Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

Kumar, Akshai; Goldman, Alan S.

doi:10.1007/3418_2015_113

Cited by 29 publications

(23 citation statements)

References 131 publications

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“…The formation of C–C bonds plays an important role in synthetic organic chemistry having varied applications in fuel, fine chemicals, medicinal, agrochemicals, pharmaceuticals, and many others . A plethora of reactions that lead to C–C bonds majorly involve transition-metal catalyzed C-H or C-X (X = Cl, Br, I) activation operating via radical or purely organometallic pathways . The direct coupling of primary and secondary alcohols to yield β-alkylated alcohols has been extensively studied in the past decade via the hydrogen borrowing pathway which has been pegged as one of the most important green chemistry research areas recently .…”

Section: Introductionmentioning

confidence: 99%

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

2021

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We report here the synthesis and characterization of a series of NNN pincer-nickel complexes of the type ( R2 NNN)-NiCl 2 (CH 3 CN) (R = i Pr, t Bu, Cy, Ph, and p-F-C 6 H 4 ) based on bis(imino)pyridine ligands. In solution, these complexes are found to be equilibrium mixtures containing one and two pincer ligands, respectively. While the crystal structure of the former was reported by us recently for R = i Pr, we report the crystal structure of the latter in this study for R = p-F-C 6 H 4 . The considered NNN pincer-Ni complexes have been successfully employed to accomplish the catalytic β-alkylation of several secondary alcohols with a variety of benzyl alcohols at 140 °C with high yields and unprecedented turnovers. A maximum of 92% yield of the β-alkylated product at 18 400 TON was obtained in the reaction of benzyl alcohol with 1-(4-(trifluoromethyl)phenyl)ethane-1-ol in the presence of 0.005 mol % of ( Ph2 NNN)NiCl 2 (CH 3 CN) and 5 mol % of NaO t Bu at 140 °C after 24 h. The reaction exhibits zero-order dependence of rate on catalyst concentration and first-order dependence on the concentration of base, benzyl alcohol, and 1-phenyl ethanol which points to the base-mediated aldol condensation as the rate-determining step. Most of the intermediates involved in catalysis have been identified by HRMS. To the best of our knowledge, this is the first report on a pincer-Ni catalyzed β-alkylation of alcohols and, hitherto, such unprecedented turnovers have not been reported with a homogeneous molecular nickel-based catalyst.

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Section: Introductionmentioning

confidence: 99%

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

2021

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“…Since the seminal report by Moulton and Shaw of t Bu 4 PCP-pincer ligated iridium complexes ( R4 PCP = κ 3 -C 6 H 3 -2,6-(CH 2 PR 2 ) 2 ), many complexes with the tridentate “PCP” motif have been synthesized and extensively investigated . Such complexes include the most effective catalysts reported to date for the dehydrogenation of alkanes, and they have been reported to effect numerous other stoichiometric and catalytic reactions involving the cleavage of C–H bonds …”

Section: Introductionmentioning

confidence: 99%

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

et al. 2018

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Several complexes of ( tBu 4 PNP)Os (1) ( tBu 4 PNP = C 5 NH 3 -2,6-(CH 2 P t Bu 2 ) 2 ) are reported. 1 is isoelectronic with ( tBu 4 PCP)Ir ( 2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the ( tBu 4 PNP)Os complexes were investigated in this context. ( tBu 4 PNP)OsH 4 (1-H 4 ) is analogous to ( tBu 4 PCP)IrH 4 (2-H 4 ), but while 2-H 4 has some character of a dihydrogen dihydride, 1-H 4 is unambiguously a tetrahydride. Ethylene reacts with 1-H 4 to afford trans-DFT calculations indicate that alkane C−H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C−H or H−H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H 2 addition to the perfectly isoelectronic [( tBu 4 PNP)Ir(I)] + cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H 2 addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H 2 (to give tetrahydrides) is much more favorable for ( tBu 4 PNP)Os. NBO analysis indicates that the MH 2 /MH 4 additions are oxidative, whereas the M/MH 2 transformations are reductive.

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“…Among them, pincer ligand, a tridentate ligand with meridional coordination mode, is one of the most promising compounds for application to catalysis by utilizing the thermal stability of pincer complexes . As one of the most important reactions in industry, dehydrogenation of alkanes to produce the corresponding alkenes has been widely studied with pincer–Ir complexes after the seminal discovery of homogeneous Ir–phosphine complexes and their improved version with a pincer ligand . To improve catalytic activity for dehydrogenation of alkane, three different modifications of the most active PCP–Ir catalyst systems have been extensively investigated, i.e., (1) tuning electronics and sterics of the pincer ligand with a variety of substituents, (2) using other metals rather than iridium, , and (3) using other coordinating atoms rather than phosphorus and carbon atoms .…”

Section: Introductionmentioning

confidence: 99%