Structural Evolution and Properties of Praseodymium Antimony Oxochlorides Based on a Chain-like Tertiary Building Unit

Unveiling the structural evolution of single-crystalline compounds based on certain building units may help greatly in guiding the design of complex structures. Herein, a series of praseodymium antimony oxohalide crystals have been isolated under solvothermal conditions via adjusting the solvents used, that is, [HN(CH2CH3)3][FeII(2,2′-bpy)3][Pr4Sb12O18Cl15]·EtOH (1) (2,2′-bpy = 2,2′-bipyridine), [HN(CH2CH3)3][FeII(2,2′-bpy)3]2[Pr4Sb12O18Cl14)2Cl]·N(CH2CH3)3·2H2O (2), and (H3O)[Pr4Sb12O18Cl12.5(TEOA)0.5]·2.5EtOH (3) (TEOA = mono-deprotonated triethanolamine anion). Single-crystal X-ray diffraction analysis revealed that all the three structures feature an anionic zig-zag chain of [Pr4Sb12O18Cl15−x]n as the tertiary building unit (TBU), which is formed by interconnections of praseodymium antimony oxochloride clusters (denoted as {Pr4Sb12}) as secondary building units. Interestingly, different arrangements or linkages of chain-like TBUs result in one-dimensional, two-dimensional layered, and three-dimensional structures of 1, 2, and 3, respectively, thus demonstrating clearly the structural evolution of metal oxohalide crystals. The title compounds have been characterized by elemental analysis, powder X-ray diffraction, thermogravimetric analysis, and UV-Vis spectroscopy, and the photodegradation for methyl blue in an aqueous solution of compound 1 has been preliminarily studied. This work offers a way to deeply understand the assembly process of intricate lanthanide-antimony(III) oxohalide structures at the atomic level.


Introduction
Hierarchical assembly is an effective strategy for rational and precise construction of new structures [1][2][3][4][5][6][7]. Building units (BUs) are essential in the hierarchical assembly. Therefore, it is important to search for new secondary building units (SBUs), and even tertiary building units (TBUs), for assembling complex structures. Meanwhile, systematical demonstration of structural evolution at the atomic level is meaningful to understand the assembly process and, therefore, help to design new intricate structures [8]; however, this remains a challenge because of the difficulty in obtaining precise or single-crystalline structures in each hierarchical level.
The asymmetric unit of the compound [HN(CH 2 (2,2 -bpy) 3 ] 2+ complexes and protonated triethylamine, as well as the lattice solvent molecules, are located in between the chains (Figure 1 and Figure S1 in Supplementary Materials). Compared with the original isolated {Pr 4 Sb 12 } cluster, four terminal Cl ions from two Pr 3+ ions are changed into the bridging mode in each cluster of 1, thus resulting in a 1-D chain.
Crystal data for (H3O)[Pr4Sb12O18Cl12.5(TEOA)0.5]·2.5EtOH (3, C8H25O23N0.5Cl12.5Sb12Pr4, M = 2964.18 g/mol): monoclinic; space group C2/m (no. 12); a = 30.385(2) Å; b = 14.9201 (12) Å; c = 11.2014(7) Å; β = 90.379(7)°; V = 5078.1(6) Å 3 ; Z = 4; T = 295(2) K; μ(MoKα) = 10.743 mm −1 ; Dcalc = 3.877 g/cm 3 ; 13,723 reflections measured (4.820° ≤ 2θ ≤ 58.366°); 6374 unique (Rint = 0.0340, Rsigma = 0.0525), which were used in all calculations. The final R1 was 0.0330 (I > 2σ(I)), and wR2 was 0.0734 (all data    2+ complex, and half of a protonated triethylamine cation, as well as half of a neutral triethylamine and one water molecule. The two types of cations in 2 are illustrated in Figure 2a. Similar to that in compound 1, there are also 1-D zig-zag chains of [Pr4Sb12O18Cl15]n present in compound 2 that are extended along the a-axis, as highlighted in Figure 2b. However, the chains in 2 are further interlinked to each other via sharing a single chloride bridge to result in 2-D layers along the ac plane ( Figure 2b). The layered structure in 2 can be regarded as a further   3 ] 2+ complex, and half of a protonated triethylamine cation, as well as half of a neutral triethylamine and one water molecule. The two types of cations in 2 are illustrated in Figure 2a. Similar to that in compound 1, there are also 1-D zig-zag chains of [Pr 4 Sb 12 O 18 Cl 15 ] n present in compound 2 that are extended along the a-axis, as highlighted in Figure 2b. However, the chains in 2 are further interlinked to each other via sharing a single chloride bridge to result in 2-D layers along the ac plane ( Figure 2b). The layered structure in 2 can be regarded as a further aggregation of the isolated chains in structure 1 as TBUs. Compared to the original isolated {Pr 4 Sb 12 } cluster, five terminal Cl ions from three Pr 3+ ions are changed into the bridging mode in each cluster of 2, and each {Pr 4 Sb 12 } cluster connects to three neighboring ones to construct the 2-D network with windows with a size of 7.46 × 8.86 Å 2 . The layers then further pack along the b-axis in a staggered form (Figures 2c and S2); the guest molecules, as well as [Fe II (2,2 -bpy) 3 ] 2+ complex groups and protonated triethylamine, are located between layers.
aggregation of the isolated chains in structure 1 as TBUs. Compared to the original isolated {Pr4Sb12} cluster, five terminal Cl ions from three Pr 3+ ions are changed into the bridging mode in each cluster of 2, and each {Pr4Sb12} cluster connects to three neighboring ones to construct the 2-D network with windows with a size of 7.46 × 8.86 Å 2 . The layers then further pack along the b-axis in a staggered form (Figures 2c and S2); the guest molecules, as well as [Fe II (2,2′-bpy)3] 2+ complex groups and protonated triethylamine, are located between layers.  Figure 3b); meanwhile, the chains further link to each other along the a-axis through another set of Cl2 bridges (Cl(4)2) to form a layer extended along the ab plane ( Figure 3b). Interchain linking via sharing a Cl bridge (Cl(5)) between a Pr atom of the {Pr4Sb12} cluster in one layer and a (Sb3O3) unit of the {Pr4Sb12} cluster from another layer finally creates a 3-D porous framework, with a channel of 11.29 × 11.29 Å 2 along the c-axis (Figure 3b,c). The connection of Pr-Cl-Sb3O3 is firstly found in the {Pr4Sb12} cluster-based structures [23][24][25][26]. Hydrated protons act as cations, and as well as ethanol solvent molecules, are located in the 1-D channels along the c-axis. Compared to the original isolated {Pr4Sb12} cluster [23], seven terminal Cl ions from all four Pr 3+ ions in each {Pr4Sb12} cluster of 3 are changed into the bridging mode, and each cluster connects to four neighboring ones to construct the 3-D framework. On the other hand, structure 3 can also be regarded as a 3-D  (5)) between a Pr atom of the {Pr 4 Sb 12 } cluster in one layer and a (Sb 3 O 3 ) unit of the {Pr 4 Sb 12 } cluster from another layer finally creates a 3-D porous framework, with a channel of 11.29 × 11.29 Å 2 along the c-axis (Figure 3b,c). The connection of Pr-Cl-Sb 3 O 3 is firstly found in the {Pr 4 Sb 12 } cluster-based structures [23][24][25][26]. Hydrated protons act as cations, and as well as ethanol solvent molecules, are located in the 1-D channels along the c-axis. Compared to the original isolated {Pr 4 Sb 12 } cluster [23], seven terminal Cl ions from all four Pr 3+ ions in each {Pr 4 Sb 12 } cluster of 3 are changed into the bridging mode, and each cluster connects to four neighboring ones to construct the 3-D framework. On the other hand, structure 3 can also be regarded as a 3-D framework that is directly constructed by the interconnection of the chain-like TBUs that are also present in compounds 1 and 2.

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Molecules 2023, 28, x FOR PEER REVIEW 5 of 12 framework that is directly constructed by the interconnection of the chain-like TBUs that are also present in compounds 1 and 2. When comparing the structures of 2 and 3, we found that the same chain TBUs aggregate into the 2-D layer in compound 2 and the 3-D framework in compound 3, respectively, which corresponds to different connecting modes of the cluster-based chain TBUs in the structural assembly. Compared to the two sets of Pr-Cl2-Pr and one set of Pr-Cl-Pr bridges in 2, there is an extra connection of Pr-Cl-Sb3O3 from the last Pr 3+ ion to result in the 3-D framework of 3 rather than the 2-D network in 2 ( Figure 4). Relatively speaking, the smaller cation of (H3O) + in structure 3 may help to induce a structure with a higher dimension than the larger ones of [Fe(2,2′-bpy)3] 2+ and triethylamine in structures 1 and 2.  When comparing the structures of 2 and 3, we found that the same chain TBUs aggregate into the 2-D layer in compound 2 and the 3-D framework in compound 3, respectively, which corresponds to different connecting modes of the cluster-based chain TBUs in the structural assembly. Compared to the two sets of Pr-Cl 2 -Pr and one set of Pr-Cl-Pr bridges in 2, there is an extra connection of Pr-Cl-Sb 3 O 3 from the last Pr 3+ ion to result in the 3-D framework of 3 rather than the 2-D network in 2 ( Figure 4). Relatively speaking, the smaller cation of (H 3 O) + in structure 3 may help to induce a structure with a higher dimension than the larger ones of [Fe(2,2 -bpy) 3 ] 2+ and triethylamine in structures 1 and 2.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 12 framework that is directly constructed by the interconnection of the chain-like TBUs that are also present in compounds 1 and 2. When comparing the structures of 2 and 3, we found that the same chain TBUs aggregate into the 2-D layer in compound 2 and the 3-D framework in compound 3, respectively, which corresponds to different connecting modes of the cluster-based chain TBUs in the structural assembly. Compared to the two sets of Pr-Cl2-Pr and one set of Pr-Cl-Pr bridges in 2, there is an extra connection of Pr-Cl-Sb3O3 from the last Pr 3+ ion to result in the 3-D framework of 3 rather than the 2-D network in 2 ( Figure 4). Relatively speaking, the smaller cation of (H3O) + in structure 3 may help to induce a structure with a higher dimension than the larger ones of [Fe(2,2′-bpy)3] 2+ and triethylamine in structures 1 and 2.  as a TBU to assemble the title compounds with distinct structural dimensionalities, that is, 1-D for 1, 2-D for 2, and 3-D for 3. As above-mentioned, the dimensionalities and hierarchy level of the cluster-based title structures are controlled by the numbers and fashions of linkage units that interconnect the 1-D chain-like TBUs. Without additional linkers, isolated 1-D chain-like anions are evidenced in 1, while additional Pr-Cl-Pr bridges in 2 and Pr-Cl 2 -Pr, as well as unique Pr-Cl-Sb 3 O 3 bridges in 3, lead to 2-D and 3-D TBU-based networks, respectively. Unlike the previously reported {Pr 4 Sb 12 } cluster-organic structures [24][25][26], in which the linkers are bi/tri functional organic ligands, herein, the title structures are formed through interconnections of TBUs by inorganic linkage units, such as Pr-Cl-Pr and Pr-Cl-Sb 3 O 3 . This is also different from the case in the assembly of single-crystalline superstructures we just reported, in which the (FeCN 6 ) and/or the Ac (acetate group) function as linkers along with Cl − to realize the aggregation of eight {Pr 4 Sb 12 } clusters into supercluster TBUs rather than chain-like TBUs in this work. Overall, by analyzing the crystal structure motifs in detail, a clear structural evolution of the cluster-based structures is achieved ( Figure 5).
Molecules 2023, 28, x FOR PEER REVIEW 6 of 12 As a whole, there are obvious relationships of structures among the three title compounds. The {Pr4Sb12} clusters act as SBUs to aggregate into a zig-zag chain, which then acts as a TBU to assemble the title compounds with distinct structural dimensionalities, that is, 1-D for 1, 2-D for 2, and 3-D for 3. As above-mentioned, the dimensionalities and hierarchy level of the cluster-based title structures are controlled by the numbers and fashions of linkage units that interconnect the 1-D chain-like TBUs. Without additional linkers, isolated 1-D chain-like anions are evidenced in 1, while additional Pr-Cl-Pr bridges in 2 and Pr-Cl2-Pr, as well as unique Pr-Cl-Sb3O3 bridges in 3, lead to 2-D and 3-D TBU-based networks, respectively. Unlike the previously reported {Pr4Sb12} cluster-organic structures [24][25][26], in which the linkers are bi/tri functional organic ligands, herein, the title structures are formed through interconnections of TBUs by inorganic linkage units, such as Pr-Cl-Pr and Pr-Cl-Sb3O3. This is also different from the case in the assembly of single-crystalline superstructures we just reported, in which the (FeCN6) and/or the Ac (acetate group) function as linkers along with Cl − to realize the aggregation of eight {Pr4Sb12} clusters into supercluster TBUs rather than chain-like TBUs in this work. Overall, by analyzing the crystal structure motifs in detail, a clear structural evolution of the cluster-based structures is achieved ( Figure 5).

The Basic Characterizations
The high purity of the title compounds was well demonstrated by the fact that the experimental PXRD pattern matched well with the calculated results from the single-

The Basic Characterizations
The high purity of the title compounds was well demonstrated by the fact that the experimental PXRD pattern matched well with the calculated results from the single-crystal X-ray diffraction data (Figure 6), along with various characterizations, such as elemental analysis (EA) and TGA. Note that the PXRD patterns feature broad and low-intensity diffraction peaks, which are common for these types of high-nuclearity cluster-based compounds [8,[23][24][25][26]. The TGA for compounds 1 and 3 was carried out under a nitrogen atmosphere in the temperature range of 20-1000 • C ( Figure S3). The UV-Vis absorption spectra of compounds 1, 2, and 3 were collected to characterize their optical absorption abilities and bandgaps. As shown in Figure 7, the optical absorption properties of the title compounds are in accordance with the colors of the compounds in each case, and the bandgaps are calculated to be 1. 84, 1.80, and 1.92 eV for compounds 1, 2, and 3, respectively.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 12 crystal X-ray diffraction data (Figure 6), along with various characterizations, such as elemental analysis (EA) and TGA. Note that the PXRD patterns feature broad and low-intensity diffraction peaks, which are common for these types of high-nuclearity cluster-based compounds [8,[23][24][25][26]. The TGA for compounds 1 and 3 was carried out under a nitrogen atmosphere in the temperature range of 20-1000 °C ( Figure S3). The UV-Vis absorption spectra of compounds 1, 2, and 3 were collected to characterize their optical absorption abilities and bandgaps. As shown in Figure 7, the optical absorption properties of the title compounds are in accordance with the colors of the compounds in each case, and the bandgaps are calculated to be 1.84, 1.80, and 1.92 eV for compounds 1, 2, and 3, respectively.

The Photodegradation Characterizations of Compound 1
In recent years, a number of scholars have used metal-oxo cluster-based compounds to study the performance of the adsorption and/or degradation of dyes [32][33][34][35]. In this work, compound 1 was chosen as the representative to study the photodegradation property of these cluster-based compounds. In determining the stability of compound 1, it was found that it can dissolve completely in water to form a red, clear solution ( Figure S4). In view of this, we investigated the photocatalytic performance of compound 1 as a homogeneous reaction catalyst for the photocatalytic degradation of MB solutions. The Figure 6. Comparison of powder X-ray diffractograms of title compounds 1 (a), 2 (b) and 3 (c) with that simulated from corresponding single-crystal X-ray diffraction data.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 12 crystal X-ray diffraction data (Figure 6), along with various characterizations, such as elemental analysis (EA) and TGA. Note that the PXRD patterns feature broad and low-intensity diffraction peaks, which are common for these types of high-nuclearity cluster-based compounds [8,[23][24][25][26]. The TGA for compounds 1 and 3 was carried out under a nitrogen atmosphere in the temperature range of 20-1000 °C ( Figure S3). The UV-Vis absorption spectra of compounds 1, 2, and 3 were collected to characterize their optical absorption abilities and bandgaps. As shown in Figure 7, the optical absorption properties of the title compounds are in accordance with the colors of the compounds in each case, and the bandgaps are calculated to be 1. 84, 1.80, and 1.92 eV for compounds 1, 2, and 3, respectively. Figure 6. Comparison of powder X-ray diffractograms of title compounds 1 (a), 2 (b) and 3 (c) with that simulated from corresponding single-crystal X-ray diffraction data.

The Photodegradation Characterizations of Compound 1
In recent years, a number of scholars have used metal-oxo cluster-based compounds to study the performance of the adsorption and/or degradation of dyes [32][33][34][35]. In this work, compound 1 was chosen as the representative to study the photodegradation property of these cluster-based compounds. In determining the stability of compound 1, it was found that it can dissolve completely in water to form a red, clear solution ( Figure S4). In view of this, we investigated the photocatalytic performance of compound 1 as a homogeneous reaction catalyst for the photocatalytic degradation of MB solutions. The

The Photodegradation Characterizations of Compound 1
In recent years, a number of scholars have used metal-oxo cluster-based compounds to study the performance of the adsorption and/or degradation of dyes [32][33][34][35]. In this work, compound 1 was chosen as the representative to study the photodegradation property of these cluster-based compounds. In determining the stability of compound 1, it was found that it can dissolve completely in water to form a red, clear solution ( Figure S4). In view of this, we investigated the photocatalytic performance of compound 1 as a homogeneous reaction catalyst for the photocatalytic degradation of MB solutions. The absorbance of the MB solution was measured by UV-Vis spectrophotometer after different illumination times. It was found that the absorption intensity of the MB solution at the maximum absorption wavelength of 668 nm decreased with the illumination duration, illustrating that compound 1 has an obvious degradation ability towards MB. After illumination of 7 h, the degradation of MB by compound 1 was close to 71.8% (Figure 8).
absorbance of the MB solution was measured by UV-Vis spectrophotometer after different illumination times. It was found that the absorption intensity of the MB solution at the maximum absorption wavelength of 668 nm decreased with the illumination duration, illustrating that compound 1 has an obvious degradation ability towards MB. After illumination of 7 h, the degradation of MB by compound 1 was close to 71.8% (Figure 8).
Preparation of 2. A mixture of Pr(Ac)3·5H2O (0.3602 g, 0.88 mmol), SbCl3 (0.5985 g, 2.625 mmol), Na4Fe(CN)6·10H2O (0.1089 g, 0.225 mmol), and 2,2′-bpy (0.0674 g, 0.43 mmol) in TEOA/TEA/ethanol (0.2 mL/0.4 mL/9 mL) was sealed in a 28 mL Teflon-lined stainlesssteel autoclave at 150 °C for 5 days, then cooled to RT. The product was washed several The {Pr 4 Sb 12 } cluster-based compounds previously reported by us were all synthesized by using 2-methylpyridine and water as solvents under solvothermal conditions [23][24][25][26]. The mixed-solvents strategy has proved to be effective in preparing new compounds [13,36]. Therefore, a mixture of TEOA, TEA, and ethanol was utilized as the solvent system to prepare the title compounds. Furthermore, in our early attempts to synthesize {Pr 4 Sb 12 } cluster-based compounds, various sources of iron, such as FeCl 3 ·6H 2 O, Fe(NO 3 ) 3 ·9H 2 O, and Na 4 [Fe(CN) 6 ]·10H 2 O, were used. However, the title compounds could only be obtained in a mixed-solvent system and by using Na 4 [Fe(CN) 6 ]·10H 2 O as the iron source. Although many attempts were made, the yield of the syntheses was still low, indicating the complexity of the reactions.
Characterizations. Powder X-ray diffraction (PXRD) patterns were obtained on a Rigaku Miniflex-II diffractometer by using Cu K α radiation (λ = 1.54178 Å) with an angular range of 2θ = 3 • − 65 • at 30 KV, 15 mA, and a step size of 0.2. The simulated PXRD pattern was calculated from the SCXRD data using the Mercury program. Elemental analyses (EAs) of C, H, and N were performed using a German Elementary Vario EL III instrument. Energy-dispersive spectroscopy (EDS) was obtained by a JEOL JSM-6700F scanning electron microscope. Thermogravimetric analyses (TGA) were performed using crystalline sample loads in Al 2 O 3 crucibles with a NETZSCH STA 449F3 unit at a heating rate of 10 K min −1 under a N 2 atmosphere and in the temperature range of 25-1000 • C (heating rate, 10 • C min −1 ). UV-Vis absorbance photometric tests were carried out on a UV-2600 and PerkinElmer Lambda 350 UV-Vis spectrophotometer at RT in the range of 200-800 nm. Solid-state optical diffuse reflectance spectra were recorded on a Shimadzu 2600 UV/vis spectrometer at RT in the range of 200-800 nm. A BaSO 4 plate was utilized as a standard, which possesses 100% reflectance. The absorption data were then obtained from the reflectance spectra by using the Kubelka-Munk function α/S = (1 − R) 2 /2R, where α refers to the absorption coefficient, S refers to the scattering coefficient, and R refers to the reflectance.
Photodegradation experiments. A methylene blue (MB) solution at a concentration of 100 ppm was prepared and diluted into MB standard solutions of 10, 8, 6, 4, 2, and 0 ppm. The absorbance of different concentrations of MB solutions was measured at the maximum absorption wavelength of 668 nm. The standard working curve of methylene blue solution was established using concentration (C) as the horizontal coordinate and absorbance (A) as the vertical coordinate ( Figure S4). Then, 100 mg of compound 1 was ground into a photocatalytic reactor containing 100 mL of methylene blue solution (10 ppm) and stirred for 30 min in the dark to reach the adsorption-desorption equilibrium. A sample was taken as a standard, and then the absorbance of the sample at 668 nm was determined spectrophotometrically at 1 h intervals under light conditions using a 0.22 µm pinhead filter, and the concentration of the sample was calculated as C t . The calculation formula is as follows: D = (C 0 − C t )/C 0 × 100%. C 0 is the concentration of MB in the mixed system at the initial equilibrium, and C t is the concentration of MB in the system at various times of the reaction.
Single-Crystal X-ray Diffraction (SCXRD). Crystals of suitable size were selected for immersion in crystal oil under an optical microscope, and then a suitable glass wire was selected to hold the crystals on top of the glass wire for SCXRD characterization. The singlecrystal X-ray diffraction data for 1 and 3 were collected on a SuperNova CCD diffractometer with graphite monochromatic Mo K α radiation (λ = 0.71073 Å) at 295 K. The single-crystal X-ray diffraction data for 2 were collected on a SuperNova CCD diffractometer with graphite monochromatic Cu K α radiation (λ = 1.54178 Å) at 298 K. The structures were solved by direct methods and refined by full-matrix least-squares on F 2 using the SHELX-2018 program package [37]. In structure 1, both [HN(CH 2 CH 3 ) 3 ] + and lattice EtOH molecules are disordered over two positions, with a refined SOF ratio of ca. 0.75: 0.25. Accordingly, some soft restraints, such as SIMU, ISOR, and DELU, were applied to the obtained reasonable geometries and displacements for disordered atoms. Another feature is that one of the capping Cl ions coexists with the (TEOA) − anion; their SOFs were refined to be close to 0.5 and 0.5, respectively, and finally were fixed at 0.5 and 0.5, respectively. The squeeze routine in the PLATON program was applied for structures 2 and 3 to squeeze out the cations (e.g., [HN(CH 2 CH 3 ) 3 ] + and H 3 O + ) and lattice molecules (i.e., EtOH, H 2 O, and N(CH 2 CH 3 ) 3 ) that could not be found from the difference-Fourier maps and/or refined properly [38]. Also of note is that, although many attempts to obtain high-quality crystals for compound 2 were made, the SCXRD data for 2 were always imperfect. However, the primary anionic structures of 2 were reliably determined. CCDC NO. 2241063 (for 1), 2241064 (for 2), and 2241065 (for 3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 18 November 2022).

Conclusions
In summary, we have constructed a chain-like TBU of [Pr 4 Sb 12 O 18 Cl 15−x ] n by aggregation of {Pr 4 Sb 12 } clusters. Through careful adjustment of the synthetic conditions, especially the solvents, three new structures based on the chain-like TBU have been obtained, including structure 1 featuring isolated 1-D anionic chains, structure 2 featuring an anionic 2-D layer, and structure 3 holding an anionic 3-D framework. A detailed analysis of the three title structures enabled us to clearly demonstrate at the atomical level the step-bystep evolution of this family of chain-like TBU-based complex structures, thus offering an excellent example for studying the precise design and controllable molecular assembly of new single-crystal cluster-based structures. The photodegradation ability of compound 1 was investigated preliminarily. Future study will focus on discovering new SBUs and TBUs towards the hierarchical assembly of single-crystal superstructures, especially on the basis of searching for new types of linkers for SBUs and TBUs.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28062725/s1. Table S1: Crystallographic data and structural refinement details for the title compounds; Figure S1: View along the b-axis of the packing of anionic chains together with the [Fe(2,2 -bpy) 3 ] 2+ cations in compound 1. For clarity, the guest molecules and H atoms are omitted; Figure S2: View along the b-axis of the packing of the anionic layers in compound 2; Figure S3: Thermogravimetric test curves for compounds 1 (a) and 3 (b); Figure S4. Photograph for the solution formed by dissolving 4 mg compound 1 in 4 mL water; Figure S5: Standard curve for methylene blue solution; the horizontal coordinate is the concentration and the vertical coordinate is the absorbance.