Rational Design and Construction of Hierarchical Superstructures Using Shape-Persistent Organic Cages: Porphyrin Box-Based Metallosupramolecular Assemblies

Abstract

We report a new approach to building hierarchical superstructures using a shape-persistent porous organic cage, which acts as a premade secondary building unit, and coordination chemistry. To illustrate the principle, a zinc-metalated porphyrin box (Zn-PB), a corner-truncated cubic porous cage, was connected by suitable dipyridyl terminated bridging ligands to construct PB-based hierarchical superstructures (PSSs). The PSSs were stabilized not only by the coordination bonds between Zn ions and bipyridyl-terminated ligands but also by π–π interactions between the corners of the Zn-PB units. By varying the length of the linker, we identified an optimum range of the linker length for construction of PSSs. The PSSs have large void volumes and an extrinsic surface area compared to the parent PBs, which can be exploited for the selective encapsulation and interior functionalization of the PSSs for various applications, including catalysis. We observed that singlet oxygen induced synthesis of the natural product, juglone, is more efficiently catalyzed by PSS-1 than its constituent component Zn-PB.

Supramolecular Tuning Enables Selective Oxygen Reduction Catalyzed by Cobalt Porphyrins for Direct Electrosynthesis of Hydrogen Peroxide

“…MALDI‐MS and UV/Vis show complete metalation for each catalyst (Figures S3–S4, S6–S10). The porous nature of Co‐PB‐1(6) was confirmed with N 2 sorption analysis giving a Brunauer‐Emmett‐Teller surface area of 562 m 2 g −1 (Figure S15), which is in agreement with our previous report on Zn‐PB‐1(6) . In contrast, N 2 sorption studies showed Co‐rPB‐1(6) to be relatively non‐porous with a surface area of 23 m 2 g −1 .…”

“…To synthesize the supramolecular catalysts, we first prepared the free‐base porphyrin box, PB‐1(6), through condensation of six tetraformylphenyl porphyrins with eight tripodal triamines bearing hexyloxy solubilizing groups . Treatment of PB‐1(6) with excess NaBH 4 reduces the twenty‐four imine bonds to amines yielding the reduced porphyrin box, rPB‐1(6), as shown in Scheme .…”

“…Furthermore, 1 H NMR spectroscopy shows the disappearance of the imine peak and subsequent broadening of the remaining resonances, which we attribute to a large degree of conformational heterogeneity (See Supporting Information). Using analogous methods to our previous work, cobalt was introduced into all six porphyrin moieties in both PB‐1(6) and rPB‐1(6) by heating with CoCl 2 and 2,6‐lutidine in THF (Scheme ). MALDI‐MS and UV/Vis show complete metalation for each catalyst (Figures S3–S4, S6–S10).…”

Elucidating heterogeneous photocatalytic superiority of microporous porphyrin organic cage

“…These unique synthetic hosts with well-defined cavities are capable of offering diverse weak interactions to recognize and encapsulate organic substrates 46 , constrain their orientations 47 , stabilize intermediates 48 , and mediate reaction kinetics, and thus used for their diverse applications 49 . Although the enzyme-like catalytic activities have been revealed for such special hosts by grafting/post-trapping catalytically active sites on cage skeletons or in cavities, respectively 25,44,45,[50][51][52] , visible light photocatalytic molecular cages toward organic reactions have been still very rarely realized because of the low quantum yield of excited states attributed to the cage-induced self-quenching effect 23,[53][54][55] . It has been envisioned that the ultrafast photo-induced energy/electron communication between hosts and cavity-confined guests in a close distance 23 may facilitate the rapid conversion of reaction reagents.…”

Supramolecular Tuning Enables Selective Oxygen Reduction Catalyzed by Cobalt Porphyrins for Direct Electrosynthesis of Hydrogen Peroxide

“…The oxygen reduction reaction (ORR) is a central chemical transformation for biological and chemical energy conversion and storage, and can proceed through either a 2 e pathway to produce H 2 O 2 or a 4 e pathway to produce H 2 O. [1] Using electricity offers the possibility to drive this process with sustainable energy input, and in the context of molecular catalysts, structural and functional mimics of cytochrome c oxidase enzymes that perform ORR in biology have focused on the selective formation of H 2 O to better understand multielectron small molecule activation and develop efficient fuel cells. [2] Alternatively, H 2 O 2 is a valuable oxidant, energy carrier, and commodity chemical, and using electricity for H 2 O 2 production presents an environmentally green alternative to the traditional anthraquinone process for its synthesis, [3] with the key challenge to avoid further reduction to more thermodynamically favored H 2 O product.…”