A manganese(II) dichloro-bridged one-dimensional polymer: Structural, fluorescence and low-temperature magnetic study

Inorganica Chimica Acta 359 (2006) 3841–3846 www.elsevier.com/locate/ica

A manganese(II) dichloro-bridged one-dimensional polymer: Structural, fluorescence and low-temperature magnetic study Arpi Majumder a, Matthias Westerhausen b, Alexander N. Kneifel b, Jean-Pascal Sutter c,*, Nathalie Daro c, Samiran Mitra a,* b c

a Department of Chemistry, Jadavpur University, Raja S. C. Mallik Road, Kolkata, West Bengal 700 032, India Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-University Jena, August-Bebel-Str. 2, D-07743 Jena, Germany Institut de Chimie de la Matie`re, Condense´e de Bordeaux, UPR 9048 CNRS, 87, Ave. Dr. Schweitzer, F-33608 PESSAC, France

Received 30 November 2005; received in revised form 31 March 2006; accepted 19 April 2006 Available online 11 May 2006

Abstract One-dimensional organic/inorganic composite coordination polymer has been synthesised by the reaction of manganese(II) chloride with the chelating bidentate ligand, 1,10-phenanthroline (1,10-phen). X-ray single crystal analysis shows a doubly chloride bridged 1-D polymer, [Mn(l-Cl)2(phen)]n (1), where manganese(II) ions possess octahedral environment. The complex is characterised by elemental analysis, different spectroscopic, electrochemical and low temperature magnetic susceptibility measurements. 1 exhibits strong fluorescence emission band at 410 nm and can serve as potential photoactive material as indicated from the characteristic fluorescence properties. Magnetic susceptibility measurements reveal a weak ferromagnetic interaction between the two high-spin Mn(II) ions of J = 0.017 cm1.  2006 Elsevier B.V. All rights reserved. Keywords: Mn(II) complex; Doubly chloro-bridged; Crystal structure; Fluorescence; Magnetic susceptibility study

1. Introduction The polymeric compounds have received much interest and importance in recent years, owing to not only their intriguing structural motifs but also due to their potential application in catalysis, medicine, host–guest chemistry and the promising photo-, electro-, and magnetic materials [1,2]. A number of one-dimensional chain coordination polymers have been reported. But, relatively few investigations of the structures and magnetic properties of halogenbridged Mn(II) polymers have been carried out [3,4]. In addition to the magnetic interest and the application of *

Corresponding authors. Present address: Laboratoire de Chimie de Coordination du CNRS, Universite´ Paul Sabatier, 205, route de, Narbonne, F-31077 Toulouse, France (J.-P. Sutter); Tel.: +91 33 2668 2017; fax: +91 33 2414 6414 (S. Mitra). E-mail address: [email protected] (S. Mitra). 0020-1693/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.04.044

manganese compounds in industrial catalysis [5], chloride-bridged Mn(II) compounds are attractive for the study of the role of chloride anions in the assembly of manganese ions in photosynthetic water oxidation systems [2]. Dichloro-bridged complexes have been used as models to understand the electron transfer process, the excited-state acid–base properties of inorganic systems and magnetic coupling interactions. The ferromagnetic exchange interactions in some of these complexes exhibit a dramatic rise in magnetic moment at low temperature and have been studied in detail [6]. Dichloro-bridged species have primarily been used as precursors for the preparation of monomeric ortho-metallated complexes [7]. Such compounds containing cyclometallating ligands have been exploited in electrophosphorescent complexes that are capable of generating pure red, green and blue light, are in high demand due to their potential use as dopants in full-colour organic light-emitting displays [8].

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The corresponding Mn(II) complexes Mn(II)X2L (L = bpy or phen, X = C1 or Br) have been described by a number of groups and are assumed to be polymeric octahedral complexes. This assumption is made on the basis of far-IR and low frequency Raman data [9]. However, the formulated polymeric octahedral structure of [Mn(lCl)2(phen)]n was never confirmed by a crystal and molecular structure determination [10]. As a part of our efforts to prepare polynuclear halogen-bridged complexes [11], we isolated crystals of [Mn(l-Cl)2(phen)]n and present here the synthesis, structural features, photoluminescence and magnetic susceptibility properties. There have been several reports on the halogen-bridged coordination polymers, however, the studies on their optical properties, such as fluorescence, are rare. 2. Experimental 2.1. Physical measurements The infrared spectrum of the complex was recorded on a Perkin–Elmer RX 1 FT-IR spectrophotometer with a KBr disc. Electrochemical study was performed on a CH 600 A cyclic voltammeter instrument using dimethylformamide as solvent for complex 1, and using tetrabutylammonium perchlorate as the supporting electrolyte. Perkin–Elmer Lambda 40 (UV–Vis) spectrophotometer and Spex Fluorolog II spectrofluorimeter were used for the electronic absorption and fluorescence measurements. The solutions of the 1,10-phen and the complex prepared in dimethylformamide and the OD for each of the solutions at the excitation wavelength was kept below 0.3. Magnetic susceptibility measurements were carried out with a Quantum Design MPMS-5S SQUID magnetometer under an applied magnetic field of 5000 Oe. Diamagnetic corrections were estimated from Pascal tables and magnetic data were corrected for diamagnetic contributions of the sample holder. 2.2. Materials All the chemicals and solvents used for the synthesis were of reagent grade. MnCl2 Æ 4H2O and 1,10-phenanthroline (Merck) were used as received.

Table 1 Crystallographic data of complex 1 Chemical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Reflections collected Independent reflections Density (calculated) (Mg/m3) Absorption coefficient (mm1) F(000) Crystal size (mm) h Range for data collection () R indices (all data) R1 wR2 Final R indices [I > 2r(I)] R1 wR2 ˚ 3) Largest differential peak and hole (e A

C12H8MnN2Cl2 306.04 monoclinic C2/c 17.1315(17) 10.4450(10) 7.0271(7) 90.00 111.402(2) 90.00 1170.7(2) 4 2577 796 1.736 1.557 612 0.3 · 0.2 · 0.1 4.66 < 2h < 46.50 0.0830 0.1497 0.0578 0.1282 1.157 and 0.533

2.4. Crystallographic data collection and refinement Data were collected on a Siemens P4 diffractometer with a Siemens SMART-CCD area detector with graphite ˚ ) using monochromated Mo–Ka radiation (k = 0.71073 A oil-coated rapidly cooled single crystals [12]. Crystallographic parameters [13], details of data collection and refinement procedures are summarised in Table 1. The structures were solved by direct methods and refined with the software packages SHELXL-93 and SHELXL-97 [14]. Neutral scattering factors were taken from Cromer and Mann [15] and for the hydrogen atoms from Stewart et al. [16]. The non-hydrogen atoms were refined anisotropically, the isotropic refinement of the hydrogen atoms gave reasonable structural parameters. 3. Results and discussion 3.1. Crystal and molecular structure of [Mn(l-Cl)2(phen)]n (1)

2.3. Synthesis The complex 1 was synthesised by adding a methanolic solution (10 ml) of 1,10-phen (0.180 g, 1 mmol) to a solution of manganese(II) chloride (0.198 g, 1 mmol) dissolved in methanol (15 ml). The resulting solution was stirred for 10 min and filtered. The mixture was kept at 10 C. After 4 days, yellow crystals suitable for X-ray crystal study were isolated. Yield: 0.26 g, 85%, Anal. Calc. for [Mn(l-Cl)2(phen)]n, C, 47.05; H, 2.61; N, 9.14. Found: C, 46.98; H, 2.58; N, 9.11%.

In the crystal structure of the title complex, [Mn(l-Cl)2(phen)]n units are linked by double l2-bridging chlorides to form one-dimensional zigzag chains that run along the c-axis. The geometry of the monomeric units in the coordination polymer is shown in Fig. 1. Relevant bond lengths and bond angles are given in Table 2. The coordination sphere of Mn(II) is a distorted octahedron with four bridging chlorides and two nitrogens from 1,10-phen as vertices – two chloride ligands reside in the equatorial plane trans to the 1,10-phen N-atoms and

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(tmen = Me2NCH2CH2NMe2) [19]. The shortest inter˚ , which is quite large chain Mn  Mn distance is 10.032 A compared with that observed in Mn(II) chain complexes in which the chains are connected by hydrogen bonds [3– 5b,20]. No classical hydrogen bonds occur in the crystal structure of [Mn(l-Cl)2(phen)]n. There may, however, be some weaker C–H  Cl interactions, as the C1–Cl1 distance is ˚. found to be 3.487(9) A 3.2. Infrared spectrum Mn–Cl stretch vibrations are found at 260 cm1 and the Mn–Cl bridging vibration in the range 220 cm1 which is also in agreement with the literature [21]. 3100, 1520, 1220, 1195, 1050, 875, 845, 785, 770, 642, 610, 520, 425, 320 cm1 are the characteristic bands of 1,10-phen observed in the spectrum of 1 [22]. Fig. 1. ORTEP view of three monomeric units within a single chain of complex 1.

The electronic spectrum of 1 was recorded in dimethylformamide and showed two strong bands at about 265 and 295 nm which can be assigned to p–p* transition in the ligand. The complex does not show any d–d transition [23].

Table 2 ˚ ) and angles () for complex 1 Bond lengths (A Mn(1)–N(1) Mn(1)–Cl(1) Cl(1)–Mn(1)–N(1) Cl(1)–Mn(1)–N(1B) Cl(1)–Mn(1)–Cl(1A) Cl(1)–Mn(1)–Cl(1B) Cl(1)–Mn(1)–Cl(1C)

2.256(6) Mn(1)–Cl(1A) 2.4821(17) 162.4(2) 92.4(2) 83.94(5) 102.82(9) 97.58(6)

3.3. Electronic spectrum

2.6499(16)

Cl(1A)–Mn(1)–N(1) 85.4(1) Cl(1A)–Mn(1)–N(1B) 92.7(1) Cl(1A)–Mn(1)–Cl(1C) 177.57(8) N(1)–Mn(1)–N(1B) 74.1(3) Mn(1)–Cl(1)–Mn(1A) 96.06(5)

the other two Cl anions occupy the apical sites with the trans-axial angle of 177.57(8). Each asymmetrically bridged chloride ligand exhibits distances of 2.4821(17) ˚ . The Mn2Cl2 diamond cores, which join and 2.6499(16) A inversion-related molecules, are strictly planar. Each 1,10-phen is bound to the manganese(II) center providing a five membered Mn–N–C–C–N-chelate ring. The bite angle value (N1–Mn1–N1a, 74.1(3)) is significantly smaller than the ideal value of 90 and is the primary source of distortion in the geometry which is in good agreement with the values found in structurally related l2-Cl complexes [10,17]. The degrees of deviation from an ideal octahedral geometry are reflected in the cisoid (74.1(3)– 102.82(9)) and transoid angles (162.38(16)–177.57(8)). The dihedral angle between each of the adjacent bridging planes is 102.82. The bridging angle Mn1–Cl–Mn1a of 96.06(6) is in accordance with those found in structurally related l2-complexes [10]. The crystal packing is shown in Fig. 2. ˚ is also comThe intrachain Mn  Mn distance 3.817 A parable to those found in double chloride-bridged diman˚ ganese(II) complexes [18], but shorter than the 3.94 A distance observed in polymeric [Mn(l-Cl)2(tmen)]n

3.4. Fluorescent properties The ligand 1,10-phen shows photoluminescence. The maximum of the structured emission for the ligand 1,10phen is located at 363 nm with a shoulder at about 425 nm. The broad structureless emission (charge transfer band) of the 1,10-phen at 525 nm corresponds to excimer emission of 1,10-phen. The emission spectrum of 1 is altogether different from that of the 1,10-phen and showing the main peak at 410 nm (Fig. 3). Absence of the eximer emission reflects a complete destruction of the stacking pattern. The excitation of the 1,10-phen along with 1 is presented in (Fig. 4), wherefrom it is evident that the complex has its own excitation spectrum different from that of 1,10-phen. The strongest excitation peak for 1 is at 370 nm, at a higher energy than that for 1,10-phen excitation peak, 390 nm. The vibrational progression of the excitation spectrum of the complex corresponds well with those of the emission spectra, reflecting that the molecule retains its ground state geometry in the excited state. 3.5. Electrochemical study Electrochemical property of the 1 has been studied by cyclic voltammetry in dimethylformamide medium (0.1 M KCl) with tetrabutylammonium perchlorate as supporting electrolyte at a scan rate of 50 mV s1. Complex 1 shows an oxidative response at 0.84 V versus SCE, which is assigned to the manganese(II)–manganese(III) oxidation and a reductive response at 0.65 V versus SCE assigned

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a

Mn Cl N C H

Emission intensity (arb. unit)

Emission Intensity (arb. unit)

Fig. 2. Packing diagram of complex 1 along the ab plane.

300

350

400

450

500

550

600

650

Wavelength/nm Fig. 3. Normalised emission spectra of 1,10-phen (m) and as-synthesised polymer (—) at room temperature (kex = 300 nm).

for manganese(III)–manganese(II) which is shown in (Fig. 5). This oxidation is quasi-reversible, characterised by a peak-to-peak separation (DEp) of 190 mV, which remains unchanged upon changing the scan rate. 3.6. Magnetic study The temperature dependence of the molar magnetic susceptibility, vM, for complex 1 was measured on polycrystalline powder sample in the temperature range 2–300 K, the

250

300

350

400

450

500

Wavelength/nm Fig. 4. Normalised excitation spectra of 1,10-phen (m) and as-synthesised polymer (—) at room temperature (monitoring wavelengths are 360 and 410 nm, respectively).

plot of vMT versus T is given in Fig. 6. At 300 K, the value found for vMT is 4.30 cm3 K mol1 in good agreement with the spin only value of 4.375 cm3 K mol1 anticipated for a Mn(II) ion (S = 5/2). This value remains constant down to 30 K and below this temperature it slightly increases to reach a maximum of 4.42 cm3 K mol1 at 5 K before decreasing. Such a behaviour suggests the existence of weak ferromagnetic interactions among the Mn ions through the double chloride bridges and weaker antiferromagnetic interactions between the chains. This latter con-

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Fig. 5. Cyclic voltammogram of compound 1 in dimethylformamide.

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For di-l-hydroxo-bridged copper compounds, a wellknown correlation holds between the J versus the bridging Cu–O–Cu angle and for a change of 1, a change of approx. 40 cm1 is expected in the J value [24]. For example, in the simpler case of certain chlorine-bridged copper(II) dimers, the balance between ferro- and antiferromagnetic exchange is reached at ca 94 [17b]. For di-l-chloride-bridged Mn(II) compounds such a quantitative correlation is not found. Ferromagnetic interactions among Mn(II) centres via chlorine bridges have been reported before [3b,24,25], but antiferromagnetic interactions in related systems are usually found [4b,26], pointing to the subtle roles played by geometry and topology in determining the nature and amplitude of chlorine mediated exchange coupling. Finally, it can be noticed that the ferromagnetic interaction found for [Mn(l-Cl)2(phen)]n, 1, with J = 0.017 cm1 is one order of magnitude smaller than for the related [Mn(l-Cl)2(bipy)]n that has been reported recently [3b].

4.5

4. Conclusion In this study, investigations by X-ray, spectroscopic and magnetic susceptibility studies of a dichloro-bridged Mn(II) polymer is reported. Magnetic susceptibility measurement is indicative for the existence of very week ferromagnetic interaction between the Mn(II) ions through the chloride bridges with weak antiferromagnetic interactions between the ions. Compound 1 possesses an intense fluorescence property at room temperature and it is suggested that it exhibits potential applications as photoactive material.

4.3 5 4

4.2 M (µ B)

M

3

-1

χ T (cm .K.mol )

4.4

4.1

3 2 1

4 0 0

10

20 30 H (kOe)

40

50

3.9

0

50

100

150

200

250

300

T (K) Fig. 6. Experimental (h) and calculated (—) vMT vs. T behavior for complex 1. (Inset. Field dependence of the magnetisation at 2 K.)

tribution is also supported by the field dependence of the magnetisation recorded at 2 K (Fig. 6 inset) with a magnetisation which tends to a saturation value of 4.5 lB, slightly below the value expected for an assembly of S = 5/2 spins without interactions. The vMT versus T data for compound 1 were analysed with the expression of the susceptibility P for a chain of n1 equally spaced classical-spin ðH ¼ J i¼1 SAi  SAi1 Þ recalled below [4a]. An excellent fit to the experimental data in the temperature domain 5–300 K was obtained (Fig. 6), confirming the extremely weak ferromagnetic exchange with J = 0.017 ± 0.0003 cm1 (g was fixed at 2.0). Ng2 b2 SðS þ 1Þ 1 þ u  3kT 1u    1 JSðS þ 1Þ JSðS þ 1Þ u ¼ coth  kT kT

v¼

5. Supplementary material Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 235763 for 1. Copy of this information may be obtained free of charge from The Director, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or http://www. ccdc.cam.ac.uk). Acknowledgements We acknowledge the financial assistance from DST (New Delhi, India). Our thanks are also extended to Dr. N. Chattopadhyay, Department of Chemistry, Jadavpur University, Kolkata-32, India, for fluorescence study and Dr. S. Bhattacharya, Department of Chemistry, Jadavpur University, Kolkata-32, India, for electrochemical study. This work was supported by the Centre FrancoIndien pour la promotion de la Recherche Avance´e/IndoFrench Centre for the Promotion of Advanced Research (Project 3108-3).

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