Cleaving Dihydrogen with Tetra(o-tolyl)diborane(4)

Article abstract of DOI:10.1021/jacs.7b00924

Tetra(o-tolyl)diborane(4), 1, was synthesized and characterized experimentally as well as theoretically by density functional theory (DFT) calculations. Exposure of 1 to H₂ (1 bar) at room temperature afforded the corresponding di(o-tolyl)hydroborane through cleavage of the H-H and B-B bonds. DFT calculations suggested a diarylboryl anion character for the transition state.

Full text of DOI:10.1021/jacs.7b00924

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Communication
Cleaving Dihydrogen with Tetra(o-tolyl)diborane(4)
Nana Tsukahara, Hiroki Asakawa, Ka-Ho Lee, Zhenyang Lin, and Makoto Yamashita
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00924 • Publication Date (Web): 06 Feb 2017
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Cleaving Dihydrogen with Tetra(o-tolyl)diborane(4)
Nana Tsukahara,† Hiroki Asakawa,† Ka-Ho Lee,‡ Zhenyang Lin,*‡ Makoto Yamashita*§¶
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†Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27, Kasuga, Bunkyo-
ku, 112-8551, Tokyo, Japan.
‡Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong
§Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya, Aichi 464-8603, Japan
¶Research Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
Supporting Information Placeholder
toward CO and tert-butylisocyanide.10a This dibo-
ABSTRACT: Tetra(o-tolyl)diborane(4) 1 was synthe-
rane(4) reacts also with alkynes to form diborylal-
kenes10b and the regioselectivity of the diboration to
afford syn- or anti-diborylalkenes can be controlled
by addition of a catalytic amount of base and 1,2-
dimethoxyethane. This diborane(4) furthermore re-
acts with 2,6-dimethylphenylisocyanide to furnish an
1,2-oxaboretane ring through a ring-contraction of
Bpin.10c This characteristic reactivity of pinB–BMes2
was attributed to the high electron affinity arising
from the two vacant p-orbitals of the two boron at-
oms in parallel arrangement.10d Similarly high elec-
tron affinity was reported for Mes2B–B(Mes)Ph,
which was synthesized from (MeO)2B–B(OMe)2 and
is capable to form the corresponding radical anion
and dianion.11 Moreover, Mes2B–B(Mes)Ph represents
the only reported example of a tetraaryldiborane(4).
Herein, we report the synthesis of tetra(o-
tolyl)diborane(4) 1 and its reactivity toward H2 at
room temperature, as well as a mechanistic study
based on DFT calculations.
sized and characterized experimentally as well as
theoretically by DFT calculations. Exposure of 1 to H2
(1 bar) at room temperature afforded the correspond-
ing di(o-tolyl)hydroborane through cleavage of the
H–H and B–B bonds. DFT calculations suggested a
diarylborylanion character for the transition state.
Since the discovery of heterogeneously catalyzed
hydrogenations by Sabatier,1 catalytic and stoichio-
metric hydrogenations with H2 have represented ma-
jor research fields in molecular transition-metal
chemistry.2 However, the limited availability of pre-
cious metals as hydrogenation catalysts has fueled
the quest for more sustainable options. Two such
recently developed alternatives are: i) the replace-
ment of the precious metals with base metals, and ii)
the use of main-group elements which can react with
H2.3 The latter can be subdivided into two classes: i)
frustrated Lewis pairs (FLPs),4 and ii) main-group
element compounds in low oxidation states. The lat-
ter class involves group 13 or 14 elements such as
C(II), Si(II), Ge(I), Sn(I), P(II), B(0), B(II), Al(I), and
Ga(I).5 Our recent report regarding the deprotona-
tion of H2 with boryllithium6 is also relevant in this
context, as boryl anions can be considered as a B(I)
species. As far as reactive trivalent group 13 com-
pounds are concerned, the reaction of an antiaro-
matic borole with H2,7 the reversible cleavage of H2
with dianionic diboraanthracenes,8 and FLP-like re-
activity of diaminogallium with cooperation of lig-
and9 have been recently reported.
By careful choice of the precursor, 1 could be ob-
tained in 30% isolated yield from the reaction be-
tween B2cat2 and o-tolMgBr (Scheme 1). The molecu-
lar structure of 1 was determined by single-crystal X-
ray diffraction analysis (Figure 1). In the crystal, 1
adopts an almost orthogonally twisted structure
[C8–B1–B2–C15 = 101.9(7)°], similar to that of Mes2B–
1
B(Mes)Ph.11a The H NMR spectrum of 1 exhibited
four magnetically equivalent o-tol groups at room
temperature, indicating free rotation around the B–B
and B–C bonds on the NMR timescale. The 11B NMR
spectrum of 1 in C6D6 showed a broad signal at 89
11
ppm. Dissolution of 1 in THF did not alter the B
Recently, we reported the synthesis of the unsym-
NMR chemical shift, indicating that THF does not
coordinate to the boron atom in 1.
10
metrical diborane(4) pinB–BMes2 and its reactivity
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Scheme 1. Synthesis of tetra(o-tolyl)diborane(4) 1. at room temperature afforded di(o-tolyl)hydroborane
1
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4
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2 in 52% yield (Scheme 2). This reaction could be
considered as a stoichiometric counterpart of the
previously reported hydrogenolysis of diborane(4) by
group 10 metal catalysts.12 The crystal structure of 2
revealed a dimeric structure with hydride bridges
(Figure 4, left), which is similar to those of previously
reported dimeric diarylhydroboranes.13 The solid
9
state IR spectrum of 2 exhibited a characteristic ꢀ-H
vibration at 1520 cm–1, which is comparable to that of
[HB(C6F5)2]2.13b Dissolution of 2 in C6D6 or CDCl3 led
to five B-bonded 1H nuclei and three 11B nuclei in the
1H{11B} and 11B NMR spectra, respectively. This behav-
ior should be attributed to a monomer/dimer equi-
librium14 and a potential coordination of solvent
molecules to the highly Lewis acidic boron center of
the monomer.13b,15 In contrast, dissolution of 2 in
THF-d8 led to a simple NMR spectra indicating a
formation of 3-d8, which could not be isolated be-
cause the coordinating THF was labile. Addition of
pyridine to isolated 2 without solvent at room tem-
perature afforded single crystals of the correspond-
ing pyridine-hydroborane complex 4, which was
completely characterized by NMR spectroscopy, X-
ray analysis, and high-resolution FAB mass spectros-
copy.
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C8
C1
C15
B1
B2
C22
Figure 1. Molecular structure of 1 (thermal ellipsoids
set at 50% probability; one of two independent mole-
cules and hydrogen atoms were omitted for clarity).
To estimate the electron affinity of 1,
electrochemical measurements and DFT calculations
were conducted. The cyclic voltammogram of a THF
solution of 1 showed a reversible reduction wave at –
2.1 V (vs. Cp2Fe/Cp2Fe+). The first reduction potential
of 1 is thus less negative than that of previously
reported pinB-BMes2 (–2.5 V). Similar to our
previous report,10d we also calculated the dependency
of the LUMO level and the free energy of 1 on the
torsion angle C–B–B–C. The optimized structure of 1
contains a torsion angle of –96.6°, which is
comparable to the crystal structure and leads to a
calculated LUMO level of –1.29 eV. Increasing the
torsion angle of 1 from –90° to 0° in increments of 10°
lowers the LUMO energy (as well as the stability) as
shown in Figure 2. When the torsion angle
approaches 0°, the LUMO energy drops to –2.20 eV
and the corresponding structure is destabilized by
8.51 kcal/mol. This relatively mild destabilization
suggests that rotation around the B–B bond should
occur at room temperature. The lowering of the
LUMO of 1 upon rotation around the B–B bond
should be attributed to the overlap of the two vacant
p-orbitals on the two boron centers (Figure 3). The
LUMO of 1 in the ground state (–96.6°) consists of
π*-orbitals of two o-tol rings and one vacant p-
orbital of the boron atom. The LUMO+1 is of similar
energy and mainly localized on the other B(o-tol)2
moiety, while the LUMO of 1 (0.0) exhibits two
completely merged vacant p-orbitals on the two
boron atoms. It should be noted that the HOMOs of
both rotational isomers exhibit a B–B σ-bond
character.
Figure 2. Dependency of the LUMO energy level (in
eV) and the relative stability (in kcal/mol) of 1 on the
torsion angle of the C(ipso)–B–B–C(ipso) moiety.
Figure 3. Frontier orbitals of the rotational isomers of 1
at C–B–B–C = 0.0° (top) and C–B–B–C = –96.6° (bot-
tom).
Tetra(o-tolyl)diborane(4) 1 reacts directly with H2.
Exposing a hexane solution of 1 for 2 h to H2 (1 bar)
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Scheme 2. Reaction of 1 with H2 and subsequent Scheme 3. Proposed reaction mechanism and ener-
1
2
3
4
reactions
gy profile for the hydrogenolysis of 1 [Ar = o-tolyl;
calculated at M06/6-31G(d) [SMD: benzene]; dashed
lines correspond to the formation/cleavage of bonds
or 3-center-2-electron bonds].a
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Figure 4. Molecular structures of 2 (left) and 4 (right)
(thermal ellipsoids at 50% probability; hydrogen atoms
except for B–H were omitted for clarity).
To get a better understanding of the detailed path-
way for the reaction of 1 with H2, DFT calculations
were performed to show that the overall reaction
consists of four steps (Scheme 3). The reaction
should be initiated by coordination of the H–H bond
to one of the vacant p-orbitals of one of the boron
atom to afford intermediate 5, in which the C–B–B–C
torsion angle changed to –55.1°, indicating that the
strong electron affinity of 1 enables the binding to
H2. Upon coordination of H2, both the H–H and B–B
bonds should be elongated and the geometry of the
H2-coordinated boron atom should become pyrami-
dalized. Subsequently, cleavage of the B–B σ-bond
should be accompanied by a proton-migrating H–H
bond cleavage via TS5-6, leading to the formation of
the dimeric hydroborane intermediate 6, which con-
tains a hydride and an o-tolyl group as bridging lig-
ands. Here, it should be noted that in TS5-6, the ipso
carbon atom of one o-tolyl group starts to interact
with the boron atom it is not directly attached to [C--
B 2.347 Å]. TS5-6 represents the rate-determining step
of the reaction sequence and requires an overall acti-
vation free energy of 19.8 kcal/mol. The resulting
intermediate (6) is thus located 8.3 kcal/mol below
the reference point (1 + H2). Intermediate 6 involves
a characteristic B–C–B three-center-two-electron
bond, which has recently been confirmed.16 Dissocia-
tion of 6 to afford monomeric hydroborane 7, and
the subsequent dimerization of 7 to afford hydride-
bridged dimer 2 through the two low-energy transi-
tion states TS6-7 and TS7-2 include relatively low acti-
vation barriers. The overall reaction from 1 + H2 to
give 6 is exergonic by 20.1 kcal/mol.
aRelative free energies and electronic energies (in pa-
rentheses) are given in kcal/mol.
Figure 5. Selected natural bond orbitals: (a) NBO91
and (b) NBO92 of TS5-6, as well as (c) schematic illus-
tration of the diarylborylanion character of B3 in TS5-6
(numbering of atoms is based on the calculations)
To characterize the transition state for the H–H
bond cleavage, an NBO analysis was performed. The
calculated natural charge on the migrating H1
(+0.306) is more positive than that on H2 (+0.200),
indicating that H1 should be transferred as a proton.
The NBO analysis on TS5-6 showed that the vacant
orbital of the migrating proton H1 interacts with
both the bonding orbitals of the H–B and B–B bonds
(NBO 91 and NBO92, Figure 5a,b), and exhibits sig-
nificant second-order perturbation energies (E2) of
352.52 and 200.49 kcal/mol. Thus, the transition
states cleaving H–H bond (TS5-6) may be described
as an intramolecular deprotonation by an sp2-sp3 di-
borane17 that exhibits a diarylborylanion character
(Figure 5c). The proton-migrating character of TS5-6
is similar to TSs for the reaction of H2 with boryllith-
ium,6 an antiaromatic borole,7 and phopshinobo-
rane.18
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439-441; (c) Peng, Y.; Brynda, M.; Ellis, B. D.; Fettinger, J. C.;
In conclusion, tetra(o-tolyl)diborane(4) 1 was syn-
thesized, and characterized experimentally as well as
theoretically by DFT calculations. Exposure of 1 to H2
at room temperature afforded the corresponding dia-
rylhydroborane via cleavage of the H–H and B–B
bonds. DFT calculations suggested a diarylborylan-
ion character for the transition state.
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/jacs.x.
Experimental and computational details (PDF)
Crystallographic data (CIF)
DFT Coordinates (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*makoto@oec.apchem.nagoya-u.ac.jp, *chzlin@ust.hk
(7) (a) Fan, C.; Mercier, L. G.; Piers, W. E.; Tuononen, H. M.;
Parvez, M., J. Am. Chem. Soc. 2010, 132, 9604-9606; (b)
Houghton, A. Y.; Karttunen, V. A.; Fan, C.; Piers, W. E.;
Tuononen, H. M., J. Am. Chem. Soc. 2013, 135, 941-947.
(8) von Grotthuss, E.; Diefenbach, M.; Bolte, M.; Lerner, H.-
W.; Holthausen, M. C.; Wagner, M., Angew. Chem. Int. Ed. 2016,
55, 14067-14071.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This research was supported by JSPS KAKENHI Grant
Number JP24109012 and 26288019, the Research Grants
Council of Hong Kong (HKUST16303614 and C5023-
14G), CREST from the JST (14529307), and The Asahi
Glass Foundation. The authors thank Profs. T. Hiyama
(Chuo U) and Y. Nishibayashi (U-Tokyo) for providing
us with access to an X-ray diffractometer and to a mass
spectrometer. This paper is dedicated to Professor Ta-
kayuki Kawashima on the occasion of his 70th birthday.
(9) Abdalla, J. A. B.; Riddlestone, I. M.; Tirfoin, R.; Aldridge, S.,
Angew. Chem. Int. Ed. 2015, 54, 5098-5102.
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Katsuma, Y.; Asakawa, H.; Lee, K.-H.; Lin, Z.; Yamashita, M.,
Organometallics 2016, 35, 2563-2566; (d) Asakawa, H.; Lee, K.-H.;
Furukawa, K.; Lin, Z.; Yamashita, M., Chem. Eur. J. 2015, 21, 4267-
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