A high-activity cobalt-based MOF catalyst for [2 + 2 + 2] cycloaddition of diynes and alkynes: insights into alkyne affinity and selectivity control?

Article abstract of DOI:10.1039/c7ra12136a

This work develops a highly efficient metal–organic framework (MOF) catalyst for the straightforward synthesis of functionalized benzenes via [2 + 2 + 2] cycloaddition. As efficient cooperative catalyst for such reactions, Co-MOF-1, shows great sustainability and efficiency. This new catalytic system can lead to the generation of a series of structurally diverse benzenes in good to excellent yields (up to 95%).

Full text of DOI:10.1039/c7ra12136a

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A high-activity cobalt-based MOF catalyst for
[2 + 2 + 2] cycloaddition of diynes and alkynes:
Cite this: RSC Adv., 2018, 8, 4895
insights into alkyne affinity and selectivity control†
Fen Xu,
Chun-Sen Liu
Xiao-Ju Si, Xiao-Ning Wang, Hao-Dong Kou, Di-Ming Chen,
*
*
and Miao Du
Received 5th November 2017
Accepted 15th January 2018
This work develops a highly efficient metal–organic framework (MOF) catalyst for the straightforward
synthesis of functionalized benzenes via [2 + 2 + 2] cycloaddition. As efficient cooperative catalyst for
such reactions, Co-MOF-1, shows great sustainability and efficiency. This new catalytic system can lead
to the generation of a series of structurally diverse benzenes in good to excellent yields (up to 95%).
DOI: 10.1039/c7ra12136a
rsc.li/rsc-advances
Cycloaddition is a promising strategy used to develop a new shown activity in [2 + 2 + 2] cycloaddition, though the dimer-
synthetic methodology with high atom economy, providing ization or trimerization of terminal diynes usually occurs.15 On
efficient routes to construct various chemical bonds resulting in this account, a porous MOF [Co2(HCOO)2(CPT)2](NMF)5(H2O)2
molecular complexity.1 For example, the [2 + 2 + 2] cycloaddition (Co-MOF-1) with high affinity of acetylene was successfully
of diynes and alkynes has been a powerful tool for the synthesis constructed based on
a
bi-functional ligand 4-(4-
of functionalized benzenes in one step.2 A variety of transition carboxyphenyl)-1,2,4-triazole (HCPT).16 As a result, a variety
metal catalysts, including Rh,3 Ru,4 Pd,5 Ir6 and Ni7 have shown of benzene derivatives can be obtained via Co-MOF catalyzed
their capacity for [2 + 2 + 2] annulations. However, in this [2 + 2 + 2] cycloaddition in high yields with great tolerance of
connection, three crucial problems need to be resolved. (1) The substrates and the crystalline defect of catalyst will account for
competing dimerization or trimerization remains a challenging the signicant selectivity of the reactions (Scheme 1).
issue for [2 + 2 + 2] cycloaddition with high selectivity. (2) These
The Co-MOF-1 was harvested by reaction of Co(NO3)2$6H2O
unrecoverable homogeneous catalysts will take major and HCPT in NMF-water. Single-crystal X-ray diffraction anal-
a
responsibility for energy waste and environmental pollution. (3) ysis indicates that Co-MOF-1 crystallizes in the trigonal space
The sensitivity to air or moisture for the known catalysts may group R3c.16 A mentionable structural feature is the presence of
ꢀ
˚
greatly reduce the reaction efficiency and make the operation distorted octahedral cages with the inner diameter of 7.9 A,
complicated. As a result, the development of easily separable constituted by twelve Co(II) and twelve CPTꢀ for each cage
and recoverable heterogeneous catalysts is of great signicance (Fig. 1a and S2†). The octahedral cages are sequentially
and interest in this respect.
extended via sharing the trigonal windows. As a result, the 3D
Metal–organic frameworks (MOFs) represent a rising class of framework displays 1D hexagonal channels along a axis with
˚
porous crystalline materials with exceptional surface areas and the pore diameter of 5.4 A (Fig. 1b, S3 and S4†) and the available
3
uniformly dispersed metal ions.8 Such inherent properties void of 16 835 A , corresponding to 48.6% of the crystal volume
˚
3
˚
make MOFs as advanced materials for gas storage and separa- (34 647.6 A ).
tion,9 drug delivery,10 heterogeneous catalysis,11 and so on.
The porosity of Co-MOF-1 was conrmed by N2 sorption at
Recently, we have designed a series of MOFs materials with 77 K. The CH2Cl2-exchanged Co-MOF-1 was activated at 70 ꢁC to
diverse pore shapes, sizes and compositions for targeted afford the desolvated material Co-MOF-10 (Fig. S5 and S6†).
applications12 and also developed diversied metal-catalyzed
cycloadditions involving alkynes.13 Thus, it will be anticipated
to explore the potentials of MOFs as the new-type catalysts for
cycloadditions, which may avoid the restrictions for the known
examples. Notably, Co(I) or Co(II)/additive14 catalytic system has
College of Material and Chemical Engineering, Zhengzhou University of Light Industry,
Zhengzhou 450002, China. E-mail: chunsenliu@zzuli.edu.cn; dumiao@zzuli.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1491616–1491623.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra12136a
Scheme 1 Catalytic strategy for [2 + 2 + 2] cycloaddition in this work.
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Table 1 Optimization of reaction conditionsa
Entry
Catalyst
Temp. (ꢁC)
Yieldb (%)
1
2
Co-MOF-1
Co-MOF-10
Co-MOF-10
Co-MOF-10
Co-MOF-10
Co-MOF-10
Co-MOF-10
Co-MOF-10
Co(NO3)2
80
80
80
80
40
RT
80
80
80
80
40
59
86
28
Trace
Trace
Trace
60
12
N.D
3c
4d
5
6
Fig. 1 (a) The serial octahedral cages in Co-MOF-1. (b) The 3D
framework of Co-MOF-1 with 1D channels running along a axis. (c) N2
sorption isotherm at 77 K for Co-MOF-10 (inset: pore size distribution
plots). (d) C2H2 adsorption isotherms of Co-MOF-10 at 273 and 298 K.
7e
8c,f
9g
10h
Co-MOF-10
a
Reaction conditions: diyne 1a (0.25 mmol), phenylacetylene 2a
(2 equiv., 0.5 mmol), Co-MOF (10 mg), dppp (6 mol%), Zn powder
Notably, Co-MOF-10 shows an initial type-I adsorption isotherm,
as a typical microporous material and then, a type-IV adsorption
with obvious hysteresis and a signicant portion of the meso-
pore volume of 0.46 cm3 gꢀ1 (Fig. 1c). This can be a consequence
of the dissociation of Co(II) ions and formation of defect sites in
b
c
(10 mol%) and DCE (2 mL) at 80 ꢁC for 24 h. Isolated yield. 2a
d
e
f
(4 equiv.).
Co-MOF-10 (1 mg).
No dppp. dppp (3 mol%).
g
h
Co(NO3)2 (5 mol%), dppp (6 mol%) and Zn powder (10 mol%). In
the absence of Zn powder, N.D (not detected).
lattice.17 The maximal N2 uptake is 307 cm3 gꢀ1, corresponding to
a BET surface area of 947 m2 gꢀ1 and pore diameter of 5.4 A. The determined as Co-MOF-10 (10 mg), dppꢁp (6 mol%) and Zn
˚
pore size distribution is consistent with the observation from its powder (10 mol%) in DCE (2.0 mL) at 80 C for 24 h.
crystal structure. The combination of high porosity and defect
The generality and functional-group tolerance was explored
sites in Co-MOF-10 promotes us to investigate its affinity toward by different substituted alkynes. A broad range of phenyl-
acetylene, showing a high C2H2 adsorption capability (Fig. 1d) of acetylenes with either electron-donating or -withdrawing
178 cm3 gꢀ1 at 273 K and 1 bar and 120 cm3 gꢀ1 at 298 K and groups, could react smoothly with NTs-diyne 1a and enable
1 bar. The isosteric heat of sorption (Qst) for C2H2 molecule was the formation of substituted benzenes 3aa–ad and 3af–ah in
calculated to evaluate the affinity between C2H2 and Co-MOF-10 86–95% yields (Scheme 2). Initially, 1a was treated with 2a,
(Fig. S7 and S8†). The result shows a high Qst value of 36 kJ molꢀ1
at zero loading, revealing a preferred binding capacity for C2H2
compared with the reported MOFs without high density of active
sites (Qst values: 13.3–32.0 kJ molꢀ1).18
Catalysis of [2 + 2 + 2] cycloaddition by Co-MOF-1 was initi-
ated from a terminal NTs-diyne 1a and phenylacetylene 2a
(Table 1), in the presence of 6 mol% 1,3-bis(diphenylphos
phino)propane (dppp) and 10 mol% Zn powder in 1,2-dichlo-
roethane (DCE). It is interesting that 3aa can be obtained
ꢁ
directly in 40% yield aer reaction at 80 C for 24 h (entry 1).
The yield was enhanced to 59% when Co-MOF-10 was used and
higher yield was achieved in the presence of 4 equiv. of phe-
nylacetylene (entries 2 and 3). Subsequent tests indicated that
reducing the temperature was greatly to the disadvantage of
annulation (entries 5 and 6) and a control experiment illus-
trated that dppp was necessary for this transformation (entry 7).
In addition, decreasing the loading of dppp also sharply cut
down the yield of target product (entries 3 and 8). It was also
found that the transformation was dramatically affected by the
catalyst, and the combination of Co(NO3)2 and dppp was
incompetent to produce 3aa in only 12% yield (entry 9). 3aa was
not detected in the absence of Zn powder (entry 10). The air-
stable and efficient Co-MOF-10 catalyst can be separated and
recycled that makes it much sustainable compared to known
examples. Therefore, the optimal reaction conditions were Scheme 2 Formation of benzenes from diynes 1a–j and alkynes 2a–j.
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Co-MOF-10 and dppp in DCE. This led to the formation of 3aa in active species of this transformation. As the presence of Co(II)
86% isolated yield. As illustrated, aromatic alkynes bearing Me– ions in the initial reaction solution (25.5 mg Lꢀ1) was detected
and F– groups served as the most proper substrates in this case by ICP-MS, we suspected that such a reaction would begin with
and showed unconspicuous effect on the reaction efficiency a sluggish release of Co(II) due to the crystal defect of catalyst,
(Scheme 2, 3ad and 3ag). In contrast, the ortho-substituted which was then reduced by Zn powder to give Co(I) species. The
1-ethynyl-2-methyl benzene showed extremely poor reactivity active Co(I) surrounded the surface of Co-MOF-10 material to
(Scheme 2, 3ae). Further, para- and meta-substitution was also facilely capture the neighboring diyne A via ligand exchange,
tolerated, while a slight reduced yield was found for aromatic forming a 5-membered metallacyclopentadiene intermediate B
alkyne with meta-substitution (Scheme 2, 3ag and 3ah). through traditional oxidative addition. However, attempt to
Notably, the reaction of pharmaceutically important aromatic isolate the species B was failed. Aer that, B will either
substituted alkynes, aiming at introducing uorine and/or undergo alkyne insertion to form 7-membered azametallacycle
uorinated building blocks into the aromatic ring, has C or [4 + 2] annulation to produce D, resulting in benzenes via
remained a challenge thus far. Herein, 2g and 2h delicately reductive elimination to afford the nal product E and also
decorated by uorine were revealed as meaningful substrates as regenerate Co(I) for the next catalytic cycle (Fig. 2).
well, affording the corresponding products 3ag and 3ah in 93%
To conrm the catalytic mechanism, two control experi-
and 86% yields. However, sterically hindered internal alkynes 2i ments were further discussed. First, subjecting N,N-di(but-2-yn-
and 2j were incompatible, forming dimer or trimer of diynes as 1-yl)-4-methylbenzenesulfonamide 1h with 2a to this annula-
side-products, where the bulky phenyl groups may prevent the tion failed to deliver the target product. In fact, 1h was largely
contact of substrates with the active sites of catalyst.
retained aer reaction, revealing that higher steric internal
Subsequently, the scope of this reaction with respect to diyne will suppress the fast access to active Co(I) species on the
diynes was further extended. A variety of N-based diynes, surface of catalyst (eq. 1 in Fig. 3). Then, a compensatory
ranging from N,N-di(prop-2-yn-1-yl)benzenesulfonamide 1c experiment involving the coupling of terminal diyne 1a with
with electronically diverse groups on phenyl ring (1b–1d), alkyne 2j showed that the dimer of 1a was achieved instead of [2
N-benzyl-N-(prop-2-yn-1-yl)prop-2-yn-1-amine 1f, to N,N- + 2 + 2] annulation. This reveals that the internal alkyne will
di(prop-2-yn-1-yl) methanesulfonamide 1g, was used in this hardly get close to the active Co(I) due to the low affinity to
catalytic system. The corresponding products were obtained in Co-MOF-10 (eq. 2 compared with eq. 3 and eq. 4 in Fig. 3). Thus,
84–95% yields (Scheme 2, 3ba–ga), which are similar to those
reactions with NTs-based diyne 1a. Notably, the electronic
nature of phenyl ring for N-based diynes turned out to be
signicant for the cyclization. With Co-MOF-10 and dppp in the
presence of Zn powder, 3ba was achieved in a yield of 95%,
whereas 3aa was produced in 86% yield. Signicantly, cyclo-
adduct 3da with F– substituent was obtained in high yield of
93%. In accordance with the observation that steric factor
intensively inuences the reactivity of alkyne substrates,
internal diyne N,N-di(but-2-yn-1-yl)-4-methylbenzene sulfon-
amide 1h was failed to afford benzene scaffold 3ha. This work
was also highlighted by substrate of ester tethered diyne,
affording corresponding product 3ia in 87% yield. In addition
to N-based or ester-tethered diyne, dipropargyl ether showing
high reactivity for dimerization to arene byproduct,19 cyclized
Fig. 2 Proposed mechanism for the transformation.
with 2a to afford the corresponding benzo[c]furan 3ja in
moderate yield (Scheme 2).
The major advantage of heterogeneous catalysts is their
reutilization, which is one of the most important indexes to
evaluate the performance of catalyst effect and stability along
the catalytic cycle. Recycling experiments were performed on
Co-MOF-10 for the annulation of 1b and 2a. The catalyst was
recycled for ve runs with no decrease of activity, giving 93–95%
isolated yields (Fig. S9a†). Aer each case, the recovered
Co-MOF-10 catalyst was characterized by powder X-ray diffrac-
tion (PXRD) technique (Fig. S9b†). Notably, even aer the h
run, the PXRD pattern evidently conrmed that the catalyst
remained excellent crystallinity and was not damaged in the
catalytic process.
To further shed some light on the reaction mechanism,
auxiliary experiment was designed and conducted to probe the Fig. 3 The auxiliary verification experiments for catalytic mechanism.
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it can be concluded that the cycloaddition would proceed
favorably near the catalyst surface with “spot-activated cobalt”,
allowing only high-affinity alkynes to approach the metal center
and thus high selectivity can be realized for cycloadditions. This
strategy also provided the potentials for the development of
template and precise catalysts.
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In conclusion, a 3D Co(II)-based MOF was designed and
prepared, showing serial window-sharing octahedral cages and
1D hexagonal channels. The activated Co-MOF-10 has high
affinity toward acetylene and serves as a heterogeneous and
recoverable catalyst for [2 + 2 + 2] cycloaddition to afford highly
substituted benzenes with remarkable efficiency, selectivity,
and functional group tolerance. Notably, this transformation is
also compatible with aromatic alkynes with uorine substituent
and thus provided products with potential pharmaceutical
activity. Co-MOF-10 with high alkyne affinity and “activated
cobalt” can preclude the fast approach of sterically hindered
substrates to the active sites, which may well account for the
selectivity of achieved products. The results witnessed the great
success of Co-MOF-10 as catalyst for annulation and encouraged
further developments of new MOFs catalysts for expedient
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Conflicts of interest
There are no conicts to declare.
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