Oxidation and β-alkylation of alcohols catalysed by Iridium(I) complexes with functionalised N-heterocyclic Carbene ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C-C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and β-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH)(cod)(MeIm(2- methoxybenzyl))][BF] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF) of 1283 h, and also in the β-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan-2-ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross-aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C-C bond that involves the reaction of an O-bound enolate generated in the basic medium with the electrophilic aldehyde.Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER, Project CTQ2013-42532-P), MICINN (Project Consolider Ingenio 2010 CSD2009-00050) and Diputación General de Aragón (DGA/FSE-E07) is gratefully acknowledged. J.F.-T. thanks the Spanish MICINN for a predoctoral fellowship.Peer Reviewe

Catalytic activity of iridium NHC complexes covalently bonded to carbon nanotubes and graphene oxide

Trabajo presentado a la Annual World Conference on Carbon, celebrada en Dresden (Alemania) del 12 al 17 de julio de 2015.Carbon nanotubes and graphene have been extensively applied as proactive supports to generate heterogeneous catalysts. Both exhibit a carbon structure composed by carbon atoms with sp2 hybridization and both are used in similar applications, as in catalysis. However, the role of each particular substrate, determined by its structural peculiarities, can be differentvshould be studied for each particular catalytic reaction and in their structure.The aim of this work is to study the differences between oxidized carbon nanotubes (CNT) and graphene oxides (GO) as proactive supports of iridium Nheterocyclic carbene (NHC) catalyst. The effect of their inherent structure in the catalytic activity in hydrogen transfer reactions was studied in detail.The authors thank MINECO/MECD (Projects CONSOLIDER INGENIO 2010 CSD2009-00050, CTQ 2010-15221, Ramón y Cajal contract of P. Alvarez and fellowship of M. Blanco) and the Diputación General de Aragón (E07) for their financial support.Peer Reviewe

Diseño de catalizadores de iridio basados en ligandos NHC para procesos de transferencia y autotransferencia de hidrógeno

La presente Memoria, estructurada en cuatro capítulos, describe la síntesis y caracterización de complejos de rodio(I) e iridio(I) con ligandos NHC con potencial comportamiento hemilábil y la evaluación de su actividad catalítica en reacciones en fase homogénea de transferencia y autotransferencia de hidrógeno. Adicionalmente, se ha estudiado la actividad catalítica de materiales basados en complejos de iridio-NHC soportados sobre nanotubos de carbono en reacciones de transferencia de hidrógeno de 2-propanol a ciclohexanona. En el capítulo 1 se presenta la síntesis de una serie de sales de imidazolio precursoras de los ligandos carbeno N-heterocíclico (NHC) que se utilizarán posteriormente. En general, la desprotonación del protón ácido H2 de las sales de imidazolio O-funcionalizadas por los grupos metoxo de los complejos dímeros [M(¿-OMe)(cod)]2 (M = Rh, Ir) ha permitido preparar los complejos neutros de rodio/iridio(I)-NHC [MX(cod)(MeImRO)]. Por otro lado, los complejos catiónicos de rodio/iridio(I) [ML(cod)(MeImRO)][X] (L = CH3CN, PPh3 y py) se han preparado por reacción de los halocomplejos neutros [MX(cod)(MeImRO)] con sales de plata con un anión no coordinante (AgBF4) en presencia de ligandos PPh3 o piridina, o de un disolvente coordinante (THF, CH3CN). Las sales de amonio-imidazolio [RImRN][X]·HX (R = metilo o t-butilo; RN = N,N-dimetilpropilamina, N-t-butilpropilamina; X = Cl, Br) presentan dos protones ácidos lo que ha permitido la síntesis secuencial de las sales de anión complejo dihalogenocicloctadieniridiato(I) y cationes amino-imidazolio, [RImRN][IrX2(cod)], y la síntesis de los carbeno NHC-amino-complejos neutros de iridio(I) [IrX(cod)(MeImRN)]. Finalmente, los amino-NHC complejos catiónicos de iridio(I) [Ir(cod) {k2-C,N-(MeImRN)}][BF4] se preparan por transmetalación del carbeno NHC por reacción de los complejos organometálicos carbeno-plata [AgX(MeImRN)]n (X = Cl, Br), aislados o generados in situ, con el solvato complejo de acetona de iridio(I) [Ir(cod)(OCMe2)2][BF4]. En el capítulo 2 se estudia la actividad y selectividad de los complejos de Ir-NHC en procesos de transferencia de hidrógeno de 2-propanol a diferentes sistemas insaturados y de formiato de sodio a cetonas y aldehídos en un sistema bifásico en agua. El complejo catiónico de iridio(I) [Ir(NCCH3)(cod)(MeIm(2-metoxibencil))][BF4] es el catalizador más activo de los complejos sintetizados en la transferencia de hidrógeno de 2-propanol a sustratos insaturados con dobles enlaces C=O y C=N alcanzando un TOF de 4622 h-1 en la reducción de ciclohexanona. Los cálculos DFT realizados para este sistema demuestran el efecto positivo de la presencia del fragmento rígido 2-metoxibencilo sobre la actividad catalítica del complejo. Los cálculos han puesto de manifiesto la existencia de una interacción del fragmento 2-metoxibencil con el protón en posición ß del grupo isopropóxido que desestabiliza el intermedio alcoxo complejo [Ir(OiPr)(cod)(MeIm(2-metoxibencil))] lo que facilitaría la formación del iridio-hidruro complejo [Ir(H)(cod)(MeIm(2-metoxibencil))], la especie catalíticamente activa y que ha sido detectada por espectrometría de masas. Por otro lado, los complejos de iridio(I) con ligandos carbeno NHC N u O-funcionalizados son moderadamente activos en la transferencia de hidrógeno de formiato de sodio a cetonas y aldehídos en un sistema bifásico con medio acuoso.En el capítulo 3 se estudia la actividad catalítica de los complejos de iridio(I)-NHC en procesos de autotransferencia de hidrógeno. Los complejos neutros y catiónicos de iridio(I) con ligandos NHC N u O-funcionalizados son activos en procesos de autotransferencia de hidrógeno como ß-alquilación de alcoholes secundarios con alcoholes primarios, y N-alquilación de aminas con alcoholes. En los primeros, se observa una selectividad y actividad elevada en condiciones de exceso de base mientras que para los segundos, la selectividad es cuantitativa a amina al aumentar los tiempos de reacción. Tanto en los procesos de autotransferencia de hidrógeno catalizados por complejos neutros como en las reacciones catalizadas por complejos catiónicos se observa un efecto positivo sobre la actividad y selectividad del proceso pero desconocemos la naturaleza de la influencia del grupo funcional del ligando carbeno NHC. En el capítulo 4 se describe la preparación de catalizadores de Ir-NHC híbridos basados en complejos Ir-NHC soportados sobre nanotubos de carbono multipared (MWCNT). Los materiales se han preparado funcionalizando los nanotubos de carbono multipared por reacción con sales de imidazolio funcionalizadas con grupos hidroxilo. La posterior desprotonación del protón H2 de dichas sales por los grupos metoxo del complejo dímero [Ir(¿-OMe)(cod)]2, dan lugar a los complejos de NHC-iridio(I) soportados. Se han utilizado dos tipos de nanotubos de carbono soportados, un material oxidado químicamente y un material oxidado químicamente pero sin grupos ácido carboxílico. Estos catalizadores híbridos son activos en la reducción de ciclohexanona por transferencia de hidrógeno de 2-propanol, y la actividad del catalizador híbrido con el ligando carbeno NHC funcionalizado con un fragmento 3-hidroxipropil soportado sobre nanotubos de carbono oxidados es comparable con la observada en fase homogénea. Además, es posible reciclar estos catalizadores híbridos en cinco ciclos catalíticos consecutivos sin observar pérdida de actividad e incluso obtener una conversión similar en presencia de aire.Peer Reviewe

Mechanistic insights into transfer hydrogenation catalysis by [Ir(cod)(NHC)2]+ complexes with functionalized N-heterocyclic carbene ligands

The synthesis of unbridged biscarbene iridium(I) [Ir(cod)(MeIm∩Z)2]+ complexes having N- or O-functionalized NHC ligands (∩Z = 2-methoxybenzyl, pyridin-2-ylmethyl, quinolin-8-ylmethyl) is described. The molecular structures of the complexes show an antiparallel disposition of the carbene ligands that minimize the steric repulsions between the bulky substituents. However, the complexes were found to be dynamic in solution, due to the restricted rotation about the C(carbene)–Ir bond that results in two interconverting diasteromers having different dispositions of the functionalized NHC ligands. A rotational barrier of around 80 kJ mol–1 (298 K) has been determined by 2D EXSY NMR spectroscopy. The iridium(III) dihydride complex [IrH2(MeIm∩Z)2]+ (∩Z = pyridin-2-ylmethyl) has been prepared by reaction of the corresponding iridium(I) complex with molecular hydrogen. These complexes efficiently catalyzed the transfer hydrogenation of cyclohexanone using 2-propanol as a hydrogen source and KOH as base at 80 °C with average TOF values of 117–155 h–1 at 0.1 mol % iridium catalyst loading. All of the catalyst precursors showed comparable activity independent of both the wingtip type at the NHC ligands and the counterion. Mechanistic studies support the involvement of diene free bis-NHC iridium(I) intermediates in these catalytic systems. DFT calculations have shown that a MPV-like concerted mechanism (Meerwein–Ponndorf–Verley mechanism), involving the direct hydrogen transfer at the coordination sphere of the iridium center, might compete with the well-established hydrido mechanism. Indirect evidence of a MPV-like mechanism has been found for the catalyst precursor having NHC ligands having with a pyridin-2-ylmethyl wingtip.Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER, Project CTQ2013-42532-P), MICINN (Project Consolider Ingenio 2010 CSD2009-00050) and Diputación General de Aragón (DGA/FSE-E07) is gratefully acknowledged. J.F.-T. thanks the Spanish MICINN for a predoctoral fellowship.Peer reviewe

Functionalized carbon nanotubes as support of Iridium-NHC complexes for catalysis applications

Trabajo presentado como póster a la Annual World Conference on Carbon celebrada en Rio de Janeiro (Brasil) del 14 al 19 de julio de 2013.A new strategy for the functionalization of multiwalled carbon nanotubes (MWCNT) for their use as support of iridium N-heterocyclic carbene (NHC) complexes has been developed. Oxidized MWCNT obtained by acid treatment were covalently modified through their OH groups by functionalization with the appropriate hydroxyl-ending imidazolium salt. Carbon nanotubes thermally reduced at 400 ºC (MWCNT-TR400) were also studied for comparative purposes. The quantification of imidazolium groups in the imidazolium-modified MWCNT samples were carried out by means of solid characterization techniques such as TGA or XPS. Moderated to good conversions were achieved. Both samples were used to generate immobilized iridium NHC complexes from the imidazolium salts. The supported complexes were active as heterogeneous catalysts for the reduction of cyclohexanone to cyclohexanol by transfer hydrogenation processes from 2-propanol/KOH. A comparative study with related homogeneous acetoxy-functionalized Ir-NHC complexes has shown that heterogeneous systems are more efficient than the homogeneous ones. Good catalyst recyclability without any loss of catalytic activity and excellent catalyst stability on air was observed.The authors thank MICINN (Projects CONSOLIDER INGENIO 2010 CSD2009-00050, MAT2010-16194 and CTQ 2010-15221), and the Diputación General de Aragón (E07) for their financial support. Dr. Patricia Álvarez thanks MICINN for her Ramón y Cajal contract. Javier Fernández-Tornos and Matias Blanco acknowledge their fellowships from MICINN and MECDPeer Reviewe

Iridium(I) complexes with hemilabile N-heterocyclic carbenes: Efficient and versatile transfer hydrogenation catalysts

16 páginas, 5 figuras, 3 tablas, 3 esquemas.A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]+ (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH3)(cod)(MeImR)]+ (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h–1. The complexes [Ir(NCCH3)(cod)(MeImR)]+ (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity.Financial support from Ministerio de Ciencia e Innovación (MICINN/FEDER) is gratefully acknowledged (Projects: CTQ2010-15221, CSD2006-0015, and CSD2009-00050). J.F. T. thanks the Spanish MICINN for a predoctoral fellowship, and S.W. thanks the Eramus program (Univ. RWTH-Aachen, Germany).Peer reviewe

Enhanced hydrogen-transfer catalytic activity of iridium N-heterocyclic carbenes by covalent attachment on carbon nanotubes

Oxidized multiwall carbon nanotubes (CNT) were covalently modified with appropriate hydroxyl-ending imidazolium salts using their carboxylic acid groups. Characterization of the imidazolium-modified samples through typical solid characterization techniques, such as TGA or XPS, allows for the determination of 16 wt % in CNT-1 and 31 wt % in CNT-2 as the amount of the imidazolic fragments in the carbon nanotubes. The imidazolium-functionalized materials were used to prepare nanohybrid materials containing iridium N-heterocyclic carbene (NHC)-type organometallic complexes with efficiencies as high as 95%. The nanotube-supported iridium-NHC materials were active in the heterogeneous iridium-catalyzed hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source. The iridium hybrid materials are more efficient than related homogeneous catalysts based on acetoxy-functionalized Ir-NHC complexes with initial TOFs up to 5550 h -1. A good recyclability of the catalysts, without any loss of activity, and stability in air was observed. © 2013 American Chemical Society.The authors thank MICINN (Projects Consolider Ingenio 2010 CSD2009-00050 and CTQ 2010-15221), and the Diputación General de Aragón (E07) for their financial support. Dr. Patricia Álvarez thanks MICINN for her Ramón y Cajal contract. Javier Fernández-Tornos and Matias Blanco acknowledge their fellowships from MICINN and MECD.Peer Reviewe

Funcionalización de nanotubos de carbono como soporte de complejos NHC de iridio para su aplicación en catálisis

Trabajo presentado a la XII Reunion del Grupo Español del Carbón celebrada en Madrid del 20 al 23 de octubre de 2013.Los autores agradecen al MICINN (Consolider Ingenio 2010 CSD2009-00050, CTQ 2010-15221, Ramon y Cajal), y a la Diputación General de Aragón (E07). Javier Fernández-Tornos y Matías Blanco agradecen sus becas de formación al MICINN y al MECD.Peer Reviewe

Graphene-NHC-iridium hybrid catalysts built through -OH covalent linkage

Graphene oxide (GO) and thermally reduced graphene oxide (TRGO) were covalently modified with imidazolium salts through their hydroxyl surface groups. The selective reaction of the -OH groups with p-nitrophenylchloroformate produced labile intermediate organic carbonate functions which were used for the covalent anchoring of a hydroxy-functionalized imidazolium salt. Nanohybrid materials containing iridium N-heterocyclic carbene (NHC)-type organometallic complexes were prepared by causing the imidazolium-functionalized materials to react with [Ir(μ-OMe)(cod)]2. The iridium content of the graphene-based hybrid catalysts, as determined by XPS and ICP-MS was the order of ∼5 and 10 wt.%, for the TRGO and GO-based materials, respectively. The graphene-supported iridium hybrid materials were active in the heterogeneous hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as the hydrogen source. The thermally reduced graphene-NHC-iridium hybrid catalyst showed the best catalytic performance with an initial TOF of 11.500 h-1, slightly better than the related acetoxy-functionalized NHC iridium homogeneous catalyst. A good catalyst recyclability and stability were achieved.The authors thank the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (Projects Consolider Ingenio 2010 CSD2009-00050 and CTQ 2013-42532-P), and the Diputación General de Aragón (E07) for their financial support. Dr. P. A. thanks MICINN for a Ramón y Cajal contract. J. F-T. and M. B. acknowledge their fellowships from MINECO and MECD (AP2010-0025).Peer Reviewe

Effect of structural differences of carbon nanotubes and graphene based iridium-NHC materials on the hydrogen transfer catalytic activity

A proper design of heterogeneous molecular catalysts supported on carbon materials requires a systematic study of >metal-carbon support interactions> and their influence on catalytic activity. In this study, hybrid materials containing covalently anchored iridium N-heterocyclic carbene (NHC) organometallic complexes have been successfully prepared from oxidized and partially reduced carbon nanotubes (CNTs). The preparation method for these supported materials relies on the selective functionalization of the superficial hydroxylic groups using the imidazolium salt, 1-(3-hydroxypropyl)-3-methyl-1H-imidazol-3-ium chloride. The hydrogen transfer catalysis activity of these nanotube-based hybrid catalysts was tested by the reduction of cyclohexanone to cyclohexanol with 2-propanol, and the results of the tests were compared with those obtained using similar hybrid graphene-based catalysts. While EXAFS analysis revealed a common first coordination shell of the iridium atom for all the hybrid materials examined, independently of whether they were either supported on carbon nanotubes or graphene materials, catalytic activity in all the reduced materials was significantly superior. Moreover, catalytic systems based on reduced CNTs exhibited a better performance than those based on reduced graphene materials. Both these facts suggest there is a positive correlation between hydrogen transfer catalytic activity, reconstruction of the aromatic carbon structure and the smaller amount of oxygen functional groups.The authors thank the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (Projects Consolider Ingenio 2010 CSD2009-00050 and CTQ2013-42532-P), and the Diputación General de Aragón (DGA) (FSE-E07 and FSE-E69) for their financial support. Dr. P. A. thanks MICINN for a Ramón y Cajal contract. J. F-T. and M. B. acknowledge their fellowships from MINECO and MECD (AP2010-0025).Peer Reviewe