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This article is published as part of the Dalton Transactions themed issue entitled:

Molecular Magnets Guest Editor Euan Brechin University of Edinburgh, UK

Downloaded by KIT on 01 March 2011 Published on 18 March 2010 on http://pubs.rsc.org | doi:10.1039/B925482J

Published in issue 20, 2010 of Dalton Transactions

Image reproduced with permission of Jürgen Schnack Articles in the issue include: PERSPECTIVES: Magnetic quantum tunneling: insights from simple molecule-based magnets Stephen Hill, Saiti Datta, Junjie Liu, Ross Inglis, Constantinos J. Milios, Patrick L. Feng, John J. Henderson, Enrique del Barco, Euan K. Brechin and David N. Hendrickson, Dalton Trans., 2010, DOI: 10.1039/c002750b Effects of frustration on magnetic molecules: a survey from Olivier Kahn until today Jürgen Schnack, Dalton Trans., 2010, DOI: 10.1039/b925358k COMMUNICATIONS: Pressure effect on the three-dimensional charge-transfer ferromagnet [{Ru2(mFPhCO2)4}2(BTDA-TCNQ)] Natsuko Motokawa, Hitoshi Miyasaka and Masahiro Yamashita, Dalton Trans., 2010, DOI: 10.1039/b925685g Slow magnetic relaxation in a 3D network of cobalt(II) citrate cubanes Kyle W. Galloway, Marc Schmidtmann, Javier Sanchez-Benitez, Konstantin V. Kamenev, Wolfgang Wernsdorfer and Mark Murrie, Dalton Trans., 2010, DOI: 10.1039/b924803j Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research www.rsc.org/dalton

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Synthesis, magnetic and photomagnetic study of new iron(II) spin-crossover complexes with N4 O2 coordination sphere† Li Zhang,a,c Guan-Cheng Xu,a Hong-Bin Xu,a Valeriu Mereacre,b Zhe-Ming Wang,a Annie K. Powell*b and Song Gao*a


Received 3rd December 2009, Accepted 1st March 2010 First published as an Advance Article on the web 18th March 2010 DOI: 10.1039/b925482j A new family of neutral mononuclear iron(II) spin crossover (SCO) compounds, Fe(L1–6 )2 (L1–6 = N¢-((pyridin-2-yl)methylene)benzohydrazide (HL1 ), N¢-(1-(pyridin-2-yl)ethylidene)-benzohydrazide (HL2 ), N¢-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL3 ), 2-hydroxy-N¢-((pyridin-2yl)methylene)benzohydrazide (HL4 ), 2-hydroxy-N¢-(1-(pyridin-2-yl)ethylidene)benzohydrazide (HL5 ), 2-hydroxy-N¢-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL6 )) with N4 O2 donor sets have been synthesized from series tridentate Schiff base ligands with N,N,O donor sets. The investigation of magnetic properties of these compounds reveal that in the measured temperature range, compound 1 is in the high-spin (HS) state, and compound 3 and 6 are mainly in the low-spin (LS) state, whereas the other compounds exhibit various SCO properties: compound 2 undergoes a gradual incomplete SCO with characteristic temperature T 1/2 higher than 350 K; compound 4 exhibits a special stepwise thermally induced SCO occurring at ~150 K (smooth) and 200 K (two-steps, with T S1↑/↓ = 204/202 K and T S2↑/↓ = 227/219 K) with a mixture of the HS and LS states yielded below 100 K; compound 5 shows a gradual and complete LS ↔ HS SCO with characteristic temperature T 1/2 = 273 K. All the three SCO compounds show the LIESST (light induced exited spin state trapping) effect with different ¨ levels of photoconversion. To thoroughly analyze these behaviours, Mossbauer spectra and DSC of 4 and 5, crystal structures of all the compounds at 290 K and 5 in the LS state at 110 K were carried out, which confirmed the structural changes accompanying the spin transition. In addition, alkyl substitution effect on the ligand field was suggested for this system.

Introduction Spin transition (ST) complexes or spin crossover (SCO) complexes are a fascinating class of compounds that can be switched between the paramagnetic high-spin (HS) and diamagnetic low-spin (LS) state by external perturbations such as temperature, pressure, light or field.1 Since the discovery of the first SCO compound, such compounds have aroused more and more attention for potential applications in molecular switches or data storage devices.2 Recent activities in this field have been devoted to the design of new molecule-based functional materials in which the SCO properties may be combined with other physical or chemical properties in a synergic fashion.3 However, to be useful in practice, abrupt and complete spin transitions exhibiting a wide hysteresis around room temperature would be of great interest.4 So far it is still a great a Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, P. R. China. E-mail: [email protected] b Institut für Anorganische Chemie der Universität Karlsruhe, Karlsruhe Institute of Technology, Engesserstr., 15, D-76131, Karlsruhe, Germany. E-mail: [email protected] c Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046, P. R. China † Electronic supplementary information (ESI) available: Fig. S1–S4, and CIF files of crystallography data for the structures in this work. CCDC reference numbers 756912 (1), 756913 (2), 756914 (3), 756915 (4), 756916 (5 110 K), 756917 (5 290 K) and 756918 (6). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b925482j

4856 | Dalton Trans., 2010, 39, 4856–4868

challenge to design and produce systems that can be tuned and exploited at room temperature. Octahedral transition metal complexes with the configuration 3dn (n = 4–7) are expected to exhibit spin crossover behaviours when the energy difference between the low state and high state is in the range of kT.5 Up to now, the influence of chemical parameters such as substituents on the ligand, counterions or additional solvent molecules on the ligand field strength and the transition temperature T 1/2 is well understood. Nevertheless, the SCO transition seems to be strongly influenced in a complicated way by many secondary factors, and the detailed cooperative mechanism of the SCO behaviour is not perfectly understood. It has been recognized that cooperative interactions are mainly responsible for the special features of the spin transition characteristics such as temperature and abruptness of the transition and hysteresis loops.1d,6 However, the question of how to design strongly cooperative SCO materials has still not been answered satisfactorily. The two normal strategies being used now are a supramolecular approach and a polymeric approach, which are based on manipulating intermolecular interactions, such as p–p stacking, hydrogen bonding or van der Waals interactions, in mononuclear compounds6,7 or on selecting suitable covalent linkages between metal centers in the case of polynuclear compounds.8 However, the preparation of promising complexes is still not predictable as a result of the complexity of the elastic interactions in real systems and the poor knowledge of the relevant lattice properties. In this context, the design of new molecular compounds exhibiting the spin crossover phenomenon This journal is © The Royal Society of Chemistry 2010


(SCO) is still one of the most relevant and challenging questions in the field of magnetic molecular materials. In the past few decades, many efforts have concentrated on the systems with FeII , FeIII and CoII , and especially on iron(II) compounds with an N6 coordination sphere.9 However, there are also some reports on iron(II) SCO compound with N4 O2 coordination spheres arising from donors in Schiff base like ligands or from separate O- and N- donor chelates, which can provide the required ligand field.10 Recently, Jäger and Weber’s research groups reported series of SCO iron(II) compounds with N4 O2 coordination spheres by combining planar tetradentate N2 O2 Schiff base like ligands with trans-ligated pyridine-type Ndonor ligands. With significantly different ligand-field strength and intermolecular contacts, especially hydrogen bonds, mononuclear and 1D chain systems with wide thermal hysteresis loop around room temperature were found.11 On the other hand, new iron(III) SCO compounds with N4 O2 coordination spheres have also been well investigated, exhibiting the unexpected lightinduced excited spin state trapping (LIESST) effect.4i,12 It is worth noting that, in such FeIII -N4 O2 systems, p–p stacking of the neighbouring Schiff base ligands with N2 O donors is one of the key parameters determining the intermolecular interactions which influence not only the SCO behaviours, but also the LIESST effect. Furthermore, it was also shown that the magnetic properties of such compounds are strongly dependent on the nature of the counter anions and/or the solvent molecules.4i,12,13 The purpose of the present work is to combine planar Schiff base ligands with iron(II) to form neutral iron(II) complexes. On one hand, with the necessary p electrons in such ligands p–p stacking interaction should be favoured. On the other hand, the formation of neutral SCO complexes may exclude the effect from the anions on the system. To this end, we have designed the series of tridentate Schiff base ligands displayed in Scheme 1. These types of ligand are always effective Fe chelators that show great promise for the treatment of Fe overload diseases and therefore have been investigated from their biological aspects.14 Herein, we present six new neutral mononuclear iron(II) complexes with N4 O2 coordination spheres with such tridentate Schiff base liked ligands. One of these shows an unusually stepwise SCO with a plateau at g HS = 0.25 at lower temperature and the other two compounds show SCO properties with transition temperatures near to or higher than room temperature. A series with alkyl substitutions on the ligands demonstrate the consequent effect on ligand field strength, which could suggest a new way to design and tailor spincrossover systems.

Experimental General All reagents and solvents in the syntheses were of reagent grade and used without further purification. All reactions were carried out in air. Caution! Iron(III) perchlorate is potentially explosive and should be handled with much care. Elemental analyses of carbon, hydrogen, and nitrogen were carried out with an Elementar Vario EL analyzer. Micro-IR spectroscopy studies were performed on a Nicolet Magna-IR 750 spectrophotometer in the 4000–650 cm-1 region (w, weak; b, broad; m, medium; s, strong). Thermal analyses were performed on SDT Q600 thermal analyzer (thermogravimetric analysis/differential thermal analysis, TGA/DTA) and a Q100 differential scanning calorimeter (DSC), under a nitrogen gas flow (50 mL min-1 ) with a constant heating or cooling rate of 5 K min-1 . Temperature and enthalpy were calibrated using the melting point of indium. Synthesis of the ligands The ligands N¢-((pyridin-2-yl)methylene)benzohydrazide (HL1 ), N¢-(1-(pyridin-2-yl)ethylidene) benzohydrazide (HL2 ), N¢-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL3 ), 2-hydroxy-N¢((pyridin-2-yl)-methylene)benzohydrazide (HL4 ), 2-hydroxy-N¢(1-(pyridin-2-yl)ethylidene)benzohydrazide (HL5 ), 2-hydroxy-N¢(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL6 ), were prepared using similar method described in the literature.15 Synthesis of the compounds 1–6 The compounds were prepared by using the following general method: the methanol solution (5 mL) of the ligand (0.2 mmol) with equimolar NaOH and iron(III) perchlorate (Fe(ClO4 )3 ·6H2 O) (0.1 mmol) in methanol solution (5 mL) were placed separately in the side arms of H-shaped tube, and the two solutions were carefully linked by methanol (5 mL) to allow slow liquid-to-liquid diffusion. Crystals were formed in several weeks. Fe(L1 )2 (1). Yield: 42%. FT-IR (cm-1 ): 3059 (b), 1598 (m), 1553 (w), 1480 (m), 1462 (m), 1359 (s), 1143 (m), 720 (m). Anal. calcd for C26 H20 N6 O2 Fe: C 61.92, H 4.00, N 16.66. Found: C 61.61, H 4.04, N 16.48%. Fe(L2 )2 (2). Yield: 36%. FT-IR (cm-1 ): 3059 (b), 1593 (w), 1567 (w), 1482 (m), 1456 (m), 1372 (s), 1154 (m), 708 (m). Anal. calcd for C28 H24 N6 O2 Fe: C 63.17, H 4.54, N 15.79. Found: C 62.92, H 4.50, N 15.82%. Fe(L3 )2 (3). Yield: 60%. FT-IR (cm-1 ): 3544 (b), 3067 (b), 1599 (m), 1585 (m), 1541 (w), 1479 (m), 1456 (w), 1358 (s), 1084 (s), 705 (s). Anal. calcd for C38 H28 N6 O2 Fe: C 69.52, H 4.30, N 12.80. Found: C 69.18, H 4.73, N 12.52%. Fe(L4 )2 ·0.5H2 O (4). Yield: 52%. FT-IR (cm-1 ): 3058 (b), 2713 (b), 1620 (w), 1597 (m), 1487 (m), 1462 (m), 1362 (s), 1252 (m), 1157 (m), 757 (m). Anal. calcd for C26 H21 N6 O4.5 Fe: C 57.26, H 3.88, N 15.41. Found: C 57.29, H 3.85, N 15.25%.

Scheme 1

Schematic representation of the ligands used in this work.

This journal is © The Royal Society of Chemistry 2010

Fe(L5 )2 (5). Yield: 55%. FT-IR (cm-1 ): 3066(b), 2721(b), 1619(w), 1596(m), 1487(m), 1460(m), 1369(s), 1254(m), 1165(m), 758(m). Anal. Calcd for C28 H24 N6 O4 Fe: C, 59.59; H, 4.29; N, 14.89. Found: C, 58.46; H, 4.45; N, 14.45%. Dalton Trans., 2010, 39, 4856–4868 | 4857

Fe(L6 )2 (6). Yield: 58%. FT-IR (cm-1 ): 3062 (b), 2926 (b), 1620 (m), 1586 (m), 1484 (m), 1437 (m), 1373 (s), 1254 (m), 1158 (m), 753 (m). Anal. calcd for C38 H28 N6 O4 Fe: C 66.29, H 4.10, N 12.21. Found: C 65.82, H 4.20, N 12.20%.


Magnetic measurements The variable temperature magnetic susceptibility measurements were carried out on samples constituted of polycrystalline crystals (10–20 mg) using a Quantum Design MPMS-XL5 SQUIDMagnetometer in a temperature range from 2 to 390 K. All measurements were carried out at two field strengths (2 kOe and 5 kOe) in the settle mode. Gelatine capsules were used as sample containers for measurements. The data were corrected for the magnetization of the sample holder, and diamagnetic corrections were estimated using Pascal’s constants.16 The photomagnetic measurements were performed using a LD Pumped All-Solid-State Laser with 532 nm wavelength coupled via an optical fiber to the cavity of a Quantum Design MPMSXL5 SQUID-Magnetometer operating with an external magnetic field of 10 kOe. The sample was located at the center of the standard sample holder produced by Quantum Design Company. The weight was estimated by comparing the thermal SCO curve with that for an accurately weighted sample. It was noted that there was no change in the data due to irradiative heating of the sample. Standardized method for determining LIESST properties was followed.17 After being cooled slowly to 10 K, the sample was irradiated and the change in magnetism followed. When the saturation point had been reached, the irradiation was ceased and the temperature increased at a rate of 0.3 K min-1 while the magnetization was measured every 1 K. T(LIESST) was determined from the minimum of the dc M T/dT vs. T curve. The data were corrected for the magnetization of the sample holder and the diamagnetic contributions, estimated from the Pascal’s constants.

X-Ray crystallography Crystallographic data for single crystals of the compounds were collected on a Nonius Kappa CCD diffractometer with a 2.0 kW sealed-tube source using graphite monochromated Mo-Ka radi˚ and an Oxford low-temperature system. ation of l = 0.71073 A Intensities were corrected for Lorentz and polarization effect and empirical absorption. The structures were solved by direct method, and refined by full-matrix least-squares on F 2 using SHELX program.18 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added by calculated geometry. Some constraints were used to model the disorder in solvent molecules.

Mössbauer spectroscopy ¨ The Mossbauer spectra were acquired using a conventional spectrometer in the constant-acceleration mode equipped with a 57 Co source (3.7 GBq) in rhodium matrix. The isomer shift values (d) are with respect to a a-iron foil at room temperature. The ¨ sample was inserted inside an Oxford Instruments MossbauerSpectromag 4000 Cryostat. The sample temperature can be varied between 3.0 and 300 K. 4858 | Dalton Trans., 2010, 39, 4856–4868

Results and discussion Synthesis Scheme 1 gives a schematic representation of the ligands used in this work. The 1 : 1 condensation reactions between pyridine-2-aldehyde, 2-acetylpyridine or 2-benzoylpyridine and benzhydrazide or salicylhydrazide yielded the Schiff base ligands HL1 –HL6 . The FeII complexes can be synthesized by the slow diffusion method by using iron(III) perchlorate as reagent in neutral or basic media. All reactions were carried out in air; no precautions to exclude air or moisture were found to be necessary. Under the reaction conditions, the ligand molecule tautomerises from the ketonic to the enolic form and the initial iron(III) can be ¨ spectra (see below) reduced into iron(II) ions.14c The Mossbauer of 4 and 5 also confirm the metal ions are iron(II) rather than iron(III). The IR spectra of the complexes show strong peaks at the region of 1358–1373 cm-1 and double bands around 1553– 1620 cm-1 , which are assigned to u(C–O- ) and u(C=N–N=C) of hydrazone, respectively.14,15 The characteristic bands of ClO4 were absent in the IR spectra. From these observations, it is concluded that the ligands react in the enol forms in stead of keto form by deprotonation of N–H proton and coordinated to the metal ions FeII . The compounds were also characterized by single-crystal X-ray diffraction and microanalyses, which confirm that the ligands HL1 –HL6 bind to iron(II) ions in two equivalent as monodeprotonated form producing charge-neutral FeII complexes. The final compounds 1–6 crystallized as black green blocks and were stable at room temperature in air without oxidization, decomposition or loss of solvent molecules. On the other hand, different methods were also used to find a universal way to synthesis this series of complexes. It was found that iron(II) complexes can also be obtained with iron(II) perchlorate as starting reagent both by slow diffusion and by the precipitation method with the same basic medium at room temperature, only with a lower yield than using iron(III) salts as starting reagents. Furthermore, the use of a suitable base was also found to be important to afford good yields of the complexes, as the same experiments conducted without base gave poor yields or no product at all. Different bases can be used in the procedure, such as triethylenetetramine and ammonia, but the optimal base was NaOH, which allows for shorter times to obtain suitable crystals and in a higher yield than using triethylenetetramine or ammonia. Magnetic, photomagnetic and thermal studies The thermal dependence of the product c M T for 1 is displayed in the ESI, Fig. S1,† where c M is the molar magnetic susceptibility and T is the absolute temperature. At 300 K, c M T is equal to 3.41 cm3 K mol-1 , which is in the range of the values expected for an iron(II) ion in the HS state. As the temperature is lowered, c M T remains almost constant and then decreases more markedly in the temperature interval 20–5 K attaining a value of 2.30 cm3 K mol-1 . This low temperature variation typically corresponds to the occurrence of zero-field splitting of the S = 2 state. The thermal dependence of the product c M T for 2 is displayed in Fig. 1. The room temperature value of c M T for 2 is 0.20 cm3 K mol-1 , which is a little higher than the value expected for LS iron(II). Upon heating the moment increases This journal is © The Royal Society of Chemistry 2010


Fig. 1 Plot of the c M T vs. T for 2. 䉱 = data recorded in the heating mode without irradiation; 䉲 = data recorded in the cooling mode without irradiation; = data recorded with irradiation at 10 K; = T(LIESST) measurement, data recorded in the warming mode with the laser turned off after irradiation reaching saturation. Inset: the larger part of the T(LIESST) measurement (left); plot of dc M T/dT vs. T indicating a T(LIESST) value (right).

until it reaches a value of 2.05 cm3 K mol-1 at 390 K, which highlights the SCO transition of the iron centers from LS to HS state. As the limit temperature of the magnetometer is reached, the completeness of the transition can not be observed. Upon cooling a gradual transition into the LS state is observed that is complete at approximately 250 K. The low-temperature value of c M T at 100 K is 0.06 cm3 K mol-1 , indicating that all iron centers are essentially in the LS state. Measurements in both heating and cooling cycles have been performed and no obvious thermal hysteresis loop was found. The characteristic temperature T 1/2 , defined as the temperature at which the sample shows a population of 50% high-spin species, may higher than 350 K for 2. The LIESST experiment was carried out on a microcrystalline sample of 2. Irradiation with the green light (l = 532 nm) at 10 K yields a slightly increase in the c M T value (Fig. 1 inset), which is attributed to the LS to HS SCO at the Fe(II) site. The minimum of the dc M T/dT vs. T curve defines the T(LIESST) limit temperature, which is approximately 43 K for compound 2. As compound 2 is dark-green in color, it is difficult to irradiate the inner part of the crystals therefore only very low amount (2% around) of photo-excitation is found. Compound 3 does not show obvious evidence for spin transition but appears to be predominantly low-spin state in the temperature range of 5-390 K, see ESI, Fig. S2.† Its c M T value of 0.46 cm3 K mol-1 at 300 K, implying it is ca. 15% high-spin at room temperature. In accordance, the X-ray diffraction studies at 290 K exhibits little longer Fe–N and Fe–O bond distances than the observed for the LS state, which can be also attributed to an admixture of LS and HS bond lengths. Fig. 2 shows the thermal dependence of the c M T product of compound 4. At 300 K, the c M T product of 4 has a value of 3.31 cm3 K mol-1 , indicating that most of the iron(II) ions are in the HS state (S = 2) at this temperature. c M T diminishes stepwise from 300 K down to 110 K with an inflection point at 190 K, and then reaching a plateau value of 1.09 cm3 K mol-1 below 100 K. The T 1/2 temperature for each step is 150 K and 220 K, respectively. Below 20 K, c M T presents a sharp decrease to reach a value of 0.65 cm3 K mol-1 at 4 K, that is probably due to a zero-field splitting This journal is © The Royal Society of Chemistry 2010

Fig. 2 Plot of the c M T vs. T for 4 in the heating (䉱) and cooling mode (䉲). Inset: Plot of dc M T/dT vs. T indicating the T S values between 150–240 K.

and possibly weak antiferromagnetic coupling of the Fe(II) ions remaining in the HS configuration. Upon subsequent heating, the similar behaviour as in the cooling mode is observed but with a slightly lower path appearing from 190 K to 240 K, leading to an 8 K wide hysteresis loop in the high-temperature step. This thermal hysteresis loop is apparent and directly related to the removal of the solvent molecules. Furthermore, the first derivatives of the c M T product relative to the temperature, dc M T/dT, in cooling and heating mode suggest that the FeII sites switch in a two-step process within the narrow temperature range of 190–240 K, with T S1↑/↓ = 204/202 K and T S2↑/↓ = 227/219 K for the so called “step 1” and “step 2” respectively (Fig. 2 inset). The magnetic susceptibility results described here suggest that approximately three-quarters of the Fe(II) ions undergo a stepwise thermal spin transition, with ca. one-quarter remaining in the high-spin state ¨ at all temperatures. Mossbauer results also confirmed a 23.5% constant residual high-spin fraction of the total amount of FeII ions, which can be assigned to structural or topological defects. Such stepwise SCO is seldom found in mononuclear Fe(II) system. In most examples, two-step SCO was observed for some FeII dinuclear complexes and for the 1D chain when there were two crystallographically different iron centers in the HS and/or LS phase.11c,19 However, two-step transitions were also observed in compounds with crystallographically equivalent metal ions.10b,20 In such cases, the two-step SCO may be explained by a competition between antagonistic short-range (inter- or intramolecular) interactions and long-range elastic interactions or structural phase transition. In the case of compound 4, the single crystal structure only shows the occurrence of one type of crystallographic iron atom at 290 K. For we have no evidence of a structural crystallographic transition at lower temperatures for this compound and the main reason for the stepwise phenomenon is still requires further research using several methods, especially detailed single-crystal X-ray studies at several temperatures between 90 and 290 K. The LIESST phenomenon for 4 was also monitored with photomagnetic measurements, as shown in Fig. 3. The excitation took place at 10 K with 532 nm laser source for 8 h until the saturation point had been reached. The c M T reached limit value of 2.7 cm3 mol-1 K after the irradiation, which did not increase when the irradiation was ceased. Comparison of the c M T value recorded at room temperature, which were as expected for a Dalton Trans., 2010, 39, 4856–4868 | 4859


Fig. 3 Plot of the c M T vs. T in the range of 10–300 K for the photomagnetic effect for 4. = data recorded with irradiation at 10 K; = T(LIESST) measurement, data recorded in the warming mode with the laser turned off after irradiation reaching saturation. For comparative purposes, the thermal variation plots are shown. 䉱 = data recorded in the heating mode without irradiation; 䉲 = data recorded in the cooling mode without irradiation. Inset: plot of dc M T/dT vs. T indicating a T(LIESST) value.

purely HS state, and the c M T value recorded with photoexcitation results in 75% photoconversion. The value recorded at higher temperature 90 K is 0.8 cm3 mol-1 K, indicating the occurrence of incomplete relaxation from the metastable HS state to the LS state. When the temperature was further increased, c M T increased stepwise reaching a room temperature value of 3.4 cm3 mol-1 K with 1 T 1/2 ↑ = 150 K and 2 T 1/2 ↑ = 200 K, respectively. Compared with the thermal magnetic measurement results, the hightemperature step (2 T 1/2 ↑ = 200 K) in thermal relaxation of the T(LIESST) measurement is steeper and the two-step process is absent. Furthermore, there are two minima in the dc M T/dT vs. T curve (Fig. 3 inset), which defines the two T(LIESST) values, at 68 K(1) and 76 K(2). In comparing the shapes of the dc M T/dT vs. T curve, we found that T(LIESST)1 at 68 K has a sharp minimum, indicative of strong cooperativity.17 In contrast, T(LIESST)2 at 76 K is small and round. This behaviour is in agreement with the thermal SCO progress. As the empirical relation T(LIESST) = T 0 - 0.3T 1/2 , with T 0 = 150 K for meridional tridentate ligands, has been defined,17 it is reasonable to connect the interactions that contribute to gradual thermal spin transition at 1 T 1/2 (= 150 K) with the light-induced T(LIESST)2 value (= 76 K) and the interactions that contribute to the sharp thermal spin transition at 2 T 1/2 (200 K) with T(LIESST)1 (= 68 K). A DSC measurement for 4 was recorded in the temperature range 195–300 K with a heating and cooling rate of 5 K min-1 and the experimental curves are given in Fig. 4. The values of the transition temperatures form the onset of the DSC peaks with 208/217 K and 222/229 K (cooling/heating). The width of the thermal hysteresis then follows as DT S1 (212.5 K step) = 9 K and DT S2 (225.5 K step) = 7 K. These values are in agreement with results from the susceptibility measurements, taking into account that the onset temperatures were determined with a scanning rate a little bit faster than for the SQUID in the settle mode. The enthalpy changes, DH, associated with each step of the spin conversion have been calculated by averaging the measurements obtained from increasing and decreasing temperatures. The average values are 4860 | Dalton Trans., 2010, 39, 4856–4868

Fig. 4 Differential scanning calorimetry (DSC) for 4 in the heating and cooling modes.

0.30 kJ mol-1 and 0.89 kJ mol-1 for 4. The corresponding entropy gains (DS), calculated from these thermal anomalies using the relation DH = TDS (T = (T cooling + T heating )/2) are 1.39 J mol-1 K-1 (T S1 ) and 3.95 J mol-1 K-1 (T S2 ), indicating that both steps are accompanied by different entropy variations. These values are far smaller than the expected values for spin transitions in FeII compounds. The c M T value of compound 5 is 3.49 cm3 K mol-1 at 380 K, which is close to the expected value for a HS state (S = 2) of an iron(II) ion (Fig. 5). Upon cooling, the c M T value starts to decrease and no plateau was observed until 10 K approaching a value of 0.095 cm3 K mol-1 . This value can be attributed to the iron(II) LS (S = 0) state. The characteristic temperature T 1/2 = 273 K was observed. Measurements were performed in both heating and cooling cycles and no obvious thermal hysteresis loop was found.

Fig. 5 Plot of the c M T vs. T for 5. 䉱 = data recorded in the heating mode without irradiation; 䉲 = data recorded in the cooling mode without irradiation; = data recorded with irradiation at 10 K; = T(LIESST) measurement, data recorded in the warming mode with the laser turned off after irradiation reaching saturation. Inset: Plot of dc M T/dT vs. T indicating a T(LIESST) value.

Fig. 5 also shows the photomagnetic behaviours of compound 5. At 10 K, irradiation results in a sharp increase in c M T. The magnetic signal reaches saturation under light irradiation at a value close to 1.2 cm3 K mol-1 . As the temperature is increased, c M T initially increases slightly before reaching a plateau and then decreases sharply to recover the magnitude before This journal is © The Royal Society of Chemistry 2010


irradiation. The small increase in the magnetic signal recorded at low temperatures corresponds to the anisotropy of the HS iron(II) ion in octahedral surroundings, also known as zero-field splitting. The minimum of the dc M T/dT vs. T curve defines the T(LIESST) limit temperature, which is 57 K for compound 5. The low level of photoexcitation, 35% around, is in agreement with the dark colour of compound 5. Differential scanning calorimetry (DSC) measurements were carried out on 5 between 200 and 350 K at heating and cooling rates of 5 K min-1 . The temperature dependence of the heat flow in the heating and cooling modes of 5 is shown in Fig. 6. In the heating and cooling mode, broad endothermic peaks and exothermic peaks at around 273 K are consistent with the magnetic measurements. The enthalpy changes, DH, associated with the spin conversion have been calculated by averaging the measurements obtained from increasing and decreasing temperatures. The average values are 4.90 kJ mol-1 for 5. The corresponding entropy gains (DS), calculated from these thermal anomalies using the relation DH = TDS (T = (T cooling + T heating )/2) is 17.88 J mol-1 K-1 , which is smaller than the other related FeII –N4 O2 compounds constructed from the tetra-dentate ligands.11a

Fig. 6 Differential scanning calorimetry (DSC) for 5 in the heating and cooling modes.

Compound 6 appears to be predominantly low-spin in the temperature range of 5–380 K (ESI, Fig. S3).† In accordance, the crystal structure at 293 K exhibits Fe–N bond distances typical for the iron(II) LS state (Table 4, see details below). In comparison with compound 4 and 5, the substituted phenyl group leads obviously to this drastically changed ST behaviour; proving high sensitivity of the spin transition to subtle changes in the molecular environment.

Mössbauer spectroscopy ¨ Mossbauer spectra on polycrystalline samples of 4 recorded in cooling mode are displayed in Fig. 7 and the corresponding spectral parameters and errors are listed in Table 1. ¨ Mossbauer spectra obtained at 240, 160, 100, 20, and 3 K for complex 4 mirror the variable temperature magnetic susceptibility data. At 240 K two sets of doublets with very different quadrupole splittings (DE Q ) are observed, typical for similar iron(II) complexes that exhibit gradual spin conversions. Henceforth, left doublet will imply turquoise line, right doublet—green line and central doublet—blue line. The left doublet with DE Q of 1.24 mm s-1 and an isomer shift (d) of 0.39 mm s-1 are assigned to the low-spin This journal is © The Royal Society of Chemistry 2010

¨ Fig. 7 The 57 Fe Mossbauer spectra of 4 at 240, 160, 100, 20 and 3 K (green line, HS FeII ; turquoise line, LS FeII ; red line, the sum of HS and LS FeII ). See Table 1 for the fitting parameters.

isomer, in accordance with the magnetic data. The right doublet, with DE Q of 2.48 mm s-1 and d of 0.95 mm s-1 , is assigned to the high-spin isomer. With decreasing temperature, the intensity of the left doublet increases with a concomitant decrease in the intensity of the right doublet, which is the dominant absorption at 100 K and suggesting that a thermally induced spin-crossover is operative in 4. Between 100 and 3 K, there is a very minor change in the relative intensities of the left and right doublets. At 3 K, the area ratio of the two quadrupole doublets of the high-spin and low-spin FeII ions changes to ~ 1 : 3.2, meaning the predominant Dalton Trans., 2010, 39, 4856–4868 | 4861

¨ Table 1 Mossbauer parameters for 4 T/K

d a /mm s-1

DE Q /mm s-1

C/mm s-1

Area (%)

Assignment

240

0.39(1) 0.955(4) 0.372(6) 0.96(1) 0.371(5) 1.02(2) 0.373(6) 1.07(1) 0.370(1) 1.074(2)

1.24(2) 2.483(8) 1.26(1) 2.56(2) 1.315(7) 2.50(3) 1.32(1) 2.42(3) 1.318(1) 2.421(3)

0.65(3) 0.32(1) 0.450(6) 0.304(8) 0.41(1) 0.40(3) 0.391(8) 0.41(2) 0.386(3) 0.358(7)

52.79 47.21 70.89 29.11 75.45 24.55 75.55 24.45 76.52 23.48

LS HS LS HS LS HS LS HS LS HS

160 100 20


3

a Relative to a-Fe at room temperature. The statistical errors are given in parentheses. HS = high-spin; LS = low-spin.

existence of the low-spin FeII species and a small fraction of highspin species (23.5%). In spite of the high transition temperature of compound 5, we ¨ ¨ collected several Mossbauer spectra, see Fig. 8. The Mossbauer data at 286 K, like those recorded for complex 4, indicate that complex 5 consists of spin isomers, with the low-spin isomer (left doublet) dominant at this temperature, displaying DE Q and d values that are consistent with S = 0 iron(II) and high-spin isomer (right doublet) displaying DE Q and d values that are consistent with S = 2 iron(II) (Table 2). A good fit for this spectrum and of spectra at lower temperatures could only be achieved only after the introduction of an additional doublet (central doublet) with parameters which might be attributed to an iron(II) with intermediated spin S = 1 or else from the occurrence of two different but closely related iron(II) sites in the LS state. From a structural viewpoint these sites might correspond to different species in slightly distinct chemical surroundings. With decreasing temperature from 286 to 158 K, the intensity of the left doublet increases with a concomitant decrease in the intensity of the right doublet, suggesting again, as in the case of 4 compound, that a thermally induced spin-crossover is operative in 5. At ¨ 3 K the Mossbauer spectrum consists of a unique quadrupolesplit doublet with an isomer shift, d, of 0.35 mm s-1 and a quadrupole splitting, DE Q , of 1.22 mm s-1 . These parameters are typical for LS iron(II). The lines are broader than those at higher temperature which might also require introduction of a small ¨ Table 2 Mossbauer parameters for 5 T/K

d a /mm s-1

DE Q /mm s-1

C/mm s-1

Area (%)

Assignment

286

0.27(1) 0.88(4) 0.61(6) 0.280(2) 0.90(1) 0.73(2) 0.289(1) 1.01(2) 0.73(2) 0.3012(6) 1.14(4) 0.74(3) 0.349(6)

1.18(2) 2.22(9) 1.53(1) 1.21(5) 2.37(3) 1.47(3) 1.230(2) 2.41(5) 1.44(4) 1.234(1) 2.37(7) 1.47(1) 1.22(1)

0.46(1) 0.53(2) 0.43(6) 0.394(6) 0.47(3) 0.37(7) 0.347(4) 0.61(8) 0.44(9) 0.344(2) 0.51(4) 0.50(4) 0.42(1)

62.35 29.72 7.92 76.12 17.75 6.12 83.03 10.80 6.16 83.92 7.67 8.40 100

LS HS IS LS HS IS LS HS IS LS HS IS LS

260 195 158 3

a Relative to a-Fe at room temperature. The statistical errors are given in parentheses. LS = low spin; HS = high spin; IS = intermediate spin.

4862 | Dalton Trans., 2010, 39, 4856–4868

¨ Fig. 8 The 57 Fe Mossbauer spectra for 5 at 286, 260, 195, 158 and 3 K (green line, HS FeII ; turquoise line, LS FeII ; blue line, IS FeII ; red line, the sum of HS, IS and LS FeII ). See Table 2 for the fitting parameters.

additional doublet with values comparable to those of iron(II) sites in the LS state. Crystal structure Crystals suitable for X-ray structure analysis have been obtained for all the complexes. The relevant crystallographic data for 1–6 are summarized in Table 3. Selected bond lengths and angles within the coordination sphere are listed in Table 4. Selected intraand intermolecular distances are reported in Table 5. In the case This journal is © The Royal Society of Chemistry 2010


Table 3 Crystallographic data and structure refinements for 1–6 Compound

1

2

3

4

5 (290 K)

5 (110 K)

6

Formula FW/g mol-1 T/K Crystal system Space group ˚ a/A ˚ b/A ˚ c/A a/◦ b/◦ g /◦ ˚3 V /A Z Dc /g cm-3 m(Mo-Ka)/mm-1 Crystal size/mm

C26 H20 FeN6 O2 504.33 290(2) Monoclinic P21 /n 9.778(2) 23.745(5) 10.281(2) 90.00 103.96(3) 90.00 2316.6(8) 4 1.446 0.688 0.30 ¥ 0.25 ¥ 0.15 0.795, 0.911 27 205 4073(0.0555) 2888 316 0.0304, 0.0714 0.0551, 0.0761 1.000 0.213, -0.280

C28 H24 FeN6 O2 532.38 290(2) Monoclinic Cc 9.982(2) 20.133(4) 12.113(2) 90.00 91.63(3) 90.00 2433.3(8) 4 1.453 0.659 0.25 ¥ 0.20 ¥ 0.20 0.853, 0.879 4160 4155(0.0072) 2836 337 0.1066, 0.2487 0.1389, 0.2654 1.000 2.617, -0.673

C38 H28 FeN6 O2 656.51 290(2) Triclinic P1¯ 11.036(2) 12.326(3) 12.517(3) 66.63(3) 83.03(3) 81.18(3) 1541.2(5) 2 1.415 0.536 0.25 ¥ 0.15 ¥ 0.12 0.878, 0.938 24 095 5392(0.0743) 3474 425 0.0374, 0.0600 0.0774, 0.0651 1.000 0.210, -0.292

C26 H21 FeN6 O4.5 545.34 290(2) Monoclinic C2/c 18.485(4) 24.520(5) 11.575(2) 90.00 104.70(3) 90.00 2537.2(1) 4 1.428 0.641 0.15 ¥ 0.15 ¥ 0.12 0.910, 0.927 34 178 4466(0.0747) 2398 341 0.0497, 0.1322 0.1103, 0.1547 0.997 0.627, -0.157

C28 H24 FeN6 O4 564.38 290(2) Orthorhombic Aba2 12.580(3) 18.805(4) 10.589(2) 90.00 90.00 90.00 2504.9(9) 4 1.497 0.651 0.12 ¥ 0.10 ¥ 0.07 0.926, 0.956 2098 2098(0.0000) 1055 181 0.0445, 0.0496 0.1407, 0.0598 0.965 0.155, -0.141

C28 H24 FeN6 O4 564.38 110(2) Orthorhombic Aba2 12.398(3) 18.807(4) 10.325(2) 90.00 90.00 90.00 2407.6(8) 4 1.557 0.677 0.12 ¥ 0.10 ¥ 0.07 0.923, 0.954 16 624 2353(0.0566) 1954 180 0.0297, 0.0631 0.0437, 0.0665 1.011 0.278, -0.183

C38 H28 FeN6 O4 688.51 290(2) Monoclinic P21 /c 10.652(2) 13.796(3) 22.181(4) 90.00 91.87(3) 90.00 3257.9(11) 4 1.404 0.515 0.25 ¥ 0.22 ¥ 0.12 0.812, 0.984 41 687 5695(0.1053) 2862 445 0.0435, 0.0585 0.1190, 0.0649 1.037 0.407, -0.302

T min and T max No. total reflections No. unique reflections (Rint ) No. observed [I ≥ 2s(I)] No. parameters R1 , wR2 [I ≥ 2s(I)] R1 , wR2 (all data) GOF ˚3 Dr, e/A

˚ ) and angles (◦ ) within the coordination sphere of FeN4 O2 octahedral geometry with spin state of 1–6 Table 4 Selected bond lengths (A T/K

S

˚ Fe–N1/N4/A

˚ Fe–N2/N5/A

˚ Fe–O1/O2/A

N2–Fe–N5/◦

∠L1,L2 a /◦

1

290(2)

2

290(2)

0

3

290(2)

~0

4

290(2)

2

5 (290 K) 5 (110 K) 6

290(2) 110(2) 290(2)

2 0 0

2.095(2) 2.103(2) 1.864(8) 1.857(8) 1.875(2) 1.884(2) 2.079(3) 2.079(3) 2.024(3) 1.870(2) 1.864(2) 1.870(2)

2.119(1) 2.083(2) 2.003(7) 1.988(8) 1.993(2) 1.975(2) 2.086(3) 2.124(3) 2.068(3) 1.995(2) 1.973(2) 1.985(2)

158.6(7)

2

2.264(2) 2.236(2) 1.947(9) 1.915(9) 1.966(2) 1.963(2) 2.236(3) 2.204(4) 2.083(4) 1.946(2) 1.927(2) 1.944(2)

b

/◦

Hc /◦

85.9

154.3

10.34

176.5(4)

85.9

82.8

5.99

174.0(1)

84.8

87.3

6.27

166.7(1)

87.0

144.8

9.68

174.6(3) 176.0(2) 179.4(0)

83.3 83.3 87.2

130.7 84.9 78.7

8.88 6.12 5.66

Dihedral angle between the least-squares planes of the tridentate ligands. b The distortion parameter , which is defined as sum of the deviations from ◦ c 90 of the 12 cis angles in the coordination sphere. The distortion parameter H, which is defined as the sum of the deviations from 60◦ of the 24 possible q angles on the dihedral angles between two triangular planes.

a

of 5, the X-ray crystal structure was measured before and after the spin transition. All the ligands HL1-6 used are tridentate ligands with the enolicO, azomethine-N and pyridine-N donor sites. Such two ligand molecules are coordinated to the Fe(II) center in monoanionic form after deprotonation of the enolic oxygen atom, producing a charge-neutral six-coordinate Fe(II) complex. The planar ligands in mer configurations are perpendicular and enforce meridional coordination geometries around metal ions. A pronounced distortion of the FeN4 O2 octahedron also induced from the inequivalence of the bond lengths. For comparison, the structures are divided into two groups, 1–3 and 4–6, according to the presence of the same lateral chain but different substituents at R1 in the ligands. This journal is © The Royal Society of Chemistry 2010

Crystal structures of 1–3 Perspective views of the molecular structure of 1–3 are shown in Fig. 9. Compound 1 crystallizes in the monoclinic space group P21 /n with Z = 4. Each of the FeII ions is surrounded by two nitrogen atoms and one oxygen atom belonging to the each ligands, forming a distorted octahedral pyramid in the N4 O2 donor set. Both tridentate ligands in mer configuration are perpendicular with a dihedron angle of 85.9◦ . The bond angle N2–Fe–N5 = 158.6(7)◦ deviated considerably from the ideal value of 180◦ . The ˚ (Fe–N2/N5), 2.10 A ˚ (Fe–O1/O2) average bond lengths are 2.10 A ˚ (Fe–N1/N4), which are similar to the characteristic and 2.25 A bond lengths of high spin state FeII of the N4 O2 doner set.11 It is also confirmed by the magnetic investigations which show high spin Dalton Trans., 2010, 39, 4856–4868 | 4863

Table 5 Main hydrogen-like interactions

1 2 3 4


5 290 K 5 110 K 6

D–H ◊ ◊ ◊ A

˚ d(D–H)/A

˚ d(H ◊ ◊ ◊ A)/A

˚ d(D ◊ ◊ ◊ A)/A

∠(DHA)/◦

C17–H17 ◊ ◊ ◊ O1a C24–H24 ◊ ◊ ◊ N3b C4–H4 ◊ ◊ ◊ O1c C18–H18 ◊ ◊ ◊ O2d C12–H12 ◊ ◊ ◊ N3e O2–H2 ◊ ◊ ◊ N3 O4–H4 ◊ ◊ ◊ N6 O2–H2A ◊ ◊ ◊ N3 C4–H4 ◊ ◊ ◊ O1f O2–H2A ◊ ◊ ◊ N3 C1–H1 ◊ ◊ ◊ O2g C4–H4 ◊ ◊ ◊ O1h O2–H2A ◊ ◊ ◊ N3 O4–H4A ◊ ◊ ◊ N6 C23–H23 ◊ ◊ ◊ O2i

0.93 0.93 0.93 0.93 0.93 0.82 0.82 0.82 0.93 0.82 0.95 0.95 0.82 0.82 0.93

2.490 2.620 2.480 2.440 2.570 1.840 1.840 1.840 2.500 1.810 2.430 2.450 1.830 1.860 2.390

3.229(3) 3.444(3) 3.399(18) 3.268(15) 3.313(3) 2.566(5) 2.548(5) 2.554(6) 3.304(8) 2.551(3) 3.274(3) 3.347(3) 2.553(3) 2.582(3) 3.183(4)

137.00 148.00 169.00 149.00 137.00 147.00 144.00 144.00 144.00 146.00 148.00 158.00 147.00 147.00 143.00

Symmetry codes: a 1 - x, -y, 1 - z. b -1 + x, y, -1 + z. c 1/2 + x, 1/2 - y, 1/2 + z. d 1/2 + x, 1/2 - y, -1/2 + z. e 2 - x, 1 - y, 1 - z. f -1/2 + x, 1 - y, -1/2 + z. g -1/2 + x, 2 - y, -1/2 + z. h 1/2 + x, 2 - y, -1/2 + z. i 1 + x, y, z.

over the whole temperature range. Intermolecular p ◊ ◊ ◊ p staking between the pyridine rings and benzene rings in the neighbouring ligands and intermolecular interactions between carbon atoms C6 and the p ring of lateral benzene rings in the neighbouring ˚ molecules, namely C6–H ◊ ◊ ◊ p, are found with a distance of 3.697 A ˚ , respectively (ESI, Fig. S4).† and 3.582(4) A Compound 2 crystallizes in the monoclinic space group Cc with Z = 4. Similar to the structure described for 1, the Fe atom is hexa-coordinated by two units of tridentate Schiff base ligands with the enolate O, azomethine N and pyridine N as donor atoms. The methyl group substituent and the aromatic lateral rings attached to the carbonyl group are close to parallel to the plane of the coordinated ligand and the least square plane of the slightly distorted coordinated ligands makes an angle of ˚ (Fe– 86.2◦ . The average bond lengths involving iron are 1.93 A ˚ ˚ N1/N4), 1.99 A (Fe–O1/O2) and 1.86 A (Fe–N2/N5), which are ˚) shorter than those in 1. Two hydrogen-bonds (C4 ◊ ◊ ◊ O1 = 3.399 A ˚ ) are found in the crystal packing, as and (C18 ◊ ◊ ◊ O2 = 3.268 A shown in Fig. 10, leading to an infinite two-dimensional layer of molecules. Attempts to record the structure in the HS form at high temperature were unsuccessful due to the formation of cracks in the crystal. Compound 3 crystallizes in the triclinic space group P1¯ with Z = 4. The phenyl ring substituents adjacent to the 2-pyridyl ring are twisted out of the plane so as to avoid H ◊ ◊ ◊ H repulsion with the proton at the 3-position of the pyridyl ring. The dihedral angle between the two mer configuration ligand planes defined by the pyridine and two consecutive five-membered chelate rings of the coordinated hydrazone is ca. 84.3◦ , thus resulting in the distorted octahedral coordination geometry. The coordination bond lengths (Table 4), which are longer than observed for the LS state but shorter than observed for typical HS ions, indicate a mixed spin state in the crystal. These results are in good agreement with results from variable-temperature magnetic measurement. The comparison of the structures and properties of compounds 1, 2 and 3 shows that the three complexes present similar molecular structures but exhibit different magnetic behaviours. Comparing the single crystal structures at 290 K of the three compounds 4864 | Dalton Trans., 2010, 39, 4856–4868

in different spin state, huge differences in bond lengths between high-spin and low-spin states are found. As the ligand field strength strongly depends on the Fe–L distance, a substitution effect on the ligand field could be suggested. On exchange of H for CH3 or Ph group at the position R1 of the ligands, a shortening of the neighbouring Fe–N and Fe–O bonds occurs and consequently an enhancement of the ligand field strength. As a result, the paramagnetic iron(II) complex becomes a spin crossover or diamagnetic complex. A pronounced distortion of the FeN4 O2 octahedron is also induced from the inequivalence of the bond lengths. In the highspin state, the Fe–O1(/O2) distance appears to be shorter than Fe– N1(/N4) distance, but longer than Fe–N2(/N5) distance, owing to the fact that the central N2 donor is involved in two chelate rings on either side. While in the low-spin state, the Fe–O1(/O2) distance tends to be the longest one in the three types of the coordination bonds, accompanied by the decreased bond distance compared with those in the HS state. The respective coordination polyhedra of compounds 1, 2 and 3 show different degrees of distortion, which may be related qualitatively in terms of the parameters 6b,21 and H22 as defined in Table 4. Distortion of the coordination sphere from ideal octahedral is related to the ligand field strength on the iron ions. A larger value, meaning a larger distortion from the Oh symmetry, corresponds to weaker values of the central ligand field strength on iron(II). Here, the ◦ ◦ iron(II) ions are 154.3 in 1, 82.8 in 2 and 87.3◦ in 3, respectively, which show that the coordination geometry of the low-spin state is much closer to a regular octahedron than that of the high-spin state. For a mixed-spin state compound, such as 3, the octahedral distortion parameter is between the values of the low-spin and high-spin state as is often the case in many other spin crossover systems. Similar trends are also found in the value of H, which represents the average trigonal distortion, varying from 5.99◦ (LS) to 10.34◦ (HS). On the other hand, the observed Nazo –Fe–Nazo angles tend to be closer to the ideal value of 180◦ in the low-spin state and to 158.6◦ in the high-spin state, which might provide another tool for determining the spin state in this series of iron(II) complexes. This journal is © The Royal Society of Chemistry 2010


Fig. 10 Packing view of 2 along c axis at 290 K.

Fig. 9 X-Ray crystal structures of 1 (a), 2 (b) and 3 (c) at 290 K. Hydrogen atoms were omitted for clarity.

Crystal structures of 4–6 X-Ray crystal structures of 4–6 are presented in Fig. 11, and show similar structures to those of compounds 1–3, except that OH This journal is © The Royal Society of Chemistry 2010

groups are attached to the lateral aryl rings at the R2 site in the corresponding ligands. Typical intramolecular hydrogen bonding between the phenolic oxygen and the hydrazide nitrogen atoms O2–H2 ◊ ◊ ◊ N3/O4-H4 ◊ ◊ ◊ N6 appear in these complexes as a result of the deprotonation of –NH-CO– group of the ligands. These hydrogen bonds contribute to the nearly planar configuration of the ligands. Compared with compound 1, the average Fe–N and Fe–O distances of 4 are shorter. Calculation of the octahedral distortion parameter, = 144.8◦ , and the trigonal distortion parameter, ◦ H = 9.68 , shows that the distortion of iron(II) coordination geometry is a bit smaller than that of 1. Nevertheless, these values are still near to the value of the typical HS FeII ion. Intramolecular hydrogen bonding between the phenolic oxygen and the hydrazide nitrogen atoms O2–H2 ◊ ◊ ◊ N3/O4–H4 ◊ ◊ ◊ N6 are found while the substituent OH groups are not involved in the formation of intermolecular interactions. The solvent molecules are disordered and do not participate in the formation of intermolecular hydrogen bonding. Instead, a series of intermolecular interactions exist between the carbon atoms C6 and the p ring of lateral benzene rings in the neighbouring molecules (Fig. 12), with C6–H ◊ ◊ ◊ p ˚ in length. These short interactions might be responsible 3.582(4) A for the stepwise spin transition occurring at low temperature. Unfortunately, repeated attempts to grow single crystals suitable for variable temperature X-ray diffraction studies have been unsuccessful. Herein, no further discussions are made. X-Ray structure analyses of compound 5 were performed at 110 K and 290 K, respectively. With decreasing temperature, the space group Aba2 is unchanged implying that there is no crystallographic phase transition, while the cell dimensions decrease ˚3 ˚ 3 at 290 K to 2407.6(8) A slightly by 3.9% from 2504.9(9) A at 110 K, in line with the observed thermal spin transition in 5. The two nearly planar ligands are perpendicular to each other around the metal ion, with the same dihedral angle of 83.0◦ at 110 K and 290 K and the Nazo –Fe–Nazo angles changed slightly from 176.0(2)◦ at 110 K to 174.6(3)◦ at 290 K. Since the coordination bond lengths around the Fe(II) centers differ for lowspin (LS) and high-spin (HS) states, LS and HS states can be identified by the metal–ligand bond lengths. In fact, at 110 K, the ˚, Fe–N coordination bond lengths are 1.946(2) and 1.870(2) A ˚ , which fall into the Fe–O coordination bond length is 1.995(2) A Dalton Trans., 2010, 39, 4856–4868 | 4865


Fig. 12 Packing view of 4 at 290 K.

Fig. 11 X-Ray crystal structures of 4 (a), 5 (b) and 6 (c) at 290 K. Hydrogen atoms were omitted for clarity.

range characteristic of LS FeII ion. In contrast, at 290 K, the ˚ and Fe–O bond Fe–N bond lengths are 2.083(4) and 2.024(3) A ˚ , respectively, typical for HS FeII ion. The length is 2.068(3) A bond length of Fe–O is between the bond lengths of the two Fe–N 4866 | Dalton Trans., 2010, 39, 4856–4868

bond distances at 290 K, while it becomes the longest one in the three types of coordination bonds at 110 K. Relatively large bond ˚ ) are observed for Fe–N bonds, while length changes (0.14–0.15 A ˚ ) of Fe–O bonds are small. The difference the changes (0.07 A in Dd(Fe–L) values in this iron(II) compound is comparable with those for other FeII N4 O2 compounds,10,11 but larger than those for SCO iron(III) complexes with N4 O2 coordination, which have ˚ average changes of Fe–N bonds and Fe–O bond lengths of 0.15 A 9,12 ˚ and 0.04 A, respectively. Moreover, the distortion parameters of the octahedron, and H, are 84.9◦ and 6.12◦ at 110 K and ◦ ◦ 130.7 and 8.88 at 290 K, which are consistent with the different spin states observed at different temperature. The crystal packing of 5 in the HS and LS state is quite different, as shown in Fig. 13a and 13b. The intermolecular C4–H4 ◊ ◊ ◊ O1 contacts with C4 ◊ ◊ ◊ O1 ˚ in the HS state to 3.351(3) A ˚ separations ranging from 3.304(8) A in the LS state appear to be slightly weaker, however, a new short ˚ ) is observed in the LS state. contact C1–H ◊ ◊ ◊ O2 (3.274(3) A Moreover, intermolecular p ◊ ◊ ◊ p stacking between the coordinated pyridine rings and the lateral benzene rings in the neighbouring ˚ in the LS state to 3.916 A ˚ in the HS ligands varies from 3.773 A state. Compound 6 has a similar molecular structure to compound 3. The dihedral angle between the two mer configuration ligand planes defined by the pyridine and two consecutive five-membered chelate rings of the coordinated hydrazone is ca. 87.0◦ . The ˚ (Fe– average bond lengths of the inner coordination sphere (1.93 A ˚ (Fe–O1/O2) and 1.87 A ˚ (Fe–N2/N5) indicate N1/N4), 1.98 A low-spin iron(II), which are in full agreement with results from variable-temperature magnetic measurements. From the above results we can see that the ligand substitution effect, which show a sequence of field strengths: Ph > CH3 > H in compound 1–3, can also be applied to explain the results observed in compound 4–6. On the other hand, comparing the structures of compound 1 with 4, or 2 with 5, we can also see that the presence of the OH group at the R2 site in the lateral phenyl ring, influences the ligand field strength but does not contribute to the enhancement of the cooperative effect in the compounds. This journal is © The Royal Society of Chemistry 2010


provide a good system for circumventing some of these problems, such as excluding the effect of anions. Furthermore, it is worth noting that different substitution of the ligands has a pronounced effect on the ST properties of the compounds. Changes at the R1 site have a pronounced effect on the ST properties of the complexes. Changes at the R2 site on the lateral benzohydrazide chain do not seem to contribute to the enhancement of the cooperative effect in the system, but do affect the spin state of the compounds. These results pave the way for learning how to control the spincrossover behaviour in new systems. Therefore, this system can be expanded even further by introducing different substituent groups, and HS or LS complexes may be targeted by correct choice of these groups. Furthermore, by careful design of the ligands, polynuclear metal complexes may be generated and this work is ongoing.

Acknowledgements This work was supported by the NSFC (20821091, 20701039) and the National Basic Research Program of China (2009CB929403, 2006CB601102). Ms Zhang and Mr Xu contributed equally to the present work. Fig. 13 Packing view of 5 along a axis at 290 K (a) and 100 K (b).

Conclusion We have structurally and magnetically characterized six new neutral mononuclear iron(II) compounds with N4 O2 donor sets. Compound 2 and 5 are SCO compounds with T 1/2 higher than or near room temperature, which favours possible practical uses. The absence of thermal hysteresis behaviours might be improved by increasing the cooperativity in the molecule, e.g. by favouring weak hydrogen or p–p bonds in the crystal lattices. The construction of such mononuclear SCO compounds is under progress. Compound 4 exhibits a special thermally induced SCO occurring in stepwise fashion with a mixture of the HS and LS states present below 100 K. Further research using several methods, especially detailed single-crystal X-ray studies at several low temperatures, are needed to discover the main reasons for the stepwise phenomenon. We have also examined the photomagnetic properties of the SCO compounds. Due to the dark colour of the compounds, none of these SCO compounds display quantitative photoexcitation. We have noticed that for the compound 2, only a very low level of photoconversion, about 2%, can occur. In contrast compound 4 and 5 display about 75% and 35% level of photoexcitation. The T(LIESST) can be easily defined from the minimum of the dc M T/dT vs. T curves, which confirmed that cooperative effect exist in the systems. All the experimental T(LIESST) values follow a trend with a relationship between T(LIESST) and the T 1/2 values, i.e. T(LIESST) = T 0 - 0.3T 1/2 , with T 0 = 150 K for meridional tridentate ligands, but are smaller than the “theoretical” T(LIESST) values.17 ” A common disadvantage with all other iron(II) SCO systems is that they sometimes show extreme sensitivity to local environmental influences, including crystal packing, inclusion of solvent and anion/cation molecules, and concomitant crystalline phase transition events. The neutral SCO complexes reported in this article with their good stability in the atmosphere appear to This journal is © The Royal Society of Chemistry 2010

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