RESUMO
Stable metal-free diradicaloids are fascinating compounds, typically based on covalent polycyclic or nitrogen-containing π-conjugated frameworks. Unfortunately, their preparation and the modulation of their diradical character require substantial synthetic efforts. The present work introduces a synthetic approach to diradicaloids by the ease and modularity of Lewis pair formation. Binding redox-active bis(catecholato)silane Lewis acids to ditopic tetraoxolene Lewis bases yields adducts with varying spin ground states. Computational analyses disclose that the diradical character increases with the electron donor ability of the catechols and the electron accepting ability of the tetraoxolene. Hence, this protocol grants access to diradicaloids with rationally adjustable diradical character of high potential for numerous applications in a single step.
RESUMO
Anionic hypercoordinated silicates with weak donors were proposed as key intermediates in numerous silicon-based reactions. However, their short-lived nature rendered even spectroscopic observations highly challenging. Here, we characterize hypercoordinated silicon anions, including the first bromido-, iodido-, formato-, acetato-, triflato- and sulfato-silicates. This is enabled by a new, donor-free polymeric form of Lewis superacidic bis(perchlorocatecholato)silane 1. Spectroscopic, structural, and computational insights allow a reassessment of Gutmann's empirical rules for the role of silicon hypercoordination in synthesis and catalysis. The electronic perturbations of 1 exerted on the bound anions indicate pronounced substrate activation.
RESUMO
Quinones are mild oxidants, the redox potentials of which can be increased by supramolecular interactions. Whereas this goal has been achieved by hydrogen bonding or molecular encapsulation, a Lewis acid-binding strategy for redox amplification of quinones is unexplored. Herein, the redox chemistry of silicon tris(perchloro)dioxolene 1 was studied, which is the formal adduct of ortho-perchloroquinone QCl with the Lewis superacid bis(perchlorocatecholato)silane 2. By isolating the anionic monoradical 1.- , the redox-series of a century-old class of compounds was completed. Cyclic voltammetry measurements revealed that the redox potential in 1 was shifted by more than 1â V into the anodic direction compared to QCl , reaching that of "magic blue" or NO+ . It allowed oxidation of challenging substrates such as aromatic hydrocarbons and could be applied as an efficient redox catalyst. Remarkably, this powerful reagent formed in situ by combining the two commercially available precursors SiI4 and QCl .
RESUMO
The reaction of ortho-quinones with silicon tetraiodide leads to neutral silicon trisdioxolenes in high yield, delivering the unknown oxidized form of triscatecholatosilicate dianions and the first example of open-shell semiquinonates connected via a single non-metal center. Silicon tris(perchloro)dioxolene is a stable diradical with a triplet ground state, as supported by X-ray diffraction; IR, resonance Raman, UV/Vis, and (VT)EPR spectroscopy; and Kohn-Sham broken-symmetry computations. Preliminary results suggest that the preferred magnetic ground state can be altered through variation of the substituents.
RESUMO
No neutral silicon Lewis super acids are known to date. We report on the synthesis of bis(perchlorocatecholato)silane and verify its Lewis super acidity by computation (DLPNO-CCSD(T)) and experiment (fluoride abstraction from SbF6- ). The exceptional affinity towards donors is further demonstrated by, for example, the characterization of an unprecedented SiO4 F2 dianion and applied in the first hydrodefluorination reaction catalyzed by a neutral silicon Lewis acid. Given the strength and convenient access to this new Lewis acid, versatile applications might be foreseen.
