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Inorganic Chemistry Communications 10 (2007) 489–493

Synthesis, characterisation and cryomagnetic studies of anovel homonuclear Nd(III) Schiff base dimer

Joy Chakraborty a, Guillaume Pilet b,*, M. Salah El. Fallah c, Joan Ribas c, Samiran Mitra a

a Department of Chemistry, Jadavpur University, Kolkata, West Bengal, 700 032, Indiab Groupe de Cristallographie et Ingenierie Moleculaire, Laboratoire des Multimateriaux et Interfaces UMR 5615 CNRS – Universite Claude

Bernard Lyon 1, Bat. Jules Raulin, 43 bd du 11 Novembre, 1918 69622 Villeurbanne, Cedex, Francec Departamento de Quımica Inorganica, Universitat de Barcelona, Martı i Franques, 1-11, 08028 Barcelona, Spain

Received 14 September 2006; accepted 20 December 2006Available online 30 December 2006

Abstract

A novel homodinuclear complex [NdIII(L)(NO3)]2 (1) has been synthesised [H2L = N1,N3- bis(salicylideneimino)diethylenetriamine, apentadentate Schiff base with N3O2 donor set] and characterised with spectroscopic and micro-analytical techniques. Single crystal X-raydiffraction study reveals a centrosymmetric binuclear neutral entity [space group, P21/n; a = 12.911(5); b = 11.938(5); c = 13.960(5) A;Z = 4] where Nd(III) metal centers are bridged together by two phenoxo oxygen atoms each coming from the two ligands. The mostinteresting fact is that two similar ‘‘salen’’ moieties of each ligand are behaving completely different in their coordination. In the doublydeprotonated ligand (L2�), one phenoxo oxygen is mono coordinated to the metal, whereas its immediate neighbour on the other endbridges the two Nd(III) centers. The distance between the Nd(III) centers is found to be 3.884(3) A. Temperature dependence (2–300 K)magnetic susceptibility study suggests the presence of an antiferromagnetic interaction operating via two phenoxo bridges.� 2007 Elsevier B.V. All rights reserved.

Keywords: Nd(III) dimer; Schiff base; Crystal structure; Characterisation; Cryomagnetic studies

Schiff base ligands are considered as ‘‘privilegedligands’’ because they can be obtained by simple or mul-tiple self-condensation of suitable formyl- or keto- andprimary amine precursors to give rise to planar or tridi-mensional complexes in one step. These ligands are quiteefficient to bind different transition, non-transition andinner-transition metals, and able to stabilise them in var-ious oxidation states. Many excellent works have beendevoted to the exploration of new synthetic pathwaysand structural aspects of the resulting systems highlight-ing the role of the ligands, suitable combination of bridg-ing groups and of the different metal ions to design achemical pathway, capable of tuning the synthesis frommononuclear to multinuclear complexes [1]. Among thesecoordination complexes, nowadays Lanthanides (Ln) [2]

1387-7003/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2006.12.015

* Corresponding author. Tel.: +33 4 72 44 82 20; fax: +33 4 72 43 11 60.E-mail address: Guillaume.pilet@univ-lyon1.fr (G. Pilet).

have become a rich topic of research due to their appli-cations as contrast agents for NMR imaging [3,4], asliquid-crystalline metal complexes (metallomesogens) [5],as active agents in cancer radiotherapy [6] or as lumines-cent stains for protein labeling and sensitive homoge-neous immunoassays [7]. Recently, the use of thesecomplexes as catalytic reagents for organic transforma-tions has increased too, and a wide variety of chiralLn(III) complexes have been used as efficient homoge-neous catalysts for asymmetric Aldol reaction, Michaeladdition and Diels–Alder reaction, etc. where the centralmetal ion of the Ln(III) catalysts often affects the enanti-oselectivity and catalytic activity [8,9]. Being large tripos-itive ions with small charge-to-radius ratios, lanthanidesusually tend to achieve higher coordination numbers(i.e. >6) by maximising donor atom ligation to formdinuclear species of the formula Ln2(SB)3 (SB = Schiffbase), containing bridging Schiff base donor atoms [10–

Fig. 1. ORTEP view of the molecular structure of the homobinuclearNd(III) entity (Hydrogen atoms have been omitted for clarity and non-hydrogen atoms have been drawn at 50% thermal ellipsoid probabilitylevel) A few selected bond lengths (A) and bond angles (�): Nd(1)–O(5)2.235(4); Nd(1)–O(27) 2.383(3); Nd(1)–O(27) 2.398(3); Nd(1)–O(2)2.553(4); Nd(1)–O(3) 2.577(4); Nd(1)–N(13) 2.601(5); Nd(1)–N(16)2.617(4); Nd(1)–N(19) 2.634(5); Nd(1)–O(27)–Nd(1#) 108.68(10); O(27)–Nd(1)–O(27#) 71.32(10); N(13)–Nd(1)–N(16) 65.85(16); N(16)–Nd(1)–N(19) 65.45(14); N(19)–Nd(1)–O(27) 69.15(14); N(13)–Nd(1)–O(5)71.53(16).

N16 N13

N19

O2

O3

O5

O27

O27

Fig. 2. Perspective disposition of the donor sites around each Nd centreshown in Neumann projection (central Nd atom shown in green colour).

490 J. Chakraborty et al. / Inorganic Chemistry Communications 10 (2007) 489–493

12]. Depending on the preparative procedures, a numberof mono-, di- or polynuclear Ln(III)-H2salen [where sale-n = N,N 0-ethylene-bis(salicylideneimine)] complexes withdifferent compositions are known, including Ln2(salen)3,Eu(Hsalen)(salen), Ln(H2salen)X3 Æ nH2O [13–20]. Costeset al. proposed the first hypothetical structure of Ln(III)with macrocyclic Schiff base purely on the basis of spec-troscopic arguments in solution [21]. Recently several newroutes have also been evolved to synthesize and charac-terize mononuclear, homo- or heterodinuclear lanthanideSchiff base complexes [11,22–28]. Template synthesis andcharacterization of the new mono- and homo- or hetero-dinuclear macrocyclic complexes of metal ions of varyingradii and electron configuration – in particular rare earthelements with polyaza and polyoxaaza Schiff bases alsogot a new dimension in the past 25 years [29]. More cur-rently, Kaczmarek et al. have established some unusualcoordination mode of Gd(III) with N,N 0-bis(salicylid-ene)-4-methyl-1,3-phenylenediamine ligand. A similartype of dimeric complexes of Gd(III) and Tb(III) havealso been reported having 8-fold bicapped trigonal-pris-matic geometry [31]. Still now, structurally characterisedsalen-type complexes of 4f – block elements are not verymuch common. Prompted by those earlier works, wereport here the synthesis, X-ray crystal structure andcharacterisation of a novel homo bimetallic Schiff basecomplex of Nd(III).

The ligand H2L [N1,N3-bis(salicylideneimino)diethylene-triamine], a pentadentate Schiff base with N3O2 donor sethas been prepared by reflux condensation of salicylalde-hyde with diethylenetriamine (2:1 milli molar ratio) indry methanol. This ligand solution (2 mmol) was addeddropwise to a solution [methanol:DMF (2:1 v/v)] ofNd(NO3)3 Æ 6H2O (2 mmol) with constant stirring. Theresulting mixture was further refluxed for 40 min. at65 �C. Transparent rhombic shaped single crystals of suit-able diffraction quality were obtained in situ during reflux.They were filtered off, washed with ethanol and dried.Anal. Calc. for C18H18N4NdO5 (%): C, 42.01; H, 3.53; N,10.89. Found: C, 41.95; H, 3.60; N, 10.80. IR (KBr,cm�1): 1638 (C@N); 1260 (C–O). One bifurcated strongpeak at 1540 cm�1 can be attributed to the vibration ofthe phenoxy oxygens present in two different environments[32]. kmax(nm)/(e dm3 mol�1 cm�1): 365 (5300), 300 (2835),265 (9690). The prepared pentadentate ligand containsstrong donors, namely phenoxo oxygen atoms as well asimine nitrogen atoms bearing an excellent coordinationability with transition/inner-transition metal ions throughits N3O2 donor set. Single crystal X-ray structure revealsthe complex as a centrosymmetric neutral homobinuclearentity. ORTEP illustration [32] of the complex (Fig. 1)shows that two adjacent [Nd(L)(NO3)] moieties are bridgedtogether via two phenoxo bridges, each of which comingfrom the individual ligands. The whole complex sits on acrystallographically imposed centre of inversion formingthe l2-diphenoxo bridged binuclear structure with bothoctacoordinated Nd(III) centers (Fig. 2). The local coordi-

nation environment is exactly identical for both the centerswhich can be best described as a distorted square antiprismwith NdN3O5 chromophore [two imine N–N(13), N(19);one secondary N–N(16) from the amine part; two bridgingphenoxo O–O(27); one singly coordinated phenoxo O–O(5)and two O–O(2) and O(3) from bidentate univalent NO�3 �as evidenced from Fig. 1. Due to the presence of severalsaturated sp3 hybridised carbons in the amine part, theligand loses the planarity and induces the Nd(III) centersto enjoy a distorted square antiprism geometry (Figs. 2and 3). The bond distances of Nd(1)–Nimine are in the range2.601(5)–2.634(5), Nd(1)–N(16): 2.617(4) and Nd(1)–Oni-

trate: 2.553(4)–2.577(4) A, respectively. The nature of coor-

N13

O2

O5

O3

O27

N19

O27

N16

Nd

Fig. 3. Distorted square antiprism geometry described by donor atomsaround the Nd centre as illustrated in the polyhedral view.

J. Chakraborty et al. / Inorganic Chemistry Communications 10 (2007) 489–493 491

dination of the two identical ‘‘salen’’ moieties of the sameligand is completely different and hence noteworthy. Outof the two phenoxo oxygen atoms coming from eachligand, one is simply monocoordinated while the otherone bridges the adjacent Nd(III) centers in a bidentatefashion as reflected in the Nd–Ophenoxo bond distances[Nd(1)–O(27), 2.383(3) and Nd(1)–O(5), 2.235(4) A]. Thedistance of Nd(1)–Nd(1#): 3.8842(4) A is relatively toolong to consider any intramolecular Nd–Nd bonding. Adetailed bonding study does not reveal any trace of inter-molecular weak or hydrogen bonding interactions withinthe system (Fig. 4). A few selected crystallographic param-eters have been provided in Ref. [33].

The use of Ln(III) ions in molecular magnetism repre-sents an active field of research. Magnetic studies of d-block transition metal polynuclear complexes have shown

Fig. 4. Packing view of the dimeric com

great progress from experimental and theoretical contribu-tions. Procedures have become fairly well established forpredicting the magnetic properties of these polynuclearcomplexes. However, magnetic investigations concerningmixed metal complexes comprising d- and f- block transi-tion metal ions or 4f–4f homometallic complexes have beenoverlooked until recently due to the very weak interactionand the large anisotropic effect of lanthanide ions. Ratherlarge and anisotropic magnetic moments of most of theLn(III) ions make these ions appealing building blocks inthe molecular approach of magnetic materials [34–38].

In the last decade a few coordination complexes havebeen investigated magnetically, though the study is largelylimited to the case where LnIII = GdIII. Indeed the purespin ground state of GdIII allows a very simple analysisof the magnetic properties with a Heisenberg Hamiltonian[21,39–41]. For the LnIII ions with first order orbitalmomentum, this situation becomes more complicated.Indeed the 4f configuration of an LnIII ion splits into2S+1 LJ states by the interelectronic repulsion and spin-orbit coupling. Each of these states further splits into Starkcomponents ðupto 2J þ 1; n ¼ even; J þ 1

2; n ¼ oddÞ due

to the crystal field perturbation.Informative magnetic susceptibility measurements for 1

has been carried out with a Quantum Design SQUIDMPMS-XL susceptometer apparatus working in the range2–300 K under a magnetic field of approximately 500 G (2–30 K) and 10,000 G (35–300 K). Diamagnetic correctionswere estimated from Pascal’s tables. For NdIII ion, the2S+1LJ ground state is well separated from the first excitedstate. At room temperature, these components arising out

plex in the bc-plane of the unit cell.

0 50 100 150 200 250 3000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 50 100 150 200 250 3000

20

40

60

80

100

(cm

mol

)

T (K)

χ

χMT

(cm

3 mol

-1K

)

T (K)

Fig. 5. Plots of observed vMT (s) and 1/vM (inset) ms T for 1 (per twoNd).

492 J. Chakraborty et al. / Inorganic Chemistry Communications 10 (2007) 489–493

of the ground state are all populated. But progressivedepopulation starts as T decreases. When the NdIII ion isexchange coupled with another NdIII magnetic center in adimeric system, the temperature dependence of vMT isdependent on both the thermal population of NdIII Starkcomponents and also NdIII–NdIII magnetic interactionsvia the exchange pathways [42]. The Nd3+ ion is a f3 elec-tronic system, with 4I9/2 ground state (g = 8/11) and 4I11/2

first excited state at ca. 2000 cm�1 from the ground state.The magnetic properties of 1 in the form of vMT and1/vM (inset) vs T plots (vM is the molar magnetic suscepti-bility for Nd2 entity) are shown in Fig. 5. At 300 K, vMT

shows a value of 3.122 cm3 mol�1 K which is slightly smal-ler than the expected for two magnetically quasi-isolatedNdIII ions (2 · 1.64 cm3 mol�1 K). The value of1.64 cm3 mol�1 K is the standard value corresponding tothe NdIII free ion, with g = 8/11 [42]. Obviously this valuecan change in a given complex, but the variation should besmall, owing to the character of the fn complexes. The vMTvalues gradually decrease from room temperature toapproximately 30 K and then, quickly decreases to0.606 cm3 mol�1 K at 2 K (Fig. 5). The global feature canindicate both weak antiferromagnetic interactions and thedepopulation of the Stark levels. The 1/vM curve (Fig. 5,

0 10000 20000 30000 40000 50000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

M/N

β

H (G)

Fig. 6. The field dependence of the reduced magnetization at 2 K forcomplex 1 (per two Nd).

inset) cut the T axis at negative temperature. In a firstapproach, this feature indicates antiferromagnetic charac-ter, but as it has been indicated by Kahn, ‘‘it is importantto point out that the deviation with respect to the Curie lawmay have other origins than the magnetic interactions’’[42]. In particular, for elements with strong spin–orbit cou-pling, like NdIII, a f3 system, the deviation of the Curie lawis not necessarily the signature of ferro- or antiferromag-netic interactions. The plot of the reduced magnetisationM/Nb curve is shown in Fig. 6, which reaches a value of2.58 Nb at 5 T. In this case, the shape for the curve indi-cates clearly the antiferromagnetic coupling. It is very dif-ficult to pinpoint the exact cause responsible for thenature of the magnetic interaction in this kind of f-com-plexes [43–45].

Acknowledgements

Joy Chakraborty is grateful to the University GrantsCommission for a Senior Research Fellowship offered tohim. This work has also been financially assisted by theDefence Research and Development Organisation, andDepartment of Science and Technology, New Delhi, Gov-ernment of India. J.C. warmly thanks Ms. Maitri Mapa ofNational Chemical Laboratory, CSIR and Ms. Lipika Royof IIT, Mumbai for their valuable support.

Appendix A. Supplementary data

CCDC 612533 contains the supplementary crystallo-graphic data for this article. These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic DataCentre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.inoche.2006.12.015.

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