Synthesis, structure and magnetic properties of a 1D coordination polymer of Cu(II) containing phenoxido and dicyanamido bridging groups

Polyhedron 81 (2014) 282–289

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Synthesis, structure and magnetic properties of a 1D coordination polymer of Cu(II) containing phenoxido and dicyanamido bridging groups Rahman Bikas a,b,⇑, Hassan Hosseini-Monfared a, Maria Korabik b,⇑, Marta S. Krawczyk b, Tadeusz Lis b a b

Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, Wroclaw 50-383, Poland

a r t i c l e

i n f o

Article history: Received 2 April 2014 Accepted 4 June 2014 Available online 19 June 2014 Keywords: Hydrazone Schiff bases Cu(II) coordination polymer Magnetic properties Crystal structure Phenoxido and dicyanamido bridges

a b s t r a c t A new CuII 1D coordination polymer, {[Cu2(L)(l1,5-dca)]22(CH3OH)}n (1), containing the carbohydrazide based ligand (H3L = bis-[(E)-N0 -(2-hydroxybenzylidene)]carbohydrazide) and the dicyanamide (dca) bridging group has been synthesized and characterized by magnetic measurements, single crystal X-ray diffraction and other spectroscopic methods. X-ray analysis reveals that the ligand coordinates to the CuII ion as a hexadentate trinegative N3O3-donor ligand. Tetranuclear copper(II) units with double phenoxido bridges are formed, connected and extended by two l1,5-dca anions, forming a 1D polymeric structure. The magnetic measurements showed global antiferromagnetic interactions with two coupling constants between ‘‘outer’’ (J = 87 ± 2 cm1) and ‘‘inner’’ (J0 = 129 ± 2 cm1) copper pairs in tetranuclear units. The dicyanamide, adopting the end-to-end bridging mode (metal–metal separation of ca. 8.062(5) Å), has presented poor efficiency in mediating magnetic interactions. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The design and synthesis of novel coordination polymers and metal–organic frameworks (MOFs) have attracted great attention in the fields of inorganic and coordination chemistry [1]. Several effective synthetic strategies have been employed to construct coordination polymers with diverse topological structures over the past several decades [2]. The most useful strategy to construct such compounds is to employ appropriate simple bridging ligands,    like pseudohalides (N 3 , N(CN)2 , SCN , OCN ) [3], or multitopic organic ligands (with a specific structure and several donor atoms) which simultaneously bind to several metal ions [4]. Dicyanamide, N(CN) 2 (dca), is one of the versatile bridging ligands used to obtain 1D, 2D or 3D coordination networks [5]. This bridging ligand also plays an important role in the magnetic exchange pathways between paramagnetic centers and in general contributes to various magnetic interactions [6]. On the other hand, phenol containing ligands are one of the most important O-donor systems in bioinorganic chemical systems ⇑ Corresponding authors. Address: Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran. Tel.: +98 241 5152576; fax: +98 241 2283203 (R. Bikas), Faculty of Chemistry, University of Wroclaw, F. JoliotCurie 14, Wroclaw 50-383, Poland. Tel.: +48 71 3757260 (M. Korabik). E-mail addresses: [email protected] (R. Bikas), [email protected] (M. Korabik). http://dx.doi.org/10.1016/j.poly.2014.06.024 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

[7] because of the widespread occurrence of tyrosyl radicals in various metalloproteins [8]. The phenolic oxygen can act as a bridging group by coordination to two metal ions. Several magnetostructural studies have reported that phenoxido bridged copper complexes show both ferromagnetic [9] and antiferromagnetic [10] interactions. Two rare tetranuclear copper clusters have been reported recently, with doubly phenoxido and pseudohalide end-on N-3 bridges [11], where global antiferromagnetic coupling is the superposition of two interactions: through double phenoxido- and azide bridges. Taking advantage of the coordination ability of carbohydrazone based ligands in forming multinuclear metal complexes [12] and the bridging ability of the dicyanamide (dca) ligand, we synthesized a new 1D coordination polymer of Cu(II) containing both phenoxido and dca bridging groups. 2. Experimental 2.1. Materials and instrumentations All chemicals and solvents were purchased from Acros and were used as received without further purification. IR spectra were recorded in KBr disks with a Bruker FT-IR spectrophotometer. The elemental analyses (carbon, hydrogen and nitrogen) of the compounds were obtained from a Carlo ERBA Model EA 1108

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analyzer. Atomic absorption analysis was carried out using Varian Spectra AA 220 equipment. 1H and 13C{1H} NMR spectra were measured with a Bruker Spectrospin spectrometer at 250 and 62.9 MHz, respectively. UV–Vis spectra of solutions of the H3L ligand and complex 1 were recorded on a thermo spectronic, Helios Alpha spectrometer. The electronic reflectance spectrum of undiluted compound 1 was measured on a Cary 5 UV–Vis–NIR spectrophotometer in the 200–900 nm range. EPR spectra of powdered samples were recorded at room temperature and 77 K on a Bruker ELEXSYS E 500 CW-EPR spectrometer operating at the X-band frequency and equipped with an ER 036TM NMR Teslameter and E 41 FC frequency counter. 2.2. Synthesis of bis-[(E)-N0 -(2-hydroxybenzylidene)]carbohydrazide (H3L) A methanol (10 mL) solution of salicylaldehyde (0.73 g, 6.00 mmol) was drop-wise added to a methanol solution (10 mL) of carbohydrazide (0.27 g, 3.00 mmol) and the mixture was refluxed for 6 h. The solution was then evaporated on a steam bath to 5 mL and cooled to room temperature. The resulting white precipitate was separated and filtered off, washed with 5 mL of cooled methanol and recrystallized from ethanol. Yield: 94% (0.84 g). M.p. 223–224.2 °C. Anal. Calc. for C15H14N4O3 (MW = 298.30): C, 60.40; H, 4.73; N, 18.78. Found: C, 60.37; H, 4.70; N, 18.81%. IR (KBr, cm1): 3417 (m, br), 3265 (vs br), 3159 (s), 1717 (vs), 1686 (s), 1624 (m), 1550 (m), 1490 (s), 1405 (s), 1352 (vs), 1271 (vs), 1217 (s), 1189 (m), 1157 (m), 1114 (m), 1034 (m), 958 (vs), 911 (m), 821 (s), 761 (s), 732 (m), 644 (m), 576 (m), 530 (m), 479 (m), 442 (w). 1H NMR (250 MHz, DMSO-d6, TMS) d: 10.88 (s, 2H, O–H, N–H), 8.43 (s, 1H, CH@N), 7.68 (s, 1H), 7.19 (t, 1H, J = 7.25 Hz), 6.85 (m, 2H). 13C{1H} NMR (62.9 MHz, DMSO-d6) d: 157.01 (C@O), 152.4 (CH@N), 143.27 (C–OH), 131.1, 128.6, 120.1, 119.6, 116.6. UV–Vis (in CH3OH, c = 5  105 M, kmax [nm] with e [M1 cm1]): 219 (29.1  103), 292 (26  103), 330 (33  103).

The effective magnetic moments were calculated from the expression:

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

leff ¼ 2:83 vcorr m  T ðB:M:Þ The best exchange parameters were obtained by fitting with a P ðvexp Tvcalc TÞ2 good agreement factor R defined as: R ¼ ni¼1 i exp i 2 . Standard ðvi

deviations [13c] parameters.

were

added

to

the

TÞ

calculated

exchange

2.5. X-ray crystallography X-ray diffraction data for 1 were collected at 100 K by the xscan technique on an Xcalibur PX with an Onyx CCD diffractometer, equipped with an Oxford Cryosystems open-flow nitrogen cryostat, using graphite-monochromated Cu Ka (k = 1.54180 Å) radiation. The structure was solved by direct methods with SHELXS-97 [14] and refined with the full-matrix least-squares techniques on F2 with SHELXL-97 [15]. All non-hydrogen atoms were refined anisotropically and the C-bonded hydrogen atoms were included in an idealized geometry and treated as riding on their parent atoms. The temperature factors Uiso of C-bonded hydrogen atoms were set as 1.2 times of the Ueq value of the appropriate carrier atom. The O- and N-bonded hydrogen atoms were located from difference maps and treated with Uiso(H) = 1.5Uiso(O) and U(H) = 1.2Uiso(N). The structure plots were prepared with DIAMOND [16]. Relevant crystal data and refinement parameters are listed in Table 1. 3. Results and discussion 3.1. Synthesis and spectroscopy Reaction of carbohydrazide with two equivalents of salicylaldehyde led to a hexadentate Schiff base ligand, H3L, in excellent yield and purity. It was found that H3L is stable in the keto form in the

2.3. Synthesis of {[Cu2(L)(l1,5-dca)]22(CH3OH)}n (1) Complex 1 was synthesized by the reaction of the carbohydrazone ligand H3L (0.149 g, 0.50 mmol), Cu(NO3)23H2O (0.241 g, 1.00 mmol) and sodium dicyanamide (0.089 g, 1.00 mmol) in methanol using the thermal gradient method in a branched tube. The above mentioned amounts of the materials were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms, the tube was sealed and the reagents containing arm was immersed in an oil bath at 60 °C, while the other arm was kept at ambient temperature. After one day, dark brown crystals were deposited in the cooler arm. Yield: 69% (0.180 g). Anal. Calc. for C18H15Cu2N7O4 (MW = 520.45): C, 41.54; H, 2.91; N, 18.84; Cu, 24.42. Found: C, 41.49; H, 2.89; N, 18.92; Cu, 24.50%. Selected IR (KBr, cm1): 3396 (w, br, O–H), 3210 (s), 3057 (w), 2320 (s), 2255 (vs), 2195 (s), 1660 (vs), 1626 (vs), 1595 (s), 1541 (m), 1506 (vs), 1470 (m), 1420 (s), 1359 (m), 1283 (m), 1250 (w), 1199 (m), 1154 (m), 1115 (m), 1012 (m), 952 (w), 849 (w), 756 (s), 619 (w), 585 (m), 493 (m), 475 (m), 423 (w). 2.4. Magnetic measurements Magnetization measurements in the temperature range 1.8– 300 K were carried out on powdered samples of the complex, at a magnetic field of 0.5 T, using a Quantum Design SQUID Magnetometer (type MPMS-XL5). Corrections for diamagnetism of the constituting atoms were calculated using Pascal’s constants [13a,b], the value of 60  106 cm3 mol1 has been used as the temperature-independent paramagnetism of the copper(II) ion.

Table 1 Crystallographic data of compound 1. 1 Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dx (g cm3) F(0 0 0) l (mm1) Crystal shape (color) Crystal size (mm) T (K) H Range (°) hkl range Measured reflections Reflections with I > 2r(I) Independent reflections Rint R[F2 > 2r(F2)] wR(F2) Parameters S Dqmax/Dqmin (e Å3)

[C17H11Cu2N7O3CH4O]n 520.45 triclinic  P1 8.572(4) 10.012(4) 11.769(4) 71.09(5) 83.73(5) 73.07(5) 914.0(6) 2 1.891 524 3.28 block (dark brown) 0.11  0.05  0.02 100(2) 4.0–70.3 10 ? 9, 12 ? 12, 12 ? 14 7276 1040 3321 0.207 0.095 0.131 281 1.01 0.82/1.02

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solid state and solution. In the 1H NMR spectrum of H3L (Fig. S1 in the Supplementary information), the presence of a broad signal at d = 10.88 ppm was assigned to resonances of the N–H and O–H groups, which are overlapped. The signal at d = 8.43 ppm was attributed to the resonance of the azomethine (CH@N) hydrogen. In the IR spectrum of H3L (Fig. S2 in the Supplementary information), the broad band at 3417 cm1 was assigned to an O-H stretch involved in hydrogen bonding. The bands at 3265 and 3159 cm1 (due to N-H bond stretches) and very strong bands at 1717 and 1686 cm1 (referred to amidic C@O stretching vibrations) confirm the existence of free H3L in the keto form in the solid state. The reaction of H3L, copper(II) nitrate trihydrate and sodium dicyanamide in a 1:2:2 M ratio in methanol led to {[Cu2(L) (l1,5-dca)]22(CH3OH)}n (1). The IR spectrum of 1, depicted in Fig. S2, shows the characteristic stretches of dicyanamide – three bands at 2195, 2255 and 2320 cm1 which are attributed to the msym(CN), masym(CN) and msym + masym (CN) modes, respectively. The shift of wavenumbers of the dca peaks of 1 when compared to those of the free dicyanamide (2181, 2229 and 2287 cm1) is consistent with coordination of dca in the complex. The masym(C–N) and msym(C–N) stretching frequencies occur at 1359 and 952 cm1, respectively. A C@O band is observed in IR spectra of 1 and free ligand (Fig. S2), but it is shifted towards lower frequencies in the case of 1. This finding suggests coordination of the ligand in the keto form. The band at 1626 cm1 expresses the coordination of the azomethine nitrogen (C@N) to the metal core. Moreover, there is a band at 3210 cm1 in the IR spectrum of 1 which is due to the presence of an NH bond in 1 and confirms the presence of an amidic hydrogen. Furthermore, the appearance of the C@O stretching vibration band, m(C@O), at 1660 cm1 suggests the absence of enolization and delocalization in the –N-C@O group in the coordinated ligand (L3) [17]. Additionally, the very broad band at about 3400 cm1 in the IR spectrum of crystals of 1 is attributed to OH groups of methanol molecules involving in hydrogen bonding. The weak band at 3057 cm1 and strong band at 756 cm1 are due to aromatic C–H vibrations. The weak band at 423 cm1 can be due to copper-ligand vibration modes. This band is observed as a new peak for the complex and is not present in the spectrum of the free ligand. UV–Vis spectra of the free ligand (H3L) and compound 1 were recorded at 300 K in methanol (Fig. S4). A solid-state UV–Vis spectrum of 1 (Fig. S5) was also recorded using the reflectance technique. The spectrum of H3L shows three bands at 219, 292 and 330 nm, which are attributed to p ? p⁄ (219, 292 nm) and n ? p⁄ (330 nm) transitions. Analogous bands corresponding to transitions of the chromophores present in the ligands are also observed in the spectrum of 1 at 216, 230, 250 and 281 nm (in methanol). In the solid state UV–Vis spectrum, similar bands are observed at 220, 246, 272, 300 and 330 nm. Compound 1 presents the most intense band at 390 nm (in solution) and 400 nm (in the solid state), which is assigned to ligand to metal charge transfer processes [18], as well as much weaker and less well defined broad asymmetric d-d bands, observed at 630, 665 nm (in solution) and 600, 640 nm with a shoulder at 720 nm (in the solid state), corresponding to different coordination environments of the CuII ions in the tetrameric units of 1. 3.2. X-ray structure of compound 1 The crystal structure and labelling of the atoms for 1 are displayed in Fig. 1a and selected bond lengths and angles are collected in Table 2. Diffraction studies reveal that the studied polymeric structure is centrosymmetric, containing two independent copper atoms. The Cu1 center is five-coordinated, [CuN2O3], with a distorted square-pyramidal coordination environment (s = 0.42 where s = difference between the two largest angles/60; s = 1 for

ideal trigonal–bipyramidal and s = 0 for ideal square-pyramidal [19]). The coordination sphere of Cu1 is formed by the ONO donor set of the hydrazone ligand L3 (azomethine nitrogen N1, phenolate oxygen O1 and amide oxygen O2 atoms), and the fourth position is occupied by N51 of the bridging dicyanamide ligand. The phenolate oxygen acts as a bridging group between two symmetry related Cu1 atoms, whereas the axial position is occupied by a phenolate oxygen of the adjacent coordinated ligand. Two Cu1 atoms and two bridging phenoxido groups create a planar four-membered Cu2O2 cyclic unit. The Cu1–O1Cu1i angle and and Cu1  Cu1i distance are 97.9(4)° and 3.267(4) Å, respectively. In 1, L3 forms a sequence of six-, five-, five- and six membered chelate rings and is almost planar. The Cu2 core has a square planar coordination environment, [CuN3O], with the oxygen and two nitrogen atoms (N2, N3 and O3) provided by the carbohydrazone ligand and the nitrogen atom (N21) from the bridging dicyanamide ligand. The dicyanamide bridging ligand connects the Cu2 atom to the Cu1ii atom and the 1D polymeric chain forms by this connection (Fig. 1b). While the Cu2Cu1 distance through the L3- ligand is 4.850(5) Å, the separation of Cu2Cu1ii through the dicyanamide bridge is 8.062(5) Å. The Cu–O and Cu–N bond lengths are in the normal range and are close to those found in other reported CuII complexes with hydrazone ligands [20]. The C8–N2 (1.354(16) Å) and C8–N9 (1.348(18) Å) bond lengths are similar to those observed in hydrazone ligands coordinated in the keto form [17] and the value of the C8–O2 bond length equals 1.230(14) Å, which is considerably shorter than the C–O bond length in reported complexes containing the enol form of N-arylidene hydrazone ligands [21]. Moreover, this bond length is comparable to the C@O bond length in free carbohydrazone based ligands [22]. All of these findings indicate the elimination of the amidic hydrogen without enolization through N2–C8–O2, which is also strongly confirmed by the IR spectrum. Although amide deprotonation without enolization is in contrast with reported aroylhydrazone complexes, recently we have noticed this phenomena for copper [17], manganese [23] and vanadium [24] complexes. The dicyanamide bridges show an approximately symmetric N„C–N–C„N unit and the C21–N31–C41 angle is 129.0(18)°, typical for the dicyanamide ligand [5]. Two molecules of uncoordinated methanol are present in the crystallized framework of 1, and these molecules are engaged in two kinds of hydrogen bonds: (a) O–H  O, where the OH group from methanol interacts with the O2 atom from L3-; (b) N–H  O, where the oxygen atom from the MeOH molecule acts as an acceptor. The 1D polymeric chains of compound 1 are joined via these types of hydrogen bonds and form a 2D supramolecular structure. Some weak C–H  N, C–H  O and C–H  p interactions cooperate to stabilize the crystal packing. Neighboring chains pack by strong p–p stacking interactions between the symmetry-related sixmembered Cu2–O3–C16–C11–C10–N3 chelate rings and salicylidene/five-membered Cu2–N2–C8–N9–N3 chelate rings (Fig. 2a). Similar interactions were previously described in the literature [25,26]. 3.3. Magnetic properties The magnetic properties of 1 were investigated over the 1.8–300 K temperature range. Tetranuclear copper units, later being interconnected by double l-1,5-dca bridges (Fig. 1), were taken into consideration to explain them. Plots of the magnetic susceptibility vm and vmT versus T (vm is the molar magnetic susceptibility per 4 CuII ions) are given in Fig. 3. The broad susceptibility maximum in the vm curve, observed at 150–200 K, may indicate that antiferromagnetic coupling between the copper(II) ions is present. There is also a paramagnetic contribution clearly observed at low temperatures. The vmT value at room temperature,

R. Bikas et al. / Polyhedron 81 (2014) 282–289

285

Fig. 1. (a) DIAMOND plot of the molecular structure of 1 with thermal ellipsoids drawn at the 30% probability level; symmetry code i = x, y + 1, z + 1, ii = x + 1, y  1, z. (b) 1D polymeric chain of compound 1.

equal to 0.980 cm3 mol1 K (leff = 2.80 B.M.), is lower than expected for 4 non-interacting CuII ions, each with S = 1/2 (leff = 3.56 B.M.). Upon cooling, vmT decreases continuously, reaching a plateau with a value of about 0.0580 cm3 mol1 K (leff = 0.68 B.M.) in the 3–50 K temperature range. This plateau would denote that the ground state is dominated by an antiferromagnetic coupling within the four CuII ions: (Cu2" – Cu1; – Cu1" – Cu2;). The magnetic susceptibility of 1 has been analyzed by a model of interaction for the tetracopper(II) unit {[Cu2(L)(l1,5-dca)]22(CH3OH)}, derived from the Hamiltonian: H ¼ Jð^ S1 ^ S2 þ ^ S3 ^ S4 Þ  J 0 ^ S2 ^ S3 [27], where J is the exchange coupling constant between Cu2  Cu1 through NN/OCNN bridges and J0 is the exchange coupling constant between the inner pair Cu1  Cu1i (Fig. 1a) through double phenoxido bridges. The global magnetic susceptibility vm has been taken as the sum of two independent contributions: tetracopper

susceptibility vm ðtetraÞ and monomeric paramagnetic impurity vm ðmonoÞ; according to the equation [27]:

vm ¼ vm ðtetraÞ  ð1  xÞ þ vm ðmonoÞ  x; where x is the mole fraction of the monomeric form, defined as:

vm ðmonoÞ ¼ Ng 2 b2 =4k T (Curie–Weiss model for S = 1/2 spins [13b]). The magnetic susceptibility per tetracopper(II) unit, was derived from the van Vleck formula [27]:

vm ðtetraÞ ¼

vm ðtetraÞ

Ng 2 b2 10expð2uÞ þ 2expð2uÞ þ 4expð2tÞ 4kðT  HÞ 5 expð2uÞ þ 3expð2uÞ þ 6expð2tÞ þ expð4tÞ

where t ¼ J=kT and u ¼ J 0 =kT. The Curie Weiss term (H) was included, taking into account the double dicyanamide bridges

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Table 2 Selected geometrical parameters (Å, °) for compound 1. Bond

Distance (Å)

Cu1O1 Cu1O1i Cu1O2 Cu1N1 Cu1N51ii Cu2O3 Cu2N3 Cu2N21 Cu2N2 O1C1 N21C21 N31C21 N31C41 N51C41

1.938(9) 2.377(9) 1.974(9) 1.983(9) 2.029(10) 1.938(9) 1.936(12) 1.936(11) 2.026(9) 1.365(14) 1.155(19) 1.286(19) 1.315(17) 1.100(15)

Atoms

Angle (°) i

Cu1O1Cu1 O1Cu1O2 O1Cu1N1 O2Cu1N1 O1Cu1N51ii O2Cu1N51ii N1Cu1N51ii O1Cu1O1i O2Cu1O1i N1Cu1O1i N51iiCu1O1i O3Cu2N3 O3Cu2N21 N3Cu2N21 O3Cu2N2 N3Cu2N2 N21Cu2N2

97.9(4) 172.7(4) 94.1(4) 81.0(4) 96.2(4) 90.8(4) 147.1(6) 82.1(4) 95.6(4) 119.4(4) 93.0(5) 91.9(4) 87.2(4) 173.3(7) 171.8(4) 80.7(5) 100.6(5)

Fig. 3. Temperature dependence of vm (d) and vmT (s) (vm is the magnetic susceptibility per tetracopper(II) unit). The solid lines represent the calculated curves (see parameters in the text).

R ¼ 7:36  106 : between the tetrameric entities and possible magnetic interactions through these paths. A least-squares fitting of the magnetic data over the whole temperature range leads to J = -87 ± 2 cm1, J0 = 129 ± 2 cm1, g = 2.14 ± 0.01, H = 0.14 ± 0.02 K and vm ðmonoÞ = 3.8%, as indicated by the solid curve in Fig. 3. The best exchange parameters were obtained by fitting with a good agreement factor R, defined as follows:

R¼

2 n X ðvexp T  vcalc TÞ i

i¼1

i

ðvexp i TÞ

2

;

The calculations show the antiferromagnetic exchange between the ‘‘outer’’ (J) as well as ‘‘inner’’ (J0 ) pairs of copper atoms. A stronger antiferromagnetic coupling J0 is found between Cu1  Cu1i (Fig 1a), through the double phenoxido bridges (J0 = 129 ± 2 cm1). Analysis of the magnetostructural correlations for the dinuclear bis(phenoxido)-bridged CuII complexes shows a large dependence of the coupling on the bridging Cu–O–Cu angle, especially for the most common planar complexes [28]. For the planar system with the dx2-y2 magnetic orbitals of Cu ions, the magnetic interaction

Fig. 2. (a) p–p interactions; (b) hydrogen bonding in the crystal of 1.

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R. Bikas et al. / Polyhedron 81 (2014) 282–289 Table 3 Relationship between the magnetic properties and the structural parameters for diphenoxido-bridged CuII complexes. J (cm1)

Addison parameter s

Angles (°) Cu–O–Cu

Distance (Å) Cu  Cu

Torsion angle Cu2O2

Ref.

129 ± 2 184.3 478.4 185.2 500.9

0.42 0.57 0.49 0.55 0.03, 0.06

3.267(4) 3.103 3.108 3.129 3.051

0 –

This work [29a]

0.33 0.05, 0.18

0 12.94 12.83 0 14.8

[29b]

140.8 614.7

97.9(4) 99.15 103.51 100.43 102.56 100.93 100.94 100.95 101.45

is antiferromagnetic in the range 90–105° for the Cu–O–Cu angles, and its absolute value increases with the Cu–O–Cu angle [28a,b]. Very strong antiferromagnetic coupling, about 1100 cm1, was found for the bridging angles 108.9 and 110.2° [10b], however, very large antiferromagnetic coupling constants (|J| > 600 cm1) are usually obtained with larger errors. The smaller dependence of the coupling on the bridging Cu–O–Cu angle was described for complexes with an out-of-plane shift of the copper ion [28a]. A reduction of antiferromagnetic coupling is observed when the geometry of the interacting copper ions becomes closer to trigonal bipyramidal, where the dz2 magnetic orbital is engaged in magnetic coupling [29]. In the analyzed compound 1, the Cu1 centers connected through l-PhO groups are five coordinated, [CuN2O3], with the Addison parameter s = 0.42, which suggests a coordination environment between distorted square-pyramidal and trigonal bipyramidal. The nature and magnitude of the magnetic couplings of 1 have been discussed on the basis of structural parameters and compared with related examples from the literature (Table 3). The obtained results confirmed that for a Cu–O–Cu angle equal to 97.9(4)° relatively high antiferromagnetic coupling is observed (J = 129 cm1) and the interactions increase with an increase of the Cu–O–Cu angle. It is clearly seen in Table 3 that an increase of the s parameter diminishes the antiferromagnetic coupling, which was proposed by Ghosh et al. [29a]. With the increasing s value, the dz2 magnetic orbital mixes with the dx2y2 one, which is not so well directed towards the Cu–O–Cu bonds. Some authors [29b,c] suggest that there is also a dependence of the magnetic interactions on the Cu–O–Cu–O torsion angles. Magnetic interactions between the ‘‘outer’’ Cu2°Cu1 pairs through the L3 ligand is weaker (J = 87 ± 2) and in accordance with the higher distance (4.850(5) Å) between the interacting pairs and the different geometries of Cu1 and Cu2. The overlap between the magnetic orbitals of the Cu2 and Cu1 atoms is significantly weaker, arising from a partially trigonal character of the Cu1 environments and electron delocalization on the dz2 magnetic orbitals. The explanation of the magnetic interactions between the Cu1 and Cu2 pairs is more complicated because the copper ions are bridged by NN and OCNN atoms, connected together by a C atom, which could add and counterbalance their effects [13b]. The coupling through the dicyanamide-bridges is very weak (H = 0.14 ± 0.02 K) and is in accord with the structural parameters. The dicyanamide bridging ligand connects the Cu2 ion to the Cu1ii ion in an end-on fashion and the Cu2  Cu1ii distance is 8.062(5) Å. A similar example was observed by Mitra et al. [30]. Dominant antiferromagnetic coupling was confirmed by the magnetization vs. magnetic field relation at 2 K (Fig. S3). The calculated magnetization curve for the four uncoupled S = 1/2 spins (Ssum = 2), derived from the equation M = gbSNBs(x), where Bs(x) is the Brillouin function and x = gbSH/kT [13b], is significantly higher than that observed for 1 (Fig. S3). The polycrystalline powder EPR spectra of compound 1 at 77 K and at room temperature are shown in Fig. 4. At 77 K a broad signal spread over 3500 G (500–6000) appears (Fig. 4), indicative for a

3.187 –

0

2000

4000

[29c]

6000

8000

B(G) Fig. 4. EPR spectra of compound 1 at RT (red) and 77 K (black). (Colour online.)

bulk concentration of copper ions. An intensive isotropic signal at about 3000 G is observed at room temperature, as well as at 77 K, confirming the presence of a monomeric impurity. The broad signal disappears at room temperature, showing only a monomeric impurity. A similar situation has already been observed for other CuII-tetranuclear species [11,31], with antiferro- as well as ferromagnetic interactions. The spread of the resonance in the spectrum at 77 K reveals that zero-field splitting is of the same order of magnitude as the Zeeman interactions [11]. 4. Conclusion In conclusion, the preparation and physicochemical characterization of a new CuII 1D coordination polymer have been presented. The hexadentate trinegative N3O3-donor ligand causes creation of tetranuclear units, connected and extended by two l1,5-dicyanamide anions. Cryomagnetic investigation reveals global antiferromagnetic behavior occurring within the tetranuclear copper(II) units. The two long dca bridges in an end-to-end fashion, which join the tetranuclear copper(II) units, transmit very weak antiferromagnetic interactions. It should be underlined that the presented complex 1 is an example where the structural dimensionality is different from the magnetic one. The careful analysis of the crystal structure is a very important step in understanding the magnetic behavior of polynuclear complexes, and in identifying the possible intermolecular exchange pathways. Acknowledgments The authors are grateful to the University of Zanjan for financial support of this study. R. Bikas thanks the Ministry of Science,

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Research and Technology of the I.R. Iran for scholarship No. 213631. Financial support received from the Polish National Science Centre (Grant No. NN204 198240) is gratefully acknowledged.

[9]

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Appendix A. Supplementary data CCDC 993645 contains the supplementary crystallographic data for 1. 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 http://dx.doi.org/10.1016/j.poly.2014.06.024.

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