A Gigantic Molecular Wheel of {Gd140}: A New Member

Dec 4, 2017 - A Gigantic Molecular Wheel of {Gd140}: A New Member of the. Molecular Wheel .... carboxyl bridge (d(Gd···Gd) = 3.789−3.876 Å); sin...
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A Gigantic Molecular Wheel of {Gd140}: A New Member of the Molecular Wheel Family Xiu-Ying Zheng, You-Hong Jiang, Gui-Lin Zhuang, Da-Peng Liu, HongGang Liao, Xiang-Jian Kong, La-Sheng Long, and Lan-Sun Zheng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11112 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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A Gigantic Molecular Wheel of {Gd140}: A New Member of the Molecular Wheel Family Xiu-Ying Zheng,†, # You-Hong Jiang,†, # Gui-Lin Zhuang,†† Da-Peng Liu,† Hong-Gang Liao,† Xiang-Jian Kong,*† La-Sheng Long,*† and Lan-Sun Zheng† †

Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surface and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. ††

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China.

Supporting Information Placeholder ABSTRACT: Nanoscale inorganic wheel-shaped structures are one of the most striking molecular aggregations. Here, we report the synthesis of a gigantic lanthanide wheel cluster 3+ containing 140 Gd atoms. As the largest lanthanide cluster reported thus far, {Gd140} features an attractive wheel-like structure with tenfold symmetry. The nanoscopic molecular wheel possesses the largest diameter of 6.0 nm and displays high stability in solution, which allows direct visualization by scanning transmission electron microscopy (STEM). The discovery of the lanthanide {Gd140} cluster represents a new member of the molecular wheel family.

The assembly of nanoscale inorganic wheel-like structures is one of the most striking research fields in coordination chemistry, because their unique internal nanospaces allow the study of new phenomena in confinement scenarios, such as synergistic catalysis in confined conditions, ionic 1-2 conductivity and molecular magnetic materials. The discovery of nanoscale molybdenum oxide clusters reported by Müller et al., for example Mo154 and Mo176, was one of the most important steps forward in research on giant wheel3 shaped nanostructures. Subsequently, much effort has been paid to synthesize analogs; however, only several transition metal compounds featuring huge wheel structures have been 4 obtained, such as Mn70 and Mn84 reported by Christou et al. 5 and Pd72 and Pd84 reported by Cronin et al. Compared to these transition metal wheels, the synthesis of giant lanthanide wheels has not been accomplished due to the high coordination numbers and versatile coordination geometries of lanthanides. In an effort to construct high-nuclearity lanthanide metal clusters, we demonstrated that the slow release of anions is 6-7 essential to the assembly of lanthanide clusters. Among 2− the method of slowly introducing CO3 anions, atmospheric 7 CO2 fixation has been widely adopted. Herein, we fortunately obtained a giant wheel cluster {Gd140} by reaction of myo-inositol, acetate and Gd(ClO4)3 under refluxing

through absorbing CO2 from the atmosphere. The gigantic molecular structure possesses an outer diameter of ∼6.0 nm and inner diameter of ∼3.4 nm (Figure 1). Interestingly, the gigantic wheel is stable in solution and can be visualized by scanning transmission electron microscopy (STEM).

Figure 1. The crystal structure of the cation cluster of [Gd140(CO3)20(μ3-OH)100(CH3COO)80(LH3)40(H2O)200]80+. Purple: Gd. Red: O. Gray: C. White: H.

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The giant {Gd140} molecule was obtained through the hydrolysis of Gd(NO3)3•6H2O in the presence of acetate and myo-inositol (LH6) as mixed ligands. Single-crystal X-ray diffraction revealed that {Gd140} with the formula [Gd140(CO3)20(μ3-OH)100(CH3COO)80(LH3)40(H2O)200] 3•(NO3)80•(H2O)x (x ≈ 80, [LH3] = deprotonated three protons from [LH6]) features a novel giant wheel-like metal 3+ core containing 140 Gd atoms. Due to disorder, NO3 counter anions and guest H2O molecules removed by SQUEEZE in structural refinement. The number of water molecules and counter ions are confirmed by elemental analysis, thermogravimetry and charge balance. The metal cationic core of {Gd140} has tenfold symmetry and consists of 8+ ten [Gd14(CH3COO)8(LH3)4(CO3)2(μ3-OH)10(H2O)20] 3+ (abbreviated as {Gd14}) subunits. In each {Gd14} unit, 14 Gd 2ions are connected together by two central templating CO3 ligands in situ generated from atmospheric CO2 fixation and 10 OH anions, forming one saddle-like [Gd14(CO3)2(μ328+ OH)10] unit, as shown in Figure 2a. The [Gd14(CO3)2(μ328+ OH)10] unit is further stabilized and protected by eight 3CH3COO , four deprotonated [LH3] and 20 aqua ligands, generating the {Gd14} unit. The four myo-inositol ligands display two kinds of coordination modes, one in a 1 2 2 2 1 0 1 2 3 2 1 0 μ5:η :η :η :η :η :η fashion and another in a μ5:η :η :η :η :η :η fashion (Figure S2). The metal framework of the {Gd14} unit can be viewed as five triangles and one tetrahedron connected by shared vertices (Figure 2b). Ten {Gd14} units are alternately linked together by rotating o 180 between two adjacent units through two μ3-OH ligands and two μ2-O atoms from two myo-inositol ligands (LH3) (Figure S3), forming the gigantic wheel structure of {Gd140}. The {Gd140} displays tenfold symmetry, which is similar to the reported wheels Mn84 shows sixfold symmetric with 4b {Mn14} subunits and Pd84 features sevenfold symmetric 5 with {Pd12} building blocks. This means that the symmetrybuilding block principle could lead to the rational assembly of gigantic wheel-like clusters. The molecular wheel possesses a diameter of ∼6.0 nm and inner diameter of ∼3.4 nm, with a thickness of ∼1.5 nm. Interestingly, the ordered arrangement of the {Gd140} molecules within the crystal forms a nanotube with a one-dimensional channel along the a-axis (Figure S4). Including all the bridging oxygen atoms, the metal-oxo core of the {Gd140} cluster can also be considered as consisting of 20 cubane-like {Gd4O4} units and 50 defected cubane-like {Gd3O4} units by shared adjacent vertexes. It is well known that cubane-like {Ln4O4} is the most common building block to construct high-nuclearity lanthanide 8 clusters. In the last two decades, a series of lanthanide clusters based on cubane-like {Ln4O4} as a building block have been reported. For example, wheel-like {Ln12} and {Ln15} were constructed from four and five {Ln4O4} building blocks, 9 respectively. The Ln60 cage clusters were generated from 24 6 cubane-like {Ln4O4} building blocks. The present results suggest that the cubane-like {Ln4O4} unit is the perfect building block to construct nanoclusters with metal atoms from dozens to hundreds (Figure S5).

Figure 2. Ball-and-stick representation of the [Gd14(CO3)2(μ3OH)10]28+ unit (a), the metal framework of the {Gd14} subunit (b) and the gigantic wheel-like structure of [Gd140(CO3)20(μ3-OH)100]280+ (c).

The larger diameter allows us to directly characterize the stability of {Gd140} by scanning transmission electron microscopy (STEM). A dilute solution of {Gd140} in ethanol was loaded onto a silicon chip with 10 nm silicon nitride membrane windows, the solvent gradually evaporated under ambient atmosphere, and then the chip was loaded into a Talos F2oo TEM instrument. As shown in Figure 3a, the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image shows a uniform nanoring shape, and Figure 3b shows a high-resolution STEM image of the nanoring, where the bright ring indicates the Gd atom positions. Figure S6 shows the side view of the assembled nanorings, which indicate the nanorings have a thickness of approximately 1.5 nm. Figure 3c contains the statistics of the ring size with an average diameter of approximately 5.0 nm, which is slightly smaller than the value measured by crystallography due to the tilting of the molecular wheels and the invisibility of the light elements C, H and O in STEM. The resulting images suggest that the gigantic wheel structure has high structural stability in ethanol solution. Although many metal clusters have been reported in the past two decades, the characterization of the 3c, 10, 11 stability of metal clusters by STEM is rare. This work is the first example reporting the stability of high-nucleariy lanthanide clusters characterized by STEM. The stability of {Gd140} in water was also measured by dynamic light

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scattering (DLS). As shown in Figure S7, the size of the cluster is 6.0 ± 0.4 nm, close to the calculated value. The temperature dependence of the magnetic susceptibility of {Gd140} shows that the observed χMT value of 3 -1 1041.9 cm K mol at 300 K is close to the expected value of 3 -1 1102.5 cm K mol for 140 uncoupled Gd(III) ions (J = 7/2, g = 2), as shown in Figure 4a. Upon lowing the temperature, the χMT value remains almost constant between 300 K and 110 K

and then decreases gradually to 25 K. Up further cooling, the χMT value decreases sharply to the minimum value of 497.6 3 -1 cm K mol at 2 K. These changes indicate that presence of a weak antiferromagnetic interaction. In the temperature -1 range of 2 K to 300 K, the χM vs T plot can be fitted with the 3 -1 Curie-Weiss law, generating C = 1065.8 cm K mol and θ = 2.4 K for {Gd140}. The negative but small θ further confirms 12 the weak antiferromagnetic interaction.

Figure 3. (a) HAADF-STEM image of {Gd140}; (b) high-resolution image of an individual molecular wheel; (c) statistics of the {Gd140} molecular wheel size.

To investigate the magnetic coupling interaction in {Gd140}, magnetic fitting of the χMT vs T curve was conducted. Owing 140 to the ultra-large spin matrix with a total dimension of 8 × 140 8 , it is impossible to apply the well-known irreducible tensor operators (ITO) method. By modifying the LOOP 13 code of the Algorithms and Libraries for Physical 14 Simulations (ALPS) project, we herein coupled the Genetic 15 Algorithm with Quantum Monte Carlo (QMC) method to search the global optimal magnetic parameters. Essentially, the entire spin exchange network (Figure 4b) was simplified as 3J models (see the Hamiltonian operator in SI) as follows: mixed interaction (J1) of μ3-OH, μ2-O and μ2-OLH3 (d(Gd···Gd) =3.709···4.108 Å); mixed interaction (J2) of μ3-OH and μ2carboxyl bridge (d(Gd···Gd) = 3.789···3.876 Å); single 2interaction (J3) of μ2-CO3 (d(Gd···Gd) = 5.976···6.484 Å). In view of the huge paramagnetic ions and guest anions, both a temperature-independent paramagnetism (TIP) parameter and inter-cluster interaction (zJ) were considered. After the evolution of 32 generations (Figure S10), the simulation -1 yielded one set of optimal parameters: J1 = ─ 0.021 cm , J2 = ─ -1 -1 3 -1 0.112 cm , J3 = ─ 0.104 cm , g = 1.97, TIP = 0.00 cm mol , zJ = -1 -5 2 0.00 cm and R = 3.786 × 10 (R = ∑[(χMT)obs ─(χMT)calcd] / 2 -1 ∑[(χMT)obs] ). The J1 of ─ 0.021 cm reveals the weak antiferromagnetic coupling in the μ3-OH, μ2-O and μ2-OLH3 -1 bridges. The J2 value of ─ 0.112 cm and J3 value of ─ 0.104 cm 1 suggest antiferromagnetic coupling in the μ2-carboxyl and μ2-CO3 bridges, respectively. The values of J1, J2 and J3 are also in good agreement with those obtained both experimentally 16 and theoretically. Moreover, the simulated magnetization curve based on the current three J values also well matched the experimental one (Figure S12).

Figure 4. (a) Plot of the temperature dependence of χMT for {Gd140} under a 1000 Oe dc field (○) and the fitting plot (red line); (b) net spin exchange of 140 Gd(III) ions using the 3 J mode.

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The field dependence of the magnetization for {Gd140} was studied in the temperature range of 2 - 11 K and magnetic field range of 0 – 7 T (Figure S13a). The value of 941 NμB at 2 K is consistent with the saturation value of 980 NμB. Due to 3+ the presence of the huge number of isotropic Gd ions with high-spin ground states, the magnetocaloric effect (MCE) of {Gd140} was investigated. The maximum magnetic entropy change (ΔSm) can be calculated using the Maxwell equation 17 of ΔSm(ΔH) = ∫[∂M(T, H)/∂T]HdH. The experimental value of -1 -1 38.0 J·kg ·K at 2 K for ΔH = 7 T is smaller than the -1 -1 theoretical value of 51.4 J·kg ·K (based on the equation −ΔSm 3+ = nR ln(2S + 1)) for 140 uncorrelated Gd ions, which may be 3+ due to the weak antiferromagnetic interactions between Gd ions (Figure S13b). In summary, we reported the largest lanthanide-exclusive cluster {Gd140} with the largest diameter of 6.0 nm. The gigantic 140-metal core showed a beautiful wheel-like structure and displayed tenfold symmetry. The {Gd140} cluster represents a new member of the molecular wheel family and provides a possibility to explore the synthesis large wheel-like lanthanide clusters upon building blocks with available symmetries. Further, based on high stability of the cluster in solution, the single molecular properties, such as molecular sensor or synergistic catalysis in confined conditions, may be expected by loading the cluster on the semiconductor or graphene as substrate.

ASSOCIATED CONTENT Supporting Information Synthesis and characterization details and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author X.-J.K ([email protected]) or L.-S.L ([email protected])

Author Contributions #

These authors (X.-Y. Z. and Y.-H. J) contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the 973 project (Grant 2014CB845601) from the Ministry of Science and Technology of China, the National Natural Science Foundation of China (Grants nos. 21422106, 21371144, 21673184, 21431005 and 21390391), the Fok Ying Tong Education Foundation (151013) and the Recruitment Program for Leading Talent Team of Anhui Province. We thank Dr. Wenming Qin and the staffs from BL17B beamline of National Center for Protein Sciences Shanghai (NCPSS) at Shanghai Synchrotron Radiation Facility for assistance during data collection and Dr. Zhenyi Zhang in Bruker(Beijing) Scientific Technology Co., Ltd for the help of structure refinement.

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