High-nuclearity nickel(II) clusters: Ni13 complexes from the use of 1-hydroxybenzotriazole

Polyhedron 28 (2009) 1903–1911

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High-nuclearity nickel(II) clusters: Ni13 complexes from the use of 1-hydroxybenzotriazole Constantina Papatriantafyllopoulou a, Eleanna Diamantopoulou a, Aris Terzis b, Vassilis Tangoulis a, Nikolia Lalioti a,*, Spyros P. Perlepes a,* a b

Department of Chemistry, University of Patras, 265 04 Patras, Greece Institute of Materials Science, NCSR ‘‘Demokritos”, 153 10 Aghia Paraskevi Attikis, Greece

a r t i c l e

i n f o

Article history: Available online 4 December 2008 Keywords: Azido-bridged clusters Crystal structures 1-Hydroxybenzotriazolate metal complexes Magnetic properties Nickel(II) Tridecanuclear clusters

a b s t r a c t The reaction of 1-hydroxybenzotriazole, btaOH, with nickel(II) acetate tetrahydrate, in the absence of an external base, has been investigated. The reaction of Ni(O2CMe)2  4H2O with one equivalent of btaOH in nPrOH leads to the the first structurally characterized tridecanuclear nickel(II) cluster and the biggest metal/btaOH or btaO cluster discovered to date, [Ni13(OH)6(O2CMe)8(btaO)12(H2O)6(nPrOH)4]  6.6H2O  4(nPrOH) (1  6.6H2O  4(nPrOH)). The centrosymmetric molecule contains the {Ni13(l3-JY)6(l3ONNNR)12}8+ core (R– = C6H4–) and is characterized by competitive antiferromagnetic exchange interactions within its pentanuclear subunits containing the l3-JY bridges. The reaction of 1 with an excess of N3  ions in refluxing MeOH has led to the azido-bridged product [Ni13(N3)6(O2CMe)8(btaO)12(MeOH)10] (2), in which the spin frustration observed in 1 seems to disappear. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The success of organic chemists in establishing methods for making large, complicated molecules in a systematic and controlled manner is one of the great collective achievements of 20th-century science. By comparison transition metal chemists have made little progress in discovering general approaches to making complexes containing large numbers of metal centres [1]. This is mainly because until the 1980s the obvious biological relevance and commercial applications of the organic compounds were not matched by properties of polynuclear metal complexes (clusters). However, in the last 20 years or so polynuclear clusters of paramagnetic metal ions have been attracting intense research interest, because it was discovered that such molecules can display the phenomenon of single-molecule magnetism [2]. SMMs are individual molecules capable of functioning as nanoscale magnetic particles, thus representing a molecular approach to nanomagnetism. Such molecules behave as magnets below their blocking temperature (TB), exhibiting hysteresis in magnetization versus dc field scans. This magnetic behaviour of SMMs results from the combination of a large ground spin state (S) with a large and negative Ising (or easy-axis) type of magnetoanisotropy, as measured by the axial zero-field splitting parameter D. Due to their small size, SMMs

* Corresponding authors. Tel.: +30 2610452988 (N. Lalioti); tel.: +30 2610 997146; fax: +30 2610 997118 (S. P. Perlepes). E-mail addresses: [email protected] (N. Lalioti), [email protected] (S.P. Perlepes). 0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.10.036

have also been shown to exhibit interesting phenomena of the quantum world, such as quantum tunneling of magnetization [3] and quantum phase interference [4]. SMMs have been proposed for several applications [5], including high-density information storage and as qubits for quantum computation. Due to the properties and application potentiality of 3d-metal clusters, there continuous to be a need for an arsenal of synthetic methods to such molecules. There are two broad synthetic approaches that are being pursued to prepare new clusters [1,6]. One route uses rigid ligands with predictable and controllable binding modes, and metal ions with preferred coordination geometry. Typical examples of this ‘designed assembly’ approach are the grids made by Lehn’s group [7], the sophisticated structures produced by Saalfrank et al. [8] or the polyhedra constructed by Fujita’s group based on the concept of ‘molecular panelling’ [9]. Other synthetic inorganic chemists use much less well-behaved ligands, typically 1,3-bridging ligands where chelation would produce the thermodynamically disfavoured four-membered chelating ring. Once formation of a five- or six-membered chelating ring is excluded, the coordinative flexibility of any polydentate ligand increases enormously [1,6]. This flexibility in turn allows stabilization of many unpredictable structures (which can lead to novel properties), almost invariably incorporating further auxiliary bridging ligands such as oxides, hydroxides or alkoxides. This approach has been termed ‘serendipitous assembly’ [10] and has proven extremely successful in the synthesis of clusters with exciting structures and interesting properties [6,10]. Serendipitous assembly often relies on creating a mismatch between the number or

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istry of btaOH has not attracted the interest of inorganic chemists and only few metal complexes have been prepared and studied. The synthetic investigation of the [Cu2(O2CMe)4(H2O)2]–btaOH reaction system in MeOH resulted in the isolation of the threedimensional (3D), diamond-like coordination polymer [Cu(btaO)2(MeOH)]n [17], which is a soft magnet exhibiting two critical temperatures at 6.4 and 4.4 K. The anion btaO is the 1-hydroxybenzotriazolate ligand (since the –OH group of 1-hydroxybenzotriazole is deprotonated, an alternative name of btaO would be 1-oxidobenzotriazolate). Later [18], the reaction of Ni(NO3)  6H2O with btaOH in aqueous NH3–DMF yielded the trinuclear complex [Ni3(btaO)6(NH3)6]. Reaction of zerovalent Mn with btaOH and NH4SCN in DMF has allowed the isolation of the one-dimensional (1D) polymer [Mn3(btaO)2(SCN)4(DMF)8]n [19a]. The use of the  btaO =MeCO2  combination in manganese(II) chemistry has recently yielded the 2D coordination polymer [MnII3(O2CMe)2(btaO)4(MeOH)2]n; the intra- and inter-magnetic exchange coupling constants for this complex were obtained from Monte Carlo calculations [19b]. The investigation of the [Ni(acac)2(H2O)2]–btaOH reaction mixture in MeCN yielded the novel heptanuclear cluster [Ni7(OH)2(acac)8(btaO)4(H2O)2] [20a]. It should be mentioned at this point that complex [Fe(btaOH)6](ClO4)3 has also been structurally characterized by Zhang [20b]. The octahedral cation contains formally neutral btaOH ligands; however, the ligands exist as zwitterions exhibiting the 1.100 ligation mode, see Scheme 2. The coordinative flexibility and versatility of the carboxylate ion and the l2, l3 or l4 potential of the deprotonated ligand (Scheme 2), btaO, prompted us to combine the two ligands to aim for new types of clusters. The loss of some degree of synthetic control may be more than compensated for by the vast diversity of structures expected using the combination of the two potentially bridging ligands. We describe herein the preparation and characterization of the complexes [Ni13(OH)6(O2CMe)8(btaO)12(H2O)6(nPrOH)4]

N N

N

OH

btaOH Scheme 1. Structural formula of 1-hydroxybenzotriazole (btaOH).

type of coordination sites available on a single metal site and the donor set supplied by the ligand. It is clear, however, that we cannot simply trust to luck in preparing 3d-metal clusters; there thus has to be considerable forethought in the ligands, metals and conditions (reaction ratio, ‘pH’, solvent, etc.) used for any significant progress to be made. As the field develops, the boundary between ‘designed’ and ‘serendipitous’ assembly is becoming blurred [1]. In the context of the several variations of ‘serendipitous assembly’, we have been exploring ‘ligand blend’ reactions involving carboxylates and the various anionic forms of di-2-pyridyl ketone [11] and 2-pyridyl oximates [12] as a means to high-nuclearity species; the latter two families of ligands have the ability to bridge two or more metal ions with the simultaneous formation of chelating rings. We relatively recently decided to extend the exploration of ligand combinations by employing ligands that cannot chelate. One such ligand is 1-hydroxybenzotriazole (btaOH, Scheme 1), a well-known peptide-coupling additive [13] which exhibits corrosion inhibitive properties towards Cu [14,15] and Fe [16]. The coordination chem-

H

CuII

M

N

N

N

N

N

N

N

N

N

O

O

O-

FeIII

η1 (1.100)

CuII 1

MnII

M'

N

N N

N

O

O

η2:η1:μ 3 (3.2100)

MnII

NiII N

N

N

MnII

η1:η1:μ (2.0110)

η :η :μ (2.1100) 1

M

N

M'

NiII

N O M'

η1:η1:η1:μ 3 (3.1110)

NiII

NiII

η2:η1:η1:μ 4 (4.2110)

Scheme 2. The up-to-date crystallographically established coordination modes of btaOH (up left) and btaO in metal complexes; M = NiII, Os and M0 = MnII, NiII. The Harris notation [21] for the description of the modes is given in parentheses.

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 6.6H2O  4(nPrOH) (1  6.6H2O  4(nPrOH)) and [Ni13(N3)6(O2CMe)8(btaO)12(MeOH)10] (2), which are the first tridecanuclear nickel(II) clusters reported to date. The last 5 years have witnessed a remarkable growth in interest in nickel(II) clusters because few of them have proven to be SMMs [22]. This work can be considered as a continuation of our interest in the chemistry of clusters of the middle and late 3d-metal ions with O,N-ligation [23]. Portions of this work have been previously communicated [23h].

2. Experimental 2.1. General and physical measurements All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received. Microanalyses (C, H, N) were performed by the University of Ioannina (Greece) Microanalytical Laboratory using an EA 1108 Carlo Erba analyzer. IR spectra (4000–400 cm1) were recorded on a Perkin– Elmer 16 PC FT-spectrometer with samples prepared as KBr pellets. Variable-temperature magnetic susceptibility measurements were carried out on powdered samples in the 2–300 K temperature range, using a Quantum Design MPMS SQUID susceptometer in a 1 kG applied magnetic field. Magnetization measurements were carried out at 2 K in the field range 0–6.5 T. All samples were packed into gelatine capsules and placed in a drinking straw sample holder. All magnetic susceptibility values were corrected with Pascal’s constants and corrections were applied for the sample holder (eicosane, gel capsule and drinking straw). Safety note! Benzotriazoles, azides and their metal complexes are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times. 2.2. Compound preparation 2.2.1. [Ni13(OH)6(O2CMe)8(btaO)12(H2O)6(nPrOH)4]  6.6H2O  4(nPrOH) (1  6.6H2O  4(nPrOH)) A colourless solution (15 ml) of btaOH (0.081 g, 0.60 mmol) in nPrOH was added to a green solution of Ni(O2CMe)2  4H2O (0.149 g, 0.60 mmol) in nPrOH (10 ml). The obtained pale green solution was filtered and allowed to stand undisturbed in a closed flask at room temperature. Green prismatic crystals of 1  6.6H2O  4(nPrOH) formed within 24–36 h and these were collected by filtration, washed with Et2O (2  3 ml) and dried in vacuo. Typical yields were in the 50–60% range. Selected IR (KBr pellet) data: m = 3422mb, 3320sb, 1592s, 1579s, 1439s, 1405m, 1200m, 1170m, 1125m. The dried solid analyzed satisfactorily as 1  5H2O  3(nPrOH). Anal. Calc. for C109H156O52N36Ni13 (3654.85): C, 36.72; H, 4.41; N, 14.14. Found: C, 37.08; H, 4.63; N, 13.88%. 2.2.2. [Ni13(N3)6(O2CMe)8(btaO)12(MeOH)10] (2) A suspension of compound [Ni13(OH)6(O2CMe)8(btaO)12H2O)6 (nPrOH) 4 ]  6.6H2 O  4(nPrOH)(1  6.6H2 O  4(nPrOH)) (1.097 g, 0.30 mmol) in MeOH (20 ml) was treated with solid NaN 3 (0.156 g, 2.40 mmol). The mixture was refluxed overnight, filtered to remove a few quantity of undissolved material, and the resulting dark green solution was allowed to stand undisturbed in a closed flask at room temperature. Green microcrystalline solid of 2 was precipitated within 4 days and this was collected by filtration, washed with Et 2O (2  3 mL) and dried in vacuo. Typical yields were in the 30–40% range. Selected IR (KBr pellet) data: m = 3310sb, 2077s, 1592s, 1578s, 1440m, 1403m, 1199m, 1173m, 1124m. The dried solid analyzed as 2  2H 2O. Anal. Calc. for C 98 H 116 O 40N 54 Ni 13 (3453.36): C, 34.09; H, 3.39; N, 21.90. Found: C, 34.37; H, 3.72; N, 22.06%.

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2.3. Single-crystal X-ray crystallography A crystal of 1  6.6H2O  4(nPrOH) with approximate dimensions 0.15  0.20  0.40 mm was mounted in capillary. Diffraction measurements were made on a Crystal Logic Dual Goniometer diffractometer using graphite-monochromated Mo radiation. Complete crystal data and parameters for data collection and processing are listed in Table 1. Unit cell dimensions were determined and refined by using the angular settings of 25 automatically centred reflections in the range 11° < 2h < 23°. Intensity data were recorded using the h–2h scan method. Three standard reflections monitored every 97 reflections showed less than 3% variation and no decay. Lorentz, polarization and W-scan absorption were applied using the Crystal Logic software package. The structure was solved by direct methods using SHELXS-86 [24] and refined by full-matrix least-squares techniques on F2 with SHEL  XL-97 [25]. Hydrogen atoms of the btaO and MeCO2 ligands were introduced at calculated positions as riding on bonded atoms. Those of the hydroxo ligands, and coordinated H2O and nPrOH molecules were located by difference maps and were refined isotropically. All non-hydrogen atoms were refined anisotropically, except those of the solvate molecules and the carbon atoms C94, C95 and C96 of a coordinated nPrOH which were refined isotropically. The monodentate MeCO2  ligand and Ow3 coordinated on Ni7 were found disordered, switching positions in the coordination sphere, thus the MeCO2  ligand was refined in two orientations with sum of occupation factors equal to one. 3. Results and discussion 3.1. Brief synthetic comments Reaction of Ni(O2CMe)2  4H2O with one equivalent of btaOH in propanol and storage of the obtained pale green solution at room temperature led to the precipitation of green prismatic crystals Table 1 Summary of crystal data, data collection and structure refinement for the X-ray diffraction study of complex 1  6.6H2O  4(nPrOH). Complex

1  6.6H2O  4(nPrOH)

Empirical formula Formula weight Colour, habit Crystal system Space group a (Å) b (Å) c (Å) b (o) V (Å3) Z qcalc (g cm3) Radiation, k (Å) l (mm1) F(0 0 0) Temperature (K) 2hmax (°) Ranges h, k, l

C112H167.2N36Ni13O54.6 3654.85 green prisms monoclinic P21/a 16.807(10) 23.815(13) 22.884(13) 93.106(18) 9146(9) 2 1.327 Mo Ka, 0.71073 1.384 3784 298 41.5 16 ? 16 23 ? 0 22 ? 0 9639 9345 (0.0427) 5796 968 1.091 0.0737 0.1479 0.629/0.620

Measured reflections Unique reflections (Rint) Reflections used (I > 2r(I)) Parameters refined Goodness-of-fit (on F2) R1a (I > 2r(I)) wR2b (I > 2r(I)) (Dq)max/(Dq)min (e Å3) a b

P P R1 = (|Fo|  |Fc|)/ (|Fo|). P wR2 = {R[w(F2o  F2c)2]/ [w(F2o)2]}1/2.

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of [Ni13(OH)6(O2CMe)8(btaO)12(H2O)6(nPrOH)4]  6.6H2O  4PrnOH (1  6.6H2O  4(nPrOH)) in 55% yield (Eq. (1)). Obviously, btaOH is deprotonated by the basic MeCO2  groups: nPrOH

13NiðO2 CMeÞ2 4H2 O þ 12btaOH þ 4ðnPrOHÞ !

 ½Ni13 ðOHÞ6 ðO2 CMeÞ8 ðbtaOÞ12 ðH2 OÞ6 ðnPrOHÞ4  þ18MeCO2 H 1

þ 40H2 O ð1Þ Complex 1 seems to be the only product from the Ni(O2CMe)2  4H2O/btaOH reaction mixture irrespective of the NiII to ligand reaction ratio. The addition of counterions such as ClO4  and PF6  (for the isolation of potentially existing cationic complexes) and the crystallization method have no influence on the identity of the product. Following our well developed strategy [26], we tried to replace some or all the OH bridges in 1 with end-on N3  bridges in order to introduce ferromagnetic interactions (the end-on azido ligand is a ferromagnetic coupler [26]) in the superexchange scheme (vide infra) Thus, reaction of pre-isolated 1 with an excess (8–10 equivalents) of NaN3 in MeOH under reflux gave a green solution from which microcrystalline [Ni13(N3)6(O2CMe)8(btaO)12(MeOH)10] (2) was precipitated in 30% yield upon storage at room temperature (Eq. (2)). Despite hundreds of experiments, we have failed to grow single crystals of the azido product suitable for crystallography due to poor diffraction and/or twinning problems (more than fifty crystals were examined). Thus, the characterization of complex 2 is based on the analytical and IR data (vide infra):

½Ni13 ðOHÞ6 ðO2 CMeÞ8 ðbtaOÞ12 ðH2 OÞ6 ðnPrOHÞ4  þ6NaN3 1

MeOH

þ 10MeOH ! ½Ni13 ðN3 Þ6 ðO2 CMeÞ8 ðbtaOÞ12 ðMeOHÞ10  T

2

þ 6H2 O þ 4ðnPrOHÞ

ð2Þ

3.2. Description of structure Partially labelled plots of the tridecanuclear molecule present in 1  6.6H2O  4(nPrOH) is shown in Fig. 1. Selected interatomic distances and angles are listed in Table 2. Table 3 summarizes important hydrogen-bonding data.

Table 2 Selected interatomic distances (Å) and angles (°) for 1  6.6H2O  4(nPrOH). Bond distances Ni(7)  Ni(7’) Ni(1)–N(3) Ni(1)–N(23) Ni(1)–N(43) Ni(2)–O(61) Ni(2)–O(62) Ni(2)–O(63) Ni(2)–N(12) Ni(2)–N(32) Ni(2)–N(52) Ni(3)–O(61) Ni(3)–O(62) Ni(3)–O(63) Ni(3)–N(2) Ni(3)–N(22) Ni(3)–N(42) Ni(4)–O(21) Ni(4)–O(62) Ni(4)–O(71) Ni(4)–O(72) Bond angles N(3)–Ni(1)–N(30 ) N(23)–Ni(1)–N(23’) N(43)–Ni(1)–N(43’) O(61)–Ni(2)–N(32) O(62)–Ni(2)–N(12) O(63)–Ni(2)–N(52) O(61)–Ni(3)–N(22) O(62)–Ni(3)–N(2) O(63)–Ni(3)–N(42) O(21)–Ni(4)–O(81) O(62)–Ni(4)–O(71) O(72)–Ni(4)–N(33) O(41)–Ni(5)–OW(1) O(75)–Ni(5)–OW(1) O(76)–Ni(4)–N(53)

21.252(2) 2.106(9) 2.105(9) 2.115(9) 2.090(7) 2.072(7) 2.065(9) 2.122(10) 2.097(10) 2.082(9) 2.074(8) 2.077(6) 2.052(9) 2.102(9) 2.080(9) 2.083(10) 2.075(7) 1.992(7) 2.207(8) 2.084(8) 180.0(2) 180.0(4) 180.0(5) 168.0(4) 166.6(3) 168.3(4) 174.1(3) 173.8(3) 173.2(4) 176.9(3) 164.2(3) 166.5(4) 179.2(3) 136.2(5) 164.2(4)

Ni(4)–O(81) Ni(4)–N(33) Ni(5)–O(41) Ni(5)–O(61) Ni(5)–O(75) Ni(5)–O(76) Ni(5)–OW(1) Ni(5)–N(53) Ni(6)–O(1) Ni(6)–O(63) Ni(6)–O(73) Ni(6)–O(74) Ni(6)–O(82) Ni(6)–N(13) Ni(7)–O(11) Ni(7)–O(31) Ni(7)–O(51) Ni(7)–O(77) Ni(7)–OW(2) Ni(7)–OW(3) O(1)–Ni(6)–O(82) O(63)–Ni(6)–O(73) O(74)–Ni(6)–N(13) O(11)–Ni(7)–OW(2) O(31)–Ni(7)–OW(3) O(51)–Ni(7)–O(77) Ni(2)–O(61)–Ni(3) Ni(2)–O(61)–Ni(5) Ni(3)–O(61)–Ni(5) Ni(2)–O(62)–Ni(3) Ni(2)–O(62)–Ni(4) Ni(3)–O(62)–Ni(4) Ni(2)–O(63)–Ni(3) Ni(2)–O(63)–Ni(6) Ni(3)–O(63)–Ni(6)

2.080(9) 2.073(10) 2.094(8) 2.000(8) 2.109(9) 2.092(8) 2.121(9) 2.048(10) 2.095(9) 2.003(9) 2.144(9) 2.091(8) 2.071(10) 2.033(11) 2.053(9) 2.062(9) 2.036(9) 2.060(11) 2.100(14) 2.051(12) 175.4(4) 166.2(4) 164.8(4) 174.8(5) 175.1(5) 174.0(4) 83.0(3) 116.1(3) 124.8(4) 83.4(3) 118.1(3) 123.4(3) 84.2(3) 118.1(5) 124.1(4)

Symmetry operation used to generate equivalent atoms: x + 1, y, z + 1.

The structure of 1  6.6H2O  4(nPrOH) consists of the tridecanuclear molecule [Ni13(OH)6(O2CMe)8(btaO)12(H2O)6(nPrOH)4] (Fig. 1), and solvate H2O and nPrOH molecules; the solvate molecules will not be further discussed. The centrosymmetric [Ni(1) is a crystallographic inversion center] tridecanuclear molecular assembly is held together by six l3 (or 3.1 using Harris notation

Fig. 1. The structure of 1 in the crystal. Primed and unprimed atoms are related by the crystallographic inversion center.

C. Papatriantafyllopoulou et al. / Polyhedron 28 (2009) 1903–1911 Table 3 Selected hydrogen-bonding interactions for complex 1  6.6H2O  4(nPrOH).a D–H  A

D  A (Å)

H  A (Å)

D–H  A (°)

Symmetry operator of A

O(61)–H(O61)  O(1) O(62)–H(O62)  O(41) O(63)–H(O63)  O(21) O(81)–H(O81)  O(71) O(82)–H(O82)  O(1)b

2.911 2.828 2.846 2.90 2.645

2.035 1.788 2.275 2.093 2.091

142.0 160.0 137.0 164.8 122.1

OW(1)– HW(O1A)  OP(1)c OW(1)– HW(O1B)  OP(2)c OW(2)– HW(O2A)  W(2)b

2.664

1.492

149.3

2.691

1.503

158.1

2.761

2.249

135.2

x, y, z x, y, z x, y, z x, y, 1  z 0.5  x, 0.5 + y, 1z 0.5  x, 0.5 + y, 1z 0.5  x, 0.5 + y, 1z 0.5  x, 0.5 + y, 1z

a b c

A = acceptor atom, D = donor atom. Atoms W(1) and W(2) belong to coordinated water molecules. Atoms OP(1) and OP(2) (not shown in Fig. 1) belong to lattice propanol.

[21]) hydroxo ligands [O(61), O(62), O(63) and their symmetry equivalents] and twelve g1:g1:g1:l3 (or 3.1110 [21], see Scheme 2) btaO groups [the deprotonated O atoms of the btaO groups are O(1), O(11), O(21), O(31), O(41), O(51) and their symmetry counterparts]. Peripheral ligation is provided by six chelating (g2 or 1.11) MeCO2  ligands, two monodentate (g1 or 1.10) MeCO2  groups, six terminal aqua ligands [OW(1), OW(2), OW(3) and their symmetry equivalents] and four terminal nPrOH molecules [O(81), O(82) and their symmetry equivalents]. Thus, the core is {Ni13(l3OH)6(l3-JNNNR)12}8+ (Fig. 2), where R– = C6H4–. As a result of the above described ligation, the distorted octahedral chromophores of the seven, crystallographically independent metal ions are Ni(1)N6, Ni(2)(Ohydroxo)3N3, Ni(3)(Ohydroxo)3N3, Ni(4)(Ohydroxo) Ni(5)(Ohydroxo)(Oacetate)2 (ObtaO ) (Oacetate)2(ObtaO )(Opropanol)N, (Oaquo)N, Ni(6)(Ohydroxo)(Oacetate)2(ObtaO )(Opropanol)N and Ni(7) (Oacetate)(ObtaO )3(Oaquo)2.

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The Ni–O(l3-hydroxo), Ni–Oacetate, Ni–ObtaO , Ni–Oaquo, Ni–Opropanol and Ni–N bond lengths are typical for high-spin, six coordinate Ni(II) complexes [23a,b,27]. Each l3-JY group forms one short and two longer bonds to the metal ions. The Ni–O bond to the monodentate acetate ligand [Ni(7)–O(77) 2.060(11) Å] is slightly shorter, as expected, than the Ni–O bonds to the bidentate acetate groups [average value: 2.115(8) Å]. There is an amount of hydrogen bonding in 1  6.6H2O  4 (nPrOH) (see Table 3). The cluster molecules are directly connected forming ‘‘chains” through an intermolecular hydrogen bond involving the nPrOH oxygen atom O(81) (and its symmetry partner) as donor and the acetate oxygen atom O(71) (x, y, z + 1), and its symmetry partner, as acceptor. The six l3 hydroxo ligands [O(61), O(62), O(63) and their symmetry equivalents] are intramolecularly hydrogen bonded with O atoms from six different btaO ligands [O(1), O(41), O(21) and their symmetry equivalents, respectively]. The crystal structure is also stabilized by a variety of hydrogen bonds between the cluster molecule and/or the lattice solvent molecule resulting in a 3D network. The metal ions Ni(7), Ni(2), Ni(3), Ni(1), Ni(30 ), Ni(20 ) and Ni(70 ) are coplanar and almost colinear; this view is emphasized in Fig. 3. Using the perspective view of this figure, Ni(5) is 3.130 Å above the Ni7 plane, while Ni(4) and Ni(6) are 2.318 and 0.812 Å, respectively, below the Ni7 plane. The shortest NiII  NiII distance within the molecule is Ni(2)  Ni(3) [2.759(2) Å]; this short distance is a consequence of the presence of three monoatomic hydroxo bridges. The Ni(7)  Ni(70 ) distance is the longest one [21.252(2) Å]. In addition to the various triangular topologies that are present in 1, several other subunits are clearly visible within the core of 1: (i) The {Ni(2)(OH)3Ni(3)}+ (and its symmetry equivalent) subcore has never been identified in Ni(II) cluster or polymer chemistry; (ii) the metal ions Ni(1), Ni(4), Ni(5), Ni(6), Ni(40 ), Ni(50 ) and Ni(60 ) adopt the topology of two tetrahedra sharing a common apex at Ni(1) (Fig. 4); this topology is extremely rare in Ni(II)

Fig. 2. The {Ni13(l3-JY)6(l3-ONNNR)12}8+ core of complex 1, where R– = C6H4–. Colour code: Ni, green; O, red; N, blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. A view of the core of complex 1 emphasizing the planarity and almost linearity of seven (out of 13) NiII ions.

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plexes that are discussed in the Introduction [17–20], the other three compounds are the organometallic complexes [Os3H(CO)10(btaO)], [Os3(C@NHPrn)(CO)10(btaO)] and [Os3(C@ NHCH2Ph)(CO)10(btaO)] [29]. The 3.1110 coordination mode found in 1 has been observed only in [MnII3(O2CMe)2(btaO)4(MeOH)2]n [19b]. Only the 2.0110 mode has been found in more than two complexes, i.e. in [Ni3(btaO)6(NH3)6] [18], [Ni7(OH)2(acac)8(btaO)4(H2O)2] [20] and in the Os organometallic compounds [29]. Complex 1 joins a small but growing family of tridecanuclear metal complexes; these complexes have various metal topologies. Since most of these complexes have been reported only recently, we felt it timely to collect them in Table 4, together with their spin ground-state values, where there are available. Inspection of Table 4 shows that complex 1 is the first structurally characterized tridecanuclear Ni(II) cluster. While the number of polynuclear transition metal complexes reported continues to grow rapidly, some nuclearities remain extremely rare. Tridecanuclear complexes are particularly uncommon. 3.3. IR spectra

Fig. 4. The vertex-sharing, double tetrahedral topology of seven NiII ions.

chemistry and has been observed only in the chiral complex [Ni7(OH)2L3], where L is the tetra-anion of (2R,3R)-1,4-bis(3,5-ditert-butylsalicylideneamino)-2,3-butanediol [28]; (iii) the {Ni(2)Ni(3)Ni(4)Ni(5)Ni(6)(O(61)H)(O62)H)(O(63)H}7+ subcore can be described as a ‘mutant’ butterfly, with Ni(2) and Ni(3) ions at the central (‘body’) and Ni(4), Ni(5), Ni(6) on its three end (‘wingtip’) positions (Fig. 5); tetranuclear Ni(II) clusters with a normal {Ni4(l3-JY)2}6+ topology have been structurally characterized [17]; and (iv) the central {Ni3(l-NNR0 )6} subcore (R0 – = C6H4NO–), which is present in 1 (Fig. 6), has been observed in the linear trinuclear cluster [Ni3(btaO)6(NH3)6] [18]; in the latter the deprotonated oxygen atoms of the btaO ligands are hydrogen bonded to the ammine ligands, whereas in the former the btaO oxygen atoms are each coordinated to a NiII ion resulting in a higher nuclearity complex. Compound 1 is the nineth structurally characterized complex of any metal containing btaO ligands. Except the Werner-type com-

A characteristic feature in the IR spectrum of complex 1 is the appearance of a strong broad band at 3320 cm1 assignable to m(JY)Y2J/nPrOH. The presence of hydroxo ligands in the complex is manifested by a medium band at 3420 cm1 [34]. The broadness and relatively low frequency of these bands are both indicative of hydrogen bonding. The spectra of the sodium and potassium salts of 1-hydroxybenzotriazole (btaONa, btaOK) exhibit bands at 1180 and ca. 1105 cm1, associated with the m(N@N) and m(N–N) modes of vibration, respectively. These bands are shifted to 1200 and 1125 cm1, respectively, in the spectrum of the complex. The medium intensity band at 1170 cm1 is assigned to m(N–O) [20a]. This band has been shifted to lower wavenumber in 1 compared with its frequency (1224 cm1) in the IR spectrum of the potassium salt of 1-hydroxybenzotriazole, btaOK. Such a large shift to lower frequencies has also been observed [17,19,20a] in the spectra of other metal complexes where the deprotonated oxygen atom of btaO is coordinated. The strong bands at 1592 and 1579 cm1 are assigned [35] to the mas(CO2) vibration; the ms(CO2) vibrations appear [35] at 1439 and 1405 cm1. The presence of two distinct bands for each mode reflects the presence of two types of acetate ligands in the complex. The 1592 and 1405 cm1 [D = mas(CO2)  ms(CO2) = 187 cm1] pair are assigned to the monodentate acetates, while the 1579 and 1439 cm1 one (D = 140 cm1) to the chelating acetates [35]. The differences D are more (187 cm1) and less (140 cm1) than the D value for NaO2CMe (164 cm1), as expected for the monodentate and bidentate of carboxylate ligation, respectively [35]. The IR spectrum of 2 is very similar to that of 1, the main differences being the presence of one strong band at 2077 cm1 and the absence of the band at 3420 cm1 in the former. This similarity supports analogous structures for 1 and 2. The band at 2077 cm1 is assigned to the asymmetric stretching mode of the azido ligand [36] and is situated at almost the same wavenumber observed in clusters containing end-on azido ligands [26d]. 3.4. Magnetic properties

Fig. 5. The ‘mutant’ butterfly {Ni5(l3-JY)3}7+ subunit present in 1.

Variable-temperature dc magnetic susceptibility studies were performed on powdered samples of compounds 1 and 2 in an applied magnetic field of 0.1 T (1.0 KG) and in the 2.0–300 K range. The data are plotted as vMT versus T in Fig. 7. vVN at 300 K is 16.24 emu mol1 K, lower than the 17.19 emu mol1 K value (for g = 2.3) expected for a cluster of 13 non-interacting NiII ions. As T is lowered, the vVN product gradually decreases to 9.88 emu mol1 K at 16 K where a plateau appears; the product

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Fig. 6. The central {Ni3(l-NNR’)6} subcore (R0 – = C6H4NO–) in 1.

increases from this value at 9 K to 10.21 emu mol1 K at 3.3 K before finally decreasing to 9.52 emu mol1 K at 2.0 K. The low temperature behaviour is consistent with a frustrated magnetic system, where there are competitive antiferromagnetic exchange interactions, mainly within the pentanuclear Ni(2)Ni(3)Ni(4)Ni(5)Ni(6)/Ni(20 )Ni(30 )Ni(40 )Ni(50 )Ni(60 ) subunits (triangular spin topologies). From now onwards, the numbering scheme that will be used for the magnetic discussion of 1 is the same with that used in the real structure, see Figs. 1 and 3–5. We define spin frustration here in its more common, general sense of competing exchange interactions of similar magnitude that prevent the preferred spin alignments, rather than the original, most specific sense that competing interactions of the same magnitude lead to a degenerate ground state. In an attempt to try to quantify the magnetic behaviour of the system, and since a phenomenological Hamiltonian of 13 S = 1 spins is a quite large system to be treated even with the ITO methods, some approximations were used. A simplified Hamiltonian of the system is given by Eq. (3):

H ¼ J 1 ðS2 S3 þ S20 S30 Þ  J 2 ½ðS2 þ S3 ÞðS4 þ S5 þ S6 Þ þ ðS20 þ S30 Þ  ðS40 þ S50 þ S60 Þ  J 3 ½S1 ðS3 þ S30 Þ  J 4 ðS2 S7 þ S20 S70 Þ

ð3Þ

To avoid parametrization, we decided to attempt to fit the susceptibility data considering that the main exchange interactions come from the pentanuclear subunits mentioned above, i.e. to use a 2-J model assuming that J3 = J4 = 0. These pentanuclear subunits contain monoatomic bridges (Fig. 5), derived from the l3JY ligands, which give more effective coupling than the diatomic

or triatomic bridges. Thus, in a simple approximation, the system can be considered as consisting of two pentanuclear subunits and three mononuclear NiII sites. Best-fit parameters are J1 = 24.5 cm1, J2 = 12.6 cm1 and g = 2.31. The model used to fit the data does not take into account the decline of vVN below 3.3 K. This steep decrease, often occurring in polynuclear nickel(II) complexes, is associated [37] with the characteristic zero-field splitting of the NiII ion and probably with the intermolecular interactions present in the crystal structure of the complex. The fitting results are in accordance with the behaviour of the system, in which the existence of antiferromagnetic interactions within the triangular Ni(2)Ni(3)Ni(4,5,6) subunits leads to frustration (Scheme 3). It should be mentioned that despite the small Ni(2)– O(H)–Ni(3) angles [20a], the interaction J1 is antiferromagnetic. We attribute this partly to the l3 character of the hydroxo ligands and partly to the frustrating behaviour of the system. Further investigation of the magnetic behaviour of 1 using Monte Carlo methodology [19b] is in progress in order to investigate the system without approximations; preliminary results indicate that our model is valid. The vVN product for 2 at 300 K is 16.87 emu mol1 K and remains practically constant in the 300–54 K range, below which it decreases drastically to a value of 5.93 emu mol1 K at 2.2 K. Assuming a structural similarity between 1 and 2, and employing

18

Complex [V13(l6-J)3(l3-J)6(l-J)2O9(l3-JY)3(l2-JY)3]3 [Co13(OH)3(chp)19(O3PPh)2(H2O)2(Hchp)2]a [Co13(OH)3(chp)19(O3PPh)2(H2O)2(EtOAc)2]a [Co13(OH)2(phth)(chp)20]a,b [Mn13(l5-O)6(l3-O)2(l3-OEt)6(O2CPh)12] [Mn13O12(Me2Bta)12F6(MeOH)10(H2O)2]c [Mn13O8(OEt)6(O2CC6H4OPh)12] [Mn13(l3-O)(OMe)8(O3CCPh)4(O2CCMe3)10] [Mn13O8(OH)6(ndc)6]d [Mn13O8(OEt)5.5(OH)0.5(ndc)6]d [Mn13O8(OEt)6(O2CPh)12] [Fe13O4F24(OMe)12]5 [Ni13(OH)6(O2CMe)8(btaO)12(H2O)6(nPrOH)4] a b c d

Hchp = 6-chloro-2-hydroxypyridine. phth = phthalate. Me2BtaH = dimethylbenzotriazole. ndc2 = 1,8-naphthalenedicarboxylate.

Spin ground state

15/2 11/2 5 9/2 11/2 11/2

Reference [30] [31a] [31a] [31b] [32a] [32b] [32c] [32d] [32e] [32e] [32e] [33] this work

14 12

12 10

10

M/NμB

Table 4 Formulae and spin ground-state value for structurally characterized non-organometallic tridecanuclear metal complexes.

χ MT/emu mol-1 K

16

8

8 6 4 2 0

6

0

2

4

6

H/T 0

50

100

150

200

250

300

T/K Fig. 7. Plot of vVT vs. T for powdered samples of 1 (open stars) and 2 (black circles); the solid lines represent the fit to the theoretical models (see text for the fit parameters). Plots of reduced magnetization (M/NlD) vs. H at 2 K in the field range 0–6.5 T are shown in the inset.

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namely [MnII3(O2CMe)2(btaO)4(MeOH)2]n, is a 2D coordination polymer [19b]. Another interesting point of this work is the rational reactivity of 1 with N3  ions which gave the – most probably – structurally analogous azido-bridged cluster 2 with different magnetic properties. Complexes of 3d-metal ions other than NiII (this work) and MnII  [19b] with the btaO =RCO2  (R = H, Me, Ph, etc.) ligand combination are not known to date, and it is currently not evident whether the structural types (clusters versus polymers) of such compounds are dependent on the nature of the metal and the R group of the carboxylate ligand. Such matters are under intense investigation in our group. Acknowledgements We thank the European Social Fund (ESF), the Operational Program for Educational and Vocational Training II (EPEAEK II) and particularly the Program PYTHAGORAS (Grant b.365.037), for funding this work. Appendix A. Supplementary data CCDC 668367 contains the supplementary crystallographic data for 1  6.6H2O  4PrnOH. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336033; or e-mail: deposit@ ccdc.cam.ac.uk. Scheme 3. Spin topologies of the pentanuclear subunits present in complex 1 (A) and assumed to be present in complex 2 (B). The monoatomic bridges in A derive from the crystallographically established l3-JY groups, while in B come from the nitrogen atoms of the l3 end-on N3 ligands which are proposed to be present in the azido product.

the same model and approximations used for 1, best-fit parameters for 2 are J1 = 12.8 cm1, J2 = 1.93 cm1 and g = 2.27. Thus, the spin frustration observed in 1 seems to disappear in 2, due to the existence of a ferromagnetic exchange interaction in the pentanuclear subunits of the latter (Scheme 3) [28]. According to Fig. 7, the ground-state spin of compound 2 is smaller than that of 1. The most probable reason for this is the spin frustration in the latter which results in a larger S value compared with that in the former; it seems that the frustrated NiII5 units have a larger spin value in the ground state than the non-frustrated ones. Thus the conversion of 1 into 2 represents an extremely rare case in which substitution of bridging hydroxo ligands by end-on bridging azide groups has led to a decrease in the ground-state S value; usually the addition of the latter is sought as a means of raising the value of S [26]. The magnetization curves of 1 and 2 are shown in the inset of Fig. 7; no saturation is observed indicating that many energy states are populated even at very low temperatures 4. Conclusions The present work extends the body of results that emphasize the ability of the anionic ligand btaO to form interesting structural types in 3d-metal chemistry. The initial use of the  btaO =MeCO2  combination in Ni(II) chemistry has provided access to cluster 1 with interesting structural and magnetic properties. This cluster is the highest nuclearity metal btaOH or btaO to date and the first Ni13 complex, showing that btaO can indeed support high nuclearity chemistry when coupled with appropriate ancillary ligands. Complex 1 is only the second complex of any metal in which btaO and MeCO2  groups coexist; the first one,

References [1] R.E.P. Winpenny, in: J.P. Sauvage (Ed.), Perspectives in Supramolecular Chemistry, vol. 5, Wiley, New York, 1999, p. 193 (Chapter 5). [2] (a) For excellent reviews concerning SMMs, see: G. Christou, D. Gatteschi, D.N. Hendrickson, R. Sessoli, MRS Bull. 25 (2000) 66; (b) D. Gatteschi, R. Sessoli, Angew. Chem., Int. Ed. 42 (2003) 268; (c) R. Birchner, G. Chaboussant, C. Dobe, H. Güdel, S.T. Ochsenbein, A. Sieber, O. Waldman, Adv. Funct. Mater. 16 (2006) 209; (d) G. Aromi, E.K. Brechin, Struct. Bond. 122 (2006) 1. [3] (a) J.R. Friedman, M.P. Sarachik, J. Tejada, R. Ziolo, Phys. Rev. Lett. 76 (1996) 3380; (b) L. Thomas, F. Lionti, R. Ballou, D. Gattechi, R. Sessoli, B. Barbara, Nature 383 (1996) 145. [4] W. Wernsdorfer, R. Sessoli, Science 284 (1999) 133. [5] (a) M.N. Leuenberger, D. Loss, Nature 410 (2001) 789; (b) W. Wernsdorfer, N. Aliaga-Acalde, D.N. Hendrickon, G. Christou, Nature 416 (2002) 406; (c) S. Hill, R.S. Edwards, N. Aliaga-Acalde, G. Christou, Science 302 (2003) 1015; (d) J. Tehada, E.M. Chudnovsky, E. del Barco, J.M. Hernandez, T.P. Spiller, Nanotechnology 12 (2001) 181; (e) R.E.P. Winpenny, Angew. Chem., Int. Ed. 47 (2008) 7992. [6] R.E.P. Winpenny, in: J.A. McCleverty, T.J. Meyer (Eds.), Comprehensive Coordination Chemistry II, vol. 7, Elsevier, Amsterdam, 2004, p. 125 (Chapter 7.3). [7] E. Breuning, U. Ziener, J.-M. Lehn, E. Wegelius, K. Rissanen, Eur. J. Inorg. Chem. (2001) 1515. [8] R.W. Saalfrank, U. Reimann, M. Goritz, F. Hampel, A. Scheurer, F.W. Heinemann, M. Buschel, J. Daub, V. Schunemann, A.X. Trautwein, Chem. Eur. J. 8 (2002) 3614. [9] M. Fujita, K. Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa, K. Biradha, Chem. Commun. (2001) 509 (Feature article). [10] For a provocative perspective, see: R.E.P. Winpenny, J. Chem. Soc., Dalton Trans. (Dalton Perspective) (2001) 1. [11] For a review, see: G.S. Papaefstathiou, S.P. Perlepes, Comm. Inorg. Chem. 23 (2002) 249. [12] For a review, see: C.J. Milios, Th.C. Stamatatos, S.P. Perlepes, Polyhedron 25 (2006) 134 (Polyhedron Report). [13] W. Konig, R. Geiger, Ber. Dtsch. Chem. Ges. 103 (788) (1970) 2024. [14] W. Qafsaoui, C. Blanc, J. Roques, N. Pebere, A. Srhiri, C. Mijoule, G. Mankowski, J. Appl. Electrochem. 31 (2001) 223. [15] R. Youda, H. Nishihara, K. Aramaki, Corros. Sci. 28 (1988) 87. [16] M.J. Collie (Ed.), Corrosion Inhibitors: Developments Since 1980, Noyes Data Corporation, NJ, USA, 1983, p. 227. [17] V. Tangoulis, C.P. Raptopoulou, V. Psycharis, A. Terzis, K. Skorda, S.P. Perlepes, O. Cador, O. Kahn, E.G. Bakalbassis, Inorg. Chem. 39 (2000) 2522.

C. Papatriantafyllopoulou et al. / Polyhedron 28 (2009) 1903–1911 [18] E. Diamantopoulou, S.P. Perlepes, D. Raptis, C.P. Raptopoulou, Transition Met. Chem. 27 (2002) 377. [19] (a) G.S. Papaefstathiou, R. Vicente, C.P. Raptopoulou, A. Terzis, A. Escuer, S.P. Perlepes, Eur. J. Inorg. Chem. (2002) 2488; (b) C.C. Stoumpos, E. Diamantopoulou, C.P. Raptopoulou, A. Terzis, S.P. Perlepes, N. Lalioti, Inorg. Chim. Acta 361 (2008) 3638. [20] (a) E. Diamantopoulou, C.P. Raptopoulou, A. Terzis, V. Tangoulis, S.P. Perlepes, Polyhedron 21 (2002) 2117; (b) X.-M. Zhang, Acta Crystallogr., Sect. E 61 (2005) m1799. [21] R.A. Coxall, S.G. Harris, D.K. Henderson, S. Parsons, P.A. Tasker, R.E.P. Winpenny, J. Chem. Soc., Dalton Trans. (2000) 2349. [22] (a) H. Andres, R. Basler, A.J. Blake, C. Cadiou, G. Chaboussant, C.M. Grant, H.-U. Güdel, M. Murrie, S. Parsons, C. Paulsen, F. Semadini, V. Villar, W. Wernsdorfer, R.E.P. Winpenny, Chem. Eur. J. 8 (2002) 867; (b) S.T. Ochsenbein, M. Murrie, E. Rusanov, H. Stoeckli-Evans, C. Sekine, H.-U. Güdel, Inorg. Chem. 41 (2002) 5133; (c) E.-C. Yang, W. Wernsdorfer, S. Hill, R.S. Edwards, M. Nakano, S. Maccagnano, L.N. Zakharov, A.L. Rheingold, G. Christou, D.N. Hendrickson, Polyhedron 22 (2003) 1727; (d) M. Moragues-Cánovas, M. Helliwell, L. Ricard, E. Riviere, W. Wernsdorfer, E. Brechin, T. Mallah, Eur. J. Inorg. Chem. (2004) 2219; (e) A. Bell, G. Aromi, S.J. Teat, W. Wernsdorfer, R.E.P. Winpenny, Chem. Commun. (2005) 2808; (f) E.-C. Yang, W. Wernsdorfer, L.N. Zakharov, Y. Karaki, A. Yamaguchi, R.M. Isidro, G.-D. Lu, S.A. Wilson, A.L. Rheingold, H. Ishimoto, D.N. Hendrickson, Inorg. Chem. 45 (2006) 529; (g) G. Aromi, S. Parsons, W. Wernsdorfer, E.K. Brechin, E.J.L. McInnes, Chem. Commun. (2005) 5038; (h) D. Venegas-Yazigi, E. Ruiz, J. Cano, S. Alvarez, Dalton Trans. (2006) 2643. [23] (a) For representative very recent papers, see: C. Papatriantafyllopoulou, G. Aromi, A.J. Tasiopoulos, V. Nastopoulos, C.P. Raptopoulou, S.J. Teat, A. Escuer, S.P. Perlepes, Eur. J. Inorg. Chem. (2007) 2761; (b) Th.C. Stamatatos, E. Diamantopoulou, C.P. Raptopoulou, V. Psycharis, A. Escuer, S.P. Perlepes, Inorg. Chem. 46 (2007) 2350; (c) Th.C. Stamatatos, D. Foguet-Albiol, S.-C. Lee, C.C. Stoumpos, C.P. Raptopoulou, A. Terzis, W. Wernsdorfer, S.O. Hill, S.P. Perlepes, G. Christou, J. Am. Chem. Soc. 129 (2007) 9484; (d) Th.C. Stamatatos, C. Papatriantafyllopoulou, E. Katsoulakou, C.P. Raptopoulou, S.P. Perlepes, Polyhedron 26 (2007) 1830; (e) C.J. Milios, R. Inglis, A. Vinslava, R. Bagai, W. Wernsdorfer, S. Parsons, S.P. Perlepes, G. Christou, E.K. Brechin, J. Am. Chem. Soc. 129 (2007) 12505; (f) C.J. Milios, A. Prescimone, A. Mishra, S. Parsons, W. Wernsdorfer, G. Christou, S.P. Perlepes, E.K. Brechin, Chem. Commun. (2007) 153; (g) C.J. Milios, P.A. Wood, S. Parsons, D. Foguet-Albiol, C. Lampropoulos, G. Christou, S.P. Perlepes, E.K. Brechin, Inorg. Chim. Acta 360 (2007) 3932;

[24] [25] [26]

[27]

[28] [29] [30] [31]

[32]

[33] [34] [35] [36] [37]

1911

(h) C. Papatriantafyllopoulou, E. Diamantopoulou, A. Terzis, N. Lalioti, V. Tangoulis, S.P. Perlepes, Inorg. Chem. Commun. 11 (2008) 454. G.M. Sheldrick, SHELXS-86, Structure Solving Program, University of Göttingen, Germany, 1986. G.M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures from Diffraction Data, University of Göttingen, Germany, 1997. (a) G.S. Papaefstathiou, A. Escuer, R. Vicente, M. Font-Bardia, X. Solans, S.P. Perlepes, Angew. Chem., Int. Ed. 40 (2001) 884; (b) G.S. Papaefstathiou, A. Escuer, R. Vicente, M. Font-Bardia, X. Solans, S.P. Perlepes, Chem. Commun. (2001) 2414; (c) A.K. Boudalis, B. Donnadieu, V. Nastopoulos, J.M. Clemente-Juan, A. Mari, Y. Sanakis, J.-P. Tuchages, S.P. Perlepes, Angew. Chem., Int. Ed. 43 (2004) 2266; (d) G.S. Papaefstathiou, A.K. Boudalis, Th.C. Stamatatos, C.J. Milios, C.G. Efthymiou, C.P. Raptopoulou, A. Terzis, V. Psycharis, Y. Sanakis, R. Vicente, A. Escuer, J.-P. Tuchages, S.P. Perlepes, Polyhedron 26 (2007) 2089; (e) A.K. Boudalis, Y. Sanakis, J.M. Clemente-Juan, B. Donnadieu, V. Nastopoulos, A. Mari, Y. Coppel, J.-P. Tuchagues, S.P. Perlepes, Chem. Eur. J. 14 (2008) 2514. Th.C. Stamatatos, E. Diamantopoulou, A. Tasiopoulos, V. Psycharis, R. Vicente, C.P. Raptopoulou, V. Nastopoulos, A. Escuer, S.P. Perlepes, Inorg. Chim. Acta 359 (2006) 4149. X. Xu, S. Gou, W. Huang, Y. Ma, Y. Li, Eur. J. Inorg. Chem. (2003) 2054. K.-L. Lu, S. Kumaresan, Y.-S. Wen, J.R. Hwu, Organometallics 13 (1994) 3170. T. Kurata, Y. Hayashi, A. Uehara, K. Isobe, Chem. Lett. 32 (2003) 1040. (a) E.K. Brechin, A. Graham, A. Parkin, S. Parsons, A.M. Seddon, R.E.P. Winpenny, J. Chem. Soc., Dalton Trans. (2000) 3242; (b) E.K. Brechin, R.A. Coxall, A. Parkin, S. Parsons, P.A. Tasker, R.E.P. Winpenny, Angew. Chem., Int. Ed. 40 (2001) 2700. (a) Z. Sun, P.K. Gantzel, D.N. Hendrickson, Inorg. Chem. 35 (1996) 6640; (b) L.F. Jones, E.K. Brechin, D. Collison, J. Raftery, S.J. Teat, Inorg. Chem. 42 (2003) 6971; (c) A. Ferguson, K. Thompson, A. Parkin, P. Cooper, C.J. Milios, E.K. Brechin, M. Murie, Dalton Trans. (2007) 728; (d) M. Shanmugam, G. Chastanet, T. Mallah, R. Sessoli, S.J. Teat, G.A. Timco, R.E.P. Winpenny, Chem. Eur. J. 12 (2006) 8777; (e) C. Lampropoulos, M. Murugesu, K.A. Abboud, G. Christou, Polyhedron 26 (2007) 2129. A. Bino, M. Ardon, D. Lee, B. Spingler, S.J. Lippard, J. Am. Chem. Soc. 124 (2002) 4578. C.J. Milios, Th.C. Stamatatos, P. Kyritsis, A. Terzis, C.P. Raptopoulou, R. Vicente, A. Escuer, S.P. Perlepes, Eur. J. Inorg. Chem. (2004) 2885. G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227. Th.C. Stamatatos, G.S. Papaefstathiou, L.R. MacGillivray, A. Escuer, R. Vicente, E. Ruiz, S.P. Perlepes, Inorg. Chem. 46 (2007) 8843. Z.E. Serna, L. Lezama, M.K. Urtiaga, M.I. Arriortua, M.G. Barandika, R. Cortés, T. Rojo, Angew. Chem., Int. Ed. 39 (2000) 344.