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Iron Complexes for Hydrogen Activation and Catalytic Hydrogenation

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Iron Complexes for Hydrogen Activation and Catalytic Hydrogenation Iron Complexes for Hydrogen Activation and Catalytic Hydrogenation THÈSE NO 6776 (2015) PRÉSENTÉE LE 25 SEPTEMBRE 2015 À LA FACULTÉ DES SCIENCES DE BASE LABORATOIRE DE SYNTHÈSE ET DE CATALYSE INORGANIQUE PROGRAMME DOCTORAL EN CHIMIE ET GÉNIE CHIMIQUE ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Simona MAZZA acceptée sur proposition du jury: Prof. K. Severin, président du jury Prof. X. Hu, directeur de thèse Prof. A. Mezzetti, rapporteur Dr Ph. Dupau, rapporteur Dr M. Mazzanti, rapporteuse Suisse 2015 "Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world" A. Einstein Acknowledges At first I would like to thank my advisor Prof. Xile Hu for giving me the opportunity to carry out my Ph.D. studies in his group. His instructive advices and guidance were extremely valuable and without them the development of this thesis would not have been possible. I would like to thank my thesis jury for accepting to examine this thesis: Prof. Kay Severin, Dr. Marinella Mazzanti, Prof. Antonio Mezzetti and Dr. Dupau Philippe. Many thanks to the LSCI secretary Christina Zamanos-Espremian for her help with the administrative work and our chitchats during the coffee breaks. A special thank goes also to the secretary of the Ph.D. school Anne-Lene Odegaard. Thanks to the past and present co-workers of the LSCI that contributed to an agreeable working environment. Besides, I really enjoyed the cheerful time spent outside the lab during our outdoor activities. Special thanks go to Dr. Heron Vrubel for his precious help in setting up experiments and for the home-made glassware, to Thomas Di Franco for the French translation of the abstract and to Pablo Marcelo Garcia and Carlos Morales for our fruitful conversations. Thanks to Dr. Euro Solari and Dr. Rosario Scoppelliti for the X-Ray and elemental analyses that contributed to this thesis and for the enjoyable and interesting talks we had together. I would like to thank the team of the ISIC chemical store: Gladys Pache, Annalise Carrupt and Benjamin Kronenberg for their excellent work and their help whenever I needed. Thanks to Francisco Sepulveda and Dr. Laure Menin for the mass-spec analysis; to Dr. Mieville Pascal for his help with the NMR spectrometers and to Patrick Favre and Donald Zbinden for the IT support. I Abstract The activation of molecular hydrogen (H2) via either homolytic or heterolytic cleavage is one of the most attractive subjects in sustainable chemistry. So far the cleavage of H-H bond is commonly achieved by using transition metal complexes based on precious metals such as ruthenium, rhodium, platinum, palladium or iridium. Due to the low availability and price issues in the last decades an unceasing amount of research has been carried out in order to find more attractive substitute metals. Looking at the first-row transition metals, iron is offering a good alternative being non-toxic, abundant, inexpensive and eco-friendly. Inspired by Nature several groups have developed structural and functional iron complexes mimicking the active site of the iron-hydrogenases, which show high reactivity in the H2 cleavage. Usually pendant bases have been incorporated onto families of Fe complexes in order to achieve active systems. In the view of these recent developments, in chapter two, we investigated the possibility of synthesizing novel iron (II) complexes bearing an amine internal base and providing an open site for substrate binding as catalysts for H2 activation. Pentacoordinated Fe(II) low spin complexes [(PhPNP)Fe(CO)(bdt)] (1), [(PhPNP)Fe- (CO)(Nbt)] (4), [(CyPNP)Fe(CO)(Nbt)] (5), [(dppe)Fe(CO)(Nbt)] (6) and the paramagnetic Cy complex [( PNP)FeCl2] (10) have been synthesized and fully characterised. Unfortunately, when these complexes were tested as catalysts for hydrogenation reaction of a wide range of unsaturated substrates, no appreciable reactivity was observed. Same behaviour was observed Cy Cy in the case of complexes [( PNP)Fe(CO)(Cp)] (11) and [( PNP)Fe(CH3CN)(Cp)] (12) where the Cp ligand was installed in order to modulate the electronic and steric properties on the iron center. In chapter three, a new class of well-defined iron pincer complexes is reported. Several Fe(II) complexes supported by a 2,6-bis(phosphinito)pyridine ligand (PONOP) have been synthesized and fully characterised. In particular, the Fe-hydride complexes iPr iPr [( PONOP)-Fe(CO)(H)Br] (14) and [( PONOP)Fe(CO)(H)(CH3CN)](OTf) (15) could activate H2 at room temperature. Moreover, complexes 14 and 15 served as catalysts for the selective hydrogenation of aldehydes at room temperature. In presence of sodium formate as hydrogen donor, 14 and 15 showed an excellent reactivity in hydrogen transfer reaction of aldehydes. The mechanism of hydrogen activation and hydrogenation is discussed based on the observed reactivity of iron complexes. The feature of being chemoselective towards II aldehydes and a broad functional-group tolerance make these iron-hydride systems remarkable in the class of the earth-abundant-metal hydrogenation catalysts. Chapter four is dedicated to the tuning of the Fe-PONOP systems reported in chapter three in order to synthesize similar iron(II) complexes exhibiting enhanced reactivity for H2 activation and hydrogenation of unsaturated substrates. As first attempt, complexes Cy Cy [( PONOP)Fe(CO)Br2] (22) and the analogous chloride [( PONOP)Fe(CO)Cl2] (23) bearing the stronger donor CyPONOP ligand were synthesized and tested as catalysts for hydrogenation reaction, but none of the substrates employed was reduced. As second attempt, the CO ligand was substituted with the better donor ligand tert-butyl isocyanide in presence of iPrPONOP as pincer ligand. Several Fe-PONOP complexes were synthesized and fully characterized. In particular the Fe-hydride complex [(iPrPONOP)Fe(tBuNC)(H)Br] (25) exhibited reactivity towards hydrogenation of aldehydes under the same reaction conditions reported for 14. No reaction was observed in presence of acetophenone, cyclohexene and 1- decene demonstrating that, although spectroscopically different, 25 did not exhibit enhanced reactivity relative to 14. Keywords: hydrogen activation, hydrogenation reaction, hydrogen transfer reaction, iron- hydride complexes, hydrogenases, pincer ligands, homogeneous catalysis, aldehydes. III Résumé L'activation d'hydrogène moléculaire (H2) via un clivage homolytique ou un clivage hétérolytique est l'un des sujets les plus attractifs de la chimie durable. Jusqu'à présent, le clivage de la liaison H-H est communément réalisé par l'utilisation de complexes basés sur des métaux de transition tels que le ruthénium, le rhodium, le platine, le palladium ou l'iridium. A cause de leur faible disponibilité et de problèmes de coût ces dernières décennies, d'incessantes recherches ont été menées afin de trouver des métaux de substitution plus attractifs. Parmi les métaux de transition de la première rangée, le fer offre une bonne alternative tant par sa non-toxicité, son abondance, son bas coût que par son caractère écologique. S'inspirant de la nature, plusieurs groupes de recherche ont développé des complexes de fer mimant par leur structure et par leur fonction le site actif des hydrogénases à fer, qui font preuve d'une réactivité élevée pour le clivage de H2. Des bases pendantes ont été généralement incorporées dans les familles de ces complexes de fer afin d'obtenir des systèmes actifs. Dans l'optique de ces récents développements, le chapitre 2 présente les recherches sur la possibilité de synthétiser de nouveaux complexes de fer (II) portant une base amine interne et fournissant un site libre pour lier les substrats en tant que catalyseurs Ph d'activation de H2. Les complexes pentacoordinés de Fe(II) bas spin [( PNP)Fe(CO)(bdt)] (1), [(PhPNP)Fe(CO)(Nbt)] (4), [(CyPNP)Fe(CO)(Nbt)] (5), [(dppe)Fe(CO)(Nbt)] (6) et le Cy complexe paramagnétique [( PNP)FeCl2] (10) ont été synthétisés et entièrement caractérisés. Malheureusement, lorsque ces complexes ont été testés comme catalyseur des réactions d'hydrogénation d'une grande variété de substrats insaturés, aucune réactivité appréciable n'a été observée. Le même comportement a été observé dans le cas des complexes Cy Cy [( PNP)Fe(CO)(Cp)] (11) et [( PNP)Fe(CH3CN)(Cp)] (12) dans lesquels le ligand Cp a été placé afin de moduler les propriétés électroniques et stériques du centre métallique fer. Dans le chapitre 3, une nouvelle classe de complexes "pincer" de fer bien définis est exposée. Plusieurs complexes de Fe(II) supportés par un ligand 2,6-bis(phosphinito)pyridine (PONOP) ont été synthétisés et entièrement caractérisés. En particulier, les complexes iPr iPr d'hydrure de fer [( PONOP)-Fe(CO)(H)Br] (14) et [( PONOP)Fe(CO)(H)(CH3CN)](OTf) (15) peuvent activer H2 à température ambiante. De plus, les complexes 14 et 15 ont servi comme catalyseur pour l'hydrogénation sélective des aldéhydes à température ambiante. En présence de formate de sodium comme donneur d'hydrogène, 14 et 15 ont montré une IV excellente réactivité pour la réaction de hydrogénation par transfert des aldéhydes. Le mécanisme d'activation de l'hydrogène et d'hydrogénation est discuté en se basant sur la réactivité observée de ces complexes de fer. Le caractère chémiosélectif envers les aldéhydes et une grande tolérance aux groupes fonctionnels rendent ces systèmes d'hydrure de fer remarquables dans la classe des catalyseurs d'hydrogénation à base de métaux abondants sur Terre. Le chapitre 4 est consacré à la mise au point de systèmes Fe-PONOP décrits au chapitre 3 afin de synthétiser des complexes de fer (II) similaires exhibant une réactivité améliorée pour l'activation de H2 et l'hydrogénation de substrats insaturés. Comme premiers Cy essais, les complexes [( PONOP)Fe(CO)Br2] (22) et l'analogue chloré Cy Cy [( PONOP)Fe(CO)Cl2] (23) portant le ligand PONOP plus fortement donneur ont été synthétisés et testés comme catalyseurs pour la réaction d'hydrogénation mais aucun des substrats employés n'a été réduits.
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