Application of Ni-based metal-organic framework as heterogeneous catalyst for disulfide addition to acetylene

Article abstract of DOI:10.1016/j.catcom.2020.106119

Several recent studies have shown unique adsorption activity of metal organic frameworks (MOFs) towards unsaturated hydrocarbons. In the current article, we explored the application of Ni-MOFs for S-functionalization of acetylene. We showed that Ni-MOF-74 catalyzed the reaction of disulfide addition to gaseous acetylene with excellent selectivity. The prime advantage of the proposed Ni-MOF-74 over other examined catalysts was its easy separation and recycling possibility. Moreover, it demonstrated no leaching of Ni species into the solution. The work was supplemented with a study on the catalyst behavior in the course of the reaction by using SEM, EDX, XRD, and FT-IR methods.

Full text of DOI:10.1016/j.catcom.2020.106119

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Application of Ni-based metal-organic framework as
heterogeneous catalyst for disulfide addition to acetylene
Evgeniya S. Degtyareva, Kirill S. Erokhin, Valentine P. Ananikov
PII:
S1566-7367(20)30195-3
DOI:
https://doi.org/10.1016/j.catcom.2020.106119
CATCOM 106119
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
28 March 2020
17 July 2020
18 July 2020
Please cite this article as: E.S. Degtyareva, K.S. Erokhin and V.P. Ananikov, Application
of Ni-based metal-organic framework as heterogeneous catalyst for disulfide addition
to
acetylene,
Catalysis
Communications
(2020),
https://doi.org/10.1016/
j.catcom.2020.106119
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Application of Ni-based metal-organic framework as heterogeneous
catalyst for disulfide addition to acetylene
Evgeniya S. Degtyareva,≠ Kirill. S. Erokhin,≠ Valentine P. Ananikov*
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
http://AnanikovLab.ru
≠ The authors contributed equally
*Corresponding author e-mail address: val@ioc.ac.ru
ABSTRACT
Several recent studies have shown unique adsorption activity of metal organic frameworks
(MOFs) towards unsaturated hydrocarbons. In the current article, we explored the application of Ni-
MOFs for S-functionalization of acetylene. We showed that Ni-MOF-74 catalyzed the reaction of
disulfide addition to gaseous acetylene with excellent selectivity. The prime advantage of the
proposed Ni-MOF-74 over other examined catalysts was its easy separation and recycling
possibility. Moreover, it demonstrated no leaching of Ni species into the solution. The work was
supplemented with a study on the catalyst behavior in the course of the reaction by using SEM,
EDX, XRD, and FT-IR methods.
Keywords: heterogeneous catalysis, acetylene, Ni-MOF-74, disulfides, addition, selectivity
1. Introduction
New materials and methods for efficient gas separation and storage are highly demanded by gas-
processing industry. Metal organic frameworks (MOFs) have a tremendous potential in this area due
to high tunability of their pores and specific sorption activity for selected molecules [1–4]. Another
promising application of MOFs is their utilization as solid catalysts for organic reactions [5,6]. The
combination of procedures of gas storage or separation with chemical reactions is a perspective field
for forthcoming research. To put the idea into practice, a MOF-based catalyst efficient in the
reactions with gaseous chemicals should be developed. Acetylene seems to be a promising starting
material in this regard. Recently, our group has described the synthesis of 1,2-bis(arylthio)ethenes
from acetylene and disulfides catalyzed by Ni(acac)2 [7]. Mechanistic studies have demonstrated the
influence of homogeneous and heterogeneous Ni-containing species on the reaction process, and it
has been concluded that the reaction has a dynamic nature. To shift the equilibrium to pure
heterogeneous catalysis, we propose utilizing Ni-containing MOFs instead of Ni(acac)2 (Scheme 1).
Scheme 1. Model reaction of MOF-catalyzed Ph2S2 addition to acetylene.
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It should be noted that the reaction of disulfide addition to alkynes is a well-studied one [8–13].
However, unsubstituted acetylene is rarely included into the scope of alkynes, because it needs an
individual approach. In contrast to traditional metal catalysts, MOF provides an outstanding
opportunity to design transformations of unsubstituted acetylene due to MOF specific structural
features [14–18]. To the best of our knowledge, here we describe the first example of MOF-mediated
addition of disulfides to the C≡C bond. The goal of the work is to reach the high selectivity towards
the formation of the demanding cis- isomer, because it cannot be achieved in regular non-catalytic
addition reactions, where a mixture of the cis-/trans- isomers with the prevailing amount of the
thermodynamically more stable trans- isomer is typically expected [19,20].
2. Experimental
2.1. Materials
Reagents were obtained from commercial sources and were checked by NMR before use. MeCN
saturation with acetylene was carried out by bubbling C2H2 through the solvent at 0 °C for 30 min.
Bis(4-bromophenyl), bis(2-chlorophenyl), bis(3-chlorophenyl), bis(2-methoxyphenyl), and bis(3-
methoxyphenyl) disulfides were prepared according to the previously published procedure [21]. The
solvents were purified according to the published methods. Unless noted otherwise, MOFs were
activated by washing with solvent and keeping under vacuum for at least 24 h.
2.2. Synthesis of Ni-MOF-74 (Ni-4)
Ni(OAc)2∙4H2O (2.3 mmol, 572 mg) was dissolved in water (17.5 mL); 2,5-dihydroxyterephtalic
acid (1.2 mmol, 232 mg) was dissolved in THF (10 mL); and DABCO (1.15 mmol, 130 mg) was
dissolved in THF (7.5 mL). Afterwards, all the three solutions were mixed together in a tube and
were stirred for 15 minutes at room temperature, followed by heating to 120 °C in the oven for 24 h.
Then the reaction mixture was cooled to room temperature. A light-brown solid of Ni-4 was
centrifuged and the liquid was poured out. The obtained solid of MOF was washed with 3x10 mL
THF. The resulting solid was dried under vacuum at room temperature for 2 days. The yield of Ni-4
was 57 % (337 mg).
The 3a, 3b, 3c, 3f, 3i products were identified according to the literature data [7].
2.3. General procedure of 1,2-bis(arylthio)ethenes synthesis
Solid reagents, including diaryl disulfide (0.2 mmol), Ni-4 (0.006 mmol, 2.2 mg) and PPh3
(0.06 mmol, 15.7 mg) were added to a screw-capped test-tube. Then the test-tube was flushed with
argon, and the C2H2-presaturated MeCN (1 mL) was added. The tube was immediately closed with a
screw cap. The reaction was carried out at 60 °C for the indicated time.
In order to test the recyclability of Ni-4, the solid was separated from the reaction mixture by
centrifugation and was washed with 3x5 mL MeCN. After washing, the catalyst was re-used.
2.4. Leaching test procedure
The reaction was carried out according to the general procedure described in Section 2.3 for 17 h,
and the product yield was determined; the test-tube was centrifuged and the liquid phase was
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carefully transferred into a new test-tube. A new portion of acetylene was bubbled through the
solution at room temperature for 10 min; afterwards, the test-tube was immediately closed. The
reaction was continued at 60 °C under stirring for additional 24 h. Then the product yield was
determined again. The increase of the product yield demonstrated the presence of catalytically active
species in the solution.
3. Results and Discussion
First of all, a series of MOFs was synthesized from nickel salts and various bridging ligands
(benzene-1,4-bicarboxylic acid (BDC), 2,5-dihydroxyterephtalic acid (DOBDC), 1,3,5-
benzenetricarboxylic acid (BTC), 2,5-furandicarboxylic acid (FDC), triethylenediamine (DABCO),
and 4,4’-bipyridine (bpy)). The structures of the obtained solids were analyzed by X-ray powder
diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy
(EDX) (see the Supporting Information).
The catalytic activity of the synthesized Ni-MOFs was examined in the target reaction of phenyl
disulfide (1a) addition to acetylene (2) (Scheme 1, Table S1). Among the evaluated catalysts, the
most promising were Ni(BDC) (Ni-1), Ni(BTC)(DABCO) (Ni-2) and two samples of Ni(DOBDC)
synthesized under different reaction conditions (Ni-3 and Ni-4), which exhibited the formation of
1,2-bis(phenylthio)ethene (3a) in yields higher than 50 % (Table 1). The only registered byproduct of
the reaction was Z,Z-1,4-bis(arylthio)butadiene, which was detected in the spectra of the reaction
mixtures in minor quantities. All the tested MOFs were activated by the solvent exchange method
and/or by heating under vacuum. In addition to the conventional method, activation by cyclohexane
freeze-drying was carried out for Ni-3 and Ni-4 samples [22]; however, application of this more
complex protocol did not lead to an increase of the catalytic activity (Table S1). Utilization of non-
activated Ni-4 MOF led to a drop of the product yield to 15 % (Table S1). Additionally, it should be
noted that increasing the catalyst amount to 10 mol% Ni-4 did not significantly improve the product
yield.
Table 1. Catalytic activity and stability of selected MOFs in model reaction.[a]
№
1
2
3
4
[a]
MOF
Ni-1
Ni-2
Ni-3
Ni-4
3a yield, %[b]
Ni:S ratio in the solid[c]
Catalyst leaching
59
65
60
56
12.6:1
0.9:1
5.0:1
8.1:1
Yes
Yes
Yes
No
The reaction conditions: Ph2S2 (0.2 mmol), MOF (3 mol%), PPh3 (30 mol%), C2H2-saturated
[b]
CH3CN (1 mL), 17 h, 60 °C.
trimethyl(phenyl)silane as an internal standard. According to EDX analysis of the solid isolated
after the synthesis (see the Supporting Information).
The yields were determined by 1H NMR, using
[c]
The four MOFs of choice were employed for further study. In all the experiments, the model
reaction visually stayed heterogeneous and the light color of the initial MOFs turned to dark brown.
That change could indicate the formation of a Ni-S bond, similar to that observed earlier in the
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reaction with Ni(acac)2 [7]. After completing the transformation, the solid was isolated and its
surface morphology and composition were compared with SEM and EDX data on the parent Ni-
MOFs (Fig. 1a, 1b and S2). The EDX spectra of all the solids isolated after the synthesis showed the
appearance of sulfur in the composition (Table 1, Fig. S3). Though all the initial MOFs had different
morphologies, during the catalytic process they mostly lost the initial structure and became covered
with a notable amount of deposit, which appeared as round-shaped particles. Structural fragments of
the parent Ni-1 – Ni-4 MOF remained only in the samples with high Ni:S molar ratios (entries 1 and
4, Table 1).
Fig. 1. SEM microphotograph of Ni-4 before the reaction (A) and after the 1st (B) and the 5th (C)
catalytic cycles. Scale bar corresponds to 2 µm.
To confirm the presence of the active catalytic species in the precipitate, but not in the solution,
hot centrifugation experiments were performed. If the tested MOF-catalyst acted heterogeneously,
then its removal from the reaction mixture should immediately stop the reaction. The leaching test
was performed with the Ni-1 – Ni-4 MOFs similarly to the published procedure [7] . A significant
increase in the product yield was registered in the reactions catalyzed by the Ni-1 – Ni-3 MOFs,
(entries 1-3, Table 1). Unexpectedly, the removal of the solid from the reaction with Ni-4 shut down
the catalysis (entry 4, Table 1), thus demonstrating the heterogeneous nature of the catalyst and
making Ni-4 the catalyst of choice. This result was very interesting, because Ni-3 and Ni-4 MOFs
were synthesized by similar reaction procedures with the only difference in the usage of the DABCO
additive in the case of Ni-4. Despite the similar morphology and crystal structures observed by SEM
and XRD methods (Fig. S1, S2), the Ni-3 and Ni-4 MOFs performed differently in the model
reaction. This effect could be related to the influence of DABCO on the appearance of defects in the
crystal structure of MOF, which provided additional catalytically active metal sites.
Results of powder XRD analysis of the initial Ni-4 and the Ni-4 isolated after the reaction are
shown in Fig. 2. The characteristic pattern corresponds to the standard Ni-MOF-74 diffractogram
[23]; the sharp peaks confirm the crystallinity of the sample. In the FT-IR spectra (Fig. 3),
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characteristic bands attributed to ν(COO) = 1559 cm-1, 1410 cm-1 of the carboxylic group and 1194
cm-1 of the attached hydroxyl group remained after the reaction [23]. At the same time, new bands
appeared at 3007 cm-1, 1008 cm-1, 740 cm-1, and 690 cm-1. We propose that the sample after the
reaction accommodates Ni–SPh fragments (Fig. S4) and, possibly, acetylene oligomers [24] formed
as byproducts of nickel catalysis.
Fig. 2. Powder XRD diffraction pattern of Ni-4 before and after the reaction.
Fig. 3. FT-IR spectra of Ni-4 before and after the reaction.
Ni-4 was successfully loaded to five sequential syntheses carried out at 60 °C for 48 h each.
Interestingly, the MOF reusability performance increased after the 1st cycle; the conversion of Ph2S2
to product 3a in the 2nd to 5th cycles was ~100 %. The solid isolated after the 5th cycle was studied by
SEM and EDX (Fig. 1c). No fragments similar to the initial Ni-4 were observed; the sample
consisted of a mixture of particles about 1 µm and 80±20 nm. EDX analysis confirmed the influence
of sulfur on the decay of MOF: the amount of sulfur after the 5th catalytic cycle was Ni:S = 73:27,
while after the 1st cycle it was Ni:S = 89:11; no sulfur was present in the initial Ni-4 sample (Fig. 1).
The obtained data clearly showed that the structure of MOF was modified by an S-containing
compound without the destruction of the structural framework.
In order to test the reactivity of substituted acetylenes phenyl acetylene, 2-methyl-3-yn-2-ol,
hexyne-1 and 4-octyne were utilized, but the product formation was not observed. It could be
explained by taking into account that the reaction can take place inside the porous catalyst structure;
thus the substituted acetylenes did not reach the Ni-metal centers. The reaction with ethylene under
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the same conditions did not proceed, thus showing the differences in the reactivity of triple and
double bonds. In overall, the process was rather substrate-selective towards the acetylene molecule.
The catalytic activity of Ni-4 was established in the reaction with various aromatic disulfides
(Scheme 2A). Products 3a – 3c were obtained with 83-94 % yields in the reaction with phenyl,
p-tolyl, and p-methoxyphenyl disulfides. It should be noted that the reaction did not proceed when
the substituent position in methoxyphenyl disulfide was changed from para to meta, while the
synthesis with o-methoxyphenyl disulfide resulted in the formation of 3e in a 45 % yield. Utilization
of the electron acceptor p-cholorophenyl disulfide in the reaction demonstrated the formation of the
target product 3f in a 77 % yield. The reaction with o-chlorophenyl disulfide provided 3h in a 65 %
yield, together with 32 % of the (1Z,3Z)-1,4-bis((4-chlorophenyl)thio)buta-1,3-diene byproduct.
m-Chlorophenyl disulfide was not reactive under the studied conditions. In the reaction with
p-bromophenyl disulfide, product 3i was formed in a 48 % yield in MeCN. In order to increase the
disulfide solubility, the reaction was carried out in other solvents and 3i was obtained in a 56 % yield
in DMF and in a 63 % yield in acetone. No product formation was observed in the reaction with
4-nitrophenyl disulfide, 4-hydroxyphenyl disulfide and diphenyl diselenide. Aliphatic disulfides
(dicyclohexyl and n-dibutyl disulfides) were not active in the studied reaction.
High stereo-selectivity is an important advantage of the developed catalytic system. For the
studied reaction, the formation of the trans- isomer was not observed. Product 3 with the
cis-arrangement of the ArS groups was obtained with excellent selectivity.
Scheme 2. Scope of diaryl (A) and alkyl aryl (B) disulfide addition to acetylene catalyzed by Ni-4
1
MOF. Yields were determined by H NMR (isolated yields are shown in parentheses). [a]The
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[b]
(1Z,3Z)-1,4-bis((4-chlorophenyl)thio)buta-1,3-diene byproduct was formed in a 32 (30) % yield.
The reaction was carried out in acetone.
Interesting results were obtained in the reaction of acetylene with methyl phenyl disulfide
(Scheme 2B), where 1,2-bis(phenylthio)ethene 3a was detected as the only product. Moreover, GC-
MS analysis of the reaction mixture showed the formation of Ph2S2 and Me2S2 as byproducts (Fig.
S5). We suppose, that these compounds appeared in course of phosphine-catalyzed disulfide
methathesis process, which was described earlier in the literature [25,26]. This observation allows to
establish the phosphine role in the reaction mechanism.
On the basis of the experimental data, the reaction mechanism was proposed (Scheme 3). The
catalysis is initiated in the course of substitution of marginal linkers by ArS groups on the surface of
the initial MOF. This stage can be promoted by the phosphine providing the thiolated form of MOF
(4). There are two possible pathways of the formation of complex 4, where the phosphine ligand
either (1) reduces nickel in the initial MOF, thus stimulating the further disulfide addition; or (2)
coordinates to Ar2S2 and weakens the S-S bond, thus facilitating the substitution of the O-containing
ligand on the MOF surface. It is noteworthy that, according to the XRD and EDX data (Fig. 1 - 3),
the linker substitution takes place only at specific MOF sites on the crystal surface without alteration
of the deep structure and without formation of a [NiSAr2]n coordination polymer. These data on the
preservation of the MOF crystallinity, together with experimental observations of the methathesis
process (Scheme 2B, Fig. S5), supports the second pathway of the formation of complex 4.
The formed MOF-based thiolate 4 is able to coordinate acetylene by different metal centers
leading to π-complex 5 and further acetylene inserts into the Ni-S bond, thus forming intermediate 6.
The involvement of the apical site into the alkyne coordination is of particular importance for Ni
complexes [12]. Thus, the coordination environment of the MOF structure provides the fascinating
opportunity for realization of such a mechanism. The cis-configuration of the resulting product 3
(Fig. S6) supports the proposed configuration of insertion complex 6. At the last step, elimination of
the product followed by addition of another disulfide molecule closes the catalytic cycle.
Scheme 3. Proposed mechanism of Ni-4-catalyzed disulfide addition to acetylene.
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4. Conclusions
In summary, current study reports utilization of Ni MOFs in the catalytic reaction with gaseous
acetylene. Ni-MOF-74 served this goal and it was used for the first time in the reaction of aromatic
disulfide addition to acetylene. The unique feature of the proposed MOF is its ability to stabilize the
heterogeneous form of the catalyst by binding nickel thiolate with the MOF structure. The study
points out that the MOF utilization for the catalytic purpose relies not only on its porosity, but also on
its ability to retain crystallinity and regularity during the reaction. The studied catalytic reaction
resulted in the exclusive formation of one product in the form of the cis- isomer. Excellent
stereoselectivity was confirmed for a variety of disulphides.
Acknowledgements
The reported study was funded by RFBR according to research project № 18-33-01075 mol_a.
Authors gratefully acknowledge Dr. Sci. Igor V. Mishin for acquisition of PXRD patterns.
Declaration of Competing Interest
The authors declare that they have no known competing financial interest of personal relationships
that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data
Supplementary data associated with the article can be found in the online version.
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
The first example of Ni-MOF-74
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catalyzed
S-functionalization of
acetylene is reported


The Ni-MOF-74 is stable to leaching
of Ni species to solution
It was found that the method of Ni-
MOF-74 synthesis effects MOF
catalytic activity

Experimental study of MOF structure
modification during catalytic process
is provided
10

Figure 1

Figure 2

Figure 3