| 1. |
- Ahlstrand, David A., et al.
(author)
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Csp(3)-H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products
- 2017
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In: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 23:8, s. 1748-1751
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Journal article (peer-reviewed)abstract
- Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino) pyridine is not chelation assisted, which has not been described before for Csp(3)-H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino) pyridine was studied by DFT calculations and kinetic measurements.
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| 2. |
- Chakrabarti, Kaushik, et al.
(author)
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Acceptorless dehydrogenation of 4-methylpiperidine by supported pincer-ligated iridium catalysts in continuous flow
- 2023
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In: Catalysis Science and Technology. - 2044-4753. ; 13:17, s. 5113-5119
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Journal article (peer-reviewed)abstract
- Finding alternative and sustainable ways to produce, store and convert energy is key for reducing fossil fuel-based CO2 emissions. In this transformation, hydrogen for energy storage and hydrogen-powered fuel cells for energy conversion can play important roles. However, storage of hydrogen itself is difficult and the concept of reversible liquid organic hydrogen carriers (LOHCs) has been proposed given the advantages of using liquid storage materials. A key part in the adaption of LOHCs is the catalyst design for efficient dehydrogenation of these hydrogen-carrying species. In this study, the use of silica- and alumina-supported POCOP-Ir systems for gas phase acceptorless dehydrogenation of 4-methylpiperidine (an LOHC with 6.1 wt% hydrogen) is investigated in a continuous-flow system with a high TON. To increase stability and reactivity, a new POCOP-Ir complex with two anchors was designed and found to be highly active in the dehydrogenation of 4-methylpiperidine with ∼91 000 turnovers in 45 h. In addition, this catalyst showed a maintained activity with a TOF of 1684 h−1 after 45 h.
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| 3. |
- Mousa, Abdelrazek H., et al.
(author)
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Carboxylation of the Ni-Me Bond in an Electron-Rich Unsymmetrical PCN Pincer Nickel Complex
- 2020
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In: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 39:9, s. 1553-1560
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Journal article (peer-reviewed)abstract
- The synthesis of a new unsymmetrical PCN ligand bearing tert-butyl groups on the phosphorus atom and isopropyl groups on the nitrogen donor atom is presented. It reacts with the commercially available Ni(DME)Br2 precursor to offer the corresponding t-BuPCNi-Pr pincer nickel bromide complex 1 together with a paramagnetic species, which was characterized as a tetrahedral nickel complex. Complex 1 reacts with MeMgCl to give the corresponding methyl complex 3. Carboxylation of complex 3 using 4 atm of CO2 gave the PCN nickel acetate complex 4 under mild reaction conditions comparable to those for the corresponding palladium complexes with PCP ligands.
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| 4. |
- Polukeev, Alexey V., et al.
(author)
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Combined Experimental and Computational Study of the Mechanism of Acceptorless Alcohol Dehydrogenation by POCOP Iridium Pincer Complexes
- 2022
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In: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 41:7, s. 859-873
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Journal article (peer-reviewed)abstract
- Iridium pincer complexes of the type (POCOP)Ir (POCOP = 2,6-(tBu2PO)2C6H3) are very productive catalysts for dehydrogenation of secondary alcohols. To our surprise, we found that turnover frequencies demonstrated by (POCOP)IrH2 (IrH2) are higher in more dilute solutions of the catalyst, which triggered a mechanistic study of alcohol dehydrogenation by IrH2. Here, we provide strong evidence that acceleration by dilution is related to the rate-limiting mass transfer of hydrogen, which, so far, has not received much attention in the literature. Using experimental and computational methods, we show that dehydrogenation has two high-barrier steps, namely the reaction of IrH2 with alcohol to give (POCOP)IrH(OR) (IrH(OR)) and subsequent β-elimination in the latter. Depending on the alcohol and reaction conditions, IrH(OR) can be formed via an associative pathway that includes proton transfer to the hydride or a dissociative mechanism that involves hydrogen elimination from IrH2 to give a 14e (POCOP)Ir species. Rapid re-hydrogenation of IrH(OR) or the 14e (POCOP)Ir by dissolved hydrogen is responsible for the rate retardation in more concentrated solutions of the catalyst. The suggested mechanism gives a satisfactory quantitative description of the catalytic cycle, such that kinetic curves and reaction orders in the catalyst can be reproduced.
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| 5. |
- Polukeev, Alexey V., et al.
(author)
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Cyclohexane-Based Phosphinite Iridium Pincer Complexes : Synthesis, Characterization, Carbene Formation, and Catalytic Activity in Dehydrogenation Reactions
- 2017
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In: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 36:3, s. 639-649
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Journal article (peer-reviewed)abstract
- Metalation of two cyclohexane-based phosphinite pincer ligands, cis-POCyPO (4) and trans-POCyPO (5) (POCyPO = {1,3-bis-[(di-tert-butylphosphinito]cyclohexane}−), is reported. In line with previously published results (Dalton Trans. 2009, 8626, DOI: 10.1039/B910798C), ligand 4 undergoes aromatization to give benzene-based complex (POCOP)IrHCl (3) at high temperatures in the presence of [Ir(COD)Cl]2. However, here we present the isolation of carbene complex (POCyOP)IrCl (6) which is an intermediate in the aromatization process; upon reaction with H2, 6 can be readily transformed to the corresponding hydrido-chloride 8. Metalation of trans-POCyOP ligand 5 gives hydrido-chloride 13 which only upon further heating can be converted to the corresponding carbene 14. A mechanistic study of hydrogenation of carbene 6 is reported, as well as interesting ambient temperature CO-induced C-H activation in β-position of 6, a process that under other circumstances takes place around 200 °C. The cis complex (POCyOP)IrHCl (8), upon activation with base, revealed moderate activity in transfer dehydrogenation of cyclooctane (144 turnover numbers (TON)), while the performance of trans analog 13 was much better (up to 1684 TON). Carbene complex 6 and in situ generated 14 demonstrated promising activity in acceptorless dehydrogenation of alcohols, presumably operating via a novel metal-ligand cooperation type mechanism. Some of the alcohol dehydrogenations generated large amounts of polystyrene.
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| 6. |
- Polukeev, Alexey V., et al.
(author)
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Iridium Catalyzed Dehydrogenation in a Continuous Flow Reactor as a Tool Towards Practical On-Board Hydrogen Generation From LOHCs
- 2022
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In: ChemSusChem. - : Wiley. - 1864-564X .- 1864-5631. ; 15:8
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Journal article (peer-reviewed)abstract
- To enable the large-scale use of hydrogen fuel cells for mobility applications, convenient methods for on-board hydrogen storage and release need to be developed. A promising approach is liquid organic hydrogen carriers (LOHCs), since these are safe, available on a large scale and compatible with existing re-fuelling infrastructure. Usually, LOHC dehydrogenation is carried out in batch-type reactors by transition metals and their complexes and suffers from slow H 2 release kinetics and/or inability to reach high energy density by weight due to low conversion or the need to dilute the reaction mixture. Here we report the use of a continuous flow reactor in combination with a heterogenized iridium pincer complex, which enables a tremendous increase in LOHC dehydrogenation rates. Thus, dehydrogenation of isopropanol is performed in a regime that in terms of gravimetric energy density, hydrogen generation rate and precious metal content is potentially compatible with applications in a fuel-cell powered car.
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| 7. |
- Polukeev, Alexey V., et al.
(author)
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Iridium complexes with aliphatic, non-innocent pincer ligands
- 2018
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In: Journal of Organometallic Chemistry. - : Elsevier BV. - 0022-328X. ; 867, s. 33-50
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Journal article (peer-reviewed)abstract
- Pincer complexes attract considerable attention both as interesting objects for fundamental studies and efficient catalysts. This review describes a relatively new offshoot of the field - namely, the chemistry of iridium complexes with aliphatic PCsp3P non-innocent pincer ligands. High flexibility, often combined with possibility to activate internal C-H bonds, offers several new patterns of metal-ligand cooperation; some of them were successfully used in catalysis of acceptorless alcohol dehydrogenation, olefin hydroformylation and deuterium exchange, others opened up for remarkable stoichiometric reactions.
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| 8. |
- Polukeev, Alexey V., et al.
(author)
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Iridium Hydride Complexes with Cyclohexyl-Based Pincer Ligands : Fluxionality and Deuterium Exchange
- 2016
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In: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 35:16, s. 2600-2608
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Journal article (peer-reviewed)abstract
- Two hydride compounds with aliphatic pincer ligands, (PCyP)IrH2 (PCyP = {cis-1,3-bis[(di-tert-butylphosphino)methyl]cyclohexane}(-) (1) and (PCyP)IrH4 (2), have been studied, with emphasis on features where such systems differ from arene-based analogues. Both compounds reveal relatively rapid exchange between alpha-C-H and Ir-H, which can occur via formation of carbene or through demetalation, with nearly equal barriers. This observation is confirmed by deuterium incorporation into the alpha-C-H position. Complex 1 can reversibly add an N-2 molecule, which competes with the alpha-agostic bond for a coordination site at iridium. The hydrogen binding mode in tetrahydride 2 is discussed on the basis of NMR and IR spectra, as well as DFT calculations. While the interpretation of the data is somewhat ambiguous, the best model seems to be a tetrahydride with minor contribution from a dihydrido-dihydrogen complex. In addition, the catalytic activity of 1 in deuterium exchange using benzene-d(6) as a deuterium source is presented.
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| 9. |
- Polukeev, Alexey V., et al.
(author)
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Oxidation-induced C-H bond activation in iridium pincer complexes
- 2023
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In: Dalton Transactions. - 1477-9226. ; 52:22, s. 7701-7708
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Journal article (peer-reviewed)abstract
- Dehydrogenation reactions that produce molecular hydrogen are thermodynamically unfavourable. Desired is to couple them with a green driving force, such as oxidation with oxygen or an electric current. This, in turn, requires understanding of the catalyst's redox properties. Here we report oxidation of the iridium pincer complexes (POCOP)IrHCl (POCOP = 2,6-(tBu2PO)2C6H3; 1a) and (PCP)IrHCl (PCP = 2,6-(tBu2PCH2)2C6H3; 1c) that induced intramolecular C-H activation, followed by the formation of complexes with a cyclometallated tert-butyl group. Based on an electrochemical study and DFT calculations, we propose a mechanism that involves H+ loss from hydrochlorides 1a and 1c to give a highly reactive (pincer)IrCl+ compound.
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| 10. |
- Polukeev, Alexey V.
(author)
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Synthesis, Structure, and Catalytic Activity of Cyclometalated Iridium Complexes with a Bidentate POC Ligand
- 2023
-
In: European Journal of Inorganic Chemistry. - 1434-1948. ; 26:31
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Journal article (peer-reviewed)abstract
- Synthesis, characterization and catalytic activity of cyclometalated iridium complexes with a bidentate POC ligand is presented. Metalation of POC-H (di-tert-butyl(phenoxy)phosphane) with [Ir(COD)Cl]2 proceeded rapidly at room temperature and afforded mixture of (POC)(POC-H)IrHCl (1 a) and (POC)(COD)IrHCl (1 b), from which complexes (POC)(L)IrHCl where L=PPh3 (1 c), bipyridine (1 d) and [2,2′-bipyridine]-6,6′-diol (1 e) were prepared through ligand exchange. The compounds were tested in acceptorless dehydrogenation of 1-phenylethanol and transfer dehydrogenation of ethanol in a context of comparison with pincer counterparts (POCOP)IrHCl and (PCN)IrHCl. An attempt to prepare a dihydride complex from 1 e led to dimeric complex [(POC)(bipy-diol−)IrH]2 (3) that could explain the low activity of 1 e. DFT studies provided insight into POC-H vs POCOP-H metalation mechanism.
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