Inorganica Chimica Acta 361 (2008) 3723–3729

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Structure and magnetic properties of a trinuclear nickel(II) complexwith benzenetricarboxylate bridge

Jerzy Mrozinski a,*, Alina Bienko a, Pavel Kopel b, Vratislav Langer c

a Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie, 50 383 Wroclaw, Polandb AVX Czech Republic, Dvorákova 328, 563 01 Lanškroun, Czech Republicc Environmental Inorganic Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 March 2008Received in revised form 3 April 2008Accepted 3 April 2008Available online 10 April 2008

Dedicated to Dante Gatteschi.

Keywords:Nickel complexBenzenetricarboxylateTrinuclear complexesTrimesic acidCrystal structureMagnetism

0020-1693/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ica.2008.04.005

* Corresponding author. Fax: +48 71 3757307.E-mail address: jmroz@wchuwr.chem.uni.wroc.pl

Novel trinuclear Ni(II) complex [Ni3(pmdien)3(btc)(H2O)3](ClO4)3 � 4H2O, 1 where pmdien = N,N,N0 ,N0,N00-pentamethyldiethylenetriamine, H3btc = 1,3,5-benzenetricarboxylic (trimesic) acid, has been preparedand structurally characterized. Three nickel atoms are bridged by btc trianion and their coordinationsphere is completed by three N atoms of pmdien and O atom of the water molecule. The three nickel(II)magnetic centers are equivalent and their coordination spheres are completed to deformed octahedrons.Magnetic susceptibility was measured over the temperature range 1.8–300 K and zJ0 = �0.19 cm�1,D = 3.79 cm�1, g = 2.18 parameters were calculated.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

A very important class of compounds in both inorganic and bio-inorganic chemistry is that containing carboxylato group. Espe-cially, the use of multifunctional ligands for the preparation ofmetal organic compounds becomes of great interest. The com-pounds can contain 2D and 3D frameworks linked by organic li-gands or hydrogen bonding. The compounds formed can bepotentially used for the preparation of inorganic porous materialsapplicable for adsorption, ion exchange or catalysis.

1,3,5-Benzenetricarboxylic acid (btcH3) is of interest because ofits symmetry and potential use of six oxygen donor atoms. Manycompounds with different degrees of deprotonation of btcH3 weresynthesized. Because of the versatility in bonding properties it isalways very important to obtain completely solved crystal andmolecular structures. The examples of coordination compoundswith uncoordinated btc anions are rather rare as follows fromCCDC database [1–4]. A very wide range of compounds involvingthese with btc bridge were obtained and their structural propertiesare known. In some cases, the acid is not completely deprotonizedand even mononuclear complex or complexes bridged by secondligand can be formed [5–9]. But most of the complexes with btc an-

ll rights reserved.

(J. Mrozinski).

ion are polynuclear and a variety of coordination modes are ob-served. These are for example: (a) chelating bis-bidentate andmonodentate [10,11]; (b) chelating-bridging bis-bidentate andchelating bidentate [12]; (c) monodentate, bridging monodentateand bidentate [10]; (d) tris-monodentate [13–17]; (e) tris-chelat-ing [14,18–21]; (f) tris-bidentate [22]. It is necessary to note thatthese modes can be combined in a crystal structure.

Nickel centers atom prefers coordination number six and thusits coordination sphere must be completed by other ligands. Btcanion and water molecules functioning as ligands are reported in[23].

The pyridines, polyfunctional alcohols or 4-picoline were usedin combination with btc anion and their structures were solvedin [24–28]. Prior et al. have used bridging ligands 4-aminopyridineand 4,40-bipyridine for the preparation of polymeric complexes[29,30]. Tetraaza and hexaazamacrocylic ligands in combinationwith btc and their structures are also known [31–35]. In our previ-ous work we have prepared and characterized series of nickel andcopper complexes with a combination of N-donor ligands or Schiffbases with btc anion [36]. Molecular structure of [Cu3(mdp-ta)3(btc)](ClO4)3 � 4H2O (mdpta = N,N-bis-(3-aminopropyl)methyl-amine) was solved. There is a btc3� bridge connecting threecopper atoms in tris-monodentate mode. Coordination sphere ofcentral atoms is completed by three N atoms of mdpta, forming de-formed square plane.

Table 1Crystal data and structure refinement for SC-2-1a

Empirical formula C36H86Cl3N9Ni3O25

Formula weight 1327.62Temperature (K) 173(2)Wavelength (Å) 0.71073Crystal system monoclinicSpace group P21/cUnit cell dimensions

a (Å) 11.7911(4)b (Å) 30.7753(11)c (Å) 17.0110(6)a (�) 90b (�) 107.637 (1)c (�) 90

Volume (Å3) 5882.7(4)Z 4Density (calc.) (Mg/m3) 1.499Absorption coefficient (mm�1) 1.170F(000) 2800Crystal size (mm3) 0.78 � 0.47 � 0.38Theta range for data collection (�) 1.82–33.15Index ranges �17 6 h 6 18,

�46 6 k 6 45,�26 6 l 6 26

Reflections collected 107680Independent reflections (R(int)) 21349 (0.0280)Completeness to theta (%) 31.00� (100.0%)Absorption correction multi-scanMaximum and minimum transmission 0.6649 and 0.4623Refinement method full-matrix least-squares on F2

Data/restraints/parameters 21349/9/553Goodness-of-fit on F2 1.012Final R indices [I > 2sigma(I)] R1 = 0.0385, wR2 = 0.1080R indices (all data) R1 = 0.0454, wR2 = 0.1112Largest difference in peak and hole (e �3) 0.742 and �0.457

3724 J. Mrozinski et al. / Inorganica Chimica Acta 361 (2008) 3723–3729

In this paper, we report on the preparation of the first nickel(II)trinuclear complex with 1,3,5-benzenetricarboxylate bridge. Wehave used bulky pmdien ligand to prevent the formation of a poly-nuclear complex and from X-ray study follows that the complexpossesses deformed coordination sphere of the nickel(II) ioncenters.

2. Experimental

2.1. Materials and methods

Safety note: Caution! Perchlorate salts of metal complexes withorganic ligands are potentially explosive. Even small amount ofthese materials should be handled with great caution.

The chemicals and solvents were supplied from Aldrich and Lac-hema (Czech Republic). The C, H, N analyses were carried out on anEA 1108 instrument (FISONS). IR spectra were recorded on a Spe-cord M 80 (Carl Zeiss, Jena).

Magnetic susceptibility measurements in the temperaturerange of 1.8–300 K were carried out on powdered sample, at themagnetic field of 5 kG using a Quantum Design SQUID Magnetom-eter (type MPMS-5). The correction for diamagnetism was calcu-lated using Pascal’s constants as �996 � 10�6 m3 mol�1, whereasthe temperature independent paramagnetism of the Ni2+ centerswas calculated as 220 � 10�6 cm3 mol�1 [37]. The effective mag-netic moment per nickel(II) atom was calculated from the equationleff ¼ 2:83

ffiffiffiffiffiffiffiffiffivMT

pB.M., where vM is the molar magnetic susceptibil-

ity corrected for diamagnetism.

2.2. Synthesis of [Ni3(pmdien)3(H2O)3(btc)](ClO4)3 � 4H2O

Pmdien (0.42 cm3, 2 mmol) was added to a solution of Ni(-ClO4)2 � 6H2O (0.74 g, 2 mmol) in ethanol (50 cm3). A solution of

Fig. 1. Numbering scheme for [Ni3(pmdien)3(btc)(H2O)3](ClO4)3 � 4H2O with atomic dispclarity.

btcH3 (0.15 g, 1 mmol) in water (10 cm3) neutralised with KOH(0.17 g, 3 mmol) was slowly added to the stirred brown solution.

lacement ellipsoids drawn at 30% probability level. Hydrogen atoms are omitted for

J. Mrozinski et al. / Inorganica Chimica Acta 361 (2008) 3723–3729 3725

The color of the solution turned to green and a little amount ofgreen precipitate appeared, which was then filtered off. After aweek dark green crystals together with a little amount of precipi-tate and KClO4 were obtained. The product was recrystallized sev-eral times from water. Yield: 46%. Anal. calc.: C, 30.8; H, 6.4; N, 9.8;S, 3.7. Found: C, 30.7; H, 6.4; N, 9.4; S, 3.4%. IR (cm�1): 1580 (mas-

COO), 1376 (msymCOO).

2.3. X-ray structure determination

Diffraction data were collected using a Siemens SMART CCD dif-fractometer with Mo Ka radiation (k = 0.71073 Å, graphite mono-chromator). The crystals were cooled to 173(2) K by a flow ofnitrogen gas using the LT-2A device. A full sphere of reciprocal spacewas scanned by 0.3 steps in x with a crystal-to-detector distance of3.97 cm. Preliminary orientation matrix was obtained from the firstframes using SMART [38]. The collected frames were integrated usingthe preliminary orientation matrix which was updated every 100frames. Final cell parameters were obtained by refinement of thepositions of 9539 reflections with I > 10r(I) after integration of allthe frames using SAINT software [38]. The data were empirically cor-rected for absorption and other effects using the SADABS program [39].The structures were solved by direct methods and refined by full-matrix least-squares on all jF2j data using SHELXTL software [40].Important crystallographic parameters are collected in Table 1.

Fig. 2. Content of the unit cell in projection along the c-axis. There are layers ofcomplexes in the (a, c) plane, connected by hydrogen bonds via squeezed perchl-orates and waters.

3. Results and discussion

3.1. X-ray crystal structure

The molecular structure is depicted in Fig. 1, whereas the viewof crystal packing is in Figs. 2 and 3.

The structure consists of a trinuclear complex cation, three unco-ordinated perchlorate anions and four lattice water molecules. Theuncoordinated solvent molecules and lattice water molecules weredisordered and unable to refine satisfactorily, so these were‘‘squeezed” out from the structure using PLATON [41] program. Eachcentral nickel(II) is coordinated by three nitrogen atoms from pmdienand oxygen atom of coordinated water molecule. Btc trianion forms abridge among the three nickel atoms. From the above it is obviousthat nickel atoms are hexacoordinated in deformed octahedral geom-etry with NiN3O3 chromophore and pmdien is in facial arrangement.The shortest Ni–N distances equal 2.1068 Å and 2.1222 Å, whereasthe Ni–O distances are equal to 2.1517 Å and 2.1842 Å. All theremaining distances and angles are provided in Table 2.

Medium strength hydrogen bonds can be found in the struc-tures (Table 3), which expand the molecules of 1 to the hydrogenbonded chain via squeezed perchlorates and waters along crystal-lographic a and c plane (Fig. 3).

3.2. Magnetic susceptibility measurements

The study of magnetic susceptibility for examined complex hasbeen performed in the field of 0.5 T within the temperature rangeof 1.8–300 K. The experimental data show the product vMT (vM isthe susceptibility per one Ni(II) magnetic center unit) as a functionof temperature (Fig. 4).

The compound reveals the magnetic behavior which is charac-teristic for weak antiferromagnetic coupling. The vMT valuesmoothly decreases in a wide range of temperatures and rapidlydecreases at the lowest temperatures. The value of vMT at roomtemperature is 1.23 cm3 mol�1 K (3.09 B.M.), which is expectedfor one Ni(II) paramagnetic ion. The complex magnetic data(Fig. 4) indicated that nickel(II) magnetic centers in the trimerare isolated as a result of 1,3,5-benzenetricarboxylic bulky ion

causes sterical hindrance preventing magnetic interaction betweenthe inside trimers. The slight decrease of vMT can be due to D (zero-fields splitting effect) of Ni(II) ions and intermolecular exchangeinteractions between trimeric units. In the temperature range of50–300 K the compound obeys the Curie–Weiss law [1/vM = C(T � h)] with calculated values C = 1.199 cm3 K mol�1 andh = �1.20 K. For the description of the susceptibility data the fol-lowing equation was used [42]:

vM ¼2Ng2l2g2

3kT2x�1 � 2 expð�xÞx�1 þ expð�xÞ

þ2 expð�xÞ

where x = D/kT and molecular field correction

vcorrM ¼ vM

1� 2zJ0vM

Ng2avb2

The values of exchange parameters were determined by least-square procedure and the minimization of the function

R ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPni¼1 ðvMÞ

expi � ðvMÞ

calci

h i2�ðvMÞ

expi

� �2

Pni¼11 ðvMÞ

expi

� �2.

vuuuut

Fig. 3. Content of the unit cell in projection along the a-axis. There are layers of complexes in the (a, c) plane, connected by hydrogen bonds via squeezed perchlorates andwaters.

3726 J. Mrozinski et al. / Inorganica Chimica Acta 361 (2008) 3723–3729

where R was the criterion used to determine the best fit. Least-square fits to the experimental data were obtained for zJ0 = �0.19cm�1, D = 3.79 cm�1, g = 2.184 and R = 6.054 � 10�5.

The field H dependence of the magnetization M (in B.M. units)at 1.7 K (Fig. 5) clearly supports the occurrence of spontaneousmagnetic moments in the complex examined. As the magnetic fieldincreases, the M versus H curve is linear up to 1000 Oe, and thenshows a sinusoidal variation before reaching saturation at a valueof Ms = 1.84 B.M. per nickel ion. In such a case, the ground stateequals Smax = 1 and the magnetization (per Ni unit) should saturateto the value of M = 2 B.M. The obtained value is clearly smaller andevidences an occurrence of zero-field splitting effect and antiferro-magnetic exchange interaction between nickel(II) trimeric units.

It seems interesting to compare magnetic behavior of the exam-ined complex with that obtained for [Ni3(pmdien)3(l-ttc)](ClO4)3

(where (ttc3�) is trithiocynanurate3� ion [43]). The conversion oftrinuclear Ni(II) complex is accompanied by an exchange of centralbridging ion (Fig. 6).

Nickel atoms are pentacoordinated and the geometry can be de-scribed best as deformed trigonal bipyramid (TBP) formed by threeN atoms of pmdien and chelating N,S set of ttc anion. In compari-son with structure 1, the Ni–N bond lengths in apical positions aresignificantly longer for 1 (Ni–N2 = 2.140 Å, Ni–N1C = 2.174 Å).When we compare bond angles in both the structures, we can con-clude that geometry about nickel atom in structure 1 is deformedoctahedral with equatorial angles of 85.25�, 82.99� and 108.83�and axial angle of 172.39�, whereas the geometry of nickel atomin [Ni3(pmdien)3(l-ttc)](ClO4)3 with equatorial angles of 120.13�,

115.48 � and 124.32� (close to the regular angle 120�) and axial an-gle of 171.21� is closer to regular TBP (deformed trigonal bipyra-mid). The differences in the crystalline structures of bothcomplexes have a significant influence on their magnetic proper-ties. [Ni3(pmdien)3(l-ttc)](ClO4)3 trimer showed strong interac-tions between the adjacent Ni(II) ions, which led to ferro-magnetic coupling (Fig. 7).

The occurrence of magnetic phase transition was established bymeasuring the FCM and ZFCM as functions of the temperature un-der a weak magnetic field. The ferromagnetic coupling has beenconfirmed by the magnetization measurements at 2 K (Fig. 8)where the hysteresis loop is well visible. The AC data show abehavior which may be appropriate for a spin glass system [43].

Replacement of trithiocynanurate3� ion by a more bulky 1,3,5-benzenetricarboxylic3� ion causes sterical hindrance preventingmagnetic interaction between nickel magnetic centers.

It should be worthwhile to compare the magnitude of the ex-change parameter in reported Ni trimer with those obtained for[Cu3(pmdien)3(l-ttc)](ClO4)3 trimer [44]. This complex containsthe same bridging ligand and terminal ions as [Ni3(pmdien)3(l-ttc)](ClO4)3 compound but different magnetic centers – copper(II).The trinuclear structure of the complex was also proved by singlecrystal X-ray analysis, but the data are not sufficient for deposit.Coordination polyhedron of copper atoms is completed by threeN atom of pmdien. In comparison with nickel complexes, the cop-per centers are in slightly distorted trigonal bipyramidal arrange-ment with basal plane formed by S, N atoms of ttc and central Natom of pmdien (Scheme 1).

Table 2Bond lengths (Å) and angles (�) for SC-2-1a

NiA–O61 2.0406(10)NiA–O1A 2.0611(11)NiA–N3A 2.1098(12)NiA–N2A 2.1152(13)NiA–N1A 2.1335(11)

NiB–O1B 2.0459(11)NiB–N3B 2.1068(12)NiB–N1B 2.1222(11)NiB–N2B 2.1404(12)NiB–O42 2.1517(9)NiB–O41 2.1842(10)

NiC–O1C 2.0716(13)NiC–O21 2.0902(11)NiC–N2C 2.1030(13)NiC–N1C 2.1307(14)NiC–N3C 2.1503(13)NiC–O22 2.2805(11)

O61–NiA–O1A 91.69(5)O61–NiA–N3A 91.16(5)O1A–NiA–N3A 95.94(5)O61–NiA–N2A 95.80(5)O1A–NiA–N2A 172.39(5)N3A–NiA–N2A 85.25(5)O61–NiA–N1A 156.87(5)O1A–NiA–N1A 89.60(5)N3A–NiA–N1A 111.68(5)N2A–NiA–N1A 82.99(5)

O1B–NiB–N3B 94.47(5)O1B–NiB–N1B 92.50(5)N3B–NiB–N1B 107.80(5)O1B–NiB–N2B 175.89(5)N3B–NiB–N2B 85.49(5)N1B–NiB–N2B 83.61(5)O1B–NiB–O42 91.32(4)N3B–NiB–O42 91.71(4)N1B–NiB–O42 159.74(4)N2B–NiB–O42 92.78(4)O1B–NiB–O41 85.34(4)N3B–NiB–O41 152.30(4)N1B–NiB–O41 99.88(4)N2B–NiB–O41 96.61(4)O42-NiB–O41 60.63(3)

O1C–NiC–O21 87.63(6)O1C–NiC–N2C 174.85(6)O21–NiC–N2C 97.44(5)O1C–NiC–N1C 96.25(6)O21–NiC–N1C 90.93(5)N2C–NiC–N1C 84.60(6)O1C–NiC–N3C 91.46(6)O21–NiC–N3C 160.19(5)N2C–NiC–N3C 83.47(5)N1C–NiC–N3C 108.83(6)O1C–NiC–O22 87.65(6)O21–NiC–O22 60.09(4)N2C–NiC–O22 94.10(5)N1C–NiC–O22 150.65(5)N3C–NiC–O22 100.11(5)

Table 3Hydrogen bonds for SC-2-1a (Å and �)

D–H� � �A d(D–H) d(H� � �A) d(D� � �A) \(DHA)

O1A–H12A� � �O41i 0.82(2) 1.95(2) 2.7446(14) 162(3)O1B–H11B� � �O62i 0.81(2) 1.96(2) 2.7418(15) 162(4)

Symmetry transformations used to generate equivalent atoms:(i) �x + 2, �y + 1, �z + 2.

Fig. 4. Plots of the magnetic data for [Ni3(pmdien)3(btc)(H2O)3](ClO4)3 � 4H2O vM (�)and vMT (�) vs temperature (K) and calculated data (full line) obtained from theleast-squares fit of the theoretical R. Vicente et al. equation [42].

Fig. 5. Field dependence of the magnetization for [Ni3(pmdien)3(btc)(H2O)3]-(ClO4)3 � 4H2O complex.

Fig. 6. The [Ni3(pmdien)3(l-ttc)](ClO4)3 complex in projection along the crystallo-graphic threefold axis.

J. Mrozinski et al. / Inorganica Chimica Acta 361 (2008) 3723–3729 3727

The compound reveals the magnetic behavior characteristic forantiferromagnetic spin coupling. The vMT value decreases gradu-ally with lowering of the temperature (Fig. 9).

Fig. 7. Plots of the magnetic data for [Ni3(pmdien)3(l-ttc)](ClO4)3vM (�) and vMT (�)vs temperature (K).

Fig. 8. Hysteresis loop M = f(H) for [Ni3(pmdien)3(l-ttc)](ClO4)3 at 2 K.

Scheme 1. The scheme of complex cation [Cu3(pmdien)3(l-ttc)]3+.

Fig. 9. Plots of the magnetic data for [Cu3(pmdien)3(l-ttc)](ClO4)3vM (�) and vMT (�)vs temperature (K).

3728 J. Mrozinski et al. / Inorganica Chimica Acta 361 (2008) 3723–3729

There is plateau between 20 and 10 K with small slope, whichcorresponds to the Curie law expected for a doublet ground stateand magnetic moment stabilizes approximately at 1.81 B.M. (char-acteristic for a single copper(II) center with one unpaired electron).This stems from an antiferromagnetic coupling among copper cen-ters inside the trimer molecule. The exchange of Ni(II) ion intoCu(II) ion causes the change of nature of magnetic interaction fromferro to antiferromagnetic coupling.

4. Conclusion

We would like to finish the present work with a brief commenton the chemical relevance of the complexes. [Ni3(pmdi-en)3(btc)(H2O)3](ClO4)3 � 4H2O compound is the next example oftrinuclear nickel(II) complex of btc with triangular Ni3 core. Thetheoretical models presented fit the experimental data quantita-tively. The models are based on weak intermolecular exchangeinteractions between trimeric units and assumptions that nickel(II)magnetic centers in the trimer are isolated as a result of 1,3,5-benzenetricarboxylic bulky ion causing sterical hindrance prevent-ing magnetic interaction between inside trimers.

5. Supplementary material

CCDC 614619 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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