Tunable Prussian blue analogues for the selective synthesis of propargylamines through A3 coupling

Article abstract of DOI:10.1039/c8cy00073e

M¹[Co(CN)₆]_2/3-type Prussian blue analogues (M¹-Co PBAs) were studied as catalysts for the synthesis of propargylamines via A³ coupling of phenylacetylene, benzaldehyde and piperidine. Cu_0.86

Full text of DOI:10.1039/c8cy00073e

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Tunable Prussian blue analogues for the selective
3
synthesis of propargylamines through A
Cite this: DOI: 10.1039/c8cy00073e
coupling†
Received 11th January 2018,
Accepted 16th March 2018
Carlos Marquez,
Francisco G. Cirujano,
Cédric Van Goethem,
DOI: 10.1039/c8cy00073e
Ivo Vankelecom, Dirk De Vos * and Trees De Baerdemaeker
*
rsc.li/catalysis
1
1
M ijCoIJCN)
6
]
2/3-type Prussian blue analogues (M –Co PBAs) were
changes in synthesis procedures (e.g. using alcohols and
other organic additives). For example, Zn–Co PBA based ma-
3
studied as catalysts for the synthesis of propargylamines via A
1
9,20
coupling of phenylacetylene, benzaldehyde and piperidine.
Cu0.86Zn0.14–Co PBA was the best catalyst for the reaction by
combining the high conversion obtained with Cu–Co PBA with the
excellent selectivity obtained with Zn–Co PBA.
terials are well-known epoxide polymerization catalysts
and have also been used as catalyst for the activation of al-
2
1
kynes in hydroamination reactions (C–N bond formation),
2
2,23
and for copolymerization of CO and epoxides.
Moreover,
2
2
+
2+
mixed metal PBAs (Fe ,Cu –Co PBA) have been employed as
The reaction between a terminal alkyne, a secondary amine
and an aldehyde, also known as the A coupling, is a
multicomponent reaction with a high atom efficiency and
water as the only by-product. Therefore, it is considered a
green process for the synthesis of pharmaceutical
intermediates or final products such as bioactive
propargylamines.
substantially short times and with high selectivity to the
propargylamine product (the A product) the use of a catalyst
normally containing transition metals – is necessary.
simplify catalyst recovery, heterogenization of the active
phase is desired.
Prussian blue analogues (PBAs), in many cases also re-
ferred to as double metal cyanides (DMCs), are cyanide-
bridged transition metal coordination polymers with the gen-
eral formula M ijM IJCN) ] ·xH O (hereafter abbreviated as
M –M PBA”). PBAs are easily synthesized by a precipitation
reaction between aqueous solutions of the cyanometalate
complex, [M IJCN) ] , and an M salt. Even though PBAs
solid catalysts for the aerobic oxidation of oximes to carbonyl
3
24
compounds. In this work, we have synthesized a series of
PBAs based on earth-abundant divalent metals (Fe, Co, Ni,
Cu and Zn) and investigated their potential for the synthesis
3
of propargylamines via C–H activation in the A coupling re-
action of phenylacetylene, benzaldehyde and piperidine
1
–5
In order to carry out the reaction in
(
Scheme 1). To the best of our knowledge, this is the first
time that PBAs are applied for C–H activation of
phenylacetylene in multicomponent reactions.
3
6
–12
–
To
A series of PBAs were synthesized by modifying previously
25,26
reported procedures
through addition of an aqueous so-
6
,13–17
1
lution of K ijCoIJCN) ] to an aqueous solution of a M Cl ·xH O
3
6
2
2
2 2 2 2 2 2 2 2
salt (FeCl ·4H O, CoCl , NiCl ·6H O, CuCl ·2H O or ZnCl )
containing PTMEG and tert-butanol. ICP analyses of selected
1
PBA samples show that the M /Co ratio obtained is higher
1
u
2
n v
2
than the stoichiometric 1.5, indicating that a slight excess of
1
2
“
1
M is present in the structure (Table S1†). This is expected,
1
considering that with a 10 to 1 ratio M Cl
2
·xH
2
O to
2
u−
1
18
1
6
3 6 2 2
K ijCoIJCN) ], an excess M Cl ·xH O was used during the syn-
were among the first reported coordination polymers, their
thesis. The crystallinity of the samples was confirmed by
powder X-ray diffraction (PXRD, Fig. S1†). All samples show
1
8
use as catalyst only dates back to the 1960s. Recently, sev-
eral studies have focused on the expansion of the catalytic ap-
plications of PBAs. This can be achieved by virtue of the mul-
tiple possible variations in the active metal as well as by
Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F,
3
001 Leuven, Belgium. E-mail: dirk.devos@kuleuven.be,
trees.debaerdemaeker@kuleuven.be
Electronic supplementary information (ESI) available: Experimental details
synthesis of the PBAs, characterization, catalytic testing), additional characteri-
†
(
3
zation (ICP, high resolution XRD patterns and Pawley fitting, N
isotherms) and catalytic data. See DOI: 10.1039/c8cy00073e
2
physisorption
Scheme 1
A
coupling between phenylacetylene, benzaldehyde and
piperidine to produce the corresponding propargylamine.
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reflections corresponding to a cubic phase typical for metal
aldehyde over the C–H activation of phenylacetylene under
these reaction conditions (Fig. S6†).
27–29
1
hexacyanocobaltates.
As expected for the different M s,
the FTIR spectra of the samples show a blue shift in the posi-
On the one hand, the highest phenylacetylene conversion
was obtained with Cu–Co PBA (Fig. 1). On the other hand,
the selectivity towards the A product was higher using
Zn–Co PBA compared to the other metals. In the latter case,
just a small amount of acetophenone was produced. This
trend is also maintained when the selectivity is assessed at
the same phenylacetylene conversion (Fig. S7†). In light of
tion of the CN stretching band compared to K ijCoIJCN) ] (Fig.
3
6
2
6,30,31
3
S2†).
The textural properties of the PBAs vary
1
depending on M , as evidenced by N
2
physisorption (Table S2
and Fig. S3†). Although this type of material is usually micro-
porous in nature, the N2 isotherms of Fe–Co, Co–Co and
Cu–Co PBA also exhibit a hysteresis loop around p/p = 0.8,
0
indicative of the presence of mesopores. The acid properties
of selected samples were studied with pyridine adsorption
followed by FTIR spectroscopy (Fig. S4 and Table S2†). The
this, a series of Cu Zn –Co multi-metal PBA complexes with
x 1−x
different Cu/Zn ratios was prepared with the aim of tuning
the catalytic performance of the system.
Elemental analysis showed that the multi-metal samples,
Cu Zn –Co PBA, contain a much larger amount of Cu than
x 1−x
Zn, compared to the initial Cu/Zn molar ratios used during
synthesis (Table S1†). High resolution X-ray diffraction mea-
surements and Pawley fitting (Fig. S8†) allowed the refine-
ment of the lattice parameters of the sample Cu0.86Zn0.14–Co
PBA, which was found to crystallize in the cubic space group
Fm ¯3 m – just like Cu–Co PBA and Zn–Co PBA (Fig. S9 and
−
1
bands at 1450, 1490 and 1610 cm are attributed to pyri-
3
2
dine adsorbed on Lewis acid sites. No band was observed
−
1
around 1540 cm , which indicates that there are no
3
2
Brønsted acid sites in the samples.
All synthesized PBA samples were investigated both in
3
terms of activity and selectivity to the A product. The reac-
1
tion rate was particularly sensitive to the nature of the M
1
metal (Fig. 1 and S5†). For the bimetallic M –Co PBAs, the
highest reaction rate was obtained with Cu–Co PBA,
exhibiting an activity one order of magnitude higher than the
other PBAs. These results are in line with the reported excel-
lent catalytic activity of Cu sites for this type of
S10†). As expected given the high Cu content, the lattice pa-
rameters of Cu0.86Zn0.14–Co PBA are in between those of
Cu–Co PBA and Zn–Co PBA, but much closer to those of
Cu–Co PBA (Table S3†). Similarly, the rest of the multi-metal
samples show reflections corresponding to a cubic phase
(Fig. S11†). Other physicochemical properties of the multi-
metal samples (ν(CN), Lewis acidity, textural properties) are
also intermediate between those of Cu–Co PBA and Zn–Co
PBA (Table S2 and Fig. S3, S4 and S12†). Furthermore,
HAADF-STEM images (Fig. 2) of the sample Cu0.86Zn0.14–Co
PBA confirm the formation of a single PBA phase and the close
1
4–17,33,34
reaction.
In the case of Fe–Co PBA, the activity for
3
the A coupling reaction was low and the predominant reac-
tion was the reduction of benzaldehyde to benzyl alcohol,
most likely following a Meerwein–Ponndorf–Verley (MPV)
mechanism due to the presence of 2-butanol as solvent and
potential reductant. Even though the use of Fe as catalytic
3
site has been reported for both A coupling and MPV reac-
1
2,35,36
tions,
this specific Fe site favors the reduction of benz-
3
Fig. 1 Conversion (■) and selectivity ( ) to the A product for the
coupling of phenylacetylene (0.05 mmol), piperidine (0.1 mmol) and
benzaldehyde (0.1 mmol) after 24 h reaction time at 383 K and initial
3
3
rate ( ) of the A coupling expressed as μmol of A product formed
per h over 10 mg of PBA. Conversion and selectivity are based on
phenylacetylene. Acetophenone was the only phenylacetylene-derived
side-product detected.
Fig. 2 HAADF-STEM image (a) and EDX composition mapping for Zn
(b), Co (c) and Cu (d) of the sample Cu0.86Zn0.14–Co PBA.
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−
1
3
Table 1 TOF (h ) in the A coupling reaction and in additional test reac-
proximity between Zn and Cu, as no segregated Zn-rich or
Cu-rich phases are observed.
1
tions for selected M –Co PBA samples
The incorporation of Cu into the Zn–Co PBA increased the
catalytic activity of the solid for the A coupling reaction in
Cu
Zn
Cu0.86Zn0.14
2.1
3
3
a
TOF A coupling
TOF phenylacetylene hydration
0.43 0.052
0.18 0.0025 0.055
TOF benzaldehyde–piperidine coupling 0.24 0.43
comparison to the original Zn–Co PBA (Fig. 3). Remarkably,
at higher Cu content, the simultaneous presence of Cu and
Zn in the structure yielded catalysts exhibiting activity supe-
rior to that of Cu–Co PBA. In fact, the TOF obtained with the
most active multi-metal sample (Cu0.86Zn0.14–Co PBA) was al-
most five times higher than the one obtained with Cu–Co
PBA (Table 1). This suggests a synergistic effect when com-
bining Zn and Cu in the same crystalline framework. Further-
b
c
0.40
a
3
Turnover frequency based on initial rates of the A coupling
reaction expressed as mmol of A product formed per mmol of M
(Cu, Zn or Cu + Zn) per h. Turnover frequency for the hydration of
phenylacetylene (0.05 mmol) at 383 K for 6 h expressed as mmol of
acetophenone formed per mmol of M (in the absence of piperidine
and benzaldehyde) per h. Turnover frequency for the piperidine
0.05 mmol) and benzaldehyde (0.05 mmol) coupling at 383 K for 24
h expressed as mmol of 1-benzylpiperidine and α-phenyl-1-
piperidinemethanol formed per mmol of M in the absence of
phenylacetylene per h.
3
1
b
1
c
(
3
more, the selectivity to the A product is also increased with
1
respect to the Cu–Co PBA, from 60% to 93% at 90% conver-
sion of phenylacetylene (Fig. S7†). This increase is attributed
to the presence of Zn sites in the PBA structure: they could fa-
cilitate the formation of the iminium ion, while also helping
in the C–C bond formation between the iminium ion and the
2
+
353 K, our results show that a specific site – Zn in this case
– is needed for this reaction to occur considerably. Without
Zn, the rapid activation of phenylacetylene (on Cu sites)
yields considerable amounts of acetophenone, whereas with-
out Cu in the structure, this activation takes place too slowly
and the coupling of phenylacetylene, piperidine and benzal-
dehyde does not occur substantially. Blank experiments of
the reactions under the same conditions did not produce de-
tectable amounts of any product, also confirming this
hypothesis.
Moreover, the selectivity to the A product was very differ-
ent when the Cu active sites were in the form of soluble spe-
cies (Table S4†). In the case of the homogeneous CuIJOAc)
CuCl and CuIJClO ) catalysts, only 1,4-diphenylbuta-1,3-diyne
(from the homocoupling of phenylacetylene) was detected as
a product after 24 h, which is not formed when PBAs are
used as catalyst. This result suggests that under these reac-
tion conditions, the Cu centers in the PBA framework are
stable and maintain their +2 oxidation state. Additionally,
3
7
Cu-coordinated alkyne (Scheme 2).
To further prove this hypothesis, additional reactions were
performed first in the absence of, and then with only
phenylacetylene in the reaction mixture (Table 1). Results
2
+
show that Zn sites seem to facilitate the coupling between
benzaldehyde and piperidine to form α-phenyl-1-piperidine-
methanol and 1-benzylpiperidine, with Zn–Co PBA exhibiting
a TOF almost double the TOF of Cu–Co PBA for this reaction.
The product 1-benzylpiperidine is believed to be formed by re-
duction of the iminium ion via a hydrogen-transfer mecha-
3
3
8
nism involving the 2-butanol solvent. In contrast, Cu sites
appear to enable the activation of phenylacetylene. The TOF
obtained with the Cu–Co PBA was two orders of magnitude
higher than the one obtained with Zn–Co PBA for the hydra-
2
,
2
4 2
1
3,39
tion of phenylacetylene. Contrary to previous reports
claiming that the formation of the iminium ion between the
aldehyde and the amine occurs almost spontaneously above
2
+
1
5
Cu0.86Zn0.14–Co PBA exhibited a similar selectivity and a
higher conversion than the homogenous ZnCl salt. A compa-
2
rable activity was obtained with respect to other heteroge-
neous Cu-containing catalysts reported in literature, such as
of [CuIJ2-pymo)
graphene under similar reaction conditions.
2
] and Cu nanoparticles supported on
1
5,40
In aprotic
solvents (both non polar like toluene and dioxane, and polar
3
and highly coordinating such as DMSO) the yields of the A
product decreased (Table S5†). The best catalytic perfor-
mance of the Cu0.86Zn0.14–Co PBA was obtained with 2-buta-
nol as a solvent at 383 K. Protic, polar solvents have been
3
found to improve the rate of A coupling reaction, presum-
ably by facilitating initial iminium ion formation due to the
4
1
stabilization of charged activation states.
3
The yield of A product vs. time is plotted in Fig. S13† for
Cu0.86Zn0.14–Co PBA. The kinetics of the A coupling reaction
3
are still a matter of debate. It has been reported that the
phenylacetylene conversion vs. time plot should follow a
trend similar to pseudo second-order kinetics because the
production of the propargylamine depends on the
3
Fig. 3 Yield of A product after 24 h reaction time for the coupling of
phenylacetylene (0.05 mmol), piperidine (0.1 mmol) and benzaldehyde
0.1 mmol) at 383 K over 10 mg of Cu
Cu + Zn) molar ratios.
(
x
Zn1−x–Co PBA with different Cu/
(
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3
Scheme 2 Proposed reaction mechanism for the A coupling reaction of phenylacetylene, piperidine and benzaldehyde catalyzed by a Cu
x
Zn1−x–Co
3
multi-metal PBA. a = α-phenyl-1-piperidinemethanol, b = 1-benzylpiperidine, c = A product (1-(1,3-diphenyl-2-propyn-1-yl)piperidine).
concentration of both phenylacetylene and the iminium Conclusions
1
5
ion. However, more recent studies fitted the data by a first-
order kinetics equation with respect to phenylacetylene.
4
2
The catalytic performance of a series of PBAs was evaluated
for the synthesis of propargylamines through A coupling.
These materials show the possibility of tuning their catalytic
performance by virtue of the multiple possible variations in
the active metal. Here, the combination of Zn and Cu yielded
3
Analysis of the conversion of phenylacetylene at different re-
action times reveals that the best fit (highest coefficient of
2
determination, R ) was obtained when the data were fitted by
first-order in the alkyne (Fig. S14†). This result was further
supported by the evaluation of the variation of the reaction
rate with respect to the concentration of phenylacetylene
a series of synergistic Cu Zn –Co multi-metal PBA com-
x
1−x
plexes, with Cu0.86Zn0.14–Co PBA proving to be an active, se-
lective and recyclable heterogeneous catalyst for the reaction.
(Fig. S15†). When the concentration of phenylacetylene was
doubled, the rate of the reaction also doubled, which sug-
gests that the reaction order with respect to this reactant is
equal to one. In contrast, when the concentration of piperi-
dine was varied instead (Fig. S16†), a lower reaction rate for
Conflicts of interest
There are no conflicts to declare.
3
the A coupling was observed as the initial concentration of
Acknowledgements
piperidine was increased. This not only shows that the reac-
tion order is not equal to one with respect of piperidine, but
also suggests inhibition caused by piperidine due to strong
This project has received funding from the European Union's
Horizon 2020 research and innovation programme under the
Marie Sklodowska-Curie grant agreement No. 641887 (project
acronym: DEFNET). F. G. C. acknowledges the European
Union's Horizon 2020 research and innovation programme
under the Marie Sklodowska-Curie grant agreement No.
750391 (project acronym SINMOF) for financial support. F. W.
O. Vlaanderen (Research Foundation Flanders) is thanked for
project funding (D. D. V.: research projects; C. V. G. and I. V.:
grant no. G.0256.14N) and a Postdoctoral Fellowship (T. D.
B.). D. D. V. thanks KU Leuven for the Metusalem grant
CASAS. Funding for the TEM through Hercules project AKUL/
13/19 is kindly acknowledged. The authors are grateful to prof.
Andrew L. Goodwin, Hanna L. B. Boström, Emily M. Reynolds
(University of Oxford) and the beamline staff of I11 (Diamond
2
+
43–47
adsorption on Cu sites or formation of Cu complexes.
Finally, the heterogeneity of the catalyst was studied by a
hot filtration test. As shown in Fig. S17,† the hot filtrate shows
no appreciable activity after stirring for an additional 20 h, in-
dicating that there is no leaching of active species from the
catalyst. This was further corroborated by elemental analyses
performed after reaction (Table S1†). Remarkably, recycling
tests show that Cu0.86Zn0.14–Co PBA largely maintains its activ-
ity after five runs (80% yield after fifth run vs. 85% for the
fresh catalyst), even though there is a phase change when
compared to the pristine sample (Fig. S17†). However, no no-
table changes were observed in the FTIR spectrum of
Cu0.86Zn0.14–Co PBA after one reaction cycle (Fig. S18†).
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Light Source) for collecting the high resolution XRD data dur-
ing Block Allocation Group beamtime (EE13284). The authors
thank Michael T. Wharmby (DESY) for useful discussions and
prof. Jin Won Seo (KU Leuven) for TEM support.
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Weckhuysen, F. G. Cirujano, D. De Vos and T. De Baerde-
maeker, J. Catal., 2017, 354, 92–99.
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