Alcohol Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium

Lü, Zhiyao; Malinoski, Brock; Flores, A. V.; Conley, Brian L.; Guess, Denver; Williams, Travis J.

doi:10.3390/catal2040412

Cited by 10 publications

(8 citation statements)

References 40 publications

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“… We chose to demonstrate it in AB dehydrogenation because coordinative saturation of its reduced form ( 23 ) should logically enable release of multiple equivalents of H 2 , potentially with the rate of Noyori-type bifunctional ruthenium-based systems. Although the latter were known to be very fast, they are limited in productivity to one equivalent of H 2 . The Shvo dimer reacts by splitting heterolytically into an oxidizing fragment ( 22 ) and a reducing fragment ( 23 ).…”

Section: Ab Dehydrogenation By Shvo’s Catalyst: Mechanism and Platfor...mentioning

confidence: 99%

Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility

et al. 2016

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One of the greatest challenges in using H as a fuel source is finding a safe, efficient, and inexpensive method for its storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered attention for its high hydrogen weight density (19.6 wt %) and ease of handling and transport. Hydrogen release from ammonia borane is mediated by either hydrolysis, thus giving borate products that are difficult to rereduce, or direct dehydrogenation. Catalytic AB dehydrogenation has thus been a popular topic in recent years, motivated both by applications in hydrogen storage and main group synthetic chemistry. This Account is a complete description of work from our laboratory in ruthenium-catalyzed ammonia borane dehydrogenation over the last 6 years, beginning with the Shvo catalyst and resulting ultimately in the development of optimized, leading catalysts for efficient hydrogen release. We have studied AB dehydrogenation with Shvo's catalyst extensively and generated a detailed understanding of the role that borazine, a dehydrogenation product, plays in the reaction: it is a poison for both Shvo's catalyst and PEM fuel cells. Through independent syntheses of Shvo derivatives, we found a protective mechanism wherein catalyst deactivation by borazine is prevented by coordination of a ligand that might otherwise be a catalytic poison. These studies showed how a bidentate N-N ligand can transform the Shvo into a more reactive species for AB dehydrogenation that minimizes accumulation of borazine. Simultaneously, we designed novel ruthenium catalysts that contain a Lewis acidic boron to replace the Shvo -OH proton, thus offering more flexibility to optimize hydrogen release and take on more general problems in hydride abstraction. Our scorpionate-ligated ruthenium species (12) is a best-of-class catalyst for homogeneous dehydrogenation of ammonia borane in terms of its extent of hydrogen release (4.6 wt %), air tolerance, and reusability. Moreover, a synthetically simplified ruthenium complex supported by the inexpensive bis(pyrazolyl)borate ligand is a comparably good catalyst for AB dehydrogenation, among other reactions. In this Account, we present a detailed, concise description of how our work with the Shvo system progressed to the development of our very reactive and flexible dual-site boron-ruthenium catalysts.

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Section: Ab Dehydrogenation By Shvo’s Catalyst: Mechanism and Platfor...mentioning

confidence: 99%

Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility

et al. 2016

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“…Further, despite their catalytic utility, no direct evidence has been collected to show a mechanistic account of the cooperative role, if any, that boron and ruthenium are playing in the reactive mechanisms of 1 or 2 . iv We suspect this is partially due to the robustness of the bridging μ-OH ligand between the boron and ruthenium centers, which inhibits access to a free borane in catalytic reactions.…”

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A dual site catalyst for mild, selective nitrile reduction

Lü

Williams

2014

Chem. Commun.

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We report a novel ruthenium bis(pyrazolyl)borate scaffold that enables cooperative reduction reactivity in which boron and ruthenium centers work in concert to effect selective nitrile reduction. The pre-catalyst compound {[κ 3 -(1-pz) 2 HB(N=CHCH 3 )]Ru(cymene)} + TfO − (pz = pyrazolyl) was synthesized from readily-available materials through a straightforward route, thus making it an appealing catalyst for a number of reactions.As part of our ongoing studies of dual site ruthenium, boron-containing catalysts for the manipulation of hydride groups, we have recently reported a series of [di(pyridyl)borate]ruthenium complexes (1, 2) i that exhibit remarkable reactivity in a number of applications, ii notably including dehydrogenation of ammonia borane. iii Although they are successful catalysts, these di(pyridyl)dimethylborate-derived complexes are cumbersome to prepare, due largely to dependence on an expensive and reactive BrBMe 2 starting material and high water and oxygen sensitivity of intermediate complexes in their syntheses. Further, despite their catalytic utility, no direct evidence has been collected to show a mechanistic account of the cooperative role, if any, that boron and ruthenium are playing in the reactive mechanisms of 1 or 2. iv We suspect this is partially due to the robustness of the bridging μ-OH ligand between the boron and ruthenium centers, which inhibits access to a free borane in catalytic reactions.We show here a conveniently prepared, borate-pendant ruthenium complex (3) that retains much of the reactivity of the original di(pyridyl)borate complexes (see ESI), show that it is an efficient and selective catalyst for nitrile reduction, and provide evidence for the cooperative role that boron and ruthenium play in the reaction, as hydride donor and activating group, respectively. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript steps without the need for materials that are cost-prohibitive or difficult to manipulate: all materials are amenable to handling using standard Schlenk techniques and/or a glove box. 3 can be crystallized from isopropanol and hexanes; its molecular structure was determined by single crystal X-ray diffraction (Scheme 1). The crystal structure of 3 shows the borate ligand in a tetrahedral geometry at boron, which has analogy to the popular tris(pyrazolyl)borohydride (Tp) ligand series. vi, vii, viii The synthesis of 3 revealed an important insight into the mechanism of its catalytic reactivity. In the conversion of 4 to 3, a hydride is transferred from a ligand B-H group to 5's coordinated nitrile in >90% NMR yield (Scheme 1). This reaction is the first example of our envisioned cooperative reactivity of ruthenium and boron. Contrary to the design concept from which we originally prepared 2, ii boron in 3 is not behaving as a Lewis acid. The structure of 3 shows that B-H addition to the nitrile proceeds in a cis fashion, and NMR evidence reveals that the selectivity for this geometry is exclusive. Thus we believe that the mech...

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“…Our lab has been involved in catalytic dehydrogenation for some time, 24,25,26,27,28,29,30 and we recently reported a catalytic, acceptorless dehydrogenation of glycerol. 31 Here we show that we can realize the proposal in scheme 1, a tandem process which has an output of high oleate FAMEs and no glycerol waste stream.…”

Section: Introductionmentioning

confidence: 99%

Upgrading Biodiesel from Vegetable Oils by Hydrogen Transfer to Its Fatty Esters

Lü

Cherepakhin

Kapenstein

et al. 2018

ACS Sustainable Chem. Eng.

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Conversion of vegetable-derived triglycerides to fatty acid methyl esters (FAMEs) is a popular approach to the generation of biodiesel fuels and the basis of a growing industry. Drawbacks of the strategy are that (a) the glycerol backbone of the triglyceride is discarded as waste, and (2) most available natural triglycerides in the U.S. are multi-unsaturated or fully saturated, giving inferior fuel performance and causing engine problems. Here we show that catalysis by iridium complex 1 can address both of these problems through selective reduction of triglycerides high in polyunsaturation. This is realized using hydrogen from methanol or those imbedded in the triglyceride backbone, concurrently generating lactate as a value-added C3 product. Additional methanol or glycerol as a hydrogen source enables reduction of corn and soybean oils to > 80% oleate. The cost of the iridium catalyst is mitigated by its recovery through aqueous extraction. The process can be further driven with a supporting iron-based catalyst for the complete saturation of all olefins. Preparative procedures are established for synthesis and separation of methyl esters of the hydrogenated fatty acids, enabling instant access to upgraded biofuels.

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Alcohol Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium

Cited by 10 publications

References 40 publications

Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility

Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility

A dual site catalyst for mild, selective nitrile reduction

Upgrading Biodiesel from Vegetable Oils by Hydrogen Transfer to Its Fatty Esters

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