Complete Dynamic Reconstruction of C60, C70, and (C59N)2 Encapsulation into an Adaptable Supramolecular Nanocapsule.

The dynamic adaptability of tetragonal prismatic nanocapsule 18+ in the selective separation of fullerenes and endohedral metallofullerenes (EMFs) remains unexplored. Therefore, the essential molecular details of the fullerene recognition and binding process into the coordination capsule and the origins of fullerene selectivity remain elusive. In this work, the key steps of fullerene recognition and binding processes have been deciphered by designing a protocol which combines 1H-1H exchange spectroscopy (2D-EXSY) NMR experiments, long time-scale Molecular Dynamics (MD) and accelerated Molecular Dynamics (aMD) simulations, which are combined to completely reconstruct the spontaneous binding and unbinding pathways from nanosecond to second time-range. On one hand, binding (k'on) and unbinding (koff) rate constants were extracted from 1H-1H exchange spectroscopy (EXSY) NMR experiments for both C60 and C70. On the other hand, MD and aMD allowed monitoring the molecular basis of the encapsulation and guest competition processes at a very early stage under nonequilibrium conditions. The receptor capsule displays dynamical adaptability features similar to those observed in the process of biomolecular recognition in proteins. In addition, the encapsulation of bis-aza[60]fullerene (C59N)2 within a supramolecular coordination capsule has been studied for the first time, showcasing the pros and cons of the dumbbell-shaped guest in the dynamics of the encapsulation process and in the stability of the final bound adduct. The powerful combination of NMR, MD, and aMD methodologies allows to obtain a precise picture of the subtle events directing the encapsulation and is thus a predictive tool for understanding host-guest encapsulation and interactions in numerous supramolecular systems.

Highly Selective Fluoride Recognition by a Small Tris-Urea Covalent Cage.

A highly selective recognition of fluoride was achieved through the design of a small hemicryptophane cage (3) presenting a southern tris-urea hosting moiety. The resulting host-guest complex has been characterized by electrospray ionization-high-resolution mass spectrometry, 1H and 19F NMR, and X-ray diffraction techniques. In particular, X-ray diffraction analysis of [3·F-] reveals that the encapsulation of one fluoride, within 3, occurs through NH···F- H-bonding with the six NH residues of the tris-urea ligand. An association constant of 1200 M-1 was extracted from 1H NMR titration experiments, indicating that efficient fluoride binding also occurs in solution. Finally, in sharp contrast with previously reported urea-based hemicryptophane hosts, the small preorganized cavity found in 3 allows for an exclusive selectivity for fluoride over other competing halides.

Mechanism of Oxidative Activation of Fluorinated Aromatic Compounds by N-Bridged Diiron-Phthalocyanine: What Determines the Reactivity?

The biodegradation of compounds with C-F bonds is challenging due to the fact that these bonds are stronger than the C-H bond in methane. In this work, results on the unprecedented reactivity of a biomimetic model complex that contains an N-bridged diiron-phthalocyanine are presented; this model complex is shown to react with perfluorinated arenes under addition of H2 O2 effectively. To get mechanistic insight into this unusual reactivity, detailed density functional theory calculations on the mechanism of C6 F6 activation by an iron(IV)-oxo active species of the N-bridged diiron phthalocyanine system were performed. Our studies show that the reaction proceeds through a rate-determining electrophilic C-O addition reaction followed by a 1,2-fluoride shift to give the ketone product, which can further rearrange to the phenol. A thermochemical analysis shows that the weakest C-F bond is the aliphatic C-F bond in the ketone intermediate. The oxidative defluorination of perfluoroaromatics is demonstrated to proceed through a completely different mechanism compared to that of aromatic C-H hydroxylation by iron(IV)-oxo intermediates such as cytochrome P450 Compound I.

Positive Cooperative Effect in Ion-Pair Recognition by a Tris-urea Hemicryptophane Cage.

The synthesis of a hemicryptophane cage combining a CTV unit with a C3 symmetrical moiety bearing three urea functions is reported. This host was found to bind anions with higher binding constants than other previously reported hemicryptophanes. Due to its heteroditopic character this cage proved to be an efficient ion-pair receptor. The best cooperativity effect was observed for the tetramethylammonium bromide (TMABr) salt, which was confirmed and rationalized by DFT calculations.

Bioinspired Oxidation of Methane in the Confined Spaces of Molecular Cages.

Non-heme iron, vanadium, and copper complexes bearing hemicryptophane cavities were evaluated in the oxidation of methane in water by hydrogen peroxide. According to 1H nuclear magnetic resonance studies, a hydrophobic hemicryptophane cage accommodates a methane molecule in the proximity of the oxidizing site, leading to an improvement in the efficiency and selectivity for CH3OH and CH3OOH compared to those of the analogous complexes devoid of a hemicryptophane cage. While copper complexes showed low catalytic efficiency, their vanadium and iron counterparts exhibited higher turnover numbers, ≤13.2 and ≤9.2, respectively, providing target primary oxidation products (CH3OH and CH3OOH) as well as over-oxidation products (HCHO and HCOOH). In the case of caged vanadium complexes, the confinement effect was found to improve either the selectivity for CH3OH and CH3OOH (≤15%) or the catalytic efficiency. The confined space of the hydrophobic pocket of iron-based supramolecular complexes plays a significant role in the improvement of both the selectivity (≤27% for CH3OH and CH3OOH) and the turnover number of methane oxidation. These results indicate that the supramolecular approach is a promising strategy for the development of efficient and selective bioinspired catalysts for the mild oxidation of methane to methanol.

Self-Assembled Cofacial Zinc-Porphyrin Supramolecular Nanocapsules as Tuneable 1 O2 Photosensitizers.

We demonstrate the benefits of using cofacial Zn-porphyrins as structural synthons in coordination-driven self-assembled prisms to produce cage-like singlet oxygen (1 O2 ) photosensitizers with tunable properties. In particular, we describe the photosensitizing and emission properties of palladium- and copper-based supramolecular capsules, and demonstrate that the nature of the bridging metal nodes in these discrete self-assembled prisms strongly influences 1 O2 generation at the Zn-porphyrin centers. The PdII -based prism is a particularly robust photosensitizer, whereas the CuII self-assembled prism is a dormant photosensitizer that could be switched to a ON state upon disassembly of the suprastructure. Furthermore, the well-defined cavity within the prisms allowed encapsulation of pyridine-based ligands and fullerene derivatives, which led to a remarkable guest tuning of the 1 O2 production.

Design of Zn-, Cu-, and Fe-Coordination Complexes Confined in a Self-Assembled Nanocage.

The encapsulation of coordination complexes in a tetragonal prismatic nanocage (1·(BArF)8) built from Zn-porphyrin and macrocyclic Pd-clip-based synthons is described. The functional duality of the guest ligand L1 allows for its encapsulation inside the cage 1·(BArF)8, along with the simultaneous coordination of ZnII, CuII, or FeIII metal ions. Remarkably, the coordination chemistry inside the host-guest adduct L1⊂1·(BArF)8 occurs in both solution solution and solid state. The resulting confined metallocomplexes have been characterized by means of UV-vis, ESI-HRMS, NMR, and EPR techniques. Furthermore, the emission of the Zn-porphyrin fluorophores of 1·(BArF)8 is strongly quenched by the encapsulation of paramagnetic complexes, representing a remarkable example of guest-dependent tuning of the host fluorescence.

Reversible C60 Ejection from a Metallocage through the Redox-Dependent Binding of a Competitive Guest.

The reversible encapsulation of a tetrapyridyl extended-tetrathiafulvalene (exTTF)-based ligand (m-Py)exTTF by a tetragonal Zn-porphyrin-based prismatic nanocage (1) is described. The reversible uptake and release of the (m-Py)exTTF guest proceeds through drastic electronic and conformational changes occurring upon oxidation of the latter. This reversible system has been explored in a guest-exchange process, by addition of (m-Py)exTTF to the host-guest complex [C60 ⊂1], leading to fullerene C60 ejection from the host cavity. Remarkably, the subsequent redox-triggered ejection of (m-Py)exTTF, leads to the recovery of the empty cage 1, which remains available for further C60 encapsulation. The C60 ejection is justified by the preferable coordination of the pyridine anchors of (m-Py)exTTF to the two Zn-porphyrin units of 1. This approach, based on the use of a switchable competitive guest, offers a promising new strategy for guest-delivery control.

Catalytic defluorination of perfluorinated aromatics under oxidative conditions using N-bridged diiron phthalocyanine.

Carbon-fluorine bonds are the strongest single bonds in organic chemistry, making activation and cleavage usually associated with organometallic and reductive approaches particularly difficult. We describe here an efficient defluorination of poly- and perfluorinated aromatics under oxidative conditions catalyzed by the µ-nitrido diiron phthalocyanine complex [(Pc)Fe(III)(µ-N)Fe(IV)(Pc)] under mild conditions (hydrogen peroxide as the oxidant, near-ambient temperatures). The reaction proceeds via the formation of a high-valent diiron phthalocyanine radical cation complex with fluoride axial ligands, [(Pc)(F)Fe(IV)(µ-N)Fe(IV)(F)(Pc(+•))], which was isolated and characterized by UV-vis, EPR, (19)F NMR, Fe K-edge EXAFS, XANES, and Kß X-ray emission spectroscopy, ESI-MS, and electrochemical techniques. A wide range of per- and polyfluorinated aromatics (21 examples), including C6F6, C6F5CF3, C6F5CN, and C6F5NO2, were defluorinated with high conversions and high turnover numbers. [(Pc)Fe(III)(µ-N)Fe(IV)(Pc)] immobilized on a carbon support showed increased catalytic activity in heterogeneous defluorination in water, providing up to 4825 C-F cleavages per catalyst molecule. The µ-nitrido diiron structure is essential for the oxidative defluorination. Intramolecular competitive reactions using C6F3Cl3 and C6F3H3 probes indicated preferential transformation of C-F bonds with respect to C-Cl and C-H bonds. On the basis of the available data, mechanistic issues of this unusual reactivity are discussed and a tentative mechanism of defluorination under oxidative conditions is proposed.

X-ray absorption and emission spectroscopies of X-bridged diiron phthalocyanine complexes (FePc)2X (X = C, N, O) combined with DFT study of (FePc)2X and their high-valent diiron oxo complexes.

µ-Nitrido diiron phthalocyanine [PcFe(+3.5)NFe(+3.5)Pc](0) is an efficient catalyst, able to catalyze the oxidation of methane under near-ambient conditions. In this work, we compared the properties of structurally similar µ-carbido (1), µ-nitrido (2), and µ-oxo (3) dimers of iron phthalocyanine. The goal was to discern the structural and electronic differences between these complexes and to propose a rationale for the exceptional activity of 2. Extended X-ray fine-structure spectroscopy, high-resolution X-ray emission spectroscopy, and resonant inelastic X-ray scattering were applied to study the geometry and electronic structure of iron species in the series 1-3. The data provided by core hole spectroscopies were compared to the results of DFT calculations and found to coherently describe the structural and electronic properties of 1-3 as having equivalent iron centers with formal iron oxidation degrees of 3, 3.5, and 4 for the µ-oxo, µ-nitrido, and µ-carbido dimers, respectively. However, the bond length to the bringing atom changed in an unexpected sequence Fe-O > Fe-N < Fe-C, indicating redox non-innocence of the brigding µ-carbido ligand in 1. According to the X-ray emission spectroscopy, the µ-nitrido dimer 2 is a low-spin compound, with the highest covalency in the series 1-3. The DFT-calculated geometry and electronic structures as well as core hole spectra of hypothetical high-valent oxo complexes of 1-3 were compared, in order to explain the particular catalytic activity of 2 and to estimate the prospects of spectroscopic observation of such species. It appears that the terminal Fe═O bond is the longest in the oxo complex of 2, due to the strong trans-effect of the nitrido ligand. The corresponding LUMO of the µ-nitrido diiron oxo complex has the lowest energy among the three oxo complexes. Therefore, the oxo complex of 2 is expected to have the highest oxidative power.

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.

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

Reproducing the key features offered by metalloprotein binding cavities is an attractive approach to overcome the main bottlenecks of current open artificial models (in terms of stability, efficiency and selectivity). In this context, this featured article brings together selected examples of recent developments in the field of confined bioinspired complexes with an emphasis on the emerging hemicryptophane caged ligands. In particular, we focused on (1) the strategies allowing the insulation and protection of complexes sharing similarities with metalloprotein active sites, (2) the confinement-induced improvement of catalytic efficiencies and selectivities and (3) very recent efforts that have been made toward the development of bioinspired complexes equipped with weakly binding artificial cavities.

A caged tris(2-pyridylmethyl)amine ligand equipped with a Ctriazole-H hydrogen bonding cavity.

A capped bioinspired ligand built from a tris(2-pyridyl-methyl)amine (TPA) unit and surmounted by a triazole-based intramolecular H-bonding secondary sphere was prepared. The resulting cage provides a well-defined cavity combining the hydrophobic nature with H-bonding properties. Its coordinating properties were explored using Zn(II) and Cu(II) metal ions.

Alkenylphosphonates: unexpected products from reactions of methyl 2-[(diethoxyphosphoryl)methyl]benzoate under Horner-Wadsworth-Emmons conditions.

Methyl 2-[(diethoxyphosphoryl)methyl]benzoate reacts with several aldehydes to produce an alkenylphosphonate as the major product, together with varying amounts of the expected Horner-Wadsworth-Emmons product, a 1,2-disubstituted E-alkene. Use of a bulky aldehyde or the tert-butyl ester favours the normal HWE product.

A tris(benzyltriazolemethyl)amine-based cage as a CuAAC ligand tolerant to exogeneous bulky nucleophiles.

The archetypal tris(benzyltriazolemethyl)amine (TBTA) ligand was equipped with a bowl-shaped cap in the cage Hm-TBTA. Hm-TBTA accelerates CuAAC reactions without suffering from product inhibition. Furthermore, this shielded ligand efficiently protects the copper center from deactivation by the CuI-chelator glutathione, opening the way to novel approaches for efficient CuAAC reactions in complex media.

Control and Transfer of Chirality Within Well-Defined Tripodal Supramolecular Cages.

The development of new strategies to turn achiral artificial hosts into highly desirable chiral receptors is a crucial challenge in order to advance the fields of asymmetric transformations and enantioselective sensing. Over the past few years, C 3 symmetrical cages have emerged as interesting class of supramolecular hosts that have been reported as efficient scaffolds for chirality dynamics (such as generation, control, and transfer). On this basis, this mini review, which summarizes the existing examples of chirality control and propagation in tripodal supramolecular cages, aims at discussing the benefits and perspectives of this approach.

Chirality transfer in a cage controls the clockwise/anticlockwise propeller arrangement of the tris(2-pyridylmethyl)amine ligand.

Predictable control of the propeller arrangement of the tris(2-pyridyl-methyl)amine (TPA) ligand was achieved through the preparation of the smallest hemicryptophane 1. This newly designed cage displays a chirality transfer from its northern unit to the TPA ligand. 1 can coordinate Cu(i), yielding a rare T-shaped complex with controlled helicity of the TPA-Cu(i) core.

Enantiopure C1-Cyclotriveratrylene with a Reversed Spatial Arrangement of the Substituents.

Cyclotriveratrylene (CTV) is a macrocyclic cyclophane presenting a bowl-shaped conformation, used as building block to construct cryptophane and hemicryptophane capsules. A method to synthesize new enantiopure CTV derivatives with an unprecedented spatial arrangement of their substituents, exhibiting C1 symmetry, is described. The absolute configuration was assigned by ECD spectroscopy coupled with modeling. A statistical model has allowed for optimization of the proportion of C1 CTV, and the modularity of this approach is also highlighted.

Synthesis and characterization of µ-nitrido, µ-carbido and µ-oxo dimers of iron octapropylporphyrazine.

Three µ-X bridged diiron octapropylporphyrazine complexes having Fe(III)-O-Fe(III), Fe(+3.5)-N[double bond, length as m-dash]Fe(+3.5) and Fe(IV)[double bond, length as m-dash]C[double bond, length as m-dash]Fe(IV) structural units have been prepared and characterized by UV-vis, EPR, X-ray absorption spectroscopy and electrochemical methods. Single crystals of all the complexes were obtained from benzene-acetonitrile and their structures were determined by X-ray diffraction. In contrast to µ-oxo complex (), µ-nitrido () and µ-carbido () dimers crystallized with one benzene molecule per two binuclear complex molecules arranged cofacially to the porphyrazine planes at Fe-Cbenzene distances of 3.435-3.725 Å and 3.352-3.669 Å for and , respectively. The short distances suggest an interaction between the iron sites and the benzene π-system which is stronger in the case of the Fe(IV)[double bond, length as m-dash]C[double bond, length as m-dash]Fe(IV) unit with a higher Lewis acidity. The Fe-X-Fe angle increases in the sequence -- from 158.52° to 168.5° and 175.10°, respectively, in agreement with the Fe-X bond order. However, the lengths of the Fe-X bonds do not follow this trend: Fe-O = 1.75/1.76 Å > Fe-C = 1.67/1.67 Å > Fe-N = 1.65/1.66 Å indicating unexpectedly long Fe-C bonds. This observation can be explained by back π-donation from the µ-carbido ligand to the Fe-C antibonding orbital thus decreasing the bond order which is confirmed by DFT calculations.