Self-assembly of a Mixed Valence Copper Triangular Prism and Transformation to Cage Triggered by an External Stimulus

Transformation to Cage Triggered by an External Stimulus

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Self-assembly of a Mixed Valence Copper Triangular Prism and

†

Ya-Liang Lai, Xue-Zhi Wang, Xian-Chao Zhou, Rui-Rong Dai, Xiao-Ping Zhou,* and Dan Li

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c02682

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ABSTRACT: A triangular prismatic metal−organic cage based on

mixed valence copper ions has been designed and synthesized by

using metallocycle panels and pillar ligands. The triangular prism

will be quickly transformed to a 10-nuclear cage upon an external

chemical stimulus, which features a bicapped square antiprism

structure. This prismatic cage can act as a catalyst for oxidation of

aromatic alcohols to their corresponding aromatic aldehydes with high yields at room temperature under O atmosphere.

INTRODUCTION

the simplest prismatic structure, which can be obtained by

combination of the end-capped metal centers as vertexes, the

tritopic planar ligand as roofs and ﬂoors, and the bidentate

■

In a biological system, the implementation of functions

depends on the robust signal transductions, which usually

involve molecular-scale responses to stimuli and conformation

changes. To mimic a biological system, chemists investigate the

stimuli-responsive behavior at a molecular scale, which will

probably help the design of advanced materials that show

functions in response to an external stimulus (e.g., light, pH, or

linear ligands as pillars in a 6:2:3 molar ratio. Alternatively, a

triangular prism can be obtained by assembly of a two-

component approach by using three 0° Pt-based molecular

clips to connect two tripodal organic panels. Furthermore,

the heteroleptic triangular prism can be synthesized by a three-

component reaction of pyridyl and carboxylate ligands with

the presence of a chemical species). Metal−organic cages

MOCs) containing well-deﬁned cavities via coordination-

(

platinum acceptors. Although many metal−organic triangular

driven self-assembly are typical models for mimicking

prisms have been reported, the method of designing a

enzymes which have a variety of applications, including in

triangular prism by the combination of metal−organic

3b,4

2,5

separation, recognition and sensing,

catalysis,

and

triangular panels and pillar ligands is rarely reported.

biomedicine. MOCs based on dynamic coordination bonds

will probably undergo structural changes after induction of

In our previous work, we had designed and synthesized a

series of hexanuclear mixed valence and heteronuclear

external stimuli, e.g., light, pH, guest, solvents, and

triangular metallocycles by using a metalloligand approach.

crystallization. For example, Clever et al. found that a series

of photoactive Pd L coordination cages based on dithienyle-

We speculated that we could probably utilize the metallocycles

as the triangular panels to construct a triangular prismatic

MOC (Scheme 1) based on following reasons. (1) The

triangular metallocycle is stable and rigid, and (2) each

thene ligands allowed triggering of guest uptake and released

,12

by irradiation.

Nitschke et al. reported that the presence of

−

^g^u^e^s^t^C^l^O4 anions promoted a structural transformation of

metallocycle contains three four- or ﬁve-coordinated Cu

the M L tetrahedron into an M L pentagonal prism.

10 15

centers with weak coordinated anions, which are possibly

coordinated by bidentate pillar ligands to fabricate a triangular

prism. In this work, we report the design and assembly of a

Crowley et al. designed and synthesized a heterobimetallic

PdPtL cage, which can reversibly transform from opened to

closed state selectively at one end upon treatment with

prismatic MOC formed through coordination of the Cu ions

diﬀerent stimuli (N,N′-dimethylaminopyridine and p-toluene-

of the metallocycles and bidentate ligands (Scheme 1),

sulfonic acid). To the best of our knowledge, however, there

are very few MOCs in which both the oxidation states of metal

ions and structure are changed after cage-to-cage trans-

formulated as [Cu (Cu L1) ] L2 ·8ClO ·2H O (denoted

3 2

as 1, H L1 = N,N′-(propane-1,3-diyl)bis(1-(1H-imidazol-4-

yl)methanimine) and L2 = triethylenediamine), which features

formation upon an external stimulus.

The rational design of coordination cages is available by

utilizing the directional feature of coordination bonds and

Received: September 8, 2020

deﬁned coordination geometry of metal ions. Many types of

prismatic cages have been successfully designed and synthe-

sized by precisely choosing metal ions and multitopic ligands

(

e.g., triangular prism, quadrangular prism, pentaprism,

and hexagonal prism ). The metal−organic triangular prism is

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Schematic Diagram of Self-assembly of the Mixed

Valence Metal−Organic Triangular Prism from Triangular

Metallocycles and Pillar Ligands

Figure 1. Metallocycle structure in prism 1 (a) and the triangular

prismatic cage structure of 1 (b). Color codes: Cu , green; Cu ,

orange; C, light gray; N, blue. All hydrogen atoms and anions are

omitted for clarity.

square pyramidal geometry, while the Cu adopts a linear

coordination geometry. The Cu −N bond distances are in the

range of 1.991−2.297 Å, and the Cu −N bond distance is

.860 Å. The coordination bond distances in prism 1 are in

good agreement with those in Cu L1 The diﬀerence between

the metallocycle in 1 and Cu L1₃is the planarity. The

triangular Cu L1 is distorted, while metallocycle of 1 becomes

planar after coordination to L2 (Figure S1).

a mixed valence triangular prism structure. The prism 1 is

composed of two mixed valence metallocycles and three L2.

We found that prism 1 can slowly transform to a metal−

organic bicapped square antiprism in organic solvent, which

Prism 1 is a regular prism, in which its height is about 7.166

Å and its width of triangle base is about 11.654 Å.

Interestingly, similar to Cu L1 the cavities of the two

−

contains ten Cu and eight L1. Interestingly, the reacting rate

triangular metallocycles in 1 are occupied by ClO anion as a

guest, as shown in Figure 2 a. The ClO ₄ ions are located in the

−

can be highly boosted by adding N-methylimidazole (NMI) to

the reaction mixture, which acts as an organic stimulus for the

structure transformation. Furthermore, prism 1 can act as a

catalyst for selectively oxidizing the aromatic alcohol to

corresponding aldehydes at room temperature.

RESULTS AND DISCUSSION

Prism 1 was synthesized by the assembly of the metallocycle

and pillar ligand L2 with a mole ratio of 2:3 in a mixed solvent

■

(

dimethyl sulfoxide DMSO/ethanol, 4:1, v/v) at 80 °C for 6 h

for details see the Supporting Information ), which was

protected under nitrogen atmosphere. The green crystalline

powder mixed with a few single crystals was obtained with a

very high yield (98%). In the reaction, the organic ligand L1

was synthesized from condensation of 4-formylimidazole with

,3-diaminopropane, while the hexanuclear mixed valence

metallocycle, Cu L1 was obtained by using a stepwise

synthesis strategy (Scheme Sl ). Furthermore, the nitrogen

3 ,

protection is critical for synthesis of prism 1. When the

reaction was not protected by N , prism 1 cannot be

successfully obtained, which is probably caused by oxidation

−

^Fⁱ^g^u^r^e²^.^Eⁿ^c^a^p^s^u^l^a^tⁱ^oⁿ^o^f^C^l^O4 anions by the metallocycles of 1

(

of the Cu centers in Cu L1 . Moreover, we also tested other

a); the “ABAB” packing structure of 1 along the c-axis (b, space-

ﬁlling mode), the “ABAB” packing structure of 1 along the b-axis (c);

and a hexagonal channel ﬁlled with ClO anions (d). Color codes:

Cu , green; Cu , orange; C, light gray; N, blue; O, red; Cl, light blue.

typical pillar ligands including pyrazine and 4,4′-bipyridine;

however, the prismatic metal−organic cage was not success-

fully obtained. The possible reason is due to that pyrazine and

−

,4′-bipyridine are relatively weaker Lewis bases.

All hydrogen atoms are omitted for clarity.

Single-crystal X-ray diﬀraction (SCXRD) analysis revealed

that prism 1 crystallizes in a hexagonal P6 /mmc space group,

center of the metallocycle panels. The encapsulation of ClO ⁻

which is obviously diﬀerent from that of Cu L1 (P1). As

shown in Figure 1, the hexanuclear triangular metallocycle is

ions is probably due to the weak C H···O interactions (H···O

bond distance of 2.602 Å, Figure S2 ) and electrostatic eﬀect

−

similar to Cu L1 , which is further pillared by L2 to form the

between ClO₄and positive skeleton of prism 1. As shown in

triangular prism 1. In prism 1, there are six Cu and six Cu

centers, six L1, three L2, and eight ClO . The skeleton of 1 is

Figure 2b and 2c, prism 1 can be described as an “ABAB” layer

packing where each prism has six nearest neighbors. Regular

hexagonal channels are formed by packing these prisms with a

diameter about 12.39 Å (deﬁned by H···H distance of diagonal

imidazole groups, without consideration of van der Waals

−

positive (+6), which is charge-balanced by the ClO .

However, there are two excess ClO₄⁻anions in the crystal

lattice, which are probably charge-balanced by two hydroniums

−

(

H O ). The Cu is ﬁve-coordinated and adopts a distorted

force), which are ﬁlled with ClO₄anions (Figure 2d). The

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

structure (Figure 4 ), whose conformation depends on the size

Article

Figure 3. PXRD patterns of the as-synthesized metallocycle Cu L1 and prism 1 and the calculated pattern from the single-crystal structure of

prism 1 (a). XPS spectrum of prism 1 with ﬁtting peaks (b, blue Cu , green Cu ).

total potential solvent area volume of 1 is 3977 Å (43.5%) in a

unit cell, which was checked by the PLATON program.

Powder X-ray diﬀraction (PXRD) analysis showed that the

PXRD pattern of prism 1 closely matched with its simulated

patterns from SCXRD data, indicating that the pure phase of

prism 1 was obtained (Figure 3a). The PXRD pattern of prism

SCXRD analysis revealed that the new green crystal crystallizes

in a C2/c space group, which features a bicapped square

antiprism cage structure. This cage, formulated as Cu L ·

0 8

4ClO (denoted 2), is identical to our previously reported

was obviously diﬀerent to the pattern of Cu L1 , indicating

6 3

the successful assembly of prism 1 from metallocycle and L2.

X-ray photoelectron spectroscopy (XPS) is an important tool

used to characterize the type and valence state of metal ions.

To conﬁrm the oxidation of the copper centers in the prism 1,

the XPS spectrum of prism 1 was measured. As shown in

Figure 3b, the spectrum of prism 1 showed a strong

asymmetric Cu 2p₃_/₂photoelectron peak with satellite peaks,

indicating the mixed valence characteristic of the copper center

Figure 4. Transformation from triangular prism 1 to the bicapped

square antiprism 2.

in prism 1. The Cu 2p₃_/₂peaks can be deconvoluted into two

contributions, which are located at 933.36 and 934.95 eV,

respectively (Figure 3b), which is in good agreement with that

of metallocycle Cu L1 . Bond valence sum (BVS) also has

of anions. Unambiguously, after the transformation, the Cu

been utilized to determine the oxidation state of copper ions in

center was oxidized to Cu by O in the air, and the triangular

prism 1. The calculations of the BVS value for the ﬁve-

coordinated Cu center gives 2.071, while that for the linear

coordination Cu center gives 0.949. The BVS results of prism 1

are consistent with that of metallocycle Cu L1 Both XPS and

prism 1 was transformed into 2. Furthermore, electrospray

ionization mass (ESI-MS) spectrometry was employed to

study the green solution. As shown in Figure S5 , the ESI-MS

spectrum showed that the green solution contained 2 and that

no 1 was detected, which indicates that the transformation was

successful. PXRD analysis showed that the pattern of cage 2

closely matches with its simulated pattern calculated from

SCXRD data (Figure S6 ), suggesting that the product was pure

and that prism 1 was completely transformed to cage 2. We

BVS calculations indicate the mixed valence states of copper

centers in prism 1, which is similar to Cu L1 .

Thermogravimetric analysis (TGA) revealed that prism 1

was thermally stable up to approximately 262 °C (Figure S3).

To study its porous property, N adsorption measurement was

−

carried out. The activated 1 could adsorb approximately about

speculated that ClO₄may play an important role (e.g., as a

−1

0 cm g N (77 K, P/P = 0.3, Figure S4). The Brunauer−

template) in the transformation process. As shown in Figure 4,

−

Emmett−Teller (BET) and Langmuir surface areas are

calculated to be 50 and 74 m /g, respectively. The lower N

after the transformation, one ClO₄anion was encapsulated in

the cavity of cage 2, which indicates that the ClO₄⁻anions

located in the triangular panels of prism 1 probably templated

adsorption for 1 is probably due to the pores of 1 being

blocked by the partial decomposition of its packing structure.

the formation of cage 2 after the oxidation of Cu to Cu .

Although the transformation from prism 1 to cage 2 is

successful, the transformation rate is very slow. In biological

systems, the conformation change is quick in response to a

stimulus. Thus, to mimic a biological system, an accelerated

transformation rate is required. In literature, NMI can

The Cu centers are presented in the prism 1, which will

probably be oxidized to Cu and induce the structure change.

This hypothesis encourages us to study the cage-to-cage

transformation property of prism 1. The solubility of prism 1 is

poor in common organic solvents, e.g., DMSO, DMF, and

acetonitrile. However, we found that the prism 1 will slowly

dissolve in DMF in 3 days, and a green solution was obtained.

Slow vapor diﬀusion of ethyl acetate into this green solution of

signiﬁcantly improve the catalytic eﬀect of lipase and is an

important co-catalyst for alcohol oxidation when the copper

complexes as the catalyst. Thus, we tried to add NMI into

the reaction system to improve the reacting rate for the

provided green rod-like crystals suitable for SCXRD analysis.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

transformation. Interestingly, with the addition of NMI (5

equiv of 1) to the DMF solution, the solid prism 1 dissolved in

DMF within about 1 min to give a green solution. The ESI-MS

study documented that prism 1 was transformed to cage 2

successfully (Figure S7 ). The possible reason for NMI

accelerating the transformation is due to its strong coordina-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

■

Corresponding Author

Xiao-Ping Zhou − College of Chemistry and Materials Science,

Guangdong Provincial Key Laboratory of Functional

Supramolecular Coordination Materials and Applications,

Jinan University, Guangzhou, Guangdong 510632, P. R.

China; orcid.org/0000-0002-0351-7326;

Email: zhouxp@jnu.edu.cn

tion ability to Cu centers. The binding of NMI on Cu will

change its coodination geomtery, which probably helps the O

oxidation of the Cu centers to Cu and then boosts the

transformation process.

In our previous studies, the mixed valence Cu /Cu

metallocylce has been a good catalyst for oxidation of

hetero)aromatic alcohols to corresponding (hetero)aromatic

(

Authors

aldehydes. Prism 1 also contains the catalytic acitve Cu sites

and has potentially catalytic ability for oxidation of alcohols by

employing (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl

Ya-Liang Lai − College of Chemistry and Materials Science,

Guangdong Provincial Key Laboratory of Functional

Supramolecular Coordination Materials and Applications,

Jinan University, Guangzhou, Guangdong 510632, P. R.

China

Xue-Zhi Wang − College of Chemistry and Materials Science,

Guangdong Provincial Key Laboratory of Functional

Supramolecular Coordination Materials and Applications,

Jinan University, Guangzhou, Guangdong 510632, P. R.

China

Xian-Chao Zhou − College of Chemistry and Materials

Science, Guangdong Provincial Key Laboratory of Functional

Supramolecular Coordination Materials and Applications,

Jinan University, Guangzhou, Guangdong 510632, P. R.

China

Rui-Rong Dai − College of Chemistry and Materials Science,

Guangdong Provincial Key Laboratory of Functional

Supramolecular Coordination Materials and Applications,

Jinan University, Guangzhou, Guangdong 510632, P. R.

China

Complete contact information is available at:

TEMPO) as a co-catalyst and NMI as the base. As shown

in Figure S8 , prism 1 can catalyze oxidation of 16

hetero)aromatic alcohols to their corresponding (hetero)-

(

aromatic aldehydes with quantitative yields (except 1-

ⁿ^a^p^h^t^h^a^l^eⁿ^e^m^e^t^h^aⁿ^o^l⁷³^%⁾^a^t^r^o^o^m^t^e^m^p^e^r^a^t^u^r^e^uⁿ^d^e^r^O2

atmosphere, when the catalyst system consists of 5 mol % 1, 10

mol % TEMPO, and 50 mol % NMI in acetonitrile. The

catalyst system is optimized by following our previous work.

Without prism 1 or TEMPO, trace benzyl aldehyde was

detected, suggesting that both the prism 1 and TEMPO are

indispensable to the oxidation reaction. The scope of this

oxidation reaction was found to be very broad, suggesting that

prism 1 is a good catalyst. However, the catalytic activity of

prism 1 is not good as the metallocycle, which can achieve the

same catalytic eﬃciency under an air condition. Furthermore,

prism 1 can also catalyze the sequential alcohol oxidation/

Knoevenagel condensation reaction to yield benzylidenemalo-

nonitrile (yield 90%) by using benzyl alcohol and malononi-

trile as starting materials (Scheme S2).

Dan Li − College of Chemistry and Materials Science,

Guangdong Provincial Key Laboratory of Functional

Supramolecular Coordination Materials and Applications,

Jinan University, Guangzhou, Guangdong 510632, P. R.

China; orcid.org/0000-0002-4936-4599

CONCLUSION

■

In summary, we have successfully designed and synthesized a

mixed valence copper−organic triangular prism by using

triangular metallocycle panels and pillar ligands. The

copper−organic triangular prism can be transformed to a

bicapped square antiprism cage by oxidation of the cuprous

ions to cupric ions by air. The NMI molecules can act as a

stimulus to accelerate this transformation process. This work

provides a new strategy for constructing a metal−organic

triangular prism by using a triangular panel and pillar ligand.

The NMI boosting the transformation rate provides a unique

example for understanding the cage-to-cage transformation.

https://pubs.acs.org/10.1021/acs.inorgchem.0c02682

Author Contributions

†

These authors contributed equally to this work.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was supported by the National Natural Science

Foundation of China (21731002, 21871172), the Major

Program of Guangdong Basic and Applied Research

ASSOCIATED CONTENT

sı Supporting Information

■

(

2019B030302009), the Fundamental Research Funds for

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02682.

the Central Universities (21619405), the Guangzhou Science

and Technology Program (202002030411), and Jinan

University.

Experimental characterizations, additional structural

presentations, and FT-IR, PXRD, and ESI-MS analyses

PDF)

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