Two One-Dimensional Copper-Oxalate Frameworks with the Jahn–Teller Effect: [(CH 3 ) 3 NH] 2 [Cu( µ -C 2 O 4 )(C 2 O 4 )] · 2.5H 2 O (I) and [(C 2 H 5 ) 3 NH] 2 [Cu( µ -C 2 O 4 )(C 2 O 4 )] · H 2 O (II)

: Two one-dimensional oxalate-bridged Cu(II) ammonium salts, [(CH 3 ) 3 NH] 2 [Cu( µ -C 2 O 4 ) (C 2 O 4 )] · 2.5H 2 O ( I ) and [(C 2 H 5 ) 3 NH] 2 [Cu( µ -C 2 O 4 )(C 2 O 4 )] · H 2 O ( II ) were obtained and characterized. They were composed of ammonium: (CH 3 ) 3 NH + in ( I ), (C 2 H 5 ) 3 NH + in ( II ), [Cu( µ -C 2 O 4 )(C 2 O 4 ) 2 − ] n and H 2 O. The Jahn–Teller-distorted Cu(II) is octahedrally coordinated by six O atoms from three oxalates and forms a one-dimensional zigzag chain. The hydrogen bonds between ammonium, the anion and H 2 O form a three-dimensional network. There is no hydrogen bond between the anion chains. They were insulated at 20 ◦ C with a relative humidity of 40%. Ferromagnetic and weak-ferromagnetic behaviors were observed in I and II , separately. No long-range ordering was observed above 2 K.

Quantum spin liquid is an intriguing magnetic state, where spin ordering or freezing prevents spin frustration in a resonating valence bond (RVB) state.In 1979, P. W. Anderson proposed the RVB state in S = 1/2, a two-dimensional triangular lattice [30].In 1987, he proposed that La 2 CuO 4 is a parent compound of cuprate superconductors.The antiferromagnetic insulator La 2 CuO 4 turns into a diamagnetic superconductor after hole doping, and a quantum spin liquid with Jahn-Teller distortion on Cu(II) is an indispensable magnetic state [31].The spin-frustrated copper-oxalate framework with Jahn-Teller distortion supports a platform for molecular-based quantum spin liquids [32,33].Strong antiferromagnetic interactions without a long-range ordering above 2 K with spin frustration were observed in two-dimensional honeycomb lattices: θ 21  , which is a quantum spin liquid with no long-range ordering was observed until 60 mK [39].In these compounds, the antiferromagnetic behavior depends on the antiferromagnetic interaction between the ferromagnetic couple.The magnetic structure of [(C 2 H 5 ) 3 NH] 2 [Cu 2 (µ-C 2 O 4 ) 3 ] is lower than the three-dimensional, with the coexistence of ferromagnetic and antiferromagnetic interactions between Jahn-Teller distorted Cu(II) [37,39].Researching the magnetic properties of Jahn-Teller distorted onedimensional copper-oxalate frameworks [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2 2− ] n without the hydrogen bond between anions will help us to quantitatively analyze the magnetic interaction in Jahn-Teller-distorted two-dimensional and three-dimensional copper-oxalate frameworks and design new candidate quantum spin liquids.Two one-dimensional copper-oxalate framework compounds, , have been obtained and characterized.The related work is presented here.Elemental analyses of carbon, hydrogen and nitrogen were performed using the Flash EA 1112 elemental analyzer.The IR spectra were recorded on a Bio-rad FTS6000/UMA500 spectrometer (Figure S1).Thermogravimeter analysis was carried out on a Shimadzu DTG-60 analyzer at a 10 • C/min heating rate from room temperature to 550 • C under N 2 gas with an Al bag.I remains stable until 40 • C, and II remains stable until 80 • C.

Experiment
X-ray powder diffraction was carried out using a Rigaku RINT2000 diffractometer at room temperature with Cu Kα radiation (λ = 1.54056Å) in a flat-plate geometry (Figures S2  and S3).
Single-crystal X-ray diffraction was carried out on an Enraf-Nonius KappaCCD diffractometer at room temperature.The crystal structure was solved using the direct method and refined using the full-matrix least square on F 2 using the SHELX program, with anisotropic thermal parameters for all non-hydrogen atoms [40].The hydrogen atoms on C and N were located through calculation, and on H 2 O they were located through a difference Fourier map.All of the H were refined isotropically.The crystallographic data are listed in Table S1.
The resistance measurement was performed on a single crystal at Tonghui TH2828.Gold wires were attached to the best developed surfaces of a single crystal with a size of 0.40 * 0.30 * 0.11 mm (I) and 0.71 * 0.60 * 0.17 mm (II) using gold paste.The two-probe conductivity was measured at 20 • C and a relative humidity (RH) of 40%.
Magnetization measurements were performed on a polycrystalline sample tightly packed into a capsule on a Quantum Design MPMS 7XL SQUID system above 2 K. Susceptibility data were corrected for the diamagnetism of the sample by Pascal constants and background by experimental measurement of the sample holder [41].Temperaturedependent magnetization was performed under an applied field of 1000 G. Isothermal magnetization was measured at 2 K from 0 to 65 kG.

Result and Discussion
I crystallizes in a triclinic system with space group P 1.There are two (CH 3 ) 3 NH + , one Cu 2+ , one oxalate anion, two half-oxalate anions and one-and-a-half H 2 O coexisting in an independent unit (Figure 1).Cu 2+ is coordinated to two O atoms from one bidentate oxalate (O1 and O2) and four O atoms from two disbidentate oxalates in the Q 3 Jahn-Teller distortion mode of a CuO 6 octahedron [8].
Magnetochemistry 2023, 9, 120 3 of 12 Susceptibility data were corrected for the diamagnetism of the sample by Pascal constants and background by experimental measurement of the sample holder [41].Temperaturedependent magnetization was performed under an applied field of 1000 G. Isothermal magnetization was measured at 2 K from 0 to 65 kG.

Result and Discussion
I crystallizes in a triclinic system with space group P1.There are two (CH3)3NH + , one Cu 2+ , one oxalate anion, two half-oxalate anions and one-and-a-half H2O coexisting in an independent unit (Figure 1).Cu 2+ is coordinated to two O atoms from one bidentate oxalate (O1 and O2) and four O atoms from two disbidentate oxalates in the Q3 Jahn-Teller distortion mode of a CuO6 octahedron [8].Susceptibility data were corrected for the diamagnetism of the sample by Pascal constants and background by experimental measurement of the sample holder [41].Temperaturedependent magnetization was performed under an applied field of 1000 G. Isothermal magnetization was measured at 2 K from 0 to 65 kG.

Result and Discussion
I crystallizes in a triclinic system with space group P1.There are two (CH3)3NH + , one Cu 2+ , one oxalate anion, two half-oxalate anions and one-and-a-half H2O coexisting in an independent unit (Figure 1).Cu 2+ is coordinated to two O atoms from one bidentate oxalate (O1 and O2) and four O atoms from two disbidentate oxalates in the Q3 Jahn-Teller distortion mode of a CuO6 octahedron [8].At last, the hydrogen bonds N-H…O and C-H…O between the ammonium and the inner O of the terminal and bridged oxalate, the O-H…O between H2O and the oxalate, and the O-H…O between the H2O molecules form a three-dimensional hydrogen-bonded network in crystal (Figure 4).There is no hydrogen bond between the anions.II crystallizes in a monoclinic system with the space group P21/c.There are two (C2H5)3NH + , one Cu 2+ , one and two half oxalates, and one H2O in an independent unit (Figure 5).The Cu 2+ is octahedrally coordinated by six O atoms from two bisbidentate oxalates and one bidentate oxalate, as in I, with Cu-O distances of 1.960(2)~1.999(2)Å on the equatorial plane, and Cu1-O8: 2.314(2) Å and Cu1-O7: 2.368(2) Å from the apex.The Cu-O distances of II in the equatorial plane are shorter than the direction of the apex as a  4).There is no hydrogen bond between the anions.separated by (CH3)3NH + (N1) along the c axis, and there are hydrogen bonds N-H…O, C-H…O between the cation and the out O atom in the oxalate.The anionic sheets composed of a [Cu(µ-C2O4)(C2O4) 2− ]n chain and (CH3)3NH + in a 1:1 ratio are separated by a cation layer composed of a zigzag (CH3)3NH + (N2) chain and a zigzag H2O chain along the c axis.There are hydrogen bonds between neighboring H2O molecules.Five H2O molecules formed a hydrogen-bond [H2O]5 linear cluster along the c axis (Figure 3).At last, the hydrogen bonds N-H…O and C-H…O between the ammonium and the inner O of the terminal and bridged oxalate, the O-H…O between H2O and the oxalate, and the O-H…O between the H2O molecules form a three-dimensional hydrogen-bonded network in crystal (Figure 4).There is no hydrogen bond between the anions.II crystallizes in a monoclinic system with the space group P21/c.There are two (C2H5)3NH + , one Cu 2+ , one and two half oxalates, and one H2O in an independent unit (Figure 5).The Cu 2+   II crystallizes in a monoclinic system with the space group P2 1 /c.There are two (C 2 H 5 ) 3 NH + , one Cu 2+ , one and two half oxalates, and one H 2 O in an independent unit (Figure 5).The Cu 2+   The zigzag chain in I and II is centrosymmetric with the inversion center located at the middle point of the oxalate bridge; thus, the metal sites have a ΔΛΔΛ configuration along the b axis in I and II (Figure 6).It is similar to [(CH3)4N]2Cu(C2O4)2(H2O) [42,43].The hydrogen bonds between ammonium and the anion and H2O and the anion influence the bond length of the CuO6 octahedron due to the Jahn-Teller distortion.Due to the magnetic orbitals of dx 2 − y 2 on Cu(II) with the unpaired electrons parallel to each other and the axial Cu to oxalate-oxygen angles, which are sensitive to magnetic interaction and smaller than 109.5°, a ferromagnetic interaction was expected [43,44].The zigzag chain in I and II is centrosymmetric with the inversion center located at the middle point of the oxalate bridge; thus, the metal sites have a ∆Λ∆Λ configuration along the b axis in I and II (Figure 6).It is similar to [42,43].The hydrogen bonds between ammonium and the anion and H 2 O and the anion influence the bond length of the CuO 6 octahedron due to the Jahn-Teller distortion.Due to the magnetic orbitals of dx 2 − y 2 on Cu(II) with the unpaired electrons parallel to each other and the axial Cu to oxalate-oxygen angles, which are sensitive to magnetic interaction and smaller than 109.5 • , a ferromagnetic interaction was expected [43,44].[29,30,34,35,37,38].A one-dimensional oxalate-bridged zigzag [Cu(µ-C2O4)(C2O4) 2− ]n chain is formed along the b axis.The zigzag chain in I and II is centrosymmetric with the inversion center located at the middle point of the oxalate bridge; thus, the metal sites have a ΔΛΔΛ configuration along the b axis in I and II (Figure 6).It is similar to [(CH3)4N]2Cu(C2O4)2(H2O) [42,43].The hydrogen bonds between ammonium and the anion and H2O and the anion influence the bond length of the CuO6 octahedron due to the Jahn-Teller distortion.Due to the magnetic orbitals of dx 2 − y 2 on Cu(II) with the unpaired electrons parallel to each other and the axial Cu to oxalate-oxygen angles, which are sensitive to magnetic interaction and smaller than 109.5°, a ferromagnetic interaction was expected [43,44].In II, the [Cu(µ-C2O4)(C2O4) 2− ]n chain is surrounded by zigzag chains of (C2H5)3NH + and H2O.A pair (C2H5)3NH + column separates two zigzag chains along the c axis.There are hydrogen bonds between the N of the ammonium and the O on the bridged oxalate: N1-H1…O5 2.10 Å/146.9°,N1-H1…O8 2.44 Å/131.7°.There are hydrogen bonds between the N of the ammonium and the O on the terminal oxalate: N2-H2…O3 2.20 Å/139°, N2-H2…O4 2.21 Å/143.1°.There are hydrogen bonds between H2O and the terminal oxalate: O9-H4…O3 2.11 Å/159°, O9-H3…O4 2.20 Å/164°.There are hydrogen bonds between ammonium and H2O: C8-H8C…O1 2.45 Å/174°; C15-H15A…O8 2.40 Å/155°.The hydrogen bond forms a two-dimensional (2D) network on the (201) plane (Figure 7).There is no hydrogen bond between the one-dimensional [Cu(-C2O4)(C2O4) 2− )]n chains.The hydrogen bond forms a two-dimensional (2D) network on the (201) plane (Figure 7).There is no hydrogen bond between the one-dimensional [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2− )] n chains.
On the basis of the hydrogen bonded cation layer, the resistance, as the proton conductivity under different relative humidities (RH), was measured.Depending on the thermal dynamic analysis, I and II dehydrate at 40 • C (I) and 76 • C (II), losing H 2 O, with a relative weight of 11.2% in I and 4% in II; therefore, the experiment should be carried out below 40 • C (Figure 8).When the RH increased, the conductivity of I and II increased.Under a relatively high RH, the surfaces of the crystal were covered with debris at first, which was solvable in gel.Although the sample was restored to a solid state when the RH decreased and reached the same value as the beginning, the sample turned out to be in a polycrystalline state but not a single crystal.When a single crystal of I or II was exposed to air under a low relative humidity, such as when the relative humidity was lower than 35%, guest molecules, such as H 2 O, in I and II would escape from the crystal, leading to crystalline collapse.The crystal surface remained transparent and clear after measurements at 20 • C and an RH of 40%.The resistance comes from the intrinsic behavior of the crystal.The resistance is 1 × 10 9 Ω•cm in I and 1 × 10 7 Ω•cm in II.They are insulators.On the basis of the hydrogen bonded cation layer, the resistance, as the proton conductivity under different relative humidities (RH), was measured.Depending on the thermal dynamic analysis, I and II dehydrate at 40 °C (I) and 76 °C (II), losing H2O, with a relative weight of 11.2% in I and 4% in II; therefore, the experiment should be carried out below 40 °C (Figure 8).When the RH increased, the conductivity of I and II increased.Under a relatively high RH, the surfaces of the crystal were covered with debris at first, which was solvable in gel.Although the sample was restored to a solid state when the RH decreased and reached the same value as the beginning, the sample turned out to be in a polycrystalline state but not a single crystal.When a single crystal of I or II was exposed to air under a low relative humidity, such as when the relative humidity was lower than 35%, guest molecules, such as H2O, in I and II would escape from the crystal, leading to crystalline collapse.The crystal surface remained transparent and clear after measurements at 20 °C and an RH of 40%.The resistance comes from the intrinsic behavior of the crystal.The resistance is 1 × 10 9 •cm in I and 1 × 10 7 •cm in II.They are insulators.On the basis of the oxalate-bridging and Jahn-Teller distortion of the Cu(II) ion, Cu(II) in an independent unit, and the magnetic properties were studied per Cu 2+ /mol.
At 300 K, the χT value of I was 0.473 cm 3 K mol −1 and g = 2.25.It is higher than 0.375 cm 3 K mol −1 for an isolated, spin only Cu(II) ion with S = 1/2, g = 2.00 and in the range of Cu 2+ compounds [34-37,45,46].The χT value remained stable at 0.478 cm 3 K mol −1 at 40 K and increased slowly, reaching 0.71 cm 3 K mol −1 at 2 K.No bifurcation is observed from zero-field-cool magnetization and field-cooled magnetization (ZFCM/FCM)  On the basis of the hydrogen bonded cation layer, the resistance, as the proton conductivity under different relative humidities (RH), was measured.Depending on the thermal dynamic analysis, I and II dehydrate at 40 °C (I) and 76 °C (II), losing H2O, with a relative weight of 11.2% in I and 4% in II; therefore, the experiment should be carried out below 40 °C (Figure 8).When the RH increased, the conductivity of I and II increased.Under a relatively high RH, the surfaces of the crystal were covered with debris at first, which was solvable in gel.Although the sample was restored to a solid state when the RH decreased and reached the same value as the beginning, the sample turned out to be in a polycrystalline state but not a single crystal.When a single crystal of I or II was exposed to air under a low relative humidity, such as when the relative humidity was lower than 35%, guest molecules, such as H2O, in I and II would escape from the crystal, leading to crystalline collapse.The crystal surface remained transparent and clear after measurements at 20 °C and an RH of 40%.The resistance comes from the intrinsic behavior of the crystal.The resistance is 1 × 10 9 •cm in I and 1 × 10 7 •cm in II.They are insulators.On the basis of the oxalate-bridging and Jahn-Teller distortion of the Cu(II) ion, Cu(II) in an independent unit, and the magnetic properties were studied per Cu 2+ /mol.
At 300 K, the χT value of I was 0.473 cm 3 K mol −1 and g = 2.25.It is higher than 0.375 cm 3 K mol −1 for an isolated, spin only Cu(II) ion with S = 1/2, g = 2.00 and in the range of Cu 2+ compounds [34-37,45,46].The χT value remained stable at 0.478 cm 3 K mol −1 at 40 K and increased slowly, reaching 0.71 cm 3 K mol −1 at 2 K.No bifurcation is observed from zero-field-cool magnetization and field-cooled magnetization (ZFCM/FCM) On the basis of the oxalate-bridging and Jahn-Teller distortion of the Cu(II) ion, Cu(II) in an independent unit, and the magnetic properties were studied per Cu 2+ /mol.
At 2 K, the isothermal magnetization (M) saturated at 1.11 Nβ (N is Avogadro's number and β is the Bohn magneton, 1 Nβ = 5585 cm −1 G mol −1 ) at 65 kG (Figure 11).The average anisotropic g-factor calculated from isothermal magnetization at 2 K is 2.22.It is in the range of 2.25 from χT at 300 K and 2.31 from Baker-Rushbrooke-Gilbert model fitting.
measurements from 2 K to 100 K under 100 G (Figure S4).The magnetic data were fitted with the Curie-Weiss law from 2 to 300 K: C = 0.4711(2) cm −1 •K/mol,  = 0.61(6) K and R = 1.37 × 10 −5 .It suggests a ferromagnetic interaction in I (Figure 9) [47,48].A one-dimensional Baker-Rushbrooke-Gilbert model was used to fit the temperature-dependent magnetization above 2 K, yielding J = 0.60(2) cm −1 , g = 2.31(1) and R = 9.2 × 10 −4 (Figure 10) [49].It shows an intrachain ferromagnetic interaction and corresponds with the Curie-Weiss fitting.At 2 K, the isothermal magnetization (M) saturated at 1.11 Nβ (N is Avogadro's number and β is the Bohn magneton, 1 Nβ = 5585 cm −1 G mol −1 ) at 65 kG (Figure 11).The average anisotropic g-factor calculated from isothermal magnetization at 2 K is 2.22.It is in the range of 2.25 from χT at 300 K and 2.31 from Baker-Rushbrooke-Gilbert model fitting.with the Curie-Weiss law from 2 to 300 K: C = 0.4711(2) cm −1 •K/mol,  = 0.61(6) K and R = 1.37 × 10 −5 .It suggests a ferromagnetic interaction in I (Figure 9) [47,48].A one-dimensional Baker-Rushbrooke-Gilbert model was used to fit the temperature-dependent magnetization above 2 K, yielding J = 0.60(2) cm −1 , g = 2.31(1) and R = 9.2 × 10 −4 (Figure 10) [49].It shows an intrachain ferromagnetic interaction and corresponds with the Curie-Weiss fitting.At 2 K, the isothermal magnetization (M) saturated at 1.11 Nβ (N is Avogadro's number and β is the Bohn magneton, 1 Nβ = 5585 cm −1 G mol −1 ) at 65 kG (Figure 11).The average anisotropic g-factor calculated from isothermal magnetization at 2 K is 2.22.It is in the range of 2.25 from χT at 300 K and 2.31 from Baker-Rushbrooke-Gilbert model fitting.In II, the χT value was 0.443 cm 3 K mol −1 at 300 K with a g-factor of 2.13.It is higher than the 0.375 cm 3 K mol −1 of an isolated, spin only Cu(II) ion with S = 1/2, g = 2.00.It is in the range of Cu 2+ compounds, as in I [34][35][36][37]45,46].As the temperature decreased, the χT value decreased slowly to 0.393 cm 3 K mol −1 around 30 K, and then increased, reaching 0.446 cm 3 K mol −1 at 2 K (Figure 12).No bifurcation was observed in the ZFCM/FCM measurement from 2 K to 100 K under 100 G (Figure S5).The magnetic data were fitted with Curie-Weiss law from 80 to 300 K with C = 0.462(1)) cm −1 •K/mol, g = −14.2(4)K and R = 3.8 × 10 −5 (Figure 12).In II, the χT value was 0.443 cm 3 K mol −1 at 300 K with a g-factor of 2.13.It is higher than the 0.375 cm 3 K mol −1 of an isolated, spin only Cu(II) ion with S = 1/2, g = 2.00.It is in the range of Cu 2+ compounds, as in I [34][35][36][37]45,46].As the temperature decreased, the χT value decreased slowly to 0.393 cm 3 K mol −1 around 30 K, and then increased, reaching 0.446 cm 3 K mol −1 at 2 K (Figure 12).No bifurcation was observed in the ZFCM/FCM measurement from 2 K to 100 K under 100 G (Figure S5).The magnetic data were fitted with Curie-Weiss law from 80 to 300 K with C = 0.462(1)) cm −1 •K/mol, g = −14.2(4)K and R = 3.8 × 10 −5 (Figure 12).In II, the χT value was 0.443 cm 3 K mol −1 at 300 K with a g-factor of 2.13.It is higher than the 0.375 cm 3 K mol −1 of an isolated, spin only Cu(II) ion with S = 1/2, g = 2.00.It is in the range of Cu 2+ compounds, as in I [34][35][36][37]45,46].As the temperature decreased, the χT value decreased slowly to 0.393 cm 3 K mol −1 around 30 K, and then increased, reaching 0.446 cm 3 K mol −1 at 2 K (Figure 12).No bifurcation was observed in the ZFCM/FCM measurement from 2 K to 100 K under 100 G (Figure S5).The magnetic data were fitted with Curie-Weiss law from 80 to 300 K with C = 0.462(1)) cm −1 •K/mol, g = −14.2(4)K and R = 3.8 × 10 −5 (Figure 12).A one-dimensional Baker-Rushbrooke-Gilbert model combined with exchange coupling was used to fit the temperature-dependent magnetization from 2 to 300 K with J = 0.87(2) cm −1 , g = 2.035(3), zJ = −0.65(2)cm −1 and R = 6.76 × 10 −5 (Figure 13) [50].It shows that intrachain ferromagnetic interaction is stronger than intrachain antiferromagnetic interaction.The g-factor is in the range of 2.13 from χT at 300 K.A one-dimensional Baker-Rushbrooke-Gilbert model combined with exchange coupling was used to fit the temperature-dependent magnetization from 2 to 300 K with J = 0.87(2) cm −1 , g = 2.035(3), zJ = −0.65(2)cm −1 and R = 6.76 × 10 −5 (Figure 13) [50].It shows that intrachain ferromagnetic interaction is stronger than intrachain antiferromagnetic interaction.The g-factor is in the range of 2.13 from χT at 300 K.At 2 K, the magnetization increases with increasing field and is saturated at 0.89 Nβ at 65 kG (Figure 14).The average anisotropic g-factor calculated from isothermal magnetization at 2 K is 1.78.Its magnetic behavior is not the same as expected.This means the Jahn-Teller effect is important to the magnetic property of the copper-oxalate framework.This is different from the compounds [CrMn(C2O4)3 − ]n, where in the ferromagnetic order, temperature and isothermal magnetization at 2 K are the same as those taken from ammonium salts to charge-transfer salts [19,51,52].Depending on the difference in magnetic behaviors between I and II, the Jahn-Teller effect will help us to obtain molecular-based candidate quantum spin liquid and to look for a new superconductor and colossal magnetoresistance material from copper-oxalate frameworks as cuprate superconductors and colossal magnetoresistance material.At 2 K, the magnetization increases with increasing field and is saturated at 0.89 Nβ at 65 kG (Figure 14).The average anisotropic g-factor calculated from isothermal magnetization at 2 K is 1.78.Its magnetic behavior is not the same as expected. Tis means the Jahn-Teller effect is important to the magnetic property of the copper-oxalate framework.This is different from the compounds [CrMn(C 2 O 4 ) 3 − ] n , where in the ferromagnetic order, temperature and isothermal magnetization at 2 K are the same as those taken from ammonium salts to charge-transfer salts [19,51,52].Depending on the difference in magnetic behaviors between I and II, the Jahn-Teller effect will help us to obtain molecular-based candidate quantum spin liquid and to look for a new superconductor and colossal magnetoresistance material from copper-oxalate frameworks as cuprate superconductors and colossal magnetoresistance material.
At 2 K, the magnetization increases with increasing field and is saturated at 0.89 Nβ at 65 kG (Figure 14).The average anisotropic g-factor calculated from isothermal magnetization at 2 K is 1.78.Its magnetic behavior is not the same as expected.This means the Jahn-Teller effect is important to the magnetic property of the copper-oxalate framework.This is different from the compounds [CrMn(C2O4)3 − ]n, where in the ferromagnetic order, temperature and isothermal magnetization at 2 K are the same as those taken from ammonium salts to charge-transfer salts [19,51,52].Depending on the difference in magnetic behaviors between I and II, the Jahn-Teller effect will help us to obtain molecular-based candidate quantum spin liquid and to look for a new superconductor and colossal magnetoresistance material from copper-oxalate frameworks as cuprate superconductors and colossal magnetoresistance material.

Figure 2 .
Figure 2. The zigzag anionic chain in I.

Figure 2 .
Figure 2. The zigzag anionic chain in I.

Figure 2 .
Figure 2. The zigzag anionic chain in I.The one-dimensional [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2− ] n zigzag chain running along the b axis is separated by (CH 3 ) 3 NH + (N1) along the c axis, and there are hydrogen bonds N-H• • • O, C-H• • • O between the cation and the out O atom in the oxalate.The anionic sheets composed of a [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2− ] n chain and (CH 3 ) 3 NH + in a 1:1 ratio are separated by a cation layer composed of a zigzag (CH 3 ) 3 NH + (N2) chain and a zigzag H 2 O chain along the c axis.There are hydrogen bonds between neighboring H 2 O molecules.Five H 2 O molecules formed a hydrogen-bond [H 2 O] 5 linear cluster along the c axis (Figure 3).
The one-dimensional [Cu(µ-C2O4)(C2O4)2− ]n zigzag chain running along the b axis is separated by (CH3)3NH + (N1) along the c axis, and there are hydrogen bonds N-H…O, C-H…O between the cation and the out O atom in the oxalate.The anionic sheets composed of a [Cu(µ-C2O4)(C2O4)2− ]n chain and (CH3)3NH + in a 1:1 ratio are separated by a cation layer composed of a zigzag (CH3)3NH + (N2) chain and a zigzag H2O chain along the c axis.There are hydrogen bonds between neighboring H2O molecules.Five H2O molecules formed a hydrogen-bond [H2O]5 linear cluster along the c axis (Figure3).

Figure 4 .
Figure 4. Packing diagram of I viewed along the b axis.Dash yellow lines are hydrogen bonds.Color code: Cu, cyan; O, red; C, white; N, blue; H, light grey.

Figure 3 .
Figure 3. Arrangement of (CH 3 ) 3 NH + and H 2 O in cation layer of I. Dashed yellow lines are hydrogen bonds between H 2 O in [H 2 O] 5 .At last, the hydrogen bonds N-H• • • O and C-H• • • O between the ammonium and the inner O of the terminal and bridged oxalate, the O-H• • • O between H 2 O and the oxalate, and the O-H• • • O between the H 2 O molecules form a three-dimensional hydrogen-bonded network in crystal (Figure4).There is no hydrogen bond between the anions.

Figure 4 .
Figure 4. Packing diagram of I viewed along the b axis.Dash yellow lines are hydrogen bonds.Color code: Cu, cyan; O, red; C, white; N, blue; H, light grey.
is octahedrally coordinated by six O atoms from two bisbidentate oxalates and one bidentate oxalate, as in I, with Cu-O distances of 1.960(2)~1.999(2)Å on the equatorial plane, and Cu1-O8: 2.314(2) Å and Cu1-O7: 2.368(2) Å from the apex.The Cu-O distances of II in the equatorial plane are shorter than the direction of the apex as a

Figure 4 .
Figure 4. Packing diagram of I viewed along the b axis.Dash yellow lines are hydrogen bonds.Color code: Cu, cyan; O, red; C, white; N, blue; H, light grey.

Figure 6 .
Figure 6.The zigzag anionic chain in II.

Figure 6 .
Figure 6.The zigzag anionic chain in II.In II, the [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2− ] n chain is surrounded by zigzag chains of (C 2 H 5 ) 3 NH + and H 2 O.A pair (C 2 H 5 ) 3 NH + column separates two zigzag chains along the c axis.There are hydrogen bonds between the N of the ammonium and the O on the bridged oxalate: N1-H1• • • O5 2.10 Å/146.9 • , N1-H1• • • O8 2.44 Å/131.7 • .There are hydrogen bonds between the N of the ammonium and the O on the terminal oxalate: N2-H2• • • O3 2.20 Å/139 • , N2-H2• • • O4 2.21 Å/143.1 • .There are hydrogen bonds between H 2 O and the terminal oxalate: O9-H4• • • O3 2.11 Å/159 • , O9-H3• • • O4 2.20 Å/164 • .There are hydrogen bonds between ammonium and H 2 O: C8-H8C• • • O1 2.45 Å/174 • ; C15-H15A• • • O8 2.40 Å/155 • .The hydrogen bond forms a two-dimensional (2D) network on the (201) plane (Figure7).There is no hydrogen bond between the one-dimensional [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2− )] n chains.On the basis of the hydrogen bonded cation layer, the resistance, as the proton conductivity under different relative humidities (RH), was measured.Depending on the thermal dynamic analysis, I and II dehydrate at 40 • C (I) and 76 • C (II), losing H 2 O, with a relative weight of 11.2% in I and 4% in II; therefore, the experiment should be carried out below 40 • C (Figure8).When the RH increased, the conductivity of I and II increased.Under a relatively high RH, the surfaces of the crystal were covered with debris at first, which was solvable in gel.Although the sample was restored to a solid state when the RH decreased and reached the same value as the beginning, the sample turned out to be in a polycrystalline state but not a single crystal.When a single crystal of I or II was exposed to air under a low relative humidity, such as when the relative humidity was lower than 35%, guest molecules, such as H 2 O, in I and II would escape from the crystal, leading to

Figure 8 .
Figure 8. Schematic TGA curves of I and II.Above 300 °C, the final residue is CuO.

Figure 8 .
Figure 8. Schematic TGA curves of I and II.Above 300 °C, the final residue is CuO.

Figure 8 .
Figure 8. Schematic TGA curves of I and II.Above 300 • C, the final residue is CuO.

Figure 10 .
Figure 10.Temperature-dependent susceptibility of I. Black empty square is experimental data.Red solid curve is the best fit from the Baker-Rushbrooke-Gilbert model.

Figure 10 .
Figure 10.Temperature-dependent susceptibility of I. Black empty square is experimental data.Red solid curve is the best fit from the Baker-Rushbrooke-Gilbert model.

Figure 10 . 12 Figure 11 .
Figure 10.Temperature-dependent susceptibility of I. Black empty square is experimental data.Red solid curve is the best fit from the Baker-Rushbrooke-Gilbert model.Magnetochemistry 2023, 9, 120 8 of 12

Figure 11 .
Figure 11.Isothermal magnetization of I at 2 K.

Figure 11 .
Figure 11.Isothermal magnetization of I at 2 K.

Figure 13 .
Figure 13.Temperature-dependent susceptibility of II.Empty solid square, is experimental data.Red solid curve is best fit from the Baker-Rushbrooke-Gilbert model with exchange coupling.

Figure 13 .
Figure 13.Temperature-dependent susceptibility of II.Empty solid square, is experimental data.Red solid curve is best fit from the Baker-Rushbrooke-Gilbert model with exchange coupling.
Two one-dimensional copper-oxalate framework compounds were obtained and characterized.The hydrogen bonds among ammonium, H 2 O and the copper-oxalate framework form a three-dimensional hydrogen-bond network, and there is no hydrogen bond between the one-dimensional [Cu(µ-C 2 O 4 )(C 2 O 4 ) 2− ] n chains.The Q 3 -mode Jahn-Teller distortion of elongated CuO 6 octahedrons is observed.They are insulators.The Jahn-Teller effect results the ferromagnetic and weak-ferromagnetic interaction between Cu(II) in I and II.No long-range ordering is observed above 2 K.
were obtained from a methanol solution of Cu(NO 3 ) 2•3H 2 O and H 2 C 2 O 4 •2H 2 O with (CH 3 ) 3 N for I and (C 2 H 5 ) 3 N for II in a 1:3:5 ratio at room temperature.Bulk blue plateful crystals of I and II were obtained after four weeks.The crystal was washed with CH 3 COOC 2 H 5 and dried.Elemental analysis calculated (%) for C 10 H 25 CuN 2 O 10.50 (I): C 29.67, H 6.22 and N 6.92 and found C 29.87, H, 6.08 and N 6.97.For C 16 H 34 CuN 2 O 9 (II): C 41.60, H 7.42, N 6.06 and found C 42.03, H 7.46 and N 6.11.