A Facile Synthesis of Robinson's NHC-Stabilised Diborane(4)

Article abstract of DOI:10.1002/ejic.202000596

Reactions of bis(pinacolato)diboron (B₂pin₂) with [(IDip)AlH₃] {IDip = (HCNDip)₂C:, Dip = 2,6-iPr₂C₆H₃} afforded both the new diborane [(IDip)BH₂B(pin)] and the known compound [(IDip)BH₂BH₂(IDip)] in a facile one-pot procedure; the latter reaction is improved by the addition of free IDip. [(IDip)AlH₃] transfers both IDip and hydride ligands to the diborane moiety in a halide-free approach. [(IDip)BH₂B(pin)] was structurally and spectroscopically characterised and additional spectroscopic data for [(IDip)BH₂BH₂(IDip)] is reported.

Full text of DOI:10.1002/ejic.202000596

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European Journal of Inorganic Chemistry
Diboranes | Very Important Paper |
A Facile Synthesis of Robinson's NHC-Stabilised Diborane(4)
[
a]
[a]
[a]
[a]
Michael Trose, David B. Cordes, Alexandra M. Z. Slawin, and Andreas Stasch*
Abstract: Reactions of bis(pinacolato)diboron (B pin ) with IDip. [(IDip)AlH ] transfers both IDip and hydride ligands to the
2
2
3
[
(IDip)AlH ] {IDip = (HCNDip) C:, Dip = 2,6-iPr C H } afforded diborane moiety in a halide-free approach. [(IDip)BH B(pin)]
3
2
2
6
3
2
both the new diborane [(IDip)BH B(pin)] and the known com- was structurally and spectroscopically characterised and addi-
2
pound [(IDip)BH BH (IDip)] in a facile one-pot procedure; the tional spectroscopic data for [(IDip)BH BH (IDip)] is reported.
2
2
2
2
latter reaction is improved by the addition of free
Diboranes[1] with a boron-boron single bond, such as the triborane species.[11] Here we report on a simple alternative
commercially available bis(pinacolato)diboron (B pin ), are preparation of Robinson's NHC-stabilised diborane(4), 1, start-
2
2
commonly employed reagents in synthetic chemistry for nu- ing from the commercially available boron(II) compound
[
1–4]
merous borylation reactions,
the resulting C–B fragments allows the transformation into a
and their further conversion of bis(pinacolato)diboron.
The reaction of stoichiometric amounts of [(IDip)AlH ] 4
[
12]
3
[
1,4]
wide range of functional groups.
dinate boron centres can be activated by the coordination of temperature was monitored by H and B NMR spectroscopy
Diboranes with three-coor- with one equivalent of B pin in deuterated benzene at room
2 2
1
11
anionic ligands or neutral σ-donors, e.g. N-heterocyclic carb- and showed the emergence of new resonances for one IDip-
2
3
[1,4,5]
1
enes (NHCs), to sp -sp diboranes.
The application of NHCs containing compound in the H NMR spectrum alongside
in boron chemistry has furthermore led to the synthesis and smaller resonances for some free IDip in an approximate 3:1
[
6]
[
7]
isolation of a range of unusual low oxidation state molecules,
including diadducts of diboron species such as B H , B H ,
and B2, see Figure 1 for selected examples involving the steri- product formed in > 70 % yield shows one imidazolylidene CH
ratio. After approximately five hours, all [(IDip)AlH ] (4) was con-
sumed according to H NMR spectroscopy, and the main new
3
[
8]
[8,9]
1
2
4
2 2
[
10]
cally demanding NHC IDip [= (HCNDip) C:, Dip = 2,6-iPr C H ]. resonance at δ = 6.37 ppm, and two doublets (δ = 1.04 and
2
2 6 3
The products 1–3 were obtained from reduction reactions in- 1.44) and one septet (δ = 2.81) for the protons of the IDip
1
1
volving very strong alkali metal-based reducing agents and the isopropyl groups. Moreover, B NMR spectroscopy revealed
BH moieties in 1 and 2 resulted from uncontrolled hydrogen one broad resonance at 40.1 ppm and a poorly resolved triplet
abstraction reactions likely of solvent molecules. Highly reactive at –38.0 ppm suggesting an asymmetrically substituted dibo-
congeners of 3 have subsequently been hydrogenated under rane species with both three- and four-coordinate boron cen-
[
9c]
[1,4,5]
mild conditions in high yields to congeners of 2, but no facile tres.
Performing a larger scale reaction in toluene at room
route to 1 has yet been reported. Related phosphine adducts temperature, followed by removal of all volatiles, extraction of
of B H have been reported that were obtained from a higher the residue into n-hexane and crystallisation at 4 °C afforded
2
4
[
(IDip)BH B(pin)] (5) in 45 % isolated yield as a colourless crys-
2
talline material, see Scheme 1. During extraction with n-hexane,
an insoluble gel-like residue formed as a by-product, presuma-
bly a polymeric material of approximate “HAlpin” composition.
Figure 1. Examples of IDip-stabilised diboron species.
[
a] Dr. M. Trose, Dr. D. B. Cordes, Prof. Dr. A. M. Z. Slawin, Dr. A. Stasch
EaStCHEM School of Chemistry, University of St Andrews,
North Haugh, St Andrews, KY16 9ST, UK
E-mail: as411@st-andrews.ac.uk
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000596.
©
2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
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.
Scheme 1. Synthesis of 5.
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Compound 5·0.5C H was studied by single-crystal X-ray dif- significant improvement in the ratio of the formation of 5 and
6
6
fraction and crystallised with a full molecule in the asymmetric 1 was observed from these experiments. Conversely, we tested
unit (see Figure 2) showing a B1–B2 bond length of 1.685(5) Å the influence of additional free carbene IDip on the formation
{
2
cf. 1.686(4) Å in [(IMes)BH B(cat)]}, IMes = [(HCNMes) C:, Mes = by reacting two equivalents of 4 with one equivalent of B pin2
2
2
2
[
13]
,4,6-Me C H , cat = catecholato],
and a B1–C1 bond and half an equivalent of IDip in deuterated benzene. Heating
3
6 2
lengths of 1.592(5) Å {cf. 1.605(3) Å in [(IMes)BH B(cat)]}. these mixtures for three days at 50 °C or two days at 80 °C
2
[
(IMes)BH B(cat)] and [(IDip)BH B(cat)] were previously prepared afforded
reaction
mixtures
that
mainly
contained
2
2
by the reaction of the respective NHC adduct of B cat with [(IDip)BH BH (IDip)] (1) in approximately 70 % in-situ yield
2
2
2
2
BCl to afford [(IMes)BCl B(cat)] or [(IDip)BCl B(cat)] followed by alongside resonances of free IDip which may contain contribu-
3
2
2
[
15]
hydrogenation using two equivalents of tri-n-butyltinhydride tions from [(IDip)AlH ] (4).
A reaction on a small preparative
scale in this stoichiometric ratio in toluene at 80 °C for two days
Also, hydride abstraction of related [(IMe)BH(Mes)B(pin)] {IMe = afforded, after extraction into n-hexane to remove the insoluble
3
[
13]
and 10 mol-% sodium tetrakis(3,5-dichlorophenyl)borate.
(
MeCNMe) C:} leads to a cationic diborane species that activates gel-like “HAlpin”, 1 in 47 % isolated yield after crystallising at
2
[
14]
dihydrogen at low temperatures.
4 °C, see Scheme 2.
Scheme 2. Synthesis of 1.
The addition of free IDip may aid in the activation of the
Figure
(IDip)BH
2.
Molecular
structures
(25 %
thermal
ellipsoid)
of
diborane, either B pin and/or 5, by coordination to a three-
2
2
[
2
B(pin)]·0.5C
6
H
6
, 5·0.5C
6
H
6
. Only B–H hydrogen atoms are shown.
[1,4,5]
coordinate boron centre.
This would lengthen and weaken
Minor disordered isopropyl methyl groups and solvent molecule not shown
for clarity. Selected bond lengths [Å] and angles (°): B1–B2 1.685(5), B1–C1
the B–O bonds and thus aid in substitution of the pinacolato
groups in combination with using an oxophilic Al reagent that
is also a good hydride source. Comparing bond lengths in struc-
turally characterised imidazolylidene adducts of B pin to those
1
.592(5), C1–N2 1.356(4), C1–N5 1.355(4), B2–O1 1.365(5), B2–O2 1.394(5);
C1–B1–B2 124.6(3).
2
2
After successfully employing [(IDip)AlH ] (4) to transfer both
3
of uncoordinated B pin shows the expected elongation of the
2
2
the IDip carbene ligand to boron and substitute hydride ligands
for the pinacolato moiety with retention of the B–B bond, we
studied other ratios of 4 with B pin to evaluate if Robinson's
NHC-stabilised diborane(4) compound [(IDip)BH BH (IDip)] (1)
can be directly prepared in a one-pot procedure. A reaction of
two equivalents of 4 with B pin2 in deuterated benzene at
room temperature afforded only traces of 1 after three days, as
supported by the reported H and B NMR resonances.
Similar experiments carried out at 50 °C or 80 °C afforded mix-
B–O bond from ca. 1.38 Å in B pin by ca. 0.1 Å (ca. 7 %) in its
2
2
mono-NHC adducts, whereas the effect on the longer B–B bond
2
2
[4,5]
is only ca. 0.03 Å (ca. 2 %).
Thus, we propose a mechanism
2
2
where [(IDip)AlH ] (4) coordinates to the pinacolato moiety on
3
an IDip coordinated boron centre followed by hydride transfer
to boron and full pinacolate substitution, see Figure 3 for a
proposed intermediate. This would form [(IDip)AlH(pin)] or a
very similar species that likely degradates to insoluble “HAlpin”
and regenerates free IDip that is catalytically employed in the
reaction. Inspecting reported RAl(pin) complexes shows that
these appear to be only soluble when an excess of alkyl to pin
groups are present, i.e. in structurally characterised
2
1
11
[8b,8d]
tures mainly containing [(IDip)AlH ] (4), [(IDip)BH B(pin)] (5) and
3
2
[
(IDip)BH BH (IDip)] (1) in varying ratios, for example with ap-
2 2
proximately 50 % of 1 after one day at 80 °C. It is notable that
no resonances of free IDip were observed unlike in reaction
mixtures for the synthesis of 5, vide supra. We have previously
found that mixtures of [(IDip)AlH ] (4) and IDip show an equilib-
3
rium in deuterated aromatic solvents and hence only one set
[
15]
of resonances for the IDip fragment is found.
As the ex-
tremely broad resonance for the AlH hydrogens cannot easily
3
be integrated in a reliable manner, it is difficult to judge the
true ratio of 4 to IDip in the reaction mixtures. Because
[
12]
[
(IDip)AlH ] (4) can be prepared from LiAlH and IDip,
we
3
4
added LiAlH to study whether an additional hydride source
4
can increase the yield of [(IDip)BH BH (IDip)] (1). However, no
Figure 3. Proposed intermediate in the formation of 1.
2
2
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[
(Me Al) (pin) (AlMe)] which was isolated from other insoluble Supporting Information (see footnote on the first page of this
2 2 2
[
16]
article): Experimental details, NMR and IR spectra, X-ray crystallo-
graphic and refinement details.
species, or when bulky tert-butyl groups are employed.
Free
IDip was also observed in reaction mixtures forming 5 and
therefore a similar mechanism is possible for its formation.
Other exchange reactions of the pinacolate groups for B pin
2
2
Acknowledgments
[
17]
are known, for example using Grignard reagents or involving
[
5b]
We thank the University of St Andrews.
Lewis acidic BBr3.
In our hands, [(IDip)BH BH (IDip)] (1) was obtained as a pale-
2
2
yellow crystalline material, cf. the closely related yellow dial- Keywords: Aluminum · Diboranes · Group 13 elements ·
[
18]
ane(4) complex [(IDip)AlH AlH (IDip)], and showed the same Hydrides · Carbene ligands
2
2
unit cell parameters from single-crystal X-ray crystallography as
[
8b,8d]
already reported.
matched those reported previously,
The NMR spectroscopic data of 1
[
8b,8d]
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3
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2
4
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3
cially available B pin , plus some additional IDip in the case
2
2
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Received: June 22, 2020
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Communication
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European Journal of Inorganic Chemistry
Diboranes
M. Trose, D. B. Cordes,
A. M. Z. Slawin, A. Stasch* .............. 1–5
A Facile Synthesis of Robinson's
NHC-Stabilised Diborane(4)
Bis(pinacolato)diboron reacts with the to the diborane moiety in a halide-free
NHC aluminium hydride [(IDip)AlH ] in approach and the ligand exchange
3
a one-step protocol to [(IDip)BH B(pin)] process to the diborane(4) complex is
2
and [(IDip)BH BH (IDip)]. [(IDip)AlH ] enabled by the addition of free NHC li-
2
2
3
transfers both hydride and NHC ligands gand.
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