Geometric complementarity in assembly and guest recognition of a bent heteroleptic cis-[Pd₂LA₂LB₂] coordination cage

Abstract

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 [Pd2LA2LB2]4+ coordination cage achieved through the geometric complementarity of two carefully designed ligands, LA and LB. With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [Pd2LA4]4+ strained helical assembly and a [Pd4LB8]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-[Pd2LA2LB2]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 [Pd2LA2LB2]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-[Pd2LA2LB2]4+ species. The robust nature of the cis-[Pd2LA2LB2]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.