Enzymnachbildungen auf der Basis supramolekularer Koordinationschemie

Riding the Wave of Monodentate Ligand Revival: From the A/B Concept to Noncovalent Interactions

The rediscovery of chiral monodentate ligands made in the period 1999-2003 had important consequences in enantioselective transition-metal catalysis, such as the introduction of the A/B concept (i.e., use of monodentate ligand mixtures) and, later, a renewed interest in supramolecular ligands capable of ligand-ligand and ligand-substrate interactions. This Personal Account summarizes the contributions made by our research group in this area in the period 2004-2015, which reflect the abovementioned developments. Within this area, we introduced some original concepts, such as 1) the use of chiral tropos ligand mixtures; 2) the development of new strategies to maximize heterocomplex formation from combinations of simple monodentate ligands; 3) the investigation of new ligand-ligand interactions to achieve selective heterocomplex formation; and 4) the development of highly efficient and synthetically accessible supramolecular ligands.

Manifesting Subtle Differences of Neutral Hydrophilic Guest Isomers in a Molecular Container by Phase Transfer

Achieving strong host-guest interactions between synthetic hosts and hydrophilic guests in solution is challenging because solvation effects overwhelm other effects. To resolve this issue, we transferred complexes of cucurbit[7]uril (CB[7]) and monosaccharides to the gas phase and report here their intrinsic host-guest chemistry in the absence of solvation effects. It was observed that effective host-guest interactions in the gas phase mediated by ammonium cations allow the differentiation of the monosaccharide isomers in complex with CB[7] upon vibrational excitation. The potential of the unique observation was extended to a quantitative supramolecular analytical method for the monosaccharide guests. The combination of host-guest chemistry and phase transfer presented in this study is an effective approach to overcome current limitations in supramolecular chemistry.

A Stimuli-Responsive Rotaxane-Gold Catalyst: Regulation of Activity and Diastereoselectivity

A rotaxane-based Au catalyst was developed and the effect of the mechanical bond on its behavior was studied. Unlike the non-interlocked thread, the rotaxane requires a catalytically innocent cofactor, the identity of which significantly influences both the yield and diastereoselectivity of the reaction. Under optimized conditions, Au(I) (the catalyst), Ag(I) (to abstract the Cl(-) ligand), and Cu(I) (the cofactor) combine to produce a catalyst with excellent activity and selectivity.

A Porphyrin Coordination Cage Assembled from Four Silver(I) Triazolyl-Pyridine Complexes

The synthesis of a zinc(II) porphyrin 1 with four appended triazolyl-pyridine chelates is reported. Complexation of the porphyrin peripheral ligands with Ag(I) ions in a 1:2 binding stoichiometry afforded quantitatively the coordination cage [Ag4 (1)2 ](4+) . The assembly and disassembly processes of the cage were investigated in solution using UV/Vis spectroscopy. The mathematical analysis of the data obtained in the UV/Vis titration of 1 with Ag(I) confirmed the assembly in CH2 Cl2 /MeOH (90:10) solution of a species having a 1:2 porphyrin/silver stoichiometry and assigned to it an overall stability constant of 5.0×10(26)  M(-5) . The use of a model system allowed an independent assessment of a microscopic binding constant value (Km ) for the interaction between the triazolyl-pyridine ligand and Ag(I) . The coincidence that existed between the Km values extracted from the model system and the titration of 1 provided an indication of the quality and fit of the data analysis. It also allowed the calculation of the average effective molarity (EM) value for the three intramolecular processes that led to the cage assembly as 2.6 mM. Simulated speciation profiles supported the conclusion that at millimolar concentration and working under strict stoichiometric control of the silver/porphyrin ratio, the cage [Ag4 (1)2 ](4+) was the species exclusively assembled in solution. On the other hand, when the concentration of added Ag(I) was approximately 2.6 mM, 50 % of the coordination cage disassembled into open aggregates.

Twisted baskets

A preparative procedure for obtaining a pair of twisted molecular baskets, each comprising a chiral framework with either right ((P)-1syn) or left ((M)-1syn) sense of twist and six ester groups at the rim has been developed and optimized. The racemic (P/M)-1syn can be obtained in three synthetic steps from accessible starting materials. The resolution of (P/M)-1syn is accomplished by its transesterification with (1R,2S,5R)-(-)-menthol in the presence of a Ti(IV) catalyst to give diastereomeric 8(P) and 8(M). It was found that dendritic-like cavitands 8(P) and 8(M), in CD2Cl2, undergo self-inclusion ((1)H NMR spectroscopy) with a menthol moiety occupying the cavity of each host. Importantly, the degree of inclusion of the menthol group was ((1)H NMR spectroscopy) found to be greater in the case of 8(P) than 8(M). Accordingly, it is suggested that different folding characteristic of 8(P) and 8(M) ought to affect the physicochemical characteristics of the hosts to permit their effective separation by column chromatography. The absolute configuration of 8(P)/8(M), encompassing right- and left-handed "cups", was determined with the exciton chirality method and also verified in silico (DFT: B3LYP/TZVP). Finally, the twisted baskets are strongly fluorescent due to three naphthalene chromophores, having a high fluorescence quantum yield within the rigid framework of 8(P)/8(M).

Modular synthesis of linear bis- and tris-monodentate fused [6]polynorbornane-based ligands and their assembly into coordination cages

A modular approach has been developed for the synthesis of rigid linear di- and tritopic ligands based on a fused [6]polynorbornane scaffold. The design provides up to three sites for installing functionality, including both "ends" and a "central" position with the advantage that each region can be independently addressed during synthesis. To illustrate the utility of the approach, both pyridyl and picolyl units were incorporated to provide six new ligands, with centers and ends either matched or mismatched. Indeed, both [M2L4] cages with endohedral functionality and [M3L4] complexes were cleanly produced from these ligands with assembled structures confirmed by using (1)H NMR spectroscopy, HRMS, and molecular modelling.

Gold(I) catalysis at extreme concentrations inside self-assembled nanospheres

Homogeneous transition-metal catalysis is a crucial technology for the sustainable preparation of valuable chemicals. The catalyst concentration is usually kept as low as possible, typically at mM or μM levels, and the effect of high catalyst concentration is hardly exploited because of solubility issues and the inherent unfavorable catalyst/substrate ratio. Herein, a self-assembly strategy is reported which leads to local catalyst concentrations ranging from 0.05 M to 1.1 M, inside well-defined nanospheres, whilst the overall catalyst concentration in solution remains at the conventional mM levels. We disclose that only at this high concentration, the gold(I) chloride is reactive and shows high selectivity in intramolecular CO and CC bond-forming cyclization reactions.

Transient substrate-induced catalyst formation in a dynamic molecular network

In biology enzyme concentrations are continuously regulated, yet for synthetic catalytic systems such regulatory mechanisms are underdeveloped. We now report how a substrate of a chemical reaction induces the formation of its own catalyst from a dynamic molecular network. After complete conversion of the substrate, the network disassembles the catalyst. These results open up new opportunities for controlling catalysis in synthetic chemical systems.

Anionic bipyridyl ligands for applications in metallasupramolecular chemistry

The facile synthesis of anionic bipyridyl ligands with dinuclear clathrochelate cores is described. These metalloligands can be obtained in high yields by the reactions of M(ClO4 )2 (H2 O)6 (M: Zn, Mn, or Co) with 4-pyridylboronic acid and 2,6-diformyl-4-methylphenol oxime or 2,6-diformyl-4-tert-butylphenol oxime, followed by deprotonation. The ligands are interesting building blocks for metallasupramolecular chemistry, as evidenced by the formation of a Pt-based molecular square and four coordination polymers with 2D or 3D network structures. Competition experiments reveal that the utilization of anionic bipyridyl ligands can result in significantly more stable assemblies.

Cerium-based M4L4 tetrahedra as molecular flasks for selective reaction prompting and luminescent reaction tracing

The application of metal-organic polyhedra as "molecular flasks" has precipitated a surge of interest in the reactivity and property of molecules within well-defined spaces. Inspired by the structures of the natural enzymatic pockets, three metal-organic neutral molecular tetrahedral, Ce-TTS, Ce-TNS and Ce-TBS (H6TTS: N',N'',N'''-nitrilotris-4,4',4''-(2-hydroxybenzylidene)-benzohydrazide; H6TNS: N',N'',N'''-nitrilotris-6,6',6''-(2-hydroxybenzylidene)-2-naphthohydrazide; H6TBS: 1,3,5-phenyltris-4,4',4''-(2-hydroxybenzylidene)benzohydrazide), which exhibit different size of the edges and cavities, were achieved through self-assembly by incorporating robust amide-containing tridentate chelating sites into the fragments of the ligands. They acted as molecular flasks to prompt the cyanosilylation of aldehydes with excellent selectivity towards the substrates size. The amide groups worked as trigger sites and catalytic driven forces to achieve efficient guest interactions, enforcing the substrates proximity within the cavity. Experiments on catalysts with the different cavity radii and substrates with the different molecular size demonstrated that the catalytic performance exhibited enzymatical catalytic mechanism and occurred in the molecular flask. These amides were also able to amplify guest-bonding events into the measurable outputs for the detection of concentration variations of the substrates, providing the possibility for metal-organic hosts to work as smart molecular flasks for the luminescent tracing of catalytic reactions.

Cooperatively enhanced receptors for biomimetic molecular recognition

The concept of preorganization suggests that organizing a receptor around its guest during binding is detrimental, because the cost of conformational change is assumed to be paid out of the binding energy. Although this concept has historically guided the synthesis of a great many synthetic hosts, in recent years, chemists have begun to synthesize receptors that resemble proteins in their cooperative conformational changes. Such changes could enhance the host-guest interactions, in particular if the binding of the guest triggers previously unengaged noncovalent interactions within the host. These hosts, referred to as cooperatively enhanced receptors, corroborate with their biological counterparts to support the approach of creating high-affinity receptors through the combined strategies of cooperativity and preorganization. Solvents, often the invisible participants of any solution-based supramolecular process, should be properly considered in the design of synthetic receptors, whether preorganized or cooperatively enhanced.