Nitrogen-Based Ligands Accelerate Ammonia Borane

Jul 29, 2015 - Iridium-based hydride transfer catalysts: from hydrogen storage to fine chemicals. Zhiyao Lu , Valeriy Cherepakhin , Ivan Demianets , P...
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Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst Xingyue Zhang, Zhiyao Lu, Lena K. Foellmer, and Travis J. Williams* Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661, United States S Supporting Information *

ABSTRACT: We previously reported that quantitative poisoning, a test for homogeneous catalysis, behaves oddly in the dehydrogenation of ammonia borane (AB) by Shvo’s catalyst, whereas the “poison” 1,10-phenanthroline (phen) accelerates catalysis and apparently prevents catalyst deactivation. Thus, we proposed a protective role for phen in the catalysis. Herein we account for the mechanistic origin of this accelerated AB dehydrogenation in the presence of phen and define the relevance boundaries of our prior proposal. In so doing, we present syntheses for novel amine- and pyridineligated homologues of the Shvo catalyst and show their catalytic efficacy in AB dehydrogenation. These catalysts are synthetically easy to access, air stable, and rapidly release over 2 equiv of H2. The mechanisms of these reactions are also discussed.

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INTRODUCTION

the catalyst is deactivated by hydroboration by borazine (6, Scheme 1).14 In that study we conducted a quantitative poisoning experiment to rule out heterogeneous nanoparticle formation by adding 1,10-phenanthroline (phen) to the reaction.15 Instead of the expected slowing, we saw rate acceleration proportional to [phen]. We further observed that when LnRu-NH3 adduct 916 was used as a catalyst precursor, it was not deactivated by borazine (Figure 2) in the same way as the parent system (1). With these two observations, we formed a hypothesis that NH3 or phen can coordinate 3, which preempts hydroboration of 3 by borazine until the complex can associate AB by hydrogen bonding to its carbonyl group,12b,17 thus avoiding catalyst deactivation (dotted line in Scheme 1).14 Herein we challenge our hypothesis of a protective role for NH3 or phen by analyzing the kinetics of AB dehydrogenation with isolated pyridine- and amine-ligated homologues of the Shvo system. In addition to refining our understanding of this thought-provoking mechanistic issue, we report here that these novel, air-stable, and easily prepared complexes are very efficient catalysts for AB dehydrogenation in their own right, releasing 2 equiv of H2 in 1 h (70 °C) at 10 mol % catalyst loading. We show data that indicate that the success of these new catalytic systems is based on our original hypothesis of a protective role for their supporting pyridine ligands, and we show reactivity data for complexes of variously substituted pyridines that inform this proposal. We further show that bidentate nitrogen ligands such as 2,2′-bipyridine (bipy) or

The dehydrogenation of ammonia borane (AB, H3N-BH3) is a popular approach to high-capacity hydrogen storage because of AB’s high hydrogen density (19.6 wt %), ability to release H2 under mild conditions, and desirable physical properties.1 Accordingly, many catalytic approaches to hydrogen release from AB have appeared, involving both aqueous2 and anhydrous systems,3 the latter offering a more facile approach to spent-fuel rehydrogenation.4 Among the homogeneous systems that are known, excellent approaches based on iron,5 molybdenum,6 iridium,7 rhodium,8 nickel,9 palladium,10 and ruthenium11,12 have been reported. Two of the first examples of reusable ruthenium catalysts have come from our lab. Particularly, we used Shvo’s catalyst 112 to dehydrogenate AB, then developed catalysts 2a and 2b (Figure 1), which are capable of releasing >2 equiv of H2 from AB and up to 4.6 wt % of H2 in high [AB].11c,13 We have proposed a detailed mechanism for AB dehydrogenation by Shvo’s catalyst wherein

Received: May 14, 2015 Published: July 29, 2015

Figure 1. Ru-based AB dehydrogenation catalysts. © 2015 American Chemical Society

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Organometallics Scheme 1. Proposed AB Dehydrogenation Mechanism by Shvo’s Catalyst

Figure 2. Left: AB dehydrogenation catalyzed by 1 (10 mol % Ru atom) (green diamond) and 1 with phen (0.1 equiv to Ru, blue triangle; 0.5 equiv, red circle). Right: AB dehydrogenation catalyzed by 14 mol % Ru atom of 1 (green diamonds) and 9 (red circles).

phen can expel the tetraphenylcyclopentadienone (CPD) ligand from the metal center, which is a previously undocumented form of reactivity for the Shvo system.

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RESULTS AND DISCUSSION Synthesis, Characterization, and Reactivity of Pyridine Complexes. Pyridine-ligated ruthenium complexes 11− 13 were prepared by dropwise addition of the appropriate substituted pyridine to ruthenium dimer 10 in benzene solvent at room temperature (Scheme 2). Treatment of the reaction mixture with hexanes yielded the corresponding products as pale gray powders. Structural data collected on the products were as expected; the molecular structure of 11 is shown in Figure 3. Therein, ruthenium adopts the predicted piano stool

Figure 3. Catalytic AB dehydrogenation Left: AB consumption (11B NMR) catalyzed by 1, 11, 12, and 13 (10 mol % Ru atom, 70 °C, 1:2 C6D6/diglyme). Right: H2 release by eudiometry with 13 (10 mol % Ru atom, 70 °C, 1:2 C6D6/diglyme with 2% EtOH).

geometry, which has analogy to the known molecular structures of 916 and [(2,5-Ph2-3,4-Tol2-CPD)Ru(CO)(PPh3)(pyr)].18 The complexes are bench-stable with stability similar to 1. Compounds 11−13 affect significantly faster AB dehydrogenation than 1 (Figure 3, left). For example, a reaction catalyzed by 13 and 2% EtOH has a rate constant of H2 evolution of 1.40(4) × 10−3 s−1, which is 4-fold faster than an analogous reaction of 1, with a rate constant of 3.7(1) × 10−4 s−1 (Figure 3, right).12 Moreover, 13 gives a slightly higher extent of H2 release, 2.1 equiv. Ethanol here is a cocatalyst: although it has known reactivity with AB, particularly when present as solvent and activated by base,19 we have shown

Scheme 2. Synthesis of Pyridine Complexes 11−13

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Organometallics previously that a catalytic portion is a first-order cocatalyst that functions primarily as a proton shuttle in the Shvo system.12a Comparable results are obtained when free pyridines are added to reaction solutions of 1; these reactions differ from those of isolated pyridine adducts by only a brief initiation delay. If free nitrogen ligands are added to a reaction mixture, they show a similar protective behavior. For example, if 1 is treated with a 4-DMAP, Et3N, or even tetramethylethylenediamine (TMEDA) in situ, saturation catalysis (the linear region in the kinetic profile) is extended from ca. 30% conversion to ca. 80% conversion.12 See the Supporting Information for time course plots. Impact of Pyridine Substitution on Reactivity. In the dehydrogenation studies shown in Figure 3, we observe increasing reaction rates (12 < 11 < 13) as the pyridine ligands become less electron rich. For example, these reactions have AB consumption rate constants of 2.04(4), 4.3(1), and 5.6(2) × 10−4 M s−1, respectively, for precatalysts 12, 11, and 13 in their saturation catalysis periods. We account for this by considering binding affinity of the pyridine ligand to ruthenium: the electron-rich 4-dimethylaminopyridine (4DMAP) ligand in 12 should bind an electrophilic metal more tightly than an electron-neutral pyridine. The complement is applicable to the least electron-rich 4-trifluoromethylpyridine (4-TFMP). Accordingly, when 13 is treated with 1 equiv of 4DMAP, we see 12 formed in solution at room temperature immediately with approximately 90% of 13 converted to 12 in 90 min at 70 °C. We rationalize from our observations that, while these pyridines can increase the rate for AB dehydrogenation, a less tightly binding pyridine enables a faster reaction by facilitating ligand/substrate exchange on the ruthenium center. This is consistent with a proposal that pyridine binding to ruthenium is protective of the catalyst, but pyridine dissociation is necessary for catalytic turnover. Interactions of Pyridine Complexes with BN Compounds in Solution. The pyridine ligand has several reactive options in solution. Beyond ruthenium, free pyridine can ligate BH3; we observe pyridine-BH3 by 11B NMR in the course of these reactions. Further evidence of pyridine ligation to boron, nitrogen species comes from the 19F NMR handle of 13: we observed over eight 4-TFMP species at the end of the reaction, including free ligand. In the corresponding 11B NMR spectra, we further observe broad signals that are consistent with polymeric boron byproducts such as polyborazylene. In contrast, AB dehydrogenation reactions catalyzed by 1 have borazine as the sole boron product. On the basis of these observations, we infer that the pyridines are ligating intermediates in dehydrogenation in a way that disfavors borazine accumulation. We also observe in reactions involving 12, the 4-DMAP adduct [Me2N-C5H4N-BH2NH2-BH3], 14, which was recently characterized by Rivard (Figure 4).20 To determine whether 14 is a necessary part of the fastest mechanistic pathway for AB dehydrogenation or merely a cul-de-sac, we dehydrogenated an independently prepared sample of 14. Treatment of 14 (0.11 M) with 1 (10% Ru atom) was comparable in rate to 1catalyzed AB dehydrogenation, if not slightly slower under the same conditions, so we find that formation of 14 is not enabling more rapid AB dehydrogenation. Since 14 and pyridine-BH3 are the only major pyridine−boron adducts that we observe during catalysis or in a ruthenium-free control reaction (vide infra), and because these dehydrogenate more slowly than AB under our catalytic conditions, we believe that pyridine−boron

Figure 4. Left: Metal-catalyzed dehydrogenation of 14. Right: Dehydrogenation of AB (blue squares) and 14 (red circles) by Shvo’s catalyst 1 (10 mol % Ru atom, 70 °C, 1:2 C6D6/diglyme).

ligation is not the major cause of reaction acceleration by added pyridine. Based on these observations, monodentate pyridine ligands can play a protective role in the dehydrogenation of AB, probably by preempting borazine hydroboration. We see that the catalysis rate is increased and catalyst deactivation is avoided in the presence of pyridines, whether these ligands are precomplexed to the catalyst or added separately to the reaction. We believe that pyridine−ruthenium ligation is important to this behavior. CPD Ligand Displacement by Bidentate Nitrogen Ligands. When AB dehydrogenation reactions involving 1 are treated with bidentate nitrogen ligands (e.g., phen, bipy, TMEDA), the active catalyst for AB dehydrogenation could involve multiple [RuLn] species. This is due to the lability of the CPD ligand in the presence of these bidentate dinitrogen compounds at elevated temperatures. For example, CPD is displaced in the attempted syntheses of a TMEDA-supported complex, 15. In conditions analogous to the syntheses of 11− 13, we observe that treatment of 10 with TMEDA results in the formation of dinuclear species 16, where one CPD ligand is expelled (Scheme 3). CPD displacement is confirmed by GCScheme 3. Synthesis and Structure of 16 and ORTEP Diagram of 16 with Ellipsoids Drawn at the 50% Probability Level and Hydrogens Omitted for Clarity

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Organometallics

On the basis of these observations, we believe that the origins of rate acceleration in our quantitative poisoning experiments include the formation of a reactive, homogeneous [(phen)Ru(CO)2] species.

MS. In addition to offering a structural suggestion for a reactive, CPD-free AB dehydrogenation catalyst monomer, [(CO)2Ru(TMEDA)], the dimeric structure of 16 presents a first-in-class Ru−Ru bond (2.7735(6) Å) formed to a Shvo-like fragment, here featuring a single CPD ligand bridging both metal centers. As expected, free CPD is detected when 10 reacts with bidentate ligands phen or bipy at 70 °C. In contrast to TMEDA, we were unable to isolate a single phen-ligated ruthenium adduct, even when treating 10 with phen at room temperature. A control experiment with pyridine and 10 at 70 °C yields only 11: no CPD liberation is observed. We therefore surmise that bidentate nitrogen ligands will expel CPD f rom the Shvo system under our reaction conditions, while pyridines will not. Kinetic Effects of CPD Displacement. Formation of a [(phen)Ru(CO)2]2+ species in situ results in a very rapid system for AB dehydrogenation, although it is unclear what the active catalyst(s) are in this system. For example, treating [(phen)RuCl2(CO)2] with 2 equiv of TlOTf afforded an excellent catalyst for AB dehydrogenation, which completed in 1.5 h under our conditions. Figure 5 (left) shows the relative

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CONCLUSIONS We show here that amine and pyridine ligands can accelerate the 1-catalyzed dehydrogenation of AB. Monodentate pyridines enable accelerated dehydrogenation of AB without CPD displacement. We propose that the reason for this is that pyridines can protect the catalyst against hydroboration by borazine. Bidentate (N−N) ligands appear to accelerate catalysis through an additional mechanism where a relatively more reactive ruthenium species is formed upon loss of CPD. Further experiments on improved H2 production using these catalysts and strategies are under way.

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EXPERIMENTAL SECTION

I. General Procedures. All air- and water-sensitive procedures were carried out either in a Vacuum Atmospheres glovebox under nitrogen (0.5−10 ppm of O2 for all manipulations) or using standard Schlenk techniques under nitrogen. Ethanol was distilled from sodium ethoxide and stored over molecular sieves. Deuterated NMR solvents were purchased from Cambridge Isotopes Laboratories. Benzene, benzene-d6, diethylene glycol dimethyl ether (diglyme, Alfa Aesar), tetraethylene glycol dimethyl ether (tetraglyme, Alfa Aesar), tetramethylethylenediamine (Alfa Aesar), and triethylamine (Et3N, Alfa Aesar) were dried over sodium benzophenone ketyl and distilled prior to use. Shvo’s catalyst was purchased from Strem Chemicals and used without further purifications. Ammonia borane (NH3BH3) was purchased from Sigma-Aldrich and used under a N2 atmosphere without further purifications. The integrity of this material was checked regularly by 1H and 11B NMR. Pyridine, 4-DMAP, 4-TFMP, and 1,10-phenanthroline were purchased from Alfa Aesar and used without further purification. 2,2-Bipyridine was purchased from Lancaster Synthesis Inc. and used without further purification. Dimer 10 was synthesized according to Mays, Morris, and Shvo.21 Ammonia adduct 9,14,16 pyridine adduct 14,20 and Ru(Phen)Cl2(CO)222 were prepared according to literature procedures. Sonication procedures were done in a VWR desktop sonic cleaner bath. GC-MS spectra were acquired with a Thermo Focus GC equipped with a DSQII mass-selective detector. 1H and 11B NMR spectra were obtained on Varian 600, 500, and 400 MHz spectrometers (600 MHz in 1H, 192 MHz in 11B) with chemical shifts reported in units of ppm. All 1H chemical shifts were referenced to the residual 1H solvent (relative to TMS). All 11B chemical shifts were referenced to BF3-OEt2 in diglyme in a coaxial external standard (0 ppm). All 19F chemical shifts were referenced to a CFCl3 external standard. NMR spectra were taken in 8-inch J-Young tubes (Wilmad) with Teflon valve plugs. All spectra were processed using MesRe Nova (v. 9.0.0-12821). FTIR were taken on KBr salt plates on a Bruker Vertex 80 FTIR. Accurate mass spectrometry was conducted by the University of California, Riverside, High Resolution Mass Spectrometry Facility. Elemental analysis was conducted by the University of Illinois Urbana−Champaign Microanalysis Laboratory. II. Preparative and Spectroscopic Details. 11: Ru-pyridine adduct 11 was prepared under air by dissolving 10 (50 mg, 0.046 mmol, 1 equiv) in benzene (5 mL). Pyridine (15 μL, 0.18 mmol, 4 equiv) was dissolved separately in benzene (0.1 mL) and added dropwise to the 10/benzene solution. The reaction mixture color lightened from an orange to a lighter yellow-orange. The reaction was stirred at room temperature overnight to ensure completion. The reaction was then passed through a pipet filter filled with cotton and Celite, and benzene was removed by rotary evaporation. The residue was dried under reduced pressure overnight. Hexanes (5 mL) was added to the residue, and the mixture was immersed in a sonication bath for 15 min. A pale yellow-white precipitate was filtered out and

Figure 5. Left: Dehydrogenation of AB by 1 (green circles), (phen)RuCl2(CO)2 with 2 equiv of TlOTf (blue diamonds), and 11 (red squares). Right: Dehydrogenation of AB by 1 (green circles), (phen)RuCl2(CO)2 with 2 equiv of TlOTf (blue diamonds), and 11 (red squares). Conditions are 10 mol % Ru atom, 70 °C, 1:2 C6D6/ diglyme.

rates of AB consumption: these conditions are significantly more reactive than 1, but less reactive than 11. As expected, no mechanism change is obvious in this time course plot. Presumably, part of the reason that phen did not poison the Shvo system as expected, but rather accelerated it, was the generation of a portion of a relatively reactive [(phen)Ru(CO)2] species in situ. In line with this proposal, the rate of AB consumption with phen-treated 1 lies nicely between the rates of dehydrogenation with 1 and with our [(phen)Ru(CO)2] species, which is consistent with a mechanism that involves a [(phen)Ru(CO)2] catalyst that is attenuated by interaction with a CPD-ligated moiety. We surmise that CPD was not involved in this accelerated reaction. Ruthenium must have been involved, however, because while we observe that phen, TMEDA, or 4-DMAP will release H2 from AB without ruthenium, these are slow and have different product selectivities: 11B NMR shows these to give a branched cyclotetraborazane1j and only traces of borazine after 4 h at 70 °C. 3735

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Tetramethylethylenediamine (14 μL, 0.18 mmol, 2 equiv) was separately dissolved in hexanes (20 mL). The 10/benzene solution was added dropwise into the hexanes solution. The reaction was stirred vigorously at room temperature overnight. The color of the heterogeneous reaction mixture lightened from orange to pale yellowwhite upon completion. Solvent was then removed by rotary evaporation. The residue was dried under reduced pressure overnight. Hexanes (5 mL) was added to the residue, and the mixture was immersed in a sonication bath for 15 min. A pale yellow-white precipitate was filtered out and washed with hexanes. The solid was then lyophilized from a benzene solution to give a dull white-yellow powder in 89% yield (49 mg). 1 H NMR (600 MHz, methylene chloride-d2): δ 7.59 (dd, J = 8.3, 1.3 Hz, 8H, Ph), 7.12 (t, J = 7.5 Hz, 8H, Ph), 7.10−7.02 (m, 24H, Ph), 2.51 (s, 4H, ethylene), 2.05 (s, 12H, tetramethyls). 13C NMR (150 MHz, benzene-d6): δ 201.79 (CO), 167.62 (C1 of Cp), 134.34 (Ph), 132.24 (Ph), 132.18 (Ph), 130.46 (Ph), 128.14 (Ph), 127.98 (Ph), 127.81 (Ph), 126.39 (Ph), 103.68 (C2,5 of Cp), 82.46 (C3,4 of Cp), 57.02 (ethylene), 53.27 (tetramethyls). FTIR (ν, cm−1): 2012.3, 1950.6 (M − CO’s), 1648.4 (CO). Anal. Calcd: C, 68.39; H, 5.08; N, 2.28. Found: C, 68.1; H, 4.72; N, 2.15. Mp: 182−191 °C, dec, black crust. Unsymmetrical Dimer 16. 16 is not readily separated from 15 and 17 (see Supporting Information). Two separate synthetic routes are described. 1. Preparation of 15, 16, and 17 from 10 and TMEDA was conducted by dissolving 10 (50 mg, 0.046 mmol, 1 equiv) in benzene (5 mL) under air. Tetramethylethylenediamine (7 μL, 0.09 mmol, 1 equiv) was added to the solution while stirring vigorously. The reaction was then stirred in a 35 °C oil bath overnight. The resulting dark red-purple solution was then passed through a filter filled with cotton and Celite, the solvent was removed by rotary evaporation, and the residue was dried under reduced pressure. Hexanes (5 mL) was added to the residue, and the resulting suspension was immersed in a sonication bath for 5 min to produce a dark red powder. The powder was isolated via filtration and washed with hexanes. Benzene was added to the powder, and the resulting solution was lyophilized to dryness. NMR analysis of the powder showed 17 as the major product, with 16 and 15 as minor products. Minimum fresh benzene was added to dissolve a portion of the powder (ca. 15 mg), and vapor diffusion was conducted with hexanes selectively to yield crystals of 16 for X-ray analysis. 2. In a separate synthesis of 16, 10 (7.62 mg, 0.014 mmol of Ru atom, 1 equiv of Ru atom) and Ru3(CO)12 (3.0 mg, 0.014 mmol of Ru atom, 1 equiv of Ru atom) were dissolved in benzene-d6 (0.5 mL) in a J-Young NMR tube in a nitrogen-filled glovebox. Tetramethylethylenediamine (2.1 μL, 0.014 mmol, 1 equiv) was added, and the tube was swirled to allow solvation of all reactants. The tube was put in a 45 °C oil bath, and after 10 min, the orange solution had darkened to a darker orange; then the tube was left to react for 36 h before NMR analysis. Although the major product (16) could not be separated from other Ru-containing compounds in the reaction, it matched 1H NMR shifts of the minor product observed above (see Supporting Information for further details). 1 H NMR (400 MHz, benzene-d6): δ 7.91, 6.99, 6.94, 6.86, 6.73, 2.18 (s, 6H), 1.65−1.68 (m, 8H, ethylene and methyl), 1.47 (m, 2H). III. Mechanistic Studies Using 11B NMR. In a typical reaction, 7.7 mg of AB was combined with catalyst (10 mol %) in a J-Young NMR tube while in a glovebox under nitrogen. The AB concentration and catalyst concentrations may be varied. Diglyme or tetraglyme (0.4 mL) and benzene-d6 (0.2 mL) were added to the tube. The sample tube was immediately inserted into a preheated, preshimmed, and prelocked NMR (70 °C) tube, and the kinetic monitoring commenced. Disappearance of AB in the solution was monitored by the relative integration of its characteristic peak in the 11B spectrum (−22 ppm) and the BF3-OEt2 standard. The acquisition involved a 1.84 s pulse sequence in which 16 384 complex points were recorded, followed by 1 s relaxation delay. To eliminate B−O peaks from the borosilicate NMR tube and probe, the 11B FIDs were processed with backward linear prediction. Safety note: caution should be used when

washed with hexanes. The solid was then lyophilized from a benzene solution to give a pale yellow-white powder in 84% yield (46 mg). 1 H NMR (600 MHz, benzene-d6): δ 8.19 (d, J = 7.7 Hz, 4H, Ph), 8.10 (d, J = 4.8 Hz, 2H, C2,6 of Pyr), 7.31 (d, J = 7.0 Hz, 4H, Ph), 7.06 (t, J = 7.6 Hz, 4H, Ph), 6.94 (t, J = 7.4 Hz, 2H, Ph), 6.86 (t, J = 7.5 Hz, 4H, Ph), 6.80 (t, J = 7.4 Hz, 2H, Ph), 6.44 (t, J = 7.7 Hz, 1H, C4 of Pyr), 6.05 (t, J = 6.7 Hz, 2H, C3,5 of Pyr). 13C{1H} NMR (150 MHz, benzene-d6): δ 201.46 (CO), 170.05 (C1 of Cp), 155.68 (C2,6 of Pyr), 137.07 (C4 of Pyr), 134.69 (Ph), 132.79 (Ph), 132.57 (Ph), 130.34 (Ph), 128.35 (Ph), 128.18 (Ph), 127.95 (Ph), 126.43 (Ph), 125.51 (C3,5 of Pyr), 104.71 (C2,5 of Cp), 80.81 (C3,4 of Cp). FTIR (ν, cm−1): 2008.2, 1956.4 (M − CO’s), 1615.8 (CO). ESI-MS for [M − H]+: calcd 622.0951 g/mol, found 622.0950 g/mol. Mp: 181−187 °C, dec, black crust. 12: Ru-4-DMAP adduct 12 was prepared under air by dissolving 10 (50.0 mg, 0.046 mmol, 1 equiv) in benzene (5 mL). 4Dimethylaminopyridine (22.6 mg, 0.18 mmol, 4 equiv) was separately dissolved in benzene (0.1 mL), immersed in a sonication bath briefly until homogeneous, and added dropwise to the 10/benzene solution. The reaction mixture color lightened from an orange to a lighter yellow-orange. The reaction was stirred at room temperature overnight to ensure completion. The reaction was then passed through a pipet filter filled with cotton and Celite, and benzene was removed by rotary evaporation. The residue was dried under reduced pressure overnight. Diethyl ether (5 mL) was added to the residue, and the resulting suspension was immersed in a sonication bath for 15 min. A pale yellow-white precipitate was filtered out and washed with hexanes. The solid was then lyophilized from a benzene solution to give a pale yellow-white powder in 60% yield (37 mg). 1 H NMR (400 MHz, benzene-d6): δ 8.33−8.30 (m, 4H, Ph), 7.80− 7.77 (m, 2H, C2,6 of DMAP), 7.39−7.36 (m, 4H, Ph), 7.12 (t, J = 7.8 Hz, 4H, Ph), 6.97 (t, J = 7.4 Hz, 2H, Ph), 6.88 (t, J = 7.4 Hz, 4H, Ph), 6.84−6.79 (m, 2H, Ph), 5.34 (d, J = 7.3 Hz, 2H, C3,5 of DMAP), 1.77 (s, 6H, methyls of DMAP). 13C NMR (101 MHz, CDCl3): δ 200.94 (CO), 169.46 (C1 of Cp), 154.51 (C2,6 of DMAP), 154.26 (C4 of DMAP), 134.04 (Ph), 132.54 (Ph), 132.26 (Ph), 130.05 (Ph), 127.69 (Ph), 127.61 (Ph), 127.46 (Ph), 125.80 (Ph), 108.63 (C3,5 of DMAP), 103.18 (C2,5 of Cp), 80.42 (C3,4 of Cp), 39.11 (methyls of DMAP). FTIR (ν, cm−1): 2009.0, 1939.3 (M − CO’s), 1627.1 (CO). ESIMS for [M − H]+: calcd 665.1373 g/mol, found 665.1376 g/mol. Mp: 199−209 °C, dec, black crust. 13: Ru-4-TFMP adduct 13 was prepared by dissolving 10 (50 mg, 0.046 mmol, 1 equiv) in benzene (5 mL) under air. 4Trifluoromethylpyridine (21 μL, 0.18 mmol, 4 equiv) was dissolved in benzene (0.1 mL) and added dropwise to the 10/benzene solution. The reaction mixture color lightened from orange to a lighter yelloworange. The reaction was stirred at room temperature overnight to ensure completion. The reaction was then passed through a pipet filter filled with cotton and Celite, and benzene was removed by rotary evaporation. The residue was dried under reduced pressure overnight. Hexanes (5 mL) was added to the residue, and the mixture was immersed in a sonication bath for 15 min. A pale yellow-white precipitate was filtered out and washed with hexanes. The solid was then lyophilized from a benzene solution to give a dull yellow-orange powder in 82% yield (52 mg). 1 H NMR (400 MHz, benzene-d6): δ 8.18−8.14 (m, 4H, Ph), 8.08 (d, J = 5.9 Hz, 2H, C2,6 of TFMP), 7.31−7.28 (m, 4H, Ph), 7.06 (t, J = 7.7 Hz, 4H, Ph), 6.96−6.91 (m, 2H, Ph), 6.87 (dd, J = 8.2, 6.4 Hz, 4H, Ph), 6.84−6.78 (m, 2H, Ph), 6.08 (d, J = 5.8 Hz, 2H, C3,5 of TFMP). 13 C NMR (101 MHz, CDCl3): δ 199.98 (CO), 168.46 (C1 of Cp), 156.78 (C2,6 of TFMP), 139.72 (q, J = 35.4 Hz, C4 of TFMP), 133.04 (Ph), 132.09 (Ph), 131.74 (Ph), 130.06 (Ph), 128.02 (Ph), 127.87 (Ph), 127.79 (Ph), 126.49, (Ph), 122.09 (q, J = 273.7 Hz, CF3), 121.69 (q, J = 3.5 Hz, C3,5 of TFMP), 103.89 (C2,5 of Cp), 81.42(C3,4 of Cp). 19 F NMR (564 MHz, benzene-d6): δ −65.69. FTIR (ν, cm−1): 2015.8, 1960.1 (M − CO’s), 1606.1 (CO). ESI-MS for [M − H]+: calcd 690.0825 g/mol, found 690.0831 g/mol. Mp: 200−207 °C, dec, black liquid. Dimer 15: Ru-TMEDA dimer 15 was prepared under air by dissolving 10 (50 mg, 0.046 mmol, 1 equiv) in benzene (5 mL). 3736

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Organometallics carrying out these reactions, as the release of hydrogen can lead to sudden pressurization of reaction vessels. IV. Hydrogen Quantification. In a typical reaction, 7.7 mg of AB (0.25 mmol) was combined with catalyst (10 mol %) in a 2 mL Schlenk bomb equipped with a Teflon stir bar while in a glovebox under nitrogen. Benzene (0.2 mL) and diglyme containing 2 mol % ethanol (0.4 mL) were added to the flask (made from a stock solution of 3.7 μL of EtOH in 5 mL of diglyme). A eudiometer was constructed as follows: the side arm of the valve of the Schlenk flask was connected to a piece of Tygon tubing, which was adapted to 20 gauge (0.03 inch) Teflon tubing with a needle. The tubing was threaded through the open end of a buret that was sealed with a Teflon stopcock on the other end. The buret was filled with water. The entire apparatus was then inverted into a 500 mL Erlenmeyer filled with water and clamped onto a metal ring stand. The reactor’s valve was opened to release gas from the reactor headspace while heating in a regulated oil bath. The volume of liberated gas was recorded periodically until gas evolution ceased. Liberated hydrogen was quantified by recording its volume displacement in the eudiometer. V. GC-MS Detection of Free CPD. In a typical reaction, 10 and a bidentate nitrogen ligand (1,10-phenanthroline, TMEDA, or 2,2′bipyridine) were added into a vial at room temperature. The reaction was placed in a 70 °C oil bath for 12−16 h. The solvent was then removed under reduced pressure, and the residue was immersed in a sonication bath in hexanes for 1 min. The supernatant was then passed through a Celite filter and analyzed by GC-MS. CPD elutes at ca. 16.8 min with this method, with the major mass peaks being 384 m/z (CPD) and 178 m/z (portion of acetylene). See the Supporting Information for spectra. CDCC 1062142 (11) and 1062141 (16) contain supplementary crystallographic data for this article.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00409. Graphical NMR data and detailed kinetic plots (PDF) X-ray crystallographic data for compound 11 (CIF) X-ray crystallographic data for compound 16 (CIF) Compounds 11 and 16 (MOL)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (T. J. Williams): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National Science Foundation (CHE-1054910) and the Hydrocarbon Research Foundation. We thank the NSF (DBI-0821671, CHE-0840366, CHE1048807) and NIH (1 S10 RR25432) for sponsorship of research instrumentation. Fellowship assistance from The Sonosky Foundation of the USC Wrigley Institute (X.Z., Z.L.) and the Johns Hopkins Center for Talented Youth (L.K.F.) is gratefully acknowledged. We acknowledge Dr. Brian Conley and Jeff Celaje, and Nicky Terrile for insightful discussions. We thank Dr. Ralf Haiges for help with X-ray crystallography.

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REFERENCES

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