Pyrrolidinium-Based Cyanides: Unusual Architecture and Dielectric Switchability Triggered by Order-Disorder Process.

Two three-dimensional metal-organic compounds of the formula Pyr2KM(CN)6, where M = Co, Fe and Pyr = pyrrolidinium ((CH2)4NH2+), have been found to crystallize at room temperature in a monoclinic structure, space group P21/c. They are cyano-bridged compounds with an unprecedented type of architecture containing pyrrolidinium cations in the voids. The materials have been investigated by X-ray diffraction, dielectric, and spectroscopic methods as a function of temperature in order to determine their properties and the mechanism of the reversible phase transitions occurring at ca. 345-370 K. The phase transitions in both crystals are first order and are associated with a symmetry increase to a rhombohedral structure (space group R3̅m) as well as a significant disorder of organic cations above Tc. On the basis of Raman scattering and IR spectroscopy it has been assumed that the phase transition in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a "click-in" of the cyanide bridges. The materials have been proposed as possible switchable dielectrics due to their respective high differences in dielectric permittivities across the phase transition.

Introduction

The hybrid organic–inorganic perovskites based on diatomic and multiatomic bridges, which are built of MIIO6 or MIIN6 octahedra (MII = Cd, Mg, Zn, Mn, Co, Fe, Cu, Ni) connected by short organic linkers (for instance, HCOO–, N3–, CN–, N(CN)2–, and HPO2–), have been extensively studied in recent years due to their magnetic, ferroelectric, multiferroic, ferroelastic, switchable dielectric, and barocaloric properties.[1−13] The characteristic feature of these materials is the presence of large voids occupied by organic ions. The properties of these compounds have been determined, among others, by the size, shape, and ability of the organic cation to form hydrogen bonds (HBs) with the anionic framework. For instance, the polar order and switchable dielectric properties appear due to ordering–disordering of these organic cations.[2,9,13−16]

Hybrids with CN– ligands have been less studied in comparison to the formate analogues. Up to now, four A2KCo(CN)6 (A = methylammonium (MA), dimethylammonium (DMA), azetidinium (AZE), formamidinium (FM)) and eight A2KFe(CN)6 (A = MA, DMA, trimethylammonium (TrMA), tetramethylammonium (TMA), guanidinium (GA), acetamidinium (Ace), imidazolium (HIm), trimethylammonium oxide ((CH3)3NOH, TrMO)) cyanides have been synthesized.[15−23] The research on MA, TrMA, TMA, AZE, HIm, TrMO, and FM analogues has shown that these compounds exhibit single order–disorder phase transitions (PTs) accompanied by a distortion of the metal cyanide frameworks.[15−21] These transitions lead to steplike dielectric anomalies with a change in the real part of the dielectric permittivity ε′ that is typically below 10.[15,19,21] Much larger changes, about 18 at 1 MHz and 30 at 2 MHz, were reported only for (Him)2KFe(CN)6 and (MA)2KCo(CN)6, respectively.[17,20] In contrast to this behavior, two-step order–disorder PTs were reported for (DMA)2KFe(CN)6, (Ace)2KFe(CN)6, and (GA)2KFe(CN)6.[18,22] The LT PTs in (DMA)2KFe(CN)6 and (Ace)2KFe(CN)6 and HT PT in (GA)2KFe(CN)6 lead to weak steplike anomalies of ε′ (less than 10).[18,22] An increase in ε′ has also been noticed at HT PT in (DMA)2KFe(CN)6, but the value of this change is not clear, since the PT occurs only a few degrees below the decomposition temperature.[18] It is worth adding that for (MA)2KCo(CN)6 the steplike anomaly strongly increases when the frequency changes from 1 kHz to 135 Hz, and this behavior has been attributed to a large contribution of the ac conductivity.[17] Furthermore, Fe analogues have often shown the presence of dielectric relaxation[17,18] but in the case of Co-based compounds dielectric relaxation has only been reported for (FA)2KCo(CN)6.[21]

Herein, we report the synthesis and detailed studies of two Co and Fe cyanide frameworks (for simplicity referred to hereafter as 1Co and 2Fe) templated with pyrrolidinium (Pyr+) cations. The Pyr+ cations have not yet been employed in a synthesis of any perovskite-like materials. We will show that these compounds crystallize in an architecture different from that other cyanides and they exhibit switchable dielectric properties and unusually large frequency dependence of the steplike anomaly. We will discuss the origin of this behavior.

Experimental Details

Synthesis

All reagents (analytical grade) used for the synthesis were commercially purchased from Sigma-Aldrich and used without further purification. In order to grow single crystals of 1Co and 2Fe a mixture containing 2.6 mL of pyrrolidine (30 mmol) and 20 mL of water was neutralized with about 3 mL of hydrochloric acid. Then 5 mmol of K3M(CN)6 was dissolved in this solution on a hot plate at 50 °C with stirring. After 3 h the heating was switched off and the solutions were allowed to stand at room temperature. The rhombic crystals (dark orange for 2Fe and pale yellow for 1Co) were harvested after 1 week. A comparison of their powder XRD patterns with the calculated patterns based on the single-crystal data attest to the phase purity of powdered samples (Figure S1 in the Supporting Information).

X-ray Diffraction

The powder X-ray diffraction was measured on an X’Pert PRO powder diffractometer operating with Cu Kα radiation. The diffractograms were collected in the Bragg–Brentano geometry using fixed-divergence slits.

The single-crystal X-ray diffraction experiments were carried out with Mo Kα radiation using Xcalibur (Atlas). The absorption was corrected by Multiscan method in CrysAlis PRO 1.171.38.43 (Rigaku Oxford Diffraction, 2015). The empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was applied. The structures were solved by direct methods in SHELXT and refined with SHELXL2014/7.[23] The hydrogen atoms were placed in calculated positions and refined as riding atoms. Due to the complex disorder of protonated amines the hydrogen atoms were not introduced during the refinement of HT phases. The data have been assigned the following CCDC (Cambridge Crystallographic Data Centre) deposition numbers: 1969449–1969452.

Calorimetric and Dielectric Measurements

The heat capacity was measured using a Mettler Toledo DSC-1 calorimeter with a high resolution of 0.4 μW. Nitrogen was used as a purging gas, and the heating and cooling rate was 5 K/min. The excess heat capacity associated with the PT was evaluated by subtraction of the baseline, representing variation in the absence of the PTs, from the data.

The dielectric measurements were performed every 1 K using a Novocontrol Alpha impedance analyzer. The temperature was controlled by the Novo-Control Quattro system, by using a nitrogen gas cryostat. The temperature stability of the samples was better than 0.1 K. The dimensions of the single crystals with crystallographic orientations perpendicular to the (011) plane were 1.7 × 1.2 × 0.7 mm3 for 2Fe and 1.9 × 1.3 × 0.6 mm3 for 1Co, respectively. Silver paste was used to ensure a good electrical contact. The ac voltage with an amplitude of 1 V and frequency in the range 1 kHz to 1 MHz was applied across the sample. Each switching cycle was registered as time-dependent dielectric permittivity for 30 min at two constant temperatures. The temperature ramp between these temperatures was kept as 5 K/min.

IR and Raman Spectroscopy

The temperature-dependent IR spectra were measured in a heating mode on a KBr pellet in the range 4000–550 cm–1 using a standalone Nicolet iN10 microscope equipped with a liquid nitrogen cooled mercury–cadmium–telluride detector, permanently aligned 15× objective, and 0.7 numerical aperture with the working distance set at 16 mm. The Raman spectra were measured in a heating mode using a Renishaw InVia Raman spectrometer equipped with a confocal DM 2500 Leica optical microscope, a thermoelectrically cooled CCD as a detector, and an Ar+ laser operating at 488 nm. Due to the bad quality of the signal from 2Fe crystals, only the 1Co sample was measured. The temperature of the samples during Raman and IR measurements was controlled applying a Linkam THMS 600 Heating/Freezing Stage. The spectral resolution in Raman and IR measurements was set at 2 cm–1.

Results and Discussion

DSC

The DSC measurements reveal one heat anomaly at 368 K (361 K) for 1Co and at 354 K (346 K) for 2Fe upon heating (cooling) (Figure and Figure S2). The symmetrical shape of these anomalies and significant thermal hysteresis point to first-order character of the PTs. The associated change in enthalpy ΔH and entropy ΔS are estimated for 1Co to be ∼4.7 kJ mol–1 and ∼13.3 J mol–1 K–1, respectively. Similar values, ∼4.3 kJ mol–1 and ∼13.2 J mol–1 K–1, respectively, are found for 2Fe. For an order–disorder transition ΔS = R ln N, where R is the gas constant and N is the ratio of the number of configurations in the disordered and ordered phases. The estimated N value associated with heat anomalies is about 4.9. A very large value of N indicates that the HT phases are strongly disordered. In the FM analogues the entropy values are on the order of 40 J mol–1 K–1, while for an HIm-cyanide crystal it amounts to ca. 6.6 J mol–1 K–1.[20,21]

Figure 1

Temperature dependence of (a) Cp and (b) ΔS related to the PTs in 1Co (circles and squares) and 2Fe (solid lines) in heating (red) and cooling (blue) runs.

Crystal Structures

The crystal structures of 1Co and 2Fe are isomorphic at RT as well as after the PT to the HT phase (Table ). Both compounds crystallize in the monoclinic system with P21/c symmetry; however, the architecture differs from those of other known cyanides. The asymmetric unit contains two M atoms located at the inversion center (Ci), one K, six cyanide C≡N ligands, and two ordered Pyr+ cations. All atoms except M occupy general positions of C1 symmetry. In the inorganic part of the structure each M(1) is surrounded by six K centers at a distance of ∼5.9 Å similarly to other cyanides,[21] forming (011) layers with a pseudotrigonal atom arrangement. The layers are linked by nodes consisting of M(2)–(CN)6, that possess only two K neighbors at a much reduced distance of 4.0 Å (Table S1). This yields a 3D arrangement with pronounced channels expanding in the b direction, which are populated with Pyr+. This architecture resembles the pillar-layered structures,[24] though the pillars here are strictly inorganic. Figure a,b shows the topology of 1Co and 2Fe; Figure c highlights the crystal voids. The cyanide ligands compose a distorted-octahedral environment around metal centers in both 1Co and 2Fe and M(2) links with two neighboring K centers via three cyanide bridges that strongly shapes the spheres of both KN6 and M(2)C6.

Table 1

Experimental Details of Single-Crystal X-ray Diffraction Experiments

	1Co phase I	1Co phase II	2Fe phase I	2Fe phase II
Crystal Data
chem formula	C14H20CoKN8	C14H20CoKN8	C14H20FeKN8	C14H20FeKN8
Mr	398.41	398.41	395.33	395.33
cryst syst, space group	trigonal, R3̅m:H	monoclinic, P21/c	trigonal, R3̅m:H	monoclinic, P21/c
temp (K)	375	295	365	281
a, b, c (Å)	8.8604(4), 8.8604(4), 41.342(2)	14.3134(5), 8.9546(2), 15.0150(5)	8.8920(4), 8.8920(4), 41.596(2)	14.4144(5), 8.9694(2), 15.0356(5)
α, β, γ (deg)	90, 90, 120	90, 107.216(3), 90	90, 90, 120	90, 107.297(3), 90
V (Å3)	2810.8(3)	1838.26(10)	2848.3(3)	1856.02(10)
Z	6	4	6	4
μ (mm–1)	1.15	1.17	1.03	1.05
cryst size (mm)	0.23 × 0.19 × 0.1	0.23 × 0.19 × 0.1	0.22 × 0.17 × 0.05	0.30 × 0.21 × 0.08
Data Collection
Tmin, Tmax	0.824, 1.000	0.927, 1.000	0.870, 1.000	0.950, 1.000
no. of measd, indep, and obsd (I > 2σ(I)) rflns	9641, 718, 612	19241, 3755, 2940	3725, 725, 517	27497, 3543, 2352
Rint	0.022	0.024	0.031	0.049
(sin θ/λ)max (Å–1)	0.609	0.625	0.610	0.625
Refinement
R(F2 > 2σ(F2)), Rw(F2), S	0.103, 0.336, 1.12	0.036, 0.098, 1.11	0.075, 0.231, 1.11	0.052, 0.135, 1.09
no. of rflns	718	3755	725	3543
no. of params	44	221	68	220
no. of restraints	8	12	6	12
H atom treatment	H atom params not defined	H atom params constrained	H atom params not defined	H atom params constrained
Δρmax Δρmin (e Å–3)	1.59, −0.91	0.66, −0.42	0.38, −1.11	0.56, −0.51

Figure 2

Pyr2KM(CN)6, LT phase II, P21/c. (a) The topology of MK cyanides. (b) A pseudotrigonal arrangement of M(1)–K layers. (c) Metal cyanide framework with an octahedral environment of metal ions. The channels in the structure are highlighted by blue shading. (d) In LT phases there are two inequivalent Pyr+ cations, both interacting with the metal cyanide framework through the N–H···N HBs.

In the RT phase (I) the cations are ordered and anchored in the structure through N–H···N HBs to the cyanide nitrogen atoms. Figure d shows the close surrounding of symmetry-independent Pyr+. In both 1Co and 2Fe each Pyr+ forms one strong HB with the metal cyanide framework with a donor–acceptor (D-A) distance of 2.88(1) Å (the detailed values are given in Table ). The remaining hydrogen atoms from NH2 groups are involved in weaker interactions with D–A distances ranging from 3.07(1) to 3.35(1) Å. The respectively large atomic displacement parameters for ring carbons indicate a certain freedom of movement of the cations in this phase, possibly thermally activated.

Table 2

Selected HB Parameters

D–H···A	D–H (Å)	H···A (Å)	D···A (Å)	D–H···A (deg)
Co Phase II
N1A–H10A···N1	0.89	2.48	3.065(3)	123.7
N1A–H10B···N6a	0.89	2.00	2.875(3)	167.1
N2A–H13A···N3a	0.89	2.03	2.876(4)	159.3
N2A–H13B···N2b	0.89	2.58	3.351(4)	145.6
Fe Phase II
N1A–H10A···N1	0.89	2.50	3.076(5)	122.7
N1A–H10B···N6a	0.89	2.02	2.890(5)	166.5
N2A–H13A···N3a	0.89	2.04	2.888(6)	160.2
N2A–H13B···N2b	0.89	2.57	3.344(6)	145.8

Symmetry code: x, −y + 1/2, z – 1/2.

Symmetry code: −x, −y + 1, −z.

The PT, recorded at Tc = 368/361 K in 1Co and Tc = 354/346 K in 2Fe, is accompanied by an increase of symmetry to rhombohedral, R3®m. The new unit cell is related to the LT cell by the [1 −1 0 1 1 0 −1/3 1/3 1/3] matrix. The Z value increases from 4 to 6 in phase I. Thus, despite the symmetry increase the asymmetric unit contains, similarly to phase II, the same number of metal atoms: two independent M ions, here at –3m (D3) symmetry, and K at a 3m (C3) site. The presence of a 3-fold axis defines the environment of the metal centers. The number of symmetry-independent C≡N groups is reduced to two, each one being linked to one individual M center. The cyanide bridges that connect M(2) nodes with two neighboring K centers have much larger atomic displacement parameters in comparison to those of M(1).

The increasing temperature activates the thermal motions of Pyr+. Similarly to LT phase, there are two different ammonium cations; however, in phase I they are both disordered. One Pyr+ may adopt at least three various positions. As the nitrogen atom is located on a 3-fold axis these positions are related by the rotation of carbon ring atoms around the axis. Additionally, the ammonium nitrogen atom significantly leans out of the plane defined by the ring carbons. In 2Fe the disorder is even larger; a satisfactory refinement is obtained with an almost spherical distribution of carbon and amine nitrogen atoms. Figure presents the packing of phase I and the resolved disorder of both ammonium cations as well as the environment of Co(2) and K metals with the triple cyanide bridges.

Figure 3

Pyr2KCo(CN)6, HT phase I. Views of the crystal structure along the (a) [110] and (b) [001] directions. (c) There are two distinct Pyr+ cations in I; both are disordered via a 3-fold axis, and the 3-fold symmetry also determines the arrangement of cyanide ligands around metal centers.

The disorder related to the presence of the 3-fold axis is apparent in many crystals accommodating nicollite or hexagonal symmetries.[2,27,28] Usually, the disorder is dynamic and is thermally activated. Freezing of the cation motions along a decrease in temperature induces PTs. The LT phases of pyrrolidinium-based organic–inorganic hybrids often possess ferroelectric or even multiferroic properties.[27,28]

Dielectric Studies

For both investigated compounds the steplike anomalies of the real part (ε′) of the dielectric permittivity are observed around Tc in the heating and cooling modes, revealing a reversible PT (Figure ). With the LT phase as a starting point, the ε′ value increases from about 15–17 before the temperature reaches the PT point, corresponding to the low dielectric state. A further temperature increase results in sharp steplike dielectric anomalies to ∼23–25 at Tc, corresponding to a high dielectric state. Such a sharp dielectric anomaly around Tc is a typical characteristic for switchable dielectric materials. For 1Co the dielectric permittivity ε′ value in the high dielectric state is roughly 1.5 times of that in the low dielectric state. Moreover, for all frequencies this difference remains unchanged. In the case of 2Fe above Tc a significant dispersion can be noticed, resulting in strong changes of ε′ with decreasing probing frequency.

Figure 4

Temperature dependence of the real part of the dielectric permittivity (ε′) measured for 1Co (a) and 2Fe (b) single crystals at selected frequencies during the heating cycle. The inset presents the heating (red) cycles and cooling (blue) cycles for f = 1 MHz.

Generally, such behavior in the frameworks based on cyanide ligands is related to structural changes starting from the phase where organic cations are frozen to the orientationally disordered phase, which consequently leads to a significant change in the dielectric constant.[14,20−32] Thus, the difference in the observed frequency dispersions of 1Co and 2Fe indicates the disparate range of resonant frequencies attributed to the dynamic motion of the organic cation. Taking the above findings into consideration, we can conclude that for 2Fe the interaction between the Pyr+ and the framework is stronger in comparison to that in the 1Co crystal, which is consistent with the crystallographic data. The PT temperatures match the DSC results well, which clearly indicates the reversible thermally triggered structural PTs. A small thermal hysteresis observed during the heating and cooling processes is comparable with those observed in the DSC results.

The desired switchable behaviors of dielectric permittivities between the high and low dielectric states in the investigated compounds classify them as potential HT switchable materials (see Figure and Figure S3). Close to discontinuous switching of the ε′ value between the two stable dielectric states is related to rapid symmetry breaking during the structural PT, resulting from the order–disorder transition of Pyr+ and the deformation of the framework. It can be noticed that both 1Co and 2Fe crystals exhibit a tendency to stable resistance to fatigue. As illustrated in Figure , ε′ switching is stable and remains unchanged after at least three cycles. In each cycle, the low and high dielectric states represent “switch off” and “switch on”, respectively. When the temperature goes through the PT point, ε′ increases quickly from the low dielectric state to the high dielectric state, and when the temperature is lowered, the ε′ value decreases reversibly from the high dielectric state to the low dielectric state. With the temperature repeatedly changing above or below the PT temperature, dielectric permittivity switching is observed. The repetition of ON/OFF cycles suggests the possible electronic application of the investigated compounds.

Figure 5

Reversible dielectric switching (ON and OFF) of ε′ between HT and LT phases measured for 1Co at a probing frequency of 1 MHz.

Raman Scattering and IR Studies

Table S2 shows the results of the factor group analysis for both 1Co and 2Fe samples, while the observed IR and Raman modes along with their assignment are given in Table S3. IR and Raman spectra in the full range are presented in Figures S4 and S5 for 1Co as well as in Figure S6 for the 2Fe analogue, respectively. The details of the IR and Raman spectra for 1Co are presented in Figures and 7, while details of IR spectra for 2Fe are given in Figure S7. The assignment has been proposed on the basis of the literature data available for other similar compounds: cyanides and the compounds containing Pyr+ as well as a related cyclopentane.[21,33−37] The thermal evolution of the bands, as described below, is based mainly on the changes of the 1Co compound due to the fact that both cyanides are isostructural and behave in a similar fashion upon temperature change. Figure and Figures S8 and S9 present the temperature-dependent evolution of the position and/or the bandwidth (fwhm) of several selected modes of 1Co and 2Fe crystals, respectively, in order to get insight into the mechanism of the transitions.

Figure 6

Details of the temperature-dependent IR spectra of 1Co.

Figure 7

Details of the temperature-dependent Raman spectra of 1Co.

Figure 8

Plots of the wavenumber and fwhm vs T of the selected modes (a) νs(NH2) (IR), (b) νas(CH2) (Raman), (c) ν(CN–) (IR), (d) ω(NH2) (IR), (e) ring + ρ(CH2) (Raman), and (f) ρ(NH2) (Raman) in 1Co.

There are two types of HBs in 2Fe and 1Co samples, i.e. medium and strong,[38] one with a donor–acceptor distance of ca. 3.07–3.35 Å being located at roughly 3200 cm–1, while the other is present at about 2800 cm–1 with a length equal to ca. 2.88 Å. The Raman and IR spectra of the studied compounds show many characteristic changes as a function of temperature.

First, narrowing of many IR and Raman bands on a decrease in temperature is observed for both compounds, with a clear discontinuous jump at the PT temperature (Figures –8 and Figures S4–S7 and S9). This is especially characteristic of the bands assigned to the NH2 vibrations, such as ν(NH2) stretching modes present in the region 3260–2730 cm–1 (Figure a and Figure S9a for 1Co and 2Fe, respectively), δ(NH2) bending modes present at ca. 1630–1590 cm–1 (Figure and Figure S7 for 1Co and 2Fe, respectively), ω(NH2) and τ(NH2) bending modes at 1400–1230 cm–1 (Figures , 7, and 8d,f for 1Co and Figures S7 and S9d for 2Fe), and ρ(NH2) bending modes at 880–820 cm–1 (Figures , 7, and 8f for 1Co and Figures S7 and S9d for 2Fe). This behavior is consistent with an ordering of the Pyr+ cations. Thus, temperature-dependent Raman and IR spectra confirm the order–disorder mechanism of the PTs, similarly to those found in DMA metal formates or the FA-based perovskites mentioned above.[21,39,40] It worth adding that the change in fwhm values is not as distinct as that observed before for other similar compounds. For instance, the Raman mode of 1Co at ca. 880 cm–1 presents a 1.3 increase in fwhm on going from the LT to HT phase against a 4-fold increase in FA2KCo(CN)6.[21] Interestingly, many bands of the studied compounds remain broad below Tc and exhibit further significant narrowing on cooling (see for instance the temperature dependence of fwhm for the ring + ρ(CH2) and ρ(NH2) Raman-active modes in Figure e,f), which indicates that the Pyr+ cations possess a large degree of motional freedom even much below Tc.

Second, many bands split into a few components with a decrease in temperature, which is consistent with the lowering of symmetry. Furthermore, many bands exhibit abrupt shifts at Tc. For some modes related to HBs these shifts are as distinct, as in the case of perovskite crystals studied by us before: e.g., formamidinium Mn-formate, (FA)2KM(CN)6, DMA-based formates, and azides.[21,39,40,42] For example, the NH2 stretching mode, observed near 3040 cm–1 in the HT phase, shifts to lower wavenumbers in the LT phase (by ca. 20 cm–1, Figure a and Figure S9a for 1Co and 2Fe, respectively), while the ω(NH2) mode exhibits an upshift of about 33 cm–1 (Figure d). The maxima of the remaining HB-related modes experience lesser shifts, though much stronger than those reported for other systems: e.g., FA-based cyanides. This implies that the phase transition leads to a significant rearrangement of the HB network. It is important to note that, in spite of rapid movements of the organic cations at elevated temperatures and the rearrangement of HBs, there is still a division of their strength into medium and strong, as the two groups of the bands originating from HBs are preserved. In contrast to the NH2 modes, the ring vibrations show weaker wavenumber shifts at Tc (see Figure b,e and Figure S9c for 1Co and 2Fe, respectively). These shifts go along with the shortening of the C–H bonds within Pyr+ cations in the LT phase. Thus, all the facts suggest that the ordering of Pyr+ cations below Tc strongly influences their conformation. This behavior has been attributed to a respectively high flexibility of the ring.[36]

Third, the characteristic feature is the continuous wavenumber shift of the bands well ahead of their phase transition point. This refers to both the pyrrolidinium cations and cyanide ions, unlike the case of FA analogues or azide systems.[40,41] (see Figure b,e and Figure S9c for 1Co and 2Fe, respectively, for C–H vibrations). This difference indicates that the change in temperature has a much stronger effect on pyrrolidinium–framework interactions in comparison to cyanide- and azide-based perovskite-like materials. Such behavior can be most likely be attributed to the conformation of the Pyr+ cation that allows large out-of-plane motions of atoms. As a result, slowing down of thermally activated motions upon cooling significantly affects HB strength and thus chemical bonds and angles in the organic cation and metal cyanide frameworks.

Fourth, the PTs in both crystals lead to pronounced vibrational changes in the metal cyanide frameworks (see Figures , 7, and 8c and Figure S8a for 1Co, and Figures S6 and S9b for 2Fe, respectively). For instance, the stretching mode observed at ca. 2125/2150 cm–1 in the IR/Raman spectrum of 1Co shifts by roughly 6/10 cm–1 in the whole temperature range with a noticeable wavenumber upshift (∼1 cm–1 and fwhm decrease at the PT point during cooling (Figure c and Figure S8a for 1Co and Figure S9b for 2Fe). The upshift informs us about the shortening of C≡N bonds, since the voids need to adjust to the ordering of the organic cations. A large shift in the whole temperature range, significantly larger in comparison to the other cyanide or azide analogues,[21,41,42] indicates a greater softness of the studied compounds. The fwhm of the stretching mode is much smaller in the HT phase (about 7.7 cm–1, Figure c) than fwhm values of Pyr+ modes (13–16 cm–1, Figure e,f). This behavior is consistent with the fact that the cyanide linkers are ordered. Interestingly, the change in fwhm at Tc is quite large, about 2.5 cm–1. This behavior indicates large thermal movements of the CN– group atoms in the HT phase and freezing of these movements in the LT phase.

Conclusions

Two metal–organic compounds of the formula [Pyr2KM(CN)6], where M = Co, Fe, have been found to crystallize in an unusual architecture, with channels expanding in the b direction filled with Pyr+ cations. They have been investigated by X-ray diffraction, dielectric, and spectroscopic methods in order to determine the mechanism of the PTs occurring within the ca. 345–370 K range. According to the obtained results they are first order and both crystals adopt isostructural architectures regarding LT and HT phases. To our knowledge this is an unprecedented representation of pyrrolidinium-templated compounds containing cyanides, related to the known cyano-bridged perovskite compounds and other metal–organic perovskite analogues.

The desired switchable behaviors of dielectric permittivities between the high and low dielectric states in the investigated compounds classify them as potential HT switchable materials. What is more important, the materials seem to be resistant to fatigue, and this indicates their possible electronic application.

Vibrational spectroscopy has been a very useful tool in the investigation of the mechanisms of PTs in these systems (though rarely used so far), as the most intense stretching band of the cyanides serves a perfect probe due to its isolation from the other bands as well as to the fact that both IR and Raman spectroscopy are sensitive to slight changes in bond lengths imposed by a crystal distortion. It has been observed that the movements of HBs are active well below the PT point, which indicates their dynamic nature even at LT. The Pyr+ cations and specifically the HBs undergo drastic modifications at Tc and significant changes on a further temperature decrease below Tc. The thermal evolution of bands assigned to the cyanide ions is more pronounced than in the case of other cyanide-based perovskites. This behavior can be attributed to a softer nature of the studied compounds and a presence of strong HBs, which binds the Pyr+ cations and one of the cyanide linkers quite tightly. Therefore, we assume that the PT in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a “click-in” of the cyanide bridges.