A PN(Pz)P ligand protected Au₂Cu₂ complex for photoluminescent ultra-low humidity detection with reversible single-crystal-to-single-crystal transformations

Abstract

Reliable and sensitive detection of sub-ppm level, ultra-low humidity is critical for a number of important technologies. We herein report the reaction of a P/N hybrid ligand 3-dppmapz-1-ol with Au(I) and Cu(I) salts resulting in a heteronuclear Au₂Cu₂ cluster complex [Au₂Cu₂(3-dppmapz)₂](PF₆)₂·4MeOH (1·4MeOH) that undergoes reversible single-crystal-to-single-crystal (SCSC) transformations into analogues 1 and 1·2H₂O. The photoluminescent emission of 1·2H₂O at 625 nm shifted to 617 nm within 24 h and 622 nm after 65 h in P₂O₅ dried air, and returned to green in MeOH/H₂O or MeOH/EtOH vapour. Crystals of 1 could rapidly detect ultralow humidity of 0.21 ppm (w/w).

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Introduction

Reliable and sensitive detection of ultra-low humidity is critical for many processes, during which the moisture content needs to be strictly monitored and/or controlled, for example under 500 ppm in microelectronic packaging,¹ 55 ppm in the aviation industry,² 10 ppm during reactions involving Grignard and organolithium reagents,³ and 0.1 ppm in lithium-ion battery manufacturing.⁴ Current technology shows good sensitivity in the detection of ultra-low humidity. The most common capacitive sensors consisting of aluminum oxide (γ-Al₂O₃) nanoporous films have achieved a humidity detection range of 3.7–100 ppm.⁵ “Enzyme-like” humidity-sensitive chromic materials have achieved naked-eye humidity detection from 0.01–100 ppm.⁴ These methods required external electronic circuits or suffered from irreversibility. It is highly desirable to develop more reversible low humidity sensing materials for these important applications.

In recent years, many photoluminescent Au/Ag/Cu complexes have been employed for the reversible detection of organic vapours such as MeOH,^6–10 MeCN,^11–13 THF,^14,15 DMSO,^16–18 and CH₂Cl₂,^19–22 as well as small inorganic molecules including H₂,²³ O₂,^24–28 CO, NO²⁹ and H₂O^8,30 with good sensitivity, selectivity, reproducibility and portability. The chromic photoluminescent responses of these complexes are attributed to their weak interactions with target molecules, via H-bonding,^31–33 C–H⋯π^9,34,35 and π⋯π interactions,^36–38 molecular deformation or formation/disruption of metal–solvent bonds.^7,39–42 For example, Yam and co-workers have reported that the green emission of the cluster [Au₁₀{μ-Ph₂PN(–CH₂-o-Py)PPh₂}₄(μ₃-S)₄](PF₆)₂ shifted to blue and red when exposed to CH₂Cl₂ and MeOH vapours, respectively, due to the changing of crystal forms. This emission shifted to yellow on exposure to trifluoroacetic acid (TFA) vapour due to pyridine nitrogen protonation.⁴³ Ito et al. have reported two gold(I) isocyanide complexes displaying reversible photoluminescence transitions between orange and green when exposed to air or saturated MeOH vapour, attributed to the reversible release or inclusion of MeOH in the crystal lattice.⁶ However, the intermolecular weak interactions responsible for these colour changes are challenging to elucidate, rendering the putative sensing mechanism less convincing. One reliable way of understanding the underlying chemistry is to observe the configuration of analyte molecules and the host during single crystal transformation.

In this work, we have prepared a new Au/Cu cluster [Au₂Cu₂(3-dppmapz)₂](PF₆)₂·4MeOH (1·4MeOH, 3-dppmapzH = N,N-bis((diphenylphosphanyl)methyl)-1H-pyrazol-3-amine). 1·4MeOH undergoes reversible single-crystal-to-single-crystal (SCSC) transformation to analogues 1 and 1·2H₂O on vacuum treatment or solvent exchange. The three compounds emitted bright red (625 nm, 1), orange (600 nm, 1·2H₂O) and green (560 nm, 1·4MeOH) phosphorescent emissions. The reversible colour change on the formation of 1·2H₂O was suitable for visible detection of ultra-low humidity changes down to 0.21 ppm (w/w, in P₂O₅ dried air). Complex 1·2H₂O also exhibited a selective response to MeOH in the presence of water or EtOH.

Results and discussion

The starting ligand 3-dppmapz-1-ol (3-(bis(((diphenylphosphanyl)methyl)amino)-1H-pyrazol-1-yl)methanol) was synthesized via the Mannich reaction of HPPh₂, 3-aminopyrazole and paraformaldehyde in refluxing methanol in a yield of 82% (Scheme S1†). The reaction of 3-dppmapz-1-ol, AuCN, Cu(MeCN)₄BF₄ and Et₄NPF₆ in MeOH/CH₂Cl₂ at ambient temperature followed by slow diffusion with hexane produced complex 1·4MeOH as yellow crystals (Fig. 1). 3-DppmapzH could not be synthesized directly, but was formed in situ and deprotonated during this reaction. It seems probable that the Au(I) and Ag(I) salts catalyzed the decomposition of the N–C bond. The PF₆⁻ counter anion was introduced by the addition of Et₄NPF₆. Attempts using Cu(MeCN)₄PF₆ instead of Cu(MeCN)₄BF₄ produced uncharacterized white precipitates.

Fig. 1 Synthesis and crystal structure of 1·4MeOH with 50% thermal ellipsoids. All H atoms except for those of the H-bonded O–H groups are omitted for clarity. Symmetry codes for A: 1 − x, 1 − y, 1 − z; B: 1 − x, −y, 1 − z.

3-Dppmapz-1-ol was insoluble in water, but readily soluble in CH₂Cl₂ and MeOH. The ¹H NMR spectrum (Fig. S1†) of 3-dppmapz-1-ol in DMSO-d₆ contained signals for the –CH₂– group (4.14, 5.16 ppm), –OH– group (6.46 ppm), –Ph (7.37–7.33 ppm) and pyrazol group (5.70, 7.50 ppm). One signal for –PPh₂ (−24.75 ppm) was also identified in the ³¹P{¹H} NMR spectrum (Fig. S3†). 1·2H₂O and 1·4MeOH were insoluble in water and nonpolar solvent Et₂O, slightly soluble in CH₂Cl₂ and DMSO. The IR spectra of 3-dppmapz-1-ol, 1·2H₂O and 1·4MeOH (Fig. S4†) showed signals attributable to the stretching vibrations of –C=N– at 1432, 1433 and 1435 cm⁻¹, respectively. The IR spectra of 3-dppmapz-1-ol and 1·4MeOH (Fig. S4†) showed stretching vibrations at 1045 and 1027 cm⁻¹ attributable to the stretching vibrations of –C–OH. The vibrations at 849 and 847 cm⁻¹ were attributable to PF₆⁻ in 1·2H₂O and 1·4MeOH.

The crystal structure of 1·4MeOH (Fig. 1) at 120 K indicated a square planar tetranuclear Au₂Cu₂ cluster, in which the Au1–Cu1 (2.8227(5) Å), Au1–Cu1A (2.8726(5) Å) and Cu1–Cu1A (2.6723(10) Å) distances are appropriate for metal–metal interactions. The Au1 atom was further coordinated with two P atoms from different dppmapz⁻ anions with a P1–Au1–P2A angle of 168.48(3)°, while Cu1 was coordinated by two N atoms vertical to the Au₂Cu₂ plane. Two weak Cu⋯F interactions were also observed in the Au₂Cu₂ plane.

The MeOH molecules in 1·4MeOH were readily replaced by H₂O in air or liquid water, transforming this complex into analogue 1·2H₂O (Fig. 2). This transition was reversible. Exposure of 1·2H₂O crystals to saturated MeOH vapour or immersion in MeOH for several hours gave 1·4MeOH. The thermogravimetric analysis (TGA, Fig. S5†) curve of 1·2H₂O indicated the loss of the two water molecules (2.0% w/w) below 110 °C, and indeed this complex gave anhydrous 1 under vacuum (2 h at 100 °C or 24 h at ambient temperature). Likewise, crystals of 1·4MeOH under vacuum at room temperature produced 1 within 1 h. The PXRD patterns (Fig. 2) of 1, 1·2H₂O and 1·4MeOH matched those simulated from the single-crystal X-ray crystal diffraction (SCXRD) data. Peaks corresponding to 1·2H₂O appeared in the PXRD spectra of 1 and 1·4MeOH on exposure to air for 5 min and 10 min, respectively, and these patterns changed completely to that of 1·2H₂O upon immersion in water within 5 and 30 min, respectively. By comparison, the reverse transformation of 1·2H₂O to 1·4MeOH required long-term (12 h) immersion of 1·2H₂O crystals in liquid MeOH.

Fig. 2 Reversible inter-conversions of 1, 1·2H₂O and 1·4MeOH, with PXRD patterns simulated from the SCXRD data and those of the samples during the inter-transformations.

Upon excitation at 280–400 nm, crystals of 1, 1·2H₂O and 1·4MeOH emitted brightly at 560 nm (1·4MeOH), 600 nm (1·2H₂O) and 625 nm (1), respectively (Fig. 3a). At room temperature, the emissions were excitation-independent and changing the excitation wavelength from 340 nm to 365 nm did not change the shapes of the emission spectra. The maximum quantum yields (QY) were 0.71 for 1·2H₂O (λ_ex = 365 nm) and 0.80 for 1·4MeOH (λ_ex = 350 nm) (the QY of 1 required measurement under vacuum and was not acquired), while the lifetimes (τ, excited at 375 nm) were 90.5 μs (1), 88.9 μs (1·2H₂O) and 94.8 μs (1·4MeOH). These relatively long lifetimes and large Stokes shifts (>220 nm) suggested that this photoluminescence was due to phosphorescence emission in each case. The long-term phosphorescence was likely caused by the radiative decay pathway that is formally symmetry-forbidden. Some or all of the relaxation pathways from the locally excited state to the emissive state might involve multiple intermediate electronic states of different symmetries, which could be formally forbidden and thus occur on the long timescale.⁴⁴

Fig. 3 (a) Excitation (dashed lines) and emission (solid lines) spectra of 1 (red), 1·2H₂O (orange) and 1·4MeOH (green) in the solid state. (b) Photos of a single crystal transitioning between the three forms excited under a 365 nm LED light. (c) Central structures of 1, 1·2H₂O and 1·4MeOH, and the distances of the Cu–N bonds, the Cu⋯F interactions and O–H⋯F hydrogen bonds from their SCXRD data. All –CH₂Ph groups and H atoms except for O–H are omitted for clarity.

The abovementioned interconversions between the three analogues could be demonstrated repeatedly using one single crystal. The emission colour of this crystal changed reversibly (Fig. 3b). The SCXRD measurements at 120 K indicated that the structures of 1 and 1·2H₂O were similar to that of 1·4MeOH, except for the distance and orientation of the O–H⋯F hydrogen bonds between the PF₆⁻ anions and the H₂O/MeOH molecules. As listed in Table S1,† although all three complexes crystallized in the triclinic crystal system with the P1̄ space group, the cell angle of 1 was different from those of 1·2H₂O and 1·4MeOH. The cell volume of 1 expanded from 1465 ų to 1490 ų (1·2H₂O) and 1629 ų (1·4MeOH) when the water and MeOH molecules were introduced. Moreover, the PXRD patterns simulated from the SCXRD data were obviously different and indicated phase transitions during the single-crystal-to-single-crystal (SCSC) transformations. Mechanical forces could not lead to such phase transitions as grinding the crystals of 1·2H₂O didn't cause any emission shift (Fig. S6†).

The photoluminescence of Cu–N complexes is usually attributed to metal-to-ligand charge transfer (MLCT) and thermally activated delayed fluorescence (TADF).⁴⁵ Temperature dependent experiments demonstrated that the emissions of 1, 1·2H₂O and 1·4MeOH were blue-shifted when the temperature was gradually decreased from 280 to 80 K (Fig. S7–S9†), while the lifetimes τ shortened slightly (1: 87.4 μs; 1·2H₂O: 73.5 μs; 1·4MeOH: 74.8 μs), which excludes the possibility of TADF. We subsequently investigated the relationship between solid-state structures and emission. As marked in Fig. 3c, the solid-state structure of 1 contained no hydrogen bonds, but the strongest Cu⋯F interactions. 1·2H₂O and 1·4MeOH possessed more and stronger O–H⋯F hydrogen bonds and weaker Cu⋯F interactions. Consequently, the Cu–N bond lengths were commensurately decreased. We speculate that the O–H⋯F hydrogen bonds between the MeOH and H₂O molecules and the PF₆⁻ anions attract the electrons of the PF₆⁻ anions, and therefore weakened the Cu⋯F interactions, and further strengthened the Cu–N bonds, influencing the energy levels of the ground and excited states of these complexes and blue-shifting the emissions of 1·2H₂O and 1·4MeOH.⁴⁶

We performed density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations using the CP2K program⁴⁷ and analyzed using the Multiwfn program.⁴⁸ The excitation of 1, 1·2H₂O and 1·4MeOH was mainly from the highest occupied molecular orbitals (HOMOs) to the lowest unoccupied molecular orbitals (LUMOs). As shown in Fig. 4 and Fig. S10,† the HOMOs were located at the d orbitals of the Cu atoms (along the Cu–N bonds), while the LUMOs were mainly located at the σ* orbitals of the Au–P bonds, which suggested that the excitation was metal to metal–ligand charge transfer (MMLCT). The energy differences ΔE(T₁–S₀) between the triplet excited state (T₁) and the singlet ground state (S₀) were 3.02 eV (1), 3.06 eV (1·2H₂O) and 3.12 eV (1·4MeOH), respectively, which were in line with the emission energies of the three compounds.

Fig. 4 The distribution of the HOMO and LUMO in 1.

Exploiting the reversible adsorption and desorption of water molecules, 1·2H₂O was employed as a photoluminescence probe for the detection of humidity in air and organic solvents. The distinguishable emission shifts were visible to the naked eye when a crystal of 1·2H₂O transformed into an analogue (Fig. 3b). At room temperature, when 1·2H₂O (2 mg) was ground and placed in a 100 mL sealed container filled with different desiccants (1 g) for 24 h, its emission (excited at 340 nm, Fig. 5a) was red-shifted to 613, 614, and 617 nm for CaCl₂, KOH,⁴⁹ and P₂O₅,⁵⁰ respectively, which correlated to a water content of 304, 17 and 0.021 ppm (w/w). These emissions were red-shifted by 12 nm (CaCl₂ and KOH) and 8 nm (P₂O₅) compared to that of anhydrous 1 (in a vacuum), indicating that 1 was capable of detecting ultra-low humidity changes even in P₂O₅ dried air. This detection limit is comparable to the lowest recorded value of 0.01–100 ppm, determined using nitro-oxazolidine as the sensory material,⁴ and is lower than those of other reported optical sensory materials,^51,52 including 1 ppm for SrF₂:Eu³⁺@TTA nanoparticles⁵³ and 248 ppm (w/w) for perovskite CH₃NH₃PbBr₃.⁵⁴ The reversibility between 1 and 1·2H₂O was further verified by alternatively placing 1·2H₂O in a vacuum and air, over 5 cycles (Fig. S11†). Time dependent emissions of 1·2H₂O in the P₂O₅ dried air (Fig. 5b) shifted to 617 nm within 25 h, which moved to 620 nm after 40 h, finally reaching 622 nm after 65 h, indicating that the total loss of solvate from 1·2H₂O was quite slow. However, the emission of anhydrous 1 recovered to that of 1·2H₂O very quickly (within seconds) when exposed to air which made the kinetic study difficult.

Fig. 5 Emission spectra of 1·2H₂O (a) in air dried by different desiccants after 24 h, and (b) in air dried by P₂O₅ after different times upon 340 nm excitation. (c) Diagram of the crystals inside and outside of the pipe wall of the return line of an Et₂O reflux equipment. Inset is a photo of the emissions of these crystals (under 365 nm LED) after several hours of reflux. (d) Photos of the equipment used to sense MeOH vapour from MeOH/H₂O and MeOH/EtOH mixtures, and the emissions of 1·2H₂O in the presence of different molar ratios of MeOH/H₂O under 365 nm LED. (e) and (f) Emission spectra of 1·2H₂O in the presence of different ratios of MeOH/H₂O and MeOH/EtOH under 360 nm excitation.

The photoluminescent responses of 1·2H₂O to other small inorganic and organic solvents were further investigated. NH₃ and trifluoroacetic acid (TFA) vapour caused emission quenching (Fig. S12†), which was irreversible. PXRD patterns of the samples showed the loss of crystallinity (Fig. S13†), which indicated that some chemical reaction occurred between 1·2H₂O and NH₃ or TFA. Emissions of 1·2H₂O on immersing in other organic molecules including EtOH, PrOH (propanol), iPrOH (isopropanol), BuOH (butanol), EG (ethyl glycol), CH₂Cl₂, CHCl₃, DMSO, DMF, THF, Et₂O, EA (ethyl acetate), n-hexane, CH₃CN, acetone and PE (petroleum ether) indicated that the chromic photoluminescence response was highly selective toward MeOH and DMSO (Fig. 6). These emission wavelengths were quite similar to those of 1 (in DMSO) and 1·2H₂O (in other solvents), suggesting that DMSO might seize the water molecules from 1·2H₂O, while other organic solvents could not replace water molecules. It is worth noting that, although 1·2H₂O was slightly soluble in CH₂Cl₂ and DMSO, the solutions in both solvents gave no obvious emission, which was probably due to the relatively low concentrations.

Fig. 6 Emission spectra of 1 2H₂O on exposure to MeOH, EtOH, PrOH, iPrOH, BuOH, EG, CH₂Cl₂, CHCl₃, DMSO, DMF, THF, Et₂O, EA, n-hexane, CH₃CN, acetone and PE under 340 nm excitation.

Both 1 and 1·2H₂O were insoluble in organic solvents such as Et₂O and THF, and so can be employed to monitor the water content in these solvents that have been dried by refluxing with sodium. As shown in Fig. 5c and Fig. S14,† several crystals of 1·2H₂O were placed inside the pipe wall of the return line, which emitted red light under a 365 nm LED after refluxing for several hours, while another crystal placed outside still emitted orange. Compared to the traditional indicator benzophenone, 1·2H₂O has the advantages of rapid sensing, lower dosage, recycling and easy operation with no need for adding a contaminant into the refluxing flask.

Moreover, 1·2H₂O could also selectively detect MeOH in the presence of H₂O and EtOH. Powdered samples of 1·2H₂O were coated on quartz slides and each slide was sealed in a quartz cell opening into a small tube containing different ratios of MeOH/H₂O and MeOH/EtOH at room temperature (Fig. 5d). The emission spectra after 10 min indicated obvious changes when the molar fraction of MeOH was more than 60% in H₂O (Fig. 5e) and 70% in EtOH (Fig. 5f). The colour change was visible to the naked eye when the molar fraction of MeOH was more than 70% in H₂O (see photos in Fig. 5d).

Conclusions

In summary, 1·2H₂O is a visible photoluminescent sensor for ultra-low humidity detection. The emissions of 1, 1·2H₂O and 1·4MeOH changed from red to green during their reversible SCSC transformations. This solvent induced photoluminescence chromism can be attributed to changes in O–H⋯F hydrogen bonding between the solvate molecule and PF₆⁻, which influences the Cu⋯F and Cu–N bond strengths. The photoluminescent chromic response to the transition from 1 to 1·2H₂O in P₂O₅ dried air (H₂O: 0.021 ppm) is currently the best reversible chemical sensor for humidity detection with potential applications in a number of important technologies.

Experimental section

Materials and methods

Elemental (C, H, and N) analyses (EA) were measured on a Carlo-Erba CHNO-S microanalyzer. Powder X-ray diffraction (PXRD) measurements were recorded on a Panalytical X'Pert-Pro MPD X-ray diffractometer with a Cu Kα source (30 kV, 10 mA) and vacuum equipment. IR spectra were obtained on a VERTEX 70 FT-IR spectrometer (4000–500 cm⁻¹) with an ATR probe. NMR spectra were measured at ambient temperature on a Varian UNITY plus-400 spectrometer, with chemical shifts (δ, ppm) referenced to the solvent signal of DMSO-d₆. Emission spectra, transient photoluminescence and quantum yield measurements were performed on a FLS1000 spectrometer (Edinburgh Instruments, U.K.). Photoluminescence spectra at low temperatures were recorded using an Optistat DN cryostat. Thermogravimetric analysis (TGA) was performed on a SDT2960 analyzer under a N₂ atmosphere and in the temperature range from room temperature to 800 °C, at a heating rate of 10 °C min⁻¹.

Synthesis of 3-dppmapz-1-ol

A mixture of HPPh₂ (1.86 g, 10 mmol), 3-aminopyrazole (0.42 g, 5 mmol) and solid paraformaldehyde (0.55 g, 18 mmol) in MeOH (50 mL) was refluxed for 24 h under nitrogen, while the reaction progress was monitored by TLC. The solvent was removed by rotary evaporation and the resulting pale yellow oily crude product was recrystallized from dichloromethane and ethyl ether to give a white solid of 3-dppmapz-1-ol. Yield: 2.09 g (82%). Anal. calcd for C₃₀H₂₉N₃OP₂: C, 70.72; H, 5.74; N, 8.25. Found: C, 70.47; H, 5.88; N, 8.18. IR (ATR): 3114 (w), 1555 (s), 1507. (w), 1480 (w), 1430 (m), 1382 (w), 1295 (w), 1263 (m), 1249 (m), 1150 (w), 1056 (s), 989(w), 946(w), 865(w), 836(m), 741(s), 723(m), 695(s), 624(w) cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, ppm) δ 7.50 (d, J = 2.4 Hz, 1H), 7.37–7.33 (m, 20H), 6.46 (t, J = 7.4 Hz, 1H), 5.70 (d, J = 2.4 Hz, 1H), 5.16 (d, J = 7.3 Hz, 2H), 4.14 (s, 4H). ¹³C NMR (101 MHz, DMSO-d₆, ppm) δ 161.31, 137.93 (d, J = 14.6 Hz), 133.46–133.10 (m), 131.47, 129.21 (d, J = 4.2 Hz), 129.14–128.90 (m), 92.01, 72.83, 52.41. ³¹P NMR (162 MHz, DMSO-d₆, ppm) δ −24.75.

Synthesis of [Au₂Cu₂(3-dppmapz)₂](PF₆)₂·4MeOH (1·4MeOH), 1·2H₂O and 1

A mixture of 3-dppmapz-1-ol (19.2 mg, 0.038 mmol) and AuCN (5.6 mg, 0.025 mmol) in CH₂Cl₂ (1 mL) and MeOH (2 mL) was stirred for 1 h at room temperature to afford a clarified solution. Cu(MeCN)₄BF₄ (12.0 mg, 0.038 mmol) and Et₄NPF₆ (3.3 mg, 0.012 mmol) were then added and stirred for 3 h to obtain a yellow turbid solution. The mixture was then centrifuged and the supernatant was slowly diffused with hexane, which afforded light yellow blocks of 1·4MeOH after 10 days. Yield: 1.4 mg (6.2% based on AuCN). IR (ATR): 3541(w), 3454 (m), 3171(w), 2932(m), 2827(w), 1550(w), 1572(m), 1437(s), 1373(m), 1331(s), 1205(m), 1100(s), 1029(s), 997(m), 896(m), 847(w), 798(m), 700(s), 629(w) cm⁻¹.

1·2H₂O was prepared by immersing 1·4MeOH in water for 24 h, while 1 was prepared from 1·4MeOH under vacuum generated by an oil pump (Preiffer Duo 3), at 100 °C for 2 h or at room temperature for 24 h. The yield was quantitative. 1·2H₂O Anal. calcd for C₅₈H₅₆Au₂Cu₂F₁₂N₆O₂P₆: C, 38.62; H, 3.13; N, 4.66. Found: C, 39.46; H, 2.95; N, 5.52. IR (ATR): 3627(w), 3117(w), 3061(w), 2920(w), 1678(w), 1641(w), 1563(w), 1488(w), 1433(m), 1385(w), 1205(w), 1095(m), 999(m), 903(m), 849(w), 798(m), 735(s), 690(s), 634(m).

Single-crystal X-ray structure determinations

Single-crystal X-ray diffraction (SCXRD) data for all crystals were recorded on a Bruker D8 VENTURE diffractometer using Ga Kα radiation (1.34138 Å). Single crystals of 1·2H₂O and 1·4MeOH were coated with Paratone oil on a Cryoloop pin, while 1 was sealed in a vacuum capillary tube. An Oxford Cryosystems cryostream at 120 K was used during data collection. Data were processed using the Bruker APEX2 software package using SAINT v8.34A and corrected for absorption by multi-scan. The structures were solved by direct methods and refined on F² by full-matrix least-squares using the SHELXTL-2016 program package.⁵⁵ All non-hydrogen atoms were refined anisotropically. H atoms of the water molecule in 1·2H₂O were first located from the difference Fourier maps and then their thermal parameters were constrained to U_iso(H) = 1.5U_eq(O). All other hydrogen atoms were added theoretically. Selected crystallographic data and refinement parameters are listed in Table S1.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Suzhou Science and Technology Plan Project (Grant No. SGC2021016) and the National Natural Science Foundation of China (Grant No. 21671144).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2241691–2241693 for 1, 1·2H₂O and 1·4MeOH. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00549f

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