An electron poor iridium pincer complex for catalytic alkane dehydrogenation

Kovalenko, Oleksandr O.; Wendt, Ola F.

doi:10.1039/c6dt01816e

Cited by 31 publications

(17 citation statements)

References 41 publications

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“…1,5-Dibromo-2,4-bis(trifluoromethyl)benzene was obtained from the deamination of 2,6-dibromo-3,5-bis(trifluoromethyl)aniline ,, and was subject to cyanation to give 4,6-bis(trifluoromethyl)isophthalonitrile (Scheme ). The CF3 Phebox pro-ligand was synthesized by the reaction of isophthalonitrile with 2-amino-2-methylpropan-1-ol…”

Section: Experimental Results and Discussionmentioning

confidence: 99%

“…The synthesis of high-value olefins from the corresponding alkanes or alkyl-substituted molecules is therefore a reaction of great potential value. The dehydrogenation of alkanes by pincer-ligated transition-metal complexes has been developed and studied extensively, largely with the use of sacrificial hydrogen acceptors (transfer dehydrogenation). − Acceptorless dehydrogenation, however, is in principle the ideal reaction for the conversion of alkanes to olefins; the lack of any added reagents is more economical (and atom economical), the reaction is operationally simpler in principle, and H 2 is produced as a potentially valuable byproduct …”

Section: Introductionmentioning

confidence: 99%

“…Preliminary kinetic experiments suggested that turnover numbers were limited by catalyst decomposition. We considered that replacing the methyl groups on the Phebox aryl ring of 1 with trifluoromethyl groups would increase stability, by eliminating the weak benzylic C–H bonds as well as by sterically protecting the aryl C–H bond and by strengthening the aryl–Ir bond . We also considered that the substitution with trifluoromethyl groups would increase the electrophilicity of the iridium center and decrease the trans influence of the aryl group, either of which might favor the key C–H activation step .…”

Section: Introductionmentioning

confidence: 99%

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Alkane Dehydrogenation Catalyzed by a Fluorinated Phebox Iridium Complex

et al. 2021

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We report an iridium acetate complex with a fluorinated Phebox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3,5-bis(trifluoromethyl)phenyl) that is a highly effective catalyst for acceptorless dehydrogenation of alkanes. Under typical acceptorless dehydrogenation conditions, a high turnover frequency is obtained, which is limited by the rate of expulsion of H 2 from the reaction solution. Rates and turnover numbers for acceptorless dehydrogenation are significantly greater than found for the nonfluorinated analogue. As in the case of the nonfluorinated analogue, Na + acts as a cocatalyst with the fluorinated catalyst again yielding greater rates and total turnovers. Computational studies shed light on the possible mechanistic pathways. The initial alkane activation is a net Ir−H/C−H bond metathesis leading to the formation of an Ir−alkyl bond and loss of H 2 ; this is the slowest chemical step in the cycle. The lowest-energy pathway is calculated to proceed via concerted metalated deprotonation (CMD) of the alkane. Pathways proceeding via transition states with oxidative addition (Ir(V)) character, however, are calculated to be only slightly higher in energy. These transition states can lead either to Ir(V) intermediates, which then lose H 2 , or connect directly to a dihydrogen complex. The role of Na + is largely to promote dechelation by coordinating to an acetate oxygen, opening a vacant coordination site that allows reaction with the alkane. This coordination by Na + prevents the CMD mechanism from operating, but it significantly lowers the energy of the Ir(V) TSs. NBO analysis shows a net transfer of charge from the alkane atoms to the metal complex in the Ir(V) TSs, with and without coordinated Na + . Thus, the oxidative addition is actually reductive in nature, driven in part by electrophilicity of the metal center. The Na + cation further increases electrophilicity in addition to promoting dechelation. The greater activity of the fluorinated catalyst compared with the parent complex can also be explained in terms of the electrophilic nature of the reaction. The fluorinated catalyst is also more resistant to decomposition than the nonfluorinated analogue.

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Section: Experimental Results and Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Alkane Dehydrogenation Catalyzed by a Fluorinated Phebox Iridium Complex

et al. 2021

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“…If the corresponding hydrido-chloride complex is used as catalyst, then 1 equiv of base, typically t BuONa, is required to generate active species. Cis complex 8 demonstrated only moderate catalytic activity with TON 144, which can be compared to arene-based (PCP)Ir ( 2 , TON 230) and (POCOP)Ir ( 3 , TON 1512 according to the original report; 1730–1918 in our hands) , and therefore was not studied any further. Trans complex 13 however showed a fairly good TON of 1223 under the same conditions.…”

Section: Resultsmentioning

confidence: 99%

Cyclohexane-Based Phosphinite Iridium Pincer Complexes: Synthesis, Characterization, Carbene Formation, and Catalytic Activity in Dehydrogenation Reactions

Polukeev

Wendt

2017

Organometallics

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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.20098626), 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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“…Metal complexes of pincer ligands, or tridentate meridionally coordinated ligands, continue to attract significant interest due to their inherent stability and modularity. − Bis(phosphinite) ligands with a resorcinol-based backbone or POCOP (POCOP = C 6 H 4 -1,3-[OP(R) 2 ] 2 , R = alkyl or aryl) ligands have been used to synthesize an interesting variety of pincer complexes . In particular, iridium and nickel complexes with POCOP ligands have been utilized in catalytic alkane transfer dehydrogenation and carbon dioxide reduction, respectively. , …”

Section: Introductionmentioning

confidence: 99%

Synthesis and Characterization of 3,5-Bis(di-tert-butylphosphinito)pyridine Pincer Complexes

Swisher

Grubbs

2020

Organometallics

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The synthesis of some "reverse pyridine" bis(phosphinite) pincer complexes of nickel and rhodium is reported. N-Functionalization of a POCOP ligand with a borane Lewis acid was found to permit cyclometalation with metal precursors, which reacted with the free base ligand in an undesired manner. Convenient removal of the coordinated Lewis acid was accomplished using polymer-supported 4-dimethylaminopyridine. The effects of borane Lewis acid coordination on the physical and spectroscopic properties of the pincer complexes were also assessed.

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An electron poor iridium pincer complex for catalytic alkane dehydrogenation

Cited by 31 publications

References 41 publications

Alkane Dehydrogenation Catalyzed by a Fluorinated Phebox Iridium Complex

Alkane Dehydrogenation Catalyzed by a Fluorinated Phebox Iridium Complex

Cyclohexane-Based Phosphinite Iridium Pincer Complexes: Synthesis, Characterization, Carbene Formation, and Catalytic Activity in Dehydrogenation Reactions

Synthesis and Characterization of 3,5-Bis(di-tert-butylphosphinito)pyridine Pincer Complexes

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