Synthesis, Crystal Structures and Fluorescence Properties of Two Nanosized Zr-Based Molecular Capsules

Citation: TAO Yan-Li, CHEN Wei-Chao, WANG Xin-Long, SU Zhong-Min. Synthesis, Crystal Structures and Fluorescence Properties of Two Nanosized Zr-Based Molecular Capsules[J]. Chinese Journal of Inorganic Chemistry, 2019, 35(11): 2108-2116. doi: 10.11862/CJIC.2019.224 shu

锆簇基纳米分子胶囊的合成、晶体结构和荧光性质

陶艳丽 ^1, ,
陈维超 ^{1,
,} ,
王新龙 ^1, ,
苏忠民 ^{1,2,
,}

1.
东北师范大学化学学院, 长春 130024
2.
长春理工大学, 长春 130024

通讯作者: 陈维超, chenwc061@nenu.edu.cn
苏忠民, zmsu@nenu.edu.cn

基金项目:
中国博士后基金资助项目 2019T120227

国家自然科学基金 21801038

中央高校基本科研专项资金 2412018QD003

吉林省教育厅 JJKH20190271KJ

国家自然科学基金 21471027

中国博士后基金资助项目 2018M630312

国家自然科学基金 21671034

国家自然科学基金（No.21801038，21471027，21671034），吉林省教育厅（No.JJKH20190271KJ），中国博士后基金资助项目（No.2018M630312，2019T120227）和中央高校基本科研专项资金（No.2412018QD003）资助

摘要: 选取4，4'-联苯二甲酸（4，4'-H₂BPDC）和2，6-萘二甲酸（2，6-H₂NDC）分别与二氯二茂锆（Cp₂ZrCl₂）反应制备了2例结构新颖的锆基纳米分子胶囊[Cp₃Zr₃（μ₃-O）（μ₂-OH）₃]₂（BPDC）₃Cl₂·H₂O（Zr-MC-1）和[Cp₃Zr₃（μ₃-O）（μ₂-OH）₃]₂（NDC）₃Cl₂·3H₂O（Zr-MC-2）。这2个分子胶囊均由3个线性羧基配体桥联2个三核茂锆建筑单元构成。利用单晶X射线衍射、粉末X射线衍射、元素分析、红外光谱、热重分析和氮气吸附-脱附等手段对化合物进行了表征。此外，荧光光谱分析表明Zr-MC-1和Zr-MC-2分别为深蓝光和蓝光发射材料，其最大发射波长分别为404和456 nm。

关键词:

English

Synthesis, Crystal Structures and Fluorescence Properties of Two Nanosized Zr-Based Molecular Capsules

1.
Northeast Normal University, Changchun 130024, China
2.
Changchun University of Science and Technology, Changchun 130024, China

Corresponding author: CHEN Wei-Chao, chenwc061@nenu.edu.cn SU Zhong-Min, zmsu@nenu.edu.cn

Received Date: 29 July 2019
Revised Date: 09 September 2019
Available Online: 10 November 2019

Abstract: Two nanosized zirconium-based molecular capsules (Zr-MCs), namely[Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂(BPDC)₃Cl₂ ·H₂O (Zr-MC-1) and[Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂(NDC)₃Cl₂·3H₂O (Zr-MC-2), have been synthesized by reacting zirconocenedichloride (Cp₂ZrCl₂) with 4, 4'-biphenyldicarboxylic acid (4, 4'-H₂BPDC) and 2, 6-naphthalenedicar-boxylic acid (2, 6-H₂NDC), respectively, which are constructed by two trinuclear zirconocene building blocks and three linear carboxylate ligands. They were characterized by single-crystal X-ray diffraction, powder X-ray diffraction, elemental analysis, IR spectroscopy, thermogravimetric analysis and N₂ adsorption-desorption tests. In addition, the fluorescence spectra indicate that Zr-MC-1 and Zr-MC-2 are deep-blue and blue emitting materials with emission peaks at 404 and 456 nm, respectively.

Key words:

0. Introduction

Metal-organic molecular capsules (MCs) remains discrete inorganic-organic molecular complexes that are built by the self-assembly of metal ions/clusters and organic linkers^[1-3]. It has attracted extensively wide investigations as a result of its fascinating structures with adjustable inner cavities and potential applications, such as gas storage-separation^[4-6], magne-tism^[7-10], drug delivery^[11-12], catalysis^{[4, 13]} together with guest molecule accommodating and release^[13-14]. As is known to all, the reported secondary building units (SBUs) during the preparation of MCs mainly focuse on early transition metal centers, and the reversible property of such coordination bonds may bring poor water stability as well as limited applications in most research area of MCs.

Recently, Zr-based metal-organic frameworks featuring high stability in both acid and alkaline aqueous solutions owing to the quite strong Zr-O bonds (chemical bond energy: 776 kJ·mol^-1) have gained extensive interests^[15]. Inspired by this aspect, in 2013, Yuan and co-workers has developed a facile strategy to access a family of Zr-based metal-organic polyhedra (MOPs) by reaction of bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂, Cp=η⁵-C₅H₅) with carboxyl ligands^[16]. Such zirconocene precursor could be easily hydrolyzed in presence of water and carboxylate acid to achieve a trinuclear ligand-bridged Zr-cation moiety, namely Cp₃Zr₃(μ₃-O)(μ₂-OH)₃, which acts as 3-connected SBUs for the construction of various water-stable coordination polyhedra. Those MOPs show potential applications in catalysis^[17], proton conduc-tivity^[18] and gas storage-separation^[19], etc. So far, only several Zr-based MOPs have been discovered, and the rational design and synthesis of more Zr-based MOPs with specific architecture and functionality still possess a considerable challenge^[15-23].

Taking into account the above backgrounds, we herein present the synthesis and crystal structures of two Zr-MCs constructed from trinuclear Cp₃Zr₃(μ₃-O) (μ₂-OH)₃ SBUs and linear dicarboxylate ligands. They were fully characterized by multiple analysis techniques and their photoluminescent properties were also described.

1. Experimental

1.1 Materials and physical measurements

All chemical reagents were derived from commerce and used directly without purification. PXRD patterns were measured from 5° to 50° through a Siemens D5005 diffractometer at 40 kV, 30 mA for Cu Kα (λ=0.154 18 nm). Thermogravimetric analysis (TGA) of the crystal samples was tested using a PerkinElmer TG-7 analyzer by heating from 25 to 800 ℃ (10 ℃·min^-1) under a dry nitrogen environment. Elemental analyses (C, H, N) were performed on a PerkinElmer 2400 CHN Elemental analyzer. The FT-IR spectra were measured from 4 000 to 400 cm^-1 using KBr pellets with an Alpha Centauri FT/IR spectrophotometer. N₂ gas adsorption-desorption and porosimetry were performed on ASIQM0G002-3. The solid-state photoluminescence (FL) spectra of both crystals and organic ligands were performed at room temperature using F-7000 FL spectrophotometer. A transient spectrofluorimeter (Edinburgh FLSP920) was used to test the excited-state lifetimes (τ) of the two compounds in solid state.

1.2 Preparations of Zr-MC-1 and Zr-MC-2

Zr-MC-1 was synthesized by reacting 4, 4′-H₂BPDC (0.02 g, 0.08 mmol) and zirconocenedichloride (Cp₂ZrCl₂) (0.03 g, 0.1 mmol) in N, N-dimethylforma-mide (DMF, 1.0 mL), CH₃OH (1.0 mL) and water (5 drops) at 80 ℃ for 18 hours. After slowly cooling to room temperature, colorless crystals were obtained, washed with DMF and dried in air. Yield: 88% based on 4, 4′-H₂BPDC. Elemental analysis Calcd. for C₇₂H₆₂ O₂₀Zr₆Cl(%): C 47.33, H 3.29; Found(%): C 47.23, H 3.30. IR (KBr, cm^-1): 3 420 (m), 1 654 (m), 1 608 (m), 1 574 (s), 1 524 (s), 1 415 (s), 1 180 (w), 1 097 (w), 1 013 (m), 808 (w), 766 (w), 695 (w), 674 (m), 603 (m), 536 (m), 447 (s).

Zr-MC-2 was prepared by reacting 2, 6-H₂NDC (0.02 g, 0.08 mmol) and zirconocenedichloride (Cp₂ZrCl₂) (0.03 g, 0.1 mmol) in N, N-dimethylformamide (DMF, 1.0 mL), CH₃OH (1.0 mL) and water (5 drops) at 80 ℃ for 12 hours. Colorless crystals were isolated (washed with DMF) when the oven was gradually cooled to room temperature. Yield: 90% based on 2, 6-H₂NDC. Elemental analysis Calcd. for C₆₆H₆₀O₂₃Zr₆Cl₂(%): C 43.11, H 3.27; Found(%): C 43.31, H 3.20. IR (KBr, cm^-1): 3 198 (m), 2 930 (w), 1 657 (s), 1 599 (s), 1 553 (s), 1 494 (w), 1 419 (s), 1 365 (s), 1 247 (w), 1 197 (w), 1 092 (w), 1 017 (w), 921 (w), 812 (m), 787 (m), 770 (w), 658 (w), 607 (m), 490 (m).

1.3 Single-crystal X-ray crystallography

Single-crystal X-ray crystallography data were collected at 298 K on Bruker D8 VENTURE with micro-focus Cu Kα radiation (λ=0.154 178 nm). Absorption corrections were achieved by a multi-scan technique. The structures were solved by direct methods, and further refinement of the structures was performed by using full-matrix least squares techniques with SHELXL-2014 program^[24]. Hydrogen atoms were fixed geometrically at calculated positions and allowed to ride on the parent non-H atoms. All the non-H atoms were refined ansiotropically during the refinement. Additionally, some restraints were used including SIMU and DFIX in the final refinement. Details of the data are summarized in Table 1.

Table 1

Table 1. Crystal data and structure refinement parameters for Zr-MC-1 and Zr-MC-2

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Compound	Zr-MC-1	Zr-MC-2
Empirical formula	C₇₂H₆₂Cl₂O₂₁Zr₆	C₆₆H₆₀Cl₂O₂₃Zr₆
Formula weight	1 881.43	1 839.36
Temperature / K	295.01	295.01
Crystal system	Monoclinic	Monoclinic
Space group	P2₁/m	C2/c
a / nm	1.350 6(2)	2.039 4(2)
b / nm	1.604 3(3)	1.988 9(2)
c / nm	1.675 7(3)	1.789 29(18)
β / (°)	90.650(8)	105.434(5)
V / nm³	3.630 6(10)	6.996 0(12)
Z	2	4
D_c / (g·cm^-3)	1.721	1.746
Absorption coefficient / mm^-1	8.108	8.419
F(000)	1 872	3 656
Reflection collected	30 261	47 019
Independent reflection	6 627	6 912
R_int	0.052 7	0.046 8
Goodness-of-fit on F²	1.066	1.042
Final R indices [I > 2σ(I)]^{a, b}	R₁=0.070 0, wR₂=0.149 3	R₁=0.040 2, wR₂=0.095 1
R indices (all data)	R₁=0.091 0, wR₂=0.165 0	R₁=0.047 9, wR₂=0.101 1
$^{\rm{a}}{R_1} = \sum {\left\\| {{F_{\rm{o}}}\| - \|{F_{\rm{c}}}} \right\\|} /\sum {\left\| {{F_{\rm{o}}}} \right\|} ;{\;^{\rm{b}}}w{R_2} = {\left\{ {\sum {\left[ {w{{\left( {F_{\rm{o}}^2 - F_{\rm{c}}^2} \right)}^2}} \right]} /\sum {\left[ {w{{\left( {F_{\rm{o}}^2} \right)}^2}} \right]} } \right\}^{1/2}}$

CCDC: 939840, Zr-MC-1; 939841, Zr-MC-2.

2. Results and discussion

2.1 Structure descriptions of Zr-MC-1 and Zr-MC-2

Reaction of Cp₂ZrCl₂ and linear dicarboxylate ligands (4, 4′-H₂BPDC for Zr-MC-1 and 2, 6-H₂NDC for Zr-MC-2) in a mixed solvent system including DMF, CH₃OH and distilled water at 80 ℃ gave rise to two nanosized molecular capsules (Zr-MCs). As previously reported by Yuan et al.^[16], the cationic Cp₃Zr₃(μ₃-O)(μ₂-OH)₃ SBUs are derived from the slow hydrolysis process of Cp₂ZrCl₂ in the existence of water (Fig. 1). The carboxylate ligands in the SBUs (close to C₃ symmetry) are oriented to one face while the μ-OH species are oriented to the other, where the C₂ axes in carboxylate ligands create an angle of approximately 60°. In this connection, the substitution of the primary carboxylate in Cp₃Zr₃(μ₃-O)(μ₂-OH)₃ by the linear 4, 4′-H₂BPDC and 2, 6-H₂NDC ligands was realized in this system.

Figure 1

Figure 1. Diagrammatic sketch of the formation of trinuclear Cp₃Zr₃(μ₃-O)(μ₂-OH)₃ SBUs

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Structural analysis (Table 1) indicates that Zr-MC-1 and Zr-MC-2 both crystallize in the monoclinic system with corresponding P2₁/m and C2/c space group, respectively. Two molecular capsules representing V₂E₃ (V=vertex and E=edge) topology could be regarded as two Cp₃Zr₃(μ₃-O)(μ₂-OH)₃ SBUs as the vertices and three associated ligands as the edges. Similar topology in Zr-MCs including the flexible sulfonate-carboxylate ligand has already been reported by Zang et al^[18]. Such type of capsules with a well-known linear connection between the metal SBUs and organic species remains excellent candidate in metal-organic supercontainers. As described in Fig. 2, the capsule inner (yellow column) pore sizes of the cages are about 1.3 and 1.1 nm (the distance between two μ₃-O atoms in both zirconocene SBUs), respectively. Just as expected, this distinction is attributed to the lengths of 4, 4′-H₂BPDC and 2, 6-H₂NDC ligands. In both crystal structures, hydrogen bonds (O-H…Cl…H-O) among Cl^-, μ₂-OH on zirconocene and free water not only form the three-dimensional supramolecular networks of MCs but also balance the framework charge. Furthermore, selected bond distances and bond angles of Zr-MC-1 and Zr-MC-2 are listed in Table 2.

Figure 2

Figure 2. Ball-and-stick representation of [Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂ SBUs (a), 4, 4′-BPDC ligand (b), 2, 6-NDC ligand (c), Zr-MC-1 (d), and Zr-MC-2 (e)

Color code: C: gray; O: red; Zr: blue. The free space in the polyhedra is filled with capsular cages. For clarity, H atoms are omitted

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Figure 3

Figure 3. Hydrogen-bonding in Zr-MC-1 (a) and 3D stacking of Zr-MC-1 (b); Hydrogen-bonding in Zr-MC-2 (c) and 3D stacking of Zr-MC-2 (d)

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Table 2

Table 2. Selected bond lengths (nm) and angles (°) for Zr-MC-1 and Zr-MC-2

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Zr-MC-1
Zr(1)-O(1)	0.207 1(8)	Zr(1)-O(11)	0.220 3(6)	Zr(1)-C(33)	0.252 7(11)
Zr(1)-O(6)#1	0.214 4(6)	Zr(1)-C(32)#1	0.250 6(10)	Zr(1)-C(33)#1	0.252 7(11)
Zr(1)-O(6)	0.214 4(6)	Zr(1)-C(32)	0.250 6(10)	Zr(1)-C(46)	0.250 7(16)
Zr(1)-O(11)#1	0.220 3(6)
O(1)-Zr(1)-O(6)	73.2(2)	O(6)-Zr(1)-C(32)#1	79.7(3)	O(11)#1-Zr(1)-C(33)	112.1(4)
O(1)-Zr(1)-O(6)#1	73.2(2)	O(6)-Zr(1)-C(32)	101.8(3)	O(11)-Zr(1)-C(33)#1	112.1(4)
O(1)-Zr(1)-O(11)	79.2(2)	O(6)#1-Zr(1)-C(32)	79.7(3)	O(11)#1-Zr(1)-C(33)#1	77.9(3)
O(1)-Zr(1)-O(11)#1	79.2(2)	O(6)-Zr(1)-C(33)	130.4(3)	O(11)-Zr(1)-C(46)	81.7(3)
O(1)-Zr(1)-C(32)	152.1(3)	O(6)-Zr(1)-C(33)#1	90.7(4)	O(11)#1-Zr(1)-C(46)	81.7(3)
O(1)-Zr(1)-C(32)#1	152.1(3)	O(6)#1-Zr(1)-C(33)	90.7(4)	C(32)#1-Zr(1)-C(32)	30.7(6)
O(1)-Zr(1)-C(33)#1	153.O(3)	O(6)#1-Zr(1)-C(33)#1	130.4(3)	C(32)-Zr(1)-C(33)#1	51.3(4)
O(1)-Zr(1)-C(33)	153.O(3)	O(6)-Zr(1)-C(46)	122.2(3)	C(32)#1-Zr(1)-C(33)	51.3(4)
O(1)-Zr(1)-C(46)	154.2(5)	O(6)#1-Zr(1)-C(46)	122.2(3)	C(32)#1-Zr(1)-C(33)#1	30.8(4)
O(6)-Zr(1)-O(6)#1	92.6(3)	O(11)-Zr(1)-O(11)#1	83.7(3)	C(32)-Zr(1)-C(33)	30.8(4)
O(6)#1-Zr(1)-O(11)#1	151.7(2)	O(11)-Zr(1)-C(32)	105.5(3)	C(32)#1-Zr(1)-C(46)	51.1(5)
O(6)-Zr(1)-O(11)	151.7(2)	O(11)-Zr(1)-C(32)#1	128.4(3)	C(32)-Zr(1)-C(46)	51.1(5)
O(6)-Zr(1)-O(11)#1	85.2(2)	O(11)#1-Zr(1)-C(32)#1	105.5(3)	C(33)-Zr(1)-C(33)#1	52.2(6)
O(6)#1-Zr(1)-O(11)	85.2(2)	O(11)#1-Zr(1)-C(32)	128.4(3)	C(46)-Zr(1)-C(33)	31.5(4)
O(6)#1-Zr(1)-C(32)#1	101.8(3)	O(11)-Zr(1)-C(33)	77.9(3)	C(46)-Zr(1)-C(33)#1	31.5(4)
Zr-MC-2
Zr(1)-O(2)	0.212 2(3)	Zr(1)-O(8)#1	0.219 6(3)	Zr(1)-C(8)	0.249 9(4)
Zr(1)-O(3)	0.213 9(3)	Zr(1)-C(6)	0.252 6(4)	Zr(1)-C(9)	0.250 0(4)
Zr(1)-O(4)	0.209 1(3)	Zr(1)-C(7)	0.251 5(4)	Zr(1)-C(10)	0.251 8(4)
Zr(1)-O(6)	0.218 4(3)
O(2)-Zr(1)-O(3)	90.75(12)	O(4)-Zr(1)-O(2)	73.47(11)	O(8)#1-Zr(1)-C(6)	125.01(14)
O(2)-Zr(1)-O(6)	151.84(11)	O(4)-Zr(1)-O(3)	72.75(11)	O(8)#1-Zr(1)-C(7)	116.91(17)
O(2)-Zr(1)-O(8)#1	85.60(13)	O(4)-Zr(1)-O(6)	78.87(11)	O(8)#1-Zr(1)-C(8)	85.77(17)
O(2)-Zr(1)-C(6)	114.45(18)	O(4)-Zr(1)-O(8)#1	79.44(11)	O(8)#1-Zr(1)-C(9)	74.00(14)
O(2)-Zr(1)-C(7)	84.79(16)	O(4)-Zr(1)-C(6)	153.60(13)	O(8)#1-Zr(1)-C(10)	97.30(17)
O(2)-Zr(1)-C(8)	81.40(14)	O(4)-Zr(1)-C(7)	152.01(14)	C(7)-Zr(1)-C(6)	30.96(8)
O(2)-Zr(1)-C(9)	108.91(17)	O(4)-Zr(1)-C(8)	151.61(13)	C(7)-Zr(1)-C(10)	51.25(12)
O(2)-Zr(1)-C(10)	132.08(14)	O(4)-Zr(1)-C(9)	152.96(13)	C(8)-Zr(1)-C(6)	51.34(12)
O(3)-Zr(1)-O(6)	85.99(12)	O(4)-Zr(1)-C(10)	154.23(13)	C(8)-Zr(1)-C(7)	31.14(8)
O(3)-Zr(1)-O(8)#1	151.82(11)	O(6)-Zr(1)-O(8)#1	84.26(12)	C(8)-Zr(1)-C(9)	31.21(8)
O(3)-Zr(1)-C(6)	81.78(14)	O(6)-Zr(1)-C(6)	92.77(17)	C(8)-Zr(1)-C(10)	51.42(12)
O(3)-Zr(1)-C(7)	90.48(17)	O(6)-Zr(1)-C(7)	123.15(16)	C(9)-Zr(1)-C(6)	51.31(12)
O(3)-Zr(1)-C(8)	121.33(17)	O(6)-Zr(1)-C(8)	123.81(15)	C(9)-Zr(1)-C(7)	51.44(12)
O(3)-Zr(1)-C(9)	133.04(14)	O(6)-Zr(1)-C(9)	93.38(16)	C(9)-Zr(1)-C(10)	31.09(8)
O(3)-Zr(1)-C(10)	105.70(17)	O(6)-Zr(1)-C(10)	75.37(13)	C(10)-Zr(1)-C(6)	30.94(8)
Symmetry codes: #1: x, -y+3/2, z for Zr-MC-1; #1: -x+1, y, -z+1/2 for Zr-MC-2

2.2 Powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA)

The phase purities of Zr-MC-1 and Zr-MC-2 were verified by PXRD. The uniform patterns between the experimental and simulated results give evidence of pure target samples, and the crystalline structures of both MCs remained unchanged after the water treatment (Fig. 4). In order to test the thermal stabilities of the two crystals, the TGA experiment was carried out between 25 and 800 ℃ under N₂ atmosphere. The weight loss below 100 ℃ indicates the loss of physically absorbed water molecules, and chloride ions and lattice water molecules were released afterwards. Finally, Zr-MC-1 and Zr-MC-2 lost weight drastically at ~500 ℃ (Fig. 5).

Figure 4

Figure 4. Experimental and simulated powder X-ray diffraction patterns for Zr-MC-1 (a) and Zr-MC-2 (b)

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Figure 5

Figure 5. TG curves for Zr-MC-1 (a) and Zr-MC-2 (b)

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2.3 Infrared spectroscopy (IR)

IR spectroscopy is an effective mean for the analysis of organic ligands and zirconocene SBUs in the crystal structures. For this purpose, IR spectra of two MCs were measured in KBr pellets. As presented in Fig. 6, the absorption peaks in 400~1 200 cm^-1 region belong to zirconium-oxygen characteristic stretching vibrations^[25].The peaks located at 3 420 cm^-1 for Zr-MC-1 and 3 198 cm^-1 for Zr-MC-2 could be assigned to the vibration of the OH group, because of the exists of μ₂-OH species and free water molecules in the crystals^[26]. There was no absorption band in the region from 1 690 to 1 730 cm^-1, showing that 4, 4′-H₂BPDC and 2, 6-H₂NDC groups in Zr-MC-1 and Zr-MC-2 are deprotonated^[26]. Furthermore, the peaks of 1 415 cm^-1 (1 608 cm^-1) for Zr-MC-1 and 1 419 cm^-1 (1 599 cm^-1) for Zr-MC-2 are attributed to the symmetric (asymmetric) C-O stretching vibrations^[27].

Figure 6

Figure 6. IR spectra for Zr-MC-1 (a) and Zr-MC-2 (b)

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2.4 N₂ adsorption measurements

In the light of capsule inner pores and pore connectivity in Zr-MC-1 and Zr-MC-2, N₂ adsorption experiments were determined at 77 K. Fresh samples (150 mg) were solvent-exchanged with dichloromethane several times for 3 days, and then the soaking samples were further activated at 60 ℃ for 12 hours for next test. As shown in Fig. 7a, a hysteresis loop was identified through the relative pressure P/P₀ ranging from 0.4 to 1.0 for Zr-MC-1. The isotherm could be assigned to the typical Ⅳ isotherm on the basis of the IUPAC classification^[20]. Similar observations are also occurred for Zr-MC-2 (Fig. 7c). From Fig. 7b and 7d, it may be concluded that both MCs are mesoporous materials. The noticeable Brunauer-Emmett-Teller (BET) and Langmuir surface areas of Zr-MC-1 were 28 and 44 m²·g^-1, respectively. The pore volume is 0.089 4 nm³ and the porosity is 2.5% according to the PLATON routine. Similarly, BET surface areas and Langmuir surface areas of Zr-MC-2 are 90 and 132 m²·g^-1, respectively. The pore volume is 0.354 nm³ and the porosity is 5.1% based on the calculated results from the PLATON routine^[20].

Figure 7

Figure 7. N₂ adsorption-desorption isotherms of Zr-MC-1 (a) and Zr-MC-2 (c) at 77 K; Pore size distributions of Zr-MC-1 (b) and Zr-MC-2 (d)

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2.5 Luminescence property

The solid-state photoluminescence (PL) spectra of Zr-MC-1 and Zr-MC-2 as well as corresponding dicarboxylate ligands were investigated at room temperature. As depicted in Fig. 8, on the principle of π^*→n transitions of the intraligands in either free ligands, 4, 4′-H₂BPDC ligand (red line, Fig. 8a) shows an emission at 407 nm (λ_ex=330 nm) while the strong emission peak of 2, 6-H₂NDC ligand (red line, Fig. 8b) is located at 424 nm (λ_ex=380 nm)^[28-29]. With respect to Zr-MC-1 (black line, Fig. 8a), the emission (404 nm, λ_ex=330 nm) is mainly derived from 4, 4′-H₂BPDC ligand. Conversely, Zr-MC-2 exhibited an apparent red-shifted emission (456 nm, λ_ex=380 nm) compared to 2, 6-H₂NDC ligand, this phenomenon is not strange and should be assigned to ligand-to-metal charge transfer (LMCT) process. What′s more, these observations indicate that Zr-MC-1 and Zr-MC-2 might be excellent candidates of deep-blue and blue-light-emitting materials, respectively.

Figure 8

Figure 8. (a) Emission spectra of 4, 4'-H₂BPDC and Zr-MC-1, (b) Emission spectra of 2, 6-H₂NDC and Zr-MC-2

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Additionally, the luminescence decay curves of Zr-MC-1 and Zr-MC-2 were also measured at room temperature. As shown in Fig. 9, the luminescence lifetimes of both MCs are quite different: the lifetime of Zr-MC-1 (λ_ex=330 nm) and Zr-MC-2 (λ_ex=380 nm) are 1.48 and 17.02 ns, respectively. This fact may be attributed to that the rigidity of 2, 6-H₂NDC ligand is stronger in contrast to 4, 4′-H₂BPDC ligand, thus the much weaker vibrations in Zr-MC-2 effectively reduce the loss of energy by radiationless decay^[30].

Figure 9

Figure 9. Solid state fluorescence decay curves of Zr-MC-1 (a) and Zr-MC-2 (b)

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3. Conclusions

In summary, two new nanosized Zr-based MCs constructed from trinuclear Cp₃Zr₃(μ₃-O)(μ₂-OH)₃ SBUs and linear dicarboxylate ligands have been successfully synthesized under solvothermal conditions. Two crystals were fully characterized by multiple analysis techniques and their photoluminescent properties indicate that both complexes might be excellent candidates of blue-light-emitting diode devices. The obtained conclusions could inspire further achievements in connection with the exploration of diverse coordination zirconocene molecular capsules with amazing architectures and desired functionalities to be realized.

Acknowledgements: This work was financially supported by the NSFC of China (Grants No.21801038, 21471027, 21671034), the Science and Technology Research Foundation of the Thirteenth Five Years of Jilin Educational Committee (Grant No.JJKH20190271KJ), the China Postdoctoral Science Founda-tion funded project (Grants No.2018M630312, 2019T120227), and the Fundamental Research Funds for the Central Univer-sities (Grant No.2412018QD003).

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Figure 1 Diagrammatic sketch of the formation of trinuclear Cp₃Zr₃(μ₃-O)(μ₂-OH)₃ SBUs

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Figure 2 Ball-and-stick representation of [Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂ SBUs (a), 4, 4′-BPDC ligand (b), 2, 6-NDC ligand (c), Zr-MC-1 (d), and Zr-MC-2 (e)

Color code: C: gray; O: red; Zr: blue. The free space in the polyhedra is filled with capsular cages. For clarity, H atoms are omitted

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Figure 3 Hydrogen-bonding in Zr-MC-1 (a) and 3D stacking of Zr-MC-1 (b); Hydrogen-bonding in Zr-MC-2 (c) and 3D stacking of Zr-MC-2 (d)

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Figure 4 Experimental and simulated powder X-ray diffraction patterns for Zr-MC-1 (a) and Zr-MC-2 (b)

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Figure 5 TG curves for Zr-MC-1 (a) and Zr-MC-2 (b)

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Figure 6 IR spectra for Zr-MC-1 (a) and Zr-MC-2 (b)

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Figure 7 N₂ adsorption-desorption isotherms of Zr-MC-1 (a) and Zr-MC-2 (c) at 77 K; Pore size distributions of Zr-MC-1 (b) and Zr-MC-2 (d)

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Figure 8 (a) Emission spectra of 4, 4'-H₂BPDC and Zr-MC-1, (b) Emission spectra of 2, 6-H₂NDC and Zr-MC-2

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Figure 9 Solid state fluorescence decay curves of Zr-MC-1 (a) and Zr-MC-2 (b)

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Table 1. Crystal data and structure refinement parameters for Zr-MC-1 and Zr-MC-2

Compound	Zr-MC-1	Zr-MC-2
Empirical formula	C₇₂H₆₂Cl₂O₂₁Zr₆	C₆₆H₆₀Cl₂O₂₃Zr₆
Formula weight	1 881.43	1 839.36
Temperature / K	295.01	295.01
Crystal system	Monoclinic	Monoclinic
Space group	P2₁/m	C2/c
a / nm	1.350 6(2)	2.039 4(2)
b / nm	1.604 3(3)	1.988 9(2)
c / nm	1.675 7(3)	1.789 29(18)
β / (°)	90.650(8)	105.434(5)
V / nm³	3.630 6(10)	6.996 0(12)
Z	2	4
D_c / (g·cm^-3)	1.721	1.746
Absorption coefficient / mm^-1	8.108	8.419
F(000)	1 872	3 656
Reflection collected	30 261	47 019
Independent reflection	6 627	6 912
R_int	0.052 7	0.046 8
Goodness-of-fit on F²	1.066	1.042
Final R indices [I > 2σ(I)]^{a, b}	R₁=0.070 0, wR₂=0.149 3	R₁=0.040 2, wR₂=0.095 1
R indices (all data)	R₁=0.091 0, wR₂=0.165 0	R₁=0.047 9, wR₂=0.101 1
$^{\rm{a}}{R_1} = \sum {\left\\| {{F_{\rm{o}}}\| - \|{F_{\rm{c}}}} \right\\|} /\sum {\left\| {{F_{\rm{o}}}} \right\|} ;{\;^{\rm{b}}}w{R_2} = {\left\{ {\sum {\left[ {w{{\left( {F_{\rm{o}}^2 - F_{\rm{c}}^2} \right)}^2}} \right]} /\sum {\left[ {w{{\left( {F_{\rm{o}}^2} \right)}^2}} \right]} } \right\}^{1/2}}$

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Table 2. Selected bond lengths (nm) and angles (°) for Zr-MC-1 and Zr-MC-2

Zr-MC-1
Zr(1)-O(1)	0.207 1(8)	Zr(1)-O(11)	0.220 3(6)	Zr(1)-C(33)	0.252 7(11)
Zr(1)-O(6)#1	0.214 4(6)	Zr(1)-C(32)#1	0.250 6(10)	Zr(1)-C(33)#1	0.252 7(11)
Zr(1)-O(6)	0.214 4(6)	Zr(1)-C(32)	0.250 6(10)	Zr(1)-C(46)	0.250 7(16)
Zr(1)-O(11)#1	0.220 3(6)
O(1)-Zr(1)-O(6)	73.2(2)	O(6)-Zr(1)-C(32)#1	79.7(3)	O(11)#1-Zr(1)-C(33)	112.1(4)
O(1)-Zr(1)-O(6)#1	73.2(2)	O(6)-Zr(1)-C(32)	101.8(3)	O(11)-Zr(1)-C(33)#1	112.1(4)
O(1)-Zr(1)-O(11)	79.2(2)	O(6)#1-Zr(1)-C(32)	79.7(3)	O(11)#1-Zr(1)-C(33)#1	77.9(3)
O(1)-Zr(1)-O(11)#1	79.2(2)	O(6)-Zr(1)-C(33)	130.4(3)	O(11)-Zr(1)-C(46)	81.7(3)
O(1)-Zr(1)-C(32)	152.1(3)	O(6)-Zr(1)-C(33)#1	90.7(4)	O(11)#1-Zr(1)-C(46)	81.7(3)
O(1)-Zr(1)-C(32)#1	152.1(3)	O(6)#1-Zr(1)-C(33)	90.7(4)	C(32)#1-Zr(1)-C(32)	30.7(6)
O(1)-Zr(1)-C(33)#1	153.O(3)	O(6)#1-Zr(1)-C(33)#1	130.4(3)	C(32)-Zr(1)-C(33)#1	51.3(4)
O(1)-Zr(1)-C(33)	153.O(3)	O(6)-Zr(1)-C(46)	122.2(3)	C(32)#1-Zr(1)-C(33)	51.3(4)
O(1)-Zr(1)-C(46)	154.2(5)	O(6)#1-Zr(1)-C(46)	122.2(3)	C(32)#1-Zr(1)-C(33)#1	30.8(4)
O(6)-Zr(1)-O(6)#1	92.6(3)	O(11)-Zr(1)-O(11)#1	83.7(3)	C(32)-Zr(1)-C(33)	30.8(4)
O(6)#1-Zr(1)-O(11)#1	151.7(2)	O(11)-Zr(1)-C(32)	105.5(3)	C(32)#1-Zr(1)-C(46)	51.1(5)
O(6)-Zr(1)-O(11)	151.7(2)	O(11)-Zr(1)-C(32)#1	128.4(3)	C(32)-Zr(1)-C(46)	51.1(5)
O(6)-Zr(1)-O(11)#1	85.2(2)	O(11)#1-Zr(1)-C(32)#1	105.5(3)	C(33)-Zr(1)-C(33)#1	52.2(6)
O(6)#1-Zr(1)-O(11)	85.2(2)	O(11)#1-Zr(1)-C(32)	128.4(3)	C(46)-Zr(1)-C(33)	31.5(4)
O(6)#1-Zr(1)-C(32)#1	101.8(3)	O(11)-Zr(1)-C(33)	77.9(3)	C(46)-Zr(1)-C(33)#1	31.5(4)
Zr-MC-2
Zr(1)-O(2)	0.212 2(3)	Zr(1)-O(8)#1	0.219 6(3)	Zr(1)-C(8)	0.249 9(4)
Zr(1)-O(3)	0.213 9(3)	Zr(1)-C(6)	0.252 6(4)	Zr(1)-C(9)	0.250 0(4)
Zr(1)-O(4)	0.209 1(3)	Zr(1)-C(7)	0.251 5(4)	Zr(1)-C(10)	0.251 8(4)
Zr(1)-O(6)	0.218 4(3)
O(2)-Zr(1)-O(3)	90.75(12)	O(4)-Zr(1)-O(2)	73.47(11)	O(8)#1-Zr(1)-C(6)	125.01(14)
O(2)-Zr(1)-O(6)	151.84(11)	O(4)-Zr(1)-O(3)	72.75(11)	O(8)#1-Zr(1)-C(7)	116.91(17)
O(2)-Zr(1)-O(8)#1	85.60(13)	O(4)-Zr(1)-O(6)	78.87(11)	O(8)#1-Zr(1)-C(8)	85.77(17)
O(2)-Zr(1)-C(6)	114.45(18)	O(4)-Zr(1)-O(8)#1	79.44(11)	O(8)#1-Zr(1)-C(9)	74.00(14)
O(2)-Zr(1)-C(7)	84.79(16)	O(4)-Zr(1)-C(6)	153.60(13)	O(8)#1-Zr(1)-C(10)	97.30(17)
O(2)-Zr(1)-C(8)	81.40(14)	O(4)-Zr(1)-C(7)	152.01(14)	C(7)-Zr(1)-C(6)	30.96(8)
O(2)-Zr(1)-C(9)	108.91(17)	O(4)-Zr(1)-C(8)	151.61(13)	C(7)-Zr(1)-C(10)	51.25(12)
O(2)-Zr(1)-C(10)	132.08(14)	O(4)-Zr(1)-C(9)	152.96(13)	C(8)-Zr(1)-C(6)	51.34(12)
O(3)-Zr(1)-O(6)	85.99(12)	O(4)-Zr(1)-C(10)	154.23(13)	C(8)-Zr(1)-C(7)	31.14(8)
O(3)-Zr(1)-O(8)#1	151.82(11)	O(6)-Zr(1)-O(8)#1	84.26(12)	C(8)-Zr(1)-C(9)	31.21(8)
O(3)-Zr(1)-C(6)	81.78(14)	O(6)-Zr(1)-C(6)	92.77(17)	C(8)-Zr(1)-C(10)	51.42(12)
O(3)-Zr(1)-C(7)	90.48(17)	O(6)-Zr(1)-C(7)	123.15(16)	C(9)-Zr(1)-C(6)	51.31(12)
O(3)-Zr(1)-C(8)	121.33(17)	O(6)-Zr(1)-C(8)	123.81(15)	C(9)-Zr(1)-C(7)	51.44(12)
O(3)-Zr(1)-C(9)	133.04(14)	O(6)-Zr(1)-C(9)	93.38(16)	C(9)-Zr(1)-C(10)	31.09(8)
O(3)-Zr(1)-C(10)	105.70(17)	O(6)-Zr(1)-C(10)	75.37(13)	C(10)-Zr(1)-C(6)	30.94(8)
Symmetry codes: #1: x, -y+3/2, z for Zr-MC-1; #1: -x+1, y, -z+1/2 for Zr-MC-2

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