Facile Synthesis of a Stable Dihydroboryl {BH2}? Anion

Article abstract of DOI:10.1002/anie.201809983

While the one-electron reduction of (CAAC^Me)BH₂Br (CAAC^Me=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields a hydride-shift isomer of the corresponding tetrahydrodiborane, a further reversible reduction leads to the first stable parent boryl anion, [(CAAC^Me)BH₂]^?, which acts as a powerful boron nucleophile.

Full text of DOI:10.1002/anie.201809983

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Accepted Article
Title: Facile Synthesis of a Stable Dihydroboryl {BH2}–
Authors: Merle Arrowsmith, James Mattock, Stephan Hagspiel, Ivo
Krummenacher, Alfredo Vargas, and Holger Braunschweig
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809983
Angew. Chem. 10.1002/ange.201809983
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Facile Synthesis of a Stable Dihydroboryl {BH2}– Anion
Merle Arrowsmith, James D. Mattock, Stephan Hagspiel, Ivo Krummenacher, Alfredo Vargas and
Holger Braunschweig*
Abstract: While the one-electron reduction of (CAACMe)BH2Br
(CAACMe = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-
ylidene) yields
a hydride-shift isomer of the corresponding
tetrahydrodiborane, a further reversible reduction leads to the first
stable parent boryl anion, [(CAACMe)BH2]–, which acts as a powerful
boron nucleophile.
Tricoordinate boron reagents generally behave as electrophiles
due to their vacant p orbital. In 2006, however, Yamashita and
Nozaki reported the first boryl anion, compound I (Fig. 1),[1] in
which boron is in its formal +1 oxidation state and reacts as a
nucleophile, thus enabling access to a wide range of novel
boron-element-bonded compounds.[2] Curran and coworkers
later succeeded in generating a highly reactive, fleeting N-
heterocyclic carbene (NHC)-supported {BH2}– parent boryl anion
(II) at low temperature and trapping it with a variety of
electrophiles,[3] while our own group isolated a nucleophilic
borolyl anion that displayed not only classical salt metathesis but
also single-electron transfer reactivity.[4]
A major breakthrough in low-valent boron chemistry came
with the isolation by Bertrand and coworkers of the first metal-
free boron(I) compound, borylene III,[5] which owes its
remarkable stability to the excellent σ donor and π acceptor
properties and the steric shielding afforded by the two
supporting cyclic (alkyl)(amino)carbene (CAAC) ligands.[6] Since
then, CAACs have been successfully employed to stabilize a
wide range of ever more reactive borylenes[7] and boryl anions.[8]
The most recent examples of these include the CAAC-CO-
stabilized derivative IV, which under photolytic conditions
liberates CO and generates a dicoordinate borylene synthon,[9]
the highly reactive chloro(hydro)boryl anion V, which was
isolated as a potassium crown ether complex,[10] and most
recently, the N2 activation compound VI, which may be viewed
as a N2-bridged borylene dimer.[11]
Figure 1. Selected examples of low-valent borylenes and boryl anions.
In this contribution we describe the facile stepwise reduction of a
CAAC-stabilized (dihydro)haloborane to the corresponding
diborane, and further to the first isolable, room-temperature-
stable {BH2}– parent boryl anion, which reacts as a strong boron
nucleophile.
The room temperature reduction of (CAACMe)BH2Br (1, see
Fig. S21 in the SI for the solid-state structure of 1) with 1.1 equiv.
lithium sand in THF (Scheme 1) yielded a pale yellow solution
exhibiting a single broad 11B NMR resonance at 21.2 ppm (fwmh
≈ 420 Hz). After removal of volatiles and extraction with hexanes,
crystallization at –25 °C yielded a crop of gold-orange crystals of
compound 2 (79% yield). The 1H{11B} NMR spectrum of 2
showed a single, symmetrical CAACMe ligand environment and a
broad 2H BH2 resonance at 2.66 ppm. The very broad 13C NMR
resonance of the carbene carbon atom was detected by an
HMBC experiment at 161.5 ppm, 67 ppm upfield from that of the
borane precursor 1 (δ13C = 128.2 ppm).
Dr M. Arrowsmith, S. Hagspiel, Dr I. Krummenacher, Prof. Dr. H.
Braunschweig
Institut für Anorganische Chemie, Julius-Maximilians-Universität
Würzburg, Am Hubland, 97074 Würzburg, Germany,
and
Institute for Sustainable Chemistry & Catalysis with Boron, Julius-
Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg,
Germany.
Scheme 1. One-electron reduction and homocoupling of 1 to form diborane 2.
Dip = 2,6-diisopropylphenyl.
E-mail: h.braunschweig@uni-wuerzburg.de
J. D. Mattock, Dr. A. Vargas
Department of Chemistry, School of Life Sciences, University of
Sussex, Brighton BN1 9QJ, Sussex, United Kingdom.
The symmetry implied by the NMR data contrasts with the
crystallographically-derived structure of 2, which shows an
Supporting information for this article can be found under: …
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10.1002/anie.201809983
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unsymmetrically μ2-hydride-bridged diborane(5) (Fig. 2). B1 is
supported by a neutral, purely σ-donating CAACMe ligand (B1-C1
1.5477(16) Å), whereas B2 is coordinated by a protonated
CAACMe ligand (CAACMeH), which displays an sp3-hybridized
C21 carbon atom (N2-C21-B2 114.15(9), C26-C21-B2
114.29(9)°) with B2-C21 and N2-C21 single bonds (1.6039(16)
and 1.4774(14) Å, respectively). The B-B bond length of
1.6394(18) Å is significantly shorter than in related μ2-hydride-
bridged diboranes(5) obtained from the reduction of mono-NHC-
stabilized 1,1-diaryl-2,2-dichlorodiboranes (ca. 1.67 – 1.68 Å),[12]
presumably due to the absence of steric repulsion from the
hydride ligands in 2.
low. Density functional theory (DFT) calculations carried out at
the OLYP/TZ2P level of theory showed that the free energy of
the isomeric tetrahydrodiborane(6) 2
՛
is only 1.1 kcal∙mol–1
above that of 2 (Scheme 2, see Fig. S25).[16] 11B NMR shift
calculations at the same level of theory provided chemical shifts
for 2 of –3.20 ppm for B1 and 45.4 ppm for B2, the average of
which is 24.3 ppm, close to the 22 ppm observed for this
resonance in solution at room temperature.
The synthesis of compound 2 is a rare example of targeted
reduction of a neutral Lewis-base-stabilized sp3-borane to a
neutral diborane. All previous examples involved NHC-stabilized
di- and trihaloboranes undergoing both reductive coupling and
exchange of all remaining halogens with hydrides, through
hydrogen abstraction from the reaction solvent by radical
intermediates.[13] The formation of 2 may be facilitated by 1
undergoing a 1,2-hydride shift from boron to the CAACMe
carbene carbon atom to form the isomeric sp2-borane
Scheme 2. Hypothesized rapid hydride-shift isomerization of 2. Relative free
energies (OLYP/TZ2P) in brackets (kcal∙mol–1).
The reduction of 1 with 2.5 equiv. lithium sand in a 1:5
THF/hexane mixture led to quantitative formation of the
dihydroboryl anion 3 (Scheme 3, workup A), which displays a
broad 11B NMR resonance at –4.7 ppm, downfield from that of
the related [(CAACMe)BH(CN)]– anion (δ11B = –10.8 ppm)[8a] and
(CAACMeH)BHBr, 1
՛
, the free energy of which was calculated to
be only 11.9 kcal∙mol–1 above that of 1 (see Fig. S24), thereby
making it accessible under the reaction conditions employed in
the reduction of 1 to 2. Such reversible boron-to-carbon hydride
shifts are well-documented in CAAC-supported hydroboron
compounds[15] and are owed to the good π acceptor properties of
CAACs, which result in a relatively low-lying LUMO and a small
HOMO-LUMO gap.[6]
7
a Li NMR singlet at 0.84 ppm. After filtration and storage of the
filtrate at –25 °C for three days, compound 3 was isolated as
large, bright orange crystals in 81% yield. The 1H{11B} NMR
spectrum of 3 displayed a broad BH2 resonance integrating for
2H with respect to the CAACMe resonances.
Scheme 3. Synthesis of the parent boryl anion 3 and its dimer 3 . Workup A:
՛
1) Filtration, 2) storage of filtrate at –25 °C for crystallization. Workup B: 1)
Removal of volatiles, 2) extraction with toluene, 3) storage at –25 °C for
crystallization.
Figure 2. Crystallographically-derived molecular structure of compound 2.
Thermal ellipsoids drawn at 50% probability level. Ellipsoids on the CAACMe
ligand periphery and most hydrogen atoms omitted for clarity. Selected bond
lengths (Å) and angles (°): B1-B2 1.6394(18), B1-C1 1.5477(16), B2-C21
1.6039(16), C1-N1 1.3169(14), C21-N2 1.4774(14), B1-H1 1.113(15), B2-H2
1.110(15), B1-H3 1.225(15), B2-H3 1.415(15), C1-B1-B2 125.07(10), C21-B2-
B1 119.81(10), Σ∠C1 359.89(10).[14]
X-ray structural analysis of 3 revealed a monomeric structure,
comprised of a planar, anionic [(CAAC)BH2]– moiety (Σ∠B1
360.0(9)°) coordinated via the two boron-bound hydrides to a Li
cation solvated by three THF residues (Fig. 3a). The B1-C1
bond length (1.440(2) Å) is similar to that in the related
(cyano)hydroboryl anion dimer (1.447(3) Å),[8a] indicating strong
π backdonation from the lone pair on boron to the π-acidic
CAACMe ligand. The lithium cation is nearly aligned with the C1-
B1 bond (C1-B1-Li1 165.45(15)°) and located at 2.293(3) Å from
the boron(I) center. To our surprise, a change in the workup
procedure involving complete removal of the reaction solvent
While the solid-state structure of 2 does not reflect the solution
data, such reversible hydrogen shifts are also likely to be the
origin of the apparent symmetry of the compound in solution,
with all four hydrogen atoms shifting back and forth between the
terminal, bridging and C1-bound positions. Since cooling to –
110 °C in d8-THF did not lead to decoalescence of the 11B NMR
resonance, the barrier for this hydride shift is likely to be very
and extraction with toluene provided the dimeric species 3
՛
upon
crystallization (Scheme 3, workup B). The solid-state
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Figure 3. a) and c) Crystallographically-derived molecular structure of compounds 3 and 3
՛
, respectively. Thermal ellipsoids drawn at 50% probability level.
Ellipsoids on the CAACMe ligand periphery and most hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°): 3 B1-C1 1.440(2), C1-N1
1.4332(15), B1-H1A 1.145(17), B1-H1B 1.140(16), B1∙∙∙Li1 2.293(3), Li1-H1A 2.000(17), Li1-H1B 1.917(17), C1-B1-Li1 165.45(15), Σ∠B1 356.0(9), Σ∠C1
՛
359.96(11); 3 B1-C1 1.441(6), C1-N1 1.421(4), B1-H1A 1.11(4), B1-H1B 1.19(4), B1∙∙∙Li1 2.301(8), B1∙∙∙Li1՛ 2.407(8), Li1∙∙∙Li1՛ 3.041(13), H1A-Li1 1.91(4), H1B-
Li1 2.07(4), Li1∙∙∙C1՛ 2.669(8), C1-B1-Li1՛162.9(4), C1-B1-Li 83.8(3), B1-Li1-B1՛ 99.6(3), Σ∠C1 359.6(3), Σ∠B1 360(2); b) Plot of the HOMO of 3 at the OLYP/TZ2P
level of theory (–1.905 eV, isovalue 0.005).
structure of 3 shows a boryl anion moiety with similar structural
՛
parameters to 3 (Fig. 3c). Its lithium cations are coordinated by a
single THF residue and bridge over the boron-bound hydrides to
the second boron center so as to form a distorted B2Li2 square
(B1∙∙∙Li1 2.301(8); B1∙∙∙Li1՛ 2.407(8) Å; B1-Li1-B1՛ 99.6(3); Li1-
B1-Li1՛ 80.4(3)°). It is noteworthy that the coordination of each Li
cation to the boron atom of the second dimer moiety occurs
perpendicularly, thereby allowing the CBH2 borylene unit to
remain planar. Unlike monomeric 3, isolated crystals of dimeric
3
՛
proved virtually insoluble in hydrocarbon solvents, thus
precluding the acquisition of NMR data. Though indefinitely
stable in THF solution up to 70 °C under inert atmosphere, both
՛
3 and 3 slowly decomposed in the solid state to (CAAC)BH3,
even when stored in the glovebox freezer at –30 °C.
Plots of the frontier molecular orbitals (MOs) of 3 show a
HOMO corresponding to the B-C π-bond, with a small
π
Scheme 4. Reactivity of 3/3 .
՛
antibonding contribution from the C1-N1 bonds (Fig. 3b), similar
to other CAAC-supported boryl anions.[8,10] Furthermore, the B-C
Mayer bond order amounts to 1.664, which is less than for the
analogous [(CAACMe)BHCl]– anion (1.703),[10] but still indicative
of substantial π backbonding from boron to CAACMe. The
calculated Hirshfeld partial charges of –0.161 for B1 and –0.077
for C1[17] confirm the negative charge accumulation on the boron
atom, which is the opposite charge distribution calculated for
[(CAACMe)BHCl]– (–0.086 for B1 and –0.102 for C1).[10] This
The nucleophilic character of 3/3
՛
was confirmed by salt
metathesis with Me3SnCl, which quantitatively yielded colorless
(CAACMe)BH2(SnMe3) (4, Scheme 4b, see Fig. S23 for the solid-
state structure of 4), characterized by a 11B NMR triplet at –29.7
(1J11B-1H = 100 Hz) and a broad 119Sn NMR quartet at –24.4
(1J11B-119Sn = 314 Hz), effectively identical to that of the previously
reported compound (CAACMe)BH(CN)(SnMe3).[8a]
The reaction of 3 with one equiv. (CAACMe)BBr3 was
accompanied by an instant color change to deep blue as well as
should make 3/3 particularly nucleophilic at boron.
՛
Interestingly, cyclic voltammetry performed on compounds 1,
2 and 3 in THF showed in all three cases a reversible reduction
wave at –3.2 V (referenced to the (Fc+/Fc) couple, see Figs. S18
– S20), suggesting the possibility of a chemically reversible
reduction of 2 to 3. Indeed the reduction of isolated 2 with Li in
THF led to the clean formation of 3, a process that most likely
the formation of a colorless precipitate, presumably LiBr. The 11
B
NMR spectrum of the reaction mixture showed quantitative
1
formation of 1 (broad triplet at δ11B = –19.0, JB-H = 103 Hz) as
well as a broad resonance at 40.0 ppm attributable to the blue-
colored dihydrodiborene (CAACMe)2B2H2 (5, Scheme 4c),[18]
which was confirmed by high-resolution mass spectrometry
([C40H64B2N2]: calc. 594.5250; found 594.5227). We postulate
that the reaction proceeds via a double salt metathesis leading
to an intermediate triborane(7), which undergoes intramolecular
bromide and hydride migration, as well as B-B bond cleavage, to
generate 5 and 1 (Scheme 5). While this is certainly not the
ensues from the B-B bond cleavage of isomer 2
՛
. Conversely,
the oxidation of 3/3 with the 2,2,6,6-tetramethylpiperidyloxyl
՛
(TEMPO) radical in a 1:1 boron-to-radical ratio resulted in
quantitative conversion back to diborane 2 (Scheme 4a), thus
confirming the chemical reversibility of the reduction as well as
of the 2/2 hydride shift process.
՛
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In conclusion, we have shown that (CAACMe)BH2Br (1)
undergoes a selective, B-B bond-forming, one-electron reduction
to
a
hydride-shift isomer of the tetrahydrodiborane
(CAACMe)2B2H4, compound 2. Subsequent two-electron
reduction of 2 yields the first isolable parent boryl anion,
[(CAACMe)BH2]–, which may be isolated in its monomeric form 3
or dimeric form 3 , depending on the crystallization conditions.
՛
Compound 3 owes its remarkable solution stability to the strong
π acceptor properties of the CAAC ligand, their HOMO being
entirely delocalized over the B-C
π bond. Furthermore,
[(CAACMe)BH2]– can be quantitatively oxidized back to 2, in an
unusual B-B bond-forming oxidation reaction with the TEMPO
[14] Two distinct polymorphs of 2 were obtained, one crystallizing in the
radical. While [(CAACMe)BH2]– reacts as
a typical boron
ꢀ
triclinic space group P 1, presented here in Fig. 2, the other in the
nucleophile towards Me3SnCl, leading to the formation of a new
B-Sn bond, it undergoes a unique B=B double-bond-forming
double salt metathesis with (CAACMe)BBr3, generating the
dihydrodiborene (CAACMe)2B2H2 and 1. In view of these
promising preliminary results, we are continuing to explore the
orthorhombic space group P c a 21, presented in Fig. S22.
[15] a) D. Auerhammer, M. Arrowsmith, H. Braunschweig, R. D. Dewhurst, J.
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[16] Attempts to calculate the energy barrier for this hydride shift failed as
optimizations of possible transition states simply reverted to compound
2.
reactivity of 3/3 towards other p-block electrophiles.
՛
[17] For the dimer 3 , the HOMO and HOMO–1 are both symmetrically
՛
distributed over the B-C π-bonds of both moieties (see Fig. S28), the B-
C Mayer bond orders amount to 1.517 and the calculated Hirshfeld
partial charges are –0.167 for B1 and –0.054 for C1.
Acknowledgements
[18] M. Arrowsmith, J. Böhnke, H. Braunschweig, M. A. Celik, T. Dellermann,
K. Hammond, Chem. Eur. J. 2016, 22, 17169-17172.
The authors thank the Deutsche Forschungsgemeinschaft (H.B.)
and the University of Sussex (A.V.) for financial support.
Conflict of interest
The authors declare no conflict of interest.
Keywords: boryl anion • diborane • reduction • nucleophilic
boron • cyclic (alkyl)(amino)carbene
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
M. Arrowsmith, J. D. Mattock, S.
Hagspiel, I. Krummenacher, A. Vargas,
H. Braunschweig*
Page No. – Page No.
Facile Synthesis of a Stable
Dihydroboryl {BH2}– Anion
Successive reductions of a cyclic (alkyl)(amino)carbene (CAAC)-supported
(bromo)dihydroborane yield a hydride-shift isomer of the corresponding
tetrahydrodiborane and the first stable dihydroboryl anion, which acts as a powerful
boron nucleophile.
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