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Solid State Sciences 12 (2010) 461–465

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Solid State Sciences

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A 2-D metal-organic framework of the iron-based 18-metallacrown-6 withN-(methyl-maleamic ester) terminal ligand

Yuting Chen a,b, Jianmin Dou a,*, Daopeng Zhang a, Dacheng Li a

a School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, PR Chinab Department of Chemistry, Dezhou University, Dezhou 253023, PR China

a r t i c l e i n f o

Article history:Received 4 September 2009Received in revised form22 November 2009Accepted 7 December 2009Available online 16 December 2009

Keywords:MetallacrownMetal-organic frameworkIntermolecular interactionsAzametallacrownMagnetic property

* Corresponding author. Tel./fax: þ86 635 8239069E-mail address: jmdou@lcu.edu.cn (J. Dou).

1293-2558/$ – see front matter � 2009 Elsevier Masdoi:10.1016/j.solidstatesciences.2009.12.008

a b s t r a c t

An 18-membered complex, [Fe(MMSHZ)(DMF)]6 (1) [MMSHZ¼N-(methyl-maleamic ester)acylsalicyl-hydrazine] was synthesized and structurally characterized. There are not only particular geometryconfigurations of N-terminal groups which result in the unique shape of the centric cavity, but also inter-and intra-molecular interactions assembling adjacent molecules into 2-D framework in complex 1.Temperature-dependent magnetic susceptibility measurements indicate that this complex exhibitssignificantly antiferromagnetic coupling among the metal centers.

� 2009 Elsevier Masson SAS. All rights reserved.

1. Introduction

Metallacrown (MC) is of current interest in the field of supra-molecular chemistry, self-assembly, host–guest chemistry, andmolecular recognition [1,2]. Included in this family of metal-lacrowns are mainly two types based on structural units. One typeof MCs mostly consists of [M–N–O] repeat units with V(V), Mn(III),Fe(III), Ga(III), Co(III/II), Ni(II), Cu(II), Zn(II) metal ions as ringmetals, utilizing hydroxamic acids and/or ketonoximic acid asbackbone ligands, and their sizes have ranged from 9-MC-3 [1a] to12-MC-4 [1b], 15-MC-5 [3a] and 18-MC-6 [3b]. While based ondihydrazide derivatives, the other type of MCs with [M–N–N]repeat units (azametallacrown, azaMC) was synthesized. The ringsof azaMCs have been expanded to 18-MC-6 [4–6,7b], 24-MC-8 [7],30-MC-10 [7b,8], 36-MC-12 [9], 45-MC-15 [10], 48-MC-16 [11],even to 60-MC-20 [12]. Also, the ligands, such as salicylaldehyde-2-pydinecarboxylhydrazone, 3, 5-diphenylpyrazolate or deriva-tives of 1, 2, 4-triazoe, have been used for the preparation ofazaMCs [13].

Interest in MCs stems not only from their aestheticallymolecular frameworks and selective recognitions to anion/cation,

.

son SAS. All rights reserved.

but also from the potential applications in the assembly of multi-dimensional metal-organic frameworks (MOFs) as secondarybuilding units (SBUs). While a variety of polymers formed byazaMCs through covalent bonds have been intensively investi-gated [14], the research of polymers constructed by azaMCs viano-covalent interactions is still rare, and only a few supramo-lecular structures based on MCs with M–N–N repeat units havebeen constructed [8b,15]. To the best of our knowledge, noexample in which azaMCs as SBUs are linked into two-dimen-sional network through intermolecular interactions is reportedup to the present.

Taking it into account that, existing in its enol form in solutionand providing with four replaceable protons, N-(maleamic acid)-acylsalicylhydrazine (H4-MSHZ) can coordinate as a potentialtetra-deprotonated ligand and be prone to forming hydrogen-bond, we made the reaction of H4-MSHZ and Fe(NO3)3$4H2O, andobtained a novel 18-metallacrown-6 complex: [Fe(MMSHZ)(DMF)]6

(1) [MMSHZ¼N-(methyl-maleamic ester)acylsalicylhydrazine].Remarkably, in complex (1), N-terminal carboxyl group of H4-MSHZwas esterified into methyl-maleamic ester, which is not only almostperpendicular to phenyl of ligand itself but also parallel to phenyl ofadjacent ligand. Furthermore, this complex is assembled into thefirst 2-D supramolecular network based on azaMC as SBU viaintermolecular edge-to-face (EF) stacking interactions and C–H/Ohydrogen-bonds.

Table 1Crystal data of refinements for complex 1.

Empirical formula C90H96Fe6N18O36

Formula weight 2340.95Crystal system Triclinicspace group P-1a (Å) 15.2531(16)b (Å) 15.7997(18)c (Å) 16.251(2)a (�) 111.190(2)b (�) 95.7780(10)g (�) 115.554(2)V (Å3) 3137.3(6)Z 1Dcalc (mg/m3) 1.239m (mm�1) 0.751Theta range for data collection 1.41–25.01Reflections collected/unique 16,354/10,893 [R(int)¼ 0.0534]Data/restraints/parameters 10,893/0/676Goodness-of-fit on F2 0.999Final R indices [I> 2q] R1¼ 0.0808, wR2¼ 0.1895Largest diff. peak hole (e/Å�3) 0.623 and �1.163

Y. Chen et al. / Solid State Sciences 12 (2010) 461–465462

2. Experiment

2.1. General

All reagents for the syntheses were obtained commercially withanalytical grade and were used without further purification. IRspectra were recorded on Nicolet-5700 spectrophotometer in therange 4000–400 cm�1 (KBr pellets). Element analyses were per-formed on a Perkin–Elmer 2400 II elemental analytical instrument.UV–Vis spectra were recorded on a Shimadzu-UV-2501 spectro-photometer. Thermogravimetry (TG) was carried out on Perkin–Elmer Diamond DSC TG-DTA 6300 thermal analyzer from 50 to500 �C. The variable-temperature magnetic susceptibility datawere collected on the polycrystalline samples over a 2–300 Ktemperature range at 2KOe using a Quantum Design SQUID MPMSXL-7 magnetometer.

2.2. Synthesis of N-(maleamic acid)acylsalicylhydrazine (H4-MSHZ)[16]

0.98 g (10 mmol) of Maleic anhydride and 1.52 g (10 mmol)salicylhydrazide were added to 100 ml of chloroform at ambienttemperature, and then refluxed for 8 h. The white precipitateobtained was filtered and washed with small quantities of coldchloroform. Yield: 2.16 g (97%). Elemental analysis (%) calc. forC11H10N2O5: C 52.80, H 4.01, N 11.20; found: C 52.62, H 4.16, N11.44%. Melting point: 439–441 K. IR (KBr, cm�1): 3423, 3328, 3194,2706, 1702, 1675, 1609, 1554, 1412, 1237, 945.

2.3. Synthesis of the complex [Fe(MMSHZ)(DMF)]6 (1)

The solution of Fe(NO3)3$4H2O (0.081 g, 0.2 mmol) in meth-anol (10 mL) was added to the mixture of H4-MSHZ (0.05 g,0.2 mmol) and NaOH (0.024 g, 0.6 mmol) in DMF (15 mL). Afterstirring for 2 h, dimethyl ether was diffused into the filtrate. Thesolution had been standing for 3 weeks, the crystals suitable forX-ray diffraction study were obtained. Yield: 0.049 g (63%),based on Fe(NO3)3$4H2O. M. P. >573 K. Elemental analysis (%)calc. for C90H96Fe6N18O36: C 46.14, H 4.1, N 10.76; found: C 46.02,H 4.26, N 10.44%. IR (KBr, cm�1): 3050, 1729, 1709, 1601, 1558,1462, 1415, 1399, 1259, 1168, 1048,1030, 632, 605. UV–Vis (l/nm):216(m), 352(w).

2.4. Powder X-ray diffraction

Sample containing only single crystals of compound of 1, wasisolated for the analysis by powder X-ray diffraction. The compo-sition and phase purity of the product were confirmed by over-laying the simulated X-ray diffraction pattern calculated fromsingle-crystal X-ray diffraction data with the observed powderpattern of the product as shown in Fig. S1.

2.5. Crystallographic data collection and processing

X-ray crystallographic data were collected on a Bruker SMARTCCD1000 diffractometer with graphite-monochromatic Mo–Karadiation (l¼ 0.71073 Å) at 298 (2) K. A semiempirical absorptioncorrection was applied to the data. The structure was solved bydirect method and expanded using Fourier difference techniquewith SHELXS-97 program package. All non-hydrogen atoms wererefined anisotropically by full matrix least-squares calculations onF2, and hydrogen atoms were located in calculated positions and/orin the positions from difference Fourier map. Crystallographic dataare summarized in Table 1.

3. Results and discussion

3.1. Synthesis and spectral characterization of complex 1

We have endeavored to synthesize one metallamacrocycle withN-COOH group in the centric hole, and made the reaction of H4-MSHZ, Fe(NO3)3$4H2O and NaOH in 1:1:3 molar ratio. Unexpectedly,the complex obtained in this process was [Fe(MMSHZ)(DMF)]6, inwhich N-terminal groups in the cavity are not N-(maleamic acid)groups but N-(methyl-maleamic ester) groups, indicating that theesterification came into being in reacting process (Scheme 1).

In the IR spectra of complex 1, the disappearance of the bandsabout -COOH and the appearance of bands at 1168, 1030 and1462 cm�1 attributed to the C–O–C and –OCH3 vibration show thatthe esterification of N-terminal ligand took place in synthesizingprocess. The absence of N–H and C]O stretching vibration bands isconsistent with the deprotonation of CONH groups and coordina-tion to the Fe ion in enol form, which is also confirmed by the bandsat 632 and 605 cm�1 attributed to M–O linkage and M–N linkage,respectively [5a]. The >C]N–N]C< framework seen at l609 cm�1

in the ligand was shifted to 1601 cm�1 upon coordination to Featom [17]. The appearance of the bands at 1259 and 632 cm�1

instead of the bands at 3194, 2708 and 1237 cm�1 in the ligandsupports the involvement of phenolic oxygen in coordination withmetal ion through deprotonation [6b]. The UV–Vis spectra wererecorded in DMF for the complex 1. The spectra of compound 1display absorption peaks at 216 and 352 nm. The 216 nm can beattributed to ring internal ligand p / p* or n / p* transition ofthe benzene rings, N-(methyl-maleamic ester) groups and pconjugation. The absorption maxima at 352 nm can be assigned tometal to ligand Fe / O/N charge transfer transitions.

3.2. Description of the compound 1

Crystal structure analysis reveals that Complex 1 is a chair-like18-membered metallamacrocycle constructed by six – [Fe(III)–N–N] – repeat units (Fig. 1, Fig. S2). Selected interatomic distancesand angles are listed in Table 2. The pentadentate ligand bridgesmetal ions using N–N group via simultaneous tridentate andbidentate connecting modes in back-to-back fashion. A phenolicoxygen atom (O2), a hydrazine nitrogen atom (N1) and a carbonyloxygen atom (O3) of the ligand are bound to one Fe3þ ina meridional mode, forming the six-membered and five-

Scheme 1. The synthesizing process of complex 1.

Y. Chen et al. / Solid State Sciences 12 (2010) 461–465 463

membered chelating rings, and the remaining hydrazine N2 andone carbonyl O1 of this ligand are chelated to the adjacent Fe3þ,resulting in a five-membered ring. The dihedral angels betweenthese chelating rings are 2.13, 1.18 and 1.46� respectively,showing that the ligand is coordinated with metals in almost co-planar pattern. However, the average dihedral angel between theplane of the tridentate-binding moiety in one ligand and that ofthe bidentate-binding moiety in adjacent ligand around the sameFe3þ is 87.52�, indicating that neighboring ligands are coordi-nated with the Fe3þ ions at nearly vertical orientation (Fig. 2).The sixth coordination site of the octahedral Fe3þ is occupied byO atom from DMF molecule. This binding mode of the ligand notonly enforces the stereochemistry of Fe3þ into propeller

Fig. 1. The molecular structure of complex 1.

configuration but also leads to alternating D=L chirality of metalcenter, resulting in two opposite chiral faces.

It is quite worth noting the geometrical configuration of N-(methyl-maleamic ester) groups, although they are alternatelyarranged into two faces of the metallamacrocycle in complex 1, asshown in most reported azaMCs [6–8]. The no-hydrogen atoms ofN-(methyl-maleamic ester) group are nearly co-planar, but thedihedrals of the N-terminal plane with the remaining salicylhy-drazine group of ligand itself and the phenyl group of adjacentligand are 87.81� and 5.32� respectively, showing that the N-endgroup is approximately arranged not only perpendicular to sali-cylhydrazine group itself but also parallel to that of adjacent ligand(Fig. S3). Noticeably, the diameter of the entrance formed by threemethyl carbon atoms (C12, C24, C36) is enlarged to 10.856 Å, ratherlarger than those of reported 18-MC-6 with N-alkyl ends, thoughthat of 4.774 Å formed by three carbon atoms (C9, C21, C33) near tometal-ring is slightly bigger than those of corresponding MCs.Consequentially, the depth of the cavity is extended to 11.344 Å(Fig. S4), which is much deeper than those of 18-MC-6 based onlinear N-alkyl sides [6,7]. Thus, the central cavity of complex 1 lookslike two loudhailers bonded in back–back pattern, and this uniqueconfiguration is distinctly different from those in reported azame-tallacrowns [4a].

Equally, some attention should be paid to the intra-molecularand intermolecular stacking interactions in complex 1. There are twointra-molecular C–H/p interactions between methyl-carbons of

Table 2Selected bond lengths (Å) and angles (�) of complex 1.

Fe(1)–O(2) 1.862(9) Fe(2)–O(8) 2.066(7)Fe(1)–O(11)#1 1.990(7) Fe(2)–N(2) 2.108(8)Fe(1)–O(3) 2.048(7) Fe(3)–O(12) 1.886(8)Fe(1)–O(16) 2.063(8) Fe(3)–O(6) 1.990(7)Fe(1)–N(1) 2.068(9) Fe(3)–N(5) 2.037(8)Fe(1)–N(6)#1 2.132(8) Fe(3)–O(13) 2.041(7)Fe(2)–O(7) 1.886(8) Fe(3)–O(18) 2.056(9)Fe(2)–O(1) 2.001(8) Fe(3)–N(4) 2.117(9)Fe(2)–N(3) 2.018(9) N(1)–N(2) 1.416(11)

Fig. 2. Coordination mode of ligands around the Fe3þ in complex 1.

Y. Chen et al. / Solid State Sciences 12 (2010) 461–465464

N-terminal groups and phenyls of adjacent ligands in each faceof metallamacrocycle, namely C36-H36/Cg1 [H36/Cg1¼3.030 Å,:C36–H36/Cg1¼130.51�, Cg1: C2–C7] and C12–H12/Cg2 [H12/Cg2¼ 3.283 Å, :C12–H12/Cg2¼124.25�, Cg1: C14–C19], whichfurther stabilizes the above peculiar shape of complex 1(Fig. S5). Moreinterestingly, each molecule is connected with two adjacent ones viatwo pairs of symmetry-related edge-to-face (EF) stacking interactionsalong the c axis [edge and face represent phenyls of C14–C19 and C2–C7, H17/Cg¼ 3.209 Å, :C17–H17/Cg¼ 141.97�, symmetry code:1� x, 1� y, �z], and this intermolecular interactions are synchro-nously strengthened by two symmetry-related intermolecular C39–H39/O7 interactions between the coordinated DMF molecules andphenolic oxygens of ligands with length of H bonds 2.674(3) Å [18].Accordingly, each metallamacrocycle as one 8-connectd SBU isarranged into 1-D chain along c axis (Fig. S6a). Significantly, thesechains are further propagated into the 2-D supramolecular networkparallel to ac plane through double C–H/O hydrogen-bonds

Fig. 3. Diagrams of the 2-D network of complex 1 along the ac p

between oxygen of N-acyl tail and the coordinated DMF molecules(H45/O5¼ 2.529 Å, :C45–H45/O5¼161.75�, symmetry code:2� x, 1� y, 1� z) (Fig. 3; Fig. S6b, S7, S8).

3.3. Thermogravimetry analysis of compound 1

The TG curve of compound 1 exhibits two steps of weight losseswith the increasing temperature (Fig. S9). The first weight loss isabout 17.87% in the range of about 55–212 �C, correspondence tothe loss of six coordinated DMF molecules (calculated value18.63%). The second weight loss is 59.03% in the range of 320–476 �C, which may be resulted from the decomposition of thecompound.

3.4. Magnetic property of compound 1

The magnetic property of complex 1 has been investigated ata field of 2 KOe in the temperature range 2–300 K. As illustrated inFig. 4, the cmT value of title compound shows the presence of thestrongly antiferromagnetic coupling between the Fe(III) centers.The value of cmT at 300 K is 25.26 emu K mol�1, which is slightlysmaller than the sum value expected for six spin-only para-magnetic systems with S¼ 5/2(cmT¼ 26.25 emu K mol�1). Withdecreasing temperature, the cmT values first decrease graduallyuntil reaching 21.61 emu K mol�1 at 150 K, and then sharplydecrease to 0.23 emu K mol�1 at 2 K. The magnetic susceptibilitiesobey the Curie–Weiss expression c�1(T)¼ C/(T� q) with Curieconstant C¼ 26.71 cm3 K mol�1 and a negative Weiss constantq¼�71.39(1) K using the data within T> 70 K, indicating theantiferromagnetic interaction between the Fe(III) ions.

The magnetic superexchange interaction would propagatebetween the neighboring centers (J1), while the near-neighboringcenters (J2) and the opposite centers (J3) are neglected because ofthe distances of them being beyond 8.295(3) and 9.609(2) Å,respectively. With analogous coordination environment of metalions in compound 1, six Fe(III) centers are simplified to be arrangedin the symmetry of D6h. a least-square fiting for the data in thetemperature range 2–300 K gives the parameters J/k¼�6.42(1) K,g¼ 2.123 and the agreement factor F¼

P[(cobs� ccald)2/

cobs]¼ 1.21�10�4. The negative value of J further demonstrates anantiferromagnetic coupling between the paramagnetic centers.This antiferromagnetic nature of the complex 1 can be attributed tothe overlap through the N–N bridging group between the singleoccupied metal d-orbitals.

lane. (a) A ball and stick diagram. (b) A schematic diagram.

Fig. 4. Temperature dependence of cmT for complex 1 (the solid lines represent thebest fitting for complexes 1).

Y. Chen et al. / Solid State Sciences 12 (2010) 461–465 465

4. Conclusion

In conclusion, we have constructed a novel 18-memberedazaMC with N-(methyl-maleamic ester) terminal. The introduc-tion of H3-MMSHZ ligand provided not only unique configurationof molecule but also inter- and intra-molecular interactions,forming two-dimensional framework based upon metallamacro-cycle as SBUs. There is antiferromagnetic coupling between high-spin Fe(III) centers through N–N bridge, which is further sug-gested by a negative Weiss constant q¼�71.39(1) K and negativevalue of J/k¼�6.421 K.

Acknowledgment

We gratefully acknowledge the financial assistance of theNational Natural Science Foundation of China (20671048).

Appendix A. Supplementary materials

Crystallographic data for the structure reported in this paperhave been deposited in the Cambridge Crystallographic DataCenter, CCDC No. 686129. Copy of this information may be obtainedfree from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ(fax: 544 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk). Supplementary data associated with this

article can be found in the online version, at doi:10.1016/j.solidstatesciences.2009.12.008.

References

[1] (a) M.S. Lah, M.L. Kirk, W. Hatfield, V.L. Pecoraro, J. Chem. Soc. Chem. Commun.(1989) 1606;(b) G. Psomas, C. Dendrinou-Samara, M. Alexiou, A. Tsohos, C.P. Raptopoulou,A. Terzis, D.P. Kessissoglou, Inorg. Chem. 37 (1998) 6556;(c) G. Mezei, C.M. Zeleski, V.L. Pecoraro, Chem. Rev. 107 (2007) 4933.

[2] (a) D. Dragancea, V.B. Arion, S. Shova, E. Rentschler, N.V. Gerbeleu, Angew.Chem. Int. Ed. 44 (2005) 7938;(b) G.A. Ardizzoia, M.A. Angaroni, G. La Monica, F. Cariati, S. Cenini, M. Moret,N. Masciocchi, Inorg. Chem. 30 (1991) 4347.

[3] (a) B.R. Gibney, D.P. Kessissoglou, J.W. Kampf, V.L. Pecoraro, Inorg. Chem. 33(1994) 4840;(b) T. Afrati, C. Dendrinou-Samara, C.M. Zaleski, J.W. Kampf, V.L. Pecoraro,D.P. Kessissoglou, Inorg. Chem. Commun. 8 (2005) 1173.

[4] (a) B. Kwak, H. Rhee, S. Park, M.S. Lah, Inorg. Chem. 37 (1998) 3599;(b) J. Song, D. Moon, M.S. Lah, Bull. Korean Chem. Soc. 23 (2002) 708;(c) B. Kwak, H. Rhee, S. Park, M.S. Lah, Polyhedron 19 (2000) 1985;(d) I. Kim, B. Kwak, M.S. Lah, Inorg. Chim. Acta 317 (2001) 12.

[5] (a) S. Lin, S.X. Liu, B.Z. Lin, Inorg. Chim. Acta 328 (2002) 69;(b) S. Lin, S.X. Liu, B.Z. Lin, Chin. J. Inorg. Chem. 18 (2002) 1205;(c) S. Lin, S.X. Liu, J.Q. Huang, C.C. Lin, J. Chem. Soc. Dalton Trans. (2002) 1595.

[6] (a) B. Li, D.D. Han, G.Z. Cheng, Z.P. Ji, Inorg. Chem. Commun. 8 (2005) 216;(b) L.F. Jin, F.P. Xiao, G.Z. Cheng, Z.P. Ji, J. Organometallic. Chem. 691 (2006) 2909;(c) F.P. Xiao, L.F. Jin, G.Z. Cheng, Z.P. Ji, Inorg. Chim. Acta 360 (2007) 3341;(d) F.P. Xiao, L.F. Jin, G.Z. Cheng, Z.P. Ji, Polyhedron 26 (2007) 2695.

[7] (a) S. Lin, S.X. Liu, Z. Chen, B.Z. Lin, S. Gao, Inorg. Chem. 43 (2004) 2222;(b) R.P. John, K.J. Lee, B.J. Kim, B.J. Suh, H. Rhee, M.S. Lah, Inorg. Chem. 44(2005) 7109;(c) M. Park, R.P. John, D. Moon, K. Lee, G.H. Kimb, M.S. Lah, J. Chem. Soc. DaltonTrans. (2007) 5412.

[8] (a) S.X. Liu, S. Lin, B.Z. Lin, C.C. Lin, J.Q. Huang, Angew. Chem. Int. Ed. 40 (2001)1084;(b) J.M. Dou, M.L. Liu, D.C. Li, D.Q. Wang, Eur. J. Inorg. Chem. (2006) 4866.

[9] (a) R.P. John, K.J. Lee, M.S. Lah, Chem. Commun. (2004) 2660;(b) R.P. John, J. Park, D. Moon, K. Lee, M.S. Lah, Chem. Commun. (2006) 3699.

[10] R.P. John, M. Park, D. Moon, K. Lee, S. Hong, Y. Zou, C.S. Hong, M.S. Lah, J. Am.Chem. Soc. 129 (2007) 14142.

[11] K. Lee, R.P. John, M. Park, D. Moon, H.C. Ri, G.H. Kim, M.S. Lah, Dalton Trans.(2008) 131.

[12] D. Moon, K. Lee, R.P. John, G.H. Kim, B.J. Suh, M.S. Lah, Inorg. Chem. 45 (2006)7991.

[13] (a) Y. Bai, D. Dang, C. Duan, Y. Song, Q. Meng, Inorg. Chem. 44 (2005) 5972;(b) R.G. Raptis, H.H. Murray, J.P. Fackler, J. Chem. Soc. Chem. Commun. (1987) 737;(c) J.-C. Liu, J.-Z. Zhuang, X.-Z. You, X.-Y. Huang, Chem. Lett. (1999) 651.

[14] (a) M. Moon, I. Kim, M.S. Lah, Inorg. Chem. 39 (2000) 2710;(b) D. Moon, J. Song, B.J. Kim, B.J. Suh, M.S. Lah, Inorg. Chem. 43 (2004) 8230;(c) D. Moon, M.S. Lah, Inorg. Chem. 44 (2005) 1934;(d) A.C. Cutland-Van Noord, J.W. Kampf, V.L. Pecoraro, Angew. Chem. Int. Ed. 41(2002) 4668;(e) J.J. Bodwin, V.L. Pecoraro, Inorg. Chem. 39 (2000) 3434.

[15] F. Xiao, L. Jin, W. Luo, G. Cheng, Z. Ji, Solid State Sci. 10 (2008) 1358.[16] H.D. Yin, J.C. Cui, Y.L. Qiao, Inorg. Chem. Commun. 11 (2008) 684.[17] H. Adams, D.E. Fenton, G. Minardi, E. Mura, A.M. Pistuddi, C. Solinas, Inorg.

Chem. Commun. 3 (2000) 24.[18] (a) L.S. Reddy, S. Basavoju, V.R. Vangala, A. Nangia, Cryst. Growth Des. 6 (2006)

161;(b) T. Steiner, G.R. Desiraju, Chem. Commun. (1998) 891.