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

Abstract

We report reversible structural transformation that occurs in two ladder compounds: Cu₂CO₃(ClO₄)₂(NH₃)₆ (1) and Cu₂CO₃(ClO₄)₂(NH₃)₅(H₂O) (2), when they are exposed to gaseous vapors. The ladder structures of both 1 and 2 consist of two Cu²⁺ ions and one CO₃²⁻ ion. In 1, the Cu²⁺ ions are coordinated by three NH₃ molecules on each side, while those in 2 are coordinated by three NH₃ molecules on one side, and two NH₃ molecules and one H₂O molecule on the other side. We demonstrated reversible transformation of 1 and 2 via the exposure of 1 to H₂O vapor and the exposure of 2 to NH₃ vapor using a simple bench-scale method. The minor structural change observed led to a significant difference in physical properties, which we observed using several methods.

1. 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: 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 Cu²⁺ ions and one CO₃²⁻ ion, with each Cu²⁺ ion being coordinated by three NH₃ 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, [(H₃N)₃Cu-CO₃-Cu(NH₃)₂(H₂O)]_n, contains two Cu²⁺ ions and one CO₃²⁻ ion and its structure is similar to that of 1. One of the two Cu²⁺ ions is, however, coordinated using two NH₃ molecules and one H₂O 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: J₁/k_B = −7.26 K, J₂/k_B = −4.42 K, and J₃/k_B = 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.

2. Materials and Methods

Sample preparation: Single crystals of Cu₂CO₃(ClO₄)₂(NH₃)₆ (1) and Cu₂CO₃(ClO₄)₂(NH₃)₅(H₂O) (2) were synthesized based on past studies [16,20]. Elemental analysis (calcd.) for 1: C: 2.38 (2.46), H: 3.60 (3.72), N: 16.88 (17.21), and for 2: C: 2.43 (2.46), H: 3.52 (3.50), N: 13.95 (14.32). To identify structural transformation, powder samples of 1 and 2 were exposed to H₂O or NH₃ vapors, respectively. In this experiment, the powder sample was spread over a glass plate, which was then placed on top of a glass stand inside a beaker containing either H₂O or 14.8 mol dm⁻³ NH₃ aqueous solution (20 mL) in a confined space at 303 K (Figure S1).

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 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).

3. Results and Discussion

3.1. 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 in a spin ladder compound doped with magnetic impurities, Sr(Cu_1−xZn_x)₂O₃, 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 (NH₃ or H₂O), these results suggest that NH₃ can be substituted by H₂O 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.

3.2. Transformation from Compound 1 to 2

To further establish the aforementioned observations and implications, we evaluated the PXRD pattern of 1 exposed to H₂O vapor for 4.5 h (1_H₂O). The PXRD pattern of 1_H₂O was a superposition of the patterns of 1 and 2 (Figure 3). This implies that in the basic formula [(H₃N)₃Cu-CO₃-Cu(NH₃)₃] of 1, one NH₃ molecule was partially replaced by one H₂O 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 H₂O (m/z = 18) across a temperature range of 340 to 450 K. Up to 400 K, the weight loss and MS intensity derived from H₂O was not observed in 1. In contrast, weight loss was observed in 1_H₂O, and MS intensity derived from H₂O was observed below 400 K. The analysis of 2 yielded results that were similar to those of 1_H₂O. In addition, the elemental analysis of 1_H₂O 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 NH₃ and H₂O ligands, for 1_H₂O is closer to that of 2 than that of 1 (Figure S6). The magnetic behavior of 1_H₂O, 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 NH₃ in 1 was substituted by H₂O after exposure to H₂O vapor, and the anomaly observed in 1 at low temperature was caused by the transformation of 1 to 2 by H₂O, corresponding to the prediction mentioned earlier.

3.3. 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.

3.4. 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 the PXRD pattern of 1_H₂O_NH₃ was similar to that of 1, we can conclude that 1 and 2 were reversibly changed by gaseous vapors.

3.5. 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 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.

4. Conclusions

This study of gas-dependent reversible structural and magnetic transformation between two ladder compounds Cu₂CO₃(ClO₄)₂(NH₃)₆ (1) and Cu₂CO₃(ClO₄)₂(NH₃)₅(H₂O) (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 H₂O 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 NH₃ 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.