Synthesis and Characterization of Hypercoordinated Silicon Anions: Catching Intermediates of Lewis Base Catalysis

Journal article

Tris(catecholato)silicate dianions, a compound class that is already known for over a century, can be readily prepared by reacting sand with catechol under basic conditions. Strikingly, the two-electron-oxidized derivative – silicon tris(perchloro)dioxolene 1Cl – has been recently accessed, representing a thermally stable, neutral triplet diradical and the first non-metal complex with redox-active and mixed-valence substituents. In the present work, the redox properties of 1Cl are investigated and by the synthesis of the corresponding monoradical anion [1Cl]•−, the redox series of tris(catecholato)silicates in general is completed. With cyclic voltammetry the redox potentials E1/2 = 0.43 V and 0.88 V (vs. Fc/Fc+) were finally determined. Comparing the redox potentials of 1Cl with free tetrachloro-o-benzoquinone, a tremendous shift of about 1.2 V becomes apparent. Moreover, 1Cl is applicable as efficient redox catalyst. By varying the quinone ligands and the silicon source, further homo- and heteroleptic derivatives are prepared. Variable temperature EPR measurements disclose the existence of diradicals with a triplet ground state. With a more profound understanding of the monomeric species, the synthesis is extended to higher nuclearity. A straightforward approach is established by introducing substituted 2,5-dihydroxy-p-benzoquinone (H2dhbqY) as [dhbqY][Na@15c5]2 (Y = Cl, Br, Ph, NO2) salts and mixing with bis(catecholato)silanes 2X (X = Cl, Br, CF3, iPr) to obtain the dinuclear species [4X,Y][Na@15c5]2, which are robust to coordinating environments. By selective combination of more electron-rich 2X and electron-poor dhbq linkers diradicaloid complexes [4X,Y]2− (X = Cl, iPr and Y = Cl, Br, NO2) were obtained and characterized. The opposite extreme with the smallest diradical character was accomplished by combining electron-poor 2CF3 and electronrich dhbqPh. The underlying design principle is further disclosed by computational analyses. Conclusively, this one-step protocol grants access to dimeric silicon polyoxolenes with control over and fine-tuning of the spin ground state. Lastly, preliminary results are obtained for the trimeric structures by implementing the six-fold deprotonated tritopic linker 2,3,6,7,10,11-hexahydroxytriphenylene (H6hhtp) with 2Cl. The results gathered in this work present a fundamental understanding of silicon-bridged polyoxolenes and thus are valuable extensions based on a non-metal main group element to known works based on transition metals

Authors: Ansmann, N; Hartmann, D; Sailer, S; Erdmann, P; Maskey, R

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Angewandte Chemie International Edition
• Volume: 61
• Issue: 27
• Pages: e202203947
• Publication date: 2022-04-19
• DOI: 10.1002/anie.202203947
• EISSN: 1521-3773
• ISSN: 1433-7851
• Language: English
• Keywords: Organic chemistry; Catalysis; Base (topology); Materials science; Characterization (materials science); Lewis acids and bases; Nanotechnology; Chemistry; Silicon