A Self‐Assembled M₈L₆ Cubic Cage that Selectively Encapsulates Large Aromatic Guests

Abstract

Biological encapsulants such as ferritin, [1] lumazine synthase, [2] and viral capsids [3] achieve their selective separation and sequestration of substrates by providing: 1) a guest microenvironment isolated from the surroundings, 2) favorable interactions complementing a size and shape match with the encapsulated guests, and 3) sufficient flexibility to allow guests to be incorporated and released.[4] These biological hosts self-assemble from multiple copies of identical protein subunits, the symmetries and connection properties [5] of which dictate the hollow polyhedral structures of the encapsulant. In order to create abiological molecular systems that are capable of expressing functions of similar complexity to biological systems and to explore new applications of synthetic hosts, [6] there is a need to create synthetic capsules capable of tightly and selectively binding large substrates.Taking inspiration from natural systems [1][2][3] and from other previously reported metal-organic capsules, [7] we report the design and synthesis of a series of metallo-supramolecular cage molecules capable of selectively encapsulating large aromatic guests. The necessary features to achieve this function are: 1) small pore sizes to isolate guests from the environment, [8] 2) large cavity sizes to ensure sufficient volume for the guests of interest, 3) enough flexibility and lability to allow guests to enter and exit the host, and 4) regions of the cage walls rich in p-electron density to provide favorable interactions with targeted guests.[9] The selective encapsulation of large aromatic molecules is an attractive goal since their physicochemical properties are similar, which can render their separation difficult. The higher fullerenes represent particularly attractive targets because their potential applications [10] remain difficult to explore because of the challenges associated with their separation, despite recent advances. [11] Employing principles of geometric analysis, [5] we determined that combination of the C 4 -symmetric tetrakis-bidentate ligand shown in Figure 1 with the C 3 -symmetric iron(II) tris(pyridylimine) center would result in the formation of an O-symmetric cubic structure of general formula M 8 L 6 , in which the corners of the cube are defined by the metal centers and the faces by the ligands (Figure 1). This cage represents the first example of a new class of closed-face metallosupramolecular cubic hosts to be synthesized. In order to provide favorable binding sites for our target guests we incorporated porphyrin moieties, which have previously been demonstrated to interact with large aromatic molecules, [11a-c, 12] into our design. This design also provides for small pore sizes and the potential to create new chemical functionality through the introduction of different metal ions into the centers of the N 4 macrocycle and by substituting these metals axial ligands. We chose to employ labile iron(II) centers with pyridylimine ligands as chelating agents to allow for the formation of the liga...

Sponge-like molecular cage for purification of fullerenes

“…Host platforms containing extended p-systems have been designed because of their affinity towards the sphere-like unsaturated structure of fullerenes. In this line, several covalent or supramolecular approaches pursued include: (a) macrocyclic arene-based receptors [18][19][20][21][22][23][24] , (b) macrocyclic structures containing aromatic heterocycles [25][26][27][28] , (c) p-extended TTF receptors [29][30][31][32][33] and (d) macrocyclic or jawlike structures containing porphyrin moieties 17,[34][35][36][37][38][39][40][41][42][43][44][45] . Interestingly, Fujita and co-workers reported the use of the empty channels of a metal-organic framework (MOF) for encapsulating fullerenes upon soaking MOF crystals in fullerene-containing toluene.…”

Photochemical H2 Evolution Catalyzed by Porphyrin-based Cubic Cages Singly and Doubly Encapsulating PtCl2(4,4′-dimethyl-2,2′-bipyridine)

“…6 Nevertheless, it was also observed that some platinum(II) complexes tend to degrade during photolysis, even due to undesirable reactions caused by the H 2 evolved, leading to dispersion of colloidal platinum nanoparticles. 7 In our hope to improve the robustness of the Pt(II) catalysts during photolysis, the present study focuses on the Pt(II) catalyst encapsulated within the title self-assembled molecular capsule, 8,9 which was reported by others from different perspectives. 1012 In this study, a molecular cube consisting of six metal-free porphyrin fragments (Cube; [C 408 H 276 (Figure 1), reported by Nitschke et al, 9 has been selected as a host framework.…”

“…9,11 Taking these into consideration, we have selected several platinum(II) complexes having π-conjugated ligand systems, 3,4 such as PtCl 2 (bpy) (PtBpy), PtCl 2 (dmbpy) (PtDmb; dmbpy: 4,4¤-dimethyl-2,2¤-bipyridine), PtCl 2 (phen) (PtPhn; phen: 1,10-phenanthroline), and [PtCl(tpy)] + (PtTpy; tpy: 2,2¤:6¤,2¤¤-terpyridine) ( Figure 2). Based on the method used to encapsulate guest molecules within Cube, 9 Cube was reacted with each catalyst (4 equiv) in DMF (N,N-dimethylformamide) at 80°C for 22 h (see SI for details) followed by nearly quantitative deposition of the Cube components by adding a large excess of ether. Although we were unable to clarify the optimum conditions affording a single product in a quantitative yield, we could successfully confirm that singly and doubly modified Cube's, i.e., Cube(PtCat) and Cube(PtCat) 2 (PtCat = PtBpy, PtDmb, or PtPhn), can be given when a neutral catalyst is employed as a guest molecule (Scheme 2).…”

Symmetry breaking in self-assembled M 4 L 6 cage complexes

“…Platonic or Archimedean metal-organic polyhedra can be constructed by a careful choice of ligands and metal ions (8,(23)(24)(25)(26)(27), where the geometries of the individual building blocks define the symmetry axes of the polyhedron. Although high-symmetry architectures represent a great achievement in terms of logical molecular design, lower-symmetry ones are possibly of still greater interest.…”

“…Besides the fundamental interest in symmetry breaking mentioned above (1), an asymmetric capsule could achieve recognition of asymmetric substrates (28) and possibly catalyze asymmetric transformations (29). So far all reported metal-organic capsules have high symmetry (27,30,31), although in some cases they are chiral (29,(32)(33)(34), and specific guest molecules are observed to stabilize lower-symmetry structures (32,35). M 4 L 6 capsules are constructed from octahedrally coordinated metal ions and linear bis-bidentate ligands.…”