Synthesis of a hydrogen-bridged tetraborane(6): A substituent effect of a diaminoboryl group toward the B-B multiple bond character

Article abstract of DOI:10.1039/c9dt01117j

A hydrogen-bridging tetraborane(6) was synthesized from boryllithium, a boron nucleophile, in three steps. The structural and spectroscopic analyses of the tetraborane(6) revealed σ-donor and π-acceptor effects of diaminoboryl substituents toward the cent

Full text of DOI:10.1039/c9dt01117j

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Yagi, H. Kisu
and M. Yamashita, Dalton Trans., 2019, DOI: 10.1039/C9DT01117J.
Volume 45 Number
1
7
January 2016 Pages 1–398
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

Page 1 of 5
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
View Article Online
DOI: 10.1039/C9DT01117J
Journal Name
COMMUNICATION
Synthesis of Hydrogen-Bridged Tetraborane(6): Substituent Effect
of Diaminoboryl Group toward B-B Multiple Bond Character
Atsumi Yagi, Haruki Kisu and Makoto Yamashita*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
hydrogen-bridging tetraborane(6) was synthesized from short bond lengths and narrower HOMO-LUMO gap, have been
boryllithium, a boron nucleophile, in three steps. The structural and studied. Connection of these species with boryl substituent(s)
spectroscopic analysis of the tetraborane(6) revealed -donor and has been known to be able to alter the electronic properties of

-acceptor effect of diaminoboryl substituents toward the central these multiple bonds [Figure 1(b)]. Introduction of dialkylboryl
B–B moiety having multiple bond character. DFT calculations also substituent to disilene (F) provided information about -
supported the experimentally observed substituent effect of boryl accepting character of dialkylboryl group toward Si=Si double
3
7, 38
group.
bond.
We reported synthesis and properties of boryl-
substituted diphosphene (G), where boryl substituents showed
Recent development of B–B multiple bond species opened a
new area of chemistry due to their characteristic structure and
reactivity of -bond(s). Two-electron reduction of diborane(4)

-donor and -acceptor character toward electronic properties
39-41
of P=P double bond.
Recently, a boryl-substituted diborene
1
(H) was reported to have an effective -conjugation between
led to a formation of the first B=B double bond species with two
B=B double bond and vacant p-orbitals of catecholatoboryl
2
, 3
anionic charge [A, Figure 1(a)]. The first diborene, a neutral
B=B double bond species (B), was synthesized by a reduction of
23
substituents. In this work, we herein report synthesis of boryl-
substituted hydrogen-bridged diborane(4), which can be
considered as tetraborane(6) species, and the substituent effect
of the diaminoboryl substituents toward multiple bond
character of the inner B-B moiety.
4
, 5
NHC-coordinated BBr
3
derivative.
After the first discovery,
propensities of diborene derivatives were extensively studied
6
-28
by an improvement of their synthesis.
diboryne, a neutral B≡B triple bond species (C),
synthesized by reduction of Lewis base-stabilized
On the other hand,
7
, 29-33
has been
a
tetrahalodiborane(4) precursor. Related hydrogen-bridging
diborane(4) species possessing B-B multiple bond character
were synthesized by using a bulky aryl substituent, Eind group
3
4-36
(D).
The H-bridged structure of D could be considered as a
doubly protonated form of dianionic B≡B triple bond compound
as supported with DFT study with a remarkably short B-B bond
distance in comparison with those observed in diborene
derivatives. Recently, an amino-substituted homocatenated
triborane(3) compound (E), which can be considered as a trimer
of aminoborylene, was reported to have a B–B multiple bond
character.
In the chemistry of multiply-bonded main group element
species, characteristic properties of the multiple bond such as
Figure 1. (a) Examples of multiboron compounds having B-B
multiple bond character and (b) multiply bonded p-block
element species bearing boryl substituent(s)
Department of Molecular and Macromolecular Chemistry, Graduate School of
Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya, 464-8603 Aichi, Japan
Electronic Supplementary Information (ESI) available: CCDC 1884516-1884518 for
Reaction of boryllithyum 1, which was prepared in accordance
2
-4. For ESI and crystallographic data in CIF or other electronic format. See
4
2, 43
with our previous contribution,
with two equivalents of
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 2 of 5
COMMUNICATION
Journal Name
2
3
trimethylborate to give an unsymmetrical diborane(4) (2) with Å), indicating an effective -conjugation between internal B-B
View Article Online
two amino and two alkoxy substituents in 78% yield (Scheme 1). bond possessing a multiple bond charaDcOtIe: 1r0.a10n3d9/Cv9aDcaT0n1t11p7J-
Treatment of 2 with two equivalents of boron tribromide orbitals of diaminoboryl substituents in 4. These results suggest
induced
ligand
exchange
reaction
to
afford 4 can be described with resonance structures in Scheme 2.
diaminodibromodiborane(4) (3) in 53% yield. Reaction of 3 with
five equivalents of potassium graphite in toluene at –35 °C
generated a hydrogen-bridged tetraborane(6) 4. Colourless
crystals 4 were obtained by recrystallization from its hexane
1
solution in 12% isolated yield. In the H NMR spectrum of 4
6 6
(C D ), the signal of bridging hydrogen atoms was observed as a
broad singlet at –2.25 ppm. This chemical shift is higher-field
shifted in comparison with that of the aryl-substituted
3
4
hydrogen-bridged diborane(4) D (2.94 ppm). However, the
NMR chemical shift calculations by using GIAO method
indicated that the higher field shifted signal of bridging Figure 2. X-ray structure of 4 (left: side view, right: top view);
hydrogen atoms in 4 would be their peculiar property of B2H2 Thermal ellipsoids set at 50% probability. Hydrogen atoms
core structure (Figure S20) rather than the effect of boryl except for the bridging hydrogen atoms are omitted for clarity.
substituents.44 In the B NMR spectrum, the signals of 4 were
11
observed at 5 and 30 ppm as a broad resonance, in which the
former could be assigned as two internal boron atoms in
3
4
Scheme 2. Possible resonance structures of 4
comparison with that in D. The IR spectrum of 4 exhibited a
–1
series of B–H–B vibrations (1321-1174 cm ) in the same region
–1
34
DFT calculations were performed to analyse electronic structure
of 4. The experimentally obtained structure of 4 by X-ray
analysis was used as an initial structure for optimization at
B3LYP/6-31+G(d,p) level of theory. Characteristic molecular
orbitals of 4 are illustrated in Figure 3. Two molecular orbitals,
HOMO–49 and HOMO–2 would correspond to two -like
orbitals at the central B–B unit. Overlapping of *-orbitals of the
central B-B unit and two vacant p-orbitals in 4 seems to produce
LUMO. Two combinations of two -type N–B–N orbitals
construct energetically degenerated HOMO–1 and HOMO. In
contrast, aryl-substituted D has slightly higher *-like LUMO
and -like HOMO, and two combinations of -orbitals of Eind
groups consist HOMO-1 and HOMO-2 (Figure S19). This result
indicates the boryl substituents in 4 contribute to lower - and
to those of D (1265-1157 cm ), confirming the existence of
the two bridging hydrogen atoms. Reaction of 3 with KC in
toluene-d resulted in an appearance of small signal (–2.30
ppm) in addition to the signal of the bridging hydrogen atoms in
8
8
4
(Figure S14). This new signal could be assigned as a bridging
2
1
hydrogen atom in partially deuterated 4-d . Although H NMR
spectrum of partially deuterated 4 did not exhibit any
detectable signal probably due to significant broadening of the
2
signal caused by coupling between two quadrupolar nuclei of H
and 1 B/ B, incorporation of deuterium atoms to 4 was also
1
10
supported by APCI-TOF MS and IR spectroscopy (Figures S13, 15,
1
6). This result indicates the origin of the bridging hydrogen
atoms in 4 could be attributed to inter- and intra-molecular
hydrogen trapping from toluene solvent and backbone or Dip
group in 3.

*-orbital of the central B-B unit through their -accepting
effect. It should be noted that a bonding combination of -type
orbitals of N–B–N unit and the central B–B unit constructs a
linear and delocalized -conjugation over four boron atoms in
HOMO-12. This -conjugation is consistent with resonance
structures in Scheme 2. The NPA charges on the central B–B unit
in 4 are negative (B1, B2: −0.239), while those of D were positive
(0.043, 0.074), supporting the σ-donating effect of boryl
substituents to increase the electron density at the central B–B
unit.
Scheme 1. Synthesis of hydrogen-bridged tetraborane(6) 4 (Dip
i
=
2,6- Pr
2 6 3
C H )
The molecular structure of 4 was unambiguously confirmed by
X-ray crystallographic analysis (Figure 2). Two bridging
hydrogen atoms were placed as a peaks in a difference Fourier
map. It was revealed that the inner B–B bond distance of
3
4
1
.488(4) Å, which is similar to that in D [1.4879(7) Å] and lies
between the reported B=B double and B≡B triple bonds. On the
other hand, the outer B–B distances of 1.666(3), 1.661(3) Å are
4
5
shorter than B–B covalent bond distance (1.75 Å), and are
similar to that of the diboryldiborene H (1.645(3) Å-1.676(6)
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 5
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
COMMUNICATION
6
7
8
9
.
.
.
.
P. Bissinger, H. Braunschweig, A. Damme, T.VKieuwpAfretircleanOdnliAne.
Vargas, Angew. Chem. Int. Ed., 2012D,O5I1: ,109.9103319-/9C993D4T.01117J
H. Braunschweig, R. D. Dewhurst, K. Hammond, J. Mies, K.
Radacki and A. Vargas, Science, 2012, 336, 1420-1422.
J. Brand, H. Braunschweig and S. S. Sen, Acc. Chem. Res.,
2
013, 47, 180-191.
P. Bissinger, H. Braunschweig, A. Damme, T. Kupfer, I.
Krummenacher and A. Vargas, Angew. Chem. Int. Ed.,
2
014, 53, 5689-5693.
1
1
0.
1.
H. Braunschweig, R. D. Dewhurst, C. Hörl, A. K. Phukan, F.
Pinzner and S. Ullrich, Angew. Chem. Int. Ed., 2014, 53,
3
241-3244.
H. Braunschweig and C. Horl, Chem. Commun., 2014, 50,
0983-10985.
1
Figure 3. Selected molecular orbitals of 4 calculated at the 12.
B3LYP/6-31+G(d,p) level of theory. Hydrogen atoms omitted for
clarity except for the bridged hydrogens.
P. Bissinger, H. Braunschweig, A. Damme, C. Hörl, I.
Krummenacher and T. Kupfer, Angew. Chem. Int. Ed., 2015,
5
4, 359-362.
1
3.
M. Arrowsmith, J. Böhnke, H. Braunschweig, M. A. Celik, T.
Dellermann and K. Hammond, Chem.-Eur. J., 2016, 22,
1
7169-17172.
Conclusions
1
1
4.
5.
M. Arrowsmith, J. Böhnke, H. Braunschweig and M. A.
Celik, Angew. Chem. Int. Ed., 2017, 56, 14287-14292.
H. Braunschweig, I. Krummenacher, C. Lichtenberg, J. D.
Mattock, M. Schäfer, U. Schmidt, C. Schneider, T.
Steffenhagen, S. Ullrich and A. Vargas, Angew. Chem. Int.
Ed., 2017, 56, 889-892.
By using boryllithium as a starting material, the hydrogen-
bridging tetraborane(6) derivative 4 was synthesized in three
steps. The structural and spectroscopic analysis revealed -
donor and -acceptor effect of diaminoboryl substituents
toward B–B multiple bonding character. DFT calculations
further supported the observed substituent effect of boryl
groups.
1
1
1
6.
7.
8.
W. Lu, Y. Li, R. Ganguly and R. Kinjo, J. Am. Chem. Soc.,
2
017, 139, 5047-5050.
W. Lu, Y. Li, R. Ganguly and R. Kinjo, Angew. Chem. Int. Ed.,
017, 56, 9829-9832.
2
S. Rixin Wang, M. Arrowsmith, H. Braunschweig, R. D.
Dewhurst, V. Paprocki and L. Winner, Chem. Commun.,
2
Conflicts of interest
There are no conflicts to declare.
017, 53, 11945-11947.
1
2
9.
0.
M. Arrowsmith, J. D. Mattock, J. Bohnke, I. Krummenacher,
A. Vargas and H. Braunschweig, Chem. Commun., 2018, 54,
4669-4672.
D. Auerhammer, M. Arrowsmith, P. Bissinger, H.
Braunschweig, T. Dellermann, T. Kupfer, C. Lenczyk, D. K.
Roy, M. Schäfer and C. Schneider, Chem.-Eur. J., 2018, 24,
Acknowledgement
The authors thank Prof. Tsukasa Matsuo (Kindai University), Mr.
Kazuya Sadamori (Kindai University), and Prof. Kazuyoshi
Tanaka (Kyoto University) for sharing the results of DFT
2
66-273.
calculations about D in their previous works.34-36 They also 21.
acknowledged Prof. Hiroshi Shinokubo for providing an access
to APCI-TOF mass spectrometer. This work was supported with
D. Auerhammer, M. Arrowsmith, R. D. Dewhurst, T. Kupfer,
J. Bohnke and H. Braunschweig, Chem. Sci., 2018, 9, 2252-
2
260.
2
2.
J. Bohnke, T. Bruckner, A. Hermann, O. F. Gonzalez-
Belman, M. Arrowsmith, J. O. C. Jimenez-Halla and H.
Braunschweig, Chem. Sci., 2018, 9, 5354-5359.
by a Grants-in-Aid for Scientific Research (A) (MEXT KAKENHI
grant number 17H01191). The theoretical calculations were
carried out using resources of the Research Center for
Computational Science, Okazaki, Japan.
2
3.
A. Hermann, J. Cid, J. D. Mattock, R. D. Dewhurst, I.
Krummenacher, A. Vargas, M. J. Ingleson and H.
Braunschweig, Angew. Chem. Int. Ed., 2018, 57, 10091-
1
0095.
Notes and references
24.
W. Lu and R. Kinjo, Chem. Commun., 2018, 54, 8842-8844.
W. Lu and R. Kinjo, Chem.-Eur. J., 2018, 24, 15656-15662.
T. E. Stennett, R. Bertermann and H. Braunschweig, Angew.
Chem. Int. Ed., 2018, 57, 15896-15901.
T. E. Stennett, J. D. Mattock, L. Pentecost, A. Vargas and H.
Braunschweig, Angew. Chem. Int. Ed., 2018, 57, 15276-
2
2
5.
6.
1
.
M. Arrowsmith, H. Braunschweig and T. E. Stennett,
Angew. Chem. Int. Ed., 2017, 56, 96-115.
2
.
A. Moezzi, M. M. Olmstead and P. P. Power, J. Am. Chem.
Soc., 1992, 114, 2715-2717.
2
2
2
7.
8.
9.
3
4
.
.
P. P. Power, Inorg. Chim. Acta, 1992, 198–200, 443-447.
Y. Wang, B. Quillian, P. Wei, C. S. Wannere, Y. Xie, R. B. King,
H. F. Schaefer, P. v. R. Schleyer and G. H. Robinson, J. Am.
Chem. Soc., 2007, 129, 12412-12413.
1
5281.
T. E. Stennett, J. D. Mattock, I. Vollert, A. Vargas and H.
Braunschweig, Angew. Chem. Int. Ed., 2018, 57, 4098-
4
102.
5
.
Y. Wang, B. Quillian, P. Wei, Y. Xie, C. S. Wannere, R. B. King,
H. F. Schaefer, P. v. R. Schleyer and G. H. Robinson, J. Am.
Chem. Soc., 2008, 130, 3298-3299.
H. Braunschweig, T. Dellermann, R. D. Dewhurst, W. C.
Ewing, K. Hammond, J. O. C. Jimenez-Halla, T. Kramer, I.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 4 of 5
COMMUNICATION
Journal Name
Krummenacher, J. Mies, A. K. Phukan and A. Vargas, Nat.
View Article Online
Chem., 2013, 5, 1025-1028.
DOI: 10.1039/C9DT01117J
3
0.
J. Böhnke, H. Braunschweig, P. Constantinidis, T.
Dellermann, W. C. Ewing, I. Fischer, K. Hammond, F. Hupp,
J. Mies, H.-C. Schmitt and A. Vargas, J. Am. Chem. Soc.,
2
015, 137, 1766-1769.
3
3
1.
2.
J. Grunenberg, Chem. Sci., 2015, 6, 4086-4088.
N. Holzmann, M. Hermann and G. Frenking, Chem. Sci.,
2
015, 6, 4089-4094.
3
3.
F. A. Perras, W. C. Ewing, T. Dellermann, J. Bohnke, S.
Ullrich, T. Schafer, H. Braunschweig and D. L. Bryce, Chem.
Sci., 2015, 6, 3378-3382.
3
3
4.
5.
Y. Shoji, T. Matsuo, D. Hashizume, H. Fueno, K. Tanaka and
K. Tamao, J. Am. Chem. Soc., 2010, 132, 8258-8260.
Y. Shoji, T. Matsuo, D. Hashizume, M. J. Gutmann, H.
Fueno, K. Tanaka and K. Tamao, J. Am. Chem. Soc., 2011,
1
33, 11058-11061.
3
6.
Y. Shoji, S. Kaneda, H. Fueno, K. Tanaka, K. Tamao, D.
Hashizume and T. Matsuo, Chem. Lett., 2014, 43, 1587-
1
589.
S. Inoue, M. Ichinohe and A. Sekiguchi, Chem. Lett., 2008,
7, 1044-1045.
3
3
3
4
4
4
4
4
7.
8.
9.
0.
1.
2.
3.
4.
3
K. Takeuchi, M. Ikoshi, M. Ichinohe and A. Sekiguchi, J. Am.
Chem. Soc., 2009, 132, 930-931.
S.-s. Asami, M. Okamoto, K. Suzuki and M. Yamashita,
Angew. Chem. Int. Ed., 2016, 55, 12827-12831.
S.-s. Asami, S. Ishida, T. Iwamoto, K. Suzuki and M.
Yamashita, Angew. Chem. Int. Ed., 2017, 56, 1658-1662.
S.-s. Asami, K. Suzuki and M. Yamashita, Chem. Lett., 2017,
4
6, 686-689.
Y. Segawa, M. Yamashita and K. Nozaki, Science, 2006, 314,
13-115.
1
Y. Segawa, Y. Suzuki, M. Yamashita and K. Nozaki, J. Am.
Chem. Soc., 2008, 130, 16069-16079.
The NMR chemical shift calculations supported the lower
1
field shift of the H NMR chemical shift for bridging
hydrogen atoms in Eind-substituted derivative D, probably
due to deshielding effect of aromatic Eind substituents.
J. Emsley, The Elements, Oxford University Press, New York,
4
5.
3
rd edn., 1998.
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT01117J