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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., ファイル公開：2019-04-23

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.

An efficient and potentially scalable method is described for the synthesis of the silica-supported complexes [(≡Si–O−)WMe5] and [(≡Si–O−)WMe2(≡CH)] obtained by in situ alkylation of the surface-grafted tungsten chloride [(≡Si–O−)WCl5] (1). [(≡Si–O−)WCl5] can be readily prepared by the reaction of commercially available and stable tungsten hexachloride WCl6 with partially dehydroxylated silica at 700 °C (SiO2-700). Further reaction with ZnMe2 at room temperature rapidly forms a mixture of surface-alkylated tungsten complexes. They were fully characterized by microanalysis, FTIR, mass balance, and solid-state NMR (1H, 13C, 1H–13C HETCOR, 1H–1H double quantum and triple quantum) and identified as [(≡Si–O−)WMe5] and another product, [(≡Si–O−)WMe2(≡CH)]. The latter might have been generated by partial decomposition of the tungsten methyl chloride compound, which is formed during the stepwise alkylation of [(≡Si–O−)WCl5]. DFT calculations were carried out to check the relative stability of the tungsten methyl ...

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.

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.

The selective aliphatic carbon–carbon activation of cyclo-octane (c-octane) was achieved via the RhII(ttp)-catalyzed 1,2-addition of Rh(ttp)H to give Rh(ttp)(n-octyl) (ttp = tetratolylporphyrinato dianion) in good yield under mild reaction conditions. This mechanism is further supported by DFT calculations. The reaction worked only with the sterically accessible Rh(ttp) porphyrin complex but not with the bulky Rh(tmp) system (tmp = tetrakismesitylporphyrinato dianion), thus showing the highly steric sensitivity of carbon–carbon bond activation by transition metal complexes.

A new arsenic-based pincer (AsOCOAs) dehydrogenation catalyst has been synthesized, IrHCl{2,6-C6H3-(O-AsBut2)} (3). Treatment with an equivalent of base (NaO-tert-butoxide) under an atmosphere of hydrogen gas affords the dihydride catalyst IrH2{2,6-C6H3-(O-AsBut2)} (4). The activity of 3 was explored under transfer dehydrogenation conditions with cyclooctane and tert-butyl ethylene, giving a maximum turnover number of 960 at 175 °C in 24 h. Acceptorless dehydrogenations were also explored with pyrrolidine-based molecules, ethylperhydrocarbazole (5), methylperhydroindole (6), and butylpyrrolidine (7), in which all results indicate 3 is roughly half as active as the analogous phosphine-based pincer catalyst 2. Akin to the phosphine pincer catalysts the activity was seen to steadily improve with increasing temperature, peaking at 175 °C, upon which thermal decomposition sets in.

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.

In the presence of an iridium 3,4,7,8-tetramethyl-1,10-phenanthroline catalyst, a methyl group on the silicon atom of alkyltrimethylsilanes undergoes selective C–H borylation with bis(pinacolato)diboron in cyclooctane at 135 °C to give alkyl(borylmethyl)dimethylsilanes. The C–H borylation of tetramethylsilane takes place efficiently at 100 °C. Permethyloligosilanes can also undergo C–H borylation without cleavage of the Si–Si bonds.

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 chlorid...

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.

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.

Deprotonation of 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride (1a) followed by reaction with chloro-1,5-cyclooctadiene Ir(I) dimer affords the anionic Ir(I) complex [K][{OCO}Ir(cod)] (2: OCO = 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl; cod = 1,5-cyclooctadiene), the first Ir complex stabilized by a diphenolate imidazolyl-carbene ligand. In the solid state 2 exhibits square-planar geometry, with only one of the phenoxides bound to the metal center. Oxidation of 2 with 2 equiv of [FeCp_2][PF_6] generates the Ir(III) complex [{OCO}Ir(cod)(MeCN)][PF_6] (3). Reaction of 3 with H_2 results in the liberation of cyclooctane and a species capable of catalyzing the hydrogenation of cyclohexene to cyclohexane. Displacement of cyclooctadiene from 3 can be achieved by heating in acetonitrile to form [{OCO}Ir(MeCN)3][PF_6] (4) or by reaction with either PMe_3 or PCy_3 to generate [{OCO}Ir(PMe_3)_3][PF_6] (5) or [{OCO}Ir(PCy_3)_2(MeCN)][PF_6] (6), respectively. 6 reacts with CO in acetonitrile to give an equilibrium mixture of 6 and [{OCO}Ir(PCy_3)_2(CO)][PF_6] (7) and with chloride to generate [{OCO}Ir(PCy_3)_(2)Cl] (8). The solid-state structure of 8 shows that the diphenolate imidazolyl-carbene ligand is distorted from planarity; DFT calculations suggest this is due to an antibonding interaction between the phenolates and the metal center in the highest occupied molecular orbital (HOMO) of the complex. 8 undergoes two successive reversible one-electron oxidations in CH_(2)Cl_2 at −0.22 and at 0.58 V (vs ferrocene/ferrocenium); EPR spectra, mass spectroscopy, and DFT calculations suggest that the product of the first oxidation is [{OCO}Ir(PCy_3)_(2)Cl]+ (8+), with the unpaired electron occupying a molecular orbital that is delocalized over both the metal center and the diphenolate imidazolyl-carbene ligand.

Novel bis(phosphine) pincer complexes of iridium based on ferrocene and ruthenocene have been synthesized. The reaction of 1,3-bis((di-tert-butylphosphino)methyl)metallocenes [t-BuP,CH,PM] (M = Fe, 10; M = Ru, 13) with [Ir(COE)2Cl]2 in refluxing toluene leads to the corresponding chloro-hydrido pincer complexes as a mixture of H-endo- (M = Fe, 11; M = Ru, 14) and H-exo-IrH(Cl)[t-BuP,C,PM] (M = Fe, 12; M = Ru, 15) isomers. Treatment of compounds 11, 12 and 14, 15 with NaH in hot cyclooctane generates corresponding dihydrido complexes IrH2[t-BuP,C,PM] (M = Fe, 6; M = Ru, 7). Reactivity of the chelated iridium atom in hydrido-iridium pincer complexes and oxidation centered at the iron atom of ferrocene-based pincer complexes were studied. Complexes H-exo-IrH(Cl)(CO)[t-BuP,C,PFe] (20), Ir(CO)[t-BuP,C,PFe] (21), and Ir(CO)[2,6-(tBu2PO)2C6H3] (30) were characterized by X-ray crystallography. On the basis of νCO values for carbonyl iridium derivatives, electronic characteristics of the corresponding fragments {I...

A series of bis(phosphinite) (p-XPCP)IrH2 pincer complexes {[PCP = η3-5-X-C6H2[OP(tBu)2]2-1,3], X = MeO (6a), Me (6b), H (6c), F (6d), C6F5 (6e), and ArF [=3,5-bis(trifluoromethyl)phenyl] (6f)} have been synthesized by dehydrochlorination of (p-XPCP)IrHCl precursor complexes 4a−f with NaOtBu in the presence of hydrogen. Dehydrochlorination of 4f in the presence of nitrogen yields {(p-ArFPCP)Ir}2{μ-N2} (11f), which was analyzed by X-ray diffraction. Complexes 6a−f exhibit identical catalytic activity in the transfer dehydrogenation of cyclooctane (COA) with tert-butylethylene (TBE) when compared to mixtures of precatalysts 4a−f and NaOtBu. The electronic properties of the fragments (p-XPCP)Ir (Aa−f) are discussed on the basis of the νCO of (p-XPCP)Ir(CO) complexes (8a−f) as well as on 1JHD coupling constants of monodeuterated complexes (p-XPCP)IrHD (6a−f-d1). Reaction of 4a−f with NaOtBu in arene solvents generates (p-XPCP)Ir(aryl)(H) complexes (9 and 10), which undergo rapid arene exchange on the NMR time...

The dehydrogenation of IrH2{C6H3-2,6-(CH2PBut2)2} (1) by tert-butylethylene followed by reaction with an excess of water leads to the isolation of IrH(OH){C6H3-2,6-(CH2PBut2)2} (2) in nearly quantitative yield. The hydrido hydroxo complex has been characterized by multinuclear NMR spectroscopy as well as a single-crystal X-ray structure determination. An isotopic labeling study with D2O indicates that 2 arises from the oxidative addition of water to the intermediate 14-electron complex Ir{C6H3-2,6-(CH2PBut2)2}. The title complex is an efficient catalyst for the transfer dehydrogenation of cyclooctane to cyclooctene but shows no catalytic activity for the hydroxylation of the alkane by water. The conversion of 1 to 2 can be reversed by placing a solution of 2 under 1 atm of H2 at 25 °C.

Complexes of type [{R2P(CH2)nPR2}Ru(2-Me-all)2] (2-Me-all = 2-methylpropenyl; R = Cy, n = 1−3, 5a−c; R = Me, n = 2, 6; R2 = −(CH2)4−, n = 2, 7) have been synthesized from the reaction of the corresponding electron-rich diphosphines with [(cod)Ru(2-Me-all)2] (4) at 50−70 °C. The new complexes were fully characterized by multinuclear NMR spectroscopy and mass spectroscopic techniques. Reacting 4 with Cy2P(CH2)nPCy2 containing hydrocarbon bridges with n = 3 (1c) and n = 4 (1d) at 95 and 50 °C, respectively, led to [κ2P,P‘-{(η3-C6H8)CyP(CH2)nPCy2}Ru(η3-C8H13)] (n = 3, 4; 8c,d) via intramolecular C−H bond activation and concomitant hydride transfer to cyclooctadiene. The molecular structure of 8c was unambiguously assigned by multinuclear 1D and 2D NMR spectroscopy and confirmed by single-crystal X-ray diffraction. The new complexes were tested as homogeneous catalyst precursors in thermal intermolecular C−H activation processes. In dehydrogenation of cyclooctane (coa), an initial turnover frequency of 1.9 h-1...

The synthesis of new metal complexes containing the keto phosphine ligands R12PCH2C(O)R2 (R1 = Ph, R2 = Ph, Me, t-Bu, p-C6H4F; R1 = i-Pr, R2 = Ph) is described. Reaction of 1 equiv of (diphenylphos...

Thermal decomposition of bis(trialkylphosphine)-3,3,4,4-tetramethylplatinacyclopentanes in hydrocarbon solvents yields two major products: 2,2,5p-tntramethylbutane and l-methyl-l-tert:butylcyclopropane. The former appears to be generated by heterogeneous process(es) catalyzed by platinum(0) (colloidal and/or solid) formed during the decomposition; the latter is the product of a homogeneous reaction sequence. The existence of competing heterogeneous and homogeneous reaction pathways complicates the study of this thermal decomposition. The addition of mercury(O) to the system selectively suppresses the heterogeneous reaction by poisoning the bulk platinum(O). This technique has been tested in one well-defined model system-the homogeneous hydrogenation of dineopentylbis(triethylphosphine)platinum(Il) to dihydridobis(triethylphosphine)platinum(Il) and neopentane in the presence of the heterogeneous platinum(0)-catalyzed hydrogenation of l-methylcyclopentene to methylcyclopentane-and found to eliminate the heterogeneous reaction while leaving the homogeneous one essentially unaffected. Application of this technique to the thermal decomposition of 3,3,4,4-tetramethylplatinacyclopentane simplifies the reaction by eliminating 2,2,3,3-tetramethylbutane as a product. Similarly, addition of mercury(0) to solutions containing H2 and (1,5-cyclooctadiene)dimethylplatinum(Il) in cyclohexane suppresses the autocatalytic, heterogeneous platinum-catalyzed conversion of the organoplatinum compound to (inter alia) cyclooctane, methane, and platinum(0). In other reactions examined-especially the high-temperature thermal decompositions of (1,5-cyclooctadiene)dineopentylplatinum(II) and of cis-bis(cyclopentyldimethylphosphine)dimethylplatinum(Il) in hydrocarbon solvents-mercuq/ poisoning does not successfully separate homogeneous and heterogeneous reactions. In the latter instance, an additional complicating processapparent reaction of mercury(O) with the organoplatinum compound-introduces reaction paths which seem to generate methyl radicals. The paper includes a simple preparation of 1,4-dihalo-2,2,3,3-tetramethylbutane and illustrates the use of this material as a reagent for the preparation of 3,3,4,4-tetramethylmetallacyclopentanes via the corresponding di-Grignard reagents.