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OxWCA - From Innocent and Ligand-forming Oxidant Salts to the Synthesis and Application of Reactive Alkaline Earth or Early Transition Metal-Arene-Cations [Mx(arene)y]n+

Subject Area Inorganic Molecular Chemistry - Synthesis and Characterisation
Term from 2019 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 431116391
 
Final Report Year 2024

Final Report Abstract

With this project, substantial deficits in two areas of oxidation = deelectronation chemistry should be advanced. Hence, with new methods and reagents from part A) using innocent or ligand forming oxidant salts access to the hitherto underdeveloped areas of metal-arene-complex cations [M(arene)y]+ with M being from Group 2 (B) as well as Groups 3 to 5 (C) should be generated and, if achieved, be tested for polymerization or catalysis. New and known methods and reagents from our work in A) were heavily investigated and quantified in terms of their deelectronation potentials against Fc+/Fc in two publications, yielding extreme formal potentials for the reagents of up to 1.89 V. Yet, our attempts to set the concept of transfer-deelectronation into experimental reality did lead for the suggested XeF2/2Li+ system to surprises that are nearing completion and will be submitted as a full paper by the end of the year. Only very recently, we isolated a system that fulfills the needs of the concept of transfer-deelectronation that will be developed further until publication. With these methods and reagents, we succeeded to prepare in area B) the first unsupported Group 2-arene complexes and fully characterized them with experimental and theoretical means. Advancing over this, we used chelating ansa-arenes to prepare the first Sr-ansa-arene complex. It was successfully applied as catalyst in a CO2-reduction to CH4 and a surprisingly controlled isobutylene polymerization. When turning to part C), the access to Group 5 arene cations with the new methods and reagents was straightforward demonstrated, but, despite considerable efforts, especially the quest for [M(arene)x]WCA salts of groups 3 and 4 was futile, due to a multitude of reasons, the most drastic of which were the limited deelectronation potentials of the hitherto available reagents. Note that extremely non-noble metals like Scandium or Ytterbium, with standard aqueous reduction potentials of –2.077 / –2.19 V (!) did not react in our non-basic and non-coordinating environment of fluorobenzenes xFB (x = 1…5 F-atoms) with very potent reagents like Ag+, NO+ or the novel innocent deelectronators with formal potentials E°’ reaching up to +1.89 V vs. Fc+/Fc in xFB solution! Hence, we did analyze the energetics with Born-Fajans-Haber Cycles and realized that even seemingly facile reactions are thermodynamically impossible due to the low contribution of the arene complexation towards especially the group 3 and 4 cations. Thus, a very important contribution to the aqueous redox potential cited above is the very high Gibbs energy of hydration of the HSAB-hard group 3 (and 4) metals. This can by far not be overcome with the reagents presented here, albeit they are at the positive potential limit of what is thermodynamically possible. To elucidate, what is possible, we enlarged our synthetic focus to the entire series of 3d metals in their univalent state and do report here several missing members for [M(arene)x]WCA salts with M being Fe, Ni, but also Au.

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