Reactions of B2(o-tolyl)4 with Boranes: Assembly of the Pentaborane(9), HB[B(o-tolyl)(μ-H)]4

Article abstract of DOI:10.1002/anie.202101054

Reactions of the diborane(4) B₂(o-tolyl)₄ and monohydridoboranes are shown to give B(o-tolyl)₃ and (o-tolyl)BR₂ (R₂=(C₈H₁₄) 3, cat 4, pin 5, (C₆F₅)₂

Full text of DOI:10.1002/anie.202101054

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Boron Chemistry
Hot Paper
Reactions of B2(o-tolyl)4 with Boranes: Assembly of the
Pentaborane(9), HB[B(o-tolyl)(m-H)]4
Karlee L. Bamford, Zheng-Wang Qu,* and Douglas W. Stephan*
Abstract: Reactions of the diborane(4) B2(o-tolyl)4 and
monohydridoboranes are shown to give B(o-tolyl)3 and (o-
tolyl)BR2 (R2 = (C8H14) 3, cat 4, pin 5, (C6F5)2 6) as the major
products. The corresponding reaction with BH3-sources gives
complex mixtures, resulting from hydride/aryl exchange,
dimerization and borane elimination. This led to the isolation
of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(m-
H)]4 8. The reaction pathways are probed experimentally and
by computations.
(MesBH2)2 yielded the triboron cation [H2B(m-H)(m-Mes)B-
(m-Mes)(m-H)BH2]+.
Targeting new avenues to unique boron reagents, our
interest focuses on the potential of diboranes(4). Though
alkoxydiboranes(4) are exploited extensively in the construc-
tion of C B bonds,[12] aryldiboranes(4) have drawn much less
À
attention. While Berndt and co-workers reported the first
aryldiborane(4) in 1988,[13] earlier reduction chemistry on
Mes2BF failed to generate B2Mes4,[14] perhaps as a conse-
quence of steric crowding. Nonetheless, in 1992 Power and co-
workers[15] isolated the diboranes(4), B2(R)(Mes)3 (R = OMe,
Ph, CH2SiMe3). Such tetraaryl-substituted species (Figure 1a)
The chemistry of boron reagents continues to be of wide-
spread interest, affording applications in complex organic
syntheses,[1] optoelectronics,[2] materials,[3] and boron cluster
chemistry.[4] Our interest in boron compounds, stems from
their utility in “frustrated Lewis pair” (FLP) chemistry.[5]
While the Lewis acidity of boranes can be directly exploited
in intermolecular FLPs, boron hydrides can also be employed
as synthons, including in the synthesis of the now classic
intramolecular FLP Mes2PCH2CH2B(C6F5)2 from the Erker
group.[6] In our own work, 9-BBN served as a precursor to
a N-heterocyclic carbene stabilized borenium cation.[7] In
a similar vein, Crudden and co-workers[8] expanded such
borenium cations to include those employing mesoionic
carbenes. Among our more recent efforts to increase the
diversity of main group Lewis acids,[9] we have explored other
electron-deficient boron reagents in small-molecule activa-
tion. For example, we used the borinium cation [Mes2B][B-
(C6F5)4], originally described by Shoji and co-workers,[10] in
reactions with H2, hydridoborane and silane, leading to the
first diboranium cation [B2(m-H)2(m-Mes)Mes3][B(C6F5)4].[11]
This species was also derived from Mes2BH and Brønsted
acid. Interestingly, the corresponding protonation of
Figure 1. a) Known tetraaryl-substituted diboranes(4), and b) hydrido-
substituted aryldiboranes(4).
remained largely unexplored, until 2017 when Yamashita and
co-workers developed
a
one-pot synthesis of the
tetraaryldiborane(4), B2(o-tolyl)4 and demonstrated its ability
to activate H2.[16] Later that same year, Yamaguchi and
Piers[17] described the ability of the dithieno-diborin to
similarly react with H2. In 2018, Erker and co-woorkers[18]
reported the synthesis of the dissymmetric tetrasubstituted
diborane(4) Ph(C6HR(C6F5)(SiMe3))BB(C6F5)2, while the
Yamashita group reported the reactions of B2(o-tolyl)4 with
CO, nitriles, azobenzene, and pyridazine.[19] Most recently,
Yamashita and co-workers have also reported the reduction
of B2(o-tolyl)4 affording a dianion which behaves as a diary-
lboryl anion equivalent.[20] In related work on boron nucle-
ophiles and diboranes(4), Yamashitaꢀs group also reported
a doubly hydride-bridged tetraborane(6) species.[21]
[*] K. L. Bamford, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George St., Toronto, Ontario, M5S3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Dr. Z.-W. Qu
Mulliken Center for Theoretical Chemistry,
Institut fꢀr Physikalische und Theoretische Chemie,
Rheinische Friedrich-Wilhelms-Universitꢁt Bonn
Beringstrasse 4, 53115 Bonn (Germany)
E-mail: qu@thch.uni-bonn.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101054.
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
An even more elusive subset of aryldiboranes(4) are
hydrido-substituted derivatives. Tamao and Matsuo used
extreme steric demands[22] to prepare the butterfly and
twisted geometries of dihydridodiboranes(4) (Figure 1b).
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The paucity of sterically unencumbered hydridodiboranes,[23]
The formation of 3–6 demonstrates substituent/hydride
suggests species of formulae B2HAr3 or B2H2Ar2 are reactive.
Herein we specifically target the generation of hydridodibor-
anes via reactions of B2(o-tolyl)4 (1) with secondary boranes
and BH3-sources. These reactions are shown to proceed via
aryl/hydride exchange while subsequent reactions of the
generated hydridodiboranes(4) prompt boron cluster forma-
tion. In the case of BH3·SMe2, the reaction with 1 gives an
unprecedented tetraaryl-pentaborane(9). The reaction path-
ways are probed both experimentally and computationally.
The combination of 1 and one of the monohydridobor-
anes, (HB(C8H14), HBcat, HBpin, or HB(C6F5)2), in a 1:1 ratio
in benzene afforded two major products after 12 h as
evidenced by NMR spectroscopy.[24] An 11B NMR signal at
72.6 ppm, common to all reactions, was unambiguously
confirmed to arise from B(o-tolyl)3 2 via independent syn-
thesis and crystallographic characterization (Figure 2a). The
second products were identified as (o-tolyl)B(C8H14) (3),[25]
(o-tolyl)Bcat (4),[26] (o-tolyl)Bpin (5),[27] and (o-tolyl)B(C6F5)2
(6),[28] respectively, based on known spectroscopic data. In the
case of 4, this was also confirmed by X-ray crystallography
(Figure 2b).
redistribution upon combination of 1 with a monohydridobor-
ane. However, as the corresponding diborane(4) product
B2H(o-tolyl)3 is not observed, further reactivity should
account for the formation of 2 and 7.
The corresponding reaction of 1 with one equivalent of
BH3·SMe2 (2 M in THF) in toluene was monitored by NMR
spectroscopy. After 16 h, 1 was consumed and major 11B
signals at 72.4, 3.0, À0.3, À4.6, À8.3 and À46.8 ppm were
observed. While the first of these resonances arises from 2,
workup afforded the isolation of a product 8 in 24% relative
yield,[29] which accounts for the 11B NMR signals at À4.6 and
À46.8 (d, 1JBH = 167 Hz) ppm.
A crystallographic study of 8 revealed it is a square-
pyramidal 2,3,4,5-substituted pentaborane(9), B5H5(o-tolyl)4
(Figure 3). The basal boron atoms have terminal o-tolyl
Figure 2. POV-ray depiction of a) 2, b) 4. B: yellow-green; C: black; O:
red. All hydrogen atoms have been omitted for clarity.
Figure 3. POV-ray depiction of 8 as viewed from a) side-on, and b) top-
down. B: yellow-green, C: dark grey, H: light grey. All hydrogen atoms
except those bound to boron centers have been omitted for clarity.
In a similar fashion, reactions of 1 with two equivalents of
either HBcat or HBpin gave two major products. The product
common to both reactions is [H2B(o-tolyl)]2 (7) which gives
substituents with bridging hydrides, while the apical boron
bears a terminal hydride. The four equivalent B–B distances
in the basal plane are each 1.834(2) ꢁ, while those to the
apical boron are 1.691(3) ꢁ, resulting in a displacement of the
apical boron from the basal plane of 1.086 ꢁ. This structure
and the strongly shielded 11B chemical shift of the apical
boron are consistent with the three-dimensional aromatic-
ity[30] of nido-pentaboranes(9), presumably accounting for the
high stability of 8. Indeed, compound 8 shows no evidence of
reaction after prolonged heating at 1108C, in toluene solution
(Figures S30, S31). These observations are consistent with the
known stability of the parent pentaborane(9), B5H9.[31] In
a related sense, compound 8 showed no reaction with D2
(1 atm) even after heating to 1108C for 24 hours (Figures S32,
S33). This behavior is parallel to that of B5H9 under base-free
thermolysis.[31,32]
a
11B signal at 19 ppm (vide infra). In addition, 4 or 5 are
observed as respective products (Scheme 1).
Compound 8 is, to our knowledge, a unique example of
Scheme 1. Reactions of 1 with hydridoboranes.
Angew. Chem. Int. Ed. 2021, 60, 8532 –8536
a tetrasubstituted pentaborane(9)[33] and the first example in
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which arylation exists on the basal boron atoms of
a pentaborane(9).[34] Perhaps more importantly, we note
that prior pentaborane(9) derivatives have been exclusively
derived from functionalization of B5H9 or higher clusters,[35]
whereas here we assemble the B5 cluster from substituted
borane and diborane species (i.e. B1 and B2 synthons).
Seeking to identify the remaining products in the reaction
mixture of 1 and BH3·SMe2, we speculated that [H2B(o-
tolyl)]2 (7) and [HB(o-tolyl)2]2 (9) were among them. Efforts
to generate these species selectively by redistribution reac-
tions[36] of 2 and BH3·L (L = SMe2 or THF) failed. However,
we noted that in describing the formation of 9, HB(o-
tolyl)2·C6D6, and HB(o-tolyl)2, Yamashita and co-workers[16]
had ascribed them to the 11B signals at 28.5, 18.6 and
72.4 ppm, respectively, in the reaction of 1 and H2. Noting
that our data unambiguously affirmed the downfield reso-
nance arises from 2, we re-examined this reaction in both
hexane and C6D6, finding no spectroscopic difference.[37]
Given the propensity of diaryl(hydrido)boranes to dimer-
ize,[38] we suggest 7 and 9 are indeed formed from reaction of
1 and H2 (4 atm) and this accounts for the 11B signals at 18.6
and 28.5 ppm, respectively (Figures S16, S17). This view was
further supported by our DFT-computed[39] 11B chemical
Scheme 2. DFT-computed free energy paths (in kcalmolÀ1, at 298 K
temperature and 1 M concentration) for the reactions of 1 (Ar=o-
tolyl) in toluene with HBcat.
aryl/hydride exchange is 1.9 kcalmolÀ1 endergonic over
a moderate free energy barrier of 20.0 kcalmolÀ1 (via
transition structure TSA) affording the product 4 and the
transient hydridodiborane(4) H(o-tolyl)BB(o-tolyl)2 (A).
Dimerization of A giving (A)2 is À16.6 kcalmolÀ1 exergonic
over a barrier of only 5.7 kcalmolÀ1 (via TSAd). This dimer
needs only 7.6 kcalmolÀ1 to eliminate the experimentally
observed species 2 and the computed by-product, H2B3(o-
tolyl)3 (Ad). While the precise fate of Ad is uncertain, further
reaction with borane or diborane(4) species in solution could
account for the minor unidentified by-products in the reaction
mixture.
Given that reactions of 1 and hydridoboranes are
computed to provide access to triboron species, it is tempting
to suggest such species react with hydridodiboranes(4) to give
the observed pentaborane(9) species where the degree of
substitution is under thermodynamic control. Alternatively,
the established nucleophilicity of sp2–sp3 diboranes[33] sug-
gests THF or SMe2 enhances disproportionation of
hydridodiboranes(4), prompting delivery of “BH” to (C)2
affording 8. This latter view is consistent with reports by
Kodama and Perry that the sp3–sp3 diborane B2H4·(PMe3)2
effects expansion of boron hydride clusters by nominal
diborane cleavage into BH3·(PMe3) and “BH·(PMe3)”.[42]
Analogous computations for the reaction of 1 and
BH3·SMe2 showed an even more complex array of possibil-
ities (see Supporting Information), such as aryl/hydride
exchange reactions, dimerization of hydrido-boron species
and subsequent elimination of boranes. Nonetheless it is
interesting to note that our DFT calculations infer triboron
intermediates may react with diboranes, affording further
thermodynamically favored aggregates such as the observed
pentaborane(9) (see Supporting Information). Certainly, we
can infer that the availability of additional hydrides in the
reactions of BH3 sources favors the generation of reactive
intermediates that are central to the formation of 8.
shifts (see Supporting Information) for 2, 4, 7, 8, and 9 (dcalc
=
73.0; 37.0; 21.3; À5.6, À44.3; 28.7 ppm) that agree well with
experimental values. These revised assignments indicate that
neither 7 nor 9 are present in the original reaction mixture of
1/BH3·SMe2. However, addition of excess SMe2 to the 1/H2
reaction mixture showed loss of the 11B signals at 18.6 and
28.5 ppm and the appearance of signals at À0.3 and À8.3 ppm
analogous to those seen in the reaction mixture of 1 and
BH3·SMe2. Thus, we attribute these respective signals to (o-
tolyl)2BH·SMe2 (10) and (o-tolyl)BH2·SMe2 (11), a view
consistent with our DFT-computed 11B chemical shifts
(dcalc = À1.5, À5.6 ppm).
Performing the reaction of 1 with neat BH3·SMe2 in THF
afforded no trace of 2, rather 8 and HB(o-tolyl)2·THF are
formed.[16] In contrast, repeating the reaction of 1 with neat
BH3·SMe2, in the total absence of THF, afforded no trace of 8.
Instead, 11B NMR data reveal a mixture of 2 in addition to
two new strong signals at 2.3 and À22.6 ppm (see Supporting
Information). Interestingly, addition of THF to this mixture
reduces the intensity of these peaks and affords 8 after 24 h,
suggesting the unassigned signals arise from species that act as
precursor(s) to 8. Collectively, these data suggest that
intermediate borane/SMe2 adducts are kinetically reactive
in the presence of THF, prompting o-tolyl/hydride exchange.
These reactions are unexpectedly complex given the
simplicity of the reagents involved. Nonetheless, the ability
of sterically unhindered aryl(hydrido)boranes[40] and
diboranes(4) to scramble substituents or aggregate via
hydride bridges, results in complex mixtures. In addition,
the presence of THF or SMe2 also induces equilibria for Lewis
adduct formations with less encumbered boron centers.
Despite these complexities, dispersion-corrected DFT calcu-
lations were performed at the PW6B95-D3 + COSMO-RS//
TPSS-D3 + COSMO level (see Supporting Information)[41] to
garner some insight into the reactions of 1 with hydridobor-
anes. In the case of 1 and HBcat in toluene (Scheme 2), initial
In summary, we have shown that transient hydridodibor-
anes generated via reactions of the diborane(4) 1 with
secondary boranes are highly reactive, providing a complex
mixture of products including the known species 2–7, in
addition to higher boron-aggregates. In the corresponding
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the products. These reactions demonstrated that hydride/aryl
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unlock avenues to the pentaborane(9) species 8. This latter
product represents the only known polyaryl pentaborane(9)
and the first to be assembled from borane and diborane(4)
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Acknowledgements
The authors thank NSERC of Canada for financial support.
D.W.S. is grateful for the award of a Canada Research Chair
and to the Guggenheim Foundation for the award of a 2020
fellowship. K.L.B is grateful for the award of an Alexander
Graham Bell Canada Graduate Scholarship. Z.W.Q is grate-
ful to the DFG (project SPP1807 and Gottfried Wilhelm
Leibnitz prize to Prof. Stefan Grimme) for financial support.
Open access funding enabled and organized by Projekt
DEAL.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: boron · cluster · diborane(4) · metathesis ·
pentaborane(9)
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Manuscript received: January 22, 2021
Accepted manuscript online: February 4, 2021
Version of record online: March 4, 2021
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