Gas-Dependent Reversible Structural and Magnetic Transformation between Two Ladder Compounds

: We report reversible structural transformation that occurs in two ladder compounds: Cu 2 CO 3 (ClO 4 ) 2 (NH 3 ) 6 ( 1 ) and Cu 2 CO 3 (ClO 4 ) 2 (NH 3 ) 5 (H 2 O) ( 2 ), when they are exposed to gaseous vapors. The ladder structures of both 1 and 2 consist of two Cu 2 + ions and one CO 32 − ion. In 1 , the Cu 2 + ions are coordinated by three NH 3 molecules on each side, while those in 2 are coordinated by three NH 3 molecules on one side, and two NH 3 molecules and one H 2 O molecule on the other side. We demonstrated reversible transformation of 1 and 2 via the exposure of 1 to H 2 O vapor and the exposure of 2 to NH 3 vapor using a simple bench-scale method. The minor structural change observed led to a signiﬁcant di ﬀ erence in physical properties, which we observed using several methods.


Introduction
Materials with the capacity to undergo reversible structural and physical changes, usually thermal phase transitions, are required in devices such as sensors and memories [1,2]. Such materials exhibit different structures above and below the phase transition temperature. However, the physical properties of each structure cannot be examined across the entire temperature range. In contrast, light irradiation-induced reversible changes allow for the detailed examination of physical properties throughout the entire temperature range, both before and after the transformation. However, transformation induced by irradiation poses certain problems, like the influence of heat generated by irradiation and the limited surface area reached by the irradiating light. Therefore, a material that undergoes reversible change in response to an external stimulus other than temperature or light is practically applicable as sensors and memories.
In recent years, spin ladders have attracted significant attention in the field of superconductivity [3][4][5][6][7][8][9][10][11][12][13][14][15] due to the theoretical possibility, for an even-leg spin ladder system, to undergo a superconducting transition with doped carriers. In the past, we successfully synthesized two molecular even-leg ladder compounds: Cu 2 CO 3 (ClO 4 ) 2 (NH 3 ) 6 (1) and Cu 2 CO 3 (ClO 4 ) 2 (NH 3 ) 5 (H 2 O) (2) [16]. The ladder structure of 1, [(H 3 N) 3 Cu-CO 3 -Cu(NH 3 ) 3 ] n is constructed by alternately stacking two Cu 2+ ions and one CO 3 2− ion, with each Cu 2+ ion being coordinated by three NH 3 molecules (Figure 1a,b). The ladder structure of 1 is magnetically isolated due to the presence of perchlorate ions between ladders. The temperature-dependent molar magnetic susceptibility of this compound can be reproduced using a magnetically isolated spin ladder model [17], whereby the magnetic exchange interactions of the ladder rung and leg were estimated to be J rung /k B = −364 K and J leg /k B = −27.4 K, respectively. Meanwhile, the ladder structure of 2, [(H 3 N) 3 Cu-CO 3 -Cu(NH 3 ) 2 (H 2 O)] n , contains two Cu 2+ ions and one CO 3 2− ion and its structure is similar to that of 1. One of the two Cu 2+ ions is, however, coordinated using two NH 3 molecules and one H 2 O molecule, thus distinguishing it from 1 (Figure 1c,d). In addition, a small structural phase transition involving the reorientation of the perchlorate ions was observed in 2 at approximately 205 K, thus inducing adjustment in the ladder structure of 2, without the destruction of the exchange interaction model. The CIF file of 2 at room temperature (HT) is attached to the supplementary materials. The temperature dependence of the molar magnetic susceptibility of 2 was reproduced using an alternating chain model [18], rather than the spin ladder model, with the following magnetic exchange interactions: 42 K, and J 3 /k B = 0 K. Furthermore, 2 exhibited an antiferromagnetic transition at 3.4 K [19].
Crystals 2020, 10, x FOR PEER REVIEW 2 of 10 undergoes reversible change in response to an external stimulus other than temperature or light is practically applicable as sensors and memories. In recent years, spin ladders have attracted significant attention in the field of superconductivity [3][4][5][6][7][8][9][10][11][12][13][14][15] due to the theoretical possibility, for an even-leg spin ladder system, to undergo a superconducting transition with doped carriers. In the past, we successfully synthesized two molecular even-leg ladder compounds: Cu2CO3(ClO4)2(NH3)6 (1) and Cu2CO3(ClO4)2(NH3)5(H2O) (2) [16]. The ladder structure of 1, [(H3N)3Cu-CO3-Cu(NH3)3]n is constructed by alternately stacking two In the present study, we investigated the reversible structural and magnetic switching between 1 and 2 by exposing the compounds to H 2 O and NH 3 vapors, respectively (Scheme 1). Such reversible magnetic switching may be applicable in sensors where they would be used to make distinctions between gases such as NH 3 and H 2 O. temperature (HT) is attached to the supplementary materials. The temperature dependence of the molar magnetic susceptibility of 2 was reproduced using an alternating chain model [18], rather than the spin ladder model, with the following magnetic exchange interactions: J1/kB = −7.26 K, J2/kB = −4.42 K, and J3/kB = 0 K. Furthermore, 2 exhibited an antiferromagnetic transition at 3.4 K [19].
In the present study, we investigated the reversible structural and magnetic switching between 1 and 2 by exposing the compounds to H2O and NH3 vapors, respectively (Scheme 1). Such reversible magnetic switching may be applicable in sensors where they would be used to make distinctions between gases such as NH3 and H2O. Scheme 1. Reversible structural transformation between 1 and 2 via the exchange of H2O and NH3.
Powder X-ray diffraction measurement: Powder X-ray diffraction (PXRD) patterns were measured using a Rigaku RINT2100 diffractometer at r.t. for all samples. The measurements were performed via Cu Kα radiation (λ = 1.5418 Å) at a scanning rate of 2.0° min −1 under an applied electric voltage of 40 kV and a current of 40 mA.
Magnetic measurement: The temperature dependence of the magnetic susceptibility and fielddependent magnetization were measured using a Quantum Design MPMS-5S ( Figure 2, Figure S2), MPMS-XL ( Figure S4), and MPMS-3 ( Figure S4) superconducting quantum interference device (SQUID) magnetometer using single crystals or powdered samples contained in a gelatin capsule.
Thermal analysis: Thermogravimetric (TG) analyses were carried out on powdered samples using a SII TG/DTA 6200N instrument under N2 flow. Measurements were conducted at a scanning rate of 5 K min −1 . Gas chromatography-mass spectrometry (GC-MS) analyses were performed using a SHIMADZU GCMS-QP2010 Ultra instrument at a scanning rate of 5 K min −1 .
Infrared spectroscopy: Infrared spectroscopy (IR) spectra were measured using KBr pellets and a JASCO FT/IR-660 Plus spectrometer within the range of 400-4000 cm −1 .
Heat capacities: The heat capacities were measured via the thermal relaxation method using a Quantum Design Physical Property Measurement System ( Figure S3).
Powder X-ray diffraction measurement: Powder X-ray diffraction (PXRD) patterns were measured using a Rigaku RINT2100 diffractometer at r.t. for all samples. The measurements were performed via Cu Kα radiation (λ = 1.5418 Å) at a scanning rate of 2.0 • min −1 under an applied electric voltage of 40 kV and a current of 40 mA.
Magnetic measurement: The temperature dependence of the magnetic susceptibility and field-dependent magnetization were measured using a Quantum Design MPMS-5S ( Figure 2 and Figure S2), MPMS-XL ( Figure S4), and MPMS-3 ( Figure S4) superconducting quantum interference device (SQUID) magnetometer using single crystals or powdered samples contained in a gelatin capsule.
Thermal analysis: Thermogravimetric (TG) analyses were carried out on powdered samples using a SII TG/DTA 6200N instrument under N 2 flow. Measurements were conducted at a scanning rate of 5 K min −1 . Gas chromatography-mass spectrometry (GC-MS) analyses were performed using a SHIMADZU GCMS-QP2010 Ultra instrument at a scanning rate of 5 K min −1 .
Infrared spectroscopy: Infrared spectroscopy (IR) spectra were measured using KBr pellets and a JASCO FT/IR-660 Plus spectrometer within the range of 400-4000 cm −1 .
Heat capacities: The heat capacities were measured via the thermal relaxation method using a Quantum Design Physical Property Measurement System ( Figure S3).

Magnetic Anomaly of Compound 1
Magnetic measurements were performed using a single crystal of 1 in magnetic fields H parallel (Figure 2a,c) or perpendicular (Figure 2b,d) to the c-axis. For H // c, the temperature dependence of the molar magnetic susceptibility (χ m -T) of 1 in a field of 5 kOe exhibited unexpected magnetic behavior, hereafter called "anomaly", at approximately 3 K (Figure 2a). Additionally, a magnetic jump considered to be a spin-flop transition was observed at approximately 11 kOe in the field-dependent magnetization (M-H) curve for 1 at 1.8 K (Figure 2c). When magnetic fields of 3, 11, and 15 kOe were applied at the temperature dependence of magnetization measurements, the magnetic anomaly of the magnetic field of 15 kOe, which was larger than the spin-flop field, vanished ( Figure S2). In contrast, such an anomaly was not observed in the χ m -T and M-H measurements for H // c (Figure 2b,d). Heat capacity measurements were performed in magnetic fields below 30 kOe, parallel to the c-axis, to determine whether the anomaly was attributable to a magnetic phase transition ( Figure S3). A peak (partially out of the measured temperature range) was observed below 3 K in the zero-field, and the peak broadened as the magnetic field increased. The results of the magnetic and heat capacity measurements showed that the anomaly of 1 at low temperature was attributable to an antiferromagnetic transition. A similar anomaly in an inorganic spin ladder, whereby an antiferromagnetic transition was detected Crystals 2020, 10, 841 5 of 9 in a spin ladder compound doped with magnetic impurities, Sr(Cu 1−x Zn x ) 2 O 3 , was reported by Azuma et al. [21][22][23]. In the present study, however, 1 was not doped, and the anomaly cannot be explained using this mechanism. To further analyze this phenomenon, we performed magnetic measurements of H // c using a single crystal of 1 exposed to air for 10 days ( Figure S4). Under this condition, the peak at 3 K became more prominent. To examine this change, PXRD measurement was performed on 1 after exposing the powder sample to air for 3 days (1_air). The PXRD pattern of 1_air corresponded to a superposition of the PXRD patterns of 1 and 2 (Figure 3). Because the compositions of 1 and 2 only differ by a molecule (NH 3 or H 2 O), these results suggest that NH 3 can be substituted by H 2 O from the air in the crystal of 1. Therefore, the anomaly may not be intrinsic in 1 but in the partial transformation of 1 into 2.

Transformation from Compound 2 to 1
In order to investigate the reverse reaction from 2 to 1, 2 was exposed to NH3 vapor from an NH3 aqueous solution for 2 h (2_NH3). The PXRD pattern of 2_NH3 exhibited characteristic peaks of both 1 and 2, implying that the expected transformation had occurred ( Figure 5). Therefore, TG analysis and GC-MS were performed for 2_NH3 ( Figure S5). Similar to 1, the weight loss and MS intensity derived from H2O was not observed until 400 K. In addition, the results of the elemental analysis of 2_NH3 also corresponded to intermediate values (elemental analysis: C: 2.68, H: 3.61, N: 16.55) between those of 1 and 2 (see the Materials and Methods section). The IR spectrum of 2_NH3 in the 3000−3700 cm -1 region also changed and was similar to the spectrum for 1 ( Figure S6). The results of the magnetic measurements show that the magnetic behavior of 2_NH3 (Figure 4d) differed from that of 2 ( Figure 4c) and approached that of 1 (see Figure 4a). These results indicate that the exposure of 2 to NH3 vapor transforms it into 1. Figure 3. PXRD patterns of 1 (blue), 1_air (1 exposed to air for 3 days; yellow), 1_H 2 O (1 exposed to H 2 O vapor for 4.5 h; green), and 2 (red).

Transformation from Compound 1 to 2
To further establish the aforementioned observations and implications, we evaluated the PXRD pattern of 1 exposed to H 2 O vapor for 4.5 h (1_H 2 O). The PXRD pattern of 1_H 2 O was a superposition of the patterns of 1 and 2 ( Figure 3). This implies that in the basic formula [(H 3 N) 3 Cu-CO 3 -Cu(NH 3 ) 3 ] of 1, one NH 3 molecule was partially replaced by one H 2 O molecule. We confirmed the replacement of the coordinating molecules through TG analysis and GC-MS. Figure S5 shows weight loss in the samples, as well as MS intensity corresponding to the presence of H 2 O (m/z = 18) across a temperature range of 340 to 450 K. Up to 400 K, the weight loss and MS intensity derived from H 2 O was not observed in 1. In contrast, weight loss was observed in 1_H 2 O, and MS intensity derived from H 2 O was observed below 400 K. The analysis of 2 yielded results that were similar to those of 1_H 2 O. In addition, the elemental analysis of 1_H 2 O yielded intermediate values (elemental analysis: C: 2.49, H: 3.68, N: 15.46) between those of 1 and 2 (see the Materials and Methods section). IR spectrum in the 3000−3700 cm −1 region, mainly derived from NH 3 and H 2 O ligands, for 1_H 2 O is closer to that of 2 than that of 1 ( Figure S6). The magnetic behavior of 1_H 2 O, as shown in Figure 4b, was drastically different compared to that of 1 (Figure 4a) and tended toward that of 2 (Figure 4c). From these results, we conclude that the NH 3 in 1 was substituted by H 2 O after exposure to H 2 O vapor, and the anomaly observed in 1 at low temperature was caused by the transformation of 1 to 2 by H 2 O, corresponding to the prediction mentioned earlier.   . PXRD patterns of 2 (red), 2_NH3 (2 exposed to NH3 vapor for 2 h; yellow) and 1 (blue).

Reversible Transformation between Compounds 1 and 2
Reversible transformation between 1 and 2 was investigated. 1_H2O, which was obtained by exposing 1 to vapor for 4.5 h, was exposed to NH3 vapor from an NH3 aqueous solution for 2 h (1_H2O_NH3). The PXRD measurement of 1_H2O_NH3 was carried out to determine whether the change between 1 and 2 occurred reversibly depending on the gaseous vapors. Figure 6 shows the PXRD pattern of 1_H2O_NH3, including the patterns of 1, 1_H2O, and 2 for comparison. Given that Figure 4. Temperature-dependent molar magnetic susceptibility in an applied field of 5 kOe for (a) 1, (b) 1_H 2 O (1 exposed to H 2 O vapor for 4.5 h), (c) 2, and (d) 2_NH 3 (2 exposed to NH 3 vapor for 2 h). The insets show the data in the low-temperature range (below 10 K).

Transformation from Compound 2 to 1
In order to investigate the reverse reaction from 2 to 1, 2 was exposed to NH 3 vapor from an NH 3 aqueous solution for 2 h (2_NH 3 ). The PXRD pattern of 2_NH 3 exhibited characteristic peaks of both 1 and 2, implying that the expected transformation had occurred ( Figure 5). Therefore, TG analysis and GC-MS were performed for 2_NH 3 ( Figure S5). Similar to 1, the weight loss and MS intensity derived from H 2 O was not observed until 400 K. In addition, the results of the elemental analysis of 2_NH 3 also corresponded to intermediate values (elemental analysis: C: 2.68, H: 3.61, N: 16.55) between those of 1 and 2 (see the Materials and Methods section). The IR spectrum of 2_NH 3 in the 3000−3700 cm −1 region also changed and was similar to the spectrum for 1 ( Figure S6). The results of the magnetic measurements show that the magnetic behavior of 2_NH 3 (Figure 4d) differed from that of 2 ( Figure 4c) and approached that of 1 (see Figure 4a). These results indicate that the exposure of 2 to NH 3 vapor transforms it into 1. Figure 4. Temperature-dependent molar magnetic susceptibility in an applied field of 5 kOe for (a) 1, (b) 1_H2O (1 exposed to H2O vapor for 4.5 h), (c) 2, and (d) 2_NH3 (2 exposed to NH3 vapor for 2 h). The insets show the data in the low-temperature range (below 10 K). Figure 5. PXRD patterns of 2 (red), 2_NH3 (2 exposed to NH3 vapor for 2 h; yellow) and 1 (blue).

Reversible Transformation between Compounds 1 and 2
Reversible transformation between 1 and 2 was investigated. 1_H2O, which was obtained by exposing 1 to vapor for 4.5 h, was exposed to NH3 vapor from an NH3 aqueous solution for 2 h (1_H2O_NH3). The PXRD measurement of 1_H2O_NH3 was carried out to determine whether the change between 1 and 2 occurred reversibly depending on the gaseous vapors. Figure 6 shows the PXRD pattern of 1_H2O_NH3, including the patterns of 1, 1_H2O, and 2 for comparison. Given that Figure 5. PXRD patterns of 2 (red), 2_NH 3 (2 exposed to NH 3 vapor for 2 h; yellow) and 1 (blue).

Reversible Transformation between Compounds 1 and 2
Reversible transformation between 1 and 2 was investigated. 1_H 2 O, which was obtained by exposing 1 to vapor for 4.5 h, was exposed to NH 3 vapor from an NH 3 aqueous solution for 2 h (1_H 2 O_NH 3 ). The PXRD measurement of 1_H 2 O_NH 3 was carried out to determine whether the change between 1 and 2 occurred reversibly depending on the gaseous vapors. Figure 6 shows the PXRD pattern of 1_H 2 O_NH 3 , including the patterns of 1, 1_H 2 O, and 2 for comparison. Given that the PXRD pattern of 1_H 2 O_NH 3 was similar to that of 1, we can conclude that 1 and 2 were reversibly changed by gaseous vapors.

Stability of Compounds 1 and 2
Based on these experiments, the transformation from compound 1 to 2 occurred when 1 was exposed to H2O vapor, and the opposite transformation from compound 2 to 1 occurred when 2 was exposed to NH3 vapor, despite the simultaneous presence of H2O vapor due to the aqueous solution. The results indicated that 1 and 2 were stable in the presence of NH3 and H2O vapors, respectively. To verify this, we exposed 1 to NH3 vapor for 2 h and 2 to H2O vapor for 4.5 h. The PXRD measurements showed that 1 exposed to NH3 vapor (1') and 2 exposed to H2O vapor (2') underwent almost no structural changes ( Figure S7). This confirmed that 1 and 2 maintained their original structure when exposed to NH3 and H2O vapor present in the air, respectively. Based on these results, the anomaly observed in the magnetic measurements of 1 at low temperature is assumed to be attributable to a contribution of the antiferromagnetic transition of 2, produced by the partial transformation of 1 to the more stable structure of 2, in the presence of H2O vapor. The transformation Figure 6. PXRD patterns of 1 (blue), 1_H 2 O (1 exposed to H 2 O vapor for 4.5 h; green), 1_H 2 O_NH 3 (1_H 2 O exposed to NH 3 vapor for 2 h; yellow), and 2 (red).

Stability of Compounds 1 and 2
Based on these experiments, the transformation from compound 1 to 2 occurred when 1 was exposed to H 2 O vapor, and the opposite transformation from compound 2 to 1 occurred when 2 was exposed to NH 3 vapor, despite the simultaneous presence of H 2 O vapor due to the aqueous solution. The results indicated that 1 and 2 were stable in the presence of NH 3 and H 2 O vapors, respectively. To verify this, we exposed 1 to NH 3 vapor for 2 h and 2 to H 2 O vapor for 4.5 h. The PXRD measurements showed that 1 exposed to NH 3 vapor (1') and 2 exposed to H 2 O vapor (2') underwent almost no structural changes ( Figure S7). This confirmed that 1 and 2 maintained their original structure when exposed to NH 3 and H 2 O vapor present in the air, respectively. Based on these results, the anomaly observed in the magnetic measurements of 1 at low temperature is assumed to be attributable to a contribution of the antiferromagnetic transition of 2, produced by the partial transformation of 1 to the more stable structure of 2, in the presence of H 2 O vapor. The transformation did not completely progress to the point where all physical properties were consistent with those of the opposite compound after the transformation, owing to the decomposition of the crystal surface.

Conclusions
This study of gas-dependent reversible structural and magnetic transformation between two ladder compounds Cu 2 CO 3 (ClO 4 ) 2 (NH 3 ) 6 (1) and Cu 2 CO 3 (ClO 4 ) 2 (NH 3 ) 5 (H 2 O) (2) was inspired by the observation of an anomaly in a single crystal of 1 at low temperature based on magnetic and heat capacity measurements. Because this anomaly became more prominent after the exposure of 1 to air, we realized that the exposure of 1 to H 2 O vapor transformed it into 2. This transformation was identified using PXRD, TG analyses, GC-MS, elemental analyses, IR spectroscopy, and magnetic measurements. Subsequently, we demonstrated the transformation of 2 to 1 by exposing 2 to NH 3 vapor. We also achieved a reversible transformation between 1 and 2. In the particular cases where the moisture sensibility of 1 could be avoided or used as advantage, the reversible switching properties demonstrated in our study could allow the development of sensors and memories.