Kinetics of −CH2CH2– Hydrogen Release from a BN-cyclohexene

Communication pubs.acs.org/Organometallics

Kinetics of −CH2CH2− Hydrogen Release from a BN-cyclohexene Derivative Zachary X. Giustra,† Lien-Yang Chou,† Chia-Kuang Tsung,* and Shih-Yuan Liu* Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States S Supporting Information *

ABSTRACT: Complete −CH2CH2− dehydrogenation of 1,2-dimethyl1,2-BN-cyclohexene (1) was achieved using a Pd/C catalyst in a gas-phase microreactor. Arrhenius analysis yielded an activation energy (Ea) of 10.3 ± 0.3 kcal mol−1 and a pre-exponential factor (A) of 2.2 ± 0.2 (log A), respectively. These terms reflect a lesser kinetic favorability in comparison to those determined for all-carbon dimethylcyclohexene (Ea = 8.6 ± 0.3 kcal mol−1, log A = 3.6 ± 0.1). Despite being isostructural and isoelectronic with a CC bond, the B−N bond of 1 thus appears to confer a different measure of activity with respect to Pd-catalyzed −CH2CH2− dehydrogenation.

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subcategory arose following the seminal work of Crabtree and co-workers,12 wherein they demonstrated that replacement of cycloalkane CH2 units with NH groups could markedly improve the thermodynamics of dehydrogenation (lower reaction endothermicity etc.). In a similar vein, our group has investigated the consequences of incorporating NH2BH2 units into cycloalkyl structures to form saturated carbon−boron−nitrogen (CBN) heterocycles (Figure 2).13 Our past studies of these mixed

ecent technological advances have brought fuel cell electric vehicles (FCEVs) to the verge of small-volume commercial production.1,2 For this new phase of development to have a serious chance of economic success, however, several additional issues must be addressed first. Foremost among these is the lack of a large-scale hydrogen storage and distribution infrastructure1−3 compatible with the current operational requirements of FCEVs: all models to date rely on state-of-the-art4 compressed gas at 700 bar as their source of H2. While several countries and organizations have plans to incrementally increase the number of compressed hydrogen refueling stations for vehicular use,1−3 there remains significant interest in developing methods to chemically store H2 in either solid or liquid materials5 compatible with extant conventional fuel distribution networks and technologies (Figure 1).

Figure 2. Saturated CBN heterocycles developed as potential hydrogen storage materials.

organic−inorganic hydrogen storage materials have focused primarily on characterizing H2 release from the amine−borane groups13c−h,14 and the overall thermodynamics of the sixmembered CBN compounds.13a,b On the other hand, prior to this work, a complementary kinetic analysis of dehydrogenation from the carbonaceous components of these CBN heterocycles remained elusive in our hands due to the lack of effective reaction conditions for this transformation. We had found that, at the elevated temperatures typically required for cycloalkane dehydrogenation, rapid BN hydrogen release occurs first to produce species that are isostructural and isoelectronic with cyclohexenes (A; Figure 3, top).15 For all-carbon compounds, it is known16 that dehydrogenation of a cycloalkene is significantly more facile

Figure 1. Representative examples of materials for chemical hydrogen storage.

Organic chemical hydrides constitute a major class of such potential hydrogen storage materials.6,7 They can be considered to include any compound in which H2 is fixated in the form of vicinal C−H, X−H bonds (X = carbon or heteroatom), although considerations of practicality primarily limit the effective scope to cyclic hydrocarbons such as methylcyclohexane8 and decalin9 and saturated nitrogen heterocycles10 such as N-ethylperhydrocarbazole.11 Particular interest in the latter © XXXX American Chemical Society

Received: May 21, 2016

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DOI: 10.1021/acs.organomet.6b00412 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

Scheme 1. Synthesis of 1,2-Dimethyl-1,2-BN-cyclohexene 1 and Dehydrogenation Product 3

Figure 3. Initial BN dehydrogenation of saturated CBN heterocycles to form BN-cyclohexenes (A) (top) and proposed comparison of −CH2CH2− dehydrogenation kinetics between CBN and all-carbon substrates (bottom).

than that of the corresponding fully saturated cycloalkane. The aforementioned difficulty in achieving similar reactivity with CBN heterocycles therefore called into question the assumed analogy between the B−N bond in A and the CC bond in a cyclohexene with respect to −CH2CH2− dehydrogenation. The present work aimed to clarify this aspect of the two bonds’ relationship by first achieving effective catalytic dehydrogenation of a model BN-cyclohexene compound (1) and then comparing the kinetics of this reaction to those of the equivalent all-carbon cyclohexene and cyclohexane substrates (Figure 3, bottom). Given that all our attempts at acceptorless −CH2CH2− dehydrogenation of CBN materials in solution were unsuccessful, we turned to gas-phase heterogeneous catalysis in an effort to access a viable mode of reactivity under flow conditions.6b,17 This shift in strategy required a sufficiently volatile substrate (bp ≤150 °C), specifically one for which analysis of the endocyclic −CH2CH2− dehydrogenation kinetics would not be complicated by the possibility of competitive H2 release from vicinal C−H, N−H or C−H, B−H bonds or exocyclic alkyl groups. To meet these criteria, we prepared 1,2-dimethyl-1,2BN-cyclohexene (1), using adaptations of methods previously developed by our group13b and others,18 as shown in Scheme 1. Aromatization of intermediate 2 by palladium on carbon (Pd/ C) also conveniently facilitated access to quantities of the desired fully dehydrogenated product 3 for characterization purposes.19 The general experimental setup for the dehydrogenation studies was as follows (see the Supporting Information for detailed parameters): neat 1 was injected via syringe pump into a stream of argon directly above a U-bend quartz tube reactor. This tube was packed on one side with layers of glass wool, loose quartz, and a mixture of quartz and catalyst above a porous glass frit. A digitally controlled tube furnace was used to heat the tube reactor to the desired reaction temperature, and the reactor outflow was sent directly to a mass spectrometer (electron ionization mode) for real-time quantification of product formation. After determining the optimal substrate injection rate and minimum reactor temperature required to ensure complete passage of 1 through the system, we performed a cursory activity screening of commercially available heterogeneous dehydrogenation catalysts. Given our past observations of the inactivity of these catalysts in solution-phase reactions, we were pleased to find that, under gas-phase conditions, simple Pd/C afforded continuous dehydrogenation of 1 beginning at 170 °C (Figure 4A).20 Significantly, H2 and 3 were consistently generated in a 2:1 ratio, indicating the former to derive entirely from complete dehydrogenation of 1 (Figure 4B). This is in

Figure 4. (A) Rates of formation of 3 and H2 from 1 at various temperatures normalized by mass catalyst. (B) Determination of consistent 2:1 ratio of H2 and 3 evolution rates at all temperatures.

accord with the typical behavior of cyclohexane in gas-phase dehydrogenation reactions, in which partially dehydrogenated products are generally not observed.21 With reliable reaction conditions for 1 thus established, we next turned to confirming the existence of an appreciable difference in the dehydrogenation kinetics of representative allcarbon cyclohexene and cyclohexane compounds in our specific B

DOI: 10.1021/acs.organomet.6b00412 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics catalytic flow system. To this end, the apparent rate constants for unsubstituted cyclohexene and cyclohexane at various temperatures22 were determined using the Basset−Habgood equation,23 and the reaction activation parameters were subsequently calculated from Arrhenius analysis. As shown in entries 1 and 2 of Table 1, the activation energy (Ea) for Table 1. Measured Arrhenius Activation Parameters for Dehydrogenation of 1 and Related All-Carbon Compounds

Figure 5. Extrapolated rate constants (k) for 1 (black), cyclohexene (green), cyclohexane (orange), and dimethylcyclohexene (purple) on the basis of experimentally determined Arrhenius parameters.

constants for 1 are also all between 2 and 3 orders of magnitude smaller than those of DMC. This again illustrates that the BN and all-carbon versions of the same substrate exhibit notably different dehydrogenation activities, in spite of their isosteric relationship. In summary, we have developed gas-phase conditions to effect acceptorless −CH2CH2− dehydrogenation of a model BN-cyclohexene (1) using a heterogeneous Pd/C catalyst. Arrhenius analysis of this reaction in comparison to the dehydrogenation of dimethylcyclohexene revealed appreciably less favorable activation parameters for 1 (higher Ea and smaller A). The B−N bond in 1 thus appears less influential in promoting efficient −CH2CH2− dehydrogenation than the analogous CC bond in an all-carbon cycloalkene.

Conditions: 50 mL min−1 Ar carrier gas, 20 mg of 10 wt % Pd/C; substrate injection rates (mmol s−1) cyclohexene (1.10 × 10−4), cyclohexane (1.03 × 10−4), 1 (1.04 × 10−4), dimethylcyclohexene (1.03 × 10−4), cis-1,2-dimethylcyclohexane (9.85 × 10−5), trans-1,2dimethylcyclohexane (1.14 × 10−4); hydrogen evolution rates measured from 70 to 110 °C at 10 °C increments for cyclohexene,22 from 170 to 190 °C at 5 °C increments for cyclohexane, 1, and 1,2dimethylcyclohexane, and from 70 to 90 °C at 5 °C increments for dimethylcyclohexene.22 All reactions were repeated in triplicate. Error ranges were calculated as the standard deviation from the mean. b Values based on quantification of dehydrogenated product mass signal. A complementary analysis based on H2 mass signal gave similar results for 1; see the Supporting Information for a representative example. cA 5:1 mixture of 1,2- and 1,6-dimethylcyclohexene was used due to synthetic limitations. dValues based on quantification of H2 mass signal. a

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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/acs.organomet.6b00412. Experimental details, characterization data, and NMR spectra of new compounds and Arrhenius plots (PDF)

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cyclohexene dehydrogenation in our system is 2.0 kcal mol−1 lower than that for cyclohexane; the pre-exponential factor (A) for cyclohexene is also 2 orders of magnitude larger.24 More importantly, the results for these two substrates are clearly distinct, lending support to our initial proposal to categorize the dehydrogenation reactivity of 1 as more or less cyclohexene-like on the basis of kinetic analysis. Significantly, comparison of the activation parameters for 1 and dimethylcyclohexene (DMC) also revealed moderate differences: the activation energy for 1 is 1.7 kcal mol−1 higher than that for DMC, and the pre-exponential factor for 1 is also 1 order of magnitude smaller (Table 1, entries 3 and 4).25,26 In this respect, the kinetic trends for 1 relative to DMC appear to mirror to an extent those for cyclohexane relative to cyclohexene. (We could not directly compare the kinetics of 1 to those of 1,2-dimethylcyclohexane, as we observed both the cis and trans isomers of the latter to rapidly deactivate the Pd/ C catalyst.27) Our experimental determination of activation energy and pre-exponential factor values also facilitates a basic comparison of each dehydrogenation reaction’s rate constants within a given temperature range. As shown in Figure 5, from 50 to 400 °C, rate constants extrapolated from the Arrhenius parameters for cyclohexane are between 3 and 4 orders of magnitude smaller than those of cyclohexene. In a similar manner, the rate

AUTHOR INFORMATION

Corresponding Authors

*E-mail for C.-K. T.: [email protected]. *E-mail for S.-Y. L.: [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the U.S. Department of Energy, Office of Energy and Renewable Energy (DE-EE-0005658). We thank Prof. David A. Dixon for helpful discussions. S.-Y.L. thanks the Dreyfus Foundation for a Teacher-Scholar award and the Humboldt Foundation for a Friedrich Wilhelm Bessel Research Award.

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DOI: 10.1021/acs.organomet.6b00412 Organometallics XXXX, XXX, XXX−XXX