Biomimetic Transformation by a Crystal of a Chiral MnIIâCrIII

Article pubs.acs.org/cm

Biomimetic Transformation by a Crystal of a Chiral MnII−CrIII Ferrimagnetic Prussian Blue Analogue Yusuke Yoshida,† Katsuya Inoue,*,† Koichi Kikuchi,‡ and Mohamedally Kurmoo*,§ †

Department of Chemistry, Institute for Advanced Materials Research, and Center for Chiral Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan ‡ Department of Chemistry, Tokyo Metropolitan University, 1-1 minami-Osawa, Hachioji 192-0397, Japan § Institut de Chimie de Strasbourg, CNRS-UMR7177, Université de Strasbourg, 4 rue Blaise Pascal, 67070 Strasbourg, France S Supporting Information *

ABSTRACT: We present a case of a concerted sequence of several characteristic biological reactions such as electron transfer, mixed valency, proton or ion motion, and dehydration−rehydration occurring in a single crystal and its associated phase transformations. It provided a unique opportunity to look for accurate pictures of sequence mimicking biological function using crystallography. Here, we report the observation of a reversible musclelike action associated with proton motion between ammonium and coordinated and uncoordinated water molecules in unison in which the whole X-ray diffraction study (including eight structure determinations as a function of temperature and cycling through two phase transformations) was performed in situ on a single crystal of the chiral molecular magnet [MnII(R-pnH)(H2O)][CrIII(CN)6]·H2O [R-pn = (R)-1,2-propanediamine]. In addition, the reversible structural change in dimensionality from two- to three-dimensional is observed with an increase in the ferrimagnetic Curie temperature from 38 to 73 K upon dehydration.

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(H2O)][CrIII(CN)6]·H2O.36 This compound has several facets to it, which are incorporated by choice. It is heterometallic class II mixed-valence (MnII−CrIII); it has paramagnetic centers with different spins (5/3 and 3/2), and it is chiral by virtue of the presence of only one enantiomer of the organic form in the structure. Because of the Mn−NC−Cr antiferromagnetic interaction, it is a ferrimagnet (TC = 38 K; Hcoercive < 10 Oe). Furthermore, μ-SR and nonlinear ac susceptibilities reveal a physical phenomenon not observed earlier that is associated uniquely with chirality and magnetic order.64−71 To verify certain findings, we have synthesized its congener containing the R enantiomer, [MnII(R-pnH)(H2O)][CrIII(CN)6]·H2O (1), and undertaken the study of its structural, thermal, optical, and magnetic properties, which reveals unknown characteristics and forms the content of this paper. What is still unknown in this compound and [MnII(R-pnH)(H2O)][MnIII(CN)6]·2H2O (TC = 21.2 K)37 is the fact that they display an amazing range of interesting chemical and physical properties that have never been encountered. Here we report on its robustness through two phase transitions, including dehydration and rehydration, and the associated characteristics, in particular the magnetic properties. The eight structures determined from one crystal while the temperature is cycled between 200 and 360 K retain complete order within the same space group, P212121.72−74 Interestingly,

INTRODUCTION The field of magnetism based on molecules and coordination networks is a rapidly advancing area of research and is of major academic and industrial importance because of the potential application in information storage and quantum computing.1−16 As the desired long-range magnetic ordering is becoming quite common and can be designed, research is being directed toward improving the critical transition temperatures [TCurie (TC) or TNéel], the magnetic hardness (Hcoercive), and the energy products (BH)max.17−23 These are thought to be accessible through design of the coordinating ligands, the choice of paramagnetic centers, and the control of the structural and magnetic dimensionalities.24 These advances have led to the realization of multiple properties within one compound. Therefore, in parallel to these works, there is a concerted effort to deliberately introduce other properties such as electrical25−29 or proton30 conduction, chirality,31−47 nonlinear optics48 or luminescence,49 and, most recently, porosity.50−60 We and others have focused on introducing chirality into modified Prussian blue by using enantiopure amine ligands in the search for synergy of magnetism and chirality. To the best of our knowledge, molecular magnetism has not yet been implicated in biomimetics as has been evidenced in several areas, such as architecture, robotics, dynamics, electrochemistry, multifunctional materials, and computing.61−63 The work presented here represents a first example. Pervious works have led to several chiral magnets that belong to different crystal classes. Among them is [MnII(S-pnH)© XXXX American Chemical Society

Received: July 13, 2016 Revised: September 15, 2016

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DOI: 10.1021/acs.chemmater.6b02852 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials Scheme 1. Protocol Used for in Situ Single-Crystal Structure Determinations

a

The color code of the frames is as follows: blue for LTP, green for HTP, and red for DP. Movies S1−S6 display six of the processes. Pyris 6 TGA instrument operating under dry nitrogen at a heating rate of 5 K/min for 21.841 mg of 1. Differential Scanning Calorimetry (DSC). DSC experiments were performed at two different warming rates using a Rigaku DSC8230 instrument under a nitrogen atmosphere. For 1, measurements with warming rates of 0.25 and 2 K/min were performed on samples of 20.009 and 4.990 mg with 20.693 and 5.442 mg of Al2O3 as references, respectively. Crystal Structures and Sample Preparations. All the diffraction data were collected by in situ measurements using a Bruker SMARTAPEX diffractometer equipped with a CCD area detector and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) in ωscan mode (0.3° steps). Semiempirical absorption corrections on Laue equivalents were applied. The structures were determined by direct methods and refined by full-matrix least squares against F2 of all data using SHELX-97.76−78 Hydrogen atoms on nitrogen and carbon atoms were included in calculated positions and not refined. The positions of the hydrogen atoms on the oxygen atom were determined from the different Fourier map unless the refinement was unstable. All the atoms except hydrogen were refined anisotropically. Data collections were performed on the virgin low-temperature phase (LTP), high-temperature phase (HTP) at 330 and 298 K, relaxed LTP, quenched HTP, relaxed LTP, dehydrated phase (DP), and rehydrated HP, sequentially, as shown in Scheme 1. The processes were registered as diffraction frames with time and made into movies. Virgin LTP was warmed from 298 to 330 K (Movie S1) at a rate of 4 K/min under nitrogen with a straw covering, and HTP (330 K) was obtained. HTP (330 K) was cooled from 330 to 298 K at a rate of 4 K/min under nitrogen, and HTP (298 K) data were collected after the straw had been removed. HTP was slowly cooled from 298 to 280 K at a rate of 0.1 K/min (Movie S2) and warmed from 280 to 298 K at a rate of 4 K/min under nitrogen, and relaxed LTP diffraction data were collected. Relaxed LTP was warmed from 298 to 330 K (Movie S3) at a rate of 4 K/min and rapidly cooled from 330 to 200 K at a rate of 10 K/min under nitrogen, and quenched HTP diffraction data were collected. Quenched HTP was warmed from 200 to 295 K at a rate of 4 K/min, slowly cooled from 295 to 290 K at a rate of 0.1 K/min (Movie S4), and warmed from 280 to 298 K at a rate of 4 K/min under nitrogen, and relaxed LTP (second) diffraction data were collected. Relaxed LTP was heated from 298 to 360 K (Movie S5) at a rate of 2 K/min and cooled from 360 to 298 K at a rate of 4 K/min under nitrogen, and dehydrated DP diffraction data were collected. DP was exposed to air for 30 min (Movie S6), and rehydrated HP diffraction data were collected. For LTP, the crystals were found to break during the temperature cycle, and this was associated with the different hardnesses when the

one of the two transitions involves a rarely observed proton transfer associated with a change in the corrugation pitch of the layer structure that resembles a biological system such as a muscle. Dehydration associates the two-dimensional (2D) layers to three-dimensional (3D) layers and causes a giant leap in TC from 38 K through 39 K and finally to 73 K and swapping of the magnetic easy axes in the a−b plane.

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EXPERIMENTAL SECTION

Synthesis. This compound was synthesized by a method slightly modified from that reported previously.36 In this synthesis, all manipulations were performed under an argon atmosphere. Two milliliters of an aqueous solution of R-pn·2HCl (368 mg, 2.5 mmol) and K3[Cr(CN)6] (203 mg, 0.625 mmol) was mixed with 0.5 mL of 8.0 M KOH (4 mmol), and the mixture was placed at the bottom of a glass tube (14 mm inner diameter); 1.5 mL of a H2O/EtOH mixture (3:1) was then layered on top of this mixture, followed by layering with 2.5 mL of a H2O/EtOH (1:3) solution of MnCl2·4H2O (372 mg, 1.88 mmol). The glass tube was sealed and kept in a dark place. After 2 months, green needle plate crystals (Figure S1) were harvested. This method also yields cubic crystals of Prussian blue, Mn3[Cr(CN)6]2· nH2O,75 as a byproduct in a proportion not exceeding 10%. The crystals were filtered and washed with H2O, EtOH, and ether. The desired crystals were separated manually under an optical microscope for further measurements. Yield of 175 mg (including the cubic crystal), 74.9% {0.468/0.625 mmol based on K3[Cr(CN)6]}. Anal. Calcd for 1, C9H15CrMnN8O2: C, 28.89; H, 4.04; N, 29.94. Found: C, 28.90; H, 3.95; N, 30.34. The original reported synthesis of green needles of [Mn(SpnH)(H2O)][Cr(CN)6]·H2O also resulted in the formation of yellow thin needles of a nonhydrate compound K0.4(S-pnH)0.6[Cr(CN)6][Mn(S-pn)] and the cubic crystals of the Prussian blue.36 The yellow form was not obtained during our preparation of [Mn(R-pnH)(H2O)][Cr(CN)6]·H2O. Elemental Analyses. Elemental analyses for C, H, and N were performed on a PerkinElmer 2400II instrument at the Natural Science Center for Basic Research and Development (N-BARD) of Hiroshima University. Thermogravimetric (TG) and Differential Thermal (DT) Analyses. TG and DT analyses were performed on 10.06 mg of 1, using a Rigaku TG8120 instrument operating under dry nitrogen while samples were being heated from 290 to 363−380 K at a rate of 1 K/ min and cooled to 290 K at a rate of 10 K/min. After being cooled, the dehydrated sample was exposed to air overnight. Further measurements were performed at higher temperatures by use of a PerkinElmer B


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Chemistry of Materials

Table 1. Crystallographic Data for Virgin LTP (1), HTP at 330 K (2) and 298 K (3), Relaxed LTP (4), Quenched HTP (5), Relaxed LTP (6), Dehydrated DP (7), and Rehydrated HP (8)

formula weight T (K) atmosphere crystal system space group a (Å) b (Å) c (Å) V (Å3) Z DC (g/cm3) μ (mm−1) total no. of reflections no. of unique reflections observed [I ≥ 2σ(I)] no. of parameters R1/wR2 [I ≥ 2σ(I)] R1/wR2 all data goodness of fit Flack Δρ (e/Å3)

LTP (virgin) (1)

HTP at 330 K HTP at 298 K (2) (3)

LTP (relaxed) (4)

374.23 298 air

374.23 330 nitrogen

374.23 298 air

374.23 298 air

7.6420(6) 14.5365(12) 14.9570(12) 1661.5(2) 4 1.496 1.429 5716

7.4503(16) 14.465(3) 15.649(3) 1686.4(6) 4 1.474 1.408 5794

3505

HTP (quenched) (5)

LTP (relaxed) (6)

DP (dehydrated) (7)

HP (rehydrated) (8)

374.23 298 air

338.20 298 nitrogen

374.23 298 air

7.4439(7) 14.4725(14) 15.6319(15) 1684.1(3) 4 1.476 1.410 5776

374.23 200 nitrogen orthorhombic P212121 7.6421(10) 7.4250(6) 14.5416(19) 14.5177(11) 14.958(2) 15.5242(12) 1662.2(4) 1673.4(2) 4 4 1.495 1.485 1.428 1.419 5688 5749

7.6459(7) 14.5442(12) 14.9647(13) 1664.1(3) 4 1.494 1.426 5686

7.712(3) 13.401(5) 14.058(5) 1452.8(9) 4 1.546 1.615 3548

7.6695(16) 14.586(3) 15.013(3) 1679.5(6) 4 1.480 1.413 5256

3583

3579

3517

3561

3530

2198i

3433

2944

2814

2923

2949

3168

2967

1592

2357

205 0.0309 0.0725 0.0387 0.0750 0.982 −0.01(3) 0.421, −0.363

192 0.0371 0.0904 0.0505 0.0942 0.981 0.02(4) 0.487, −0.503

200 0.0364 0.882 0.0455 0.913 0.963 −0.01(3) 0.510, −0.562

209 0.0322 0.0743 0.0402 0.0772 0.978 −0.04(3) 0.416, −0.489

204 0.0303 0.0733 0.0352 0.0749 1.003 −0.05(2) 0.665, −0.381

204 0.0311 0.0727 0.0390 0.0751 0.982 −0.03(3) 0.450, −0.346

174 0.0707 0.1594 0.1096 0.1817 1.136 0.14(12) 0.588, −0.621

192 0.0897 0.1937 0.1497 0.2300 1.256 −0.08(11) −0.756, 1.097

crystals were glued to the glass fiber using Araldite. Furthermore, a direct flow of nitrogen onto the crystal was used during intensity measurement for HTP at 309 K where the structural phase transition was observed to gradually affect the quality of the diffraction peaks. This occurs only in the first transformation from virgin LTP to HTP. To circumvent the two difficulties, two precautions were taken. One is to wrap the selected crystal with a thin polyethylene sheet (cling film) without using glue, and this is then glued to the glass fiber with Araldite at one end and the other end left open; otherwise, it was not possible to properly dehydrate the crystal in situ. The second problem was resolved by shielding with a clear drinking straw around the crystal. Neither of these appears to affect the diffracted intensities. The results of the protocol (Scheme 1) for in situ single-crystal structure determinations are listed in Table 1 (summary of crystallographic data). Powder X-ray Diffraction. PXRD patterns were measured on a Rigaku Rint 2000 system using Cu Kα radiation and employing a scan rate of 4.0°/min and a step of 0.02°. The sample was sequentially transformed from virgin LTP to quenched HTP, relaxed LTP, dehydrated DP, and rehydrated HP. After virgin LTP diffraction data had been collected, virgin LTP was placed into a heated desiccator and kept at 315 K for 1 h and quenched HTP diffraction data were collected. Quenched HTP was placed in a refrigerator at 283 K and kept there for 2 h, and relaxed LTP diffraction data were collected. Relaxed LTP was placed in a heated desiccator and kept at 363 K for 30 min under nitrogen, and dehydrated DP diffraction data were collected. Dehydrated DP was exposed to air for 30 min, and rehydrated HP diffraction data were collected. The simulated patterns were calculated from the single-crystal X-ray data using RIETAN2000.79 Infrared Spectroscopy. IR spectra were recorded by transmission through KBr disks containing ∼0.5% of the compounds using a HORIBA model FT-720 FT-IR spectrometer. For HTP, the KBr pressed disk was warmed to 323 K for 2 days before measurement. For DP, some powdered KBr containing the virgin sample was vacuumed

at 383 K for 2 h and then pressed into a disk. For HP, the virgin sample was heated to 365 K at a rate of 1 K/min, exposed to air overnight, then mixed with KBr, and pressed into a disk. UV−Vis Spectroscopy. UV−vis absorption spectra of LTP were recorded by transmission through a single crystal (1.5 mm × 5 mm and a thickness of ∼0.2 mm) held on cut slits between two aluminum sheets by use of a UVIKON-XL apparatus from BIOTEK Instruments. Because of the high absorption below 400 nm, the transmission was too low to allow any measurement to be performed. Magnetic Properties of Powdered Samples. In situ magnetic measurements were performed on 15.023 mg of a LTP sample using a Quantum Design MPMS-5S SQUID magnetometer. Diamagnetic corrections were estimated using Pascal constants80 (−1.891 × 10−4 cm3 mol−1 for LTP and HTP and −1.631 × 10−4 cm3 mol−1 for DP) and background correction by experimental measurement of an empty gelatin capsule. Virgin LTP was warmed from 300 to 312 K at a rate of 0.25 K/min and cooled to 100 K at a rate of 10 K/min to produce quenched HTP. Quenched HTP was warmed from 280 to 295 K and cooled to 280 K at a rate of 0.25 K/min and warmed to produce relaxed LTP. Relaxed LTP was warmed from 310 to 360 K at a rate of 0.5 K/min and purged for 1 h at 360 K to produce dehydrated DP. Dehydrated DP was exposed to air for 1 week to produce rehydrated HP. Zero-field-cooled (ZFC) and field-cooled (FC) measurements were performed in an applied field of 5 Oe, and magnetic susceptibility was measured in an applied field of 100 Oe. Magnetic susceptibilities for quenched HTP were collected upon cooling from 100 to 2 K, followed by warming from 106 to 300 K. For the dehydrated DP sample, magnetic susceptibilities were collected upon cooling from 360 to 2 K. For the other phases, magnetic susceptibilities were collected upon cooling from 300 to 2 K. Isothermal magnetization at 2 K was measured for each phase from −50 to 50 kOe. Magnetic Properties for Single-Crystal Samples. Isothermal initial magnetizations were performed at 2 K, where the crystals were aligned along the crystallographic axes determined by the aid of singleC


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Chemistry of Materials crystal X-ray diffraction, from 0 to 50 kOe. Two single crystals were used to complete the full set of measurements. Measurements were sequentially taken for the b-axis (LTP), c-axis (LTP), c-axis (HTP), baxis (HTP), a-axis (LTP), a-axis (HTP), a-axis (DP), b-axis (DP), and c-axis (DP). The crystallographic axes were determined more accurately from the minimum or maximum point of angular dependence of the magnetization. The phases were determined by their magnetic critical temperatures. The two crystals were covered with methyl methacrylate polymer (PMMA) using CH2Cl2. The first single crystal (2.622 mg, LTP) was set on the sample rotor with the a-axis parallel to the rotation axis (perpendicular to the applied field). Then, b-axis (LTP) and c-axis (LTP) MH curves were measured. Virgin LTP was then warmed from 290 to 312 K at a rate of 0.25 K/min and cooled to 100 K at a rate of 10 K/min to produce quenched HTP. Then, c-axis (HTP) and b-axis (HTP) MH curves were measured. Quenched HTP was warmed from 280 to 295 K and cooled to 280 K at a rate of 0.25 K/min to produce relaxed LTP. The same crystal (LTP) was remounted on the sample rotor with the c-axis parallel to the rotation axis. Then MH curve was measured for the aaxis (LTP). Relaxed LTP was twice warmed from 290 to 312 K at a rate of 0.25 K/min and cooled to 100 K at a rate of 10 K/min to produce quenched HTP. Then the MH curve for the a-axis (HTP) was measured. Quenched HTP was heated from 310 to 360 K at a rate of 2 K/min to produce dehydrated DP. Then a-axis (DP) and b-axis (DP) MH curves were measured. The second crystal (1.3450 mg) was set on the sample rotor with the a-axis parallel to the rotation axis. The crystal was heated from 310 to 360 K at a rate of 2 K/min to produce dehydrated DP. Then the c-axis (DP) MH curve was measured.

Structures of Virgin LTP (1), Relaxed LTP (4), Relaxed LTP (6), and Rehydrated HP (8) at 298 K. LTP has been found to be the same for several crystals and at several stages throughout the protocol used for a single crystal, and their structural determinations confirm the reversibility at all stages. We have determined all the structures at 298 K to be consistent for easy and reliable comparison of the parmeters. It is clear from the reliability factors that direct comparisons of the geometries can be performed with complete confidence. The key feature of the structure of LTP is the 2D square grid of Mn and Cr(CN)4, where Mn and Cr alternate in the a−b plane and are bridged by cyanide (Figure 1). However, the layers are not

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RESULTS AND DISCUSSION Thermogravimetric and Differential Thermal Analyses. TG and DT measurements on the virgin crystals gave the first indication of reversibility of a phase transition at 305 K followed by dehydration at 360 K [observed weight loss of 9.8%, calculated value of 9.63% (Figure S2)]. They are labeled as LTP (low-temperature phase) to HTP (high-temperature phase) and HTP to DP (dehydrated phase). When the temperature of the samples was decreased to 290 K after they had been heated to 380 K and exposed to air (≈50% humidity) for ≈8 h, the crystals re-absorb water and recover their initial weights, which we label as DP to HP (rehydrated phase). These processes are reversible and have been performed four times consecutively. Above 450 K, the phases decompose to yield MnO2 and Cr2O3. Differential Scanning Calorimetry. DSC at two different scan rates (0.25 and 2 K/min) confirms the endothermic transformation at 305 K (Figure S3). The presence of a hysteresis at a low rate suggests a first-order transition, and the absence of a recovery of LTP upon a fast rate scan indicates that it is possible to quench the high-temperature phase, which was subsequently confirmed by crystallographic and magnetic measurements (see later). We therefore label another phase to identify the quenched state by quenched HTP. The crystal structures in the different states were determined using several single crystals, all confirming that the transition is accompanied by a change in coordination of pn from its α-nitrogen atom for LTP to the β-one for HTP and DP. Given the existence of several thermodynamically stable states and the fact that interconversions between them are reversible, the protocol in Scheme 1 was adopted, where all experiments were performed on a single crystal. Crystallographic data for all phases of this crystal are listed in Table 1, and the bond distances and angles are listed in Table S1. X-ray structural analyses of all the intensity data collections indicate the conservation of space group P212121 in all the states.

Figure 1. Projections of the structures along the a-axis (left) showing the corrugation of the layers and the c-axis (right) showing the relative orientations of adjacent layers: LTP (top), HTP (middle), and DP (bottom). The two water molecules are differentiated by different colors (red and green).

planar but are corrugated and propagated along the b-axis in phase with neighboring layers. Viewing along the c-axis, which is perpendicular to the layers, we find that adjacent layers are staggered. The two remaining cyanide groups complete the octahedral coordination of the Cr atom and are pointing out of the layers while the amine and one water molecule are coordinated trans to each other to the Mn atom. Another water molecule is situated in the galleries between the layers. To balance the charges, a proton is needed, and on the basis of chemical grounds, it was thought to be on the noncoordinated nitrogen of the amine as in the previous report of the compound with the S enantiomer.36 We find that this proton, involved in unison by strong hydrogen bonding among the diamine, the coordinated water molecules, and the noncoordinated water molecules, is coupled to the transition from LTP and HTP. Important to note with respect to the structure of HTP is the fact that the amine is coordinated to the Mn atom via the nitrogen on the α-carbon. An important point to note is that during the transformation from LTP to HTP and vice versa new Bragg reflections appear next to certain existing reflections, indicating the presence of a D


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amine is coordinated to the Mn as that observed in the HTP structure. Mn(1) has four bridging cyanide nitrogen atoms [N(1)−N(4)] in equatorial positions, atom N(7) from pn, and atom N(5) of the bridging cyanide to complete its octahedral geometry. Heterometal (Cr···Mn) separations for DP are considerably short [5.092(3) and 5.189(4) Å], while LTP and HTP showed lengths of >5.24 Å. HTP and DP have the same space group but different structural motifs. The length of the caxis and the cell volume for DP [14.058(5) Å and 1452.8(9) Å3, respectively] are smaller than those of HTP [15.6319(15) Å and 1684.1(3) Å3, respectively]. The unit cell volume is decreased by 231.3 Å3 upon dehydration. Powder X-ray Diffraction. In situ measurements of powder X-ray diffraction confirm the observations made on the single crystals, and in addition, the measurements show the time dependence of the transformation of HTP to LTP (Figure S4). The time dependence of the diffraction peaks during rehydration showed a decreasing 002 peak of DP (2θ = 12.4°) and LTP (2θ = 11.7°) growing, but no peaks from HTP were observed. This experiment was performed at 302 K where HTP is stable, indicating hydration transforms DP to LTP directly (Figure S4f). Infrared Spectroscopy. The bands in the IR spectra of the three phases can be assigned using group frequencies (Figure S5 and Table S2). The asymmetric and symmetric stretching modes of the water molecules are observed as the highestenergy peaks at 3635 and 3541 cm−1, respectively. Its bending mode is observed at 1680 cm−1. All three bands are absent from the spectrum of DP, confirming the dehydration of LTP and HTP. The corresponding NH stretching modes and those of CH are observed at lower energies. We note that some sharpening of these bands is observed in the spectrum of DP compared to those of LTP, HTP, and HP, which may be associated with the presence of hydrogen bonds in the latter three. Only two sharp stretching modes of the cyanide are observed in the spectra of LTP, HTP, and HP, and the energies are the same for the three. However, for DP, there are two extra lower-intensity peaks that may be due to the difference between those within and between layers. A very strong peak at 474 cm−1 is seen in all spectra, which may be due to the Cr−C mode. UV−Vis Spectroscopy. The transmission UV−vis spectrum of a single crystal is shown in Figure S6. In contrast to the colors of the individual components, yellow for Cr(CN)6 and colorless for MnON5, that of LTP is green. However, the color is not so intense. The transmission spectrum of a single crystal shows three bands in the visible region. The green color is due to the window at 19 kK. The three bands are in addition to those of the individual components and, thus, are intervalence bands (Mn → Cr or Cr → Mn).81,82 Their presence can be explained by a simple molecular orbital scheme by considering that the crystal field splitting is much larger for the Cr(CN)6 center. The weak intensities of the bands suggest that they are spin-forbidden. The most likely spin-forbidden feature will be the Cr → Mn intervalence transitions. Therefore, we assign the lowest-energy band to the t2g (Cr) → t2g (Mn) transition centered at 13 kK, the intermediate-energy band to the t2g (Mn) → eg (Cr) transition at 16.5 kK, and the higher-energy band at 21 kK to the t2g (Mn) → eg (Mn) d−d transition. The fine structure in the lower-energy band (12826 and 12444 cm−1) may be due to splitting of the t2g levels as a consequence of the departure from octahedral symmetry.

new phase. The intensities of the new reflections grow at the expense of the original one. The presence of two superposed diffraction patterns of continuously changing intensities may be interpreted as the crystal contains both forms with possibly a traveling amorphous mixed state between them through the transition. Structures of HTP at 330 K (2), HTP at 298 K (3), and Quenched HTP at 200 K (5). The structure of the HTP phase has been determined at different temperatures to verify the reversibility as well as to confirm the thermodynamic order of the phase transition. Heat capacity has suggested that the transition is of first-order, and it was, therefore, possible to quench the crystal in the high-temperature phase by fast cooling; this allowed us to study the low-temperature magnetic properties (see below). The main feature of the structure of HTP remains the same as that of LTP for all three determinations, while the most striking difference is an increase in the c-axis [from 14.9 to 15.6 Å (4.6%)] from LTP to HTP (Table 1). The structural phase transition from LTP to HTP was then easily identified by collecting a 2D frame on the CCD as a function of temperature (see Movie 2). Within the 2D sheets, the average Mn−Cr separation of 5.3249(18) Å for HTP is slightly shorter than that for LTP [5.3457(16) Å]; the shortest intersheet metal separations are observed between the Mn and Cr atoms [7.3217(6) and 7.4696(8) Å for LTP and HTP, respectively], while the shortest intersheet homometallic contacts are greater than 8 Å. To our surprise, the most striking change was the way in which the amine is coordinated to the Mn atom. In contrast to LTP in which the nitrogen on the αcarbon makes a bond with the Mn, for HTP it is the one on the β-carbon. Structural phase transition is assumed to involve hydrogen bonding, which can be seen between 2D sheets. The noncoordinating nitrogen atom forms two hydrogen bonds with the oxygen atom of the coordinating water and crystal water. The oxygen atom of the coordinating water forms two more hydrogen bonds between nitrogen atoms of the cyanide group in neighboring layer. The consequence of the change in amine coordination is a change in pitch of the corrugation by 90°, which must be concerted, and therefore, the layer acts as a muscle in its relaxed and excited (stretched) states. Structure of DP (7) at 298 K. Preparation of the crystal for data collection of the dehydrated phase (DP) was performed by heating the single crystal to 360 K at a rate of 2 K/min and cooling it to 298 K in a nitrogen stream. Recording of the diffraction pattern as a function of temperature shows the consecutive transitions from LTP to HTP and then from HTP to DP. Interestingly, the Bragg reflections are doubled at the two transition temperatures, where the intensity of the original spots decreases at the expense of the new ones; this suggests the existence of both phases within the crystal. It may be regarded as a traveling front separating the two phases. While the Bragg spots are of the same size and shape on going from LTP to HTP, they are severely elongated for DP and HP. However, after data reduction and analyses, we were able to refine the structure for DP to an acceptable reliability factor. The X-ray structural analysis of DP reveals the same space group, P212121, but instead, it displays three-dimensional connectivity between Mn and Cr via the previously nonbridging cyanides (Figure 1). Both the coordinating and noncoordinating water molecules are absent from the structure, while the free cyanide takes the place of the departed water. This causes severe movements of the layers, and eventually, the layers stack in eclipse form when viewed along the c-axis. The E


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Figure 2. Proposed mechanism for the structural changes involving (top) a concerted proton transfer and water exchange (red and green oxygen atoms) and (bottom) evolution of the structure during the dehydration and the associated bond breaking and bond forming steps.

Mechanisms for the Structural Transformations. The structural transformations encountered in this study are unprecedented. We therefore provide in the following section a possible mechanism to explain the one major difference between the structures of LTP and HTP, that is, the way the amine is coordinated to the layer, to the α-carbon nitrogen for LTP and to the β-one for HTP (Figure 2). In the possible mechanism, a proton on a nitrogen atom moves along the aaxis by chain electrostatic collision, dissociating Mn−N and Mn−O coordination bonds. Subsequently, the primary nitrogen of amine and crystal water binds to Mn, resulting in the HTP structure. The “loose” proton is the key, and its hydrogen bonding with the water molecules is quite important and also defines the energy barrier (Figure 3). This transition is experimentally found to be of first-order, and therefore, all the events happen in a concerted fashion. In the dehydration process, all the water molecules leave the crystal and noncoordinating cyanide groups of the adjacent layer

coordinate to the vacant Mn site. The dehydrated form retains the connections to the amine as in the HTP structure, but when it is rehydrated at room temperature, it appears to return to LTP without passing through HTP, in agreement with the stability of the two phases above and below the transition temperature of 310 K. The phase transitions are represented pictorially in Figure 4.

Figure 4. Pictorial phase transformation diagram for the interconversion among LTP, HTP, and DP.

Magnetic Properties. All the magnetic data were collected by in situ measurements. At first, the magnetic measurements for virgin LTP were taken. The same experiments with quenched HTP, relaxed LTP, DP, and rehydrated HP were repeated, in turn. The temperature dependencies of ZFC−FC magnetization of all compounds are displayed in Figure 5. Under cooling conditions, χMT values at 300 K decrease to minimum values, increase to reach maximum values, and decrease again. Dehydrated DP shows a lower χMT value in the high-temperature region and a maximum value higher than that

Figure 3. Potential energy curves for the reversible transformation of LTP to HTP. F


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Figure 5. Temperature dependence of the ZFC−FC magnetizations in an applied field of 5 Oe (top) and isothermal magnetization at 2 K (bottom) for virgin LTP, quenched HTP, relaxed LTP, dehydrated DP, and rehydrated HP.

shortening distances between Mn and Cr atoms through the cyanide group. Hysteresis loops were observed for DP and HP at 2 K but not for LTP, HTP, or relaxed LTP. The observed saturation magnetization values for all phases are in the range of 2.00−2.07 μB, which are in good agreement with the theoretical value of antiferromagnetic coupling between Cr3+ and Mn2+ ions (5/2 − 3/2 = 1; MS = 2 μB).83−85 The largest observed coercive field among all phases is only 40 Oe in DP, describing all phases are soft ferrimagnets. Isothermal Magnetization on Single Crystals. All phases have different magnetic anisotropies. For LTP, the initial magnetization curves for the three principal crystallographic axes are shown in Figure S7. The easy, intermediate, and hard axes are the a-, b-, and c-axes, respectively. This magnetic anisotropy relationship is different from the two other phases. Quenched HTP and DP show a-, c-, and b-axes and b-, a-, and c-axes, respectively (Table 2). One noticeable change is the large field required to align the moment along the hard axis for DP compared to those of LTP and HTP. The reason for the intermediate axis rotation from LTP to HTP is not expected, while a change in the easy axis upon dehydration may not be so unusual because of the large structural change that accompanies the process where the homometal distance along the b-axis in DP is shortened. During the measurement, the hard and intermediate axis change from LTP to HTP was observed from magnetization angular dependence reversal, which was collected by in situ measurement. Dehydration was easily elucidated from the decrease in the magnetization value while the sample was being heated (Figure S8).

of the other phases, indicating stronger antiferromagnetic interaction between spin centers (Table 2). In the ZFC−FC Table 2. Magnetic Parameters for All Phases

C (cm3 K mol−1)a θ (K)b χMT (300 K) (emu K mol−1) Tmin (K)c χMT (Tmin) (emu K mol−1) Tmax (K)d TC (K)e M (2 K) (emu G mol−1)f Msat (μB)g HC (Oe)h MREM (μB)i easy axis [Hsat (Oe)]j intermediate axis [Hsat (Oe)] hard axis [Hsat (Oe)]

LTP

HTP

relaxed LTP

DP

5.98 −64.6 4.94

Powder 6.07 −73.0 4.94

5.91 −62.5 4.90

5.93 −83.3 4.69

6.25 −68.3 5.06

84 3.64

96 3.65

84 3.65

126 3.83

84 3.71

33 38 285

33 39 344

33 38 344

54 73 1471

33 38 614

2.05 40 1.128

2.00 20 0.267

2.07 5 0.062

2.06 2.07 4 6 0.055 0.074 Single Crystal a, 100 a, 50 b, 3000 c, 3500

b, 400 a, 2500

c, 4500

c,

b, 4500

rehydrated HP

16000

Curie constant. bWeiss constant. cTemperature at minimum χMT. d Temperature at maximum χMT. eCritical temperature based on FC measurements. fMagnetization at 2 K in the FC measurement (H = 5 Oe). gMagnetization saturation value at 2 K. hCoercive field at 2 K. i Remnant magnetization at 2 K. jField needed to saturate the magnetization at 2 K. a

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CONCLUSION [Mn(R-pnH)(H2O)][Cr(CN)6]·H2O proved to be a compound with a large number of functionalities, which include heterometallic mixed valency, chirality, ferrimagnetism, proton transfer, and surviving two single-crystal-to-single-crystal phase transitions reversibly without a great loss of crystallinity. One of the phase transitions is promoted by a rare proton transfer between amine/ammonium and water/hydroxonium couples that changes the coordination of the amine and water and requires a concerted change in the curvature of the corrugation of the Mn−CN−Cr layers. The initial change progresses through the crystal, mimicking the function of a muscle. Dehydration of the crystals introduces bridges between the layers, resulting in an increase in magnetic dimensionality, which in turn increases the Curie temperature from 38 to 73 K

measurement, a long-range magnetic ordering is observed below 38, 39, 38, 73, and 38 K for LTP, HTP, relaxed LTP, DP, and HP, respectively. While the structural phase transition marginally increases TC, dehydration increases it dramatically. A correlation is seen between Weiss constants θ and critical temperature TC, except in the case of rehydrated LTP. The number of interaction pathways explains the difference in the TC values. For LTP, the average metal separations between Mn and Cr atoms in 2D sheets for HTP is shorter than that for LTP. Therefore, HTP has more overlap between metal and cyanide ligand orbital than LTP, which gives a higher TC value for HTP. Meanwhile, the nearly doubled TC for DP is thought to be due to the increasing number of magnetic neighbors and G


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and adds a small amount of magnetic hardness. All these phenomena are absolutely reversible and partly confirmed by neutron scattering experiments.86

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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.chemmater.6b02852. Figures of TG and DT analyses and DSC, PXRD, IR, UV−vis, and magnetization data and tables of bond lengths and angles and IR band energies and assignments (PDF) Crystallographic data (CIF) CheckCIF/PLATON reports and ORTEP diagrams (PDF) movies of transformation (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written by Y.Y. and M.K. and revised by all authors, who have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by (a) a Grant-in-Aid for Scientific Research (S) (25220803, “Toward a New Class Magnetism by Chemically controlled Chirality”), (b) the Center for Chiral Science at Hiroshima University (MEXT program for promoting the enhancement of research universities, Japan), and (c) the JSPS Core-to-Core Program, A. Advanced Research Networks. M.K. is funded by the CNRS, France.

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