Synthesis, structure and magnetic properties of phosphate-bridged polynuclear copper(II) complexes

Inorganica Chimica Acta 388 (2012) 37–45

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Synthesis, structure and magnetic properties of phosphate-bridged polynuclear copper(II) complexes Hong-Lin Zhu, Ling Jin, De-Yi Cheng, Yue-Qing Zheng ⇑ Center of Applied Solid State Chemistry Research, Ningbo University, Ningbo 315211, PR China

a r t i c l e

i n f o

Article history: Received 8 June 2011 Received in revised form 22 February 2012 Accepted 3 March 2012 Available online 10 March 2012 Keywords: Polynuclear Copper(II) complexes Phosphate Crystal structures Magnetic properties

a b s t r a c t Three new phosphate-bridged copper(II) complexes, [Cu4(H2O)2(phen)4(l4-PO4)2(l2-O)]11H2O (1), [Cu4(bpy)4(l4-PO4)2(l2-Cl)2]18H2O (2) and [Cu2(bpp)4(H2PO4)2](HPO4)H2O (3) (phen = 1,10-phenanthroline, bpy = 2,2-bipydine and bpp = 1,3-bis(4-pyridyl)-propane), have been synthesized under ambient conditions. The tetranuclear Cu(II) butterfly cores in 1 are aggregated to 2D supramolecular layer via intermolecular p–p stacking interactions. In 2, the dinuclear copper units are bridged by the phosphate and Cl anion to generate centrosymmetric tetranuclear complex molecules [Cu4(bpy)4(l4-PO4)2 (l2-Cl)2], which are aggregated to 2D layers via p–p stacking interactions. The Cu atoms in 3 are interlinked by a pair of bpp ligands to generate 2D (4,4) network which is through twofold ‘2D parallel/2D parallel’ mode inclined interpenetration to induce 3D motifs. The variable temperature magnetic characterizations suggest antiferromagnetic coupling interactions with J = 9.38 and 28.03 cm1 for 1, J = 2.41 cm1 for 2, respectively. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, many research activities have focused on the synthesis of inorganic–organic hybrid frameworks owing to their diverse structural chemistry and potential applications [1–5]. Inorganic frameworks are highly attractive in the search for new materials because of stability and superior electronic, magnetic and optical properties, while organic components built upon molecular building-blocks hold great promises for process ability, flexibility, structural diversity and geometrical control. Incorporation of the inorganic and organic ligands into a single structure may generate inorganic–organic hybrid composites that enhance or combine the useful properties. As yet, the synthesis of transition metal-phosphate complexes containing organic ligands have been investigated [6–13]. As compared with inorganic phosphates, the organic molecules can greatly affect the connecting patterns of inorganic polyhedra, providing a method for the synthesis of new materials [14,15]. Very recently, a series of polynuclear compounds with transition metal–organic phosphates have been published. For example, Robert P. Doyle and co-workers reported the synthesis and characterization of tetranuclear copper(II) complexes [Cu4(H2O)2(L)4 (l4-PO4)2(l2-CO3)] (L = 2,20 -bipyridine and 1,10-phenanthroline), which possessed tetranuclear butterfly cores and a l4-PO4 modes of bridging phosphate [10].

⇑ Corresponding author. Tel./fax: +574 87600747. E-mail address: [email protected] (Y.-Q. Zheng). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.03.014

One of our research interests has focused on a systematic study of the polynuclear copper(II) complexes. By employing the HnPO4(3n) oxoanions and copper(II) ions along with didentate ligand such as 2,20 -bipyridine, 1,10-phenanthroline and 1,3-bis(4pyridyl)-propane, we now extended the work to obtain complexes with different nuclearity and different coordinating or bridging modes of HxPO4(3x) ion. In the present contribution, we report three new polynuclear copper(II)-phosphate complexes, [Cu4 (H2O)2(phen)4(l4-PO4)2(l2-O)]11H2O (1), [Cu4(bpy)4(l4-PO4)2 (l2-Cl)2]18H2O (2) and [Cu2(bpp)4(H2PO4)2](HPO4)H2O (3). The common features of the compounds 1 and 2 are the tetranuclear unit, and the compound 3 is a twofold ‘2D parallel/2D parallel’ mode of inclined interpenetration structure. 2. Experimental section 2.1. Materials All chemicals of reagent grade were commercially available and used without further purification. 2.2. Physical methods Powder X-ray diffraction measurements were carried out with a Bruker D8 Focus X-ray diffractometer to check the phase purity. The FT–IR spectra were recorded from KBr pellets in the range 4000–400 cm1 on a Shimadzu FTIR-8900 spectrometer. The C, H and N microanalyses were performed with a PE 2400II CHNS

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H.-L. Zhu et al. / Inorganica Chimica Acta 388 (2012) 37–45

Table 1 Summary of crystal data, data collection, structure solution and refinement details for 2–4 (T = 293(2)). Compounds

1

2

3

Empirical formula Formula weight Description Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) F(0 0 0) Absorption coeff. h (°) Total number of data collected Number of observed data [I P 2r(I)] Number of variables Goodness-of-fit on F2 R1, wR2 [I P 2r(I)]a R1, wR2 (all data)a dqmax, dqmin (e Å3)

C48H58Cu4N8O22P2 1415.12 blue platelet triclinic  P1

C40H68Cl2Cu4N8O26P2 1464.02 blue block triclinic  P1

C52H65Cu2N8O13P3 1246.11 blue block tetragonal P43212

11.294(2) 13.978(3) 19.677(4) 80.74(3) 75.15(3) 76.86(3) 2907(1) 2 1.617 1448 1.583 3.01–25.00 12613 9955 4415 0.951 0.0926, 0.2120 0.1891, 0.2634 1.015, 0.524

10.451(2) 11.739(2) 12.451(3) 79.41(3) 88.17(3) 74.34(3) 1446(1) 1 1.682 752 1.688 3.04–25.00 11453 5072 4390 1.044 0.0500, 0.1366 0.0572, 0.1460 1.098, 0.953

18.521(3) 18.521(3) 15.732(3)

5397(2) 4 1.534 2592 0.952 3.02–27.48 27394 6167 4035 1.046 0.0795, 0.1471 0.1274, 0.1653 0.547, 0.456

P P P P a R1 = (Fo  Fc)/ Fo, wR2 = [ w(Fo2  Fc2)2/ w(Fo2)2]1/2, and w = [r2(Fo2) + (aP)2 + bP]1 where P = (Fo2 + 2 Fc2)/3. For 1, a = 0.0885 and b = 0. For 2, a = 0.0807 and b = 1.7041. For 3, a = 0.0627 and b = 7.1647.

elemental analyzer. The electronic transmittance spectra were recorded on a Shimadzu UV-2501/PC UV–Vis spectrophotometer. Single crystal X-ray diffraction data were collected by Rigaku Raxis Rapid X-ray diffractometer. The temperature-dependent magnetic susceptibilities were determined with a Quantum Design SQUID magnetomer (Quantum Design Model MPMS-7) in the temperature range 2–300 K with an applied field of 5 KOe. 2.2.1. Synthesis of [Cu4(H2O)2(phen)4(l4-PO4)2(l2-O)]11H2O (1) 0.171 g (1.0 mmol) CuCl22H2O and 0.198 g (1.0 mmol) 1,10phenanthroline (phen) were dissolved in 20 mL MeOH/H2O (1:3 V/V), and to this 1.0 mL 1 M phosphoric acid (H3PO4) was added under continuous stirring, showing a deep blue clear solution. Then the pH of the mixture solution was adjusted to 5.5– 5.6 by addition of NaOH (1 M), and the resulting blue solution was allowed to stand at room temperature. Slow evaporation for several weeks afforded a small amount of blue plate-like crystals of 1 (yield 50% based on the initial CuCl22H2O input). Anal. Calc. for C48H58Cu4N8O22P2: C, 40.74; H, 4.13; N, 7.92. Found: C, 40.03; H, 4.81; N, 7.26%. IR spectroscopic analysis (KBr, m/cm1): 3396 s, 3084 w, 1628 m, 1583 m, 1521 m, 1492 w, 1429 s, 1342 m, 1224 w, 1074 s, 1003 m, 850 s, 721 s, 648 m, 535 m (see Fig. S2 in Supporting Information). UV–Vis [H2O; k/nm (abs)]: 266 (p–p⁄). (see Fig. S3 in Supporting Information). 2.2.2. Synthesis of [Cu4(bpy)4(l4-PO4)2(l2-Cl)2]18H2O (2) A similar method to that used for the preparation of 1 was followed for the preparation of 2 by replacing 1,10-phenanthroline with 2,20 -bipyridine. Slow evaporation of the solution for several weeks at room temperature afforded blue block-like crystals of 2 (yield 40% based on the initial CuCl22H2O input). Anal. Calc. for C40H68Cl2Cu4N8O26P2: C, 32.82; H, 4.68; N, 7.65. Found: C, 32.41; H, 5.03; N, 7.38%. IR data (cm1, KBr): 3410 s, 3110 w, 1641 m, 1610 s, 1573 m, 1497 m, 1473 m, 1448 s, 1316 m, 1252 m, 1159 m, 1090 s, 1054 s, 998 s, 924 m, 773 s, 730 m, 642 w, 553 w, 416 m (see Fig. S2 in Supporting Information). UV–Vis [H2O; k/nm (abs)]: 244 (p–p⁄), 298 (n–p⁄). (see Fig. S3 in Supporting Information).

2.2.3. Synthesis of [Cu2(bpp)4(H2PO4)2](HPO4)H2O (3) 1.0 mL (1.0 M) NaOH was added to an aqueous solution of 0.170 g (1.0 mmol) CuCl22H2O in 5.0 mL H2O to yield blue precipitate, which was separated by centrifugation and washed with distilled water for seven times, then transferred into a mixed solvent consisted of 5.0 ml methanol and 15.0 ml water. To the resulting suspension 0.199 g (1.0 mmol) 1,3-bis(4-pyridyl)-propane and 1.0 mL 1 M phosphoric acid were successively added. The mixture was continuously stirred for 30 min and the indiscerptible substance was then filtered off. The sky-blue filtrate was allowed to stand at room temperature and slow evaporation for several weeks afforded deepblue block-like crystals of 3 (yield 15% based on the initial CuCl22H2O input). Anal. Calc. for C52H65Cu2N8O13P3: C, 50.12; H, 5.26; N, 8.99. Found: C, 49.84; H, 5.45; N, 8.78%. IR data (cm1, KBr): 3437 w, 3095 vw, 3017 vw, 2932 w, 1616 s, 1557 w, 1509 m, 1430 s, 1228 m, 1163 m, 1104 m, 1068 m, 1028 w, 935 m, 872 w, 831 m, 510 s (see Fig. S2 in Supporting Information). UV–Vis [H2O; k/nm (abs)]: 254 (p–p⁄). (see Fig. S3 in Supporting Information).

2.3. X-ray crystallography Suitable single crystals were selected under a polarizing microscope and fixed with epoxy cement on respective fine glass fibers which were then mounted on a Rigaku R-Axis Rapid IP X-ray diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71073 Å) for cell determination and subsequent data collection. The reflection intensities in suitable h ranges were collected at 293 K using the x scan technique. The employed single crystals exhibit no detectable decay during the data collection. The data were corrected for Lp and empirical absorption effects. The SHELXS-97 and SHELXL-97 programs [16,17] were used for structure solution and refinement. The structure were solved by using direct methods. Subsequent difference Fourier syntheses enabled all non-hydrogen atoms to be located. After several cycles of refinement, all hydrogen atoms associated with carbon atoms were geometrically generated, and the rest of the hydrogen atoms were located from the successive difference Fourier syntheses, while the water H

H.-L. Zhu et al. / Inorganica Chimica Acta 388 (2012) 37–45

atoms of split oxygen atom OW9 in 2 could not be positioned reliably and were omitted from a difference Fourier map. Finally, all non-hydrogen atoms were refined with anisotropic displacement parameters by the full-matrix least-squares technique and hydrogen atoms with isotropic displacement parameters set to 1.2 times of the values for the associated heavier atoms. Detailed information about the crystal data and structure determination is summarized in Table 1. Selected interatomic distances and bond angles are tabulated in Tables S1–S3. 3. Results and discussion 3.1. Syntheses As shown in Scheme 1, reaction of CuCl22H2O, phosphoric acid and 1,10-phenanthroline (phen) in methanol–water at room temperature afforded tetranuclear Cu(II) butterfly cores [Cu4(H2O)2 (phen)4(l4-PO4)2(l2-O)]11H2O 1 with adjusting pH to 5.6 by addition of NaOH. Substitution of 2,20 -bipyridine(bpy) for phen also yielded the tetranuclear units [Cu4(bpy)4(l4-PO4)2(l 2-Cl)2]18H2O 2, but a similar method to that used for the preparation of 1 was followed by replacing 1,10-phenanthroline with 1,3-bis(4-pyridyl)propane (bpp) to afford the complex [Cu(bpp)3Cl2]2H2O [18], which was reported by Carlucci in 2002. If the crystal of the compound 1 dissolved in 20 mL MeOH/H2O (1:1 V/V) and adjusted the pH to alkaline solution with NaOH, the reported tetranuclear butterfly compound [Cu4(H2O)2(phen)4(l4-PO4)2(l2-CO3)]xH2O [10] was obtained in several weeks. Moreover, reaction of Cu(OH)2, phosphoric acid and 1,10-phenanthroline(phen) with pH to 7.8 also afforded tetranuclear Cu(II) butterfly compound [Cu4(H2O)2 (phen)4 (l4-PO4)2(l2-CO3)]xH2O. When the fresh precipitated Cu(OH)2 was used as Cu(II) source materials in the place of CuCl26H2O, and it reacted with bpp and phosphoric acid in an methanolic aqueous solution to produce the compound 3. The powder XRD patterns of 1–3 match well with the corresponding ones simulated on the basis of the single crystal data (Fig. S1). The present synthetic procedures could be summarized as follows in Scheme 1. 3.2. Description of the crystal structures 3.2.1. [Cu4(H2O)2(phen)4(l4-PO4)2(l2-O)]11H2O The asymmetric unit of complex 1 consists of four independent copper atoms (namely Cu1, Cu2, Cu3 and Cu4, respectively), two bis-monodentate phosphate ions, four phenanthroline ligands, two aqua ligands, one l2-O ion and 11 lattice water molecules. As shown in Fig. 1, four [Cu(phen)]2+ units are bridged by two tetradentate phosphate ions and a l2-O2- ion, affording a tetranuclear butterfly unit [Cu4(H2O)2(phen)4 (l4-PO4)2(l2-O)]. Each copper ion is located in a CuN2O3 square pyramidal coordination environment. The basal plane of Cu2 and Cu3 is defined by two N atoms of phenanthroline ligand and two O atoms of a phosphate ion,

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and the axial position is occupied by an aqua ligand. While for Cu1 and Cu4, the two O atoms in the basal plane are from a phosphate ion and a l2-O ion, respectively, and the axial position is occupied by another O atom of phosphate ion. The axial Cu–O bond lengths fall in the range 2.276(8)–2.422(7) Å, distinctly larger than the basal Cu–O lengths. The s parameters (s = 0 indicates ideal square pyramidal environment and s = 1 indicates ideal trigonal bipyramidal environment) indicated the slightly distortion of the square pyramid (0.15 for Cu1, 0.06 for Cu2, 0.01 for Cu3 and 0.08 for Cu4) [19]. Two phosphate ions adopt l4-O,O’,O’’ coordination pattern which have been found in some other compounds to bridge the copper ions into a planar tetranuclear butterfly copper skeleton [10]. The coordinated P–O bond lengths of P1 and P2 locate in the range 1.531(6)–1.539(8) and 1.523(8)–1.532(7) Å, respectively, larger than corresponding ones in the reported complexes [10]. The phenanthroline ligands are well coplanar and the dihedral angles between the phenanthroline rings are 62.1° (for the phenanthroline coordinated to Cu1 and Cu4) and 60.5° (for the phenanthroline coordinated to Cu2 and Cu3), respectively. The distance of 3.32 and 3.41 Å between the neighboring phenanthroline ligands indicate strong intramolecular p–p stacking interactions. The phen ligands also participate in a continuous intermolecular p–p stacking interaction with separation of 3.35–3.54 Å. Such intermolecular interactions are regarded as the driving forces to assemble the tetranuclear complex molecules into 2D supramolecular layers parallel to (1 1 0) as demonstrated in Fig. 2a. The resulting 2D supramolecular sheet contains cavities in which some of the lattice water molecules reside. The lattice water molecules interact with each other to form layers which are alternately arranged with the complex molecular layers. The water layers are further formed a 3D array via hydrogen bonding between the water layers and the water molecules reside in the above-mentioned cavities (Fig. 2b). Obviously, the extensively hydrogen bonding interactions have a dramatic effect on the stabilization of crystal structures. 3.2.2. [Cu4(bpy)4(l4-PO4)2(l2-Cl)2]18H2O The asymmetric unit of complex 2 contains two copper ions, a phosphate ion, two 2,2-bipyridine, a chloride ion and nine lattice water molecules. As displayed in Fig. 3, both Cu1 and Cu2 are disposed in CuN2O2Cl square pyramidal environment, of which the basal-plane is defined by two N atoms of a bpy ligand and two O atoms of two phosphate ions while the apical position is occupied by a bridging Cl ion. The bond lengths and angles indicate the significantly distortion of the coordination spheres, which is identical to the s parameters (0.20 for Cu1 and 0.50 for Cu2). The phosphate ions take the same coordination mode as in compound 1, but the uncoordinated P–O bond length is 1.511(2) Å, smaller than the coordinated P–O lengths. The two pyridine rings of the bpy ligand chelating Cu1 forms a dihedral angle of 8.83°, and the dihedral angle is only 2.3° for the ligand chelating Cu2, indicating a nearly perfect coplanarity. The dinuclear copper unit is pairwise bridged by the phosphate and Cl anion to generate centrosymmetric

Scheme 1. Schematic synthesis of 1–3.

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Fig. 1. Ortep view of the tetranuclear [Cu4(H2O)2(phen)4(l4-PO4)2(l2-O)] complex molecule in 1 together with atom numbering scheme (The thermal ellipsoids are drawn at 30% probability level).

tetranuclear complex molecules [Cu4(bpy)4(l4-PO4)2(l2-Cl)2]. As similar to compound 1, the intramolecular and intermolecular p–p stacking interactions make a great contribution to the formation of the 2D supramolecular layers with the stacking distances of 3.66 and 3.44–3.46 Å, respectively (Fig. 4a). The lattice H2O molecules reside in cavities in the 3D supramolecular architecture (Fig. 4b), which form hydrogen bonds to the phosphate O atoms as well as to the lattice H2O molecules. 3.2.3. [Cu2(bpp)4(H2PO4)2](HPO4)H2O 3 Complex 3 is isostructurally to the reported complex [Cu2(bpp)2(HSO4)2](SO4)H2O [20]. The Cu(II) ion is five-coordinated by four N atoms of four bpp ligands in the basal plane and an O atom of a terminal phosphate ion in the axial position to give a CuN4O square pyramidal geometry. The Cu–N bond lengths range in 2.025(5)–2.047(5) Å, significantly larger than corresponding ones in [Cu2(bpp)2(HSO4)2](SO4)H2O, while the axial Cu–O bond length of 2.107(4) Å is significantly smaller than the corresponding ones (2.289(5) Å). The s parameter of 0.15 indicates that the square pyramidal geometry is slightly distorted. As shown in Fig. 5, the different of two bpp molecules both act as a bis-monodentate bridging mode and adopt a TG conformation. The di-protonated phosphate ion just functions as a counterion with an O-donor role in ligating copper centers via the Cu–O covalent bond. The Cu atoms are alternately bridged by a pair of different bpp ligands to form 2D21 [Cu2(bpp)4(H2PO4)2] layer parallel to (0 1 1) plane (Fig. 6a). The Cu  Cu separation through two of bpp ligand are 12.25 and 12.26 Å, respectively. Topologically, if the bpp ligands are considered as twofold linker, the copper centers are treated as the fourfold connector, and the whole layer can be regarded as a (4, 4) network. The (4, 4) nets have enough empty space to provide for interpenetration. The 2D sheets are inclined parallel into 3D network with an inclined angle of 70.2(1)° as shown in Fig. 6b, to the best of our knowledge, such a inclined (4, 4) interpenetration structure have been defined the ‘2D parallel/2D parallel’ mode. The mono-protonated phosphate ion and lattice molecules form extensively hydrogen bonds each other. 3.3. Magnetic properties Temperature-dependent magnetic susceptibility measurements for compounds 1 and 2 are performed on the polycrystalline sample in the temperature range of 2–300 K in a fixed magnetic field 5 KOe, and the magnetic behaviors in the form of vmT products and

vm1 versus T plots are depicted in Fig. 7 (vm being the magnetic susceptibility both per four Cu unit for 1 and 2, respectively). For compound 1, the value of vmT at 300 K is 1.407 cm3 mol1 K, which is less than the value of 1.497 cm3 mol1 K for the spin-only value of four uncoupled CuII ions, indicating an orbital quenching. Starting from room temperature, the resultant moment per copper decreases from 1.407 to 0.168 cm3 mol1 K at 300 and 2 K, respectively, revealing the occurrence of a very weak net antiferromagnetic interaction operating between the four copper centers. The increase of the vm value at low temperature could be attributed to the presence of a small amount of monomeric CuII impurities. At first sight, modeling the magnetic data for 1 appears quite complicated because of the coexistence of three different exchange coupling constants (Ji) coexist within the tetranuclear copper (II) core of 1 via phosphate and l2-O ion (Scheme 2a). When the phosphate is the bridging ligand, two coordination pathways to the copper atoms are found: one through a single oxygen atom (l2-OPO3), and the other through a O–P–O triatomic set of atoms (l2-PO4) that is the classical coordination motif for the bridging phosphate. The Cu1 and Cu2 ions as well as Cu3 and Cu4 ions both are bridged by the l2-OPO3 with J1. The only a l2-O ion exchange pathway is dealt with J2 between Cu2 and Cu4 ions. In the case of J3, a coordination mode of the l2-PO4 occurs between Cu1 and Cu3, Cu1 and Cu4, Cu2 and Cu3. Strong magnetic exchange coupling are predicted that the value of J1 and J2 is larger than the J3, because the extremely weak magnetic interactions in J3 are expected through the two phosphate bridges pathways with small values of the coupling constant. Therefore we intend to fit the magnetic data of 1 in a linear tetranuclear Cu4 coupling system as shown in Scheme 2b [12]. Assuming J3 = 0, the spin exchange Hamiltonian can be represented as H = 2J1(S1S2 + S3S4) 2J2S2S3. The expression of magnetic susceptibility is as follows:

vm ¼ Ng2 b2 =kT  x=y  ð1  qÞ þ q  Ng2 b2 =kT þ TIP

ð1Þ

x ¼ 10exp½ðJ1  1=2J2 Þ=jT þ 2exp½ðJ1  1=2J2 Þ=jT þ 2expf½1=2J2 þ ðJ21 þ J22 Þ

1=2

=jTg þ 2expf½1=2J2  ðJ21 þ J22 Þ

1=2

=jTg

y ¼ 5exp½ðJ 1  1=2J 2 Þ=jT þ 3exp½ðJ 1  1=2J 2 Þ=jT þ 3expf½1=2J 2 þ ðJ 21 þ J 22 Þ1=2 =jTg þ 3expf½1=2J 2  ðJ 21 þ J 22 Þ1=2 =jTg þ expf½J1 þ 1=2J 2 þ ð4J 21  2J 1 J 2 þ J 22 Þ1=2 =jTg þ expf½J 1 þ 1=2J 2  ð4J 21  2J 1 J 2 þ J 22 Þ1=2 =jTg

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Fig. 2. (a) Packing diagram of 1 showing the 2D layer formed by adjacent complex molecules (b) Packing diagram of 1 in spacefilling mode showing the alternately array of lattice water layers and complex molecule layers.

where N, g, and b are Avogadro’s number, the g-factor, and the Bohr magneton, respectively, and k is Boltzman’s constant. The parameter q denotes the fraction of paramagnetic impurity in the sample and a temperature independent paramagnetism (TIP) was also considered and fixed at 6.0  105 cm3 mol1. The experimental magnetic susceptibility data were fit to the theoretical expression in Eq. (1), and the an excellent fit was obtained with the following parameters: J1 = 9.38 cm1, J2 = 28.03 cm1, g = 2.11, TIP = 6.0  105 emu mol1 per Cu, with 3.7% ‘‘monomer’’ impurity. The relatively large negative J2 value indicates that the l2-O ion bridges the copper atoms considerable antiferromagnetic coupling as expected, and the value of coupling constant is obviously stronger than the J1 with l2-OPO3 exchange pathway in axial-equatorial positions. Actually, according to some studies about O–P–O triatomic phosphatebridged copper(II) compounds, the three-atom phosphate-bridged can’t simply be regarded as being a weak bridge producing intrinsically weak coupling. The research of Ainscough [21] demonstrates that such bridges should present very small J values. A dinuclear compound [{Cu2(HL)(H2PO4)2}2][NO3]22H2O, [H2L = 2,20 -bis[1-(2-pyridyl) methylidene] thiocarbonyl bis(hydrazine)]

was reported by Ranford and co-workers [22], which possessed a three atom bridging H2PO42 disposed in axial–equatorial positions with respect to adjacent copper(II) centers, gave a J value attributed to this bridging moiety of 16 cm1. Moreover, Doyle and co-worker found similar weak antiferromagnetic coupling within dimer Cu units with bridging by H2AsO4 [23]. In contrast, Spiccia represents stronger antiferromagnetic coupling through l2-HPO4 in {[Cu3L(l-OH)(l3-HPO4)(H2O)] [PF6]33H2O}n, where L = 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene [24]. Clearly, the exact bridging mode, bridging angles and orbitals involved all play a role in determining coupling strength, which can be significant. The magnetic properties of compound 2 under the form of vmT vs. T plot is shown in Fig. 10, the value of vmT for Cu2+ ions at room temperature is 0.969 cm3 mol1 K. Upon cooling, the vmT value gradually decreases to 0.259 cm3 K mol1 at 2 K, which is typical of an overall antiferromagnetic interactions between Cu(II) ions. In the light of the structural data of 2, the Cu1 and Cu2 and symmetry-related Cu1#1 and Cu2#1 are bridged by the Cl anion and the l2-OPO3 with J1. The coordination mode of the l2-PO4 occurs

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Fig. 3. Ortep view of the tetranuclear [Cu4(bpy)4(l4-PO4)2(l2-Cl)2] complex molecule in 2 together with atom numbering scheme (The thermal ellipsoids are drawn at 30% probability level).

Fig. 4. (a) Packing diagram of 2 showing the 2D layer formed by adjacent complex molecules (b) Packing diagram of 2 in spacefilling mode showing the alternately array of lattice water layers and complex molecule layers.

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Fig. 5. Ortep drawing of complex 3 with atom numbering scheme. (The thermal ellipsoids are drawn at 30% probability level).

Fig. 6. (a) 2D layer generated from the Cu atoms bridged by bpp ligands in 3. (b) Two interpenetrating 2D topological representation of 3.

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which is derived through the Hamiltonian H = 2J1S1S2, where J1 is the magnetic coupling between Cu1 and Cu2, g is the average Landé factor and N, b, k, TIP and T have theirs usual meanings. Best fit parameters through Equation (2) are J1 = 2.41 cm1, g = 2.04 P and R = 8.9  106 (R = [(vm)obs  (vm)calc]2/[(vm)obs]2). The temperature-independent paramagnetism (TIP or Na) was set to 6.0  105 cm3 mol1 for each Cu atom. The calculated curve matches very well the experimental data in the whole temperature range explored. The antiferromagnetic coupling should be correlated to a significant overlap between the two orbitals of the Cu1 and Cu2 ions through the l2-OPO3 and Cl anion. The short Cu– O bond and the large Cu–O–Cu angles of thel2-OPO3, relative to the long Cu–Cl bond points at a dominant coupling through the single oxygen atom of phosphate bridge. According to the above structure description, the Addison’s s value of Cu1 and Cu2 in 2 is 0.20 and 0.50, respectively, indicating that the coordination sphere around Cu1 atoms is a square-pyramidal environment, while the Cu2 has a intermediate between a square pyramid and a trigonal bipyramid. In the latter case, the unpaired electron of copper(II) occupies either the dz2 or the dx2 y2 orbital, which would both point along the Cu–O bond. Short Cu–O bond and large Cu–O–Cu angles in square-pyramidal phosphate-bridged dimmer results in a strong overlap of dx2 y2 orbitals and thus produce to antiferromagnetic coupling. 4. Conclusions

Fig. 7. vm and vmT vs. T plots for complexes 1 (a) and 2 (b) (vm being the magnetic susceptibility per a Cu48+ unit). Solid lines represent the best fits.

between Cu1 and Cu1#1, Cu1 and Cu2#1, Cu2 and Cu1#1, Cu2 and Cu2#1 with the J2 coupling constant, which is predicted a very weak magnetic coupling because of the small spin density on the O–P–O triatomic linking and the long bonds of metals. Therefore, the magnetic structure can be model to dinuclear via bridging of l2-OPO3 and Cl anion. We have analyzed the magnetic data by a simple Bleaney–Bowers law for two interacting spin doublets (Equation (2)) [25],

vm ¼

 1 2Nb2 g 2 1 þ TIP 1 þ expð2J 1 =kTÞ 3 3kT

ð2Þ

We have successfully synthesized three new phosphate-bridged copper(II) complexes [Cu4(H2O)2(phen)4(l4-PO4)2(l2-O)]11H2O (1), [Cu4(bpy)4(l4-PO4)2(l2-Cl)2]18H2O (2) and [Cu2(bpp)4 (H2PO4)2](HPO4)H2O (3) (phen = 1,10-phenanthroline, bpy = 2,2bipydine and bpp = 1,3-bis(4-pyridyl)-propane). The compound 1 possess a tetranuclear butterfly units containing phosphate with l4-O,O0 ,O00 -PO4 coordination pattern. In 2, the dinuclear units are pairwise bridged by PO4 and Cl to generate centrosymmetric tetranuclear complex molecules. The compound 3 exhibits a 2D (4,4) network with adopting twofold ‘2D parallel/2D parallel’ mode to form inclined interpenetration structure. These complexes 1 and 2 with a high degree of hydration demonstrate the importance of weak intermolecular interactions on the resultant solid-state structures. We are extending our synthetic approach to incorporate other transition metals and phosphate anion with multi-nuclear compounds. Acknowledgements This project was supported by the National Natural Science Foundation of China (Grant No. 20072022) and the Scientific Research Fund of Ningbo University (Grant Nos. XKL11058 and XYL11005). The honest thanks are also extended to K.C. Wong Magna Fund in Ningbo University.

Scheme 2. Magentic exchange coupling in 1.

H.-L. Zhu et al. / Inorganica Chimica Acta 388 (2012) 37–45

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