Clathrochelate Metalloligands in Supramolecular Chemistry and Materials Science

Abstract

The term "clathrochelate" describes a complex in which a coordinatively saturated metal ion is surrounded by a macropolycyclic ligand. First examples of clathrochelate complexes were reported 50 years ago. Meanwhile, the synthesis and reactivity of clathrochelates have been investigated in detail, and numerous applications have been explored. In this Account, we summarize work on the utilization of transition metal clathrochelates as metalloligands in supramolecular chemistry and materials science, with special focus on results from our group. First, we discuss the chemistry of boron-capped clathrochelates. These complexes are facile to synthesize by metal-templated condensation reactions. The synthesis is modular, and it is straightforward to implement structural variations. Importantly, it is possible to attach functional groups such as amines, pyridines, or carboxylic acids to the ligand periphery. Other noteworthy features of boron-capped clathrochelates are high thermodynamic and kinetic stability, tunable redox potential, and good solubility. Next, we show that clathrochelate-based metalloligands can be used to build molecularly defined metal-ligand assemblies of nanoscale dimensions. Different molecular architectures are described, including coordination cages with unusual gyrobifastigium or square orthobicupola-like structures. Metalloligands containing multiple clathrochelate complexes are particularly well suited to build large metal-ligand assemblies (>3 nm) with minimal synthetic efforts. Boron-capped clathrochelates have also been investigated in the context of materials chemistry. Linear or cross-linked clathrochelate polymers were found to display permanent porosity. Furthermore, such polymers were used to prepare conducting films on electrodes. Clathrochelate metalloligands are well suited to prepare metal-organic frameworks (MOFs). The high stability of clathrochelates ensures compatibility with harsh reaction conditions, and it mitigates potential problems such as exchange reactions. Boron-capped clathrochelates can be decorated with functional groups in lateral and apical position, and it is possible to use these complexes as multiconnected nodes in polymeric structures. Overall, we hope to convey the utility of clathrochelate complexes in supramolecular chemistry and materials science. The work published thus far gives a first glimpse of the potential of these compounds, but there are other directions, which are waiting to be explored. For example, it will be interesting to study the properties of nanostructures based on chiral clathrochelate complexes. Furthermore, the redox and magnetic properties of clathrochelates may give rise to novel functional materials. Given that clathrochelates are straightforward to prepare, we hope that others will join the efforts to explore the supramolecular and materials chemistry of these interesting molecular building blocks.

Unveiling the Occurrence of Co(III) in NiCo Layered Electroactive Hydroxides: The Role of Distorted Environments

“…This behavior known as “ cis distortion ”, [70, 71] has been reported before for six‐fold‐coordinated Cu(II) coordination compounds [72, 73] as well as several clathrochelates metalloligands, [74, 75] providing a versatile tool to tune the electronic properties of the metallic centers [76] . Moreover, this approach was recently employed to effectively stabilize highly oxidized centers as Fe(IV) in aqueous solution, under ambient conditions [77] .…”

Site-Specific Metal Chelation Facilitates the Unveiling of Hidden Coordination Sites in an FeII/FeIII-Seamed Pyrogallol[4]arene Nanocapsule

“…The design of metal-organic supramolecular structures with well-defined shapes and dimensions continues to attract intense interest, a primary reason being their ability to mimic biologically active sites. [1][2][3][4][5][6][7][8] These inorganic-organic hybrids are typically formed by the metal-directed assembly of suitably functionalized organic ligands, [9][10][11][12][13][14][15][16][17] and recent years have witnessed the synthesis of such species with paramagnetic metal ions; this is a popular strategy due to the inherent potential for one to control or influence the resulting magnetic properties of a system. 9,[18][19][20][21][22][23][24] Our efforts to synthesize such supramolecular hybrids primarily utilize C-alkylpyrogallol [4]arenes (PgCn, where n is the number of carbon atoms in pendant alkyl chains) as organic ligands.…”

Cages vs. Prisms: Controlling the Formation of Metallosupramolecular Architectures with Ligand Side‐Chains

“…[25] A unique feature of 1 is the presence of three bulky and rigid phenanthrene groups, which point away from the ligand center. Compared to other clathrochelate complexes described in the literature, [27,28] the conformational flexibility of the central part of complex 1 is particularly low. The rigidity of the core of complex 1 is evident when comparing its solid-state structure (determined by X-ray diffraction) with that of metalloligand 2 [25] (Figure 2).…”

“…For our investigations, we have prepared the new metalloligand 1 (Scheme ). This ligand contains an inert Fe II cage complex (′clathrochelate complex′), in its center, and it features four terminal 4‐pyridyl groups. The synthesis of 1 was accomplished in two steps.…”