] In this context, we employed a self-assembly approach that incorporates the three methods described above to construct racemic helical metallacyclophanes, (P ,M)- [Pd 3 X 6 (L 1 ) 2 ], by the reaction of K 2 [PdX 4 ] (X = Cl, Br) with the C 3 -symmetric tridentate ligand L 1 as a programmed discrete helical component. A subsequent partial substitution reaction of (P ,M)-[Pd 3 X 6 (L 1 ) 2 ] with another C 3 -symmetric tridentate ligand L 2 , or direct self-assembly of K 2 [PdX 4 ] with both L 1 and L 2 , produced unprecedented conglomerate crystals forming a ball-joint-type host–guest system, (P)- [Pd 3 X 6 (L 1 ) 2 ]@(M)-[Pd 3 X 6 (L 1 )(L 2 )] and (M)-[Pd 3 X 6 (L 1 ) 2 ]@ (P)-[Pd 3 X 6 (L 1 )(L 2 )] (X = Cl, Br; L 1 = N,N’,N’’-tris(2-pyridi- nylethyl)-1,3,5-benzenetricarboxamide; [44] L 2 = N,N’,N’’-tris- (3-pyridinylpropyl)-1,3,5-benzenetricarboxylate; P = right- handed helix; M = left-handed helix). The synthesis of this host–guest system is an effective method for obtaining useful aggregates. Herein, we present a very effective strategy for the synthesis of such a system. Its crystal structures, the driving aggregative force behind it, and the reversible equilibrium between the aggregate and its dissociated species in solution are also discussed. The self-assembly of K 2 [PdX 4 ] with L 1 at room temper- ature produced racemic crystalline products of helical trime- tallacyclophanes, (P ,M)-[Pd 3 X 6 (L 1 ) 2 ] (X = Cl: (P ,M)-1-Cl; X = Br: (P ,M)-1-Br). Subsequent reaction of (P ,M)-1-X with L 2 at 70 8C yielded unprecedented conglomerate crystals of chiral (P)-[Pd 3 X 6 (L 1 ) 2 ]@(M)-[Pd 3 X 6 (L 1 )(L 2 )] (X = Cl: (P)-1- Cl@(M)-2-Cl; X = Br: (P)-1-Br@(M)-2-Br) and its enantio- meric (M)-[Pd 3 X 6 (L 1 ) 2 ]@(P)-[Pd 3 X 6 (L 1 )(L 2 )] (X = Cl: (M)-1- Cl@(P)-2-Cl ; X = Br : (M)-1-Br@(P)-2-Br ; Scheme 1). At this temperature, a partial substitution reaction was achieved in N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). A change in the molar ratio of the reactants did not have a significant effect on product formation in any case. Direct reaction of K 2 [PdX 4 ] with both L 1 and L 2 in appro- priate molar ratios produced the same conglomerate crystals, even at room temperature. However, at room temperature, the second step of the two-step reaction does not occur, which indicates that the partial substitution reaction of (P ,M)-1-X with L 2 requires slightly more vigorous conditions than the direct reaction. The crystalline solids are soluble in DMSO and DMF, but are almost insoluble in common organic solvents such as acetone, chloroform, and tetrahydrofuran. Recrystallizations of all of the products from DMF or DMSO yielded the same results irrespective of the co-solvent (acetone, EtOH, and MeOH), which indicates that all of the products were thermodynamically stable. The carbonyl stretching frequencies of (P ,M)-1-Cl (1656 cm À1 ) and (P ,M)- [*] H. Lee, T. H. Noh, Prof. Dr. O.-S. Jung Department of Chemistry and Chemistry Institute for Functional Materials Pusan National University, Pusan 609-735 (Korea) E-mail: oksjung@pusan.ac.kr [**] This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST; 2010- 0026167 and 2011-0013576). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201306674. . Angewandte Communications 11790  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2013, 52, 11790 –11795