Dehydrogenation Of Cyclooctane Research Articles

Bridging Dehydrogenation and Hydrogenation in Heterogeneous Catalysis: A Demonstration of a Unified Catalytic Approach.

In the pursuit of enhancing the applications of hydrogen as an energy carrier, this research delved into the utilization of a singular hybrid catalyst capable of performing both dehydrogenation and hydrogenation processes for Liquid Organic Hydrogen Carriers (LOHCs). This study presents the synthesis and characterization of a hybrid catalyst, combining an organometallic pincer complex with Pd-Ru heterostructures supported on γ-alumina. Unlike conventional transition and noble metal nanoparticles, the use of a pincer complex offers exceptional thermal stability due to its aryl backbone, which is advantageous for various endothermic dehydrogenation reactions of hydrocarbons in LOHCs. This pioneering hybrid catalyst is a novel approach, demonstrating a proof of concept. In this study, we utilized the hybrid catalyst to investigate the dehydrogenation and hydrogenation of a lower enthalpic system, specifically the cyclooctane-cyclooctene system. The dehydrogenation of cyclooctane was conducted at 443 K using tertiary butyl ethylene as a sacrificial hydrogen acceptor, while the hydrogenation of cyclooctene reaction catalyzed by Pd-Ru nanostructures occurred at 298 K and 1 atm H2. The results showed successful tandem dehydrogenation-hydrogenation reactions. However, challenges were noted in terms of catalytic activity and recyclability, providing valuable insights for further refinement and optimization.

Low-Temperature Dehydrogenation of Cyclooctane by Using Pincer Iridium Complexes Bearing an N,N′-Diarylated PNCNP Ligand

PNCNP pincer iridium complexes possessing arylnitrene linkers (−N(Ar)−) catalyze transfer dehydrogenation of cyclooctane (COA) under low-temperature conditions, giving 200 turnovers at 125 °C using tert-butylethylene (TBE) and 60 turnovers at 50 °C using cyclohexene (CHE) as a sacrificial hydrogen acceptor. Despite prior reports dealing with the synthetic difficulty of pincer iridium complexes bearing di(isopropyl)phosphoramine sidearms, our design using chemically inert (3,5-disubstituted-aryl)nitrene linkers allowed for the successful complexation with iridium. The structural characterization and computational study revealed that steric congestion provided by arylnitrene linkers destabilizes the resting internal alkene adducts, thereby reducing the overall activation barrier of the rate-limiting C–H addition. The steric congestion also hinders undesirable disproportionation of CHE, allowing for its use as an effective hydrogen acceptor. With their straightforward synthesis coupled with fine tunability through the 3,5-substituents on the aryl groups, the N,N′-diarylated PNCNP ligands presented in this work will be a valuable addition to the highly successful PCP pincer framework.

Synthesis of A Pincer-IrV Complex with A Base-Free Alumanyl Ligand and Its Application toward the Dehydrogenation of Alkanes.

A pincer-iridium complex bearing a Lewis-base-free X-type alumanyl ligand has been synthesized. X-ray diffraction, NMR and IR spectroscopy, as well as XANES analysis confirmed its tetrahydrido-IrV structure and Lewis acidity at the Al center as supported by DFT calculations. The resulting complex was applied as a catalyst for the transfer dehydrogenation of cyclooctane.

Synthesis of A Pincer‐IrVComplex with A Base‐Free Alumanyl Ligand and Its Application toward the Dehydrogenation of Alkanes

AbstractA pincer‐iridium complex bearing a Lewis‐base‐free X‐type alumanyl ligand has been synthesized. X‐ray diffraction, NMR and IR spectroscopy, as well as XANES analysis confirmed its tetrahydrido‐IrVstructure and Lewis acidity at the Al center as supported by DFT calculations. The resulting complex was applied as a catalyst for the transfer dehydrogenation of cyclooctane.

Synthesis and Application of Pyrrole-Based PNP–Ir Complexes to Catalytic Transfer Dehydrogenation of Cyclooctane

A series of tBu- and iPr-substituted PNP–pincer Ir and Rh complexes with pyrrole-based core were synthesized and characterized. The structures of the obtained complexes were varied depending on the size of alkyl substituents and ligands other than PNP ligand. All of them exhibit low activity toward transfer dehydrogenation of cyclooctane.

Synthesis and coordination chemistry of new asymmetric donor/acceptor pincer ligands, 1,3-C6H4(CH2PtBu(Rf))2 (Rf = CF3, C2F5).

Syntheses of new asymmetric pincer precursors 1,3-C6H4{CH2P(tBu,X)}2 (tBu,XPCPH; X = Cl, SiMe3, OPh) and a new class of hybrid donor/acceptor pincer ligands 1,3-C6H4{CH2P(tBu,Rf)}2 (tBu,RfPCPH; Rf = CF3, C2F5) are reported. All tBu,XPCPH compounds are obtained as mixtures of meso and rac diastereomers in varying ratios (meso : rac ∼ 4 : 1 to 3 : 2) which were used without separation. Treatment of Ru(cot)(cod) with tBu,CF3PCPH under 1 atm H2 in acetone at 20 °C produced the hydride solvate (tBu,CF3PCP)Ru(acetone)xH which was not isolated, but could be trapped as stable diene complexes (tBu,CF3PCP)Ru(L)2H (L2 = cod (1[combining low line]), nbd (2[combining low line])). Catalytic cyclooctane dehydrogenation studies demonstrate that 2[combining low line] has ∼50% the activity of (CF3PCP)Ru(cod)(H), but significantly higher catalyst stability and is able to operate at higher catalyst loading concentrations without deactivation via bimolecular decomposition.

Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis(phosphine) PBP pincer ligand. The previously reported complexes (PBP)Ir(Ph)(Cl) (1) and (PBP)Ir(H)(Cl) (2) were converted to the new complexes (PBP)IrH4 (3) and (PBP)Ir(Ph)(H) (4). Complexes 3 and 4 serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs tert-butylethylene. The dicarbonyl complex (PBP)Ir(CO)2 (6) was also synthesized, by the reaction of CO with either 3 or 4. Intermediates (PBPhP)Ir(H)(CO)2 (5) and (PBP)IrH2(CO) (7) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of 3 and 6. Solid-state structures of 3 and 6 were determined by X-ray crystallography.

Synthesis and complexes of imidazolinylidene-based CCC pincer ligands

A series of imidazolinium-based CCC pro-ligands featuring N-Mes, Dipp, iPr and tBu substituents (1·2HCl) have been prepared. The corresponding free carbenes are readily generated through deprotonation by strong bases and, in addition to being characterised in situ by 1H and 13C NMR spectroscopy, were trapped through reaction with CuCl. Iridium pincer compounds of the N-Mes (5a) and N-Dipp (5b) substituted ligands, viz. [Ir(1)HCl(NCMe)], were obtained through reaction between the respective pro-ligand, [Ir(COE)2Cl]2, and Et3N in acetonitrile at ca. 80°C. Under similar conditions the N-iPr and N-tBu analogues were not formed. The new iridium pincer complexes 5a and 5b were fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state by X-ray diffraction. Under relatively forcing reaction conditions neither 5a nor 5b, in combination with KOtBu, show any significant catalytic activity for the transfer dehydrogenation of cyclooctane to cyclooctene using tert-butylethylene as the sacrificial hydrogen acceptor (ca. 2 TONs, 150°C, 24h).

Dehydrogenation of cycloalkanes at rhodium complexes bearing fluorinated cyclopentadienyl ligands

Abstract The fluorinated cyclopentadienyl complexes [Rh(η5-C5H4CH2CH2C6F13)(CO)2] (2), [Rh(η5-C5H4CH2CH2C8F17)(CO)2] (3), [Rh(η5-C5H4CH2CH2C10F21)(CO)2] (4), [Rh(η5-C5H4CH2CH2CnF2n+1)(CO)(PEt3)] (7: n = 6; 8: n = 8; 9: n = 10) and [Rh(η5-C5H4CH2CH2CnF2n+1)(CO){P(CH2CH2C6F13)3}] (10: n = 8; 11: n = 10) were synthesized and their reactivity was investigated. Compound 3 converts at room temperature into the trinuclear complex [Rh2(η5-C5H4CH2CH2C8F17)2(μ-CO)2Rh(η5-C5H4CH2CH2C8F17)(CO)] (6). The complexes 9 and 11 were employed in dehydrogenation reactions of cycloalkanes in C6F14/hydrocarbon mixtures. Photocatalytic conversions were observed and a turnover number of 16 for the dehydrogenation of cyclooctane with complex 11 was obtained.

Synthesis of Pincer Iridium Complexes Bearing a Boron Atom and iPr-Substituted Phosphorus Atoms: Application to Catalytic Transfer Dehydrogenation of Alkanes

An iPr-substituted PBP-pincer ligand was synthesized and introduced to iridium metal to give iPr-(PBP)Ir(H)Cl and iPr-(PBP)Ir(C2H4) complexes. Both of these complexes were found to be moderately active for the catalytic transfer dehydrogenation of cyclooctane.

Iridium PCsp3P-type Complexes with a Hemilabile Anisole Tether

A series of iridium PCsp3P complexes based on bis(2-diisopropylphosphinophenyl)-2-anisoylmethane (PCanisHP) is reported. (PCanisP)Ir(H)Cl was generated from the C–H activation of the backbone by [Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene), while the dihydride (PCanisP)Ir(H)2 was generated by hydride metathesis from (PCanisP)Ir(H)Cl. Both complexes are 18e octahedral complexes and water stable. The hemilability of the anisole tether was probed using CO and PMe3; multiple isomers, in which the anisole substituent was displaced, were generated, showing the flexibility of the ligand backbone. (PCanisP)Ir(H)2 showed deuterium incorporation in the hydride, backbone, and anisole positions upon moderate heating in C6D6. Both (PCanisP)Ir(H)Cl and (PCanisP)Ir(H)2 were precatalysts for transfer dehydrogenation of cyclooctane under moderate conditions.

Catalytic dehydrogenation of cyclooctane and triethylamine using aliphatic iridium pincer complexes

The majority of the known pincer iridium based catalysts for dehydrogenation of alkanes has arene-based backbones. Here, the catalytic activity of aliphatic iridium pincer complexes, viz. the cyclohexane-based phosphine complex (PCyP)IrHCl (4) (PCyP={cis-1,3-bis-[(di-tert-butylphosphino)methyl]cyclohexane}−) and the 2-methylpropane-based phosphinite complex (POCOP)IrHCl (5) (POCOP=1,3-bis-(di-tert-butylphosphinito)-2-methylpropane−), in dehydrogenation of cyclooctane and triethylamine was studied. They give TONs that are in the range of 0–200. In addition, improved procedures for synthesis and metallation of the PCyP ligand (3) are presented.

Acceptorless Alkane Dehydrogenation Catalyzed by Iridium CCC-Pincer Complexes

Iridium complexes of CCC-pincer bis-N-heterocyclic carbenes, including a newly synthesized trifluoromethyl-substituted complex, were examined as catalysts for the acceptorless dehydrogenation of cyclooctane and n-undecane. Up to 103 turnovers were observed for the dehydrogenation of cyclooctane, and up to 97 turnovers were observed for the dehydrogenation of n-undecane. The catalysts showed high initial turnover frequencies, followed by a gradual loss of activity over 24 h. Experiments indicate that this loss of activity is due to catalyst decomposition rather than product inhibition. Stoichiometric reactivity was investigated for the precatalysts, focusing on the synthesis of dihydride and trihydride complexes as well as the dissociation and addition of neutral ligands.

Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes: Synthesis and Application of Novel 7–6–7 Ring‐Based Pincer Iridium Complexes

A series of Ir-PCP pincer precatalysts [(7-6-7-(R) PCP)Ir(H)(Cl)] and [(7-6-7-(Ar) PCP)Ir(H)(Cl)(MeCN)] bearing a novel "7-6-7" fused-ring skeleton have been synthesized based upon the postulate that the catalytic species would have durability due to their rather rigid structure and high activity owing to the low but sufficient flexibility of their backbones, which are not completely fixed. Treatment of these precatalysts with NaOtBu gave rise to the active 14 electron (14e) species [(7-6-7-(iPr) PCP)Ir] and [(7-6-7-(Ph) PCP)Ir], which can trap hydrogen and were spectroscopically characterized as the tetrahydride complexes. Both [(7-6-7-(iPr) PCP)Ir] and [(7-6-7-(Ph) PCP)Ir] were found to be highly effective in the transfer dehydrogenation of cyclooctane with tert-butylethylene as the hydrogen acceptor, the initial reaction rate at high temperature (230 °C) being higher for [(7-6-7-(iPr) PCP)Ir] than [(7-6-7-(Ph) PCP)Ir], and the turnover number (TON) of the overall hydrogen transfer being higher for the latter. Nonetheless, the estimated TONs were as high as 4600 and 4820 for the two complexes at this temperature, respectively, which are unprecedented absolute values. In terms of durability, the [(7-6-7-(Ph) PCP)Ir] complex is the catalyst of choice for this reaction. Structural analysis and computational studies support the importance of the low flexibility of the ligand core.

N-Heterocyclic Dicarbene Iridium(III) Pincer Complexes Featuring Mixed NHC/Abnormal NHC Ligands and Their Applications in the Transfer Dehydrogenation of Cyclooctane

The reaction of 1,3-bis(imidazolyl)benzene with excess 1-bromoadamantane at 170 °C for 14 h afforded 1,3-bis(1-adamantylimidazolium)benzene dibromide (1), which was characterized by IR, NMR, and X-ray diffraction. Metathetical anion exchange with excess sodium iodide yielded the N-heterocyclic dicarbene precursor 1,3-bis(1-adamantylimidazolium)benzene diiodide, (CHimidCHCHimid)I2 (2), in high yield. The reaction of [Ir(μ-Cl)2(cod)]2 (cod = 1,5-cyclooctadiene) with 2 in the presence of 2.2 equiv of Cs2CO3 in refluxing acetonitrile led to the formation of the unsymmetrical iridium(III) hydride pincer complex [Ir(H)I(CNHCCCaNHC)(NCMe)] (3), which contains a C2-bound NHC ligand and a C5-bound NHC ligand. Recrystallization of 3 from ClCH2CH2Cl/Et2O generated the dihalide Ir(III) pincer complex [IrX2(CNHCCCaNHC)(NCMe)], where X = mixture of I and Cl (4). Its formation probably involves initial formation of an iridium chloro iodo intermediate via solvent-induced substitution of the hydride ligand of 3 by chloride, followed by partial chloride/iodide anion exchanges at both the Ir–Cl and Ir–I sites. Treatment of 4 with excess KI in acetonitrile afforded the diiodide Ir(III) pincer complex [IrI2(CNHCCCaNHC)(NCMe)] (5), which was characterized by IR, NMR, and X-ray diffraction. Dehydroiodination of the monohydride complex 3 with sodium tert-butoxide (NaO-t-Bu) in the presence of hydrogen led to the formation of the new hydrido iridium pincer complex 6, which shows detectable catalytic activity toward the transfer dehydrogenation reaction of cyclooctane.

Acceptor Pincer Chemistry of Ruthenium: Catalytic Alkane Dehydrogenation by (CF3PCP)Ru(cod)(H)

The synthesis and reactivity of a series of Ru(II) complexes based on the strongly π-accepting pincer ligand 1,3-C6H3(CH2P(CF3)2)2 (CF3PCP) is reported. Thermolysis of [Ru(cod)(η3-2-methylallyl)2] with CF3PCPH under H2 affords a mixture of the three complexes (μ-CF3PCPH)Ru(H)(μ-H)(μ-η6,κ3-CF3PCP)Ru(H), [(CF3PCP)Ru(H)]2(μ-CF3PCPH)2, and (CF3PCP)Ru(cod)H, which were structurally characterized and individually prepared in moderate yields. (CF3PCP)Ru(cod)H reacts with (C2F5)2PCH2CH2P(C2F5)2 (dfepe) to give (CF3PCP)Ru(dfepe)H. (CF3PCP)Ru(cod)H is moderately active as an alkane dehydrogenation catalyst. Thermolysis in 1:1 mixtures of cyclooctane and tert-butylethylene at 150 and 200 °C resulted in initial rates of 180 and 1000 turnovers h–1 of cyclooctene, respectively. Acceptorless dehydrogenation of cyclooctane also occurs, with an initial rate of 14 turnovers h–1. The decrease of catalyst activity over time was found to be due to thermal catalyst decomposition rather than product inhibition by cyclooctene.

Iridium Complexes of CCC-Pincer N-Heterocyclic Carbene Ligands: Synthesis and Catalytic C−H Functionalization

A series of four meta-phenylene-bridged bis-benzimidazolium chlorides were synthesized as precursors to rigid, monoanionic, CCC-pincer N-heterocyclic carbene ligands. For ligands with mesityl, 3,5-xylyl, or 3,5-di-tert-butylphenyl side groups, reaction with [Ir(1,5-cyclooctadiene)Cl]2 in acetonitrile with either excess triethylamine or stoichiometric cesium fluoride as base gave neutral, iridium(III) pincer complexes of the formula Ir(CCC)HCl(MeCN), which were purified by chromatography on silica gel. Metalation failed under these conditions for a 2,6-diisopropylphenyl-substituted derivative. In combination with NaOtBu, these complexes form active catalysts for the acceptorless dehydrogenation of cyclooctane and for arene C−H borylation in neat arene solvent.

Catalytic dehydrogenation of cyclooctane with titanium, zirconium and hafnium metallocene complexes

Abstract Metallocene complexes in combination with cocatalysts like methylalumoxane (MAO) are not only excellent catalysts for olefin polymerization but also appropriate catalysts for the activation of alkanes in homogeneous (autoclave) and heterogeneous (fixed bed reactor) reactions. The activities of the catalysts depend on the temperature, the cocatalysts, additives, the central metal and the ligand structure. Generally, complexes with low steric demands and MAO as cocatalyst gave the highest activities. The comparison of different π-ligands resulted in the following activity order: cyclopentadienyl > indenyl > fluorenyl. The influence of σ-ligands and n-donor ligands gave the following activity order: –Cl > –PMe3 > –CH2Ph > –(CH2)4CH3 > –NPh3. The activities depended on the nature of the cocatalyst and decreased in the following order: MAO ≫ AlMe3 > AlEt3. The addition of aluminum powder and the Lewis base NPh3 increased the activity of the Cp2ZrCl2/MAO catalyst. The Cp2ZrCl2/MAO/NPh3 catalyst showed the highest activity in homogeneous reactions with 458 turnovers in 16 h at 300 °C. The Cp2ZrCl2/MAO/NPh3/SI1102 catalyst gave the highest activity in heterogeneous catalysis with 206 turnovers in 5 h at 350 °C. None of the catalysts required a hydrogen acceptor like an external olefin.

Catalytic dehydrogenation of cyclooctane in homogeneous solution with titanium, zirconium and hafnium complexes containing N,O-chelating ligands

A series of new titanium, zirconium and hafnium complexes with the heteroatom chelating ligands hydroxyquinolines, hydroxypyridines and hydroxyimines were synthesized. The complexes were activated with MAO and successfully tested for the catalytic CH activation reactions of cyclooctane. They gave TONs of 1.7–18.7 (300 °C, 16 h) in homogeneous solution. The TONs were clearly influenced by the ligand structure. The endothermic CH activation reaction with group 4 metal catalysts is preferably carried out at higher reaction temperatures (300–400 °C).

Catalytic dehydrogenation of cyclooctane with neutral iridium(I) complexes

A series of new 1,5-cyclooctadiene iridium(I) complexes with chelating ligands has been synthesized. The ligands are naphthoxyimines, carboxylates and alcoholates. The complexes catalyze the homogeneous dehydrogenation of cyclooctane to give cyclooctene and hydrogen without an external hydrogen acceptor up to rates of 75 turnovers. The catalysts are active for at least 48 h at a temperature of 300 °C. The ligand structure has an influence on the activity and selectivity of the corresponding catalysts.