Ruthenium-Catalyzed Ammonia Borane Dehydrogenation

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Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility Xingyue Zhang, Lisa Kam, Ryan Trerise, and Travis J. Williams* Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661, United States CONSPECTUS: One of the greatest challenges in using H2 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.

1. INTRODUCTION As hydrogen fuel has enjoyed popularity as a clean, abundant energy source, much research has been devoted to its safe and efficient storage. One approach is chemical storage, optimally involving reversible hydrogenation of a substrate to form an airand water-stable chemical. Ammonia borane (AB or H3N-BH3) is a stable, solid material at room temperature that is very dense in hydrogen (19.6 wt %).1 Theoretically, up to three equivalents of H2 can be released from AB, although a catalyst is required to enable desirable rates of hydrogen release, extent of reaction, and product selectivity. There are two general methods for the catalytic dehydrogenation of AB. The first is hydrolysis: many catalysts are known to release 2.8−3.0 equiv of H2 from aqueous solutions of AB in minutes via hydrolysis. Transition metal complexes of cobalt, iron, copper, molybdenum, vanadium, rhodium, and iridium2 and nanoparticles of cobalt, nickel, iron, copper, gold, ruthenium, rhodium, palladium, and platinum are popular catalysts for mild AB hydrolysis, often proceeding at ambient temperature and low catalyst loadings.3 However, hydrolysis © 2016 American Chemical Society

poses drawbacks including the formation of ammonia, a poison for proton exchange membrane (PEM) fuel cells,4 and multiple borate products. If regeneration of spent fuel is the ultimate goal for implementation of AB in hydrogen storage, then the boron and nitrogen products of dehydrogenation should be compounds where rehydrogenation is possible.5 In the case of hydrolysis, a large amount of energy is required to rereduce the borate B−O bonds, thus rendering regeneration thermodynamically unfeasible.1b Therefore, a second, nonhydrolytic strategy of dehydrogenating AB is more conducive to spent-fuel regeneration because borate byproducts are avoided. Many homogeneous catalyst systems dehydrogenate AB nonhydrolytically; these include complexes of iron,6 molybdenum,7 iridium,8 rhodium,9 nickel,10 palladium,11 and ruthenium (Figure 1).12 A limited number of homogeneous metal catalysts can produce two equivalents of H2 and even fewer can surpass this two equivalent mark.6c,7,10,13 Our lab has introduced a Received: September 25, 2016 Published: December 29, 2016 86

DOI: 10.1021/acs.accounts.6b00482 Acc. Chem. Res. 2017, 50, 86−95

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Accounts of Chemical Research

Figure 1. Selected representative catalysts for AB dehydrogenation.

number of ruthenium catalysts that dehydrogenate AB, releasing multiple equivalents of H2 (Figure 2).14

Scheme 1. Nonhydrolytic Hydrogen Release from AB

2. AB DEHYDROGENATION BY SHVO’S CATALYST: MECHANISM AND PLATFORM FOR DEVELOPMENT Dehydrogenation of AB by Shvo’s Catalyst

Shvo’s catalyst (6) is a ligated ruthenium dimer that was first reported by Youval Shvo in the 1980s (Scheme 2).19 We chose Scheme 2. Shvo’s Catalyst (6)15 and Its Oxidizing (22) and Reducing (23) Fragments Figure 2. Ruthenium catalysts from the Williams laboratory that dehydrogenate AB past 2 equiv of H2.15

Known homogeneous catalysts for AB dehydrogenation can be divided according to those that release only one equivalent of H2 and those that release more than one. Among the catalysts that evolve only one equivalent of H2, coordination of aminoborane (NH2BH2), the first dehydrogenation product of NH3BH3, to the catalyst limits productivity to one equivalent of H2.16 These are apparently limited to one equivalent because the NH2BH2-catalyst coordination promotes formation of insoluble oligomers, [NH2BH2]n, that leave the reaction solution. It is also possible that metal-catalyzed polymerization of aminoborane with these catalysts simply outcompetes its offmetal oligomerization.17 In fact, this later view has enabled the development of a very nice iron-based system for AB dehydrogenation.18 Catalyst systems that liberate more than one equivalent of H2 from NH3BH3 include 1−5 in Figure 1. BN intermediates in the dehydrogenation pathway of AB commonly observed with these catalysts are shown in Scheme 1. Rehydrogenation of polyborazylene (21) and other NxByHz byproducts needs to be reasonable to achieve to enable fuel recycling.

to demonstrate it in AB dehydrogenation because coordinative saturation of its reduced form (23) should logically enable release of multiple equivalents of H2,16a 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 H2.14 The Shvo dimer reacts by splitting heterolytically into an oxidizing fragment (22) and a reducing fragment (23). This reducing fragment consists of a metal hydride and acidic proton, contributing to its metal− ligand cooperative reactivity. Shvo’s catalyst enables liberation of 2.0 equiv of H2 from AB (Scheme 2).14a Upon completion, the 1H and 11B NMR spectra of the dehydrogenated solution reveal borazine as the exclusive boron product. Boron (11B) NMR provides an excellent tool to monitor the kinetics of this reaction, but the data do not 87


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Accounts of Chemical Research Scheme 3. Shvo-Catalyzed AB Dehydrogenation Mechanism

product of BH3 dissociation from ammonia borane, and [NH2BH2] generated by the first dehydrogenation of ammonia borane.1b,21 AB must therefore release its complementary NH3, which itself is a competitive inhibitor22 in the slow catalysis cycle, along with borazine inhibition, part of a kinetic double whammy. Identifying the two mechanisms of catalysis inhibition (borazine hydroboration and NH3 ligation) led us to develop superior second-generation catalysis. One strategy was clear: design a scaffold that could not be deactivated by hydroboration. This strategy is discussed in section 4. Ammonia inhibition led us to a more circuitous plan: we observed that AB dehydrogenation with 24 was slower than the same reaction with 6 (Figure 3), but curiously, the reaction of 24 maintained its fast kinetics regime longer than that of 6, ultimately enabling it to reach completion first. Somehow, 24 is immune to death by hydroboration, as if NH3 binding the ruthenium site of the catalyst is protecting the oxygen site from deactivation. Therefore, we envisioned a strategy in which we would find a weakly binding NH3 surrogate that would give this protective behavior without kinetic inhibition of AB dehydrogenation. Lacking a better term, we called this a “semi-site protection” plan: inhibit one of the catalyst’s binding sites to protect the other.

conform to a simple kinetic regime. The data feature three distinct rate regimes: 1. A brief initiation period (ca. 2% conversion), 2. A fast linear regime (through ca. 30% conversion), and 3. A slower regime that fits to first order exponential decay.14d Decoding these separate regimes and understanding the molecular events that enable the transitions from one to the next involved an in-depth reaction kinetics study that revealed key insights that ultimately enabled the design of successful second generation catalysts.

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MECHANISM OF AB DEHYDROGENATION BY SHVO’S CATALYST The mechanism of AB dehydrogenation with the Shvo catalyst begins with a short initiation period (∼2% conversion) in which 6 dissociates to 22 and 23 (Scheme 3, upper left). Compound 22 then converts quickly to 23 because its reduction by AB is rapid. After the catalyst initiation, the second regime of the reaction displays fast, linear kinetics through ∼20−30% conversion (Scheme 3, left cycle). In this phase, the reaction has a zero order dependence on [AB] and first order dependence on [Ru]. Through this phase, 1H NMR spectroscopy shows a persistent monomeric ruthenium hydride at 1H δ = −10 ppm, which is the resting state of the catalyst. This is consistent with H−H bond formation as the rate-determining step. The last section of the mechanistic regime is catalyst deactivation, or “death”, in which catalysis becomes slow (Scheme 3, right cycle). It is characterized by the appearance of first order dependence on [AB] and a rise in [borazine] (20). Several κ1-Ru-H hydride peaks emerge between 1H δ = −9 and −10 ppm14d simultaneously with exponential decay behavior in [AB]. These things happen because borazine (20) hydroborates intermediate 22 to give deactivated borazine complex 26. This hydroboration is analogous to the reaction of an analogue of 22 with catecholborane reported by Clark.20,14d We also observe μ-aminodiborane 17 in this slow catalysis regime, a

The “Semi-Site Protection” Strategy

Another case of this apparently protective phenomenon arose while testing the catalysis for homogeneity via a quantitative poisoning experiment,23 wherein the reaction was run in the presence of a small portion of a catalyst poison, e.g., 1,10phenanthroline (27, Figure 4). We were surprised to find that this poison accelerates the reaction by prolonging the fast catalysis phase of the reaction, much like NH3. Scheme 4 sketches a mechanistic proposal to account for the protective nature that amine ligands have on the Shvo catalyst. We proposed at the time that a nitrogen ligand binds reversibly to the open coordination site of 22, thus protecting it from 88


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ation with the Shvo system if a ligand that binds 22 with a rapid on/off rate and appropriate equilibrium constant could be found. Pyridine presents a systematically tunable ligand scaffold with appropriate ruthenium binding affinity, so we isolated a series of pyridine-ligated ruthenium complexes (Figure 5) to test in the catalysis. Compounds 7−9 effect AB dehydrogenation faster than that of parent complex 6 (Figure 6). For example, a reaction catalyzed by 9 is 4-fold faster than an analogous reaction of 6. Moreover, 9 gives a higher extent of H2 release: 2.1 rather than 2.0 equiv. In the dehydrogenation studies shown in Figure 6, there is an increase in reaction rate (8 (py-NMe2) < 7 (py-H) < 9 (py-CF3)) as the pyridine ligands become less electron rich. For precatalysts 8, 7, and 9, these reactions have AB consumption rate constants of 2.0, 4.3, and 5.6 × 10−4 M s−1, respectively, in their saturation catalysis periods. These data are consistent with a view that a less-tightly binding pyridine enables a faster reaction. They also are consistent with the proposal that pyridine binding to ruthenium is protective of the catalyst; however, pyridine dissociation is necessary for catalytic turnover. Unlike reactions with pyridine ligands, when AB dehydrogenation reactions of 6 are treated with bidentate nitrogen ligands (e.g., 1,10-phenanthroline, 2,2′-bipyridine, or tetramethylethylenediamine), the ligand displaces the catalyst’s tetraphenylcyclopentadienone (CPD), which leads to a situation in which multiple differently ligated, potentially reactive ruthenium species are present in the reaction. This is inconsistent with the protective hypothesis and will be discussed in section 3; however, it did enable the discovery of catalyst systems that delivered hydrogen beyond the second equivalent.

Figure 3. AB dehydrogenation with 6 (diamonds) and 24 (circles).15 AB (0.25 mol) and 0.035 mol [Ruatom] are added to 0.6 mL of diglyme/benzene-d6 at 70 °C.

3. BEYOND THE SECOND EQUIVALENT Maximizing the efficiency of H2 release is highly desirable. However, only a handful of homogeneous catalysts achieve a dehydrogenation of 2.5 equiv of H2 or more (Figure 1, right). Currently, Baker’s nickel-based system (3) supported by Ender’s carbene holds the record for highest extent of H2 release from AB reported to date.10 The Guan Fe-POCOP catalyst (4) uses inexpensive iron but dehydrogenates relatively slowly while reacting through 2.5 equiv of H2.6c The Agapie molybdenum catalyst (5) has the same productivity, but its practicality is limited by air and water sensitivity.7 Further, Wegner has recently presented a metal-free catalyst that is capable of releasing ∼2.5 equiv of H2.13 Catalyst Development

We saw that the Shvo catalyst loses its CPD ligand in the presence of bidentate nitrogen ligands (like 27) to generate a species that is a faster AB dehydrogenation catalyst than the parent. We reasoned that simple (phen)Ru(CO)2X2 complexes like 3025 and 1126 (Figure 7) could exceed this reactivity without a CPD group. We find that catalyst 11 dehydrogenates AB efficiently at low catalyst loading, down to 1 mol %, producing 2.4−2.7 equiv of hydrogen (Figure 7). The catalyst system is stable to air and robust, capable of producing a similarly high extent of H2 release (2.6, 2.5, 2.4) in each of three AB reloadings. Treatment of AB with 11 results in the formation of a family of AB dehydrogenation products (Scheme 1) like those formed with the Shvo catalyst (6), except that the reaction does not stop at borazine but rather goes on to polyborazylene materials

Figure 4. [AB] dehydrogenation with 6 in the presence of 1,10phenanthroline, 27. AB (0.25 mol) and 5 mol % 6 are added to 0.6 mL of diglyme/benzene-d6 at 70 °C.

hydroboration. When this ligand dissociates, the catalyst is again accessible for hydroboration. The Shvo system is known to form robust hydrogen bonds to many N−H groups,24 so we thought this type of precoordination interaction (29 in Scheme 4) could affix AB to the catalyst prior to phenanthroline dissociation, thus making hydroboration by borazine less competitive. A Test of the Protection Proposal

If the argument of Scheme 4 is to be believed, then it should be possible to realize fast saturation kinetics in AB dehydrogen89


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Accounts of Chemical Research Scheme 4. A “Semi-Site Protection” Mechanism

Figure 5. Pyridine complexes 7−9; ORTEP diagram of 7.

Figure 7. CPD-free Ru complexes (phen)RuCl2(CO)2 (30)15 and (phen)Ru(OAc)2(CO)2 (11) and application in H2 production from 1% of 11 at 70 °C in diglyme, releasing 2.7 equiv.

Figure 6. Catalytic AB dehydrogenation: AB consumption (11B NMR) catalyzed by 6−9 (10 mol % Ru atom, 70 °C, 1:2 C6D6/diglyme).

could plausibly have been found in a large enough systematic screen of reaction conditions, the fundamental insights into

(21). Additionally, unlike 6 and its derivatives, 11 will catalyze the cross-linking isolated borazine itself. While the route to discovery of the remarkable reactivity of 11 from the original observation of prolonged catalysis in the presence of NH3 was circuitous, it shows an example of mechanism-led reaction optimization. Although catalyst 11

dehydrogenation and the reactivity of B−H groups with catalysts in this class have also enabled us to make important discoveries in formic acid dehydrogenation,27 CO2 reduction,28 and other areas. 90


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4. NEW CATALYST ARCHITECTURES FOR AB DEHYDROGENATION

Scheme 5. A Second-Generation Catalyst

Development of Second Generation AB Dehydrogenation Catalysts

Apart from ammonia borane, we sought to develop general systems and strategies for hydride abstraction using the Shvo platform as inspiration (Figure 8).14c Our general approach

the catalytic species is not deactivated. Consistent with this view, we observe that 12 can liberate >2 equiv of H2 from ammonia borane even with repeated use and exposure to the atmosphere. The best performance in AB dehydrogenation with 12 is realized under highly concentrated suspension conditions (Figure 10). For example, a slurry of AB and tetraglyme Figure 8. Strategic design of metal/Lewis acid two-site catalysts with Ru/B used in this illustration.

involved cooperation of a transition metal and a ligand-centered coordination-directing element. This is analogous to hydrogen bond-directed oxidation catalysts such as Noyori’s (Ts-DPEN) (cym)RuCl systems29 and the parent Shvo19 and Casey30 systems. We devised a series of ruthenium complexes in which the boron center is affixed to ruthenium through C−B bonds using pyridine linkers.31,14b The first of these was tris(acetonitrile) μhydroxide di(pyridyl)borate ruthenium triflate 12 (Figure 9).

Figure 9. Structure and ORTEP diagram of Ru/B complex 12.

Figure 10. AB dehydrogenation with 1215 (2.0 mol % in tetraglyme under air at 70 °C). H2 release = 4.2 system wt %.

Complex 12 is formed from dimethylborate complex 13 (Figure 2) and places a labile hydroxide ligand between the boron and ruthenium centers. This intent was to enable release of hydroxide to give access to both coordination sites. Reactivity of 12 with organic molecules has been limited,14i,32 but its most useful application has been in AB dehydrogenation.

(100.0 mg of AB, 202.6 mg of tetraglyme, 5.8 system wt % stored H2) was treated with 2.0 mol % 12 at 70 °C to yield 2.0 equiv of H2 in 4 h and a cumulative total of 2.2 equiv (TON = 110) upon completion of the reaction. This corresponds to the release of 4.2 wt % H2. This catalyst system is reusable, and it works well when open to air. With a catalyst loading of 0.1 mol %, we observe a TON of ∼5700 over three runs and liberation of up to 4.6 wt % H2. Unlike Shvo’s catalyst, 12 demonstrates first order kinetics in [AB] through 3 half-lives.

Reactivity of 12 with AB

In addition to the design concept laid out in Figure 8B, 12 was built so that its hydroxyl group could be abstracted by boranes, thus providing a strategy to avoid the problem of catalyst Oborylation as in the Shvo catalyst. Scheme 5 highlights this concept: if 12 is borylated by analogy to the Shvo system (26), the resulting borylated oxygen atom can be dissociated so that 91


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Accounts of Chemical Research Mechanism of AB Dehydrogenation with 12

Ankan Paul has recently published a computational study on the mechanism of AB dehydrogenation with 12;36 however, the findings contradict most of the physical data that have been collected on the system.14c,d,e,i These investigators propose a monomeric catalyst with persistence of 12’s μ−OH and acetonitrile ligands intact throughout catalysis. Both of these functionalities are known to be derivatized quickly in the presence of AB to give a dimeric catalytic species. Further, the computational data have 15’s trifluoroacetate moiety metalbound during catalysis, thus predicting a single TFA peak when the catalyst is resting. However, two peaks are observed in the 19 F NMR spectrum after the catalyst has turned over. Thus, we find no relationship between this computational study and the catalytic reaction.

The mechanism of AB dehydrogenation with 12 is not known. The study of this mechanism is frustrated by the rapid transformation of 12 into a diversity of species, including an apparently dimeric active species that is deprived of most of the functionality of 12: its bridging hydroxyl group appears to be abstracted by AB, and its acetonitrile ligands are reduced. 11B NMR data show disappearance of 12’s −0.8 ppm singlet immediately upon AB treatment, and 1H NMR spectra show reduction of all of 12’s acetonitrile ligands to ethylamine in minutes at room temperature.33 Despite these issues, some mechanistic information can be collected on this reaction from detailed kinetic and synthetic studies. Kinetics data collected by 11B NMR indicate a rate law that is first order in [AB] and half order in catalyst. These indicate a rate-limiting transition state that involves AB coordination or activation and the presence of a dinuclear (Ru)2 intermediate, although we have not observed this species. Possible structures include those with bridging hydride or ethylamido structures, although we have not observed a persistent metal hydride, which argues against a Ru−H moiety as the resting state of catalysis. This is in line with observations of the (NHC)Ni systems,10 but contrasts with our own14 and others’ findings for ruthenium12 and iridium9 catalysts. Catalyst 12 seems to interact concurrently with both the proton and hydride groups of AB in a bifunctional transition state. We probed this by recording isotope effects for isotopologues of AB.34 kH/kD values for the B−H and N−H bonds are 1.2 and 1.6, respectively. A combined isotope effect measured using D3N−BD3 (kNHBH/kNDBD = 1.7) is within measurement error of the product of the two independent isotope effects (1.9). This is consistent with a concerted, asynchronous transition state in the rate-determining step. Plausible mechanistic scenarios include one in which 12 donates its OH proton, thus initiating an acid-catalyzed reaction35 or the so-formed oxide bridge providing an internal base. To probe these, we synthesized bridging carboxylate complexes 14 and 15 (Figure 11), which are devoid of the

Optimization of Boron Lewis Acid-Containing Ligands for Ruthenium

Although successful catalysts, these di(pyridyl)dimethylboratederived complexes are cumbersome to prepare, largely due to dependence on an expensive and reactive bromodimethylborane starting material and high water and oxygen sensitivity of intermediate complexes in their syntheses. Therefore, we turned our attention to alternate ligand sets like the bis(pyrazolyl)borate (34) that employs the Lewis acidic boron but avoids the μ−OH ligand of 12 and its BrBMe2 precursor. Complex 16 (Scheme 6) is a conveniently prepared, Scheme 6. Syntheses and ORTEP Diagram of Catalyst 16

borate-pendant ruthenium complex that retains much of the reactivity of 12 in AB dehydrogenation. The synthesis of 16 proceeds in two smooth steps without the need for materials that are cost-prohibitive or difficult to manipulate. Catalyst 16 has superior reactivity to that of 12 in a number of transformations relevant to the manipulation of energy carriers; particularly, it is a similarly effective catalyst for ammonia borane dehydrogenation with an H2 release extent of 2.1 equiv at kobs = 3.77(8) × 10−4 s−1 compared to 12’s 3.60(8) × 10−4 s−1 at 5 mol % [Ru] loading. Complex 16 has further valuable reactivity: in addition to being an excellent AB dehydrogenation catalyst, we also find it to have reactivity in formic acid dehydrogenation and oxygen evolution from aqueous ceric ammonium nitrate. More impressive than these, however, is its reactivity in borohydride-mediated nitrile reduction. Because 16’s ruthenium and boron centers can respectively activate a nitrile and deliver a hydride to it, 16 enables remarkably mild and selective synthesis of primary amines. Because of this reactivity, it is currently being commercialized.37

Figure 11. Bridging acetate (14) and TFA (15) and ORTEP diagram of 15.15

protic functionality. Surprisingly, 5.0 mol % solutions of 12 and 14 share the same rate of AB consumption, which is consistent with our observation of hydroxide abstraction from 12 upon initiation of catalysis. To interrogate the role of boron, we attempted AB dehydrogenation with complex 15, which has different electronic characteristics from 12 or 14. This gives catalysis about twice as fast as 12. Thus, the occupancy of the bridging coordination site between ruthenium and boron has an important influence on dehydrogenation rate, but 12’s proton cannot be important.

5. CONCLUSIONS This Account tells a story of how mechanistic work on ammonia borane dehydrogenation with the Shvo catalyst led to several new catalytic systems. Identifying two mechanisms for 92


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metallics 2013, 32, 3769−3772. (b) San Nacianceno, V.; Azpeitia, S.; Ibarlucea, L.; Mendicute-Fierro, C.; Rodríguez-Diéguez, A.; Seco, J. M.; San Sebastian, E.; Garralda, M. A. Stereoselective Formation and Catalytic Activity of Hydrido(acylphosphane) (chlorido)-(pyrazole)rhodium(III) Complexes. Experimental and DFT Studies. Dalton Trans 2015, 44, 13141−13155. (c) Lapin, N. V.; D’yankova, N. Y. Hydrogen Evolution Kinetics during Transition Metal OxideCatalyzed Ammonia Borane Hydrolysis. Inorg. Mater. 2013, 49, 975−979. (3) (a) Jiang, H.-L.; Xu, Q. Catalytic Hydrolysis of Ammonia Borane for Chemical Hydrogen Storage. Catal. Today 2011, 170, 56−63. (b) Yan, J. M.; Zhang, X. B.; Akita, T.; Haruta, M.; Xu, Q. One-Step Seeding Growth of Magnetically Recyclable Au@Co Core−Shell Nanoparticles: Highly Efficient Catalyst for Hydrolytic Dehydrogenation of Ammonia Borane. J. Am. Chem. Soc. 2010, 132, 5326−5327. (c) Jiang, H. L.; Umegaki, T.; Akita, T.; Zhang, X. B.; Haruta, M.; Xu, Q. Bimetallic Au−Ni Nanoparticles Embedded in SiO2 Nanospheres: Synergetic Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane. Chem. - Eur. J. 2010, 16, 3132−3137. (d) Ramachandran, P. V.; Gagare, P. D. Preparation of Ammonia Borane in High Yield and Purity, Methanolysis, and Regeneration. Inorg. Chem. 2007, 46, 7810− 7817. (4) Halseid, R.; Vie, P. J. S.; Tunold, R. Effect of Ammonia on the Performance of Polymer Electrolyte Membrane Fuel Cells. J. Power Sources 2006, 154, 343−350. (5) (a) Davis, B. L.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Matus, M. H.; Scott, B.; Stephens, F. H. Efficient Regeneration of Partially Spent Ammonia Borane Fuel. Angew. Chem., Int. Ed. 2009, 48, 6812. (b) Sutton, A. D.; Burrell, A. K.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Nakagawa, T.; Ott, K. C.; Robinson, J. P.; Vasiliu, M. Regeneration of Ammonia Borane Spent Fuel by Direct Reaction with Hydrazine and Liquid Ammonia. Science 2011, 331, 1426−1429. (6) (a) Vance, J. R.; Robertson, A. P. M.; Lee, K.; Manners, I. Photoactivated, Iron-Catalyzed Dehydrocoupling of Amine−Borane Adducts: Formation of Boron−Nitrogen Oligomers and Polymers. Chem. - Eur. J. 2011, 17, 4099−4103. (b) Baker, R. T.; Gordon, J. C.; Hamilton, C. W.; Henson, N. J.; Lin, P. H.; Maguire, S.; Murugesu, M.; Scott, B. L.; Smythe, N. C. Iron Complex-Catalyzed Ammonia− Borane Dehydrogenation. A Potential Route toward B−N-Containing Polymer Motifs Using Earth-Abundant Metal Catalysts. J. Am. Chem. Soc. 2012, 134, 5598−5609. (c) Bhattacharya, P.; Krause, J. A.; Guan, H. Mechanistic Studies of Ammonia Borane Dehydrogenation Catalyzed by Iron Pincer Complexes. J. Am. Chem. Soc. 2014, 136, 11153−11161. (7) Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. Molybdenum Catalyzed Ammonia Borane Dehydrogenation: Oxidation State Specific Mechanisms. J. Am. Chem. Soc. 2014, 136, 11272− 11275. (8) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, M.; Goldberg, K. I. Efficient Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128, 12048−12049. (9) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Transition Metal-Catalyzed Formation of Boron−Nitrogen Bonds: Catalytic Dehydrocoupling of Amine-Borane Adducts to Form Aminoboranes and Borazines. J. Am. Chem. Soc. 2003, 125, 9424−9434. (b) Jaska, C. A.; Manners, I. Catalytic Dehydrocoupling of Amine-Borane and Phosphine-Borane Adducts: The Mechanism Is Heterogeneous in One Case and Homogeneous in the Other. J. Am. Chem. Soc. 2004, 126, 1334−1335. (c) Shrestha, R. P.; Diyabalanage, H. V. K.; Semelsberger, T. A.; Ott, K. C.; Burrell, A. K. Catalytic Dehydrogenation of Ammonia Borane in Non-Aqueous Medium. Int. J. Hydrogen Energy 2009, 34, 2616−2621. (d) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. Amine-Borane σ-Complexes of Rhodium. Relevance to the Catalytic Dehydrogenation of Amine-Boranes. J. Am. Chem. Soc. 2008, 130, 14432−14433. (e) Alcaraz, G.; Sabo-Etienne, S. Coordination and Dehydrogenation of Amine−Boranes at Metal Centers. Angew. Chem., Int. Ed. 2010, 49, 7170−7179.

deactivation of the Shvo system, amine coordination and borazine hydroboration, led to two complementary strategies for catalyst development. Our insights into the impact of ammonia coordination to the Shvo catalyst enabled us to devise much more reactive pyridine and phenanthroline-ligated catalysts. Our approach of designing catalysts inert to deactivation by hydroboration gave us two structurally novel systems, 12 and 16, that have enabled exciting new reactivity, both in hydrogen release from AB and in organic synthesis. Opportunities and challenges remain in the ammonia borane field;38 excellent work is being done in spent fuel regeneration,5 heterogeneous catalysis, and “liquefaction” of ammonia borane for fuel cycle applications.39 Rather than these, we are turning our attention to moving from a boron/nitrogen fuel carrier to a CO2/methanol economy.27,28,40

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Travis J. Williams: 0000-0001-6299-3747 Notes

The authors declare no competing financial interest. Biographies Xingyue Zhang goes by Lily and was born in Kunming, China. She received her B.S. in Chemistry from the University of California, Berkeley in 2010 and Ph.D. in Chemistry at the University of Southern California in 2016. Her research interests include tackling current environmental problems with green chemistry. Lisa Kam was born in Austin, Texas in 1996. She is currently pursuing a B.S. in Biochemistry at the University of Southern California. Under the guidance of the Williams lab, her research interests include studying the practical applications of transition metal catalysts. She plans to graduate in 2018 and pursue medical school. Ryan Trerise was born in Glendora, CA in 1995. He is currently an undergraduate at the University of Southern California and is on track to graduate with a B.S. in Human Biology in 2018. He plans to attend a Physician’s Assistant program in the future. Travis J. Williams was an undergraduate with Michael Richmond (University of North Texas) and Erick Carreira (Caltech, B.S. 1998) before completing a Ph.D. with Paul Wender at Stanford (2005). He returned to Caltech as a postdoc with John Bercaw before starting his independent career at USC in 2007. He still has not decided what he wants to do when he grows up.

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ACKNOWLEDGMENTS The work highlighted in this Account was sponsored by the National Science Foundation (CHE-1054910, CHE-1566167), the ACS Petroleum Research Fund (47987-G1), the Hydrocarbon Research Foundation, and USC.

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REFERENCES

(1) (a) Staubitz, A.; Robertson, A. P. M.; Manners, I. AmmoniaBorane and Related compounds as Hydrogen Donors. Chem. Rev. 2010, 110, 4079−4124. (b) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B-N Compounds for Chemical Hydrogen Storage. Chem. Soc. Rev. 2009, 38, 279−293. (2) (a) Nelson, D. J.; Truscott, B. J.; Egbert, J. D.; Nolan, S. P. Exploring the Limits of Catalytic Ammonia−Borane Dehydrogenation Using a Bis(N-heterocyclic carbene) Iridium(III) Complex. Organo93


Article

Accounts of Chemical Research (10) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalysts for Dehydrogenation of Ammonia-Borane for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844−1845. (11) (a) Kim, S.-K.; Han, W.-S.; Kim, T.-J.; Kim, T.-Y.; Nam, S. W.; Mitoraj, M.; Piecoś, Ł.; Michalak, A.; Hwang, S.-J.; Kang, S. O. Palladium Catalysts for Dehydrogenation of Ammonia Borane with Preferential B−H Activation. J. Am. Chem. Soc. 2010, 132, 9954−9955. (b) Kim, S.-K.; Hong, S.-A.; Son, H.-J.; Han, W.-S.; Michalak, A.; Kang, S. O. Dehydrogenation of Ammonia-Borane by Cationic Pd(II) and Ni(II) Complexes in a Nitromethane Medium: Hydrogen Release and Spent Fuel Characterization. Dalton Trans. 2015, 44, 7373−7381. (12) (a) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K. Ruthenium-Catalyzed Dehydrogenation of Ammonia Boranes. J. Am. Chem. Soc. 2008, 130, 14034−14035. (b) Kaβ, M.; Friedrich, A.; Drees, M.; Schneider, S. Ruthenium Complexes with Cooperative PNP Ligands: Bifunctional Catalysts for the Dehydrogenation of Ammonia−Borane. Angew. Chem., Int. Ed. 2009, 48, 905−907. (c) Rossin, A.; Rossi, A.; Peruzzini, M.; Zanobini, F. Chemical Hydrogen Storage: Ammonia Borane Dehydrogenation Catalyzed by NP3 Ruthenium Hydrides (NP3 = N(CH2CH2PPh2)3). ChemPlusChem 2014, 79, 1316−1325. (d) Boulho, C.; Djukic, J.-P. The Dehydrogenation of Ammonia−Borane Catalysed by Dicarbonylruthenacyclic(II) Complexes. Dalton Trans. 2010, 39, 8893−8905. (e) Staubitz, A.; Sloan, M. E.; Robertson, A. P. M.; Friedrich, A.; Schneider, S.; Gates, P. J.; Günne, J.; auf der, S.; Manners, I. Catalytic Dehydrocoupling/Dehydrogenation of NMethylamine-Borane and Ammonia-Borane: Synthesis and Characterization of High Molecular Weight Polyaminoboranes. J. Am. Chem. Soc. 2010, 132, 13332−13345. (13) Lu, Z.; Schweighauser, L.; Hausmann, H.; Wegner, H. A. MetalFree Ammonia−Borane Dehydrogenation Catalyzed by a Bis(borane) Lewis Acid. Angew. Chem., Int. Ed. 2015, 54, 15556−15559. (14) (a) Conley, B. L.; Williams, T. J. Dehydrogenation of AmmoniaBorane by Shvo’s Catalyst. Chem. Commun. 2010, 46, 4815−4817. (b) Conley, B. L.; Williams, T. J. Thermochemistry and Molecular Structure of a Remarkable Agostic Interaction in a Heterobifunctional Ruthenium-Boron Complex. J. Am. Chem. Soc. 2010, 132, 1764−1765. (c) Conley, B. L.; Williams, T. J. Dual Site Catalysts for Hydride Manipulation. Comments Inorg. Chem. 2011, 32, 195−218. (d) Lu, Z.; Conley, B. L.; Williams, T. J. A Three-Stage Mechanistic Model for Ammonia−Borane Dehydrogenation by Shvo’s Catalyst. Organometallics 2012, 31, 6705−6714. (e) Conley, B. L.; Williams, T. J. A Robust, Air-Stable, Reusable Ruthenium Catalyst for Dehydrogenation of Ammonia Borane. J. Am. Chem. Soc. 2011, 133, 14212−14215. (f) Lu, Z.; Williams, T. J. A Dual Site Catalyst for Mild, Selective Nitrile Reduction. Chem. Commun. 2014, 50, 5391−5393. (g) Zhang, X.; Lu, Z.; Foellmer, L. K.; Williams, T. J. Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst. Organometallics 2015, 34, 3732−3738. (h) Zhang, X.; Kam, L.; Williams, T. J. Dehydrogenation of Ammonia Borane through the Third Equivalent. Dalton Trans. 2016, 45, 7672−7677. (i) Lu, Z.; Malinoski, B.; Flores, A. V.; Guess, D.; Conley, B. L.; Williams, T. J. Alcohol Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium. Catalysts 2012, 2, 412−421. (15) CCDC 953038 (6), 1062142 (7), 738031 (12), 738030 (13), 830936 (15), 963291 (16), and 723730 (24) contain supplementary crystallographic data for the respective compounds. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (16) (a) Pons, V.; Baker, R. T.; Szymczak, N. K.; Hildebrant, D. J.; Linehan, J. C.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Coordination of Aminoborane, NH2BH2, Dictated Selectivity and Extent of H2 Release in Metal-catalysed Ammonia Borane Dehydrogenation. Chem. Commun. 2008, 12, 6597−6599. (b) Kalviri, H. A.; Gartner, F.; Ye, G.; Korobkov, I.; Baker, R. T. Probing the Second Dehydrogenation Step in Ammonia-borane Dehydrocoupling: Characterization and Reactivity of the Key Intermediate, B-(cyclotriborazanyl)amine-borane. Chem. Sci. 2015, 6, 618−624.

(17) Staubitz, A.; Sloan, M. E.; Robertson, A. P.; Friedrich, A.; Schneider, S.; Gates, P. J.; Schmedt auf der Günne, J.; Manners, I. Catalytic Dehydrocoupling/Dehydrogenation of N-MethylamineBorane and Ammonia-Borane: Synthesis and Characterization of High Molecular Weight Polyaminoboranes. J. Am. Chem. Soc. 2010, 132, 13332−13345. (18) Glüer, A.; Förster, M.; Celinski, V. R.; Schmedt auf der Günne, J.; Holthausen, M. C.; Schneider, S. Highly Active Iron Catalyst for Ammonia Borane Dehydrocoupling at Room Temperature. ACS Catal. 2015, 5, 7214−7217. (19) Shvo, Y.; Czarkie, D.; Rahamim, Y. A New Group of Ruthenium Complexes: Structure and Catalysis. J. Am. Chem. Soc. 1986, 108, 7400−7402. (20) Koren-Selfridge, L.; Query, I. P.; Hanson, J. A.; Isley, N. A.; Guzei, I. A.; Clark, T. B. Synthesis of Ruthenium Boryl Analogues of the Shvo Metal-Ligand Bifunctional Catalyst. Organometallics 2010, 29, 3896−3900. (21) Wright, W. R. H.; Berkeley, E. R.; Alden, L. R.; Baker, R. T.; Sneddon, L. G. Transition Metal Catalysed Ammonia-Borane Dehydrogenation in Ionic Liquids. Chem. Commun. 2011, 47, 3177− 3179. (22) Hollmann, D.; Jiao, H.; Spannenberg, A.; Bähn, S.; Tillack, A.; Parton, R.; Altink, R.; Beller, M. Deactivation of the Shvo Catalyst by Ammonia: Synthesis, Characterization, and Modeling. Organometallics 2009, 28, 473−479. (23) Widegren, J. A.; Finke, R. G. A Review of the Problem of Distinguishing True Homogeneous Catalysis from Soluble or Other Metal-Particle Heterogeneous Catalysis Under Reducing Conditions. J. Mol. Catal. A: Chem. 2003, 198, 317−341. (24) (a) Samec, J. S.; Éll, A. H.; Åberg, J. B.; Privalov, T.; Eriksson, L.; Bäckvall, J.-E. Mechanistic Study of Hydrogen Transfer to Imines from a Hydroxycyclopentadienyl Ruthenium Hydride. Experimental Support for a Mechanism Involving Coordination of Imine to Ruthenium Prior to Hydrogen Transfer. J. Am. Chem. Soc. 2006, 128, 14293−14305. (b) Casey, C. P.; Clark, T. B.; Guzei, I. A. Intramolecular Trapping of an Intermediate in the Reduction of Imines by a Hydroxycyclopentadienyl Ruthenium Hydride: Support for a Concerted Outer Sphere Mechanism. J. Am. Chem. Soc. 2007, 129, 11821−11827. (25) (a) Black, D.; Deacon, G.; Thomas, N. Ruthenium Carbonyl Complexes. I. Synthesis of [Ru(CO)2(bidentate)2]2+ Complexes. Aust. J. Chem. 1982, 35, 2445−2453. (b) Frediani, P.; Bianchi, M.; Salvini, A.; Guarducci, R.; Carluccio, L. C.; Piacenti, F. Ruthenium Carbonyl Carboxylates with Nitrogen-Containing Ligands: II. Synthesis and Characterization of Mononuclear Compounds. J. Organomet. Chem. 1994, 476, 7−11. (26) Frediani, P.; Bianchi, M.; Salvini, A.; Guarducci, R.; Carluccio, L. C.; Piacenti, F. Ruthenium Carbonyl Carboxylate Complexes with Nitrogen-Containing Ligands III. Catalytic Activity in Hydrogenation. J. Organomet. Chem. 1995, 498, 187−197. (27) Celaje, J. J. A.; Lu, Z.; Kedzie, E. A.; Terrile, N. J.; Lo, J. N.; Williams, T. J. A Prolific Catalyst for Dehydrogenation of Neat Formic Acid. Nat. Commun. 2016, 7, 11308. (28) Lu, Z.; Williams, T. J. Di(carbene)-Supported Nickel Systems for CO2 Reduction Under Ambient Conditions. ACS Catal. 2016, 6, 6670−6673. (29) Noyori, R.; Sandoval, C. A.; Muñiz, K.; Ohkuma, T. Metal− Ligand Bifunctional Catalysis for Asymmetric Hydrogenation. Philos. Trans. R. Soc., A 2005, 363, 901−912. (30) Casey, C. P.; Guan, H. Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes. J. Am. Chem. Soc. 2009, 131, 2499−2507. (31) Hodgkins, T. G.; Powell, D. R. Derivatives of the Dimethylbis(2-pyridyl)borate(1−) Ion: Synthesis and Structure. Inorg. Chem. 1996, 35, 2140−2148. (b) Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N. Facile Arene C−H Bond Activation and Alkane Dehydrogenation with Anionic LPtIIMe2- in Hydrocarbon−Water Systems (L = Dimethyldi(2-pyridyl)borate). J. Am. Chem. Soc. 2006, 128, 13054−13055. 94


Article

Accounts of Chemical Research (32) Pennington-Boggio, M. K.; Conley, B. L.; Williams, T. J. A Ruthenium-Catalyzed Coupling of Alkynes with 1,3-Diketones. J. Organomet. Chem. 2012, 716, 6−10. (33) Smurthwaite, T. D. In Situ 11B NMR Kinetic Study of the Dehydrogenation of Ammonia Borane in Solution. M.S. Dissertation, Washington State University, Pullman, WA, 2011. (34) Hu, M. G.; Van Paasschen, J. M.; Geanangel, R. A. New Synthetic Approaches to Ammonia-Borane and its Deuterated Derivatives. J. Inorg. Nucl. Chem. 1977, 39, 2147−2150. (35) Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Acid Initiation of Ammonia−Borane Dehydrogenation for Hydrogen Storage. Angew. Chem., Int. Ed. 2007, 46, 746−749. (36) Bhunya, S.; Roy, L.; Paul, A. Mechanistic Details of Ru− Bis(pyridyl)borate Complex Catalyzed Dehydrogenation of Ammonia−Borane: Role of the Pendant Boron Ligand in Catalysis. ACS Catal. 2016, 6, 4068−4080. (37) Williams, T. J.; Lu, Z. A Dual Site Catalyst for Mild, Selective Nitrile Reduction. U. S. Patent 9,403,753, August 2, 2016. (38) Rossin, A.; Peruzzini, M. Ammonia−Borane and Amine−Borane Dehydrogenation Mediated by Complex Metal Hydrides. Chem. Rev. 2016, 116, 8848−8872. (39) Mal, S. S.; Stephens, F. H.; Baker, R. T. Transition Metal Catalysed Dehydrogenation of Amine-Borane Fuel Blends. Chem. Commun. 2011, 47, 2922−2924. (40) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy; Wiley-VCH, 2006.

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