Poly-sigma-hole systems for molecular recognition, host-guest chemistry and catalysis

• Applicant: Professor Dr. Norbert W. Mitzel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 575052936

The aim of this project is to combine several sigma-hole functions in one molecule (poly-sigma-hole systems, PSH systems) in order to enhance their effect on substrates through their co-operative binding. Sigma holes are regions of positive electrostatic potential that occur on atoms opposite (strongly) electron-withdrawing substituents and - like Lewis acids - act as electron acceptors. Chalcogen and pnictogen bridges have recently been added to the long-known halogen bridges. In the planned PSH systems, a spatially defined arrangement of their functions ensures the selectivity of substrate binding. The aim is to create systems for optimised recognition and activation of Lewis-based substrates. The planned PSH systems consist of rigid or flexible organic backbones that are provided with chalcogen or pnictogen bridge donor functions based on antimony, selenium and tellurium (e.g. the functions Sb(C2F5)2, SeCF3 and TeCF3). The frameworks include anthracene, triptycene and biphenylene, each with two alkynyl functions in the 1,8-position (2 binding sites), as well as anthracene photodimers with four alkynyl functions in the same direction (4 binding sites), but also flexibly bridged systems with simple alkane or organosilane chains between the functions. Chalcogen and pnictogen bridge donors as electron acceptor catalysts offer several advantages over poly-Lewis acids and systems based on hydrogen bridge donors: they are particularly suitable for use in non-polar, hydrophobic systems, they are generally rather mild electron acceptors and therefore tolerant to many functional groups. As they usually contain heavy elements, their large acceptor orbitals tend to bind soft substrates in the HSAB sense. Therefore, they partly cover a complementary substrate spectrum. We will develop syntheses for such chelate-like binding poly-sigma-hole systems and test their complexation properties and selective recognition of different Lewis-based substrates. For this purpose, the binding of different anions (e.g. halides) or small base molecules will be tested. In NMR titration experiments we quantify binding strengths and cooperativity of the functions. The PSH systems are to be used as catalysts in more complex test reactions. As multiple electron acceptors, they should activate substrate molecules in a similar way to Lewis acid catalysis. We expect some of our systems to be stable in water, so that catalytic reactions should be possible under simpler preparative conditions or even in (partially) aqueous systems. The influence of the following parameters will be investigated in order to optimise the cooperative effect of the systems on the catalytic activity: a) type and number of functions, and b) spatial arrangement and rigidity.

• DFG Programme: Research Grants