PAPER www.rsc.org/dalton | Dalton Transactions

Isomorphous replacement of MII ions in MII–GdIII dimers (MII = CuII, MnII,Q2

NiII, CoII, ZnII): magnetic studies of the products†

Anastasia N. Georgopoulou,a Rosa Adam,b Catherine P. Raptopoulou,a Vassilis Psycharis,a Rafael Ballesteros,b

Belen Abarcab and Athanassios K. Boudalis*a

Received 3rd December 2009, Accepted 24th February 2010First published as an Advance Article on the web ??????DOI: 10.1039/b925525g

Complexes [MIIGdIII{pyCO(OEt)pyC(OH)(OEt)py}3](ClO4)2·EtOH [MII = CuII (1), MnII (2), NiII (3),CoII (4) and ZnII (5)] crystallize in the monoclinic Cc space group and contain one hexacoordinate MII

ion and one enneacoordinate GdIII ion, bridged by three {pyCO(OEt)pyC(OH)(OEt)py}- ligands.Magnetic susceptibility measurements indicate a ferromagnetic interaction for 1 and antiferromagneticinteractions for 2–4. Using the H = -JSGd

III SMII spin Hamiltonian formalism, fits to the magnetic

susceptibility data yielded J values of +0.32 cm-1 for 1, -1.7 cm-1 for 2, and -0.22 cm-1 for 3. In complex4, the orbital contributions of CoII precluded the determination of the magnetic coupling. The complexfollows the Curie–Weiss law with q = -2.07 K (-1.44 cm-1).

Introduction

The progress in molecular magnetism over the past two decadeshas included, among other things, the development of het-erometallic complexes, which encouraged us to study the mag-netic exchange occurring between different spin carriers. In that5category of complexes a very important family is that comprisingtransition metal (3d) and lanthanide (4f) ions.

Simple 3d–4f dinuclear complexes have proved to be usefulin studying magnetic exchange between transition metals andlanthanides. Magnetically isolated and amenable to controlled10chemical and structural modifications, they allow us to identifykey parameters of the problem and carry out exact calculations.The first study of this kind was carried out by Bencini et al.,1

which evidenced a ferromagnetic CuII–GdIII interaction. This waslater corroborated by the first structurally characterized, discrete15CuII–GdIII dimer, reported by Costes et al.2 An important numberof dinuclear CuII–GdIII3 and other 3d–4f4 complexes have beenprepared since.

The study of the magnetic exchange between 3d and 4f metalions presents several complications: (i) the orbital contributions20observed in LnIII ions, render the spin-Hamiltonian inapplicable.The magnetic behaviour of an isolated LnIII complex dependscritically on the ligand field imposed by the ligands’ donor atoms.This dependence is complicated and there is no simple theoreticalmodel to account for it. (ii) Due to the effective shielding of the254f electrons by the outer-shell electrons, the 3d–4f (and also the4f–4f) magnetic exchange is very weak. Thus, its effect on thetemperature dependence of the magnetic susceptibility is masked

aInstitute of Materials Science, NCSR “Demokritos”, 153 10, AghiaParaskevi Attikis, Greece. E-mail: tbou@ims.demokritos.gr; Fax: (+30)210-6503365; Tel: (+30) 210-6503346bDepartamento de Quımica Organica, Faculdad de Farmacia, Universidadde Valencia, Avda., Vicente Andres Estelles s/n, 46100, Burjassot, Valencia,Spain† CCDC reference numbers 755832 for 1, 756139 for 2 and 755833 for3. For crystallographic data in CIF or other electronic format see DOI:10.1039/b925525g

by the effect of the thermal depopulation of the excited Starklevels upon cooling. Although these former complications do not 30arise in GdIII-containing complexes, the rest of the paramagneticLnIII ions have a first-order angular momentum and their magneticinterpretation is not straightforward.

Of the reported 3d–4f complexes, the vast majority comprisesCuII and GdIII. Due to the small spin of CuII (S = 1/2) and its 35lack of single-ion anisotropy, the interpretation of its magneticbehaviour is quite straightforward compared to that of othertransition-metal ions. Similarly, the lack of orbital contributionsof GdIII makes it the most attractive LnIII ion. The intrinsicallyferromagnetic CuII–GdIII interaction is an additional attractive 40aspect. The result is that studies comprising other LnIII ions orother transition metal ions are significantly scarcer.

Some years ago we started exploring the coordination chemistryof the ligand di-2,6-(2-pyridylcarbonyl)pyridine (pyCOpyCOpy,dpcp, Scheme 1),5 which led to several interesting polynuclear 45transition-metal complexes with various divalent transition-metalions (CoII, CuII, NiII)6–10 and FeIII.11 Given its potential to bridgeseveral metal ions, we wondered whether we could use it to prepareheterometallic 3d–4f clusters. Although this would imply a self-assembly process with significantly less operator control than that 50achieved with Schiff-base ligands, we considered it worthwhileto investigate this possibility. In this work we present a series ofisostructural MII–GdIII [MII = CuII (1), MnII (2), NiII (3), CoII (4)and ZnII (5)] complexes exhibiting isomorphous coordination ofthe MII ions. 55

Scheme 1 Structure of ligand dpcp.

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 1–8 | 1

Results and discussion

Syntheses of the complexes

The first member of this family that was isolated was the CuII–GdIII complex [CuIIGdIII{pyCO(OEt)pyC(OH)(OEt)py}3](ClO4)2

(1). Initially, we employed sodium acetate in our reaction mix-5tures, hoping to obtain acetato-bridged complexes, but also inorder to assist the deprotonation of the hemiacetal form ofthe pyCOpyCOpy ligand. However, seeing that acetates do notparticipate in the formation of the complex we decided to carryout subsequent reactions in its absence. These reactions led10to a microcrystalline product, whose IR spectrum revealed thepresence of dpcp and perchlorates. However, no single crystals ofadequate quality could be isolated for structural characterizationwith X-ray crystallography. Therefore, we decided to includesodium acetate in all subsequent preparations, to ensure the15highest possible crystallinity of our products.

Intrigued by the isolation of 1 and its relatively rigid coordina-tion environment, we decided to replace CuII with other divalentparamagnetic metal ions. To our satisfaction, this was possiblefor MnII (2), NiII (3) and CoII (4). Considering the number of20different metal(II) ions giving access to this family of complexes, wewere interested in whether this structural type could be stabilizedby other such ions. Since a commonly studied such ion is ZnII,we were interested to see whether the ZnII analogue would bealso accessible. A concomitant interest in such a case would be25the possibility of extending our studies to LnIII ions other thanGdIII, through the empirical approach suggested by Costes.12 Thus,repeating the preparation using Zn(ClO4)·6H2O successfully led tothe desired ZnII–GdIII complex (5).

The reactions leading to complexes 1–5 can be summarized in30eqn (1).

M ClO H O pyCOpyCOpy Gd NO H O EtOH

MGd

EtOH( ) ( )

[4 2 2 3 3 26 3 6 6⋅ + + ⋅ + →D

{{ ( ) ( )( ) } ]( )pyCOH OEt pyC OH OEt py ClO HNO H O3 4 2 3 23 12+ +(1)

Attempts to prepare the nitrate complexes using the correspond-ing nitrate salts yielded no solids, even at high concentrations.35Slow evaporation of these solutions led to solids only when it wasallowed to proceed to dryness. In those cases the solids were notcrystalline.

The stability of this series of complexes deserves a finalcomment. Given the fact that different metal(II) ions exhibit widely40varying chemistries, the stabilization of 1–5 cannot be attributedto the MII ions themselves. Instead, we would rather speculatethat it is the ligand that infers a thermodynamic stability to thestructure, and the ability for it to accommodate a wide varietyof MII ions. Something similar has been observed with the related45ligand di-2-pyridyl ketone [(py)2CO, dpk], which has yielded thefamily of complexes [M9(X)2(O2CMe)8{(py)2CO2}4] (MII = FeII,CoII, NiII; X- = OH-, N3

-, NCO-) which retain the same structuredespite the differences in the MII ions.13

Description of the structures50

All complexes are isostructural and crystallize in the Cc spacegroup as ethanol solvates. Due to their structural similarities,

complete structural determinations were not carried out for all, butonly for 1–3, whose magnetic properties were analysed in detail.

Partially labelled plots of the cations of 1 and 3 are shown in 55Fig. 1 and 2, respectively. Important structural parameters areshown on Table 1. For brevity, the structural description will becarried out generically for all complexes, unless otherwise required.

Fig. 1 Partially labelled POV-Ray plot of the cation of complex 1.

Fig. 2 Partially labelled POV-Ray plot of the cation of complex 3.

The M atom in the complexes is hexacoordinate adopting anoctahedral geometry, which in the case of CuII exhibits Jahn– 60Teller elongation along the O(61)–N(1) axis. The equatorial bonddistances range narrowly between 1.981 and 2.022 A, with the axialbonds being 2.316 and 2.202 A [for O(61) and N(1), respectively].The coordination sphere around the Mn atom in 2 is moresymmetrical, with the bonding distances spanning the 2.145– 652.222 A range. The coordination sphere around the Ni atomin 3 exhibits the narrowest bond-length distribution, which is2.034–2.066 A. The Gd atoms are enneacoordinate. The M andGd atoms are connected by three monatomic alkoxo bridges

2 | Dalton Trans., 2010, 39, 1–8 This journal is © The Royal Society of Chemistry 2010

Table 1 Selected bond distances (A) and angles (◦) for 1·EtOH, 2·EtOH and 3·EtOH

1·EtOH 2·EtOH 3·EtOH 1·EtOH 2·EtOH 3·EtOH

M–O(61) 2.316(3) 2.186(4) 2.060(6) M–N(61) 2.022(4) 2.180(5) 2.034(8)M–N(1) 2.202(4) 2.222(5) 2.066(8) Gd–O(61) 2.288(4) 2.354(4) 2.335(5)M–N(31) 1.981(5) 2.167(5) 2.034(8) Gd–O(1) 2.353(3) 2.337(4) 2.321(6)M–O(1) 2.000(4) 2.145(4) 2.055(6) Gd–O(31) 2.380(3) 2.341(4) 2.340(5)M–O(31) 2.009(4) 2.168(4) 2.041(6) M ◊ ◊ ◊ Gd 3.031(1) 3.125(1) 2.987(2)N(31)–M–O(1) 168.0(2) 157.1(2) 166.2(3) N(61)–M–N(1) 99.8(2) 100.2(2) 97.7(3)N(31)–M–O(31) 81.7(2) 75.8(2) 80.8(3) N(31)–M–O(61) 96.8(1) 98.2(2) 96.7(3)O(1)–M–O(31) 86.3(1) 81.3(1) 85.4(2) O(1)–M–O(61) 80.6(1) 79.4(1) 83.1(2)N(31)–M–N(61) 100.4(2) 108.1(2) 100.4(3) O(31)–M–O(61) 83.8(1) 82.0(1) 85.8(2)O(1)–M–N(61) 90.5(2) 93.5(2) 93.2(2) N(61)–M–O(61) 76.8(1) 76.0(2) 80.3(3)O(31)–M–N(61) 160.6(2) 157.9(2) 166.1(3) N(1)–M–O(61) 158.8(1) 154.8(2) 163.1(3)N(31)–M–N(1) 104.3(2) 106.6(2) 100.2(3) M–O(1)–Gd 87.9(1) 88.3(1) 85.9(2)O(1)–M–N(1) 78.5(1) 75.9(2) 80.3(3) M–O(31)–Gd 87.0(1) 87.6(1) 85.7(2)O(31)–M–N(1) 98.3(1) 99.3(2) 95.7(3) M–O(61)–Gd 82.3(1) 86.9(1) 85.4(2)

belonging to the three dpcp ligands, with the M–O–Gd anglesvarying between 82.3 and 87.9◦ for 1 and with the respectiveangles exhibiting a much smaller dispersion for 2 (86.9–88.3◦) and3 (85.4–85.9◦). The MGdO2 rings show marked deviations fromplanarity, with the M/O(i)/O(j) and Gd/O(i)/O(j) planes forming5dihedral angles of 53.76 [O(1)/O(31)], 56.75 [O(31)/O(61)] and59.95◦ [O(61)/O(1)] for 1, 54.00 [O(1)/O(31)], 54.10 [O(31)/O(61)]and 57.32◦ [O(61)/O(1)] for 2 and 55.95◦ [O(1)/O(31)], 55.96[O(31)/O(61)] and 58.96◦ [O(61)/O(1)] for 3.

The dpcp ligand has undergone ethanolysis on both10its carbonyl atoms and is present in its singly deproto-nated {pyC(O)(OEt)pyC(OH)(OEt)py}- form. From each ligandmolecule, one pyridyl ring is coordinated to the M atom andanother to the Gd atom, while the third is non-coordinated. Thehydrogen atom of the protonated hydroxyl group participates in15H-bonding with the N-atom of non-coordinated pyridyl ring. Thisbridging mode is new for dpcp and is shown in Scheme 2.

Scheme 2 Bridging mode of dpcp in the 1–5 series of complexes.

The molecules are well separated from each other [e.g. smallestGd(x, y, z) ◊ ◊ ◊ Gd(x, 2 - y, -0.5 + z) separation of 11.45 A in 1),thus ensuring an effective magnetic insulation.20

IR spectroscopy

The IR spectra of complexes 1–5 are practically identical. Thedescribed absorption positions are those of 1. A series of broadand strong peaks at 3546, 3477 and 3413 cm-1 are attributed tostretching vibrations of the hydroxyl groups of the dpcp ligands,25hydrogen-bonded to the N-atoms of the uncoordinated pyridylgroups of adjacent dpcp molecules.

The medium-intensity bands at 2975 and 2874 cm-1 are assignedto the asymmetric vas[C–H(CH3)] and to the symmetric vsym[C–

H(CH3)] stretching vibration of the ethyl functions of the dpcp 30ligands. The medium bands at 2932 and 2874 cm-1 are tentativelyassigned to the v[(CH2)] vibration. A series of bands between 1600–1400 cm-1 (1599, 1574, 1474, 1454, 1436 cm-1) are attributed tothe stretching v(C ◊ ◊ ◊ C) and v(C ◊ ◊ ◊ N) vibrations of the pyridylrings of dpcp. A series of bands between 1300–1100 cm-1 (1300, 351272, 1228, 1209 cm-1) are assigned to the in-plane d [C–H(py)]deformations of the pyridyl groups of dpcp.

A strong and broad band at 1091 cm-1 and the medium bandat 623 cm-1 are attributed to the v3[F 2] and v4[F 2] modes of unco-ordinated perchlorates, respectively.14 The v[(C–O)dpcp] bands from 40C–O stretching of the dpcp alkoxo groups, expected around 1050–1100 cm-1, are probably overlapped with the perchlorate broadperchlorate vibrations. The out-of-plane v[C–H(py)] vibrations forthe pyridyl rings of dpcp appear split to two strong bands at 792and 759 cm-1. 45

Magnetic properties

Given the strictly dinuclear structure of the complexes, a simpleisotropic-exchange spin-Hamiltonian could be employed for theinterpretation of their magnetic properties:

H = -JSGdIII SM

II (2) 50

Since the g-factors are not necessarily the same for both ions,the resulting coupled S states will have global g-values that willdepend on the individual ones. These are calculated as:

gS = c1gA + c2gB (3)

where c1 = (1 + c)/2, c2 = (1 - c)/2 and c = [SA(SA + 1) + SB(SB 55+ 1)]/S(S + 1).15

For CuII no single-ion zero-field splitting (zfs) parameters aredefined, while for MnII and GdIII the axial (D) zfs parameters wereconsidered negligible. However, for NiII this parameter is known tobe quite important, so calculations taking it into account were also 60carried out. The experimental data of complexes 1–4 and best-fitcurves for 1–3 are shown in Fig. 3. Magnetization isotherms at2 K for all complexes, along with calculated curves, are shown inFig. 4.

Complex 1. The cMT value for 1 at 300 K is 8.32 cm3 mol-1 K, 65corresponding to the value predicted for two non-interacting SA =7/2 and SB = 1/2 spins (gGd = 2, gCu = 2.16). This remains

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 1–8 | 3

Fig. 3 cMT vs. T experimental data for complexes 1–4 and calculatedcurves corresponding to the best-fit solutions (see text).

Fig. 4 M vs. H experimental data for complexes 1–4 at 2 K. Solidlines correspond to simulations carried out using MAGPACK, employingbest-fit parameters derived from fits to the magnetic susceptibility. Thedashed line is a Brillouin curve corresponding to a pair of non-interactingSA = 7/2 and SB = 3/2 spins (g = 2).

constant upon cooling down to 20 K, increasing upon furthercooling up to 9.19 cm3 mol-1 K at 2 K. This behaviour suggests aweak ferromagnetic interaction.

For SA = 7/2 and SB = 1/2, Hamiltonian (2) gives rise to two Sspin states with energies ES, E4 = 0 and E3 = 4J, whose g-values,5gS, are g3 = (9gGd - gCu)/8 and g4 = (7gGd + gCu)/8. Applicationof the van Vleck equation yields an analytical expression for themagnetic susceptibility:

c b=

++

−

−

N

kT

g e g

e

J kT

J kT

232 4

42

4

28 60

7 9

/

/(4)

Fits according to this equation yielded best-fit parameters10J = +0.32 cm-1, gGd = 2.01, gCu = 2.13, with R = 1.7 ¥ 10-5,suggesting an S = 4 ground state. The magnetization isothermat 2 K shows a saturation value of 8.06 NAmB at 5.5 T and wassuccessfully simulated using the best-fit set of parameters (Fig. 4),thus corroborating our findings.15

Complex 2. The cMT value for 2 at 300 K is 12.50 cm3 mol-1 K,very close to the value predicted for two non-interacting SA =7/2 and SB = 5/2 spins (12.26 cm3 mol-1 K, gGd = gMn = 2).This decreases gradually upon cooling down to ~50 K and moreabruptly below that temperature, reaching 1.34 cm3 mol-1 K at 202 K. This behaviour suggests an antiferromagnetic interaction.

For SA = 7/2 and SB = 5/2, Hamiltonian (2) gives rise to sixspin states S, with energies ES, E6 = 0, E5 = 6J, E4 = 11J, E3 =15J, E2 = 18J, E1 = 20J, whose g-values, gS, are calculated from(3) as g1 = (9gGd - 5gMn)/4, g2 = gGd, g3 = (4gGd + gMn)/5, g4 = 25(2gGd + gMn)/3, g5 = (5gGd + 3gMn)/8, g6 = (3/5)gGd + (3/7)gMn.Application of the van Vleck equation leads to the expression:

c b= ×

+ + +− − − −

N

kT

g e g e g e g eJ kT J kT J kT

2

12 20

22 18

32 15

42 112 10 28 60/ / / JJ kT J kT

J kT J kT J kT

g e g

e e e e

/ /

/ / /

+ ++ + +

−

− − −

110 182

3 5 7 952 6

62

20 18 15 −− −+ +11 611 13J kT J kTe/ /

(5)

Fits according to this equation yielded best-fit parameters J = 30-1.7 cm-1, gGd = 2.04, gMn = 2.02, with R = 1.2 ¥ 10-5, suggestingan S = 1 ground state. The magnetization isotherm at 2 K doesnot show evidence of saturation, but a constant increase uponincreasing fields, indicative of the progressive stabilization of themagnetic states of higher spin multiplicities. Simulation with the 35set of best-fit parameters successfully reproduces the M vs. H data,thus corroborating our solution (Fig. 4).

Complex 3. The cMT value for 3 at 300 K is 9.33 cm3 mol-1 K,close to the value predicted for two non-interacting SA = 7/2 andSB = 1 spins (9.23 cm3 mol-1 K gGd = 2, gNi = 2.25). This remains 40constant upon cooling down to ~50 K, decreasing upon furthercooling down to 7.52 cm3 mol-1 K at 2 K. This behaviour suggestsa very weak antiferromagnetic interaction possibly combined withzfs contributions from NiII.

For SA = 7/2 and SB = 1, Hamiltonian (2) gives rise to six spin 45states S, with energies ES, E9/2 = 0, E7/2 = 9J/2, E5/2 = 8J, whoseg-values, gS, we calculate from (3) as g5/2 = (9gGd - 2gNi)/7, g7/2 =gGd, g9/2 = (7gGd + 2gNi)/9. Application of the van Vleck equationleads to the expression:

c b=

+ ++ +

− −

− −

N

kT

g e g e g

e e

x x

x x

25 22 16

7 22 9

9 22

16 92

35 84 165

6 8 10/ / / (6) 50

where x = J/2kT .Fits according to this equation yielded best-fit parameters

J = -0.22 cm-1, gGd = 2.04, gNi = 2.15, with R = 3.5 ¥ 10-6

(solution A).However, this expression does not take into account the zero- 55

field splitting of NiII, which is known to be quite important.15

In order to assess its magnitude, we undertook full-matrixcalculations, employing the spin-Hamiltonian:

ˆ ˆ ˆ ˆ ( ˆ ˆ ),H J DS gGd Ni z Ni Gd Ni=− + + +S S H S S2 b (7)

which assumes gGd = gNi = g. 60Best-fit parameters using this model were J = -0.23 cm-1, D =

1.51 cm-1, g = 2.05, with R = 3.7 ¥ 10-6 (solution B). This solutionis of comparable quality to solution A, yielding practically thesame J value and a relatively small DNi parameter.

4 | Dalton Trans., 2010, 39, 1–8 This journal is © The Royal Society of Chemistry 2010

The magnetization isotherm at 2 K shows a saturation value 65of 9.21 NAmB at 5.5 T and was successfully simulated using theset of parameters of solution A (Fig. 4), thus corroborating ourconclusions.

Complex 4. The cMT value for 4 at 300 K is 10.67 cm3 mol-1 K,higher than the value predicted for two non-interacting SA = 7/25and SB = 3/2 spins (9.76 cm3 mol-1 K gGd = gCo = 2). Thisis attributed to the orbital contributions, which are known tobe important in octahedral environments. This remains relativelyconstant upon cooling down to ~60 K, decreasing upon furthercooling down to 6.89 cm3 mol-1 K at 2 K.10

This overall behaviour suggests antiferromagnetic interactions.However, due to the complications arising from the orbital contri-butions of CoII, the magnetic exchange could not be determinedfrom fits of the magnetic susceptibility data using a simple spinHamiltonian. The cM

-1 vs. T curve follows the Curie–Weiss law,15with a constant q = -2.07 K (1.44 cm-1), favouring our qualitativeestimate of antiferromagnetic interactions. The magnetizationisotherm of 4 approaches saturation near 5.5 T, reaching a valueof 9.68 NAmB. This falls below the Brillouin curve of a pair ofnon-interacting SA = 7/2 and SB = 3/2 spins (g = 2), consistent20with our estimate of antiferromagnetic interactions.

Discussion of magnetic susceptibility data

Complex 1. As we said, significant work has been carried outin the preparation and magnetic study of CuII–LnIII heterometalliccomplexes and of CuII–GdIII in particular. The common feature25among these studies has been the usually ferromagnetic CuII–GdIII

interaction. Gatteschi explained this ferromagnetism by a spin-polarization mechanism involving orbital interaction between the6s orbital of GdIII and the 3d orbital of CuII. Kahn explainedit by a configuration interaction of the states arising from an30electron jump from the CuII 3d orbital to the GdIII 5d orbital.Using quantum chemical calculations, Paulovic et al. suggestedthat this ferromagnetism is intrinsic to the CuII–GdIII pair, alsoproposing that the mechanisms of Gatteschi and Kahn are notmutually exclusive, but rather interconvertible by application of35proper transformations to the magnetic orbitals.16 However, thisconclusion is at variance with experimental data by Costes et al.,revealing an antiferromagnetic interaction within a CuII–GdIII

pair.17 This finding suggests that this interaction is not intrinsicallyferromagnetic but that it must depend on several factors.40

More recently, Cirera and Ruiz, through density-functionalmethods, determined a relationship between J and the Cu–O–Gd–O torsion angle, studying a series of dinuclear complexeswith GdCuO2 cores. These correlations revealed that planar coreslead to stronger ferromagnetic interactions, which decrease upon45deviations from planarity,3 something that had previously beenconcluded by Costes on purely empirical data.18

Based on the above correlation, we may explain the weakferromagnetic interaction in 1 as a consequence of the availablesuperexchange pathways, which highly deviate from planarity50(see Description of the Structures and Scheme 3). Consideringthat atom O(61), which sits on the Jahn–Teller axis, occupiesa non-magnetic orbital (of dz2 character), superexchange willbe effective mainly through O(1) and O(31), occupying orbitalsof the equatorial positions (of dx2-y2 ). The large Cu–O(31)-Gd–55

O(1) torsion angle (36.06◦) agrees nicely with the small J valuedetermined above.

Scheme 3 Superexchange pathways in complex 1 showing the Jahn–Telleraxis (grey bonds) and dihedral angle of the CuGdO2 core.

Complex 2. Contrary to the case of CuII-GdIII complexes, thenumber of respective complexes with other metal ions is muchsmaller. Relevant examples of MnII, NiII and CoII complexes are 60presented on Table 2. In particular for MnII, examples involvingMnII–GdIII complexes are very rare. A MnII

3GdIII2 pentanuclear19

and a MnII2GdIII

2 tetranuclear complex20 have been reported, withthe smallest having been reported to date being two trinuclearMnII

2GdIII trinuclear complexes, reported by Clerac21 and Costes.22 65The only determination of the JMnGd coupling has been carriedout by Costes.22 Thus, complex 2 is the first dinuclear MnII–GdIII complex, and only the second whose MnII–GdIII magneticexchange has been determined. Our determination of an antiferro-magnetic MnII–GdIII interaction, as well as the qualitative findings 70of Wu,19 suggest that the MnII–GdIII interaction is not intrinsicallyferromagnetic, as is the case with CuII. More examples would bewelcome to further elucidate the MnII–GdIII interaction.

Complex 3. Concerning NiII, several examples of dinuclearNiII–LnIII complexes have been reported, partly because square- 75planar NiII is diamagnetic and it allows the estimation of theorbital contribution of orbitally non-degenerate LnIII ions.12 Someof the reported NiII–GdIII complexes contain high-spin NiII.These examples invariably reveal the interplay of ferromagneticinteractions, as is presented on Table 2. Complex 3 is the first to 80exhibit an antiferromagnetic NiII–GdIII interaction.

Complex 4. There have been several reports of CoII–GdIII

complexes, in some of which a determination of the JCoGd couplingconstant has been attempted by use of an isotropic model assumingSCo = 3/2. As has been pointed out by Benelli and Gatteschi,1 not 85taking into account the orbital contributions of CoII reduces thequantitative reliability of these determinations. Nevertheless, anincrease of cMT upon cooling is a good qualitative indicator offerromagnetic exchange. This being said, complex 4 joins a groupof CoII–GdIII complexes32,33,36 not showing such an increase, for 90which the magnetic interaction may be antiferromagnetic.

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 1–8 | 5

Table 2 Dinuclear and selected oligonuclear MII–GdIII complexes (MII = MnII, high-spin NiII and CoII) and reported magnetic exchange parameters

MII MII–GdIII complexa JM–Gd/cm-1b ,c Ref.

MnII [Mn2GdL2](NO3) NR (> 0) 21[Mn2[Gd(CH3OH)2]2(OH)(TC4A)2](OH) NR (> 0) 20[Mn3Gd2(CMe3CH2CO2)12(bipy)2] NR (<0) 19[Mn2GdL2(NO3)3] +1.56/0 22

NiII [NiGdL(H2O)2(NO3)3] +3.6 23[(NiL)Gd(hfac)2(EtOH)]·1.5EtOH +0.34 24[NiGdL(DMF)](ClO4)2·MeCN +0.56 25[(LNi(H2O))2Gd(H2O)](CF3SO3)3 +4.8/+0.05 (DNi = 12.4 cm1) 26[(LNi)2Gd](NO3) +0.91 (DNi = 4.5 cm-1) 26[Ni2GdL2][ClO4] +1.08 27[Ni2Gd(L)2(NO3)2(MeOH)4]NO3 +1.6 28[Ni2Gd(bcn)2]ClO4 +11.0 29[Ni2Gd(trn)2]NO3 NR (> 0) 30

CoII [LCo(MeOH)Gd(NO3)3] +0.90d 31[Gd(DMF)4(H2O)3(m-CN)Co(CN)5]·H2O NR (< 0) 32[(TPP)Co{P(CH2NC6H4-2-CO2Me)3}Gd] -2.1d 33[L2Co(H2O)2Gd(NO3)3] +1.0d 34[CoGd(piv)5(C9H7N)(H2O)] NR (> 0) 35[Co2Gd(piv)6(C9H7N)2(NO3)] NR (> 0) 35[Co2Gd(NO3)L6(bipy)2] NR (q = -1.15 K)e 36[Co2Gd(L)2(H2O)4][Cr(CN)6] NR (q = +0.76 K)e 37

a For the full formulae of the ligands, the interested reader is referred to the original publications. b According to the -JM–GdSMSGd spin Hamiltonianformalism. c NR = not reported (qualitative estimate). d Derived using a spin-only Hamiltonian and tabulated only indicatively. e Fit to a Curie–Weisslaw.

Conclusions and perspectives

During this work we found a way to isomorphously replace MII

ions in a series of isostructural MII–GdIII complexes. We werethus able to prepare a series of complexes of similar structuresand different MII ions and to study their magnetic properties.In the case of 1 (CuII–GdIII) the results verified the intrinsictendency of CuII to couple ferromagnetically with GdIII. In the5case of 2 (MnII–GdIII) our studies allowed the second determi-nation of a MnII–GdIII coupling. Consideration of our resultsin conjunction to those found in the literature suggests thatprobably there is no such intrinsic tendency for the MnII–GdIII pair.Similarly, the NiII–GdIII complex 3 did not exhibit a ferromagnetic10interaction, constituting the first such case and suggesting apossible subject for future theoretical studies. Similarly, theCoII–GdIII complex 4 did not show evidence of ferromagneticinteractions.

There are several perspectives arising both from a synthetic15and from a magnetic viewpoint. The first is to attempt theisomorphous replacement of MII with additional metal ions,e.g. FeII, which have been studied even less with LnIII ions.The second is the replacement of GdIII with other LnIII ions,in order to assess the MII–LnIII magnetic exchange. This will20be most interesting in the case of MnII, with which only ahandful of heterometallic LnIII complexes have been reported,21

and whose magnetic exchange is not well understood. A thirdperspective concerns the three uncoordinated pyridyl groups.Deprotonation of the hydroxyl functions of the three dpcp25ligands may create an O3N3 coordination compartment, in whichan additional MII ion may be accommodated, leading to alinear MII

2LnIII cluster. We are currently trying to deprotonatethe three hydroxyl groups, without affecting the integrity of thestructure.30

Experimental

Materials

All reagents and solvents were of analytical grade and usedas received expect dpcp, which was synthesized according to aliterature procedure.5 All syntheses were carried out under aerobic 35conditions. Caution! Although no such tendency was observedduring the current work, perchlorate salts are potentially explosiveand should be handled with caution and in small quantities.

Syntheses of the complexes

[CuIIGdIII{pyCO(OEt)pyCOH(OEt)py}3](ClO4)2·EtOH (1· 40EtOH). Solid dpcp (87 mg, 0.30 mmol) and NaOAc·3H2O(20 mg, 0.15 mmol) were added to a light blue solution ofCu(ClO4)2·6H2O (55 mg, 0.15 mmol) in EtOH (40 mL). Thesolution changed to dark blue and stirred under reflux for 20 min.Solid Gd(NO)3·6H2O (22 mg, 0.05 mmol) was then added to the 45solution, which was stirred under reflux for 20 min. During thattime the colour changed to light blue. After cooling, the solutionwas left for slow evaporation. Green prisms of 1·EtOH formedafter 2 d, which were filtered off and dried in vacuo. The yieldwas ~29 mg (~ 37%). The dried complex analyzed as solvent-free. 50Elemental analysis calcd for C63H66Cl2CuGdN9O20: C 48.47, H4.26, N 8.08. Found C 48.55, H 4.19, N 7.98%.

[MnIIGdIII{pyCO(OEt)pyCOH(OEt)py}3](ClO4)2·EtOH (2·EtOH). Solid dpcp (87 mg, 0.30 mmol) and NaOAc·3H2O(20 mg, 0.15 mmol) were added to a colourless solution of 55Mn(ClO4)2·6H2O (54 mg, 0.15 mmol) in EtOH (40 mL). Thesolution changed to yellow-brown and stirred under reflux for20 min. Solid Gd(NO)3·6H2O (22 mg, 0.05 mmol) was thenadded to the solution, which was stirred under reflux for 45 min.

6 | Dalton Trans., 2010, 39, 1–8 This journal is © The Royal Society of Chemistry 2010

During that time the colour changed to dark yellow-brown. Aftercooling, the solution was filtered off and layered with double thevolume of Et2O (10 ml). Yellow-brown prisms of 2·EtOH formedafter 1 d, which were filtered off and dried in vacuo. The yieldwas ~35 mg (~ 45%). The dried complex analyzed as solvent-free.5Elemental analysis calcd for C63H66Cl2GdMnN9O20: C 48.74, H4.28, N 8.12. Found C 48.72, H 4.34, N 8.08%.

[NiIIGdIII{pyCO(OEt)pyCOH(OEt)py}3](ClO4)2·EtOH (3·EtOH). Solid dpcp (87 mg, 0.30 mmol) and NaOAc·3H2O(20 mg, 0.15 mmol) were added to a light green solution of10Ni(ClO4)2·6H2O (55 mg, 0.15 mmol) in EtOH (30 mL). Thesolution changed to slurry brown-yellow and stirred under refluxfor 20 min. Solid Gd(NO)3·6H2O (22 mg, 0.05 mmol) was thenadded to the solution, which was stirred under reflux for 45 min.During that time the color changed to light green slurry. After15cooling, the solution was filtered off and layered with doublethe volume of a mixture of Et2O–n-hexane (10 mL, 1 : 1 v/v).Purple prisms of 3·EtOH formed after 1 d, which were filteredoff and dried in vacuo. The yield was ~21 mg (~ 26%). The driedcomplex analyzed as solvent-free. Elemental analysis calcd for20C63H66O20N9Cl2NiGd : C 48.63, H 4.27, N 8.10. Found C 48.55,H 4.29, N 8.12%.

[CoIIGdIII{pyCO(OEt)pyCOH(OEt)py}3](ClO4)2·EtOH (4·EtOH). Solid dpcp (87 mg, 0.30 mmol) and NaOAc·3H2O(20 mg, 0.15 mmol) were added to a light pink of Co(ClO4)2·6H2O25(55 mg, 0.15 mmol) in EtOH (40 mL). The solution changedto dark orange and stirred under reflux for 10 min. SolidGd(NO)3·6H2O (22 mg, 0.05 mmol) was then added to thesolution, which was stirred under reflux for 20 min. During thattime the colour changed to dark pink-orange. After cooling, the30solution was filtered off and layered with double the volume ofEt2O (10 mL). Light orange prisms of 4·EtOH formed after 2 d,which were filtered off and dried in vacuo. The yield was ~22 mg(~ 28%). The dried complex analyzed as solvent-free. Elementalanalysis calcd for C63H66O20N9Cl2CoGd : C 48.62, H 4.27, N 8.10.35Found C 48.74, H 4.32, N 8.03%.

[ZnIIGdIII{pyCO(OEt)pyCOH(OEt)py}3](ClO4)2·EtOH (5·EtOH). Solid dpcp (29 mg, 0.1 mmol) and NaOAc·3H2O(20 mg, 0.05 mmol) were added to a colourless solution ofZn(ClO4)2·6H2O (19 mg, 0.05 mmol) in EtOH (25 mL). The 40solution changed to light yellow and stirred under reflux for10 min. Solid Gd(NO)3·6H2O (7 mg, 0.01 mmol) was then addedto the solution, which was stirred under reflux for 20 min. Aftercooling, the solution was layered with double the volume of Et2O(10 mL). Light yellow prisms of 5·EtOH formed after 5 d, which 45were filtered off and dried in vacuo. The yield was ~6 mg (~ 24%).The dried complex analyzed as solvent-free. Elemental analysiscalcd for C63H66O20N9Cl2ZnGd : C 48.42, H 4.26, N 8.06. FoundC 48.49, H 4.20, N 8.11%.

X-Ray crystallography 50

Crystals of 1 (0.24 ¥ 0.28 ¥ 0.55 mm), 2 (0.10 ¥ 0.20 ¥ 0.25mm) and 3 (0.10 ¥ 0.20 ¥ 0.25 mm) were taken from the motherliquor and immediately cooled to -93 ◦C (1, 2) and -113 ◦C (3).Diffraction measurements were made on a Rigaku R-AXIS SPI-DER Image Plate diffractometer using graphite monochromated 55Cu-Ka radiation. Data collection (w-scans) and processing (cellrefinement, data reduction and Empirical absorption correction)were performed using the CrystalClear program package.38 Im-portant crystallographic and refinement data are listed in Table 3.The structures were solved by direct methods using SHELXS-9739 60and refined by full-matrix least-squares techniques on F 2 withSHELXL-97.40 Further experimental crystallographic details for1: 2qmax = 130◦; reflections collected/unique/used, 44 868/10 120[Rint = 0.0488]/10 120; 895 parameters refined; (D/s)max = 0.002;(Dr)max/(Dr)min = 0.899/-0.577 e/A3; R1/wR2 (for all data), 650.0435/0.0924. Further experimental crystallographic details for2: 2qmax = 129◦; reflections collected/unique/used, 41 812/10 288[Rint = 0.0577]/10 288; 894 parameters refined; (D/s)max = 0.008;(Dr)max/(Dr)min = 0.893/-1.060 e/A3; R1/wR2 (for all data),0.0497/0.1132. Further experimental crystallographic details for 703: 2qmax = 130◦; reflections collected/unique/used, 43 530/10 380[Rint = 0.0810]/10 380; 897 parameters refined; (D/s)max = 0.001;

Table 3 Crystallographic data for complexes 1·EtOH, 2·EtOH, 3·EtOH, 4·EtOH and 5·EtOH

1·EtOH 2·EtOH 3·EtOH 4·EtOH 5·EtOH

Formula C65H72Cl2CuGdN9O21 C65H72Cl2GdN9MnO21 C65H72Cl2GdN9NiO21 C65H72Cl2GdN9CoO21 C65H72Cl2GdN9ZnO21

FW 1607.01 1598.43 1602.18 1602.43 1608.87Space group Cc Cc Cc Cc Cca/A 15.3013(2) 15.338(2) 15.2166(2) 15.273(2) 15.303(3)b/A 20.5209(3) 20.491(2) 20.4545(3) 20.503(2) 20.509(4)c/A 22.6575(4) 23.224(4) 22.7423(4) 22.952(4) 22.908(4)a/◦ 90 90 90 90 90b/◦ 105.363(1) 106.096(7) 105.032(1) 105.276(8) 105.432(8)g /◦ 90 90 90 90 90V/A3 6860.1(2) 7013 6836.3(2) 6933 6949Z 4 4 4 4 4T/◦C -93 -93 -113 -93 -83Radiation Cu-Ka Cu-Ka Cu-Ka Cu-Ka Cu-Karcalcd/g cm-3 1.556 1.514 1.557m/mm-1 7.975 8.892 7.962Reflections with I > 2s(I) 9312 9522 8044R1

a 0.0405 0.0459 0.0690wR2

a 0.0899 0.1085 0.1454

a w = 1/[s 2(F o2) + (aP)2 + bP] and P = [max (F o

2,0) + 2F c2]/3, R1 = R (|F o| - |F c|)/R (|F o|) and wR2 = {R [w(F o

2 - F c2)2]/R [w(F o

2)2]}1/2.

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 1–8 | 7

(Dr)max/(Dr)min = 1.825/-1.501 e/A3; R1/wR2 (for all data),0.0808/0.1607. All hydrogen atoms were introduced at calculatedpositions as riding on bonded atoms. All non-hydrogen atomswere refined anisotropically; except of the oxygen atoms of theperchlorate counterions which were found disordered over two5positions and were refined isotropycally with occupation factorssumming one. Plots of all structures were drawn using theDiamond 3.1 program package.41

The identity of complexes 4 and 5 was confirmed by unitcell determinations. Relative parameters for these complexes are10shown on Table 3.

Physical measurements

Elemental analysis for carbon, hydrogen, and nitrogen wasperformed on a PerkinElmer 2400/II automatic analyzer. Infraredspectra were recorded as KBr pellets in the range 4000–400 cm-115on a Bruker Equinox 55/S FT-IR spectrophotometer. Variable-temperature magnetic susceptibility measurements were carriedout on powdered samples in the 2–300 K temperature rangeusing a Quantum Design MPMS SQUID susceptometer operatingunder a magnetic field of 0.1 T. Magnetization isotherms between200 and 5.5 T were collected at 2 K. Diamagnetic correctionsfor the complexes were estimated from Pascal’s constants. Themagnetic susceptibilities for 1–3 were computed by analyticalexpressions derived as described in the text. For 3, the magneticsusceptibility was also computed by exact calculation of the25energy levels associated with the spin Hamiltonian throughdiagonalization of the full matrix with the enhanced version ofa general program for axial symmetry.42 Least-squares fittingswere accomplished with an adapted version of the function-minimization program MINUIT.43 The error-factor R is defined30

as RN

=−∑ ( )exp

exp

c cc

calc2

2, where N is the number of experimental

points. Simulations of M vs. H were carried out with theMAGPACK program package.44

Acknowledgements

We are grateful to the Greek General Secretariat for Research35and Technology (PEP ATTIKIS 2000-06 project ATT_28), theMinisterio de Educacion y Ciencia (Spain, Project CTQ2006-15672-C05-03) for financial support, and the European RegionalDevelopment Fund for co-financing.

References40

1 A. Bencini, A. Benelli, A. Caneschi, R. L. Carlin, A. Dei and D.Gatteschi, J. Am. Chem. Soc., 1985, 107, 8128.

2 J.-P. Costes, F. Dahan, A. Dupuis and J.-P. Laurent, Inorg. Chem., 1996,35, 2400.

3 See for example:J. Cirera and E. Ruiz, C. R. Chimie, 2008, 11, 1227,Q345and references therein.

4 See for example:(a) M. Andruh, J.-P. Costes, C. Diaz and S. Gao, Inorg.Chem., 2009, 48, 3342; (b) C. Benelli and D. Gatteschi, Chem. Rev.,2002, 102, 2369, and references therein.

5 B. Abarca, R. Ballesteros and M. Elmasnaouy, Tetrahedron, 1998, 54,5015287.

6 A. K. Boudalis, C. P. Raptopoulou, B. Abarca, R. Ballesteros, M.Chadlaoui, J.-P. Tuchagues and A. Terzis, Angew. Chem., Int. Ed., 2006,45, 432.

7 A. K. Boudalis, C. P. Raptopoulou, V. Psycharis, Y. Sanakis, B. Abarca, 55R. Ballesteros and M. Chadlaoui, Dalton Trans., 2007, 3582.

8 A. K. Boudalis, C. P. Raptopoulou, V. Psycharis, B. Abarca and R.Ballesteros, Eur. J. Inorg. Chem., 2008, 3796.

9 A. K. Boudalis, M. Pissas, C. P. Raptopoulou, V. Psycharis, B. Abarcaand R. Ballesteros, Inorg. Chem., 2008, 47, 10674. 60

10 A. N. Georgopoulou, C. P. Raptopoulou, V. Psycharis, R. Ballesteros,B. Abarca and A. K. Boudalis, Inorg. Chem., 2009, 48, 3167.

11 A. N Georgopoulou, R. Adam, Y. Sanakis, C. P. Raptopoulou, V.Psycharis, R. Ballesteros, B. Abarca and A. K. Boudalis, Polyhedron,2009, 28, 3251. 65

12 J.-P. Costes, F. Dahan, A. Dupuis and J.-P. Laurent, Chem.–Eur. J.,1998, 4, 1616.

13 See:A. K. Boudalis, Y. Sanakis, J. M. Clemente-Juan, B. Donnadieu, V.Nastopoulos, A. Mari, Y. Coppel, J.-P. Tuchagues and A. P. Perlepes,Chem.–Eur. J., 2008, 14, 2514, and references therein. 70

14 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordina-tion Compounds, 4th edn, John Wiley & Sons, 1986, pp. 130, 138.

15 A. Bencini and D. Gatteschi, Electron Paramagnetic Resonance ofExchange Coupled Systems, Springer Verlag, 1990, pp. 48–55.

16 See: J. Paulovic, F. Cimpoesu, M. Ferbinteanu and K. Hirao, J. Am. 75Chem. Soc., 2004, 126, 3321, and references therein.

17 J.-P. Costes, F. Dahan, A. Dupuis and J.-P. Laurent, XX, 2000, 39, 169. Q418 J.-P. Costes, F. Dahan and A. Dupuis, Inorg. Chem., 2000, 39, 165.19 B. Wu, Dalton Trans., 2006, 5113.20 Y. Bi, Y. Li, W. Liao, H. Zhang and D. Li, Inorg. Chem., 2008, 47, 9733. 8021 V. Chandrasekhar, B. M. Pandian, R. Boomishankar, A. Steiner and

R. Clerac, Dalton Trans., 2008, 5143.22 J.-P. Costes, J. Garcia-Tojal, J.-P. Tuchagues and L. Vendier, Eur. J. In-

org. Chem., 2009, 3801.23 J.-P. Costes, F. Dahan, A. Dupuis and J.-P. Laurent, Inorg. Chem., 1997, 85

36, 4284.24 T. Yamaguchi, Y. Sunatsuki, H. Ishida, M. Kojima, H. Akashi, N. Re,

N. Matsumoto, A. Pochaba and J. Mrozinski, Inorg. Chem., 2008, 47,5736.

25 Q.-Y. Chen, Q.-H. Luo, L.-M. Zheng, Z.-L. Wang and J.-T. Chen, Inorg. 90Chem., 2002, 41, 605.

26 J.-P Costes, T. Yamaguchi, M. Kojima and L. Vendier, Inorg. Chem.,2009, 48, 5555.

27 V. Chandrasekhar, B. M. Pandian, R. Boomishankar, A. Steiner, J. J.Vittal, A. Houri and R. Clerac, Inorg. Chem., 2008, 47, 4918. 95

28 T. Shiga, N. Ito, A. Hidaka, H. Ohkawa, S. Kitagawa and M. Ohba,Inorg. Chem., 2007, 46, 3492.

29 C. A. Barta, S. R. Bayly, P. W. Read, B. O. Patrick, R. C. Thompsonand C. Orvig, Inorg. Chem., 2008, 47, 2280.

30 S. R. Bayly, Z. Xu, B. O. Patrick, S. J. Rettig, M. Pink, R. C. Thompson 100and C. Orvig, Inorg. Chem., 2003, 42, 1576.

31 J.-P. Costes, F. Dahan and J. Garcia-Tojal, Chem.–Eur. J., 2002, 8, 5430.32 G. Li, T. Akitsu, O. Sato and Y. Einaga, J. Coord. Chem., 2004, 57, 189.33 R. Raturi, J. Lefebvre, D. B. Leznoff, B. R. McGarvey and S. A.

Johnson, Chem.–Eur. J., 2008, 14, 721. 10534 J. P. Costes, F. Dahan, A. Dupuis and J.-P. Laurent, C. R. Acad. Sci.

Paris, Ser. II, 1998, 417. Q535 Y. Cui, G. Chen, J. Ren, Y. Qian and J. Huang, Inorg. Chem., 2000, 39,

4165.36 B. Wu, J. Coord. Chem., 2008, 61, 2558. 11037 T. Shiga, H. Ohkawa, S. Kitagawa and M. Ohba, J. Am. Chem. Soc.,

2006, 128, 16426.38 CrystalClear, Rigaku/MSC Inc., The Woodlands, Texas, USA, 2005.39 G. M. Sheldrick, SHELXS-97: Structure Solving Program, University

of Gottingen, Germany, 1997. 11540 G. M. Sheldrick, SHELXL-97: Crystal Structure Refinement Program,

University of Gottingen, Germany, 1997.41 DIAMOND–Crystal and Molecular Structure Visualization, Ver. 3.1,

Crystal Impact, Rathausgasse 30, 53111, Bonn, Germany.42 J.-M. Clemente-Juan, C. Mackiewicz, M. Verelst, F. Dahan, A. 120

Bousseksou, Y. Sanakis and J.-P. Tuchagues, Inorg. Chem., 2002, 41,1478.

43 F. James and M. Roos, MINUIT Program, a System for FunctionMinimization and Analysis of the Parameters Errors and Correlations,Comput. Phys. Commun., 1975, 10, 345. Q6125

44 (a) J. J. Borras-Almenar, J. M. Clemente-Juan, E. Coronado and B. S.Tsukerblat, Inorg. Chem., 1999, 38, 6081; (b) J. J. Borras-Almenar, J. M.Clemente-Juan, E. Coronado and B. S. Tsukerblat, J. Comput. Chem.,2001, 22, 985.

8 | Dalton Trans., 2010, 39, 1–8 This journal is © The Royal Society of Chemistry 2010