Dibismuthanes in catalysis: From synthesis and characterization to redox behavior towards oxidative cleavage of 1,2-diols

Article abstract of DOI:10.1039/d1ob00367d

A family of aryl dinuclear bismuthane complexes has been successfully synthesized and characterized. The two bismuth centers are bonded to various xanthene-type backbones, which differ in ring-size and flexibility, resulting in complexes with different intramolecular Bi?Bi distances. Moreover, their pentavalent Bi(v) analogues have also been prepared and structurally characterized. Finally, the synergy between bismuth centers in catalysis has been studied by applying dinuclear bismuthanes5-8to the catalytic oxidative cleavage of 1,2-diols. Unfortunately, no synergistic effects were observed and the catalytic activities of dinuclear bismuthanes and triphenylbismuth are comparable.

Full text of DOI:10.1039/d1ob00367d

Organic &
Biomolecular Chemistry
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Dibismuthanes in catalysis: from synthesis and
characterization to redox behavior towards
oxidative cleavage of 1,2-diols†
Cite this: Org. Biomol. Chem., 2021,
19, 4922
Marc Magre, ‡ Jennifer Kuziola,‡ Nils Nöthling and Josep Cornella
*
A family of aryl dinuclear bismuthane complexes has been successfully synthesized and characterized.
The two bismuth centers are bonded to various xanthene-type backbones, which differ in ring-size and
flexibility, resulting in complexes with different intramolecular Bi⋯Bi distances. Moreover, their penta-
valent Bi(V) analogues have also been prepared and structurally characterized. Finally, the synergy
between bismuth centers in catalysis has been studied by applying dinuclear bismuthanes 5–8 to the
catalytic oxidative cleavage of 1,2-diols. Unfortunately, no synergistic effects were observed and the cata-
lytic activities of dinuclear bismuthanes and triphenylbismuth are comparable.
Received 25th February 2021,
Accepted 24th April 2021
DOI: 10.1039/d1ob00367d
rsc.li/obc
Among the known examples, a common denominator is the
high ligand flexibility and the long distances between the two
Introduction
Organobismuthanes are a class of organometallic reagents Bi atoms (Fig. 1A). Yet, access to dibismuthanes bearing a
where the Bi(III) is connected to C atoms. Such compounds more rigid ligand backbone would enable a systematic study of
have been largely studied and a wide variety of examples have the Bi–Bi distance. Herein we report the synthesis, characteriz-
been reported.1 In particular, the triarylbismuthanes subclass ation and structural analysis of a family of new triaryl dinuc-
has attracted the attention of chemists due to their rather high lear bismuthanes, which bear distinct aromatic backbones on
stability and facile preparation. As a result, several monometal- the tether unit. Systematic structural variations on the back-
lic organobismuthanes have been explored as reagents for bone permit an evaluation on how the rigidity of the tether
organic synthesis.2 Compared to the vast literature on mono- influences the Bi–Bi distance (Fig. 1B).
metallic triarylbismuthanes, examples of bi- and dimetallic Bi
Oxidation of the dibismuthanes with SO2Cl2 afforded high
complexes are much rarer and are mainly relegated to the low- yields of dimetallic Bi(V) dichloro compounds, a class of com-
valent counterparts Bi(I) and Bi(II). Dimerization in these com- pounds that remained elusive to date. The structure of both Bi
plexes is highly favored as a result of the unpaired electron in (III) and Bi(V) dimetallic complexes was elucidated by single
Bi(II)3,4 or the stabilization of the highly reactive lone-pair in Bi crystal X-ray diffraction and their behavior in solution analyzed
(I).3,5 On the other hand, examples of dimetallic organobis- by NMR. Due to our growing interest in exploring the redox
muth complexes have been comparatively much less explored, properties of Bi,12 the catalytic redox properties of these com-
and only a handful of examples exist in the literature.6 Indeed, pounds have also been explored in the context of the oxidative
compared to the lighter counterparts in the group 15 (e.g. N7 cleavage of 1,2-diols.
and P8), examples of dimetallic heavy pnictogens are really
limited and mainly focus on As and Sb,9,10 leaving dimetallic
Bi compounds as boutique examples (Fig. 1A).6 This can be
Results and discussion
Synthesis of heteroleptic triaryldibismuthanes 5–8
attributed to the classical ligand redistribution of heteroleptic
triarylbismuthanes,11 which poses severe hurdles in the selec-
tive synthesis of unsymmetrical dinuclear bismuthanes.
Based on the precedents in the synthesis of diphosphines and
the availability of methods to obtain dihalogenated precur-
Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr 45470, Germany.
E-mail: cornella@kofo.mpg.de
sors,13 we set out to explore the heavy analogs of Xantphos,
Homoxantphos, DBFphos and DPEphos, where the P atom has
been replaced by Bi. Capitalizing on the important work from
Hyvl on the efficient synthesis of heteroleptic triarylbis-
muthanes,14 we were able to access the desired dibismuthanes
(5–8) in good yields (Scheme 1). Dinuclear bismuthanes
†Electronic supplementary information (ESI) available: Crystallographic data for
compounds 5–12. CCDC 2063973 (5), 2063975 (6), 2063978 (7), 2063976 (8), 2063977
(9), 2063980 (10), 2063974 (11) and 2063979 (12). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/d1ob00367d
‡These authors contributed equally to this work.
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8, bearing a more flexible backbone, required in situ Grignard
formation followed by transmetalation to Ph2BiOTs
(Scheme 1B). It is important to mention that 5–8 were reluc-
tant to hydrolysis and were purified by flash chromatography
through silica gel. Crystals suitable for X-ray diffraction were
obtained for all the compounds prepared. By analogy to the P
counterparts and based on the already reported nomenclature
for BINABi,6b we named the new dibismuthanes Xantbis,
Homoxantbis, DPEbis and DBFbis.
With dibismuthanes 5–8 in hand, we can structurally
compare the effect of the ring size of the backbone (5 vs. 6 and
7) and the flexibility (5 vs. 8). Dibismuthanes 5–8 are stable
towards air and moisture, both in solution and in solid state.
By means of NMR spectroscopy, we could observe that dinuc-
lear bismuthanes 5–8 present a C2-symmetric backbone in
solution, which suggest both Bi atoms are equivalent.
The solid state structure of dibismuthane 5 (Fig. 2) bears a
close resemblance to the monometallic triphenylbismuth,
both in the Bi–C distances and the trigonal pyramidal geome-
try around the Bi atoms.15 The resemblance in structure
between triphenylbismuth and 5 is attributed to the rigidity of
the backbone in 5, which limits the torsion and results in a
large Bi⋯Bi distance of 5.544(1) Å. This led us to speculate
that both Bi in 5 do not interact with each other, and the
complex can be conceived as two independent Bi centers, both
electronically and structurally. However, as the Bi get closer in
space, this resemblance to the monometallic Ph3Bi slowly
disappears.
For example, the solid state structure of dibismuthane 6
(Fig. 3) shows the expected trigonal pyramidal geometry at
both Bi atoms. However, compared to the geometry of triphe-
nylbismuth,15 dinuclear bismuthane 6 shows a slight distor-
tion comparing both Bi atoms [C2–Bi1–C16: 96.16(19), C2–
Bi1–C22: 93.60(19), C16–Bi1–C22: 93.82(18) compared to C12–
Bi2–C28: 90.36(19), C12–Bi2–C34: 94.80(2), C28–Bi2–C34:
93.06(19)], probably as a result of the steric hindrance between
both centers. A closer look at the xanthene backbone revealed
Fig. 1 (A) Representative examples of dimetallic triarylbismuthanes. (B)
Synthesis of dimetallic Bi(III) and Bi(V) complexes.
Scheme 1 Synthesis of dibismuthanes 5–8.
Fig. 2 Structure of 5. Ellipsoids are drawn at the 50% probability level
and H atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Bi1–C2: 2.262(7), Bi1–C13: 2.244(6), Bi1–C19: 2.253(7), Bi2–
C11: 2.252(7), Bi2–C25: 2.243(7), Bi2–C31: 2.248(7), C2–Bi1–C13: 93.7
(2), C2–Bi1–C19: 96.3(3), C13–Bi1–C19: 93.8(2), C11–Bi2–C25: 95.8(2),
bearing a rigid xanthene-like backbone (5–7) could be isolated
in good yields as white solids via an organozinc-transmetala-
tion protocol (Scheme 1A). On the other hand, dibismuthane C11–Bi2–C31: 94.0(2), C25–Bi2–C31: 96.4(2); Bi1⋯Bi2: 5.544(1) Å.
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benzoxepine derivative [C5–C7–C8: 113.54°, C9–C8–C7:
118.20°].17 We attribute this high structural torsion to the
steric constraint between both bismuth centers, resulting in a
longer C1–C12 distance [4.324(1) Å of 7 vs. 4.039 Å of benzoxe-
pine derivative]. However, the most remarkable feature in 7 is
a Bi⋯Bi distance of 3.807(1) Å: much below the sum of the van
der Waal radius (4.14 Å) and among the shortest distances
between two Bi(III) atoms reported.18 Additionally, examples of
short Bi⋯Bi distances between two Bi(III) atoms can be found
in an intermolecular fashion in some diarylbismuth–(III)
halides [3.965(4)18a and 3.973(9) Å18b] and also in BiMe3
[3.899(1) Å].18c
The increasing differences when moving from 5 to 7 are
ascribed mainly due to the steric repulsion between the Bi
atoms, which translates into rather unique torsions of the
shared ligand backbones.
Fig. 3 Structure of 6. Ellipsoids are drawn at the 50% probability level
and H atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Bi1–C2: 2.268(6), Bi1–C16: 2.246(5), Bi1–C22: 2.262(5), Bi2–
C12: 2.255(5), Bi2–C28: 2.242(5), Bi2–C34: 2.252(5), C2–Bi1–C16: 96.16
(19), C2–Bi1–C22: 93.60(19), C16–Bi1–C22: 93.82(18), C12–Bi2–C28:
90.36(19), C12–Bi2–C34: 94.80(2), C28–Bi2–C34: 93.06(19); Bi1⋯Bi2:
4.187(1) Å.
Finally, the structure of dibismuthane 8 (Fig. 5) shows that
when the ether backbone is not tethered, the Bi–C remain
unaltered and are comparable to those from dibismuthanes
5–7. In the solid state structure the two aryl groups from the
ligand backbone are placed almost perpendicular to each
other [angle between phenyl planes 72.05(1)°], probably due to
the steric hindrance of the –BiPh2 moieties. In fact, this struc-
tural behavior results in a Bi⋯Bi distance of 5.430(1) Å.
Similarly as observed with 5 and 6 (compared to their corres-
ponding P-analogues), the Bi⋯Bi distance in 8 is larger than
its P-analog DPEphos (4.876 Å).19
the characteristic bending, comparable to that observed for
the phosphine analogue Xantphos.16 The structure of dibis-
muthane 6 shows a distance between both bismuth atoms of
4.187(1) Å, longer than its phosphorus homonym (4.045(1)
Å).16b In terms of Bi–C distances, 6 compares well with its
monometallic analogue, showing that the presence of two
bismuth units does not have a detrimental effect on the Bi–C
distances.
Synthesis of pentavalent dibismuth(V) 9–12
After the successful synthesis and analysis in the solid state of
a family of structurally different dibismuthanes 5–8, we
decided to study their reactivity towards oxidation. Whereas
the oxidation of triarylbismuthanes has been widely studied
Differences from the monometallic triarylbismuth are more
exacerbated in dinuclear bismuthane 7 (Fig. 4).
The presence of the BiPh2-moieties in 7 influences dramati-
cally the backbone, increasing its torsion [C5–C7–C8: 108.8
(3)°, C9–C8–C7: 115.0(3)° of 7] compared to non-substituted
Fig. 4 Structure of 7. Ellipsoids are drawn at the 50% probability level
and H atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Bi1–C1: 2.251(3), Bi1–C15: 2.255(3), Bi1–C21: 2.247(4), Bi2–
C11: 2.252(3), Bi2–C27: 2.259(4), Bi2–C33: 2.262(3), C1–Bi1–C15: 92.32
(12), C1–Bi1–C21: 97.62(12), C15–Bi1–C21: 94.52(13), C11–Bi2–C27:
92.72(12), C11–Bi2–C33: 93.21(12), C27–Bi2–C33: 94.34(12), C5–C7–
C8: 108.8(3), C9–C8–C7: 115.0(3); Bi1⋯Bi2: 3.807(1) Å.
Fig. 5 Structure of 8. Ellipsoids are drawn at the 50% probability level
and H atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Bi1–C1: 2.2624(17), Bi1–C13: 2.2494(19), Bi1–C19: 2.2605(18),
Bi2–C7: 2.2426(18), Bi2–C25: 2.2491(18), Bi2–C31: 2.2525(19), C1–Bi1–
C13: 97.61(6), C1–Bi1–C19: 92.13(6), C13–Bi1–C19: 92.20(7), C7–Bi2–
C25: 94.66(7), C7–Bi2–C31: 97.14(7), C25–Bi2–C31: 91.50(7); Bi1⋯Bi2:
5.430(1) Å.
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using strong oxidants,20,21 it is worth mentioning that oxi- position and the aromatic rings in equatorial, in agreement
dation of dinuclear bismuthanes have not been reported. with the apicophilicity of electronegative ligands.22 The struc-
Then, we decided to oxidize 5–8 with the commonly utilized ture of pentavalent dibismuthane 9 bears a close resemblance
SO2Cl2. In all cases, smooth conversion of all dibismuthanes to Ph3BiCl2.23 Due to the larger distance between both bismuth
to the parent pentavalent dimetallic 9–12 were obtained centers (Bi⋯Bi: 6.290(1) Å) the steric pressure between them is
(Scheme 2). Due to the high yields obtained, evaporation of highly diminished. This structural feature is reflected on the
the solvent led to pure Bi(V) compounds, whose structures distances between the Bi and the C in the backbone [Bi1–C2:
were elucidated by single crystal X-ray analysis.
2.184(2) Å and Bi2–C11: 2.186(3) Å] showing minimal differ-
As depicted in Fig. 6, each Bi atom in 9 adopts a trigonal ences among them and to Ph3BiCl2 (Fig. 6).
bipyramidal geometry with the two chloride ligands in apical
Compared to Ph3BiCl2,23 the geometry of complex 10
(Fig. 7) is slightly more distorted, probably due the steric hin-
drance between both Bi units. This geometry distortion is
more evident when comparing both Bi centers. Whereas Bi1
atom adopts a geometry similar to the Ph3BiCl2 [C12–Bi1–
C34A: 137.74(16)°, C12–Bi1–C28: 116.05(10)°, C34A–Bi1–28:
106.19(16)°], the geometry of Bi2 atom presents a larger degree
of distorsion [C2–Bi2–C16: 155.63(9)°, C2–Bi2–C22: 102.11(9)°,
C16–Bi2–C22: 102.17(9)°]. The large angle observed between
C2–Bi2–C16 is the consequence of the effort of Bi2 to accom-
modate the incoming Cl3 from the neighboring Bi1. The steric
hindrance between both Bi centers is also manifested in the
distance between the Bi and the carbon at the backbone. For
example, whereas Bi1–C12 distance is 2.190(2) Å, the distance
of Bi2–C2 is elongated up to 2.212(3) Å. This repulsion is also
reflected in the torsion between the Bi2 and the ligand back-
bone. For example, the C1–C2–Bi2 angle is 128.98(18)°, com-
pared to the C13–C12–Bi1 angle of 124.27(19)°. Hence, the Bi2-
Scheme 2 Synthesis of pentavalent dibismuth(V) compounds 9–12.
Fig. 7 Structure of 10. Ellipsoids are drawn at the 50% probability level
and disordered parts, solvent and H atoms are omitted for clarity.
Selected bond lengths (Å) and angles (°): Bi2–C2: 2.212(3), Bi2–C16:
2.223(2), Bi2–C22: 2.212(2), Bi2–Cl2: 2.5702(7), Bi2–Cl1: 2.5977(6), Bi1–
C12: 2.190(2), Bi1–C34A: 2.189(5), Bi1–C28: 2.214(3), Bi1–Cl4: 2.5825(7),
Bi1–Cl3: 2.5828(7), C2–Bi2–C16: 155.63(9), C2–Bi2–C22: 102.11(9),
C16–Bi2–C22: 102.17(9), Cl1–Bi2–Cl2; 176.01(2), C2–Bi2–Cl2: 89.03(7),
C2–Bi2–Cl1: 87.53(7), C16–Bi2–Cl2: 92.14(7), C16–Bi2–Cl1: 90.16(7),
C22–Bi2–Cl2: 92.19(7), C22–Bi2–Cl1: 90.51(7), C12–Bi1–C34A: 137.74
(16), C12–Bi1–C28: 116.05(10), C34A –Bi1–C28: 106.19(16), Cl3–Bi1–
Cl4: 172.07(2), C12–Bi1–Cl4: 85.19(7), C12–Bi1–Cl3: 87.13(7), C34A
–Bi1–Cl4: 93.92(16), C34A –Bi1–Cl3: 90.43(15), C28–Bi1–Cl4: 93.43(7),
C28–Bi1–Cl3: 91.72(7); Bi1⋯Bi2: 5.031(1) Å.
Fig. 6 Structure of 9. Ellipsoids are drawn at the 50% probability level
and H atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Bi1–C2: 2.184(2), Bi1–C13: 2.213(2), Bi1–C19: 2.213(2), Bi1–
Cl1: 2.5816(6), Bi1–Cl2: 2.5827(6), Bi2–C11: 2.186(3), Bi2–C25: 2.197(3),
Bi2–C31: 2.194(2), Bi2–Cl3: 2.5496(6), Bi2–Cl4: 2.5769(6), C2–Bi1–C13:
130.24(9), C2–Bi1–C19: 113.08(9), C13–Bi1–C19: 116.51(8), Cl1–Bi1–
Cl2; 177.46(2), C2–Bi1–Cl1: 86.64(7), C2–Bi1–Cl2: 91.17(7), C13–Bi1–
Cl1: 89.26(7), C13–Bi1–Cl2: 91.19(7), C19–Bi1–Cl1: 90.29(7), C19–Bi1–
Cl2: 91.75(7), C11–Bi2–C25: 112.86(9), C11–Bi2–C31: 128.63(9), C25–
Bi2–C31: 118.32(9), Cl3–Bi2–Cl4: 174.32(2), C11–Bi2–Cl3: 92.41(7),
C11–Bi2–Cl4: 87.53(7), C25–Bi2–Cl3: 93.52(7), C25–Bi2–Cl4: 91.74(7),
C31–Bi2–Cl3: 88.73(6), C31–Bi2–Cl4: 86.88(6); Bi1⋯Bi2: 6.290(1) Å.
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center is out-of-plane due to the steric interaction with the tuated, reflected in the angle between the planes of the benzo
neighboring Bi1. The high repulsion and steric hindrance in groups (in 11 is 46.39° and in 7 is 56.09°). This steric inter-
10 results in the increase of the Bi⋯Bi distance (5.031(1) Å vs. action between both bismuth centers is also shown in the dis-
4.187(1) Å from the trivalent analogue 6). Further insights into tance between the Bi and the carbon at the backbone. For
the repulsion and torsion between both Bi centers in 10 can be example, although attached to the same aryl unit, the Bi1–C2
obtained from the VT-NMR analysis (from 25 °C to −90 °C, see distance is 2.192(2) Å whereas the distance of Bi2–C13 is
ESI† for details). In solution at ambient temperature, the flux- elongated up to 2.218(3) Å. This Bi–C elongation is also
ionality between both bismuth centers is apparent, as 10 observed in its 6-membered ring analogue 10 (Fig. 7) but not
reveals itself as a C2-symmetric compound, with both Bi being in the more rigid 5-member ring 9 (Fig. 6). The high repulsion
equivalent. However, at lower temperatures, the two Bi atoms and steric hindrance in 11 results in the increase of the Bi⋯Bi
are no longer equivalent, consistent with the solid-state ana- distance [5.195(1) Å vs. 3.807(1) Å from the trivalent analogue
lysis of 10. The solution behavior of 10 can be explained by a 7]. Due to the higher degree of backbone flexibility in 11 com-
chloride-induced steric repulsion between the two TBP pared to 10, the Bi⋯Bi distance in 11 is slightly longer than in
bismuth centers (Scheme 3). At low temperature, the Cl- 10, an opposite structural feature observed in the less sterically
induced structural strain can be observed.
congested trivalent analogues (7 and 6).
As represented in Fig. 8, the presence of bulky BiPh2Cl2-
The structure of pentavalent dibismuthane 12 (Fig. 9)
moieties in 11 has an influence on the backbone, as observed shows similarities to compound 9 and Ph3BiCl2.23 Therefore,
also in its analogue Bi(III) 7 (Fig. 4). In agreement with the each bismuth center adopts a slightly distorted trigonal bipyra-
increase of steric hindrance of the Bi-centers (–BiPh2Cl2 in 11 midal geometry with the Cl in the apical positons. Similar to
vs. –BiPh2 7) the torsion in the ligand backbone is more accen- the parent Bi(III) (8), the flexibility of the ligand backbone
allows to diminish the steric constraints between both
bismuth centers. Indeed, the two aryl groups from the back-
bone are placed almost perpendicular to each other [86.28
(1)°]. Nevertheless, despite the free rotation of the ligand, the
distance between both bismuth atoms is 6.288(1) Å, very
similar to that found for 9, which contains a rigid 5-member
ring scaffold in the backbone.
Catalytic oxidative cleavage of 1,2-diols
Scheme 3 Proposed structural distortion observed in 10 at low
temperature.
We have seen that dinuclear bismuthanes 5–8 show excellent
air and moisture stability and high reactivity with strong oxi-
Fig. 8 Structure of 11. Ellipsoids are drawn at the 50% probability level
and solvent and H atoms are omitted for clarity. Selected bond lengths
(Å) and angles (°): Bi1–C2: 2.192(2), Bi1–C15: 2.200(2), Bi1–C21: 2.212(2),
Bi1–Cl1: 2.5972(6), Bi1–Cl2: 2.5962(6), Bi2–C13: 2.218(3), Bi2–C27:
2.216(3), Bi2–C33: 2.206(3), Bi2–Cl3: 2.5881(7), Bi2–Cl4: 2.5856(7), C2–
Bi1–C15: 119.38(9), C2–Bi1–C21: 127.71(9), C15–Bi1–C21: 112.80(9),
Cl1–Bi1–Cl2: 170.725(19), C2–Bi1–Cl1: 88.91(7), C2–Bi1–Cl2: 85.48(7),
C15–Bi1–Cl1: 88.26(6), C15–Bi1–Cl2: 88.06(6), C21–Bi1–Cl1: 96.08(7),
C21–Bi1–Cl2: 93.19(7), C13–Bi2–C27: 138.46(10), C13–Bi2–C33: 110.94
(10), C27–Bi2–C33: 110.60(11), Cl3–Bi2–Cl4: 175.59(2), C13–Bi2–Cl3:
91.39(7), C13–Bi2–Cl4: 89.29(7), C27–Bi2–Cl3: 90.69(9), C27–Bi2–Cl4:
91.67(9), C33–Bi2–Cl3: 87.62(7), C33–Bi2–Cl4: 88.07(7), C9–C8–C7:
109.6(2), C6–C7–C8: 110.1(2); Bi1⋯Bi2: 5.195(1) Å.
Fig. 9 Structure of 12. Ellipsoids are drawn at the 50% probability level
and H atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Bi1–C1: 2.184(3), Bi1–C13: 2.199(3), Bi1–C19: 2.204(3), Bi1–
Cl1: 2.5862(8), Bi1–Cl2: 2.5892(8), Bi2–C7: 2.189(3), Bi2–C25: 2.205(3),
Bi2–C31: 2.214(3), Bi2–Cl3: 2.5677(8), Bi2–Cl4: 2.6191(8), C1–Bi1–C13:
130.36(11), C1–Bi1–C19: 109.56(12), C13–Bi1–C19: 120.09(12), Cl1–Bi1–
Cl2: 175.42(3), C1–Bi1–Cl1: 86.12(8), C1–Bi1–Cl2: 90.01(8), C13–Bi1–
Cl1: 92.04(8), C13–Bi1–Cl2: 88.59(8), C19–Bi1–Cl1: 92.47(9), C19–Bi1–
Cl2: 91.13(9), C7–Bi2–C25: 128.79(12), C7–Bi2–C31: 114.23(12), C25–
Bi2–C31: 116.77(12), Cl3–Bi2–Cl4: 176.06(3), C7–Bi2–Cl3: 93.06(9), C7–
Bi2–Cl4: 83.75(9), C25–Bi2–Cl3: 89.99(9), C25–Bi2–Cl4: 90.22(9), C31–
Bi2–Cl3: 91.45(8), C31–Bi2–Cl4: 91.96(9); Bi1⋯Bi2: 6.288(1) Å.
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Table 2 Dibismuthane 8-catalyzed oxidative cleavage of 1,2-diolsa
dants such as SO2Cl2, affording the corresponding Bi(V) com-
plexes 9–12 in almost quantitative yields. Barton and co-
workers reported one of the first examples of bismuth redox
catalysis, showing that catalytic amounts of triphenylbismuth
together with NBS were able to catalyze the oxidative cleavage
of 1,2-diols in excellent yields.24 The application of the rare
examples of dibismuthanes reported has been relegated
mainly to their use as ligands for transition metal complexes.6
To the best of our knowledge, there is no precedent on the
catalytic redox behavior of such dimetallic compounds.
Consequently, we decided to test our family of dibismuthanes
5–8 in the known oxidative cleavage of 1,2-diols.25 We began
our investigations by screening dibismuthanes 5–8 towards the
oxidative cleavage of 1,2-diphenylethane-1,2-diol 13 using the
reaction conditions already developed by Barton and co-
workers (Table 1). It was immediately revealing that the ligand
backbone had an important effect on the catalytic perform-
ance. Whereas the catalytic activity of dibismuthanes 5–6 and
8 compared well with BiPh3 (Table 1, entry 1 vs. entries 2–3
and 5), dinuclear bismuthane 7 showed much lower activity.
This was attributed to the close distance between both
bismuth atoms (Fig. 4), increasing the steric hindrance and
preventing catalytic activity.
Entry
1
1,2-Diol
Product
Yieldb (%)
97 (88)
2c
3c
73 (68)
70 (66)
4
5
98 (91)
98 (94)
Kinetic experiments showed that dibismuthane 8, bearing
the most flexible ligand, outperformed the remaining dinuc-
lear bismuthanes, performing similarly to BiPh3 (see ESI† for
details). Among dibismuthanes with rigid backbones (5–7) we
found out that 5 performed slightly better than 6 (Bi⋯Bi dis-
tance of 5.544(1) and 4.187(1) Å, respectively) and it was
superior to 7 (Bi⋯Bi distance of 3.807(1) Å). This trend is in
perfect agreement with a lower catalytic activity when the
dinuclear bismuthane bears a rigid backbone and a short
a Reaction conditions: 1,2-diols (0.12 mmol), 8 (2 mol%), NBS (1.2
equiv.), K2CO3 (5 equiv.) in 1.2 mL of CD3CN [0.1 M] at 23 °C for
30 min. b Yields were determined by 1H NMR using mesitylene as
internal standard. Isolated yields in parenthesis. c Reaction time of
1 hour.
Bi⋯Bi distance, two features that would increase the steric hin- diol 13 underwent oxidative cleavage quantitatively (Table 2,
drance in the bismuth active centers. With the optimized reac- entry 1), more sterically hindered 1,1,2,2-tetraphenylethane-
tion conditions in hand, we decided to study the ability of 1,2-diol 15 required longer reaction time. Similarly, decane-
dibismuthane 8 towards the catalytic oxidative cleavage of 1,2-diol 17 required longer time to be converted to nonanal 18
different 1,2-diols (Table 2). Whereas 1,2-diphenylethane-1,2- (Table 2, entry 3) compared to 1-phenylethane-1,2-diol 19,
which converted to the desired benzaldehyde 14 quantitatively
in 30 min.
Table 1 Catalyst screening for the oxidative cleavage of 13a
Application of the Bi-catalyzed protocol to the naturally
occurring (1S,2S,3R,5S)-(+)-2,3-pinandiol (20) led to the quanti-
tative conversion towards the dicarbonyl compound 21. This
last example is worth commenting further. Whereas most of
the examples have freedom of rotation along the C–C bond,
pinanediol has a rigid backbone, and therefore, the confor-
mation of the diol is locked. However, this does not pose a
Entry
Bi-catalyst
Time (min)
Yieldb (%)
problem in the C–C scission. This is in agreement with pre-
vious observations by Barton with a cis-decaline diol system.24
In cases where NBS might be problematic in certain synthetic
endeavors, the reaction can also be carried out with stoichio-
metric amounts of the corresponding Ar3Bi(V)Cl2, obtaining
virtually quantitative yields of the C–C cleavage (see ESI† for
details). We believe that this reactivity could have interesting
applications in the realm of natural product synthesis and
further studies along these lines are currently under investi-
gation in our laboratory.
1
2
3
4
5
6
7
BiPh3 (10 mol%)
5 (5 mol%)
6 (5 mol%)
7 (5 mol%)
8 (5 mol%)
8 (2 mol%)
—
10
10
10
10
10
30
30
>95
>95
>95
55
>95
>95
<10
a Reaction conditions: 13 (0.12 mmol), 5–8 (5 or 2 mol%), NBS (1.2
equiv.), K2CO3 (5 equiv.) in 1.2 mL of CD3CN [0.1 M] at 23 °C for 10 or
30 min. b Yields were determined by 1H NMR using mesitylene as
internal standard.
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3 For a review on bimetallic Bi(I) and Bi(II), see: H. J. Breunig,
Z. Anorg. Allg. Chem., 2005, 631, 621–631.
Conclusion
In summary, a variety of structurally different dinuclear bis-
muthanes have been synthesized and structurally character-
ized by single crystal X-ray diffraction. Fine-tuning of the
ligand scaffold permitted a systematic evaluation of the influ-
ence of the backbone on the bismuth geometry and more
importantly, on the Bi⋯Bi distance. Among these compounds,
7 revealed itself as a dibismuthane with an extremely short
intramolecular Bi⋯Bi distance. Moreover, the oxidation of
dibismuthanes 5–8 has been accomplished using SO2Cl2, iso-
lating the corresponding pentavalent dibismuth tetrachlorides
9–12 in excellent yields. Studies on the catalytic redox pro-
perties of 5–8 revealed that the ligand backbone has a dra-
matic effect on the catalytic activity towards the oxidative clea-
vage of 1,2-diols. In this regard, dinuclear bismuthane 8 (with
a flexible backbone and a long Bi⋯Bi distance) does not
surpass the catalytic activity of triphenylbismuth. Further
studies on the possible synergistic effects of two Bi atoms in
the same complex towards catalytic redox processes are cur-
rently ongoing.
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The authors declare no conflict of interest.
7 J.-C. Kizirian, Chem. Rev., 2008, 108, 140–205.
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Acknowledgements
Financial support for this work was provided by Max-Planck-
Gesellschaft, Max-Planck-Institut für Kohlenforschung and
Fonds der Chemischen Industrie (FCI-VCI). This project has
received funding from European Union’s Horizon 2020
research and innovation programme under Agreement No.
850496 (ERC Starting Grant, J.C.). We thank Prof. Dr A.
Fürstner for in-sightful discussions and generous support. We
thank MS, GC and X-ray departments of Max-Planck-Institut
für Kohlenforschung for analytic support. We thank Dr R.
Goddard for X-ray crystallographic analysis. Open Access
funding provided by the Max Planck Society.
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