12 Search Results

The triphosphorus-coordinating pincer iridium fragment (^tBuPPP)Ir was recently reported to be highly active for the catalytic dehydrogenation of n-alkanes. Dehydrogenation is calculated to be highly regioselective for the terminal position of n-alkanes. Here, the extremely high intermolecular selectivity observed in n-alkane/cycloalkane competition experiments supports the prediction of extremely high regioselectivity for dehydrogenation of n-alkanes. The use of sterically unhindered hydrogen acceptors is key to observing the high activity of the (^tBuPPP)Ir fragment. 4,4-Dimethylpent-1-ene (TBP) is found to be particularly convenient for this purpose. With the commonly used hydrogen acceptor 3,3-dimethylbut-1-ene (TBE), (^tBuPPP)Ir affords n-alkane dehydrogenation at a rate no differentmore » than that obtained with the well-known fragment (^iPrPCP)Ir. However, with the use of TBP as acceptor, (^tBuPPP)Ir shows much greater activity for n-alkane transfer dehydrogenation than previously reported catalysts, affording appreciable rates even at 50 °C, an unprecedentedly low temperature for catalytic alkane transfer dehydrogenation. Also critical to the identification of (^tBuPPP)Ir as a highly effective catalyst is the use of n-alkane substrate rather than the commonly used “model” dehydrogenation substrate, cyclooctane, with which dehydrogenation rates are much lower than those with n-alkanes.« less

The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2-di-t-butyl-phosphinophenyl)phosphine (^tBuP^HPP), has been synthesized. When complexed to iridium, it adopts a meridional (“pincer”) configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based–pincer (^tBuPPP) complex. Stoichiometric reactions of the (^tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded; one coordination site cis to the phosphido is even more crowded; andmore » the remaining site is particularly open. The (^tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous (^RPCP)Ir catalysts. Accordingly, DFT calculations predict that (^tBuPPP)Ir and (^tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (^RPCP)Ir proceeds through an intermediate trans-(PCP)IrH₂(alkene), (^tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH₂(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (^tBuPPP)Ir and (^tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Furthermore, the resting state of the (^tBuPPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.« less

Dehydrogenation to give olefins offers the most broadly applicable route to the chemical transformation of alkanes. Furthermore, transition-metal-based catalysts can selectively dehydrogenate alkanes using either olefinic sacrificial acceptors or a purge mechanism to remove H₂; both of these approaches have significant practical limitations. Here, we report the use of pincer-ligated iridium complexes to achieve alkane dehydrogenation by proton-coupled electron transfer, using pairs of oxidants and bases as proton and electron acceptors. Up to 97% yield was achieved with respect to oxidant and base, and up to 15 catalytic turnovers with respect to iridium, using t-butoxide as base coupled with variousmore » oxidants, including oxidants with very low reduction potentials. Mechanistic studies indicate that (pincer)IrH₂ complexes react with oxidants and base to give the corresponding cationic (pincer)IrH⁺ complex, which is subsequently deprotonated by a second equivalent of base; this affords (pincer)Ir which is known to dehydrogenate alkanes and thereby regenerates (pincer)IrH₂.« less

Chemical upcycling of waste polyolefins via hydrogenolysis offers unique opportunities for selective depolymerization compared to high temperature thermal deconstruction. Here, we demonstrate the hydrogenolysis of polyethylene into liquid alkanes under mild conditions using ruthenium nanoparticles supported on carbon (Ru/C). Reactivity studies on a model noctadecane substrate showed that Ru/C catalysts are highly active and selective for the hydrogenolysis of C(sp³)–C(sp³) bonds at temperatures ranging from 200 to 250 °C. Under optimal conditions of 200 °C in 20 bar H₂, polyethylene (average M_w ~ 4000 Da) was converted into liquid n-alkanes with yields of up to 45% by mass after 16more » h using a 5 wt % Ru/C catalyst with the remaining products comprising light alkane gases (C₁–C₆). At 250 °C, nearly stoichiometric yields of CH₄ were obtained from polyethylene over the catalyst. The hydrogenolysis of long chain, low-density polyethylene (LDPE) and a postconsumer LDPE plastic bottle to produce C₇–C₄₅ alkanes was also achieved over Ru/C, demonstrating the feasibility of this reaction for the valorization of realistic postconsumer plastic waste. By identifying Ru-based catalysts as a class of active materials for the hydrogenolysis of polyethylene, this study elucidates promising avenues for the valorization of plastic waste under mild conditions.« less

Molecular metal oxide cluster, K₅[α-1,2-PV₂W₁₀O₄₀] (PV₂W₁₀), was found to have intrinsic catalytic activity for the oxidative dehydrogenation of propane with high selectivity (> 80%) to propylene at low propane conversion (0.3%). Synthesis of dispersed PV₂W₁₀ in non-reducible supports, γ-Al₂O₃ and SiO₂, was done by incipient wetness impregnation. The supported catalysts were characterized by IR, Raman spectroscopy, nitrogen adsorption, x-ray powder diffraction (PXRD), elemental analysis, hydrogen temperature-programmed reduction (H₂–TPR), and ammonia temperature-programmed desorption (NH₃–TPD). Catalytic testing of the supported PV₂W₁₀ at equimolar cluster concentration revealed that when supported in γ-Al₂O₃ it is more active (sevenfold increase in propane conversion) but inmore » SiO₂ it is more selective to propylene (94%). The observed performance was due to both an increase in reducibility and higher concentration of strong acid sites for PV₂W₁₀ supported in γ-Al₂O₃ versus SiO₂. Lastly, PV₂W10 was shown to remain intact under reaction conditions indicating its thermal and oxidative stability.« less

Isomer search or molecule enumeration refers to the problem of finding all the isomers for a given molecule. Many classical search methods have been developed in order to tackle this problem. However, the availability of quantum computing architectures has given us the opportunity to address this problem with new (quantum) techniques. This paper describes a quantum isomer search procedure for determining all the structural isomers of alkanes. We first formulate the structural isomer search problem as a quadratic unconstrained binary optimization (QUBO) problem. The QUBO formulation is for general use on either annealing or gate-based quantum computers. We use themore » D-Wave quantum annealer to enumerate all structural isomers of all alkanes with fewer carbon atoms (n < 10) than Decane (C₁₀H₂₂). The number of isomer solutions increases with the number of carbon atoms. We find that the sampling time needed to identify all solutions scales linearly with the number of carbon atoms in the alkane. We probe the problem further by employing reverse annealing as well as a perturbed QUBO Hamiltonian and find that the combination of these two methods significantly reduces the number of samples required to find all isomers.« less

Owing to the strong nonpolar bonds involved, selective C-H functionalization of methane and ethane to esters remains a challenge for molecular homogeneous chemistry. Here, we report that the computationally predicted main-group p-block Sb^V (TFA)₅ complex selectively functionalizes the C-H bonds of methane and ethane to the corresponding mono and/or diol trifluoroacetate esters at 110-180 °C with yields for ethane of up to 60 % with over 90 % selectivity. Additionally, experimental and computational studies support a unique mechanism that involves Sb^V -mediated C-H activation followed by functionalization of a Sb^V -alkyl intermediate.

The cuticle plays important roles in plant development, growth and defense against biotic and abiotic attacks. Crystallized epicuticular wax, the outermost layer of cuticle, is visible as white-bluish glaucousness. In crops like barley and wheat, glaucousness is trait of adaption to the dry and hot cultivation conditions, and hentriacontane-14,16-dione (β-diketone) and its hydroxy derivatives are the major and unique components of cuticular wax in the upper parts of adult plants. But their biosynthetic pathway and physiological role largely remain unknown. In the present research, we identified a novel wax mutant in wheat cultivar Bobwhite. The mutation is not allelic tomore » the known wax production gene loci W1 and W2, and designated as W3 accordingly. Genetic analysis localized W3 on chromosome arm 2BS. The w3 mutation reduced 99% of β-diketones, which account for 63.3% of the total wax load of the wild-type. W3 is necessary for β-diketone synthesis, but has a different effect on β-diketone hydroxylation because the hydroxy-β-diketones to β-diketone ratio increased 11-fold in the w3 mutant. Loss of β-diketones caused failure to form glaucousness and significant increase of cuticle permeability in terms of water loss and chlorophyll efflux in the w3 mutant. Transcription of 23 cuticle genes from five functional groups was altered in the w3 mutant, 19 down-regulated and four up-regulated, suggesting a possibility that W3 encodes a transcription regulator coordinating expression of cuticle genes. Biosynthesis of β-diketones in wheat and their implications in glaucousness formation and drought and heat tolerance were discussed.« less