Sequential Synthesis of 3d–3d′–4f Heterometallic Heptanuclear

Article pubs.acs.org/IC

Sequential Synthesis of 3d−3d′−4f Heterometallic Heptanuclear Clusters in between Lacunary Polyoxometalates Rinta Sato, Kosuke Suzuki, Takuo Minato, Kazuya Yamaguchi, and Noritaka Mizuno* Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: In this work, we have successfully created several unprecedented discrete 3d−3d′−4f heterotrimetallic clusters in between lacunary polyoxometalates (POMs). By the three-step sequential introduction of metal cations into a trivacant lacunary POM TBA4H6[A-α-SiW9O34] (TBA = tetra-n-butylammonium) in organic media, five kinds of sandwich-type POMs with doublediamond-shaped 3d−3d′−4f heptanuclear clusters (IIIFeM4Ln2, TBAnHm[FeM4{Ln(L)2}2O2(A-α-SiW9O34)2], M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+; L = acac (acetylacetonate), hfac (hexafluoroacetylacetonate)) were successfully synthesized for the first time. By introduction of two [Ln(L)2]+ units on the ends of pentanuclear clusters [FeMn4O18(OH)2]23− and [FeCu4O18(OH)2]27−, the magnetic interactions between Mn3+−Mn3+ and Cu2+−Cu2+ could be modulated. Among a series of the heterometallic heptanuclear compounds, IIIFeMn4Lu2 exhibited the slow magnetic relaxation characteristic for a single-molecule magnet under the zero applied magnetic fields.

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INTRODUCTION Polyoxometalates (POMs) are a class of anionic metal-oxide clusters with versatile structural topologies, and their chemical and physical properties can be modulated by the molecular design. These features make them attractive materials in a wide range of fields including catalytic chemistry, analytical chemistry, medicinal chemistry, electrochemistry, and magnetochemistry.1 In particular, lacunary POMs serve as multidentate O-donor ligands, and various metal cations can be incorporated into the lacunary sites for construction of not only single metal sites but also discrete multinuclear clusters. Recently, heterometallic complexes consisting of 3d and 4f metal cations have shown to display remarkable magnetic, catalytic, and optical properties owing to the interaction and/or cooperative effects between these metal cations.2 Although lacunary POMs are useful multidentate ligands for both 3d and 4f metal cations, indeterminacy of competing reactions between oxophilic 4f and less reactive 3d metal cations toward nucleophilic POM ligands makes the precise design and synthesis of 3d−4f heterometallic complexes very difficult. Consequently, strict control of synthetic conditions should be required to construct precisely arranged clusters, and a small number of bimetallic 3d−4f core-containing POMs have been reported to date;3,4 [{α-P2W15O56}6{Ce3Mn2(μ3-O)4(μ2-OH)2}3(μ2-OH)2(H 2 O) 2 (PO 4 )] 47− , 4a [{α-P 2 W 16 O 57 (OH) 2 }{CeMn 6 O 9 (O2CCH3)8}]8−,4b [{(VO)2Dy(H2O)4K2(H2O)2Na(H2O)2}(B-α-AsW9O33)2]8−,4c [K⊂{FeCe(AsW10O38)(H2O)2}3]14−,4d [{Ce(H 2 O) 2 } 2 Mn 2 (B-α-GeW 9 O 34 ) 2 ] 8− , 4e [{DyMn 4 (μ 3 O) 2 (μ 2 -OH) 2 (H 2 O)(CO 3 )}(β-SiW 8 O 31 ) 2 ] 13− , 4g [Dy 6 Fe 6 (H2O)12(SiW10O38)6]26−,4h and [{(GeW9O34)2Dy3(μ-OH)3(H2O)}6{Co2Dy3(μ3-OH)6(OH2)6}4]56−,4i for example. Therefore, © XXXX American Chemical Society

development of synthetic methods applicable to various types of 3d and 4f metal cations is still indispensable. Furthermore, introduction of additional transition metal cations (3d′) into 3d−4f cores to form 3d−3d′−4f heterometallic cores would lead to unique magnetic materials, while POMs possessing such clusters have never been reported so far, to the best of our knowledge. We have recently developed “non-aqueous methods” for synthesis of various multinuclear metal-containing POMs using organic solvent-soluble lacunary precursors in organic media, and these POMs exhibit remarkable magnetic, catalytic, and photocatalytic properties.5 Compared with conventional synthetic methods in aqueous media, the incorporation of metal cations into lacunary POMs in organic media does not suffer from difficulty in controlling the reaction conditions and/or unexpected isomerization of POMs during metal introduction, which makes design of heterometallic arrangements much easier.6 Herein, we report the successful synthesis of 3d-3d′-4f heterometallic clusters by three-step sequential introduction of metal cations into trivacant lacunary Keggin-type POM TBA4H6[A-α-SiW9O34] (SiW9, TBA = tetra-n-butylammonium)7 in organic media (Figure 1). By this method, five kinds of sandwich-type POMs possessing discrete 3d−3d′−4f heterotrimetallic heptanuclear clusters (IIIFeM4Ln2, TBAnHm[FeM4{Ln(L)2}2O2(A-α-SiW9O34)2], M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+; L = acac (acetylacetonate), hfac (hexafluoroacetylacetonate)) were synthesized for the first time. Furthermore, their magnetic properties have been investigated. Received: October 13, 2015

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DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

should sequentially be introduced into the lacunary sites of [A-α-SiW9O34]10− units; thus, mononuclear 3d metal-containing POMs I M (TBA 7 H n [M(A-α-SiW 9 O 34 ) 2 ]·2H 2 O·C 2 H 4 Cl 2 , Figure 1b) were first synthesized by the reaction of SiW9 and M,8 and then IIMMn4 containing 3d−3d′ heterodimetallic pentanuclear clusters could be formed by introduction of 3d′ (Mn3+) cations into the remaining coordination sites of IM.6d On the basis of these results, we envisioned that synthesis of precisely arranged 3d−3d′−4f heterotrimetallic heptanuclear clusters would be possible by the sequential synthetic method. As we expected, the three-step introduction of metal cations into lacunary SiW9 is quite effective, and the desired 3d−3d′−4f (Fe3+−Mn3+−Gd3+) heterotrimetallic heptanuclear cluster could selectively be obtained by the following sequential procedure; first, (1) mononuclear Fe3+-containing POM IFe and (2) heterodimetallic pentanuclear Fe3+−Mn3+-containing POM IIFeMn4 were sequentially synthesized, and subsequently, (3) 3d−3d′−4f heterotrimetallic heptanuclear cluster-containing POM IIIFeMn4Gd2 could be synthesized by the reaction of IIFeMn4 and Gd(acac)3 in 1,2-dichloroethane (Figure 1, see Experimental Section for the detailed procedure). It should also be noted that IIIFeMn4Gd2 could not be formed by the reaction of IFe, Mn(acac)3, and Gd(acac)3 in one-step (Figure S1). The IR spectrum of IIIFeMn4Gd2 showed the bands assignable to CO stretching of acac (1502 and 1609 cm−1), and the bands in the fingerprint region of POMs were quite similar to those of IIFeMn4 (Figure S2). Fortunately, single crystals of IIIFeMn4Gd2 for X-ray analysis were successfully obtained by vapor diffusion of diethyl ether into an acetonitrile solution of IIIFeMn4Gd2 (Table 1). The structure of the anion part of IIIFeMn4Gd2 is shown in Figure 1d. Two [Gd(acac)2]+ units were introduced on the ends of heteropentanuclear [FeMn4O18(OH)2]23− cluster to form an unprecedented 3d−3d′−4f heterometallic heptanuclear [FeMn4{Gd(acac)2}2O20]23− cluster. Four Mn3+ cations were arranged around the central Fe3+, and two [Gd(acac)2]+ units were arranged at each of the two shorter edges of the {Mn4} rectangle, forming the doublediamond-shaped heptanuclear cluster (Figure 1f). Two oxygen atoms of each acac ligand and four oxygen atoms of POM frameworks (pseudo vacant sites) coordinated to each Gd3+. In the [FeMn4{Gd(acac)2}2O20]23− cluster, the {FeO6} octahedron and the {MnO5} pyramid shared their corners, and the {MnO5} pyramid and the {GdO8} square antiprism shared their edges. The bond valence sum (BVS) values of silicon (4.23), manganese (2.92, 3.02), iron (2.83), gadolinium (2.97), and tungsten (5.77−6.36) indicated that the respective valences were +4, +3, +3, +3, and +6 (Table S1). The BVS values of the μ3-oxygen atoms (O35, O35*) bridging two Mn3+ and one Gd3+ were 1.96, indicating that there are no protons on these oxygen atoms, thus oxo ligands. Seven TBA cations per anion were crystallographically assigned in accord with the elemental analysis data. The CSI-mass spectrum of the single crystals of IIIFeMn4Gd2 dissolved in acetonitrile showed the sets of signals centered at m/z 3827 and 7412 assignable to [TBA9FeMn4Gd2(acac)4O2(SiW9O34)2]2+ and [TBA8FeMn4Gd2(acac)4O2(SiW9O34)2]+, respectively (Figure 2a), indicating that the heterotrimetallic structure is stable and IIIFeMn4Gd2 exists as a single species in this solution. The X-ray crystallography, CSI-mass spectrum, and elemental and thermogravimetric analyses data showed that the molecular formula of IIIFeMn4Gd2 is TBA 7 [FeMn 4 {Gd(acac) 2 } 2 O 2 (A-α-SiW 9 O 34 ) 2 ]·2H 2 O· C2H4Cl2.

Figure 1. (a) Three-step sequential synthesis of 3d−3d′−4f heterotrimetallic heptanuclear cluster-containing POMs IIIFeM4Ln2 (M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+) from SiW9 via a mononuclear Fe3+-containing POM (IFe) and a heterometallic pentanuclear cluster-containing POM (IIFeM4). (b−d) Polyhedral and ball-and-stick representations of the anion parts of (b) IFe, (c) IIFeM4, and (d) IIIFeM4Ln2. Orange, purple, green, and red spheres represent Fe, M, Ln, and oxygen atoms, respectively. Orange, purple, green, gray, and light blue polyhedra represent {FeO6}, {MO5}, {LnO8}, {WO6}, and {SiO4}, respectively. (e) ORTEP representation of the anion part of IIIFeMn4Gd2 with thermal ellipsoids drawn at the 50% probability level. (f) Polyhedral and ball-and-stick representation of the heptanuclear core in IIIFeM4Ln2.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization of 3d−3d′−4f Heterotrimetallic Clusters. Initially, we attempted to synthesize 3d−3d′−4f heterotrimetallic clusters by simply mixing 3d, 3d′, and 4f metal cations with SiW9 simultaneously in organic media. However, the attempt ended in failure. For example, the cold-spray ionization (CSI)-mass spectrum of the reaction solution containing Fe(acac)3, Mn(acac)3, Gd(acac)3, and SiW9 in 1,2-dichloroethane showed sets of signals centered at m/z 6421, 6510, 6597, 6652, 6701, and 7004 assignable to [TBA8H6Mn(SiW9O33)2]+, [TBA8H7Mn2(SiW9O34)2]+, [TBA8H8Mn3O2(SiW9O34)2]+, [TBA8H5Mn4O2(SiW9O34)2]+, [TBA8H 2 Mn 5 O 2 (SiW 9 O 34 ) 2 ] + , and [TBA 8 H 4 Mn 4 Gd(acac) 2 O 2 (SiW9O34)2]+, respectively, thus indicating that the complicated mixture was obtained (Figure S1). We have very recently reported that the sequential introduction of metal cations into lacunary POMs in organic media is very effective for the selective synthesis of 3d−3d′ heterodimetallic core-containing POMs IIMMn4 (TBA7Hn[MMn4(OH)2(A-α-SiW9O34)2]·2H2O· C2H4Cl2, M = Fe3+, Co2+, etc., Figure 1c).6d These POMs could not be synthesized by simple mixing of 3d (M) and 3d′ (Mn3+) metal cations and SiW9 simultaneously, and these metal cations B

DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for IIIFeMn4Gd2, IIIFeMn4Dy2, IIIFeCu4Gd2, and IIIFeCu4Dy2 formula FW (g mol−1) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z temp (K) ρcalcd (g cm−3) GOF R1 [I > 2σ(I)] R2

IIIFeMn4Gd2

IIIFeMn4Dy2

IIIFeCu4Gd2

IIIFeCu4Dy2

C140FeGd2Mn4N11O82Si2W18 7103.1 monoclinic C2/c (No. 15) 30.3095(5) 23.7850(4) 33.7132(6) 112.5396(9) 22447.7(7) 4 113(2) 2.102 1.071 0.1005 (for 20046 data) 0.3123 (for all 30522 data)

C144Dy2FeMn4N13O80Si2W18 7157.66 monoclinic C2/c (No. 15) 30.2746(7) 23.7787(6) 33.6356(8) 112.2021(11) 22418.6(9) 4 113(2) 2.121 1.116 0.1067 (for 20992 data) 0.2822 (for all 29021 data)

C122Cl12Cu4F24Fe Gd2N6O78Si2W18 7668.67 monoclinic P21/n (No. 14) 19.6894(2) 20.1629(2) 26.5399(3) 100.5584(4) 10357.81(19) 2 113(2) 2.459 1.011 0.0556 (for 25142 data) 0.1688 (for all 28660 data)

C122Cl12Cu4Dy2F24FeN6O80Si2W18 7711.17 monoclinic P21/n (No. 14) 19.74030(10) 20.1328(2) 26.5264(2) 100.6942(4) 10359.22(14) 2 113(2) 2.472 1.013 0.0526 (for 24518 data) 0.1466 (for all 28691 data)

In a similar way, IIIFeMn4Dy2 with the [FeMn4{Dy(acac)2}2O20]23− cluster and IIIFeMn4Lu2 with the [FeMn4{Lu(acac)2}2O20]23− cluster could be synthesized by the reaction of IIFeMn4 with Dy(acac)3·2H2O and Lu(acac)3·2H2O, respectively (Tables 1 and S1, Figures 2, S2, and S3). The crystal structure of IIIFeMn4Dy2 could successfully be determined by X-ray analysis and was intrinsically isostructural with that of IIIFeMn4Gd2. Although the crystal structure of IIIFeMn4Lu2 could not be determined due to the insufficient quality of the single crystals, the CSI-mass and IR spectra, powder X-ray diffraction patterns, and elemental analysis data suggested that the IIIFeMn4Lu2 possessed essentially the same structure as IIIFeMn4Gd2 and IIIFeMn4Dy2 (Figures 2, S2, and S4). The anion structures of IIFeMn4 and IIIFeMn4Ln2 possessed intrinsically isostructural {FeMn4} core. By the introduction of [Ln(acac)2]+ units on the ends of the {FeMn4} core, the average Mn−O bond length in Mn1−O35−Mn2 of the central {FeMn4} core in IIIFeMn4Ln2 became somewhat shorter (1.86 and 1.87 Å for IIIFeMn4Gd2 and IIIFeMn4Dy2, respectively) than that of IIFeMn4 (1.96 Å), and the average bond angle of Mn1−O35−Mn2 of IIIFeMn4Ln2 became slightly larger (129.6 and 128.7° for IIIFeMn4Gd2 and IIIFeMn4Dy2, respectively) than that of IIFeMn4 (128.3°). The selection of constituent metal cations in the clusters is an important factor to control the properties of POMs, and thus we next investigated applicability of the present synthetic method to other metal clusters. POM II FeCu4 with [FeCu4O18(OH)2]27− cluster was synthesized by the reaction of IFe with Cu(OAc)2·2H2O in 1,2-dichloroethane. The singlecrystal X-ray analysis showed that IIFeCu4 was intrinsically isostructural with IIFeMn4 except that IIFeCu4 possessed four Cu2+ cations instead of Mn3+ cations (Table S2, Figures S2 and S5). The molecular formula of IIFeCu4 is TBA7H4[FeCu4(OH)2(SiW9O34)2]·C2H4Cl2. In our attempt to synthesize a heterometallic heptanuclear cluster by the reaction of IIFeCu4 with Dy(acac)3 in 1,2-dichloroethane, the desired heptanuclear cluster could not selectively be formed, and the CSI-mass spectrum indicated formation of a complex mixture. The number of exchangeable protons with Dy3+ cations in IIFeCu4 is six and larger than that in IIFeMn4 (two protons), which likely resulted in the different reactivity of IIFeCu4 and IIFeMn4 to Dy3+ cations. By using Dy(hfac)39 with highly electron-withdrawing

Figure 2. Positive ion CSI-mass spectra of (a) IIIFeMn4Gd2, (b) IIIFeMn4Dy2, and (c) IIIFeMn4Lu2 in acetonitrile. The sets of signals centered at m/z 3827, 7412, 3832, 7422, 3845, and 7447 were assignable to [TBA9FeMn4Gd2(acac)4O2(SiW9O34)2]2+, [TBA8FeMn4Gd2(acac)4O2(SiW9O34)2]+, [TBA9FeMn4Dy2(acac)4O2(SiW9O34)2]2+, [TBA8FeMn4Dy2(acac)4O2(SiW9O34)2]+, [TBA9FeMn4Lu2(acac)4O2(SiW9O34)2]2+, and [TBA8FeMn4Lu2(acac)4O2(SiW9O34)2]+, respectively.

CF3 groups, IIIFeCu4Dy2 was successfully obtained. The X-ray crystallographic analysis revealed that the anion part of IIIFeCu4Dy2 was intrinsically isostructural with that of IIIFeMn4Dy2 except the type of ligands on Dy3+ cations (Tables S1 and S3, Figure S3). Two [Dy(hfac)2]+ units coordinated to the bridging C

DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry oxygen atoms between Cu2+ cations. Six TBA cations were crystallographically assigned in accord with the elemental analysis data. Six dichloromethane molecules per anion existed neighboring to the hfac ligands on Dy3+ cations. The distances between adjacent fluorine atoms in hfac and chlorine atoms in dichloromethane (3.11 Å between F11 and Cl11; 3.42 Å between F12 and Cl31) were close to the sum of van der Waals radii of fluorine and chlorine (3.22 Å), indicating the presence of halogen−halogen interactions.10 These results suggest that IIIFeCu4Dy2 is likely stabilized by multiple interactions among the hfac ligands on the anion and dichloromethane. The X-ray crystallography and elemental and thermogravimetric analyses data showed that the molecular formula of IIIFeCu4Dy2 is TBA6H5[FeCu4{Dy(hfac)2}2O2(A-α-SiW9O34)2]·6CH2Cl2. By the reaction of IIFeCu4 with Gd(hfac)3, IIIFeCu4Gd2 with the [FeCu4{Gd(hfac)2}2O20]27− cluster could also be synthesized (Tables S1 and S3, Figures S2 and S3). The BVS values of O35 bridging two Cu2+ and one Ln3+ in IIIFeCu4Dy2 (1.27) and IIIFeCu4Gd2 (1.28) indicated that these oxygen atoms were hydroxo ligands (μ3-OH). By introduction of [Ln(hfac)2]+ units, the average Cu−O bond length in Cu1−O35−Cu2 became slightly longer (2.00 and 2.00 for IIIFeCu4Dy2 and IIIFeCu4Gd2, respectively) than that of IIFeCu4 (1.94 Å), and the bond angle of Cu1−O35−Cu2 became slightly larger (121.0 and 120.4° for IIIFeCu4Dy2 and IIIFeCu4Gd2, respectively) than that of IIFeCu4 (119.5°). Usually, POMs are easily polymerized into “infinite” or “oligomeric” structures in the presence of additional metal cations, and several two and three-dimensional structures based on 3d metal-containing POMs and 4f metal linkers have previously been reported.11 In the present POMs IIIFeM4Ln2, β-diketonato ligands located on Ln3+ cations could prevent from formation of infinite structures and maintain the “discrete” clusters. Although several 3d−3d′−4f heterometallic multinuclear complexes have been reported,12 most of them possess “infinite” structures bridged by cyanometalates. In addition, IIIFeM4Ln2 are the first discrete POMs that contain three types of transition metal or lanthanoid cations other than constituent metals of the original frameworks (Si and W). Although there are many reports on POMs with exogeneous organic ligands such as acetate,4b,f phosphate,13a−c ethylenediamine,13d and bipiridyne,13e IIIFeM4Ln2 represents the first series of POMs in which β-diketonato ligands are introduced as exogenous ligands, to the best of our knowledge. Magnetic Properties of Heterotrimetallic Clusters in between POMs. The direct current (dc) magnetic susceptibilities of IIIFeM4Ln2 were measured on the polycrystalline samples. The χmT values of IIIFeMn4Gd2 and IIIFeMn4Dy2 at 300 K were 26.25 and 38.94 cm3 K mol−1, respectively (Figure 3). These values were in good agreement with the sums of two Ln3+ cations (Gd3+, S = 7/2, L = 0, J = 7/2, g = 2, 7.87 cm3 K mol−1; Dy3+, S = 5/2, L = 5, J = 15/2, g = 4/3, 14.16 cm3 K mol−1) and the experimental value for IIFeMn4 (11.90 cm3 K mol−1).6d In addition, the χmT value of IIIFeMn4Lu2 with diamagnetic Lu3+ cations at 300 K was 10.80 cm3 K mol−1 and close to that of IIFeMn4. The χmT values of these POMs continuously decreased with decrease in temperature probably because of the intramolecular antiferromagnetic interactions and the magnetic anisotropy of Ln3+ cations. The temperature dependence of χmT for IIIFeMn4Gd2 and IIIFeMn4Lu2 were fitted with the Heisenberg−Dirac−Van Vleck Hamiltonian. Three types of intramolecular exchange interactions J1 (Fe3+−Mn3+), J2 (Mn3+−Mn3+), and J3 (Mn3+−Gd3+) were considered for

Figure 3. (a) Temperature dependence of χmT of IIIFeMn4Dy2, IIIFeMn4Gd2, IIIFeMn4Lu2, IIIFeCu4Gd2, and IIIFeCu4Dy2 under the applied field of 0.1 T. Solid lines represent the best fits for IIIFeMn4Gd2, IIIFeMn4Lu2, and IIIFeCu4Gd2 by adopting the Heisenberg−Dirac−Van Vleck Hamiltonian.

IIIFeMn4Gd2, and J1 and J2 were considered for IIIFeMn4Lu2. The best fit for IIIFeMn4Gd2 afforded the exchange interactions of J1 = −8.29 cm−1, J2 = −9.51 cm−1, and J3 = −0.02 cm−1. In particular, antiferromagnetic interactions between Mn3+−Mn3+ (J2) became stronger than those in IIFeMn4 (J2 = −1.17 cm−1)6d by the introduction of [Gd(acac)2]+ units, whereas the J1 value hardly changed (J1 = −7.77 cm−1 in IIFeMn4). The enhancement of the antiferromagnetic interactions between Mn3+− Mn3+ was also observed in IIIFeMn4Lu2 (J1 = −8.45 cm−1, J2 = −7.10 cm−1). As mentioned above, by introduction of the third 4f cations, the Mn−O−Mn bond angle became somewhat larger (129.6 and 128.7° for IIIFeMn4Gd2 and IIIFeMn4Dy2, respectively) than that of IIFeMn4 (128.3°). These changes in the coordination geometries of the {FeMn4} core resulted in modulation of the exchange interactions between Mn3+ cations.14 The χmT values of IIFeCu4, IIIFeCu4Gd2, and IIIFeCu4Dy2 at 300 K were 4.18, 19.4, and 24.4 cm3 K mol−1 and significantly smaller than sums of the spin only values, 5.87, 21.6, and 34.2 cm3 K mol−1, respectively (Cu2+, S = 1/2, g = 2.00, 0.38 cm3 K mol−1; high spin Fe3+, S = 5/2, g = 2.00, 4.37 cm3 K mol−1; Gd3+, S = 7/2, L = 0, J = 7/2, g = 2, 7.87 cm3 K mol−1; Dy3+, S = 5/2, L = 5, J = 15/2, g = 4/3, 14.17 cm3 K mol−1) likely because of the presence of considerable antiferromagnetic interactions (Figures 3 and S6). As expected, the analysis of the temperature dependence of χmT for IIFeCu4 indicated strong antiferromagnetic interactions between Fe3+−Cu2+ (J1 = −29.2 cm−1) and Cu2+−Cu2+ (J2 = −28.3 cm−1). By introduction of [Gd(hfac)2]+ units between Cu2+−Cu2+, the exchange interactions became J1 = −26.7 cm−1 (Fe3+−Cu2+), J2 = −15.46 cm−1 (Cu2+−Cu2+), and J3 = 0.35 cm−1 (Cu2+−Gd3+). With regard to IIIFeCu4Gd2, the antiferromagnetic interactions J2 between Cu2+− Cu2+ were weaker than those in IIFeCu4 and these changes are different from the enhanced antiferromagnetic interactions between Mn3+−Mn3+ by the introduction of [Ln(acac)2]+ units (IIIFeMn4Ln2). Both Cu1−O35−Cu2 and Mn1−O35−Mn2 angles became slightly larger by the introduction of [Ln(L)2]+ units as described above, and one of the possible reasons for the different effects on magnetic interactions is the difference of protonation behavior of O35 bridging atoms (oxo ligands in IIIFeMn4Ln2 ; hydroxo ligands in IIIFeCu4Gd2). Finally, the alternating current (ac) magnetic susceptibility measurements of IIIFeM4Ln2 were carried out under the zero applied dc fields for analysis of single-molecule magnet D

DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Frequency dependence of the (a) in-phase and (b) out-of-phase ac magnetic susceptibility of IIIFeMn4Lu2 under the zero applied dc field in the temperature range of 1.9−4.0 K.

Figure 5. (a) Cole−Cole plots for ac magnetic susceptibility of IIIFeMn4Lu2. The solid curves are theoretical calculations on the basis of the generalized Debye model. (b) Plots of ln τ versus T−1 for IIIFeMn4Lu2 under the zero applied dc field. The solid line represents the best fit with the Arrhenius law.

properties (Figures 4 and S7−S11).15 Ac magnetic susceptibility data showed that IIIFeMn4Lu2 exhibited frequency dependence of in-phase (χ′) and out-of-phase components (χ″), indicating the presence of slow magnetic relaxation characteristic for a single-molecule magnet (Figure 4). In contrast, IIIFeMn4Gd2 and IIIFeMn4Dy2 hardly showed slow magnetic relaxation, while IIFeMn4 exhibited single-molecule magnet properties.6d One of the possible reasons is that the magnetic relaxation process of other POMs was changed by magnetic interactions between Mn3+ and Ln3+ cations. The Cole−Cole plots for IIIFeMn4Lu2 in the form of χ″ versus χ′ were fitted using the generalized Debye model,16 and the small α values of 0.19−0.28 showed the small distribution of the relaxation process (Figure 5a). The relaxation time (τ) of IIIFeMn4Lu2 at each temperature was deduced from the frequency dependence of χ″ signals of ac magnetic susceptibility. The ln τ verus T−1 plots showed that the data between 1.9 and 2.9 K, followed a thermally activated behavior with energy barrier for magnetization reversal (Ueff) of 19.7 K and pre-exponential factor (τ0) of 1.4 × 10−7 s (Figure 5b).

was introduced into trivacant lacunary silicotungstate, and the second 3d′ (M) and the third 4f metal cations (Ln, lanthanoid) were successively introduced in organic media, resulting in the selective formation of 3d−3d′−4f heterotrimetallic heptanuclear {FeM4Ln2} core-containing POMs IIIFeM4Ln2 (TBAnHm[FeM4{Ln(L)2}2O2(A-α-SiW9O34)2], M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+; L = acac (acetylacetonate), hfac (hexafluoroacetylacetonate)). The magnetic interactions between Mn3+−Mn3+ and Cu2+−Cu2+ could be modulated by the introduction of [Ln(L)2]+ units, and the antiferromagnetic interactions became stronger and weaker compared with those of the original pentanuclear clusters IIFeMn4 and IIFeCu4. Among a series of the heterometallic heptanuclear clusters, IIIFeMn4Lu2 exhibited the slow magnetic relaxation characteristic for a single-molecule magnet under the zero applied dc fields (Ueff = 19.7 K).

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EXPERIMENTAL SECTION

Materials. TBA4H6[A-α-SiW9O34]·2H2O (SiW9),7 TBA7H10[Fe(A-α-SiW9O34)]·2H2O·C2H4Cl2 (IFe),8 and TBA7H2[FeMn4O2(A-αSiW9O34)]·2H2O·C2H4Cl2 (IIFeMn4)6d were synthesized according to the reported procedures. Gd(hfac)3·2H2O and Dy(hfac)3·2H2O were synthesized according to the reported procedure.9 Fe(acac)3 and Mn(acac)3 were obtained from TCI. Cu(OAc)2·2H2O was obtained from Kanto Chemical. Dy(acac)3·2H2O was obtained from Strem Chemicals. Gd(acac)3·2H2O and Lu(acac)3·2H2O were obtained from Aldrich. Solvents were obtained from Wako Pure Chemical Industries and Kanto Chemical and used as received.

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CONCLUSION We have for the first time synthesized structurally well-defined 3d−3d′−4f heterometallic heptanuclear clusters by the threestep sequential introduction of metal cations into the lacunary pockets of trivacant lacunary POMs. The pillared Fe3+ cation E

DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Instruments. IR spectra were measured on JASCO FT/IR-4100 using KBr disks. CSI-mass spectra were recorded on JEOL JMST100CS and JMS-T100LP. Thermogravimetric and differential thermal analysis (TG-DTA) were performed on Rigaku Thermo plus TG 8120. ICP-AES analyses were performed on Shimadzu ICPS8100. Elemental analyses for C, H, and N were performed on Yanaco MT-6 and Elementar vario MICRO cube at the Elemental Analysis Center of School of Science, the University of Tokyo. X-ray Crystallography. Diffraction measurements were made on a Rigaku MicroMax-007 Saturn 724 CCD detector with graphite monochromated Mo Kα radiation (λ = 0.71069 Å) at 113 K. The data were collected and processed using CrystalClear17 and HKL2000.18 Neutral scattering factors were obtained from the standard source. In the reduction of data, Lorentz and polarization corrections were made. The structural analyses were performed using CrystalStructure,19 WinGX,20 and Yadokari-XG.21 All structures were solved by SHELXS and refined by full-matrix least-squares methods using SHELXL.22 The metal atoms (Si, Mn, Fe, Cu, Gd, Dy, W) and oxygen atoms in the POM frameworks were refined anisotropically. The highly disordered solvent molecules of crystallization (acetonitrile) were omitted by the use of SQUEEZE program.23 The high R2 values for IIIFeMn4Gd2 and IIIFeMn4Dy2 were probably because of the highly disordered solvent molecules and/or the quality of the single crystals. Residual electron densities were observed near W, Gd, and Dy atoms for IIIFeCu4Gd2 and IIIFeCu4Dy2 probably because of disorder of the anion frameworks. CCDC-1429850 (IIFeCu4), CCDC-1429851 (IIIFeMn4Gd2), CCDC1429852 (IIIFeMn4Dy2), CCDC-1429853 (IIIFeCu4Gd2), and CCDC1429854 (IIIFeCu4Dy2) contain the supplementary crystallographic data for this paper. The data can be obtained free of charge via 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 [email protected]). BVS Calculations. The BVS values were calculated by the expression for the variation of the length rij of a bond between two atoms i and j in observed crystal with valence Vi

Vi =

crystals was covered by aluminum foil with tiny holes, and the vial was stored in a 50 mL screw cap vial containing diethyl ether for a few days. Elemental analysis, calcd (%) for C 132 H 28 0 FeGd 2 Mn4N7O78Si2W18 (TBA7[FeMn4{Gd(C5H7O2)2}2O2(SiW9O34)2]· 5H2O: C, 21.84; H, 4.03; N, 1.35; Si, 0.77; Mn, 3.03; Fe, 0.77; Gd, 4.33; W, 45.59. Found: C, 21.57; H, 4.10; N, 1.35; Si, 0.75; Mn, 3.07; Fe, 0.81; Gd, 4.52; W, 45.55. Positive ion MS (CSI, acetonitrile) m/z 3827 [TBA9FeMn4Dy2(C5H7O2)4O2(SiW9O34)2]2+ (m/z 3827), m/z 7412 [TBA8FeMn4Dy2(C5H7O2)4O2(SiW9O34)2]+ (m/z 7412). IR (KBr pellet, cm−1): 1632, 1595, 1519, 1484, 1459, 1401, 1383,1261, 1153, 1107, 1053, 1014, 960, 900, 789, 753, 669, 618, 534, 368, 334. Synthesis of IIIFeMn4Dy2. IIIFeMn4Dy2 was synthesized through the same procedure as that for IIIFeMn4Gd2 using Dy(acac)3·2H2O as the starting material (dark brown crystals, 47% yield based on IIFeMn4). Elemental analysis, calcd (%) for C132H280Dy2FeMn4N7O78Si2W18 (TBA7[FeMn4{Dy(C5H7O2)2}2O2(SiW9O34)2]·5H2O: C, 21.81; H, 4.02; N, 1.35; Si, 0.77; Mn 3.02; Fe, 0.77; Dy, 4.47; W, 45.52. Found: C, 21.49; H, 4.09; N, 1.32; Si, 0.82; Mn, 3.09; Fe, 0.82; Dy, 4.46; W, 46.21. Positive ion MS (CSI, acetonitrile) m/z 3823 [TBA9FeMn4Dy 2 (C 5 H 7 O 2 ) 4 O 2 (SiW 9 O 34 ) 2 ] 2 + (m/z 3823), m/z 7422 [TBA8FeMn4Dy2(C5H7O2)4O2(SiW9O34)2]+ (m/z 7422). IR (KBr pellet, cm−1): 1631, 1593, 1520, 1484, 1460, 1403, 1383, 1361, 1263, 1152, 1105, 1055, 1015, 960, 937, 890, 791, 754, 633, 554, 532, 385, 366, 361. Synthesis of IIIFeMn4Lu2. IIIFeMn4Lu2 was synthesized through the same procedure as that for IIIFeMn4Gd2 using Lu(acac)3·2H2O as the starting material (dark brown crystals, 30% yield based on IIFeMn4). Elemental analysis, calcd (%) for C132H280FeLu2Mn4N7O78Si2W18 (TBA7[FeMn4O2(SiW9O34)2{Lu(C5H7O2)2}2]: C, 21.73; H, 4.01; N, 1.34; Si, 0.77; Mn 3.01; Fe, 0.77; Lu, 4.80; W, 45.36. Found: C, 21.47; H, 4.12; N, 1.33; Si, 0.77; Mn, 3.07; Fe, 0.81; Lu, 4.81; W, 45.71. Positive ion MS (CSI, acetonitrile) m/z 3845 [TBA9FeMn4Lu 2 (C 5 H 7 O 2 ) 4 O 2 (SiW 9 O 3 4 ) 2 ] 2 + (m/z 3845), m/z 7447 [TBA8FeMn4Lu2(C5H7O2)4O2(SiW9O34)2]+ (m/z 7447). IR (KBr pellet, cm−1): 1631, 1613, 1522, 1484, 1463, 1406, 1383, 1362, 1267, 1153, 1107, 1061, 1015, 960, 937, 925, 890, 790, 758, 633, 538, 365. Synthesis of IIFeCu4. To a 1,2-dichloroethane solution (8 mL) of IFe (156 mg, 25 μmol), Cu(OAc)2·2H2O (20 mg, 100 μmol, 4 equiv with respect to IFe) was added, and the resulting solution was stirred for 24 h at room temperature (ca. 20 °C). After diethyl ether (5 mL) was added, the solution was kept for 2 days at 4 °C. The yellow crystals of IIFeCu4 for X-ray crystallographic analysis were obtained (138 mg, 85% yield based on IFe). Elemental analysis, calcd (%) for C114H262Cl2Cu4FeN7O70Si2W18 (TBA7H4[FeCu4(OH)2(SiW9O34)2]· C2H4Cl2): C, 20.75; H, 4.00; N, 1.49; Si, 0.85; Fe, 0.85; Cu 3.85; W, 50.16. Found: C, 20.84; H, 4.05; N, 1.43; Si, 0.83; Fe, 0.88; Cu, 3.80; W, 50.74. Positive ion MS (CSI, DCE) m/z 3492 [TBA 9 H 6 FeCu 4 O 2 (SiW 9 O 3 4 ) 2 ] 2 + (m/z 3492), m/z 6741 [TBA8H6FeCu4O2(SiW9O34)2]+ (m/z 6741). IR (KBr pellet, cm−1): 1635, 1484, 1383, 1153, 1106, 1065, 1008, 998, 970, 955, 929, 905, 881, 797, 768, 737, 649, 528, 442, 419, 375, 354, 348, 336, 303, 290, 279, 254. Synthesis of IIIFeCu4Gd2. To a dichloromethane solution (7 mL) of IIFeCu4 (40 mg, 6.1 μmol), Gd(hfac)3·2H2O (14 mg, 17 μmol, 2.8 equiv. with respect to IIFeCu4) was added, and the resulting solution was stirred for 30 min at room temperature (ca. 20 °C). After diethyl ether (1.4 mL) was added, the solution was kept for 4 days at 30 °C. The yellow crystals of IIIFeCu4Dy2 for X-ray crystallographic analysis were obtained (20 mg, 42% yield based on IIFeCu4). Elemental analysis, calcd (%) for C122H237Cl12Cu4F24FeGd2N6O78Si2W18 (TBA 6 H5[FeCu 4 {Gd(C 5 HF6O 2) 2} 2 O2(SiW 9O34 )2 ]·6CH2 Cl2): C, 18.53; H, 3.02; N, 1.06; Si, 0.71; Fe, 0.71; Cu 3.21; Gd, 3.98; W, 41.85. Found: C, 18.24 H, 2.97; N, 1.01; Si, 0.77; Fe, 0.80; Cu 3.34; Gd, 3.72; W, 43.18. IR (KBr pellet, cm−1): 1653, 1559, 1528, 1506, 1484, 1383, 1259, 1202, 1149, 1003, 962, 907, 883, 795, 660, 535, 370. Synthesis of IIIFeCu4Dy2. IIIFeCu4Dy2 was synthesized through the same procedure as that for IIIFeCu4Gd2 using Dy(hfac)3·2H2O as the starting material (yellow crystals, 73% yield based on IIFeCu4). Elemental analysis, calcd (%) for C122H237Cl12Cu4Dy2F24FeN6O78Si2W18 (TBA6H5[FeCu4{Dy(C5HF6O2)2}2O2(SiW9O34)2]·6CH2Cl2): C,

⎛ r0′ − rij ⎞ ⎟ ⎝ B ⎠

∑⎜ j

where B is constant equal to 0.37 Å and r′0 is bond valence parameter for a given atom pair.24 Magnetic Measurements. Magnetic susceptibility data of the polycrystalline samples were measured on Quantum Design MPMSXL7. Dc magnetic susceptibility measurements were carried out under the applied field of 0.1 T in the temperature range of 1.9−300 K. Ac magnetic susceptibility measurements were carried out under the 3.96 Oe ac oscillating field. Diamagnetic corrections were applied by using Pascal constants and diamagnetisms of the sample holder and TBA4H6[A-α-SiW9O34]·2H2O. By using the PHI program,25 the magnetic interactions in the [FeM4{Ln(L)2}2O20]m− cluster were analyzed by fitting the temperature-dependence of magnetic susceptibilities with the following isotropic spin Heisenberg−Dirac− Van Vleck Hamiltonian (eq 1), where J1, J2 and J3 represent exchange interactions of Fe3+−Mn+, Mn+−Mn+, and Mn+−Ln3+, respectively: H = − 2J1(SFe·SM1 + SFe·SM2 + SFe·SM3 + SFe·SM4) − 2J2 (SM1·SM2 + SM3·SM4) − 2J3(SM1·SLn1 + SM2·SLn1 + SM3·SLn2 + SM4 ·SLn2)

(1)

Synthesis of IIIFeMn4Gd2. To a 1,2-dichloroethane solution (9 mL) of IIFeMn4 (196 mg, 30 μmol), Gd(acac)3·2H2O (60 mg, 122 μmol, 4 equiv. with respect to IIFeMn4) was added, and the resulting solution was stirred for 12 h at room temperature (∼20 °C). After diethyl ether (3 mL) was added, the solution was kept for 1 day at 30 °C. The brown crystals of IIIFeMn4Gd2 were obtained (88 mg, 41% yield based on IIFeMn4). The single crystals suitable for X-ray crystallographic analysis were obtained by vapor diffusion of diethyl ether into an acetonitrile solution of the crude crystals of IIIFeMn4Gd2 at 25 °C: a 6 mL screw cap vial containing the acetonitrile solution of the crude F

DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Int. Ed. 2014, 53, 5356. (e) Ishimoto, R.; Kamata, K.; Suzuki, K.; Yamaguchi, K.; Mizuno, N. Dalton Trans. 2015, 44, 10947. (6) (a) Suzuki, K.; Kikukawa, Y.; Uchida, S.; Tokoro, H.; Imoto, K.; Ohkoshi, S.; Mizuno, N. Angew. Chem., Int. Ed. 2012, 51, 1597. (b) Suzuki, K.; Sato, R.; Mizuno, N. Chem. Sci. 2013, 4, 596. (c) Sato, R.; Suzuki, K.; Sugawa, M.; Mizuno, N. Chem. - Eur. J. 2013, 19, 12982. (d) Suzuki, K.; Sato, R.; Minato, T.; Shinoe, M.; Yamaguchi, K.; Mizuno, N. Dalton Trans. 2015, 44, 14220. (7) Minato, T.; Suzuki, K.; Kamata, K.; Mizuno, N. Chem. - Eur. J. 2014, 20, 5946. (8) Sato, R.; Suzuki, K.; Minato, T.; Shinoe, M.; Yamaguchi, K.; Mizuno, N. Chem. Commun. 2015, 51, 4081. (9) Richardson, M. F.; Wagner, W. F.; Sands, D. E. J. Inorg. Nucl. Chem. 1968, 30, 1275. (10) Pedireddi, V. R.; Reddy, D. S.; Goud, B. S.; Craig, D. C.; Rae, A. D.; Desiraju, G. R. J. Chem. Soc., Perkin Trans. 2 1994, 2353. (11) (a) Zhang, S.; Zhao, J.; Ma, P.; Chen, H.; Niu, J.; Wang, J. Cryst. Growth Des. 2012, 12, 1263. (b) Compain, J.-D.; Mialane, P.; Dolbecq, A.; Mbomekallé, I. M.; Marrot, J.; Sécheresse, F.; Duboc, C.; Rivière, E. Inorg. Chem. 2010, 49, 2851. (c) Yao, S.; Zhang, Z.; Li, Y.; Lu, Y.; Wang, E.; Su, Z. Cryst. Growth Des. 2010, 10, 135. (12) (a) Kou, H. Z.; Zhou, B. C.; Gao, S.; Wang, R. J. Angew. Chem., Int. Ed. 2003, 42, 3288. (b) Gheorghe, R.; Andruh, M.; Costes, J.-P.; Donnadieu, B. Chem. Commun. 2003, 2778. (c) Gheorghe, R.; Madalan, A. M.; Costes, J.-P.; Wernsdorfer, W.; Andruh, M. Dalton Trans. 2010, 39, 4734. (13) (a) Godin, B.; Chen, Y.-G.; Vaissermann, J.; Ruhlmann, L.; Verdaguer, M.; Gouzerh, P. Angew. Chem., Int. Ed. 2005, 44, 3072. (b) Botar, B.; Ellernb, A.; Kögerler, P. Dalton Trans. 2009, 5606. (c) El Moll, H.; Dolbecq, A.; Marrot, J.; Rousseau, G.; Haouas, M.; Taulelle, F.; Rogez, G.; Wernsdorfer, W.; Keita, B.; Mialane, P. Chem. - Eur. J. 2012, 18, 3845. (d) Nohra, B.; Mialane, P.; Dolbecq, A.; Rivière, É.; Marrot, J.; Sécheresse, F. Chem. Commun. 2009, 2703. (e) Wang, J.; Ma, P.; Shen, Y.; Niu, J. Cryst. Growth Des. 2008, 8, 3130. (14) Magnetic interactions in IIIFeMn4Dy2 and IIIFeCu4Dy2 could not be analyzed because of the orbital angular momentum of Dy3+ cations. (15) Copper-containing POMs IIFeCu4, IIIFeCu4Gd2, and IIIFeCu4Dy2 did not show slow magnetic relaxation likely because of their small magnetic anisotropies. (16) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, U.K., 2006. (17) (a) CrystalClear, version 1.3.6; Rigaku and Rigaku/MSC: The Woodlands, TX. (b) Pflugrath, J. W. Acta Crystallogr., Sect. D: Biol. Crystallogr. 1999, D55, 1718. (18) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data Collected in Oscillation Mode. In Methods in Enzymology; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276, Macromolecular Crystallography, Part A, pp 307−326. (19) CrystalStructure, version 3.8; Rigaku and Rigaku/MSC: The Woodlands, TX. (20) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (21) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analysis, release 97-2; University of Göttingen: Göttingen, Germany, 1997. (22) Wakita, K. Yadokari-XG, Software for Crystal Structure Analyses; Yadokari-XG Project, 2001; Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses: Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. Nippon Kessho Gakkaishi 2009, 51, 218. (23) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, A46, 194. (24) (a) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B: Struct. Sci. 1985, B41, 244. (b) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, B47, 192. (c) Trzesowska, A.; Kruszynski, R.; Bartczak, T. J. Acta Crystallogr., Sect. B: Struct. Sci. 2004, B60, 174. (25) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. J. Comput. Chem. 2013, 34, 1164.

18.51; H, 3.02; N, 1.06; Si, 0.71; Fe, 0.71; Cu 3.21; Dy, 4.10; W, 41.79. Found: C, 18.21; H, 3.00; N, 1.00; Si, 0.71; Fe, 0.76; Cu 3.22; Dy, 3.90; W, 41.98. IR (KBr pellet, cm−1): 1654, 1559, 1528, 1506, 1484, 1417, 1383, 1259, 1203, 1149, 1112, 1055, 1005, 962, 908, 883, 795, 785, 761, 741, 694, 672, 661, 587, 536, 368, 350, 332, 289, 280, 257.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02358. Crystallographic data, IR and CSI-mass spectra, and magnetic data (PDF) Crystallographic information file for IIIFeMn4Gd2 (CIF) Crystallographic information file for IIIFeMn4Dy2 (CIF) Crystallographic information file for IIIFeCu4Gd2 (CIF) Crystallographic information file for IIIFeCu4Dy2 (CIF) Crystallographic information file for IIFeCu4 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-3-5841-7272. Fax: +81-3-5841-7220. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports, and Technology of Japan.

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REFERENCES

(1) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. (b) Thematic issue on polyoxometalates: Hill, C. L. Chem. Rev. 1998, 98, 1. (d) Hill, C. L. In Comprehensive Coordination Chemistry II, Vol. 4; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier Pergamon: Amsterdam, 2004; pp 679−759. (e) Long, D.-L.; Tsunashima, R.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 1736. (2) (a) Benelli, C.; Gatteschi, D. Chem. Rev. 2002, 102, 2369. (b) Andruh, M.; Costes, J.-P.; Diaz, C.; Gao, S. Inorg. Chem. 2009, 48, 3342. (c) Matsunaga, M.; Shibasaki, M. Chem. Commun. 2014, 50, 1044. (3) (a) Reinoso, S. Dalton Trans. 2011, 40, 6610. (b) Zhao, J.-W.; Li, Y.-Z.; Chen, L.-J.; Yang, G.-Y. Chem. Commun. DOI: 10.1039/ C5CC10447E. (4) (a) Fang, X.; Kögerler, P. Angew. Chem., Int. Ed. 2008, 47, 8123. (b) Fang, X.; Kögerler, P. Chem. Commun. 2008, 3396. (c) Merca, A.; Müller, A.; Van Slageren, J.; Läge, M.; Krebs, B. J. Cluster Sci. 2007, 18, 711. (d) Chen, W.; Li, Y.; Wang, Y.; Wang, E.-B.; Zhang, Z. Dalton Trans. 2008, 865. (e) Reinoso, S.; Galán-Mascarós, J. R. Inorg. Chem. 2010, 49, 377. (f) Reinoso, S.; Galán-Mascarós, J. R.; Lezama, L. Inorg. Chem. 2011, 50, 9587. (g) Wu, H.-H.; Yao, S.; Zhang, Z.-M.; Li, Y.-G.; Song, Y.; Liu, Z.-J.; Han, X.-B.; Wang, E.-B. Dalton Trans. 2013, 42, 342. (h) Zhang, Z.-M.; Li, Y.-G.; Yao, S.; Wang, E.-B. Dalton Trans. 2011, 40, 6475. (i) Ibrahim, M.; Mereacre, V.; Leblanc, N.; Wernsdorfer, W.; Anson, C. E.; Powell, A. K. Angew. Chem., Int. Ed. 2015, 54, 15574. (5) (a) Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2010, 49, 6096. (b) Kikukawa, Y.; Suzuki, K.; Sugawa, M.; Hirano, T.; Kamata, K.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2012, 51, 3686. (c) Kikukawa, Y.; Kuroda, Y.; Suzuki, K.; Hibino, M.; Yamaguchi, K.; Mizuno, N. Chem. Commun. 2013, 49, 376. (d) Suzuki, K.; Tang, F.; Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem., G

DOI: 10.1021/acs.inorgchem.5b02358 Inorg. Chem. XXXX, XXX, XXX−XXX