Mixed-Ligand Metal–Organic Frameworks and Heteroleptic Coordination Cages as Multifunctional Scaffolds—A Comparison

Abstract

Conspectus Porous nanostructures and materials based on metal-mediated self-assembly have developed into a vibrantly studied subdiscipline of supramolecular chemistry during the past decades. In principle, two branches of such coordination compounds can be distinguished: Metal–organic frameworks (MOFs) on the one side represent infinite porous networks of metals or metal clusters that are connected via organic ligands to give solid-state materials. On the other hand, metal–organic cages (MOCs) are discrete and soluble systems with only a limited number of pores. Formation of a particular structure type is achieved by carefully balancing the donor site angles within the ligands as well as the nature and coordination geometry of the metal component. Years of research on MOFs and MOCs has yielded numerous types of well-defined porous crystals and complex supramolecular architectures. Since various synthetic routes and postsynthetic modification methods have been established, the focus of recent developments has moved toward the preparation of multifunctional systems that are able to mimic the structural and functional complexity of natural enzymes. This Account compares different strategies to prepare multifunctional MOFs and heteroleptic MOCs and gives a perspective on where to move forward. While the preparative toolbox for multifunctional MOFs is already quite mature, pore accessibility and substrate diffusion within the crystal have been identified as major challenges yet to be overcome. Only recently have a set of different strategies for the assembly of heteroleptic MOCs been developed. Such multifunctional cages can be formed from either partially protected or “naked” metal cations. Controlled assembly, producing single products rather than statistical mixtures, leans on assembly-dependent approaches making use of either steric effects or shape complementarity between the ligands. Further strategies include coordination-site engineering and hierarchical assembly of preformed components. The main challenge with heteroleptic, functional MOCs is to find a balance between the required dynamic assembly fidelity and the stability of the resulting system under operating conditions. If these limitations can be overcome in the future, chemists will be able to design multifunctional systems of similar activity and complexity as nature’s enzymes from simple and easily accessible synthetic building blocks. Major impacts on chemical sensing, small-molecule recognition and sequestration, drug delivery, and catalysis will be achieved by these materials.

Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes

“…In addition, ligand derivatization with sterically more demanding quinoline donors yielded an unprecedented [Pd 2 L 2 3 X 2 ] n + bowl structure (X = CH 3 CN or Cl – ), which allowed us to extend the guest scope to C 70 and use it as a fullerene-protecting group in cycloaddition reactions with anthracene. The bowl’s sterically congested coordination environment allows clean conversion into a heteroleptic assembly 11 via hierarchical reaction with bridging carboxylates, 8c,12 thus yielding a pill-shaped dimer, capable of binding two fullerenes. 13 All findings are supported by NMR and mass spectrometric results as well as single crystal X-ray structures of one ligand, both empty hosts, and three different host–guest complexes, achieved with a combination of cryogenic crystal handling, the use of highly brilliant synchrotron radiation, 14 and advanced macromolecular refinement protocols.…”

Self-assembled conjoined-cages

“…In contrast to the multiple binding sites of the multi-3D-cavity cages, there exist single-3D-cavity systems capable of accommodating multiple variety of guests in site-specific manner 28,29 . It is pertinent to note that a variety of 3D-metal−organic frameworks possessing an infinite number of conjoined-cages in their architectures are known 7 . As described above, only a handful of multi-3D-cavity self-assembled cages are known in literature, where the cavities are usually arranged in a linear fashion in their superstructures and the maximum number of cavities is three.…”

New Heteroleptic 3D Metal Complexes: Synthesis, Antimicrobial and Solubilization Parameters

“…In coordination chemistry, nitrogen-donor ligands, with strong redox stability and quite easy access to functionalization, have been extensively explored for the formation of heteroleptic complexes, that is, metal species which have more than one type of ligand [ 11 , 12 ]. The biological potentials including cytotoxicity, genotoxicity, antineoplastic, antitumor and bactericidal attributes have been reported for the heteroleptic chelates encompassing the bidentate ligands [ 2 , 13 ].…”