Article pubs.acs.org/IC
Spin Crossover in Fe(II)−M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines Olesia I. Kucheriv,†,∇ Sergii I. Shylin,†,‡,∇ Vadim Ksenofontov,‡ Sebastian Dechert,§ Matti Haukka,∥ Igor O. Fritsky,† and Il’ya A. Gural’skiy*,† †
Department of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska St. 64, 01601 Kyiv, Ukraine Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University of Mainz, Staudingerweg 9, D-55099 Mainz, Germany § Institute of Inorganic Chemistry, Georg August University of Göttingen, Tammannstr. 4, D-37077 Göttingen, Germany ∥ Department of Chemistry, University of Jyväskylä, PO Box 35, FI-40014 Jyväskylä, Finland ‡
S Supporting Information *
ABSTRACT: Discovery of spin-crossover (SCO) behavior in the family of FeII-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe2+, square planar tetracyanometalates MII(CN)42− (MII = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes FeII ion has a pseudo-octahedral coordination environment supported by four μ4tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and MII = Ni (1), Pd (2), Pt (3); X = Me and MII = Ni (4), Pd (5)) or one 2X-pyrazine and one water molecule (X = I and MII = Ni (7), Pd (8), Pt (9)), or, alternatively, two distinct FeII positions with either two pyrazines or two water molecules (X = Me and MII = Pt (6)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN6 units (1−6) display cooperative spin transition, while FeII ions in N5O or N4O2 surrounding are high spin (HS). Structural changes in the nearest FeII environment upon low-spin (LS) to HS transition, which include ca. 10% Fe−N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of 1−9 (see abstract figure). Effects of a pyrazine substituent and MII nature on the hyperfine parameters in both spin states are established.
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chromic materials,11 microthermometry,12 switchable metalomesogens, 13 chemical sensors, 5 actuators, 7,8 chiral switches,14,15 anticounterfeit paper,16 etc. Hofman clathrates and their analogues are among the most known, well studied, and perspective for practical applications SCO compounds.17 They are bimetallic three-dimensional (3D) and two-dimensional (2D) coordination frameworks constructed of FeII ions supported with cyanometallic anions [M(CN)x]y− (where M = Ni, Pd, Pt, Cu, Ag, Au, Nb)17,18 and N-donor heterocyclic ligands. They represent a group of materials with exceptionally abrupt and hysteretic SCO; some of them have been prepared as single crystals, nanoparticles,19 thin films,20 patterns,21 and gratings.22 As well, because of the presence of big guest accessible pores, which is typical for some of Hofman clathrates and their analogues, the SCO behavior of these compounds is sensitive to the inclusion of guest
INTRODUCTION Spin crossover (SCO) is a spectacular ability of some 3d metal complexes to exist in two different spin states that can be triggered under the influence of such external stimuli as temperature,1 pressure,2 light irradiation,3 magnetic field,4 or absorption5 of different chemical compounds. The rational choice of ligands is a key factor to design coordination compounds with such switchable behavior. In Fe(II) two different electronic configurations (t2g6eg0 for a low spin (LS) and t2g4eg2 for a high spin (HS)) can be stabilized depending on the iron-ligand bond length and associated ligand field strength.6 Because of the cooperativity of spin transition a hysteretic behavior is frequently observed. Switching between two states may involve drastic change of magnetic, optical,6 mechanical,7,8 and electrical properties.9,10 In this way, a presence of the hysteresis loop leads to a molecular memory of the material that is the main requirement for construction of molecular devices. Recent achievements in study of SCO offer numerous potential applications of this phenomenon: thermo© 2016 American Chemical Society
Received: February 23, 2016 Published: April 27, 2016 4906
DOI: 10.1021/acs.inorgchem.6b00446 Inorg. Chem. 2016, 55, 4906−4914
Article
Inorganic Chemistry molecules.17 Such sensitivity gives characteristics required for novel chemoresponsive device components to these porous metal−organic frameworks. In fact, the main feature responsible for the variety of these complexes is a vast majority of potential organic ligands to design these frameworks. Pyridine,23 halogenopyridines,24 aminopyridine,25 pyrazine,26 bis(4-pyridyl)acetylene,27 4,4′di(pyridylthio)methane,28 azopyridine,29 1,2-di(4-pyridyl)ethylene,30 pyrimidine,31 5-bromopyrimidine,32 and some others33−44 (that are mostly represented by pyridine derivatives) have been reported as coligands in SCO Hofmann clathrates analogues. Pyrazine is one of the simplest μ2-bridging systems among these ligands. Its potential of 1,4-binding leads to the separation of FeM(CN)4 (M = Ni, Pd, Pt) layers by ∼7 Å26 and formation of compact frameworks with pores accessible for small molecules.10,32,46 Transition temperature in [Fe(pz)Pt(CN)4] has been significantly increased by partial oxidation of Pt via chemical adsorption of halogens (T1/2 = 264 K for Cl, 309 K for Br, 382 K for I) in the framework.47,48 Some other Hofman clathrates analogues are not guest sensitive in their SCO behavior; for example, in case of aminopyridine and M = Ni (T1/2 = 161 K), Pd (T1/2 = 188 K), Pt (T1/2 = 198 K) formation of dense frameworks without guest inclusion has been observed.25 The main advantage of the complexes with this feature is highly reproducible SCO behavior that depends on neither guest incorporation and solvent loss upon numerous thermal cycles nor solvent-related polymorphism. Curiously enough, in some selected clathrates analogous two-step49 or even three-step transition28 accompanied by hysteresis is observed that offers a way to “multistability” in these SCO systems. Here we describe new SCO analogues of Hofman clathrates that exploit 2-substituted pyrazines50 (chloro-, iodo-, and methyl-), taking into account that the modification of pyrazine can influence not only the structure of a final complex but also a spin state of Fe via tuning the resulting ligand field on the metal ion.
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18.77; found C, 25.47; H, 1.88; N, 18.41%; for C8H5IFeN6NiO (7): calcd. C, 21.71; H, 1.14; N, 18.99; found C, 21.85; H, 1.23; N, 19.15%; for C8H5IFeN6PdO (8): calcd. C, 19.60; H, 1.03; N, 17.14; found C, 19.45; H, 1.11; N, 17.03%; for C8H5IFeN6PtO (9): calcd. C, 16.60; H, 0.87; N, 14.51; found C, 16.81; H, 0.97; N, 14.59%. IR spectra are shown in Figure S1. Crystallization. Single crystals of 1 were obtained by a slowdiffusion method in a 10 mL U-shaped tube. Four milliliters of water containing 1 mg of ascorbic acid were poured on the bottom of the tube. A water−isopropanolic solution (1:1, 2 mL) containing a mixture of Fe(OTs)2·6H2O (0.2 mmol, 102 mg) and chloropyrazine (0.4 mmol, 46 mg) was poured in one arm of the tube. The opposite arm was filled with a water−isopropanolic solution (1:1, 2 mL) of K2[Ni(CN)4] (0.2 mmol, 48 mg). Yellow crystals were formed after a period of four weeks; they were collected and kept in the mother solution prior to the measurements. Crystals of 6 were obtained by the slow-diffusion method within three layers in a 20 mL tube: first layer was a solution of Fe(OTs)2·6H2O (0.2 mmol, 102 mg) and Mepz (0.4 mmol, 38 mg) in water (4 mL); second layer was a mixture of water and isopropanol (2:1, 8 mL); third layer was a water−isopropanolic solution (1:2, 4 mL) of K2[Pt(CN)4] (0.2 mmol, 75 mg). In two weeks this yielded yellow crystals in the second layer; they were collected and kept in the mother solution prior to the measurements. Crystals of 7−9 were obtained by the slow-diffusion method within three layers in 10 mL tubes: first layer was a solution of Fe(OTs)2· 6H2O (0.1 mmol, 51 mg) and Ipz (0.2 mmol, 41 mg) in water (2 mL); second one was water−isopropanolic mixture (1:1, 5 mL); third one was a solution of K2[M(CN)4] M = Ni (7), Pd (8), Pt (9) (0.1 mmol, 24 mg (7), 29 mg (8), 38 mg (9) in water−isopropanolic solution (2 mL, 1:3). After two weeks orange crystals grew in the second layer; they were collected and kept in the mother solution prior to the measurements. Single Crystal X-ray Diffraction. X-ray data for 1 at 170 and 250 K, 6 at 293 K, 8 and 9 at 170 K were collected using a Bruker AXS CCD Smart 1000 diffractometer, and for 7 at 133 K using STOE IPDS II diffractometer (graphite monochromated Mo Kα radiation, λ = 0.710 73 Å) by use of ω scans at −140 °C. The crystal structures were solved by direct methods (SHELXS-97) and refined on F2 using all reflections with SHELXL-2014.52 All non-hydrogen atoms except for the guest 2-methylpyrazine atoms in 6 were refined anisotropically. Most hydrogen atoms were placed in calculated positions and assigned to an isotropic displacement parameter of 1.2 Ueq(C). The symmetryrelated oxygen-bound hydrogen atoms were refined using DFIX restraints (dO−H = 0.82 Å) and with a fixed isotropic displacement parameter of 0.08 Å2. The selected crystal of 7 was nonmerohedrically twinned (twin law: 1 0 0.01, 0 1 0.02, −0.01−0.05 1) with a fractional contribution of 0.395(3) for the minor domain. Face-indexed absorption corrections were performed numerically with the program X-RED.53 The mentioned atoms of the guest in 6 were treated isotropically to ensure a stable refinement. The hydrogen atoms of the water molecules in 6−9 were located from the difference Fourier map. Other H atoms were positioned geometrically and constrained to ride on their parent atoms. Physical Measurements. Powder X-ray diffraction (PXRD) patterns were acquired on a Siemens D5000 diffractometer (λ = 1.540 56 Å) over the 2Θ range of 10°−80°. Elemental analyses (CHN) were performed with a Vario EL III element analyzer. IR spectra were recorded with a PerkinElmer spectrometer BX II (4000− 400 cm−1) in KBr pellets. Temperature-dependent magnetic susceptibility measurements were performed with a Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with a 5 T magnet operating in 1.8−400 K temperature range. Cooling and heating rate was 1 K min−1, and magnetic field was 0.5 T. Diamagnetic corrections derived from Pascal constants were applied. Fitting of the experimental data was performed using JulX program. 57 Fe Mö ssbauer spectra were recorded with a 57Co source embedded in a rhodium matrix using a conventional constantacceleration Mö ssbauer spectrometer equipped with a nitrogen cryostat. Absorbers were prepared by placing the powdered samples
EXPERIMENTAL SECTION
Materials and General Procedures. Chloro-, iodo-, and methylpyrazines, anhydrous potassium tetracyanometallates (M = Ni, Pd, Pt), metallic iron, ascorbic acid, and p-toluenesulfonic acid were purchased from Sigma-Aldrich and used as received. Iron(II) p-toluenesulfonate hexahydrate was prepared by the reaction between iron powder and ptoluenesulfonic acid according to the procedure described previously.51 Synthesis of 1−9 Powders. Complexes 1−9 were obtained in form of powders as follows. Isopropanolic solution (1 mL) of 2substituted pyrazine (Xpz) [X = Cl (1, 2, 3), Me (4, 5, 6), and I (7, 8, 9)] (0.5 mmol, 57.3 mg (1, 2, 3), 47 mg (4, 5, 6), 103 mg (7, 8, 9) was added to an aqueous solution (3 mL) of Fe(OTs)2·6H2O (0.25 mmol, 127 mg), and 1 mg of ascorbic acid was used to prevent oxidation. Afterward, a solution of K2[M(CN)4] [M = Ni (1, 4, 7), Pd (2, 5, 8), Pt (3, 6, 9)], (0.25 mmol, 60 mg (1, 4, 7), 72 mg (2, 5, 8), 94 mg (3, 6, 9) in water (3 mL) was added to the mixture. This yielded yellow precipitates, which were separated by centrifugation, washed with water, centrifuged, and dried in air. Elemental analysis for C12H6Cl2FeN8Ni (1): calcd. C, 32.19; H, 1.35; N, 25.03; found C, 32.05; H, 1.41; N, 24.89%; for C12H6Cl2FeN8Pd (2): calcd. C, 29.09; H, 1.22; N, 22.62; found C, 28.91; H, 1.28; N, 22.47%; for C12H6Cl2FeN8Pt (3): calcd. C, 24.68; H, 1.04; N, 19.19; found C, 24.57; H, 1.09; N, 19.13%; for C14H12FeN8Ni (4): calcd. C, 41.33; H, 2.97; N, 27.54; found C, 41.50; H, 3.03; N, 27.47%; for C14H12FeN8Pd (5): calcd. C, 36.99; H, 2.66; N, 24.65; found C, 36.89; H, 2.71; N, 24.67%; for C10.75H10.1FeN6.70OPt (6): calcd. C, 25.82; H, 2.04; N, 4907
DOI: 10.1021/acs.inorgchem.6b00446 Inorg. Chem. 2016, 55, 4906−4914
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Inorganic Chemistry (50 mg) in poly(methyl methacrylate) holders. Fitting of the experimental data was performed with Recoil software. Hyperfine parameters uncertainties were evaluated from the covariance matrix of the fit. Isomer shifts are given relatively to iron metal at ambient temperature.
Table 1. Selected Bond Distances [Å] and Angles [deg] of 1 at 170 and 250 K 170 K
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Fe1N1 Fe1N3 Ni1C2 Cl1···centroid
RESULTS AND DISCUSSION Crystal Structure. Complexes 1−6 precipitate immediately as fine yellow-orange powders when an aqueous solution of corresponding tetracyanometallate is added to a water−alcohol solution of Fe(OTs)2·6H2O with either 2-chloropyrazine or 2methylpyrazine. Single crystals of clathrates were obtained by the slow diffusion between these reagents. The crystal-structure of 1 was acquired at 170 and 250 K. Similarly to analogous with aminopyridine25 and [Fe(2cloropyridine)2Pt(CN)4]24 it crystallizes in the monoclinic space group C2/m with two formula units per cell. Crystallographic data are summarized in Table S1. The FeII site is situated on the inversion center. It has a coordination environment of a slightly elongated octahedron [FeN6] (Figure 1a). Equatorial Fe−N bonds [Fe1−N3 = 1.986(2) Å (170 K)
N2Fe1N1
distances 2.024(3) 1.986(2) 1.872(3) 3.5957(3) angles 89.03(10)
250 K 2.206(3) 2.1232(18) 1.870(2) 3.4854(5) 88.81(7)
change of bond lengths upon SCO in FeII is ∼0.2 Å,17 a slightly underestimated value observed for 1 may be related to a noncomplete spin transition at 170 K. This transition leads to a crystal lattice expansion by 1.2%, 3.0%, and 1.7% along a, b, and c axes, respectively, and by 7.3% in cell volume similarly to the analogous with substituted pyridines.24,25 The distances between FeII cation and [Ni(CN)4]2− are considerably shorter than those between FeII and chloropyrazine ligands. The latter should be related to the higher affinity of FeII cation to negatively charged tetracyanonickelate comparing with neutral organic ligand. The coordination environment around FeII atoms at 250 K is more distorted than that at 170 K (octahedral distortion parameters∑Fe|90° − Θ| = 11.64° at 170 K and 14.36° at 250 K). The Fe1−N3−C5 angle is 169.2(2)° at 250 K and 172.6(2)° at 170 K. The smaller deviation from linearity in Fe−N−C linkage indicates stronger σ-bonding and therefore stronger ligand field strength in the LS state comparing to the HS. Additionally, both LS and HS structures of 1 include weak intermolecular π···Cl contacts (Figure 1b) that are responsible for the interaction between 2D [FeNi(CN)4]∞ layers. Single crystals of 2−5 could not be obtained by slowdiffusion method, and a judgment on their structures is based on PXRD data. Indeed, 1−5 show the similar set of PXRD patterns (Figure S2) suggesting that the compounds adopt a general topology of 2D {FeL2[M(CN)4]} (L = methylpyrazine, chloropyrazine; M = Ni, Pt, Pd) cyanide-bridged FeII−MII bimetallic networks. This idea is supported by the analytical data (IR spectra are given in Figure S1, CHN analysis is summarized in the Supporting Information) as well as by Mössbauer spectroscopy. In contrast, 6 shows essentially different PXRD patterns, which is related to its completely different crystal structure and thus its SCO behavior. The compound 6 crystallizes in triclinic P1̅ space group with two formula units per cell. The crystallographic parameters are summarized in Table S2. Two different types of FeII ions are present in the structure. Equatorial coordination sites of Fe1 are occupied by four N atoms from four different [Pt(CN)4]2− groups, while the axial positions are occupied by two water molecules forming FeHSN4O2 polyhedron. Simultaneously, Fe2 is coordinated by six N atoms resulting in a slightly elongated FeN6 octahedral environment, with the axial positions occupied by two methylpyrazine ligands and equatorial coordination sites occupied by four [Pt(CN)4]2− groups (Figure 2a). The average Fe2−N bond length in the compound is 2.164 Å (Table 2), which indicates that FeII ions are in HS state at 293 K, a fact consistent with the magnetic and Mössbauer measurements. Each FeII ion is coordinated by four [Pt(CN)4]2− groups, and each [Pt(CN)4]2− group coordinates four FeII, forming 2D [FePt(CN)4]∞ polymer sheets. These layers stack along the [022] direction. Two consecutive layers are separated from
Figure 1. (a) Fragment of the crystal structure of 1 at 170 K showing atom-labeling scheme. Atomic displacement parameters are drawn at 50% probability. (b) Crystal structure of 1 showing π···Cl contacts as dashed green lines. Hydrogen atoms are omitted for clarity. (c) View of the crystal structure of 1 in ac plane. H atoms are omitted. [Fe: black, Ni: brown, Cl: green, N: blue, C: gray, H: light-gray].
and Fe1−N3 = 2.123(2) Å (250 K)] (Table 1) belong to four [Ni(CN)4]2− groups, while axial N atoms are from two chloropyrazine molecules [Fe1−N1 = 2.024(3) Å (170 K) and Fe1−N1 = 2.206(3) Å (250 K)] (Figure 1a). The coordination network is further connected by these bridging cyanonickelate ligands into a 2D square grid that lies in the [100] plane (Figure 1b,c). The change of an average Fe−N bond length by 0.167(6) Å is caused by a temperature-induced spin transition in FeII ions6 that is further confirmed by magnetic and Mössbauer measurements, though small contribution from the thermal expansion should be also considered. Since a typical 4908
DOI: 10.1021/acs.inorgchem.6b00446 Inorg. Chem. 2016, 55, 4906−4914
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Inorganic Chemistry
Table 3. Selected Bond Distances [Å] and Angles [deg] of 7 (133 K) and 8−9 (170 K) 7 (M = Ni) Fe1N1 Fe1N2 Fe1N3 Fe1O1 M1C1 M1C2 H1b(O1)···N1 H1a(O1)···N2 I1···centroid O1Fe1N1 O1Fe1N2 N1Fe1N2 O1Fe1N3 N1Fe1N3 N2Fe1N3 O1H1b···N1 O1H1a···N2
Figure 2. (a) Fragment of crystal structure of 6 at 293 K with atomlabeling scheme. (b) Guest Mepz molecules occupy voids in the framework. Nonwater H atoms are omitted for clarity. (c) Crystal structure of 6 showing N(pz)···H(water) hydrogen bonds as dashed blue lines. Guest Mepz molecules and nonwater H atoms are omitted for clarity. [Fe: black, Pt: yellow, O: red, N: blue, C: gray, H: lightgray].
a
distances 2.140(7) 2.148(7) 2.220(11) 2.126(11) 1.868(9) 1.873(9) 2.48a 3.6955(2) angles 92.9(3) 92.6(3) 88.3(3) 179.8(4) 87.3(3) 87.2(3) 174(18)a
8 (M = Pd)
9 (M = Pt)
2.145(2) 2.148(2) 2.240(2) 2.123(2) 1.998(2) 1.994(2) 2.47 2.61 3.7712(1)
2.140(3) 2.140(3) 2.239(4) 2.124(3) 1.986(3) 1.984(3) 2.45 2.49 3.7665(1)
92.90(5) 92.00(5) 88.31(6) 179.64(9) 87.35(5) 97.75(5) 140.8 132.3
92.99(10) 91.82(11) 88.1(1) 179.50(14) 87.36(10) 87.83(10) 148.4 144.8
For 7 parameters of O1H1···N1 hydrogen bond are reported.
and O atoms of water molecules, respectively (Figures 3a, S3, and S4). Noteworthy, as well as in the case of Clpz and Mepz,
Table 2. Selected Bond Distances [Å] and Angles [deg] of 6 at 293 K Fe1O1 Fe1N1 Fe1N2 Fe2N3 Fe2N4 Fe2N5 N3Fe2N5 N4Fe2N3 N4Fe2N5 N1Fe1O1
distances 2.159(9) Pt1C2 2.14(1) Pt1C1 2.156(10) Pt1C4 2.142(9) Pt1C3 2.141(9) H1b(O1)···N6 2.208(9) angles 88.8(4) N1Fe1N2 91.2(4) N2Fe1O1 86.7(3) O1H1b···N6 92.8(4)
1.967(12) 1.982(11) 1.980(12) 1.970(12) 1.92 1.967(12) 90.9(4) 88.9(4) 165.60
each other by a distance of 8.6384(1) Å, measured from the average plane defined by FeII ions belonging to the respective layers. This space is partly filled in with 0.35 noncoordinated guest molecules of 2-methylpyrazine per one Fe atom (Figure 2b). In addition, the N atoms of coordinated methylpyrazine molecules are involved in intermolecular hydrogen bonding with water hydrogens (H1b(O1)···N6 distance is 1.918(2) Å, and O1−H1b−N6 angle is 165.589(7)°). Polymer layers connected into supramolecular framework via weak interactions are shown in Figure 2c. Formation of slightly different frameworks was observed in the case of 2-iodopyrazine as a ligand. Single-crystal X-ray analysis reveals that compounds 7−9 crystallize in orthorhombic Pnma group with four formula units per cell and are isostructural. Crystal data for them are summarized in Table 3. In the structures, FeII has a distorted octahedral N5O coordination environment. Similarly as in the analogues with Clpz and Mepz, equatorial positions are occupied by N atoms of four [M(CN)4]2− (M = Ni, Pt, Pd) groups, while axial Fe1− N3 and Fe1−O1 are due to N atoms of iodopyrazine molecules
Figure 3. (a) Fragment of crystal structure of 9. Atomic displacement parameters are shown at 50% probability. (b) Structure of 9 showing π···I contacts. (c) View of crystal structure of 9 along b direction showing interlayer H(water)···N(cyanide) hydrogen bonds. [Fe: black, Pt: yellow, I: violet, O: red, N: blue, C: gray, H: light-gray].
Ipz does not act as a bridging ligand, and coordinates FeII ion by N1 only. The similar coordination behavior of 2-substituted pyrazines has been observed in FeII mononuclear complexes.50 The average distances Fe−N = 2.169 Å (7), 2.177 Å (8), 2.172 Å (9), and Fe−O = 2.126(11) Å (7), 2.123(2) Å (8), 2.124(3) Å (9) that confirms the HS state of FeII going in agreement with magnetic and Mössbauer measurements. The Fe−N−C linkages show a significant deviation from linearity (20.9°, 21.2°, and 21.7° on average for 7−9), and the thus-formed 2D {Fe[M(CN) 4]2−} layers are strongly corrugated. Weak 4909
DOI: 10.1021/acs.inorgchem.6b00446 Inorg. Chem. 2016, 55, 4906−4914
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Inorganic Chemistry
hysteresis loop of ∼25 K. Such a difference in the hysteresis width in Xpz and pz complexes can be explained by the binding of the {Fe[MII(CN)4]}∞ layers into the rigid 3D framework. However, in case of Xpz formation of nonporous 2D complexes is observed making their SCO behavior independent of the guest inclusion. The stability of SCO behavior in the discussed materials is confirmed in reproducible thermal cycles (Figure S5). In the complexes based on tetracyanonickelate, 1, and 4, the FeII ions undergo two-step spin transition with a half of metallic centers undergoing crossover process at each step. The temperatures of spin transition for 1 are Tup1 = 182 K and Tup2 = 197.5 K in heating mode. Because of the cooperativity of SCO the occurrence of hysteresis loops is observed (ΔT1 = 9 K and ΔT2 = 7.5 K), consequently in cooling mode SCO is detected at lower temperatures: Tdown1 = 173 K and Tdown2 = 190 K. In case of 4 the difference between SCO steps is tiny; however, two subsequent LS to HS transitions can be found at Tup1 = 193.5 K and Tup2 = 197.5 K, and the return to the LS state occurs at Tdown1= 185.5 K and Tdown2 = 190 K, denoting two hysteresis loops of 7 K (ΔT1) and 7.5 K (ΔT2). A similar plateau with 50% conversion around 150 K has been observed for the related complex [Fe(Clpy)Pd(CN)4], in which both HS and LS FeII have been detected in the crystal structure at the intermediate temperature.24 At room temperature the χMT value for 2 is 3.3 cm3 K mol−1 and remains constant until approaching the spin transition at 260 K. When further cooled, the gradual two-step spin transition takes place (Tdown2 = 205 K and Tdown1 = 145 K). In heating mode the compound undergoes spin transition at Tup1 = 150 K and Tup2 = 215 K; therefore, despite the gradual nature of SCO, hysteresis of spin transition is detected (ΔT1 = 5 K and ΔT2 = 10 K). Such gradual or sometimes even more continuous SCO is very often observed for Hofmann clathrates;27,54−56 however, in general, it is difficult to discuss the influence of composition of the complex on the cooperativity of SCO. For 6 above 165 K the χMT has a value of 3.3 cm3 K mol−1, which indicates that all FeII ions are in the HS state. When cooled, the magnetic susceptibility abruptly decreases to ∼1.8 cm3 K mol−1 (Tdown = 144 K) because of the thermally induced SCO in a half of FeII centers. A further drop of χMT value is assigned to the zero-field splitting (ZFS) in FeII ions, which remain HS at low temperatures. The heating mode (Tup = 158 K) denotes the emerged SCO hysteresis loop of ΔT = 14 K. Such a half-transition behavior corroborates well the structural data, which evidence the presence of two crystallographically different FeII sites. Apparently, only FeII ions in N6 coordination surrounding undergo a cooperative spin transition, while those
interactions between H atoms of water molecules and N atoms of cyanide anions of the neighboring 2D grid lead to the formation of O−H···N hydrogen bonds with H···N distances of ∼2.5 Å (Figure 3b,c Table S3). As well, the structure includes weak interlayer I···π interactions (Figure 3b), and π···I contacts occur from both sides of the aromatic ring. Thereby, the frameworks 7−9 are stabilized within the combination of I···π and hydrogen interactions between 2D layers. Magnetic Properties. Measuring the magnetic susceptibility χ as a function of temperature is a standard method to study SCO systems because the change in the number of unpaired electrons between two spin states is reflected in a drastic change of χ.6 The thermal dependences of χMT for the complexes with Clpz and Mepz axial ligands (where χM stands for molar susceptibility) are shown in Figure 4.
Figure 4. Magnetic properties of 1−6 in the form of χMT vs T recorded in both heating and cooling modes at a scan rate of 1 K min−1 and magnetic field of 0.5 T. (upper) Magnetic curves in the whole temperature range of the study. Two-step SCO hysteresis loops for 1 and 4 are scaled in the lower panel in a highlighted temperature range. Red and blue points correspond to Clpz and Mepz complexes respectively; cycles − Ni, squares − Pd, triangles − Pt complexes.
Classic abrupt one-step SCO takes place in 3 and 5. For 3, Tdown = 100.5 K and Tup = 115 K with the hysteresis loop of 14.5 K, while in case of 5, SCO occurs at Tdown = 229 K in cooling mode and at Tup = 241.5 K in heating mode, with a hysteresis loop of 12.5 K. The values of magnetic susceptibility at low temperatures (χMT ≈ 0.2 cm3 mol−1 K) indicate that spin transition in 3 and 5 is practically complete. Comparing these compounds with 3D [Fe(pz)MII(CN)4], the latter undergo slightly more cooperative spin transition with
Table 4. Characteristics of the Spin Transition in 1−6 Derived from Magnetic Susceptibility Measurements complex
formula
1
Fe(Clpz)2Ni(CN)4
2
Fe(Clpz)2Pd(CN)4
3 4
Fe(Clpz)2Pt(CN)4 Fe(Mepz)2Ni(CN)4
5 6
Fe(Mepz)2Pd(CN)4 {Fe(Mepz)2Pt(CN)4}{Fe(H2O)2Pt(CN)4}· 0.7(Mepz)
step
Tup
Tdown
Tc
ΔT
transition
I II I II
182 K 197.5 K 150 K 215 K 100.5 193.5 K 197.5 K 229 K 158 K
173 K 190 K 145 K 205 K 115 K 185.5 K 190 K 241.5 K 144 K
177.5 193.75 147.5 K 210 K 107.75 K 189.5 K 193.75 235.25 151 K
9K 7.5 K 5K 10 K 14.5 K 7K 7.5 K 12.5 K 14 K
abrupt abrupt gradual gradual abrupt abrupt abrupt abrupt abrupt, half
I II
4910
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molecular symmetry and distortions, and lattice dynamics to name a few important properties.59 The spin transition could be easily monitored through hyperfine parameters (isomer shift δ60 and electric quadrupole splitting ΔEQ), which significantly differ in two spin states. In certain cases, Mö ssbauer spectroscopy was the only technique, which allowed to solve some complicated tasks regarding the nature of SCO.61,62 At 293 K the spectrum of 1 shows a unique doublet with an isomer shift of δHS = 1.042(1) mm s−1 and a quadrupole splitting of ΔEQHS = 1.591(1) mm s−1 in agreement with an FeII species in the HS state (Figure 7). At 193 K, that
in N4O2 environment are HS, a fact well-known for polynuclear triazole complexes.57 SCO properties deduced from magnetic measurements of 1− 6 are summarized in Table 4. Two photographs of powder 1 taken at room temperature and when emerged in liquid nitrogen demonstrate a pronounced thermochromic effect that accompanies spin transition (Figure 5).
Figure 5. Photographs of sample 1 in both spin states.
Magnetic measurements show that Ipz complexes 7−9 possess typical for HS FeII Curie behavior in the whole temperature range. Down to 100 K χMT has a value of ∼3.5 cm3 K mol−1, and when further cooled it drops to ∼2.6 cm3 K mol−1 at 5 K, which is a consequence of ZFS (Figure 6). The Figure 7. Mössbauer spectra of 1 and 2 at different temperatures showing HS and LS doublets in red and blue, respectively.
corresponds to a χMT(T) plateau achieved in the heating regime, an additional doublet with δLS = 0.436(2) mm s−1 and ΔEQLS = 0.423(4) mm s−1 is observed, representative of the appearance of the LS species. At this temperature the spectrum shows a presence of HS and LS forms in ratio of 49(2):51(3). When cooled to 80 K, the spectrum reveals complete disappearance of HS fraction, and instead only a LS doublet is detected (δ LS = 0.455(1) mm s−1 and ΔEQLS = 0.409(2) mm s−1). The spectra of 2 and 4, which also exhibit a two-step SCO, in the LS, HS, and LS/HS states, are similar to those of 1 (Figures 7 and 8). Hyperfine parameters for the studied systems are summarized in Table 5. At 293 K the Mössbauer spectrum of 3 consists of one doublet that should be attributed to FeII in a HS state (δHS = 1.059(3) mm s−1 and ΔEQHS = 1.277(5) mm s−1; Figure 8). When cooled to 80 K the spectrum reveals the presence of LS fraction with δ LS = 0.460(2) mm s−1 and ΔEQLS = 0.398(4) mm s−1 at the expense of the residual HS doublet. Similarly, the spectra of 5 acquired at 293 and 80 K do show characteristic HS and LS doublets, respectively, and corroborate the complete spin transition. (Figure 9). The Mössbauer spectrum of 6 at 293 K, which at first glance contains two identical absorption lines, cannot be accurately fitted with the regular Lorentzian doublet. Therefore, a model with two doublets was applied (Figure 9). When the relative intensities of the doublets are fixed to 1:1, hyperfine parameters δHS = 1.126(10) and 1.014(10) mm s−1, ΔEQHS = 1.141(12) and 1.156(11) mm s−1 can be evaluated. The doublets correspond to two different FeII sites observed in the crystal lattice of the compound. When the temperature decreased to 80 K the spectrum shows disappearance of one of the HS
Figure 6. Magnetic properties of 7−9 in the form of χMT vs T. Fits of the experimental data are performed using spin Hamiltonian (see text).
absence of SCO in 7−9 is caused by one water molecule in the coordination environment of FeII, which provides relatively weaker ligand field strength. Magnetic data for HS FeII (S = 2) with Zeeman splitting and ZFS can be described using spin Hamiltonian: ⃗ ⃗ + D(Sẑ 2 − 1/3S(S + 1)) Ĥ = gμB BS
where g is the Landé factor, μB is the Bohr magneton, B is the magnetic field, Sz is the projection of the spin moment along the field direction, and D is an axial ZFS parameter. The calculations give Landé factors g = 2.21 (7), 2.18 (8), and 2.16 (9) and ZFS parameters |D| = 15.9 (7), 15.2 (8), and 13.9 (9) cm−1.The values of g-factors are typical for previously reported high-spin FeII complexes;58 D parameters are relatively high, even though a correct comparison is complicated because of the lack of corresponding values for cyanoheterobimetallic analogues. 57 Fe Mö ssbauer Spectroscopy. Mössbauer spectroscopy is a local sensitive probe that provides direct information about Fe atoms: their oxidation and spin states, magnetic behavior, 4911
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Figure 9. Mössbauer spectra of 5 and 6 recorded at 80 and 293 K. In 6, two different FeII centers are identified, only one of them undergoes SCO.
Figure 8. Mössbauer spectra of 3 and 4 recorded at different temperatures. The minor quadrupole doublets at 80 K indicate residual (non-SCO) HS fractions.
doublets, and instead an LS doublet is detected (δ LS = 0.431(5) mm s−1 and ΔEQLS = 0.305(9) mm s−1). This experiment shows that at 80 K HS FeN4O2 and LS FeN6 metal centers are present in ratio of 1:1. Mössbauer spectra of 7−9 show very similar quadrupole doublets with δ ≈ 1.1 mm s−1 at 293 K and 1.2 mm s−1 at 80 K (Figures S6 and S7). Although no SCO is observed, a slight increase of δ upon cooling is caused by a second-order Doppler shift. Enhancement of ΔEQ at low temperatures is also typical for HS FeII in an axial electric field. Similar δ and ΔEQ behavior have been reported for the related clathrates.27,63
Comparing hyperfine parameters of the discussed SCO systems, one must note that for the LS forms of Clpz series (1− 3) ΔEQ ≈ 0.4 is noticeably higher than ΔEQ ≈ 0.3 mm s−1 in Mepz series (4−6). The difference allows us to assume that the complexes with Clpz ligands have relatively higher axial distortion of FeN6 polyhedra compared to those with Mepz. For the reported LS forms of 3D Hofmann clathrates [Fe(L)MII(CN)4] so far, the ΔEQ values vary from 0.3 for L = pz64 to 0.2 mm s−1 for L = bpac.65 For the 2D clathrates [Fe(NH2py)2MII(CN)4], only a single line (i.e., ΔEQ = 0 mm s−1) is observed in the LS state pointing to the spherically
Table 5. Hyperfine Parametersa for 1−9 complex
formula
1
Fe(Clpz)2Ni(CN)4
2
Fe(Clpz)2Pd(CN)4
3
Fe(Clpz)2Pt(CN)4
4
Fe(Mepz)2Ni(CN)4
5
Fe(Mepz)2Pd(CN)4
6
{Fe(Mepz)2Pt(CN)4} {Fe(H2O)2Pt(CN)4}·0.7(Mepz)
temperature
spin state
δ
ΔEQ
content
80 K 193 K
LS LS HS HS LS LS HS HS LS HS LS LS HS HS LS HS LS HS HS HS HS HS HS HS HS HS
0.455(1) 0.436(2) 1.087(3) 1.042(1) 0.455(2) 0.458(5) 1.090(6) 1.070(10) 0.460(2) 1.059(3) 0.449(1) 0.430(2) 1.081(3) 1.023(1) 0.445(1) 1.052(1) 0.431(5) 1.165(8) 1.126(10) 1.014(10) 1.227(2) 1.123(3) 1.219(3) 1.071(3) 1.222(6) 1.105(7)
0.409(2) 0.423(4) 2.151(7) 1.591(1) 0.416(3) 0.367(10) 1.773(11) 1.395(18) 0.398(4) 1.277(5) 0.317(1) 0.289(3) 2.013(6) 1.611(2) 0.333(2) 1.376(3) 0.305(9) 1.584(15) 1.141(12) 1.156(11) 1.771(8) 0.984(6) 1.683(8) 0.924(7) 1.597(11) 1.101(13)
100% 51(3)% 49(2)% 100% 100% 49(4)% 51(2)% 100% 96(1)% 100% 95(1)% 52(3)% 48(1)% 100% 100% 100% 52(3)% 48(2)% 50% 50% 100% 100% 100% 100% 100% 100%
293 K 80 K 165 K 293 K 80 K 293 K 80 K 187 K 293 K 80 K 293 K 80 K 293 K
a
7
Fe(Ipz)Ni(CN)4
8
Fe(Ipz)Pd(CN)4
9
Fe(Ipz)Pt(CN)4
80 K 293 K 80 K 293 K 80 K 293 K
The values are given in millimeters per second. 4912
DOI: 10.1021/acs.inorgchem.6b00446 Inorg. Chem. 2016, 55, 4906−4914
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symmetrical nature of the electric charge at Fe nuclei.25 At the same time,45 ΔEQ for the HS forms of Clpz and Mepz complexes is comparatively similar. However, it significantly decreases for both series 1−3 and 4−6 from Ni to Pt species, which indicates higher electric field gradient for the Ni derivatives. In contrast, behavior of δ does not show any noticeable features. It is in the vicinity of ∼0.45 mm s−1 for LS (80 K) and ∼1.0−1.1 mm s−1 for HS (293 K) species, which is typical for related clathrates.26
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +38 044 239 33 93. Author Contributions ∇
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by State Fund for Fundamental Research of Ukraine through Grant No. F61/93-2015 and German Academic Exchange Service (DAAD) through Leonhard Euler Program.
CONCLUSIONS Two-dimensional Hofmann-type coordination polymers can be obtained starting from FeII, tetracyanometalates, and 2substituted pyrazines. The structure of the target complex predictably depends on the nature of pyrazine derivative; hence, the properties of the material can be tuned via varying the ligand. In case of 2-chloro- and 2-methylpyrazine a formation of Fe(Xpz)2M(CN)4 (M = Ni, Pd, Pt) complexes is observed, except for X = Me, M = Pt, which gives a complex with two different FeII centers. These materials exhibit reproducible and cooperative spin transition below room temperature. In addition, the temperature of SCO, as well as its abruptness and completeness, is modified by M(CN)42− anion. The lowest Tc values are observed for the complexes with M = Pt, which thereby possess an interest for SCO quenching, kinetics, and pressure studies. Complexes with M = Ni exhibit abrupt twostep transitions and provide a new stage toward multistable SCO materials. A use of 2-iodopyrazine as a ligand leads to a formation of Fe(Ipz)(H2O)M(CN)4, which consists of unprecedentedly corrugated heterometallocyanide layers. They are HS down to the lowest temperatures due to replacement of one pyrazine ligand by water in the FeII surrounding. However, all three substituted pyrazines discussed here possess the same feature: they coordinate FeII ion in a pyridine-like manner; that is, the potential donor N atom closest to the Me, Cl, or I substitute is nonactive. Contrary to 3D Hofmann clathrates, it results in a dense packing of 2D layers interconnected through different weak interactions (e.g., hydrogen bonds, π-halogen contacts); thus, guest inclusion in the framework is omitted. Taking into account simplicity of the synthesis and attractive properties the complexes discussed here may be interesting for the development of materials with tunable SCO properties. Influence of other pyrazine substitutes as well as of other cyanometallates on spin transition will be the topics of our further investigation.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00446. IR spectra of 1−9, X-ray powder diffraction patterns of 1−9, crystal structures of 7 and 8, magnetic properties of 1, 3−5, Mössbauer spectra of 7−9. (PDF) X-ray crystallographic data for 1. (CIF) X-ray crystallographic data for 6. (CIF) X-ray crystallographic data for 7. (CIF) X-ray crystallographic data for 8. (CIF) X-ray crystallographic data for 9. (CIF) X-ray crystallographic data. (CIF) 4913
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