Thermal Dehydrogenation of Dimethylamine Borane Catalyzed by a

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Article Cite This: Organometallics 2019, 38, 2602−2609

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Thermal Dehydrogenation of Dimethylamine Borane Catalyzed by a Bifunctional Rhenium Complex Veeranna Yempally,† Salvador Moncho,† Yanyan Wang,‡ Samuel J. Kyran,‡ Wai Yip Fan,§ Edward N. Brothers,† Donald J. Darensbourg,*,‡ and Ashfaq A. Bengali*,† †

Department of Chemistry, Texas A&M University at Qatar, Doha, Qatar Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States § Department of Chemistry, National University of Singapore, Singapore 117543 Downloaded via BUFFALO STATE on July 18, 2019 at 11:04:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: The reaction of a bidentate pyrrole based rhenium tetracarbonyl complex with dimethylamine borane at room temperature results in rapid dehydrogenation. Hydrogen generation was detected at temperatures as low as 238 K, and NMR spectroscopy provided evidence for the initial formation of a Re− H−B σ bound adduct at these temperatures. The rate of the dehydrocoupling reaction was significantly influenced by the electron density on the metal center and the identity of the alkyl group (CH3 or CF3) on the ketone carbon of the pyrrole ligand. The bifunctional nature of this complex incorporating both an acidic Re center and a basic oxygen center, coupled with the hemilability of the organic carbonyl group, are key motifs of this catalyst that result in the thermal dehydrogenation of dimethylamine borane.

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INTRODUCTION The drive toward a green economy has incentivized research into the development of hydrogen storage materials, since hydrogen is the ultimate clean fuel.1 Due to their high volumetric and gravimetric hydrogen densities, amine-boranes NHnR3−nBH3 (n = 0−3), are considered to be attractive materials for this purpose.2 Metal catalyzed hydrogen release from amine-boranes with the assistance of an external activating agent or with photoirradiation is well-known.2d,e Recently, we and others reported the stoichiometric and catalytic dehydrogenation of amine-boranes by group 7 metal (Mn, Re) complexes both photochemically3 and thermally4 at elevated temperatures. While there are instances of thermal hydrogen release from amine-boranes by transition metal catalysts under ambient conditions,5 similar examples for this dehydrocoupling reaction (in the absence of external activating agents) employing group 7 metal complexes are rare. Recently, our group investigated the unusual lability of CO ligands from a rhenium carbonyl complex (1) containing a bidentate pyrrolyl ligand.6 Surprisingly, CO substitution by the weakly coordinating THF solvent occurred readily, even at room temperature (Scheme 1). While the Re−CO bond is intrinsically weak in these complexes, it was postulated that the unusual reactivity of this species is due to the presence of a weak ketone Re−O link that can easily dissociate to open a coordination site on the metal center and accommodate an incoming ligand prior to CO loss. This reactivity suggested to us that complex 1 may have utility as an amine-borane © 2019 American Chemical Society

Scheme 1. Facile Substitution of a CO Ligand in 1 by an Incoming Ligand

dehydrogenation catalyst for the following reason. A desirable catalyst is required to efficiently promote the heterolytic coupling of the hydridic (B−H) and protic (N−H) hydrogens of amine-boranes. Further, several studies have shown that the initial step in the catalytic process involves the coordination of amine-borane to a vacant site on the metal center to form a σ bound M−H−BH2NHnR3−n adduct.7,8 We therefore reasoned that a suitable catalyst should be bifunctional with both an acidic metal center (for hydride transfer) and a basic site (for proton transfer) and that a vacant metal coordination site be generated under mild conditions for initial amine-borane binding. The facile decoordination of the weak Re−O linkage in complex 1 to generate a coordinatively unsaturated intermediate with a Lewis acidic site on the rhenium center, and the resulting presence of a basic oxygen near the vacant site, Received: February 20, 2019 Published: June 26, 2019 2602

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Variable Temperature NMR. Compound 1a (0.027 mmol) was transferred into a NMR tube under argon, and 0.5 mL of tolune-d8 was syringed in. The tube was sealed under an argon atmosphere and immersed in an acetonitrile/dry ice bath (−60 °C) following which DMAB (0.169 mmol) was dissolved in 0.3 mL of toluene-d8 and added to the NMR tube. The spectrometer was precooled to −60 °C, and 1H NMR spectral acquisition was initiated by raising the temperature to the desired setting. Catalytic Studies. A NMR tube was charged with 0.5 mL of a toluene-d8 solution containing 10 mol % of catalyst (1a or 1b) under an argon atmosphere. DMAB was dissolved in 0.3 mL of tolune-d8 and added to a precooled NMR tube (−60 °C) containing compound 1a or 1b. The spectrometer was warmed and maintained at 25 °C, and spectra were acquired at regular time intervals. The integral values of the product species dimer monitored by 11B NMR (triplet, δ = 5.5 ppm) were recorded for up to 2 h at regular intervals and plotted with respect to time to obtain the relative rates of reaction for both compounds 1a and 1b. X-ray Crystal Structure Analyses. Single crystals of 2 were obtained by the slow evaporation of a mixture of dichloromethane and hexane solution. A Leica MZ7.5 stereomicroscope was used to identify suitable crystals of the same habit. Each crystal was coated in paratone, affixed to a nylon loop, and placed under streaming nitrogen (150 K) in a SMART Apex CCD diffractometer (see details in the CIF files). The space groups were determined on the basis of systematic absences and intensity statistics. The structures were solved by direct methods and refined by full matrix least-squares on F2. Anisotropic displacement parameters were determined for all nonhydrogen atoms. Hydrogen atoms were placed at idealized positions and refined with fixed isotropic displacement parameters. The following is a list of programs used: data collection and cell refinement, APEX2;9 data reductions, SAINTPLUS, version 6.63;10 absorption correction, SADABS;11 structural solutions, SHELXS97;12 structural refinement, SHELXL-97;13 graphics and publication materials, SHELXTL.14 The crystal data table (Table S1) and atomic coordinates are available in the Supporting Information.

motivated us to investigate the use of 1 as a catalyst for the dehydrogenation of amine-boranes. Compound 1 was visualized as a bifunctional catalyst with the potential ability to activate both the hydridic and protic hydrogens of dimethylamine borane (DMAB) at ambient temperatures without assistance from external activation agents. As discussed below, reaction of 1a with DMAB resulted in hydrogen evolution at temperatures as low as 238 K. Experiments, supported by theoretical calculations, were conducted to better understand the mechanism of the dehydrogenation reaction. Hydrogen release is proposed to involve the initial generation of a Re−H−B σ bound adduct, concurrent hydride and proton abstraction from DMAB to the catalyst, and subsequent H−/H+ heterolytic coupling to release H2.

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EXPERIMENTAL SECTION

Unless otherwise stated, all synthetic manipulations were carried out using Schlenk techniques. Complexes 1a, 1b, and 3 were synthesized following published protocols.6 Heptane and dichloromethane solvents were of anhydrous grade (Aldrich) and 99% purity and used as received. 1H and 11B (128 MHz) NMR spectra were recorded using a Bruker 400 MHz NMR spectrometer. The 1H chemical shifts are reported with reference to toluene-d8, and 11B NMR spectral signals are reported with respect to the unreacted borane used. An NMR spectrum of dimethylamine borane (Sigma-Aldrich, 97%) was obtained prior to the catalytic runs to rule out the presence of contaminants such as free amine. FTIR spectra were obtained using a Bruker Vertex 80 FTIR instrument at 4 cm−1 resolution employing a 0.5 mm path length cell with CaF2 windows. Synthesis of {2-(CH3O)C4H3N}Re(CO)3(NHMe2) (2). A solution of 1a (0.039 g, 0.09 mmol) in 10 mL of heptane was treated with an excess amount of DMAB (0.053 g, 0.9 mmol) and refluxed for 6 h under an argon atmosphere. Upon cooling to room temperature, the formation of a yellow precipitate was observed. The solution was evaporated to dryness, affording a solid yellow residue which was redissolved in dichloromethane (5 mL) and filtered through a small silica plug to obtain a clear yellow solution. The solution was concentrated to 1 mL, and after addition of a few drops of hexane, it was left in a 4 °C freezer to yield yellow crystals of 2 (0.027 g, 70% yield). IR data in CH2Cl2 (νCO): 1562 (w), 1892 (s), 1907 (s), 2023 (s). IR data in heptane (νCO): 1562 (w), 1904 (s), 1921 (s), 2027 (s). 1 H NMR data in CDCl3: δ 2.47 (s, methyl (pyrrole)), 2.77 (d, J = 4 Hz, 3H), 2.60 (d, 4 Hz, 3H), 6.38 (d, J = 4.1 Hz, 1H), 7.12 (d, J = 4 Hz, 1 H), 7.45 (s, 1H), 13C{1H} δ 22.14 (Me), 45.86, 44.82 (methyl on nitrogen), 123.13, 116.9 (s, 2 C), 141.9 (s, 1 C), 143.5, 143.9, (s, 2 C), 192.6 (s, 1 C, CH3O), 193.1 (s, 1 C, CO), 198.0 (s, 1 C,CO), 197.8 (s, 1 C, CO). Quantification of Hydrogen. In all cases, solutions of the catalyst 1a and amine-borane were prepared in an Ar filled glovebox, placed in 2 and 10 mL volumetric flasks, respectively, capped with a rubber septa, parafilmed, copper wired, removed from the glovebox, and filled to the appropriate level with anhydrous toluene. A degassed 5 mL pyrex glass vial with a small stir bar and rubber septa was loaded with 0.3 mL of the amine-borane solution (0.055 g of DMAB) as well as 0.5 mL of the catalyst 1a solution corresponding to 10 mol % loading of the rhenium catalyst. Prior to commencing the dehydrogenation reaction, the vial was connected via PTFE tubing to a 50 mL buret filled with distilled water connected to a variable height funnel and the pressure was equilibrated before the start of reaction. Bubbles of a released gas were seen to displace water in the buret. Volume measurements were taken every 5 min by first adjusting the level of the meniscus of the funnel to match the meniscus of the water in the buret followed by reading the volume level of the buret. The volume of hydrogen gas collected was recorded periodically for up to 2 h. A similar procedure was followed for catalyst 1b.

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COMPUTATIONAL DETAILS All DFT calculations were performed with the development version of the Gaussian suite of programs.15 The methodology was based upon the performance obtained in previous calculations in similar complexes. 4c,6,16 We used the ωB97XD functional that includes different fractions of exact exchange in the long and short ranges as well as a dispersion correction.17 All atoms were described with the triple-ζ basis set def2-TZVPP, which includes effective core potentials for Re.18 An integration grid larger than the default is used to ensure numerical accuracy (“ultrafine” grid with 99 radial shells of 590 points). The reported geometries were confirmed as minima or transition states according to their number of imaginary frequencies. Except as stated otherwise, all of the energies reported in this paper are enthalpies computed at 298.15 K and 1 atm and are expressed in kcal/mol. The figures of DFT optimized geometries included in this work were rendered using CYLview.19

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RESULTS AND DISCUSSION a. Reaction of Catalyst 1a with DMAB. Compound 1a is air-stable in solution at room temperature for several hours. This catalyst was chosen for most of the studies described herein due to its stability in comparison to complex 1b. Addition of an excess amount of DMAB (0.9 mmol) to a room temperature heptane solution of 1a (0.04 mmol) results in the immediate evolution of gas bubbles which, on the basis of GC and 1H NMR analysis (δ = 4.54 ppm), were confirmed to be due to hydrogen gas. In addition to H2, 11B NMR spectroscopy 2603

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(CO)3(NHMe2), 2. This complex was isolated in moderate yield from the reaction between 1a and an excess amount of DMAB in dichoromethane solution. Complex 2, which is airstable at room temperature for several days without decomposition, was characterized by a combination of NMR spectroscopy (Figure S2) and single crystal X-ray diffraction (Figure 2). Evidently the free amine, NHMe2, resulting from

revealed that the major product of this reaction was the cyclic amine-borane, [Me2NBH2]2 (Scheme 2). Thus, this reaction proceeded with high selectivity, as only minor amounts of other amine-borane products were observed under the conditions employed. Scheme 2. Dehydrocoupling of DMAB Mediated by Catalyst 1a/1b

An attempt was made to monitor the progress of the reaction at room temperature with the use of time-resolved FTIR spectroscopy, specifically with the aim of identifying key reactive intermediates involved in the dehydrogenation reaction. The initial IR spectrum displayed CO stretching absorptions at 2111, 2009, 1991, and 1953 cm−1 assigned to 1a. Surprisingly, no change in the IR spectrum was observed upon addition of DMAB for up to 2 h (Figure 1). The

Figure 2. X-ray structure of complex 2.

the cleavage of the B−N bond in DMAB is generated during the dehydrogenation reaction and reacts with 1a in situ to form 2. The generation of free amine in the course of metal catalyzed dehydrogenation of amine-boranes has been observed before.20 The dehydrogenation reaction slows down after 2 h at room temperature, and a 11B NMR spectrum of the solution obtained at this stage of the reaction shows the formation of ∼60% [Me2NBH2]2 with complete conversion after 9 h. Consistent with the IR studies, after 2 h, a new set of resonances in the 1H NMR spectrum matching those of 2 are observed in the aromatic region next to the pyrrole C−H protons and in the N-methyl region (Figure S1). These observations suggest that the side reaction of 1a with liberated free amine hinders the progress of the dehydrogenation process by reducing the concentration of the active catalyst. Complex 1a (5 mol %) maintained activity for up to three cycles of amine-borane addition with slow degradation of catalytic efficiency for additional cycles, most likely due to the conversion of the catalyst to 2. Catalyst 1a maintains decent activity with loadings as low as 1 mol % with 90% conversion to the cyclic dimer, although due to competitive catalyst deactivation, the reaction takes almost 30 h for completion. The IR spectrum obtained after reaction completion shows the presence of a significant amount of catalyst 1a remaining (50%) which suggests that the CO ligands in the primary coordination sphere remain bound to the metal center during the reaction. This observation provides a useful benchmark for the DFT studies discussed later and indicates an important role for the hemilability of the Re−O linkage in generating an open site on the metal center for initial binding of the amine-borane. b. VT-NMR Monitoring of the Reaction of 1a with DMAB. Since IR experiments aimed at identifying the initial complex formed upon reaction of 1a with DMAB were unsuccessful, an attempt was made to observe the expected Re−H−B intermediate by low temperature 1H NMR spec-

Figure 1. FTIR spectrum of a reaction mixture containing 1a and DMAB obtained after 2 h of reaction at room temperature. Peak positions (cm−1) labeled in black are due to 1a, while those in red are assigned to 2. Arrows indicate whether the peaks grow or decrease in intensity relative to the initial spectrum.

observed facile release of H2 and the absence of IR evidence for the presence of the expected initial Re−H−B intermediate suggests that this complex (if formed) is kinetically unstable and therefore present in a steady state concentration that is too low to detect in situ using IR spectroscopy. After longer reaction times (>2 h), a new set of IR bands at 2028, 1922, and 1905 cm−1 with an intensity pattern consistent with a facial LnM(CO)3 geometry were observed and are assigned to the amine complex {2-(CH3CO)C4H3N}Re2604

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resonance at δ = 4.54 ppm. This observation suggests that Int1 is a key species in the dehydrogenation reaction. Remarkably, these VT-NMR studies show that H2 evolution occurs at temperatures as low as 238 K. c. Ancillary Ligand Effects. c.i. Primary Coordination Sphere. Experiments were conducted to measure the influence of metal electron density upon the rate of the dehydrogenation reaction. The primary coordination sphere around the Re center was modified by replacing the π-acid CO ligand with the σ donor ligands, PPh3 and NHMe2. As shown in Scheme 4, complexes 2 and {2-(CH3CO)C4H3N}Re(CO)3(PPh3) (3) were synthesized by the reaction of 1a with either PPh3 or DMAB at room temperature in THF solvent. Consistent with an increase in metal electron density, the CO stretching absorption bands of 2 and 3 were lower in cm−1 compared to 1a. In dramatic contrast to 1a, under similar catalytic conditions at room temperature, use of either complex 2 or 3 resulted in negligible conversion of DMAB to the cyclic amine-borane product [Me2NBH2]2 for up to 48 h, as monitored by 11B NMR spectroscopy. This observation suggests that hydride transfer from the amine-borane to a Lewis acidic Re center may be an important, rate-determining step in the mechanism. Thus, an increase in the electron richness of the metal center is expected to reduce the efficiency of the dehydrogenation reaction, as observed for complexes 2 and 3. c.ii. Secondary Coordination Sphere. To further elucidate the mechanism of the dehydrocoupling reaction, the influence of substituents on the ketone carbon of 1 (R = CH3, CF3) upon the catalytic release of hydrogen gas from DMAB was investigated. The relative rates of the catalytic reaction were studied using two methods. In the first instance, the growth of the cyclic amine-borane, [Me2NBH2]2, was followed by 11B NMR spectroscopy and in a separate experiment, but under the same reaction conditions, also by measuring the volume of H2 produced. As shown in Figure 4, varying the R group has a dramatic effect on the rate of H2 and [Me2NBH2]2 production. For example, NMR spectra taken after 2 h of reaction time under similar reaction conditions show almost 60% and negligible conversion of DMAB to the cyclic dimer product when 1a and 1b are employed as catalysts, respectively (Figure 5). Similarly, the rate of H2 production is significantly faster in the case of 1a. Averaged over the two experiments, the rate of the catalytic reaction is almost a factor of 10 greater for R = CH3 compared to R = CF3. It is clear from this observation that electron donating substituents increase the rate of the dehydrogenation reaction, and this suggests that the ketone group plays an important role in the activation of the amine-borane, likely by increasing the basicity of the carbonyl oxygen and facilitating N−H proton transfer. Importantly, the observed influence of the R group upon the rate of the dehydrocoupling reaction suggests that the catalytic process is homogeneous in nature

troscopy (218−273 K). The 1H NMR spectrum of the reaction mixture containing 1a with an excess amount of DMAB (5 equiv) in toluene-d8 was obtained at 218 K and indicates the presence of a species with a metal hydride resonance at δ = −13.72 (Figure 3). This complex is tentatively assigned to the

Figure 3. Stack plot of 1H NMR spectra as a function of temperature showing intermediate Int1 in the reaction of complex 1a with DMAB in toluene-d8. Intense resonances associated with DMAB are not shown for clarity.

Re−H−B σ adduct, Int1, since the position of the hydride resonance is consistent with those reported for Ru,21 Rh,22 and group 7 metal complexes of this type (Scheme 3).23 Further Scheme 3. Initial Intermediate Formed upon Reaction of 1a with DMAB

support for the proposed structure for Int1 also comes from the 1H NMR spectrum which provides evidence for the proposed hydrogen bonding interaction between the amineborane proton and the organic carbonyl in Int1. When the temperature is increased from 248 to 278 K, a broad resonance ascribed to the amine-borane N−H proton at 3.42 ppm shifts downfield to 2.85 ppm (Figure S3). This strong shift in δH(NH) as a function of temperature is indicative of an intramolecular amine proton−base interaction, as described by Belkova et al.20a As discussed later, DFT calculations also predict the presence of a similar intermediate structure along the reaction energy profile. As shown in Figure 3, the intensity of the metal hydride resonance decreases as the temperature is warmed from 248 to 273 K with a commensurate increase in the intensity of the H2 Scheme 4. Synthesis of Complexes 2 and 3

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predict a lower activation barrier for the rate-determining step in the case of 1a (R = CH3) compared to 1b (R = CF3). d. DFT Studies. The reaction mechanism for the amineborane dehydrogenation reaction was investigated by employing the model complex 1c (R = H). These calculations were guided by the experimental findings which indicate that (a) CO ligands remain intact during the reaction, (b) metal electron density influences the rate of hydrogen release, and (c) electron donating substituents on the ketone carbon enhance the reaction rate. The energy profile for the reaction shown in Figure 6 reveals that the overall catalytic cycle is exergonic and exothermic by 11.4 and 9.9 kcal/mol, respectively, and involves three main steps: (a) coordination of the amine-borane to the metal center, (b) activation of the B−H and N−H bonds, and (c) release of the hydrogen molecule. Consistent with the experimental studies, initial coordination of the amine-borane to form Int1 does not require loss of a carbonyl ligand from the metal center. Instead, a vacant coordination site is generated by the cleavage of the aldehyde Re−O linkage. Two different isomers were found for this intermediate depending upon the orientation of the N−H bond and are shown in Figure 7 (Int1 and Int1′). Int1 in which the protic N−H bond is oriented toward the basic oxygen atom of the pyrrole ligand was calculated to be 5.5 kcal/mol more stable than the alternate conformation (Int1′). This intermediate, which exhibits a Re−H−B σ bonding interaction, is formed with a very low enthalpic barrier of 5.2 kcal/mol and is 5.1 kcal/mol more stable than the reactants. We propose that this

Figure 4. Relative rates of dehydrogenation upon reaction of complexes 1a and 1b with DMAB employing two different methods.

and that heterogeneous species are not involved, as has been observed previously for some iron based catalysts.24 The results of these experiments also provide an important benchmark for DFT studies which, as discussed below, also

Figure 5. Room temperature 11B NMR spectra obtained after 2 h upon reaction of DMAB with (a) 1a and (b) 1b. The inset shows an expanded view and indicates the presence of minor amounts of other amine-borane species formed during this time. 2606

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Figure 6. Calculated energy profile for the dehydrogenation reaction. Free energy and enthalpy values (kcal/mol) are in parentheses and bold, respectively.

of H2 is better described as one step involving the formation of the H−H bond and recovery of the original catalyst. d.i. Influence of the R Group. Calculations were conducted to probe the effect of varying the R group upon the barrier of the rate-determining step which was computed for R = CH3, CF3, and H (Figure 8). The computational results suggest that

Figure 7. Calculated structures for the initial Re−H−B σ adduct.

species was detected in the low temperature NMR experiments (1H, δ = −13.2 ppm) described earlier. Alternative reaction pathways requiring the initial decoordination of a CO ligand were also explored, but they were not considered due to the calculated instability of the resulting species. In the second, rate-determining step of the cycle, Int1 undergoes “double” activation of the coordinated amineborane B−H and N−H bonds through a concerted transition state in which the boron hydrogen is transferred to the metal as a hydride and the nitrogen hydrogen is transferred to the nearby aldehyde oxygen of the pyrrole ligand as a proton to form Int2. The barrier for this rate-determining step is calculated to be 22.9 kcal/mol, and the low activation enthalpy is consistent with the observed facile room temperature dehydrogenation of DMAB by the rhenium catalyst. The intermediates following the rate-determining step, Int2 and [Me2N = BH2], are 7.5 kcal/mol less stable than the separated reactants, but the system is stabilized by 4.9 kcal/mol after separation of the dehydrogenated amine-borane and the consecutive formation of the product dimer [BH2NMe2]2. The third step of the cycle involves heterolytic coupling of the two hydrogens on Int2 to release H2 with a low barrier of 10.3 kcal/mol and regeneration of 1c. Formally, the exploration of the potential energy surface describes this final process composed of two different steps: formation of a complex with dihydrogen coordinated to the rhenium center followed by release of H2. However, the dihydrogen complex is not stable with the applied vibrational corrections and the release

Figure 8. Calculated barriers for the rate-determining step as a function of R group.

both the reaction barrier and the energy of the corresponding intermediate increase with the electron-withdrawing ability of the R group. For example, the calculated barrier for R = CF3 is 6.4 kcal/mol higher than that for R = CH3. Thus, consistent with the experimental results, the calculations predict that the relative rates of dehydrogenation increase in the order CF3 < H < CH3. It may be reasonably assumed that this trend results from a decrease in the basicity of the oxygen atom due to the inductive effects of the R group leading to a higher barrier for the proton transfer when R = CF3 relative to CH3. Put together, the experimental and computational findings lead us to propose the catalytic cycle shown in Figure 9 for the dehydrogenation of DMAB by catalyst 1a. As was the case with the facile release of CO from 1 reported earlier,6 the hemilability of the ketone group plays a crucial role in promoting the rapid release of hydrogen from DMAB. Overall, 2607

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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.9b00115. NMR spectra and X-ray crystal structure data (PDF) Accession Codes

CCDC 1898002 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Veeranna Yempally: 0000-0003-2465-2743 Salvador Moncho: 0000-0003-1631-5587 Wai Yip Fan: 0000-0002-9963-0218 Donald J. Darensbourg: 0000-0001-9285-4895 Ashfaq A. Bengali: 0000-0002-6765-8320

Figure 9. Catalytic cycle for the dehydrogenation of DMAB by complex 1a. The dotted path accounts for the possibility that H2 release can occur without formation of a Re-(η2-H2) intermediate.

Notes

The authors declare no competing financial interest.

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the dehydrogenation of DMAB by complex 1 is made possible by the bifunctional nature of the rhenium catalyst and suggests that this motif should be an important consideration when designing such systems.

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REFERENCES

(1) (a) Eberle, U.; Felderhoff, M.; Schüth, F. Chemical and Physical Solutions for Hydrogen Storage. Angew. Chem., Int. Ed. 2009, 48, 6608−6630. (b) Huang, Z.; Autrey, T. Boron−nitrogen−hydrogen (BNH) compounds: recent developments in hydrogen storage, applications in hydrogenation and catalysis, and new syntheses. Energy Environ. Sci. 2012, 5, 9257−9268. (c) Stern, A. G. A new sustainable hydrogen clean energy paradigm. Int. J. Hydrogen Energy 2018, 43, 4244−4255. (2) (a) Hügle, T.; Hartl, M.; Lentz, D. The Route to a Feasible Hydrogen Storage Material: MOFs versus Ammonia Borane. Chem. Eur. J. 2011, 17, 10184−10207. (b) Colebatch, A. L.; Weller, A. S. Amine−Borane Dehydropolymerization: Challenges and Opportunities. Chem. - Eur. J. 2019, 25, 1379−1390. (c) Smythe, N. C.; Gordon, J. C. Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration. Eur. J. Inorg. Chem. 2010, 2010, 509−521. (d) Staubitz, A.; Robertson, A. P. M.; Manners, I. Ammonia-Borane and Related Compounds as Dihydrogen Sources. Chem. Rev. 2010, 110, 4079− 4124. (e) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B− N compounds for chemical hydrogen storage. Chem. Soc. Rev. 2009, 38, 279−293. (3) (a) Kakizawa, T.; Kawano, Y.; Naganeyama, K.; Shimoi, M. Dehydrocoupling Reactions of Secondary and Primary Amine− Borane Adducts Catalyzed by Half-sandwich Carbonyl Complexes, [CpMn(CO)3], [(η6-C6H6)Cr(CO)3], and [CpV(CO)4]. Chem. Lett. 2011, 40, 171−173. (b) Muhammad, S.; Moncho, S.; Brothers, E. N.; Bengali, A. A. Dehydrogenation of a tertiary amine-borane by a rhenium complex. Chem. Commun. 2014, 50, 5874−5877. (c) Sohail, M.; Moncho, S.; Brothers, E. N.; Darensbourg, D. J.; Bengali, A. A. Estimating the strength of the M−H−B interaction: a kinetic approach. Dalton Trans. 2013, 42, 6720−6723. (d) Kawano, Y.; Yamaguchi, K.; Miyake, S.; Kakizawa, T.; Shimoi, M. Investigation of the Stability of the M-H-B Bond in Borane σ Complexes [M(CO)5(η1-BH2R·L)] and [CpMn(CO)2(η1-BH2R·L)] (M = Cr, W; L = Tertiary Amine or Phosphine): Substituent and Lewis Base Effects. Chem. - Eur. J. 2007, 13, 6920−6931.

CONCLUSIONS

The reaction of a bidentate pyrrole based rhenium tetracarbonyl complex (1a) with dimethylamine borane at room temperature results in rapid dehydrogenation. Hydrogen generation was detected at temperatures as low as 238 K, and NMR spectroscopy provided evidence for the initial formation of a Re−H−B σ bound adduct at these temperatures. The rate of the dehydrocoupling reaction was significantly influenced by the electron density on the metal center and the identity of the alkyl group (CH3 or CF3) on the ketone carbon of the pyrrole ligand. Thus, increasing the electron density on the metal center by substituting the π acid CO ligand with σ donor groups resulted in significantly decreased reactivity, while replacement of the electron donating CH3 group on the ketone carbon with CF3 yielded a 10-fold reduction in reaction rate. DFT calculations supported the experimental findings and suggested that the overall reaction occurs in three main steps to include dechelation of the organic carbonyl group from the metal center and coordination of the amine-borane to the resulting vacant site, activation of the B−H and N−H bonds, and finally metal mediated heterolytic coupling of H+ and H− to yield H2. The bifunctional nature of 1a incorporating both an acidic Re center and a basic oxygen center, coupled with the hemilability of the organic carbonyl group, are key motifs of this catalyst that result in the thermal dehydrogenation of dimethylamine borane. 2608

DOI: 10.1021/acs.organomet.9b00115 Organometallics 2019, 38, 2602−2609

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Organometallics (4) (a) Jiang, Y.; Berke, H. Dehydrocoupling of dimethylamineborane catalysed by rhenium complexes and its application in olefin transfer-hydrogenations. Chem. Commun. 2007, 3571−3573. (b) Jiang, Y.; Blacque, O.; Fox, T.; Frech, C. M.; Berke, H. Development of Rhenium Catalysts for Amine Borane Dehydrocoupling and Transfer Hydrogenation of Olefins. Organometallics 2009, 28, 5493−5504. (c) Sharpe, H. R.; Geer, A. M.; Blundell, T. J.; Hastings, F. R.; Lewis, W.; Fay, M. W.; Rance, G. A.; Blake, A. J.; Kays, D. J. Dehydrocoupling of Dimethylamine-Borane Promoted by Manganese(II) m-Terphenyl Complexes. Catal. Sci. Technol. 2018, 8, 229−235. (5) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Rhodiumcatalyzed formation of boron−nitrogen bonds: a mild route to cyclic aminoboranes and borazines. Chem. Commun. 2001, 962−963. (b) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. Efficient Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128, 12048−12049. (6) (a) Mirebeau, J.-H.; Bideau, F. L.; Marrot, J.; Jaouen, G. Synthesis of Rhenium Carbonyl Complexes Bearing Substituted Pyrrolyl Ligands. Organometallics 2008, 27, 2911−2914. (b) Muhammad, S.; Yempally, V.; Muhammad, A.; Savador, M.; Samuel, J. K.; Edward, N. B.; Darensbourg, D. J.; Bengali, A. A. Mechanism of CO Displacement from an Unusually Labile Rhenium Complex: An Experimental and Theoretical Investigation. Inorg. Chem. 2012, 51, 13041−13049. (7) (a) Rossin, A.; Peruzzini, M. Ammonia−Borane and Amine− Borane Dehydrogenation Mediated by Complex Metal Hydrides. Chem. Rev. 2016, 116, 8848−8872. (b) Bhunya, S.; Malakar, T.; Ganguly, G.; Paul, A. Combining Protons and Hydrides by Homogeneous Catalysis for Controlling the Release of Hydrogen from Ammonia−Borane: Present Status and Challenges. ACS Catal. 2016, 6, 7907−7934. (c) Staubitz, A. Generation of High-MolecularWeight Polymers with Diverse Substituents: An Unusual Metal-Free Synthesis of Poly(aminoboranes). Angew. Chem., Int. Ed. 2018, 57, 5990−5992. (d) Bhattacharya, P.; Krause, J. A.; Guan, H. Mechanistic Studies of Ammonia Borane Dehydrogenation Catalyzed by Iron Pincer Complexes. J. Am. Chem. Soc. 2014, 136, 11153−11161. (e) Lunsford, A. M.; Blank, J. H.; Moncho, S.; Haas, S. C.; Muhammad, S.; Brothers, E. N.; Darensbourg, M. Y.; Bengali, A. A. Catalysis and Mechanism of H2 Release from Amine-Boranes by Diiron Complexes. Inorg. Chem. 2016, 55 (2), 964−973. (8) Rosello-Merino, M.; Rama, R. J.; Diez, J.; Conejero, S. Catalytic dehydrocoupling of amine-boranes and amines into diaminoboranes: isolation of a Pt(II), Shimoi-type, η1-BH complex. Chem. Commun. 2016, 52, 8389−8392. (9) APEX2, version 2009.7-0; Bruker AXS, Inc.: Madison, WI, 2007. (10) SAINTPLUS: Program for Reduction of Area Detector Data 1034, version 6.63; Bruker AXS, Inc.: Madison, WI, 2007. (11) Sheldrick, G. M. SADABS: Program for Absorption Correction of 1036 Area Detector Frames; Bruker AXS, Inc.: Madison, WI, 2001. (12) Sheldrick, G. M. SHELXS-97: Program for Crystal Structure 1038 Solution; Universitat ̈ Gotẗ ingen: Gotẗ ingen, Germany, 1997. (13) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure 1040 Refinement; Universitat ̈ Gotẗ ingen: Gotẗ ingen, Germany, 1997. (14) Sheldrick, G. M. SHELXL, version 6.10; AXS, Inc.: Madison, WI, 2000. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;

Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Parandekar, P. V.; Mayhall, N. J.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian Development Version, revision H.32; Gaussian, Inc.: Wallingford, CT, 2009. (16) (a) Raju, R. K.; Bengali, A. A.; Brothers, E. N. A unified set of experimental organometallic data used to evaluate modern theoretical methods. Dalton Trans. 2016, 45, 13766−13778. (b) Yempally, V.; Savador, M.; Muhammad, S.; Edward, N. B.; Arndtsen, B. A.; Bengali, A. A. Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation. Organometallics 2014, 33, 3591−3595. (17) Chai, J. D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom−atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (18) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (19) Legault, C. Y. CYLview, 1.0b; Université de Sherbrooke: 2009 (http://www.cylview.org). (20) (a) Titova, E. M.; Osipova, E. S.; Pavlov, A. A.; Filippov, O. A.; Safronov, S. V.; Shubina, E. S.; Belkova, N. V. Mechanism of Dimethylamine−Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex. ACS Catal. 2017, 7 (4), 2325−2333. (b) Kalviri, H. A.; Gärtner, F.; Korobkova, G. Y. I.; Baker, R. T. Probing the second dehydrogenation step in ammonia-borane dehydrocoupling: characterization and reactivity of the key intermediate, B-(cyclotriborazanyl)amine-borane. Chem. Sci. 2015, 6, 618−624. (21) Kumar, R.; Jagirdar, B. R. B−H Bond Activation Using an Electrophilic Metal Complex: Insights into the Reaction Pathway. Inorg. Chem. 2013, 52, 28−36. (22) (a) Douglas, T. M.; Chaplin, A. B.; Weller, A. S.; Yang, X.; Hall, M. B. Monomeric and Oligomeric Amine−Borane σ-Complexes of Rhodium. Intermediates in the Catalytic Dehydrogenation of Amine− Boranes. J. Am. Chem. Soc. 2009, 131, 15440−15456. (b) Johnson, H. C.; McMullin, C. L.; Pike, S. D.; Macgregor, S. A.; Weller, A. S. Dehydrogenative Boron Homocoupling of an Amine-Borane. Angew. Chem., Int. Ed. 2013, 52, 9776−9780. (23) Schlecht, S.; Hartwig, J. F. σ-Borane Complexes of Manganese and Rhenium. J. Am. Chem. Soc. 2000, 122, 9435−9443. (24) Vance, J. R.; Schafer, A.; Robertson, A. P. M.; Lee, K.; Turner, J.; Whittell, G. R.; Manners, I. Iron-Catalyzed Dehydrocoupling/ Dehydrogenation of Amine−Boranes. J. Am. Chem. Soc. 2014, 136, 3048.

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DOI: 10.1021/acs.organomet.9b00115 Organometallics 2019, 38, 2602−2609