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Iron-iron oxide core–shell nanoparticles are active and magnetically recyclable olefin and alkyne hydrogenation catalysts in protic and aqueous media

Chemical Communications (Impact Factor: 6.38). 01/2012; 48:3360-3362.

ABSTRACT Iron-iron oxide core–shell nanoparticles are active and magnetically recyclable olefin and alkyne hydrogenation catalysts in protic and aqueous mediaw We report for the first time the use of iron-iron oxide core–shell nanoparticles for the hydrogenation of olefins and alkynes under mild conditions in ethanol and in an aqueous medium. This catalyst proves robust towards the presence of oxidants, such as oxygen and water, is magnetically recoverable and shows selectivity towards the less activated double bonds. Hydrogenation is a ubiquitous reaction used in all fields of chemistry, from petrochemistry to drug synthesis. 1 Transition metals, such as Pd, Pt, Ru, Rh or Ni, both homogenous and heterogeneous, are catalysts of choice for this reaction. However, in an effort to develop a more sustainable approach, 2,3 their cost, toxicity and potential depletion has fuelled the development of alternative hydrogenation catalysts. Very recently, several catalysts 4–6 have been designed to avoid the use of precious metals, among which iron is a very attractive option. Iron is non-toxic, naturally abundant, cheap and potentially amenable to magnetic recovery. 7 Iron complexes were shown to be active catalysts 8 for the hydrogenation of olefins, 9 carbonyl bonds 10,11 and the selective hydrogenation of alkynes to alkenes. 12,13 Such complexes can also hydrogenate carbonates 14 and dehydrogenate formic acid. 15 Besides these developments in homogenous catalysis, iron in the form of suspendable nanoparticles has been investigated as a catalyst. 16,17 The de Vries group has evidenced that ligand-free iron nanoparticles (Fe NPs) are active catalysts for the hydrogenation of alkenes and alkynes under very mild conditions. 4,18 These particles proved very active, however, they could not be separated from the reaction medium magnetically because of their small size. Breit and co-workers overcame this limitation by stabilizing Fe NPs made by decomposition of Fe(CO) 5 onto graphene sheets. Although the resulting particles were active hydrogenation catalysts, 19 they were prone to oxidation in the presence of either oxygen or water. Growth of the oxide shell in the presence of an oxidant was suggested to be an absolute limitation to catalysis in terms of reactivity. Herein, we present the use of simple and stabilizer-free iron-iron oxide core–shell nanoparticles (Fe CSNPs) for the hydro-genation of alkenes and alkynes. These nanoparticles represent the first iron-based catalyst in ethanol, and in water-ethanol mixtures (Scheme 1). These nanoparticles are either synthesized in an aqueous medium, or produced in large scale commercially and suspended in water. These nanoparticles are recoverable magnetically and recyclable up to 10 times. Our results indicate that a thin shell of iron oxide surrounding the zero-valent core can protect the nanoparticles against excessive oxidation without obstructing hydrogenation reactivity, in protic and aqueous environments. Fe CSNPs were produced by the reduction of FeSO 4 in a water/methanol mixture using NaBH 4 . 20,21 Such particles have been investigated as stoichiometric reductants for water remediation 22–25 and also studied as magnetic seeds for Pd C–C couplings catalysts. 26 We were thus intrigued to see if these particles could also be active as hydrogenation catalysts. These Fe CSNPs featured an average core diameter of 44 AE 8.3 nm and a shell thickness of 6 AE 2 nm, which is comparable to what has been reported in the literature (Fig. 1). 25,27 Alternatively, we used commercial iron core–shell nanoparticles (C–Fe CSNPs) which also presented iron oxide sheets at their surface.w

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May 29, 2014