Cofactor-Mediated Nucleophilic Substitution Catalyzed by a Self-Assembled Holoenzyme Mimic

In-Situ Growth of Metallocluster Inside Heterometal-Organic Cage to Switch Electron Transfer for Targeted CO2 Photoreduction

Construction of metal-organic cages (MOCs) with internal modifications is a promising avenue to build enzyme-like cavities and unlocking the mystery of highly catalytic activity and selectivity of enzymes. However, current interests are mainly focused on single-metal-node cages, little achievement has been expended to metallocluster-based architectures, and the in situ endogenous generation of metal clusters. Herein, based on the hard-soft-acids-bases (HSAB), the metallocluster-based heterometallic MOC (Cu3VMOP) constructed of [Cu3OPz3]+ and [V6O6(OCH3)9(SO4)(CO2)3]2- clusters was obtained by one-pot method. In addition, Cu4I4 was generated in situ in the cage to form Cu4I4@Cu3VMOP by the coordination-driven hierarchical self-assembly strategy. As catalysts for CO2 reduction, Cu3VMOP produces HCOOH and CH3COOH as the main reduction product with yield of CH3COOH up to 0.9 mmol g-1, ranking among the highest value of reported materials, whereas Cu4I4@Cu3VMOP exhibited targeted CO2-to-HCOOH conversion with 100 % formic acid selectivity and the yield outperforms that of Cu3VMOP by 5 fold. Theoretical calculations and femtosecond time-resolved transient absorption reveal that endogenous Cu4I4 not only regulates orbital arrangements and enhances localized electron states to generate a long-lived charge-separated state, but also raises *CO coupling energy barrier, resulting in the targeted conversion of CO2 to formic acid.

Solvent Effects and Internal Functions Control Molecular Recognition of Neutral Substrates in Functionalized Self-Assembled Cages

A suite of internally functionalized Fe4L6 cage complexes has been synthesized with lipophilic end groups to allow dissolution in varied solvent mixtures, and the scope of their molecular recognition of a series of neutral, nonpolar guests has been analyzed. The lipophilic end groups confer cage solubility in solvents with a wide range of polarities, from hexafluoroisopropanol (HFIP) to tetrahydrofuran, and the hosts show micromolar affinities for neutral guests, despite having no flat panels enclosing the cavity. These hosts allow interrogation of the effects of an internal functional group on guest binding properties, as well as solvent-based driving forces for recognition. Introducing polar effects to the interior of the cavity enhances guest binding affinity in nonpolar solvents; adding space-filling aliphatic groups reduces affinity in all cases. While high dielectric solvents such as acetonitrile strongly favor guest binding, "low dielectric, high polarity" solvents such as HFIP strongly occupy the cavity and prevent guest recognition. Analysis of the cage optical transitions shows that the guests interact with the central ligand cores and reside in close proximity to the internal functions. These results have implications for supramolecular catalysis: balancing directed host:guest interactions (e.g., H-bonds) with entropic effects from solvent displacement is essential for reactions in these (and related) biomimetic hosts.

Catalytic Inhibition of Base-Mediated Reactivity by a Self-Assembled Metal-Ligand Host

Spacious M4 L6 tetrahedra can act as catalytic inhibitors for base-mediated reactions. Upon adding only 5 % of a self-assembled Fe4 L6 cage complex, the conversion of the conjugate addition between ethylcyanoacetate and β-nitrostyrene catalyzed by proton sponge can be reduced from 83 % after 75 mins at ambient temperature to <1 % under identical conditions. The mechanism of the catalytic inhibition is unusual: the octacationic Fe4 L6 cage increases the acidity of exogenous water in the acetonitrile reaction solvent by favorably binding the conjugate acid of the basic catalyst. The inhibition only occurs for Fe4 L6 hosts with spacious internal cavities: minimal inhibition is seen with smaller tetrahedra or Fe2 L3 helicates. The surprising tendency of the cationic cage to preferentially bind protonated, cationic ammonium guests is quantified via the comprehensive modeling of spectrophotometric titration datasets.

Modifying the internal substituents of self-assembled cages controls their molecular recognition and optical properties

Self-assembled Fe₄L₆ cage complexes with variable internal functions can be synthesized from a 2,7-dibromocarbazole ligand scaffold, which orients six functional groups to the cage interior. Both ethylthiomethylether and ethyldimethylamino groups can be incorporated. The cages show strong ligand-centered fluorescence emission and a broad range of guest binding properties. Coencapsulation of neutral organic guests is favored in the larger, unfunctionalized cage cavity, whereas the thioether cage has a more sterically hindered cavity that favors 1 : 1 guest binding. Binding affinities up to 10⁶ M^-1 in CH₃CN are seen. The dimethylamino cage is more complex, as the internal amines display partial protonation and can be deprotonated by amine bases. This amine cage displays affinity for a broad range of neutral organic substrates, with affinities and stoichiometries comparable to that of the similarly sized thioether cage. These species show that simple variations in ligand backbone allow variations in the number and type of functions that can be displayed towards the cavity of self-assembled hosts, which will have applications in biomimetic sensing, catalysis and molecular recognition.

Beyond Platonic: How to Build Metal-Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes

The field of metallosupramolecular chemistry has advanced rapidly in recent years. Much work in this area has focused on the formation of hollow self-assembled metal-organic architectures and exploration of the applications of their confined nanospaces. These discrete, soluble structures incorporate metal ions as 'glue' to link organic ligands together into polyhedra.Most of the architectures employed thus far have been highly symmetrical, as these have been the easiest to prepare. Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed. Herein we summarize recent approaches, which taken together form the first draft of a handbook for the design of higher-complexity, lower-symmetry, self-assembled metal-organic architectures.

Size- and Shape-Selective Catalysis with a Functionalized Self-Assembled Cage Host

A self-assembled Fe₄L₆ cage with internally oriented carboxylic acid functions was shown to catalyze a variety of dissociative nucleophilic substitution reactions that proceed via oxocarbenium ion or carbocation intermediates. The catalytic behavior of the cage was compared to that of other small acid catalysts, which illustrated large differences in reactivity of the cage-catalyzed reactions, dependent on the structure of the substrate. For example, only a 5% cage confers a 1000-fold rate acceleration of the thioetherification of vinyldiphenylmethanol when compared to the rate with free carboxylic acid surrogates but only a 52-fold acceleration in the formation of small thioacetals. Multiple factors control the variable reactivity in the host, including substrate inhibition, binding affinity, and accessibility of reactive groups once bound. Simple effective concentration increases or the overall charge of the cage does not explain the variations in reactivity shown by highly similar reactants in the host: small differences in structure can have large effects on reactivity. Reaction of large spherical guests is highly dependent on substitution, whereas flat guests are almost unaffected by size and shape differences. The cage is a promiscuous catalyst but has strong selectivity for particular substrate shapes, reminiscent of enzymatic activity.

Selective, cofactor-mediated catalytic oxidation of alkanethiols in a self-assembled cage host

A spacious Fe(ii)-iminopyridine self-assembled cage complex can catalyze the oxidative dimerization of alkanethiols, with air as stoichiometric oxidant. The reaction is aided by selective molecular recognition of the reactants, and the active catalyst is derived from the Fe(ii) centers that provide the structural vertices of the host. The host is even capable of size-selective oxidation and can discriminate between alkanethiols of identical reactivity, based solely on size.

No, Not That Way, the Other Way: Creating Active Sites in Self-Assembled Host Molecules

This Account describes our efforts over the last decade to synthesize self-assembled metal–ligand cage complexes that display reactive functional groups on their interiors. This journey has taken us down a variety of research avenues, including studying the mechanism of reversible self-assembly, analyzing ligand self-sorting properties, post-assembly reactivity, molecular recognition, and binding studies, and finally reactivity and catalysis. Each of these individual topics are discussed here, as are the lessons learned along the way and the future research outlook. These self-assembled hosts are the closest mimics of enzymes to date, as they are capable of size- and shape-selective molecular recognition, substrate activation and turnover, as well as showing less common ‘biomimetic’ properties such as the ability to employ cofactors in reactivity, and alter the prevailing mechanism of the catalyzed reactions.

1 Introduction

2 Paddlewheels and Self-Sorting Behavior

3 First-Row Transition-Metal-Mediated Assembly: Sorting and Stereochemical Control

4 Post-Assembly Reactivity

5 Molecular Recognition and Catalysis

6 Conclusions and Outlook