Diversity of Metal—Fullerene Framework Structures Regulated by Metal Salts

Taking into account the diversity of fullerene ligands and metal salts, metal–fullerene frameworks (MFFs) present a variety of structures. Currently, the structural control of MFFs mainly relies on the design and synthesis of fullerene ligands, while the influence of metal building units on the structures has been rarely studied. The present work represents a systematical investigation of fullerene-linked supramolecular architectures incorporating different metal salts. Treatment of a bidentate N,N-donors fullerene ligand (L1) with six metal salts ([Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, Cu(NO3)2·3H2O, Cu(OAc)2·H2O, FeCl2·4H2O and FeCl3·6H2O]) produced six one-dimensional MFFs, i.e., ZnL1(NO3)2(H2O)2 (1), CdL1(NO3)2 (2), Cu(L1)(H2O)2(NO3)2 (3), CuL1(OAc)(CH3O) (4), FeL1Cl2 (5) and FeL1Cl2(FeCl4) (6). Compounds 1–3, built with nitrates with different metal centers (M(NO3)2, M = Zn, Cd, Cu), present a 1D stair-like, 1D zigzag, and 1D linear chain structure, respectively. Compound 4, synthesized with another Cu(II) salt, Cu(OAc)2, displays a dinuclear Cu-Cu connected 1D stair-like chain structure, rather than the single Cu linked 1D linear chain obtained from Cu(NO3)2. Compounds 5 and 6, assembled from iron chloride of different oxidation states (Fe(II)Cl2 and Fe(III)Cl3) reveal a 1D zigzag and a 1D stair-like chain structure, respectively. The results demonstrate the significant influences of metal salts on the structures of metal–fullerene frameworks.

Synthesis of ZnL1(NO3)2(H2O)2 (1): L1 (0.5 mg, 1 equiv.) was dissolved in 0.5 mL dichloromethane in a glass tube, then 0.1ml dichloromethane/methanol (1:1 v/v) mixed solution was carefully added as a buffer layer. After that, 1 mL methanol solution of Zn(NO3)2·6H2O (1.76 mg, 20 equiv.) was added on the buffer layer. The glass tube was sealed and left undisturbed. Yellow crystals of 1 suitable for single-crystal X-ray diffraction were obtained after a period of a month.
Synthesis of CdL1(NO3)2 (2): Yellow crystals of 2 suitable for single-crystal X-ray diffraction were obtained by following the same procedures as described for 1, except for using Cd(NO3)2·4H2O (1.83 mg, 20 equiv.) as the metal source.
Synthesis of FeL1Cl2 (5): Green crystals of 5 were obtained by following the same procedures as described for 1 except for using FeCl2·4H2O (0.75 mg, 20 equiv.) as the building unit.
Synthesis of FeL1Cl2(FeCl4) (6): Red crystals of 6 were prepared by following the same procedures as described for 1 except for using FeCl3 (0.96 mg, 20 equiv.) as the metal source.
Single-Crystal XRD Measurements of L1 and Compounds 1-6. Experimental data for the X-ray analyses are described in the Supplementary Materials. Single-crystal X-ray diffraction data of 1 were collected at 100 K in beamline station BL17B at Shanghai Synchrotron Radiation Facility. Single-crystal X-ray diffraction data of L1 and 2-6 were collected on an XtaLAB PRO MM007HF diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The CrystalClear software package (Rigaku) was used for data collection, cell refinement, and data reduction. Crystal structures were solved by the intrinsic phasing method and Synthesis of compounds 1-6: First, 0.5 mg L1 was dissolved in 0.5 mL dichloromethane (CH 2 Cl 2 ) in a glass tube, then 0.1 mL dichloromethane/methanol (1:1 v/v) mixed solution was carefully added into the glass tube as a buffer layer. After that, 1 mL methanol (CH 3 OH) solution of metal salts (20 equiv.) was discreetly added. The glass tube was sealed and kept undisturbed at room temperature. Crystals suitable for single-crystal X-ray diffraction were obtained after a month. The photographs of crystals can be found in Figure S3 in the supporting information.
Synthesis of ZnL1(NO 3 ) 2 (H 2 O) 2 (1): L1 (0.5 mg, 1 equiv.) was dissolved in 0.5 mL dichloromethane in a glass tube, then 0.1ml dichloromethane/methanol (1:1 v/v) mixed solution was carefully added as a buffer layer. After that, 1 mL methanol solution of Zn(NO 3 ) 2 ·6H 2 O (1.76 mg, 20 equiv.) was added on the buffer layer. The glass tube was sealed and left undisturbed. Yellow crystals of 1 suitable for single-crystal X-ray diffraction were obtained after a period of a month.
Synthesis of CdL1(NO 3 ) 2 (2): Yellow crystals of 2 suitable for single-crystal X-ray diffraction were obtained by following the same procedures as described for 1, except for using Cd(NO 3 ) 2 ·4H 2 O (1.83 mg, 20 equiv.) as the metal source.

Synthesis of CuL1(H 2 O) 2 (NO 3 ) 2 (3):
Green crystals of 3 suitable for single-crystal X-ray diffraction were attained by following the same procedures as described for 1, except for using Cu(NO 3 ) 2 ·3H 2 O (1.43 mg, 20 equiv.) as the metal building unit.
Synthesis of CuL1(OAc)(CH 3 O) (4): Green crystals of 4 were obtained by following the same procedures as described for 1 except for using Cu(OAc) 2 ·H 2 O (1.19 mg, 20 equiv.) as the metal salt.
Synthesis of FeL1Cl 2 (5): Green crystals of 5 were obtained by following the same procedures as described for 1 except for using FeCl 2 ·4H 2 O (0.75 mg, 20 equiv.) as the building unit.
Synthesis of FeL1Cl 2 (FeCl 4 ) (6): Red crystals of 6 were prepared by following the same procedures as described for 1 except for using FeCl 3 (0.96 mg, 20 equiv.) as the metal source.
Single-Crystal XRD Measurements of L1 and Compounds 1-6. Experimental data for the X-ray analyses are described in the Supplementary Materials. Single-crystal Xray diffraction data of 1 were collected at 100 K in beamline station BL17B at Shanghai Synchrotron Radiation Facility. Single-crystal X-ray diffraction data of L1 and 2-6 were collected on an XtaLAB PRO MM007HF diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The CrystalClear software package (Rigaku) was used for data collection, cell refinement, and data reduction. Crystal structures were solved by the intrinsic phasing method and refined using full-matrix least-squares based on F 2 with the programs SHELXT-2014 and SHELXL-2018 within OLEX2, respectively [27]. All of the non-hydrogen atoms were refined anisotropically, and the positions of the hydrogen atoms were generated geomet-

Results
Fullerene ligands, L1 and L2, were synthesized as described in the experimental section. Although L1 was reported in our previous work [23], the growth of single-component crystals is first reported here (Figure 1a). L1 crystallizes in the P2/c space group with the asymmetric unit consisting of two halves of molecules. The crystal structure of L1 confirmed that it is a hexa-adduct of C 60 with two 4,5-diazafluorene groups (dinitrogen chelating ligands) located at the trans-1 position and four malonate groups located at the equatorial position of C 60 . Fullerene ligand L2 (Figure 1b) was synthesized according to the procedure as described for L1. It is also a hexa-adduct of C 60 with two trans-1 phenyl oxazole groups (N,O-chelating ligands) and four equatorial malonates groups. As is well known, transition metals have empty orbitals available for accepting electrons from Oand N-donors to form coordinate bonding, thus both of the fullerene ligands were used as building units to assemble with Zn(NO 3 Six one-dimensional (1D) metal-fullerene frameworks (MFFs) were successfully synthesized from L1. Unexpectedly, no crystalline polymeric structures or discrete complexes were obtained from L2, which can possibly be ascribed to the O-based groups with higher electronegativity and less donating property. L1, as a bidentate N-containing ligand, has been reported to assemble with AgSO 3 CF 3 and AgBF 4 , forming a 1D coordination polymer with 4-coordinate silver centers, and a discrete fullerene complex with 6-coordinate silver centers, respectively [23]. The results indicate that metal salts play an important role in metal-fullerene polymeric structures. To further explore the influence of metal salt properties on MFFs, Zn(II)/Cd(II)/Cu(II) nitrates, Cu(II) acetate, as well as Fe(II)/Fe(III) chlorides, was selected to coordinate with L1 under the same conditions. Single crystals of compounds 1-6 were grown from the same solvent combination, and their structures were studied in this work. Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 13 refined using full-matrix least-squares based on F 2 with the programs SHELXT-2014 and SHELXL-2018 within OLEX2, respectively [27]. All of the non-hydrogen atoms were refined anisotropically, and the positions of the hydrogen atoms were generated geometrically. The SQUEEZE program [28]

Results
Fullerene ligands, L1 and L2, were synthesized as described in the experimental section. Although L1 was reported in our previous work [23], the growth of single-component crystals is first reported here ( Figure 1a). L1 crystallizes in the P2/c space group with the asymmetric unit consisting of two halves of molecules. The crystal structure of L1 confirmed that it is a hexa-adduct of C60 with two 4,5-diazafluorene groups (dinitrogen chelating ligands) located at the trans-1 position and four malonate groups located at the equatorial position of C60. Fullerene ligand L2 (Figure 1b) was synthesized according to the procedure as described for L1. It is also a hexa-adduct of C60 with two trans-1 phenyl oxazole groups (N,O-chelating ligands) and four equatorial malonates groups. As is well known, transition metals have empty orbitals available for accepting electrons from Oand N-donors to form coordinate bonding, thus both of the fullerene ligands were used as building units to assemble with Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, Cu(NO3)2·3H2O, Cu(OAc)2·H2O, FeCl2·4H2O and FeCl3·6H2O. Six one-dimensional (1D) metal-fullerene frameworks (MFFs) were successfully synthesized from L1. Unexpectedly, no crystalline polymeric structures or discrete complexes were obtained from L2, which can possibly be ascribed to the O-based groups with higher electronegativity and less donating property. L1, as a bidentate N-containing ligand, has been reported to assemble with AgSO3CF3 and AgBF4, forming a 1D coordination polymer with 4-coordinate silver centers, and a discrete fullerene complex with 6-coordinate silver centers, respectively [23]. The results indicate that metal salts play an important role in metal-fullerene polymeric structures. To further explore the influence of metal salt properties on MFFs, Zn(II)/Cd(II)/Cu(II) nitrates, Cu(II) acetate, as well as Fe(II)/Fe(III) chlorides, was selected to coordinate with L1 under the same conditions. Single crystals of compounds 1-6 were grown from the same solvent combination, and their structures were studied in this work.   Three divalent transition metal nitrates, Zn(NO 3 ) 2 ·6H 2 O, Cd(NO 3 ) 2 ·4H 2 O and Cu(NO 3 ) 2 · 3H 2 O, were firstly assembled with L1 to show the diverse supramolecular structures based on different metal centers. Crystals of 1-3 are described and compared together, since they are formed from nitrate salts with different divalent metal ions. Notably, the differences in the crystal structures should be caused by the unique aspects of the metal ions. Light yellow crystals of ZnL1(NO 3 ) 2 (H 2 O) 2 (1) were obtained by slow diffusion of Zn(NO 3 ) 2 ·6H 2 O in methanol into a solution of L1 in dichloromethane. Compound 1 crystallized in the P2 1 /c space group and the asymmetric unit was composed of one molecule of L1, one Zn ion, and two water molecules. The assembly of L1 with zinc ions formed an unexpected 1D stair-like chain structure (Figure 2a,b) instead of the expected linear chain as in the case of [Zn(dafo) 2 (H 2 O) 2 ](NO 3 ) 2 (dafo = 4,5-diazafluoren-9-one) [29], which could be ascribed to the attachment of the fullerene cage on the diazafluorene in L1. The counter anions NO 3 − are not connected with the Zn ions and are not observed in the structure, since they are in the disordered volume. This phenomenon has been previously reported [8]. Each fullerene unit coordinates with two zinc ions through two diazafluorene appendages. The local coordination environment of the zinc center, as shown in Figure 2b, is hexa-coordinated with four nitrogen atoms from two diazafluorene units and two oxygen atoms from two water (H 2 O) molecules. The bond distances between the zinc ion and the nitrogen/oxygen atoms are 2.131(6) Å for Zn1-N1, 2.245(7) Å for Zn1-N2, 2.280(8) Å for Zn1-N3, 2.134(6) Å for Zn1-N4, 2.115(9) Å for Zn1-O w 1, and 2.089(9) Å for Zn1-O w 2, respectively. The angles centered on the zinc ion are 93.
respectively. The sum of these four angles is 360.5 • , indicating that Zn1, N1, N2, N3 and O W 1 are almost on the same plane. The angle of the N2-Zn1-O W 2 axis is 174.0(3) • , which is very close to being linear. Therefore, the coordination geometry of the zinc center can be considered as a distorted octahedron (shown in green in Figure 2b). The stair-like chains are arranged in a staggered manner and repeated in every two adjacent chains, resulting in an ABAB-type stacking (Figure 2c,d).
3.1. MFFs from Three Metal Nitrates: Crystal Structures of ZnL1(NO3)2(H2O)2 (1), CdL1(NO3)2 (2) and Cu(L1)(H2O)2(NO3)2 (3) Three divalent transition metal nitrates, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O and Cu(NO3)2·3H2O, were firstly assembled with L1 to show the diverse supramolecular structures based on different metal centers. Crystals of 1-3 are described and compared together, since they are formed from nitrate salts with different divalent metal ions. Notably, the differences in the crystal structures should be caused by the unique aspects of the metal ions. Light yellow crystals of ZnL1(NO3)2(H2O)2 (1) were obtained by slow diffusion of Zn(NO3)2·6H2O in methanol into a solution of L1 in dichloromethane. Compound 1 crystallized in the P21/c space group and the asymmetric unit was composed of one molecule of L1, one Zn ion, and two water molecules. The assembly of L1 with zinc ions formed an unexpected 1D stair-like chain structure (Figure 2a,b) instead of the expected linear chain as in the case of [Zn(dafo)2(H2O)2](NO3)2 (dafo = 4,5-diazafluoren-9-one) [29], which could be ascribed to the attachment of the fullerene cage on the diazafluorene in L1. The counter anions NO3 -are not connected with the Zn ions and are not observed in the structure, since they are in the disordered volume. This phenomenon has been previously reported [8]. Each fullerene unit coordinates with two zinc ions through two diazafluorene appendages. The local coordination environment of the zinc center, as shown in Figure 2b, is hexa-coordinated with four nitrogen atoms from two diazafluorene units and two oxygen atoms from two water (H2O) molecules. The bond distances between the zinc ion and the nitrogen/oxygen atoms are 2.131(6) Å for Zn1-N1, 2.245(7) Å for Zn1-N2, 2.280(8) Å for Zn1-N3, 2.134(6) Å for Zn1-N4, 2.115(9) Å for Zn1-Ow1, and 2.089(9) Å for Zn1-Ow2, respectively. The angles centered on the zinc ion are 93.1(3)° for OW1-Zn1-N1, 92.7(3)° for N1-Zn1-N3, 81.5(3)° for N3-Zn1-N4, and 93.2(3)° for N4-Zn1-OW1, respectively. The sum of these four angles is 360.5°, indicating that Zn1, N1, N2, N3 and OW1 are almost on the same plane. The angle of the N2-Zn1-OW2 axis is 174.0(3)°, which is very close to being linear. Therefore, the coordination geometry of the zinc center can be considered as a distorted octahedron (shown in green in Figure 2b). The stair-like chains are arranged in a staggered manner and repeated in every two adjacent chains, resulting in an ABAB-type stacking (Figure 2c,d).  To investigate the effect of metal ions on the structure of MFFs, Cd(NO 3 ) 2 ·4H 2 O and Cu(NO 3 ) 2 ·3H 2 O were adopted to assemble with the L1 ligand as well. Accordingly, the synthesis of compounds 2 and 3 are useful in comparing the possible structural changes that may occur due to the alternation of the metal ions. Yellow crystal of polymeric Cd(L1)(NO 3 ) 2 (2) was obtained by slow diffusion of Cd(NO 3 ) 2 ·4H 2 O in methanol into a solution of L1 in dichloromethane. Compound 2 crystallized in the I2/a space group with the asymmetric unit consisting of half of molecule L1, half of Cd ion and one NO 3− counter anion. The structural examinations reveal that the connection of the Cd(II) and diazafluorene of L1 forms a 1D zigzag chain, propagating with an approximately per-pendicular folding angle in the crystallographic a direction (Figure 3a). Each Cd ion is octa-coordinated by four nitrogen atoms (Cd1-N1/N1A, 2.525(4) Å, and Cd1-N2/N2A, 2.369(4) Å) from two diazafluorenes of different fullerene linkers and four oxygen atoms (Cd1-O1/O1A, 2.317(4) Å, and Cd1-O2/O2A, 2.611(5) Å) from two nitrate anions. The octahedrally coordinated Cd ion exhibit η 2 coordination to both nitrate anions. The sum of the bond angles of N1-Cd1-N2, N2-Cd1-O1A, O1A-Cd1-O2A and O2A-Cd1-N1 is 359.8 • , suggesting that Cd1, N1, N2, O1A and O2A atoms are almost coplanar. Similarly, Cd1, N1A, N2A, O1 and O2 atoms are located on the same plane. The two planes crossed each other at the point of the Cd ion, presenting a butterfly-shaped structure (Figure 3b). Interestingly, large voids of~5066 Å 3 , shown in bright yellow and occupying approximately 43% of the unit cell, are regularly distributed within and between the one-dimensional polymer chains (Figure 3c), suggesting the potential applications for solvent or gas absorption, isolation and storage. Each 1D zigzag chain runs one over the next in an ABAB sequence along the c-axis (Figure 3d). synthesis of compounds 2 and 3 are useful in comparing the possible structural changes that may occur due to the alternation of the metal ions. Yellow crystal of polymeric Cd(L1)(NO3)2 (2) was obtained by slow diffusion of Cd(NO3)2·4H2O in methanol into a solution of L1 in dichloromethane. Compound 2 crystallized in the I2/a space group with the asymmetric unit consisting of half of molecule L1, half of Cd ion and one NO 3-counter anion. The structural examinations reveal that the connection of the Cd(II) and diazafluorene of L1 forms a 1D zigzag chain, propagating with an approximately perpendicular folding angle in the crystallographic a direction (Figure 3a). Each Cd ion is octa-coordinated by four nitrogen atoms (Cd1-N1/N1A, 2.525(4) Å, and Cd1-N2/N2A, 2.369(4) Å) from two diazafluorenes of different fullerene linkers and four oxygen atoms (Cd1-O1/O1A, 2.317(4) Å, and Cd1-O2/O2A, 2.611(5) Å) from two nitrate anions. The octahedrally coordinated Cd ion exhibit η 2 coordination to both nitrate anions. The sum of the bond angles of N1-Cd1-N2, N2-Cd1-O1A, O1A-Cd1-O2A and O2A-Cd1-N1 is 359.8°, suggesting that Cd1, N1, N2, O1A and O2A atoms are almost coplanar. Similarly, Cd1, N1A, N2A, O1 and O2 atoms are located on the same plane. The two planes crossed each other at the point of the Cd ion, presenting a butterfly-shaped structure (Figure 3b). Interestingly, large voids of ~5066 Å 3 , shown in bright yellow and occupying approximately 43% of the unit cell, are regularly distributed within and between the one-dimensional polymer chains (Figure 3c), suggesting the potential applications for solvent or gas absorption, isolation and storage. Each 1D zigzag chain runs one over the next in an ABAB sequence along the c-axis (Figure 3d). Interestingly, the reaction of L1 with Cu(NO3)2 yielded the expected 1D straight line (Figure 4a) instead of the stair-like (Zn(II)-based MFF, 1) or the zigzag (Cd(II)-based MFF, 2) structure. Green crystals of Cu(L1)(H2O)2(NO3)2 (3) crystallized in the P-1 space group with the asymmetric unit making up of half L1 molecule, half of Cu ion, one coordinated water, and a free NO 3− counter anion. The Cu ion is located at the crystallographic inversion center and is hexa-coordinated with four nitrogen atoms occupying the equatorial The linear chains run in parallel, and no intermolecular hydrogen bonding interactions were observed due to the large fullerene units (Figure 4c). that the diazafluorene shows asymmetric chelation, with one Cu-N bond (2.557(2) Å) being much longer than the other one (1.998(2) Å). The sum of four equatorial angles around copper is exactly 360.0°, indicating a square-planar arrangement of the Cu-N4 unit. The O1-Cu1-O1A (180.0°, Cu-O distance of 1.981(2) Å) coordination perpendicular to the equatorial plane fills the vacant axial position and results in an octahedral geometry of the Cu center. Additionally, the free nitrate anion (NO3 − ) is linked to the coordinated H2O by a hydrogen bond (Ow1-H1···O2) with a distance of 2.65(4) Å and a corresponding angle of 177.1(3)°. The linear chains run in parallel, and no intermolecular hydrogen bonding interactions were observed due to the large fullerene units (Figure 4c). The coordination between H2O and Cu ion is not very strong, and could be substituted by a second building unit (SBU), such as di-or tetrapyridine-based pillars [30], which suggests that compound 3 stands out as an attractive building block for 2D and 3D metal-fullerene frameworks. Based on the above results, it can be concluded that the introduction of different metal ion centers accounts for the diversities of the structures. As a result, the polymeric structure has evolved from the 1D stair-like chain The coordination between H 2 O and Cu ion is not very strong, and could be substituted by a second building unit (SBU), such as di-or tetrapyridine-based pillars [30], which suggests that compound 3 stands out as an attractive building block for 2D and 3D metalfullerene frameworks. Based on the above results, it can be concluded that the introduction of different metal ion centers accounts for the diversities of the structures. As a result, the polymeric structure has evolved from the 1D stair-like chain [ZnL1(NO 3

O) (4)
Our previous studies on the reactions of L1 with AgSO 3 CF 3 and AgBF 4 showed the dependence of coordination modes and structures of MFFs on the counter anions [27]. Therefore, we take Cu(II) acetate [Cu(OAc) 2 ·H 2 O] as a contrast to Cu(NO 3 ) 2 to further survey the structure regulation by counter anions. The reaction of L1 and Cu(OAc) 2 ·H 2 O produced green crystals of compound 4 with the composition of CuL1(OAc)(CH 3 O). The space group of 4 was determined to be P-1, which is the same as compound 3. Its asymmetric unit contains half of molecule L1, one Cu ion, one CH 3 COOanion and one CH 3 O − anion. Unexpectedly, the polymeric structure of compound 4 is a 1D stair-like chain (Figure 5a) instead of a 1D straight line as shown in compound 3. The fullerene units in 4 are unprecedentedly connected by a bimetallic Cu(II) center (Figure 5b) instead of a single Cu(II) as presented in crystal 3. Further crystallographic analysis reveals that Cu1 and Cu1A possess the identical coordination environments of two nitrogen atoms from one diazafluorene unit (Cu-N distances of 2.013(3) Å and 2.630(2) Å), two oxygen atoms from one acetate anion, and two bridging oxygen from CH 3 Oanions generated from methanol (a solvent for crystallization). The coordinated Cu ions exhibit an η 2 coordination mode to the acetate anions. The distance of Cu1-Cu1A is 2.992(9) Å, longer than the van der Waals contact (2.8 Å) for Cu-Cu [31,32], suggesting that no metal-metal bond is formed between the two copper ions. To the best of our knowledge, compound 4 represents the first example of a dinuclear Cu(II)-linked fullerene coordination polymer. The packing mode of 4 is shown in Figure 5c.

MFFs from Copper Salts with Different Counter Anions: Crystal Structure of CuL1(OAc)(CH3O) (4)
Our previous studies on the reactions of L1 with AgSO3CF3 and AgBF4 showed the dependence of coordination modes and structures of MFFs on the counter anions [27]. Therefore, we take Cu(II) acetate [Cu(OAc)2·H2O] as a contrast to Cu(NO3)2 to further survey the structure regulation by counter anions.
The reaction of L1 and Cu(OAc)2·H2O produced green crystals of compound 4 with the composition of CuL1(OAc)(CH3O). The space group of 4 was determined to be P-1, which is the same as compound 3. Its asymmetric unit contains half of molecule L1, one Cu ion, one CH3COO -anion and one CH3O − anion. Unexpectedly, the polymeric structure of compound 4 is a 1D stair-like chain (Figure 5a) instead of a 1D straight line as shown in compound 3. The fullerene units in 4 are unprecedentedly connected by a bimetallic Cu(II) center (Figure 5b) instead of a single Cu(II) as presented in crystal 3. Further crystallographic analysis reveals that Cu1 and Cu1A possess the identical coordination environments of two nitrogen atoms from one diazafluorene unit (Cu-N distances of 2.013(3) Å and 2.630(2) Å), two oxygen atoms from one acetate anion, and two bridging oxygen from CH3O -anions generated from methanol (a solvent for crystallization). The coordinated Cu ions exhibit an η 2 coordination mode to the acetate anions. The distance of Cu1-Cu1A is 2.992(9) Å, longer than the van der Waals contact (2.8 Å) for Cu-Cu [31,32], suggesting that no metal-metal bond is formed between the two copper ions. To the best of our knowledge, compound 4 represents the first example of a dinuclear Cu(II)-linked fullerene coordination polymer. The packing mode of 4 is shown in Figure 5c. The use of Cu(NO 3 ) 2 and Cu(OAc) 2 as the building blocks led to different coordination configurations of the Cu centers and accordingly the different polymeric structures. Specifically, the NO 3 − anion was not coordinated with the Cu ion in compound 3, thereby facilitating the addition of two fullerene linkers on one Cu ion. Meanwhile, the CH 3 COO − anion, as a bidentate chelating ligand, binds to the Cu ion in 4, preventing the coordination of another diazafluorene unit with the same Cu ion. As a result, the structure was adjusted from a monometallic 1D linear chain to a bimetallic 1D stair-like chain upon changing the counter anions. In addition, it is worth noting that compound 4 is the only metal-fullerene framework in which the metal center is coordinated with only one fullerene ligand. Heretofore, metal atoms from Group IB (Cu and Ag) [23] and Group IIB (Zn and Cd) have been studied for supramolecular assembly with fullerene ligand L1. Each metal ion exhibits distinct coordination configurations. Notably, in addition to Group IB and IIB metal elements, Group VIII elements, such as iron, are excellent candidates to coordinate with L1. A more important reason for picking iron salts is that they have two oxidation states, making it possible for us to seek the effect of the oxidation states in preparing MFF structures.
Bulk green crystals of FeL1Cl 2 (5) were grown by slow diffusion of methanol solution of FeCl 2 ·4H 2 O into a dichloromethane solution of L1. Compound 5 crystallized in the I2/a space group with the asymmetric unit containing half of molecule L1, half of Fe(II) ion, one Cl − anion, and one CH 2 Cl 2 molecule. The structural analysis shows that the packing mode of compound 5 is analogous to that of compound 2, forming a 1D zigzag chain along the c-axis, which runs one over the next in parallel and stacks in an ABAB sequence along the b-axis (Figure 6a The use of Cu(NO3)2 and Cu(OAc)2 as the building blocks led to different coordination configurations of the Cu centers and accordingly the different polymeric structures. Specifically, the NO3 − anion was not coordinated with the Cu ion in compound 3, thereby facilitating the addition of two fullerene linkers on one Cu ion. Meanwhile, the CH3COOanion, as a bidentate chelating ligand, binds to the Cu ion in 4, preventing the coordination of another diazafluorene unit with the same Cu ion. As a result, the structure was adjusted from a monometallic 1D linear chain to a bimetallic 1D stair-like chain upon changing the counter anions. In addition, it is worth noting that compound 4 is the only metal-fullerene framework in which the metal center is coordinated with only one fullerene ligand. (5) and FeL1Cl2(FeCl4) (6) Heretofore, metal atoms from Group IB (Cu and Ag) [23] and Group IIB (Zn and Cd) have been studied for supramolecular assembly with fullerene ligand L1. Each metal ion exhibits distinct coordination configurations. Notably, in addition to Group IB and IIB metal elements, Group VIII elements, such as iron, are excellent candidates to coordinate with L1. A more important reason for picking iron salts is that they have two oxidation states, making it possible for us to seek the effect of the oxidation states in preparing MFF structures.

MFFs from Iron Chloride with Different Oxidation States: Crystal Structures of FeL1Cl2
Bulk green crystals of FeL1Cl2 (5) were grown by slow diffusion of methanol solution of FeCl2·4H2O into a dichloromethane solution of L1. Compound 5 crystallized in the I2/a space group with the asymmetric unit containing half of molecule L1, half of Fe(II) ion, one Clanion, and one CH2Cl2 molecule. The structural analysis shows that the packing mode of compound 5 is analogous to that of compound 2, forming a 1D zigzag chain along the c-axis, which runs one over the next in parallel and stacks in an ABAB sequence along the b-axis (Figure 6a  As a comparison, trivalent FeCl 3 ·6H 2 O was adopted to assemble with L1 as well. The reaction of FeCl 3 ·6H 2 O and L1 generated red crystals of Fe(L1)Cl 2 (FeCl 4 ) (6). As evidenced by the single-crystal structural analysis, compound 6 has the I2/a space group, the same as that of compound 5. The asymmetric unit consists of half of the L1 molecule, half of Fe(III) ion, one Cl − anion and half of [FeCl 4 ] − counter anion. Figure 7a shows that a 1D chain structure is formed, which is different from the zigzag shape in compound 5. The packing mode as displayed in Figure 7c,d is also different from that of compound 5. The central Fe(III) ion is hexa-coordinated by two chlorine atoms with a Fe-Cl distance of 2.333(5) Å, and four nitrogen atoms from two diazafluorene units with the Fe-N distances of 2.238(6) Å and 2.242 (7)  ion; (c) voids between chains shown along the b-axis; (d) interlayer relationship within the crystal lattice as seen from the a-axis; hydrogen atoms and malonate addends have been omitted for clarity.
As a comparison, trivalent FeCl3·6H2O was adopted to assemble with L1 as well. The reaction of FeCl3·6H2O and L1 generated red crystals of Fe(L1)Cl2(FeCl4) (6). As evidenced by the single-crystal structural analysis, compound 6 has the I2/a space group, the same as that of compound 5. The asymmetric unit consists of half of the L1 molecule, half of Fe(III) ion, one Clanion and half of [FeCl4] -counter anion. Figure 7a shows that a 1D chain structure is formed, which is different from the zigzag shape in compound 5. The packing mode as displayed in Figure 7c,d is also different from that of compound 5. The central Fe(III) ion is hexa-coordinated by two chlorine atoms with a Fe-Cl distance of 2.333(5) Å, and four nitrogen atoms from two diazafluorene units with the Fe-N distances of 2.238(6) Å and 2.242 (7)

Discussion
Based on the above studies, metal salts exhibit remarkable capabilities to regulate the structures of metal-fullerene frameworks. For compounds 1, 2 and 3, prepared from different metal nitrates (Zn(NO3)2, Cd(NO3)2 and Cu(NO3)2), Zn and Cu ions exhibited a stronger affinity for H2O rather than the multidentate ligand NO3 − , while Cd ions preferred to coordinate with NO3 -instead of H2O. Surprisingly, when Cu(NO3)2 was replaced by Cu(OAc)2, the ligands of Cu(II) were accordingly changed from H2O in 3 to CH3O -and

Discussion
Based on the above studies, metal salts exhibit remarkable capabilities to regulate the structures of metal-fullerene frameworks. For compounds 1, 2 and 3, prepared from different metal nitrates (Zn(NO 3 ) 2 , Cd(NO 3 ) 2 and Cu(NO 3 ) 2 ), Zn and Cu ions exhibited a stronger affinity for H 2 O rather than the multidentate ligand NO 3 − , while Cd ions preferred to coordinate with NO 3 − instead of H 2 O. Surprisingly, when Cu(NO 3 ) 2 was replaced by Cu(OAc) 2 , the ligands of Cu(II) were accordingly changed from H 2 O in 3 to CH 3 O − and AcO − anions in 4. Compounds 5 and 6 were synthesized from iron chlorides with different valence states. Both Fe(II) and Fe(III) ions are inclined to bind monodentate Cl − rather than H 2 O or CH 3 O − anions. These results could be interpreted as the preferred coordination modes of different metal ions, including coordination ability toward the ligands, coordination number, and coordination geometry.

Conclusions
In summary, six new 1D metal-fullerene frameworks (1-6) based on L1 and different metal salts were successfully synthesized, and their structures were unambiguously confirmed by single-crystal diffraction. The viability of L1 in fabricating fullerene-based supramolecular architectures was established. The structural diversity of compounds 1 to 6 revealed the regulation of metal salts on the MFF structures. The methodical changes to the metallic building block delivered more knowledge of fullerene coordination chemistry, and more examples were added to the limited database available on crystalline metalfullerene frameworks. Three factors in the coordination chemistry of MFFs were reviewed: the metal cation, the counter anion, and the oxidation state of the metal. The distinct coordination modes and coordination numbers of each metal ion (Zn 2+ , Cd 2+ , Cu 2+ ), the different coordination abilities of counter anions (NO 3 − , AcO − ) to metal centers and the presence of [FeCl 4 ] − units due to the higher oxidation state account for the great structural changes of MFFs. Interestingly, compound 3 may become a promising building unit for 2D and 3D MFFs if the coordinated waters are replaced by di-or tetrapyridine-based pillars. Compound 4 is the first example of a dinuclear metal-linked MFF. This study not only contributes to the basic science of MFFs, but is also valuable for the design and synthesis of next-generation multi-dimensional metal-fullerene frameworks.

Data Availability Statement:
The data presented in this study are available on a reasonable request from the corresponding author.