Bioinspired complexes confined in well-defined capsules: getting closer to metalloenzyme functionalities

Oxidation-Deformylation Cascade Catalyzed By a Mononuclear Copper Complex

In this study, two copper complexes were synthesized using N3 (arising from two pyridines and one amide group) containing ligands N-(2-picolyl)picolinamide (L1H) and bis(2-pyridylcarbonyl)amine (L2H), forming [(L1)CuII(OH2)(NO3)] (1) and [(L2)CuII(OH2)2](NO3) (2). The reaction of complex 1 with hydrogen peroxide in alcoholic solvents yielded a formate-bound complex. Studies with isotopically labeled 13C ethanol indicated that formate originates from the C1 of ethanol after C-C bond cleavage. Complex 1 was found to catalytically convert primary alcohols into formic acid probably following a two-step process: (i) alcohol oxidation to aldehyde and (ii) aldehyde deformylation. Further experiments with 2-phenylpropionaldehyde (2-PPA) confirm the ability of complex 1 to catalyze aldehyde deformylation. Both steps of the reaction are associated with significant kinetic deuterium isotope effects (KDIE), suggesting that hydrogen atom abstractions (HAA) occur during the rate-determining steps of both conversions. Overall, this system proposes a clean catalytic process for alcohol-to-formic acid conversion, operating under mild conditions, and offering potential synthetic applications.

Tuning Reactivity in Cu/TEMPO Catalyzed Alcohol Oxidation Reactions

A dinuclear copper(I) complex Cu2L22 (L2 = 3,3-dimethyl-1-(1-methyl-1H-benzo[d]imidazole-2-yl)-N-(propan-2-ylidene)butan-2-amine) containing benzimidazole and imino donors was previously reported by some of us as an efficient catalyst for the aerobic oxidation of alcohols to aldehydes in presence of TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) and an external base NMI (N-methyl imidazole). Cu(III)2(bis-μ-oxo) and Cu(II)2(bis-μ-hydroxo) cores were trapped as viable intermediates in the reaction, which provided deeper mechanistic insights. Here, we report two new ligand systems L3 (N-isopropyl-3,3-dimethyl-1-(1-methyl-1H-benzol[d]imidazole-2-yl)butane-2-amine) and L4 ((Z)-2,4-di-tert-butyl-6-(((3,3-dimethyl-1-(1-methyl-1H-benzol[d]imidazole-2-yl)butane-2-yl)imino)methyl)phenol), which are designed to perturb the overall electronics of the complexes and the resulting effects on their O2 activation mechanisms. The stronger donation of the secondary amine group stabilizes a mononuclear CuIL3 core, which nevertheless follows a dinuclear O2 activation mechanism as in Cu2L22. Notably, the CuIL3/TEMPO catalyst system performs the aerobic oxidation of alcohols to aldehydes with good yields and turnover numbers, even in the absence of NMI. The dinuclear CuI 2L42 complex involving a non-innocent phenolate group, in contrast, exhibits depleted catalytic activity, because of the instability of the Cu(III)2(bis-μ-oxo) core against intramolecular H-atom abstraction to form an alkoxo bridged dicopper(II) complex.

Mimicking the CuD Site of pMMO via a Copper Cage-Complex

The mechanism of action of particulate monooxygenase (pMMO) has yet to be determined. The CuD site with two histidines and an asparagine coordinating to copper has been identified as a potential active site of pMMO. Here, we present a copper cage complex, that assembles this coordination sphere, being a structural mimic of the pMMO. The cage is capable to catalyze aerobic oxidations of organic substrates such as benzylic alcohols to aldehydes and hydroquinones to quinones. This is inspired by the reactivity that is observed for enzymatic active sites possessing copper.

Light-Induced Reactivity Switch at O2-Activating Bioinspired Copper(I) Complexes

Using light to unveil unexplored reactivities of earth-abundant metal-oxygen intermediates is a formidable challenge, given the already remarkable oxidation ability of these species in the ground state. However, the light-induced reactivity of Cu-O2 intermediates still remains unexplored, due to the photoejection of O2 under irradiation. Herein, we describe a photoinduced reactivity switch of bioinspired O2-activating CuI complexes, based on the archetypal tris(2-pyridyl-methyl)amine (TPA) ligand. This report represents a key precedent for light-induced reactivity switch in Cu-O2 chemistry, obtained by positioning C-H substrates in close proximity of the active site. Open and caged CuI complexes displaying an internal aryl ether substrate were evaluated. Under light, a Cu-O2 mediated reaction takes place that induces a selective conversion of the internal aryl ether unit to a phenolate-CH2- moiety with excellent yields. This light-induced transformation displays high selectivity and allows easy postfunctionalization of TPA-based ligands for straightforward preparation of challenging heteroleptic structures. In the absence of light, O2 activation results in the standard oxidative cleavage of the covalently attached substrate. A reaction mechanism that supports a monomeric cupric-superoxide-dependent reactivity promoted by light is proposed on the basis of reactivity studies combined with (TD-) DFT calculations.

Molecular vessels from preorganised natural building blocks

Supramolecular vessels emerged as tools to mimic and better understand compartmentalisation, a central aspect of living matter. However, many more applications that go beyond those initial goals have been documented in recent years, including new sensory systems, artificial transmembrane transporters, catalysis, and targeted drug or gene delivery. Peptides, carbohydrates, nucleobases, and steroids bear great potential as building blocks for the construction of supramolecular vessels, possessing complexity that is still difficult to attain with synthetic methods - they are rich in functional groups and well-defined stereogenic centers, ready for noncovalent interactions and further functions. One of the options to tame the functional and dynamic complexity of natural building blocks is to place them at spatially designed positions using synthetic scaffolds. In this review, we summarise the historical and recent advances in the construction of molecular-sized vessels by the strategy that couples synthetic predictability and durability of various scaffolds (cyclodextrins, porphyrins, crown ethers, calix[n]arenes, resorcin[n]arenes, pillar[n]arenes, cyclotriveratrylenes, coordination frameworks and multivalent high-symmetry molecules) with functionality originating from natural building blocks to obtain nanocontainers, cages, capsules, cavitands, carcerands or coordination cages by covalent chemistry, self-assembly, or dynamic covalent chemistry with the ultimate goal to apply them in sensing, transport, or catalysis.

Cage-based sensors for circular dichroism analysis

Quantitative chiral sensing relying on circular dichroism (CD) is very important for determining the enantiomeric excess or concentration of small molecules without strong chromophores, because they form chiral complexes with sensors, yielding strong CD signals. Three-dimensional cages are promising platforms for chiral CD due to their stereochemical flexibility and their variety of cavity and external binding sites that can be used as chiral CD sensors. In this minireview, we discuss recent advances, future challenges, and opportunities in the quantitative sensing of small molecules in host-guest and peripheral complexes with cage sensors by chiral CD. We aim to provide inspiration for the rational design of cage sensors for quantitative chiral sensing of small molecules based on CD.

Lewis acid-driven self-assembly of diiridium macrocyclic catalysts imparts substrate selectivity and glutathione tolerance

Molecular inorganic catalysts (MICs) tend to have solvent-exposed metal centers that lack substrate specificity and are easily inhibited by biological nucleophiles. Unfortunately, these limitations exclude many MICs from being considered for in vivo applications. To overcome this challenge, a strategy to spatially confine MICs using Lewis acid-driven self-assembly is presented. It was shown that in the presence of external cations (e.g., Li+, Na+, K+, or Cs+) or phosphate buffered saline, diiridium macrocycles spontaneously formed supramolecular iridium-cation species, which were characterized by X-ray crystallography and dynamic light scattering. These nanoassemblies selectively reduced sterically unhindered C[double bond, length as m-dash]O groups via transfer hydrogenation and tolerated up to 1 mM of glutathione. In contrast, when non-coordinating tetraalkylammonium cations were used, the diiridium catalysts were unable to form higher-ordered structures and discriminate between different aldehyde substrates. This work suggests that in situ coordination self-assembly could be a versatile approach to enable or enhance the integration of MICs with biological hosts.