Inorganica Chimica Acta 362 (2009) 2205–2212

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Inorganica Chimica Acta

journal homepage: www.elsevier .com/locate / ica

Synthesis of Fe(II) and Cu(II) building blocks for metal–organic frameworks

Rebecca H. Laye b, E. Carolina Sañudo a,*

a Departament de Química Inorgànica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spainb The University of Manchester, Oxford Road, M13 9PL Manchester, UK

a r t i c l e i n f o

Article history:Received 10 July 2008Received in revised form18 September 2008Accepted 23 September 2008Available online 8 October 2008

Keywords:Ferromagnetic couplingBuilding blocksImine-based ligandsCoordination complexes

0020-1693/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ica.2008.09.052

* Corresponding author. Tel.: +34 93 4039144; fax:E-mail address: esanudo@ub.edu (E.C. Sañudo).

a b s t r a c t

In situ reaction of the aminobenzoic acids 2-aminobenzoic acid and 3,5-diaminobenzoic acid with salicyl-aldehyde provide easy access to the ligands 2-[{(2-hydroxyphenyl)methylene}amino]benzoic acid (L1)and 3,5-bis[{(2-hydroxyphenyl)methylene}amino]benzoic acid (L2). Addition of a Fe(II) or Cu(II) salt tothe solution of the ligand yields the corresponding Fe and Cu complexes. The species synthesized havebeen structurally characterized by single-crystal X-ray diffraction. The Fe(II) complex [Fe(L1)(MeOH)3](1) crystallizes in the triclinic space group P�1. The Cu(II) complex [Cu(L1)] (2) is a one-dimensional chainand crystallizes in the monoclinic space group P21. The Cu(II) complex [Et3NH]2[Cu2(L2)2] (3) crystallizesin the monoclinic space group P21/n. The magnetic properties of 1, 2 and 3 have been studied, showingthat the Cu(II) ions of 2 and 3 are ferromagnetically coupled. Complexes 1 and 3 have strong potential asmetal-bearing building blocks for the synthesis of metal–organic frameworks.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The design and synthesis of new magnetic materials is a hotfield of research nowadays. Possible applications like high densityinformation storage and molecular spintronics have attracted theinterest of the scientific community in the last few years [1]. Thecombination of appropriate ligands with transition metals thatpossess unpaired electrons has afforded a large number of specieswith interesting magnetic properties, the structural diversity ofwhich ranges from single-molecule magnets [2] to extended sys-tems [3]. Bridging ligands that can mediate ferromagnetic couplingbetween the metals are particularly sought after, since this ensuresa large spin ground state. Among these, the most studied are azide[4] in the end-on bridging mode [5] and the alkoxides and oxideswith bridging angles around 90�. However, in most cases the bridg-ing mode of the azide cannot be predicted and the very interestingclusters synthesized have been obtained by so-called ‘serendipi-tous assembly’ [6]. This is even more so with alkoxides and oxides,which are usually provided for by the solvent (an alcohol for thealkoxides or the adventitious water of an organic solvent for theoxides) [7]. A group of ligands that can be designed and offer rigidbinding modes are the aromatic imine-based ligands, these havebeen widely used in the synthesis of new coordination complexes,some of them with interesting magnetic properties [8]. They aresynthesized by the condensation reaction between an aldehydeand a primary amine. This type of ligands offers a unique combina-tion of possibilities for coordination chemists. On the one hand, the

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synthesis is straightforward, and can be done in situ without need-ing to isolate and purify the ligand beforehand. The second andgreatest asset of imine ligands is the possibility of functionaliza-tion. The two reagents needed are an aldehyde and a primaryamine, which can be functionalized practically at will: a large num-ber of aromatic aldehydes are available commercially, as well asprimary amines. Additionally, the fact that the two ‘buildingblocks’ of the ligand can be functionalized independently leads tothe possibility of preparing libraries of ligands. In this work, givenour interest in magnetic materials, functional groups such ascarboxylates and alkoxides were sought after. Thus, 2-hydroxy-benzaldehyde – also known as salicylaldehyde – was chosen asthe aldehyde reagent, and 2-aminobenzoic acid – also known asanthranillic acid – and 3,5-diaminobenzoic acid as the amine do-nors. In this way, ligands that contain one or two alkoxides and acarboxylate group, as well as the imino N donor are obtained. Reac-tion of the ligand, prepared in situ, with metal salts leads to the for-mation of the Cu(II) and Fe(II) complexes reported here.

2. Experimental

All chemicals were purchased from Aldrich and were used with-out further purifications. The imine ligand was prepared in situ foreach reaction following standard procedures. Magnetic data werecollected at the Unitat de Mesures Magnètiques at the Universitatde Barcelona using crushed crystals of the sample on a QuantumDesign MPMS-XL SQUID magnetometer equipped with a 5 T mag-net. The data were corrected for TIP and the diamagnetic correc-tions were calculated using Pascal’s constants; an experimentalcorrection for the sample holder was applied.

2206 R.H. Laye, E.C. Sañudo / Inorganica Chimica Acta 362 (2009) 2205–2212

2.1. [Fe(L1)(MeOH)3] (1)

2-Aminobenzoic acid (0.27 g, 2.0 mmol) is dissolved in 50 mLof MeOH in a Schlenck flask. 2-hydroxybenzaldehyde (0.21 mL,2.0 mmol) and Et3N (0.55 mL, 4.0 mmol) are added to the metha-nolic solution of 2-aminobenzoic acid. The resulting orange solu-tion is purged under N2 for 1 h. Solid FeCl2 � 4H2O (0.39 g,2.0 mmol) is added with stirring. The solution turns dark red. Stir-ring is maintained for 1 h and the solution is left undisturbed, un-der N2, for five days. Crystals of the product can be filtered off in53% yield. The sample is stored under N2. Elemental Anal. Calc.for 1�H2O: C, 49.86; H, 5.66; N, 3.42. Found: C, 50.11; H, 5.39; N,3.63%.

2.2. [Cu(L1)]n (2)

2-Aminobenzoic acid (0.27 g, 2.0 mmol) is dissolved in 50 mL ofMeOH. 2-hydroxybenzaldehyde (0.21 mL, 2.0 mmol) and Et3N(0.55 mL, 4.0 mmol) are added to the methanolic solution of 2-ami-nobenzoic acid. To the resulting orange solution is added Cu(BF4)2

(0.47 g, 2.0 mmol) with stirring. The solution turns dark green witha light green precipitate. The precipitate is filtered off and the darkgreen solution is left undisturbed. After seven days dark greencrystals of 2 can be isolated by filtration in 33% yield. ElementalAnal. Calc.: C, 55.63; H, 3.00; N, 4.63. Found: C, 55.49; H, 2.95; N,4.42%.

2.3. [Et3NH]2[Cu2(L2)2] (3)

3,5-Diaminobenzoic acid (0.19 g, 1.2 mmol) is dissolved in50 mL of MeOH. To this solution are added 2-hydroxybenzalde-hyde (0.26 mL, 2.5 mmol) and Et3N (0.69 mL, 5.0 mmol). The colourchanges to orange, indicating the formation of the di-imine L2.

Table 1Crystallographic data collection and refinement parameters for complexes 1, 2 and 3.

[Fe(L1)(MeOH)3] (1)

Temperature (K) 100 (3)Wavelength (nm) 0.71073Crystal system triclinicSpace group P�1a (Å) 9.4113 (8)b (Å) 9.9020 (9)c (Å) 10.0051 (7)a (�) 71.879 (7)b (�) 84.018 (6)c (�) 75.980 (7)Z 2Volume (Å3) 859.32 (12)Absorption coefficient (mm�1) 0.911F(000) 408Crystal size (mm) 0.20 � 0.08 � 0.04h Range for data collection (�) 2.59–32.13Index ranges �13 < h < 13

�10 < k < 10,�14 < l < 14

Reflections collected 9064Independent reflections (Rint) 5396 (0.0533)Completeness (h) (%) 89.5 (32.13�)Absorption correction AnalyticalMaximum and minimum transmission 0.9645 and 0.8388Data/restraints/parameters 5396/0/229Goodness-of-fit 1.044Final R indices [I > 2r(I)] R1 = 0.0459

wR2 = 0.1284R indices (all data) R1 = 0.0667

wR2 = 0.1419Largest difference peak and hole (e Å3) 0.774 and �0.765

Refinement method: Full-matrix least-squares on F2.

Cu(BF4)2 (0.59 g, 2.5 mmol) is added to the ligand solution inMeOH. The colour turns brown and a green precipitate forms.The precipitate is filtered off and the solution left undisturbed.Dark green crystals of 3 form after seven days in 28% yield. Elemen-tal Anal. Calc.: C, 62.05; H, 5.59; N, 8.04. Found: C, 61.77; H, 5.71; N,7.99%.

3. Crystallography

Crystal data for complexes 1 and 2 were collected at the Univer-sity of Manchester on an Oxford Diffraction Excalibur 2 diffractom-eter. Crystal data for complex 3 were collected on a Bruker ApexIIdiffractometer at the Unidade de Raios X at the University of San-tiago de Compostela. Hydrogen atoms were placed geometricallyand refined isotropically using a riding model. The structures weresolved by direct methods (SHELXS-97), and refined on F2 (SHELXL-97).Collection parameters can be found in Table 1. The hydrogen atomson the protonated triethylamine cations of 3 were found in the dif-ference map and refined isotropically.

4. Results and discussion

4.1. Syntheses

The ligand L1 was synthesized in situ in the reaction pot by thecondensation reaction of 2-hydroxybenzaldehyde and 2-amino-benzoic acid, in basic MeOH, as shown in Fig. 1. Enough base wasused to keep the ligand deprotonated to avoid precipitation and fa-vour coordination upon addition of the transition metal salt. Infact, the addition of FeCl2 to the solution of the ligand under N2

atmosphere caused a sudden change of colour, from yellow to deeppurple, indicating the coordination of the Fe(II) ions to the iminoligand. The solution was left undisturbed and kept under N2 to

[Cu(L1)] �MeOH (2) [Et3NH]2[Cu2(L2)2] � 4MeOH (3)

100 (3) 296 (1)0.71073 0.71073monoclinic monoclinicP21 P21/n9.5867 (2) 15.0478 (3)7.0487 (2) 12.0824 (3)9.8425 (2) 16.0325 (3)90 9098.256 (2) 99.4910 (10)90 902 2658.20 (3) 2875.02 (11)1.674 0.81342 12360.37 � 0.20 � 0.03 0.22 � 0.18 � 0.072.77–31.44 1.72–26.39�13 < h < 13 �18 < h < 18�10 < k < 10 �155 < k < 15�14 < l < 14 �19 < l < 1991243884 (0.039) 5885 (0.046)94.7 (31.44�) 99.8 (26.39�)Multi-scan Numerical0.799 and 1.000 0.843 and 0.9463884/1/185 5885/0/3601.05 1.04R1 = 0.037 R1 = 0.0279wR2 = 0.103 wR2 = 0.0695R1 = 0.041 R1 = 0.0348wR2 = 0.107 wR2 = 0.07290.95 and �0.70 0.452 and �0.335

OHO

NH2

H

OHO

N

O OHOH

OHO

NH2NH2

H

OHO

OHO

NN

OH OH

+

+

L1

L2

2

Fig. 1. The ligands L1 and L2.

Table 2Selected bond distances and angles for [Fe(L1)(MeOH)3] (1).

Distance (Å) Angle (�)

Fe1–O1 2.0033(17) O1–Fe1–O2 165.90(7)Fe1–O2 2.0557(16) O1–Fe1–O6 93.18(7)Fe1–O6 2.0914(16) O2–Fe1–O6 91.59(6)Fe1–N1 2.1220(19) O1–Fe1–N1 87.68(7)Fe1–O5 2.1586(17) O2–Fe1–N1 87.60(7)Fe1–O4 2.3027(17) O2–Fe1–N1 87.60(7)N1–C1 1.290(3) O6–Fe1–N1 179.13(7)

O1–Fe1–O5 94.58(7)O2–Fe1–O5 99.01(7)O6–Fe1–O5 85.68(6)N1–Fe1–O5 94.14(7)O1–Fe1–O4 88.86(6)O2–Fe1–O4 78.62(6)O6–Fe1–O4 82.43(6)N1–Fe1–O4 97.70(7)O5–Fe1–O4 167.80(6)

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avoid the oxidation of the Fe(II) to Fe(III). Crystals of the new com-plex [Fe(L1)(MeOH)3] (1) started to grow after a few hours. Thecrystals were collected after five days and were of good qualityfor single-crystal X-ray diffraction, which allowed the structuraldetermination of the complex.

The synthesis of the Cu(II) polymer [Cu(L1)]n (2) is similar. Inthis case, upon addition of the Cu(II) salt to the solution of the li-gand in MeOH, the colour changed from yellow to dark greenand after a few minutes a green precipitate formed. The precipitatewas filtered off and the dark green solution was left undisturbed toslowly evaporate. After a few days, dark green crystals could beisolated by filtration and single-crystal X-ray diffraction analysisrevealed the structure of complex 2. The precipitate obtainedwas analyzed by infra-red spectroscopy: comparison with the in-fra-red spectra of the crystals of 2 showed that both were the samematerial.

A variety of binding modes can be predicted for the imino li-gand L1, and two different binding modes are actually found inthe complexes reported herein. In both complexes 1 and 2, the li-gand takes three coordination positions around the metal in ameridional (mer) fashion, taking three neighbour positions in theequatorial plane of an octahedral complex such as 1, or three posi-tions in a distorted square-planar complex such as 2. Both reac-tions are performed in MeOH, a strongly coordinating solvent. Inthe formation of complex 1, given the preference of Fe(II) for hex-acoordination, the three free coordination positions left by the li-gand are occupied by neutral MeOH ligands, affording a neutralcomplex that crystallizes easily from the polar MeOH solvent.Cu(II), on the other hand, is usually found to be tetracoordinated.In this case, the tridentate ligand forces a distorted square-planargeometry around the Cu(II) ion. The fourth coordination site is ta-ken by an oxygen atom from the neighbouring [Cu(L1)] unit linkingthe monomers into a zigzag one-dimensional polymer, which ex-plains the low solubility of 2.

To prepare polimine ligands, 3,5-diaminobenzoic acid was cho-sen due the 1,3,5 pattern of substituents in the aromatic ring. Thetwo amines, meta to each other, will result in a meta-substitutedaromatic ring with two imine donor groups, which will favour fer-romagnetic coupling in the complexes synthesized [9]. Addition-ally, the carboxylate might extend the coordination networkfurther. The synthetic procedure to prepare L2 was similar to theone followed to prepared L1 in situ. In both cases, the ligand is pre-pared in situ, to the best of our knowledge, these organic moleculeshave not been reported per se in the literature, although L1 hasbeen prepared in situ and used to prepare coordination complexesin the past [11]. Reaction of 3,5-diaminobenzoic acid with twoequivalents of 2-hydroxybenzaldehyde in basic MeOH afforded a

brown-green solution of the ligand L2 in its deprotonated form.Addition of hydrated Cu(BF4)2 to the solution of L2 in basic MeOHafforded a very dark green solution. A precipitate was filtered off.After a few days undisturbed, well formed crystals of the new spe-cies [Et3NH]2[Cu2(L2)2] (3) were isolated by filtration from thesolution. The crystals were suitable for single-crystal X-ray diffrac-tion and the crystal structure was determined. The precipitate iso-lated was identified as compound 3 by comparison of the infra-redspectrum of the powder with that of the crystals. In the formationof compound 3 the ligand L2 is not using all of its possible bindingpositions. In fact, the carboxylate group of the central aromaticmoiety is not coordinated. This opens up the possibility of usingthe complex anion 3 as a metal-bearing building block for polyme-tallic compounds and metal–organic frameworks. This possibilityis currently being explored.

4.2. Description of crystal structures

[Fe(L1)(MeOH)3] (1) crystallizes in the triclinic space group P�1.Data collection and structural parameters are found in Table 1. Se-lected bond distances and angles are found in Table 2. The asym-metric unit consists of a full molecule of complex 1, shown inFig. 2 The Fe centre is hexacoordinated in a axially elongated octa-hedral fashion, with oxidation state Fe(II), as shown by close exam-ination of the structural parameters and bond-valence sumcalculations [10]. One imino ligand is coordinated to the Fe(II)ion taking up three equatorial positions, the Fe–N(1) bond distanceis 2.122 Å, Fe–O(1) is 2.003 Å and Fe–O(2) is 2.056 Å, where N(1) is

Fig. 2. Crystal structure of complex 1, with atom labels. The hydrogen atoms have been omitted for clarity (top). Packing diagram for complex 1 along the a axis of the unitcell. The intermolecular H-bonds are shown as dashed lines (bottom).

Table 3Selected bond distances and angles for [Cu(L1)]n �MeOH (2).

Distance (Å) Angle (�)

Cu1–O1 1.890(2) O1–Cu1–O3 151.58(10)Cu1–O3 1.933(2) O1–Cu1–N1 95.17(10)Cu1–N1 1.933(2) O3–Cu1–N1 93.23(10)Cu1–O2(a) 1.945(2) O1–Cu1–O2(a) 86.41(9)N1–C1 1.285(4) O3–Cu1–O2(a) 93.38(9)

N1–Cu1–O2(a) 162.88(10)

(a) refers to atoms generated by a symmetry operation.

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the nitrogen atom of the imino group, O(1) is one of the oxygenatoms of the carboxylate and O(2) is the phenoxide oxygen. The li-gand is not planar, in fact the O(1)–Fe–O(2) angle is 165.90�, whereO(1) and O(2) are the oxygen atoms of the ligand bound to the Fecentre trans to each other. Occupying the fourth position of theequatorial plane trans to the imino N–Fe bond is one MeOH ligand,with Fe(1)–O(6) of 2.091 Å. The two axial positions are taken bytwo MeOH ligands, with Fe–O bond distances of 2.159 Å to O(5)and 2.303 Å to O(4), clearly elongated from the normal Fe(II) bonddistance to an oxygen atom. The monomers of 1 are organized intolinear chains by short hydrogen bond interactions along the a axisof the unit cell, as shown in Fig. 2, with O(1)–O(5)0 contacts of2.652 Å, O(3)–O(6)0 at 2.587 Å and O(2)–O(4)0 at 2.798 Å.

The crystal structure of complex 2 at 293 K has been reportedpreviously by Wang et al. [11], but its magnetic properties hadnot been studied. New data have been collected at 100 K. Data col-lection and structural parameters are found in Table 1. Selectedbond distances and angles are found in Table 3. Complex 2 crystal-lizes in the monoclinic space group P21. The structure of 2 consistsof linear 1-D chains of the repeating asymmetric unit [Cu(L1)].Each Cu(II) ion is coordinated to two oxygen atoms (O(1) and

O(3)) and the nitrogen (N(1)) of the ligand, as shown in Fig. 2.The Cu(II) ions in 2 are tetracoordinated, in a distorted square-pla-nar fashion, in fact, the O(1)–Cu–O(3) angle is 130.40�. The fourthcoordination position is occupied by an oxygen atom (O(2)) fromthe next [Cu(L1)] unit, linking the [Cu(L1)]n into a one-dimensionalzigzag chain along the b axis of the unit cell, as shown in Figs. 2 and3.

[Et3NH]2[Cu2(L2)2] (3) crystallizes in the monoclinic spacegroup P21/n. Data collection and structural parameters can befound in Table 1. Selected bond distances and angles can be found

Fig. 3. Crystal structure of the asymmetric unit of 2. The hydrogen atoms have been omitted for clarity (top). Packing diagram of 2, showing the zigzag chains of the repeatingunit along the b axis of the unit cell (bottom).

R.H. Laye, E.C. Sañudo / Inorganica Chimica Acta 362 (2009) 2205–2212 2209

in Table 4. The asymmetric unit consists of one Cu(II) ion and oneligand L2, along with two MeOH molecules and one protonated tri-ethylamine. The full complex anion is shown in Fig. 4. The Cu(II)ion is tetracoordinated to two N atoms N(1) and N(2) (Cu–N(1) dis-tance is 1.974 Å and Cu–N(2) is 1.971 Å) and two oxygen atomsfrom the alkoxide group of the di-imine ligand, O(1) and O(2), withCu–O distances of 1.896 Å and 1.904 Å, respectively. The coordina-tion geometry of the Cu centre is very distorted from ideal square-planar: the L–Cu–L angles of two atoms coordinated in cis positions

Table 4Selected bond distances and angles for [Et3NH]2[Cu2(L2)2] � 4MeOH (3).

Distance (Å) Angle (�)

Cu1–O2 1.8962(11) O2–Cu1–O1 91.75(5)Cu1–O1 1.9034(11) O2–Cu1–N1 144.96(5)Cu1–N1 1.9710(13) O1–Cu1–N1 93.27(5)Cu1–N2 1.9742(13) O2–Cu1–N2 93.80(5)N1–C14 1.300(2) O1–Cu1–N2 147.47(5)N2–C1 1.300(2) N1–Cu1–N2 100.22(5)

with respect to the Cu centre range from 91.75� to 100.22� (for anideal square-planar complex these angles would be 90�) and the L–Cu–L angles for two atoms bound trans to each other are 147.46�and 144.97�, respectively. For an ideal square-planar complexthese angles should be 180�, and the deviation from this value indi-cates a distortion to a non-planar species. A tetrahedral Cu com-plex would require all the L–Cu–L angles to be 109�, thus, theCu(II) in 3 is in a coordination environment between square-planarand tetrahedral. The two ligands in complex 3 use the imino andalkoxo N and O atoms to coordinate to the Cu(II) ions, but the car-boxylato group of the central aromatic residue remains non-coor-dinated, as can be clearly seen in Fig. 4. In the crystal packing,the free carboxylate group is hydrogen bonding to one of the solva-tion methanol molecules (the O(3)–O(1s) distance is 2.641 Å) andto the protonated triethylamine cation (O(4)–N(3) distance is2.648 Å). The triethylamine molecule of the asymmetric unit isprotonated, and the proton was found and its position refined.The Cu–Cu distance in complex 3 is 7.437 Å. The two ligands arep–p stacking, with the two central aromatic groups at an averagedistance of 3.38 Å.

Fig. 4. Crystal structure of the anion complex of 3. Hydrogen atoms have been removed for clarity.

2210 R.H. Laye, E.C. Sañudo / Inorganica Chimica Acta 362 (2009) 2205–2212

4.3. Magnetic properties

Dc magnetic susceptibility data were collected for complex 1 atan applied field of 0.5 T in the 300 to 2 K temperature range. ThevMT value at 300 K was 3.40 cm3 K mol�1, in agreement with ex-pected value of 3.0 cm3 K mol�1 for one Fe(II) ion with S = 2 andg = 2.0. As temperature decreases, the vMT value remains nearlyconstant, until a drop to a vMT value of 1.1 cm3 K mol�1 is observedat temperatures below 50 K. This drop can be due to zero-fieldsplitting of the Fe(II) ion, to intermolecular antiferromagneticinteractions, or a partial spin crossover to an intermediate (S = 1)or low spin (S = 0) state. In order to model the magnetic data, thezero-field splitting of an S = 2 ion and the intermolecular interac-tions (as a Curie–Weiss constant) are included in the Hamiltonian,which is solved using the Van Vleck formula. The best fitting ob-tained is shown in Fig. 5 as a solid line, the fitting parameters were

T (K)0 50 100 150 200 250 300

χMT

(cm

3 K m

ol-1

)

0

1

2

3

4

Field (Oe)0 10000 20000 30000 40000 50000

M/N

β

0.0

0.5

1.0

1.5

2.0

2.5

Fig. 5. vT vs. T plot for complex 1. The solid line is the best fit to the experimentaldata (see text for fitting parameters). Inset: field dependence of the magnetizationof complex 1 at 2 K, the solid line is the Brillouin function for S = 1 and g = 2.0.

g = 2.11, D = 0.01 cm�1 and H = �4.63 K. The negative Curie–Weissconstant indicates antiferromagnetic interactions between themonomers of 1. This fitting is in disagreement with the magnetiza-tion, measured at 2 K (Fig. 5). The magnetization tends to satura-tion at a value of 2.2, which would be the expected for anintermediate spin Fe(II) ion with S = 1. Spin crossover to the inter-mediate S = 1 spin state could explain magnetization data, as wellas the sudden drop observed in the susceptibility curve. Hexacoor-dinated imine complexes of iron have been claimed to show spincrossover before [12].

Magnetic susceptibility data for a sample of complex 2 werecollected in the 2–300 K temperature range with an applied dcfield of 1.0 T, shown in Fig. 6. The vMT product per Cu(II) ion hasa value of 0.39 cm3 K mol�1 at 300 K, slightly larger than the ex-

T (K)0 50 100 150 200 250 300

χMT

(cm

3 K m

ol-1

)

0.3

0.4

0.5

0.6

0.7

0.8

M/N

β

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Field (Oe)0 10000 20000 30000 40000 50000

Field (Oe)0 10000 20000 30000 40000 50000

Fig. 6. vT vs. T plot for complex 2. The solid line is the best fit to the experimentaldata (see text for fitting parameters). Inset: field dependence of the magnetizationof complex 2 at 2 K, the solid line is the Brillouin function for S = 1/2 and g = 2.0.

R.H. Laye, E.C. Sañudo / Inorganica Chimica Acta 362 (2009) 2205–2212 2211

pected 0.375 cm3 K mol�1 for an isolated Cu(II) ion with g = 2.0 andS = 1/2. As temperature decreases, the vMT product is practicallyconstant, until below 30 K it starts to rise, up to a value of0.64 cm3 K mol�1 at 2 K. This rise is not field dependent, and isindicative of ferromagnetic interaction between the Cu(II) ions in2. The experimental data were modelled using the equation de-rived by Baker [13] for chains of S = 1/2 and only one exchangeconstant between the spins. The best fit was obtained for g = 2.05and J = 1.3 cm�1, it is shown in Fig. 6 as a solid line. The fielddependence of the magnetization was studied for 2 at 2 K, aM/Nb versus Field plot is shown in Fig. 6. The dots are the experi-mental data, while the solid line is the calculated magnetization at2 K using the Brillouin function for S = 1/2 and g = 2.0. As can beclearly seen, the magnetization of 2 is at all times larger than thecalculated for an isolated S = 1/2 with g = 2.0, as expected due tothe ferromagnetic interactions between the Cu(II) ions in the chain.The material does not order in the long range, this has been provedby the lack of an out-of-phase ac signal, as well as the coincidenceof the measurements at low temperature at different fields. Exam-ining the solid-state structure of compound 2, the very distortedsquare-planar Cu(II) ions are bridged by syn,anti-carboxylategroups from the ligand. In this bridging mode, relatively uncom-mon for carboxylate ligands, there is poor overlap between theorbitals on the bridge with those bearing the unpaired electronson each copper ion, and as predicted by Alemany et al., thecoupling should be weakly ferromagnetic, [14] as observed forcompound 2.

Magnetic susceptibility data for a crushed crystalline sample ofcomplex 3 were collected in the 30–300 K temperature range at anapplied field of 1.0 T and below that temperature at 500 G. The vTvalue at 300 K is 0.80 cm3 K mol�1, in agreement with the expectedvalue for two Cu(II) ions (0.750 cm3 K mol�1, 2S = 1/2 and g = 2.0).As temperature decreases, the vT value remains practically con-stant, but below 30 K it starts to increase, up to a value of0.91 cm3 K mol�1 at 2 K, indicating weak ferromagnetic interactionbetween the Cu(II) ions of complex 3 (Fig. 7). The magnetizationversus field data supports this conclusion. At 2 K the magnetizationtends to a saturation value of 2.0, and follows the Brillouin law foran S = 1 with g = 2.0, in agreement with a ferromagnetic interactionbetween the two Cu(II) ions that form the anion complex 3. A VanVleck equation can be easily derived to model the experimentalsusceptibility data. The fitting parameters were g = 2.09 and

T (K)0 50 100 150 200 250 300

χMT

(cm

3 K m

ol-1

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Field (Oe)0 10000 20000 30000 40000 50000

M/N

β

0.0

0.5

1.0

1.5

2.0

2.5

Fig. 7. vT vs. T plot for complex 3. The solid line is the best fit to the experimentaldata (see text for fitting parameters). Inset: field dependence of the magnetizationof complex 3 at 2 K, the solid line is the Brillouin function for S = 1 and g = 2.0.

J = 0.66 cm�1, and the calculated susceptibility is shown in Fig. 7as a solid line. The calculated ferromagnetic constant results inan S = 1 spin ground state for compound 3. As can be seen inFig. 4, the two imine donors on the ligand are meta with respectto each other in the aromatic ring. Thus, the magnetic orbital ofthe Cu(II) ion will cause spin polarization of the appropriate p-symmetry orbitals on the imine nitrogen atom and due to themeta-substitution on the aromatic ring, weak ferromagnetic cou-pling results [11].

5. Conclusion

Imine ligands have been used to obtain metal-bearing buildingblocks of Fe(II) and Cu(II). The complex [Fe(L1)(MeOH)3] (1), withthree labile MeOH ligands, is a promising material for the designof Fe(II) metal–organic frameworks. The magnetic properties ofthe coordination polymer [Cu(L1)]n (2) have been studied and ithas been found to display ferromagnetic coupling. We have de-signed a ligand with three distinct metal-binding sites, 3,5-bis[{(2-hydroxyphenyl)methylene}amino]benzoic acid (L2), thethree bonding sites are in the 1,3,5 positions of an aromatic ring,thus facilitating ferromagnetic coupling between the metal ionscoordinated to the ligand. This is the case of the compound[Et3NH]2[Cu2(L2)2] (3), which possesses a S = 1 spin ground state.Additionally, compound 3 has a free and accessible carboxylatearm and its properties as good building block for the preparationof metal–organic frameworks are currently being investigated.

Acknowledgements

E.C.S. acknowledges the financial support from Spanish Govern-ment, (Grant CTQ2006/03949BQU and Juan de la Cierva fellow-ship) and the Unidade de Raios X (RIAIDT. University of Santiagode Compostela, Spain) for the crystallography of complex 3.

Appendix A. Supplementary material

CCDC 681123, 681121 and 681122 contain the supplementarycrystallographic data for 1, 2 and 3. These data can be obtained freeof charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-ated with this article can be found, in the online version, atdoi:10.1016/j.ica.2008.09.052.

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