Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition Yi-Fei Deng, Yi-Nuo Wang, Xin-Hua Zhao and Yuan-Zhu Zhang Yi-Fei Deng Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Yi-Nuo Wang Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Xin-Hua Zhao Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 and Yuan-Zhu Zhang *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202101407 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The engineering of switchable materials with controllable stimuli-responsive multistability remains challenging in materials science. Herein, we present syntheses and structural and magnetic studies of a one-dimensional cobalt(II) coordination polymer [(enbzp)Co(bpy)](ClO4)2·MeOH·H2O ( 1; enbzp = N,N′-(ethane-1,2-diyl)bis(1-phenyl-1-(pyridin-2-yl)methanimine, bpy = 4,4′-bipyridine) and its desolvated analogue [(enbzp)Co(bpy)](ClO4)2 ( 2), obtained by reversible single-crystal-to-single-crystal (SCSC) transformation. Both complexes feature a rigid cationic chain with alternate enbzp-chelated Co(II) units and bpy linkers and exhibit incomplete and gradual spin crossover (SCO) behavior while, interestingly, additional thermally induced nonspin bistability was only observed in 2. Remarkably, the nonspin bistability shows an unprecedented scan-rate selectivity with a 75 K shift of center temperatures. At a rate above 5 K/min (fast-cooling), the transition takes place solely in the temperature range of 225–240 K centered at about 230 K with a hysteresis loop of 14 K, while at rates below 0.5 K/min (slow-cooling), the dynamic bistability moves to the room-temperature region (∼305 K) with wider hysteresis loops (∼26 K). Further studies revealed that the "slow-cooling" coupled transition occurs synchronously with the conformational swing of the cationic [(enbzp)Co(bpy)]2+ units and displacement of the [ClO4]− anions. The "fast-cooling" situation, however, could not be followed due to the rapid and irreversible structural rearrangement toward the more thermodynamically stable phase, as confirmed by time-dependent structural characterization and magnetic relaxation studies. These physical properties strongly corroborate that the two separated thermal bistability states as well as the multimagnetic states can be selected at will, based on the kinetics of subtle structural changes or rearrangements with different temperature-scan rates, which may be promoted to the memory devices armed with multichannels and functionalized in a desired manner. Download figure Download PowerPoint Introduction The engineering of switchable molecules with dramatic magnetic changes has been one of the most active areas of research worldwide , due to their potential applications for the next generation of switching, sensor, and memory devices.1–3 Among the most studied systems are spin crossover (SCO) compounds,4–6 in which the dramatic magnetic changes originate from spin transition between high-spin (HS) and low-spin (LS) states, which may be triggered by external stimuli such as temperature,7,8 light,9,10 and pressure.11,12 In addition to significant changes in magnetism, modulation of spin states may also synergize with other functionalities, such as electrical conductivity,13,14 ferroelectric property,15,16 negative thermal expansion,17,18 and luminescence,19,20 indicating the suitability of these molecules for multifunctional materials. Moreover, magnetic changes may be also achieved in a nonspin switching manner, induced by the so-called structural phase transition (SPT), such as order/disorder rearrangement,21,22 dynamic coordination bond breakage/formation,23 ligand rotation,24 and cis–trans isomerism.25 Consequently, the incorporation of both spin and nonspin transitions (SCO + SPT) in one molecule has provided a new promising strategy for the construction of multifunctional systems, which are useful for multiswitching and ternary memory devices.26,27 Representative examples can be found in a series of mononuclear SCO complexes equipped with long alkyl chains, known as soft SCO compounds,28,29 in which the SCO event is associated with the molecular-level motions of the flexible alkyl chains in either a nonsynchronous way at different temperature regimes or by being strongly synergized at the same temperature region, leading to unexpected physical properties. For example, Hayami et al.30,31 presented the unique SCO behavior of "reverse spin transition" or "re-entrant SCO," triggered by the thermal motion of the long alkyl chains that are attached to the terpyridine-like ligands of the cationic cobalt(II) complexes. Real and co-workers32 demonstrated the sweeping-dependent SCO processes caused by the isostructural phase transition in an alkyl-tailed mononuclear iron(II) complex. Recently, Clérac and co-workers33 observed the tristability in an iron(II) SCO compound, in which the non-SCO bistability was caused by the anti/gauche change of the decorated alkyl chains. Progress notwithstanding, systems featuring such synergy are extremely limited. With our long-standing interest in magnetically switchable materials,34,35 we recently prepared a few [FeIII2FeII] SCO complexes based on a series of tetradentate ligands armed with different aromatic rings, interestingly, those of the right size may rotate and thus induce an order/disorder rearrangement.36,37 We proposed extending this strategy into a rigid polymeric crystal lattice in which the effect of such atomic-level structural changes on the magnetic behavior may be significantly amplified due to the enhanced cooperativity38,39 between the SCO centers. Herein, we report the syntheses and structural and magnetic studies of a one-dimensional (1D) cobalt(II) coordination polymer, [(enbzp)Co(bpy)](ClO4)2·MeOH·H2O ( 1; enbzp = N,N′-(ethane-1,2-diyl)bis(1-phenyl-1-(pyridin-2-yl)methanimine, bpy = 4,4′-bipyridine) and its desolvated phase [(enbzp)Co(bpy)](ClO4)2 ( 2), obtained through reversible single-crystal-to-single-crystal (SCSC) transformation. Both 1 and 2 showed gradual SCO behavior, while additional nonspin bistability with bulk hysteretic behavior during the SCO event was clearly observed in 2, which exhibited unique selectivity with the temperature scan rates. Playing with the slow kinetics of related structural changes, four distinguishable magnetic states including the high-temperature (HT) state (S1), the intermediate state (S2), the dynamically metastable low-temperature (LT) state (S3), and the thermodynamically stable LT state (S4) are discussed. A slow (<0.5 K/min) or fast scan rate (> 5 K/min) would initialize the S1→S4 or S2→S3 magnetic transition, thus leading to the two distinct thermal bistabilities centered at about 305 and 230 K, respectively. Moreover, both the intermediate and dynamically metastable phases would convert to the thermodynamically stable phase via a structural rearrangement-induced relaxation process, as corroborated by the time-dependent structural characterization and magnetic relaxation studies. Experimental Methods Materials and syntheses Enbzp was prepared according to the literature.40 All other chemicals and reagents were commercially available and used without further purification. Caution: Although no such issues were observed during the present work, perchlorate salts are potentially explosive and should be handled in small quantities and with great care. Synthesis of [(enbzp)Co(bpy)](ClO4)2·MeOH·H2O (1) A methanolic solution (5 mL) containing Co(ClO4)2·6H2O (36.5 mg, 0.10 mmol) and enbzp (39.1 mg, 0.10 mmol) was allowed to stir for 30 min at 45 °C in an ambient atmosphere. After cooling to room temperature, the mixture was filtered; 4,4′-bipyridine (15.6 mg, 0.10 mmol) in 4 mL MeOH was subsequently added into the purple filtrate, and the resulting solution was left to evaporate slowly to yield the red crystals of 1. The product was collected by filtration and washed with methanol. Yield 66.7 mg (78%). Anal. Calcd for C37H36Cl2CoN6O10: C, 52.00; H, 4.25; N, 9.83. Found: C, 51.84; H, 4.46; N, 9.77. Fourier transform infrared (FT-IR) data (cm−1): 3751 (w), 3649 (w), 3076 (w), 2945 (w), 2825 (w), 2021 (w), 1599 (s), 1541 (m), 1444 (m), 1408 (s), 1359 (m), 1336 (m), 1267 (m), 1220 (m), 1164 (m), 1076 (vs), 817 (s), 794 (s), 742 (s), 700 (s), 621 (vs). It should be mentioned that complex 1 was also obtained by inserting an open vial with the crystalline sample of 2 in a closed cell with the humid methanol vapor atmosphere overnight. The resulting crystals [email protected] were found to be the same as 1, confirmed by both the single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD). Anal. Calcd for C37H36Cl2CoN6O10: C, 52.00; H, 4.25; N, 9.83. Found: C, 51.76; H, 4.34; N, 9.68. FT-IR data (cm−1): 3750 (w), 3649 (w), 3078 (w), 2945 (w), 2827 (w), 2019 (w), 1596 (s), 1539 (m), 1442 (m), 1410 (s), 1361 (m), 1337 (m), 1267 (m), 1218 (m), 1164 (m), 1076 (vs), 815 (s), 792 (s), 742 (s), 700 (s), 621 (vs). Synthesis of [(enbzp)Co(bpy)](ClO4)2 (2) Single crystals of 2 were quantitatively prepared by thermal desolvation of the crystalline sample of 1 at 350 K and under continuous nitrogen flow for 1 h. Anal. Calcd for C36H30Cl2CoN6O8: C, 53.75; H, 3.76; N, 10.45. Found: C, 53.69; H, 3.81; N, 10.35. FT-IR data (cm−1): 3750 (w), 3649 (w), 3075 (w), 2945 (w), 2827 (w), 2156 (m), 2005 (w), 1596 (s), 1541 (m), 1444 (m), 1408 (s), 1360 (m), 1335 (m), 1261 (m), 1220 (m), 1163 (m), 1074 (vs), 817 (s), 792 (s), 742 (s), 700 (s), 621 (vs). Physical measurements X-ray crystallographic data SCXRD measurements for 1 at 110–300 K ( Supporting Information Table S1) and 2 at 400–200 K ( Supporting Information Tables S2–S4) were performed using a Bruker D8 VENTURE diffractometer (Germany) with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Select bond distances and angles are listed in Supporting Information Tables S5–S8. The fully desolvated crystal of 2 was obtained via in situ thermal desolvation of 1 at 350 K on the diffractometer for 30 min, and it was then measured successively at 360, 370, 400, 360, 350, 330, 300, 290, 280, 270, 230, and 200 K, respectively, using a sweeping rate of 0.5 K/min between the measurements. Furthermore, to probe the relaxation-coupled structural rearrangement, the crystal of 2 was cooled from 350 to 270 K at 6 K/min, and then one dataset ( 2270-1) was quickly collected. After standing for an additional 5 h at 270 K, another dataset ( 2270-2) was then collected. A similar procedure at 200 K was applied for 2200-1 and 2200-2. All the structures were solved by the direct method of SHELXT41,42 and refined by full-matrix least squares (SHELXL) on F2, and empirical absorption corrections (SADABS)43 were applied. Anisotropic thermal parameters were used for the nonhydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model. CCDC 2070353 ( 1300), 2070354 ( 1110), 2070355 ( [email protected]300), 2070356 ( 2360h), 2070357 ( 2370), 2070358 ( 2400), 2070359 ( 2360c), 2070360 ( 2350), 2070361 ( 2330), 2070362 ( 2300), 2070363 ( 2290), 2070364 ( 2280), 2070365 ( 2270), 2070366 ( 2230), 2070367 ( 2200), 2070368 ( 2270-1), 2070369 ( 2270-2), 2070370 ( 2200-1), and 2070371 ( 2200-2) contain the crystallographic data that can be obtained via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or [email protected]). Elemental analysis Elemental analyses (EA) (C, H, N, and S) were measured by a vario electroluminescence (EL) cube CHNOS Elemental Analyzer (Elementar Analysensysteme GmbH, Germany). FT-IR spectroscopy FT-IR spectra were recorded in the range 600–4000 cm−1 using a Bruker TENSOR II spectrophotometer (Germany). PXRD Variable-temperature PXRD measurements were performed at 300–400 K (heating) and 400–150 K (cooling) using a Rigaku SmartLab X-ray diffractometer (Japan) with Cu Kα radiation (45 kV, 200 mA) between 5° and 50° (2θ). Thermogravimetric analysis Thermogravimetric analysis (TGA) experiments were performed with fresh samples using a METTLER TOLEDO TGA2 instrument (Switzerland). The TGA curves were measured under an argon atmosphere from 300 to 800 K with a heating rate of 5 K/min. Differential scanning calorimeter experiment Differential scanning calorimeter (DSC) measurements were recorded at 150–380 K and 260–330 K with a temperature scan rate of 5 and 0.5 K/min, respectively, using a TA Instruments Discovery DSC2500 (USA). Specific heat experiment Specific heat measurements were performed with the desolvated sample using the N-grease as the heat conductor, recorded with a PPMS DynaCool-9T system. The average sweeping rate was estimated as 0.1 K/min based on the log data file. To eliminate the perturbation from the N-grease, the heat capacity of different sets of samples as well as the related backgrounds have been carefully measured for several times, and the overall specific heat data for the sample was well reproduced after subtracting the contributions of the background. Magnetic measurements Magnetic measurements were performed using a superconducting quantum interference device (SQUID) MPMS3 magnetometer (USA). Specifically, the freshly prepared crystals were slightly ground and then placed into a capsule for SQUID brass, and the measured magnetic properties were consistently found in different batches of crystal samples. In the sweeping mode the magnetic data were recorded in the temperature range 350–150 K at rates 10, 5, 2, 0.5, and 0.25 K/min, respectively; while in the settle mode the magnetic data were further recorded in the temperature range 350–200 K with an average scan rate of 0.51 K/min. For the additional scan-rate-dependent magnetic measurements in two different procedures: (1) the sample was always cooled from 350 to 150 K at 10 K/min, while heated back to 350 K at 5, 1, 0.25, and 0.1 K/min, respectively; (2) the sample was always cooled from 350 to 150 K at 0.25 K/min, while heated back to 350 K at 10 and 1 K/min, respectively. For the relaxation experiments, the sample was initially cooled at 10 K/min from 350 K to the desired temperature, and then the susceptibility was monitored over time at the desired temperature. Magnetic data were corrected for the diamagnetism of the sample holder and for the diamagnetism of the sample using Pascal's constants.44 Results and Discussion Syntheses and crystallographic studies Treatment of Co(ClO4)2·6H2O and enbzp in a 1∶1 ratio in methanol followed by addition of equivalent 4,4′-bipyridine in an ambient atmosphere gave the dark red crystals of 1 in quantitative yield in a few days. TGA analysis ( Supporting Information Figure S1) revealed a fast weight loss of 5.9% from 300 to 350 K, corresponding to the removal of one MeOH and one H2O molecule per formula unit (calculated 5.9%); the following broad platform up to 500 K before the complete decomposition at 550 K well indicated the robust thermal-stability of the desolvated phase, which was found to be a new crystalline state ( 2), confirmed by the in situ desolvation of 1 on the X-ray diffractometer as well as the powder XRD analysis (vide infra). Accordingly, the bulk sample of 2 was prepared by heating the crystalline sample of 1 at 350 K for 1 h under a nitrogen atmosphere. Moreover, the single crystals of 2 may be resolvated and turn back to 1 when they are maintained overnight within the humid MeOH vapor atmosphere, suggesting a reversible SCSC transformation between 1 and 2. The SCXRD study revealed that complex 1 crystallized in the orthorhombic space group Pbca; and the asymmetric unit contained a cationic [(enbzp)Co(bpy)]2+ fragment, two [ClO4]−, one MeOH and one water molecule (Figure 1). The Co(II) ion adopts a distorted octahedral [CoN6] environment, with four equatorial N atoms of enbzp, and two axial N atoms of bpy ligands that linked the adjacent metal centers into a 1D coordination chain along the c-axis. Owing to the rigidity of the chelating enbzp ligand, the equatorial Co–N bond length (1.891(3)–2.020(3) Å) at 300 K were significantly shorter than the Co–N axial bonds (2.347(3) and 2.240(3) Å), leading to a pronounced elongation along the axial sites. These bond distances with the average value of 2.068 Å were considerably shorter than those of pure HS Co(II) compounds but in good agreement with a mixture of both HS and LS Co(II) species (vide infra). Upon cooling to 110 K, the Co–N bond lengths shrank slightly to 1.872–2.315 Å with an average value of 2.047 Å, suggesting the possible presence of the SCO process. In the crystal packing, the adjacent chains were coupled by significant π···π interaction (3.656–4.073 Å) between the aromatic rings in enbzp–enbzp or enbzp–bpy ligands to form a quasilayer structure ( Supporting Information Figure S2); the layers were further weakly bridged by extensive anion···π coupling (3.100–3.758 Å) dictated by the [ClO4]− anions and bpy rings, and edge-to-egde π···π interactions (3.768 Å) between enbzp ligands to generate a 3D network ( Supporting Information Figure S3). The remaining crystal volume was occupied by the interstitial solvent molecules of MeOH and H2O. Figure 1 | Crystal structures of 1 (top) and 2 (bottom, with disorders in the aromatic rings of enbzp) along the reversible SCSC transformation and conformational changes in bpy C, N, Hydrogen atoms and are for Download figure Download PowerPoint 2 crystallized in the same orthorhombic space group which the change to at above K, and when cooled back in situ to 350 K and no further change further cooling to 200 within the the coordination including the and Co–N bond lengths in 2 are similar to those in 1 while the were different (Figure 1). Specifically, a significant of bpy to a chain structure in 1, which and in 2, as indicated by the change of angles ( 1, 2, and the angles for bpy ligands ( 1, 2, were also found for 2 the 3D the anion···π Å), Å), and Å) π···π interactions ( Supporting Information and Magnetic studies Variable-temperature magnetic susceptibility for 1 and 2 was collected in the K temperature range with an applied direct of 1 (Figure For 1, the product of K at 300 K a mixture of HS and LS Co(II) Upon the product to K at K, in agreement with the pure LS the temperature the product followed the same as observed in the cooling mode before a between K K and K K and then to K at K, suggesting the gradual and incomplete SCO transition over the temperature range (Figure In of the TGA the can be to the loss of the interstitial solvent The was caused by the rigidity of the chelating enbzp ligand the Co(II) centers. in the of the product was observed at K the transition due to the structural and thermal changes, as indicated by the SCXRD and studies (vide infra). Figure 2 | Variable-temperature for 1 recorded on with a scan rate of 5 K/min, and for 2 at 350–150 K with different scan rates. states are in the are for the Download figure Download PowerPoint the sample of 1 was maintained at K for min in the magnetometer to the complete desolvation to 2. Upon the temperature with the scan rate of 5 K/min, the product from K at K to K at 2 K, indicating the overall gradual SCO process. an from K at K to K at K was observed ( Supporting Information Figure which toward the temperature range at between K and K K on heating back at the same scan resulting in a thermal bistability = K, = K) with a hysteresis loop of about 14 K 1). was most caused by the changes in the or magnetic than to a spin transition (vide infra). Remarkably, this thermal bistability showed an unprecedented on the temperature scan rates (Figure Specifically, at a scan rate the showed a similar thermal hysteretic behavior with change in at between = K and = after a scan rate of 2 K/min, the transition centered at about K = K, = K) significant with another hysteresis loop at range = K, = K). With more scan rates or 0.25 the thermal bistability only at the range K/min, = K, = 0.25 K/min, = K, = K) with the loops of and K, respectively. It should be that the hysteretic of 2 at a scan rate (5 and 0.5 K/min) were over two of measurements ( Supporting Information Figure Table 1 | The Transition for the of 2 at 10 230 5 14 230 2 300 0.5 305 0.25 305 For most of the hysteretic SCO systems, the transition observed in the "fast-cooling" are caused by a temperature associated with the effect of HS state during the which would the transition heating and thus the thermal a few SCO complexes were recently to hysteretic properties this in which additional were For example, a few a the scan rates to the transition temperature and wider and revealed the of two LS states, which may be through a We that the present has four possible thermal states Figure the for the intermediate for the dynamically metastable LT and for the thermodynamically stable LT The transitions between these four states for the magnetic properties. For the cooling process, with slow temperature-scan rates (<0.5 the S1→S4 magnetic transition is due to the time during the leading to the scan rates K/min) would this transition leading to the LT transition S2→S3 state to dynamically metastable With a temperature-scan rate the two thermal bistabilities can in a temperature range both the transitions that the gradual SCO is with another nonspin magnetic transition caused by the subtle structural changes featuring slow two thermal bistability are Accordingly, the is by the of measurements using different scan rates for the cooling and heating In the the sample was always cooled from 350 to 150 K at 10 K/min to the phase, and then the magnetic susceptibility data were recorded on back with different scan rates (Figure A heating in the phase at 5 K/min shows a at about K that to the reversible With the scan rates, this in to at a scan rate of 0.1 K/min, which is of the the other the cooling was always at 0.25 K/min to the and then the sample was heated at different scan rates (Figure the thermodynamically stable phase was it would not convert to other states for the reversible transition at Moreover, the relaxation kinetics were further by following the time of the intermediate states (Figure and Supporting Information Figure a the time significant in the for the at These clearly the and which may be caused by the structural rearrangements toward the more thermodynamically stable phase (vide infra). These that both the "fast-cooling" dynamic metastable state and the intermediate state to the more stable "slow-cooling" phase, the that the transitions are coupled with dynamic and states that are associated with the fast and slow scan rates, (Figure Figure | Variable-temperature for 2 cooling at 10 K/min and heating at 5, 1, 0.25, and 0.1 K/min, respectively. Variable-temperature for 2 cooling at 0.25 K/min and heating at 10 and 1 K/min, respectively. of the magnetic susceptibility for 2 at the selected temperatures. of the magnetic transitions between the are for the Download figure Download PowerPoint PXRD studies The reversible SCSC transformation for bulk crystalline samples between 1 and 2 was by a PXRD At room temperature, the PXRD for 1 and 2 well with their SCXRD indicating the desolvation for 1 and the good of both samples. After the crystalline sample of 2 to the humid methanol vapor atmosphere the PXRD well with that found for 1, the of the desolvation and processes ( Supporting Information Figure Moreover, the desolvation structural transformation and the of structural for the bulk samples was well by of in situ PXRD experiments (Figure 4 and Supporting Information Figure Upon heating from 300 to 350 K, the at a continuous shift to due to the loss of interstitial solvent molecules at these At above K, the at and due to the the cooling process, the reversible transition was at 350 K as indicated by the of the at and the continuous shift of the at K) to K) can be to the gradual structural changes over the temperature It should be that the and of the PXRD presented as the of

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