A new family of Fe2Ln complexes built from mononuclear anionic Schiff base subunits

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Tuning of spin crossover behaviour in iron(III) complexes involvingpentadentate Schiff bases and pseudohalides†

Ivan Nemec,*a,c Radovan Herchel,b Roman Boca,a Zdenek Travnıcek,c Ingrid Svoboda,d Hartmut Fuessd andWolfgang Linerte

Received 18th April 2011, Accepted 2nd August 2011DOI: 10.1039/c1dt10696a

Investigations on a series of eight novel mononuclear iron(III) Schiff base complexes with the generalformula [Fe(L5)(L1)]·S (where H2L5 = pentadentate Schiff-base ligand, L1 = a pseudohalido ligand, andS is a solvent molecule) are reported. Several different aromatic 2-hydroxyaldehyde derivatives wereused in combination with a non-symmetrical triamine 1,6-diamino-4-azahexane to synthesize the H2L5

Schiff base ligands. The consecutive reaction with iron(III) chloride resulted in the preparation of the[Fe(L5)Cl] precursor complexes which were left to react with a wide range of the L1 pseudohalidoligands. The low-spin compounds were prepared using the cyanido ligand: [Fe(3m-salpet)(CN)]·CH3OH(1a), [Fe(3e-salpet)(CN)]·H2O (1b), while the high-spin compounds were obtained by the reaction of thepseudohalido (other than cyanido) ligands with the [Fe(L5)Cl] complex arising from salicylaldehydederivatives: [Fe(3Bu5Me-salpet)(NCS)] (2a), [Fe(3m-salpet)(NCO)]·CH3OH (2b) and [Fe(3m-salpet)-(N3)] (2c). The compounds exhibiting spin-crossover phenomena were prepared only when L5 arosefrom 2-hydroxy-1-naphthaldehyde (H2L5 = H2napet): [Fe(napet)(NCS)]·CH3CN (3a, T 1/2 = 151 K),[Fe(napet)(NCSe)]·CH3CN (3b, T 1/2 = 170 K), [Fe(napet)(NCO)] (3c, T 1/2 = 155 K) and[Fe(napet)(N3)], which, moreover, exhibits thermal hysteresis (3d, T 1/2↑ = 122 K, T 1/2↓ = 117 K). Thesecompounds are the first examples of octahedral iron(III) spin-crossover compounds with thecoordinated pseudohalides. We report the structure and magnetic properties of these complexes. Themagnetic data of all the compounds were analysed using the spin Hamiltonian formalism including theZFS term and in the case of spin-crossover, the Ising-like model was also applied.

Introduction

Spin-crossover (SCO) materials can be reversibly switched betweenreference electronic states with different spin multiplicity by exter-nal stimuli, such as temperature, pressure, and light irradiation.1,2

aInstitute of Inorganic Chemistry, Slovak University of Technology,Radlinskeho 9, SK-812 37, Bratislava, Slovakia. E-mail: [email protected], [email protected] of Inorganic Chemistry, Faculty of Science, Palacky Univer-sity, Tr. 17. listopadu 12, CZ-77900, Olomouc, Czech Republic E-mail:[email protected] Centre of Advanced Technologies and Materials, Department of In-organic Chemistry, Faculty of Science, Palacky University, Tr. 17. Listopadu12, CZ-77146, Olomouc, Czech Republic. E-mail: [email protected] Science, Darmstadt University of Technology, D-64287, Darm-stadt, Germany. E-mail: [email protected] of Applied Synthetic Chemistry, Vienna University of Technology,1060, Vienna, Austria. E-mail: [email protected]† Electronic supplementary information (ESI) available: Thermal ellipsoidgraphics, selected crystallographic data, calculation of Hirschfield surfacesand fingerplots,29 cif files and details of magnetic data interpretation.CCDC reference numbers 823940–823952 and 836987. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c1dt10696a

In hexacoordinated mononuclear complexes with the d5 electronicconfiguration of the central metal ion, there are three possible spinisomers: low-spin (LS) with S = 1/2, intermediate spin (IS) with S =3/2 and high-spin (HS) with S = 5/2 with the spin-only effectivemagnetic moment meff = 1.7, 3.9, and 5.9 mB, respectively. With theregular octahedral geometry, only the S = 1/2 to S = 5/2 transitioncomes into play.

In previous work3 it was shown that chlorido-containingprecursor complexes [FeL5Cl] of the pentadentate Schiff-bases L5

(Fig. 1) are appropriate compounds for the “bottom-up” syntheticstrategy aimed at the synthesis of dinuclear4 and tetranuclear5

complexes. Even heptanuclear spin-crossover compounds areachievable when the ferrocyanide anion is used as a bridging unit.6

Recently, we have reported on a new group of dinuclear cyanido-bridged iron(III) complexes with gradual SCO behaviour.7 Thespin transition is significantly affected by the strong exchangeinteraction in these compounds and thereby it is of a gradualcharacter. However, our main task is to find appropriate ligandsor complexes, which preserve the SCO behaviour of the compoundand exhibit pronounced abruptness of the spin transition.

Reactions of various 2-hydroxybenzaldehydes with aliphatictriamines, such as dpt or pet (dpt = N-(3-aminopropyl)propane-1,

10090 | Dalton Trans., 2011, 40, 10090–10099 This journal is © The Royal Society of Chemistry 2011

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Fig. 1 Scheme showing the preparation of pentadentate ligands. Abbre-viations used in the text: ligands arising from salicylaldehyde derivatives –H2salpet (R1 = H, R2 = H), H2-3m-salpet (R1 = -OCH3, R2 = H),H2-3e-salpet (R1 = -OCH2CH3, R2 = H); H23Bu5Me-salpet (R1 = -C(CH3)3,R2 = -CH3), ligand arising from 2-hydroxynaphtaldehyde – H2napet.

3-diamine, pet = N-(2-aminoethyl)propane-1,3-diamine), yieldpentadentate Schiff-base ligands with the {N3O2} donor set. Theseeasily form complexes with FeCl3 giving rise to the [FeL5Cl]mononuclear complexes. In the final step, the chlorido ligand issubstituted by the appropriate monodentate ligand to obtain thedesired compounds of the general formula [FeL5L1] where L1 isan anionic ligand, or [FeL5L1]A (A refers to an anion, in most ofthe cases BPh4

-), if L1 is a neutral ligand. In the case of neutralheterocyclic N-donor ligands, such as derivatives of pyridine orimidazole, SCO behaviour is often observed – mostly showing agradual character.8 The use of anionic monodentate ligands is rareand refers only to either HS or LS complexes, depending on thetype of the used ligand.9

Herein we report on the synthesis of novel pseudohalido iron(III)mononuclear complexes. The pseudohalido ligands were chosen toimitate the properties of the cyanido bridging ligand and also be-cause there is a possibility to tune their ligand field strength gentlyby the alteration of the ligands with respect to the known spectro-chemical series:10 NCO- < NCS- < NCSe- < NCBH3

- � CN-.

Results and discussion

In order to develop a comparative magneto-structural study,two slightly different series of compounds were prepared: (a)the compounds whose ligands originate from the derivatives ofsalicylaldehyde, (b) compounds with the ligands prepared from 2-hydroxy-1-naphthaldehyde. The overall structure is very similarfor both groups of compounds. The pentadentate Schiff-baseligand along with the pseudohalido terminal ligand creates thedistorted octahedral environment around the iron centre with theoxygen atoms coordinated in the cis fashion, which is typical forthis kind of compounds bearing a small monodentate ligand.11

The molecule can be divided into two parts with respect to theasymmetry of the ligand’s aliphatic part (Fig. 1). These two partsdiffer in their geometries significantly, which becomes obviouswhen inspecting torsion angles in the coordination polyhedron orthe angles between the least square planes of the aromatic ringsand a plane defined by the triad of atoms FeNimO found in thesame part (in part II this angle is labelled as t(II), in part III ast(III)). This parameter expresses a deviation of the aromatic ringfrom the FeNimO plane. The value of t(III) is significantly higherthan t(II) and this documents higher flexibility of the ligand in

part III (see ESI, Tab. S1†). Generally, it can be said that part II ismore rigid having slightly shorter Fe–Nim and Fe–O bond lengthsthan those found in part III (see ESI, Tab. S1†).

In order to analyse the molecular geometry change resultingfrom the SCO phenomena, the previously reported parameterssuch as octahedral distortion12 and the dihedral angle (a) betweenthe least squares planes of aromatic rings were used. The parametera has been successfully employed to distinguish the moleculeswhich do or do not undergo SCO in the series of iron(III) complexeswith a hexadentate Schiff base ligand.13 One of the topics of thiswork was to check if there is a possibility to use the a parameterfor the same purpose in the complexes with pentadentate ligandsreported in this work.

The cyanido complexes (1a, 1b)

Complexes containing the cyanido terminal ligand with thegeneral formula [Fe(L5)(CN)] were prepared from a methanolsolution of the chlorido precursor [Fe(L5)Cl] and were refluxedwith a 2-fold molar excess of KCN. The immediate change from adark-violet to blue-green colour indicated a change in the spin stateof the complex which occurred after coordination of the CN group.Single-crystal X-ray analyses revealed that compounds [Fe(3m-salpet)(CN)]·CH3OH (1a) and [Fe(3e-salpet)(CN)]·H2O (1b) arein the LS state at the temperature of the experiment (293 K).

The cyanido ligand is coordinated through the carbon atom andtherefore the Fe-C bond length cannot be utilized for comparisonwith other compounds with the {N4O2} donor set. The LScharacter of the complex is documented also by the value of the Rparameter (27.38, 24.57◦ 1a; 20.80◦ 1b; complex 1a contains twomolecules in the asymmetric unit). The values of a differ slightly– 66.43◦ in 1b to 69.13◦ and 71.47◦ in 1a (Table 1). Unfortunately,crystals of the cyanido complexes were of a very poor quality, andwere moreover affected by the solvent loss, and therefore theirstructures are reported only in the ESI†.

High-spin compounds arising from salicylaldehyde derivatives(2a–2c)

Three novel compounds were prepared: [Fe(3Bu5Me-salpet)(NCS)] (2a), [Fe(3m-salpet)(NCO)]·CH3OH (2b) and[Fe(3m-salpet)(N3)] (2c) (Fig. 2). From the single-crystal X-rayanalysis it is obvious that all three compounds are in the HS stateat the temperature of the experiment (293 K).

In Table 2, there are selected structural parameters. The bondlengths, their average values and selected parameters have been

Table 1 Selected structural parameters for 1a and 1b

Fe–Nam Fe–Nima Fe–Ob ac R d

1a mol1 2.028 1.931 1.903 69.13 27.381a mol2 2.028 1.939 1.905 71.47 24.571b 2.010 1.944 1.904 66.43 20.80[Fe(salpet)CN]6 2.000 1.922 1.880 76.02 24.62avge 2.018 1.925 1.897 — 25.40

a The average value calculated from the iron–imino nitrogen bond lengths.b The average value calculated from the Fe–O bond lengths. c The dihedralangle between the least square planes of aromatic rings. d The octahedraldistortion calculated from 12 cis angles found in the coordinationpolyhedron. e The average value calculated from the data given above.

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 10090–10099 | 10091

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Fig. 2 Top: depiction of the molecular structures of 2a–c (hydrogen atoms and solvent molecules are omitted for clarity). Bottom: magnetic data for2b (left) and 2c (right): temperature dependence of the effective magnetic moment (calculated from the temperature dependence of magnetization atB = 0.1 T) and field dependence of magnetization at T = 2.0, 4.6 K. Empty circles: experimental points. Full line: the best fit to the experimental datacalculated with parameters gHS = 1.95, D = +0.61 cm-1, zj = -0.20 cm-1 for 2b; gHS = 2.05, D = +0.96 cm-1, zj = -0.25 cm-1 for 2c.

Table 2 Bond lengths and structural parameters for 2a–c

Fe–Nam Fe–Nterma Fe–Nim

b Fe–Oc ad R e

2a 2.191 2.100 2.096 1.916 75.64 56.042b mol1 2.215 2.094 2.080 1.939 73.28 54.832b mol2 2.220 2.067 2.076 1.952 82.81 53.162c 2.218 2.078 2.087 1.936 58.11 57.37avgf 2.211 2.085 2.085 1.936 — 55.35

a The length of the iron–nitrogen (from terminal ligand) bond. b Theaverage value calculated from the iron–imino nitrogen bond lengths. c Theaverage value calculated from the Fe–O bond lengths. d The dihedral anglebetween the least square planes of aromatic rings. e Octahedral distortioncalculated from 12 cis angles found in the coordination polyhedron. f Theaverage value calculated from the data given above.

calculated for four LS (1a,b, Table 1) and four HS (2a–c, Table 2)molecules.

Obviously, longer metal–ligand bond lengths are found forcomplexes in the HS state and also the distortion from the ideal oc-tahedral geometry in these compounds is higher – approximatelytwo times (Table 1, Table 2).

The most remarkable difference between molecules of distinctspin states is in the longest bond between the iron atom andamine nitrogen atom of the L5 ligand – avg(Fe–Nam)HS = 2.211A; avg(Fe–Nam)LS = 2.018 A; Fe–Nterm and Fe–Nim bonds differ lessexpressively – by ca. 0.16 A. The difference in the iron-oxygen bondlengths is the smallest (0.04 A between the average values, see Table1 and Table 2), which is in agreement with previous observationsfor iron(III) complexes with Schiff-base ligands possessing the{FeN4O2} chromophor.3

The magnetic data for 2a–c are essentially similar. The effectivemagnetic moments at room temperature saturate around the spin-only value characteristic for S = 5/2 and g = 2.0 (5.92 mB)mononuclear complexes; they remain almost constant down to30 K. Then meff drops down to what is referred to as the fingerprintof the zero-field splitting (ZFS). Modelling of magnetic propertiesof the complexes is described in more detail below.

Complexes arising from 2-hydroxy-1-naphthaldeyde (3a–3f)

The reaction of [Fe(napet)Cl] with a slight excess of the selectedpseudohalide in the methanol/acetonitrile solution led to theformation of five novel compounds. When the precursor reactedwith KNCS, KNCSe and NaBH3CN, then the resulting com-pounds contained the acetonitrile molecule yielding the generalformula [Fe(napet)(NCR)]·CH3CN (R = S (3a), Se (3b), BH3

(3f)); no solvent was present in [Fe(napet)(NCO)] (3c) and in[Fe(napet)(N3)] (3e).

[Fe(napet)(NCR)]·CH3CN, R = S (3a), Se (3b)

Two structures of the complexes involving acetonitrile as thecrystal solvent molecules (3a, 3b) were determined by single-crystal X-ray analysis. They are isomorphous, both crystallizingin the monoclinic Cc space group (please see ESI, Tab. S2†). Thesolvent molecule of acetonitrile is capping the amine nitrogenatom, which is the only present donor of significant hydrogenbond strength. Besides this contact the acetonitrile molecule isstabilized by several short CH ◊ ◊ ◊ p contacts which are locatedin between of the aromatic rings (see ESI Fig. S13†). Despitethe rich aromatic system of the napet ligand, no ring–ring


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stacking is observed. From the present intermolecular contactsthe most interesting are the CH ◊ ◊ ◊ S, CH ◊ ◊ ◊ Se and CH ◊ ◊ ◊ O ones(see ESI, Fig. S13, S18 and S19†). Magnetic measurementsrevealed that both compounds exhibit spin-crossover (Fig. 3)with T 1/2 = 151 K (3a) and 170 K (3b). The spin-only valuesfor HS and LS Fe(III) complexes are 5.9 mB (HS) and 1.7 mB (LS)and it is apparent that both compounds do not reach the lowerspin only value, probably due to the existence of a residual HSfraction along with a significant contribution of the orbital angularmomentum (see below). The shift of the critical temperature (3avs. 3b) is in agreement with the position of terminal ligandsin the spectrochemical series of ligands. However, they cannotbe compared with other iron(III) compounds exhibiting the spintransition with the coordinated pseudohalido ligand – as far as weknow they are the first examples of such compounds. Crystal datacollection was performed on the same single crystal at 150, 180 and240 K for 3a; 100 K and room temperature for 3b to analyse thepresent structural changes accompanied with the spin transition.In both cases, the monoclinic space group remains preserved.For 3b, the temperature dependence of the lattice parameterswas measured in the range of 100–300 K and the results areshown in Fig. 4. In agreement with the previous reports on Fe(II)complexes,1,12,15 a significant contraction of the unit cell volume(V uc) upon the spin transition is observed. V uc = 2782 A3 at 230 Kand on cooling a slight volume decrease is observed; below 180 Kthe volume contraction becomes more abrupt. At 120 K, V uc =2704 A3 and it stays nearly constant on further cooling (Fig. 4).A subtraction V uc(230 K) - V uc(120 K) yields the volume of thespin-crossover V SCO = 78 A3 that is 2.8% of V uc(230 K). For 3aV SCO was calculated from V uc(240 K) - V uc(100 K) = 84 A3 thatis 3.0% of V uc(240 K). These values are slightly higher than thoseusually reported (ª 2%) for the Fe(II) spin-crossover compoundswith pseudohalide ligands.12 The unit cell parameters in 3b are

Fig. 3 Magnetic properties of 3a and 3b. Left: temperature dependence ofeffective magnetic moment (calculated from the temperature dependenceof magnetization at B = 0.1 T) with the inset showing temperaturedependence of the high-spin molar fraction. Right: the isothermalmagnetizations measured at T = 2.0 and 4.6 K. Experimental data – blackcircles 3a, gray triangles 3b, full lines – the best fit calculated with gLS =2.25, gHS = 2.08, D = 1.8 cm-1, Deff = 231 K, g = 87 cm-1, nLS = 539 cm-1,xrHS = 12% for 3a (calculated DH = 1918 J mol-1 and DS = 12.5 J K-1

mol-1); gLS = 2.14, gHS = 2.03, D = 1.8 cm-1, Deff = 276 K, g = 99 cm-1, nLS =576 cm-1, xrHS = 6.0% for 3b (calculated DH = 2295 J mol-1 and DS = 13.3J K-1 mol-1).

Fig. 4 Temperature dependence of lattice parameters and the integralintensity of the three strongest reflections (3b): red – solid (-2 0 0); black –dashed (2 0 0); blue – dot-dashed (-1 -1 0). Lines serve only as a guide foreyes. For volume the fitted data are also involved (triangles).

changing analogously to V – except for a and b, from which theonset and the end of SCO are not so obvious (Fig. 4).

The intensity of several reflections is also temperature depen-dent. As a selected example, the (-2 0 0) reflection was monitored:it shows a most drastic change at I(300 K) ª 3 ¥ I(120 K). Thischange is not completely reversible; this feature could be due to apartial damage of the crystal caused by the contraction associatedwith SCO.

The temperature-induced volume change for 3b has beenrecovered by using the Ising-like model with parameters takenfrom the susceptibility fit: Deff = 276 K, g = 99 cm-1, nLS = 576 cm-1;the only variables were the V LS and V HS limits. It can be seen(Fig. 4) that the calculated data pass firmly through the experi-mental points. To this end, the volume expansion during SCO canbe treated by the same model as developed for the susceptibilitydata.14 The bond lengths within the chromophore depend onthe temperature during experiments (see Table 3). The room-temperature structure of 3b can be attributed to the pure HSstate. From the fit of magnetic data (Fig. 3) it can be seen thatat room temperature almost 94% of the molecules are in the HSstate. Therefore, it can be assumed that bond lengths should bevery close to the average calculated from the pure HS molecules.However, the distance of the amine nitrogen atom from the ironatom (Fe–Nam = 2.182(5) A) does not reach the average value ofthe HS compounds and it remains 0.03 A shorter.

Other bond parameters are close to the average values (3b at300 K: Fe–Nim = 2.072 A, Fe–O = 1.931 A; avg(Fe–Nim)HS = 2.085 A,avg(Fe–O)HS = 1.936 A, Table 2). An analogous situation (Fe–Nam =2.182(5) A) has been found in 3a: from the magnetic measurements


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Table 3 Selected structural parameters for 3a–d

Fe–Nam Fe–Nterma Fe–Nim

b Fe–Oc ad R e

3a 150 K 2.073 2.025 1.988 1.903 77.1 28.63a 180 K 2.168 2.094 2.064 1.937 76.6 47.03a 240 K 2.182 2.093 2.072 1.937 76.4 50.53b 100 K 2.022 1.964 1.936 1.882 78.5 20.53b 300 K 2.182 2.123 2.072 1.931 77.6 52.33c 100 K 2.010 1.961 1.929 1.886 83.7 21.83c 190 K 2.165 2.057 2.064 1.944 85.5 47.53d 240 K 2.206 2.088 2.077 1.941 83.2 60.0

a The length of the iron–nitrogen (from terminal ligand) bond. b Theaverage value calculated from the iron-imino nitrogen bond lengths. c Theaverage value calculated from the Fe–O bond lengths. d The dihedralangle between the least square planes of aromatic rings. e The octahedraldistortion calculated from 12 cis angles found in the coordinationpolyhedron.

(Fig. 3) it is apparent that the compound is almost fully convertedto the HS state at 240 K and the bond lengths are similar tothose found in 3b; only Fe–Nterm (2.093(3) A) is shorter (3b: Fe–

Nterm = 2.123(7) A), but this can be caused by different bindingproperties of the isoselenocyanate ligand in comparison with theisothiocyanate moiety.

In contrast to the compounds 3a and 3b and in agreement with2a–c, the Fe–Nam bond length found in the crystal structure of[Fe(napet)(NCS)]·(CH3)2CO (3d) is close to the average value of2a–c (Fe–Nam = 2.206(2) A, Table 3). Other structural and bondparameters resemble those found for the HS compounds arisingfrom salicylaldehyde very closely (Table 3). Magnetic measurement(see ESI, Fig.S2†) shows that 3d is in the HS state over the wholetemperature range (2–300 K). So it can be concluded that theshorter Fe–Nam bond length in the SCO compounds with the napetligand is not a general property of the complexes with the napetligand. The origin of this difference remains unclear and will bethe subject of the further research.

The crystal structure of 3b, which was measured at 100 K,exhibits bond parameters similar to the average values calculatedfor the LS compounds. The length of the Fe–Nam bond and Fe–Nterm value (2.022(5) A, 1.964(5) A; Table 3) are very close tothe LS average; the iron-oxygen distance is even a bit shorter

Fig. 5 Molecular structures of [Fe(napet)(NCS)]·CH3CN (3a, top left), [Fe(napet)(NCS)]·CH3CN (3b, top center) and [Fe(napet)(NCO)] (3c, top right);a view on the packing of [Fe(napet)(NCO)] (3c, middle left) and [Fe(napet)(NCS)]·CH3CN (3a, down left), the molecules of the solvent are displayed in aspacefill style (at 45% of the van der Waals radius for clarity); the overlay drawing of the 1-D chain fragment in [Fe (napet)(NCO)] at two temperatures,green at 100 K, violet at 190 K (down right). Hydrogen atoms (except the contact ones) are omitted for clarity.


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(Fe–O = 1.882 A). The crystal structures of 3a measured at 150 Kand 180 K possess metal–ligand bond lengths between the valuestypically found for LS and HS molecules (Table 3). The presentspin transition at these temperatures can be also documentedby a parameter of the octahedral distortion R which changedsignificantly with temperature (from 28.6◦ at 150 K to 47.9◦ at180 K, Table 3).

[Fe(napet)(NCO)] (3c)

The [Fe(napet)(NCO)] (3c) compound was obtained in the form ofblack prismatic crystals, and it shows spin-crossover at T 1/2 = 155 K(Fig. 6), which is higher critical temperature than in the case of 3a.However, the isocyanato ligand is supposed to provide a weakerligand field than the isothiocyanato group. This discrepancy canbe explained on the basis of the differences found in the molecularand packing structure of 3c with respect to 3a and 3b. The crystalstructure of 3c was measured at two different temperatures (100 Kand 190 K, according to the fit of magnetic data: xHS(100 K) =0, xHS(190 K) = 0.87). It crystallizes in the orthorhombic P212121

space group with a different packing in comparison to 3a and 3b(Fig. 5).

Fig. 6 Magnetic properties of 3c. Left: temperature dependence of ef-fective magnetic moment (calculated from the temperature dependence ofmagnetization at B = 0.1 T) with the inset showing temperature dependenceof the high-spin molar fraction. Right: the isothermal magnetizationsmeasured at T = 2.0 and 4.6 K. Experimental data - circles, full lines - thebest fit calculated with: gLS = 2.24, gHS = 2.03, Deff = 306 K, g = 102 cm-1,nLS = 420 cm-1 (calculated DH = 2542 J mol-1 and DS = 16.3 J K-1 mol-1).

The amino group is not capped by the solvent molecule, butit is involved in the hydrogen bonding with the oxygen atomfrom the isocyanato group (Fig. 5). This contact is shorter whenmolecules are in the LS state (at 100 K, d(N2 ◊ ◊ ◊ O3) = 2.964 A)than when they are predominantly in the HS state (at 190 K,d(N2 ◊ ◊ ◊ O3) = 3.080 A). When comparing this contact with theN2 ◊ ◊ ◊ N1S contact (between the amine group and nitrogen atom(N1S) from acetonitrile) in the structures of 3a and 3b, a basicdifference can be found – the distance of the contact is increasingupon cooling (3a: 3.199 A at 150 K, 3.190 A at 240 K; 3b: 3.186 Aat 100 K, 3.153 A at 300 K). Other present intermolecular contactsare of a weaker character (CH ◊ ◊ ◊ O and CH ◊ ◊ ◊ p contacts, ESI fig.S14, S17. S20†). A feature which indicates the distinct moleculartopology of 3c with respect to 3a and 3b is the angle between theleast square planes of aromatic rings. The value of the a angle is

by approximately 7–9◦ higher in 3c than in 3a and 3b (Table 3). Incontrast to the compounds 3a and 3b, the angle a decreases uponcooling and while the difference in a (between HS and LS form)in 3b is 0.73◦, in 3c this difference becomes greater (Da = 1.8◦).The metal–ligand bond lengths in 3c at 100 K are very similarto those found for 3b at the same temperature, only the Fe–Nam

bond is a bit shorter 2.010(4) A. At 190 K 3c is almost in the HSstate and bond lengths closely resemble those found in 3a at 180 K(according to the fit of magnetic data xHS = 0.84). Therefore, it canbe proposed that the Fe–Nam bond length in 3c at xHS = 1.0 will bealso shorter than the average value found for the HS compounds(Table 2) as it was discussed in the case of 3a and 3b above.

[Fe(napet)(N3)] (3e) and [Fe(napet)(BH3CN)] (3f)

[Fe(napet)(N3)] (3e) and [Fe(napet)(BH3CN)] (3f) were preparedas crystalline materials, but we were not able to grow single crystalsof good quality for X-ray. The compound 3e precipitates rapidlyafter the addition of sodium azide into the methanol/acetonitrilesolution of the [Fe(napet)Cl] precursor complex as a brownpowder, but it may contain a small amount of impurity. A sampleof better quality or even single-crystals can be prepared by usingthe acetone solution. Unfortunately, the single-crystals preparedby this way are still of a poor quality and they are shaped as verythin needles. However, we were able to solve the crystal structureof this compound, but due to the bad quality of the measured dataand high R-factor (R1 = 19.8%, ESI Table S2†) the structure of 3eis reported only in ESI (Fig. S12†). From the crystal structure it isobvious that the compounds 3c and 3e are isostructural, despitethe different crystal system and space group (P21/c for 3e, ESItable S2†). Similarly to 3c the crystal structure of 3e consist of1-D chains formed of [Fe(napet)(N3)] molecules interconnectedby NH ◊ ◊ ◊ N hydrogen bond between the amine group and thenitrogen atom of azide moiety (ESI, Fig. S15†). Other presentintermolecular contacts are short CH ◊ ◊ ◊ N and CH ◊ ◊ ◊ p hydrogenbonds (ESI, Fig. S15, S16†) and offset p–p interaction of aromaticrings (ESI, Fig. S17 right†). This kind of stacked ring arrangementis found also for 3c but it is significantly twisted in comparisonwith 3e (ESI, fig. S17 left†). The metal–ligand bond lengths at160 K are very similar to those found for 3a at 240 K and they areconfirming HS state for 3e at this temperature.

Compound 3f crystallizes as blue–green prismatic crystalsstacked in a bunch and no proper single crystal could be separated.The mid-infrared spectrum of 3f confirms the presence of stretch-ing vibrations of the hydroxy moiety (3641 and 3404 cm-1) andof the nitrile moiety (2248 cm-1). However, the elemental analysiswas not performed on freshly prepared crystals and a possiblesolvent loss has to be considered. The elemental analysis fits bestto the composition [Fe(napet)(BH3CN)]·0.8CH3CN·0.2CH3OH(3f¢). Magnetic data were taken also on an aged sample andtherefore the molar mass of 3f¢ has been applied for the dataanalysis.

From the magnetic point of view, the compound with the azideterminal ligand is the most interesting. Compound 3e exhibits anabrupt spin transition with a hysteresis width of 5 K (T 1/2↑ = 122 K,T 1/2↓ = 117 K) (Fig. 7). Such a transition is more common for Fe(II)mononuclear complexes;15 sometimes, however, it has also beenreported for Fe(III) compounds.16 Usually, a significant molecularreorganization upon transition, such as a large transformation


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Fig. 7 Magnetic properties of 3e and 3f¢. Temperature dependence of theeffective magnetic moment (calculated from the temperature dependenceof magnetization at B = 0.1 T) with the inset showing temperaturedependence of the high-spin molar fraction. Right: the isothermalmagnetizations measured at T = 2.0 and 4.6 K. Experimental data – blackcircles 3e, gray triangles 3f¢, full lines – the best fit calculated with gLS =2.11 gHS = 2.03, Deff = 184 K, g = 99 cm-1, nLS = 428 cm-1, xrHS = 3.0% for3e (calculated DH = 1532 J mol-1 and DS = 11.2 J K-1 mol-1); gLS = 2.27,cTIP = 2.51 ¥ 10-9 m3 mol-1 for 3f¢.

of the shape of the ligand is observed along with a rich systemof strong intermolecular contacts.17 As a good example of suchbehaviour, the [Fe(sal2trien)][Ni(dmit)2] salt (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato) could be mentioned. In this compoundvery abrupt SCO with hysteresis of 30 K is accompanied by achange of the a angle by 34◦.18 We suppose that a similar kind oftransformation (on a smaller scale) can also occur in the case of3e. The observed difference in the level of reorganization of themolecular and packing structure in 3c (relatively abrupt SCO) withrespect to less significant structural changes in 3a and 3b (gradualSCO) may serve as a clue.

Magnetic measurement for compound 3f¢ revealed that thecompound behaves as a paramagnet with S = 1/2 in the rangeof 2–300 K. The value of the magnetic moment at 100 K is 2.01 mB

and its temperature dependence was fitted with the g-factor equalto 2.27. This is due to the orbital angular momentum present inthe 2T1g local electronic state for the octahedral pattern. This valuewas used as a rough estimate of gLS for fitting of the magnetic dataof other SCO compounds in the whole series.

Interpretation of magnetic properties

The spin-crossover behaviour of the compounds under studyhas been interpreted in terms of the Ising-like model withvibrations19–21 – details are presented in the ESI†. As a result,there are three free parameters of this model, namely Deff – theeffective energy difference between the HS and LS states, g –the cooperativeness, nLS – the averaged vibration for the LSchromophore. Therefore, the HS mole fraction x¢HS is a functionof temperature and the above mentioned parameters. Now, theexperimental magnetic data for spin-crossover compounds canbe analysed as Mmol = (x¢¢HS + xrHS)MHS + (1 - x¢¢HS - xrHS)MLS,where xrHS is residual mole fraction of HS state observed at lowtemperature, and x¢¢HS is the rescaled HS fraction due to SCO asx¢¢HS = x¢HS (1 - xrHS). The overall HS mole fraction xHS is thenequal to xHS = x¢¢HS + xrHS. In general, the LS and HS molar

magnetizations MLS and MHS, respectively, were calculated usingthe spin Hamiltonian [eqn (1)]

ˆ ˆ ˆ / ˆ ˆ ˆH gBS D S S zj S Sa z a a= + −( )−mB2 21 3 (1)

comprising the spin Zeeman term, ZFS term and the molecular-field correction term for the parallel (a = z) and perpendicular (a =x, y) directions.19,21

In the case of the LS state, the ZFS term was skipped. Theabove described model may result in the following set of freeparameters: Deff, g , nLS, xrHS, gLS and gHS. (For the purely HScomplex, only gHS, D, and zj need to be considered.) The aboveparameters are effective in different parts of the susceptibilityand/or magnetization curve and thus the model cannot be viewedas overparametrized. For instance, the low temperature part ofthe spin transition curve defines gLS and xrHS whereas the hightemperature part determines gHS. The central part is modulatedby Deff, nLS, and g : The site formation energy determines theenthalpy of SCO, nLS influences the entropy change, and thecooperativeness g causes deviations from the Boltzmann lawresulting in abruptness of SCO.

To ensure that the desired parameters are unambiguous, boththe temperature and magnetic field dependencies of magnetizationwere fitted simultaneously using the simulated annealing and/orthe genetic algorithms for finding minima of the error functional.

In the cases of compounds 3a and 3b (Fig. 3), the residualfractions of the HS states were non-zero (xrHS π 0), which enabledthe determination of the ZFS parameter DHS ª 1.8 cm-1; thisis an acceptable value for the distorted octahedral coordinationof iron(III). The inclusion of the residual HS fraction into thefitting procedure was also essential for 3e (Fig. 7), but its rathersmall value (xrHS ª 3.0%) did not allow determination of the D-parameter. The fitted g-values for the HS state are close to theexpected values of gHS = 2.0 and the LS values of gLS = 2.11–2.25owing to the contribution from the orbital angular momentum.

With the aim to compare the Ising-like model parameters forall reported spin-crossover compounds, the van’t Hoff plot ln Kvs 1/T is presented in Fig. 8; where K is the equilibrium constant

Fig. 8 The van’t Hoff plots for 3a (full black line), 3b (dashed dark grayline), 3c (dotted gray line) and 3e (dot-dashed light gray line) based uponthe Ising-like model with parameters in the text.


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K = x¢HS/(1 - x¢HS) for the LS↔HS transition. The greater thecooperativeness, the greater the deviation from linearity aroundln K = 0.

The HS compounds 2b, 2c, 3f, which do not exhibit SCO,were fitted by using eqn (1). The resulting g-values are similarto the spin-crossover compounds mentioned above. In the caseof the HS complexes 2b and 2c, D values were found to be a bitsmaller (|D| < 1 cm-1) than for 3a and 3b. For these complexes itwas essential to include the molecular-field correction parameterzj, which was found to be close to -0.2 cm-1 indicating a smallantiferromagnetic intermolecular interaction in the solid state(Fig. 2). The LS compound 3f¢ was successfully analysed by gLS

and the temperature-independent paramagnetic contribution cTIP

(Fig. 7).

Conclusions

A series of novel mononuclear iron(III) Schiff-base complexeswas structurally and magnetically characterized. Remarkably, thecompounds with the pentadentate ligand napet exhibit spin-crossover phenomena of different types – from gradual andincomplete (3a, 3b) to complete and relatively abrupt (3c), orabrupt with thermal hysteresis (3e). Observation of abrupt SCO inthis series of compounds is important and promising feature fromtwo different aspects:

a) Cooperative transitions have not been observed for a series ofiron(III) complexes with pentadentate Schiff bases yet. Previouslyreported [FeL5L1]BPh4 SCO complexes exhibited gradual spintransitions. They were built of the symmetrical L5 ligands andcrystallized using the bulky tetraphenylborate anions. Theseprevent occurrence of significant intermolecular interactions be-tween the SCO molecules and hinders cooperativeness. On thecontrary, neutral [FeL5L1] species gave rise to significant level ofcooperativity as was reported in this article. It must be also notedthat the asymmetric shortening of the aliphatic part of L5 ligandand therefore, the higher rigidity of SCO molecule may also playrole in the occurrence of the cooperative phenomena in this groupof compounds.

b) Involvement of the pseudohalido ligands allows gentle tuningof the transition temperature as was previously reported for ferrouscompounds.

From the structural point of view, in compounds 3a–c there isa lack of p–p stacking interactions, which are usually responsiblefor the higher cooperativness and thus abrupt SCO.16 However,the compound 3c shows assembling into the 1D supramolecularchain held together by hydrogen bonds and therefore enhancingcooperativeness in this system. This kind of structural motif wasalso found for compound 3e and together with the presence ofp–p stacking interactions probably lead to the rather cooperativebehaviour and abrupt SCO with narrow hysteresis. For the HScomplexes of the SCO compounds, the different coordinationpolyhedron geometry with shorter Fe–Nam bond was observedwith respect to the pure HS complexes. In contrast to previouslyreported series of [Fe(saltrien)]+ derivatives13 there is no evidenceof an influence of the a parameter on the spin state. In thepure LS compounds this parameter varies in the range of 52–76◦ (Table 1), the molecular shape of the HS complexes iseven more variable (58-83◦, Table 2). Less variable values of awere found for compounds exhibiting SCO (76–86◦, Table 3).

Interestingly, the larger difference between the LS and HS valueof the a parameter was observed for the complex exhibiting morecooperative transition (3c) than in the complexes with gradualtransitions (3a, 3b), but no final conclusions can be drawn at themoment.

Further research will be focused on the more detailed studyof compounds formed by a combination of the [Fe(napet)(NCS)]molecule and suitable solvents to see how the solvent inclusioncan affect magnetic behaviour.

Experimental

Synthesis

All reagents and solvents were purchased from commercial sources(Sigma Aldrich) and used as received.

The precursor complexes [Fe(L5)Cl] and cyanido complexes[Fe(L5)(CN)] were prepared in similar ways as previouslyreported.7

Synthesis of [Fe(3Bu-5Me-salpet)(NCS)] (2a)

The precursor complex [Fe(3Bu-5Me-salpet)Cl] (0.1 g, 0.18 mmol)was dissolved in 25 ml of methanol. Into the violet solution, 20 mgof KNCS (0.21 mmol) was added and the solution was stirred at itsboiling temperature for 20 min. The obtained solution was filteredthrough a paper filter and left to cool down and to evaporate slowly.After one day dark green–brown crystals precipitated. They werefiltered off with a fritted funnel, washed twice with small amountsof cold methanol and finally with diethyl ether. (Found: C, 62.3;H, 7.0; N, 9.7. C30Fe1H41N4O2S1 (M = 577.58) requires C, 62.4; H,7.2; N, 9.7%). IR mid: n(N–H) = 3216 cm-1 (w), n(C–H)aromatic =3032, 3014 cm-1 (vw), n(C–H)aliphatic = 2955, 2905, 2867 cm-1 (m),n(NCS) = 2068 cm-1 (vs), n(C N) and n(C C) = 1626, 1608,1540 cm-1 (vs).

Synthesis of [Fe(3m-salpet)(NCO)]·CH3OH.H2O (2b)

A crystalline powder of 2b was prepared similarly to 2a using0.1 g of [Fe(3m-salpet)Cl] (0.21 mmol), however, a mixture ofacetonitrile (10 ml) and methanol (15 ml) was used for preparationof the starting solution. Into this solution 17 mg of KNCO(0.21 mmol) was added. Black single crystals were obtained byslow evaporation of the methanol/acetonitrile (1 : 6) solution.(Found: C, 51.7; H, 5.7; N, 10.3. C23Fe1H31N4O7 (M = 531.36)requires C, 52.0; H, 5.9; N, 10.5%). IR mid: n(O–H) = 3417 cm-1

(w), n(N-H) = 3235 cm-1 (w), n(C–H)aromatic = 3050, 3023 cm-1 (vw),n(C–H)aliphatic = 2928, 2871 cm-1 (m), n(NCO) = 2194 cm-1 (vs),n(C N) and n(C C) = 1622, 1598, 1541 cm-1 (vs).

Synthesis of [Fe(3m-salpet)(N3)] (2c)

Crystalline powder of 2c was prepared similarly to 2b, but intoa methanol/acetonitrile solution, 14 mg of NaN3 (0.21 mmol)was added. Black single crystals were obtained by slow evapo-ration of acetonitrile solution. (Found: C, 52.3; H, 5.2; N, 17.3.C21Fe1H25N6O4 (M = 481.31) requires: C, 52.4; H, 5.2; N, 17.5%).


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Synthesis of [Fe(napet)(NCR)]·CH3CN (R = S (3a), Se (3b))

Brown crystals of 3a and 3b were obtained similarly to 2b, using0.1 g of [Fe(napet)Cl] (0.195 mmol) in methanol/acetonitrile towhich 25 mg of KNCS (0.25 mmol, 3a) or 33 mg of KNCSe(0.25 mmol, 3b) was added.

3a, (Found: C, 62.3; H, 4.9; N, 12.0. C30Fe1H28N5O2S1 (M =578.49) requires C, 62.3; H, 4.7; N, 12.1%). IR mid: n(N-H) =3229 cm-1 (m), n(C–H)aromatic = 3049 cm-1 (w), n(C–H)aliphatic = 2958,2924, 2872 cm-1 (m), n(C N) = 2292, 2248 cm-1 (w), n(NCS) =2048 cm-1 (vs), n(C N) and n(C C) = 1611, 1600, 1535 cm-1

(vs).3b, (Found: C, 57.3; H, 4.5; N, 11.0. C30Fe1H28N5O2Se1 (M =

625.38) requires C, 57.6; H, 4.5; N, 11.2%). IR mid:n(N-H) =3228 cm-1 (m), n(C–H)aromatic = 3052 cm-1 (w), n(C–H)aliphatic = 2956,2923, 2871 cm-1 (m), n(C N) = 2291, 2248 cm-1 (w), n(NCSe) =2053 cm-1 (vs), n(C N) and n(C C) = 1613, 1601, 1537 cm-1 (vs).

Synthesis [Fe(napet)(NCO)] (3c)

Black crystals of 3c were prepared similarly to 3a but a mixture ofacetone (20 ml) and water (3 ml) was used for the preparation of thestarting solution. Into this solution, 20 mg of KNCO (0.25 mmol)was added.

(Found: C, 64.4; H, 4.9; N, 10.8. C28Fe1H25N4O3 (M = 521.37)requires C, 64.5; H, 4.8; N, 10.8%). IR mid: n(N–H) = 3247 cm-1

(m), n(C–H)aromatic = 3054 cm-1 (vw), n(C–H)aliphatic = 2951, 2919,2862 cm-1 (w), n(NCO) = 2183, 2131 cm-1 (vs), n(C N) andn(C C) = 1614, 1601, 1537 cm-1 (vs).

Synthesis [Fe(napet)(NCS)](CH3)2O (3d)

Black crystals of 3d were prepared similarly to 3a but pure acetone(20 ml) was used for the preparation of the starting solution. Intothis solution, 25 mg of KNCS (0.25 mmol) was added. (Found:C, 62.6; H, 5.4; N, 9.5. C31Fe1H31N4O3S1 (M = 595.51) requiresC, 62.5; H, 5.3; N, 9.4%). IR mid: n(N–H) = 3241 cm-1 (m), n(C–H)aromatic = 3050 cm-1 (vw), n(C–H)aliphatic = 2973, 2926, 2871 cm-1

(w), n(NCS) = 2056 cm-1 (vs), n(C O) = 1698 cm-1 (vs), n(C N)and n(C C) = 1614, 1603, 1538 cm-1 (vs).

Synthesis of [Fe(napet)(N3)] (3e)

Brown crystalline powder of 3e was prepared similarly to 3abut pure acetone (30 ml) was used for the preparation of thestarting solution. Single crystals were prepared by slow evapora-tion (approximately 2 weeks) of a solution prepared by similarprocedure, where the composition of the starting solution wasslightly modified (40 ml acetone + 5 ml CH3NO2). The identity ofthe powder sample was confirmed by a powder X-ray diffractionstudy and following comparison with the calculated single-crystalpowder pattern (ESI, fig. S23†) (Found: C, 62.2; H, 4.8; N, 16.1.C21Fe1H25N6O4 (M = 481.31) requires C, 61.9; H, 4.9; N, 16.2%).IR mid: n(N–H) = 3228 cm-1 (m), n(C–H)aromatic = 3055 cm-1 (vw),n(C–H)aliphatic = 2980, 2957, 2932, 2861 cm-1 (w), n(N3) = 2046 cm-1

(vs), n(C N) and n(C C) = 1616, 1606, 1539 cm-1 (vs).

Synthesis of [Fe(napet)(NCBH3)]·0.8CH3CN·0.2CH3OH (3f¢)

Blue–green crystals of 3f were prepared similarly to 3a and intothe starting solution, 16 mg of NaNCBH3 (0.25 mmol) was added.

(Found: C, 64.4; H, 5.6; N, 11.9. B1C29.8Fe1H31.2N4.8O2.2 (M =558.49) requires C, 64.1; H, 5.6; N, 12.0%). IR mid: n(O–H) = 3641,3404 cm-1 (m), n(N–H) = 3222 cm-1 (w), n(C–H)aromatic = 3045 cm-1

(vw), n(C–H)aliphatic = 2983, 2931, 2872 cm-1 (m), n(C N) =2288, 2248 cm-1 (w), n(NCBH3) = 2115 cm-1 (vs), n(C N) andn(C C) = 1614, 1535 cm-1 (vs).

Equipment, measurements and software

Elemental analysis was carried out on a FlashEA 1112, Ther-moFinnigan. IR spectra were measured in KBr pellets (MagnaFTIR 750, Nicolet) in the 4000–400 cm-1 region.

Magnetic susceptibility and magnetization measurements weredone using a SQUID magnetometer (MPMS, Quantum Design)from T = 2 K at B = 0.1 T. The magnetization data were taken atT = 2.0 and 4.6 K, respectively. The effective magnetic momentwas calculated as usual: meff/mB = 798(c¢T)1/2 when SI units areemployed. Analysis of magnetic data was done with the packagePOLYMAGNET.28

Single crystal X-ray diffraction data were collected using Oxforddiffraction Xcalibur with Sapphire CCD detector (1a, 1b, 2a, 2b,2c, 3a, 3b, 3d) and Xcalibur2 (3c) CCD diffractometer with aSapphire CCD detector installed at a fine-focus sealed tube (Mo-Ka radiation, l = 0.71073 A) and equipped with an OxfordCryosystems nitrogen gas-flow apparatus. All structures weresolved by direct methods using SHELXS9722 and SIR-9223 incor-porated into the WinGX24 program package. For each structureits space group was checked by the ADSYMM procedure of thePLATON27 software. All structures were refined using full-matrixleast-squares on F o

2 - F c2with SHELXTL-9722 with anisotropic

displacement parameters for non-hydrogen atoms. The hydrogenatoms were placed into calculated positions and they were includedinto riding-model approximation with U iso = 1.2 U (atom ofattachment). All the crystal structures were visualized usingthe Mercury software.25 In several compounds the SQUEZZE26

procedure was used to remove reflections of disordered andsuperimposed solvent molecules (in 1a: 82 electrons per unit cellwere removed from 222.6 A3 cavity per unit cell; 1b: 92 electronsper unit cell were removed from the 545.5 A3 cavity per unit cell,2b: 146 electrons per unit cell were removed from 567.0 A3 cavityper unit cell).

Acknowledgements

Grant Agencies (Slovakia: VEGA 1/0213/08, APVV 0006-07,VVCE 0004-07; Czech Republic: Operational Program Researchand Development for Innovations - European Regional Devel-opment Fund (CZ.1.05/2.1.00/03.0058), GACR P207/11/0841and Operational Program Education for Competitiveness - Euro-pean Social Fund (project CZ.1.07/2.3.00/20.0017), Germany:DAAD-ID50725741, Austria: Fonds zur Forderung der Wis-senschaftlichen Forschung in Osterreich (Project 19335-N17), EU:COST-D35) are acknowledged for the financial support.

Notes and references

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1994, 33, 2024–2054.


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A new family of Fe2Ln complexes built from mononuclear anionic Schiff base subunits

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