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Phosphines Modulating the Catalytic Silane Activation on Nickel-Cobalt Nanoparticles, Tentatively Attributed to Frustrated Lewis Pairs in Colloidal Solution

Abstract : The discovery of frustrated Lewis pairs (FLPs) represented a major breakthrough in modern organic chemistry, allowing the transition metal-free activation of chemical bonds in mild conditions. Consequently, FLP chemistry involving molecular Lewis acids and Lewis bases has witnessed an impressive advancement in the last decade. Nevertheless, the range of viable catalytic reactions is limited due to the formation of too stable adducts between the reaction substrate and the Lewis acid/base couple and to the high distance between catalytic sites borne by separated molecular pairs in solution. In contrast, traditional heterogeneous catalysts made of transition metal nanoparticles present adjacent active sites, although they usually require harsher conditions to be activated. The design of an FLP on metallic nanoparticles could allow the activation of small molecules in milder conditions. Herein, we propose the concept of a NanoFLP in a colloidal solution where one partner is a phosphine Lewis base and the other is the Lewis acid surface of a NiCo nanoparticle. We attempt to apply this concept to the hydrosilylative reduction of benzaldehyde. While a classical surface organometallic path through the oxidative addition of the silane on the sole metal center cannot be disproved at this stage, the reaction requires the presence of at least 5 mol % phosphine in conjunction with the NiCo nanoparticles, which represent a tenfold excess of phosphines vs metal surface atoms, a situation that should disfavor the presence of free metal sites apt to perform the oxidative addition. We identify a correlation between the Tolman cone angle and the silane conversion, consistent with both mechanisms; however, we found no clear correlation between the Tolman electronic parameter and the reaction outcome. The influence of silane bulkiness was also investigated and a less bulky phosphine was required for the reaction to work with a bulkier silane. Structural analyses evidenced that the nanoparticles are not altered during the reaction, which led us to propose the formation of a NanoFLP as a transient species in solution.
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Submitted on : Wednesday, September 22, 2021 - 1:45:41 PM
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Alberto Palazzolo, Sophie Carenco. Phosphines Modulating the Catalytic Silane Activation on Nickel-Cobalt Nanoparticles, Tentatively Attributed to Frustrated Lewis Pairs in Colloidal Solution. Chemistry of Materials, American Chemical Society, 2021, ⟨10.1021/acs.chemmater.1c03105⟩. ⟨hal-03351474⟩

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