WO2014028281A1

WO2014028281A1 - B-alkyl-substituted ammonia boranes

Info

Publication number: WO2014028281A1
Application number: PCT/US2013/053914
Authority: WO
Inventors: Shih-Yuan Liu; Patrick Campbell
Original assignee: University of Oregon; Oregon State
Current assignee: University of Oregon; Oregon State
Priority date: 2012-08-14
Filing date: 2013-08-07
Publication date: 2014-02-20
Anticipated expiration: 2015-02-14

Description

B-ALKYL-SUBSTITUTED AMMONIA BORANES

This application claims the benefit of U.S. Provisional Application No. 61/683,073, filed August 14, 2012, which is incorporated herein by reference in its entirety.

BACKGROUND

Hydrogen has the potential to replace petroleum as the primary fuel for transportation and remote power applications. One significant hurdle that must be overcome is the development of safe (for example, ambient pressure and temperature, non-toxic), efficient (high gravimetric hydrogen density), and convenient (mild H₂ release conditions) storage methods for hydrogen. Boron-nitrogen containing chemical hydride compounds have received considerable recent attention as possible hydrogen storage materials due to their high gravimetric density and favorable kinetics of hydrogen release. Ammonia borane (AB, H₃N-BH₃, 19.6 wt % H₂) and /V-substituted derivatives of AB have been the focus numerous literature reports detailing hydrogen release conditions, and characterization and regeneration of spent-fuel material.

SUMMARY Disclosed herein is a compound having a structure of:

H H

R¹ N B R²

H H wherein R¹ is selected from H, or an optionally- substituted alkyl; and

R is an optionally-substituted alkyl.

Further disclosed herein is a hydrogen storage system that includes at least one of the compounds disclosed herein. Also disclosed herein are methods for releasing hydrogen from any one of the above- described compounds or hydrogen storage systems.

The foregoing will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing catalytic dehydrogenation of two prior art compounds (AB and MeAB) compared to two novel compounds 1 and 2 disclosed herein.

DETAILED DESCRIPTION

The following explanations of terms and methods are provided to better describe the present compounds, compositions and methods, and to guide those of ordinary skill in the art in the practice of the present disclosure. It is also to be understood that the terminology used in the disclosure is for the purpose of describing particular embodiments and examples only and is not intended to be limiting.

An "R-group" or "substituent" refers to a single atom (for example, a halogen atom) or a group of two or more atoms that are covalently bonded to each other, which are covalently bonded to an atom or atoms in a molecule to satisfy the valency requirements of the atom or atoms of the molecule, typically in place of a hydrogen atom. Examples of R-groups/substituents include alkyl groups, hydroxyl groups, alkoxy groups, acyloxy groups, mercapto groups, and aryl groups.

"Substituted" or "substitution" refer to replacement of a hydrogen atom of a molecule or an R-group with one or more additional R-groups such as halogen, alkyl, alkoxy, alkylthio, trifluoromethyl, acyloxy, hydroxy, mercapto, carboxy, aryloxy, aryl, arylalkyl, heteroaryl, amino, alkylamino, dialkylamino, morpholino, piperidino, pyrrolidin-l-yl, piperazin-l-yl, nitro, sulfato or other R-groups.

The term "alkyl" refers to a branched or unbranched saturated hydrocarbon group of 1 to 24 carbon atoms, such as methyl, ethyl, w-propyl, isopropyl, w-butyl, isobutyl, i-butyl, pentyl, hexyl, heptyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl and the like. A "lower alkyl" group is a saturated branched or unbranched hydrocarbon having from 1 to 6 carbon atoms. Preferred alkyl groups have 1 to 4 carbon atoms. Alkyl groups may be "substituted alkyls" wherein one or more hydrogen atoms are substituted with a substituent such as halogen, cycloalkyl, alkoxy, amino, hydroxyl, aryl, alkenyl, or carboxyl. For example, a lower alkyl or (C₁-C₆)alkyl can be methyl, ethyl, propyl, isopropyl, butyl, iso-butyl, sec-butyl, pentyl, 3-pentyl, or hexyl; (C₃-C₆)cycloalkyl can be cyclopropyl, cyclobutyl, cyclopentyl, or cyclohexyl; (C₃-C₆)cycloalkyl(C₁-C₆)alkyl can be cyclopropylmethyl, cyclobutylmethyl, cyclopentylmethyl, cyclohexylmethyl, 2-cyclopropylethyl, 2-cyclobutylethyl, 2-cyclopentylethyl, or 2-cyclohexylethyl; (C₁-C₆)alkoxy can be methoxy, ethoxy, propoxy, isopropoxy, butoxy, iso-butoxy, sec-butoxy, pentoxy, 3-pentoxy, or hexyloxy; (C₂-C₆)alkenyl can be vinyl, allyl, 1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 1,- pentenyl, 2-pentenyl, 3-pentenyl, 4-pentenyl, 1- hexenyl, 2-hexenyl, 3-hexenyl, 4-hexenyl, or 5- hexenyl; (C₂-C₆)alkynyl can be ethynyl, 1-propynyl, 2-propynyl, 1-butynyl, 2-butynyl, 3-butynyl, 1-pentynyl, 2-pentynyl, 3-pentynyl, 4-pentynyl, 1- hexynyl, 2-hexynyl, 3-hexynyl, 4-hexynyl, or 5- hexynyl; (Ci-C₆)alkanoyl can be acetyl, propanoyl or butanoyl; halo(C₁-C₆)alkyl can be

iodomethyl, bromomethyl, chloromethyl, fluoromethyl, trifluoromethyl, 2-chloroethyl, 2- fluoroethyl, 2,2,2-trifluoroethyl, or pentafluoroethyl; hydroxy(C₁-C₆)alkyl can be hydroxymethyl, 1-hydroxyethyl, 2-hydroxyethyl, 1-hydroxypropyl, 2-hydroxypropyl, 3-hydroxypropyl, 1- hydroxybutyl, 4-hydroxybutyl, 1-hydroxypentyl, 5-hydroxypentyl, 1 -hydroxyhexyl, or 6- hydroxyhexyl; (C₁-C₆)alkoxycarbonyl can be methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, isopropoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, or hexyloxycarbonyl; (C₁-C₆)alkylthio can be methylthio, ethylthio, propylthio, isopropylthio, butylthio, isobutylthio, pentylthio, or hexylthio; (C₂-C₆)alkanoyloxy can be acetoxy, propanoyloxy, butanoyloxy, isobutanoyloxy, pentanoyloxy, or hexanoyloxy.

Disclosed herein are B-substituted ammonia borane compounds having a structure of:

H H

R¹ N B R²

H H

Formula I

wherein R¹ is selected from H, or an optionally- substituted alkyl; and

R is an optionally-substituted alkyl.

In certain embodiments, R¹ is H. In certain embodiments, R¹ is a CrC₆ alkyl, particularly methyl. In certain embodiment, R is a C -C alkyl, particularly methyl.

1 2

The compound in which R is H and R is methyl has a gravimetric hydrogen capacity of

1 2

8.9 wt hydrogen. The compound in which R is methyl and R is methyl has a gravimetric hydrogen capacity of 6.8 wt hydrogen.

The compounds disclosed herein are useful as hydrogen storage materials. In further embodiments disclosed herein, there are provided methods for storing and/or releasing hydrogen from the compounds described herein. For example, disclosed herein are hydrogen storage methods that include releasing hydrogen from at least one B-alkyl substituted ammonia borane under conditions sufficient to produce at least one borazine, and optionally hydrogenating the borazine. The hydrogen may be released and/or added during the hydrogen storage cycle in any form. For example, the hydrogen may be released and/or added as a formal equivalent of dihydrogen. A formal equivalent of dihydrogen is two hydrogen atoms, whether the hydrogen atoms are added to the substrate as dihydrogen (during hydrogenation), as hydride ions, or as protons. For example, the combination of a hydride ion and a proton formally constitutes one equivalent of dihydrogen.

Release of hydrogen from the compounds disclosed herein may be accomplished by several approaches. For example, the compounds are capable of releasing hydrogen both thermally and/or catalytically. Thermal release includes heating the compound at a sufficiently high temperature to affect release of at least one dihydrogen equivalent. For instance, the compound may be heated at a temperature of at least 50°C, particularly at least 150°C. Catalytic release of hydrogen includes contacting the compound with a metal halide catalyst at conditions sufficient for causing hydrogen release. The catalytic dehydrogenation optionally is conducted with heating such as at a temperature from 50 to 200°C, more particularly 50 to 80°C. The metal species of the metal halide catalyst may be selected, for example, from a transition metal, particularly a first-row transition metal. Illustrative metals include iron, cobalt, copper, nickel and illustrative halides include fluorine, chlorine, bromine, and iodine.

The dehydrogenated product is a borazine compound having a structure of Formula II:

Formula II

1 2 wherein each R is independently selected from H, or an optionally- substituted alkyl; and each R is independently an optionally- substituted alkyl.

In certain embodiments, R¹ is H. In certain embodiments, R¹ is a CrC₆ alkyl, particularly methyl.

In certain embodiment, R is a C -C alkyl, particularly methyl.

1 2

The structure of R and R in the fully-dehydrogenated product is dependent upon the structure of the fully-charged (i.e., saturated) compound.

The dehydrogenated product(s) may be regenerated by hydrogenating the dehydrogenated product(s). The dehydrogenated product(s) are also referred to herein as "spent fuel."

The hydrogen storage system may include at least one of the compounds described above. The hydrogen storage system may include a port for the introduction of hydrogen for subsequent storage. Similarly, it may include a tap or port for the collection of regenerated hydrogen gas.

Such a hydrogen storage system may be incorporated into a portable power cell, or may be installed in conjunction with a hydrogen-burning engine. The hydrogen storage system may be used in or with a hydrogen-powered vehicle, such as an automobile. Alternatively, the hydrogen storage device may be installed in or near a residence, as part of a single-home or multi-home hydrogen- based power generation system. Larger versions of the hydrogen storage device may be used in conjunction with, or in replacements for, conventional power generating stations.

The hydrogen storage system may also utilize one or more additional methods of hydrogen storage in combination with the presently disclosed compounds, including storage via compressed hydrogen, liquid hydrogen, and/or slush hydrogen. Alternatively, or in addition, the hydrogen storage system may include alternative methods of chemical storage, such as via metal hydrides, carbohydrates, ammonia, amine borane complexes, formic acid, ionic liquids, phosphonium borate, or carbonite substances, among others. Alternatively, or in addition, the hydrogen storage system may include methods of physical storage, such as via carbon nanotubes, metal-organic frameworks, clathrate hydrates, doped polymers, glass capillary arrays, glass microspheres, or keratine, among others.

Disclosed herein is the first synthesis of two novel boron-substituted AB derivatives 1 and 2 with 6.8 wt% H₂ and 8.9 wt% H₂, respectively. It was found that at 150 °C compounds 1 and 2 quickly and cleanly release 2 equivalents of H₂ per molecule. Catalytic dehydrogenation conditions were developed (5 mol% CoCl₂, 80 °C) and found that 1 and 2 each release ca. 2 equivalents of H₂ in less than 20 minutes (Figure 1). Under identical catalytic dehydrogenation conditions, AB release ca. 2 equivalents in >150 minutes and N-Me-AB (MeAB) released ca. 1.5 equivalents H₂ in >300 minutes. The favorable H₂ release properties and high gravimetric hydrogen capacity of compounds 1 and 2 substantiate these compounds as potential hydrogen storage materials.

A synthesis scheme (Scheme 1) for the compounds disclosed herein is shown below.

Treating Li[MeBH₃] with the appropriate commercially available amine-hydrochloride salt led to the loss of one equivalent of H₂ and generated compounds 1 and 2 in 88% and 75% isolated yield, respectively.

MeB(OH)₂ -

R = Me, H 1 2

88% 75%

Scheme 1

Examples

7

Synthesis of Compound 1. To a stirring solution of Li[MeBH₃] (0.135 H H

g, 3.77 mmol) in Et₂0 (10 mL) was added MeNH Cl (0.255 g, 3.77 e-N-B- e

H H

mmol). The slurry was allowed to stir for 1 hour, and then filtered 1

through an Acrodisc. The majority of the solvent (75%) was removed under reduced pressure and pentane was added to cause precipitation of the product. The solvent layer was removed by pipet and the solid residue was washed two times with pentane to give 1 as a white crystalline solid (0.191 g, 88% yield). 1H NMR (300 MHz, CD₂C1₂): δ 3.35 (t br, 2H, N-H), 2.54 (t, ³J_HH = 4.0 Hz, 3H, N-Me), 1.82 (q br, _BH = 90 Hz, 2H, B-H), 0.20 (s br, 3H, B-Me). ¹³C NMR (125 MHz, THF-d8): δ 30.64, 0.49 (br). ^UB NMR (96.27 MHz, CD₂C1₂): δ -11.17 (t, ³J_BH = 95 Hz). HRMS (EI+) calcd. for C₂H₈NB (-H₂) 57.074980 found 57.074987.

H H

Synthesis of Compound 2. To a stirring solution of Li[MeBH₃] (0.075 g, ^_|\j_g_^

2.10 mmol) in Et₂0 (10 mL) was added NH₄C1 (0.112 g, 2.10 mmol). H H

The slurry was allowed to stir for 1 hour, and then filtered through an ^

Acrodisc. The majority of the solvent (75%) was removed under reduced pressure and pentane was added to cause precipitation of the product. The solvent layer was removed by pipet and the solid residue was washed two times with pentane to give 2 as a white crystalline solid (0.072 g, 75% yield).

1H NMR (300 MHz, CD₂C1₂): δ 3.37 (t br, 3H, N-H), 2.93 (q br, ⁴J_BH = 90 Hz, 2H, B-H), -0.17 (s br, 3H, B-Me). ^UB NMR (96.27 MHz, CD₂C1₂): δ -14.56 (t, ³J_BH = 93 Hz).

Comparison of Rates and Extent of Dehydrogenation

Dehydrogenation experiments were performed using an automated gas burette apparatus, which enabled determination of both the rate and amount of hydrogen released. CoCl₂ was selected as the catalyst for these studies. The results of the burette experiment are depicted in Figure 1. The B-methyl substituted linear AB derivatives 1 and 2 displayed a rapid rate of hydrogen release; in the case of 2, releasing ca. 1.75 equivalents of H₂. Notably, both of the non-B -alkyl substituted compounds, MeAB and AB, displayed markedly slower rates of hydrogen release under these conditions. Ammonia borane released ca. 2 equiv. H₂ in 150 minutes and MeAB released only ca. 1.25 equiv. H₂ in 240 minutes. These results indicate that the novel 5-substituted AB derivatives 1 and 2 could potentially be useful as hydrogen storage materials, storing 6.8 wt.% and 8.9 wt.% hydrogen, respectively.

Regeneration

Facile recharging of spent material with fresh hydrogen equivalents is a critical for the widespread adoption of any chemical platform for hydrogen storage. In the case of ammonia borane, researchers have recently developed methods for the regeneration of a model AB spent fuel material, polyborazylene (PB), using digestion strategies based on benzenedidthol/Sn-H or hydrazine/NH . However, PB is just one of several potential products that can result from AB dehydrogenation. It was found that under both thermal and catalytic conditions, 1 and 2 initially form a mixture of dehydrogenated compounds. Extended heating of the mixture (36 hours) causes the intermediate species to converge to a single product, which is identified as the trimeric species hexamethylborazene 3 in the case of 1, and 2,4,6-trimethylborazene 4 in the case of 2, on the basis of their ^UB NMR chemical shifts. After catalytic dehydrogenation of 1 in refluxing THF using our standard catalyst and loading (5 mol CoCl₂), compound 3 was isolated in 71 % yield.

Me

^R-N-^N-^R

3 RH₂N-BH₂Me

catalytic (5 mol% CoCI₂, 80°C)

or thermal (150 °C, no cat)

36 hours

1 R = Me 3 R = Me, isolated in 71 % yield

2 R = H 4 R = H, observed by ^{1 1} B NMR

Scheme 2. Single product formation in the dehydrogenation of 1 and 2.

A route to regenerate the fully charged compound 1 from the spent material 3 was developed and is illustrated in Scheme 3. Activation of 3 with 3 equiv. formic acid generates the BN-cyclohexane analog 5. A crystal of 5 suitable for single X-ray diffraction was isolated, which unambiguously confirms its structural assignment. Interestingly, additional equivalents of formic acid do not add to boron or break the central BN ring, even in 10-fold excess. However, addition of LiAlH₄ to 5 does break apart the trimer and cleaves the B-0 bonds to install the B(H)₂ moieties. (Treatment of 3 directly with L1AIH₄ without pre-treatment with formic acid was not sufficient to break the ring; only unreacted starting material was recovered.) Workup using wet THF as a proton source gives 1, which was isolated in 46% yield.

Scheme 3. Regeneration of spent fuel trimer 3 to fully charged compound 1.

In view of the many possible embodiments to which the principles of the disclosure may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the disclosure and should not be taken as limiting the scope of the disclosure.

Claims

What is claimed is:

1. A compound having a structure of:

H H

R¹ N B R² H H wherein R¹ is selected from H or an optionally-substituted alkyl; and

R is an optionally-substituted alkyl.

2. The compound of claim 1, wherein R¹ is selected from H and CrC₆ alkyl.

3. The compound of claim 1, wherein R¹ is H.

4. The compound of claim 1, wherein R¹ is CrC₆ alkyl.

5. The compound of claim 1, wherein R¹ is methyl.

6. The compound of any one of claims 1 to 5, wherein R is CrC₆ alkyl.

7. The compound of any one of claims 1 to 6, wherein R is methyl.

8. A hydrogen storage system comprising at least one compound of any one of claims 1 to

7.

9. A method for releasing hydrogen from at least one compound of any one of claims 1 to

7, the method comprising heating the at least one compound, contacting the at least one compound with a catalyst, or simultaneously heating the at least one compound and contacting the at least one compound with a catalyst.

10. The method of claim 9, wherein the catalyst is CoCl₂.

11. The method of claim 9 or 10, wherein the method produces a compound having the structure of:

wherein each R 1 is independently selected from H, or an optionally- substituted alkyl; and each R 2 is independently an optionally-substituted alkyl.