Cobalt Clusters with Cubane-Type Topologies Based on Trivacant

Article pubs.acs.org/IC

Cobalt Clusters with Cubane-Type Topologies Based on Trivacant Polyoxometalate Ligands Yan Duan, Juan M. Clemente-Juan, Carlos Giménez-Saiz,* and Eugenio Coronado* Instituto de Ciencia Molecular (ICMol), Parque Científico, Universidad de Valencia, Valencia, Spain S Supporting Information *

ABSTRACT: Four novel cobalt-substituted polyoxometalates having cobalt cores exhibiting cubane or dicubane topologies have been synthesized and characterized by IR, elemental analysis, electrochemistry, UV−vis spectroscopy, X-ray single-crystal analysis, and magnetic studies. The tetracobalt(II)-substituted polyoxometalate [Co4(OH)3(H2O)6(PW9O34)]4− (1) consists of a trilacunary [B-α-PW9O34]9− unit which accommodates a cubane-like {CoII4O4} core. In the heptacobalt(II,III)containing polyoxometalates [Co7(OH)6(H2O)6(PW9O34)2]9− (2), [Co7(OH)6(H2O)4(PW9O34)2]n9n− (3), and [Co7(OH)6(H2O)6(P2W15O56)2]15− (4), dicubanelike {CoII6CoIIIO8} cores are encapsulated between two heptadentate [B-αPW9O34]9− (in 2 and 3) or [α-P2W15O56]15− (in 4) ligands. While 1, 2, and 4 are discrete polyoxometalates, 3 exhibits a polymeric, chain-like structure that results from the condensation of polyoxoanions of type 2. The magnetic properties of these complexes have been fitted according to an anisotropic exchange model in the lowtemperature regime and discussed on the basis of ferromagnetic interactions between Co2+ ions with angles Co−L−Co (L = O, OH) close to orthogonality and weakly antiferromagnetic interactions between Co2+ ions connected through central diamagnetic Co3+ ion. Moreover, we will show the interest of the unique spin structures provided by these cubane and dicubane cobalt topologies in molecular spintronics (molecular spins addressed though an electric field) and quantum computing (spin qu-gates).

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interactions in Co2+ clusters. When octahedrally coordinated by the weak ligand field created by POMs, this d7 ion shows a high spin S = 3/2 with an unquenched orbital momentum. However, the first order spin−orbit coupling, together with the octahedral distortion, results in an extensive splitting of the 4T2g ground term leading to an effective ground spin doublet (S = 1/2), which exhibits a large anisotropy.55 In the past we studied in detail the anisotropic nature of the exchange interactions in Co2+ clusters with different topologies and nuclearities encapsulated in POMs. The most significant results were (i) the first direct evidence for Co2+−Co2+ exchange anisotropy in the ferromagnetically coupled POM [Co 4 (H 2 O) 2 (PW9O34)2]10−, which encapsulates the tetranuclear rhomblike Co2+ cluster depicted in Figure 1e, using inelastic neutron scattering (INS),56,57 and (ii) the effect of the relative orientation of the exchange anisotropy axes in two closely related trinuclear Co2+ clusters (Figure 1c,d) encapsulated in [(NaOH 2)Co3(H2O)(P2W15O56 )2]17− and [Co 3W(H2O)(ZnW9O34)2]12− respectively.58,59 In addition, we also reported higher nuclearity clusters formed by seven or nine Co2+ centers showing coexisting ferromagnetic and antiferromagnetic interactions as a consequence of the topologies depicted in Figure 1n,p.60,61

INTRODUCTION Polyoxometalates (POMs) are molecular metal-oxo clusters with W, Mo, or V in their highest oxidation states.1,2 The ability of these inorganic species to incorporate almost any kind of metal or nonmetal addenda heteroatoms, together with their enormous structural and electronic diversity, allows POMs to have applications in many fields such as catalysis, medicine, material science, and molecular magnetism.3−20 In molecular magnetism, POMs play an important role because their diamagnetic structures can encapsulate clusters of exchangecoupled paramagnetic metal ions with large nuclearities and high spins that become well separated from each other leading to an ideal magnetic insulation. This feature was first exploited to check the validity of the exchange Hamiltonians in magnetic clusters of increasing nuclearities,8,21 to study the interplay between magnetic exchange and electron transfer in mixedvalence POMs22−28 and, more recently, to obtain POM-based single-molecule magnets (SMMs) based either on 3d or 4f metal ions.21,29−53 The series of cobalt-containing POMs is one of the most extended and studied systems in this field since Baker reported the first examples in 1956.54 Table 1 contains a literature survey of the cobalt clusters incorporated in polyoxowolframates, and Figure 1 shows their polyhedral representations. It can be seen that, from 2 to 16, almost all cobalt nuclearities have been achieved. All these clusters represent an excellent opportunity to conduct detailed studies on the magnetic exchange © 2016 American Chemical Society

Received: November 3, 2015 Published: January 5, 2016 925

DOI: 10.1021/acs.inorgchem.5b02532 Inorg. Chem. 2016, 55, 925−938

Article

Inorganic Chemistry

some cobalt-containing POMs have been found to be potent species for water oxidation,96,108−119 including the hexadecanuclear [{Co4(OH)3PO4}4(XW9O34)4]n− (X = SiIV, GeIV, n = 32; X = PV, AsV, n = 28) which contains a central, nonisolated {Co4O4} cubane cluster.105 Here we will show that using the heptadentate lacunary ligand [B-α-PW9O34]9−, the POM formulated as [Co4(OH)3(H2O)6(PW9O34)]4− (1) can be obtained, which contains an isolated cubane core {Co4O4} with all Co2+ ions. Regarding the dicubane topology, we will show that three different POMs incorporating a {Co7O8} core can be obtained: [Co7(OH)6(H 2 O) 6 (PW 9 O 3 4 ) 2 ] 9 − (2) and [Co 7 (OH) 6 (H 2 O) 4 (PW9O34)2]n9n− (3) (both containing the trilacunary [B-αPW9O34]9− ligand) and [Co7(OH)6(H2O)6(P2W15O56)2]15− (4) (containing the trilacunary [α-P2W15O56]12− ligand). The main difference between POMs 2 and 3 is that 2 is an isolated polyanion, while 3 is an unprecedented polymeric POM in which individual neighboring units are linked through two W− O−Co2+ bonds. POMs 2−4 all contain Co3+ in the central position of the dicubane core and Co2+ in the other six metal positions. Owing to this feature, these heptanuclear cobalt clusters can be viewed as formed by two triangular groups (Figure 1c) linked by a diamagnetic Co3+ ion. POMs encapsulating Co2+/Co3+ clusters are not common. As far as we know, there are only three previously reported examples: the [CoII(H2O)(CoIIIW11O39)]7− polyanion, which contains a dinuclear cobalt group similar to the one shown in Figure 1a,54 [CoII2CoIII4(OH)5(H2O)2(CH3CO2)(Si2W18O66)]6−, which contains a hexanuclear cobalt cluster (Figure 1k) comprising four Co3+ and two Co2+ ions apparently disordered between the six cobalt positions,99 and the polyoxoniobate [H2Co8O4(Nb6O19)4]18− which encapsulates an octanuclear cobalt cluster comprising a central {CoIII4O4} cubane core surrounded by four Co2+ ions.120 Here we will present the synthetic strategies to obtain compounds 1−4, their crystal structures, their stabilities in solution using UV−vis spectroscopy and cyclic voltammetry, and their magnetic properties. The interest of the unique spin structures provided by these cubane and dicubane cobalt topologies in molecular spintronics (molecular spins addressed though an electric field) and quantum computing (spin qu-gates) will be shown.

Figure 1. (a−u) Polyhedral representations of the previously reported cobalt clusters incorporated in polyoxowolframates. (t, u) Cubane and dicubane topologies reported in the present work. The POM formulas and nuclearities of these clusters can be found in Table 1.

One can note that Co clusters exhibiting cubane-type topology (Figure 1t,u) have not been reported yet in polyoxowolframate chemistry, although a few examples are known for other metals. Thus, the cubane topology shown in Figure 1t has been reported for nickel and manganese in [Ni4(OH)3(H2O)6(H2PW9O34)]2−106 and [(α-P2W15O56)MnIII3MnIVO3(CH3COO)3]8−,34,42,107 which contain the trilacunary Keggin-type and Dawson-type moieties, respectively. On the other hand, the heptanuclear dicubane core shown in Figure 1u has only been obtained for manganese in [(αP2W15O56)2MnIII6MnIVO6(H2O)6]14−, a POM exhibiting SMM behavior with a S = 21/2 ground state.41 In this work we will explore the chemical conditions that could lead to the formation of isolated cobalt cubane or dicubane cores encapsulated by POM moieties. From a magnetic point of view, this would provide the opportunity to study the anisotropic nature of the exchange interactions between Co2+ ions in such topologies. From the synthetic point of view, this work will be of interest to illustrate that such species can be building blocks for constructing POMs with larger cobalt nuclearities by linking these cubane or dicubane units with phosphate, acetate, carbonate or other ligands. The interest could also span the field of catalysis as, in the last years

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EXPERIMENTAL SECTION

General Methods and Materials. All reagents were of high purity grade quality, obtained from commercial sources, and used without further purification. Pure water (ρ > 18 MΩ·cm) was used throughout. It was obtained using an Elix-3/Millipore-Q Academic water purification system. IR spectra were recorded with KBr pellets on a Thermo NICOLET-5700 FT-IR spectrophotometer. The UV−vis spectra of the relevant POM were recorded on an Agilent 8453 UV− vis spectrophotometer from 190 to 400 nm using 1.000 cm-opticalpath quartz cuvettes in three different buffer solutions: 0.2 M sodium sulfate (pH 3), 0.4 M sodium acetate (pH 5), and 25 mM sodium borate (pH 9). Elemental analysis was performed in the Universidad Complutense de Madrid (CAI de Técnicas Geológicas) by inductively coupled-plasma optical emission spectroscopy (ICP-OES) on solutions prepared by treating the POMs in a hydrofluoric acid/ hydrochloric acid mixture of ratio 1:8 and diluted with water to a known volume. Thermogravimetric analyses were performed on a Mettler Toledo TGA/SDTA851e analyzer. Cyclic voltammetry measurements were carried out on an Autolab PGSTAT12 potentiostat using a three-electrode single compartment cell supplied by IJ Cambria. It was equipped with a 3 mm glassy carbon disc working electrode (which was polished sequentially with 0.3, 0.1, and 0.05 μm alumina powders and washed with distilled water before each 926


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Inorganic Chemistry Table 1. List of Polyoxowolframates Incorporating Cobalt Clustersa polyhedral representationb 1a 1b

1c

1d 1e

1f 1g 1h 1id 1j 1k 1l 1m 1n 1o 1p 1q 1r 1s

1t 1u

polyanion

Con+c

[Co (H2O)(Co W11O39)] [CoII(H2O)(CoIIIW11O39)]7− [Na2CoII2(PW9O34)2]12− [(NaOH2)2CoII2(P2W15O56)2]18− [{CoIII(en)(OH)2CoIII(en)}{PW10O37CoIII(en)}2]8− [{CoIIW5O18H}2]6− [(P4W6O34)2CoII2Na2(H2O)2]18− [CoII2(P2W15O56)2]20− [{CoII(H2O)2(OH)}2{Zn(H2O)}2{HSiW10O36}2]8− [(NaOH2)CoII3(H2O)(P2W15O56)2]17− [{CoII3(SiW9O33(OH))(SiW8O29(OH)2)}2]22− [CoII6(H2O)2(PW9O34)2(PW6O26)]17− [{(SiW9O33(OH))(SiW8O29(OH)2)CoII3(H2O)}2CoII(H2O)2]20− [(SiW10O36)(SiW8O30(OH))CoII4(OH)(H2O)7]10− [(CoII(H2O)CoII2VW9O34)2(VW6O26)]17− [CoII3W(H2O)2(GeW9O34)2]8− [WCoII3(D2O)2(ZnW9O34)2]12− [CoII4(H2O)2(PW9O34)2]10− [CoII4(H2O)2(SiW9O34)2]12− [CoII4(H2O)2(GeW9O34)2]12− [CoII4(Hen)2(SiW9O34)2]10− [CoII4(Hen)2(GeW9O34)2]10− [CoII4(Hdap)2(HGeW9O34)2]8− [CoII4(Hdap)2(PW9O34)2]8− [CoII4(H2O)2(P2W15O56)2]16− [CoII4(H2O)2(As2W15O56)2]16− [{CoII(H2O)}2(OH)2{CoII(H2O)2}2(H2SiW10O36)2]6− [CoII4(H2O)2(VW9O34)2]10− [{SiCoII2W10O36(OH)2(H2O)}2]12− [(SiW9O34)CoII4(OH)L2(CH3COO)3]8− (L = OH−, N3−) [(SiW9O34)CoII4(OH)2(H2O)3(β-alanine)2]4− [{CoII4(OH)(H2O)3}(Si2W19O70)]11− [CoII3W(L)2(CoIIW9O34)2]12− (L = H2O, D2O) [CoII2CoIII4(OH)5(H2O)2(CH3CO2)(Si2W18O66)]6− [CoII6(OH)3(H2O)9(L)(PW9O34)] (L = 4,4′-bis(1,2,4-triazol-ylmethyl)biphenyl) [{(PW9O34)CoII3(OH)(H2O)2(Ale)}2CoII]14− (H5Ale = alendronic acid) [CoII7(H2O)2(OH)2P2W25O94]16− [(SiW9O34)2CoII8(OH)6(H2O)2(CO3)3]16− [CoII9(OH)3(H2O)6(HPO4)2(PW9O34)3]16− [CoII9(OH)3(H2O)6(HPO4)2(P2W15O56)3]25− [CoII9Cl2(OH)3(H2O)9(SiW8O31)3]17− [Co14P10W60O232(OH)9(H2O)6]35− [{CoII4(OH)3PO4}4(PW9O34)4]28− [{CoII4(OH)3PO4}4(SiW9O34)4]32− [{CoII4(OH)3PO4}4(GeW9O34)4]32− [{CoII4(OH)3PO4}4(AsW9O34)4]28− [CoII4(OH)3(H2O)6(PW9O34)]4− [CoII6CoIII(OH)6(H2O)6(PW9O34)2]9− [CoII6CoIII(OH)6(H2O)4(PW9O34)2]n9n− [CoII6CoIII(OH)6(H2O)6(P2W15O56)2]15−

2

54

2

62 63 64 65 66 67 68 59, 63 69 70, 71 72 72 73 74 58 57, 75−81 82−84 74, 83, 85 86 87 88 89 76 90, 91 92 93 94 72 95 96 97, 98 99 100

II

II

8−

3

3 4

4 4 4 4 5 6 6 7 7 8 9 9 14 16

4 7

reference

40 61 72 60, 81, 101, 102 45 103 45 104, 105 105 105 105 this work this work

a

en = ethylenediamine; dap = 1,2-diaminopropane. bRefers to the representations shown in Figure 1. cThe number of Con+ ions incorporated in the POM structure. dThis POM contains a tetranuclear cobalt cluster disordered with two different arrangements (see Figure 1i). Synthesis of Na 1 . 5 Cs 2 . 5 [Co 4 (OH) 3 (H 2 O) 6 (PW 9 O 3 4 )]·9H 2 O (Na1.5Cs2.5-1). 2.05 g (25 mmol) of CH3COONa and 1.24 g (4.41 mmol) of CoSO4·7H2O were dissolved in 50 mL of water, producing an aqueous solution with a pH of 7.5. Then, 2.42 g (1 mmol) of freshly prepared, solid Na8H[B-α-PW9O34] (synthesized according to a literature method121) were added in small portions, with stirring, to the previous solution. When the addition was completed, the solution

experiment), a platinum wire counter electrode and a Ag/AgCl (3 M KCl) reference electrode. All cyclic voltammograms were recorded at a scan rate of 10 mV·s−1 using the same media and concentrations as for UV−vis spectroscopy. The solutions were deaerated during at least 15 min with argon and kept under a positive pressure of this gas during the experiments. 927


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Inorganic Chemistry was stirred during 30 min, heated to 80 °C for 20 min, and then allowed to cool back to room temperature (pH 6.6). At this point, 0.854 g (5.07 mmol) of CsCl was added in small portions producing a violet precipitate which was filtered out (dry weight 1.84 g) and identified as a mixed cesium/sodium salt of the well-known tetracobalt sandwich polyoxoanion [Co4(H2O)2(PW9O34)2]10− by IR spectroscopy (see Figure S2). The filtrate was kept in an open container at room temperature, and, after 5 days, pink, needle-like crystals were obtained, filtered, washed with cold water, and air-dried (yield: 0.143 g; 5% based on [B-α-PW9O34]9−). Anal. Calcd (Found) for Na1.5Cs2.5[Co4(OH)3(H2O)6(PW9O34)]·9H2O: P, 0.98 (0.90); W 52.47 (50.4); Co 7.48 (6.8); Cs 10.54 (11.0); Na 1.09 (0.84). IR (2% KBr pellet 1100−400 cm−1; see Figure S1): 1077 (m), 1026 (s), 951(m, sh), 933 (s), 887 (s), 793 (w), 728 (s), 588 (w), 484 (w). The TG curve of 1 (Figure S3) shows a total weight loss of 10.34% in the range 30−500 °C, which agrees with the loss of 15 water molecules and 3 hydroxyls in the structure (calcd 10.19%). Synthesis of Cs7Na2[Co7(OH)6(H2O)6(PW9O34)2]·20H2O (Cs7Na22). The synthetic procedure was similar to that in Na1.5Cs2.5-1, but after the filtration of the violet precipitate, 0.024 g (0.09 mmol) of K2S2O8 was added to the filtrate, and the solution was heated again to 80 °C for 1 h. The mixture was allowed to cool back to room temperature and filtered again. The resultant orange solution was kept in an open container, and, after 1 day, orange, thin, layered crystals were obtained, filtered, washed with cold water, and air-dried (yield: 0.224 g; 7% based on [B-α-PW9O34]9−). Anal. Calcd (Found) for Cs7Na2[Co7(OH)6(H2O)6(PW9O34)2]·20H2O: P, 0.97 (0.84); W 51.56 (51.4); Co 6.43 (5.9); Cs 14.50 (13.5); Na 0.72 (0.57). IR (2% KBr pellet 1100−400 cm−1, see Figure S1): 1062 (m), 1027 (s), 977 (m, sh), 952 (m, sh), 935 (s), 878 (s), 806 (s), 779 (m, sh), 721 (s), 588 (w), 540 (w), 514 (w), 484 (w), 444 (w), 422 (w). The TG curve of 2 (Figure S4) shows a total weight loss of 9.25% in the range 30−500 °C, which agrees with the loss of 26 water molecules and 6 hydroxyls in the structure (calcd 8.89%). Synthesis of K5Na2[Co7(OH)6(H2O)4(PW9O34)2]Co(H2O)2·20H2O (K5Na2-3). 21.60 g (65.6 mmol) of Na2WO4·2H2O and 0.84 g (5.92 mmol) of Na2HPO4 were dissolved in 60 mL of water, and the pH of the solution adjusted to 5.5 using glacial acetic acid. Another aqueous solution containing 5.16 g (20.72 mmol) of Co(CH3COO)2·4H2O in 100 mL of water was added dropwise to the first one, and the pH of the resultant solution was adjusted again to 5.5 using glacial acetic acid. Then, the solution was refluxed for 2 h and hot filtered. To the hot filtrate, 8.96 g (91.2 mmol) of potassium acetate and 0.44 g (1.63 mmol) of potassium persulfate were successively added in small portions. After the addition of the solids, the solution was concentrated at 80 °C until a final volume of 120 mL was attained. Then, the solution was allowed to cool back to room temperature, and a large amount of black precipitate was formed which was filtered, washed with cold water, and air-dried (9.42 g). The IR spectra of this precipitate (see Figure S2) is almost identical to the corresponding spectra of the tetracobalt sandwich polyoxoanion [CoII4(H2O)2(PW9O34)2]10−, although its black color indicates that one or more Co ions bear an oxidation state of +3. The filtrate was allowed to stand at room temperature in an open container, and, after 1 day, a brown precipitate was formed which was filtered and recrystallized in water to afford 35 mg (yield < 1%) of orange-brown, plate-like crystals. Anal. Calcd (Found) for K5Na2[Co7(OH)6(H2O)4(PW9O34)2]Co(H2O)2· 20H2O: P, 1.08 (1.30); W 57.63 (57.4); Co 8.21 (7.5); K 3.40 (3.6); Na 0.80 (0.61). IR (2% KBr pellet 1100−400 cm−1; see Figure S1): 1062 (m), 1028 (s), 952 (m, sh), 935 (s), 880 (s), 804 (s), 780 (m, sh), 721 (s), 588 (w), 539 (w), 508 (w), 481 (w), 415 (w). The TG curve of K5Na2-3 (Figure S5) shows a total weight loss of 10.14% in the range 30−380 °C, which agrees with the loss of 26 water molecules and 6 hydroxyls in the structure (calcd 9.93%). Synthesis of Na15[Co7(OH)6(H2O)6(P2W15O56)2]·38H2O (Na15-4). A solution of 0.192 g (1.30 mmol) of CH2(COONa)2 in 10 mL of water was added to 15 mL of an aqueous solution containing 0.527 g (1.81 mmol) of Co(NO3)2·6H2O. To the resulting pink solution (pH 6.8), 0.800 g (0.181 mmol) of solid Na12[α-P2W15O56]·24H2O (synthesized according to a literature method121) were added in small portions with

stirring causing a color change, from pink to light red. The solution was stirred during 2 h (pH 5.9), heated to 80 °C for 30 min, and then allowed to cool back to room temperature. At this point, 0.018 g (0.011 mmol) of CsCl were added in small portions producing a reddish precipitate which was filtered out and identified as a cesium/ sodium salt of [Co4(H2O)2(P2W15O56)2]16− by IR spectroscopy (see Figure S2). The filtrate was kept in an open container at room temperature, and, over the next days, its color changed gradually from red to orange. After 14 days, orange, plate-like crystals were obtained, filtered, washed with cold water, and air-dried (yield: 0.128 g; 8% based on [α-P2W15O56]12−). Anal. Calcd (Found) for Na15[Co7(OH)6(H2O)6(P2W15O56)2]·38H2O: P, 1.36 (1.30); W 60.7 (57.2); Co 4.54 (5.2); Na 3.80 (2.2). IR (2% KBr pellet 1100−400 cm−1, see Figure S1): 1085 (s), 1043 (m), 1007 (m, sh), 976 (m, sh), 947 (m, sh), 934 (s), 908 (s), 881 (s), 823 (s), 728 (s), 599 (w), 561(w), 528 (w), 456 (w). The TG curve of Na15-4 (Figure S6) shows a total weight loss of 9.78% in the range 30−500 °C, which agrees with the loss of 44 water molecules and 6 hydroxyls in the structure (calcd 9.85%). X-ray Crystallography. Suitable crystals of Na1.5Cs2.5-1, Cs7Na22, K5Na2-3, and Na15-4 were coated with Paratone N oil, suspended on small fiber loops, and placed in a stream of cooled nitrogen (120 K) on an Oxford Diffraction Supernova diffractometer equipped with a graphite-monochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). The data collection routines, unit cell refinements, and data processing were carried out using the CrysAlis software package,122 and structure solution and refinement were carried out using SHELXS97 and SHELXL-97.123 In Na1.5Cs2.5-1, four well-behaved water molecules of solvation and 2.5 cesium counter-cations could be found in difference electron density maps. One of the cesium ions was located on the b-glide plane (Cs1), a second one in general position (Cs2), and a third one was disordered between two positions (Cs3 and Cs4). The crystal structure of Na1.5Cs2.5-1 contains channels parallel to the c axis in which further disordered water molecules and sodium countercations (detected by chemical analysis) lay. According to the TGA and chemical analysis results, 5 water molecules and 1.5 sodium countercations per polyoxometalate reside in these channels. These additional water molecules and sodium ions were added to the final formula of Na1.5Cs2.5-1 using the SQUEEZE procedure implemented in the PLATON program.124 The X-ray diffraction pattern of Cs7Na2-2 reveals diffuse diffraction spots which gave rise to diffuse streaks along the c*-axis. This fact is indicative of the presence of stacking disorders of the POMs, which form layers in the crystallographic ab plane. These stacking disorders are the origin of the high R factors obtained for the crystal structure of Cs7Na2-2, which must then be considered as an “average” structure. According to the TGA and chemical analysis results, this compound contains 10 additional water molecules per POM, which were added to the formula of Cs7Na2-2 using the SQUEEZE procedure implemented in the PLATON program.124 In K5Na2-3, apart from the cobalt atoms forming the dicubane core, a further cobalt atom was found in Fourier maps, which is bonded to four surface oxygen atoms of the POM and two water molecules. This cobalt atom (Co5) and the water molecules bonded to it have 50% refined occupancies, which (due to the centrosymmetric nature of the POM) gives a total of one such cobalt atom per POM. Ten water molecules of crystallization were found in the asymmetric unit, and no significant voids with disordered solvent were detected. The refinement of the crystal structure of Na15-4 reveals that the polyoxoanion exhibits an 8% disorder consisting in a 60° rotation of the full POM around a hypothetical pseudosenary axis coincident with the long axis of the POM. This disorder was only evident for the W atoms of the capping triads of the two [α-P2W15O56]12− lacunary units and also for the Co atoms which occupy the three vacant sites in both units. Therefore, the disorder was modeled only for those W and Co atoms. The disorder is not revealed in the remaining W atoms belonging to the six-membered belts of the lacunary [α-P2W15O56]12− unit because a 60° rotation brings these W atoms into coincidence. The disorder is neither apparent for the much lighter O atoms of the 928


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Table 2. Crystallographic Data for Na1.5Cs2.5[PW9Co4O34(OH)3(H2O)6]·9H2O (Na1.5Cs2.5-1), Cs7Na2[Co7(OH)6(H2O)6(PW9O34)2]·20H2O (Cs7Na2-2), K5Na2[Co7(OH)6(H2O)4(PW9O34)2]Co(H2O)2·20H2O (K5Na2-3), and Na15[Co7(OH)6(H2O)6(P2W15O56)2]·38H2O (Na15-4) compound

Na1.5Cs2.5-1

Cs7Na2-2

K5Na2-3

Na15-4

empirical formula formula weight space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z T/K λ/Å ρcalcd/g cm−3 μ/mm−1 R [F02 > 2σ(F02)]a Rw [F02 > 2σ(F02)]b

Co4Cs2.50H33Na1.50O52PW9 3153.36 Pbcn 24.7139(2) 21.6677(2) 20.02760(10) 90 90 90 10724.65(14) 8 120.00(10) 0.71073 3.906 22.243 0.0470 0.1346c

Co7Cs7H58Na2O100P2W18 6418.56 P21/c 12.4982(5) 12.6768(6) 33.332(2) 90 96.416(5) 90 5248.0(5) 2 120.00(10) 0.71073 4.062 23.249 0.1495 0.3933d

Co8H58K5Na2O100P2W18 5742.62 P1̅ 10.2222(3) 12.6115(4) 17.8646(6) 77.041(3) 83.369(3) 73.088(3) 2144.23(11) 1 120.00(10) 0.71073 4.447 25.952 0.0519 0.1184e

Co7H94Na15O162P4W30 9083.49 P1̅ 12.7112(3) 12.7409(4) 23.7998(6) 101.199(2) 93.011(2) 105.768(2) 3616.04(17) 1 120.00(14) 0.71073 4.171 24.745 0.0470 0.1044f

a R = Σ(||F0| − |Fc||)/Σ|F0|. bRw = {Σ[w(F02 − Fc2)2]/Σ[w(F02)2]}1/2. w = 1/[σ2(F02) + (AP)2 + BP], where P = (F02 + 2Fc2)/3. cA = 0.0899, B = 0. dA = 0.2000, B = 0. eA = 0.0537, B = 44.75. fA = 0.0345, B = 189.1526.

POM due to the small extent of the disorder. The disorder was modeled separately for the W and Co atoms giving exactly the same extent (8%) for both of them, which points to a rotational disorder of the whole POM and rules out the possibility of a α → β conformational disorder. Fifteen sodium countercations per POM (in full agreement with the anionic charge of 5) and 32 water molecules of solvation were found in difference electron density maps. However, there are regions of the unit cell of Na15-4 which contain additional disordered water molecules, which, according to the TGA and chemical analysis results, were estimated as six additional water molecules per POM. These water molecules were added to the formula of Na15-4 using the SQUEEZE procedure implemented in the PLATON program.124 All atoms were refined anisotropically in the four crystal structures except some sodium cations and water molecules of solvation having partial occupancies. Absorption corrections were applied to the four crystal structures. Hydrogen atoms of water molecules or hydroxyl anions were not located. The crystallographic data for the four structures are summarized in Table 2. Magnetic Measurements. Samples of Na1.5Cs2.5-1, Cs7Na7-2, K5Na2-3, and Na15-4 were prepared by compacted powder molded from ground crystalline samples. Each sample was covered with the minimum amount of liquid eicosane (40 °C) in order to prevent crystallite torquering. Variable-temperature susceptibility measurements were carried out in the temperature range 2−300 K on a magnetometer equipped with a SQUID sensor (Quantum Design MPMS-XL-5). The data were corrected for diamagnetic contribution from eicosane and for the diamagnetic contributions of the polyanions as deduced by using the Pascal’s constant tables. Isothermal magnetization measurements at low temperature (2 and 5 K) were performed up to a field of 5 T in the same apparatus.

■

tion. However, the addition of cesium cations to an aqueous [B-α-PW9O34]9−/Co2+ system in similar conditions results in a bulky precipitate of the well-known tetranuclear Co2+ cluster sandwiched by two POM moieties of [B-α-PW9O34]9−of formula [Co4(H2O)2(PW9O34)2]10−, originally reported by Weakley.75 To obtain the tetracobalt cubane-containing POM (1) one needs first to filter this cesium salt. Then, 1 crystallizes from the mother liquor after 5 days of standing, as a minor product with very low yield (7%). The reason for the different behavior between Co2+ and Ni2+ ions lies in the higher rate of formation of [Co4(H2O)2(PW9O34)2]10− in the pH range in which the heptadentate [B-α-PW9O34]9− ligand is stable (approximately 5−8).76 This hinders the formation of 1, which must be considered as a byproduct. In contrast, reaction of [B-α-PW9O34]9− with Ni2+ ions does not produce significant amounts of the corresponding Weakley’s POM, allowing the formation of other nickel-substituted POMs in higher yields, such as the aforementioned cubane-containing [Ni4(OH)3(H2O)6(H2PW9O34)]2− polyanion (at pH 4.8) or the trinickel POM [Ni3(H2O)3PW10O39H2O]7− (at pH 6−7).125 The nickel-containing Weakley’s POM is more conveniently obtained by direct condensation of wolframate and phosphate ions at pH 6.5 rather that from the preformed [B-α-PW9O34]9− ligand.126 An additional difference between cobalt and nickel systems is that 1 is obtained at pH value of 6.6, while significantly lower value (4.8) is required for the synthesis of the analogous nickel derivative.106 A POM similar to 1, i.e., containing a cubane cobalt cluster, can be envisioned using the trilacunary Dawson unit [αP2W15O56]12− instead of the Keggin one. In this case, the formation of the tetranuclear cobalt sandwich [Co4(H2O)2(P2W15O56)2]16−, first reported by Finke,76,127 is also highly favored in the reaction system [α-P2W15O56]12−/Co2+. Therefore, the addition of a small amount of cesium chloride produces a bulky precipitate containing Finke’s POM, while other minor POM species remain in solution (larger amounts of cesium cations cause the precipitation of these minor species too). Unexpectedly, POM 4 was obtained from the mother

RESULTS AND DISCUSSION

Synthetic Approach. In order to obtain cobalt clusters with the topologies shown in Figure 1t,u, we fixed our attention on the synthetic procedure of the previously known [Ni4(OH)3H2O)6(H2PW9O34)]2− anion, which contains a cubane {Ni4O4} unit.106 This POM was obtained in moderate yield (35%) by the reaction of the trilacunary Keggin anion [Bα-PW9O34]9− with Ni2+ ions at pH 4.8 and 80 °C and isolated by precipitation as a cesium salt and subsequent recrystalliza929


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Inorganic Chemistry liquor resulting after the filtration of the precipitate, instead of the hypothetical “[Co4(OH)3(H2O)6(P2W15O56)]7−”. POM 4 encapsulates a {Co7O8} dicubane cobalt cluster (shown in Figure 1u) in which the central cobalt atom is in a trivalent state. This oxidation state is probably obtained because the central cobalt atom is in a coordination environment of six hydroxyl anions. This fact reduces the E° value of the Co3+/ Co2+ couple, thereby facilitating oxidation of the central cobalt atom to the +3 state by atmospheric O2. This oxidation process takes place during the 14 days of slow evaporation of the solvent required to obtain the crystals of Na15-4 and is evidenced by the gradual change of the solution from red to orange. Crystals of Na1.5Cs2.5-1, however, were obtained after only 4 days of evaporation at room temperature, which is probably too short time to get a significant concentration of an oxidized cobalt POM in solution by the action of atmospheric O2. Then, when using the ligand [B-α-PW9O34]9− the lower solubility of Na1.5Cs2.5-1 seems to be the reason POM 1 (containing a Co2+ cubane cluster) is obtained instead of a POM containing a Co2+/Co3+ dicubane cluster. In order to deliberately obtain this last compound the oxidation process must be accelerated by the addition of a one-electron oxidant (such as potassium persulfate). In these conditions the main POM species in solution are the oxidized ones, which crystallize after 1 day as Cs7Na2-2. To obtain the cubane-containing POM with the lacunary ligand [α-P2W15O56]12− we attempted two strategies: (i) addition of larger amounts of cesium cations in the synthesis of 4, and (ii) decreasing the pH of the reaction mixture. However, larger amounts of cesium always led to a mixed precipitate containing Finke’s POM ([Co 4 (H 2 O) 2 (P2W15O56)2]16−), which was impossible to purify, while a decrease of pH produced exclusively Finke’s POM. We also attempted to obtain cubane and dicubane compounds from direct condensation of wolframate and phosphate anions instead of using the preformed lacunary ligands. Only compound K3Na2-3 was obtained following this strategy. POM 3 consists of a polymeric one-dimensional chain made of units that are structurally similar to POM 2 (see structural description). The reason for such a condensation of POMs 2 into 3 likely lies in the reflux and later concentration process followed in this synthesis. It is interesting to note that the formation of a similar polymeric chain made from the condensation of POMs of type 4 is not possible, probably due to the steric hindrance imposed by the larger ligand [αP2W15O56]12−. Crystal Structure of Na1.5Cs2.5-1. The novel polyoxoanion [Co4(OH)3(H2O)6(PW9O34)]4− (1) consists of one heptadentate [B-α-PW9O34]9− ligand which incorporates a {Co4O4} cubane unit arising from the tetrahedral arrangement of four edge-shared CoO6 octahedra (see Figure 2). This type of arrangement gives rise to an idealized C3v symmetry for anion 1. This architecture was first reported by Kortz et al. for the nickel(II)-containing tungstophosphate [H2PW9Ni4O34(OH)3(H2O)6]2−.106 As far as we know, 1 is the second example of a POM exhibiting this topology. Both compounds crystallize in the same space group with similar unit cell parameters and are, therefore, isostructural. In the nickel derivative however two hydrogen atoms were assumed to be bonded directly to the surface of the POM (without specific binding sites) for charge balance considerations, as only two cesium countercations were found in the crystal structure. In Na1.5Cs2.5-1 a total of 2.5 cesium cations per POM were found in Fourier difference

Figure 2. Polyhedral and ball-and-stick representation of the [Co4(OH)3(H2O)6(PW9O34)]4− polyoxoanion (1). Gray octahedra, [WO6]; yellow tetrahedra: [PO4]; blue spheres, Co; pink spheres, O.

maps. Hence, the electroneutrality is achieved by 1.5 extra sodium cations which are disordered (along with solvation water molecules) inside infinite channels running along the crystallographic c direction. The presence of these sodium atoms inside the channels was assessed by chemical analysis, making unnecessary the assumption of hydrogen atoms bonded to the surface of the POM. As a result of the tetrahedral arrangement of the four CoO6 octahedra in 1, two types of chemically different Co atoms arise (see Figure 3a): the apical cobalt atom, which is not directly coordinated by the trivacant [B-α-PW9O34]9− unit (Co1), and the three central cobalt atoms, which share one common oxygen (O10) with the phosphorus atom of the POM (Co2, Co3, and Co4). Co1 has three terminal ligands (O1, O2, and O3), while the other three cobalt atoms have only one terminal ligand (O7, O8, and O9 for Co2, Co3, and Co4, respectively). According to bond valence sum (BVS) calculations,128 all these terminal ligands correspond to water molecules, while the three μ3-bridging oxygens of the cubane unit correspond to hydroxyl groups (see Figure S7). BVS calculations also confirm that all four cobalt atoms in 1 are Co2+ ions. All bond distances and angles involved in the cubane cluster of 1 are listed in Table S1. Crystal Structure of Cs7Na2-2. The novel polyoxoanion [Co7(OH)6(H2O)6(PW9O34)2]9− (2) consists of two heptadentate [B-α-PW9O34]9− ligands which encapsulate a heptanuclear {Co7O8} cluster. This cluster can be conceptually constructed from two fused Co4O8 cubanes that share a common cobalt atom, which sits at an inversion center. This type of arrangement lends an overall D3d symmetry to anion 2 (see Figure 4 for a representation of complex 2) and gives rise to two chemically different cobalt atoms in 2 (see Figure 3b): the central cobalt atom at the inversion center (Co1), which is not directly coordinated by the trivacant [B-α-PW9O34]9− unit, and the six cobalt atoms arranged in two separated triads, each of them sharing a common oxygen with a phosphorus atom of the POM (Co2, Co3, and Co4). BVS calculations strongly indicate that Co1 exhibits an oxidation state of +3, while all other cobalt atoms are divalent (see Figure S8). Moreover, the μ3-O ligands coordinating Co1 (O4, O5, and O6) correspond to hydroxyl groups, while the terminal ligands (O7, O8, and O9) bonded to the divalent cobalt ions correspond to water molecules (see Figure S8). All bond distances and angles involved in the dicubane cluster of 2 are listed in Table S1. 930


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Figure 3. Ball-and-stick representations and atom labeling of the cubane (a) and dicubane (b) cobalt clusters contained in the POMs reported in these work.

molecules that coordinate two cobalt atoms in 2 (labeled as Co4, one in each triad) have been substituted in 3 by terminal oxygen atoms of two neighboring polyoxoanions, giving rise to polymeric chains running along the a axis (see Figure 5). This type of polymeric chain formed by POM units with this topology has not been reported previously. In these chains every POM unit is linked to the two nearest polyanions via four Co−Obridge−W connections (two connections with each neighbor). BVS calculations give the same results as for 2: Co1 bears an oxidation state of +3, while all other cobalt ions are divalent; the μ3-O ligands bonded to Co1 correspond to hydroxyl ions, and all the terminal oxygen ligands of the divalent cobalt ions correspond to water molecules. The bridging O atom (O26) is nonprotonated (see Figure S9). A further difference between 2 and 3 is that each monomeric POM unit in 3 is monocapped by one additional cobalt atom (Co5), which sits in one of the tetragonal sites of the parent Keggin anion. Co5 is bonded to four surface oxygen atoms of the POM and two water molecules. Co5 has a refined occupancy of 50% and, due to the centrosymmetric nature of 3, is disordered between two positions located at both ends of the POM. From the magnetic point of view, this external Co5 atom is well separated from the cobalt atoms belonging to the {Co7O8} cluster. Therefore, superexchange pathways would involve at least one O−W−O bridge and rather long cobalt-to-

Figure 4. Polyhedral and ball-and-stick representation of the [Co7(OH)6(H2O)6(PW9O34)2]9− polyoxoanion (2). Gray octahedra, [WO6]; yellow tetrahedra: [PO4]; blue spheres, Co; pink spheres, O.

Crystal Structure of K 5Na 2-3. Polyoxoanion 3 is topologically identical to 2 with the difference that two water

Figure 5. Polyhedral and ball-and-stick representation of the polymeric chain [Co7(OH)6(H2O)4(PW9O34)2]n9n− (3). Gray octahedra, [WO6]; yellow tetrahedra: [PO4]; blue spheres, Co; pink spheres, O. 931


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the central cobalt ion, due to the decrease of the reduction potential of the Co3+/Co2+ pair. Stabilities in Aqueous Solution. The aqueous solution stabilities of POMs 1, 2, and 4 were assessed by using UV−vis spectroscopy in 0.2 M sodium sulfate (pH 3.0), 0.4 M sodium acetate (pH 5.0), and 25 mM sodium borate (pH 9) buffer solutions over a period of 24 h. POM 3 was not included in the study due to its polymeric nature. All three compounds exhibit absorption bands in the range 200−250 nm which are attributed to ligand-to-metal charge-transfer transitions (O → W) characteristic of the presence of POMs.129 A comparison between the initial spectrum of POM 1 in a pH 3 buffer solution and the spectrum recorded after 24 h of standing is shown in Figure 7. It can be seen that the last spectrum

cobalt distances (minimum distance of 5.66 Å Co3−Co5, with the rest over 7 Å), so they are expected to be negligible. All bond distances and angles involved in the dicubane cluster of 3 are listed in Table S1. Crystal Structure of Na15-4. The novel polyoxoanion [Co7(OH) 6(H 2O) 6(P 2W15O56 )2]15− (4) consists of two heptadentate [α-P2W15O56]12− ligands which encapsulate a heptanuclear {Co7O8} cluster (see Figures 3b and 6). This

Figure 7. Evolution of the UV spectrum of a solution of POM 1 (2.5 × 10−6 M) with time, recorded in a pH 3 medium (red and blue lines) and UV spectrum of [PCo(H2O)W11O39]5− (2.5 × 10−7 M) in the same medium. Inset: UV spectrum of a solution of CoSO4 (7.4 × 10−4 M) in the same medium.

develops a new peak at 252 nm, a situation reminiscent of the behavior of the isostructural POM [Ni4(OH) 3(H2O) 6(H2PW9O34)]2− which was shown to be unstable in a wide pH range, decomposing into [PNi(H2O)W11O39]5− as the major product.130 This also seems to be the case for POM 1, as a comparison of the UV spectra of 1 at pH 3, recorded after 24 h of standing, and the spectrum of [PCo(H2O)W11O39]5− (which is stable in these conditions) reveals that both are very similar (see Figure 7). The remaining differences can be attributed to the presence of other minor, unidentified decomposition compounds and to Co2+(aq) ions, which are released from 1 as it decomposes. The same conclusion can be deduced from the UV spectra of 1 in a pH 5 acetate medium (see Figure S13), while at pH 9, the evolution of the UV spectra of 1 (Figure S14) shows that, although the positions of the absorption bands do not change significantly after 24 h, the characteristic bands become weaker, suggesting that 1 also decomposes partially in this medium. The same behavior is observed for POM 2 in aqueous solution, as the evolution of the UV spectra of 2 after 24 h of standing (Figures S15−S17) is the same as for 1 at the three pH values studied. POM 4, however, has a higher stability in aqueous solutions than 1 and 2 at pH 5 and 9, as deduced from the minimal variation observed in the UV spectra of 4 after 24 h (Figures S18−S20). The partial decomposition of POM 1 into [PCo(H2O)W11O39]5− as the major product has also been confirmed by cyclic voltammetry experiments. The initial cyclic voltammogram of 1 in pH 3 (sulfate medium) exhibits a composite reduction wave comprised between −0.75 and −1.00 V and a

Figure 6. Polyhedral and ball-and-stick representation of the polyoxoanion [Co7(OH)6(H2O)6(P2W15O56)2]15− (4). Gray octahedra, [WO6]; yellow tetrahedra: [PO4]; blue spheres, Co; pink spheres, O.

cobalt cluster is structurally similar to the one described for 2, and BVS calculations yield the same results concerning the oxidation state of the cobalt atoms and the protonation level of the O ligands (see Figure S10). Therefore, POMs 2 and 4 are similar, the only difference being the phosphotungstate ligands [B-α-PW9O34]9− and [α-P2W15O56]12− used in each case, respectively. The POM architecture exhibited by 4 was first reported by Fang, Kögerler et al. for the tungstophosphate [MnIII6MnIVO6(H2O)6(P2W15O56)2]14−.107 As far as we know, 4 is the second example of a POM exhibiting this topology, although there are two important differences between 4 and the Mn derivative. On the one hand, while in 4 the cobalt atom located at the inversion center exhibits an oxidation state of +3 and the other cobalt atoms are divalent, in the Mn derivative the Mn atom at the inversion center exhibits a + 4 oxidation state, while all other Mn atoms are trivalent. On the other hand, while the six μ3-O ligands bonded to the trivalent cobalt ion correspond to hydroxyl groups, in the Mn derivative they are reported to be oxo ligands. Therefore, the degree of protonation of these oxygen ligands seems to be related to the oxidation state of the central 3d metal ion. As explained in the previous section, the presence of these six ligands as hydroxyl ions is likely the key of the trivalent oxidation state of 932


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Inorganic Chemistry small oxidation shoulder at ca. −0.6 V (vs Ag/AgCl) which correspond to redox processes involving W (see Figure 8). At

Figure 9. Thermal behavior of product susceptibility and temperature for compound 1 at 0.2 and 1.0 T. Expansion of low temperature range and fitting results are shown at inset. Solid lines represent the best fitting with the anisotropic model proposed.

Figure 8. Comparison of an initial cyclic voltammogram of POM 1 (5 × 10−5 M) with the cyclic voltammogram recorded after 0.5 h (pH = 3). Inset: cyclic voltammograms of [PCo(H2O)W11O39]5− (3 × 10−5 M, solid line) and [Co(H2O)6]2+ (2.5 × 10−5 M, dotted line) in the same medium.

more negative potentials than −1.00 V a proton reduction wave, not shown, takes place and is accompanied by the deposition of a solid thin film on the platinum working electrode. After 30 min, this shoulder becomes better and better defined in the subsequent cyclic voltammograms, developing in a separated wave centered at −0.65 V. The resulting cycle voltammogram is very similar to that of a solution of the “true” [PCo(H2O)W11O39]5− in the same buffer solution (see inset in Figure 8). The same results are observed in the cyclic voltammograms of 1 at pH 5 (see Figure S21), although the voltammetric waves experience a shift to more negative potentials as a result of the pH increase, a general feature in POM electrochemistry. For POMs 2 and 4 the cyclic voltammograms between 0.1 and −0.7 V are almost featureless, as a proton reduction wave (accompanied by a thin film deposition on the working electrode) takes place at lower potentials, avoiding the observation of the W reduction processes. Therefore, while 4 appears to be quite stable during the first 24 h between pH 5 and 9, POMs 1 and 2 quickly decompose upon standing in aqueous solutions in a wide pH range. The result is a mixture of decomposition products in which [PCo(H2O)W11O39]5− appears to be the major species. Decomposition of 1 and 2 can be roughly described by the following overall chemical equations:131

Figure 10. Magnetization vs field for compound 1 at 2 and 5 K. Solid lines represent the simulation of properties with the parameter set obtained from susceptibility fit.

plot of the isothermal magnetization vs H measured from 0 to 5 T at 2 and 5 K. The χmT versus T curve exhibits a smooth and continuous decrease from room temperature (χmT = 16.0 emu K mol−1) to a minimum at 43 K. Below this temperature, a sharp peak is observed with a maximum at 6.0 K (χmT = 17.6 emu K mol−1). The maximum clearly depends on the magnetic field, and when the magnetic field increases, it decreases in value, shifts to higher temperature, and becomes broader. The decrease in χmT is due to the depopulation of the single-ion Co2+ doublets upon cooling down, while the sharp increase at lower temperature indicates a ferromagnetic Co−Co interaction within the cluster (vide infra). Finally, the presence of a maximum is indicative of the anisotropic nature of the exchange interactions. The magnetic behavior of the cobalt samples containing heptameric units 2, 3, and 4 are displayed in Figure 11 as a plot of χmT vs T. Figure 12 shows a plot of the isothermal magnetization vs H measured from 0 to 5 T at 2 and 5 K for 4. The χmT versus T curves exhibit a smooth and continuous decrease from room temperature (χmT = 24.4 emu K mol−1 for 2, χmT = 27.9 emu K mol−1 for 3, χmT = 24.5 emu K mol−1 for 4) and tend toward a leveling around 30−40 K. Below this temperature, a sharp decrease is observed indicating strong antiferromagnetic interactions inside the heptameric units. Let us now analyze the experimental results using a quantitative approach. High-spin octahedral Co2+ is an orbitally degenerate ion with a 4T1 ground electronic term. Because of spin−orbit coupling and the low-symmetry crystal field, this ground term splits into six Kramers doublets. The smooth decrease from room temperature in all measurements can be attributed to the depopulation of the higher Kramers doublets.

11[Co4 (OH)3 (H 2O)6 (PW9O34 )]4 − + 63H+ + 105H 2O → 9[Co(H 2O)(PW11O39)]5 − + 35[Co(H 2O)6 ]2 + + 2(PO4 )3 −

(1)

44[Co7(OH)6 (H 2O)6 (PW9O34 )2 ]9 − + 460H+ + 862H 2O → 72[Co(H 2O)(PW11O39)]5 − + 236[Co(H 2O)6 ]2 + + 16(PO4 )3 − + 11O2

(2)

Magnetic Properties. The magnetic behavior of the tetranuclear cobalt(II) cluster isolated in 1 is displayed in Figure 9 as a plot of the product χmT vs T and in Figure 10 as a 933


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Figure 11. Thermal behavior of product susceptibility and temperature for compound 2 (a), 3 (b), and 4 (c) at 0.1 T. Expansion of low temperature range and fitting results are shown as insets. Solid lines represent the best fitting with the anisotropic model proposed.

Ĥ = −2

∑

2 3

2 4

3 4

{Ji (Sî Sî + Sî Sî + Sî Sî )

i=x ,y,z 1

2

3

4

+ rJi Sî (Sî + Sî + Sî )}

where Ji refers to the exchange components associated with the coupling between the three magnetically equivalent Co2+ coordinated to the POM (see Figure 13). In order to avoid overparametrization, the exchange coupling between these three central cobalts with the apical one has been considered to be proportional to Ji, J′i = rJi (r is the proportionality parameter). A simultaneous fit of the two magnetic susceptibilities at two fields (0.2 and 1.0 T) was performed by numerical diagonalization of the full eigenmatrix134,135 and gives the following sets of parameters: Jx = 15.0 cm−1, Jy = 12.6 cm−1, Jz = 9.3 cm−1, r = 0.123, gav = 5.02 where gav = (gx + gy + gz)/3 (R = 4.6 × 10−5). Again, to reduce adjustable parameters, the three components of the tensor g were considered proportional to the square root of the corresponding component of the tensor exchange. This proportionality factor is the same for all components and the only adjustable parameter. The ferromagnetic sign of the exchange parameters for edge-sharing CoO6 octahedra can be associated with the orthogonality of the magnetic orbitals, Co−O−Co angles close to 90°. The validity of the exchange model and fit is confirmed by the magnitude of the exchange parameters, which is in good agreement with previously published analogous edge-sharing oxo-cobalt clusters encapsulated by POMs.57−59,61,77,78,98 The comparatively small value of J′ (r ≈ 0.123) can be associated with the different nature of the ligand bridge (a double bridge formed by −O and −OH for the pairwise interaction between basal cobalts, and a single −OH bridge for the interaction between the basal cobalts and the apical one), and to small changes in averaged angle Co−L−Co, (values very close to

Figure 12. Magnetization vs field for compound 4 at 2 and 5 K. Solid lines represent the simulation of properties with the parameter set obtained from susceptibility fit.

Moreover, the cobalt atom in the center of the heptameric units presents typical distances of oxidation state +3. This central cobalt should be diamagnetic due to the strong field associated with the hydroxyl groups and only plays a role of exchange pathway between the other spins. Typically, magnetic interactions between paramagnetic sites through diamagnetic metal ions are negligibly small or weakly antiferromagnetic. At low temperature (below 30−40 K) only the lowest Kramers doublet is significantly populated so that the exchange interaction between two octahedral Co2+ ions can be conveniently described by assuming a coupling between these fully anisotropic Kramers doublets with fictitious spins 1/2.132,133 Taking into account all the aspects mentioned above, the effective exchange Hamiltonian for compound 1 can be written as eq 1: 934


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explained above and with similar magnitude to other previously studied cobalt systems. In this case, the antiferromagnetic interaction between trimeric units through the Co3+ leads to antiferromagnetic S = 0 ground state for the heptameric cluster. This also results in the fact that the susceptibility is far less sensitive to anisotropy; therefore, the extracted ferromagnetic exchange tensor is more isotropic in these clusters than in 1. Compound 3 has not been fitted since the presence of an additional isolated cobalt atom outside heptanuclear cluster leads to overparametrization. However, the magnetic behavior of this cluster is similar to that observed in the previous compounds suggesting that the exchange parameters inside the heptanuclear unit should be very close to those of compound 2.

■

CONCLUSIONS Four new cobalt-substituted POMs have been synthesized containing cobalt cores exhibiting cubane (for 1) or dicubane topologies (for 2, 3, and 4). These cobalt clusters are encapsulated by the heptadentate lacunary POM units [B-αPW9O34]9− (in 1, 2, and 3) or [α-P2W15O56]15− (in 4) and represent the first isolated cobalt clusters of this type in POM chemistry. Additionally, the dicubane clusters contain six Co2+ ions and one central Co3+ ion, and then represent rare examples of cobalt clusters encapsulated by POMs having cobalt in both oxidation states. The occurrence of the +3 oxidation state is due to the coordination environment of the central cobalt, which is made up of six hydroxyl ligands. The preparation of all these POMs has been elusive until now because they are formed as minor byproducts during the synthesis of the more stable and well-known tetracobalt sandwich POMs [Co 4 (H 2 O) 2 (PW9O34)2]10− and [Co4(H2O)2(P2W15O56)2]16−. Hence, these species must be removed from the reaction mixture (for example, by precipitation with Cs+ cations) to achieve the isolation and crystallization of 1−4. From the structural point of view, 1, 2, and 4 crystallize as alkaline ion salts of isolated polyoxoanions, while 3 consists of polymeric chains formed by individual POM units similar to the ones found in 2 that are connected through two Co−O−W bridges. The stability of POMs 1, 2, and 4 in aqueous solution has been assessed using UV−vis spectroscopy suggesting that, while 1 and 2 partially decompose into the species [PCo(H2O)W11O39]5−, 4 is more stable in the pH range 5−9. From the magnetic point of view, complex 1 exhibits ferromagnetic interactions between the three magnetically equivalent Co2+ coordinated to the polyoxometalate (trimer) and also between each of them and the apical Co2+. Its magnetic behavior has been modeled assuming a dominant anisotropic exchange interaction within the trimer, and a much weaker exchange interaction of this trimer with the apical Co2+. The ferromagnetic nature of these exchange interactions are consistent with the angles Co−L−Co (L = O, OH), which are close to orthogonality. Complexes 2, 3, and 4 exhibit an antiferromagnetic behavior at very low temperature due to the antiferromagnetic interaction between the two trimeric units, which are ferromagnetically coupled. At this point one can notice that the spin structures exhibited by these two cluster topologies show some unique features. Thus, since the coupling within the basal Co3 triangle is much stronger than that between these cobalts and the apical one, the ground state of the tetranuclear Co2+ cluster (compound 1) can be viewed as resulting from a weak interaction between two different spin doublets: a doublet, |S, M⟩ = |3/2, ± 1/2|, coming from a ferromagnetic anisotropic interaction between the three

Figure 13. Schematic representation of tetranuclear {CoII4O4} and heptanuclear {CoII6CoIIIO8} cores (color code: Co2+ blue, Co3+ red, O2− gray, and OH− white). Exchange networks of the corresponding magnetic clusters show a dominant intratrimer exchange pathway, J (depicted as full rod) and a weaker exchange interaction (depicted as a dashed rod in the tetramer, and as a dotted line in the heptamer), which corresponds to the magnetic coupling between the basal triangular unit and the apical one.

orthogonality, 91°, for the basal cobalts, and larger values, 97.1° and 98.3°, for the angles connecting the basal cobalts with the apical one). Further confirmation of the model is obtained by the low-temperature behavior of the magnetization as a function of external field, which can be fitted with the same set of parameters (Figure 10). Deviations from experimental curves at higher field than 3 T can be due to contributions coming from excited Kramers doublets not included in the model. In the case of the heptanuclear systems, 2, 3, and 4, the effective exchange Hamiltonian can be written as eq 2: Ĥ = −2

∑

1 2

1 3

2 3

4 5

4 6

{Ji (Sî Sî + Sî Sî + Sî Sî + Sî Sî + Sî Sî

i=x ,y,z 5 6 1 2 3 4 5 6 + Sî Sî ) + 2J ′(S ̂ + S ̂ + S ̂ )(S ̂ + S ̂ + S ̂ )}

where Ji is the exchange components associated with the pathway between the three magnetically equivalent cobalts on each trinuclear units. On the other hand, J′ is associated with the exchange interaction between cobalts from different trinuclear units through the diamagnetic central Co3+. In order to simplify the number of parameters, this last interaction has been considered isotropic and equal between all of them. For compounds 2 and 4, a fit of magnetic susceptibilities at 0.1 T was performed by numerical diagonalization of the full eigenmatrix and gives the following sets of parameters: Jx = 12.7 cm−1, Jy = 11.0 cm−1, Jz = 10.23 cm−1, J′ = −0.333 cm−1, gav = 4.29 (R = 2.5 × 10−4) for 2 and Jx = 10.6 cm−1, Jy = 7.7 cm−1, Jz = 9.9 cm−1, J′ = −0.380 cm−1, gav= 4.41 (R = 2.9 × 10−4) for 4. Again, the exchange coupling between the Co2+ ions belonging to the trinuclear units is ferromagnetic due to the same reasons 935


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basal cobalts, and the fundamental Kramers doublet coming from the apical cobalt. This feature is in sharp contrast with that encountered in other cobalt trimers (with triangular or linear topologies)58,59 and in rhomb-like centrosymmetrical cobalt tetramers encapsulated by POMs.78 In the trimeric clusters, direct access to energy levels by INS spectroscopy shows that these systems have a doublet ground state well separated in energy from the first excited one.58,59 As for the rhomblike tetramer formed by four coplanar edge-sharing CoO 6 octahedra, its energy scheme consists of a doublet ground state very stable compared to other levels due to strong ferromagnetic anisotropic coupling between the four cobalts.136 In contrast, complex 1 presents, as a result of the interaction between two nonequivalent spin doublets coming from two different units (mononuclear and trinuclear), an energy scheme with four levels ranging over 5 cm−1. This situation may be useful to provide a 2-qubit quantum gate.137 Obviously, a detailed description of its low-lying energy levels and wave functions, as that provided by INS, will be also very useful to check this possibility. In the case of the heptanuclear systems, its magnetic properties can be interpreted on the basis of an antiferromagnetic coupling between two identical ferromagnetically coupled trinuclear units. As discussed above, each trinuclear unit has an energy scheme with only doubly degenerate levels. The antiferromagnetic coupling between these two units results in a complex level scheme with a diamagnetic ground state (S = 0) quasidegenerate with an antisymmetric combination of ± M functions. In view of this result, this molecule can be a candidate of electrically responsive magnetic molecule in which its ground spin state can be changed by application of an external electric field.138−141

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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.5b02532. Crystallographic data in CIF format for 1, 2, 3, and 4 (CSD No. 430365) (CIF) FT-IR spectra; thermograms; bond distances and angles involved in the cobalt clusters; bond valence sum calculations; powder X-ray diffraction patterns; UV−vis spectra in solution; cyclic voltammograms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.G.-S.) E-mail: [email protected]. *(E.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work has been funded by the EU (ERC Advanced Grant SPINMOL and COST Action CM1203 “Polyoxometalate Chemistry for Molecular Nanoscience, PoCheMon”), the Spanish MINECO (Grants CTQ2014-52758-P, MAT201456143-R and Marı ́a de Maeztu Project MDM-2015-0538), and Generalitat Valenciana (Prometeo and ISIC Programmes of excellence). We thank José Ma Martı ́nez-Agudo for performing some of the physical measurements. 936


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