Synthesis, crystal structures and magnetic behaviors of two dicyanamide bridged di- and polynuclear complexes of cobalt(II) derived from 2,4,6-tris(2-pyridyl)1,3,5-triazine and imidazole

Polyhedron 28 (2009) 2436–2442

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Synthesis, crystal structures and magnetic behaviors of two dicyanamide bridged di- and polynuclear complexes of cobalt(II) derived from 2,4,6-tris(2-pyridyl)1,3,5-triazine and imidazole Anamika Das a, Christoph Marschner b, Joan Cano c,d, Judith Baumgartner b, Joan Ribas c, M. Salah El Fallah c,*, Samiran Mitra a,* a

Department of Chemistry, Jadavpur University, Raja S.C. Mallik Road, Kolkata, West Bengal 700032, India Institut für Anorganische Chemie, Technische Universität Graz, Austria Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès, 1-11, 08028-Barcelona, Spain d Centre de Recerca en Química Teórica, Universitat de Barcelona, Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain b c

a r t i c l e

i n f o

Article history: Received 27 February 2009 Accepted 20 April 2009 Available online 3 May 2009 Keywords: Cobalt(II) Tptz Dca Imz Magnetic property

a b s t r a c t Two new dicyanamido-bridged di- and polynuclear complexes of Co(II), [Co(dca)(tptz)(H2O)]22(ClO4) (1) and [Co(dca)2(imz)2]n (2) [dca, dicyanamide; tptz, 2,4,6-tris(2-pyridyl)1,3,5-triazine; and imz, imidazole] have been synthesized and characterized structurally, as well as magnetically. The X-ray single crystal structure determination of complex 1 shows that two symmetry related octahedral Co(II) ions are separated by dca ligand and other coordination sites are satisfied by tptz and aquo ligands. Each dinuclear unit is associated with each other by intramolecular hydrogen bonding interactions, giving rise to a 1D chain structure. On the other hand complex 2 is a 1D coordination polymer having [Co(II)(imz)2] units connected by double bridging dca ligands. These 1D chains interact through face-to-face p–p stacking interactions of the imz rings extending the dimensionality to a 2D supramolecular network. The variable temperature (300–2 K) magnetic measurements of both compounds reveal that dicyanamide exhibits a weak antiferromagnetic interaction between the metal centers. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In the past few decades, tremendous progress has been made in supramolecular chemistry. A large number of beautiful and functional supramolecular architectures have been constructed, starting with well-defined building blocks via self-assembly process [1–4]. In this process, the small molecular building blocks that recognizes each other through direction specific molecular interactions such as coordination bonds, hydrogen bond, p–p stacking and C–H–p interaction etc. to form the extended architecture [5]. The particular attractive is the novel type of structural motifs, like honeycomb, grid, interdigitation, different degree of interpenetration, three dimensional channels are obtained depending upon the metal ions and organic linkers with different tunable properties [1–5]. One strategy to build an extended network is to utilize certain features of the bridging ligands, such as azide, [6] dicyanamide [7], carboxylate [8] and different neutral organic spacer

* Corresponding authors. Tel.: +91 33 2668 2017; fax: +91 33 2414 6414 (S. Mitra). E-mail addresses: [email protected] (C. Marschner), baumgartner@ tugraz.at (J. Baumgartner), [email protected] (M.S. El Fallah), smitra_2002@ yahoo.com (S. Mitra). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.04.024

[9] having versatile binding modes, conformational flexibility and the ability to form non-covalent interactions [10]. Among the abovementioned bridging ligand, dca acts as a remarkable building blocks for the construction of various functional coordination architectures such as discrete mononuclear, dinuclear, as well as 1D chains, tubes, or ladders, 2D sheets and 3D supramolecular networks, when terminal or bridging coligands (L) are introduced to form M–dca–L system having important magnetic property [11–16]. The different coordination modes of dca such as terminal, bidentate, tridentate and unusual l4-dca towards metal ions plays an important role for the construction of extended architecture [7]. The magnetic properties of polynuclear architecture depend on the nature of the paramagnetic metal center i.e. the spin state, the dimensionality of the system and on the type of bridging ligands. Although dca can mediate only weak interactions between paramagnetic metal centers and a few multidimensional dca bridged compounds exhibit long-range ferromagnetic ordering [7]. Among the first row transition metal ions containing an unpaired electron, the magnetic properties of Co(II) complexes are interesting and are affected by the several factors such as single-ion effects, spin–orbit coupling, distortion from regular stereochemistry, electron delocalization, crystal field mixing of the excited states into the ground

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state etc. [17]. Thus, the interpretation of these magnetic results is challenging. To date, the synthesis of high spin polynuclear Co(II) complexes and their magnetic study at low temperature have been reported by several groups [18,19]. In contrast, there are only a few examples of structurally as well as magnetically characterized dca bridged discrete dinuclear compounds [15,16]. In this paper, we report the synthesis, structural characterizations and hence magnetic properties of two cobalt coordination compounds [Co(dca)(tptz)(H2O)]22(ClO4) (1), [Co(dca)2(imz)2]n (2) [where dca = dicyanamide; tptz = 2,4,6-tris(2-pyridyl)1,3,5-triazine; and imz, imidazole]. Here, dca is used as a bridging ligand and coordination environment is varied by N-donor ligands i.e. tptz for complex 1, and imz for complex 2. In complex 1, two symmetry related Co(II) atoms are separated through nitrile nitrogen atoms of dca ligand and tptz act as a chelating ligand. Each dinuclear unit is connected to each other by H-bonding forming 1D stair-case like structure. To the best of our knowledge this is the first example of dinuclear Co(II) complex bridged by dca ligand. Whereas complex 2 is a doubly end-to-end dca bridged 1D coordination polymer with trans coordinated imz ligands. The overall structure is stabilized by face-to-face p–p stacking and H-bonding interactions arising among the imz ligands. Variable temperature magnetic study of these complexes reveals weak antiferromagnetic coupling between the metal centers.

2. Experimental 2.1. Physical techniques Elemental analyses were carried out using a Perkin–Elmer 2400 II elemental analyzer. Infrared spectra were recorded on a Perkin– Elmer RX I FT-IR spectrophotometer using KBr pellets. Magnetic measurements were carried out in the ‘‘Servei de Magnetoquímica Universitat de Barcelona” on polycrystalline sample with a Quantum Design MPMS SQUID susceptometer operating at a magnetic field of approximately 1000 G between 2 and 300 K. The diamagnetic corrections were evaluated from Pascal’s constants for all the constituent atoms. 2.2. Materials High-purity 2,4,6-tris(2-pyridyl)-1,3,5-triazine and imidazole were purchased from the Aldrich Chemical Co. Inc. and used without further purification. High purity sodium dicyanamide was obtained from Fluka. All other chemicals were of AR grade. Caution: Perchlorate salts are potentially explosive in the presence of organic compounds. Only a small amount of the materials should be prepared and handled with care. 2.3. Syntheses 2.3.1. Synthesis of [Co(dca)(tptz)(H2O)]22(ClO4) (1) A methanolic solution of tptz (0.5 mmol, 0.15 g) was added to an aqueous solution (5 mL) of Co(ClO4)26H2O (1 mmol, 0.365 g) and sodium dicyanamide (2 mmol, 0.17 g) with continuous stirring for about 20 min. The resulting solution was filtered and the filtrate was kept at room temperature. After five days, orange crystals suitable for X-ray diffraction were obtained from the filtrate. Yield 84%. Anal. Calc. for 1 C40H28Co2N18O22(ClO4): C, 43.33; H, 2.34; N, 22.75. Found: C, 43.35; H, 2.33; N, 22.71%. 2.3.2. Synthesis of [Co(dca)2(imz)2]n (2) An aqueous solution (5 mL) of CoCl26H2O (1 mmol, 0.237 g) and sodium dicyanamide (2 mmol, 0.17 g) were mixed with constant stirring. Then an ethanolic solution of imidazole (1 mmol,

0.06 g) was added to the solution and stirred for about 30 min at room temperature. After three days, pink crystals suitable for Xray diffraction were obtained from the filtrate. Yield 95%. Anal. Calc. for 2 C10H8CoN10: C, 36.67; H, 2.44; N, 42.78. Found: C, 36.26; H, 2.45; N, 42.75%. 2.3.3. X-ray data collection and structure refinement Single crystal X-ray diffraction study for 1 was carried out on a Bruker Smart Apex CCD diffractometer using graphite-monochromatizd Mo Ka radiation (0.71073 Å). The data were corrected for absorption by using the multi-scans program SADABS [20]. The structure was solved by direct and difference Fourier methods and refined by full-matrix least-squares on F2 using SHELXL-97 [21] and SHELXS-97 program package [22], respectively. The single crystal data of complex 2 were collected on Enraf Nonius, CAD4 diffractometer and the structure was solved by direct methods and subsequently refined by full-matrix least-squares procedures on F2 with allowance for anisotropic thermal motion of all non-hydrogen atoms employing the WINGX package with the relevant programs SIR-97 [23] and SHELXL-97 [22]. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed geometrically and refined with isotropic thermal parameters. All calculations were carried out using PLATON and ORTEP-32 programs [24,25]. The unit-cell parameters, crystal system, space group and refinement details are summarized in Table 1. 3. Results and discussion 3.1. Infrared spectra Complexes 1 and 2 display strong absorption bands at 2280, 2242 and 2162 cm1, respectively, which correspond to the m(CN) of dca ligand [7]. The large shift towards high frequencies, compared with these of free dca (2232, 2179 cm1) confirmed that dca bridged between the metal centers. In complex 1, the bands in the region 1645–1390 cm1 and 950–900 cm1 are observed

Table 1 Crystal data and structure refinements of 1 and 2. Sample

1

2

Empirical formula M Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å) Z kMo Ka (Å) Dc [g cm3] l (mm1) F(0 0 0) h Range (°) Total data Unique data Observed data [I > 2r(I)] Nref; Npar Ra Rwb Rint Dqmax. (e Å3) Dqmin. (e Å3)

C40H28Co2N18O22(ClO4) 1107.55 triclinic  (No.2) P1 7.7755(16) 10.577(2) 13.985(3) 70.55(3) 79.30(3) 82.02(3) 1062 2 0.71073 1.732 0.992 560 1.6–24.7 7409 3548 2838 3548;333 0.0521 0.1167 0.045 0.91 0.40

C10H8CoN10 327.19 monoclinic P21/c (No.14) 6.5800(10) 14.1540(10) 7.395(2) 90 94.920(10) 90 686.2(2) 2 0.71069 1.584 2.758 3328 2.426.3 2759 2571 1017 257;1135 0.0575 0.2033 0.129 0.55 0.83

a b

P P R = (|FoFc|)/ |Fo|. P P Rw = { [w(|FoFc|)2]/ [w|Fo|2]}1/2.

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due to the C–N and C–C stretching and bending modes of the coordinated tptz ligand [26]. Besides this, IR spectrum of complex 1 shows absorption bands in the region of 3552–3200 cm1 and 1160–1085 cm1 which clearly indicate the presence of crystal water and perchlorate moiety respectively. Apart from this complex 2 a strong absorption band is observed in the range 1270– 750 cm1, which may be assigned as C–C, C–N stretching and bending modes of the coordinated imz ligand. 3.2. Description of the crystal structures

Table 2 Selected bond lengths (Å) and bond angles (°) for 1. Co1–O1 Co1–N4 Co1–N7 O1–Co1–N1 O1–Co1–N5 O1–Co1–N8 N1–Co1–N5 N1–Co1–N8 N4–Co1–N7 N5–Co1–N7 N7–Co1–N8

2.168(3) 2.176(3) 2.108(4) 83.75(12) 86.40(12) 87.43(13) 73.77(12) 170.50(14) 94.51(13) 93.02(13) 85.97(14)

Co1–N1 Co1–N5 Co1–N8 O1–Co1–N4 O1–Co1–N7 N1–Co1–N4 N1–Co1–N7 N4–Co1–N5 N4–Co1–N8 N5–Co1–N8

2.087(3) 2.196(3) 2.030(4) 89.64(12) 173.10(13) 73.53(12) 102.71(13) 147.30(12) 110.02(13) 102.22(14)

3.2.1. Crystal structure of [Co(dca)(tptz)(H2O)]22(ClO4) 1 The ORTEP diagram of 1 (Fig. 1) shows that this is a discrete dinuclear complex where two symmetry related Co(II) atoms are separated by two dca anions in an end-to-end fashion. Each asymmetric unit contain one cobalt(II) atom which is octahedrally coordinated by three nitrogen atoms of tptz ligand, two nitrogen atoms of dca ligands and one oxygen atoms of aqua ligand. The equatorial coordination sites are composed by two nitrogen atoms (N7, N8) of bridging dca, one oxygen atom (O1) of coordinated water molecule and one nitrogen (N1) atom of central triazine ring whereas the axial coordination sites are occupied by two pyridyl nitrogen atoms (N4, N5) of tptz ligand with bond angle between them 147.30(12). Between Co-N(tptz) and Co-N(dca) bond distances (2.087(3)–2.196(3) and 2.030(4)–2.108(4) Å, respectively) the former is significantly larger. The cobaltcobalt separation within dinuclear unit is 7.377(1) Å and comparable to similar type of [Cd(dca)(tptz)(H2O)]22(ClO4) system [27]. The deviation from ideal octahedral geometry is reflected by bond length and angles given in Table 2. Each dimer is connected to each other by H-bonding forming a stair-case 1D chain running along the crystallographic a axis (Fig. 2). The uncoordinated nitrogen atom (N6) of pyridyl ring of tptz ligand and coordinated water (H15wN6, 2.22(6) Å, Table 3) play a role in the formation of this type of architecture. The two perchlorate counter anions present in the crystal lattice balance the electronic charge of the complex and also involved in H-bonding with coordinated water molecule (Table 3).

3.3. Magnetic study and magneto-structural correlation of 1 and 2

3.2.2. Crystal structure of [Co(dca)2(imz)2]n 2 The molecular structure of 2 displays 1D polymeric pattern built up by cobalt ions, pairs of bridging dca and imz ligands (Fig. 3). The asymmetric unit contain one cobalt(II) atom which is coordinated through three nitrogen atoms coming from two bridging dca and one imz ligand, respectively. For symmetry reasons, the nitrogen atom N3, N4 and its symmetry related counter parts lie in the equatorial plane along with the cobalt atom whereas imz nitrogen atom (N1) and its symmetry related counter

The magnetic property of complex 1 in the form of vMT versus T plot (vM is the molar magnetic susceptibility for two Co(II) ions) is shown in Fig. 5. The value of vMT at 300 K is 6.019 cm3 K mol1 which is greater than that expected for two isolated spin-only ions (vMT = 3.74 cm3 K mol1 for two isolated S = 3/2 ions) indicating that an important orbital contribution is involved. The vMT value of complex 1 is continuously decreased from room temperature to 4.41357 cm3 K mol1 at 20 K. The shape of these curves until 20 K can indicate not only antiferromagnetic coupling but also

part lie in the trans-axial position with bond angle between them 180°. The CoN(nitrile) bond distances of 2.161(2) and 2.164(3) Å are slightly higher than that involving the imz of 2.101(2) Å and those are comparable to similar type of [Mn(dca)2(im)2]n system where as im = imidazole [28]. The slight deviation from ideal octahedral geometry is due to the equatorial bond lengths and angles (Table 4). Similar to 1 here, dca anions coordinate through the nitrile nitrogen atoms and span the metals at 7.395(1) Å corresponding to the length of the crystallographic c axis. The metal–metal separation through the dca anions is in good agreement with the distance of 7.370 (1) Å found in the anionic dicyanamidometallate ladder–like 1D polymer [(Ph4As)2{Co2(dca)6(H2O)}H2O CH3OH]n [29]. An interesting Hbonding network is formed by the amide nitrogen atom of the bridging dca ligands with the hydrogen atom of the secondary amine of imz ligands of a neighbouring unit (N2– H2, 0.86; H2N5, 2.13 Å, Table 3) that leads to a supramolecular 2D arrangement. Similar type of hydrogen bonding interactions are also observed in Co(biz)2(dca)2 (biz = 2,20 biimidazole) system, where 1D chain structure is formed [30]. This 2D arrangement is further stabilized by face-to-face p–p stacking interactions arising among the imz rings and the chains are interlocked at a distance of 3.851(2) Å which is shown in Fig. 4.

Fig. 1. ORTEP diagram (40% thermal ellipsoids) of 1 with atom labeling scheme. (Hydrogen atoms are removed for clarity.)

A. Das et al. / Polyhedron 28 (2009) 2436–2442

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Fig. 2. H-bonded 1D chain structure of 1. Perchlorate moieties are omitted for clarity.

spin–orbit coupling as typically observed for octahedral Co(II) ions. At low temperatures, there are significant differences in vMT values applying small magnetic fields which indicates the presence of a small amount of ferromagnetic or canted antiferromagnetic-ordered impurities, like a- or b-[Co(dca)2] [7]. This feature (presence of magnetic ordered impurities) is very frequent in most of the CodcaL derivatives [18] and all calculations given below refer to the sample with the smallest amount of impurities. On the other hand, for many years, the Lines’ theory has been utilized to obtain the exchange coupling in dinuclear and trinuclear Co(II) complexes [31]. In this theory, it is assumed a perfect octahedral high-spin Co(II) complex where the single-ion excited states are well separated (>9000 cm1) from the ground state and can be neglected. The 4T1g ground state is split into a sextet, a quartet and a Kramers’ doublet by spin–orbit coupling. The eigen values and eigen functions have been found by the diagonalization of the operator given in Table 3 Hydrogen bonding interactions (Å, °) for 1 and 2. Complex

D–HA

D–H

HA

DA

\D–HA

1

O1H15.N6i O1–H14O2 N2H2N5i

0.77(6) 1.07(6) 0.8600

2.22(6) 1.67(6) 2.1300

2.947(5) 2.736(4) 2.942(4)

159(7) 172(5) 158.00

2

Symmetry code: i = 1x, 1y, z for 1 and i = 2x, 1/2 + y, 1/2z for 2.

H¼

3 jk L  S 2

where j and k are the orbital reduction factor and the spin–orbit coupling parameters, respectively. The occurrence of the factor 3/2 is due to the analogy between the matrix elements of L within the 4T1 state and that of the –3/2L between the p functions. Applying Lines’ formula to fit the magnetic data for dinuclear Co(II) complexes, the poor results are obtained. This is due to the lack of consideration of typical distortion with regard to the perfect octahedral geometry. Herrera et al. [32] have adapted a new method to solve this problem in mononuclear systems assuming axial distortion. In this case, the triplet orbital 4T1g ground state splits into a singlet 4A2 and a doublet 4E levels with a D energy gap. The Hamiltonian involving the spin–orbit coupling, axial distortion and Zeeman interaction is given in

  1 H ¼ AjkL  S þ D L2z  LðL þ 1Þ þ lB ðAjL þ g e SÞH 3 where A, is the general factor that corresponds to 3/2 factor in Lines-equation. The factor A in the frame of T and P term isomorphism, which allows us to distinguish between the matrix elements of the orbital angular momentum operator calculated with the wave function of the ground 4T1g term with those calculated with P term basis. In the weak crystal field limit (B  Dq), c = 0 and A = 1.5 whereas in the strong crystal field limit (B  Dq), c = 1/2

Fig. 3. The coordination environment of Co(II) ions in 2 with atom labeling scheme, thermal ellipsoids are shown in 40% probability. (Hydrogen atoms are removed for clarity.)

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Table 4 Selected bond lengths (Å) and bond angles (°) for 2.

*

2.101(2) 2.164(3) 2.161(2) 89.53(9) 180.00 90.14(9) 90.47(9) 90.41(9) 90.41(9) 89.53(9) 89.59(9)

Co1N3 Co1N1* Co1N4* N1Co1N4 N1Co1N3* N3Co1N4 N3Co1N3* N1*Co1N4 N4Co1N4* N1*Co1N4*

6

2.161(2) 2.101(2) 2.164(3) 89.86(9) 90.47(9) 89.59(9) 180.00 90.14(9) 180.00 89.86(9)

χM T / cm3 K mol-1

Co1N1 Co1N4 Co1N3* N1Co1N3 N1Co1N1* N1Co1N4* N1*Co1N3 N3Co1N4* N3*Co1N4 N1*Co1N3* N3*Co1N4*

Symmetry code: 1x, y, 1z.

and A = 1. No analytical expression for the magnetic susceptibility, which depends on A, j, k and D can be derived. The values of these parameters have to be determined through numerical matrix diagonalization [31]. Although not yet fully developed in the literature about Co(II) dinuclear complexes, the mononuclear system can be extrapolated to a dinuclear one, introducing the J coupling parameter between the two pure S = 3/2 states [32]. In Fig. 6, we illustrate this new situation. Now, instead of four parameters A, j, k and D we will deal with five parameters A, j, k, D and JCoCo, respectively. The values of these parameters have to be determined, again through numerical matrix diagonalization and FORTRAN program adapted to do these calculations which has been performed by one of us [32]. The best-fit values found using the experimental data for T > 25 K were: Aj = 1.36, D = 430 cm1; JL–S = 163.5 cm1 with R = 4.4  104. Taking into account that Aj = 1.36 and JL–S = Ajk the real k value in this complex will be 120 cm1 (k for the free ion is 180 cm1). As it is shown in the Fig. 5, the calculated curve matches well the experimental data in the temperature range 28300 K. At T < 25, the magnetic data deviate from the calculated curve due to the presence of a small amount of ferromagnetic or canted antiferromagnetic-ordered impurities. Consequence of this, we cannot determine the exchange pathways JCoCo between the Co(II) atoms, which normally manifested in this range of temperatures. The field dependence of the reduced molar magnetization, M/Nb, at 2 K (Fig. 7), shows a saturation value at 50 kOe (2.29 Nb per cobalt(II) ion). The extrapolated saturation value is ca. 4.58 Nb, lie slightly above the expected value for two Co(II) centers.

5

4

3 0

50

100

150

200

250

300

T/ K Fig. 5. Plot of the vVN vs T for complex 1. Solid line represents the best fit obtained according to the method described in the text.

Real system JCo-Co L, S

L, S

Model system S(A) = 1 S(B) = 3/2 JL-S = Aκλ

JCo-Co

S(B) = 3/2 S(A) = 1 JL-S = Aκλ

Δ

g = -Aκ

Δ

ge = 2.00

ge = 2.00

g = -Aκ

Fig. 6. Schematic representation of the spin–orbit coupling and exchange coupling parameter (JCoCo) in a dinuclear Co(II) complex.

Although for a Co(II) the ground term is 4T1g (L = 1; S = 3/2) the experimental Nb value at 2 K does not correspond to three unpaired electrons, because at this low temperature only the J = 1/2 doublet (MJ = ±1/2) is populated behaving, thus, as an effective spin S0 = 1/

Fig. 4. Crystal packing of complex 2 showing three adjacent p interacting polymeric chains. (Hydrogen atoms are removed for clarity.)

A. Das et al. / Polyhedron 28 (2009) 2436–2442

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5

M / Nβ

4 3 2 1 0 0

15000

30000

45000

6000 0

H/G Fig. 7. Plot of the reduced magnetization (M/Nb) for 1 at 2 K (for two cobalt(II) ions). Solid line represents the Brillouin formula for two Co(II) ions isolated with S = 1, T = 2 K and Aj = 1.36.

2 with g = (10 + 2 Aj)/3 [33]. The noticeable D (distortion) parameter agrees with the great distortion created by the chelated dca and tptz ligands (N7CoN8 angle = 85.9° and N5CoN4 = 147.3°, instead of 90° and 180° for a regular octahedron). The magnetic properties of complex 2, in the form of vMT versus T plot (vM is the molar magnetic susceptibility for one Co(II) ion) is shown in Fig. 8. The value of vMT at 300 K is 3.26 cm3 K mol1, which is large than that expected for spin-only case (vMT = 1.87 cm3 K mol1 K, S = 3/2). The vMT values are continuously decreased from room temperature to 1.795 cm3 K mol1 at 2 K. The global feature is characteristic of weak antiferromagnetic interactions. The small polynuclear systems can also be fitted through sophisticated computer programs, based on full diagonalization methods at low temperature region (where the effective spin S0 = 1/2) [35,36]. One-dimensional systems of Co(II) atoms are frequently associated with anisotropic Ising systems, and they can be fitted in the low temperature zone assuming an effective spin S0 = 1/2 [37]. Rueff et al. [38] have proposed a phenomenological approach for some low-dimensional Co(II) systems that allows to have an estimate the strength of the antiferromagnetic exchange interactions. They have postulated the phenomenological equation:

vM T ¼ A expðE1 =kTÞ þ B expðE2 =kTÞ in which A + B equals the Curie constant [2.8–3.4 cm3 mol1 K for octahedral cobalt(II) ions], and E1, E2 represent the ‘‘activation energies” corresponding to the spin–orbit coupling and the antiferromagnetic exchange interaction, respectively. This equation adequately describes the spin–orbit coupling, which results in a splitting between discrete levels, and the exponential low-temperature divergence of the susceptibility. Very good results have been reported in one- and two-dimensional Co(II) complexes [39]. An important experimental feature in almost all octahedral Co(II) complexes is that vMT (or leff) values at room temperature are greater than one isolated spin-only ion (vMT = 1.87 cm3 mol1 K for a S = 3/2 ion), indicating that an important orbital contribution is involved. Typical values of vMT (or leff) are 2.753.4 cm3 mol1 K (4.75.2 lb) [32]. Lower values at r.t. indicate perturbation from ideal octahedral geometry [40]. An estimated J value has been calculated using the two exponential Rueff expression, mentioned above, which is suitable for any temperature greater than the possible Tc [34,35]. The fit values obtained with this procedure are: A + B = 3.43 cm3 K mol1, (A = 1.05 and B = 2.38) which perfectly agrees with those given in the literature for the Curie constant (C  2.8–3.4 cm3 K mol1), E1/k = 46.71 K, is of the same magnitude

Fig. 8. Plot of vMT vs T of polycrystalline complex of 2. Solid line corresponds to the best fit (see text).

than those reported by Rueff et al. for several one- and two-dimensional Co(II) complexes [38]. As for the value found for the antiferromagnetic exchange interaction, it is very week (E2/k = 0.500 K), corresponding to J = 1.0 K (=0.7 cm1) according to the Ising chain approximation, vMT / exp(J/2kT). The small J value is compatible with double l1,5-dca bridge ligand which always give almost negligible coupling parameters [18]. Thus we can conclude that complex 2 behaves as a one-dimensional system which is weakly magnetically coupled by dca ligand and also intramolecular p–p interactions. 4. Conclusions In this paper, we present structural and magnetic aspects of two novel dca bridged Co(II) complexes having two different coligands, namely tptz and imz. Complex 1 is a dinuclear Co(II) system extended to 1D chain through intermolecular H-bonding interactions. To the best of our knowledge, this is the first reported dicyanamido-bridged dinuclear Co(II) complex. In fact, dca bridged dinuclear complexes are few in literature, since dca has a strong tendency to form polynuclear structure. Probably, the bulkiness of the tptz ligand prevents the dinuclear moiety to propagate in a polynuclear fashion. Complex 2 is a 1D chain interlocked by face-to-face p–p stacking interactions to generate a 2D sheet. The magnetic behaviors of both the high-spin Co(II) complexes exhibit weak antiferromagnetic coupling. Further scope of this work aims at synthesis and study of different properties of various dca bridged complexes varying metal centers, as well as coligands. Supplementary data CCDC 697126 and 697128 contain the supplementary crystallographic data for 1 and 2. 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]. Acknowledgments Anamika Das is grateful to the Council of Scientific and Industrial Research, New Delhi, for providing a Research Associate ship to her. This work is also supported by AICTE New Delhi and by grants given by the Spanish (CTQ2006-01759, CTQ2006-03949) and Catalan (2005SGR-00593) governments.

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