Hydroboration of carbon dioxide with catechol- and pinacolborane using an Ir–CNP* pincer complex. Water influence on the catalytic activity

Sánchez, Práxedes; Hernández‐Juárez, Martín; Rendón, Nuria; López‐Serrano, Joaquín; Álvarez, Eleuterio; Paneque, Margarita; Suárez, Ana Gutiérrez

doi:10.1039/c8dt03951h

Cited by 19 publications

(18 citation statements)

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“…Recent research from our group has focused on the study of nonsymmetric lutidine-derived pincer ligands, i.e., having two inequivalent flanking donor groups, particularly of the CNP class (Figure ). − The presence of two significantly different strong σ-donors in the CNP pincer, such as a phosphine and a NHC, allows a larger electronic and steric diversity of the ligands, and facilitates a direct comparison of the relative acid/base reactivities of the NHC- and phosphine-bound methylene arms.…”

Section: Introductionmentioning

confidence: 99%

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

et al. 2021

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Metal catalysts based on ligands containing protonresponsive sites have found widespread applications in the hydrogenation of polar unsaturated substrates. In this contribution, Ir complexes incorporating lutidine-derived CNP (C = N-heterocyclic carbene, NHC; P = phosphine) pincer ligands with two nonequivalent Brønsted acid/base sites have been examined in the hydrogenation of aldehydes. To this end, Ir(CNP)H 2 Cl complexes were synthesized in two steps from the CNP ligand precursors and Ir(acac)(COD). These derivatives react with an excess of NaH to yield the trihydride derivatives Ir(CNP)H 3 , which were assessed as catalyst precursors in the hydrogenation of a series of aldehydes. The catalytic reactions were performed using commercial-grade substrates under neutral, mild conditions (0.1 mol % Ir-CNP; 4 bar H 2 , room temperature) with high conversions and selectivities for the reduction of the carbonyl function in the presence of other readily reducible groups such as CC, nitro, and halogens. Reaction of an Ir(CNP)H 2 Cl complex with base in the presence of an aromatic aldehyde produces the reversible formation of alkoxide Ir complexes in which the aldehyde is bound to the deprotonated pincer framework (CNP*) through the CH-NHC arm of the ligand. These species, along with a carboxylate complex resulting from the Ir mediated oxidation of the aldehyde by water, is observed in the reaction of Ir(CNP)H 3 with benzaldehyde. Finally, investigation of the mechanism of the hydrogenation of aldehydes has been carried out by means of DFT calculations considering the involvement of each arm of the Ir-CNP/CNP* derivatives. Calculations support a mechanism in which the catalyst switches its metal−ligand cooperation sites to follow the lowest energy pathway for each step of the catalytic cycle.

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Section: Introductionmentioning

confidence: 99%

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

et al. 2021

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“…Upon deprotonation of the methylene CH 2 –NHC arms, these organometallics are catalytically active in ligand-assisted processes. 22 − 25 …”

Section: Introductionmentioning

confidence: 99%

Ammonia–Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNN^H Complexes

et al. 2021

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Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H 2 molecule. In this contribution, a series of Ir III complexes based on lutidine-derived CNN H pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph ( 4a ), t Bu ( 4b ), benzyl ( 4c )] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia–borane (NH 3 BH 3 , AB) in the presence of substoichiometric amounts (2.5 equiv) of t BuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a , 1.0 equiv of H 2 per mole of AB was released in 8.5 min (turnover frequency (TOF 50% ) = 1875 h –1 ), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF 50% = 55–60 h –1 ). 4a is currently the best performing Ir III homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (−d[AB]/d t = k [ 4a ]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (−d[AB]/d t = k [ 4b ][AB] 2 ). Moreover, the reactions of the derivatives 4a and 4b with an excess of t BuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b , a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with t BuOK. Complex 6 is capable of activating H 2 reversibly to yield the trihydride derivative 7 . Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal–ligand cooperation mode.

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“…In 2018, Rendón, Suarez and co-workers showed that Ir carbonyl pincer complexes stabilized by a deprotonated lutidine-derived CNP* ligand selectively catalyzed the hydroboration of CO 2 under 1-2 bar, 30°C either to methoxyborane using HBcat (TOF = 56 h À 1 , maximum yield = 28 %, TON = 84) or to formoxyborane with the more sterically demanding HBpin (TOF = 1245 h À 1 ) in either THF/H 2 O or THF, respectively. [37] One year later, Hazari and co-workers examined the correlations between catalyst and borane structure and the level and rates of catalytic CO reduction using Pd and Ni complexes, supported by PSiP and PCP pincer ligands, varying systematically their steric and electronic properties ( Figure 1). [38] Results showed that a complex network of effects is active upon varying, together with the catalyst and the borane, the reaction conditions (CO 2 pressure, reaction time, catalyst concentration), ruling the activity and selectivity of the catalytic system.…”

Section: Introductionmentioning

confidence: 99%

“…More recently, the potential of Ir, Pd and Ni pincer complexes was examined in detail, focusing on the effects of type of ligands and hydroboranes in determining the selectivity of the reaction. In 2018, Rendón, Suarez and co‐workers showed that Ir carbonyl pincer complexes stabilized by a deprotonated lutidine‐derived CNP* ligand selectively catalyzed the hydroboration of CO 2 under 1–2 bar, 30 °C either to methoxyborane using HBcat (TOF=56 h −1 , maximum yield=28 %, TON=84) or to formoxyborane with the more sterically demanding HBpin (TOF=1245 h −1 ) in either THF/H 2 O or THF, respectively . One year later, Hazari and co‐workers examined the correlations between catalyst and borane structure and the level and rates of catalytic CO 2 reduction using Pd and Ni complexes, supported by PSiP and PCP pincer ligands, varying systematically their steric and electronic properties (Figure ) .…”

Section: Introductionmentioning

confidence: 99%

Mild and Selective Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Mn(I) PNP Pincer Complexes

et al. 2020

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Well-defined Mn(I)-PNP pincer-type complexes were tested as non-precious transition metal catalysts for the selective reduction of CO 2 to boryl-protected MeOH in the presence of hydroboranes (HBpin, 9-BBN) and borates as Lewis acids (LA) additives. The best performance was obtained under mild reaction conditions (1 bar CO 2 , 60°C) in the presence of the hydridocarbonyl complex [MnH(PNP NH-iPr)(CO) 2 ] and B(OPh) 3 as co-catalyst. Preliminary mechanistic studies suggest that the initial activation step may occur by cationization of the metal center by the strong LA, and that both metal-catalyzed and metal-free steps are present in the overall catalytic system.

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Hydroboration of carbon dioxide with catechol- and pinacolborane using an Ir–CNP* pincer complex. Water influence on the catalytic activity

Abstract: Lutidine-derived CNP*–Ir complexes catalyze the hydroboration of CO2 to methoxyborane and formoxyborane in the presence of small amounts of water.

Cited by 19 publications

References 119 publications

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Ammonia–Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNN^H Complexes

Mild and Selective Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Mn(I) PNP Pincer Complexes

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Hydroboration of carbon dioxide with catechol- and pinacolborane using an Ir–CNP* pincer complex. Water influence on the catalytic activity

Abstract: Lutidine-derived CNP*–Ir complexes catalyze the hydroboration of CO2 to methoxyborane and formoxyborane in the presence of small amounts of water.

Cited by 19 publications

References 119 publications

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Ammonia–Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes

Mild and Selective Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Mn(I) PNP Pincer Complexes

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Ammonia–Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNN^H Complexes