Geometric Complementarity in Assembly and Guest Recognition of a Bent Heteroleptic cis-[Pd2L2AL2B] 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 [(Pd2L2L2B)-L-A](4+) coordination cage achieved through the geometric complementarity of two carefully designed ligands, LA and L-B. With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [Pd2L4A](4+) strained helical assembly and a [Pd4L8B](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-[(Pd2L2L2B)-L-A](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 [(Pd2L2L2B)-L-A](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-[(Pd2L2L2B)-L-A](4+) species. The robust nature-of the cis-[(Pd2L2L2B)-L-A](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 [Pd2L4] cages and opens up possibilities of introducing multiple functionalities into a single self-assembled architecture.