In Situ Formed Catalytically Active Ruthenium Nanocatalyst in Room

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Letter pubs.acs.org/Langmuir

In Situ Formed Catalytically Active Ruthenium Nanocatalyst in Room Temperature Dehydrogenation/Dehydrocoupling of AmmoniaBorane from Ru(cod)(cot) Precatalyst Mehmet Zahmakiran,*,†,‡ Tuğcȩ Ayvalı,‡,§ and Karine Philippot‡,§ †

Department of Chemistry, Yüzüncü Yıl University, 65080, Van, Turkey CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, BP44099, F-31077 Toulouse, Cedex 4, France § Université de Toulouse, UPS, INPT, F-31077 Toulouse, Cedex 4, France ‡

S Supporting Information *

ABSTRACT: The development of simply prepared and effective catalytic materials for dehydrocoupling/dehydrogenation of ammonia-borane (AB; NH3BH3) under mild conditions remains a challenge in the field of hydrogen economy and material science. Reported herein is the discovery of in situ generated ruthenium nanocatalyst as a new catalytic system for this important reaction. They are formed in situ during the dehydrogenation of AB in THF at 25 °C in the absence of any stabilizing agent starting with homogeneous Ru(cod)(cot) precatalyst (cod = 1,5-η2cyclooctadiene; cot = 1,3,5-η3-cyclooctatriene). The preliminary characterization of the reaction solutions and the products was done by using ICPOES, ATR-IR, TEM, XPS, ZC-TEM, GC, EA, and 11B, 15N, and 1H NMR, which reveal that ruthenium nanocatalyst is generated in situ during the dehydrogenation of AB from homogeneous Ru(cod)(cot) precatalyst and B−N polymers formed at the initial stage of the catalytic reaction take part in the stabilization of this ruthenium nanocatalyst. Moreover, following the recently updated approach (Bayram, E.; et al. J. Am. Chem. Soc. 2011, 133, 18889) by performing Hg(0), CS2 poisoning experiments, nanofiltration, time-dependent TEM analyses, and kinetic investigation of active catalyst formation to distinguish single metal or in the present case subnanometer Run cluster-based catalysis from polymetallic Ru(0)n nanoparticle catalysis reveals that in situ formed Run clusters (not Ru(0)n nanoparticles) are kinetically dominant catalytically active species in our catalytic system. The resulting ruthenium catalyst provides 120 total turnovers over 5 h with an initial turnover frequency (TOF) value of 35 h−1 at room temperature with the generation of more than 1.0 equiv H2 at the complete conversion of AB to polyaminoborane (PAB; [NH2BH2]n) and polyborazylene (PB; [NHBH]n) units.

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INTRODUCTION Over the past decades, there has been rapidly growing interest in the catalytic dehydrocoupling/dehydrogenation of ammoniaborane1 and amine-borane adducts,2 because of the potential use of these materials in the chemical hydrogen storage.1,2 Among these materials, ammonia-borane (NH3BH3, AB) appears to be the most promising solid hydrogen carrier due to its high stoichiometric hydrogen content (19.6 wt %),3 which is greater than the 2015 target of the U.S. Department of Energy (9 wt % hydrogen for a material to be practically applicable).1−3 Moreover, the recent Science paper4 has described that the effective regeneration of AB is possible through a chemical route under mild conditions,4 which amplifies the importance of the catalytic dehydrogenation of AB. To this date, many transition metals in various catalytic systems have been tested in the catalytic dehydrogenation of AB (please see Table S-1 for the detailed list of catalyst systems used in the dehydrogenation/dehydrocoupling of AB).5−13 Although homogeneous catalysts5−12 have provided more notable catalytic activities than heterogeneous ones,13 the difficulties met during their laborious synthesis protocols and/ © 2012 American Chemical Society

or isolation−recovery hinder their practical use. At this point, the development of simply prepared catalytic materials that provide notable activity under mild conditions with painless product−catalyst separation remains a challenge for this important catalytic reaction. In this letter, we report a new active nanocatalyst formed in situ during the dehydrogenation of AB starting with homogeneous Ru(cod)(cot) (cod = 1,5-η2-cyclooctadiene; cot = 1,3,5-η3-cyclooctatriene) precatalyst in THF at room temperature that provides notable activity (120 total turnovers over 5 h with an initial TOF = 35 h−1) with the generation of more than 1.0 equiv of H2 (see Supporting Information for details). The preliminary characterization of both the catalytically active solution and the solid material isolated at the end of the reaction were done by ICP-OES, ATR-IR, XPS, TEM, ZCTEM, GC, EA, and 15N, 11B, and 1H NMR analyses. The sum of their results clearly evidence (i) the in situ generation of Received: December 14, 2011 Revised: February 21, 2012 Published: February 22, 2012 4908

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ruthenium nanocatalyst from the reduction of Ru(cod)(cot) by H2 generated from the concomitant dehydrogenation of AB, and (ii) the contribution of polyaminoboranes as dehydrogenation product to the stabilization of ruthenium nanocatalyst. Moreover, the results obtained from Hg(0), CS2 poisoning experiments, nanofiltration, time-dependent TEM monitoring of catalytically active reaction solution, and kinetic investigation of active catalyst formation/AB-dehydrogenation indicate that in situ formed polyaminoborane stabilized Run clusters existing in the resulting ruthenium nanocatalyst act as kinetically dominant catalytically active species in the dehydrogenation of AB. To the best of our knowledge, this is the first report of a kinetically competent ruthenium (Run) cluster based nanocatalyst system formed in situ during the dehydrogenation of AB without requiring any additional stabilizing agent.14

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RESULTS AND DISCUSSION In Situ Formation of Catalytically Active Nano-Sized Ruthenium Species during the Catalytic Dehydrogenation of Ammonia-Borane Starting with Ru(cod)(cot) Precatalyst; Kinetic Evidence and TEM Monitoring. When THF solutions of Ru(cod)(cot) (6.0 mol %) and AB were mixed at room temperature under an atmosphere of argon, vigorous hydrogen evolution was observed after a short induction time period (∼10 min), and concurrent with this, the yellow color of reaction solution gradually changed to darkbrown. All these observations indicate the formation of an active catalyst under in situ conditions. Ru(cod)(cot) being a well-known precursor for the synthesis of RuNPs under dihydrogen in mild conditions,15 the formation of nanoparticles could be expected if dihydrogen is present. Ruthenium metal is in its zerovalent oxidation state in Ru(cod)(cot) precatalyst; thus, the short induction time period observed at the initial stage of the reaction is probably required for the dihydrogen reduction of the unsaturated cyclooctadiene (cod) and cyclooctatriene (cot) ligands. Expectedly, the gas chromatography (GC) analysis of the reaction solutions at the end of the induction time period confirmed the presence of cyclooctane (at a level of 92% of the initial cyclooctadiene (cod) and cyclooctatriene (cot) present) resulting from the reduction of Ru(cod)(cot) ligands by dihydrogen produced from the dehydrogenation of AB. The progress of active catalyst formation and concomitant dehydrogenation of AB were first followed by monitoring the change in amount of hydrogen evolved. Figure 1 corresponds to the plot of the number of equivalents of H2 evolved versus time for the dehydrogenation of AB starting with Ru(cod)(cot) precatalyst in THF at 25 ± 0.1 °C. The sigmoidal kinetics observed in Figure 1 fits well to the Finke-Watzky two-step in situ active catalyst formation mechanism;16 A → B (rate constant k1) and A + B → 2B (rate constant k2). The rate constants determined from the nonlinear least-squares curve fit in Figure 1 are k1 = 7.0 × 10−3 min−1 and k2 = 0.21 M−1 min−1, in which the mathematically required correction has been made to k2 for the stoichiometry factor of 15.6, as described elsewhere.17 This is an evidence for the starting complex A = Ru(cod)(cot) is not the true catalyst but, instead, is a precatalyst en route to the catalytically active ruthenium species “B”.18,19 The in situ formation of active catalyst from Ru(cod)(cot) precatalyst during the dehydrogenation of AB was also investigated by time-dependent TEM analyses. This study was started by taking TEM images of a THF solution of

Figure 1. Equivalency of H2 evolved vs time (min) graph for the dehydrogenation of AB (0.5 mmol) starting with Ru(cod)(cot) precatalyst (0.032 mmol) in THF (2.0 mL) at 25 ± 0.1 °C and its curve fit to Finke-Watzky two step nanoparticle formation kinetics; k1 = 7.0 × 10−3 min−1 and k2 = 0.21 M−1 min−1 with R2 = 0.99.

Ru(cod)(cot) precatalyst on account of the previous results reported from Finke18 and Manners20 groups who reported that the TEM beam can induce the formation of rhodium nanoparticles from soluble homogeneous rhodium precatalysts. TEM analysis of the Ru(cod)(cot) solution in THF failed to show any soluble nanoparticles; as shown in Figure 2a, only micrometer-sized particles were observed (see additional images given in Figure SI-1) even if the sample was exposed to a 100 kV electron beam over 10 min. However, under the same conditions (Vacc = 100 kV; texposure = 10 min), the TEM image (Figure 2b; see additional images given in Figure SI-2) of the sample harvested from the solution after 40 min in the dehydrogenation reaction started with AB and Ru(cod)(cot) in THF depicts the absence of microsized particles. Moreover, TEM image of the same sample at higher magnification (Figure 2c) reveals the presence of well-dispersed and nanosized ruthenium particles in the range 0.6−2.2 nm (Figure 2d) with a mean particle size of 1.42 ± 0.72 nm. These results demonstrate that ruthenium nanocatalyst is formed in situ during the dehydrogenation of AB starting with Ru(cod)(cot) precatalyst in THF at room temperature. Characterization of In Situ Generated Nano-Sized Ruthenium Species and the Dehydrogenation Products. The progress of the catalytic dehydrogenation of AB in THF at room temperature was also monitored by 11B NMR spectroscopy. Figure 3 depicts 11B-{1H}-NMR spectra of the reaction solution recorded at different reaction times. It supports the catalytic dehydrogenation pathway of AB (δ = −23 ppm)3,21 involving first the intermolecular dehydrocoupling reaction between hydridic B−H hydrogens and protonic N−H hydrogens of AB to form polyaminoborane polymers (PAB; δ = −8 and −15 ppm)7,21 and then further dehydrogenation of polyaminoborane units to unsaturated (BN) polyborazylene (PB; δ = 22−28 ppm).3b,22 Under our reaction conditions, the formation of borazine (δ = 30 ppm) was not detected. This is an important result as the borazine is a volatile compound and potential poison for fuel cell catalysts. The formation of polyborazylene units from the complete dehydrogenation of PAB normally produces more than 2 equiv of H2 in the dehydrogenation of AB (Scheme 1).3,21 In our case, 1.54 equiv 4909

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Figure 2. (a) TEM image of Ru(cod)(cot) precatalyst solution in THF (scale bar = 250 nm) indicating the existence of micrometer-sized particles; (b) and (c) TEM images of reaction solution (at different magnifications scale bars = 250 and 20 nm, respectively) harvested from the solution after 40 min in the dehydrogenation of AB started with Ru(cod)(cot) in THF; (d) the size histogram of ruthenium nanoparticles measured from TEM image given in (c) by counting ≥150 nontouching particles.

cyclooctatriene ligands, which requires 0.32 × 0.92 = 0.29 equiv of H2 under our reaction conditions. Therefore, the total amount of hydrogen evolved is equal to 1.83 equiv of H2 per AB. The observation of less than 2 equiv of H2 generation at the complete consumption of AB implies that all PAB units do not undergo further dehydrogenation to PB. It should also be noted that, as shown in 11B-{1H}-NMR spectra of the reaction solution (Figure 3), there was no change in the intensity of PB resonance signals even if PAB signals gradually disappeared during the reaction. In this context, TEM, ZC-TEM, 11B NMR, ICP-OES, ATR-IR spectroscopy, and elemental analysis were performed to gain more information about the reaction solution and the isolated material at the end of the dehydrogenation of AB. Figure 4 shows TEM and zero contrast (ZC)-TEM images of the sample harvested at the end of the dehydrogenation of AB, which clearly indicate the formation of ruthenium aggregates (∼60 nm) encapsulated in a layer with ∼16 nm thickness (see additional images given in Figure SI-3). 11B NMR spectrum of THF/CH2Cl2 (1:1) washings of the bulk material resulting from the dehydrogenation of AB (Figure SI-4) showed the weak resonance signals at −8 and −14 ppm that can be assigned to PAB.3,21 In addition to that, ICP-OES and elemental analyses of the same solid indicated the existence of B and N elements (in 0.92:1.05 ratio)

of H2 generation was quantified, but as aforementioned, GC analyses showed 92% of hydrogenation of cyclooctadiene and

Figure 3. 11B-{1H}-NMR spectra of the reaction solution taken at different reaction times during the dehydrogenation of AB (0.5 mmol) starting with Ru(cod)(cot) precatalyst (0.032 mmol) in THF at 25 ± 0.1 °C. 4910

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Scheme 1. Possible Pathway for the in Situ Generated Ruthenium Nanoparticles Catalyzed Dehydrogenation of AB (1) Starting with Ru(cod)(cot) Precatalyst, which Produces PAB (2), PB (3), and 1.78 equiv of H2

Figure 4. (a,b) TEM images in different magnifications (scale bars = 500 and 250 nm, respectively); (c) (zero contrast) ZC-TEM image of the same region given in Figure 2b (scale bar = 250 nm); (d) ZC-TEM image of the region indicated in Figure 2c (scale bars = 80 nm) for the reaction solution harvested at the end of the dehydrogenation of AB (0.5 mmol) starting with Ru(cod)(cot) precatalyst (0.032 mmol) in THF at 25 ± 0.1 °C.

also explain the generation of less than 2 equiv of H2. Nevertheless, these PAB units as a sole stabilizing agent do not provide sufficient stabilization to these in situ generated ruthenium nanocatalyst for reusability, as they agglomerated and precipitated out of the solution as bulk ruthenium within a short time period (5−6 h). Initial Studies for the Identification of Kinetically Competent Catalyst: Is it Ru(0) n Nanoclusters or Subnanometer Run Cluster-Based Ammonia-Borane Dehydrogenation Catalysis? In a recent solid work, Finke et al.26 reported an updated approach to distinguish singlemetal homogeneous catalysis from polymetallic heterogeneous catalysis. They show that the true catalyst formed in situ during

with Ru. ATR-IR spectrum of this bulk material (Figure SI-5) shows two significant absorption bands in the region 1600− 4000 cm−1 centered at 3244 cm−1 and 2354 cm−1 respectively, which can be assigned to N−H and B−H stretching in PAB.23 Additionally, 15N-MAS NMR spectrum of the same sample shows a signal around −350 pm, which can be assignable to polyaminoborane.24 Moreover, XPS survey scan of the same solid sample gives two prominent signals of N 1s and B 1s at 400 and 191 eV,25 respectively, that exist in polyaminoboranes (Figure SI-6). All these results indicate the presence of PAB around the in situ generated ruthenium nanocatalyst. The interaction of PAB units with the ruthenium nanoparticles may prevent their further dehydrogenation into PB, which would 4911

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analysis of the linear portion of the relative rate vs equivs of CS2 plot. That plot yielded Xintercept = 2.0, implying that the amount of poison required to totally poison the active ruthenium catalyst is ≥2.0 equiv of CS2 per Ru(total). This finding strongly supports that subnanometer Run clusters existing in the in situ generated ruthenium nanocatalyst act as kinetically dominant catalytically active species.26 Initial Kinetic Studies, Reusability, and Catalytic Lifetime Experiments. In order to establish the rate law of the catalytic dehydrogenation of AB starting with Ru(cod)(cot) precatalyst, we performed two different sets of experiments. In the first set, the concentration of AB was held constant (0.25 M) and the precatalyst concentration was varied in the range of 16−25 mM ([Ru(cod)(cot) = 15.8, 19.0, 22.2, and 25 mM) at 25.0 ± 0.1 °C. A fast dehydrogenation starts after an induction time of 10−16 min. The dehydrogenation rate, determined from the nearly linear portion of the plots, increases with the catalyst concentration (0.017, 0.033, 0.045, and 0.050 equiv. H2/min for [Ru] = 15.8, 19.0, 22.2, and 25 mM, respectively). Plotting the dehydrogenation rate versus ruthenium concentration (both on logarithmic scales) gives a straight line with a slope of 2.38 ± 0.43 (Figure SI-10). The effect of the substrate concentration on the dehydrogenation rate was also studied by performing a series of experiments starting with variation of the initial concentration of AB (0.25, 0.50, 1.0, and 2.0 M) while keeping the precatalyst concentration (25 mM) constant at 25.0 ± 0.1 °C. The initial dehydrogenation rates were found to be 0.05, 0.52, 0.56, and 0.55 equiv H2/min for [AB] = 0.25, 0.50, 1.0, and 2.0 M. Plotting the dehydrogenation rate versus AB concentration (both on logarithmic scales) shows that, in a substrate concentration ≥0.50 M, the catalytic dehydrogenation of AB appears to be zero-order in the substrate concentration, while at lower substrate concentrations, one observes a firstorder dependence (Figure SI-11). As previously indicated, the ruthenium nanocatalyst formed in situ during the dehydrogenation of AB starting with Ru(cod)(cot) precipitates out of the reaction solution after ∼5−6 h. To shed some light on the reusability of this ruthenium-containing precipitate, it was used as catalyst in the dehydrogenation of the same amount of fresh AB, but it showed only negligible activity (0.27 equiv of H2 generation with a TOF value of 3.8 h−1) within the same period of time in the dehydrogenation of AB (Figure SI-12). A catalyst lifetime experiment starting with 0.032 mmol of Ru(cod)(cot) and 1.0 mol of AB at 25 ± 0.1 °C reveals a total turnover (TTO) value of 120 in the dehydrogenation of AB over 5 h before deactivation by aggregation into bulk ruthenium occurs. An initial TOF value of 35 h−1 was obtained; however, the average TOF value was calculated to be 24 h−1. The decrease in TOF value as the reaction proceeds indicates the deactivation of the ruthenium nanocatalyst, evidenced also by the precipitation of solid material at the end of the experiment. This initial activity value (35 h−1 at 25 °C) is higher than that obtained with [Rh(1,5-cod)(μ-Cl)]2 (3.3 h−1 at 40 °C),5 Ni(Enders’ NHC)2 (10 h−1 at 60 °C),7 [η5-C5H3-1,3(SiMe3)2)2Ti]2 (μ2-N2) (0.23 h−1 at 65 °C),9 Shvo catalyst (16 h−1 at 70 °C)10 and [RuH2(η2-H2)2(PCy3)2] (23 h−1 at 25 °C) 11 catalysts but still lower than the current best homogeneous [Pd(MeCN)4][BF4] (2660 h−1 at 25 °C)12 and heterogeneous Rh(0) NPs (340 h−1)13 catalysts employed in the dehydrogenation of AB (see Table-S1).

the hydrogenation of benzene starting with [RhCp*Cl2]2 is subnanometer Rh 4 clusters with average stoichiometry “Rh4Cp*2.4Cl4Hc”; even the formation of active catalyst and concomitant hydrogenation profile followed the sigmoidal kinetics and active catalyst can be killed by Hg(0) addition. Moreover, in a separate study Linehan et al.27 demonstrated that in situ formed amine-borane stabilized Rh4−6 clusters formed from [Rh(cod)Cl]2 precatalyst during the dehydrogenation/dehydrocoupling of amine boranes (NH3BH3 and (CH3)2NHBH3) are kinetically competent catalysts by using EXAFS analyses although ex situ characterization suggested a much larger colloidal metallic rhodium species.20 Therefore, we adopted the similar approach in our study to distinguish kinetically competent catalyst (subnanometer-Run clusters or polymetallic Ru(0)n nanoparticles) formed in situ during the dehydrogenation of AB starting with Ru(cod)(cot) precatalyst. Our initial results obtained from the time-dependent TEM monitoring of catalytically active reaction solution and kinetic investigation of active catalyst formation/AB-dehydrogenation indicate that in situ formed nanosized ruthenium species act as active catalyst. Additionally, we found that the addition of excess Hg(0) (850 equiv per Ru) into the reaction solution after an induction time period under argon atmosphere completely quenched reactivity of active catalyst (Figure SI7). In addition, we performed filtration experiments to investigate the possibility of any homogeneous complex that can contribute to the observed activity in the dehydrogenation of AB. For this purpose, the reaction solution obtained after the formation of bulk ruthenium was filtered through a micropore filter (200 nm) to remove any traces of bulk ruthenium metal. Then, the catalytic activity of the filtrate was tested in the dehydrogenation of AB by the addition of fresh substrate, but no activity in the dehydrogenation of AB was observed, thus evidencing the absence of any active homogeneous catalyst under our reaction conditions. However, the sum of these results is still not enough to distinguish whether subnanometer Run clusters or Ru(0)n nanoparticles act as catalytically active species.26,27 In this context, it has recently been suggested that26 the quantitative poisoning experiments with ligands such as CS2, PPh3, 1,10-phenanthroline, and thiophene are a strong tool to identify true catalyst apart from operando EXAFS analysis.26,27 For this reason, we performed a series of carbon disulfide (CS2)28 poisoning experiments as detailed in the Supporting Information. The catalytic activities were determined as a function of the addition of 0.1, 0.2, 0.5, 0.7, 0.9, and 2.0 equiv of CS2 (per total Ru). In each of these experiments, carbon disulfide was added after ∼40% of conversion was achieved to effect fully evolved sample of catalyst. It should be noted that the catalytic activity did not change within the experimental error upon the addition of 0.1 and 0.2 equiv of CS2 per Ru(total) present (initial dehydrogenation rate = 0.0342, 0.0341, and 0.0340 mL H2/min for CS2/Ru = 0, 0.1, and 0.2, respectively). However, the addition of 0.5, 0.7, 0.9, and 2.0 equiv of CS2 per Ru(total) present did drastically slow the initial catalytic activity from 0.0342 mL H2/min to 0.0276, 0.0182, 0.122, and 0.0044 mL H2/min, respectively (see Figure SI-8). The relative rates (initial activity in the presence of poison over the one in the absence of poison) were determined for each experiment and given as relative rate vs equiv of CS2 plot in Figure SI-9, which reveals that the addition of 2.0 equiv of CS2 per Ru(total) present kills most, ca. 87%, of the catalyst’s activity. As it is regular for such quantitative poisoning plots,19,28 an intercept (Xintercept) was calculated from a linear regression 4912

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CONCLUSIONS The primary findings and conclusions from this work can be summarized as follows: (i) For the first time, ruthenium nanocatalyst was simply and reproducibly prepared in situ during the catalytic dehydrocoupling/dehydrogenation of AB starting with Ru(cod)(cot) precatalyst at room temperature. (ii) The advanced characterization of the resulting ruthenium nanoparticles by a combination of techniques (ICP-OES, ATRIR, XPS, TEM, ZC-TEM, GC, EA, and 15N, 11B, and 1H NMR) as well as gas chromatography and elemental analyses revealed the formation of nanosized ruthenium species, and PAB formed at the initial stage of the dehydrogenation of AB takes part in the stabilization of these in situ generated ruthenium nanocatalyst system. (iii) One can simply, however, generate catalytically active nanosized ruthenium clusters in situ during the dehydrogenation of AB starting with Ru(cod)(cot). The resultant catalyst provides conversion of AB to PAB and PB units with a quite active (initial TOF = 35 h−1) lifetime (TTO = 120) in the dehydrogenation of AB in THF even at room temperature. (iv) It was shown that monitoring the hydrogen evolution in the dehydrogenation of AB provides an indirect route to follow the formation of active ruthenium nanocatalyst from homogeneous Ru(cod)(cot) precatalyst. (v) Our initial attempts using quantitative CS2 poisoning experiments to distinguish whether Run subnanometer clusters or Ru(0)n nanoclusters are the true catalyst show that coordinatively unsaturated Run clusters act as kinetically competent catalyst for the dehydrogenation of AB starting with Ru(cod)(cot) precatalyst. (vi) The stabilization of these in situ formed and short-lived ruthenium nanocatalyst by using hexadecylamine (CH3(CH2)15NH2) gave rise to more stable colloids in THF. However, their activity (TOF = 3.5 h−1) appeared 10-fold lower than that of ligand-free ruthenium nanocatalyst in the dehydrogenation of AB, as the surface coverage by hexadecylamine decreases their accessibility by substrate.29 Presently, two separate studies related to this work are in progress to (i) clarify the exact structure of catalytically active Run clusters by EXAFS analysis and (ii) increase the stability of the resulting nanocatalyst system by using ligands or hydrophobic solid frameworks. They will be the subject of another article.

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Ru(cod)(cot) precatalyst (1) and bulk ruthenium (2) isolated at the end of the reaction (Figure SI-12). Catalyst systems and conditions employed in the catalytic dehydrogenation/dehydrocoupling of ammonia-borane, tabulated from SciFinder literature search (Table S-1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the CNRS and Idecat REX project for financial support and to UPS-TEMSCAN for TEM facilities. M.Z. thanks the Yüzüncü Yıl University for allowing him to conduct research in CNRS/LCC (France).

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REFERENCES

(1) Staubitz, A.; Robertson, A. P. M; Manners, I. Ammonia-Borane and Related Compounds as Dihydrogen Sources. Chem. Rev. 2010, 110, 4079. (2) Staubitz, A.; Robertson, A. P. M; Sloan, M. E.; Manners, I. Amine− and Phosphine−Borane Adducts: New Interest in Old Molecules. Chem. Rev. 2010, 110, 4023. (3) Stephens, F. H.; Pons, V.; Baker, R. T. Ammonia−borane: the Hydrogen Source par Excellence? Dalton Trans. 2007, 25, 2613. (4) 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. (5) 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. (6) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. Efficient Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128, 12048. (7) Blacquiere, 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. (8) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalyzed Dehydrogenation of Ammonia−Borane for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844. (9) Pun, D.; Lobkovsky, E.; Chirik, P. J. Amineborane Dehydrogenation Promoted by Isolable Zirconium Sandwich, Titanium Sandwich and N2 Complexes. Chem. Commun. 2007, 3297. (10) Conley, B. L.; Williams, T. J. Dehydrogenation of AmmoniaBorane by Shvo’s Catalyst. Chem. Commun. 2010, 46, 4815. (11) Alcaraz, G.; Vendier, L.; Clot, E.; Sabo-Etienne, S. Ruthenium bis(B-H) Aminoborane Complexes from Dehydrogenation of AmineBoranes: Trapping of H2B-NH2. Angew. Chem., Int. Ed. 2010, 49, 918. (12) Kim, S.-K.; Han, W.-S.; Kim, T.-J.; Kim, T.-Y.; Nam, S. W.; Mitoraj, M.; Piekos, L.; 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. (13) Ayvalı, T.; Zahmakıran, M.; Ö zkar, S. One-Pot Synthesis of Colloidally Robust Rhodium(0) Nanoparticles and Their Catalytic Activity in the Dehydrogenation of Ammonia-Borane for Chemical Hydrogen Storage. Dalton Trans. 2011, 40, 3584. (14) Zahmakıran, M.; Ö zkar, S. Metal Nanoparticles in Liquid Phase Catalysis; from Recent Advances to Future Goals. Nanoscale 2011, 3, 3462. (15) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante, P.; Casanove, M.-J. Ligand-Stabilized Ruthenium Nano-

ASSOCIATED CONTENT

S Supporting Information *

Experimental section and additional TEM images of Ru(cod)(cot) (Figure SI-1), in situ generated ruthenium nanoparticles (Figure SI-2), the reaction solution at the end of the dehydrogenation of ammonia-borane starting with Ru(cod)(cot) (Figure SI-3), 11B-{1H}-NMR spectrum of the THF/ CH2Cl2 washing of black bulk material isolated at the end of the dehydrogenation of AB (Figure SI-4), ATR-IR spectrum of black bulk material isolated at the end of the dehydrogenation of AB (Figure SI-5), N 1s and B 1s XPS spectra of the bulk material isolated at the end of the reaction (Figure SI-6), Hg(0) poisoning of active catalyst formed in situ during the dehydrogenation of AB (Figure SI-7), volume of hydrogen (mL) vs time (min) plot for CS2 poisoning experiments (Figure SI-8), relative rate vs equiv of CS2 plot for CS2 poisoning experiments (Figure SI-9), ln kobs vs ln [Ru] plot (Figure SI-10), ln kobs vs ln [Ru] plot (Figure SI-11), plot of equiv of H2 evolved versus time for the catalytic dehydrogenation of AB (0.5 mmol) in THF at 25 ± 0.1 °C starting with 4913

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dx.doi.org/10.1021/la2049162 | Langmuir 2012, 28, 4908−4914