Reversible Single-Crystal-to-Single-Crystal Transformation in

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Cite This: Inorg. Chem. 2017, 56, 12914-12919

Reversible Single-Crystal-to-Single-Crystal Transformation in Photomagnetic Cyanido-Bridged Cd4M2 Octahedral Molecules Tomasz Korzeniak,* Robert Jankowski, Marcin Kozieł, Dawid Pinkowicz, and Barbara Sieklucka Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland S Supporting Information *

ABSTRACT: Two new hexanuclear octahedral cyanidobridged clusters, {[Cd II (bpy) 2 ] 4 [W IV (CN) 8 ] 2 }·10H 2 O (Cd 4 W 2 ) and {[Cd I I (bpy) 2 ] 4 [Mo IV (CN) 8 ] 2 }·10H 2 O (Cd4Mo2), have been obtained and characterized structurally and photomagnetically. Both compounds show a very rare and reversible single-crystal-to-single-crystal transformation upon dehydration accompanied by marked color changes in the case of Cd4W2. Moreover, irradiation of Cd4Mo2 using 436 nm light induces a reversible photomagnetic effect due to the LIESST-like singlet−triplet transition at the MoIV center. Analogous photomagnetic experiments for Cd4W2 did not lead to any significant change of its magnetic moment.

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INTRODUCTION The design and synthesis of new light-responsive compounds is a hot topic in materials chemistry that opens up new opportunities for the rational construction of molecule-based optical sensors and molecular switches. One of the most interesting groups of such photoactive compounds is photomagnets, molecular assemblies that change their spin state and magnetic properties in response to light. 4d and 5d cyanidometallates have played the leading role in the field of photomagnetism since the late 1990s.1−3 Cyanidometallate-based photomagnets can be grouped into one of the two classes: metal-to-metal charge transfer (MMCT) and light-induced excited-spin-state trapping (LIESST). In the case of octacyanidometallates, the MMCT class comprises multidimensional coordination networks based on [M(CN)8]n− (M = Mo, W) and various 3d metal complexes4−6 with a strong focus on CuII[MoIV(CN)8] systems.7−15 The occurrence of the LIESST effect16−19 centered at the octacyanidometallate, on the other hand, was suggested for the first time only recently based on X-ray magnetic circular dichroism (XMCD) studies of the bimetallic complexes [CuII(Meen)2]2[MoIV(CN)8]·xH2O (Meen = N,N′-dimethylethylenediamine) and [CuII(tren)]6[MoIV(CN)8](ClO4)8·4.5H2O [tren = tris(2-aminoethyl)amine].20 XMCD studies of Cs0.5Cu1.75[Mo(CN)8] nanoparticles21 led to similar conclusions about the LIESST origins of the photomagnetic behavior in some [MoIV(CN)8]-based compounds. Finally, the LIESST mechanism was unequivocally demonstrated and confirmed by Bridonneau et al. in 2015.22 In this mechanism, the MoIV center of d2 configuration reveals the transition from the S = 0 singlet state to the S = 1 triplet state due to the small energy barrier between the two d orbitals of lowest energy. This barrier is controlled mainly by the geometry of the octacyanidomolybdate moiety and displays particularly low values in the case of a distorted square © 2017 American Chemical Society

antiprism. Recently, similar LIESST-type behavior has been observed for the first time also for octacyanidotungstate(IV) systems.23 In order to further explore the opportunities offered by the intrinsic photomagnetic behavior of [MoIV(CN)8]4−, we have designed and characterized two new cyanide-bridged assemblies belonging to the family of hexanuclear octahedral clusters:24 {[CdII(bpy)2]4[WIV(CN)8]2}·10H2O (Cd4W2; bpy = bipyridine) and {[CdII(bpy)2]4[MoIV(CN)8]2}·10H2O (Cd4Mo2). So far, only a few members of the MII4MIV2 family have been obtained and characterized, {[MnII(bpy)2]4[WIV(CN)8]2} and {[MnII(bpy)2]4[MoIV(CN)8]2},25−27 {[MnII(bpy)2]4[NbIV(CN)8]2} exhibiting a high-spin (HS) ground state,28 and recently developed tetramethylphenanthroline-decorated compounds, 29 {[Mn II (tmphen) 2 ] 4 [Nb IV (CN) 8 ] 2 }, {[Co II (tmphen)2]4[NbIV(CN)8]2}, and {[FeII(tmphen)2]4[NbIV(CN)8]2}, of which the latter shows FeII-centered photomagnetism. The main goal of our studies was the incorporation of diamagnetic 4d cations into the [MIV(CN)8]-based hexanuclear clusters and a detailed investigation of their LIESST-based photomagnetic properties. In the course of this study, we have also found that both Cd4W2 and Cd4Mo2 show a single-crystal-to-single-crystal (SCSC) transformation upon reversible dehydration/rehydration, which is a very rare phenomenon among discrete supramolecular systems with only weak intermolecular interactions between the neighboring units.30 The SCSC transformation was studied using multiple techniques including single-crystal X-ray diffraction (XRD), powder XRD (PXRD), and IR spectroscopy. Received: July 5, 2017 Published: October 9, 2017 12914

DOI: 10.1021/acs.inorgchem.7b01708 Inorg. Chem. 2017, 56, 12914−12919

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Inorganic Chemistry

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EXPERIMENTAL SECTION

and crystallize in the monoclinic P21/n centrosymmetric space group. The selected structural parameters are presented in Table S1, and the asymmetric units are depicted in Figure S1. The molecular geometry of the hexanuclear core of Cd4W2 and Cd4Mo2 can be described as a distorted octahedron (Figure 1)

Physical Techniques. The UV−vis spectra were recorded in reflectance mode using a PerkinElmer Lambda 35 spectrophotometer equipped with an RSA-PE-20 diffuse-reflectance accessory. The IR spectra were recorded using a Nicolet iN10 MX FT-IR microscope equipped with a Linkam PE95/T95 temperature controller and a THMS350 V low-temperature accessory. The magnetic properties were measured using a MPMS3 SQUID-type magnetometer (Quantum Design). The photomagnetic properties were studied with the use of a Quantum Design fiber-optic sample holder coupled with a 300 W xenon lamp (LOT Quantum Design) as the light source equipped with a 436 nm bandpass filter (20 nm bandwidth). The light power measured at the sample was 0.9 mW cm−2. The finely ground diamagnetic sample was spread on a piece of adhesive tape (5 mm diameter) located ca. 2 cm from the end of the optical fiber. The experimental data were corrected for the diamagnetism of the sample and the sample holder measured before irradiation. The cooling rate of the sample inside the squid cavity was 10 K min−1. All photomagnetic measurements of the magnetic moment versus temperature were performed in a sweep mode during heating at a rate of 2 K min−1. The PXRD patterns were recorded at 25 °C (pristine state) and 80 °C (dehydrated state) using a PANalytical X’Pert PRO MPD diffractometer equipped with an Anton Paar TTK 450 low-temperature chamber. The measurements were performed in Bragg−Brentano geometry using Cu Kα X-ray radiation (λCu Kα = 1.541874 Å). Rehydration was performed outside the diffractometer under ambient conditions (ca. 70−80% relative humidity at 23 °C) for 30−40 min and remounted for the final data collection. Elemental analysis was performed using an Elementar Vario Micro Cube CHNS analyzer. Thermogravimetric analysis (TGA) studies were performed using a Mettler Toledo TGA/SDTA 851e microthermogravimeter. Single-Crystal X-ray Analysis. Diffraction data for single crystals of Cd4W2 and Cd4Mo2 were collected using a Bruker D8QuestEco diffractometer equipped with a CMOS Photon 50 area detector, a Bruker Kryoflex II low-temperature device, a Mo Kα (0.71073 Å) sealed-tube radiation source, and a graphite monochromator. The data were collected for monocrystals at 110 and 120 K, respectively. The measurements were repeated for both samples after drying them in a stream of dry nitrogen gas at 313 K for ca. 30 min. Data reduction, cell refinement, and absorption correction were performed using the Apex3 suite of programs (SAINT and SADABS). Both structures were solved using direct methods and refined anisotropically using SHELX.31 Syntheses. K4[WIV(CN)8]·2H2O and K4[MoIV(CN)8]·2H2O were prepared according to published methods.32−34 All other reagents and solvents were purchased from commercial sources (Sigma-Aldrich and Alfa Aesar) and used as received. {[CdII(bpy)2]4[WIV(CN)8]2}·10H2O (Cd4W2). A solution of Cd(ClO4)2·6H2O (10.6 mg, 0.025 mmol) in 3 mL of water was slowly added to an aqueous solution (17 mL) of 2,2′-bipyridine (16.0 mg, 0.10 mmol) and K4[WIV(CN)8]·2H2O (14.5 mg, 0.025 mmol), resulting in a yellow suspension. After 1 week, yellow needle-shaped crystals formed. The crystals were washed several times with water, collected, and dried in air. Elemental analysis was performed 24 h after collecting the crystals. Elem anal. Calcd for C96H84Cd4N32O10W2: C, 43.29; H, 3,18; N, 16.83. Found: C, 43.63; H, 3,011; N, 16.93. Yield: 21%. {[CdII(bpy)2]4[MoIV(CN)8]2}·10H2O (Cd4Mo2). The preparation was analogous to that of Cd4W2 using K4[MoIV(CN)8]·2H2O (12.4 mg, 0.025 mmol) instead of K4[WIV(CN)8]·2H2O. Light-yellow needleshaped crystals precipitated from a yellow suspension after 1 week. The crystals were washed with water, collected, and dried in air. Elem anal. Calcd for C96H84Cd4N32O10Mo2: C, 46.35; H, 3.40; N, 18.02. Found: C, 46.81; H, 3.213; N, 18.40. Yield: 41%.

Figure 1. Hexanuclear octahedral core of Cd4W2 (top) and Cd4Mo2 (bottom). H atoms are omitted for the sake of clarity. Color code: Cd, green; W, orange; Mo, purple; N, blue; C, gray.

and reveals an inversion center. The four CdII centers are located at the corners of a square defining its equatorial plane. The axial positions are occupied by the WIV or MoIV ions, respectively. The selected bond lengths and angles (Table 1) Table 1. Selected Bond Lengths (Å) and Angles (deg) for Cd4W2, Cd4Mo2, and Their Dehydrated Forms Cd4W2deh and Cd4Mo2deh Cd4W2

Cd4W2deh

Cd4Mo2

Cd4Mo2deh

Cd1−N1 Cd1−N5 Cd2−N3 Cd2−N7

2.265(5) 2.266(5) 2.296(5) 2.240(5)

2.229(7) 2.234(6) 2.244(6) 2.270(6)

2.253(3) 2.260(3) 2.300(3) 2.233(3)

2.200(7) 2.196(7) 2.231(7) 2.230(7)

Cd1−N1−C1 Cd1−N5−C5 Cd2−N3−C3 Cd2−N7−C7 N1−Cd1−N5 N7−Cd2−N3

154.8(5) 167.3(5) 142.4(5) 177.4(6) 91.22(18) 90.4(2)

155.4(6) 160.0(7) 152.0(5) 174.9(6) 94.7(3) 95.1(2)

154.7(3) 167.1(3) 140.0(3) 176.8(3) 91.46(10) 90.50(11)

156.6(6) 160.2(7) 151.8(6) 175.5(6) 94.3(3) 94.6(3)

are slightly longer than those in other 3d-metal-based polycyanidometallate-based systems.35 The coordination geometries of both [MIV(CN)8] moieties in both compounds are close to the square antiprism (SAPR-8), as determined from the continuous-shape-measure values36 presented in Table S2. Each [MIV(CN)8]4− forms four bridges toward the CdII centers, which display a distorted octahedral geometry with two N

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RESULTS AND DISCUSSION Crystal Structures and the SCSC Transformation upon Dehydration. Single-crystal XRD studies of Cd4W2 and Cd4Mo2 have shown that both compounds are isostructural 12915

DOI: 10.1021/acs.inorgchem.7b01708 Inorg. Chem. 2017, 56, 12914−12919

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Inorganic Chemistry atoms of the cyanido bridges located in cis positions and N atoms of the chelating bpy ligands occupying the remaining four sites. The Cd−NCN bonds and the Cd−NCN−CCN angles fall within the 2.240−2.300 Å and 177.4−140.0° ranges, respectively, and the Cd−Nbpy bonds are longer, 2.345−2.403 Å, than the Cd−NCN bonds. Distortion of the coordination sphere of CdII from the ideal octahedron is caused mainly by a small bite angle (ca. 70°) of the bpy ligands. The Cd−Cd distances within each Cd4M2 molecule are longer (8.6−8.8 Å) than the W−W or Mo−Mo distances (close to 6.6 Å), implying an axial compression of the octahedron along the W−W and Mo−Mo directions, respectively. The Cd4M2 molecules are quite well separated from each other. The crystallization water molecules (five per asymmetric unit) and the terminal cyanido ligands are involved in the hydrogen bonding, shown in Figure S2. The aromatic rings of the bpy ligands participate in the π−π-stacking interactions within the cluster molecules as well as between the neighboring ones (Figure S3). Careful observation of single crystals of Cd4W2 revealed that they reversibly change color from yellow to orange when heated (Figure 2) and do not crack in the process. This

Figure 3. Overlay view of the molecular structures of Cd4W2 (left) and Cd4Mo2 (right) in the pristine (blue) and dehydrated (red) forms. C atoms of 2,2′-bpy are omitted for the sake of clarity.

respectively, corresponding to the removal of 10 crystallization water molecules per Cd4M2 molecule observed in single-crystal XRD structural analysis. In order to confirm the reversibility of the dehydration process for bulk samples of Cd4W2 and Cd4Mo2 and its consistency with the single-crystal XRD structural analysis, we also performed variable-temperature PXRD experiments. The PXRD patterns of the polycrystalline samples of Cd4W2 and Cd4Mo2 recorded at 25 and 80 °C reveal significant differences resulting from the thermal dehydration process. The changes observed in the PXRD experiments are in perfect agreement with those obtained from the single-crystal XRD analysis (Figure S5). After heating, the samples were exposed to ambient air for about 1 h and remeasured at 298 K after rehydration (Cd4W2reh and Cd4Mo2reh; Figure 4 for Cd4W2

Figure 2. Single crystal of Cd4W2 in the pristine state (left), after dehydration at 313 K (center), and after rehydration (right). Dehydration was performed without removing the crystal from the goniometer head in a stream of dry nitrogen gas at 313 K.

prompted us to perform single-crystal XRD experiments for both Cd4W2 and Cd4Mo2 dried in a stream of hot dry nitrogen gas at 313 K. Structural analysis revealed that structural transformation related to the color change involves the removal of all 10 crystallization water molecules per Cd4M2 unit and yields completely anhydrous forms of the following formulas: {[CdII(bpy)2]4[WIV(CN)8]2} (Cd4W2deh) and {[CdII(bpy)2]4[MoIV(CN)8]2} (Cd4Mo2deh). Both compounds show the same space group as that in the pristine state; however, their unit-cell volumes decrease significantly by 3% and 5%, respectively (see the crystallographic details in Table S1). The dehydration-driven structural deformations of both clusters are presented in Figure 3, and the selected bond lengths and angles are compared in Table 1 with those obtained for the pristine compounds. The cyanide skeletons of both molecules undergo remarkable and unprecedented transformations. All Cd−NCN distances except Cd2−N7 are significantly shortened upon dehydration. Some of the Cd−C−N angles of the “Cd−NC−M” bridges change very little by less than 2° (Cd1−C1−N1 and Cd2−C7− N7) and others become more bent by ca. 7° (Cd1−N5−C5) or straighten up by almost 10° (Cd2−N3−C3). The squareantiprismatic coordination geometries of the [MIV(CN)8]4− anions in both compounds undergo similar deformations toward the dodecahedral geometry upon dehydration (Table S2). TGA, PXRD, and IR Spectroscopy. TGA of Cd4W2 and Cd4Mo2 (Figure S4) revealed single-step mass losses between 25 and 80 °C of 7.4% and 8.2% for Cd4W2 and Cd4Mo2,

Figure 4. Experimental PXRD patterns for Cd4W2 (red), Cd4W2deh (blue), and Cd4W2reh (green), confirming the reversibility of its dehydrations.

and Figure S6 for Cd4Mo2). The diffraction patterns of Cd4W2reh and Cd4Mo2reh appear to be identical with those of the pristine forms, thus confirming the full reversibility of the SCSC transformation upon dehydration/rehydration. The SCSC transformation of Cd4W2 and Cd4Mo2 was also followed using IR spectroscopy in the 20−80 °C temperature range shown in Figures 5 and S7. The most important changes upon heating/cooling involve the stretching and bending vibrations of the OH groups of the water molecules of crystallization. Heating leads to a complete disappearance of the corresponding bands at around 3440 and 1610 cm−1, which indicates the complete removal of all water molecules and supports the changes observed directly using single-crystal XRD structural analysis. These bands appear again upon cooling in a 12916

DOI: 10.1021/acs.inorgchem.7b01708 Inorg. Chem. 2017, 56, 12914−12919

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Figure 6. Solid-state UV−vis spectra of Cd4W2 (black) and Cd4Mo2 (red).

irradiation of the sample at 10 K, followed by M vs H at 2 K and χT vs T in the heating mode; (iii) after the sample was cooled to 2 K, the data for the thermally relaxed sample were measured again also in the heating mode. In the case of Cd4W2, only the photomagnetic response was negligible. Cd4Mo2, on the other hand, shows a clear increase of the χT product (Figure 7), reaching 0.39 cm3 K mol−1 after 26 h of irradiation (at 10 K and 0.1 T). Figure 5. IR spectra of Cd4W2 (a) and Cd4Mo2 (b) during three consecutive dehydration/rehydration cycles, confirming the reversibility of the dehydration process.

stream of humid air (ca. 95% relative humidity), which confirms the reversibility of the dehydration process. The IR bands related to the stretching vibrations ν(CN) of the cyanide ligands are also affected by the dehydration/ rehydration processes and are shown in Figure S7. The peaks observed for Cd4W2 at 2116, 2142, and 2158 cm−1 and for Cd4Mo2 at 2118, 2145, and 2158 cm−1 move toward lower wavenumbers upon dehydration (Cd4W2deh, 2106, 2126, 2136, and 2150 cm−1; Cd4Mo2deh, 2108, 2115, 2129, and 2141 cm−1), confirming distortion of the coordination spheres of WIV and MoIV. Noticeably, after three consecutive cycles of dehydration/rehydration, a weak new band at around 2200 cm−1 appeared with the intensity increasing slightly with each cycle, which may indicate slight irreversibility and decomposition. Photomagnetic Properties of Cd4W2 and Cd4Mo2. The investigation of the photomagnetic properties of Cd4W2 and Cd4Mo2 was preceded by recording of the solid-state reflectance UV−vis spectra and is shown in Figure 6. The most intensive bands in the 200−350 nm region originate from the metal-to-ligand and ligand-to-metal chargetransfer transitions involving 2,2′-bpy37 and cyanide ligands of the octacyanidometallate(IV) ions.1 The lowest-energy bands in the 360−550 nm region are related to the ligand-field transitions of the [M(CN)8]4− ions. In order to study the photomagnetic properties, irradiation experiments were performed with the use of 436 nm light. The following sequence for the photomagnetic measurements was used: (i) magnetization versus temperature and then versus field were measured and treated as reference data (zero level); (ii)

Figure 7. χT versus time of irradiation for Cd4Mo2 (λirr = 436 nm; HDC = 0.1 T; T = 10 K).

Figures 8 and 9 show the χT(T) and M(H) plots after 26 h of irradiation (violet points) and after heating to 300 K (gray points). The χT(T) (Figure 8) measured directly after irradiation shows a maximum of 0.52 cm3 K mol−1 at 30 K, which is well below the expected value of 2.0 cm3 K mol−1, assuming complete photoconversion leading to two noninteracting HS MoIV centers (S = 1; g = 2.0). On that basis, photoconversion for Cd4Mo2 can be estimated as 26%. Moreover, the low-temperature profile of the χT(T) plot measured directly after irradiation suggests the presence of weak antiferromagnetic interactions between the photoinduced MoIV centers. We presume that the interaction between the MoIVHS centers is negligible because of the statistically estimated lower concentration of the HS−HS clusters compared to HS−LS ones as well as the long distance between the Mo centers, namely, 6.62 Å. The distance between the molybdenum centers of different cluster units in the case of 12917

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moment is quenched in the HS MoIV center. On the other hand, the photoconversion ratios observed in these kinds of systems are usually below 50%.22 The main factors that intervail the photoconversion are the low absorption of light as well as the surface character of the photomagnetic effect. In our case, the sample has low absoption of the 436 nm light, which results in the slow increase of the magnetic susceptibility upon irradiation, as shown in Figure 7. The fact that even after 24 h of irradiation the saturation is not reached suggests that the maximal photoconversion ratio is much higher.

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CONCLUSIONS We have obtained two hexanuclear clusters, Cd4W2 and Cd4Mo2, with a distorted octahedral cyanido-bridged metal core. Both molecular systems reveal a reversible SCSC transformation upon dehydration, which is very rare among discrete polynuclear systems.30 The rapid SCSC transformation is connected with the removal of loosely bound water molecules and leads to significant deformations of the cluster’s cyanido skeleton. The diamagnetic Cd4Mo2 shows a photomagnetic effect upon irradiation with 436 nm light. On the basis of the experimental data, we have ascribed this phenomenon to the spin transition at the MoIV center. Cd4Mo2 is a unique system that combines a very rare dehydration-related SCSC transformation with a photomagnetic functionality.

Figure 8. χT(T) at 0.1 T (right) for Cd4Mo2 after irradiation using 436 nm light (violet points) and after heating to 300 K (gray points).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01708. Additional structural diagrams, crystallographic details, and TGA, PXRD, IR, and UV−vis data plots (PDF)

Figure 9. M(H) at 2.0 K for Cd4Mo2 after irradiation using 436 nm light (violet points) and after heating to 300 K (gray points).

Accession Codes

HS-LS interacting clusters is longer and exceeds 12 Å, which excludes the presence of through-space magnetic interactions between MoIVHS centers. Both χT(T) dependences merge gradually above 200 K which marks the relaxation temperature of the photoinduced state, which is relatively high compared to the other LIESST systems based on octacyanidomolybdates. The diamagnetic ground state is recovered almost quantitatively. However, it is noteworthy that both the χMT and M(H) signals after relaxation are positive. The shape of the χMT curve after relaxation follows that of the photogenerated state, but the magnetization signal of the thermally relaxed sample shows a broad and low maximum near 1.5 T. Due to the extremely small mass of the sample and low concentration of the paramagnetic centers it is not clear whether it originates from the little amount of the persisting photogenerated state or an experimental error. The M(H) dependences after irradiation (violet points in Figure 9) and after relaxation (gray points in Figure 7) confirm the presence of a significant photomagnetic effect for Cd4Mo2. The maximum magnetization value at 7 T measured directly after irradiation is 0.53 μB, which is well below the expected value of 4.0 μB assuming complete photoconversion leading to two noninteracting HS MoIV centers without any orbital contribution to the magnetic moment (S = 1, g = 2.0). On that basis, the photoconversion for Cd4Mo2 can be estimated as 13%. This value, however, is probably underestimated because of the antiferromagnetic interactions operating between the MoIV centers and the assumption that the orbital contribution to the magnetic

CCDC 1559307−1559310 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tomasz Korzeniak: 0000-0002-5667-7809 Dawid Pinkowicz: 0000-0002-9958-3116 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was financed by the Polish National Science Center within the OPUS Project 2014/15/B/ST5/04465. D.P. gratefully acknowledges the financial support of the Polish National Science Center within the Sonata Bis Project 2016/ 22/E/ST5/00055. The research was carried out with the equipment purchased thanks to the financial support of the 12918

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Inorganic Chemistry

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European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract POIG.02.01.00-12-023/08).

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DOI: 10.1021/acs.inorgchem.7b01708 Inorg. Chem. 2017, 56, 12914−12919