Air-stable (CAAC)CuCl and (CAAC)CuBH4 complexes as catalysts for the hydrolytic dehydrogenation of BH3NH3

Article abstract of DOI:10.1002/anie.201500224

The first stable copper borohydride complex [(CAAC)CuBH₄] [CAAC=cyclic(alkyl)(amino)carbene] bearing a single monodentate ligand was prepared by addition of NaBH₄ or BH₃NH₃ to the corresponding [(CAAC)CuCl] comp

Full text of DOI:10.1002/anie.201500224

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Chemie
International Edition: DOI: 10.1002/anie.201500224
German Edition: DOI: 10.1002/ange.201500224
Hydrides
Air-Stable (CAAC)CuCl and (CAAC)CuCuBH4 Complexes as
Catalysts for the Hydrolytic Dehydrogenation of BH3NH3**
Xingbang Hu,* Michꢀle Soleilhavoup, Mohand Melaimi, Jiaxiang Chu, and Guy Bertrand*
Abstract: The first stable copper borohydride complex
[(CAAC)CuBH4] [CAAC = cyclic(alkyl)(amino)carbene]
Herein, we report the isolation of a copper borohydride
complex supported by a single ligand. We also describe the
serendipitous discovery that [(CAAC)CuCl] complexes
[CAAC = cyclic(alkyl)(amino)carbene][17] are highly efficient
and recyclable catalysts for the hydrolytic dehydrogenation of
BH3NH3.
In the course of our ongoing search for the preparation of
a hitherto unknown stable monomeric copper hydride,[18] we
attempted the reduction of the [(CAACEt)CuCl] (1) with
NaBH4 (Figure 1). The 11B NMR spectrum of the resulting
bearing a single monodentate ligand was prepared by addition
of NaBH4 or BH3NH3 to the corresponding [(CAAC)CuCl]
complex. Both complexes are air-stable and promote the
catalytic hydrolytic dehydrogenation of ammonia borane. The
amount of hydrogen released reaches 2.8 H2/BH3NH3 with
a turnover frequency of 8400 molH molcatÀ1 hÀ1 at 258C. In
2
a fifteen-cycle experiment, the catalyst was reused without any
loss of efficiency.
M
etal borohydride M(BH4)n (n = 1, 2, and 3)[1] and
ammonia borane (BH3NH3) are known as excellent hydrogen
storage materials because of their high hydrogen capacity.[2]
Among M(BH4)n complexes, CuBH4 is unstable,[3] and only
a few copper(I) borohydride complexes stabilized by two or
three ligands (L),[4] such as the commercially available
[(Ph3P)2CuBH4], have been isolated. So far, all attempts to
isolate CuBH4 complexes supported by a single monodentate
ligand have failed, as exemplified by the rapid decomposition
of [(Ph3P)CuBH4] at À208C.[5] Releasing H2 at room temper-
ature with a high hydrogen generation rate is vital to making
significant use of BH3NH3.[6] Rh,[7] Ru,[8] Pd,[9] and Ir[10]
complexes are representative catalysts for the pyrolytic
dehydrogenation, whereas Pt[11] and Au[12] are the most
efficient for the hydrolytic dehydrogenation of BH3NH3.
Developing catalysts based on cheap transition metals,[13,14]
especially earth-abundant metals such as Fe[15] and Cu,[16] is
one of the most important steps for the large-scale application
of these dehydrogenation reactions. Additionally, because of
decomposition or particle agglomeration, most of the cata-
lysts (both noble and non-noble metal catalysts) suffer from
a short lifetime and are hardly recyclable.[2]
Figure 1. Synthesis of the [(CAACEt)CuBH4] complex 2 and the [(CAAC)-
CuCl] complexes 1, 3, and 4 used in this study. Dipp=2,6-diisopropyl-
phenyl.
product showed a quintet at d = À38.2 ppm, thus indicating
the presence of a BH4 fragment, which is different from
NaBH4 (d11B: À43.5 ppm). After work up, the complex 2 was
isolated as a white solid in 58% yield, and its structure was
confirmed by
a single-crystal X-ray diffraction study
(Figure 2).[19] Two hydrogen atoms of BH4 interact with the
metal, with Cu···H distances of 1.679(2) and 1.717(18) ꢀ. The
Cu···Ccarbene distance [1.8879(15) ꢀ] is comparable to that of
1 [1.8752(13) ꢀ]. The complex 2 is air stable at room
temperature, both in solution and in the solid state, and
decomposes only above 1628C. Surprisingly, we found that 2
could also be prepared in 91% yield by addition of excess
ammonia borane to the copper chloride complex 1. In this
case, the formation of 2 might result from an ion exchange
between 1 and [(NH3)2BH2]+[BH4]À, a postulated unstable
intermediate in the dehydrogenation of BH3NH3.[2f,g,20]
Since the copper chloride 1 cleanly reacts at room
temperature with BH3NH3 to produce 2, we wondered
whether 1 could be an efficient catalyst for the hydrolytic
dehydrogenation of BH3NH3.[21] When a 1 mol% acetone/
water solution (20 wt% of water) of 1 was added to BH3NH3,
[*] Dr. X. Hu, Dr. M. Soleilhavoup, Dr. M. Melaimi, Dr. J. Chu,
Prof. G. Bertrand
UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
Dr. X. Hu
School of Chemistry and Chemical Engineering, Nanjing University
Nanjing 210093 (P.R. China)
[**] We gratefully acknowledge financial support from the DOE (DE-
FG02-13ER16370). Acknowledgements are due to the NNSF of
China (No. 21176110), Jiangsu Province NSF (BK20141311)
(X.B.H.), and SIOC (J.X.C.) for fellowships. CAAC=cyclic (alkyl)-
(amino)carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500224.
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Other than the TOF, an important aspect for the large-
scale application of catalysts is their robustness in recycling
processes. The 1:1 mixture of 1 and KBArF was reused 15
times without any noticeable loss of catalytic activity
(Figure 4). For each cycle, the dehydrogenation reaction was
finished within 5 minutes and the release of H2 fluctuated
around 2.8 moles. The procedure for recycling the catalyst is
Figure 2. X-ray structure of 1 (left) and 2 (right). All hydrogen atoms,
except those bonded to boron, are omitted for clarity.
2.6 moles of H2 were released per mole of BH3NH3 over
5 minutes, and corresponds to a turnover frequency (TOF) of
3100 molH molcatÀ1 hÀ1 (Figure 3). As observed with other
2
catalysts, NH4BO2 is produced as the hydrolytic byprod-
uct.[2,11,12]
Figure 4. Recycling of catalyst in the BH3NH3 dehydrogenation at
258C. Catalyst: 1 mol% (1 + KBArF). Reaction time for each cycle:
5 min.
straightforward. After the dehydrogenation reaction was
over, the suspension was filtered and the solution was directly
reused without any treatment. The recyclability of the catalyst
solution greatly enhances the hydrogen storage capacity when
taking solvent into consideration.[2b]
In summary, the peculiar electronic properties of CAACs
allow the isolation of the first monoligated CuBH4 complex.
Both [(CAAC)CuBH4] and their precursors [(CAAC)CuCl]
efficiently promote the hydrolytic dehydrogenation of
BH3NH3. TOFs reach 8400 at 258C, and compare well with
those reported for noble-metal-based catalysts. Additionally,
the catalytic solution can be handled in air and be recycled
without significant loss of activity.
Figure 3. Amount of H2 released in the hydrolytic dehydrogenation of
BH3NH3 using 1 mol% 1, 1 + KBArF, 2, 3, 4, and 5 at 258C.
The TOF for 1 is about 500 times higher than that
reported using 30 mol% of CuCl2.[16] We found that
[(Ph3P)2CuCl] (5) was also a poor catalyst (TOF = 850),
which is not surprising since this complex does not react with
BH3NH3 when the reaction is carried out at room temper-
ature. These observations prompted us to try optimizing the
structure of the CAAC-supported copper catalyst. Introduc-
ing bulkier substituents on the a-carbon atom of the CAAC
ligand, as shown by 3 and 4 (Figure 1), resulted in a decrease
of activity (TOF = 1770 and 1340, respectively).
We then focused on the smallest CAAC ligand. Having
already shown that the presence of a chloride scavenger often
enhances the reactivity of CAAC-coinage metal complexes,[22]
we sought to augment the reactivity of 1 by first using a 1:1
mixture of 1 and potassium tetrakis(pentafluorophenyl)bo-
rate (KBArF). The amount of H2 released increased to
2.8 moles, with a TOF of 3360. Then, we realized that 2 was
probably either the active catalytic species or the resting state
of the catalyst. Indeed, by using 1 mol% of 2, the hydrolytic
dehydrogenation reaction of BH3NH3 was finished within
2 minutes at 258C, with 2.8 moles of H2 released per mole of
BH3NH3, thus giving a TOF of 8400. These results compared
well with those obtained in the hydrolytic dehydrogenation of
ammonia borane promoted by noble metal catalysts.[11,12]
Experimental Section
Synthesis of 1: Diethyl cyclic iminium tetrafluoroborate salt (1204 mg,
3.00 mmol), potassium hexamethyldisilazide (628 mg, 3.15 mmol),
and copper(I) chloride (312 mg, 3.15 mmol) were loaded in a Schlenk
tube under argon. THF (40 mL) was added to the solids at À788C and
the mixture was stirred for 1 h at the same temperature. The mixture
was warmed to room temperature and stirred for another 5 h. The
volatiles were removed under vacuum and the residue was washed
with hexanes (30 mL). After removing the volatiles, the residue was
extracted with benzene (40 mL). The solution was evaporated and
dried under vacuum, affording 1 as a white solid (963 mg, 78% yield).
Single crystals were obtained from a diethyl ether solution stored at
À388C. m.p. 241–2428C; 13C NMR (126 MHz, CDCl3): d = 250.26
(Ccarbene), 145.03 (Caro), 134.59 (Caro), 129.72 (CHaro), 124.74 (CHaro),
81.04 (Cq), 62.55 (Cq), 42.39 (CH2), 31.11 (CH2), 29.28 (CH3), 29.19
(CH3), 27.30 (CH), 22.37 (CH3), 9.64 ppm (CH3); HRMS (m/z):
[M+NH4]+ calcd for [C22H35NCuCl·NH4]+: 429.2092, found 429.2084.
Synthesis of 2: Method A: The complex 1 (411 mg, 1.00 mmol)
and NaBH4 (151 mg, 4.00 mmol) were loaded in a Schlenk tube under
argon. THF (30 mL) was added and the mixture was stirred for 24 h at
room temperature. The suspension was filtered under argon. The
2
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filtrate was evaporated under vacuum and the residue was extracted
with ether (50 mL). The solution was evaporated and dried under
vacuum, thus affording 2 as a white solid (225 mg, 58% yield).
Method B: Compound 1 (411 mg, 1.00 mmol) and BH3NH3 (185 mg,
6.00 mmol) were loaded in a Schlenk tube under argon. THF (30 mL)
was added to the solids and the mixture was stirred for 24 h at room
temperature. The suspension was filtered under argon. The filtrate
was evaporated under vacuum and the residue was extracted with
ether (50 mL). The solution was evaporated and dried under vacuum,
thus affording 2 as a white solid (356 mg, 91% yield). Single crystals
were obtained at À388C from an ether solution. m.p. 162–1638C
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1
(dec.); H NMR (500 MHz, C6D6): d = 7.12 (t, J = 7.5 Hz, 1H), 7.00
(d, J = 7.5 Hz, 2H), 2.75 (sept, J = 6.0 Hz, 2H), 1.65 (m, 2H), 1.56 (m,
2H), 1.43 (s, 2H), 1.36 (d, J = 6.0 Hz, 6H), 1.09 (d, J = 6.0 Hz, 6H),
0.88 (br, 12H), 0.56 ppm (m, J = 83.0 Hz, 4H); 11B NMR (96 MHz,
CDCl3): d = À38.21 (quint, J = 83.0 Hz).
Syntheses of 3 and 4 can be found in the Supporting Information.
Procedure for the hydrolytic dehydrogenation of BH3NH3
catalyzed by 1, 3, 4, or 5: The dehydrogenation reactions were
carried out at 258C under air. BH3NH3 (15.4 mg, 0.5 mmol) was
loaded in a tube equipped with a magnetic stirrer. A solution of
catalyst (1 mol% based on BH3NH3) in 1.0 mL of acetone/water
(20 wt% of water) was quickly added by a syringe. The amount of H2
released was measured with a water-filled burette.
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Procedure for the hydrolytic dehydrogenation of BH3NH3
catalyzed by 2: The dehydrogenation reaction was carried out at
258C under air. BH3NH3 (15.4 mg, 0.5 mmol) and
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Keywords: boranes · carbenes · copper · hydrides ·
structure elucidation
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Received: January 9, 2015
Published online: && &&, &&&&
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Hydrides
Copper works! [(CAAC)CuCl] reacts with
both NaBH4 and BH3NH3 to afford
[(CAAC)CuBH4] as a thermally and air-
stable complex. Both complexes effi-
ciently promote the hydrolytic dehydro-
genation of ammonia borane at room
temperature, with a turnover frequency of
X. Hu,* M. Soleilhavoup, M. Melaimi,
J. Chu, G. Bertrand*
&&&& — &&&&
Air-Stable (CAAC)CuCl and
(CAAC)CuCuBH4 Complexes as Catalysts
for the Hydrolytic Dehydrogenation of
BH3NH3
up to 8400 molH molcatÀ1 hÀ1. In addition,
2
these air-stable catalysts are readily recy-
clable. CAAC=cyclic (alkyl)-
(amino)carbene.
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