A Novel M₈L₆ Cubic Cage That Binds Tetrapyridyl Porphyrins: Cage and Solvent Effects in Cobalt‐Porphyrin‐Catalyzed Cyclopropanation Reactions

Abstract

Confinement of a catalyst can have a significant impact on catalytic performance and can lead to otherwise difficult to achieve catalyst properties. Herein, we report the design and synthesis of a novel caged catalyst system Co−G@Fe8(Zn−L ⋅ 1)6, which is soluble in both polar and apolar solvents without the necessity of any post‐functionalization. This is a rare example of a metal‐coordination cage able to bind catalytically active porphyrins that is soluble in solvents spanning a wide variety of polarity. This system was used to investigate the combined effects of the solvent and the cage on the catalytic performance in the cobalt catalyzed cyclopropanation of styrene, which involves radical intermediates. Kinetic studies show that DMF has a protective influence on the catalyst, slowing down deactivation of both [Co(TPP)] and Co−G@Fe8(Zn−L ⋅ 1)6, leading to higher TONs in this solvent. Moreover, DFT studies on the [Co(TPP)] catalyst show that the rate determining energy barrier of this radical‐type transformation is not influenced by the coordination of DMF. As such, the increased TONs obtained experimentally stem from the stabilizing effect of DMF and are not due to an intrinsic higher activity caused by axial ligand binding to the cobalt center ([Co(TPP)(L)]). Remarkably, encapsulation of Co−G led to a three times more active catalyst than [Co(TPP)] (TOFini) and a substantially increased TON compared to both [Co(TPP)] and free Co−G. The increased local concentration of the substrates in the hydrophobic cage compared to the bulk explains the observed higher catalytic activities.

Controlling the Activity of a Caged Cobalt‐Porphyrin‐Catalyst in Cyclopropanation Reactions with Peripheral Cage Substituents

“…Our efforts to grow single crystals suitable for X‐ray diffraction were unsuccessful (solvent layering and vapor diffusion at different temperatures only led to solid powders, not suitable for X‐ray). Cage Fe 8 (Zn‐L ⋅ 1) 6 was synthesized by following the same synthetic protocol and all analytical data are in line with the formation of a species identical to the previously reported cage [15] …”

“…For the purpose of studying effects of the cage periphery, we explored three different caged catalyst with different exo‐polarities (Figure 1). The previously reported Fe 8 (Zn‐L ⋅ 1) 6 cage is functionalized with apolar aliphatic tails (Figure 1, left), [15] the Fe 8 (Zn‐L ⋅ 3) 6 cage is decorated with polar groups at the periphery (Figure 1, right) and the reference Fe 8 (Zn‐L ⋅ 2) 6 cage (Figure 1, middle). We envisioned that the different peripheries of these caged systems could be used to control the affinity of caged catalyst for the substrate and thus lead to different activities.…”

“…We previously demonstrated the effect of the second‐coordination sphere on catalysis, wherein a caged catalyst functionalized with apolar groups was shown to increase the catalytic activities in the cobalt‐catalyzed cyclopropanation of styrene when compared to the free catalyst [15] . In this work, we demonstrate an unprecedented strategy for controlling catalytic performance, in which differences in peripheral functionalization of cubic supramolecular cages hosting a cobalt catalyst influences the catalytic activity for cyclopropanation of styrene.…”

Stereoconvergent, photocatalytic cyclopropanation reactions of β-substituted styrenes with ethyl diazoacetate

“…using metal-bound or free triplet carbene intermediates. In this context, the groups of de Bruin [32][33][34] and Zhang [35][36][37] recently reported on Co-based catalyzed cyclopropanation reactions of styrene that involve a metal-bound triplet carbene intermediate (Scheme 1b). In a related context, our group reported that simple acceptor-only diazoalkanes would undergo the formation of a free triplet carbene under photocatalytic conditions to achieve gem-difluoroolefination reactions.…”

Solvent Drives Switching between Λ and Δ Metal Center Stereochemistry of M8L6 Cubic Cages

“…Metal-porphyrin-containing supramolecular architectures have been used to bind pyridine-containing guests, driven by the coordination of pyridine nitrogen donors to porphyrin metal centers. 29 Taking advantage of such coordination-mediated host–guest chemistry, bidentate 4,4′-dipyridyl naphthalenediimide guest G1 , with a N···N distance of 15.4 Å, was investigated. The mixture of equimolar amounts of G1 and 1 in acetonitrile led to the formation of host–guest complex G1 ⊂ 1 ( Figure 4 , left).…”