Implementation of the HSAB concept in the chemistry of frustrated Lewis couples

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If Lewis acids and bases are prevented from directly interacting with each other, e.g. by bulky substituents, an unused reaction potential remains with which small molecules can be polarized, bound or activated. Applications of this concept of Frustrated Lewis Acid/Base Pairs (FLP) range from the heterolytic cleavage of hydrogen and thus their use as hydrogenation catalysts to the activation of carbon dioxide and other greenhouse gases. FLP enable novel reactions and catalytic processes and the stabilization of highly reactive particles. Most FLP systems employed so far use boranes and phosphanes as acid or base functions, respectively. Acids of other elements [Al, Ga, Zn, P(V), Si, Sn] have also been used. In this project, we want to use heavy main group elements such as tin, lead, antimony, indium and thallium in Lewis acid functions, which then form relatively soft acid binding sites according to the HSAB concept. On the one hand, this should make them particularly affine to soft binding sites in substrates, and on the other hand, they should bind hard substrates such as CO2 more weakly or reversibly. The fine-tuning of such binding strengths can be crucial when developing catalytic systems, since the products must be removable for regeneration. For more than two decades, we have been intensively investigating intramolecular or geminal Lewis acid/base compounds, including those of heavy elements. We have developed active FLP such as the neutral silicon or tin-phosphorus FLP, (C2F5)3E-CH2-P(CMe3)2 (E = Si, Sn) but also numerous other FLP systems. For the tin compound we could already prove some of the desired heavy atom FLP properties. Relevant preliminary work has been performed for tin and antimony. Now we want to continue this work and use these findings to extent the research to the elements lead, indium and thallium. For this purpose, we are developing reagents that enable the introduction of functions of these elements with sterically demanding and strongly electronegative groups to rigid hydrocarbon frameworks with phosphane functions. Thus FLP systems with different, defined distances between acid and base functions will be synthesized. We want to systematically understand the dependence of reactivity on the geometric parameters, on the Lewis acid function (element and ligand), in order to derive trends that can be used to optimize reactivities. To this end, the systems mentioned above will be tested on a number of substrates for their activity for binding, activation and their catalytic suitability (e.g. hydrogenation of unsaturated substrates), among other things also by means of a simultaneous conversion of hydrogen and carbon dioxide for its reduction.