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.

All five coordinating atoms of the amino-acid dianion l-aspartate (l-asp = NH2CH(COO)CH2COO2–) are found to be involved in coordination with MnII in the presence of [CrIII(CN)6]3– to self-assemble into a chiral three-dimensional cyanide-bridged K3[Mn(l-asp)]6[Cr(CN)6]·2H2O containing the highest ratio of Mn:Cr of 6:1. It adopts the chiral P3 (no. 143) space group consisting of zigzag Mn–OCO–Mn chains sharing edges of hexagonal channels with central [Cr(CN)6]3–, while K+ and H2O occupy another parallel star-shaped channel. Its magnetic susceptibility above 100 K is dominated by the nearest neighbor (Mn–Cr at 5.08 and 5.31 Å) antiferromagnetic (AF) exchange interactions (θ = −15(1) K) and below 40 K by further AF interaction between Mn and Mn at 5.32 Å. It finally reaches a steady value at 4.5 K, where a bifurcation of the zero-field-cooled and field-cooled magnetizations is observed in small fields (<1 kOe). The isothermal magnetization is linear in field and deviating toward saturation above 60 kOe at 2 K. No imaginary component of the ac susceptibilities is observed. This behavior is associated with long-range antiferromagnetic order of a helical or conic nature where the magnetic sublattices are numerous [2n × (6Mn + 1Cr)], leading to a domain of sufficient size to allow for the presence of the bifurcation. A model is proposed based on the local anisotropy and symmetry multiplicity of the space group.

We report the exceptional observation of two different magnetic ground states (MGS), spin glass (SG, TB = 7 K) and ferrimagnet (FI, TC = 18 K), for one crystal structure of [{MnII(D/L-NH2ala)}3{MnIII(CN)6}]·3H2O obtained from [Mn(CN)6]3– and D/L-aminoalanine, in contrast to one MGS for [{MnII(L-NH2ala)}3{CrIII(CN)6}]·3H2O. They consist of three Mn(NH2ala) helical chains bridged by MIII(CN)6 to give the framework with disordered water molecules in channels and between the MIII(CN)6. Both MGS are characterized by a negative Weiss constant, bifurcation in ZFC-FC magnetizations, blocking of the moments, both components of the ac susceptibilities, and hysteresis. They differ in the critical temperatures, absolute magnetization for 5 Oe FC (lack of spontaneous magnetization for the SG), and the shapes of the hysteresis and coercive fields. While isotropic pressure increases both Tcrit and the magnetizations linearly and reversibly in each case, dehydration progressively transforms the FI into the SG as followed by concerted in situ magnetic measurements and single-crystal diffraction. The relative strengths of the two moderate MnIII–CN–MnII antiferromagnetic (J1 and J2), the weak MnII–OCO–MnII (J3), and Dzyaloshinkii–Moriya antisymmetric (DM) interactions generate the two sets of characters. Examination of the bond lengths and angles for several crystals and their corresponding magnetic properties reveals a correlation between the distortion of MnIII(CN)6 and the MGS. SG is favored by higher magnetic anisotropy by less distorted MnIII(CN)6 in good accordance with the Mn–Cr system. This conclusion is also born out of the magnetization measurements on orientated single crystals with fields parallel and perpendicular to the unique c axis of the hexagonal space group.

The effect of external pressure on the magnetic properties was studied for the first time for heterospin crystals based on the Cu(II) complex with nitroxide [Cu(hfac)2NN-PzMe], which exhibits a spin-crossover-like phenomenon. An increase in the hydrostatic pressure to 0.14 GPa caused a significant shift of the magnetic anomaly temperature (from 150 K to 300 K). This complex actually functions as a highly sensitive external pressure sensor.

An unusual high magnetic hardness for the layered perovskite-like (C2H5NH3)2[FeIICl4], in addition to its already found canted antiferromagnetism, ferroelasticity, and ferroelectricity, which are absent for (CH3NH3)2[FeIICl4], has been observed. The additional CH2 in the ethylammonium compared to methylammonium allows more degrees of freedom and therefore numerous phase transitions which have been characterized by single-crystal structure determinations from 383 to 10 K giving the sequence from tetragonal to orthorhombic to monoclinic (I4/mmm ↔ P42/ncm ↔ Pccn ↔ Pcab ↔ C2/c) accompanied by both tilting and rotation of the FeCl6 octahedra. The magnetic properties on single crystal and powder samples at high temperature are similar for both compounds, but at TN (C2H5NH3)2[FeIICl4] is a proper canted antiferromagnet unlike the hidden canting observed for (CH3NH3)2[FeIICl4]. The canting angle is 0.6° toward the c-axis, and thus the moments lie in the easy plane of the iron-chloride layer defined by a critical exponent β = 0.18. The isothermal magnetizations for the field along the three orthogonal crystallographic axes show wider hysteresis for H ∥ c and is present at all temperature below 98 K. The coercive field increases as the temperature is lowered, and at T < 20 K it is difficult to reverse all the moments with the available 50 kOe of the SQUID for both single crystal and powder samples. The shape of the virgin magnetization after zero-field-cool (ZFC) indicates that the high coercive field is due to domain wall pinning. Thus, there are unusual associated anomalies such as asymmetric hysteresis and history dependence. The difference in magnetic hardness of the two compounds suggests that magnetic, electric, and elastic domains are intricately manifested and therefore raise the key question of how the different domains interact.

We report the synthesis, crystal structure, and thermal and magnetic properties of the two-dimensional achiral soft ferrimagnet [MnII(enH)(H2O)][CrIII(CN)6]·H2O (1), en = 1,2-diaminoethane, as well as the recyclability of the dehydration and rehydration and their influence on the crystal structure and its magnetic properties. Unlike [Mn(S-pnH)(H2O)][Cr(CN)6]·H2O (2S, pn = 1,2-diaminopropane), which is a chiral (P212121) enantiopure ferrimagnet (TC = 38 K), 1 crystallizes in the achiral orthorhombic Pcmn space group, having a similar two-dimensional square network of Mn−Cr with bridging cyanide, and 1 behaves also as a soft ferrimagnet (TC = 42 K). X-ray diffraction experiments on a single crystal of 1 indicate a transformation from a single crystal to an amorphous phase upon dehydrataion and partial recovery of its crystallinity upon rehydration. The dehydrated phase 1-DP exhibits long-range ordering at 75 K to a ferrimagnetic state and coercive field at 2 K of 100 Oe, which are a higher critical temperature and coercive field than for the virgin sample (HC = 60 Oe). Thermogravimetric analyses indicate that the crystallinity deteriorates upon hydration−dehydration cycling, with persistence toward the amorphous phase, as also seen by magnetization measurements. This effect is associated with an increase of statistical disorder inherent in the dehydration−rehydration process. X-ray powder diffraction suggests that 1-DP may retain order within the layers but loses coherence in the stacking of the layers.

The achiral coordination polymer [MnII(rac-pnH)(H2O)CrIII(CN)6]·H2O, (rac-pn = racemic 1,2-diaminopropane), 1·2H2O, has been prepared, and its crystal structures, optical and magnetic properties have been studied before and after dehydration followed by rehydration. The in situ X-ray diffraction, performed on one selected single crystal, shows an unusual irreversibility from the as-prepared 1·2H2O to the dehydrated [Mn(rac-pnH)Cr(CN)6], 1, and reversibility from 1 to rehydrated [Mn(rac-pnH)(H2O)Cr(CN)6]·H2O 1-HP. Virgin 1·2H2O crystallizes in the monoclinic achiral P21/m space-group having a two-dimensional (2D) square-network of Mn−Cr with bridging cyanide, and behaves as a soft ferrimagnet (TC = 36 K). Dehydrated 1 has a three-dimensional (3D) network with an additional cyanide bridge between layers and adopts the orthorhombic achiral Pmnb space-group exhibiting a ferrimagnetic behavior (TC = 70 K). Rehydrated 1-HP (TC = 36 K) is poorly crystallized having the same unit-cell as 1·2H2O and reversibly transforms to the crystalline 1 (TC = 70 K). The dehydration is associated to a change in the coordination of the amine from one layer to its neighboring one involving a proton transfer, going from {Mn-NH2CH(CH3)CH2N′H3+·····(H2O)Mn′} for 1·2H2O to {Mn-NC·····NH3+CH(CH3)CH2H2N′−Mn′} for 1. The irreversible transformation of virgin single crystal 1·2H2O to single-crystal 1 is promoted by the availability of only one Mn in the vicinity of the cyanide while during the rehydration process the reversible single-crystal 1 to a glassy 1-HP is due to the presence of two equidistant Mn atoms, which is the cause of the disorder. The change in magnetism, that is, the increase of the Curie temperature and coercive field, is associated to the structural transformation from 2D to 3D.

The effect of external pressure on the magnetic properties was studied for the first time for heterospin crystals based on the Cu(II) complex with nitroxide [Cu(hfac)2NN-PzMe], which exhibits a spin-crossover-like phenomenon. An increase in the hydrostatic pressure to 0.14 GPa caused a significant shift of the magnetic anomaly temperature (from 150 K to 300 K). This complex actually functions as a highly sensitive external pressure sensor.