Subcomponent Self-Assembly of Covalent Metallacycles Templated by Catalytically Active Seven-Coordinate Transition Metal Centers

Article abstract of DOI:10.1021/jacs.0c01035

Coordination geometries of transition metals play vital roles in the self-assembly process of supramolecular coordination complexes. Herein, seven-coordinate 3d metal ions were applied as templates and catalytically active sites for subcomponent self-assembly that resulted in a new category of covalent metallacycles. Single-crystal structures showed that the sizes, configurations, and functionalization of covalent metallacycles could be tuned by the selection of rigid dihydrazide, transition metal ions, and prefunctionalized subcomponents, respectively. Moreover, metallacycles decorated with carboxylic groups could be employed as precursors to prepare aerogels through hierarchical self-assembly, which also exhibited high catalytic activity for cycloaddition of CO2 into cyclic carbonates.

Full text of DOI:10.1021/jacs.0c01035

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Communication
Subcomponent Self-Assembly of Covalent Metallacycles Templated
by Catalytically Active Seven-Coordinate Transition Metal Centers
Zhi-Wei Li, Xin Wang, Lian-Qiang Wei, Ivana Ivanovi#-Burmazovi#, and Gao-Feng Liu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c01035 • Publication Date (Web): 03 Apr 2020
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Subcomponent Self-Assembly of Covalent Metallacycles Templated
by Catalytically Active Seven-Coordinate Transition Metal Centers
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†
†
†
, ‡
, †
Zhi-Wei Li, Xin Wang, Lian-Qiang Wei, Ivana Ivanović-Burmazović,* and Gao-Feng Liu*
†MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou
510275, China.
‡Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nuremberg, 91058 Erlangen, Germany.
Supporting Information Placeholder
attention for their outstanding ability for superoxide
dismutation, CO reduction and H generation. A kind of
2 2
ABSTRACT: Coordination geometries of transition metals play
vital roles in the self-assembly process of supramolecular
coordination complexes. Herein, seven-coordinate 3d metal ions
were applied as templates and catalytically active sites for
subcomponent self-assembly that resulted in a new category of
covalent metallacycles. Single crystal structures showed that
the sizes, configurations and functionalization of covalent
metallacycles could be tuned by the selection of rigid
dihydrazide, transition metal ions and prefunctionalized
subcomponents, respectively. Moreover, metallacycles
decorated with carboxylic groups could be employed as
precursors to prepare aerogels through hierarchical self-
assembly, which also exhibited high catalytic activity for
[21]
[22]
[23]
mononuclear seven-coordinate compounds, in which the
pentadentate ligands resulted from condensation reaction between
[24]
2
,6-diacetylpyridine and monohydrazide, attracted our interest.
The angle (α) between the residual monohydrazide was found to be
variable in the range from 80° to 90°, which is dependent on the
ligated transition metal center (Scheme 1). Based on the edge-
[25]
directed principles, we conjectured that these motifs, serving as
a new type of vertices and combining with rigid edges, may provide
access to covalent metallacycles. Meanwhile, reversible acyl
hydrazone bonds have been employed to construct
thermodynamically favored supramolecules, including covalent
[26]
[27]
organic macrocycles and catenanes. With the above strategy,
we successfully prepared a series of covalent metallacycles through
2
cycloaddition of CO into cyclic carbonates.
II
II
II
II
self-assembly of transition metal ions (Mn , Zn , Co , and Fe )
with 2,6-diacetylpyridine (or its derivatives) and rigid dihydrazide
(Scheme S3), as well as their hierarchical assembly and application
Self-assembly, a ubiquitously spontaneous physicochemical
process in nature, has enabled accesses to supramolecular
2
in CO fixation. The choice of metal templates was based on the
fact that these 3d metal ions are capable of formation of seven-
[1]
structures capable of various applications. Over the previous few
[24,28]
coordinate complexes.
decades, Lehn, Stang, Fujita, Mirkin,[5] Raymond, and
other groups
[
2]
[3]
[4]
[6]
[7]
Scheme 1. Strategy for constructing covalent metallacycles
based on PBP metal templates. (Axial coordinated solvent
molecules are omitted for clarity).
have conducted impressive researches on
coordination-driven self-assembly, which is based on highly
directional metal-ligand bonds based on the predictable
coordination geometries of metal ions. An extension of this
approach called “subcomponent self-assembly”,[8] involving in situ
formation of reversible covalent and coordination bonds around
metal ion templates, was utilized to construct various sophisticated
[9-12]
metallosupramolecular architectures.
Among them, various
[13]
metallacycles
with well-defined sizes and shapes or their
[
14]
hierarchical assemblies
have found versatile applications in
[15]
[16]
[17]
[18]
recognition,
catalysis,
sensing
and biomedical fields,
respectively. Recently, Nitschke and co-workers reported several
covalent metallacycles and metallacages, in which a well-designed
II
[19]
bis(imino)pyridyl-Pd motif acted as a 90° building block.
A
synthesis of desired architectures requires mutual adaptations
between geometries of metal ions and ligands generated from
carefully selected precursors. However, there are only limited
examples of covalent metallacycles synthesized in one-pot
The assembly is rather facile. For example, compound 1 was
synthesized by a reaction of 2,6-diacetylpyridine, manganese(II)
perchlorate and terephthalic dihydrazide in a 1:1:1 ratio in DMF.
By subsequent addition of diethyl ether, 1 precipitated as yellow
solid in 94 % yield. In the high-resolution electrospray ionization
[10,19,20]
reactions.
To our knowledge, square metallacycles
containing catalytically active 3d transition metal centers have not
been reported in the literature. Thus, strategies for synthesizing
covalent metallacycles consisting of catalytically active 3d metal
centers, in high yields, are currently in demand.
(HR-ESI) mass spectrum, peaks at m/z = 300.24222, m/z =
Somewhat less conventional seven-coordinate complexes,
possessing pentagonal bipyramid (PBP) geometries have earned
375.05124, m/z = 499.73206 and m/z = 533.04999, corresponding
to [1-3H-8ClO ] , [1-4H-8ClO ] , [1-5H-8ClO ] , and [1-4H-
4 4 4
-
5+
- 4+
- 3+
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7
ClO
4
-]3+, respectively, were in an excellent agreement with the
Crystallographic analyses revealed that 3, 4 and 5 all adopt a
saddle-like configuration, with different degrees of distortion
(Figures S18-S22). As shown in Figures 2b and S18b, the distance
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theoretically predicted isotopic patterns (Figure S3). The successful
ESI-MS analysis also demonstrated the high stability of the
skeleton under MS conditions. [19]
II
between the adjacent Co centers is 11.3 Å, similar to that in 1,
II
while the distances between the opposite Co centers are 14.9 Å
X-ray-quality single crystals of 1 were obtained from diffusion
of aqueous NaOH solution into a methanol solution of 1. Due to the
presence of a base in the solution, the metallacycle 1 crystallizes in
a form with doubly deprotonated pentadentate moieties (denoted as
and 14.3 Å, respectively, being shorter than those observed for 1.
The bite angles (O-Co-O, 75.27°, 75.97°) observed in 3 are much
smaller than those (O-Mn-O, 86.26°, 87.76°) observed for 1, which
results in a decrease of the angle α and ultimately leads to the
saddle-like configuration. The continuous-shape measurements of
1
’ in SI). As shown in Figure 1, crystallographic analysis
II
unambiguously confirmed a fully covalent [4+4] metallacycle
the seven-coordinate Co centers in 3 was calculated to be 0.107,
II
structure, where four Mn ions sit on the vertices, while four
which is much closer to zero of the ideal D5h geometry than that of
other metallacycles presented here (Table S8). Comparing the
relevant parameters of these metallacycles (Table S9), analogous
general structural characteristics are observed for 4. Thus,
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phenyls occupy the edges. The angles (α) between the phenylene
II
rings at each corner are quite close to 90° and all four Mn centers
are in the same plane. The distances between the adjacent and
II
II
II
opposite Mn centers are 11.4 Å and 16.2 Å, respectively (Figures
considering that Co and Zn have similar ionic radii, we infer that
the configurations of covalent metallacycles are affected by the size
II
S14-S15). All Mn centers are seven-coordinated within a distorted
II
pentagonal bipyramidal geometries, as confirmed by the
of metal-ion templates for PBP vertices. Larger Mn cations
[
29]
II
II
continuous-shape measurements analyses.
The equatorial
chelating
support planarity, whereas smaller Co and Zn are suitable for
saddle-like self-assembly of supramolecular metallacycles
templated by seven-coordinate pentagonal-bipyramidal 3d metal
coordination sites are occupied by a pentadentate N
3
O
2
moieties from the covalent supramolecular skeleton and the axial
positions are occupied by two methanol molecules. The five donor
atoms from the skeleton form an ideal planar structure, as the sum
of the chelate angles and the bite angle measures 360.16° and
360.50° for Mn1 and Mn2, respectively (Table S3).
[30]
ions.
We then also synthesized metallacycles with prefunctionalized
moieties. Metallacycles 6 and 7 resulted from either methyl 2,6-
diacetylisonicotinate or 2,6-diacetylisonicotinic acid, respectively,
II
whereas 8 was obtained by a similar procedure using Co salt and
2
,6-diacetylisonicotinic acid (Scheme S3). HR-ESI-MS analysis
verified their compositions, analogous to those of 1-5 (Figures S8-
S10). Crystallographic analysis of 6 demonstrated that its structure
is similar to that of 1, while decorated with four ester groups at the
corners (Figure 2c). These results show that functionalization of
PBP vertices does not influence the self-assembly, enabling the
facile construction of covalent metallacycles with specific
functions.
To date, hierarchical self-assembly of macrocycles has been
extensively investigated in the fields of metal-organic
[
31]
[32]
frameworks,
metal-organic nanotubes,
covalent organic
[33]
[34]
frameworks, gels, etc. The free carboxylic groups tethered on
Figure 1. Crystal structure of 1 (in a form with deprotonated
pentadentate moieties) with 60% thermal ellipsoids. All hydrogen
atoms are omitted for clarity and the PBP coordination spheres are
highlighted in green (C: gray, N: blue, O: red, Mn: orange).
the corners of the metallacycles 7 and 8 allowed facile reactions
3+
3+
with metal ions such as Al and Fe (Scheme S4), resulting in the
cross-linking of the metallacycles. Consequently, gelation was
successfully achieved through solvothermal method and gels
formation could be verified by turning the sample upside down
(Figures S27-S30). Subsequently, solvent exchange and extraction
In order to construct larger covalent metallacycles, longer 1,1'-
diphenyl-4,4'-dicarbohydrazide was used in the multicomponent
reaction with 2,6-diacetylpyridine and Mn(ClO
4
)
2
·6H
2
O. HR-ESI-
2
with supercritical CO afforded the aerogels (Figure 2d), denoted
as 7-Al, 7-Fe, 8-Al and 8-Fe, respectively.
MS analyses demonstrated the formation of a complete [4+4]
architecture (Figure S4, denoted as 2). Crystallographic analyses
(Figures 2a, S16, S17) revealed the larger square structure, in which
Nitrogen-sorption isotherms of the aerogels measured at 77 K
exhibited typical type IV isotherms (Figure 3a), which is attributed
to their mesopores (Figure 3b). The Brunauer–Emmett–Teller
(BET) surface areas of 7-Al, 7-Fe, 8-Al and 8-Fe are determined to
II
the distances between the adjacent and opposite Mn centers are
II
1
5.7 Å and 22.2 Å, respectively, and the axial sites of Mn centers
are occupied by pyridines. It is interesting that even with the
2
−1
increased size of the covalent [4+4] metallacycles in 2, the four
be 61, 226, 207 and 122 m g , respectively (Table S10). Scanning
electron microscopy and transmission electron microscopy (TEM)
were used to observe the morphologies and microstructures of the
aerogels, which showed that the aerogels have porous network
structures consisting of interconnected nanoparticles (Figures S27-
S30). Powder X-ray diffraction disclosed that these materials are
amorphous (Figure S31). Scanning electron energy-dispersive X-
ray measurements demonstrated the homogeneous distribution of
bimetal ions in the aerogels (Figure S32). As yet, a number of
porous frameworks comprising of salen-Co moieties have been
II
seven-coordinate Mn ions at the vertices are still in the same plane.
II
This demonstrates that seven-coordinate Mn ions are appropriate
templates for the synthesis of planar covalent metallacycles.
To explore the effects of other transition metal ions as templates,
we performed multicomponent reactions of 2,6-diacetylpyridine
and terephthalic dihydrazide, with different metal salts in methanol,
which yielded 3, 4 and 5, respectively (Scheme 1). Chemical
compositions of the metallacycles were corroborated by HR-ESI-
1
MS (Figures S5-S7). Additionally, H NMR spectrum of 4
reported to show catalytic activities toward the fixation of CO
2
to
suggested a highly symmetrical structure (Figure S11) and the
signals were unambiguously assigned by COSY (Figure S12).
[35]
cyclic carbonates.
On account of accessible generation of
coordinatively unsaturated metal sites upon removing the axially
coordinated solvent molecules, we speculated that seven-
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coordinate metal ions incorporated into metallacycles and aerogels
As next, 3 and 8-Al were selected as heterogeneous catalysts for
1
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4
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may not only increase the affinity to CO
as Lewis acid catalytic centers.
2
(Figure S33) but also act
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Figure 2. (a) Crystal structures of 1 and 2. (b) saddle-like metallacycles 3-5 represented by the crystal structure of 3. (c) Crystal structure of
. (d) Schematic representation of the formation of the aerogels. (Axial coordinated solvent molecules and all hydrogen atoms are omitted
6
for clarity.)
2
performance in chemical fixation of CO by epoxides with variable
structures (Table S11, entries 4-8 and 12-18), where a decrease of
yields by increasing molecular size of epoxides was observed.
2
Moreover, taking the catalytic CO cycloaddition with propylene
oxide to produce propylene carbonate as an example, 3 could be
reused for five cycles without decline in catalytic performance
(Figures S34). The ESI-MS results of the recycled 3 demonstrate
its robustness during the catalytic reactions (Figure S35). However,
8
-Al exhibited unsatisfied performances in its recyclability test,
2
Figure 3. (a) N adsorption (solid circles) and desorption (open
circles) isotherms measured at 77 K. (b) Pore size distributions of
the corresponding aerogels based on NL-DFT method.
with yields dropping from 95% to 75% (Figures S39 and S40),
which may be attributed to the aggregation of nanoparticles (Figure
[37]
S36b).
To our best knowledge, a related post assembly of
covalent metallacycles and construction of corresponding porous
materials has not previously been reported. Compared with other
2
the formation of cyclic carbonates from CO and epoxides in the
presence of co-catalyst (TBAB) under solvent-free conditions at
room temperature and 1 atm. In order to avoid an influence of free,
III
III
reported heterogeneous catalysts comprising salen-M (Co , Al ,
III
II
Cr , Cu ) moieties (Table S12), the two catalysts exhibited
excellent efficiency in converting CO into cyclic carbonates at
[36]
non-coordinated, carboxylic group on the catalysis, we selected
2
3
and not 8 to be compared with 8-Al. As shown in Table
ambient temperature and pressure. Hence, we infer that the cavities
and catalytically active sites of our metallacycles could be
transcribe into higher-order porous materials, in which seven-
S11(entries, 1 and 2), metallacycle 3 and corresponding Co aerogel
8-Al both demonstrated catalytic activity towards the cycloaddition
of CO to propylene oxide and produced propylene carbonate with
2
yields of 74% and 95%, respectively. By way of comparison with
the control experiment (Table S11, entry 3), this represents a
significant catalytic performance. The better performance of 8-Al
may be attributed to its mesoporous structures, which could
facilitate to form more exposed catalytic centers as well as the mass
transfer process. In addition, they also demonstrated a satisfied
II
2
coordinate Co centers are capable of CO conversion into cyclic
II
carbonates. By way of comparison, a related Co mononuclear
complex, is soluble in propylene oxide, which precludes its use as
a heterogeneous catalyst (Figure S56). Furthermore, metallacycles
II
II
II
containing Mn , Zn , and Fe centers also exhibited good catalytic
performances (Table S11, entries 9-11). Further catalytic activity
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tests and detailed mechanistic studies of the cycloaddition reaction
catalyzed by our materials are ongoing in our laboratory.
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(
In summary, we have constructed a new class of covalent
metallacycles through subcomponent self-assembly, in which the
seven-coordinate 3d metal ions were introduced as templates and
catalytically active sites for the first time. By way of comparison,
literature known metallacycles are based on square-planar,
tetragonal-pyramidal, square pyramidal or octahedral geometries
of the metal centers (Table S13). In the view of the economic
synthesis, tunable structures and tailored functionalization of the
covalent metallacycles, our strategies could provide new access to
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X-ray crystallographic data for 2 (CIF)
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X-ray crystallographic data for 4 (CIF)
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Corresponding Author
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Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
This research was supported by the National Natural Science
Foundation of China (NFSC, No. 21272285) and the Natural
Science Foundation of Guangdong Province (2014A030313170).
The authors thank Prof. Dr. Xiao-Ming Chen in Sun Yat-Sen
University for his supports and valuable suggestions, Dr. Peisen
(13) Cook, T. R.; Stang, P. J. Recent Developments in the Preparation
and Chemistry of Metallacycles and Metallacages via Coordination. Chem.
Rev. 2015, 115, 7001-7045.
(14) Sun, Y.; Chen, C.; Stang, P. J. Soft Materials with Diverse
Suprastructures via the Self-Assembly of Metal−Organic Complexes. Acc.
Chem. Res. 2019, 52, 802-817, and references therein.
(15) Omoto, K.; Tashiro, S.; Kuritani, M.; Shionoya, M. Multipoint
Recognition of Ditopic Aromatic Guest Molecules via Ag−π Interactions
within a Dimetal Macrocycle. J. Am. Chem. Soc. 2014, 136, 17946-19749.
Liao for help in the extraction with supercritical CO
Wu for help in the crystal data collection.
2
, Dr. Si-Guo
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