Breaking thiacalix[4]arene into pieces – a novel synthetic approach to higher calixarenes bearing mixed (–S–, –CH2–) bridges

A novel approach to calix[5–7]arenes possessing mixed (S and CH₂) bridges within the skeleton is based on the reaction of thiacalix[4]arene monosulfoxide with BuLi leading to a linear phenolic tetramer in essentially quantitative yield. This key intermediate is then cyclized with suitable building blocks to give macrocyclic calixarene analogues. Compared to the traditional stepwise construction of similar systems, this procedure based on thiacalixarene cleavage represents a scalable, robust, and straightforward synthesis and enables the preparation of larger calixarenes on a gram scale. As shown by ¹H NMR and UV-vis titration experiments, the mixed-bridge calix[7]arene is able to recognize fullerenes C₆₀ and C₇₀, thus showing possible applications of such systems. The structures of the mixed bridge systems were confirmed by single crystal X-ray analysis, and the behavior of novel macrocyclic skeletons in solution was studied using dynamic NMR techniques.

A novel approach is based on the cleavage of thiacalix[4]arene monosulfoxide with BuLi providing a linear tetramer in an essentially quantitative yield which is used as a building block for further cyclization.

The sulfonate group as a ligand: a fine balance between hydrogen bonding and metal ion coordination in uranyl ion complexes

Nine uranyl ion complexes have been synthesized using two kinds of sulfonate-containing ligands, i.e. 2-, 3- and 4-sulfobenzoic acids (2-, 3- and 4-SBH₂), which include additional carboxylic donors, and p-sulfonatocalix[4]arene (H₈C4S), with additional phenolic groups, and [Ni(cyclam)]²⁺, [Cu(R,S-Me₆cyclam)]²⁺ or PPh₄⁺ as counterions. [Ni(cyclam)][UO₂(4-SB)₂(H₂O)₂]·2CH₃CN (1) and [Ni(cyclam)][UO₂(3-SB)₂(H₂O)₂] (2) are molecular species in which only the carboxylate groups are coordinated to uranyl, the sulfonate groups being essentially hydrogen bond acceptors. In contrast, uranyl κ¹-O(S);κ¹-O(C)-chelation is found in the four complexes involving 2-SB^2-, different bridging interactions producing diverse geometries. [UO₂(2-SB)₂Ni(cyclam)]·H₂O (3) crystallizes as a two-dimensional (2D) assembly with fes topology, in which uranyl ion dimeric subunits are bridged by six-coordinate Ni^II cations. Complexes [UO₂(2-SB)₂Cu(R,S-Me₆cyclam)]₂·2H₂O (4) and [(UO₂)₂(2-SB)₂(C₂O₄)Cu(R,S-Me₆cyclam)] (5), obtained together from the same solution, are a molecular tetranuclear complex and a 2D species with fes topology, respectively, depending on the coordination number, 5 or 6, of the Cu^II cation. The complex [PPh₄]₂[(UO₂)₂(2-SB)₃(H₂O)]·H₂O (6) is a one-dimensional (1D), ribbon-like coordination polymer with a layered packing of alternate cationic and anionic sheets. No heterometallic complex was obtained with H₈C4S, but the copper-only compound [{Cu(R,S-Me₆cyclam)}₅(H₃C4S)₂]·17H₂O (7) displays mixed coordination/hydrogen bonding association of the copper azamacrocycle complex with the phenolic groups. The complexes [PPh₄]₅[UO₂(H₄C4S)(H₂O)₄][UO₂(H₃C4S)(H₂O)₄]·14H₂O (8) and [PPh₄]₃[UO₂(H₃C4S)(H₂O)₃]·9H₂O (9) were crystallized from the same solution and are a molecular complex and a 1D polymer, respectively, with monodentate sulfonate coordination to uranyl, while [PPh₄]₂[UO₂(H₄C4S)(H₂O)₃]·11H₂O (10) is also a 1D polymer. The anionic complexes in the last three complexes form layers (9) or double layers (8 and 10) separated from one another by hydrophobic layers of PPh₄⁺ cations. The balance between coordination and hydrogen bonding interactions with the macrocyclic ligands provides an indication of the energy of the sulfonate coordinate bond. Complex 6 is the only luminescent species in this series, albeit with a low quantum yield of 3%, and its emission spectrum is typical of a uranyl complex with five equatorial donors.

Strain effects determine the performance of artificial allosteric systems: calixarenes as models

It is shown that the performance of allosteric systems regarding the efficiency and the speed of response depends critically on the strain energy of the equilibrating conformers and of the corresponding interconversion transition state. The affinity of a substrate A can be large enough to overcome in the absence of an effector E by induced fit the strain involved in the formation of an optimal conformation for binding A. The efficiency as given by the ratio KAE/KA of binding constants in the presence or absence of an effector is, for many published synthetic allosteric systems, relatively low; in practice this means that these only function within rather limited concentration ranges. A small KAE/KA ratio means that the binding strength of A or the corresponding signal will increase only little by adding an effector, and may need higher concentration of E. Implementation of steric distortion in the minor conformer can lead to reduced binding of A in the absence of the effector E. Destabilization of conformers can also result from the inclusion of high energy water molecules within a cavity. Furthermore, until now it has been overlooked that strain in the transition state can lead to reaction times of up to days, and thus to the neglect of experimental observation. The role of conformational changes within an allosteric molecule is characterized with a variety of calixarenes and other compound classes, offering a clue for the design of more efficient synthetic systems with high cooperativity.