Dalton Transactions PAPER Cite this: Dalton Trans., 2017, 46, 5082 Received 21st February 2017, Accepted 17th March 2017 DOI: 10.1039/c7dt00649g rsc.li/dalton Kinetic investigation into the chemoselective hydrogenation of α,β-unsaturated carbonyl compounds catalyzed by Ni(0) nanoparticles† Laíze Zaramello, a,b Brunno L. Albuquerque, a Josiel B. Domingos * a and Karine Philippot * b A series of Ni(0) nanocatalysts was prepared from a Ni(COD) 2 complex in the presence of different stabil- izers (hexadecylamine, polyvinylpyrrolidone (PVP), PVP/triphenylphosphine, octanoic acid and stearic acid) for their evaluation in the selective hydrogenation reaction of α,β-unsaturated carbonyl compounds by H 2 under mild reaction conditions, i.e., low H 2 pressure, temperature and catalyst loading. All nanoca- talysts were active in reducing only the CvC bond and this chemoselectivity was attributed to the reduced nature of the Ni-NPs surface. Moreover, the hydrogenation reaction rate appeared to be sensitive to ligand type, with the carboxylic acid-stabilized systems showing the best performances. A full kinetic investigation into the t-chalcone chemoselective reduction of the CvC bond, with the best catalyst (Ni- octanoic acid) revealed that the rate-determining step isthe hydrogenation of the adsorbed substrate on the NPs surface, following a Horiuti–Polanyi type of mechanism. Regarding sustainable chemistry con- cerns, the best catalyst could be reused up to 10 times without significant loss of activity. Introduction In the last few decades, intense efforts have been dedicated to the search for new catalysts for the chemoselective reduction of olefinic bonds in α,β-unsaturated carbonyl compounds, a challenging reaction of high interest in the manufacture of industrially/pharmaceutically relevant chemicals. 1–3 More efficient processes working under mild conditions (even ambient conditions) and employing easily recoverable catalysts are necessary. The classic H 2 pressure method and catalytic transfer hydrogenation (CTH) are the most used protocols reported for this reaction, 4 and most catalytic systems involve precious metals, such as ruthenium, rhodium, platinum, iridium and palladium. 2,5,6 To overcome the high cost of these metals, catalysts containing cheaper, abundant first-row metals like iron, cobalt and nickel have been explored. 7–10 An interesting alternative is the use of nickel nanoparticles (Ni- NPs), which has steadily increased for the hydrogenation of olefins, carbonyl and nitro-aromatic compounds, 9–17 but few reports explore the double bond discrimination in α,β-unsaturated carbonyl compounds reduction. 3,18–20 When Ni-NPs are applied in the reduction of unsaturated bonds, the CTH method is the most used, where alcohols are the most common hydrogen source. NaBH 4 and hydrazine have also been reported, but under these conditions carbonyl groups can also be reduced. 10,21 The application of Ni-NPs as catalysts in CTH methods normally requires high metal loading to achieve good conversion, with values varying from 0.2 to 1 molar equivalent of Nickel in respect to the substrate. 9 The association of high molecular weight hydrogen sources with high catalyst loading usually leads to the generation of numer- ous by-products, making product separation difficult. For that reason, reactions based on Ni-NPs as catalysts and H 2 pressure still have the advantage in making the process cleaner; the NPs can be recovered from the reaction medium given their magnetic properties, and the use of H 2 avoids the formation of by-products, and consequently the poisoning of the nano- particles’ surface. To the best of our knowledge, only a few works describe selective CvC hydrogenation using Ni-NPs to reduce functionalized alkenes under H 2 pressure. Mokhov et al. have reported some selective hydrogenation of functiona- lized alkenes such as cinnamic nitrile and coumarin, under 1 atm of H 2 with Ni-NPs. However, the Ni-NPs prepared by the reduction of a nickel salt by NaBH 4 were colloidal suspensions and no catalyst recovery was performed. 17 Ionic liquid-immobi- lized Ni-NPs have also shown good results for the hydrogen- ation of 4-phenyl-3-buten-2-one among other functionalized † Electronic supplementary information (ESI) available: Complementary microscopy, spectroscopy and kinetic data. See DOI: 10.1039/c7dt00649g a Chemistry Department (LaCBio), Universidade Federal de Santa Catarina, Florianópolis, SCC.P. 476, 88040-900, Brazil. E-mail: josiel.domingos@ufsc.br b CNRS, LCC (Laboratoire de Chimie de Coordination), Université de Toulouse, 205 Route de Narbonne, Toulouse, France. E-mail: karine.philippot@lcc-toulouse.fr 5082 | Dalton Trans. , 2017, 46, 5082–5090 This journal is © The Royal Society of Chemistry 2017 Published on 17 March 2017. Downloaded by Universidade Federal de Santa Catarina (UFSC) on 10/04/2017 16:34:12. View Article Online View Journal | View Issue