This Review covers design strategies, synthetic challenges, host-guest chemistry, and functional properties of interlocked supramolecular cages. Some dynamic covalent organic structures are discussed, as are selected examples of interpenetration in metal-organic frameworks, but the main focus is on discrete coordination architectures, that is, metal-mediated dimers. Factors leading to interpenetration, such as geometry, flexibility and chemical makeup of the ligands, coordination environment, solvent effects, and selection of suitable counter anions and guest molecules, are discussed. In particular, banana-shaped bis-pyridyl ligands together with square-planar metal cations have proven to be suitable building blocks for the construction of interpenetrated double-cages obeying the formula [M4 L8 ]. The peculiar topology of these double-cages results in a linear arrangement of three mechanically coupled pockets. This allows for the implementation of interesting guest encapsulation effects such as allosteric binding and template-controlled selectivity. In stimuli-responsive systems, anionic triggers can toggle the binding of neutral guests or even induce complete structural conversions. The increasing structural and functional complexity in this class of self-assembled hosts promises the construction of intelligent receptors, novel catalytic systems, and functional materials.

A breath of fresh air is sufficient for the eightfold S-monooxygenation of an interpenetrated double cage based on eight phenothiazine ligands and four square-planar-coordinated Pd(II) cations. Besides these two cages, which were both characterized by X-ray crystallography, an eightfold S-dioxygenated double-cage was obtained under harsher oxidation conditions.

A strategy to implement four members of the classic coal‐tar dye family, Michler's ketone, methylene blue, rhodamine B, and crystal violet, into [Pd2L4] self‐assemblies is introduced. Chromophores were incorporated into bis‐monodentate ligands using piperazine linkers that allow to retain the auxochromic dialkyl amine functionalities required for intense colors deep in the visible spectrum. Upon palladium coordination, ligands with pyridine donors form lantern‐shaped dinuclear cages while quinoline donors lead to strongly twisted [Pd2L4] helicates in solution. In one case, single crystal X‐ray diffraction revealed rearrangement to a [Pd3L6] ring structure in the solid state. For nine examined derivatives, showing colors from yellow to deep violet, CD spectroscopy discloses different degrees of chiral induction by an enantiomerically pure guest. Ion mobility mass spectrometry allows to distinguish two binding modes. Self‐assemblies based on this new ligand class promise application in chiroptical recognition, photo‐redox catalysis and optical materials.

Photon-powered charge separation is achieved in a supramolecular architecture based on the dense packing of functional building blocks. Therefore, self-assembled dimers of interpenetrated coordination cages consisting of redoxactive chromophors were synthesized in a single assembly step starting from easily accessible ligands and Pd(II) cations. Two backbones consisting of electron rich phenothiazine (PTZ) and electron deficient anthraquinone (ANQ) were used to assemble either homo-octameric or mixed-ligand double cages. The electrochemical and spectroscopic properties of the pure cages, mixtures of donor and acceptor cages and the mixedligand cages were compared by steady-state UV−vis and transient absorption spectroscopy, supported by cyclic voltammetry and spectroelectrochemistry. Only the mixedligand cages, allowing close intra-assembly communication between the donors and acceptors, showed the evolution of characteristic PTZ radical cation and ANQ radical anion features upon excitation in the transient spectra. In contrast, excitation of the mixtures of the homo-octameric donor and acceptor cages in solution did not lead to any signs of electron transfer. Densely packed photo-and redox-functional self-assemblies promise molecular-level control over the morphology of the charge separation layer in future photovoltaic applications.

Previously, we introduced a series of anion-binding interpenetrated double-cages based on phenothiazine and its mono- and di-S-oxygenated derivatives. Here, we complete the structural comparison of the three related assemblies by an X-ray single crystal analysis of the sulfone derivative. We further show that the three palladium cages coexist in solution upon post-assembly mixing due to the very slow ligand exchange whereas treatment of binary mixtures of the corresponding ligands with Pd(II) leads to the formation of mixed cages comprising a statistical ligand distribution. In contrast, mixtures of one of these ligands with a shorter ligand derivative lead to narcissistic self-assembly into a double-cage and a coexisting small monomeric cage, regardless of the order of mixing and Pd(II) addition.