Sunlight-Induced Covalent Marriage of Two Triply Interlocked Pd 6 Cages and Their Facile Thermal Separation Dipak Samanta and Partha Sarathi Mukherjee* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India * S Supporting Information ABSTRACT: A template-free triply interlocked Pd 6 cage (2) was synthesized by two-component self-assembly of cis-blocked 90° acceptor cis-(tmen)Pd(NO 3 ) 2 (M) and 1,3,5-tris((E)-2-(pyridin-3-yl)vinyl)benzene (L). Assembly 2 was characterized by 1 H NMR and ESI-MS, and the structure was confirmed by X-ray crystallography, which revealed a parallel conformation of the olefin double bonds belonging to the adjacent cages in the solid state at a distance of 3.656 Å, thereby indicating the feasibility of [2+2] photochemical reaction. Two adjacent interlocked cages were covalently married together by intermolecular [2+2] cycloaddition in a single crystal-to-single crystal fashion upon exposure to sunlight/UV irradiation. Most surprisingly, the covalently married pair was easily separated thermally in aqueous medium under mild reaction conditions. M echanical interlocking is a crucial structural feature of proteins 1 and DNA 2 that significantly affects their stability and function and provides insight into protein folding mechanisms. 1b Various attempts to mimic such molecular principles of interlocked systems employed by nature, template strategy, 3 and self-assembly 4 have become powerful protocols using hydrogen-bonding, 5a hydrophobic interactions, 5b anions, 5c,d and π-π interactions 5e between subunits as driving forces. The interesting topological and physical properties of these interwoven systems make them attractive for a range of potential applications, such as biomaterials and molecular machines. 6 Sauvage, Fujita, Stoddart, and others have reported various interlocked systems such as rotaxanes, 7a catenanes, 7b-d Trefoil knots, 8a,b Borromean rings, 8c-e Solomon knots, 8c etc. with highly sophisticated architectural complexity. Recently, a complex non-DNA pentafoil knot consisting of a closed-loop pentameric cyclic double helicate was prepared by combining the use of metal helicates, anion template, and reversible imine bond formation assisted by the gauche effect. 9 Nevertheless, most such systems are designed from monocyclic molecules. Linking crossover points of bicyclic or tricyclic species with appropriate connectivity to construct mechanically linked architectures is synthetically challenging and relatively rare. 4e Moreover, understanding the principles of crystal engineer- ing 10 provides a map to design functional crystalline solids with remarkable stereocontrol. Intermolecular interactions in the solid state often play a crucial role by organizing the molecules with a precise geometry to produce regio- and stereoselective products that are inaccessible in solution. Subsequently, proximity between reaction centers of two substrates in the solid state may induce reactivity in otherwise unreactive molecules. 11 In this context, solid-state structural trans- formations by photochemical [2+2] reaction of CC bonds in various coordination polymers (CPs) and organic com- pounds have been extensively investigated in recent times, but there are very few reports on single crystal-to-single crystal (SCSC) manner involving only simple linear olefins. 12 The directional property and strength of noncovalent interactions including halogen bonds, hydrogen bonds, and π-π stacking have been carefully employed in organic systems to accomplish the prerequisite of [2+2] cycloaddition. 13 The role of the cation-π interaction, for instance, in solid-state photodimeriza- tion has been extensively investigated by Yamada and co- workers. 14 MacGillivray et al. have successfully introduced several hydrogen-bonded templates such as resorcinol and its derivatives to synthesize a number of interesting cyclobutane derivatives. 15a-c As observed in a recent report by Vittal et al., an organic polymer containing cyclobutane rings has been incorporated into a metal-organic framework (MOFs) by [2+2] cycloaddition. 15d In contrast, photodimerization of discrete coordination cages in both solution and solid state is extremely rare. 16 Despite the success in CPs and organic compounds, the majority of the reported systems are essentially irreversible due to their low thermal stability. 17 Herein we report the formation of a new triply interlocked Pd 6 -cage, [{(tmen)Pd} 6 (L) 3 ](NO 3 ) 12 (2), by two-component self-assembly of cis-blocked 90° Pd(II) acceptor M and tritopic donor L [where tmen = N,N,N′,N′-tetramethylethylenedi- amine, L = 1,3,5-tris((E)-2-(pyridin-3-yl)vinyl)benzene] (Scheme 1). Inspection of crystal packing shows that one pair of olefin double bonds of the adjacent cages is perfectly aligned parallel to each other in head-to-tail fashion, satisfying Received: November 4, 2014 Published: November 25, 2014 Scheme 1. Synthesis of (M 3 L 2 ) 2 Interlocked Cage 2 by Two- Component Self-Assembly Communication pubs.acs.org/JACS © 2014 American Chemical Society 17006 dx.doi.org/10.1021/ja511360e | J. Am. Chem. Soc. 2014, 136, 17006-17009