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A manganese(II) coordination polymer with a 3D porous structure: Synthesis, structure, and spectroscopic and magnetic behaviour
Polyhedron 28 (2009) 860–864
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
A manganese(II) coordination polymer with a 3D porous structure: Synthesis, structure, and spectroscopic and magnetic behaviour Alfonso Castiñeiras, Isabel García-Santos *, José M. Varela Departamento de Química Inorgánica, Facultad de Farmacia, Universidad de Santiago de Compostela, Avda. de las Ciencias s/n, E-15782 Santiago de Compostela, Spain
a r t i c l e
i n f o
Article history: Received 1 December 2008 Accepted 19 December 2008 Available online 24 January 2009 Keywords: Manganese complex Dithiooxamide-based ligands Coordination polymer Crystal structure Magnetic behaviour
a b s t r a c t A manganese(II) complex with N,N0 -bis(carboxymethyl)dithiooxamide (H4GLYDTO), [Mn(H2GLYDTO)(H2O)2]n, has been synthesized and characterized by elemental analysis and thermogravimetric analysis, as well as by infrared, electronic and EPR spectroscopy and magnetic susceptibility measurements. The crystal and molecular structure of this complex was determined by single-crystal X-ray structure analysis. The compound shows a 3D porous framework with alternate left- and right-handed helical channels where the manganese(II) ions have an octahedral environment. Variable temperature magnetic measurements reveal the existence of very weak antiferromagnetic interactions through the syn-anti carboxylate bridge, with an exchange parameter of J/k = 0.12 K. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The study of polynuclear complexes is one of the most active areas in coordination chemistry. These compounds constitute common ground for two areas of current interest – molecular magnetism and metal sites in biology. Furthermore, in recent years coordination polymers have also been associated with considerable progress in the field of supramolecular chemistry and crystal engineering, mainly because of their intriguing structural motifs and other functional properties, such as molecular adsorption and luminescence [1]. Transition metal carboxylate complexes represent an example of these studies and, in particular, the synthesis and physical characterization of polynuclear manganese carboxylate complexes is now an area of great interest for two reasons. Firstly, for synthetic modelling of manganese metalloenzymes [2], e.g. photosynthetic oxidase or manganese catalase [3], and secondly, for the study of high ground state spin systems as new magnetic materials [4]. Polycarboxylate ligands are an attractive prospect in the synthesis of the latter materials as they provide an array of possible binding modes, including monodentate, bidentate, bridging and terminal. Consequently, a number of different structural types have been observed. These include discrete polynuclear clusters [5], two-dimensional chains [6] and three-dimensional layer structures [7] where the manganese ions are in the +2 and/or +3 oxidation state. Our research has focused on the controlled aggregation of transition metal centres using N,N0 -disubstituted dithiooxamide deriv* Corresponding author. Tel.: +34 981 528074; fax: +34 981 547163. E-mail address: [email protected] (I. García-Santos). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.030
atives of amino acids as ligands. These dithiooxamides are centrosymmetric and contain four labile hydrogen atoms [8]. These compounds generally act as hexadentate ligands through the nitrogen, sulfur and carboxylate oxygen atoms, forming dinuclear transition metal complexes with metal centres that are antiferromagnetically coupled [9]. In addition, the carboxylate groups are capable of acting as bridging groups between neighbouring molecules to form coordination polymers. In an effort to synthesize polynuclear compounds with high ground state spins, we have investigated the behaviour of these ligands in the presence of manganese and report here the synthesis, structure and magnetic behaviour of a three-dimensional Mn(II) coordination polymer with H4GLYDTO [N,N0 -bis(carboxymethyl)dithiooxamide] (Fig. 1), where the manganese centres are held together by two bridging carboxylato moieties. In contrast to the aforementioned dinuclear complexes, H4GLYDTO in the present compound serves only as a syn-anti carboxylate tetradentate bridge. This unexpected coordination fashion of the ligand leads to a 3D neutral framework. Magnetic measurements revealed that the compound has dominant antiferromagnetic coupling. 2. Experimental 2.1. Materials and physical measurements All reagents (commercial reagents) were purchased from Aldrich and used without further purification. The ligand H4GLYDTO was synthesized as described previously [10]. Elemental analyses were carried out on a Carlo Erba EA-1108 microanalyser. Infrared spectra in the 4000–400 cm1 range were recorded using
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A. Castiñeiras et al. / Polyhedron 28 (2009) 860–864 Table 1 Crystal data and structure refinement for [Mn(H2GLYDTO)(H2O)2]n.
Fig. 1. N,N0 -bis(carboxymethyl)dithiooxamide (H4GLYDTO).
KBr pellets on a Perkin–Elmer 597 spectrophotometer and in the 700–100 cm1 range using polyethylene-sandwiched Nujol mulls on a Bruker IFS-66v spectrophotometer. TGA analyses (25– 750 °C) were recorded under an inert atmosphere (N2) on a HiRes TGA 2950 TA Instruments. Evolved gases were detected by IR spectroscopy on a Bruker FTIR TENSOR-27 spectrophotometer. The electronic spectra of microcrystalline samples were obtained by the diffuse reflectance method on Shimadzu UV-3101PC spectrophotometers over the range 900–250 nm. EPR spectra were recorded between room temperature and 120 K on a Bruker ESP 300 spectrometer operating at X-band. The magnetic susceptibilities of polycrystalline samples were measured in the temperature range 2–300 K using a SQUID (Quantum Design) magnetometer. Diamagnetic corrections were made with Pascal’s constants. Caution: although difficulties were not encountered while working with metal perchlorates, they are potentially explosive and should be handled with care.
Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dcalc (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size h range (°) Limiting indices (h, k, l) Reflections collected/unique (Rint) Completeness h (°) Absorption corrected Max./min. transmission Data/parameters Goodness-of-fit on F2 Final R indices R indices (all data) Largest difference peak/hole
C6H10MnN2O6S2 325.22 293(2) 0.71973 monoclinic P21/c (No. 14) 11.526(2) 6.080(1) 8.726(2) 90 119.695(4) 90 572.4(2) 2 1.887 1.535 330 0.25 0.23 0.06 1.89–28.28 13/15, 0/7, 11/0 5458/1376 (0.0228) 96.8/28.28 SADABS
0.9135/0.7002 1376/79 1.119 R1 = 0.0257, wR1 = 0.0656 R1 = 0.0397, wR1 = 0.0684 0.271/0.278
2.2. Synthesis [Mn(H2GLYDTO)(H2O)2]n. A mixture of H4GLYDTO (1.25 mmol, 0.30 g) and Mn(ClO4)2 6H2O (2.25 mmol, 0.80 g) in water (50 ml) was stirred at room temperature for 30 min and then NaOH (1.25 mmol, 0.05 g) was added to the solution. The resulting pale brown powder was filtered off and washed with water and ethanol. Single crystals suitable for X-ray diffraction were obtained by slow evaporation (over a week) of the yellow mother liquor at room temperature. Their IR spectra show that the crystals and the original crystalline powder are the same compound. The yield for this reaction was 0.572 g (>95% based on total Mn). Mp. 141 °C (Dec). Anal. Calc. for C6H10MnN2O6S2 (325.22): C, 22.2; H, 3.1; N, 8.6; S, 19.7. Found: C, 21.8; H, 3.1; N, 8.5; S, 20.4%. IR (mmax/cm1): 3469m m(OH), 3137m m(NH), 1591s mas(COO), 1527s, 1437m, 1395s ms(COO), 1368s, 1301s, 1258w, 1078s, 980m, 923w, 879s, 865s, 763m, 711w, 646w, 547w, 502w, 468w, 394vw, 319w, 231w, 168w, 116w. 2.3. Crystal structure determination A yellow plate crystal of [Mn(H2GLYDTO)(H2O)2]n was mounted on a glass fibre and used for data collection. Crystal data were collected at 293(2) K using a Bruker SMART CCD 1000 diffractometer. 0 A) was Graphite monochromated Mo Ka radiation (k = 0.71073 Å used throughout. The data were processed with SAINT [11] and corrected for absorption using SADABS (transmissions factors: 1.000– 0.792) [12]. The structure was solved by direct methods using the program SHELXS-97 [13] and refined by full-matrix least-squares techniques against F2 using SHELXL-97 [14]. Positional and anisotropic atomic displacement parameters were refined for all nonhydrogen atoms. Hydrogen atoms were located in difference maps and included as fixed contributions riding on attached atoms, with isotropic thermal parameters 1.2 times those of their carrier atoms. Criteria for a satisfactory complete analysis were ratios of rms shift to standard deviation of less than 0.001 and no significant features in final difference maps. Atomic scattering factors were obtained from ‘‘International Tables for Crystallography” [15]. A summary
of the crystal data, experimental details and refinement results is given in Table 1. 3. Results and discussion [Mn(H2GLYDTO)(H2O)2]n was prepared using manganese(II) perchlorate hexahydrate as the metal source and [H2GLYDTO]2 as a dianionic ligand in an aqueous medium. The product is air stable and is insoluble in common organic solvents. An exhaustive comparison of the IR spectra of the ligand [10] and complex gave information about the mode of bonding of the ligand in the manganese(II) complex. The IR spectrum of the complex shows a broad absorption at 3469 cm1, corresponding to m(OH) vibrations of the coordinated water molecules, and another medium intensity band at 3137 cm1 that can be assigned to the – NH– vibrations. Three absorption bands at 1527, 1300 and 1078 cm1 are the so-called thioamide bands, which are essentially related to the C@S stretching vibration strongly coupled to that of the C–N part of the thioamide group. The m(C@S) band appears at 879 cm1. The band due to the carbonyl group at 1710 cm1, which is present in the spectrum of H4GLYDTO, is absent in the spectrum of the complex and two bands due to asymmetric and symmetric stretching vibrations of COO are observed at 1591 and 1394 cm1, respectively. The Dm = mas ms separation value is 197 cm1, indicating that the carboxyl groups of the complex are coordinated with the manganese(II) ions in a syn-anti carboxylato-bridging mode [16]. The band at 335 cm1 is assigned to the Mn–O stretching frequency. Thermogravimetric analysis revealed that, under an inert atmosphere, the compound is stable up to 141 °C and undergoes the first weight loss (12.4%) from 141 to 231 °C, which may be due to the release of the coordinated water molecules (calcd. 11.1%) and partial decomposition of the ligand. The relatively high temperature for the loss of water molecules implies that hydrogenbonding interactions occur in the crystal packing. The weight loss upon further heating to 740 °C occurs in three stages. This is due
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to complete decomposition of the compound and corresponds to the production of CO, CH3CHO and HNCS. However, other gases such as CO2, CH4 or NH3 were also identified depending on the stage in question. The final residue of 24.9% is close to the calculated value of 21.9% based on MnO. The diffuse reflectance spectrum of [Mn(H2GLYDTO)(H2O)2]n showed absorption bands at around 15 000, 20 850 and 21 900 cm1, which can be assigned to 6A1g ? 4T1g, 6A1g ? 4A1g and 4 Eg(G) transitions, respectively. Other absorption bands at 25440, 28530 and 30210 cm1 can also be assigned to 6A1g ? 4T2g(D), 4 Eg(D) and 4T1g(P) transitions. These transitions are compatible with a distorted octahedral geometry around high-spin Mn(II) [17]. The X-band EPR spectrum of [Mn(H2GLYDTO)(H2O)2]n shows an isotropic signal with a g-value of 2.0 at room temperature. This signal increases in intensity and becomes broad upon cooling to 120 K.
Fig. 3. Perspective view of the H2GLYDTO2 anion in [Mn(H2GLYDTO)(H2O)2]n illustrating its tetradentate character as a ligand. Symmetry transformations: a = x, ½ + y, ½ z; b = 1 x, ½ + y, 3/2 z; c = 1 + x, y, z.
4. Structure description A single-crystal X-ray diffraction study revealed that the asymmetric unit consists of one MnII, half a H2GLYDTO2 dianion and one water molecule (Fig. 2). Each MnII ion is six-coordinated to four carboxylate oxygen atoms from four different H2GLYDTO2 ligands and two symmetrically related water oxygen atoms (Fig. 3). As shown in Figs. 2 and 3, the Mn centre assumes a slightly distorted octahedral coordination geometry with the Mn–O bond distances 0 in the range 2.169(1)–2.188(2) Å A (Table 2). The carboxylate oxygen atoms are nearly coplanar and define the equatorial plane of the octahedron, with the bond angles around the neighbouring atoms close to 90° (89.26–90.74°). The oxygen atoms from symmetrically related aqua ligands occupy the axial positions of the octahedron, with bond angles to equatorial oxygen atoms in the range from 87.65(6)° to 92.36(6)°. H2GLYDTO2 acts as a tetradentate ligand and the carboxylate functionality links the metal centres in a syn-anti fashion (Fig. 4). The carboxylate groups link the neighbouring manganese centres to form infinite one-dimensional helical channels that run along the crystallographic0 b-axis with an intrachannel MnMn separation of 5.3176(9) Å A (Fig. 5). These Mn atoms are shared by neighbouring helical channels that are alternately left- and right-handed. In addition, these channels are interconnected by the O2C–C–N–C–C–N–C–CO2 groups of H2GLYDTO2 to form new alternate left- and right-handed0 helical chains, both with a distance between Mn atoms of 11.213 Å A, and this gives rise to an infinite three-dimensional framework (Fig. 6). When viewed along the a-axis, such a 3D framework can also be viewed
Fig. 2. Perspective view of the asymmetric unit of [Mn(H2GLYDTO)(H2O)2]n, showing the local coordination environment of the manganese. Symmetry transformations: a = x, 3/2 y, ½ + z; b = x, ½ + y, ½ z; c = x, 2 y, 1 z; e = 1 x, 2 y, 1 z.
Table 2 Selected bond distances and angles (Å/°) for [Mn(H2GLYDTO)(H2O)2]n. Distances Mn(1)–O(12)a Mn(1)–O(12)b Mn(1)–O(11) Mn(1)–O(11)c Mn(1)–O(1)c Mn(1)–O(1) Mn(1)Mn(1)a Mn(1)Mn(1)b Mn(1)Mn(1)d
2.1688(13) 2.1688(13) 2.1860(13) 2.1860(13) 2.1883(15) 2.1883(15) 5.3176(9) 5.3176(9) 5.3176(9)
Angles O(12)a–Mn(1)–O(12)b O(12)a–Mn(1)–O(11) O(12)b–Mn(1)–O(11) O(12)a–Mn(1)–O(11)c O(12)b–Mn(1)–O(11)c O(11)–Mn(1)–O(11)c O(12)a–Mn(1)–O(1)c O(12)b–Mn(1)–O(1)c O(11)–Mn(1)–O(1)c O(11)c–Mn(1)–O(1)c O(12)a–Mn(1)–O(1) O(12)b–Mn(1)–O(1) O(11)–Mn(1)–O(1) O(11)c–Mn(1)–O(1) O(1)c–Mn(1)–O(1)
180.0 90.73(5) 89.26(5) 89.26(5) 90.74(5) 180.0 87.65(6) 92.35(6) 88.90(5) 91.10(5) 92.35(6) 87.65(6) 91.10(5) 88.90(5) 180.0
Symmetry transformations: a = x, y + 3/2, z + 1/2; y + 2, -z + 1; d = x, y + 1/2, z + 1/2.
b
= x, y + 1/2, z + 1/2; c = x,
Fig. 4. Cross-sectional view down the b-axis showing the carboxylate functionality linking the MnII in a syn-anti fashion.
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Fig. 5. View of a fragment of the structure illustrating the helical channels.
Fig. 7. View of the [Mn(H2GLYDTO)(H2O)2]n.
3D
porous
framework
along
the
a-axis
in
Table 3 Hydrogen bonding interactions for [Mn(H2GLYDTO)(H2O)2]n.
Fig. 6. Perspective view, along the b-axis, of a fragment of the 3D structure showing left-handed (blue) and right-handed (green) helical channels. The gross helical pores are marked in yellow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
as a 3D porous coordination polymer containing new rectangular-0 shaped channels with Mn–Mn distances of 6.060 and 8.726 Å A (Fig. 7). Each Mn atom is therefore connected to four neighbouring Mn atoms through four H2GLYDTO2 units in an undulated shape and the metal centres thus serve as linkers to produce 2D layers parallel to the (1 0 0) plane. Consequently, the carboxylate groups also act as pillars to support these layers, leading to an extended open 3D framework. In addition, an X-ray crystallographic study revealed a supramolecular assembly that is the result of hydrogen-bonding interactions between coordinated water molecules and the organic ligands, and between organic ligands (Table 3). 4.1. Magnetic properties The magnetic susceptibility, v, of [Mn(H2GLYDTO)(H2O)2]n was investigated between 2 and 300 K. The room temperature effective magnetic moment leff [=(8vMT)1/2] is 5.93lB/Mn, which is consistent with the value of 5.92lB/Mn expected for a magnetically isolated high-spin MnII (S = 5/2) ion with g = 2.00. From the magnetic point of view, the complex can be considered as dinuclear, where
D–HA
d(D–H) (Å)
d(HA) (Å)
d(DA) (Å)
\(DHA) (°)
O(1)–H(1A)S(1)f O(1)–H(1B)O(12)c N(1)–H(1)S(1)e N(1)–H(1)S(1)a
0.96 0.87 0.88 0.88
2.34 2.00 2.42 2.72
3.292(2) 2.755(2) 2.930(2) 3.469(2)
171.7 144.0 116.9 144.0
Symmetry transformations: a = x, y 3/2, z + 1/2; y + 2, z + 1; f = x, y + 5/2, z + 1/2.
c
= x, y + 2, z + 1;
e
= x + 1,
two Mn(II) ions are linked by one carboxylate bridge. The magnetic susceptibilities are represented as a plot of vMT and vM versus T in Fig. 8. The experimental value at room temperature (4.40 cm3 K mol–1) decreases on decreasing the temperature, first slowly and then more rapidly. This behaviour is consistent with the presence of a weak antiferromagnetic interaction between the Mn(II) ions. In order to quantitatively evaluate these magnetic interactions the data were fitted to an S = 5/2 dinuclear spin model (Eq. (1)) based on the isotopic Heisenberg exchange Hamiltonian H = 2JS1S2 (where S1 = S2 = 5/2 for MnII high-spin d5 ion systems) [18]. N is Avogadro’s number, b is Bohr magneton, g is the Landé g-value, k is the Boltzmann constant.
vM ¼
Ng 2 b2 A 3kT B
A ¼ 6exp½2J=kT þ 30exp½6J=kT þ 84exp½12J=kT þ 180exp½20J=kT þ 330exp½30J=kT B ¼ 1 þ 3exp½2J=kT þ 5exp½6J=kT þ 7exp½12J=kT þ 9exp½20J=kT þ 11exp½30J=kT
ð1Þ
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awarded to I.G.-S. by the Xunta de Galicia. We thank Dr. T. Rojo (País Vasco) for helping with the magnetic studies.
Appendix A. Supplementary data CCDC 705437 contains the supplementary crystallographic data for [Mn(H2GLYDTO)(H2O)2]n. 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) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2008.12.030. References Fig. 8. Temperature dependence of vm and vmT for [Mn(H2GLYDTO)(H2O)2]n, solid lines represent the theoretical curves (see text).
The least-squares analysis of magnetic susceptibility data led to J/k = 0.12 K (0.08 cm1) for g = 2.01. The agreement factor is around 6.58 105, which corresponds to an excellent agreement between theory and experiment. This J value suggests that weak antiferromagnetic interactions between neighbouring Mn(II) ions are mediated through the double l2-carboxylate bridges. The J value is comparable to those reported for other manganese(II) complexes with the syn-anti bridging nature of the carboxylate, which generally mediates very weak magnetic interactions [19]. Due to the long superexchange pathway, the coupling through the compound is negligible. 5. Conclusions A novel hexa-coordinate 3D manganese(II) polymer containing alternate left- and right-handed helical channels was synthesized and structurally characterized. Weak antiferromagnetic interactions between MnII ions were found with J = 0.18 K and g = 2.00. Further investigations are currently underway into magnetic materials based on N,N0 -disubstituted dithiooxamide derivatives of amino acids as ligands with transition metals. Acknowledgements This work was supported by ERDF (EU) and DGI-MCYT (Spain) (Ref. CTQ2006-15329-C02), and by an Isidro Parga Pondal contract
[1] (a) J.L. Serrano, T. Sierra, Coord. Chem. Rev. 242 (2003) 73; (b) Y.F. Zhou, F.L. Jiang, D.Q. Yuan, B.L. Wu, R.H. Wang, Z.Z. Lin, M.C. Hong, Angew. Chem., Int. Ed. 43 (2004) 5665. [2] F. Wieghardt, Angew. Chem., Int. Ed. Engl. 28 (1989) 1153. [3] G.C. Dismukes, Chem. Rev. 35 (1996) 2024. [4] R.E.P. Winpenny, Coord. Chem. Rev. 216–217 (2001) 1 (SMMs). [5] S.M.J. Aubin, M.W. Wemple, D.M. Adams, H.-L. Tsai, G. Christou, D.N. Hendrickson, J. Am. Chem. Soc. 118 (1996) 7746. [6] R. Wang, D. Yuan, F. Jiang, L. Han, S. Gao, M. Hong, Eur. J. Inorg. Chem. (2006) 1649. [7] M. Tong, J. Wang, S. Hu, J. Solid. State Chem. 178 (2005) 1518. [8] M.C.F. Vidal, I. Lens, A. Castiñeiras, A. Matilla, J.M. Tercero, J. Niclós, Polyhedron 18 (1999) 3313. [9] (a) A. Castiñeiras, M.C.F. Vidal, J. Romero, R. Sáez, A. Matilla, J. Niclós, J.M. Tercero, Z. Anorg. Allg. Chem. 627 (2001) 1553; (b) J. Borrás, R. Carballo, A. Castineiras, M.C. Gómez-Conde, M.C. FernándezVidal, J. Niclós, Z. Anorg. Allg. Chem. 631 (2005) 2221. [10] A. Castiñeiras, M.C.F. Vidal, R. Carballo, Transition Met. Chem. 20 (1995) 477. [11] Bruker, SMART and SAINT, Area Detector Control and Integration Software, Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin, USA, 1997. [12] G.M. Sheldrick, SADABS, Program for Empirical Absorption Correction of Area Detector Data, University of Göttingen, Germany, 1997. [13] G.M. Sheldrick, Acta Crystallogr. A46 (1990) 467. [14] G.M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures, University of Göttingen, Germany, 1997. [15] A.J.C. Wilson, International Tables for Crystallography, vol. C, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1995. [16] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1997. [17] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd Ed., Elsevier, Amsterdam, 1986. [18] R.L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, 1986. [19] (a) S. Konar, S.C. Manna, E. Zangrando, T. Mallac, J. Ribas, N.R. Chaudhuri, Eur. J. Inorg. Chem. (2004) 4202; (b) J.-Y. Yhang, Y. Ma, A.-L. Cheng, Q. Yue, Q. Sun, E.-Q. Gao, Dalton Trans. (2008) 2061.
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
A manganese(II) coordination polymer with a 3D porous structure: Synthesis, structure, and spectroscopic and magnetic behaviour Alfonso Castiñeiras, Isabel García-Santos *, José M. Varela Departamento de Química Inorgánica, Facultad de Farmacia, Universidad de Santiago de Compostela, Avda. de las Ciencias s/n, E-15782 Santiago de Compostela, Spain
a r t i c l e
i n f o
Article history: Received 1 December 2008 Accepted 19 December 2008 Available online 24 January 2009 Keywords: Manganese complex Dithiooxamide-based ligands Coordination polymer Crystal structure Magnetic behaviour
a b s t r a c t A manganese(II) complex with N,N0 -bis(carboxymethyl)dithiooxamide (H4GLYDTO), [Mn(H2GLYDTO)(H2O)2]n, has been synthesized and characterized by elemental analysis and thermogravimetric analysis, as well as by infrared, electronic and EPR spectroscopy and magnetic susceptibility measurements. The crystal and molecular structure of this complex was determined by single-crystal X-ray structure analysis. The compound shows a 3D porous framework with alternate left- and right-handed helical channels where the manganese(II) ions have an octahedral environment. Variable temperature magnetic measurements reveal the existence of very weak antiferromagnetic interactions through the syn-anti carboxylate bridge, with an exchange parameter of J/k = 0.12 K. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The study of polynuclear complexes is one of the most active areas in coordination chemistry. These compounds constitute common ground for two areas of current interest – molecular magnetism and metal sites in biology. Furthermore, in recent years coordination polymers have also been associated with considerable progress in the field of supramolecular chemistry and crystal engineering, mainly because of their intriguing structural motifs and other functional properties, such as molecular adsorption and luminescence [1]. Transition metal carboxylate complexes represent an example of these studies and, in particular, the synthesis and physical characterization of polynuclear manganese carboxylate complexes is now an area of great interest for two reasons. Firstly, for synthetic modelling of manganese metalloenzymes [2], e.g. photosynthetic oxidase or manganese catalase [3], and secondly, for the study of high ground state spin systems as new magnetic materials [4]. Polycarboxylate ligands are an attractive prospect in the synthesis of the latter materials as they provide an array of possible binding modes, including monodentate, bidentate, bridging and terminal. Consequently, a number of different structural types have been observed. These include discrete polynuclear clusters [5], two-dimensional chains [6] and three-dimensional layer structures [7] where the manganese ions are in the +2 and/or +3 oxidation state. Our research has focused on the controlled aggregation of transition metal centres using N,N0 -disubstituted dithiooxamide deriv* Corresponding author. Tel.: +34 981 528074; fax: +34 981 547163. E-mail address: [email protected] (I. García-Santos). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.030
atives of amino acids as ligands. These dithiooxamides are centrosymmetric and contain four labile hydrogen atoms [8]. These compounds generally act as hexadentate ligands through the nitrogen, sulfur and carboxylate oxygen atoms, forming dinuclear transition metal complexes with metal centres that are antiferromagnetically coupled [9]. In addition, the carboxylate groups are capable of acting as bridging groups between neighbouring molecules to form coordination polymers. In an effort to synthesize polynuclear compounds with high ground state spins, we have investigated the behaviour of these ligands in the presence of manganese and report here the synthesis, structure and magnetic behaviour of a three-dimensional Mn(II) coordination polymer with H4GLYDTO [N,N0 -bis(carboxymethyl)dithiooxamide] (Fig. 1), where the manganese centres are held together by two bridging carboxylato moieties. In contrast to the aforementioned dinuclear complexes, H4GLYDTO in the present compound serves only as a syn-anti carboxylate tetradentate bridge. This unexpected coordination fashion of the ligand leads to a 3D neutral framework. Magnetic measurements revealed that the compound has dominant antiferromagnetic coupling. 2. Experimental 2.1. Materials and physical measurements All reagents (commercial reagents) were purchased from Aldrich and used without further purification. The ligand H4GLYDTO was synthesized as described previously [10]. Elemental analyses were carried out on a Carlo Erba EA-1108 microanalyser. Infrared spectra in the 4000–400 cm1 range were recorded using
861
A. Castiñeiras et al. / Polyhedron 28 (2009) 860–864 Table 1 Crystal data and structure refinement for [Mn(H2GLYDTO)(H2O)2]n.
Fig. 1. N,N0 -bis(carboxymethyl)dithiooxamide (H4GLYDTO).
KBr pellets on a Perkin–Elmer 597 spectrophotometer and in the 700–100 cm1 range using polyethylene-sandwiched Nujol mulls on a Bruker IFS-66v spectrophotometer. TGA analyses (25– 750 °C) were recorded under an inert atmosphere (N2) on a HiRes TGA 2950 TA Instruments. Evolved gases were detected by IR spectroscopy on a Bruker FTIR TENSOR-27 spectrophotometer. The electronic spectra of microcrystalline samples were obtained by the diffuse reflectance method on Shimadzu UV-3101PC spectrophotometers over the range 900–250 nm. EPR spectra were recorded between room temperature and 120 K on a Bruker ESP 300 spectrometer operating at X-band. The magnetic susceptibilities of polycrystalline samples were measured in the temperature range 2–300 K using a SQUID (Quantum Design) magnetometer. Diamagnetic corrections were made with Pascal’s constants. Caution: although difficulties were not encountered while working with metal perchlorates, they are potentially explosive and should be handled with care.
Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dcalc (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size h range (°) Limiting indices (h, k, l) Reflections collected/unique (Rint) Completeness h (°) Absorption corrected Max./min. transmission Data/parameters Goodness-of-fit on F2 Final R indices R indices (all data) Largest difference peak/hole
C6H10MnN2O6S2 325.22 293(2) 0.71973 monoclinic P21/c (No. 14) 11.526(2) 6.080(1) 8.726(2) 90 119.695(4) 90 572.4(2) 2 1.887 1.535 330 0.25 0.23 0.06 1.89–28.28 13/15, 0/7, 11/0 5458/1376 (0.0228) 96.8/28.28 SADABS
0.9135/0.7002 1376/79 1.119 R1 = 0.0257, wR1 = 0.0656 R1 = 0.0397, wR1 = 0.0684 0.271/0.278
2.2. Synthesis [Mn(H2GLYDTO)(H2O)2]n. A mixture of H4GLYDTO (1.25 mmol, 0.30 g) and Mn(ClO4)2 6H2O (2.25 mmol, 0.80 g) in water (50 ml) was stirred at room temperature for 30 min and then NaOH (1.25 mmol, 0.05 g) was added to the solution. The resulting pale brown powder was filtered off and washed with water and ethanol. Single crystals suitable for X-ray diffraction were obtained by slow evaporation (over a week) of the yellow mother liquor at room temperature. Their IR spectra show that the crystals and the original crystalline powder are the same compound. The yield for this reaction was 0.572 g (>95% based on total Mn). Mp. 141 °C (Dec). Anal. Calc. for C6H10MnN2O6S2 (325.22): C, 22.2; H, 3.1; N, 8.6; S, 19.7. Found: C, 21.8; H, 3.1; N, 8.5; S, 20.4%. IR (mmax/cm1): 3469m m(OH), 3137m m(NH), 1591s mas(COO), 1527s, 1437m, 1395s ms(COO), 1368s, 1301s, 1258w, 1078s, 980m, 923w, 879s, 865s, 763m, 711w, 646w, 547w, 502w, 468w, 394vw, 319w, 231w, 168w, 116w. 2.3. Crystal structure determination A yellow plate crystal of [Mn(H2GLYDTO)(H2O)2]n was mounted on a glass fibre and used for data collection. Crystal data were collected at 293(2) K using a Bruker SMART CCD 1000 diffractometer. 0 A) was Graphite monochromated Mo Ka radiation (k = 0.71073 Å used throughout. The data were processed with SAINT [11] and corrected for absorption using SADABS (transmissions factors: 1.000– 0.792) [12]. The structure was solved by direct methods using the program SHELXS-97 [13] and refined by full-matrix least-squares techniques against F2 using SHELXL-97 [14]. Positional and anisotropic atomic displacement parameters were refined for all nonhydrogen atoms. Hydrogen atoms were located in difference maps and included as fixed contributions riding on attached atoms, with isotropic thermal parameters 1.2 times those of their carrier atoms. Criteria for a satisfactory complete analysis were ratios of rms shift to standard deviation of less than 0.001 and no significant features in final difference maps. Atomic scattering factors were obtained from ‘‘International Tables for Crystallography” [15]. A summary
of the crystal data, experimental details and refinement results is given in Table 1. 3. Results and discussion [Mn(H2GLYDTO)(H2O)2]n was prepared using manganese(II) perchlorate hexahydrate as the metal source and [H2GLYDTO]2 as a dianionic ligand in an aqueous medium. The product is air stable and is insoluble in common organic solvents. An exhaustive comparison of the IR spectra of the ligand [10] and complex gave information about the mode of bonding of the ligand in the manganese(II) complex. The IR spectrum of the complex shows a broad absorption at 3469 cm1, corresponding to m(OH) vibrations of the coordinated water molecules, and another medium intensity band at 3137 cm1 that can be assigned to the – NH– vibrations. Three absorption bands at 1527, 1300 and 1078 cm1 are the so-called thioamide bands, which are essentially related to the C@S stretching vibration strongly coupled to that of the C–N part of the thioamide group. The m(C@S) band appears at 879 cm1. The band due to the carbonyl group at 1710 cm1, which is present in the spectrum of H4GLYDTO, is absent in the spectrum of the complex and two bands due to asymmetric and symmetric stretching vibrations of COO are observed at 1591 and 1394 cm1, respectively. The Dm = mas ms separation value is 197 cm1, indicating that the carboxyl groups of the complex are coordinated with the manganese(II) ions in a syn-anti carboxylato-bridging mode [16]. The band at 335 cm1 is assigned to the Mn–O stretching frequency. Thermogravimetric analysis revealed that, under an inert atmosphere, the compound is stable up to 141 °C and undergoes the first weight loss (12.4%) from 141 to 231 °C, which may be due to the release of the coordinated water molecules (calcd. 11.1%) and partial decomposition of the ligand. The relatively high temperature for the loss of water molecules implies that hydrogenbonding interactions occur in the crystal packing. The weight loss upon further heating to 740 °C occurs in three stages. This is due
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to complete decomposition of the compound and corresponds to the production of CO, CH3CHO and HNCS. However, other gases such as CO2, CH4 or NH3 were also identified depending on the stage in question. The final residue of 24.9% is close to the calculated value of 21.9% based on MnO. The diffuse reflectance spectrum of [Mn(H2GLYDTO)(H2O)2]n showed absorption bands at around 15 000, 20 850 and 21 900 cm1, which can be assigned to 6A1g ? 4T1g, 6A1g ? 4A1g and 4 Eg(G) transitions, respectively. Other absorption bands at 25440, 28530 and 30210 cm1 can also be assigned to 6A1g ? 4T2g(D), 4 Eg(D) and 4T1g(P) transitions. These transitions are compatible with a distorted octahedral geometry around high-spin Mn(II) [17]. The X-band EPR spectrum of [Mn(H2GLYDTO)(H2O)2]n shows an isotropic signal with a g-value of 2.0 at room temperature. This signal increases in intensity and becomes broad upon cooling to 120 K.
Fig. 3. Perspective view of the H2GLYDTO2 anion in [Mn(H2GLYDTO)(H2O)2]n illustrating its tetradentate character as a ligand. Symmetry transformations: a = x, ½ + y, ½ z; b = 1 x, ½ + y, 3/2 z; c = 1 + x, y, z.
4. Structure description A single-crystal X-ray diffraction study revealed that the asymmetric unit consists of one MnII, half a H2GLYDTO2 dianion and one water molecule (Fig. 2). Each MnII ion is six-coordinated to four carboxylate oxygen atoms from four different H2GLYDTO2 ligands and two symmetrically related water oxygen atoms (Fig. 3). As shown in Figs. 2 and 3, the Mn centre assumes a slightly distorted octahedral coordination geometry with the Mn–O bond distances 0 in the range 2.169(1)–2.188(2) Å A (Table 2). The carboxylate oxygen atoms are nearly coplanar and define the equatorial plane of the octahedron, with the bond angles around the neighbouring atoms close to 90° (89.26–90.74°). The oxygen atoms from symmetrically related aqua ligands occupy the axial positions of the octahedron, with bond angles to equatorial oxygen atoms in the range from 87.65(6)° to 92.36(6)°. H2GLYDTO2 acts as a tetradentate ligand and the carboxylate functionality links the metal centres in a syn-anti fashion (Fig. 4). The carboxylate groups link the neighbouring manganese centres to form infinite one-dimensional helical channels that run along the crystallographic0 b-axis with an intrachannel MnMn separation of 5.3176(9) Å A (Fig. 5). These Mn atoms are shared by neighbouring helical channels that are alternately left- and right-handed. In addition, these channels are interconnected by the O2C–C–N–C–C–N–C–CO2 groups of H2GLYDTO2 to form new alternate left- and right-handed0 helical chains, both with a distance between Mn atoms of 11.213 Å A, and this gives rise to an infinite three-dimensional framework (Fig. 6). When viewed along the a-axis, such a 3D framework can also be viewed
Fig. 2. Perspective view of the asymmetric unit of [Mn(H2GLYDTO)(H2O)2]n, showing the local coordination environment of the manganese. Symmetry transformations: a = x, 3/2 y, ½ + z; b = x, ½ + y, ½ z; c = x, 2 y, 1 z; e = 1 x, 2 y, 1 z.
Table 2 Selected bond distances and angles (Å/°) for [Mn(H2GLYDTO)(H2O)2]n. Distances Mn(1)–O(12)a Mn(1)–O(12)b Mn(1)–O(11) Mn(1)–O(11)c Mn(1)–O(1)c Mn(1)–O(1) Mn(1)Mn(1)a Mn(1)Mn(1)b Mn(1)Mn(1)d
2.1688(13) 2.1688(13) 2.1860(13) 2.1860(13) 2.1883(15) 2.1883(15) 5.3176(9) 5.3176(9) 5.3176(9)
Angles O(12)a–Mn(1)–O(12)b O(12)a–Mn(1)–O(11) O(12)b–Mn(1)–O(11) O(12)a–Mn(1)–O(11)c O(12)b–Mn(1)–O(11)c O(11)–Mn(1)–O(11)c O(12)a–Mn(1)–O(1)c O(12)b–Mn(1)–O(1)c O(11)–Mn(1)–O(1)c O(11)c–Mn(1)–O(1)c O(12)a–Mn(1)–O(1) O(12)b–Mn(1)–O(1) O(11)–Mn(1)–O(1) O(11)c–Mn(1)–O(1) O(1)c–Mn(1)–O(1)
180.0 90.73(5) 89.26(5) 89.26(5) 90.74(5) 180.0 87.65(6) 92.35(6) 88.90(5) 91.10(5) 92.35(6) 87.65(6) 91.10(5) 88.90(5) 180.0
Symmetry transformations: a = x, y + 3/2, z + 1/2; y + 2, -z + 1; d = x, y + 1/2, z + 1/2.
b
= x, y + 1/2, z + 1/2; c = x,
Fig. 4. Cross-sectional view down the b-axis showing the carboxylate functionality linking the MnII in a syn-anti fashion.
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Fig. 5. View of a fragment of the structure illustrating the helical channels.
Fig. 7. View of the [Mn(H2GLYDTO)(H2O)2]n.
3D
porous
framework
along
the
a-axis
in
Table 3 Hydrogen bonding interactions for [Mn(H2GLYDTO)(H2O)2]n.
Fig. 6. Perspective view, along the b-axis, of a fragment of the 3D structure showing left-handed (blue) and right-handed (green) helical channels. The gross helical pores are marked in yellow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
as a 3D porous coordination polymer containing new rectangular-0 shaped channels with Mn–Mn distances of 6.060 and 8.726 Å A (Fig. 7). Each Mn atom is therefore connected to four neighbouring Mn atoms through four H2GLYDTO2 units in an undulated shape and the metal centres thus serve as linkers to produce 2D layers parallel to the (1 0 0) plane. Consequently, the carboxylate groups also act as pillars to support these layers, leading to an extended open 3D framework. In addition, an X-ray crystallographic study revealed a supramolecular assembly that is the result of hydrogen-bonding interactions between coordinated water molecules and the organic ligands, and between organic ligands (Table 3). 4.1. Magnetic properties The magnetic susceptibility, v, of [Mn(H2GLYDTO)(H2O)2]n was investigated between 2 and 300 K. The room temperature effective magnetic moment leff [=(8vMT)1/2] is 5.93lB/Mn, which is consistent with the value of 5.92lB/Mn expected for a magnetically isolated high-spin MnII (S = 5/2) ion with g = 2.00. From the magnetic point of view, the complex can be considered as dinuclear, where
D–HA
d(D–H) (Å)
d(HA) (Å)
d(DA) (Å)
\(DHA) (°)
O(1)–H(1A)S(1)f O(1)–H(1B)O(12)c N(1)–H(1)S(1)e N(1)–H(1)S(1)a
0.96 0.87 0.88 0.88
2.34 2.00 2.42 2.72
3.292(2) 2.755(2) 2.930(2) 3.469(2)
171.7 144.0 116.9 144.0
Symmetry transformations: a = x, y 3/2, z + 1/2; y + 2, z + 1; f = x, y + 5/2, z + 1/2.
c
= x, y + 2, z + 1;
e
= x + 1,
two Mn(II) ions are linked by one carboxylate bridge. The magnetic susceptibilities are represented as a plot of vMT and vM versus T in Fig. 8. The experimental value at room temperature (4.40 cm3 K mol–1) decreases on decreasing the temperature, first slowly and then more rapidly. This behaviour is consistent with the presence of a weak antiferromagnetic interaction between the Mn(II) ions. In order to quantitatively evaluate these magnetic interactions the data were fitted to an S = 5/2 dinuclear spin model (Eq. (1)) based on the isotopic Heisenberg exchange Hamiltonian H = 2JS1S2 (where S1 = S2 = 5/2 for MnII high-spin d5 ion systems) [18]. N is Avogadro’s number, b is Bohr magneton, g is the Landé g-value, k is the Boltzmann constant.
vM ¼
Ng 2 b2 A 3kT B
A ¼ 6exp½2J=kT þ 30exp½6J=kT þ 84exp½12J=kT þ 180exp½20J=kT þ 330exp½30J=kT B ¼ 1 þ 3exp½2J=kT þ 5exp½6J=kT þ 7exp½12J=kT þ 9exp½20J=kT þ 11exp½30J=kT
ð1Þ
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awarded to I.G.-S. by the Xunta de Galicia. We thank Dr. T. Rojo (País Vasco) for helping with the magnetic studies.
Appendix A. Supplementary data CCDC 705437 contains the supplementary crystallographic data for [Mn(H2GLYDTO)(H2O)2]n. 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) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2008.12.030. References Fig. 8. Temperature dependence of vm and vmT for [Mn(H2GLYDTO)(H2O)2]n, solid lines represent the theoretical curves (see text).
The least-squares analysis of magnetic susceptibility data led to J/k = 0.12 K (0.08 cm1) for g = 2.01. The agreement factor is around 6.58 105, which corresponds to an excellent agreement between theory and experiment. This J value suggests that weak antiferromagnetic interactions between neighbouring Mn(II) ions are mediated through the double l2-carboxylate bridges. The J value is comparable to those reported for other manganese(II) complexes with the syn-anti bridging nature of the carboxylate, which generally mediates very weak magnetic interactions [19]. Due to the long superexchange pathway, the coupling through the compound is negligible. 5. Conclusions A novel hexa-coordinate 3D manganese(II) polymer containing alternate left- and right-handed helical channels was synthesized and structurally characterized. Weak antiferromagnetic interactions between MnII ions were found with J = 0.18 K and g = 2.00. Further investigations are currently underway into magnetic materials based on N,N0 -disubstituted dithiooxamide derivatives of amino acids as ligands with transition metals. Acknowledgements This work was supported by ERDF (EU) and DGI-MCYT (Spain) (Ref. CTQ2006-15329-C02), and by an Isidro Parga Pondal contract
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