Synthesis, characterization and bioactivity of a novel iron(III) 18-metallacrown-6 complex with S-donor ligands

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Inorganica Chimica Acta 361 (2008) 2109–2114 www.elsevier.com/locate/ica

Synthesis, characterization and bioactivity of a novel iron(III) 18-metallacrown-6 complex with S-donor ligands Longfei Jin a

a,*

, Lijiao Cheng

b

College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan, Hubei 430074, PR China b College of Chemistry, Central China Normal University, Wuhan, Hubei 430079, PR China Received 4 July 2007; received in revised form 21 October 2007; accepted 22 October 2007 Available online 26 November 2007

Abstract The reaction of 4-salicyloyl thiosemicarbazide (H3st) (1) with FeCl3 Æ 6H2O in methanol solution gives a novel 18-metallacrown-6 [Fe(st)(H2O)]6 Æ 21H2O (2). The structure containing S–Fe bonds was determined for 2 by using X-ray crystallography. The ring of the metallacrown was consisted of six interlink [Fe–N–N] repeated units. And the ligand enforces the metal ions to form the stereochem˚ istry as a propeller configuration with alternation K/D form. The largest diameters of the disc-shaped hexanuclear ring are about 7.44 A ˚ at entrance, 9.91 A at the central of the cavity, respectively. The structural integrity and stability of the metallacrown ring were studied by electronspray ionization ESI-MS and UV–Vis spectroscopy. The results show that it is stable and soluble in methanol. Antibacterial screening data show that the complex 2 weakened dramatically the antibacterial activity of the ligand H3st.  2007 Elsevier B.V. All rights reserved. Keywords: Metallacrown; S-Donor ligand; Crystal structure; Integrity; Stability; Bioactivity

1. Introduction Metallacrowns and its analogies are a class of molecules with features distinct from the simple organic crowns, such as strong visible absorption spectra, redox activity, magnetism, molecular recognition and bioactivity [1–4]. In the structure, metallacrowns and its analogies exhibit a cyclic hole generally analogous to crown ethers with transition metal ions and a nitrogen atom replacing the methylene carbons [5]. Metallacrowns have two types of molecules. Among them, one type has a cyclic structure with interlink [M– N–O] repeat unit, the other has a [M–N–N] repeat unit. In second type of metallacrown, nitrogen atoms replace all oxygen atoms in the cyclic structure. One, two or three dimensions of networks of metallacrown could be connected via facial interactions and anion bridging [6]. To this

*

Corresponding author. Tel.: +86 2762812177; fax: +86 2767842752. E-mail address: jlfi[email protected] (L. Jin).

0020-1693/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.10.027

day, metallacrowns, with Mn(III), Fe(III), Ni(II), Cu(II), Zn(II), Ga(III) and V(V)O metal ions, [9-MC-3] [7], [12-MC-4] [8–12], [15-MC-5] [13–16], [12-MC-6] [17], [16-MC-8] [18], [18-MC-6] [19], [18-MC-8] [17], [30-MC10] [20], [36-MC-12] [21], stacking metallacrowns [22,23] as well as a variety of dimers and fused metallacrowns [9,13,24,25] have been reported. Metallacrowns are typically prepared using hydroxamic acids and/or ketonoximic acids as constructing ligands (Scheme 1a), while suitable organic molecules such as salicylhydrazides [19,20,26], picoline-tetrazolylamides [27], diethyl ketipinate [23], 3-hydroxy-2-pyridone [28] or 3-thione-1,2-dithione-4,5dithiolato [29] have also been used. Although some metallacrowns containing N–M and O– M bonds in center are known, there has yet been no report of any S–M bonds. Taking the limitations of metallacrowns based solely on shi3 and generally shz3 templates into account, the types of precursor ligands have been greatly expanded with the intention of modifying the ring type, as well as the electronic and other physical properties of the metallacrowns. In the present paper, a new potential

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2.3. Synthesis of the ligand H3st (1) M O

O N

OH M

H N

O O

O

S N H 3

Scheme 1. Basic biding sites of shi sites in compound 2 (c).

Fe S N

NH 2 O

N

NH2

Fe

(a), ligand H3st (b), and basic biding

pentadentate ligand 4-salicyloyl thiosemicarbazide (H3st) (1) (Scheme 1b) and a novel iron(III) 18-metallacrown-6 compound, [Fe(st)(H2O)]6 Æ 21H2O (2) (Scheme 1c) are reported. The triply deprotonated 4-salicyloyl thiosemicarbazide (st3) of the compound 2 may bridge the neighboring ions through its N–N group shown in Scheme 1c. In addition, the meridional coordination of the st3 to the Fe3+ cation forces the neighboring Fe3+ cations into a propeller configuration. To examine the mode of bonding and possible antagonistic or synergetic effects, minimum inhibitory concentrations (MIC) against four different bacteria species of the metallacrown complex were also measured. 2. Experimental 2.1. Materials Chemicals for the synthesis of the compounds were used as purchased. Methanol and ethanol were used without any further purification. Salicylic acid, sulfuric acid, hydrochloric acid, hydrazine hydrate, acetic anhydride, diethyl ether, KSCN, sodium acetate and FeCl3 Æ 6H2O were purchased from China Sinopharm Group Chemical Reagent Co., Ltd. All chemicals and solvents were reagent grade. Staphylococcus aureus (Staph. aureus), Escherichia coli (E. coli), Bacillus subtilis (Bac. subtilis) and Proteus vulgaris (Prot. vulgaris) were purchased from China Center for Type Culture Collection. 2.2. Physical measurements 1

H NMR and 13C NMR spectra were recorded on a Varian Inova 400 MHz NMR spectrometer at 25 C. Chemical shifts are referenced to residual solvent peak. Infrared spectra were measured on a Thermo Nicolet Corporation 470 NEXUS FT-IR Spectrometer as KBr pellets in the 4000–400 cm1 region. UV–Vis spectra were recorded on a Perkin Elmer Lambda 35 UV/Vis Spectrometer. C, H, N and S elemental analysis were performed on a Perkin Elmer 2400 Series II CHNS/O Analyzer. Fe was determined by atomic absorption spectroscopy on a Shimadzu AA-6300 Spectrophotometer. Positive and negative electrospray ionization mass spectra (ESI-MS) were performed on a Applied Biosystems API-2000 LC/MS/MS system for the compounds dissolved in methanol.

Ligand 4-salicyloyl thiosemicarbazide (H3st) (1) was synthesized according to the literature procedure [30]. Salicylhydrazide (15.22 g, 0.10 mol) and KSCN (14.40 g, 0.20 mol) were added to 100 mL of water at 0 C. Stirred for 10 minutes, then 20 mL of concentrated hydrochloric acid were added, and stirred for 1 h. The reaction mixture was slowly warmed to 93 C and stirred for 8 h. After staying for 1 h in refrigerator, the resulting light-yellow precipitate was filtered and rinsed with water to pH 6. A light-yellow solid formed was recrystallized from water at 50 C to give 18.20 g (86% yield) of ligand H3st (1). M.p. 213–216 C. Anal. Calc. for C8H9N3O2S: C, 45.48; H, 4.30; N, 19.89; S, 15.18. Found: C, 45.32; H, 4.19; N, 19.68; S, 14.82%. ESI-MS, m/z: 211 [M]; 210 [MH]; 151. IR (KBr pellet, cm1): mO–H, 3306 vs, broad; mN–H, 3200 vs, broad; mC@O, 1663 vs; mC@N, 1625 vs; mC@N– C@N, 1607 vs; mN–C@O, 1541 vs; dN–H, 1473 vs; dCS– NH2, 1353 vs; m(C–OH)al, 1298 vs; mC@S, 1277 vs; m(C–OH)phenolic, 1233 vs, 1126 s; dAr, 750 s. UV–Vis, kmax (nm) (e, l mol1 cm1): 210 (1 774), 245 (1333), 311 (636). 1 H NMR (DMSO-d6), d ppm: 11.90 (s, 1H, Ar-OH); 10.57 (s, 1H, Ar-CO–NH–); 9.46 (s, 1H, –CS–NH–); 7.97 (d, 1H, o-ArH); 7.84 (m, 1H, p-ArH); 7.72 (d, 1H, m-ArH–ArC(–OH)–); 7.47 (m, 1H, m-ArH); 7.02 (m, 2H, –NH2). 13C NMR (DMSO-d6), d ppm: 182.16 (–CS–); 168.68 (–CO–); 159.56 (ArC–OH); 134.14 (p-ArC); 128.89 (o-ArC); 118.82 (m-ArC); 117.23 (ArC–CO–); 114.99 (m-ArC–ArC(–OH)–). 2.4. Synthesis of [Fe(st)(H2O)]6 Æ 21H2O (2) H3st (0.42 g, 2.0 mmol) and sodium acetate (0.82 g, 6.0 mmol) were dissolved in 40 mL of methanol, and 0.54 g (2.0 mmol) of FeCl3 Æ 6H2O was dissolved in 10 mL of methanol in another flask. The two solutions were mixed and stirred for 1 h and the color of the mixture changed to dark brown, then filtered. After slow evaporation of the mother liquid in two weeks, dark brown block crystals were obtained from the filtrate (0.43 g, 62% yield). Anal. Calc. For C48H90Fe6N18O39S6: C, 27.84; H, 4.39; N, 12.18; S, 9.29; Fe, 16.18. Found: C, 27.63; H, 4.21; N, 12.00; S, 9.01; Fe, 16.38%. IR (KBr pellet, cm1): mH– OH, 3407 vs, broad; mC@N–C@N, 1602 vs; mN–C@O, 1530 vs; mN–C@S, 1483 vs; mAr–O, 1333 vs; mFe–O, 473 m. UV–Vis, kmax (nm) (e, l mol1 cm1): 218 (17 460), 293 (9898). 2.5. Biological activity The antimicrobial activity of 1 and 2 were assessed by their ability to inhibit the growth of Staph. aureus, E. coli, Bac. subtilis and Prot. vulgaris in Mueller–Hinton broth medium. The minimum inhibitory concentration in lg/mL against the four bacteria species was measured. Bacteria concentration was 5000–8000 cfu/mL and concen-

L. Jin, L. Cheng / Inorganica Chimica Acta 361 (2008) 2109–2114

trations of 1600, 800, 400, 200, 100, 50, 25 lg/mL of 1 and 2 in methanol were tested. The solvent showed no antimicrobial action. 2.6. X-ray crystal structure determination A crystal of 2 with dimensions of 0.30 · 0.14 · 0.12 mm was mounted in a glass capillary with the mother liquor to prevent the loss of the structural solvents during X-ray diffraction data collection. Intensity data were collected with a graphite monochromatic Mo Ka radiation (k = ˚ ) at 288(2) K on a Bruker Smart APEX diffrac0.71073 A tometer. From a total of 26 103 reflections corrected by SADABS [31,32] in the 1.79 6 h 6 25.49 range, 4005 were independent with Rint = 0.0600, of which 2839 observed reflections with I > 2r(I) were used in the structural analysis. The structure was solved by direct methods. All nonhydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms were located in calculated positions and/or in the positions from difference Fourier map. The positions and anisotropic thermal parameters of all non-hydrogen atoms were refined on F2 by full-matrix least-squares techniques with SHELXTL program package [32,33]. The final refinement converged at R1 = 0.0617, wR2 = 0.1642 (w = 1/[r2(Fo2) + (0.0899P)2 + 0.0000P], where P = (Fo2 + 2Fc2)/3) (for 4005 unique reflections), (D/r)max = 0.000, (Dq)max = 0.459 and ˚ 3. Difference electron density maps (Dq)min = 0.254 e/A revealed the presence of twenty-one disordered lattice solvate water molecules which were ultimately modeled by use of the SQUEEZE subroutine of the PLATON program suite [34]. The crystallographic data are given in Table 1.

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3. Results and discussion 3.1. Synthesis of the complex Iron metallacrown 2 was synthesized via the reaction of FeCl3 Æ 6H2O with the deprotonated 4-salicyloyl thiosemicarbazide in methanol solution (Fig. 1; Scheme 2). The compound was dark brown crystalline solids. 3.2. Spectral characterization In the IR spectra, the ligand 1 shows stretching bands attributed to C@O, C@N, C–OH (phenolic) and NH at 1663, 1625, 1233, and 3200 cm1, respectively [35]. Band at 3306 cm1 is assigned to m(O–H) vibrations which may be involving intramolecular hydrogen bonding, while band at 1233 cm1 is attributed to m(O–H) (phenolic) [36,37]. In addition, band at 1277 cm1 is attributed to m(C@S). A strong band found at 1607 cm1 is assigned to C@N– N@C group [35–37]. In the compound 2, the absence of the N–H and C@O stretching vibration bands is consistent

Table 1 Crystallographic data Empirical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A c () ˚ 3) V (A Z l (mm1) F(0 0 0) Crystal size (mm) h Range () Index ranges

Observed reflections Independent reflections (Rint) R1 wR2

C48H90Fe6N18O39S6 2071.02 rhombohedral R3 18.8277(13) 18.8277(13) 31.774(5) 120.00 9754.3(17) 3 0.789 2574 0.30 · 0.14 · 0.12 1.79–25.49 22 6 h 6 22, 22 6 k 6 22, 38 6 l 6 38 26 103 4005 (0.0600) 0.0617 0.1642

Fig. 1. Perspective view of compound 2.

OH 6

H N O

S N H

+

6 FeCl3 + 18 NaCH 3COO + 27 H2O

NH2

CH3OH

(2) + 18 NaCl + 18 CH3COOH Scheme 2.

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with the deprotonation of the CONH and CSNH groups and coordination to the Fe(III) ion. The broad band at 3407 cm1 is reasonably assigned to OH stretching vibrations, and the band is attributable to coordinated H2O molecules [38]. The C@N–N@C framework seen at l607 cm1 in the ligand shifted to 1602 cm1 upon coordination to Fe atom. The disappearance of the bands at 3306 cm1 and the appearance of the bands at 1333 cm1 support the involvement of phenolic oxygen in coordination through deprotonation. This is confirmed by the band at about 473 cm1 assigned to Fe–O (phenolic). The UV–Vis spectra were recorded in methanol for 1 and 2. The spectra of the ligand display absorption peaks at 210 nm (e, 1774), 245 nm (e, 1333) and 311 nm (e, 636). The band at 210 nm can be attributed to p ! p* transition of the benzene rings. The bands near 245 and 311 nm may be attributed to charge transfer transitions of the carbonyl and thiocarbonyl groups and p conjugation, respectively. The spectra of compound 2 display absorption peaks at 218 (17 460) and 293 (9898) nm. The intense transition near 210 nm can be attributed to ring internal ligand p ! p* or n ! p* transition of the benzene rings, the substituted group of thiosemicarbazide and p conjugation. The absorption maxima at 293 nm can be assigned to metal to ligand Fe(dp) ! O(p) or Fe(dp) ! S(p) charge transfer (MLCT) transitions.

Fe3+ cation, and one carbonyl oxygen plus the other thiosemicarbazide nitrogen in the same ligand are chelated to an adjacent Fe3+ cation. The specific connectivity of atoms forming the ring is –Fe1–N1A–N2A–Fe1A–N1C–N2C– Fe1C–N1E–N2E–Fe1E–N1D–N2D–Fe1D–N1B–N2B– Fe1B–N1–N2–. Therefore, the ligand is forcing all Fe3+ cations into a propeller configuration with alternating K/D stereochemistry as DKD or KDK forms (Fig. 1). Three water groups coordinated at the metal centers with K configuration are found on one face of the metallacrown, and the remaining three water groups coordinated to the other metal centers with D configuration are found on the other face of the metallacrown. The two faces of the disc-shaped hexanuclear ring have opposite chiralities to each other. This organization results in the 18-membered hexanuclear core ring system with an [–Fe–N–N–] repeat unit. The approximate dimensions of the oval-shaped cavity are ˚ in diameter at entrance, about 9.91 A ˚ at its about 7.44 A largest diameter at the center of the cavity. It is also observed that the all atoms in the ligand are almost in co-plane and the all manganese atoms in 2 are in an octahedral FeN2O3S environment (Fig. 2). The average neigh˚ . There are not boring Fe  Fe separation is of 5.003 A any solvent molecules in the ‘host’ cavity of metallcrown in the compound 2. As listed in Table 3, there are six of intremolecular hydrogen bonds in the compound 2. All hydrogen bonds are between the N–H group from

3.3. Description of structure 2 The metallacrown compound 2 crystallizes in the rhombohedral system and space group R 3. A diagram of the crystal structure of complex 2 is presented in Fig. 1. Important bond distances and angles are presented in Table 2. The structure exhibits a hexanuclear ring of iron atoms linked by six thiosemicarbazide N–N groups. The deprotonated ligand st3 acts as a trianionic pentadentate ligand, one phenolate oxygen, one thiocarbonyl sulfur and one thiosemicarbazide nitrogen in the ligand are bound to one

Table 2 ˚ ) and angles () in compound 2 Selected bond lengths (A Bond lengths Fe1–O2A Fe1–O3 Fe1–N2 C2–O2 C7–N1 C8–S1 Bond angles O2A–Fe1–O1 O1–Fe1–O3 O1–Fe1–N1A O2A–Fe1–N2 O3–Fe1–N2 O2A–Fe1–S1 O3–Fe1–S1 N2–Fe1–S1 N1–N2–Fe1

1.895(3) 2.068(3) 2.112(3) 1.321(5) 1.333(5) 1.647(5) 102.82(13) 84.75(13) 169.60(13) 91.21(14) 159.26(14) 166.27(10) 92.49(13) 90.46(10) 114.1(2)

Fe1–O1 Fe1–N1A Fe1–S1 C7–O1 C8–N3 N1–N2 O2A–Fe1–O3 O2A–Fe1–N1A O3–Fe1–N1A O1–Fe1–N2 N1A–Fe1–N2 O1–Fe1–S1 N1A–Fe1–S1 C8–S1–Fe1 C7–O1–Fe1

1.987(3) 2.099(3) 2.3917(15) 1.288(4) 1.473(5) 1.384(4) 90.76(16) 87.46(13) 93.64(14) 74.68(13) 107.07(14) 90.77(10) 79.02(10) 96.82(17) 118.6(2)

Fig. 2. The coordination environments of Fe(III) ion.

Table 3 ˚ , ) Hydrogen bond geometry (A D–H  A N3–H3A  N1

a

D–H

H  A

D  A

D–H  A

0.86

2.44

3.244(5)

154.8

Symmetry codes: (a) y, x  y, z.

L. Jin, L. Cheng / Inorganica Chimica Acta 361 (2008) 2109–2114

amino-group of one coordinated thiosemicarbazide and the nitrogen atom of the other coordinated thiosemicarbazide in the same Fe core. The N–H  N hydrogen bond dis˚ . These interactions undoubtedly give tances is 3.244(5) A more effective influence to the molecular structure.

0.6

0.5

0.4

A 293

3.4. Structural integrity and stability of metallacrown ring in methanol

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0.3

0.2

Electrospray ionization mass spectroscopy has been proved useful in providing the solution molecular weights of the formally neutral metallacrown salt complexes [39– 41]. The methanol solution of the Fe-containing cluster gave some peaks at m/z 1749.7, 1707.6, 875.4, 861.3, 845.3 in ESI-MS measurements in the positive mode. All of these peaks correspond to the hexanuclear cluster ion, [Fe6(st)6(H2O)2(CH3OH)4+H]+, [Fe6(st)6(H2O)5(CH3OH)+ H]+, [Fe6(st)6(H2O)2(CH3OH)4+2H]2+, [Fe6(st)6(H2O)4(CH3OH)2+2H]2+ and [Fe6(st)6(H2O)4(CH3OH)+2H]2+, respectively. To explore the structural stability of complex in methanol solution, the concentration-dependent absorbance was measured at 218 nm and 293 nm for 2, respectively (Figs. 3 and 4). The absorbance increases linearly with the concentration at the range of 1.05 · 105 and 5.25 · 105 mol/L. No change can be observed for the solutions’ absorption after one week. The results indicate that complex 2 retain integrity of the metallacrown ring [Fe6(st)6] and are stable at least at the concentration ranges in methanol. 3.5. Antimicrobial activity Minimum inhibitory concentrations of 1 and 2 against Staph. aureus, E. coli, Bac. subtilis and Prot. vulgaris are listed in Table 4. As can clearly be seen from Table 4, the compounds 1 and 2 are not strong antimicrobial activities against all of tested microorganisms, but the compound 1 has better behavior than 2. Moreover, in this study, the

0.1

0.0 0

1

2

3

4

5

6

-5

C(10 mol/L)

Fig. 4. The concentration-dependent absorbance of complex 2 in methanol was measured at 293 nm.

Table 4 Minimum inhibitory concentration (MIC) of compounds 1 and 2 in lg/ mL Microorganisms Staph. Aureus (Gram+) E. coli (Gram) Bac. subtilis (Gram+) Prot. vulgaris (Gram)

Compounds 1

2

100 400 100 200

400 800 200 400

compounds 1 and 2 against Gram+ bacteria Staph. aureus Rosenbach and Bac. subtilis (Ehrenberg) Cohn are better than against Gram bacteria E. coli (Migula) Castellani Chalmers and Prot. vulgaris Hauser in antimicrobial activity. The results are generally expected. While compound 2 has formed relatively weak antimicrobial effect against all tested microorganisms, compound 1 has shown better potent antibacterial activity against Staph. aureus and Bac. subtilis. 4. Conclusions

1.0

0.8

A 218

0.6

0.4

0.2

0.0

0

1

2

3

4

5

6

-5

C(10 mol/L)

Fig. 3. The concentration-dependent absorbance of complex 2 in methanol was measured at 218 nm.

The compound 2 is the first example of 18-metallacrown-6 compounds containing bond S–Fe. Due to the meridional coordination of the ligand to the Fe3+ ion, the ligand enforces the stereochemistry of the Fe3+ ions as a propeller configuration with alternating K/D forms. The other important structural feature in [Fe(st)(H2O)]6 Æ 21H2O is that there is not only a vacant cavity in the center of 18-metallacrown-6 core ring, but also the opposite chiralities on the two faces of the metallacrown ring system. Although the available data may be indirectly we can stress that the structural integrity and stability of metallacrown ring in methanol were stable which confirmed via ESI-MS and UV–Vis spectra. The metallacrown 2 against Gram+ bacteria Staph. aureus Rosenbach and Bac. subtilis (Ehrenberg) Cohn are better than against Gram bacteria Escherichia coli (Migula) Castellani

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Chalmers and Prot. vulgaris Hauser in antimicrobial activity. Antibacterial screening data show that the complex 2 weakened dramatically the antibacterial activity of the ligand H3st. Acknowledgements This work was supported by the Natural Science Foundation of Hubei Province (2007ABA121) and the Natural Science Foundation of South-Central University for Nationalities (YZZ07005). Appendix A. Supplementary material CCDC 648655 contains the supplementary crystallographic data for this paper. 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.2007.10.027. References [1] M.S. Lah, V.L. Pecoraro, J. Am. Chem. Soc. 111 (1989) 7258. [2] D.J. Cram, Angew. Chem., Int. Ed. Engl. 27 (1988) 1009. [3] M.S. Lah, M.L. Kirk, W. Hatfield, V.L. Pecoraro, J. Chem. Soc., Chem. Commun. (1989) 1606. [4] C.J. Pederson, Angew. Chem., Int. Ed. Engl. 27 (1988) 1021. [5] C.T. Wu, Chemistry of Crown Ethers, Science, Beijing, 1992. [6] J.J. Bodwin, A.D. Cutland, R.G. Malkani, V.L. Pecoraro, Coord. Chem. Rev. 216 (2001) 489. [7] B.R. Gibney, A.J. Stemmler, S. Pilotek, J.W. Kampf, V.L. Pecoraro, Inorg. Chem. 32 (1993) 6008. [8] A.J. Stemmler, J.W. Kampf, M.L. Kirk, V.L. Pecoraro, J. Am. Chem. Soc. 117 (1995) 6368. [9] G. Psomas, A.J. Stemmler, C. Dendrinou-Samara, J. Bodwin, M. Schneider, M. Alexiou, J. Kampf, D.P. Kessissoglou, V.L. Pecoraro, Inorg. Chem. 40 (2001) 1562. [10] D.P. Kessissoglou, J. Bodwin, J. Kampf, C. Dendrinou-Samara, V.L. Pecoraro, Inorg. Chim. Acta 331 (2002) 73. [11] A.J. Stemmler, J.W. Kampf, V.L. Pecoraro, Inorg. Chem. 34 (1995) 2271. [12] M. Alexiou, C. Dendrinou-Samara, C.P. Raptopoulou, A. Terzis, D.P. Kessissoglou, Inorg. Chem. 41 (2002) 4732. [13] D.P. Kessissoglou, J. Kampf, V.L. Pecoraro, Polyhedron 13 (1994) 1379. [14] A.J. Stemmler, A. Barwinski, M.J. Baldwin, V. Young, V.L. Pecoraro, J. Am. Chem. Soc. 118 (1996) 11962.

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