Two Polyoxophosphotungstates Formed by Wells−Dawson Cores

Nov 19, 2008 - Chemistry, Huaqiao UniVersity, Quanzhou 362021, China. ReceiVed May 1, 2008; ReVised Manuscript ReceiVed September 13, 2008...
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Two Polyoxophosphotungstates Formed by Wells-Dawson Cores Linked through W-O-W Linkages Bi-Zhou Lin,* Li-Wen He, Bai-Huan Xu, Xiao-Li Li, Zhen Li, and Pei-De Liu Key Laboratory for Functional Materials of Fujian Higher Education, Department of Applied Chemistry, Huaqiao UniVersity, Quanzhou 362021, China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 273–281

ReceiVed May 1, 2008; ReVised Manuscript ReceiVed September 13, 2008

ABSTRACT: Two polyoxophosphotungstates, [Hen][Cu(en)2(H2O)]2[Cu(en)2(H2O)2]0.5[{Cu(en)2(H2O)}2 {Cu(en)2}(P2W18O61)2] · en · nH2O (en ) ethylenediamine, n ) 16.35), 1, and [HenMe]2 [Cu(enMe)2][{Cu(enMe)2}P2W18O61] · 9H2O (enMe ) 1,2diaminopropane), 2, have been synthesized under hydrothermal conditions. It was revealed that two Wells-Dawson cores in 1 are condensed through sharing two common O atoms into an unprecedented polyoxometalate dimer in a belt-to-belt fashion, which are further fused by a copper(II) bridging fragment and grafted by two copper(II) complexes. Every two polyoxometalate units in 2 are interconnected through an oxo and a complex bridged linkers into a chainlike structure. It suggested that the pH value of the reactive mixture and the steric interference of the methyl group of enMe ligand play crucial roles on the resultant structures. Compounds 1 and 2 represent the first condensed structures formed by unsubstituted POM clusters through sharing common atoms. Along with well-separated Cu2+ centers, their magnetic behaviors can be explained based on tetranuclear and dinuclear antiferromagnetic coupling models, respectively. It was indicated that the compounds have catalytic activity in epoxidation of maleic anhydride. Introduction Polyoxometalates (POMs) are one kind of significant metal oxide clusters made of an assembly of MO6 octahedra and possess an abundant structural variety and interesting properties.1 Keggin, Wells-Dawson, Anderson, and Silverton are wellknown POM types, in which the Wells-Dawson cluster structure was first postulated by Wells in 1945,2 and determined by Dawson in 1953.3 The Wells-Dawson cluster [P2W18O62]nis close to the D3h point symmetry and contains 18 WO6 octahedra surrounding two PO4 tetrahedra (Scheme 1). There are two structurally distinct types of W atoms in the cluster: six “cap” atoms on vertical mirror planes and grouped in two sets of three, and 12 “belt” atoms that do not lie on mirror planes and grouped in two sets of six. POMs have attracted an extensive attention in the past decades because of their widely potential applications in diverse areas such as catalysis, magnetism, bioand nanotechnology, and materials science.1 POMs are conventionally isolated from the self-assembly processes controlled by oxidation-reduction reactions and/or condensation-polymerization reactions in aqueous, acidic solution.4 Although the mechanism remains elusive, a large number of novel POMs with unexpected shapes and sizes have been reported.5 A fascinating and challenging aspect of the POMs lies in the rational synthesis of large clusters and the exploration of their utilization as supramolecular nanometer models.6 In recent years, one of the important advances in the design of new POMs is linking molecular cluster subunits through secondary metal compounds to produce POM-supported metal complexes or metal-complex-bridged POM extended structures.7-12 To date, most known structures are derived from Keggin8and Anderson type polyanions.9 The hybrids derived from Wells-Dawson type anions are much less common.10 The other development is self-assembly of the lacunary POMs to reconstitute the complete (saturated) substituted derivatives or to form high-nuclearity cluster structures and extended structures through direct condensation with M-O-M′ linkages (M is W, Mo, V and so on, M′ is the substituted rare earth or transition * To whom correspondence should be addressed. E-mail: [email protected].

Scheme 1. Polyhedral Representation of a Wells-Dawson Structure

metal).6a,11-17 It have to be mentioned that the oxo-bridged POM extended structures are often based on monosubstituted polyoxotungstate anions through W-O-M linkages.14-17 In most cases, the monosubstituted metal atoms are disordered in the two positions, such as [Cu(en)2(H2O)]2[H2en][{Cu(en)2}P2CuW17O61] · 5H2O,14 [ET]8[PMnW11O39] · 2H2O,15 and [NEt3H]5[PCoW11O39] · 3H2O.16 Sometimes, the monosubstituted metal atoms are in clearly determined positions, like [Co(dpa)2(H2O)2]2[Hdpa][PCoW11O39]14 and [H2bpy]2[Nd2(H2O)9P2W17O61] · 4.5H2O.17 It is noteworthy that many POMs have been condensed from simple starting materials, rather than lacunary POMs, under hydrothermal conditions.7,14 As a continuation of the synthesis of various new POM compounds,18 we are trying to isolate novel polyoxotungstates through W-O-W direct condensation using simple starting materials. Herein, we report the hydrothermal synthesis and characterization of two novel polyoxotungstates containing Wells-Dawson units, [Hen][Cu(en)2(H2O)]2[Cu(en)2(H2O)2]0.5[{Cu(en)2(H2O)}2{Cu(en)2}(P2W18O61)2] · en · nH2O (n ) 16.35), 1, and [HenMe]2[Cu(enMe)2][{Cu(enMe)2}P2W18O61] · 9H2O, 2. Two Wells-Dawson units are condensed through sharing common O atoms and a copper(II) bridging fragment into a POM dimer in 1 and into a chainlike structure in 2. To the best of our knowledge, compounds 1 and 2 represent the first condensed

10.1021/cg8004486 CCC: $40.75  2009 American Chemical Society Published on Web 11/19/2008

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structures formed by unsubstituted POM clusters through sharing common atoms.

Table 1. Crystallographic Data for Compounds1 and2

formula

Experimental Section Materials and Instrumentation. All chemicals were commercially purchased and used without further purification. The hydrothermal reaction was carried out in 17 mL Teflon-lined stainless steel autoclaves under autogenous pressure. The reactants were stirred briefly before heating. Elemental analyses were performed on a Perkin-Elmer 2400 II element analyzer and inductively coupled plasma analysis on a Perkin-Elmer Optima 2100DV ICP spectrometer. The infrared spectrum was recorded at room temperature on a Nicolet 470 FTIR spectrophotometer as KBr pellets in the 4000-400 cm-1 region. Thermogravimetric analysis was performed in flowing N2 with a heating rate of 10 °C min-1 on a Shimadzu TGA-60H instrument. The X-ray powder diffraction patterns were recorded at ambient temperature on a Bruker D8 Advance diffractometer using Cu KR radiation (λ ) 1.5418 Å) in the 2θ range from 10 to 60°. UV-vis spectra were measured using a Shimadzu UV-2550 spectrometer equipped with a diffuse reflectance accessory in the 200-800 nm range using BaSO4 as the reference. Cyclic voltammetry measurements were carried out on a CHI 660B electrochemical station using a conventional three-electrode single compartment cell at room temperature. The working electrode was a modified CPE (carbon paste electrode). A platinum gauze was used as the counter electrode and an Ag/AgCl (saturated KCl) was used as the reference electrode. All potentials were measured and reported versus the Ag/AgCl electrode. The magnetic susceptibility data were obtained on polycrystalline samples using a Quantum Design MPMS-5S superconducting quantum interference device (SQUID) magnetometer in the temperature range from 2 to 300 K at an applied magnetic field of 5 kG, and the diamagnetic contributions were estimated from Pascal’s constants. The measurements of melting point were carried out on a XT4 digital display binocular melting-point microscope.13C NMR was performed at room temperature on a Bruker Avance 400 instrument with D2O as the solvent and TMS as the internal standard. Synthesis of [Hen][Cu(en)2(H2O)]2[Cu(en)2(H2O)2]0.5[{Cu(en)2(H2O)}2{Cu(en)2} (P2W18O61)2] · en · nH2O (1). A mixture of 0.392 g of Na2WO4 · 2H2O, 0.203 g CuCl2 · 2H2O, 0.25 mL of H3PO4 (85%), 0.10 mL of ethylenediamine () en), and 5 mL of water in the molar ratio of 1:1:3.2:1.5:231 was heated at 160 °C for 5 days. The pH value of the reactive system was adjusted to 3.9 with 4 M HCl before heating. On cooling to room temperature at a rate of 10 °C · h-1, purple block crystals of 1 were obtained in a ca. 65% yield based on W. Found: C, 3.12; H, 1.60; N, 3.87; P, 1.09; Cu, 3.48; W, 64.72%. C26H147.70Cu5.5N26O143.35P4W36 requires: C, 3.06; H, 1.46; N, 3.57; P, 1.21; Cu, 3.42; W, 64.81%. Synthesis of [HenMe]2[Cu(enMe)2][{Cu(enMe)2}P2W18O61] · 9H2O (2). A mixture of 0.396 g of Na2WO4 · 2H2O, 0.206 g of CuCl2 · 2H2O, 0.25 mL of H3PO4 (85%), 0.16 mL of 1,2-diaminopropane () enMe) and 5 mL of water in the molar ratio of 1:1:3.2:1.6:231 was heated at 160 °C for 5 days. The pH value of the reactive system was adjusted to 3.3 with 4 M HCl before heating. On cooling to room temperature at a rate of 10 °C h-1, brownish red block crystals of 2 were obtained in a ca. 70% yield based on W. Found: C, 4.15; H, 1.78; N, 3.35; P, 1.31; Cu, 2.31; W, 65.06%. C18H80Cu2N12O70P2W18 requires: C, 4.25; H, 1.59; N, 3.31; P, 1.22; Cu, 2.48; W, 65.14%. X-ray Crystallography. The intensity data were collected on a Bruker SMART APEX II CCD diffractometer using graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 295(2) K. A total of 66538 (-16 e h e 16, -26 e k e 26, -38 e l e 38) and 19494 (-15 e h e 15, -31 e k e 31, -40 e l e 40) reflections were collected for 1 and 2, respectively, of which independent reflections 33269 (Rint ) 0.0315) for 1 and 9747 (Rint ) 0.0284) for 2 were used to structural elucidation. The data were corrected for Lorentz-polarization effects, and the SADABS program was used for the absorption corrections.19 The structures were solved by direct methods, successive Fourier difference synthesis, and refined by full-matrix least-squares techniques on F2 using the SHELXL-97 software.20 All non-hydrogen atoms were refined anisotropically. The occupancy of the disordered lattice water molecule O22w in 1 was refined as 0.85(2). Bond valence sum calculations21 suggested that there are two reduced W(V) and 16 oxidized W(VI) sites in each Wells-Dawson core, with the valence sums of 4.89-5.01 (av. 4.93 for 1 and 5.01 for 2) and 5.76-6.29 (av.

fw crysta syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) unique data obsd data [I > 2σ(I)] params GOF on F2 R1, wR2a (I > 2σ(I)) R1,wR2 (all data) a

1

2

C26H147.70Cu5.5N26 O143.35P4W36 10210.87 triclinic P1j 13.0990(5) 21.4937(8) 30.6223(11) 72.139(2) 79.695(2) 84.270(2) 8064.4(5) 2 4.205 26.420 33269 22957 2175 1.051 0.0362, 0.0888 0.0617, 0.0935

C18H80Cu2N12O70 P2W18 5083.10 monoclinic C2/c 11.690(2) 24.187(5) 31.111(7) 90 95.556(4) 90 8755(3) 4 3.856 24.157 9747 9204 551 1.060 0.0601, 0.1711 0.0652, 0.1754

R1 ) ∑|Fo| - |Fc|/∑|Fo|, wR2 ) { ∑[w(Fo2 - Fc2)2 ]/∑[w(Fo2)2 ]}1/2.

6.09 for 1 and 6.11 for 2), close to the ideal values for W(V) and for W(VI), respectively. The assignment of the oxidation states for the W atoms was further supported by the redox titration of the acidic solution against a standardized KMnO4 solution. The W(V) atoms are W4, W10, W25, and W31 in 1, and W5 in 2. The hydrogen atoms riding on C and N atoms were located in calculated positions, and those on O atoms were in the positions from difference Fourier maps. All hydrogen atoms were refined isotropically using a riding mode. Crystallographic and refinement details are summarized in Table 1. Catalytic Reaction. The reaction was carried out in a 100 mL, threenecked quartz flask under magnetic stirring. After 50 mmol maleic anhydride was dissolved in 16 mL of water, 5 mL of sodium hydroxide solution (15 mol/L) was added. The flask was then heated up. When the temperature was increased to 55 °C, 5 × 10-3 mmol of the asprepared compounds were added into the flask. After that, 9.4 mL of hydrogen peroxide (30%) was added dropwise; this process was last about 30 min. The reaction was carried out at 65 °C for 1.5 h. When the reaction was completed, the solution was cooled down, and the product of catalytic reaction was separated from the solution.

Results and Discussion Synthesis. Compounds 1 and 2 were obtained from the W/Cu/L (L ) diamine) system under hydrothermal conditions. In principle, the hydrothermal synthesis methodology has many variables, such as starting materials, templates, pH, temperature and pressure,7 which can affect the result significantly. The diamine ligands were chosen for their ability to chelate copper ions with two free positions translocated so that [CuL2]2+ can act as a linking agent between two POM clusters or a grafting group bound to the surface of the POM cluster.7,18,22 Compounds 1 and 2 of Wells-Dawson clusters were isolated from the reactions with higher P:W ratios of (3-4):1, whereas lower ratios lead to Keggin derivatives.23 It was observed that the initial pH value of the reaction mixture plays a very important role on the present reactive systems. Although the heteroelement P was present in our W/Cu/en system, under the same conditions as those employed for 1, we found that the paradodecatungstate [H2W12O42]10- unit was stabilized when the pH value was greater than 7, and that the previously reported compounds (H2en)4[{Cu(en)2}H2W12O42] · 6H2O24 and [Cu(en)2]3[{Cu(en)2}2H2W12O42] · 12H2O18b were isolated at pH 7.8-9.0 and 9.5-11.5, respectively. It is consistent with the observations on isopolyoxotungstates.1a A mixed-valence Keggin compound [Cu(en)2H2O]2[{Cu(en)2}HPW12O40] · 2H2O was isolated at pH

POMs Formed by Wells-Dawson Cores through W-O-W Linkages

4.5-7 under the lower P:W conditions,25 and compound 1 can be prepared as a monophase product only from a narrow pH range of 3.5-4.2 with the optimum of 3.9 in a good yield under the molar ratio of Na2WO4 CuCl2, H3PO4, enMe and water being 1:1:3.2:1.6:231 at 160 °C for 5 days. After reducing the pH value, a previously reported Wells-Dawson hybrid (H2en)3[P2W18O62] · nH2O26 was isolated at pH 1.2-2.8. It is noted that the W sites in (H2en)3[P2W18O62] · nH2O are in a WVI state, but that in 1 are in a mixed WV/WVI state. It appeared that a higher pH value under acidic, hydrothermal conditions is helpful to prepare a mixed-valance Wells-Dawson polyoxotungstate. For the W/Cu/enMe system, the steric interference of the methyl group of enMe ligand on the nature of isopolytungstates and on the anchoring modes of the [Cu(enMe)2]2+ groups on the POM clusters may be significant, the paradodecatungstate derivatives and POM dimers were not observed in our W/Cu/ enMe system in the whole pH explored range of 3-11.5. One compound derived from Keggin [H2W12O40]6- units was isolated at pH 3.6 in the absence of heteroelement, which was reported previously.18c In contrast, compound 2 of Wells-Dawson units was obtained at pH3-3.6 (opt. 3.3) in the presence of the heteroelement P. It is noteworthy that in our experiments, the fully oxidized Na2WO4 · 2H2O was used as the W source, but one-ninth of W atoms in the resulting compounds are in a reduced +5 state. Because no other reducing agents were introduced in our reactive mixtures, it suggests that along with organic ligands to chelate copper ions, templating agents with nucleophilicity, the organic diamines, en and enMe, also act as effective agents to reduce partial WVI to WV. Organic diamine ligands acting as reducting agents under hydrothermal conditions have is not unusual in the preparation of many other POM-based organicinorganic hybrid materials.7,27 However, compared with polyoxomolybdates and polyoxovanadates, reduction of partial metal sites in hydrothermally generated polyoxotungstates is rare. Only four such materials have been reported.25,28 The formation of compounds 1 and 2 can be proposed as the following synthesis scheme (eqs 14b). 3+ 618WO2+ 18H2O 4 + 2PO4 + 36H f [P2W18O62] (1)

Cu2+ + 2L + nH2O f [CuL2(H2O)n]2+

(2)

3[Cu(en)2(H2O)n]2+ + 2[P2W18O62]6- + 0.4en f [{Cu(en)2(H2O)}2{Cu(en)2}(P2W18O61)2]6- + 0.8CO2 + 0.8NH3 + 0.4H2O (3a) [Cu(enMe)2]2+ + [P2W18O62]6- + 0.125enMe f [{Cu(enMe)2}P2W18O61]4- + 0.375CO2 + 0.25NH3 + 0.25H2O (3b) [{Cu(en)2(H2O)}2{Cu(en)2}(P2W18O61)2]6- + 2.5[Cu(en)2(H2O)n]2+ + H++ 2en + nH2O f 1

(4a)

[{Cu(enMe)2}P2W18O61]4-+ [Cu(enMe)2]2++2H+ + 2enMe + 9H2O f 2 (4b) The precursors WO42- and PO43- in aqueous, acidic solution are condensed into Wells-Dawson-type polyanions [P2W18O62]6- (eq 1).4 Deduced from the resultant compounds (1 and 2), the copper complexes containing different number of water molecules are formed (eq 2). After finishing the

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hydrothermal reactions, CO32- was detected in the resultant filtrate by the examination of an Ba(NO3)2 solution, and trace NH3 was detected by a gas chromatography. This suggests that some of the organic diamines were oxidized into CO2 and NH3 under our employed hydrothermal conditions. During the organic diamines are oxidized, partial WVI sites of the Wells-Dawson polyanion [P2W18O62]6- are reduced, and the parent clusters should be synchronously condensed into a supported POM dimer (eq 3a) or a one-dimensional anionic chain (eq 3b). At the finial step (eq 4), the dimers or the chains are assembled with counterions, diamine molecules and water molecules, and are isolated out from the solution. Crystal Structures. Both of the structures are built up on Wells-Dawson polyoxoanions [P2W18O62]8- and Cu(II) complex groups. The parent Wells-Dawson cluster is close to the D3h point symmetry and contains two [R-PW9O31]4- units, which are derived from the well-known Keggin type anion [R-PW12O40]4- by removal of a set of three corner-sharing WO6 octahedra and these two [R-PW9O31]4- units are linked through corner-sharing with the elimination of six oxygen atoms.1a,3 In compounds 1 and 2, the W-O bond distances are in the range of 1.679(8)-2.477(9) Å (avg 1.960 Å for 1 and 1.987 Å for 2). The P-O distances range from 1.474(8) to 1.612(6) Å (avg 1.534 Å for 1 and 1.538 Å for 2). These bond distances are in accord with those in the known polyoxotungstates.3,10,12-17 As mentioned above, bond valence sum calculations21 suggested that the tungsten oxidation states for each Wells-Dawson core in 1 and 2 should be assigned as {W16VIW2V}, where the W-O distances around the WV centers (av. 2.020 Å for 1 and 1.991 Å for 2) are slightly longer than those around the WVI centers (avg 1.952 Å for 1 and 1.955 Å for 2). The formalism is well consistent with the magnetic behaviors of the compounds. The crystal structure of 1 is composed of bisupporting [{Cu(en)2(H2O)}2{Cu(en)2} (P2W18O61)2]6- polyoxoanions, [Cu(en)2(H2O)]2+, [Cu(en)2(H2O)2]2+ and Hen+ cations, en molecules and lattice water molecules. As shown in Figure 1, the novel polyoxoanion [{Cu(en)2(H2O)}2{Cu(en)2}(P2W18O61)2]6- consists of two Wells-Dawson [P2W18O61]6cores, one [Cu(enMe)2]2+ bridging group and two [Cu(enMe)2(H2O)]2+ decoratedfragments.ThetwoWells-Dawson [P2W18O61]6- cores are connected to each other through two common oxygen atoms into a [(P2W18O61)2]12- dinuclear polyoxophosphotungstate cluster, which can be described as a dimer through two W-O-W linkers. The two common oxygen atoms, O17 and O35, are from two belt positions in the two Wells-Dawson cores. Due to the direct condensation, as expected, the W-O bond distances of 1.955(7)-2.029(8) Å (avg 1.979 Å) for the two W-O-W linkers are longer than those of 1.688(8)-1.757(7) Å (avg 1.716 Å) for Ot atoms (bonding to one W atom). The bridging copper(II) site, Cu1, exhibits covalent attachment to the two cluster cores through two terminal O atoms of belt tungsten atoms in the two Wells-Dawson cores with Cu-O bond lengths of 2.580(7) and 2.683(7) Å. The dimeric polyoxotungstate is further coordinated to two [Cu(en)2(H2O)]2+ fragments through two trans-O atoms from two cluster subunits with Cu-O distances of 2.715(7) and 2.937(6) Å. The copper(II) sites, Cu1, Cu2 and Cu3, in the decorated or the bridging groups thus exhibit distorted octahedral environments, each defined by four equatorial N atoms of two en molecules and two apical O atoms. These Cu-O distances are comparable to the reported values, for example, Cu-O ) 2.67 Å,8a 2.87 Å,23 2.36-2.96 Å,18c 2.83-3.05 Å,29 and 2.98 Å.30 There are three crystallographically independent copper(II) sites for the discrete cop-

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Figure 1. (a) Ball-and-stick and (b) combined polyhedral/ball-and-stick representations of dimeric polyoxoanion in 1.

per(II) complexes in 1. The copper(II) in the discrete [Cu(en)2(H2O)2]2+ cation (Cu6), located in a centrosymmetric site, is also in a CuN4O2 octahedral geometry with Cu-O distance of 2.574(9) Å. In contrast, each copper(II) atom in the discrete [Cu(en)2(H2O)]2+ cations presents a distorted square pyramidal geometry, completed by four basal N atoms from two amine ligands and one apical O atom from an aqua ligand with Cu-O distance of 2.333(8) or 2.428(7) Å. The condensation fashion of two Wells-Dawson cores in the [{Cu(en)2(H2O)}2{Cu(en)2}(P2W18O61)2]6- polyanion is distinctly different from those in the previously reported dimeric Wells-Dawson type anions. All of the reported dimers are formed by two lanthanide-ion-substituted Wells-Dawson framework units, in which the lanthanide cations are positioned in the vacant cap sites and hold two Wells-Dawson cores together through two W-O-Ln linkages. [{Ce(H2O)4(P2W17O61)}2]14-,12 [{Lu(H2O)3P2W17O61}2]17-,31 and [{Yb6(µ6-O)(µ3OH)6(H2O)6}(P2W15O56)2]14- 13 are in a cap-to-cap fashion, whereas [{Nd(H2O)3P2W17O61}2]14- 17 and [{Eu(H2O)3P2W17O61}2]14- 32 are in cap-to-belt dimers. However, the two WellsDawson cores in the dimer of 1 are unsubstituted species, and are condensed through two W-O-W linkages in a belt-to-belt fashion. Moreover, along with the common O atoms, the Wells-Dawson cores are further fused through a bridging group. To best of our knowledge, such POM dimer is unprecedented in POM chemistry. POM-supported transition or rare earth metal compounds have been reported in the past decade, such as monosupporting [Ni(bpy)3]1.5[{Ni(bpy)2(H2O)}PW12O40] · 0.5H2O,8b bisupporting [{Cu2(O2CMe)2(bpy)2}{Cu(bpy)2}SiW12O40],8e trisupporting [Cu(phen)2]2- [{Cu(phen)}2Mo8O26] · H2O,33 and tetrasupporting [{Ag2(bpy)3}2{Ag(bpy)}2PW9V3O40].34 It should be pointed out that the previously reported POM-supported metal

compounds are only restricted to discrete polyoxoanion clusters (with one core), with an exception of K2Na6[{Nd(H2O)7}2{Nd(H2O)3P2W17O61}2] · 22H2O.17 Compound 1 and this exception are based on POM dimers. However, the supporting modes in these two compounds are distinctly different. The two [Nd(H2O)7]3- groups in the latter compound are linked to the dimeric polyoxoanion [{Nd(H2O)3P2W17O61}2]14- through two terminal O atoms of two cap tungsten atoms. In contrast, in the [{Cu(en)2(H2O)}2{Cu(en)2}(P2W18O61)2]6- polyoxoanion of 1, the two [Cu(en)2(H2O)]2+ fragments decorate the dimer through one terminal O atom and one bridging O atom in two belt positions of the Wells-Dawson cores. Although both of the terminal O atoms and the bridging O atoms of the POM clusters can act as donor atoms to coordinate to the heterometal complexes in POM-supported metal complexes or metalcomplex-bridged POM extended structures, the phenomenon that in a compound some heterometal complexes are linked to the POM clusters through terminal oxo atoms and some are linked through bridging O atoms is rare.8e,34 The linkages between the decorate fragments and the polyoxoanion cores in 1 may be relevant with the relatively high concentration of the precursors under the employed hydrothermal conditions, along with the steric effects and the interactions between the building blocks. As depicted in Figure 2, each two Wells-Dawson units in 2 are connected with each other through a common O atom and a [Cu(enMe)2]2+ bridging group into a one-dimensional infinite chain parallel to the a axis. The bridging O21 linking two opposite clusters is a terminal O atom of a belt tungsten atom in a Wells-Dawson unit (Figure 3). Similar to compound 1, the W-O bond distances of 1.978(2) Å for the W-O-W linkers in 2 are longer than those of 1.678(8)-1.783(9) Å (avg 1.721 Å) for Ot atoms (bonding to one W atom), because of the direct condensation. The octahedral Cu(II) site in the

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Crystal Growth & Design, Vol. 9, No. 1, 2009 277

Figure 2. Combined polyhedral/ball-and-stick representation of the one-dimensional chain in 2.

Figure 3. Fragment of the one-dimensional chain in 2.

bridging group is completed by four N atoms from two enMe ligands and two trans-O atoms, which are from the terminal O atoms in two adjacent Wells-Dawson clusters with Cu-O distances of 2.534(8) Å. Although the chemistry of polyoxometalates interconnected through metal-organic groups into an extended structure has been well developed in the past decades,7 polyoxometalates sharing common O atoms into extended structures is very rare. The chains in [ET]8[PMnW11O39] · 2H2O,15 [NET3H]5[PCoW11O39] · 3H2O16 and [Co(dpa)2(H2O)2]2[Hdpa] [PCoW11O39]14 are built up of Keggin units, and those in [Me2NH2]8[P2CoW17O61] · 11H2O,35 [Cu(en)2(H2O)]2[H2en][{Cu(en)2}P2CuW17O61] · 5H2O, [Cu(en)2(H2O)]2[Cu(en)2]0.5[Hen]0.5 [{Cu(en)2}P2CuW17O61] · 5H2O,14 and [H3O][Nd3(H2O)17P2W17O61] · 6.75H2 O17 are built up of Wells-Dawson units. All of them are based on substituted or lacunary polyoxometalates. However, unlike these compounds, the infinite chain in 2 is fused on unsubstituted polyoxotungstate clusters. It should be pointed out that the chain in 2 is analogous with those in [Cu(en)2(H2O)]2[H2en][{Cu(en)2}P2CuW17O61] · 5H2O and [Cu(en)2(H2O)]2[Cu(en)2]0.5[Hen]0.5[{Cu(en)2}P2CuW17O61] · 5H2O.14 In these two compounds, each of the two metal sites connected to the common O atom in a Wells-Dawson cluster is occupied by Cu and W with an occupancy of one-half in a belt position. But the situation in 2 can be clearly determined as a W. In the crystal structure of 2, the discrete [Cu(enMe)2]2+ cations are located in the interchain region with Cu2 · · · O7 distance of 3.125(8) Å. These [Cu(enMe)2]2+ groups are at an equal distance between two chains. Although these interactions are quite weak, as shown in Figure 4, the adjacent chains are thus linked into a two-dimensional superamolecular layer parallel to the ac plane. The layers are in turn connected by the

HenMe through hydrogen bonds to form a three-dimensional superamolecular network, with the lattice water molecules are located in the interstice. Spectroscopic and Thermal Analysis. The IR spectra of compounds 1 and 2 (see the Supporting Information, Figures S1 and S2) exhibit a strong band at 1080 cm-1, attributed to the asymmetric stretching vibrations of P-O. The characteristic bands of the terminal W-Ot vibrations of the Wells-Dawson units appear at 943 cm-1, those of the corner-sharing W-Oc-W modes at 900 (1) and 903 (2) cm-1, and those of the edgesharing W-Oe-W vibrations at 789 (1) and 773 (2) cm-1, respectively. The features between 1130-510 cm-1 are due to the diamine ligands, and the peak at 1580 cm-1 is associated with the water molecules δ(HOH). In addition, the wide bonds between 3240-3450 cm-1 can be assigned to ν(N-H) and ν(O-H). The UV-vis diffuse reflectance spectra of 1 and 2 are displayed in Figure 5, where Kubelka-Munk function is defined as F(R) ) (1 - R)2/2R. They possess similar behaviors. The two strong bands near 225 and 303 nm are attributed to the ligand-to-metal charge transfers of Ot f W and µ2-O f W, respectively, where electrons are promoted from the low energy electronic states, mainly comprised of oxygen 2p orbitals, to the high-energy states, which are mainly comprised of metal d orbitals.36 It was evidenced that the d-d band of copper centers with different coordination geometries show a broad peak in their electronic spectra at 650-800 nm,37 the d-d transition brand of WO6 octahedra at 440-500 nm and the WVI f WV intervalence charge-transfer band at 650-720 nm.36-38 Mingling these transitions makes the brand centered near 450 nm broad. The thermal gravimetric curve of compound 1 (see the Supporting Information, Figure S3) gives a total weight loss of 11.47% in the range of 92-555 °C, accordant with the calculated loss of 11.51%. The weight loss of 4.12% at 92-205 °C corresponds to the removal of two free en molecules and 16.35 lattice water molecules per formula (calcd 4.06%). The loss of 7.28% at 240-555 °C arises from the release of eleven coordinated en ligands and five coordinated aqua molecules (calcd 7.35%). The TG curve of 2 (see the Supporting Information, Figure S4) shows a weight loss of 6.02% at 87-290 °C corresponds to the removal of two enMe molecules and nine lattice water molecules per formula (calcd 6.10%). The loss of 6.05% at 290-645 °C comes from the release of four coordinated enMe ligands (calcd 5.83%). The X-ray powder pattern of the post-TGA residue of 1 and 2 are possibly W18P2O59 (JCPDS: 41-0371), W8P4O32 (JCPDS: 43-0390), CuWO4 (JCPDS: 70-1732) and other unidentified phase. Voltammetric Behaviors. Compounds 1 and 2 are insoluble in common solvents; therefore, we employed them to fabricate bulk-modified carbon paste electrodes (CPEs) as the working electrode, according to the method described in ref 39. The cyclic voltammetric behaviors of 1- and 2-CPEs in 1 mol/L

278 Crystal Growth & Design, Vol. 9, No. 1, 2009

Lin et al.

Figure 4. Combined polyhedral/ball-and-stick representation of the two-dimensional superamolecular layer in 2, where the dashed lines denote the Cu · · · O weak interactions. Table 2. Electrochemical Data for Compounds 1and 2a compd

wave

Epc (mV)

Epa (mV)

E1/2 (mV)

∆Ep (mV)

1

1 2 3 1 2 3

-169 -323 -620 -287 -512 -610

84 -302 -547 36 -262 -554

-42 -312 -584 -126 -387 -582

253 21 73 323 250 56

2

a The electrode potentials versus Ag/AgCl were obtained at a scan rate of 150 mV s-1. The half-wave potentials, E1/2, are defined as (Epc + Epa)/2, and the peak separations, ∆Ep, are presented as Epa - Epc.

Figure 5. UV-vis diffuse reflectance spectrum of compounds 1 and 2.

Figure 6. Cyclic voltammograms of 1- and 2-CPEs versus an Ag/AgCl reference electrode in 1 mol/L H2SO4 at the scan rate of 150 mV s-1.

H2SO4 solution in the range from +400 to -700 mV at a scan rate of 150 mV s-1 are displayed in Figure 6. 1-CPE shows three redox peaks with the half-wave potentials E1/2 being -42, -312, and -584 mV, corresponding to three consecutive one-, three-, and one-electron processes with the peak separations ∆Ep being 253, 21, and 73 mV (Table 2), respectively. In contrast, the three redox peaks of 2-CPE are associated with three oneelectron processes. Their half-wave potentials E1/2 and peak separations ∆Ep are -126 and 323 mV, -387 and 250 mV,

-582 and 56 mV, respectively. The first wave for each CPE is attributed to the redox processes of the Cu centers. Its anodic peaks of the Cu+ f Cu2+ reoxidation are broad, unlike the sharp anodic peak for the Cu° f Cu2+ reoxidation in some Wells-Dawson-type compounds containing Cu atoms.40,41 The last two waves for our CPEs are assigned to the redox processes of the W centers. Their potential locations are comparable to those of the reported [P2W17O61(CuOH2)]2- electrode42. As listed in Table 2, it seems that the interconnectivity between the clusters into one-dimensional chains in 2 makes its redox process more easily, which promotes its half-wave potentials to shift toward the negative direction. The reduction of compounds immobilized in the CPEs is accompanied by the evolution of protons from solution to maintain charge neutrality. The extended structure of 2 may slow down the penetration and diffusion rates of the protons from solution into the clusters and decrease the electron exchange rate to some extent. It makes the involving electron number during the second redox process of 2-CPE decrease, and the separation between the corresponding anodic and cathodic peaks increase. As shown in Figure 6, the anodic currents regarding the W centers are greater than their corresponding cathodic ones. This should be derived from their mixed valance nature (WV/WVI) of compounds 1 and 2, whereas the cathodic currents are generally greater than the anodic ones for fully oxidized polyoxotungstates.40-42 Magnetic Measurements. The magnetic susceptibilities of powdered samples of 1 and 2 were measured in the temperature range of 2-300 K. At room temperature, the χmT values are equal to 3.64 and 1.56 emu K mol-1 for 1 and 2 (Figures 7 and 8), respectively, which are close to the expected values of 3.56 and 1.50 emu K mol-1 for 9.5 and 4 noninteracting S ) 1/2 magnetic centers. On lowering the temperature, the χmT value of 1 decreases slowly to about 75 K and then more rapidly below

POMs Formed by Wells-Dawson Cores through W-O-W Linkages

Figure 7. Temperature dependence of the magnetic susceptibility χm, and χmT product and inverse susceptibility (the insert) for 1, where the solid line is the best fit of the data based on the model described in the text.

Figure 8. Temperature dependence of the magnetic susceptibility χm, and χmT product and inverse susceptibility (the insert) for 2, where the solid line is the best fit of the data based on the model described in the text.

75 K, reaching a value of 2.24 emu K mol-1 at 2 K. Such a magnetic behavior might be considered as a result of the antiferromagnetic interaction within the {WV4} unit of two W-O-W bridges in the dimeric structure. In the range from 20 to 300 K, the data fit well to the Curie-Weiss law, χm) C/(T-θ), with a negative Weiss constant (θ) of -5.71 K and the Curie constant (C) of 3.71 emu K mol-1. The negative Weiss constant further indicates that there exists antiferromagnetic coupling in the structure. Considering the fact that the Cu(II) centers are well separated, and that the magnetic superexchange would most likely occur within the µ2-O bridged tetranuclear {WV4} cluster with W · · · W distances of 3.6165(3)-3.8240(4) Å, the magnetic susceptibility data of 1 were analyzed approximately based on the expression, χm ) 5.5Ng2Cuµ2BS(S + 1)/3kT + 30exp(2J/kT) + 6exp(-2J/kT) + 12 Ng2Wµ2B/3kT 5exp(2J/kT) + 3exp(-2J/kT) + exp(-4J/kT) + 7 where the first term is for the 5.5 spin-only Cu(II) centers with S ) 1/2 and the second for the coupling system of the tetranuclear {WV4} in a square skeleton derived from the isotropic Hamiltonian H ) -2J(Sˇ1 · Sˇ2 + Sˇ2 · Sˇ3 + Sˇ3 · Sˇ4 +

Crystal Growth & Design, Vol. 9, No. 1, 2009 279

Sˇ4 · Sˇ1). The best fit for 1 leads to gCu ) 2.08, gW ) 1.98 and J ) -8.9 cm-1 with the agreement factor F ) [Σ(χobs - χcal)2/ 2 1/2 χobs ] ) 2.68 × 10-4. The negative J value suggests an antiferromagnetic interaction between W(V) centers. The thermal variation in χmT of 2 is similar to that of 1, the χmT value of 2 at 2 K is 0.85 emu K mol-1. The magnetic susceptibility of 2 obeys the Curie-Weiss law in the range of 16-300 K with a negative Weiss constant of -3.64 K and the Curie constant of 1.58 emu K mol-1 (Figure 8). The Cu(II) centers in 2 are also well separated. It is expected that the ability of µ2-O bridge in the W-O-W linkers with W · · · W distances of 3.9125(8) Å to mediate magnetic coupling is stronger than that through the Wells-Dawson cores in the chains. Considering two spin-only Cu(II) centers and one coupling system of dinuclear {WV2} skeleton with the Hamiltonian H ) -2JSˇ1 · Sˇ2, the magnetic susceptibility data of 2 were analyzed on the basis of the following expression 2 2 χm ) 2NgCu µBS(S + 1)/3kT + Ng2WµB2/3kT

6 3 + exp(-2J/kT)

A least-squares fit obtains the parameters gCu ) 2.09, gW ) 2.01, and J ) -7.9 cm-1 with the agreement factor F ) 3.43 × 10-4. The negative J value suggests an antiferromagnetic interaction between WV centers. Each two Wells-Dawson cores are condensed through two W-O-W linkers in 1 and through one W-O-W linker in 2, respectively, which makes the antiferromagnetically exchange coupling in 1 is stronger than that in 2. In reduced or mixed-valance POMs, the exchange interactions between magnetic centers are often antiferromagnetic,28a even the unpaired electrons within clusters are delocalized or partially delocalized.18a The antiferromagnetic exchange J values between WV centers in our compounds are greater than the estimated value in [{Cu(terpy)(H2O)2}2{W12O36(PO4)}] · 6H2O,28a but they are comparable to the values in tris(pyrazolyl)borato-oxo-tungsten(V) complexes.43 Catalytic Test. A probe reaction of the epoxidation of maleic anhydride was carried out to investigate the oxidative catalytic activity of compounds. IR spectrum of the catalytic products (see the Supporting Information, Figure S5) exhibits two strong bands at 1279 and 952 cm-1 and the weak peak at 3043 cm-1, assigned to epoxy group, and a feature near 1670 cm-1, associated with νas(COO-). The observations imply that the product should be sodium hydrogen epoxysuccinate. It was further evidenced by the reaction of sodium hydrogen epoxysuccinate to epoxysuccinic acid. According to the method reported in ref 44, we used the product, from the epoxidation of maleic anhydride using 1 or 2 as the catalyst, as the starting material to form epoxysuccinic acid. The latter was identified by its melting point and 13C NMR. Its melting point was observed to be 147-149 °C, in well agreement with the melting point of epoxysuccinic acid (147 °C). The product exhibits two chemical shifts of 173.56 and 53.44 ppm (see the Supporting Information, Figure S6), which is well consistent with the Sadtler standard 13C NMR spectrum of epoxysuccinic acid (173.04 and 52.18 ppm). The catalytic activity of the catalyst was evaluated by the overall conversion of maleic anhydride. Under the above-mentioned experimental conditions, the overall conversions of maleic anhydride were 17.30% for 1 and 18.21% for 2, respectively. In three comparative experiments, Na2WO4 · H2O and (NH4)6P2W18O62 · nH2O were used as the catalysts to replace compound 1 or 2, their overall conversions were 8.43% and 11.47%, respectively. It was found that the activities of the employed catalysts were kept almost unchanged, decreased

280 Crystal Growth & Design, Vol. 9, No. 1, 2009

only less 0.3% in the eighth repetitive experiments. On the other hand, when the as-prepared compounds were replaced by CuCl2 · 2H2O as the catalyst, the overall conversion was too low to be measured. The results suggest that condensing Wells-Dawson type polyoxotungstates into high-nuclearity cluster or extended structures will enhance their catalytic activity in epoxidation of maleic anhydride. Conclusions In summary, compounds 1 and 2 provide the first examples that unsubstituted POM cores can directly condensed through sharing O atoms and heterometal complex fragments into a dimeric and a one-dimensional chain structures, respectively. It demonstrates that unsubstituted POM clusters can be directly condensed into new high-nuclearity metal-oxide derivates or oxo-bridged extended structures, like that has encountered in substituted POM anions. It indicated that the condensation and the connectivity of POM cores were significantly influenced by the pH value of the reactive mixture and the steric interference of the employed ligands. It is expected that through changing the preparation conditions, including starting materials, templates, pH values, temperature, secondary metals and their ligands, and then through controlling the variations in interaction effect and/ or connectivity between the discrete clusters, a vast chemistry of POM with different topologies is accessible, which may provide a rational approach in modifying their electronic, electrochemical and catalytic properties. Acknowledgment. The authors thank the Natural Science Foundation of Fujian Province of China (E0420001, 2005HZ014) and NCETFJ for financial support.

Lin et al.

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Supporting Information Available: Crystallographic data in CIF format, selected bond distances, IR, TG, and identification of the product from the catalytic test (IR and 13C NMR) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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