Geometric Complementarity in Assembly and Guest Recognition of a Bent Heteroleptic cis -[Pd 2 L A 2 L B 2 ] Coordination Cage

Due to the inherent difficulties in achieving a defined and exclusive formation of multicomponent assemblies against entropic predisposition, we present the rational assembly of a heteroleptic [Pd 2 L A 2 L B 2 ] 4+ coordination cage achieved through the geometric complementarity of two carefully designed ligands, L A and L B . With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [Pd 2 L A 4 ] 4+ strained helical assembly and a [Pd 4 L B 8 ] 8+ box-like structure, both of which were characterized by X-ray analysis. Combined, however, the two ligands could be used to cleanly assemble a cis -[Pd 2 L A 2 L B 2 ] 4+ cage with a bent architecture. The same self-sorted product was also obtained by a quantitative cage-to-cage transformation upon mixing of the two homoleptic cages revealing the [Pd 2 L A 2 L B 2 ] 4+ assembly as the thermodynamic minimum. The structure of the heteroleptic cage was examined by ESI-MS, COSY, DOSY, and NOESY methods, the latter of which pointed toward a cis -conformation of ligands in the assembly. Indeed, DFT calculations revealed that the angular ligands and strict Pd(II) geometry strongly favor the cis -[Pd 2 L A 2 L B 2 ] 4+ species. The robust nature of the cis -[Pd 2 L A 2 L B 2 ] 4+ cage allowed us to probe the accessibility of its cavity, which could be utilized for shape recognition toward stereoisomeric guests. The ability to directly combine two different backbones in a controlled manner provides a powerful strategy for increasing complexity in the family of [Pd 2 L 4 ] cages and opens up possibilities of introducing multiple functionalities into a single self-assembled architecture.