Catalyzed Dehydrocoupling and Hydrolysis of Amine-Boranes

Research Article pubs.acs.org/acscatalysis

Experimental Evidence Supporting Related Mechanisms for Ru(II)Catalyzed Dehydrocoupling and Hydrolysis of Amine-Boranes Ainara Telleria,† Cristian Vicent,‡ Virginia San Nacianceno,† María A. Garralda,† and Zoraida Freixa*,†,§ †

Department of Applied Chemistry, Faculty of Chemistry, University of the Basque Country (UPV-EHU), San Sebastián, 20018, Spain ‡ Serveis Centrals d’Instrumentació Científica, Universidad Jaume I, Castelló, 12071, Spain § IKERBASQUE, Basque Foundation for Science, Bilbao, 48013, Spain S Supporting Information *

ABSTRACT: A family of ruthenium(II) half-sandwich complexes was tested for the hydrolytic decomposition of amine-boranes. The analysis of the catalytic results, together with a multilateral approach based on 1H, 11B NMR, and ESIMS were used to propose a plausible and conceptually unified mechanism for both the hydrolysis and competitive dehydrogenation of amine-boranes. We propose the intermediacy of solvent-stabilized borenium cations during the catalytic cycle, evolving toward dehydrogenation products in distilled THF or releasing amine-hydroxyboranes in aqueous media. Both reaction pathways would liberate up to 1 equivalent of hydrogen through a metal-catalyzed process, but an out-ofcycle low-barrier hydrolysis of amine-hydroxyboranes would produce the 2 additional equivalents of hydrogen in aqueous solutions. Metal-catalyzed deuteration of (non hydrogen-productive) trisubstituted amine-boranes by using D2O as deuterium source was observed, and included as part of the mechanism proposal. KEYWORDS: hydrogen generation, catalysis, deuteration, mechanism, amine-boranes

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INTRODUCTION Amine-borane adducts have been widely studied as potential hydrogen storage materials, due to their stability, low molecular weight and the presence of neighboring protic N−H and hydridic B−H hydrogen atoms. Ammonia-borane (AB) is the simplest representative of the class and contains the highest gravimetric hydrogen density (19.6 wt %).1−7 Initially, thermal evolution of hydrogen from AB was pursued through various approaches (i.e., nanoconfinement, catalysis, dispersion in organic solvents or ionic liquids, etc.), but the required temperatures were too high to make the process viable.8 The discovery by Manners et al. of a metal-catalyzed process for the dehydrocoupling of amine-boranes, using Rh(I) or Rh(III) precatalyts established a milestone in the area.9 Although the initial interest was on the oligomeric compounds formed, it soon shifted toward the liberated hydrogen. From then, many studies have been devoted to the development of metalcatalysts for a controlled hydrogen liberation from amineborane adducts based on both, homogeneous and heterogeneous catalysts.1,3,8,10 Apart from thermal decomposition, two different catalytic routes have been investigated for hydrogen evolution: dehydrocoupling and solvolysis in protic solvents (see Scheme 1). The former has been widely studied; it is promoted by several metallic nanoparticles and metallic clusters11−13 as well as by discrete and well-defined organo© 2017 American Chemical Society

Scheme 1. Dehydrocoupling and Solvolytic Routes for the Metal-Catalyzed Hydrogen Generation from ABa

a

Solvolytic route has been exemplified for H2O (4) and CH3OH (5).

metallic complexes based on different metals (i.e. Rh,14,15 Ru,16−18 Ir,19−21 Os,22 Fe,13,23 Pt,24 Ni,25 or Pd.26) Most of these systems generate only 1 equivalent of hydrogen per mole of AB (according to eq 1 in Scheme 1), and only few are able to promote further dehydrogenation releasing up to 2.8 hydrogen equivalents.18,25,27−30 Compared to the dehydrocoupling, the Received: August 30, 2017 Revised: October 26, 2017 Published: October 26, 2017 8394

DOI: 10.1021/acscatal.7b02958 ACS Catal. 2017, 7, 8394−8405

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Scheme 2. Synthetic Route toward Complexes 1−10a

solvolytic route presents the advantage that generally the hydrogen evolved is close to the maximum 3 equivalents expected (even when mono- and di-N-substituted amineborane are used as substrates!). Formally, the liberated gas originates from solvolysis of the BH3 fragment, and not from the coupling of protic and hydridic hydrogen atoms of the substrate (eqs 4 and 5 in Scheme 1). In the case of a hydrolytic route the total H2 evolved corresponds to 8.9 wt % of the starting reactants (2 × H2O and H3N·BH3).31 Although many heterogeneous metal-based catalysts are known to render very active systems for the solvolytic reaction,4,5,31−45 the homogeneously catalyzed process has been much less studied. The first reports on homogeneous metal-based catalysts for the solvolytic dehydrogenation of AB were published in 2010 based on Ir(III) and Ru(II) complexes.46−48 Since then, other examples of organometallic complexes active for this reaction appeared in the literature.49−55 In contrast with the overwhelming number of mechanistic studies on the homogeneous metal-catalyzed hydrogen generation by dehydrocoupling of amine-borane adducts,10,56 to the best of our knowledge, the mechanism operating in the less-explored homogeneous hydrolysis of such substrates has never been studied. It is generally accepted that it proceeds through metal-catalyzed cleavage of the amine-borane, and a much faster (noncatalyzed) hydrolysis of the liberated BH3 in the aqueous reaction media.52 The latter being actually the process responsible for the generation of the 3 equivalents of hydrogen per mole of substrate. This reaction pathway is analog to one originally postulated for the heterogeneously catalyzed hydrolysis of AB.32,33,45 Additionally, some recent reports on the heterogeneously catalyzed process also point to other plausible mechanisms:8 a direct reaction facilitated by the metal surface forming BH3OH− (and NH4+), which is further hydrolyzed,57 the attack of a H2O molecule on a metalactivated AB forming BH2(OH), NH3 and a transient M−H bond,58,59 or more recently the metal dissociative cleavage of one B−H bond in AB forming M−H and an activated H3N· BH2−M, which is hydrolyzed in a stepwise manner through H3N·B(OH)H2, H3N·B(OH)2H, and H3N·B(OH)3 species with the concomitant hydrogen release.42,44 It is worth mentioning that in some homogeneous and heterogeneous systems originally developed for AB hydrolysis, dehydrocoupling mechanisms were also operative (in the absence of water).32,33,45,47 Herein, we present a detailed mechanistic study on the hydrolysis of AB using a family of readily available and robust (no nitrogen protection needed) precatalysts of the type [Ru(p-Cym)(Ln)Cl]Cl (p-Cym = para-cymene, Ln = 1−10, see Scheme 2). A set of systematic catalytic conditions were tested, and a multilateral approach based on 1H, 11B NMR, and ESI-MS was used to propose a plausible and conceptually unified mechanism for both the hydrolysis and competitive dehydrogenation of amine-boranes using ruthenium(II) catalysts. This mechanism opposes the one generally accepted for the solvolytic route, but it is consistent with the observed reactivity and allowed us to rationalize some (until now unexplained) observations. Namely, the lack of reactivity of trisubstituted amine-boranes through metal-catalyzed solvolytic processes,46,50 and the competitive dehydrogenation occurring when dimethylamine-borane (DMAB) was used as a substrate were rationalized.50 Additionally, the intermediacy of solventstabilized borenium cations as key intermediates in both

a (i) acetone, CH2Cl2 or EtOH. Yields: 1 63%; 2, 92%; 3, 71%; 4, 77%; 5, 85%; 6, 93%; 7, 78%; 8, 53%; 9, 60%; 10, 49%.

processes, opens the door for further studies on new catalytic possibilities. In this context, metal-catalyzed deuteration of trisubstituted amine-boranes using D2O as the deuterium source was also demonstrated.

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RESULTS AND DISCUSSION Syntheses and Catalytic Experiments. In order to get insight into the actual reaction mechanism, we focused on optimization of catalysts. We intended to analyze the effect that variations on the electronic nature of the coordinated 2,2′bipyridyl ligand have on the activity of the corresponding [Ru(p-Cym)(bipy)Cl]Cl as precatalyst for the solvolytic dehydrogenation of amine-borane adducts. For this purpose, a series of Ru(II) complexes, of the aforementioned composition, derived from 4,4′-disubstituted bipyridines (L1− L10) were synthesized. This substitution pattern on the bipyridine ligands was chosen to maximize the electronic influence of the ligand substituents on the metal center while minimizing steric differences (due to the relatively long distance between the substituent and the putative active site of the catalyst). These compounds were obtained by reaction of 2 equivalents of the corresponding 4,4′-disubstituted bipyridine (L1−L10) with [Ru(p-Cym)Cl2]2, following the standard methodology.60 All the complexes 1−10 (Scheme 2) were obtained in moderate to high yields (49−92%) and characterized by NMR, EA, HR-MS, and X-ray diffraction analysis for selected members. A stack-plot of the 1H NMR spectra of complexes 2−10 is presented in Figure 1 (1 is not included due to its low solubility

Figure 1. 1H NMR spectra of compounds 2−10 (CD3OD, 300 MHz). 8395


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Using the experimental conditions described above, complexes 1−10 were studied as precatalysts. The reaction progress was continuously monitored by automatic gas-pressure measurement. The reaction profiles obtained are presented in Figure S7 in the SI, and the results are summarized in Table S3. The independence of the reaction rate on the gas pressure was confirmed by independent experiments (see Figure S6 in SI), validating the experimental setup. In all the cases, except for the slowest system (9), nearly 3 equivalents of hydrogen were liberated at the end of the reaction (within the experimental error ±0.1 bar). 11B NMR analysis of the reaction mixtures using an internal D2O capillary confirmed that no starting AB was present at the end of the reaction. Only a very broad singlet between 10 and 19 ppm, attributed to the different borates in equilibrium in solution,33,47,52,62 was present in the spectra. As can be observed from the results in Table S3, precatalysts 1−10 showed quite different catalytic activities. To rationalize these results, a logarithmic representation of the activity of systems 1−9 (normalized k) vs Hammett parameter (σp+)63,64 was constructed (Figure 2). This parameter was used before by

in CD3OD). These spectra show the three expected aromatic signals for 4,4′-disubstited bipyridyl compounds between 6.5 and 10.0 ppm, except in the case of the derivative of the unsubstituted 2,2′-bipyridine (6), that, as expected, presents four aromatic signals due to the bipyridine ligand. Two aromatic doublets and three aliphatic signals (septuplet, singlet, and doublet, respectively) account for the coordinated pcymene fragment in all complexes. (Note: In the case of derivative 10, the asymmetry of the ligand is also reflected in the signals of the coordinated p-cymene in the 1H NMR spectrum. In this case, it presents four doublets (for the aromatic C−H protons) in the region of 5.8−6.2 ppm and a septuplet, a singlet, and two doublets in the aliphatic area of the spectrum.) The number of signals observed reflects the overall Cs symmetry of these compounds in solution because of the well-documented low-energy rotation barrier of the paracymene ligand in half-sandwich complexes.61 In the case of derivative 10, the number of signals observed in the 1H NMR spectra reflects the asymmetry of the molecule, and in the aromatic region, it could be described as superposition of the spectra of compounds 5 and 8 (see Figure S3 in SI). The chemical shift of all the signals present in the 1H NMR spectra of compounds 2−9 were analyzed and plotted against the Hammett parameter (σp+) of the bipyridine substituents (see Figure S4 and Table S2 in the SI), obtaining a linear correlation for the aromatic signals of the coordinated bipyridine and para-cymene fragment. This observation reflects the different electronic nature of the series of compounds and confirms the consistency of the interpolated σp+ values. (Note: The σp+ Hammett parameters for the different substituents have been extracted from ref 64. The values for the azide and diethylphosphonate (not reported in the literature) have been calculated on the basis of the 1H NMR chemical shifts of the free ligands (see SI).) In the case of compounds 4, 8, and 10, crystals suitable for Xray diffraction were obtained by slow diffusion of Et2O into MeOH (4) or CDCl3 (8, 10) solutions of the complexes. ORTEP representations and the main geometric features are given in Figure S50 and Table S4 in the SI. In general, no influence of the different electronic properties of the substituent on the bipyridine on the bonding distances was observed. In an earlier report we described the use of compounds of the type [Ru(p-Cym)(bipy)Cl]Cl as homogeneous catalysts for hydrolytic decomposition of amine-borane adducts.50 Using catalyst 6, nearly 3 equivalents of H2 per mole of AB were obtained when alcohols, water, or THF/H2O mixtures were used as reaction solvent.50 We also observed that an activation of the catalyst was necessary to obtain reproducible results. This preactivation could be done by mixing the precatalyst in neat AB, where a clear transformation of the catalysts was accompanied by an evident gas evolution (see Figure S5 in SI). This transformation could suggest nanoparticles formation. Nevertheless, mercury poisoning experiments, together with a clear first order dependence on the metal loading are consistent with a homogeneous catalytic system.50 More effectively the preactivation can be done in the corresponding volume of freshly distilled THF. Approximately 1% of the total amount of H2 is liberated during this procedure. (Note: It is worth mentioning that, in spite of the lack of gas evolution, the insoluble precatalyst transformed into a new dark insoluble material during the initial reaction period.) Subsequent addition of the required quantity of H2O was considered initial reaction time.

Figure 2. Logarithmic representation of k/k6 vs σp+ for the hydrolytic dehydrogenation of AB using precatalysts 1−9 (the substituent of the bipyridine has been used for labeling).

other authors to confirm the influence of electronic substituents on the activity using a family of bipyridines-based organometallic catalysts.65 Hammett parameters extracted from ref 64 or interpolated from 1H NMR chemical shifts of free ligands (see SI). A value of σp+ = −0.02, corresponding to a carboxylate group has been used for compound 5, due to the basic pH of the reaction media. Although a clear trend can be induced from Figure 2, some of the compounds deviate from this behavior. Complex 4, containing ethynyl substituents on the bipyridine presented rather slow activity compared to the one expected according to its Hammett parameter. We considered that in situ hydrogenation of the alkyne during the reaction course, generating the ethenyl (σ+ = −0.16) or ethyl (σ+ = −0.30) bipyridine derivatives could be responsible for this effect. This side reaction would render less-active precatalysts (red dots in Figure 2). (Note: 1H NMR of an equimolecular mixture of AB and complex 4 in THF-d8/D2O 1/3 revealed a new set of signals for the p-cymene and bipyridine ligands concomitant with the disappearance of the characteristic alkyne resonance at 4.38 ppm (Figure S8 in SI). As inferred by comparison with the spectrum of [Ru(p-Cym)(L6)(NH3)]2+, the new compound could be [Ru(p-Cym)(L)(NH3)]2+ (L = 4,4′-CD3CD2-2,2′8396


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ACS Catalysis bipyridine).49 The coordination of an amine ligand was not surprising since the NMR experiment was performed after venting the tube.) In the case of the derivative of unsubstituted 2,2′-bipyridine (6), the activity observed is slightly larger than expected according to its Hammett parameter. Since steric effects of the substituents in 4,4′-positions of the bipyridyl ligand should not have any influence on the reaction rate, at this point we do not have an explanation for the exceptional activity observed for this compound. In spite of these deviations, the observed correlation points to an important electronic influence on the activity of the catalyst. The positive slope observed in the linear adjustment of the data, ρ = +0.78, clearly indicates that the reaction is accelerated by electronwithdrawing groups.65,66 In the case of complex 10 (containing an asymmetrically substituted bipyridine), if the observed electronic effect on the reaction rate was just related to a change on the electrophilic character of the metal, an average activity of that observed for 5 and 8 containing bromide and azide substituents, respectively, would be expected. Surprisingly it was nearly identical to the former (Figure S9 in SI), suggesting that maybe other factors could also play a role (vide infra). As the best results were obtained with complex 1, containing the most electron-withdrawing NO2 substituent, several reaction parameters (i.e., solvents screening and effect of temperature) were studied with this system (see Figures S10− 12 SI). The results obtained are consistent with those reported for the model compound 6. At the highest temperature assayed (60 °C), 2.62 equivalents of H2 per mol of AB (3 half-lives) were liberated in only 1.8 min, slightly faster than the reported values for 2,2′-bipyridyl derivative 6. Although more efficient systems have been reported using a bis(N-heterocyclic carbene) iridium(III) complex,51,52 both 1 and 6, based on ruthenium, represent readily available compounds that can be prepared in a multigram scale from commercially available reagents, and they are active even in pure water and without nitrogen protection. Using the same reaction conditions, the catalytic process was also studied for several substituted ammine-borane adducts, using precatalysts 1 and 6.50 Tert-buthylamine-borane (TBAB), dimethylamine-borane (DMAB), trimethylamine-borane (TMAB), triethylamine-borane (TEAB), and pyridine-borane (pyB) were selected as model substrates (see Figures S13,S14 in SI). More than 2.8 equivalents of hydrogen were liberated when mono- and disubstituted amine-borane adducts were used as substrates. Since only 1 and 2 equivalents of hydrogen can be liberated from DMAB and TBAB, respectively, by dehydrocoupling of these adducts, the amount of hydrogen evolved with these substrates confirms that a solvolytic mechanism is operating in aqueous solutions. No appreciable hydrogen liberation was detected from TMAB, TEAB, or pyB, in line with our former observations,50 suggesting that at least one N−H functionality on the substrate is required for the reaction to proceed. (Note: In the case of TEAB and pyB, and due to their low miscibility in the THF/H2O 1/3 (v/v) reaction mixture, a homogeneous less-polar THF/H2O 3/1 (v/ v) mixture was used, but no gas evolution was detected either.) In spite of the lack of hydrogen generation observed with these substrates, as in the case of hydrogen-productive ones, darkening of the originally light-yellow solutions and formation of a dark precipitate suggests some reactivity. Also when these substrates were mixed with precatalyst 1 in neat, the original mixture transformed into a dark sticky solid concomitant with some gas evolution (see Figure S5, SI).

Reaction Intermediates. Electrospray ionization mass spectrometry (ESI-MS) studies were conducted to assess the speciation during the activation period and the catalytic process using the precatalyst 6 as a representative and efficient member of the whole series. This technique should be particularly fruitful in the present system because the metal-containing species are inherently charged, so it is expected that the molecular integrity of the species after their ionization is preserved.67 We first addressed the chemical speciation of compound 6 in THF in the presence of AB to simulate the activation period. ESI-MS analysis of 6 and AB in THF displayed a dominant peak due to the hydride [Ru]+−H ([Ru] = [Ru(p-Cym)(L6)]) cation (m/z 393) (see Figure 3a).

Figure 3. (a) ESI mass spectrum of a mixture of AB and 6 (10%) in THF further diluted to 5 × 10−6 M relative to the initial amount of 6 in THF; (b) Simulated (top) and experimentally observed isotopic pattern for [Ru]2+ (m/z 196), [Ru]2+−AB (m/z 211) and [Ru]+−H (m/z 393); (c) CID mass spectrum of mass-selected [Ru]2+−AB at a CEElab 10 eV.

Dications resulting from Ru−Cl bond breaking and solvent or AB coordination, namely, [Ru]2+ (m/z 196), [Ru]2+−AB (m/z 211) and [Ru]2+−THF (m/z 232) were also observed. Other species detected were the starting material [Ru]+−Cl (m/z 427), the hydroxo [Ru]+−OH (m/z 409), and [Ru]2+− THF:H2O (m/z 241), most likely formed through adventitious water present during the ESI process. In the lower m/z region, a solvent-stabilized borenium cation formulated as [H3N· BH2(THF)]+ was also detected at m/z 102. It was clearly distinguished from the solvent adducts typically found at low m/z values on the basis of its m/z value and characteristic isotopic pattern. Further insights on the composition and reactivity of the ESIdetected species were provided by collision-induced dissociation (CID) experiments. CID experiments are commonly used to prove the intrinsic reactivity of ionic species in the gas-phase that may parallel the reactivity found in the solution.68−71 In the present case, the CID mass spectrum of mass-selected [Ru]2+−AB (m/z 211) dication produced the [Ru]+−H 8397


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cation [Ru(p-Cym)(bipy)Cl]+ is in equilibrium with the corresponding aquo biscationic [Ru(p-Cym)(bipy)(H2O)]2+.73,74 When isolated [Ru(p-Cym)(L6)(H2O)](OTf)2 was studied as precatalyst for the hydrolysis of AB, identical activity to that obtained with 6 was observed.50 For this reason, this precatalyst was used for these studies to simplify the number of signals observed in the 1H NMR spectra. (Note: When these experiments were run using 6 instead of [Ru(pCym)(L6)(H2O)](OTf)2, the same results were obtained, but in the first spectra, a mixture of both 6 and [Ru(pCym)(L6)(H2O)]2+ was observed.) Initially, [Ru(p-Cym)(L6)(H2O)](OTf)2 was reacted with 10 equivalents of AB in THF-d8/D2O (1/3). 11B and 1H NMR spectra were recorded at regular time-intervals. Already in the first spectra, acquired immediately after addition of the solvents mixture, the 1H NMR spectrum confirms that all the ruthenium is in the form of [Ru(p-Cym)(L6)H]+, in agreement with the ESI-MS experiments.60 The reaction was accompanied by an evident effervescence due to the hydrogen evolution (which caused a broadening of the signals in the spectra), and the concomitant consumption of the substrate observed in both 1H and 11B NMR spectra (Figure 5). After 2 h, the high-field

product ion (see Figure 3c) according to eq 6, concomitant with [BNH5]+. This borenium cation was in fact detected as THF adduct in the single stage ESI mass spectrum, thus indicating that the AB ligated [Ru]2+−AB species is an intermediate en route to the hydride [Ru]+−H. A second fragmentation pathway was also evidenced that correspond to the liberation of neutral AB according to eq 7. [Ru]2 + −AB → [Ru]+ − H + [BNH5]+

(6)

[Ru]2 + −AB → [Ru]2 + + AB

(7)

The hydride [Ru] −H species was not observed upon dissolving 6 in THF/H2O 1/3 (v/v), which reinforces the hypothesis of the involvement of AB in the formation of such species. The positive ESI mass spectrum of 6 in THF/H2O = 1/3 (v/v) revealed the coexistence of the dominant cations [Ru]+-Cl (m/z 427), [Ru]+−OH (m/z 409) accompanied by minor signals due to doubly charged [Ru]2+ (m/z 196). This speciation is consistent with the Ru−Cl cleavage followed by water coordination upon dissolving 6 in water, a welldocumented equilibrium previously studied in aqueous media (vide infra).50,72−74 Finally, the chemical speciation under nearly catalytic conditions was explored, namely, THF/H2O = 1/3 (v/v), [AB] = 0.46 M, catalyst 10%. A higher catalyst load was used to detect Ru-containing species and to prevent ion suppression effects. The ESI-MS study in the presence of AB under catalytic conditions provided strong evidence of the intermediacy of hydride species. When a mixture of 6 and AB in THF/H2O (3/ 1) was monitored at different time intervals, the ESI mass spectra were consistent with the dominant formation of the [Ru]+−H cation at m/z 393 (see Figure 4). +

Figure 5. Sequential 1H and 11B NMR spectra of a solution of AB and [Ru(p-Cym)(L6)(H2O)](OTf)2 (10/1) in THF-d8/D2O = 1/3 (v/v).

Figure 4. ESI mass spectrum of a mixture of AB and 6 (10%) in THF/ H2O further diluted to 5 × 10−6 M relative to the initial amount of 6 in THF/H2O (3:1).

hydride signal due to the [Ru(p-Cym)(L6)H]+ decayed due to H−D exchange (in D2O),72 but the rest of signals attributed to this compound remained unaltered. After 9 h, the complete consumption of the substrate was confirmed by 11B NMR (decay of the original quartet at −22.8 ppm, 1JH−B = 93.8 Hz) and formation of a broad singlet due to borates at 19.7 ppm. In the 1H NMR new signals attributed to the [Ru(p-Cym)(L6)(NH3)]2+ were observed, in agreement with our previous publications.49 It is worth mentioning that a very weak triplet (δ = −12.58 ppm, 1JH−B = 98.9 Hz) was observed in the 11B NMR spectra, (see Figure S15 in SI). This signal could be attributed to a minor amount of a THF- or H2O-stabilized borenium cation (H3N·BH2(solvent)+), vide infra.76 The same experiment was performed using (non H2 productive) TMAB as substrate (Figure 6). According to 1H NMR analysis, already in the first spectra acquired, part of the starting [Ru(p-Cym)(L6)(H2O)]2+ transformed into the [Ru(p-Cym)(L6)H]+ species, which was the only ruthenium species after 2 h of reaction. After 6 h, the Ru−H signal disappeared, due to H−D exchange with D2O. Noticeably, within this process, the substrate (TMAB) transformed into

Minor amounts of [Ru]+−OH (m/z 409) and [Ru]+−Cl (m/ z 427) cations are also observed. Species formulated as [Ru]+− H2BO3 (m/z 453) and [Ru]+−H2BO2 (m/z 437) were observed as judged by their m/z value, isotopic pattern and CID spectra (see Figure S23). These species correspond formally to the coordination of H2BO3− and H2BO2− anions resulting from the AB hydrolysis that are in large excess in the reaction media. Recent studies from Weller and Macgregor groups revealed that dehydrogenation and oligomerization steps of amine-boranes can be conveniently monitored by ESIMS using a cationic Ir complex.75 Direct evidence of Ir-bound oligomers was given in agreement with the occurrence of the dehydrogenation and oligomerization of amine-borane. In the present work, no additional amine-borane oligomers, either as free or Ru-bound species were detected on the basis of ESI-MS, thus suggesting that the dehydrocoupling mechanism, is not operative using 6 in aqueous media. We also addressed progress of the reaction by multinuclear 1 H and 11B NMR spectroscopy. As it is well-known for compound 6 and related species, in aqueous solutions, the 8398


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the latter also shows the formation of a broad singlet of low intensity (≈7% of the total boron signal) at around 19 ppm, probably due to a small quantity of borates or Me3N·B(OH)H2 (vide infra) in solution. In the 1H NMR spectra, a small singlet close to the one of the methyl groups of TMAB was observed at 2.73 ppm (see Figure S16 in SI). The relative integration of both signals is coherent with the integration of the two species observed in the 11B NMR spectra. Addition of H2O to this sample induced a recovery on the multiplicity in the 11B NMR spectra, evidencing the reversibility of the deuteration process (Figure S17 in SI). The same behavior described for TMAB was observed when TEAB or pyB were used as substrates, although a less polar solvent mixture THF-d8/D2O = 3/1 (v/v) was used in these cases to guarantee the homogeneity of the samples (see Figures S18−S19 in SI). TMAB is known to experience boron-bonded H−D exchange in acidic media (DCl/D2O),77 together with competing acid-catalyzed substrate hydrolysis.78 Nevertheless, blank experiments confirmed that, under our experimental conditions, in absence of catalysts, none of the substrates experienced H−D exchange at the terminal boron−hydrogen (see Figure S20 in SI), which confirms that the observed substrate-deuteration is a metal-catalyzed process. Deuterium

Figure 6. Sequential 1H and 11B NMR spectra of a solution of TMAB and [Ru(p-Cym)(L6)(H2O)](OTf)2 (10/1) in THF-d8/D2O = 1/3 (v/v).

Me3N·BD3, as concluded from the decay of the signals assigned to the hydridic hydrogen atoms of the substrate in the 1H NMR spectra, and the collapse of the corresponding quadruplet into a singlet in the 11B NMR spectra (see Figure 6). A closer look at

Scheme 3. Mechanistic Proposal for Hydrolytic and Dehydrogenation Mechanisms

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of an unidentified species, and the concomitant gas evolution.49 All the efforts to identify the nature of the gas formed were unsuccessful, but most probably it was H2 coming from an AB dehydrogenative pathway (vide infra). Similar reactivity has been identified now, even with the non-hydrogen productive substrates. In view of the ESI-MS analysis we propose that during the activation period, a small amount of compound 6 present in solution may undergo chlorine dissociation (or displacement by a solvent molecule) forming an electrophilic [Ru]2+ compound (A in Scheme 3). It is well established that the reaction of AB with a Lewis acid (i.e., B(C6F5)3) in Et2O results in the formation of the boronium salt [H3N· BH2(Et2O)][HB(C6F5)3].88 Recently, this boronium cation (or Et2O-estabilized borenium cation) has been claimed as a key intermediate in metal-catalyzed AB dehydrocoupling processes.24 In analogy to that precedent, we suggest that the reaction mechanism could start by the activation of AB by the Lewis acidic [Ru]2+ (A in Scheme 3), forming an ABcoordinated species [Ru]2+−AB (B), in which the electrophilic character of boron would be enhanced. These Shimoi-type compounds have been described before for several organometallic complexes and borane-adducts (including phosphine boranes),89−94 and in many instances, they are claimed to be involved in the activation step for related dehydrogenation of amine-boranes processes.13,24,25,47,76,95−102 This species, in distilled THF, would suffer a nucleophilic attack on the activated boron center by a solvent molecule, probably through a SN2-type transition state (TS1-THF in Scheme 3), releasing a THF-stabilized borenium cation [H3N·BH2(THF)]+ (D-THF) in solution and the observed [Ru]+−H (C). Both [H3N· BH2(THF)]+ (D-THF) and [Ru]+−H (C) species were identified in the present work by ESI-MS upon dissolving 6 and AB in the presence of THF (vide supra). In addition, a closely related reactivity was experimentally determined for the mass-selected [Ru]2+−AB dication in the gas-phase. In this case, CID experiments of the [Ru]2+−AB dication revealed the proclivity of AB to be released as [BNH5]+ concomitant with the formation of the hydride [Ru]+−H (see Figure 3c and eq 6). Subsequently, an attack of the protic N−H hydrogen of [H3N·BH2(THF)]+ on [Ru]+−H, would release aminoborane (H2NBH2), and a ruthenium-dihydrogen compound (E), that eventually would regenerate the active species A liberating H2. It is likely that this reaction does not progress further in distilled THF due to precipitation of activated species B. According to this proposal, the activation process would consist of a one-cycle dehydrogenation of AB following a mechanism analogous to the one proposed by Conejero et al. for a coordinatively unsaturated Pt(II) compound.24 This hypothesis is also consistent with our former observation that compound 6 is an active catalyst for the dehydrogenation of DMAB, generating the cyclic [Me2BNH2]2.50 At this stage, we consider Shimoi-type compounds B as true active species for both dehydrogenation and hydrolysis of amine-borane adducts, both processes being operative although at different rates depending on the catalyst, substrate and solvent mixture (vide infra). Addition of water to this system, solubilizes all the compounds, and initiates the hydrolytic process. In aqueous solvent-mixtures, species B suffers a nucleophilic attack, not by the THF, but preferentially by a more nucleophilic H2O molecule, rendering the ruthenium(II)hydride (C) and a water-stabilized borenium cation (D-H2O). In this case, the most protic atom of the formed boronium

exchange at B−H bonds of amine-borane adducts using several Ru, and Ir catalysts is a process known since the middle 70s,79,80 but high temperatures and the use of D2 gas as deuterium source were required. In 1999 Shimoi described a rhenium polyhydride-mediated deuteration of TMAB and H3N· PPh3 by H−D exchange from C6D6.81 More recently, Nolan et al. reported that their bis(N-heterocyclic carbene)-ligated iridium(III) complex (a very active catalyst for the hydrolysis of AB) was also efficient for the D2-promoted deuteration of several boranes at room temperature.82 Nevertheless, to the best of our knowledge, the use of D2O as the only deuterium source for this metal-catalyzed process has not been reported to date. Kinetic Isotope Effect. Kinetic isotope effect (KIE) experiments were conducted using precatalysts 1 and 6. Analogous results were obtained with both precatalysts. Whether a hydrolytic or dehydrocoupling mechanism was responsible of the H2 evolution observed, the cleavage of a B− H bond should necessarily occur at some stage of the catalytic process. Accordingly, primary KIEs have been reported in AB dehydrocoupling reactions using H3N·BD3.25 In our case, when H3N·BD3 was used as substrate in THF/H2O mixtures, the measured reaction profile was very similar to the one obtained with H3N·BH3 under the same catalytic conditions (kH3B·NH3/ kD3B·NH3 = 1.16 for 1 and kH3B·NH3/kD3B·NH3 = 1.11 for 6) (see Figures S21−S22 in SI). This result suggests that either B−H cleavage is not involved in the RDS, or that (even being the RDS) it is a reversible process.83,84 Several experiments using isotopically labeled solvents (D2O/THF 3:1 (v/v) and H2O/THF-d8 3:1 (v/v)) were also conducted. As expected no KIE was observed by substituting THF by THF-d8. When catalysts 1 or 6 were used in mixtures of D2O/THF, much slower reactions were observed (see Figures S21−S22 in SI). Analysis of the kinetic data permitted us to determine a kH/kD ∼ 4, in both cases. This result was rather surprising since, according to the generally assumed hydrolytic reaction pathway, H2O was supposed to be involved only in a noncatalytic (and barrierless) hydrolysis of the liberated BH3.52 Although the initial reasoning would be that cleavage of an O−H/O−D bond was involved in the RDS, the whole picture is far more complicated. On the one hand, it is well-known that AB is converted to D3N·BH3 in D2O solutions,85,86 and also that [Ru(p-Cym)(bipy)H]+ undergoes H-to-D exchange when dissolved in D2O, as mentioned before.72 KIE due to protic solvents is a well-known phenomena, often observed if the species involved in the RDS contain solvent-exchangeable hydrogen atoms, or if solvation of the TS is a key factor determining reactivity.87 These phenomena made us unable to unravel, at this point, if the origin of the observed KIE was the rupture of O−D (D2O), N−D (D3N·BH3), or Ru−D bond in the RDS, or it could be better explained as solvent KIE. Mechanistic Proposal. Taking into account all the abovementioned experimental data, we proposed a plausible reaction mechanism compatible with the facts observed. As mentioned before, in spite of the low solubility of precatalyst 6 (a yellow crystalline solid) in THF, during the activation period the formation of an insoluble dark precipitate and residual gas evolution (accounting for less than 1% of the total amount of gas expected for the full hydrolysis of the substrate) was observed. Former studies on the reactivity between AB and 6 as solids (no solvent) showed the formation 8400


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important solvent KIE observed in the catalytic experiments is not surprising. Following a similar reasoning, the presence of somehow protic hydrogen atoms in the coordinated ligands could affect the structure of the hydrogen-bonded water molecules network around the metal center. In view of the enhanced reactivity observed by the unsubstituted compound 6 most probably, the reaction may be somehow favored by the presence of acidic H4 and H6 protons on the bipyridine, which could facilitate the approach of the substrate to the metal center and also the subsequent attack of a H2O molecule on it. The observation that compound 10 shows a catalytic activity virtually identical to that of 5 could indicate that creating the appropriate electronic microenvironment on only “half of the compound” suffices to induce an effective reactivity. Nevertheless, DFT calculations including explicit solvent molecules would be necessary to confirm this hypothesis. As described before, when trisubstituted amine-boranes were used as substrates, no hydrogen liberation was observed, but the precatalyst was converted into the corresponding [Ru]+−H species. When the reactions were conducted in D2O, fully deuterated-at-boron substrates (D3B·NR3) and [Ru]+−D were formed at extended reaction times. This observation can be explained according to the proposed mechanism assuming that hydrogen liberation from [Ru]2+−H2 (E) is the only irreversible step of the whole process and that hydrolysis of H3N·B(OH)H2 is a facile process,94 assisted by multiple hydrogen bonds among solvent molecules (H2O) and the boron-bonded hydridic and nitrogen-bonded protic hydrogen atoms of H3N·B(OH)H2 (F). In the case of trisubstituted amine-boranes, (R3N·BH3) the lack of protic N−H hydrogen atoms could convert it into a high-energy process, hampering the hydrolytic cycle to go to completion due to a build-up in the concentration of such hydroxyborane adducts, not consumed in the hydrolytic process. According to this hypothesis, trisubstituted substrates would be “trapped” into a reversible equilibrium A + H3B·NR3 ⇄ B′ ⇄ C + D′-H2O ⇄ E + F′, (B′, D′, and F′ represent compounds analogous to B, D, and F, respectively, but derived from trisubstituted amineboranes). In D2O media, this equilibrium will eventually lead to a deuterated [Ru]+−D intermediate C resulting in a stepwise deuteration of the hydridic boron-bonded hydrogen atoms of the substrate. The fact that [Ru]+−H and eventually [Ru]+−D were identified as the resting state of the catalyst during both the H2-productive hydrolysis substrates, as well as deuteration of non-H2-productive trisubstituted amine-borane adducts, is consistent with the above-described mechanism. Its observation as major ruthenium compound during in situ NMR experiments suggests that [Ru]+−H is the lowest energy species of the ones in equilibrium with the one actually involved in the RDS.83 The dehydrogenative cycle should be competitive with the proposed hydrolytic one (as shown in Scheme 3), and its contribution to the total amount of H2 liberated is not easy to predict. The nucleophilicity of THF is expected to be inferior to that of H2O, and consequently, a lower activity is expected for the dehydrogenative cycle. Additionally, different THF/ H2O ratios could drive the reaction to proceed toward one or other cycle preferentially (especially when extreme proportions were used), letting apart solubility issues. The fact that we obtained nearly quantitatively 3 equivalents of H2 from the catalytic experiments with mono or disubstituted amineboranes, confirms that most of the hydrogen comes from a

cation is located at the oxonium ion in D-H2O, which protonates C rendering the ruthenium-dihydrogen species (E) and liberating an amine-hydroxyborane adduct (F). This type of compounds have been also postulated for the heterogeneous hydrolysis of AB,42,44 and similar reactivity was already observed by Shimoi et al. for [CpRu(PMe3)2(η1-BH3·XMe3)][BArF24] (X = N, P) compounds, being hydrolyzed by trace amount of water, to form, in that case, Ru-dihydride compounds, also with the intermediacy of a water-stabilized borenium cation (see Scheme 4).94 Additionally, studies by Scheme 4. Hydrolysis of Ru-η1-BH3·NMe3 Shimoi Compound As Proposed in Ref 94

Jalón et al. point to a facile and reversible protonation of [Ru]+−H in acidic media forming a Ru-dihydride.72 Considering the expected enhanced acidity of oxonium hydrogens in water-stabilized borenium cations, we do not expect this step of the cycle as having a high-energy barrier. Finally, substitution of coordinated H2 by an incoming substrate molecule (either in one step or through the intervention of species A) would reinitiate the organometallic cycle, with a total release of 1 equivalent of hydrogen per mole of substrate. The additional 2 equivalents of hydrogen observed in the catalytic experiments are proposed to be formed through hydrolysis of aminehydroxyborane adducts (F) formed as side product during the organometallic cycle. Eventually the proposed mechanistic pathway would generate 1 equivalent of hydrogen through a metal-catalyzed process, and two additional ones though an out-of-cycle nearly barrierless hydrolysis of the liberated hydroxyborane. This latter step was evidenced in the ESI mass spectra at extended reaction times as species due to hydrolyzed hydroxyborane, namely [Ru]+−H2BO2 and [Ru]+− H2BO3 were observed. The intermediacy of ruthenium species on this latter process cannot be ruled out. The observed electronic influence of the bipyridine substituents on the catalytic activity points to AB activation or nucleophilic attack of H2O on Shimoi-type compounds as being the RDS, as they would be accelerated by more electrophilic metal centers. The first-order dependency of the reaction rate on both catalyst and AB suggests that the former has a higher barrier. In 2014, Jalón et al. described the use of analogous ruthenium(II) compounds as catalysts in hydrogen-generation processes in aqueous media. 72 On the basis of DFT calculations, they demonstrated the key role that solvation plays in the reaction progress, including an extended network of hydrogen-bonded molecules in their calculations. In view of the similarity among both catalytic systems (which include some common steps, as activation through Ru−Cl/Ru−H2O ligand exchange, or the intermediacy of a [Ru]−H+ species), it seems logical to presume also here that the intermediates would be in fact solvated species. Taking into account that solvation can affect not only the activation of the substrate but also the subsequent nucleophilic attack of a solvent molecule on it, the 8401


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distilled THF for 5−10 min. Addition of 1.125 mL of distilled water to this mixture was considered initial reaction time. Synthetic Procedures. [Ru(p-Cym)(4,4′-dinitro-2,2′bipyridine)Cl]Cl (1). Under a N2 atmosphere, [Ru(p-Cym)Cl2]2 (0.1 g, 0.16 mmol) and 4,4′-dinitro-2,2′-bipyridine (0.08 g, 0.326 mmol) were dissolved in 10 mL of CH2Cl2. The reaction mixture was refluxed for 15 h. It was cooled to room temperature, and the brown solid that precipitated was filtered off. Yield 63%. Elemental Analysis: calculated for (C20H20Cl2N4O4Ru·2CH2Cl2): C, 36.59; H, 3.35; N, 7.76. Found: C, 36.39; H, 3.19; N, 8.24. Exact Mass: HR-ESI-MS [C20H20ClN4O4Ru]+: calculated: m/z = 517.0217, found: m/z = 517.0228. 1H NMR (300 MHz, THF-d8): δ 9.20 (d, J = 2.3 Hz, 2H), 9.11 (d, J = 4.8 Hz, 2H), 8.25 (dd, J = 2.2 Hz, J = 5.3 Hz, 2H), 7.11 (s, 2H), 7.10 (s, 2H), 2.87 (m, 1H), 2.31 (s, 3H), 1.25 (d, J = 6.9 Hz, 6H). Low solubility hampered full characterization in solution. [Ru(p-Cym)(4,4′-bis(diethylphosphonate)-2,2′-bipyridine)(Cl)]Cl (2). Under a N2 atmosphere, [Ru(p-Cym)Cl2]2 (0.043 g, 0.070 mmol) and 4,4′-bis(diethylphosphonate)-2,2′-bipyridine (0.060 g, 0.140 mmol) were dissolved in 4 mL of CH2Cl2. The reaction mixture was refluxed for 15 h. It was cooled to room temperature, the solvent was evaporated, and the desired compound was obtained as a dark green solid. Yield 92%. Elemental Analysis: calculated for (C28H40Cl2N2O6P2Ru· CH2Cl2): C, 42.50; H, 5.17; N, 3.42. Found: C, 42.31; H, 5.46; N, 3.25. Exact Mass: HR-ESI-MS [C28H40ClN2O6P2Ru]+: calculated: m/z = 699.1094, found: m/z = 699.1103. 1H NMR (300 MHz, CDCl3): δ 10.33 (brdd, J = 4.1 Hz, J = 5.2 Hz, 2H), 8.46 (d, J = 13.4 Hz, 2H), 8.09 (ddd, J = 0.9 Hz, J = 5.5 Hz, J = 12.3 Hz, 2H), 6.56 (d, J = 6.1 Hz, 2H), 6.40 (d, J = 6.1 Hz, 2H), 4.36−4.12 (m, 8H), 2.77 (sep, J = 6.9 Hz, 1H), 2.33 (s, 3H), 1.39 (brdd, J = 7.2 Hz, J = 14.7 Hz, 12H), 1.06 (d, J = 6.9 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 157.46 (d, J = 12.7 Hz, 2CH), 153.39 (d, J = 13.5 Hz, 2Cquat), 141.05 (d, J = 187.5 Hz, 2Cquat), 129.10 (d, J = 7.5 Hz, 2CH), 124.23 (d, J = 10.5 Hz, 2CH), 105.57 (s, Cquat), 105.09 (s, Cquat), 87.64 (s, 2CH), 84.74 (s, 2CH), 63.27 (s, 4CH2), 30.68 (s, CH), 21.78 (s, 2CH3), 18.69 (s, CH3), 15.96 (s, 4CH3). 31P NMR (162 MHz, CDCl3): δ 11.26 (s, 2P). [Ru(p-Cym)(4,4′-bis(ethynyl)-2,2′-bipyridine)(Cl)]Cl (4). Under a N2 atm, [Ru(p-Cym)Cl2]2 (0.130 g, 0.214 mmol) and 4,4′-bis(ethynyl)-2,2′-bipyridine (0.087 g, 0.428 mmol) were dissolved in 10 mL of acetone. The reaction mixture was refluxed for 15 h. It was cooled to room temperature, the solvent was evaporated, and the desired compound was obtained as a light brown solid. Yield 77%. Elemental Analysis: calculated for (C24H22Cl2N2Ru): C, 56.48; H, 4.34; N, 5.49. Found: C, 56.44; H, 4.88; N, 5.74. Exact Mass: HR-ESI-MS [C24H22ClN2Ru]+: calculated: m/z = 475.0515, found: m/z = 475.0517. 1H NMR (300 MHz, MeOD-d4): δ 9.47 (d, J = 5.9 Hz, 2H), 8.68 (s, 2H), 7.81 (dd, J = 1.7 Hz, J = 5.9 Hz, 2H), 6.17 (d, J = 6.4 Hz, 2H), 5.92 (d, J = 6.4 Hz, 2H), 4.47 (s, 2H), 2.70 (sep, J = 6.9 Hz, 1H), 2.29 (s, 3H), 1.10 (d, J = 6.9 Hz, 6H). 13C NMR (75 MHz, MeOD-d4): δ 156.79 (2CH), 156.03 (2Cquat), 135.84 (2Cquat), 130.97 (2CH), 127.64 (2CH), 106.86 (Cquat), 105.94 (Cquat), 89.63 (2CH), 88.31 (2CH), 85.92 (2CH), 80.15 (Cquat), 32.38 (CH), 22.32 (2CH3), 18.92 (CH3). [Ru(p-Cym)(4,4′-dibromo-2,2′-bipyridine)Cl]Cl (5). Under a N2 atmosphere, [Ru(p-Cym)Cl2]2 (0.1 g, 0.16 mmol) and 4,4′dibromo-2,2′-bipyridine (0.10 g, 0.32 mmol) were dissolved in 10 mL of acetone. The reaction mixture was refluxed for 15 h.

hydrolytic pathway, with the standard reaction solvent mixture (THF/D2O = 1/3). In the case of dimethylamine-borane, both mechanisms proceed at competitive rates (as mentioned in a previous report).50

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CONCLUSIONS On the basis of experimental evidence, a related mechanism has been proposed, for the first time, for competing dehydrocoupling and hydrolysis of amine-borane adducts using Ru(II) catalysts. In both mechanisms, solvent-stabilized borenium cations are formed, evolving toward dehydrogenation products in distilled THF and releasing amine-hydroxyboranes in aqueous media. Both reaction pathways would liberate up to 1 equivalent of hydrogen through a metal-catalyzed process, but an out-of-cycle low-barrier hydrolysis of amine-hydroxyboranes would produce the 2 additional equivalents of hydrogen in aqueous solutions. No hydrogen generation was observed when trisubstituted amine-boranes were used as substrates, although during the process, the precatalyst was converted into a Ru−H species. The lack of H2 productivity with these substrates was attributed to the absence of protic N−H moieties in the liberated aminehydroxyboranes, which could convert its hydrolysis into a highenergy process, hampering the hydrolytic cycle to go to completion. According to this hypothesis, trisubstituted substrates would be “trapped” into a reversible equilibrium A + Substrate ⇄ B′ ⇄ C + D′-H2 O ⇄ E + F′, resulting in the deuteration of their hydridic boron-bonded hydrogen in D2O media. To the best of our knowledge, this would constitute the first example of a catalyst for the deuteration of amine-borane adducts at room temperature, using D2O as the only deuterium source.

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EXPERIMENTAL SECTION General Considerations. [Ru(p-Cym)Cl2]2, 2,2′-bipyridine (L4), 4,4′-dibromo-2,2′-bipyridine (L3), 4,4′-dimethyl2,2′-bipyridine (L7), amine-boranes, and other general chemicals were obtained from commercial sources and used without further purification. 4,4′-Dinitro-2,2′-bipyridine (L1),103,104 4,4′-bis(ethynyl)-2,2′-bipyridine (L2),105−107 4,4′dicarboxy-2,2′-bipyridine (L5),108 4,4′-diazido-2,2′-bipyridine (L6),109 4,4′-diamin-2,2′-bipyridine (L8),103,104,110 [Ru(pCym)(2,2′-bipyridine)Cl]Cl (4),60 [Ru(p-Cym)(4,4′-dicarboxy-2,2′-bipyridine)Cl]Cl (5),111 and [Ru(p-Cym)(4,4′-dimethyl-2,2′-bipyridine)Cl]Cl (7)112 were prepared following published methodologies or slight variations of the same. All solvents were dried and purified by known procedures and freshly distilled under nitrogen from appropriate drying agents prior to use. All manipulations and reactions involving air- and/or moisture-sensitive organometallic compounds were performed under an atmosphere of dry nitrogen using standard Schlenk techniques. H2 evolution was measured using the kinetic kit (MOTM) series X102 kit from manonthemoontech which permits electronic monitoring of the variation of pressure and temperature of the gas phase inside a closed glass reactor. Optimized Procedure for the Solvolytic Dehydrogenation. Catalytic reactions were carried out in a glass reactor connected to an electronic pressure transducer (MOTM). 0.69 mmol (21.3 mg) of the substrate and 0.5 mol % of the ruthenium precatalyst were stirred in 0.375 mL of freshly 8402


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Hz, 1H), 9.31 (d, J = 2.2 Hz, 1H), 8.93 (d, J = 2.1 Hz, 1H), 8.30 (d, J = 2.5 Hz, 1H), 8.02 (dd, J = 2.1 Hz, J 6.1 Hz, 1H), 7.49 (dd, J = 2.4 Hz, J = 6.3 Hz, 1H), 6.14 (d, J = 4.3 Hz, 1H), 6.12 (d, J = 4.2 Hz, 1H), 5.88 (d, J = 4.1 Hz, 1H), 5.86 (d, J = 4.1 Hz, 1H), 2.70 (sep, J = 6.9 Hz, 1H), 2.30 (s, 3H), 1.12 (d, J = 1.9 Hz, 3H), 1.10 (d, J = 1.9 Hz, 3H). 13C NMR (75 MHz, MeOD-d4): δ 157.40 (CH), 157.03 (CH), 156.82 (Cquat), 156.38 (Cquat), 154.89 (Cquat), 138.04 (Cquat), 132.27 (CH), 128.76 (CH), 119.37 (CH), 116.16 (CH), 106.35 (Cquat), 105.59 (Cquat), 88.03 (CH), 87.83 (CH), 85.51 (CH), 85.31 (CH), 32.35 (CH), 22.36 (CH3), 18.96 (CH3), 15.43 (CH3).

It was cooled to room temperature, and the yellow solid that precipitated was filtered off. Yield 85%. Elemental Analysis: calculated for (C20H20Br2Cl2N2Ru): C, 38.73; H, 3.25; N, 4.52. Found: C, 38.25; H, 3.41; N, 4.52. Exact Mass: HR-ESI-MS [C20H20Br2ClN2Ru]+: calculated: m/z = 584.8705, found: m/z = 584.8706. 1H NMR (300 MHz, MeOD-d4): δ 9.33 (d, J = 6.1 Hz, 2H), 8.91 (d, J = 2.1 Hz, 2H), 8.05 (dd, J = 2.1 Hz, J = 6.1 Hz, 2H), 6.16 (d, J = 6.4 Hz, 2H), 5.91 (d, J = 6.4 Hz, 2H), 2.72 (sep, J = 6.9 Hz, 1H), 2.29 (s, 3H), 1.12 (d, J = 6.9 Hz, 6H). 13C-APT-NMR (75 MHz, MeOD-d4): δ 157.14 (2CH), 156.19 (2Cquat), 138.12 (2Cquat), 132.52 (2CH), 129.08 (2CH), 106.95 (Cquat), 105.70 (Cquat), 88.02 (2CH), 85.72 (2CH), 32.39 (CH), 22.36 (2CH3), 18.91 (CH3). [Ru(p-Cym)(4,4′-diazido-2,2′-bipyridine)Cl]Cl (8). Under a N2 atmosphere, [Ru(p-Cym)Cl2]2 (0.1 g, 0.16 mmol) and 4,4′diazido-2,2′-bipyridine (0.077 g, 0.32 mmol) were dissolved in 10 mL of acetone. The reaction mixture was refluxed for 15 h. It was cooled to room temperature, and the solvent was evaporated. The product was purified by column chromatography (alumina, 100% CH2Cl2 to 1/100 MeOH/CH2Cl2). After reducing the volume of solvent of the collected fraction, the product was precipitated with ether, and obtained as a yellow solid. Yield 53%. Elemental Analysis: calculated for (C20H20Cl2N8Ru·CH2Cl2): C, 40.08; H, 3.52; N, 17.81. Found: C, 40.26; H, 3.30; N, 17.73. Exact Mass: HR-ESI-MS [C20H20ClN8Ru]+: calculated: m/z = 503.0575, found: m/z = 503.0571. 1H NMR (300 MHz, MeOD-d4): δ 9.33 (d, J = 6.3 Hz, 2H), 8.28 (d, J = 2.2 Hz, 2H), 7.48 (dd, J = 2.4 Hz, J = 6.3 Hz, 2H), 6.11 (d, J = 6.1 Hz, 2H), 5.85 (d, J = 6.1 Hz, 2H), 2.68 (sep, J = 6.9 Hz, 1H), 2.30 (s, 3H), 1.10 (d, J = 6.9 Hz, 6H). 13C-APT-NMR (75 MHz, MeOD-d4): δ 157.26 (2CH), 157.08 (2Cquat), 154.83 (2Cquat), 119.14 (2CH), 115.85 (2CH), 105.78 (Cquat), 105.50 (Cquat), 87.84 (2CH), 85.12 (2CH), 32.35 (CH), 22.34 (2CH3), 18.97 (CH3). [Ru(p-Cym)(4,4′-diamin-2,2′-bipyridine)Cl]Cl (9). Under a N2 atmosphere, [Ru(pCym)Cl2]2 (0.1 g, 0.16 mmol) and 4,4′diamin-2,2′-bipyridine (0.06 g, 0.326 mmol) were dissolved in 10 mL of acetone. The reaction mixture was refluxed for 15 h. It was cooled to room temperature and product was obtained as a brown solid that was filtered off. Yield 60%. Elemental Analysis: calculated for (C20H24Cl2N4Ru): C, 48.78; H, 4.91; N, 11.38. Found: C, 48.17; H, 4.88; N, 11.46. Exact Mass: HRESI-MS [C20H24ClN4Ru]+: calculated: m/z = 451.0765, found: m/z = 451.0768. 1H NMR (300 MHz, MeOD-d4): δ 8.70 (d, J = 6.6 Hz, 2H), 7.22 (d, J = 2.5 Hz, 2H), 6.77 (dd, J = 2.5 Hz, J = 6.6 Hz, 2H), 5.90 (d, J = 6.4 Hz, 2H), 5.63 (d J = 6.1 Hz, 2H), 2.61 (sep, J = 6.9 Hz, 1H), 2.26 (s, 3H), 1.08 (d, J = 6.9 Hz, 6H). 13C APT NMR (75 MHz, MeOD-d4): δ 158.16 (2Cquat), 156.61 (2Cquat), 155.09 (2CH), 112.55 (2CH), 107.61 (2CH), 104.12 (Cquat), 102.91 (Cquat), 87.07 (2CH), 83.93 (2CH), 32.24 (CH), 22.28 (2CH3), 19.02 (CH3). [Ru(p-Cym)(4-bromo-4′-azido-2,2′-bipyridine)(Cl)]Cl (10). Under a N2 atmosphere, [Ru(p-Cym)Cl2]2 (0.1 g, 0.16 mmol) and 4-bromo-4′-azido-2,2′-bipyridine (0.084 g, 0.30 mmol) were dissolved in 10 mL of acetone. The reaction mixture was refluxed for 15 h. It was cooled to room temperature, the solvent was evaporated, and the desired compound was obtained after precipitated with CH2Cl2/Et2O as a dark red solid. Yield 49%. Elemental Analysis: calculated for (C20H20BrCl2N5Ru): C, 41.25; H, 3.46; N, 12.03. Found: C, 40.76; H, 3.63; N, 11.81. Exact Mass: HR-ESI-MS [C20H20BrClN5Ru]+: calculated: m/z = 545.9632, found: m/z = 545.9642. 1H NMR (300 MHz, MeOD-d4): δ 9.33 (d, J = 2.4

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02958. X-ray crystallographic data for compound 10 (CIF) X-ray crystallographic data for compound 8 (CIF) X-ray crystallographic data for compound 4 (CIF) Synthetic procedures, Hammett parameters interpolation, reaction profiles of catalytic experiments, KIE effect studies, in situ NMR experiments, images and NMR spectra of the reactivity in neat with non H2-productive substrates, molecular structures of compounds 4, 8, and 10 according to X-ray structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zoraida Freixa: 0000-0002-2044-2725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS UPV-EHU SGIker and SCIC from the University Jaume I are acknowledged for technical assistance in the NMR and HR-MS analyses. This work was supported by Ikerbasque, Spanish MINECO (CTQ2013−23333), MINECO/FEDER (CTQ2015−65268-C2−1-P), UPV-EHU (GIU13/06), and Diputación Foral de Gipuzkoa (OF 215/2016).

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REFERENCES

(1) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079−4124. (2) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613−2626. (3) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc. Rev. 2009, 38, 279−293. (4) Wang, P.; Kang, X.-D. Dalton Trans. 2008, 5400−5413. (5) Umegaki, T.; Yan, J. M.; Zhang, X.-B.; Shioyama, H.; Kuriyama, N.; Xu, Q. Int. J. Hydrogen Energy 2009, 34, 2303−2311. (6) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116−8118. (7) Huang, Z.; Autrey, T. Energy Environ. Sci. 2012, 5, 9257−9268. (8) Zhan, W.-W.; Zhu, Q.-L.; Xu, Q. ACS Catal. 2016, 6, 6892−6905. (9) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun. 2001, 962−963. (10) Bhunya, S.; Malakar, T.; Ganguly, G.; Paul, A. ACS Catal. 2016, 6, 7907−7934 and references therein. (11) Zahmakiran, M.; Ayvali, T.; Philippot, K. Langmuir 2012, 28, 4908−4914. (12) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2003, 125, 9424−9434. 8403


Research Article

ACS Catalysis (13) Vance, J. R.; Schäfer, A.; Robertson, A. P. M.; Lee, K.; Turner, J.; Whittell, G. R.; Manners, I. J. Am. Chem. Soc. 2014, 136, 3048−3064. (14) Sloan, M. E.; Clark, T. J.; Manners, I. Inorg. Chem. 2009, 48, 2429−2435. (15) Sewell, L. J.; Huertos, M. A.; Dickinson, M. E.; Weller, A. S.; Lloyd-Jones, G. C. Inorg. Chem. 2013, 52, 4509−4516. (16) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K. J. Am. Chem. Soc. 2008, 130, 14034−14035. (17) Schreiber, D. F.; O’Connor, C.; Grave, C.; Ortin, Y.; MüllerBunz, H.; Phillips, A. D. ACS Catal. 2012, 2, 2505−2511. (18) Duman, S.; Ö zkar, S. Int. J. Hydrogen Energy 2013, 38, 180−187. (19) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048−12049. (20) Staubitz, A.; Presa Soto, A.; Manners, I. Angew. Chem., Int. Ed. 2008, 47, 6212−6215. (21) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.; Linehan, J. C. Inorg. Chem. 2008, 47, 8583−8585. (22) Esteruelas, M. A.; Fernández, I.; López, A. M.; Mora, M.; Oñate, E. Organometallics 2014, 33, 1104−1107. (23) Coles, N. T.; Mahon, M. F.; Webster, R. L. Organometallics 2017, 36, 2262−2268. (24) Roselló-Merino, M.; López-Serrano, J.; Conejero, S. J. Am. Chem. Soc. 2013, 135, 10910−10913. (25) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007, 129, 1844−1845. (26) Rossin, A.; Bottari, G.; Lozano-Vila, A. M.; Paneque, M.; Peruzzini, M.; Rossi, A.; Zanobini, F. Dalton Trans. 2013, 42, 3533− 3541. (27) Conley, B. L.; Williams, T. J. Chem. Commun. 2010, 46, 4815− 4817. (28) Conley, B. L.; Guess, D.; Williams, T. J. J. Am. Chem. Soc. 2011, 133, 14212−14215. (29) Lu, Z.; Conley, B. L.; Williams, T. J. Organometallics 2012, 31, 6705−6714. (30) Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. J. Am. Chem. Soc. 2014, 136, 11272−11275. (31) Xu, Q.; Chandra, M. J. Alloys Compd. 2007, 446−447, 729−732. (32) Xu, Q.; Chandra, M. J. Power Sources 2006, 163, 364−370. (33) Chandra, M.; Xu, Q. J. Power Sources 2006, 156, 190−194. (34) Caliskan, S.; Zahmakiran, M.; Durap, F.; Ö zkar, S. Dalton Trans. 2012, 41, 4976−7984. (35) Ozhava, D.; Ozkar, S. Int. J. Hydrogen Energy 2015, 40, 10491− 10501. (36) Yao, Q.; Lu, Z.-H.; Jia, Y.; Chen, X.; Liu, X. Int. J. Hydrogen Energy 2015, 40, 2207−2215. (37) Yang, Y.; Lu, Z.-H.; Hu, Y.; Zhang, Z.; Shi, W.; Chen, X.; Wang, T. RSC Adv. 2014, 4, 13749−13752. (38) Li, X.; Zeng, C.; Fan, G. Int. J. Hydrogen Energy 2015, 40, 3883− 3891. (39) Ning, X.; Li, Y.; Dong, B.; Wang, H.; Yu, H.; Peng, F.; Yang, Y. J. Catal. 2017, 348, 100−109. (40) Lapin, N. V.; D’yankova, N. Y. Inorg. Mater. 2013, 49, 975−979. (41) Basu, S.; Brockman, A.; Gagare, P.; Zheng, Y.; Ramachandran, P. V.; Delgass, W. N.; Gore, J. P. J. Power Sources 2009, 188, 238−243. (42) Chen, W.; Li, D.; Wang, Z.; Qian, G.; Sui, Z.; Duan, X.; Zhou, X.; Yeboah, I.; Chen, D. AIChE J. 2017, 63, 60−65. (43) Ç elik, B.; Erken, E.; Eriş, S.; Yıldız, Y.; Şahin, B.; Pamuk, H.; Sen, F. Catal. Sci. Technol. 2016, 6, 1685−1692. (44) Ma, H.; Na, C. ACS Catal. 2015, 5, 1726−1735. (45) Mahyari, M.; Shaabani, A. J. Mater. Chem. A 2014, 2, 16652− 16659. (46) Ciganda, R.; Garralda, M. A.; Ibarlucea, L.; Pinilla, E.; Torres, M. R. Dalton Trans. 2010, 39, 7226−7229. (47) Boulho, C.; Djukic, J.-P. Dalton Trans. 2010, 39, 8893−8905. (48) Graham, T. W.; Tsang, C.-W.; Chen, X.; Guo, R.; Jia, W.; Lu, S.M.; Sui-Seng, C.; Ewart, C. B.; Lough, A.; Amoroso, D.; Abdur-Rashid, K. Angew. Chem., Int. Ed. 2010, 49, 8708−8711. (49) Freixa, Z.; Garralda, M. A. Inorg. Chim. Acta 2015, 431, 184− 189.

(50) Muñoz-Olasagasti, M.; Telleria, A.; Pérez-Miqueo, J.; Garralda, M. A.; Freixa, Z. Dalton Trans. 2014, 43, 11404−11409. (51) Nelson, D. J.; Truscott, B. J.; Egbert, J. D.; Nolan, S. P. Organometallics 2013, 32, 3769−3772. (52) Fortman, G. C.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2011, 30, 5487−5492. (53) Garralda, M. A.; Mendicute-Fierro, C.; Rodríguez-Diéguez, A.; Seco, J. M.; Ubide, C.; Zumeta, I. Dalton Trans. 2013, 42, 11652− 11660. (54) San Nacianceno, V.; Ibarlucea, L.; Mendicute-Fierro, C.; Rodríguez-Diéguez, A.; Seco, J. M.; Zumeta, I.; Ubide, C.; Garralda, M. A. Organometallics 2014, 33, 6044−6052. (55) San Nacianceno, V.; Azpeitia, S.; Ibarlucea, L.; MendicuteFierro, C.; Rodríguez-Diéguez, A.; Seco, J. M.; San Sebastian, E.; Garralda, M. A. Dalton Trans. 2015, 44, 13141−13155. (56) Luo, Y.; Ohno, K. Organometallics 2007, 26, 3597−3600. (57) Peng, C.-Y.; Kang, L.; Cao, S.; Chen, Y.; Lin, Z.-S.; Fu, W.-F. Angew. Chem., Int. Ed. 2015, 54, 15725−15729. (58) Kalidindi, S. B.; Sanyal, U.; Jagirdar, B. R. Phys. Chem. Chem. Phys. 2008, 10, 5870−5874. (59) Yang, X.; Cheng, F.; Tao, Z.; Chen, J. J. Power Sources 2011, 196, 2785−2789. (60) Dykeman, R. R.; Luska, K. L.; Thibault, M. E.; Jones, M. D.; Schlaf, M.; Khanfar, M.; Taylor, N. J.; Britten, J. F.; Harrington, L. J. Mol. Catal. A: Chem. 2007, 277, 233−251. (61) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T. A. Chem. Rev. 1982, 82, 499−525. (62) Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46, 7810−7817. (63) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96−103. (64) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165−195. (65) Himeda, Y. Green Chem. 2009, 11, 2018−2022. (66) Lupp, D.; Christensen, N. J.; Fristrup, P. In Understanding Organometallic Reaction Mechanisms and Catalysis; Ananikov, V. P., Ed.; Wiley-VCH: Weinheim, 2015; pp 187−214. (67) Yunker, L. P. E.; Stoddard, R. L.; McIndoe, J. S. J. Mass Spectrom. 2014, 49, 1−8. (68) O’Hair, R. A. J. Chem. Commun. 2006, 1469−1481. (69) Putau, A.; Brand, H.; Koszinowski, K. J. Am. Chem. Soc. 2012, 134, 613−612. (70) Vikse, K. L.; McIndoe, J. S. Pure Appl. Chem. 2015, 87, 361− 377. (71) Vicent, C.; Gusev, D. G. ACS Catal. 2016, 6, 3301−3309. (72) Carrión, M. C.; Ruiz-Castañeda, M.; Espino, G.; Aliende, C.; Santos, L.; Rodríguez, A. M.; Manzano, B. R.; Jalón, F. A.; Lledós, A. ACS Catal. 2014, 4, 1040−1053. (73) Canivet, J.; Karmazin-Brelot, L.; Süss-Fink, G. J. Organomet. Chem. 2005, 690, 3202−3211. (74) Aliende, C.; Pérez-Manrique, M.; Jalón, F. A.; Manzano, B. R.; Rodríguez, A. M.; Espino, G. Organometallics 2012, 31, 6106−6123. (75) Kumar, A.; Johnson, H. C.; Hooper, T. N.; Weller, A. S.; Algarra, A. G.; Macgregor, S. A. Chem. Sci. 2014, 5, 2546−2553. (76) Kumar, R.; Jagirdar, B. R. Inorg. Chem. 2013, 52, 28−36. (77) Davis, R. E.; Brown, A. E.; Hopmann, R.; Kibby, C. L. J. Am. Chem. Soc. 1963, 85, 487. (78) Kelly, H. C.; Marriott, V. B. Inorg. Chem. 1979, 18, 2875−2878. (79) Hoel, E. L.; Hawthorne, M. F. J. Am. Chem. Soc. 1974, 96, 4676−4677. (80) Hoel, E. L.; Talebinasab-Savari, M.; Hawthorne, M. F. J. Am. Chem. Soc. 1977, 99, 4356−4367. (81) Kakizawa, T.; Kawano, Y.; Shimoi, M. Chem. Lett. 1999, 28, 869−870. (82) Nelson, D. A.; Egbert, J. D.; Nolan, S. P. Dalton Trans. 2013, 42, 4105−4109. (83) Simmons, E. M.; Hartwig, J. F. Angew. Chem., Int. Ed. 2012, 51, 3066−3072. (84) Huertos, M. A.; Weller, A. S. Chem. Sci. 2013, 4, 1881−1888. (85) Yang, X.; Fox, T.; Berke, H. Chem. Commun. 2011, 47, 2053− 2055. 8404


Research Article

ACS Catalysis (86) Hu, M. G.; Van Paasschen, M. J.; Geanangel, R. A. J. Inorg. Nucl. Chem. 1977, 39, 2147−2150. (87) Gómez-Gallego, M.; Sierra, M. A. Chem. Rev. 2011, 111, 4857− 4963. (88) Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Angew. Chem., Int. Ed. 2007, 46, 746−749. (89) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704−11712. (90) Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001, 20, 3211−3213. (91) Merle, N.; Frost, C. G.; Kociok-Köhn, G.; Willis, M. C.; Weller, A. S. J. Organomet. Chem. 2005, 690, 2829−2834. (92) Merle, N.; Koicok-Köhn, G.; Mahon, M. F.; Frost, C. G.; Ruggerio, G. D.; Weller, A. S.; Willis, M. C. Dalton Trans. 2004, 3883− 3892. (93) Yasue, T.; Kawano, Y.; Shimoi, M. Angew. Chem., Int. Ed. 2003, 42, 1727−1730. (94) Kawano, Y.; Hashiva, M.; Shimoi, M. Organometallics 2006, 25, 4420−4426. (95) Alcaraz, G.; Vendier, L.; Clot, E.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010, 49, 918−920. (96) Roselló-Merino, M.; Rama, R. J.; Diéz, J.; Conejero, S. Chem. Commun. 2016, 52, 8389−8392. (97) Tang, C. Y.; Thompson, A. L.; Aldridge, S. J. Am. Chem. Soc. 2010, 132, 10578−10591. (98) Alcaraz, G.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010, 49, 7170−7179. (99) Sewell, L. J.; Chaplin, A. B.; Weller, A. S. Dalton Trans. 2011, 40, 7499−7451. (100) Stevens, C. J.; Dallanegra, R.; Chaplin, A. B.; Weller, A. S.; Macgregor, S. A.; Ward, B.; McKay, D.; Alcaraz, G.; Sabo-Etienne, S. Chem. - Eur. J. 2011, 17, 3011−3020. (101) Douglas, T. M.; Chaplin, A. B.; Weller, A. S.; Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2009, 131, 15440−15456. (102) Wallis, C. J.; Dyer, H.; Vendier, L.; Alcaraz, G.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2012, 51, 3646−3648. (103) Zhang, D.; Telo, J. P.; Liao, C.; Hightower, S. E.; Clennan, E. L. J. Phys. Chem. A 2007, 111, 13567−13574. (104) Arzoumanian, H.; Bakhtchadjian, R.; Agrifoglio, G.; Atencio, R.; Briceno, A. Transition Met. Chem. 2006, 31, 681−689. (105) James, P. V.; Yoosaf, K.; Kumar, J.; Thomas, K. G.; Listorti, A.; Accorsi, G.; Armaroli, N. Photochem. Photobiol. Sci. 2009, 8, 1432− 1440. (106) Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org. Chem. 1996, 61, 6535−6546. (107) Kelly, T. R.; Lee, Y.-J.; Mears, R. J. J. Org. Chem. 1997, 62, 2774−2781. (108) Lin, K. F.; Ni, J.-S.; Tseng, C.-H.; Hung, C.-Y.; Liu, K.-Y. Mater. Chem. Phys. 2013, 142, 420−427. (109) Fabbrizzi, P.; Cecconi, B.; Cicchi, S. Synlett 2011, 2011 (2), 223−226. (110) Maury, O.; Guégan, J.-P.; Renouard, T.; Hilton, A.; Dupau, P.; Sandon, N.; Toupet, L.; Le Bozec, H. New J. Chem. 2001, 25, 1553− 1566. (111) Kopecky, A.; Liu, G.; Agushi, A.; Agrios, A. G.; Galoppini, E. Tetrahedron 2014, 70, 6271−6275. (112) Welby, C. E.; Armitage, G. K.; Bartley, H.; Wilkinson, A.; Sinopoli, A.; Uppal, B. S.; Rice, C. R.; Elliott, P. I. P. Chem. - Eur. J. 2014, 20, 8467−8476.

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Catalyzed Dehydrocoupling and Hydrolysis of Amine-Boranes

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