Heterocyclic bismuth(III) compounds with transannular N→Bi interactions as catalysts for the oxidation of thiophenol to diphenyldisulfide

Article abstract of DOI:10.1039/c7cy00521k

The reactions between the diorganobismuth(iii) bromides [RCH₂N(CH₂C₆H₄)₂]BiBr [R = C₆H₅ (1), C₆H₅CH₂ (2)] and appropriate silver salts resulted in new diorganobismuth(iii) compounds of the general formula [RCH₂N(CH₂C₆H₄)₂]BiX [R = C₆H₅, X = ONO₂ (3), OSO₂CF₃ (4), OSO₂C₆H₄ (CHCH₂)-4 (5); R = C₆H₅CH₂, X = ONO₂ (6)], based on a butterfly-like tetrahydro-dibenzo[c,f][1,5]azabismocine heterocyclic framework. The new species were structurally characterized in solution by ¹H, ¹³C{H} and ¹⁹F{H} NMR and in the solid state by IR spectroscopy and single-crystal X-ray diffraction. The nitrogen atom is intramolecularly coordinated to bismuth, thus resulting in hypercoordinated species of type 12-Bi-5 (3 and 6) and 10-Bi-4 (4 and 5). In addition, compound 4 shows bismuth?π arene and compounds 3 and 6 bismuth?oxygen intermolecular interactions, thus leading to dimers in the solid state. These compounds were investigated as catalysts for the oxidation of thiophenol to diphenyl disulfide by using air as an oxidizing agent, both in cyclohexane and in an ionic liquid (1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide), at temperatures below 100 °C, affording high reaction rates (TON 34.8, with 100% conversion after 5 h) and a total selectivity to the targeted product.

Full text of DOI:10.1039/c7cy00521k

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Catalysis Science & Technology
ARTICLE
Heterocyclic bismuth(III) compounds with transannular N→Bi
interactions as catalysts for the oxidation of thiophenol to
diphenyldisulfide
Received 00th January 20xx,
Accepted 00th January 20xx
Ana M. Toma,a Ciprian I. Raț,a Octavian D. Pavel,b Christopher Hardacre,c,d Tobias Rüffer,e Heinrich
Lang,e Michael Mehring,f Anca Silvestru,a,* and Vasile I. Pârvulescub,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The reactions between the diorganobismuth(III) bromides [RCH2N(CH2C6H4)2]BiBr [R = C6H5 (1), C6H5CH2 (2)] and
appropriate silver salts resulted in new diorganobismuth(III) compounds of the general formula [RCH2N(CH2C6H4)2]BiX [R =
C6H5, X = ONO2 (3), OSO2CF3 (4), OSO2C6H4 (CHCH2)-4 (5); R = C6H5CH2, X = ONO2 (6)], based on a butterfly-like tetrahydro-
dibenzo[c,f][1,5]azabismocine heterocyclic framework. The new species were structurally characterized in solution by 1H,
13C{H} and 19F{H} NMR and in solid state by IR spectroscopy and single-crystal X-ray diffraction. The nitrogen atom is
intramolecularly coordinated to bismuth, thus resulting in hypercoordinated species of type 12-Bi-5 (3 and 6) and 10-Bi-4
(4 and 5). In addition, compound 4 shows bismuth∙∙∙ π arene and compounds 3 and 6 bismuth∙∙∙oxygen intermolecular
interactions, thus leading to dimers in solid state. These compounds were investigated as catalysts for the oxidation of
thiophenol to diphenyl disulfide by using air as oxidizing agent, both in cyclohexane and in ionic liquid (1-butyl-1-
methylpyrrolidinium bis(trifluoromethanesulfonyl)imide), at temperatures below 100 °C, affording high reaction rates
(TON 34.8, with 100% conversion after 5 h) and a total selectivity into the targeted product.
appropriate systems consisting of the oxidant, solvent and
catalyst, in order to increase the selectivity, i.e. to avoid the
over-oxidation to by-products such as sulfoxides,
Introduction
The conversion of thiols to disulfides has attracted a
continuously increased interest in recent years1 due to the
importance of disulfides in biology, mainly based on their
antioxidant properties and their interconversion reactions in
living systems,2 fine chemicals synthesis, e.g. pharmaceuticals
and pesticides,3 as well as in refinery processes and rubber
vulcanization.4 Various methods, including catalysis
(homogeneous,5 heterogeneous6 or photo-catalysis7), or non-
catalytic oxidation procedures, e.g. with sodium periodate in a
solid state reaction,8 or benzyltriphenylphosphonium
peroxymonosulfate under reflux in acetonitrile9, as well as
electrochemical techniques10 were employed (Table 1).
thiosulfinates, thiosulfonates or sulfonic acids, but also the
contamination with heavy metals or the toxicity, or to reduce
the reaction time and the effective costs of the process.7,11
Thus, the replacement of halogens,12 transition metal
containing
systems,13
ammonium
persulfate14
or
15
Bu3SnOMe/FeCl3
with air, molecular oxygen,16 H2O2,17
functionalized ionic liquids [HSO3N(C2H4OSO3H)3]11a or even
nitric oxide18 is highly desired. Among these, of special interest
is the air oxidation which is a sustainable, cheap, easy to
handle and an environmentally friendly process.19
Generally, chlorinated solvents (dichloromethane),20
methanol, water, water–alcohol mixtures, or acetonitrile were
used as a reaction medium in homogeneous processes.
Different catalysts based on transition metals (Fe, Co, Pd, Pt,
Au, Mo, Ru, Rh, etc.), despite their high selectivity and short
reaction times,6a,21 are not always appropriate for
pharmaceutical or biologically active products,22 mainly due to
the toxicity of heavy metals. Therefore, heterogeneous
systems using either nanoparticles of metal oxides (Fe2O3,11a
Constant efforts were made for the identification of
7
NiFe2O4,23 Bi2WO6 ), graphite oxide,6b Fe(BTC) (BTC: 1,3,5-
benzenetricarboxylate)6a or iron phthalocyanine immobilized
on graphene oxide24 are preferred. Enzymes25 and biomimetic
iron(III)-tetraphenylporphyrin26 were also investigated as
selective catalysts in these reactions.
These state of the art examples demonstrate that there is
still need for the optimization of the procedures for the
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Journal Name
oxidation of thiols to disulfides and for readily available, safe, Table 1. Performances of catalysts reported for the oxidation
stable and cheap catalysts. In this respect bismuth in the of thiols.
Entry
Catalysts
Yiel
d, %
99
TON
0.02
8.2
198
Source
readily available, non-toxic and easy to handle oxidation states
Bi(0) or Bi(III) has already demonstrated its capability as
catalyst for a large variety of organic transformations including
oxidation, reduction, C−C and C−E (E = O, N, S, halogen) bond
formation.27 Aromatic and aliphatic thiols were oxidized to
disulfides with Bi(NO3)3 in high yields also,28 but only few
reports indicate organobismuth(III) compounds as catalysts,29
due to a supposed low stability of the C−Bi bond.27d
Bulky ligands30 or organic groups bearing donor atoms
capable for intramolecular coordination31 were used to
stabilize main group metals, including bismuth in low oxidation
1
2
Al2O3
RSC Adv., 2013,
3
, 10680.
RSC Adv., 2016,
6, 95753.
Silica
Supported iron oxide
NiO/SiO2 nanocluster
95
99
Trans. Mat. Res.
Soc. Japan 2012,
37, 177.
Org.
Chem., 2011,
3
Graphite oxide
100
0.3
Biomol.
9,
7292.
CuI
90
0.09
RSC Adv., 2013,
, 21369.
Synthesis 1999,
, 49.
3
states.
Neutral
or
cationic
species
hypercoordinated
based on
4
5
6
Fe (III)/NaI
100
10
2360
6.7
diorganobismuth(III)
a
1
Cobalt–iron magnetic 100
composites (Co0.5%)
Fe(BTC) BTC:1,3,5-
benzenetricarboxylate
Appl. Catal.,
2011, 392, 151.
Chem.
Commun., 2010,
46, 6476.
Chem.
A
tetrahydrodibenzo[c,f][1,5]thiabismocine framework were
reported as active species in Mannich reactions,32 or for the
stereoselective synthesis of (E)-α,β-unsaturated ketones,33
98
while
tetrahydrodibenzo[c,f][1,5]azabismocine framework, e.g. ionic
[tBuN(CH2C6H4)2Bi]+[B(C6F5)4]−
and neutral
compounds
based
on
a
7
Fe(BTC)
Cu3(BTC)2
Al(OH)(BDC)
Fe(NO3)3∙9H2O
1,3,2-oxazaphosphole
97
27
4
56
94
5.7
1.2
0.18
3.1
Commun., 2012,
48, 11275.
[C6H11N(CH2C6H4)2]BiL (L = BF4, OSO2C8F17) were used in
allylation reactions of aldehydes and Mukaiyama aldol
reactions.34
8
9
86
RSC Adv., 2015,
5
, 45983.
Diazenecarboxamide
100
98
2
J. Chem. Soc.,
Perkin Trans. 1,
1998, 3917.
Catal.Commun.,
2007, 8, 697.
A special attention was paid to the environmental impact
of the solvents used in the catalytic processes35,36 and different
alternatives to the organic solvents were proposed, including
ionic liquids (ILs).37
10
Manganese(III) Schiff-
base complex
20
Besides other advantages, ILs contribute to the
stabilization and the reuse of the catalyst and allow a good
separation of the organic product by distillation. Selective Results and discussion
catalytic oxidations, including the S−S coupling of thiols to
Preparation and spectroscopic characterization
disulfides took the advantage of using ILs.37a,38 They have also
been used for the S−S coupling of thiols to disulfides in
combination with various oxidizing agents in the following
Diorganobismuth(III) compounds of the general formula
[RCH2N(CH2C6H4)2]BiX [R = C6H5, C6H5CH2, X = ONO2, OSO2CF3,
OSO2C6H4(CHCH2)-4] based on a butterfly-like tetrahydro-
dibenzo[c,f][1,5]azabismocine heterocyclic framework were
prepared by ligand exchange reactions between the
systems: [bmim][methylselenite] with air,39 [bmim]Br with
42
K2S2O8,40 [bmim][BF4]41 and [BPy]BF4
with H2O2 or
WO(O2)2∙Phen∙H2O,43 ([Hnmp]BF4] with H2O2,44 [(CH3)N(n-
C8H17)3]2Mo2O11 and [(CH3)N(n-C8H17)3]2W2O11 with H2O2,45
diorganobismuth(III) bromides
1 and 2 and the appropriate
silver salt in a 1:1 molar ratio, as depicted in Scheme 1.
([C4mim]Cl/2ZnCl2
and
[C4mim]Cl/ZnCl2),
[CH2COOHmim][HSO4], [SO3H–C4mim][HSO4], [Hmim][HSO4]
and [C4mim][HSO4] with H2O2.46
We have recently reported diorganobismuth(III) halides of
the type [RCH2N(CH2C6H4)2]BiX (R = C6H5, C6H5CH2, CH3OCH2, X
= halogen).47 As a continuation of our studies regarding the
compounds
based
on
a
tetrahydrodibenzo[c,f][1,5]azabismocine
heterocyclic
framework we used the bromides RCH2N(CH2C6H4)2]BiBr [R =
C6H5 ), C6H5CH2 )] to prepare the new species
[RCH2N(CH2C6H4)2]BiX [R = C6H5, X = ONO2
OSO2C6H4(CHCH2)-4 ); C6H5CH2,
Compounds
oxidation with air of thiophenol to diphenyl disulfide in both
cyclohexane and IL (1-butyl-1-methylpyrrolidinium
(
1
(2
(
3
), OSO2CF3
(
4
),
Scheme 1. Synthesis of diorganoBi(III) complexes
3 – 6.
(
5
R
=
X
=
ONO2
(
6
)].
1
– 6 were investigated as catalysts in the
The bismuth(III) compounds
3 – 6 are microcrystalline
solids, soluble in common organic solvents. The H, 13C{H} and
19F{H} NMR spectra are consistent with the presence of only
one species in solution. The non-equivalent CH2NCH2 protons
in the heterocyclic skeleton appear as an AB spin system in the
1
bis(trifluoromethanesulfonyl)imide).
2 | J. Name., 2012, 00, 1-3
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1H NMR spectra, similarly with the situation found for the
ARTICLE
starting diorganobismuth(III) bromides
The IR spectra of the compounds
1
and 2 47
and 6 show strong,
.
3
broad bands around 1445 cm-1, characteristic for the νas(NO2)
stretching vibration,48 thus suggesting an unisobidentate
−
coordination of the anionic NO3 ligand, while the spectra of
the compounds
4 and 5 show strong bands in the ranges
1240−1295, 1160−1170 and 1010−1020 cm−1, which were
assigned to the νas(SO2), νs(SO2) and ν(SO) vibrations,
respectively.49
Single-crystal X-ray diffraction studies
Compounds
respect to the diorganobismuth(III) fragment, excepting that
the crystal of contains two independent molecules (3a and
3 – 6 have similar molecular structures, with
Fig. 1. Thermal ellipsoids representation at 30% probability in a dimer of
S1,S2-3a and R1,R2-3b isomers (hydrogen atoms are omitted for clarity).
3
3b) in the asymmetric unit. Several differences appear in the
coordination behavior of the oxo-ligands and in the types of
intermolecular associations evidenced in the crystal structures.
Thermal ellipsoids representations of the molecular structures
In 3b the benzyl group in the pendant arm is twisted in
such a position that a Bi∙∙∙ π arene interaction of 3.895 Å
between Bi2 and the phenyl ring of the pendant arm might
also be considered.54
of [
interatomic distances and bond angles for the new compounds
, as well as for the starting bromides and
47 are given in
the supporting information (see ESI, Table S1).
Strong transannular N→Bi interacꢀons (range 2.402(3) -
2.479(5) Å, are found in compounds and they are of a
3]2, [4]2, 5 and [6] are depicted in Fig. 1 – 4. Relevant
2
‾
In compounds 4 and 5 the CF3SO3 and the 4-(CH2CH)C6H4SO3‾
3
–
6
1
2
ligands, respectively, are monodentate coordinated to bismuth by
oxygen and these compounds can be described as
hypercoordinated 10-Bi-4 species with
a distorted see-saw
3
– 6
coordination geometry about bismuth [O1−Bi1−N1 147.27(13)° in 4
and 144.79(14)° in 5].
similar magnitude with those observed in the related
[RN(CH2C6H4)2]BiX (R = Ph, Cy; X = Cl, OC(O)CH2CH2GePh3,
BF4,50,51 tBuN(CH2C6H4)2BiX (X = halogen)34b or the starting
Strong Bi∙∙∙π arene interactions [Bi1∙∙∙Cg1'(C9'-C14') 3.535 Å vs.
4.1 Å were evidenced and the upper value reported for other
bismuth compounds displaying such interactions54 results in dimeric
associations in 4.
diorganobismuth(III)
bromides
{2.531(4)
Å
in
in
[C6H5CH2N(CH2C6H4)2]BiBr
(1
)
and 2.532(13)
Å
[C6H5CH2CH2N(CH2C6H4)2]BiBr
(2
),47}, thus resulting in the
A closer look at the crystal packing of the four compounds
revealed further intermolecular interactions, thus leading to
formation of two five-membered NC3Bi rings in each case,
respectively Bi1C1C6C7N1 (1) and Bi1C14C9C8N1 (2) in 3a and
supramolecular networks in compounds
3 – 6. But only in
4
–
6
and Bi2C22C27C28N3 (1) and Bi2C35C30C29N3 (2) in 3b
These rings are not planar, but folded along the imaginary
Bi1∙∙∙C7 and Bi1∙∙∙C8 axes in 3a and and about the
imaginary Bi2∙∙∙C28 and Bi2∙∙∙C29 axes in 3b
.
compound 3 the bismuth atoms in the molecules 3a are involved in
further associations of dimers by π Bi1∙∙∙Cg1' interactions of 3.586 Å
(see ESI, Fig. S1), thus determining a more congested environment
about the metal centre, while in the crystals of 4 – 6 only non-
classical hydrogen bonds, O∙∙∙H or π H∙∙∙Cg contacts, contribute to
the supramolecular networks.
4
– 6
.
Due to the planar chirality52 induced by the N→Bi
intramolecular coordination, the compounds crystallize as 1:1
mixtures of R1,R2 and S1,S2 isomers (the superscript indices “1“
and “2“ refer to the two five-membered rings, as designated
above).
In
3 and 6
the NO3‾ ligand is unisobidentate coordinated to
bismuth, thus resulting in hypercoordinated 12-Bi-5 species.53
In both compounds the molecules are associated in dimers by
‾
bridging NO3 ligands which behave as bimetallic triconnective
moieties in
(Fig. 4).
3 (Fig. 1) and bimetallic tetraconnective moieties in
6
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Fig. 4. Thermal ellipsoids representation at 30% probability and atom
numbering scheme in a dimer of R1,R2-6 and S1,S2-6 isomers (hydrogen
atoms are omitted for clarity). Symmetry equivalent position –x, -y, -z is
given by “prime”.
Fig. 2. Thermal ellipsoids representation at 50% probability and atom
numbering scheme in a dimer of S1,S2-4 and R1,R2-4 isomers (hydrogen
atoms are omitted for clarity). Symmetry equivalent position 1–x, -y, 1-z is
given by “prime”.
Catalytic behavior
In the absence of the catalyst some air oxidation of thiophenol
to diphenyl disulfide (Scheme 2) occurred albeit with a low
conversion (below 6% after 5 h), as can be seen from Fig. 5,
irrespective of the solvent nature (cyclohexane or IL). This is in
agreement with previous reports mentioning that in neutral media
in which there are no radical initiators slow air oxidation occurs.55
Scheme 2. Oxidative coupling of thiophenol to diphenyl disulfide.
Simple addition of NaOH led to an increase in the conversion to
16% after 5h, which might be explained by the fact that the
autooxidation of thiols to disulfides, proceeding via the thyil radical
(|RS|) and ‾O−O‾ species, is enhanced in the presence of an excess
of base.55
In the presence of the diorganobismuth(III) bromides
[RCH2N(CH2C6H4)2]BiBr a further increase of the catalytic activity
was observed, thus reaching a conversion of 21% for R = C6H5 (1)
and more than double (51%) for R = C6H5CH2 (2), corresponding to
TON values of 7.3 and 17.8, respectively. These different behaviors
might be explained by the observation that in solid state the
bismuth atom in 1 is involved in weak Bi∙∙∙Br (4.227 Å) and Bi∙∙∙π
arene (4.097 Å) intermolecular interactions (see ESI, Fig. S2), whilst
Fig. 3. Thermal ellipsoids representation at 30% probability and atom
numbering scheme in R1,R2-5 isomer (hydrogen atoms are omitted for
clarity).
in
2 the Bi is not involved in any intra- or intermolecular
interactions, except the transannular N→Bi one (see ESI, Fig. S3).
The latter is present in both 1 and 2 and of a similar magnitude
[2.531(4) Å in 1 and 2.532(13) Å in 2].47
Therefore, it is likely to be a significant difference in the
accessibility of the active site for the organic substrates to bind.
Furthermore, the activity of the diorganobismuth(III) compounds 3 -
6 resulted in high conversions and TON.
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polarizability of the metal−ligand bond and the accessibility of the
organic substrate.
blank
3
NaOH
4
1
2
5
6
100
80
60
40
20
0
Table 3. N→Bi transannular interacꢀons (Å), Hammett’s constants* and
calculated charge on Bi and N in compounds 1 and 3 – 5.
1
3
4
5
N→Bi
2.531(4)
2.473(8)
2.402(3) 2.454(4)
2.453(7)
0.55
Hammett‘s constant, σ56a
0.39
0.56
-
q-Bi
q-N
0.790
-0.240
0.915
0.925
-0.232
0.929
-0.237
-0.239
* Hammett’s constants for compounds 1, 3 and 4 are given as previously
reported in ref. 56a for Br, ONO2 and CF3SO3.
1
3
5
In case of compounds 3 and 6 it seems that the length of the
aliphatic chain in the organic group attached to nitrogen does not
influence the catalytic behaviour by itself, but it is responsible for
the structural differences in the two compounds in solid state.
Changing the molecular solvent from cyclohexane to 1-butyl-1-
methylpyrrolidinium bis(trifluoromethanesulfonyl)imide) lead to an
increase of the catalytic activity (Fig. 6).
Time (h)
Fig. 5. The activity of the heterocyclic bismuth(III) compounds 1 − 6 (the
experiments were performed in cyclohexane).
After 5h the conversions (%)/TONs obtained were in the
following order 3 (100/34.8) > 4 (94.8/33.0) > 6 (92.9/32.4) > 5
(86.1/30.0).
Table 2 compiles activity results of the compound 4 for the
oxidation of various thiols.
6
4/IL
5
3/IL
2
6/IL
100
90
80
70
60
50
40
30
20
10
0
Table 2. The oxidative coupling of several thiols to disulfides using
compound 4.
Entry Thiols
Conv. (%)
94.8
100
100
66.4
Sel. (%)
100
100
100
100
TON
33
35
35
23
1
2
3
4
Thiophenol
4-Methylbenzenethiol
1-Butanethiol
1-Octanethiol
The relative activity of these molecular catalysts should be
associated to the polarizability of bismuth in the presence of
different X‾ ligands and with the Lewis acidity (Bi) / basicity (N) of
the catalytic species.
0
1
2
3
4
5
DFT calculations revealed the variation of the positive charge on
bismuth (q-Bi, Table 3) in the order 5 > 4 > 3 >> 1, while the
negative charge on nitrogen varies in the order 4 > 5 > 3 >> 1 (q-N,
Table 3). The latter sequence is correlated with the experimentally
determined N→Bi distances (4 < 5 < 3 << 1).
Time (h)
Fig. 6. Time evolution of the conversion of thiophenol to disulfide in the
presence of 2, 5 and 6 as catalysts. The experiments were performed in
cyclohexane
(open
element)
and
1-butyl-1-methylpyrrolidinium
Anyway, the N→Bi interatomic distances in compounds 3 – 6
are of a similar magnitude, but shorter than in the bromides 1 and
2. This behaviour correlates well with the variation of the charge
calculated for nitrogen and with the variation of the Hammett’s
constant for the substituents attached to bismuth, which is a
measure of the electronic effects and the polarizability of the
Bi−ligand bond (Table 3).56
bis(trifluoromethanesulfonyl)imide (close element).
It can be observed that complete conversion was reached for
catalyst 6 in approx. 1h. It is noteworthy that this change in solvent
did not affect the final selectivity to diphenyl disulfide. This increase
in activity is thought to be due to the increased solubility of the
molecular oxygen in the ionic liquid compared with the
cyclohexane, as well as to the complete ionization of the catalyst in
solution.57
As a consequence, the increased catalytic activity of the
complexes with oxygenated ligands in comparison with the
diorganobismuth bromides is explained by the lower polar
character of the Bi−Br bonds in comparison with the Bi−O bonds. In
addition, the differences in the catalytic behavior of the complexes
3 – 6 might be determined by the secondary interactions in the
coordination sphere of bismuth and their influence on both the
These experiments clearly demonstrate the positive influence
of the IL as solvent. Moreover, the use of IL allowed a good
recycling of the catalysts with only a small decrease in conversion
from 92.9 to 85.8% for catalyst 4 over two consecutive reactions.
This is in good agreement with previous reports on the stabilizing
capacity of ILs in various catalytic reactions.58
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Literature reported several mechanisms for this reaction. Based
Journal Name
on EPR measurements Corma et al. concluded that on oxide
heterogeneous catalysts the reaction is controlled by a radical
pathway implying a homolytic S–H scission from the original thiol.59
The same conclusion has been drawn by Garcia et al for MOF-type
catalysts.60 A radical mechanism has also been proposed using
inorganic complexes.61,62
Taking into account the experimental data collected in this
study, a reaction pathway based on a two steps addition of PhSH to
bismuth might be proposed (Fig. 7). In the first step it is formed a
pentacoordinated Bi(III) species, with
a trigonal bipyramidal
coordination geometry, while the addition of the second PhSH
molecule resulted in an octahedral Bi(III) species. We assume that
the ONO2 ligand acts as
a monodentate moiety in the
phenylthiolato bismuth(III) intermediates. In order to bring
evidences for the proposed mechanism we performed a NMR tube
scale reaction and we studied the in situ solution behavior of a
mixture of compound 6 and thiophenol (1 : 2 molar ratio, in
toluene-d8) by a combination of 1H NMR and MS experiments.
Fig. 9. Detail of the aromatic region in the 1H NMR spectra of a mixture of
PhSH and compound 6, showing the formation of Ph2S2. (a) immediately
after adding the solvent, at 20 °C, (b) after 2 hours at 20 °C, (c) after 24
hours at 20 °C, (d) at 80 °C, (e) again at 20 °C. Spectra a - c are recorded on a
600 MHz NMR spectrometer, while spectra d and e on a 400 MHz NMR
spectrometer.
At room temperature the 1H NMR spectrum suggested that only
about 10% of the catalyst 6 has been solubilized (Fig. 8). During the
first 2 h, it showed no change of the mixture (Figure 9, and ESI, Fig.
S4), when only characteristic resonances for the pure PhSH and 6
were found in the spectrum. However, the solution turned to
slightly yellow, thus indicating the formation of some bismuth
organothiolates.63 Also, after 2h, the ESI+ mass spectrum of the
solution has shown a peak of very low intensity (1%) at m/z 618.17,
thus
suggesting
the
formation
of
the
species
[{C6H5CH2CH2N(CH2C6H4)2}BiSPh+]. The base peak at m/z 508.15
corresponds to the cation [{C6H5CH2CH2N(CH2C6H4)2}Bi+] (see ESI,
Fig. S7). After 24h the intensity of the peak at m/z 618.17 increased
1
to about 20%, while the H NMR spectrum has shown the decrease
Fig. 7. Possible reaction pathway for the oxidation of thiophenol to diphenyl
in intensity of the resonance corresponding to the PhSH proton in
disulfide.
the alifatic region and the formation of Ph2S2 (see ESI, Figs. S4 – S6).
We noticed that the C6H5-ortho protons in Ph2S2 are strongly shifted
at lower field in comparison with those in PhSH. The increase of the
temperature up to 80 °C has been accompanied by several changes
in the spectrum of the bismuth(III) complex, but in the range 20 –
80 °C the resonance associated to the CH2NCH2 protons appears as
an AB spin system, thus suggesting that nitrogen remains
coordinated to bismuth. However, Ph2S2 has been detected only
with a low intensity in the ESI+ mass spectrum, due to the poor
ionization. To improve the ionization capacity of this species, we
+
changed to APCI+, when a peak corresponding to [Ph2S2 ] appeared
at m/z 218.02 (43%), along with the cation at m/z 508.15 (see ESI,
Fig. S8). At this point, the 1H NMR spectrum of the final solution
suggests the existence of at least two bismuth containing species,
but unambiguously could be identified only the starting compound
6. To identify the nature of the insoluble species we added 5 mL of
toluene and we heated the mixture at reflux (110 °C) for 15 min.,
with stirring, when the solution became clear. Then, decreasing
again the temperature to 20 °C, the colourless precipitate was
formed again and it has been identified by 1H NMR as the starting
catalyst 6. The APCI+ MS spectrum of the final solution shows a
Fig. 8.1H NMR spectra (toluene-d8) of a reaction mixture of PhSH and
compound 6 (*) showing the formation of Ph2S2: (a) during the first 2 hours,
at 20 °C.; (b) the solution behaviour at 80 °C; (c) a mixture of pure Ph2S2 and
6.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
peak at m/z 427.14 (52%), corresponding to the cation [(PhS)2Bi+]
(Fig. S9), which might suggest the formation of an intermediate
containing two phenylthiolato groups attached to the bismuth
complex, thus confirming the pathway described in Fig. 7. This is
also in line with the previous reports considering
mechanism.59-61
a radical
Up to 70°C no NMR resonance characteristic for water
formation has been observed. We attribute this fact to its physical
adsorption on the remained solid catalyst, which only at higher
temperature is desorbed (see ESI, Fig. S6).
Scheme 3.
The IR spectra of the pure compounds 4 and 6 show no
significant differences of the bands assigned to the Bi complex in
comparison with the solid separated after the oxidation of
thiophenol in cyclohexane. We could only notice the formation of
Ph2S2 and water (see ESI, Fig. S10).
Synthesis of [C6H5CH2N(CH2C6H4)2]BiONO2 (3)
A mixture of [C6H5CH2N(CH2C6H4)2]BiBr (0.143 g, 0.249 mmol)
and AgNO3 (0.05 g, 0.294 mmol) in acetone (20 mL), was left under
stirring, in the absence of light, for 2 h. The solvent was removed in
vacuum and the remained solid was treated with CH2Cl2 (20 mL).
AgBr was removed by filtration and from the clear solution, after
the removal of the solvent, the title compound was isolated as a
colorless solid. Yield: 0.155 g (93%). M.p. 151 °C (dec.). Anal. Calcd.
Experimental
General
for C21H19BiN2O3 (MW = 556.37): C 45.33, H 3.44, N 5.04; Found C
1
Organic solvents were dried and distilled prior to use. The ionic 45.51, H 3.58, N 5.13. H NMR (CDCl3, 500 MHz): δ 4.08 (s, 2H, H8),
liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) 4.36 (AB spin system with δA 4.26 and δB 4.47, 4H, H7, 2JHH 14.4 Hz),
imide, was prepared following standard procedures.64 Initial, 1- 7.30-7.32 (m, 2H, C6H5-ortho), 7.35 (dt, 2H, C6H4-H4, 3JHH 7.3, 4JHH 1.3
butyl-1-methylpyrrolidinium bromide [Bmpyrr][Br] was synthesized Hz), 7.42-7.45 (m, 3H C6H5-meta+para) 7.53 (d, 2H C6H4-H5, 3JHH 8.02
3
3
by mixing 1-methylpyrrolidine (200 mmoles) in acetone (75 mL) Hz), 7.56 (t, 2H, C6H4-H3, JHH 7.3 Hz), 8.22 (d, 2H, C6H4-H6, JHH 7.1
with equal molar amounts of 1-bromobutane in a round-bottomed Hz). 13C{H} NMR (CDCl3, 125.72 MHz): δ 60.45 (C8), 63.84 (C7),
flask. The reaction flask was immediately covered with aluminum 128.67 (C4), 128.92 (C3), 129.44 (C6H5-meta), 129.75 (C6H5-para),
foil. The mixture was stirred for 2 days until a white semi-solid was 131.16 (C6H5-ortho), 131.66 (C5), 133.11 (C6H5-ipso), 138.39 (C6),
formed. The obtained solid was washed with cold acetone and 148.97 (C2), 180.3 (C1). IR (ν, cm-1): 1453, 1436.
separated by filtration. The anion exchange was performed further
by using lithium bis(trifluoromethanesulfonimide) [LiN(CF3SO2)2]. An Compounds 4 – 6 were prepared similarly.
aqueous solution of LiN(CF3SO2) (slight excess) was mixed with an
aqueous solution of [Bmpyrr][Br] for 2h. The obtained ionic liquid [C6H5CH2N(CH2C6H4)2]BiOSO2CF3 (4)
[Bmpyrr][NTf2] was extracted from the aqueous phase three times
From [C6H5CH2N(CH2C6H4)2]BiBr (0.188 g, 0.327 mmol) and AgOTf
with dichloromethane. The solvent was removed using a rotary
(0.089 g, 0.34 mmol) in acetone (20 mL), as a colorless solid. Yield:
evaporator at 60 °C and the ionic liquid was dried for 24 h under
0.210 g (94%). M.p. 155 °C (dec.). Anal. Calcd. for C22H19BiF3NO3S
high vacuum.
(MW = 643.43): C 41.07, H 2.98, N 2.18%. Found C 40.91, H 2.77, N
2.09%. 1H NMR (CDCl3, 500 MHz): δ 4.13 (s, 2H, H8), 4.49 (AB spin
Experiments involving air sensitive compounds were carried out
system with δA 4.39 and δB 4.59, 4H, H7, 2JHH 14.5 Hz), 7.28-7.30 (m,
under argon atmosphere. Elemental analyses were performed on a
Flash EA 1112 analyzer. Melting points were measured on an
Electrothermal 9200 apparatus and are not corrected. 1H, 13C{H}
and 19F{H} NMR spectra for compounds 3 – 6 were recorded in
CDCl3 on a Bruker 500 equipment, while the NMR spectra of the
3
4
2H, C6H5-ortho), 7.39 (dt, 2H, C6H4-H4, JHH 7.6, JHH 1.4 Hz), 7.44-
3
7.47 (m, 3H C6H5-meta+para) 7.61 (d, 2H C6H4-H3, JHH 7.7 Hz), 7.65
(t, 2H, C6H4-H5, 3JHH 7.3 Hz), 8.39 (d, 2H, C6H4-H6, 3JHH 7.5 Hz). 13C{H}
NMR (CDCl3, 125.72 MHz): δ 61.13 (C8), 65.25 (C7), 119.65 (q, CF3,
1JFC 318.9 Hz), 128.92 (C3), 129.02 (C4), 129.73 (C6H5-meta), 130.23
(C6H5-para), 131.10 (C6H5-ortho), 132.34 (C5), 132.74 (C6H5-ipso),
138.55 (C6), 149.45 (C2), 181.65 (C1). 19F{H} NMR: δ -77.44. IR (ν, cm-
1): 1299, 1197, 1020.
reaction mixture of PhSH and
6 were recorded at room
temperature on a Bruker 600 and at higher temperature on a
Bruker 400 equipment. The NMR spectra were processed with the
MestReNova software.65 The chemical shifts are reported in
δ
units
(ppm) relative to TMS (1H and 13C) and CFCl3
(
19F). The 1H and 13C
chemical shifts were assigned based on 2D NMR experiments and
they are given according to the numbering in Scheme 3. ESI+ and
APCI+ mass spectra were recorded in CH3CN on a LTQ Orbitrap - XL
instrument. The IR spectra were recorded in the range 4000 – 400
cm−1 on a Jasco spectrometer. Starting materials were commercially
available or were prepared according to literature procedures,
[C6H5CH2N(CH2C6H4)2]BiBr (1) and [C6H5CH2CH2N(CH2C6H4)2]BiBr
(2).47 The synthesis of silver 4-vinylbenzenesulfonate is given in the
electronic supporting information.
[C6H5CH2N(CH2C6H4)2]BiOSO2C6H4CHCH2-4 (5)
From a mixture of [C6H5CH2N(CH2C6H4)2]BiBr (0.103 g, 0.179 mmol)
and silver 4-vinylbenzenesulfonate (0.052g, 0.179 mmol) in acetone
(20 mL), as a colorless solid. Yield: 0.118g (98%). M.p. 181 – 183 °C.
Anal. Calcd. for C29H26BiNO3S (MW = 677.57): C 51.41, H 3.87, N
1
2.07%; Found: C 51.21, H 3.68, N 2.01%. H NMR (CDCl3, 500 MHz):
δ 4.07 (s, 2H, H8), 4.35 (AB spin system with δA 4.22 and δB 4.48, 4H,
2
3
H7, JHH 13.6 Hz), 5.33 (d, 1H, CH=CHAHB, HA, JHH 10.8 Hz), 5.82 (d,
3
3
1H, CH=CHAHB, HB, JHH 17.3 Hz), 6.73 (dd, 1H, CH=CHAHB, JHH 17.6
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Journal Name
3
Hz, JHH 10.8 Hz), 7.26 – 7.29 (m, 2H, C6H5-ortho), 7.30 (t, 2H C6H4- GC/MS/MS VARIAN SATURN 2100 T spectrometer equipped with a
H4, 3JHH 7.5 Hz), 7.38 – 7.42 (m, 2H C6H4-H3 + 1H C6H5-para), 7.45 (d, CP-SIL 8 CB Low Bleed/MS column of 30 m length and 0.25 mm
3
2H, C6H4-meta, JHH 7.1 Hz), 7.48 – 7.52 (m, 2H, C6H5-meta, 2H, diameter.
3
3
C6H4-H5), 7.93 (d, 2H, C6H4-ortho, JHH 7.5 Hz), 8.37 (d, C6H4-H6, JHH
7.7 Hz). 13C{H} NMR (CDCl3, 125.72 MHz): δ 60.16 (C8), 63.87 (C7), NMR tube scale reaction
115.83 (CH=CHAHB), 126.33 (C6H4-meta), 126.80 (C6H4-ortho),
A mixture of [C6H5CH2CH2N(CH2C6H4)2]BiONO2 (6) (0.036 g, 6.3 x 10-5
128.54 (C4), 128.70 (C6H5-meta), 129.35 (C3), 129.62 (C6H5-para),
131.18 (C6H5-ortho), 131.63 (C5), 132.93 (C6H4, C−CH), 136.11
(CH=CHAHB), 138.60 (C6), 139.85 (C6H5-ipso), 142.62 (C6H4, C −SO2),
149.22 (C2), 180.51 (C1). IR (ν, cm-1): 1246, 1197, 1020.
mmol), thiophenol (0.013 ml, 12.7 mmol) and toluene-d8 (0.5 ml)
was realized directly in the NMR tube. The solution became slightly
yellow in approx. 10 min. Most of the compound 6 remained
undisolved. The evolution of the oxidation reaction was followed by
VT 1H NMR in the range 20 – 80 °C and MS experiments.
[C6H5CH2CH2N(CH2C6H4)2]BiONO2 (6)
From [C6H5CH2CH2N(CH2C6H4)2]BiBr (0.208 g, 0.353 mmol) and
AgNO3 (0.060 g, 0.353 mmol), as a colorless solid. Yield: 0.19 g
(95%). M.p. 150 °C (dec.). Anal. calcd. for C22H21BiN2O3 (MW =
570.39): C 46.33, H 3.71, N 4.91%; Found: C 46.38, H 3.74, N 4.89%.
1H NMR: δ 2.90 - 2.93 (m, 2H, CH2, CH2C6H5), 3.34 – 3.37 (m, 2H,
Theoretical Calculations
The geometry optimizations and frequency calculations on
compounds 1 and 3 - 5 were performed with Gaussian 09,70
Revision E.01 using B3LYP functional,71 and def2-TZVP basis set.72
The dispersion corrections were accounted using D3 version of
Grimme’s dispersion with Becke-Johnson damping.73 Cartesian
coordinates of the calculated molecular structures 1 and 3 – 5 are
given in ESI, Tables S3 – S6.
2
CH2, H8), 4.39 (AB spin system with δA 4.33 and δB 4.45, 4H, H7, JHH
3
14.7 Hz), 7.02 (d, 2H, C6H5-ortho, JHH 7.2 Hz), 7.27 - 7.31 (m, 3H,
C6H5-meta+para), 7.35 (t, 2H, H4, 3JHH 7.3 Hz), 7.53 – 7.56 (m, 4H, H3
3
+ H5), 8.17 (d, 2H, H6, JHH 7.5 Hz). 13C{H} NMR: δ 34.50 (C9), 60.33
(C8), 66.41 (C7), 127.80 (C6H5-para), 128.47 (C3), 128.59 (C4), 128.90
(C6H5-ortho), 129.30 (C6H5-meta), 131.62 (C5), 136.59 (C6H5-ipso),
138.13 (C6), 149.10 (C2), 181.59 (C1). IR (ν, cm-1): 1451, 1443.
Electronic Supplementary Information
CCDC 1520802 (3), 1520801 (4) 1520800 (5) and 1520799 (6)
contain the supplementary crystallographic data for the
investigated compounds. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The supplementary material
also contains the crystal and structural refinement data for the
crystal structures of 3 – 6, interatomic distances and bond angles in
compounds 1 - 6, Figures representing the coordination sphere of Bi
in the crystals of 1 - 3, Figures representing selected 1H NMR and
MS spectra, as well as cartesian coordinates of the calculated
molecular structures 1 and 3 – 5.
X-ray structure determination
Single-crystals of compounds 3 – 6 were obtained from a
mixture of CH2Cl2 and n-hexane (1/5 v/v) at room temperature. The
data were collected on an Oxford Gemini S (3 and 4, at 110 K) or a
Smart Apex CCDC (5 and 6, at room temperature) diffractometer
using Mo Kα radiation (λ = 0.71073 Å). The calculations were
performed using the SHELXS-2013, SHELXL-2016-466 or the SHELXTL
program.67 The structures were solved by direct methods and
refined by full-matrix least-square procedures on F². All non-
hydrogen atoms were refined anisotropically and a riding model
was employed in the refinement of the hydrogen atom positions.
Crystal and structural refinement data for the crystal structures of
compounds 3 - 6 are given in ESI, Table S2. Intermolecular
interactions were identified with PLATON.68 The drawings were
created with the Diamond software package.69
Conclusions
Diorganobismuth(III)
[C6H5CH2N(CH2C6H4)2]BiX [X = Br, ONO2, OSO2CF3, OSO2C6H4
(CHCH2)-4], based on butterfly-like tetrahydro-
compounds
of
type
a
dibenzo[c,f][1,5]azabismocine heterocyclic framework, with
anionic ligands with different electron-withdrawing ability
proved to be efficient catalysts for the oxidation of thiophenol
to diphenyl disulfide under mild conditions with high
conversions and chemoselectivity. The catalytic activity in
cyclohexane was observed to increase going from the starting
diorganobismuth(III) bromides to the derivatives with
Catalysis
The oxidation of thiophenol to diphenyl disulfide was
performed in a glass batch reactor equipped with a condenser
system. Thiophenol (0.001 moles) dissolved in either 10 mL
cyclohexane
or
4
mL
1-butyl-1-methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide was stirred at 600 rpm in a
silicon bath at 100 °C under reflux. The catalyst in a loading of 5%
wt. Bi (2.8 mol% Bi) from the reaction mixture was added after the
stabilization of the temperature. In order to check the progress of
the reaction, samples of 5 μL were periodically removed from the
oxygenated ligands, as
a consequence of the stronger
electron-withdrawing character of the latter systems. This
behaviour was also correlated with a decrease of the N→Bi
distance by changing Br (about 2.531 Å) with oxygenated
ligands [range 2.402(3) - 2.479(5) Å]. It is clear that the overall
catalytic activity is strongly influenced by a combination of
factors including the acid/base character of the
diorganobismuth(III) complex, the polarizability of the Bi-X (X =
O, Br) bond and the accessibility to the metal centre.
reactor, analyzed by GC using
a
K072320 Termo-Quest
chromatograph equipped with a FID detector and a capillary
column of 30 m length and 0.324 mm in diameter and a DB-5
stationary phase. Highly pure N2 (99.999%) was used as carrier gas.
The reaction product was identified also by GS-MS, using a
8 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
9
A.R. Hajipour, S.E. Mallakpour, H. Adibi, J. Org. Chem., 2002,
67, 8666.
Catalytic experiments carried out in 1-butyl-1-
methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide
10 S.L.S. Leite, V.L. Pardini, H. Viertler, Synth. Commun., 1990,
20, 393.
11 (a) A. Sabet, A. Fakhraee, M. Ramezanpour, N. Alipour,
International Journal of Chemical, Molecular, Nuclear,
Materials and Metallurgical Engineering 2014, 8, 1184; (b) V.
showed a positive influence of the ionic liquid leading to
higher reaction rates with the same selectivity to diphenyl
disulfide. This effect is explained by the higher solubility of
oxygen in the ionic liquid than in cyclohexane, as well as by the
ionization of the catalytic species promoted by the IL.
Cimpeanu, V.I. Pârvulescu, P. Amoros, D. Beltran, S. Johnson,
C. Hardacre, Chem-Eur J., 2004, 10, 4640; (c) V. Cimpeanu,
A.N. Parvulescu, V.I. Pârvulescu, D.T. On, S. Kaliaguine, J.M.
Thompson, C. Hardacre, J. Catal., 2005, 232, 60.
Taking into account the calculated charge on bismuth
(0.790 for Br and over 0.9 for the oxygenated ligands), as well
as the experimental catalytic results, it might be concluded
that the increased polarity of the Bi–O bond compared to the
Bi–Br one, make the complexes with oxygenated ligands more
suitable for catalysis, either in cyclohexane or in ionic liquid.
The 1H NMR and ESI+ MS experiments carried out for a 1 :2
(molar ratio) mixture in toluene-d8 solution support a two
steps reaction pathway, involving the intermediary formation
of diorganobismuth(III) phenylthiolates.
12 (a) T. Aida, T. Akasaka, N. Furukawa, S. Oae, Bull. Chem. Soc.
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14 R.S. Varma, H.M. Meshram, R. Dahiya, Synth. Commun.,
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15 S. Tsuneo, O. Junzo, N. Hitosi, Tetrahedron Lett., 1990, 31
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,
,
Acknowledgements
Financial support from National University Research Council of
Romania CNCSIS (Research Project PNII-ID-PCCE-2011-2-
0050/Partner 3), from DFG SPP1807 “Control of London
Dispersion Interactions in Molecular Chemistry” and from the
Fonds der Chemischen Industrie (FCI) is greatly acknowledged.
A.T. is grateful for financial support from DAAD (fellowship,
“Eastern Europe Partnership” program and STIBET).
Theoretical calculations were performed on high-performance
computational facility MADECIP, POSCCE, COD SMIS
48801/1862 co-financed by the European Regional
Development Fund of the European Union.
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Synopsys
Aerial oxidation of thiophenol to diphenyl disulfide proceeds with high reaction rates, total conversion and
selectivity in ionic liquids by using diorganobismuth(III) catalysts based on a tetrahydro-
dibenzo[c,f][1,5]azabismocine heterocyclic framework.
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