Tetranuclear zinc complexes of ligands containing the 2-pyridyl oxime chelating site

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Inorganica Chimica Acta 361 (2008) 2677–2682 www.elsevier.com/locate/ica

Tetranuclear zinc complexes of ligands containing the 2-pyridyl oxime chelating site Javier Martinez, Iolinda Aiello, Anna Bellusci, Alessandra Crispini, Mauro Ghedini * Centro di Eccellenza CEMIF. CAL-LASCAMM, CR-INSTM Unita` della Calabria, Dipartimento di Chimica, Universita` della Calabria, I-87036 Arcavacata di Rende (CS), Italy Received 2 October 2007; received in revised form 12 December 2007; accepted 15 December 2007 Available online 28 December 2007

Abstract 1–3 Three tetranuclear zinc(II) complexes of the general formula ½Zn4 ðl3 -OHÞ2 ðLR1–3 N;N; O Þ4 ðLRN;N Þ2  (1–3) were formed from the reaction of ZnCl2 with 2 equiv. of the respective 2-pyridyl ketone oximes, HLR13 (R1–3 = H, Ph and Py). The synthesis and the characterization (elemental analysis, IR, 1H NMR and UV–Vis) are described. Complexes 2 and 3 were structurally characterized by single-crystal X-ray crystallography, revealing new examples of anti 12-metallocrown-4 complexes where anions such as hydroxyl groups are encapsulated into the cavity of the tetranuclear cluster. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Zinc complexes; Zinc metallocrowns; Oximato complexes; X-ray diffraction

1. Introduction The synthesis of metal complexes featuring photo or electro-driven emitting properties, useful for applicative purposes, is the present focus of several investigations [1– 3]. In this context, as examples for applications in electroptical devices, zinc complexes formed by 8-hydroxyquinoline (I, Chart 1) [4] or salen-like ligands (II, Chart 1) [5,6] emissive in the blue region of the electromagnetic spectrum have been recently proposed as molecular materials for LED (Light Emitting Diodes) [4,7]. The molecular structure of both the blue emitting species consists of a Zn(II) ion co-ordinated to two nitrogen and two oxygen atoms. The available X-ray molecular structures have proved that the Zn(II) centre is in a distorted square planar or tetrahedral geometry. In addition to the above-mentioned cases, another class of ligands such that pyridyl aldo or keto hydroxylamine can in principle exhibit a chelating N,O site and, among *

Corresponding author. Tel.: +39 (0) 984492062; fax: +39 (0) 984 492066. E-mail address: [email protected] (M. Ghedini). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.12.017

the different co-ordination mode displayed [8], they could also undergo deprotonation of the OH group with the consequent chelation of the Zn(II) ion (structure III in Chart 1). Oximes and in particular 2-pyridyl ketone oximes are a wide series of synthetically very versatile molecules, extensively used in co-ordination chemistry for the preparation of homo- and hetero-polymetallic complexes [9–11]. Moreover, pyridyl ketone oximes have an important role in the synthesis of metallocrown complexes some of them containing both Ni(II) and Zn(II) ions [12,13]. In particular, recently some studies by Kessissoglou group appeared. The di-2-pyridyl ketone oxime (HLR3, Chart 2) chemistry has been investigated in reactions with the ZnCl2 salt, finalized to the synthesis of new metallocrowns [8,14]. Mononuclear Zn(HLR3)Cl2 or tetranuclear 12-membered metallocrowns with [Zn4(LR3)4](OH)2Cl2 stoichiometry are the described products obtained from reactions carried in a ligand to metal 1:1 molecular ratio in neutral or alkaline medium [14]. The present paper reports an explorative study carried on the co-ordination chemistry of three commercially available 2-pyridyl ketone oximes, HLR13 (R13 = H, Ph and

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(CH2)n O

N

R N

N

N

Zn

N

Zn

O

Zn O

O

O

N

O N

N R

I

III

II

Chart 1.

H N

N N

N OH

HLR

1

N

N N

OH

HLR2

OH

HLR3

Chart 2.

Py, respectively, Chart 2), in reactions with ZnCl2 in alkaline medium. This investigation has been planned to test the possible synthesis of mononuclear complexes of the general formula Zn(LR13)2 (like III in Chart 1); therefore, the reactions have been performed in a 1:2:2 ZnCl2: HLR13:KOH molar ratio. The obtained results prove that, regardless of the nature of the R substituent of the oxime ligand, tetranuclear zinc complexes of the general formula ½Zn4 ðl3 -OHÞ2 ðLR1–3 N;N;O Þ4 1–3 ðLRN;N Þ2  (1–3) are formed. Herein, we describe the synthesis of 1–3 and the single crystal X-ray analysis of the 2 (R = Ph) and 3 (R = Py) derivatives. 2. Experimental 2.1. Materials and physical measurements All the commercially available chemicals were purchased from Aldrich Chemical Co. and were used without further purification. 1 H NMR spectra were measured in CDCl3, with a Bruker AC 300 spectrometer at 25 °C, and TMS was used as the internal standard. IR spectra were recorded from KBr pellets using a Perkin–Elmer Spectrum One FT-IR spectrometer. Absorption spectra were recorded with a Perkin–Elmer Lambda 900 spectrophotometer. The thermal behaviour was monitored with a Zeiss Axioscope microscope equipped with a Linkam C0600 heating stage. Elemental analyses were performed using a Perkin–Elmer CHN 2400 Analyzer.

air. Yield: 0.188 g (54%). M.p. > 300 °Cdec.. Anal. Calc. for C36H32N12O8Zn4 (1022.27): C, 42.30; H, 3.16; N, 16.44. Found: C, 42.05; H, 3.20; N, 16.40%. 1H NMR spectrum not available because 1 is insoluble in common solvents. IR (KBr) m = 3408 (s, broad) [O–H], 1604 (s) [C@N], 1225 (w) [N–O] cm1. UV–Vis (CH2Cl2): kmax (e, M1 cm1) = 247 (5784), 270 (5223), 314 (2750). Complexes 2 and 3 were synthesized following the procedure described for 1; purification, colour, yield and analytical data are as follows. 2.2.2. ½Zn4 ðl3 -OH Þ2 ðLR2N ;N ;O Þ4 ðLR2N ;N Þ2  (2) The crude product was recrystallised from chloroform/ ethanol. The crystals appeared to be suitable for X-ray structural determination. Yellow solid. Yield: 0.177 g (57%). M.p. 210 °C. Anal. Calc. for C72H56N12O8Zn4 (1478.85): C, 58.48; H, 3.82; N, 11.37. Found: C, 58.30; H, 3.70; N, 11.20%. 1H NMR (300 MHz, CDCl3, 25 °C): d = 8.65 (d, 3JHH = 4.70 Hz, 2H, H3), 8.36 (d, 0 3 JHH = 4.70 Hz, 1H, H3 ), 7.71 (d, 3JHH = 7.63 Hz, 4H, 0 8 12 H and H ), 7.54 (m, 5H, H9, H11 and H10 ), 7.48 (d, 3 10 3 JHH = 7.63 Hz, 2H, H ), 7.42 (td, JHH = 7.63 Hz, 0 0 4 JHH = 1.76 Hz, 2H, H5), 7.27 (m, 2H, H8 and H12 ), 6 50 60 90 110 7.20 (m, 1H, H ), 7.06 (m, 5H, H , H , H and H ), 0 7.00 (dd, 3JHH = 7.04, 2H, H4) 6.92 (m, 1H, H4 ), 5.4 (s, 1H, l3-OH). IR (KBr) m = 3429 (s, broad) [O–H], 1595 (s) [C@N], 1207 (w) [N–O] cm1. UV–Vis (CH2Cl2): kmax (e, M1 cm1) = 247 (47125), 275 (41851), 314 (34730). 2.2.3. ½Zn4 ðl3 -OH Þ2 ðLR3N ;N ;O Þ4 ðLR3N ;N Þ2 ] (3) The crude product was recrystallised from chloroform/ ethanol. The crystals appeared to be suitable for X-ray structural determination. Yellow solid. Yield: 0.180 g (58%). M.p. 282 °C. Anal. Calc. for C66H50N18O8Zn4 (1484.78): C, 53.39; H, 3.39; N, 16.98. Found: C, 53.17; H, 3.20; N, 17.10%. 1H NMR (300 MHz, CDCl3, 25 °C): d = 8.74 (d, 3JHH = 4.70 Hz, 2H, H3), 8.61 (d, 0 3 JHH = 4.70 Hz, 1H, H3 ), 8.51 (d, 3JHH = 4.70 Hz, 2H, 0 9 3 H ), 8.44 (d, JHH = 4.70 Hz, 1H, H9 ), 8.23 (d, 3 6 3 JHH = 8.23 Hz, 2H, H ), 7.92 (td, JHH = 7.63 Hz, 0 0 0 4 JHH = 1.76 Hz, 2H, H5), 7.64–7.46 (m, 4H, H6 , H5 , H11 11 4 40 and H ), 7.43–7.31 (m, 4H, H and H ), 7.18 (m, 3H, 0 0 H12 and H12 ), 7.08 (m, 1H, H10 ), 6.94 (dd, 3JHH = 7.63 Hz, 3 10 JHH = 4.70 Hz, 2H, H ), 5.4 (s, 1H, l3-OH). IR (KBr) m = 3419 (s, broad) [O–H], 1595 (s) [C@N], 1219 (w) [N– O] cm1. UV–Vis (CH2Cl2): kmax (e, M1 cm1) = 242 (72506), 275 (75763), 314 (49969). 2.3. Crystal structure analyses

2.2. Synthesis of complexes 1, 2 and 3 2.2.1. ½Zn4 ðl3 -OH Þ2 ðLR1N ;N ;O Þ4 ðLR1N ;N Þ2  (1) HLR1 (250 mg, 2.05 mmol) and KOH (115 mg, 2.05 mmol) were dissolved in 15 mL of ethanol, and ZnCl2 (140 mg, 1.02 mmol) was added. The mixture was stirred at room temperature for 24 h. The white solid that precipitated was filtered off, washed with ethanol, and dried in

Details of the crystal data collection are listed in Table 1. X-ray data for 2  0.75CHCl3 and 3  2CHCl3 were collected on a Bruker–Nonius X8 Apex CCD area detector equipped with graphite monochromator and Mo Ka radi˚ ), and data reduction was performed ation (k = 0.71073 A using the SAINT programs; absorption corrections based on multiscan were obtained by SADABS [15]. Very low qual-

J. Martinez et al. / Inorganica Chimica Acta 361 (2008) 2677–2682 Table 1 Crystal data and structure refinement for compounds 2 and 3 Compound

2

with C–H bond lengths appropriate to the carbon atom hybridization.

3

Empirical C72H56N12O8Zn4  1.4CHCl3 formula Formula 1645.88 weight Temperature 293(2) (K) Wavelength 0.71073 ˚) (A Crystal monoclinic system Space group C2/c Unit cell dimensions ˚) a (A 26.4585(1) ˚) b (A 17.3909(1) ˚) c (A 16.5090(1) a (°) 90 b (°) 92.151(3) c (°) 90 Z 4 Dcalc (Mg/ 1.440 m3) Absorption 1.458 coefficient (mm1) Index ranges 27 6 h 6 27, 17 6 k 6 17, 17 6 l 6 17 Reflections collected Independent reflections [Rint] Data/ restraints/ 9664/0/505 Goodnessof-fit on F2 Final Ra,b indices [I > 2r(I)] R indices (all data)

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C66H50N18O8Zn4  4CHCl3

3. Results and discussion

1962.19

As previously stated, our first goal was the synthesis of the mononuclear Zn(LR13)2 complexes. For this purpose, syntheses were carried out through reaction between ZnCl2 with 2 equiv. of the appropriate HLR13 ligand in alkaline medium at room temperature. Under these reaction conditions, instead of the expected Zn(LR13)2 compounds, yellow solids (complexes 1–3) with elemental analysis accounting for the [Zn4(LR13)6(OH)2] stoichiometry were obtained in satisfactory yields. The IR spectra of 1–3 exhibit a strong broad band between 3408 and 3429 cm1 assignable to m(O–H)OH [17]. Moreover, two bands, at about 1600 cm1 (1604 cm1 for 1 or 1595 cm1 for 2 and 3) and in the 1207–1225 cm1 range, respectively of strong and medium intensities, are observed in the 1–3 spectra. It is worthy to note, although tentatively, that the former band could be assigned to the oximate m(C@N) and the latter to the m(N–O) vibrational modes [14]. Noteworthily, while complex 1 is insoluble in most of the common organic solvents, it has been possible to record the 1H NMR spectra for complexes 2 and 3. The spectra are consistent with the obtained supramolecular structure resolved by X-ray analysis on single crystals of 2;3 the general formula ½Zn4 ðl3 -OHÞ2 ðLR2;3 N;N;O Þ4 ðLRN;N Þ2 . Two sets of signals in a 1:2 ratio corresponding to the two different ways of co-ordination N,N,O and N,N (Fig. 1 and Section 2) are observed for the oxime ligands and the presence of a sharp singlet signal at 5.4 ppm corresponding to the l3-OH proves the high stability of the whole structure in solution. Crystals of complexes 2 and 3 suitable for single crystal X-ray analysis were obtained from slow evaporation of a chloroform/ethanol solution. Molecules of chloroform are found in the asymmetric unit. A view of the structures of 2 and 3 is shown in Fig. 2a and b.

100(1) 0.71073 monoclinic P2(1)/c 10.4768(3) 20.8414(7) 17.8933(6) 90 94.1160(1) 90 2 1.672 1.695

26 125

13 6 h 6 13 27 6 k 6 27, 23 6 l 6 23 39 606

4374 [0.0728]

9664 [0.0502]

parameters

4374/0/438

1.029

1.021

R1 = 0.0667, wR2 = 0.1737

R1 = 0.0352, wR2 = 0.0704

R1 = 0.1061, wR2 = 0.1996

R1 = 0.0583, wR2 = 0.0787

Largest 1.023 and 0.586 difference in HLR3 and hole ˚ 3) (e A P P (|Fo|  |Fc|)/ |FoP |. [a] R1 = P 2 [b] wR2 ¼ ½ wðF o  F 2c Þ2 = wðF 2o Þ2 1=2 .

0.423 and 0.523 5

11

9

N

8

5

11 4

10

3

9

6

12

O

LR

N,N,O 5'

11' 6'

12'

10' 9'

N

8'

N

3

N,N,O 5'

11' 4'

10'

3'

9'

6'

12'

LR

4'

N

N

3'

N O

2

3

N O

2

4

N

N

N

LR

ity crystals for complex 2 have imposed data collection at low 2h value. Both structures were solved by Patterson method (SHELXS/L program in the SHELXTL-NT software package) [16] and refined by full-matrix least-squares based on F2. All non-hydrogen atoms were refined anisotropically (with the exception of the C and Cl atoms of the solvent molecule in complex 2), and all hydrogen atoms (with the exception of the hydrogen atoms OH bridge) were included as idealized atoms riding on the respective carbon atoms

6

12

10

N,N

O

LR

3

N,N

Fig. 1. N,N,O and N,N ligand co-ordination mode and proton numbering scheme of LR2 and LR3.

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Fig. 2. Molecular structures of complexes 2 (a) and 3 (b) with labelling scheme. Hydrogen atoms and solvent have been omitted for clarity.

Crystallographic data and selected interatomic distances and angles are listed in Tables 1 and 2. The presence of the two slightly different ligands HLR2 and HLR3, respectively, in complexes 2 and 3, does not imply a different co-ordination mode towards the Zn(II) ion, as demonstrated by the structural analysis herein reported. In both

Table 2 ˚ ) and angles (°) for compounds 2 and 3 Selected bond distances (A Complex Zn(1)–N(1) Zn(1)–N(2) Zn(1)–N(3) Zn(1)–N(4) Zn(2)–N(5) Zn(2)–N(6) Zn(1)–O(4) Zn(2)–O(1) Zn(2)–O(2a)a Zn(2)–O(4) N(1)–Zn(1)–N(2) N(1)–Zn(1)–N(4) N(1)–Zn(1)–O(4) N(3)–Zn(1)–N(4) N(3)–Zn(1)–O(4) O(4a)–Zn(1)–N(2) O(4a)–Zn(1)–N(3) O(4)–Zn(2)–N(5) N(6)–Zn(2)–N(5) O(2a)–Zn(2)–O(1)a O(1)–Zn(2)–N(6) O(4)–Zn(2)–N(6) O(1)–Zn(2)–N(5)

2

3 2.219(7) 2.109(8) 2.233(8) 2.114(8) 2.131(7) 2.083(7) 2.068(5) 1.984(6) 1.977(6) 2.036(5)

73.9(3) 157.7(3) 159.4(3) 74.0(3) 90.4(2) 107.2(3) 158.5(3) 162.6(3) 75.6(3) 103.7(3) 126.3(3) 87.0(3) 91.3(3)

2.179(2) 2.190(2) 2.213(2) 2.177(2) 2.144(2) 2.073(2) 2.068(2) 1.980(2) 1.985(2) 2.046(2) 74.3(7) 152.6(7) 162.0(6) 73.9(7) 92.9(7) 113.2(7) 158.9(7) 162.8(7) 75.9(7) 112.6(7) 115.2(7) 86.9(7) 88.6(7)

a Symmetry transformation used to generate equivalent atoms: complex 2: x + 1/2, y + 3/2, z + 1; complex 3: x + 1, y, z + 1.

cases, the asymmetric unit contains two crystallographically independent Zn(II) cations, three deprotonated oxime ligands, one bridging hydroxide ion, and molecules of chloroform. The repetition of the two Zn(II) ions through an inversion centre gives rise to a tetranuclear zinc cluster unit as shown in Fig. 2a and b. The zinc atoms exhibit two different co-ordination geometries. Zn(1) and Zn(1a) are in a distorted octahedral environment, being bound to two deprotonated oxime bridging ligands by their pyridyl nitrogen atoms (N(1), N(2) and N(1a), N(2a), N(3), N(4) and N(3a), N(4a)) in two five-membered chelated rings. The remaining two co-ordination sites of the octahedron are filled by two l3-bridging hydroxide ions (O4 and O(4a)) accommodated in the centre of the metallocrown ring. The co-ordination about the O(4) atom is pyramidal and the sum of the angles of 320.1° accounts for a l3-hydroxyl ion more than a bridging l3-oxo one [18]. The other two zinc ions, Zn(2) and Zn(2a), show a distorted trigonal bipyramidal geometry. Each ion is co-ordinated to one of the l3-bridging hydroxide ion, to two deprotonated oxime ligands through their oxygen atoms (O1 and O(2a) or O(1a) and O2), and to one deprotonated oxime ligand through the pyridyl nitrogen atoms (N5 and N6 or N(5a) and N(6a)) with the formation of five-membered metallacycles. In both cases the Zn(2)–N bonds are slightly shorter than the corresponding Zn(1)–N bonds, consistent with the lower co-ordination number for Zn(2) (Table 1). The orientation of the rotationally free rings bound to the C(6), C(17), C(18), C(28) and C(30) carbon atoms reflects the difference between the two ligands HLR2 and HLR3 due to the presence of a phenyl ring and a pyridyl ring, respectively, in complexes 2 and 3. In fact, while the phenyl rings in complex 2, C(7)/C(12), C(19)/C(24) and

J. Martinez et al. / Inorganica Chimica Acta 361 (2008) 2677–2682

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Fig. 3. Hydrogen bonds of the O–H—O and C–H—N type (a) intramolecular and (b) intermolecular in complex 3.

C(31)/C(36) are nearly orthogonal to the three Zn(1) and Zn(2) metallacycles (dihedral angles ranging from 59 to 70°), in the case of complex 3, the three pyridyl rings N(7)/C(11), N(8)/C(22) and N(9)/C(33) show a less pronounced tilt. The presence of weak intramolecular hydrogen bond of the C–H—N type is responsible for a slight tendency towards coplanarity between the rotationally free pyridyl rings and the ZnNN metallacyles in complex 3 (Fig. 3a). Moreover, the N(9) pyridyl ring of complex 3 shows an intermolecular C–H—N hydrogen bond with a symmetrically related molecule, generating in the crystal packing the formation of rings as repetition molecular motifs (Fig. 3b). In both cases, the oxime oxygen atoms (O(3) and O(3a)) of the deprotonated ligands form intramolecular hydrogen bonds with the bridging hydroxyl group (O(4) and O(4a)), ˚ and 2.741(2) A ˚, with O3. . .O4 separations of 2.772(8) A O(4)–H(4)—O(3) bond angles of 147.1(4)° and 155.7(1)°, in complexes 2 and 3, respectively (Fig. 2a). With reference to the crystalline molecular structure, the newly synthesized complexes 2 and 3 are new examples of anti 12-metallocrown-4 complexes. Metallocrowns are a variety of metallomacrocycles. They are a new class of multinuclear clusters that are analogous to crown ethers in both structure and function [19–24]. Structurally, metallocrowns1 exhibit a cyclic structure generally analogous to crowns ethers in their repeating pattern of O–X–X–O with the oxygen atoms oriented toward the centre of a cavity and with the methylene carbons replaced with M and N 1 The nomenclature for metallocrowns is as follows: M0 mAa[XMCM(ox)H(z)-Y], where X and Y indicate ring size and number of oxygen donor atoms, MC specifies a metallocrown, M and (ox) are the ring metal and its oxidation state, H is the identity of the remaining heteroatom bridge, and (z) is an abbreviation for the organic ligand containing the N/O functionality. There are m captured metals (M0 ) and a bridging anions (A) bound to the ring oxygens and metals, respectively. For the metallocrowns characterized as ‘‘inverse”, the nomenclature proposed is similar except the meaning of Y, indicating the number of metal ions oriented toward the central cavity.

[25,26]. For 12-metallocrown-4 complexes, two structural motifs have been identified: regular [27–35] and inverse [8,14,36,37]. Like reported for the first example of inverse 12-metallocrown-4, also defined ‘‘anti metallocrown” [18], both compounds 2 and 3 are characterized as anti metallocrowns since the zinc atoms, rather than the oximate oxygen atoms, are oriented towards the centre of the cavity, and the connectivity around the ring is N–O–Zn–O–N– Zn rather than N–O–M–N–O–M (Fig. 4). The dimension of the ring is defined as the two diagonals of the rhombus corresponding to the interatomic distances Zn(1)—Zn(1a) and Zn(2)—Zn(2a). While the Zn(1)— ˚ in complex 2 and 3.15 A ˚ in complex 3 Zn(1a) of 3.18 A are slightly shorter than those found in similar tetranuclear zinc(II) complexes containing the HLR3 N,N,O co-ordinated ligand, the corresponding Zn(2)—Zn(2a) values of ˚ in 2 and 3, respectively, are a little bit 5.91 and 5.98 A longer than those reported in the literature [14,18].

Fig. 4. Subcore of the tetranuclear complex 2.

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4. Conclusion The expected goal of the present investigation was the synthesis of Zn(LR13)2 complexes containing a tetracoordinated zinc(II) ion in a bis-chelated N,O co-ordination mode. Surprisingly, the isolated complexes 2 and 3 proved that the co-ordination chemistry of the 2-pyridyl ketone oximes, HLR2,3 (R2,3 = Ph and Py, respectively, Chart 2) in alkaline medium conveniently allows to the synthesis of metallocrown complexes, a new class of co-ordination compounds which during the last years has represented the subject of considerable interest for their potentially unique properties [38–43]. In particular, as confirmed by single crystal X-ray diffraction analysis performed on complexes 2 and 3, anti 12-metallocrown-4 complexes are easily obtained following the described synthetic routine, where anions such as hydroxyls are encapsulated into the cavity of the tetranuclear cluster. Moreover, since the elemental analysis of all the 1–3 products accounts for the same stoichiometry, even in the absence of the single crystal analysis of complex 1, we can easily assess that all 1–3 compounds feature the same molecular structure. Acknowledgements Financial support received from the Ministero dell’Istruzione, dell’Universita` e della Ricerca (MiUR) through the Centro di Eccellenza CEMIF.CAL (CLAB01TYEF) and FIRB (RBNE01P4JF) is gratefully acknowledged. J.M. is pleased to acknowledge the Marie Curie development host fellowship Contract HPMD-CT-2001-00073 for financial support. We wish to thank Dr. Nicolas Godbert (Universita` della Calabria) for 1H NMR spectra. Appendix A. Supplementary material CCDC 648534 and 648535 contain the supplementary crystallographic data for 2 and 3. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ica.2007.12.017. References [1] E. Holder, B.M.W. Langeveld, U.S. Schubert, Adv. Mater. 17 (2005) 1109. [2] R.C. Evans, P. Douglas, C.J. Winscom, Coord. Chem. Rev. 250 (2006) 2093. [3] P.-T. Chou, Y. Chi, Chem. Eur. J. 13 (2007) 380. [4] G. Giro, M. Cocchi, P. De Marco, E. Di Nicolo`, V. Fattori, J. Kalinowski, M. Ghedini, Synth. Met. 102 (1999) 1018. [5] M. La Deda, M. Ghedini, I. Aiello, A. Grisolia, Chem. Lett. 33 (2004) 1060. [6] S. Mizukami, H. Houjou, K. Sugaya, E. Koyama, H. Tokuhisa, T. Sasaki, M. Kanesato, Chem. Mater. 17 (2005) 50.

[7] T. Sano, Y. Nishio, Y. Hamada, H. Takahashi, T. Usuki, H. Shibata, J. Mater. Chem. 10 (2000) 157. [8] M. Alexiou, E. Katsoulakou, C. Dentrinou-Samara, C.P. Raptopoulou, V. Psycharis, E. Manessi-Zoupa, S.P. Perlepes, D.P. Kessissoglou, Eur. J. Inorg. Chem. (2005) 1964. [9] V.Y. Kukushkin, A.J.L. Pombeiro, Coord. Chem. Rev. 181 (1999) 147. [10] P. Chaudhuri, Coord. Chem. Rev. 243 (2003) 143. [11] C.J. Milios, T.C. Stamatatos, S.P. Perlepes, Polyhedron 25 (2006) 134. [12] J.J. Bodwin, A.D. Cutland, R.G. Malkani, V.L. Pecoraro, Coord. Chem. Rev. 216 (2001) 489. [13] V.V. Pavlishchuk, S.V. Kolotilov, A.W. Addison, M.J. Prushan, D. Schollmeyer, L.K. Thompson, E.A. Goreshnik, Angew. Chem., Int. Ed. 40 (2001) 4734. [14] M. Alexiou, C. Dentrinou-Samara, C.P. Raptopoulou, A. Terzis, D.P. Kessissoglou, Inorg. Chem. 41 (2002) 4732. [15] SMART, SAINT, and SADABS; Bruker AXS Inc., Madison, WI, 1997. [16] SHELXTL-NT Crystal Structure Analysis Package, Version 5.1; Bruker AXS Inc., Madison, WI, 1999. [17] A.K. Boudalis, N. Lalioti, G.A. Spyroulias, C.P. Raptopoulou, A. Terzis, A. Bousseksou, V. Tangoulis, J.P. Tuchagues, S.P. Perlepes, Inorg. Chem. 41 (2002) 6474. [18] A.J. Stemmler, J.W. Kampf, V.L. Pecoraro, Inorg. Chem. 34 (1995) 2271. [19] C.J. Pedersen, J. Am. Chem. Soc. 89 (1967) 2495. [20] C.J. Pedersen, J. Am. Chem. Soc. 89 (1967) 7017. [21] C.J. Pedersen, J. Am. Chem. Soc. 92 (1970) 386. [22] M.S. Lah, V.L. Pecoraro, Inorg. Chem. 30 (1991) 878. [23] M. Hiraoka (Ed.), Crown Ethers and Analogous Compounds, vol. 45, Elsevier, New York, 1992, p. 485. [24] P. Dastidar, Z. Stein, I. Goldberg, C.E. Stouse, Supramol. Chem. 7 (1996) 257. [25] V.L. Pecoraro, Inorg. Chim. Acta 155 (1989) 171. [26] M.S. Lah, V.L. Pecoraro, Comments Inorg. Chem. 11 (1990) 59. [27] M.S. Lah, M.L. Kirk, W. Hatfield, V.L. Pecoraro, J. Chem. Soc., Chem. Commun. (1989) 1606. [28] M.S. Lah, V.L. Pecoraro, J. Am. Chem. Soc. 111 (1989) 7258. [29] B.R. Gibney, J.W. Kampf, D.P. Kessissiglou, V.L. Pecoraro, Inorg. Chem. 33 (1994) 4840. [30] A.J. Stemmler, J.W. Kampf, M.L. Kirk, V.L. Pecoraro, J. Am. Chem. Soc. 117 (1995) 6368. [31] B.R. Gibney, H. Wang, J.W. Kampf, V.L. Pecoraro, Inorg. Chem. 35 (1996) 6184. [32] J.A. Halfen, J.J. Bodwin, V.L. Pecoraro, Inorg. Chem. 37 (1998) 5416. [33] G. Psomas, A.J. Stemmler, C. Dendrinou-Samara, J.J. Bodwin, M. Schneider, M. Alexiou, J.W. Kampf, D.P. Kessissoglou, V.L. Pecoraro, Inorg. Chem. 40 (2001) 1562. [34] C. Dendrinou-Samara, G. Psomas, L. Iordanidis, V. Tangoulis, D.P. Kessissoglou, Chem. Eur. J. 7 (2001) 5041. [35] D.P. Kessissoglou, J.J. Bodwin, J.W. Kampf, C. Dendrinou-Samara, V.L. Pecoraro, Inorg. Chim. Acta 331 (2002) 73. [36] V.B. Shur, I.A. Tikhonova, in: J.L. Atwood, J.W. Steed (Eds.), Encyclopedia of Supramolecular Chemistry, Marcell Dekker, New York, 2004, p. 68. [37] T.J. Wedge, M.F. Hawthorne, Coord. Chem. Rev. 240 (2003) 111. [38] D.J. Cram, J.M. Lehn, J. Am. Chem. Soc. 107 (1985) 3657. [39] C.J. Pedersen, Angew. Chem., Int. Ed. Engl. 27 (1988) 1021. [40] D.J. Cram, Angew. Chem., Int. Ed. Engl. 27 (1988) 1009. [41] E.C. Constable, in: K.D. Karlin (Ed.), Progress in Inorganic Chemistry, vol. 42, John Wiley & Sons, New York, 1994, p. 67. [42] G.M. Gray, Comments Inorg. Chem. 17 (1995) 95. [43] V.L. Pecoraro, A.J. Stemmler, B.R. Gibney, J.J. Bodwin, H. Wang, J.W. Kampf, A. Barwinski, in: K.D. Karlin (Ed.), Progress in Inorganic Chemistry, vol. 45, John Wiley & Sons, New York, 1997, p. 83.