PAPER www.rsc.org/dalton | Dalton Transactions

Syntheses, characterisation, magnetism and photoluminescenceof a homodinuclear Ln(III)-Schiff base family†

Joy Chakraborty,a Aurkie Ray,a Guillaume Pilet,b Guillaume Chastanet,b Dominique Luneau,b

Raymond F. Ziessel,c Loıc J. Charbonniere,c Luca Carrella,d Eva Rentschler,d M. S. El Fallahe andSamiran Mitra*a

Received 5th May 2009, Accepted 14th September 2009First published as an Advance Article on the web 16th October 2009DOI: 10.1039/b908910a

A novel family of homodinuclear complexes of the general formula [Ln2L2(X)2] (where Ln = Nd3+, Pr3+,Sm3+ and Tb3+ for 1, 2, 3 and 4, respectively and X, the coordinated NO3

- or Cl- anion) has beensynthesised from the corresponding lanthanide(III) salts with the pentadentate dianionic Schiff baseligand, H2L [N1,N3-bis(salicylideneimino)diethylenetriamine], that exhibits a N3O2 donor set. Singlecrystal X-ray diffraction studies evidenced the isostructurality of this family of centrosymmetric neutraldinuclear entities where the Ln(III) metal centres are coupled together by two phenolato oxygen atomsbelonging to two units of ligand (H2L). Interestingly, the two other phenolato groups of H2L aremono-coordinated to the metal ions. Temperature dependence (2-300K) magnetic susceptibility studiessuggest the presence of an antiferromagnetic interaction operating via double phenolato bridges.Photoluminescence activities of the complexes have been studied and compared with their precursorligand. All the complexes have been characterised with microanalytical and several spectroscopictechniques.

Introduction

The coordination chemistry of lanthanide ions has been widelyinvestigated over the last two decades because of both the usefulmagnetic and optical behaviours of these ions and their highcoordination numbers which afford high complexity and dimen-sionality of the resulting networks. According to the magneticaspect, such ions are involved in molecular architectures thatexhibit Single Molecule Magnet1 and Single Chain Magnet2

behaviours with fascinating potentialities in term of magneticanisotropy, as well as in Nuclear Magnetic Resonance Imaging3

that use such paramagnetic systems as contrast agents to enhanceimage quality. The optical aspect of the lanthanides lies in theirluminescent properties widely used in the lighting industry,4

electroluminescent materials and optical fibres,5 luminescenceimaging6 or biological assay.7 Most trivalent lanthanide ionsexhibit long-lived and line like emission bands at characteristic

aDepartment of Chemistry, Jadavpur University, Raja S. C. Mullick Road,Kolkata, 700032, India. E-mail: smitra_2002@yahoo.com; Fax: +91-33-2414 6414; Tel: + 91-33-2414 6193bGroupe de Cristallographie et Ingenierie Moleculaire, Laboratoire desMultimateriaux et Interfaces, UMR 5615, Universite Claude Bernard Lyon1, Bat. Jules Roulin, 43 Bd du 11 Novembre, 1918-69622, VilleurbanneCedex, FrancecLaboratoire de Chimie Moleculaire, UMR 7509 CNRS, ECPMULP, 25,rue Becquerel, 67087, Strasbourg Cedex 02, FrancedInstitut fur Anorganische und Analytische Chemie, Johannes GutenbergUniversitat Mainz, Duesberweg 10-14, D-55128, Mainz, GermanyeDepartament de Quımica Inorganica, Facultat de Quımica, Universitat deBarcelona, Martı i Franques, 1-11, E-08028, Barcelona, Spain† CCDC reference numbers 612533, 672292, 672291, 664111 for,[Nd2L2(NO3)2], [Pr2L2(NO3)2], [Sm2L2(NO3)2], [Tb2L2(Cl)2], respectively.For crystallographic data in CIF or other electronic format see DOI:10.1039/b908910a

wavelengths ranging from green light [Tb(III)] to near-infrared(NIR) light [Nd(III) and Er(III)].8 The Tb(III) and Eu(III) ionshave been extensively involved in luminescent probes due to theiremission in the visible region whereas, the Nd(III), Pr(III), Er(III)or Ho(III) ions emit in the Near Infrared Region (NIR).9 The studyof NIR luminescence is of specific interest because the emissionaround 900–1600 nm, which is highly transparent to biologicalsystems and fibre media, is valuable for fluoro-immunoassay,10

lasers and optical telecommunication.11

However, for lanthanide ions, because the f –f transitions areparity forbidden, the absorption coefficients are usually verylow with slow emissive rates. In order to overcome this, well-designed organic chromophores are used to excite lanthanide ionsas sensitisers (antenna effect).12 In this regard Schiff base metalcomplexes have played a key role to the gradual development of theLn(III) coordination chemistry, ranging from pure synthetic workto modern physicochemical and biochemically relevant studies ofmetal complexes.13 There are several reports which describe the useof modified “salen” type Schiff base ligands for the stabilisationof heteronuclear 3d–4f complexes.14 Many of these studies havefocused on magnetic behaviour while relatively few have describedthe photophysical properties of the compounds.15 In addition thelanthanide cations can promote Schiff base condensation and cangive access to complexes of otherwise inaccessible ligands throughthe template effect. This ability, combined with the applicationsof lanthanide macrocyclic complexes emerging from biology andmedicine has boosted research in these areas.16 Therefore, a largenumber of lanthanide complexes with the multidentate Schiffbase derived from the condensation of different aldehyde/ketoneand primary amine pairs has been published.17 However, onlyrecently, research work dealing with the various aspects involvingdifferent physicochemical properties and complexation behaviour

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of tetradentate Schiff bases has appeared in the literature, pri-marily focusing in the separation of actinides from lanthanides innuclear reprocessing18 and catalytic properties.19

As a potential starting material in all these fields of application,designing of the metal complex is the most crucial step in order toget the optimum performance from them. Being large tripositiveions with small charge-to-radius ratios, lanthanides tend to achievehigher coordination numbers (i.e. > 6) by maximising donoratom ligation. However, it is difficult to predict or control thelanthanide cluster architectures and nuclearities since these ionshave no strong stereochemical preferences and the aggregate ge-ometries are based mainly on steric interactions between ligands.8

Obviously, one strategy to gather ions in one finite structure is touse ligands exhibiting multidentate ability with chemical bridgesable to coordinate several metal centres. In that sense, one of themost successful chelate involved in Ln(III) dinucleation are thephenolic Schiff bases.8e Among them, bipyridyl macrocycles haverecently shown remarkable potentialities with Ln3+–Ln3+ tunableelectronic behaviour which is not only of interest in its ownright,20,21 but is also of potential benefit for photonic devices andnuclear magnetic resonance imaging (MRI) techniques. However,factors governing the formation of Ln3+–Ln3+ couples and theirsubsequent entrapment by dinucleating chelates remain largelyobscure. With phenolic ligands the importance of a large chelatenegative charge to lanthanide(III) ion ratio (r) for the dinucleationprocess was indicated by the greater stability of acyclic dinuclearcomplexes [{LnL(NO3)}2], in which r = 2:1, compared todinuclear macrocyclic complexes with a lower r = 1:1.21 Thisdependency of the Ln3+ dinucleation process on r is reasonablebecause of the hard Lewis acidity of Ln3+ ions. So, r could indeedbe an important molecular design parameter for new types ofphotonic devices and MRI contrast agents which take advantageof the tunable co-operative electronic behaviour of Ln3+–Ln3+

couples. Owing to the growing interest in the fundamental natureof Ln3+–Ln3+ coupling processes and the development of devicesbased on them, we have employed a doubly charged anionic chelatefor the purpose as a further course of investigation in this line.

Following this strategy, we have recently reported a newdinuclear Nd(III) complex [Nd2L2(NO3)2] with H2L = N1,N3-bis(salicylideneimino)diethylenetriamine, a pentadentate Schiffbase that exhibits a N3O2 donor set.22 This ligand has been widelyused in d ions coordination chemistry but scarcely with 4f ions.Its ability to generate dinuclear lanthanide complexes has beenexplored with several ions, thus standardising the preparationprocedure. The homodinuclear [Ln2L2(X)2] family, where X is acoordinated anion, is reported in this paper with Ln = Pr3+, Sm3+,Tb3+ and Nd3+ chosen for Near Infrared (NIR) Luminescencestudies. All the structures have been invariably established fromsingle crystal X-ray diffraction and analysed spectroscopically.Cryomagnetic investigations have been performed on all thecomplexes and the photoluminescence activities of the ligand andits complexes have been explored.

Results and discussion

Strategy

The doubly deprotonated ligand L2- contains strong donors,namely phenolato oxygen atoms as well as imine nitrogen

atoms bearing excellent coordination ability with transition/inner-transition metal ions through its N3O2 donor set. This ability hasbeen explored principally with d-block ions and our recent workon the [Nd2L2(NO3)2] homodinuclear species has encouraged us todevelop the lanthanide-based dinuclear complexes involving thisligand. The H2L synthesis already reported,22 is based on a doublecondensation of two salicylaldehyde with one diethylenetriamine(Scheme 1). The lanthanum nitrate or chloride salts were addedto a ligand solution and refluxed without any additional baseto deprotonate the ligand. The crystals obtained were thencharacterised.

Scheme 1 Schematic diagram showing the synthesis of ligand H2L.

This kind of multidonor ligand exhibiting phenolato groupscould favour the coordination of several lanthanide ions andespecially into dinuclear complexes.22,23 From the magnetic pointof view, the presence of phenolato groups able to bridge lan-thanide ions could be an asset to induce significant magneticexchange thanks to short Ln–Ln distances. In addition, therigidity of such linear ligand could favour the observation ofSingle-Molecule Magnet behaviour.23b According to the opticalproperties, complexes optimised for NIR emission, should complyseveral requirements. Among them, a strongly absorbing antennagroup is required to overcome direct excitation in the f –f forbiddentransitions by energy transfer from the chromophore to the metalions. In addition, this chromophore must be close to the metal tofavour this energy transfer. Therefore, the ligand should exhibitan absorption maximum around 500 nm to support the NIRluminescence.

The potentialities of the H2L ligand have then been exploredin its abilities to coordinated lanthanide ions, mediate magneticexchange and act as optical antenna.

Structural description of the complexes

The crystal structures of the [Ln2L2(X)2] family, where X isa coordinated anion (NO3

- or Cl-) and Ln = Nd, Pr, Smand Tb for 1, 2, 3 and 4, respectively, have been solved in amonoclinic space group (crystallographic data for the complexesare provided in Table 1). Complexes 1, 2, and 3 are isostructuraland differ from complex 4 since their coordinated anions arenitrate instead of chloride ion. Therefore, all the complexes areneutral dinuclear units with each cationic centre coordinated to allfive heteroatoms of L2- and terminal anions. One of the phenolato

10264 | Dalton Trans., 2009, 10263–10272 This journal is © The Royal Society of Chemistry 2009

Table 1 Crystallographic details and results for 1, 2, 3 and 4

1 2 3 4

Formula C36H36N8Nd2O10 C36H38N8Pr2O10 C36H38N8Sm2O10 C36H38Cl2N6O4Tb2

Formula weight/g mol-1 1029.20 1024.56 1043.55 1007.47Crystal dimension/mm 0.03 ¥ 0.12 ¥ 0.18 0.06 ¥ 0.17 ¥ 0.18 0.04 ¥ 0.09 ¥ 0.11 0.20 ¥ 0.26 ¥ 0.27Crystal system Monoclinic Monoclinic Monoclinic MonoclinicSpace group P21/n (no. 14) P21/n (no. 14) P21/n (no. 14) P21/c (no. 14)a/A 12.911(5) 12.8085(4) 12.8175(6) 11.149(1)b/A 11.938(5) 11.9086(6) 11.8739(4) 13.4793(9)c/A 13.960(5) 13.8970(5) 13.7328(7) 13.623(1)a/◦ 90 90 90 90b/◦ 110.732(5) 110.368(2) 110.205(2) 94.018(4)g /◦ 90 90 90 90V/A3 2012(1) 1987.2(1) 1961.4(2) 2042.3(3)Z 2 2 2 2T/K 293(1) 120(2) 120(1) 290(1)lMo-Ka/A 0.71069 0.71073 0.71073 0.71073rcalcd/g cm-3 1.692 1.645 1.767 1.638m/mm-1 2.615 2.485 3.030 3.608Ind. reflections (Rint) 6550 (0.050) 10558 (0.1000) 4589 (0.104) 4343 (0.035)R [I/s(I) > 3] 0.0385 0.0528 0.0433a 0.0572Rw 0.0685b 0.1223b 0.0789b 0.0866b

No. reflections used 4052 5396 2522 2885No. of refined parameters 253 253 253 226GOF on F 1.10 1.07 1.13 1.09Drmax,min/e A-3 1.00/-1.01 3.47/-2.37 1.54/-1.09 2.24/-1.79

a I/s(I) > 2. b All data.

Fig. 1 ORTEP illustrations of the complexes with atoms labelling: (a) for 1, 2 and 3 (NO3- ligand) and (b) for 4 (Cl- ligand). Hydrogen atoms have been

removed for clarity. Ellipsoids are represented with a probability of 25%.

oxygen atoms of each ligand bridges the adjacent metal centres.Labeled ORTEP24 pictures of 1, 2, 3 and 4 are shown in the Fig. 1.

The structures consist of crystallographically independentLn(III) cations surrounded by one deprotonated N3O2 donorSchiff base ligand and one anion giving, by symmetry, a cen-trosymmetric neutral homodinuclear entity with the followingformula: [Ln2L2(X)2]. ORTEP illustrations of the complexesshow that two adjacent {LnLX} moieties are coupled togethervia two phenolato bridges belonging to the two ligands. Thecomplex sits on a crystallographically imposed centre of inversionforming the m2-diphenolato bridged dinuclear structure withboth octacoordinated Ln(III) centres lying on a distorted squareantiprism geometry in 1-3 [Fig 2] whereas a distorted pentagonal

bipyramidal geometry is obtained for 4. The bonding parametersof all the complexes have been compared in Tables 2 and 3.An important point should be notice here. The ability to satisfythe coordination requirements of the Ln(III) centres with onlycoordinating atoms and without resort to additionally boundedsolvent molecules, especially water or alcohol, is an importantconsideration in the design of Ln(III) photonic devices since suchcoordinated solvent molecules are efficient quenchers of Ln(III)luminescence.

The coordination nature of the two identical “salen” moietiesof the same ligand is noteworthy. Out of the two phenolato oxygenatoms coming from each ligand, one is monocoordinated whilethe other one bridges the adjacent Ln(III) centres in a bidentate

This journal is © The Royal Society of Chemistry 2009 Dalton Trans., 2009, 10263–10272 | 10265

Fig. 2 (a) Perspective disposition of the donor sites around each Ln3+ centre shown in Neumann projection and (b) distorted square antiprism geometrydescribed by donor atoms around the Ln3+ center as illustrated in the polyhedral view (Central Ln atom shown in green colour) for 1-3.

Table 2 Comparison of the bond lengths (A) and Ln ◊ ◊ ◊ Ln distances (A)

1 2 3 4

Ln1-O27# 2.398(3) 2.429(3) 2.366(6) 2.313(5)Ln1-O27 2.383(3) 2.396(4) 2.358(6) 2.340(5)Ln1-N19 2.634(5) 2.653(4) 2.603(7) 2.555(7)Ln1-N16 2.617(4) 2.636(4) 2.591(7) 2.526(6)Ln1-N13 2.601(5) 2.615(4) 2.560(7) 2.511(7)Ln1-O5 2.235(4) 2.249(3) 2.220(5) 2.189(6)Ln1-Cl1 — — — 2.665(3)Ln1-O3 2.577(4) 2.578(4) 2.518(7) —Ln1-O2 2.553(4) 2.594(4) 2.548(5) —Ln1 ◊ ◊ ◊ Ln1 3.884 3.911 3.842 3.745

Symmetry codes to the equivalent position (#): -x, -y, -z.

fashion. Due to the inversion centre, the local coordinationenvironments are exactly identical for both Ln(III) centres withLnN3O5 coordination sphere in 1-3 constituted by two imine N13and N19, one secondary N16 from the amine part, two bridgingphenolato O27, one singly coordinated phenolato O5 and two O2and O3 from bidentate univalent NO3

-. Complex 4 differs from theother complexes as a chloride Cl1 ion completes the coordinationsphere instead of the nitrate ion [Fig. 1b]. Hence the geometry ofthe central Tb3+ can be described as pentagonal bipyramid with thebasal plane constructed by a N3O2 chromophore, constituted bytwo imine N13 and N19, one secondary N16 from the amine part,one bridging phenolato O27 and one singly coordinated phenolatoO5 and the axial sites of the pentagonal bipyramid are coordinatedby other bridging phenolato O27 and a terminally coordinatedchloride anion Cl1.

The metal ligand bond-lengths follow the lanthanide contrac-tion along the Pr, Nd, Sm, Tb series. In addition, the distancefrom the Ln(III) to the monochelating phenolate oxygen atom issignificantly shorter than the one with the bichelating oxygen atomand its symmetry related counterpart (Table 2). Within the Ln2O2

ring, the non-bonded transannular Ln–Ln and O–O distancesare ~3.884 and ~2.787 A, respectively. Comparison of this Ln2O2

geometry with that in the related macrocyclic dinuclear complexesreported earlier in the literature,20 shows that the intramolecularLn–Ln distances here are significantly contracted to 3.88 A (3.97 Areported for macrocyclic complexes) while there is a commensurateincrease in the O–O distance from 2.55 (reported) to 2.79 A.

Table 3 Comparison of important bond angles (deg)

1 2 3 4

O27-Ln1-N19 69.2(1) 69.1(1) 69.7(2) 71.2(2)O27-Ln1-N16 128.7(1) 128.0(1) 129.7(2) 134.6(2)N19-Ln1-N16 65.5(2) 64.9(1) 66.0(2) 67.2(2)O27-Ln1-N13 149.2(1) 149.0(1) 148.0(2) 148.1(2)N19-Ln1-N13 131.3(2) 130.7(1) 132.5(2) 134.7(2)N16-Ln1-N13 65.8(2) 65.9(2) 66.6(2) 67.6(2)O27-Ln1-O5 91.2(1) 92.4(1) 93.1(2) 98.1(2)N19-Ln1-O5 157.2(1) 158.2(1) 155.3(2) 151.7(2)N16-Ln1-O5 137.1(2) 136.6(1) 138.3(2) 140.4(2)N13-Ln1-O5 71.5(2) 71.1(1) 72.1(2) 73.4(2)O27-Ln1-O3 79.3(1) 124.2(1) 126.3(2) —N19-Ln1-O3 82.4(2) 82.8(1) 85.3(2) —N16-Ln1-O3 116.1(1) 72.6(1) 72.4(2) —N13-Ln1-O3 121.7(2) 85.2(1) 82.9(2) —O5-Ln1-O3 82.5(2) 99.4(1) 97.6(2) —O27-Ln1-O2 124.6(1) 79.4(1) 78.7(2) —N19-Ln1-O2 83.8(2) 83.8(1) 81.9(2) —N16-Ln1-O2 72.8(1) 117.2(1) 116.2(2) —N13-Ln1-O2 84.2(2) 121.7(1) 122.0(2) —O5-Ln1-O2 99.0(2) 81.5(1) 81.3(2) —O27-Ln1-O27# 71.3(1) 71.6(1) 71.2(2) 72.8(2)N19-Ln1-O27 91.4(1) 91.8(1) 92.3(2) 87.1(2)N16-Ln1-O27 86.9(1) 86.7(1) 87.9(2) 87.7(2)N13-Ln1-O27 84.0(1) 83.0(1) 83.7(2) 88.4(2)O5-Ln1-O27 93.3(1) 93.1(1) 89.4(2) 83.9(2)O3-Ln1-O2 49.0(1) 49.5(1) 50.5(2) —O3-Ln1-O27 150.2(1) 159.0(1) 159.4(2) —O2-Ln1-O27 159.3(1) 150.3(1) 149.4(2) —O27-Tb1-Cl1 — — — 167.4(2)Cl1-Tb1-O27 — — — 106.0(1)Cl1-Tb1-N19 — — — 80.7(2)Cl1-Tb1-N16 — — — 84.6(2)Cl1-Tb1-N13 — — — 97.8(2)Cl1-Tb1-O5 94.2(2)Tb1-O27-Tb1# 108.7(1) 108.4(1) 108.8(2) 107.2(2)

The shorter Ln–Ln separation in all the dinuclear complexesreported herein, compared to that of macrocyclic crown ethertype complexes, is due probably largely to the reduction in Ln(III)coordination number from ten/twelve in those macro complexesto seven/eight in 1-4 with a consequent ‘opening up’ of the bondangles at each Ln(III) cation. Each acyclic Schiff base ligand hasan essentially flat conformation with the Ln(III) cation and thecoordinating atoms are coplanar to within 0.05 A. The slightdeviation is due to the rotation of the phenolate ring out of the

10266 | Dalton Trans., 2009, 10263–10272 This journal is © The Royal Society of Chemistry 2009

plane of the rest of the ligand by approximately 26.64◦ aboutthe C(sp3)-C(sp3) bond to facilitate the bridging of the phenolateoxygen to the adjacent Ln(III) centre.

An investigation of the spatial distribution of the Ln(III) cationsthroughout the crystal shows the oxygen-bridged Ln(III) pairsto be well separated from their symmetry and lattice translatedneighbours. The closest interdimer Ln–Ln distances are 7.98and 8.15 A. Detailed bonding study did not reveal any trace ofintermolecular weak ligand-ligand or aryl-aryl interactions norhydrogen bonding interactions within the system.

Fourier transform infrared spectra

Fourier transform infrared spectra of all the complexesare fully consistent with their crystal structures establishedthrough the X-ray diffraction analyses. The ligand H2L [N1,N3-bis(salicylideneimino)diethylenetriamine] has also been charac-terised by FT-IR spectroscopy. Significant broad bands wereobtained at 1640 cm-1 corresponding to imine (C=N) stretchingfrequency. Condensation of all the primary amine groups has beenconfirmed from the absence of the N–H stretching bands in theregion 3150–3450 cm-1. The FT-IR spectra of all the synthesisedcomplexes contain strong peaks characteristic of C=N bands,confirming the complexation of the lanthanide ions. Indeed,upon coordination to the metal center, the imine C=N stretchingfrequency in the IR spectra dropped from 1640 cm-1 in thefree ligand, to 1617, 1616, 1617 and 1620 cm-1 in the relatedNd(III), Pr(III), Sm(III), and Tb(III) complexes. Deprotonationof all phenolic functions was confirmed by the lack of O–Hstretching bands in the IR region 2500–3500 cm-1 for all thecomplexes. The phenolic nstr(C-O) stretching frequency appearedat 1260 cm-1. One bifurcated strong peak at 1540 cm-1 can beattributed to the vibration of the phenoxy oxygens present in twodifferent environments.25 Ligand coordination to the Ln(III) metalcentre is substantiated by two bands at approximately 462 and448 cm-1 corresponding to nstr(Ln–N) and nstr(Ln–O) respectively.Comparison of the infrared spectra of the prepared complexes canlead us rather safely to the conclusion that the three complexes aremore or less isostructural or at least that the Schiff base ligands arecoordinated in the same manner. Assignment of the nitrate bandshas been made as reported previously for the complexes 1 to 3.22,26

Magnetic properties

The magnetic properties of rare-earth ions are governed bystrong orbital angular momentum contribution and the spin–orbitcoupling (SOC) that splits the 2S+1L multiplets of the 4f n ions into2S+1LJ J-states. The SOC constant ranges from 600 to 3000 cm-1,which ensures a good separation between the ground state and thefirst excited state. Therefore, in most cases, the magnetic behaviourof lanthanide complexes results from the ground state property.27,28

As the ions are embedded in a crystal field, the 2S+1LJ states aresplit into Stark components (up to 2J +1, n = even; J + 1

2,

n = odd). The magnetic behaviour will then be governed by thethermal population of these set of levels, preventing the applicationof a spin-only Hamiltonian for quantitative investigations ofexchange interactions between lanthanide ions. One exceptionshould be notice. The Gd(III) ion exhibits a quenched orbitalmomentum which can be therefore treated as a pure spin ground

state. This has led to numerous studies of Gd-Gd or Gd-3dcoordination complexes to investigate the exchange interactionsinvolved in such molecular systems, whereas the interactionsbetween the other Ln(III) ions and 3d ions are still very few.27

The magnetic behaviours of complexes 1-4 are reported inFigs. 3–6. The general thermal dependences of the molar magneticsusceptibilities cM present a decrease of the cMT value uponcooling. Compounds 1, 2 and 4 exhibit a similar magneticbehaviour while compound 3 deserves a particular comment. ThecMT product of 3 (Fig. 5) exhibits an almost linear decreasebetween 300 K to 20 K from 0.82 to 0.2 cm3 K mol-1, followedby a further decrease to 0.13 emu K mol-1 reached at 2 K. Thehigh temperature value agrees with the ones usually encounteredfor similar compounds.28,29 This particular feature is typical forsamarium complexes whose zero-field splitting is low enough(around 1000 cm-1) to permit the thermal population of the firstexcited state 6H7/2 from the 6H5/2 ground state. The decrease ofthe cMT product, as well as the change of slope in the c-1 vs Tcurve (Fig. 5), is indicative of the depopulation upon cooling ofthis two J-levels and the Stark sub-levels arising from the ligandfield effect. At low temperature (5–50 K) the cMT value (0.13–0.23 cm3 K mol-1) is close to the expected value of 0.18 cm3 K mol-1

Fig. 3 Thermal dependence of the cMT product. The inset presents thefield dependence of the reduced magnetisation at 2 K for 1.

Fig. 4 Thermal dependence of the cM-1 (�) and cMT (�) product for 2.

The straight line stands for the Curie–Weiss fit.

This journal is © The Royal Society of Chemistry 2009 Dalton Trans., 2009, 10263–10272 | 10267

Fig. 5 Thermal dependence of the cM-1 (�) and cMT (�) product for 3.

Fig. 6 Thermal dependence of the cMT product. The inset presents thefield dependence of the reduced magnetisation at 2 K for 4.

for two Sm(III) ions (J = 5/2, gJ = 2/7) indicating that only theJ = 5/2 ground state is populated. However, one cannot excludethe presence of small magnetic interaction between the Sm(III)ions.

The high temperature cMT values of 3.12 (1, Fig. 3), and 24.11(4, Fig. 6) cm3 K mol-1 agree with the 3.28 (Nd(III), 3H4, gJ =4/5) and 23.6 (Tb(III), 7F6, gJ = 3/2) cm3 K mol-1 values whichare expected for two isolated ions but for 2 at 300 K, cMT shows avalue of 4.51 cm3 mol-1 K (Fig. 4) which is larger than the expectedvalue of 3.21 cm3 mol-1 K for two magnetically independent Pr(III)ion (2 ¥ 1.60 cm3 mol-1 K). Pr(III) has a f 2 configuration givinga 3H4 ground state (L = 5, S = 1). Upon cooling, the cMTproducts are almost constant from 300 to 80 K for 2 and 4 whileit exhibits a smooth decrease for 1. Below 80 K a pronounceddrop of cMT is observed for all these three compounds down to0.61 (1), 0.35 (2) and 7.16 (4) cm3 K mol-1 at 2 K. Satisfactory fitsof the experimental data with the Curie-Weiss law (cM = C/(T -q)) yield to negative q values of -30.5 (1) and -4.7 (4) K, thatusually indicate antiferromagnetic exchanges while for 2 the c-1

vs T curve (depicted in Fig. 4) shows a clear deviation from theCurie law at low temperatures. As expected for Pr(III), being anon-Kramers ion in a distorted square-antiprism geometry, thecrystal field leads to the significant deviation in the Curie law from

the signature of a free ion due to the prominent population of thesinglet ground state. In accordance with the observed high valuefor the cMT product at room temperature also the determinedCurie constant, C, with 4.72 cm3 mol-1 K is slightly higher thanthe expected value for two uncoupled ions. The Weiss constant, q =-7.42 K, calculated from the c-1 vs T curve at temperatures above40 K however may indicate the presence of an antiferromagneticinteraction between the two Pr(III) ions. As discussed above thedeviation from the simple Curie law also may have the originsimply in the mixing of states within in the same J-levels dueto spin orbit interaction, thus leading to the break down of theLande interval rule. For Nd(III), Pr(III) and Tb(III), the magneticbehaviours result only from the 2S+1LJ ground states since the spin–orbit coupling is strong enough (nearly 2000 cm-1) to separateefficiently the first excited state. Therefore, in 1, 2 and 4, thedecrease of the cMT curves is typical of the thermal depopulationof the Stark components that results from the crystal field effect,associated with the possible presence of exchange interactionwithin such dimeric units.

Some information on the nature of this exchange interactioncould be extracted from magnetisation measurements. The insetsof Figs. 3 and 6 present the reduced magnetisation M/Nb curves.Despite the absence of clear saturation, the values of 2.58 (1) and9.23 (4) Nb are reached at 5 T. Concerning Nd(III) containingcomplexes, the fundamental Kramer doublet is the only state tobe populated at low temperature and the Nd(III) ion could betreated as an effective S = 1

2with a g value around 2.5.30 The

2.58 Nb saturation value observed at 2 K for 1 agrees with thispicture. In the opposite, the crystal field effect on Tb(III) ionssplits the 7F6 ground state into 13 levels and among them onlythe lowest energy level is populated at low temperature. Therefore,the saturation is rarely reached in Tb(III) containing complexes,as illustrated by the magnetisation curve of 4 that reaches 9.3 Nbinstead of ~18 Nb. Concerning the shape of curves, the curvatureobserved at low applied magnetic field for 1 clearly indicates thepresence of antiferromagnetic interaction between the two Nd(III)ions. Whereas the structural similitude between 1-4 could favoursimilar interaction in the other complexes, such features werenot observed in the magnetisation curves of 2 and 4. This mayindicate that the exchange interactions in these later systems areweaker than in 1. This point is illustrated by the differences in qvalues. The neodymium complex exhibits a large q value of -30 Kcompare to nearly -7 K for terbium and praseodymium complexes.However, it is difficult to explain this difference since the exactcause responsible of the nature of the magnetic interaction in thiskind of f - complexes is hardly identifiable.31 A simple magneto-structural analysis does not allow to draw some trends as theLn-O-Ln angles are nearly the same along the family and theLn–Ln distances tends to get shorter from praseodymium com-plexes to terbium ones.

UV-Vis spectroscopy and photoluminescence studies

Ligand in methanol. Fig. 7 shows the UV-Vis absorptionspectrum of the ligand in methanol and its main absorptionand emission properties are gathered in Table 4. The absorptionspectra of the ligand is composed of two main absorption bands.The high energy one (312 nm), is probably related to p→p*transitions centred on the benzene moiety. The assignment of

10268 | Dalton Trans., 2009, 10263–10272 This journal is © The Royal Society of Chemistry 2009

Table 4 Photophysical properties of the ligand in methanol

Absorption Emission

lmax/nm (e/M-1 cm-1) lmax/nm tfluorescence/ns

312 nm (4850 M-1 cm-1) 450 nm 3.6403 nm (1500 M-1 cm-1)

Fig. 7 UV-Vis absorption spectrum of the ligand H2L in methanol.

the low energy band is more difficult but on the basis of itsmedium absorption coefficient, it may have a pronounced chargetransfer transition character. Upon excitation into the UV region,the fluorescence spectrum of the ligand displays a pronouncedemission band with maxima at 450 nm (Fig. 8). Surprisingly, thecorresponding excitation spectra displayed the maxima to be at358 nm, a region corresponding to a well in the absorption spectra.The corresponding Stokes shifts is very large (ca. 5800 cm-1),indicating a highly polarized excited state with a large electronicreorganisation between the ground and excited states. The excitedstate decay is mono-exponential and its lifetime amounts to 3.6 ns(Table 4), suggesting emission from a non-conventional singletstate.

Fig. 8 Excitation (a, lem = 500 nm) and fluorescence (b, lexc = 360 nm)spectra of H2L in MeOH.

Luminescence spectra of the complexes in the solid state

Complexes of Sm and Tb (3 and 4). Among the two complexespossibly emitting in the visible region (3 and 4), only the Tbcomplex 4 gave rise to a measurable emission, the complex 3 being

non-luminescent. Fig. 9 displayed the emission spectra of 4 in thesolid state at room temperature upon excitation at 300 nm.

Fig. 9 Luminescence spectrum of [Tb2L2(Cl)2] (lexc = 300 nm, solidstate, r.t.).

The solid state spectrum displayed the characteristic 5D4→7FJ

(J = 3 to 6) transitions of Tb together with a residual broademission spanning from ca. 450 to 600 nm and probably arisingfrom a residual ligand centred emission. While measuring theemitted intensity at 545 nm (5D4→7F5 transition of Tb), theexcitation spectra displayed a broad excitation band in the UVregion, with a shoulder at around 430 nm (Fig. 10). In accordancewith the weak emission intensity of the terbium complex, theluminescence lifetime of the complex was very short (< 1 ms). Fromthe fluorescence spectrum of the ligand in solution (vide supra) onecan reasonably attribute the weak luminescence intensity and shortlifetime of the excited state to a Tb to ligand back energy transferprocess favoured by the weak energy difference between the ligandexcited state and the Tb 5D4 level.32

Fig. 10 Excitation spectrum of [Tb2L2(Cl)2] in the solid state(lem = 545 nm, solid diluted in a KBr matrix).

Complexes of Nd and Pr (1 and 2). Upon excitation in the UVregion (300 nm), complex 2 (Pr) displayed no emission neither inthe visible, nor in the NIR region.

The emission spectra of complex 1 (Nd) were similarly measuredin the solid state and are presented in Fig. 11. The emission bandsobserved in the NIR emission spectra of the Nd complexes canbe ascribed to the 4F3/2→4IJ transitions of Nd at 1050–1150 nm(J = 11/2) and 1300–1400 (J = 13/2).33

This journal is © The Royal Society of Chemistry 2009 Dalton Trans., 2009, 10263–10272 | 10269

Fig. 11 NIR emission spectrum of [Pr2L2(NO3)2], in the solid state(lexc = 300 nm, cut off filter at 645 nm, r.t.).

Conclusion

A pentadentate Schiff base ligand and the related neutralneodymium(III), praseodymium(III), samarium(III) and ter-bium(III) complexes have been synthesised and characterisedwith various spectroscopic methods and also photoluminescencestudies. Cryomagnetic investigations revealed an antiferromag-netic interaction between the Nd(III) ions in 1. Despite theisostructurality observed in these complexes, this interaction wasnot clearly evidenced in 2-4. X-ray diffraction studies confirmthat the geometry of the Ln3+ ions in the complexes is slightlydistorted square antiprism for 1-3, while it is pentagonal bipyramidfor complex 4. High negatively charged anionic ditopic ligandsfavour short Ln3+–Ln3+ contacts and are therefore of great interestwhen complexes with strong Ln3+–Ln3+ electronic interactionsare desired. These findings should be helpful in the design andpreparation of complexes with potential applications as tunableLn–Ln luminescent and diagnostic MRI contrast media andnovel diagnostic/therapeutic agents employing two techniquessimultaneously (e.g. radiology and fluorescence).

Experimental

Syntheses

All manipulations were carried out under aerobic conditionsat room temperature using dry commercial grade solvents. Allthe chemicals used for the syntheses were of analytical grade.99.999% pure neodymium nitrate hexahydrate, praseodymiumnitrate hexahydrate, samarium nitrate hexahydrate, terbium chlo-ride hexahydrate, salicylaldehyde and diethylenetriamine were allpurchased from Aldrich Chemical Co. Inc. and used withoutany further purification. Methanol and N,N-dimethylformamide(HCONMe2) were used as solvents and both dried before use.The solvent dimethyl formamide (AR) was distilled over di-phosphorus pentoxide before use in order to minimise the moisturecontent in the solvent.

(OH)C6H4CH=N(CH2)2NH(CH2)2N=CHC6H4(OH) (H2L).The Schiff base chelating ligand H2L was prepared following theconventional procedures (Scheme 1). Salicylaldehyde (0.244 g,2 mmol) was refluxed with diethylenetriamine (0.103 g, 1 mmol)in dehydrated methanol. After 1 h, a deep orange yellow solution

was obtained. It was subjected to TLC which revealed thepresence of some unreacted starting materials along with theproduct. Therefore, the Schiff base ligand was isolated by columnchromatography over silica gel (E. Merck) 60–120 mesh size,using a mixture of light petroleum and ethyl acetate (v/v, 1:1);subsequent evaporation of this eluate yielded pure ligand in liquidform. The solution of the purified ligand was then evaporatedunder reduced pressure to yield a gummy mass, which was driedand stored in vacuo over CaCl2 for subsequent use. Yield: 0.264 g(85%). Elemental analysis calcd for C18H21N3O2 (found): C 69.43(69.40); H 6.80 (6.82); N 13.49 (13.50)%. Main FT-IR bands(KBr, cm-1): n(C=N) 1640. UV-Vis (l, nm): 242, 369.

[Ln(L)(NO3)]2 (where Ln = Nd3+, Pr3+ and Sm3+ for 1, 2 and 3respectively). The appropriate quantity of solid Schiff base ligandH2L (0.622 g, 2 mmol) was dissolved in dry methanol (20 ml). Thisligand solution was added dropwise to a solution [methanol:DMF(2:1 v/v)] of Ln(NO3)3·6H2O (2 mmol) with constant stirring. Thesolution was refluxed at ca. 65 ◦C for approximately 1 h to completethe crystal precipitation process the onset of which was visiblewithin 40 min. The crystalline products were harvested and driedon filter paper. Fine needle shaped X-ray diffraction quality singlecrystals were manually separated out under microscope. Yield: 85,72 and 78% (with respect to the Ln ion) for 1, 2 and 3 respectively.Elemental analysis calcd for C36H38N8Nd2O10 (found): C 42.01(41.95), H 3.53 (3.60), N 10.89 (10.85)%. Elemental analysis calcdfor C36H38N8Pr2O10 (found): C 42.29 (42.23), H 3.55 (3.57), N,10.96 (10.98)%. Elemental analysis calcd for C36H38N8Sm2O10

(found): C 41.52 (41.55), H 3.48 (3.51), N 10.76 (10.75)%. Otherspectroscopic characterizations have been reported under theirrespective headings.

[Tb(L)Cl]2 (4). The appropriate quantity of solid Schiff baseligand H2L (0.622 g, 2 mmol) was dissolved in dry methanol(25 ml). This ligand solution was added dropwise to a methanolicsolution of TbCl3·6H2O (2 mmol) with constant stirring. Themixture was refluxed for 1 h at ca. 65 ◦C. The resulting solution wasfiltered and the filtrate kept undisturbed. Single crystals appearedfrom the bulk solution after 5 days. The crystalline products wereharvested and dried on filter paper. Square block shaped X-raydiffraction quality single crystals were manually separated outunder microscope. Yield: 80% (with respect to the Tb). Elementalanalysis calcd for C36H38N6Tb2Cl2O4 (found): C 45.93 (45.90), H3.85 (3.90), N 8.93 (8.95)%. Other spectroscopic characterizationshave been reported under their respective headings.

Physical measurements

Elemental analyses. C, H, N analyses were carried out using aPerkin-Elmer 2400 II elemental analyser.

FTIR. The Fourier Transform infrared spectra (4000–200 cm-1) of the ligand and the complexes were recorded on aPerkin-Elmer Spectrum RX-I FT-IR spectrometer with a solidKBr disc.

Fluorescence spectra. Fluorescence spectra in solutions weremeasured in 1 cm2 quartz suprasil cells on a FL 920 EdinburghInstrument equipped with a Hamamatsu R928 photomultiplier forthe visible domain. Steady state measurements were performedusing a 450 W Xe arc lamp while fluorescence lifetimes were

10270 | Dalton Trans., 2009, 10263–10272 This journal is © The Royal Society of Chemistry 2009

determined in the TCSPC mode using a variable frequency 310 nmpicosecond pulsed LED. Decay curves were deconvoluted usingthe program provided by the supplier, using a solution of silicasuspended in water or the measurement of the scattered light.

Absorption spectra. Absorption spectra were recorded in 1 cm2

quartz suprasil cuvettes on a Shimadzu UV3600 spectrometer after1000 fold dilution of the mother samples (2 ¥ 10-4 M in methanol).

NIR emission spectra. NIR emission spectra were measuredas solid samples in 2 mm diameter quartz tubes using a nitro-gen cooled Hamamatsu R5509-72 photomultiplier. Steady statemeasurements were performed using a 450 W Xe arc lamp, whilelifetimes were determined using a pulsed Xe flashlamp.

Cryomagnetic investigation. Cryomagnetic susceptibility mea-surements for the complexes were carried out with a QuantumDesigned SQUID MPMS-XL susceptometer apparatus whichworks in the range 2–300 K under magnetic field of 0.5 T on~20 mg of corresponding polycrystalline samples. Diamagneticcorrections were estimated from Pascal’s tables.

Single crystal X-ray diffraction studies. Each of a good qualityair stable single crystal with dimensions (0.03 ¥ 0.12 ¥ 0.18 mm) for1, (0.06 ¥ 0.17 ¥ 0.18 mm) for 2, (0.04 ¥ 0.09 ¥ 0.09 mm) for 3, and(0.20 ¥ 0.26 ¥ 0.27 mm) for 4 was mounted on a Nonius KappaCCD diffractometer equipped with a graphite monochromatedfocused X-ray tube bearing a molybdenum target (l/Mo-Ka =0.71069 A for 1 and 0.71073 A for the other complexes). Intensitydata were collected at room temperature for 1 and 4 and at 120 Kfor 2 and 3. The comparatively higher value of Rint is a consequenceof the smaller dimension of the chosen crystals in the case of 2and 3. The crystals showed almost no decomposition under X-rayexposure during data collection. Preliminary orientation matricesand unit cell parameters were obtained from the peaks of the first10 frames respectively and refined using the whole data set. Frameswere integrated and corrected for Lorentz and polarisation effectsusing DENZO.34 The scaling as well as the global refinement ofcrystal parameters was performed by SCALEPACK.34 Reflectionsthat were partly measured on previous and following frames wereused to scale the frames on each. The whole data processing wasperformed using the Kappa CCD analysis software.35 An empiricalabsorption correction36 has been performed for all data of the fourstructures. The structure of 1 has been solved by direct methodswith the SIR-92 program37 while that of the other complexeswith SIR-97,38 combined to Fourier difference syntheses andrefined against F using reflections with [I/s(I) > 3] for 1, 2and 4 and [I/s(I) > 2] for 3 (in order to keep a reasonablereflections/parameters ratio) by CRYSTALS program.39 All non-hydrogen atoms were successfully refined with anisotropic thermalparameters. The H atoms were all located in a difference map, butthose attached to carbon atoms were repositioned geometricallyand refined with isotropic thermal parameters. The H atomswere initially refined with soft restraints on the bond lengths andangles to regularise their geometries (C–H in the range 0.93–0.98,N-H in the range 0.86–0.89, O-H in the range 0.82 A) and Uiso(H)(in the range 1.2–1.5 times Ueq of the parent atom), after whichthe positions were refined with riding constraints. The unit-cellparameters, crystal system, space group and refinement detailsare summarised in Table 1. All calculations and graphical works

have been done using WinGX,40 CAMERON41 and PLATON24,42

programs.

Acknowledgements

Dr. Joy Chakraborty is grateful to the University Grants Commis-sion, New Delhi, Govt. of India for providing a Senior ResearchFellowship to him. Aurkie Ray is thankful to the Council ofScientific and Industrial Research New Delhi, Govt. of India, forproviding her with the financial support needed to carry out thework. The authors also acknowledge the financial help obtainedfrom All India Council for Technical Education, New Delhi, Govt.of India. Mathieu Stark and Dr. Gilles Ulrich from the LCOSA inFrance are warmly acknowledged for their careful NIR emissionmeasurements in quartz tubes. RZ is indebted towards the CNRSand the Region Alsace for the NIR equipment.

References

1 (a) R. Sessoli and A.K. Powell, Coord. Chem. Rev., 2009, 253, 2328;(b) N. Ishikawa, M. Sugita, T. Ishikawa, S. Koshihara and Y. Kaizu,J. Am. Chem. Soc., 2003, 125, 8694; (c) N. Ishikawa, Polyhedron, 2007,26, 2147; (d) C. Benelli and D. Gatteschi, Chem. Rev., 2002, 102, 2369;(e) J. Tang, I. Hewitt, N. T. Madhu, G. Chastanet, W. Wernsdorfer,C. E. Anson, C. Benelli, R. Sessoli and A. K. Powell, Angew. Chem.,Int. Ed., 2006, 45, 1729; (f) X. Gu, R. Clerac, A. Houri and D. Xue,Inorg. Chim. Acta, 2008, 361, 3873; (g) M. T. Gamer, Y. Lan, P. W.Roesky, A. K. Powell and R. Clerac, Inorg. Chem., 2008, 47, 6581.

2 (a) L. Bogani, C. Sangregorio, R. Sessoli and D. Gatteschi, Angew.Chem., Int. Ed., 2005, 44, 5817; (b) K. Bernot, L. Bogani, A. Caneschi,D. Gatteschi and R. Sessoli, J. Am. Chem. Soc., 2006, 128, 7947.

3 (a) E. J. Werner, A. Datta, C. J. Jocher and K. N. Raymond, Angew.Chem., Int. Ed., 2008, 47, 8568; (b) P. Caravan, J. Ellison, T. McMurryand R. Lauffer, Chem. Rev., 1999, 99, 2293; (c) V. Comblin, D. Gilsoul,M. Hermann, V. Humblet, V. Jacques, M. Mesbahi, C. Sauvage andJ. F. Desreux, Coord. Chem. Rev., 1999, 185–186, 451.

4 T. Justel, H. Nikol and C. Ronda, Angew. Chem., Int. Ed., 1998, 37,3084.

5 (a) S. Cappechi, O. Renault, D.-G. Moon, M. Halim, M. Etchells, P.J.Dobson, O. V. Salata and V. Christou, Adv. Mater., 2000, 12, 1591; (b) J.Kido and Y. Okamoto, Chem. Rev., 2002, 102, 2357.

6 S. Faulkner, S.J.A. Pope and B.P. Burton-Pye, Appl. Spectrosc. Rev.,2005, 40, 1.

7 V. W. W. Yam and K. K. W. Lo, Coord. Chem. Rev., 1999, 184, 157.8 (a) C. Piguet and J. -C. G. Bunzli, Chem. Soc. Rev., 1999, 28, 347; (b)

J.-C. G. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048; (c) G. F.de Sa, O. L. Malta, C. de Mello Donega, A. M. Simas, R. L. Longo,P. A. Santa-Cruz and E. F. da Silva Jr., Coord. Chem. Rev., 2000, 196,165; (d) C. Adachi, M. A. Baldo and S. R. Forrest, J. Appl. Phys.,2000, 87, 8049; (e) J.- C. Bunzli and C. Piguet, Chem. Rev., 2002, 102,1897.

9 (a) M. H. V. Wert, R. H. Woudenberg, P. G. Emmerink, J. W. VanGassel, J. W. Hofstraat and J. W. Verhoeven, Angew. Chem., Int. Ed.,2000, 39, 4542; (b) M. H. V. Wert, J. W. Verhoeven and J. W. Hofstraat,J. Chem. Soc., Perkin Trans., 2000, 2, 433; (c) S.-G. Roh, M.-K. Nah,J. B. Oh, N. S. Baek, K.-M. Park and H. K. Kim, Polyhedron, 2005, 24,137.

10 (a) I. Hemmila and S. Webb, Drug Discovery Today, 1997, 2, 373; (b) J.-C. G. Bunzli, Metal Complexes in Tumor Diagnosis and as AnticancerAgents, in Metal Ions in Biological systems, Ed. M. Dekker, New-York,2004, 42.

11 J.W. Stouwdam, G.A. Hebbink, J. Huskens and F.C.J.M. Van Veggel,Chem. Mater., 2003, 15, 4604.

12 (a) N. Sabbatini, M. Guardigli and J. M. Lehn, Coord. Chem. Rev.,1993, 123, 201; (b) S. I. Klink, H. Keizer and F. C. J. M. van Veggel,Angew. Chem., Int. Ed., 2000, 39, 4319.

13 (a) M. A. V. Ribeiro da Silva, M. D. M. C. Ribeiro da Silva, M. J. S.Monte, J. M. Goncalves and E. M. R. Fernandes, J. Chem. Soc., DaltonTrans., 1997, 1257, and references therein; (b) M. T. Kaczmarek, I.Pospieszna-Markiewicz, M. Kubicki and W. Radecka-Paryzek, Inorg.

This journal is © The Royal Society of Chemistry 2009 Dalton Trans., 2009, 10263–10272 | 10271

Chem. Commun., 2004, 7, 1247; (c) M. G. B. Drew, M. R. StJ. Foreman,M. J. Hudson and K. F. Kennedy, Inorg. Chim. Acta, 2004, 357, 4102;(d) H. C. Aspinall, Chem. Rev., 2002, 102, 1807; (e) S. A. Schuetz, V. W.Day, J. L. Clark and J. A. Belot, Inorg. Chem. Commun., 2002, 5, 706;(f) S. A. Schuetz, V. W. Day, R. D. Sommer, A. L. Rheingold and J. A.Belot, Inorg. Chem., 2001, 40, 5292; (g) Y. P. Cai, H. Z. Ma, B. S. Kang,C. Y. Su, W. Zhang, J. Sun and Y. L. Xiong, J. Organomet. Chem., 2001,628, 99.

14 M. Sakamoto, K. Manseki and H. Okawa, Coord. Chem. Rev., 2001,219–221, 379.

15 (a) M. P. Hogerheide, J. Boersma and G. V. Konten, Coord. Chem. Rev.,1996, 155, 87; (b) C. Piguet, C. Edder, S. Rigault, J.-C. G. BernardelliBunzli and G. Hopfgartner, J. Chem. Soc., Dalton Trans., 2000, 3999;(c) J.-P. Costes, F. Dahan and A. Dupuis, Inorg. Chem., 2000, 39, 165;(d) J.-P. Costes, F. Dahan, A. Dupuis and J.-P. Laurent, Inorg. Chem.,2000, 39, 169; (e) J.-P. Costes, F. Dahan, B. Donnadieu, J. Garcıa-Tojaland J.-P. Laurent, Eur. J. Inorg. Chem., 2001, 363; (f) J.-P. Costes, J.-P.Laussac and F. Nicodeme, J. Chem. Soc., Dalton Trans., 2002, 2731;(g) J.-P. Costes, F. Dahan and F. Nicodeme, Inorg. Chem., 2003, 42,6556; (h) R. Gheorghe, M. Andruh, A. Muller and M. Schmidtmann,Inorg. Chem., 2002, 41, 5314.

16 V. Alexander, Chem. Rev., 1995, 95, 273, and references therein.17 (a) F. Benetollo, A. Polo, G. Bombieri, K. K. Fonda and L. M.

Vallarino, Polyhedron, 1990, 9, 1411; (b) G. Bombieri, F. Benetollo,A. Polo, K. K. Fonda and L. M. Vallarino, Polyhedron, 1991, 10, 1385;(c) S. W. A. Bligh, N. Choi, W. J. Cummins, E. G. Evagorou, J. D. Kellyand M. McPartlin, J. Chem. Soc., Dalton Trans., 1994, 3369; (d) F.Benetollo, G. Bombieri, K. K. Fonda and L. M. Vallarino, Polyhedron,1997, 16, 1907; (e) K. K. Fonda, D. L. Smailes, L. M. Vallarino,G. Bombieri, F. Benetollo, A. Polo and L. De Cola, Polyhedron,1993, 12, 549; (f) S. W. A. Bligh, N. Choi, E. G. Evagorou, M.McPartlin and K. N. White, J. Chem. Soc., Dalton Trans., 2001,3169.

18 M. G. B. Drew, M. R. St. J. Foreman, M. J. Hudson and K. F. Kennedy,Inorg. Chim. Acta, 2004, 357, 4102.

19 S. Kano, H. Nakano, M. Kojima, N. Baba and K. Nakajima, Inorg.Chim. Acta, 2003, 349, 6.

20 (a) Z. Wang, J. Reibenspies and A. E. Martell, Inorg. Chem., 1997, 36,629; (b) I. A. Kahwa, J. Selbin, T. -C. Y. Hsieh and R. A. Laine, Inorg.Chim. Acta, 1986, 118, 179; (c) P. Guerriero, P. A. Vigato, J. -C. G.Bunzli and E. Moret, J. Chem. Soc., Dalton Trans., 1990, 647; (d) K. D.Matthews, S. A. Bailey-Folkes, I. A. Kahwa, C. A. OMahoney, S. V.Ley, D. J. Williams and C. J. O Connor, J. Phys. Chem., 1992, 96, 7021;(e) K. D. Matthews, R. A. Fairman, A. Johnson, K. V. N. Spence, I. A.Kahwa, G. L. McPherson and H. Robotham, J. Chem. Soc., DaltonTrans., 1993, 1719; (f) R. C. Howell, K. V. N. Spence, I. A. Kahwa,A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 1996,961.

21 R. C. Howell, K. V. N. Spence, I. A. Kahwa, A. J. P. White and D. J.Williams, J. Chem. Soc., Dalton Trans., 1996, 961.

22 J. Chakraborty, G. Pilet, M. S. El. Fallah, J. Ribas and S. Mitra, Inorg.Chem. Commun., 2007, 10, 489.

23 (a) X.-H. Bu, M. Du, L. Zhang, X.-B. Song, R.-H. Zhang and T.Clifford, Inorg. Chim. Acta, 2000, 308, 143; (b) P.-H. Lin, T.J. Burchell,R. Clerac and M. Murugesu, Angew. Chem., Int. Ed. Engl., 2008, 47,8848.

24 L. J. Farrugia, ORTEP-3 for Windows, 2.02, J. Appl. Crystallogr., 1997,30, 565.

25 (a) R. T. Conley, Infrared Spectroscopy, Allyn and Bacon Inc., Boston,1966; (b) K. Nakamoto, Infrared and Raman Spectra of Inorganic andCoordination Compounds, Part A and B, John Wiley, 5th Edn., 1997.

26 (a) S. Gourbatsis, J. C. Plakatouras, V. Nastopoulos, C. J. Cardin andN. Hadjiliadis, Inorg. Chem. Commun., 1999, 2, 468.

27 C. Benelli and D. Gatteschi, Chem. Rev., 2002, 102, 2369.28 O. Kahn, Molecular Magnetism, VCH Publishers Inc., New York, 1993.29 A-Q. Wu, Fa-K. Zheng, X. Liu, G.-C. Guo, L.-Z. Cai, Z.-C. Dong, Y.

Takano and J.-S. Huang, Inorg. Chem. Comm., 2006, 9, 347.30 R. L. Carlin, Magnetochemistry, Springer-Verlag, 1986.31 (a) S. T. Hatscher and W. Urland, Angew. Chem., Int. Ed. Engl., 2003, 42,

2862; (b) J. -P. Costes, J. M. Clemente-Juan, F. Dahan and F. Nicodeme,Dalton Trans., 2003, 272; (c) M. Hernandez-Molina, C. Ruiz-Perez, T.Lopez, F. Lloret and M. Julve, Inorg. Chem., 2003, 42, 5456.

32 (a) M. Latva, H. Takalo, V. M. Mukkala, C. Mataschescu, J. C.Rodrigues-Ubis and J. Kankare, J. Lumin., 1997, 75, 149; (b) S. Combyand J.-C. G. Bunzli, Handbook on the Physics and Chemistry of Rareearth, K. A. Jr Gschneider, J.-C. G. Bunzli and V. Pecharsky, Eds,Elsevier B.V., Amsterdam, 2007, Vol. 37, Chap. 235.

33 R. Ziessel, G. Ulrich, L. Charbonniere, D. Imbert, R. Scopelliti andJ.-C. G. Bunzli, Chem. Eur. J., 2006, 12, 5060.

34 Z. Otwinowski and W. Minor, Processing of X-ray Diffraction DataCollected in Oscillation Mode, Methods in Enzymology, Volume 276:Macromolecular Crystallography, part A, p.307-326, 1997, C.W. Carter,Jr. & R. M. Sweet, Eds., Academic Press.

35 Nonius COLLECT, DENZO, SCALEPACK, SORTAV: Kappa CCDProgram; us B.V.: Delft, The Netherlands, 1997-2001.

36 R. H. Blessing, Acta Crystallogr., Sect A, 1995, 51, 33.37 (a) A. Altomare, G. Cascarano, C. Giacovazzo and A. Guagliardi,

J. Appl. Crystallogr., 1993, 26, 343; (b) A. Altomare, G. L. Cascarano,C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori and M. Camalli,SIR92: A new tool for crystal structure determination and refinement,J. Appl. Crystallogr., 1994, 27, 435.

38 (a) A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.Giacovazzo, A. Guagliardi, A. Grazia, G. Moliterni, G. Polidori andR. Spagna, SIR97: A new tool for crystal structure determination andrefinement, J. Appl. Crystallogr., 1999, 32, 115.

39 D. J. Watkin, C. K. Prout, J. R. Carruthers and P. W. Betteridge,CRYSTALS, Issue 11, Chemical Crystallography Laboratory, Oxford,UK, 1999.

40 L. J. Farrugia, WinGX: A program for crystal structure analysis, J. Appl.Crystallogr., 1999, 32, 837.

41 D. J. Watkin, C. K. Prout and L. J. Pearce, CAMERON, ChemicalCrystallography Laboratory, Oxford, UK, 1996.

42 (a) A. L. Spek, PLATON for Windows, 1.12, Acta Crystallogr., 1990,A46, C34.

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