H/D scrambling in a chromium-catalyzed dehydrocoupling reaction of a borane-dimethylamine adduct

Article abstract of DOI:10.1039/c7dt02345f

H/D scrambling took place in a chromium-catalyzed dehydrocoupling reaction of a deuterium-labeled borane-dimethylamine adduct. In the hydrogen elimination of BH₃·NDMe₂ (1a-d_N), H₂, HD and D₂ were generated in 65:30:5 ratio, and 62% of deuterium atoms were incorporated into the major product, the dimethylaminoborane dimer. Proton and deuteron nuclei were thus concentrated into the evolved dihydrogen and aminoborane dimer, respectively. The mechanism of H/D scrambling is understood based on the reaction pathway of the dehydrocoupling of 1a, which was previously proposed based on DFT calculations. The H/D distribution in the products is explained by the energy difference according to the deuterated position in an intermediate of the dehydrocoupling reaction.

Full text of DOI:10.1039/c7dt02345f

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DOI: 10.1039/C7DT02345F
H/D Scrambling in a Chromium-catalyzed Dehydrocoupling
Reaction of Borane–dimethylamine Adduct
Yasuro Kawano*a,b and Mamoru Shimoi a
Received 00th January 20xx,
Accepted 00th January 20xx
H/D scrambling took place in a chromium-catalyzed dehydrocoupling reaction of deuterium-labeled borane–
dimethylamine adduct. In the hydrogen elimination of BH3·NDMe2 (1a-dN), H2, HD and D2 were generated in 65:30:5 ratio,
and 62 % of deuterium atom was incorporated into the major product, dimethylaminoborane dimer. Proton and deuteron
nuclei were thus concentrated into the evolved dihydrogen and aminoborane dimer, respectively. The mechanism for the
H/D scrambling is understood based upon the reaction pathway of the dehydrocoupling of 1a, which was previously
proposed based on DFT calculations. The H/D distribution in the products is explained by the energy difference according
to the deuterated position in an intermediate of the dehydrocoupling reaction.
DOI: 10.1039/x0xx00000x
www.rsc.org/
to the active catalyst [Cr(CO)4] through a BH and the NH
Introduction
hydrogen atoms, and the resulting chelate complex
undergoes NH and BH activation to generate an
aminoborane(dihydride) complex via an intermediate
Complex readily releases an aminoborane molecule , and
the remained dihydride [Cr(CO)4H2] liberates H2 to
regenerate [Cr(CO)4] after isomerization to a dihydrogen
aduuct When the amine substituents are small,
aminoborane dimerizes to yield a cyclic dimer [BH2NR2]2 ( ).
During the dehydrogention reaction, a borane complex
[Cr(CO)5(η1-BH3·NHR2)] is observed by NMR spectroscopy. This
species acts as a reservoir of [Cr(CO)5], which is readily
converted to the active catalyst [Cr(CO)4] in the reaction
system.
4
Much attention is focused on transition metal-catalyzed
dehydrocoupling reactions of borane–amine adducts.1-14 The
products of this reaction, aminoboranes and borazines, can be
good precursors of BN ceramics. In addition, the large
hydrogen content and its effective release in this reaction
strongly suggest the potential utility of amine–boranes (in
particular ammoniaborane) as chemical hydrogen storage
materials.15-22 Thus, research in this area can influence the
development of new energy systems such as fuel cells.
6
5.
6
3
(7)
8.
3
2
σ
To further confirm this reaction mechanism and to know what
occurs on the metal atom during the dehydrocoupling process,
we carried out deuterium labeling experiments. We thought
that the intramolecular 1,2-H elimination could be ascertained
by the dehydrogeation of BH3·NDMe2. However, we observed
unexpected H/D scrambling to yield an H-rich dihydrogen
isotopomeric mixture and D-introduced dimethylaminoborane
dimer. We report here this observation and discuss the
mechanism of the H/D scrambling on the basis of a DFT study.
Scheme 1
Results and discussion
Previously, we reported borane dehydrocoupling reactions
catalyzed by group-6 metal carbonyls, [M(CO)6] (M = Cr, Mo,
Dehydrocoupling of BH3·NDMe2. The H/D scrambling was found
to occur when the Cr-catalyzed dehydrocoupling reaction of
W).3 When a borane–secondary amine adduct BH3·NHR2 (
1
) is
photolyzed in the presence of catalytic amount of [M(CO)6], a
dimeric or monomeric aminoborane [BH2NR2]n ( : n = 2, : n =
BH3·NDMe2 (1a-dN) was monitored using H, H, and 11B NMR
spectroscopy. Thus, a benzene solution and a benzene-d6
solution of 1a-dN containing [Cr(CO)6] (5 mol %) were sealed in
Pyrex NMR sample tubes, and were photolyzed for 40 min and
then left at room temperature for 2 days to complete the
dehydrocoupling reaction. On the basis of the intramolecular
1,2-dehydrogenation mechanism, only HD should be expected
to evolve as a dihydrogen isotopomer. In actual, however, all
1
2
2
3
1) is produced according to the bulkiness of the amine
substituents. If a primary adduct BH3·NH2R is employed as a
precursor, a borazine derivative [BHNR]3 is provided. The
mechanism for the dehydrocoupling of secondary adducts was
investigated by DFT calculations. This reaction proceeds in an
intramolecular stepwise mechanism, and the dehydrogenation
follows 1,2-elimination (Scheme 1). Borane 1a is coordinated
2
the isotopomers, H2, HD and D2 were observed. The H NMR
spectrum of the resulting benzene solution exhibited
1
resonances of HD and D2 as a doublet (4.50 ppm, JHD = 42.7
a.Department of Basic Science, Graduate School of Arts and Sciences, The University
of Tokyo, Meguro-ku, Tokyo, 153-8902
Hz) and a singlet (4.45 ppm) signal, respectively. Their relative
quantity was 64:11 as judged by comparing the signal intensity.
Likewise, the relative quantity of H2 and HD evolved in this
b.Present Address: Kita-ku, Ukima 3-5-38-408, Tokyo, 115-0051.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: Cartesian coordinates, reaction was determined to be 140:64 based on their signal
1
intensity in the H NMR spectrum of the benzene-d6 solution
electronic energies and Gibbs free energies of the geometry-optimized species. See
DOI: 10.1039/x0xx00000x

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1
(H2: singlet, 4.46 ppm; HD: triplet, 4.43 ppm, JHD = 42.7 Hz). deuterated aminoborane dimers, 2a-d and 2a-d2 (Chart 1). The
Hence, H2, HD, and D2 were generated in 65:30:5 ratio in the deuterium incorporation into the aminoborane was further
dehydrocoupling of 1a-dN
content of 1H nucleu was much higher than that of 2H. The in the 11B{1H} NMR spectrum (Figure 2). When the signal of
.
In generated dihydrogen, the confirmed by the appearance of a 2H-coupled triplet resonance
1
2
percentage of H nuclei amounts to 80 %. Also, the observed C6H5D in the solvent is used as the internal standard in the H
isotopomer distribution almost equals the statistical NMR spectrum (Figure 1), it has been shown that 62 % of
distribution of proton and deuteron under this 1H content deuterium has been introduced into the aminoborane product.
(64:32:4).
Thus, in the dehydrocoupling of 1a-dN, proton and deuteron
Previously, Pacchioni has reported that H/D scramling occurs nuclei were concentrated into the produced dihydrogen and
via [Cr(CO)4(H2)(D2)] when [Cr(CO)6] is photolyzed under H2/D2 dimethylaminoborane dimer, respectively.
atmosphere.23 In our reaction system, redistribution of
dihydrogen isotopomers would also occur through
[Cr(CO)4(H2)(D2)]. D2 can be formed via this process.
Figure 2. 11B{1H} NMR spectra of 2a-d in the reaction mixture of the dehydrocoupling of
Chart 1
1a-dN. (a) a common spectrum (b) a resolution-enhanced spectrum obtained by the use
of sin-bell type window function.
Computational Approach to the Intramolecular H/D
Scrambling. Here we consider the mechanism for the H/D
scrambling on the basis of the reaction pathway of the
chromium-catalyzed borane dehydrocoupling. Figure 3 exhibits
the free energy profile for the transformation of 1a-dN on
[Cr(CO)4], which was suggested by DFT calculations using the
PBE0 functional.24 Its left half is essentially the same as the
dehydrogenation pathway of 1a, which we reported in the
previous paper.3 However, the relative free energy values are
slightly perturbed by the deuterium substitution. After
[Cr(CO)4] was chelated by 1a-dN with a BH and the ND atoms,
the ND and BH bond activation steps proceed successively,
forming a BH2=NMe2 ligand on the metal atom. In the first
step, the chromium atom is inserted into the ND bond to
generate the amidoborane(deuteride) complex 5-dN. The
BH2NMe2 moiety in the amidoborane ligand then rotates 90°
around the Cr–N axis, leaving the metal-bound BH hydrogen
atom, thereby the BH activation is accomplished. Note that in
the resulting intermediate 6-dCr, the hydride and deuteride
ligands, which were originally attached to boron and nitrogen
respectively, are located at mutually equivalent positions with
respect to the BH2NMe2 ligand.
Figure 1. 2H NMR spectral change (76.7 MHz, C6H6) during the Cr-catalyzed
dehydrocoupling of 1a-dN. (a) BH3·NDMe2 (1a-dN)
/ [Cr(CO)6] (5 mol%), before
<<Figure 3>>
irradiation. (b) After irradiation (40 min) + RT 2d.
The borane dehydrocoupling is achieved by dissociation of the
aminoborane ligand from 6-dCr and subsequent release of HD
from the resulting hydride(deuteride) complex. The
aminoborane dissociation from 6-dCr is 13.8 kcal mol-1
endothermic in electronic energy and only 0.2 kcal mol-1
endergonic in free energy. Remarkably, the reversion of
complex 6-dCr to the reactants, [Cr(CO)4] plus 1a-dN, can also
occur easily. The highest barrier for the reversion process is
Figure 1 exhibits the 2H NMR spectral change during the
dehydrocoupling of 1a-dN Before the reaction, only
.
a
resonance of the nitrogen-attached deuterium is observed at
3.28 ppm as a broad singlet. After the completion of the
dehydrogenation reaction, however, this signal disappeared
and a boron-coupled quartet resonance appeared alternatively
at 3.00 ppm. This resonance is assigned to the BHD moiety of
2 | J. Name., 2012, 00, 1-3
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13.1 kcal mol-1, which is taken for the conversion of 5-dN to Evolution of H2 and formation of 3a-d can also take place
TS1-dN. On the initial step of the reversion, if the aminoborane through rotation of the BH2D group of intermediate 5-dB(b)
ligand of 6-dCr rotates so that the boron end approaches the followed by the BH activation of resulting 5-dB(t). This proceeds
hydride ligand, 4-dN will only be reproduced via TS2-dN
,
5-dN
,
via TS3, which involves an η2-interacting borane group (Figure
and TS1-dN. On the contrary, if the aminoborane group rotates 5).25 However, the energy barrier for the BH2D rotation, 19.5
so that the boron end is brought close to the deuteride ligand, kcal mol-1, is substantially higher than that for the
the boron-deuterated intermediate 5-dB(b) will be formed. This aforementioned mechanism including dissociation and
species can readily be converted to 4-dB(b), which then releases recoordination of 1a-dB
BH2D·NHMe2 1a-dB). In fact, the boron-attached deuteron
.
(
2
assignable to 1a-dB is observed in the H NMR spectrum (2.04
ppm, a broad quartet) of the reaction mixture just after the
photolysis. This boron-deuterated adduct can coordinate to
[Cr(CO)4] through a BH and the NH hydrogen atoms to release
H2, yielding BHD=NMe2 (3a-d), via an intermediate 6-dB (Figure
4). Product 3a-d dimerizes (or associates with 3a) to produce
2a-d2 (or 2a-d).
Conclusions
In this work, we found intramolecular H/D scrambling in the
chromium-catalyzed dehydrocoupling reaction of borane–
dimethylamine adduct. Their mechanisms can be explained
with the reaction pathway of the dehydrocouplng. These
findings embody a dynamic behavior of the metal-coordinated
amine–borane molecule and its hydrogen atoms, which move
around the coordination sphere of the metal centre.
Experimental
All manipulations were carried out under high vacuum or dry
nitrogen atmosphere. Reagent-grade THF was distilled under
nitrogen atmosphere from sodium-benzophenone ketyl
immediately before use. Benzene and benzene-d6 were dried
over a potassium mirror and vacuum transferred into NMR
tubes directly before use. BH3·NHMe2 (1a) were prepared by
treatment of NaBH4 with the corresponding ammonium salt in
Figure 4. The boron-deuterated intermediate 6-dB.
The barriers for interconversion among the intermediates for
the H/D scrambling are sufficiently low, that the intermediates
can be in a quasi equilibrium. Accordingly, the isotope
distribution
dimethylaminoborane should be prescribed by the relative free
energy of the isotopomers of intermediate , which finally
in
generated
dihydrogen
and
THF according to the literature.26 BH3·NDMe2
(1a-dN) was
prepared by dissolving 1a in D2O.27 [Cr(CO)6] (Strem), NaBD4
6
(Aldrich), [NH2Me2]Cl and D2O (Wako Pure Chemicals) were
2
release aminoborane and dihydrogen. In the intramolecular
H/D scrambling, 6-dCr releases 3a and HD while 6-dB liberates
3a-d and H2. Frequency calculations showed that 6-dB is 0.33
kcal mol-1 stable relative to 6-dCr in free energy. Based on this
energy difference, the relative abundance of 6-dB and 6-dCr is
expected to be 63.4:36.6 in the reaction system. This value is
well consistent with the observed deuteration percentage in
the obtained aminoborane 2a (62 %). Furthermore, it is also
expected that a dihydrogen isotopomeric mixture with H:D =
81.7:18.3 is produced on the basis of this value. This also well
accords with the observation.
purchased and used as received. 1H, H, and 11B NMR spectra
were recorded on a JEOL α-500 spectrometer.
Dehydrocoupling Reaction of BH3·NDMe2 (1a-dN). A Pyrex
NMR sample tube equipped with a Teflon stop cock was
charged with 1a-dN (15 mg. 0.25 mmol) and [Cr(CO)6] (2.8 mg,
1.3
×
10-2 mmol), and connected to a vacuum line. Dry
benzene (0.4 mL) was introduced onto the sample, and the
NMR tube was flame-sealed under high vacuum. This solution
was irradiated using a 450W medium pressure Hg lamp for 2
hours, and was then allowed to stand at room temperature for
2 days to complete the dehydrocoupling reaction. The 2H NMR
spectrum of the reaction mixture showed resonances of HD
1
(4.50 ppm, doublet, JHD = 42.7 Hz), D2 (4.45 ppm, singlet), and
deuterated dimethylaminoborane dimers (3.00 ppm, broad
quartet, 1JBD = 101.4 Hz). By comparing the signal intensity, the
relative quantity of HD and D2 was determined to be 64:11. In
a similar manner, a benzene-d6 solution of 1a-dN (16 mg, 0.27
mmol) containing [Cr(CO)6] (2.9 mg, 1.3
×
10-2 mmol) was
prepared, photolyzed and then left at room temperature. The
1H NMR spectrum of this reaction mixture showed the
formation of H2 and HD along with aminoborane dimer (H2:
singlet, 4.46 ppm; HD: triplet, 4.43 ppm, 1JHD = 42.7 Hz). In this
solution, the ratio of H2 and HD was 140:64 as determined by
the signal intensity. Thus, the relative quantity of H2, HD, and
Figure 5. Rotation of the BH2D group of 5-dB(b) involving site exchange of the deuterium
atom. The relative free energy values are given in kcal mol-1. The zero-point corrected
electronic energies are also given in parentheses.
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T. Jurca and I. Manners, Nat. Chem., 2013,
Waterman, Chem. Soc. Rev., 2013, 42, 5629.
Y. Kawano, M. Uruichi, M. Shimoi, S. Taki, T. Kawaguchi, T.
Kakizawa and H. Ogino, J. Am. Chem. Soc., 2009, 131, 14946.
T. Kakizawa, Y. Kawano, K. Naganeyama and M. Shimoi,
Chem. Lett., 2011, 40, 171.
Y. Kawano, T. Asaka and M. Shimoi, Chem. Lett., 2017, 46, in
Press.
A. Kumar, I. K. Priest, T.N. Hooper and A. S. Weller, Dalton
Trans., 2016, 45, 6183.
C. A. Jaska, K. Temple, A. J. Lough and I. Manners, Chem.
Commun., 2001, 962; C. A. Jaska, K. Temple, A. J. Lough and I.
Manners, J. Am. Chem. Soc., 2003, 125, 9424.
A. Staubitz, A. P. Soto and I. Manners, Angew. Chem., Int. Ed.
Engl., 2008, 47, 6212.
5, 817; R.
D2 evolved in the dehydrocoupling reaction was estimated to
be 13:6:1 by the normalization using HD as a standard.
3
4
5
6
7
2
In addition, before the reaction the H NMR signal intensity of
the ND atom of 1a-dN was 15.01 relative to the signal of C6H5D
(natural abundance), while after the reaction, the relative
intensity of BHD signal of deuterated aminoboranes was 9.04.
Thus, 62 % of deuterium migrated onto the boron atom.
Computational Details. To obtain thermochemical parameters,
frequency calculations were carried out on deuterated
isotopomers of the species that participate in the
dehydrocoupling reaction of 1a, whose structures were
previously optimized and reported.3 This computation was
performed at the DFT/PBE0 level of theory.24 Chromium was
described with the effective core potentials (ECP) of Hay and
8
9
T. J. Clark, C. A. Russell and I. Manners, J. Am. Chem. Soc.,
2006, 128, 9582.
Wadt with
augmented with f-polarization functions (
double plus polarization valence basis set augmented with
a
double-
ζ
valence basis set (LANL2DZ)28
= 1.941).29
10 M. E. Sloan, T. J. Clark and I. Manners, Inorg. Chem., 2009,
48, 2429.
11 Y. Chen, J. L. Fulton, J. C. Linehan and T. Autrey J. Am. Chem.
Soc., 2005, 127, 3254; J. F. Fulton, J. C. Linehan, T. Autrey, M.
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Soc., 2007, 129, 11936.
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15 M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and K. I.
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16 R. J. Keaton, J. M. Blacquiere and R. T. Baker, J. Am. Chem.,
Soc. 2007, 129, 1844.
17 T. J. Clark, G. R. Whittell and Manners, Inorg. Chem., 2007,
46, 7522.
18 M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and K. I.
Goldberg, J. Am. Chem. Soc., 2006, 128, 12048.
19 N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A. Black and K.
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α
A
ζ
diffuse functions, 6-31++G(d,p) was employed for B, N, BH, CrH
and NH hydrogen atoms, which directly participated in the
dehydrocoupling. For the other atoms, a standard 6-31G(d)
basis set was applied. These functional and basis sets are the
same as those employed for the geometry optimizations.
Thermochemical parameters thus obtained, including zero
point energies and Gibbs free energy contributions, were then
added to the solvation free energy obtained by the SCRF (self
consistent reaction field) calculations using the CPCM model.30
As reported previously, on the SCRF calculations, Stuttgart-
,
Dresden ECP and valence triple-ζ plus f-polarization basis set
(SDD) were used for chromium,31 and Dunning’s aug-cc-pVDZ
was employed to describe the other atoms.32 The free energy
values are given as those at 298 K and 1 atm in benzene. All
calculations were performed with the Gaussian 03 package of
programs.33
20 M. Käß, A. Friedrich, M. Drees and S. Schneider, Angew.
Chem. Int. Ed., 2009, 48, 905.
21 M. Vogt, B. de Bruin, H. Berke, M. Trincado and H.
Conflict of interest
Grützmacher, Chem. Sci., 2011, 2, 723.
22 W. Grochala, P. Peter and P. P. Edwards, Chem. Rev., 2004,
104, 1283; T. B. Marder, Angew. Chem., Int. Ed. Engl., 2007,
46, 8116.
There are no conflicts of interest to declare.
23 G. Pacchioni, J. Am. Chem. Soc., 1990, 112, 80.
24 J. P Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996,
77, 3865.
Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research (No. 21550056) from Ministry of Education, Culture,
Sports, Science and Technology, Japan.
25 We have reported BH exchange of borane
-
σ complexes via an
η1 η2 η1 mechanism: M. Shimoi, S. Nagai, M. Ichikawa, Y.
-
Kawano, K. Katoh, M. Uruichi and H. Ogino, J. Am. Chem.
Soc., 1999, 121, 11704; T. Kakizawa, Y. Kawano and M.
Shimoi, Organometallics, 2001, 20, 3211; Y. Kawano, T.
Kakizawa, K. Yamaguchi and M. Shimoi, Chem. Lett., 2006,
35, 568; Y. Kawano, M. Hashiva and M. Shimoi,
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1
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C. A. Jaska and I. Manners, Inorganic Chemistry in Focus II
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Figure 3. Free energy profile of the H/D scrambling of 1a-dN on [Cr(CO)4]. The energy values are given in kcal mol-1. Indigo = Cr, green = B, blue gray = N, red = O, brown = C, pink =
H, orange = D.
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