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 (CF 3 PCP) is reported. Thermolysis of Ru(cod)(η3-2-methylallyl)2 ...with CF 3 PCPH under H2 affords a mixture of the three complexes (μ-CF 3 PCPH)Ru(H)(μ-H)(μ-η6,κ3-CF 3 PCP)Ru(H), (CF 3 PCP)Ru(H)2(μ-CF 3 PCPH)2, and (CF 3 PCP)Ru(cod)H, which were structurally characterized and individually prepared in moderate yields. (CF 3 PCP)Ru(cod)H reacts with (C2F5)2PCH2CH2P(C2F5)2 (dfepe) to give (CF 3 PCP)Ru(dfepe)H. (CF 3 PCP)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.
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The preparation of a series of four-coordinate complexes (CF3 PCP)Ir(L) (L = CO, DBU, nbe, coe, MeP(C2F5)2 (dfmp)) and five-coordinate complexes (CF3 PCP)Ir(L)(L′) (L = L′ = CO, dfmp, nbd, cod, ...(C2F5)2PCH2CH2P(C2F5)2 (dfepe); L = PhCN, L′ = C2H4) from dehydrohalogenation of (CF3 PCP)Ir(C2H4)(H)Cl with Et3N in the presence of trapping ligands is reported. (CF3 PCP)Ir(L) and (CF3 PCP)Ir(L)2 for L = CO, dfmp have been structurally characterized and establish a distorted-trigonal -bipyramidal coordination geometry for (CF3 PCP)Ir(L)2 with a bent PCP unit and inequivalent axial and equatorial L coordination sites. (CF3 PCP)Ir(L)(L′) systems (L = L′ = CO, C2H4; L = PhCN, L′ = C2H4) are highly fluxional, with ligand site interconversion free energy barriers determined by VT NMR of 9.7 kcal mol−1 (L = L′ = CO), 12.2 kcal mol−1 (L = L′ = C2H4), and 16.1 kcal mol−1 (L = C2H4, L′ = PhCN). A dissociative site exchange mechanism is proposed. (CF3 PCP)Ir(L) complexes readily undergo oxidative addition reactions. Addition of H2 to (CF3 PCP)Ir(CO) reversibly forms trans-(CF3 PCP)Ir(CO)(H)2 at ambient temperatures. In contrast, addition of H2 to (CF3 PCP)Ir(dfmp) affords fac,cis-(CF3 PCP)Ir(dfmp)(H)2 as the major product, with an unusual facially coordinated pincer group. VT NMR monitoring of the reaction of (CF3 PCP)Ir(CO) with H2 established the initial formation of fac,cis-(CF3 PCP)Ir(CO)(H)2 followed by conversion to mer,cis-(CF3 PCP)Ir(CO)(H)2 prior to isomerization to mer,trans-(CF3 PCP)Ir(CO)(H)2. The unusual stability of (CF3 PCP)Ir(L)2 and fac,cis-(CF3 PCP)Ir(L)(H)2 complexes is attributable to the increased stability of nonplanar (PCP)M moieties possessing strongly π-accepting phosphorus groups.
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Syntheses of osmium analogues of acceptor pincer (CF3 PCP)Ru(II) systems are reported. Treatment of Et4N2OsCl6 with CF3 PCPH at 130 °C in ethanol in the presence of Et3N gave the coordinatively ...saturated anionic carbonyl complex HNEt3 +(CF3 PCP)Os(CO)Cl2−, which subsequently may be converted to cis-(CF3 PCP)Os(CO)2Cl or cis-(CF3 PCP)Os(CO)2H by reaction with Me3SiOTf or (Et3Si)2(μ-H)+B(C6F5)4 –, respectively. (CF3 PCP)Os(cod)H was obtained in modest yields by thermolysis of Os(cod)(η3-2-methylallyl)2 with CF3 PCPH in neat cod under 3 atm of H2 at 130 °C. The alkane dehydrogenation activity of (CF3 PCP)Os(cod)H was examined: under identical conditions to previously studied (CF3 PCP)Ru(cod)H (1:1 cyclooctane/tert-butylethylene, 200 °C), the initial turnover rate for cyclooctene production was 1520 h–1, 75% the rate observed for the ruthenium analogue, but with significantly enhanced catalyst lifetime. Acceptorless cyclodecane dehydrogenation under reflux conditions gave 125 turnovers of cyclodecenes in one hour. Spectroscopic evidence on the nature of the catalyst resting state and catalyst thermal and air stability is presented.
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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&cmb.b.line;), nbd (2&cmb.b.line;)). Catalytic cyclooctane dehydrogenation studies demonstrate that 2&cmb.b.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.
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 (1combining low line), nbd (2combining low line)). Catalytic cyclooctane dehydrogenation studies demonstrate that 2combining 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.
The ligand-to-metal charge transfer state (LMCT) of (dmpe)3Re2+ (dmpe = 1,2-bis(dimethylphosphino)ethane) has been demonstrated to be a potent oxidant (E 0(Re2+*/Re+) = 2.61 V vs standard calomel ...electrode). This complex has been traditionally prepared by nontrivial routes in low yields, and very little has been achieved in optimizing the ground state and emission energy properties of the general class of complexes (PP)3Re2+ (PP = chelating diphosphine) through phosphine modification. Improved syntheses for Re(I) tris-homoleptic diphosphine complexes (PP)3Re+ (PP = 1,2-bis(dimethylphosphino)ethane (dmpe), 1,2-bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)methane (dmpm), bis(diphenylphosphino)methane (dppm), Me2PCH2PPh2, 1,3-bis(dimethylphosphino)propane (dmpp), or 1,2-bis(dimethyl-phosphino)benzene (dmpb)) were achieved by single-pot reactions exploiting the reducing potential of the phosphines when reacted with ReV oxo-complexes in 1,2-dichlorobenzene at 160–180 °C. Single-electron chemical oxidation of (PP)3Re+ yields luminescent ReII analogues; appropriate use of Ph3C+, Cp2Fe+, or (4-BrC6H4)3N+ B(C6F5)4 – salts produced (PP)3Re2+ complexes in good yields. Crystallographic trends for the Re+/Re2+ pairs show significantly lengthened Re2+–P bonds for (PP)3Re2+ relative to the corresponding (PP)3Re+ system. The redox and luminescence behavior of the complexes indicates the luminescence is from a ligand P(σ)-to-metal (Re(dπ)) charge transfer (2LMCT) state for all the complexes. Structured luminescence at 77 K is postulated to originate from relaxation of the 2LMCT state into two spin–orbit coupled states: the ground state and a state ∼3000 cm–1 above the ground state. The excited-state reduction potential (Re(II*/I)) for (depe)3Re2+ was determined from the free energy dependence of luminescence quenching rate constants. Yields for formation of charge separated ions were determined for three of the complexes with a variety of electron donors. Despite favorable electrostatics, no charge separated ions were observed for radical ion pairs for which the energy of back electron transfer exceeded 1.1 V.
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Treatment of cis-(dfmp)2PtMe2 (dfmp = (C2F5)2PMe) with the mesitylenium acid (C6Me3H4)+B(C6F5)4 – in 1,2-difluorobenzene cleanly produces an unusually stable arene complex, ...(η6-C6Me3H3)Pt(dfmp)(CH3)+(B(C6F5)4)− (1). Facile arene exchange and competitive binding equilibria have been quantified for mesitylene relative to toluene (K = 0.0030(3)) and durene (K = 20(2)). Reaction of 1 with H2 at 80 °C results in hydrogenolysis to form the arene hydride (η6-C6Me3H3)Pt(dfmp)(H)+ (2), while treatment of 1 with CO gives trans-(dfmp)2Pt(CO)Me+ as the major phosphine product. Addition of excess Me3P to 1 results in both arene and dfmp displacement to form (Me3P)3PtMe+. (η6-C6Me3H3)Pt(dfmp)(CH3)+ is a moderately active ethylene dimerization catalyst to form 2-butenes (∼7 TO h–1, 20 °C).
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•Catalyzed CLDR process for efficient syngas production and intensive CO2 reduction.•Synergistic La-Ce effect improves Fe2O3/Al2O3 performance in catalyzing the CLDR process.•Designed ...LaxCe1-x-Fe2O3/Al2O3 composites are promising redox catalysts for CLDR.•Novel insight to deposited carbon as a profoundly important reaction intermediate in CLDR.
Chemical-looping dry (CO2) reforming (CLDR) of CH4 over the LaxCe1-x-Fe2O3/Al2O3 (x = 0, 0.33, 0.67, and 1) redox catalysts paves a novel path for efficient syngas production and intensive CO2 reduction. The isolation of CO2 splitting (CS) from partial oxidation of CH4 (POM) via the proposed CLDR process makes it possible to economically address the carbon deposition of significant concern in conventional dry reforming and other related applications, and meanwhile enable a straightforward determination of active phases involved in the cyclic CLDR operation. Owing to the rare earth (i.e., La and Ce) incorporations and intimate contacts among the active Fe species, a large amount of perovskite (LaFeO3 and CeFeO3)-derived oxygen defects along with CeO2-assisted surface dispersion improvement hammer out tunnels beneficial for lattice oxygen migration, hence constituting the synergistic La-Ce effect. Moreover, our findings reveal that such La-Ce effect is advantageous for enhancing the resistance of Fe2O3/Al2O3 redox catalyst toward particle sintering and formation of inactive carbon, which guarantees catalyst tolerance against accumulated carbon deposition and more importantly the effective CO2 activation for both lattice oxygen replenishment and carbon removal. Herein, our findings demonstrate the potential of utilizing LaxCe1-x-Fe2O3/Al2O3 (x = 0.33) as a most promising redox catalyst for the proposed CLDR process.
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The protolytic stability of (dfepe)PtMe2 (dfepe = (C2F2)2PCH2CH2P(C2F5)2) and cis-(dfmp)2PtMe2 (dfmp = (C2F5)2PMe) and NMR characterization of their corresponding products in SbF5–HF superacid ...solvent mixtures are reported. Dissolution of (dfepe)Pt(Me)2 in 10 mol % of SbF5–HF at −60 °C resulted in the clean protonolysis of a single Pt–Me bond to form the cationic methyl complex (dfepe)Pt(Me)+; further conversion of (dfepe)Pt(Me)+ to (dfepe)Pt2+ occurred upon warming to −20 °C and followed pseudo-first-order kinetics (k = 1.4(2) × 10–2 min–1). In contrast, dissolution of the nonchelating analogue cis-(dfmp)2PtMe2 in 10 mol % of SbF5–HF at 20 °C evolved methane and cleanly produced the stable monomethyl complex trans-(dfmp)2Pt(Me)+. trans-(dfmp)2Pt(Me)+ is the most protolytically stable organometallic known: 33% conversion to the cis dicationic product cis-(dfmp)2Pt2+ requires 2 weeks in 10 mol % of SbF5–HF at 20 °C, whereas >90% conversion was observed in 30 h in 50 mol % of SbF5–HF. Dissolution of cis-(dfmp)2Pt(CD3)2 cleanly generated trans-(dfmp)2Pt(CD3)+, which subsequently underwent complete proton incorporation to produce trans-(dfmp)2Pt(CH3)+ within 1 h at 25 °C. This labeling study supports the reversible formation of the methane complex intermediate trans-(dfmp)2Pt(CH4)2+ under these conditions. Treatment of trans-(dfmp)2Pt(Me)+ in 10 mol % of SbF5–HF at −100 °C with 200 psi of H2 resulted in the clean formation of the dihydrogen complex trans-(dfmp)2Pt(Me)(η2-H2)+, which upon warming to −20 °C underwent methane loss and generated the hydride product trans-(dfmp)2Pt(H)+. The dihydrogen complex trans-(dfmp)2Pt(H)(η2-H2)+ has not been directly observed but has been implicated in exchange bradening behavior observed for trans-(dfmp)2Pt(H)+ under H2. Treatment of trans-(dfmp)2Pt(CD3)+ in 10 mol % of SbF5–HF at −40 °C with 200 psi of H2 cleanly produced trans-(dfmp)2Pt(CD3)(η2-H2)+ No significant H/D exchange into the Pt–CD3 group prior to trans-(dfmp)2Pt(H)+ formation was observed.
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The synthesis of cationic adducts (dfepe)Pt(Me)(L)+ (dfepe = (C2F5)2PCH2CH2P(C2F5)2; L = MeCN, CO, C2H4, C5F5N, μ-Cl) are reported. Treatment of (cod)Pt(Me)Cl with AgSbF6 in acetonitrile followed by ...the addition of dfepe afforded (dfepe)Pt(Me)(CH3CN)+SbF6 −. Addition of B(C6F5)3 to (dfepe)Pt(Me)(O2CCF3) in methylene chloride afforded the structurally characterized borane association product (dfepe)Pt(Me)(O2CCF3)B(C6F5)3 in high yield. Attempts to displace the (O2CCF3)B(C6F5)3− anion with donor ligands resulted in loss of borane and regeneration of (dfepe)Pt(Me)(O2CCF3). Addition of the mesitylenium acid (1,3,5-C6H4Me3)+B(C6F5)4 − to (dfepe)PtMe2 in methylene chloride at ambient temperatures resulted in chloride abstraction and the precipitation of the chloride-bridged dimeric complex {(dfepe)Pt(Me)}2(μ-Cl)+B(C6F5)4 −, which has been structurally characterized. In contrast, treatment of (dfepe)PtMe2 with (1,3,5-C6H4Me3)+B(C6F5)4 − in pentafluoropyridine at ambient temperature resulted in the precipitation of the structurally characterized pentafluoropyridine adduct (dfepe)Pt(Me)(NC5F5)+B(C6F5)4 − in good yield. Exposure of (dfepe)Pt(Me)(NC5F5)+B(C6F5)4 − to 1 atm of CO in o-difluorobenzene gave the carbonyl complex (dfepe)Pt(Me)(CO)+B(C6F5)4 −. In marked contrast to previously reported platinum systems, (dfepe)Pt(Me)(NC5F5)+B(C6F5)4 − is a very active ethylene dimerization catalyst at ambient temperature (600 psi ethylene, 22 °C in ortho-difluorobenzene, 150 turnovers h−1). The ethylene adduct (dfepe)Pt(Me)(η2-C2H4)+B(C6F5)4 − has been spectroscopically characterized at −20 °C.
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