Masooma Ibrahim

Hexadecacobalt(II)-Containing Polyoxometalate-Based Single-Molecule Magnet†

Polyoxometalates (POMs) are a remarkable class of inorganic compounds with enormous structural and compositional diversity and potential applications in various fields, such as catalysis, analytical chemistry, magnetism, nanotechnology, and medicine. In particular lacunary heteropolytungstates of the Keggin and Wells–Dawson type are useful inorganic, diamagnetic ligands allowing for encapsulation of various multinuclear metal-oxo assemblies. Examples of such kinds of structurally, catalytically, and magnetically interesting POMs are the manganese-containing derivative {Mn14W36}, [2a] the iron-containing derivatives {Fe9W12}, [3a] {Fe13W36}, [3b,c] {Fe16W48}, [3d] {Fe28W48}, [3e] the nickel-containing derivatives {Ni9W27}, [4a] {Ni8/Ni9W18}, [4b] {Ni12W35}, [4c] and {Ni20W34}, [4d] and the copper-containing derivatives {Cu14W36} [5a] and {Cu20W48X} (X=Cl, Br, I). [5b,c] Some transition-metal-containing polyoxomolybdates are also known, such as {Fe30Mo72}, [6a] {V30Mo72}, [6b] as well as {Co16Mo16} (two types of structures containing 16 cobalt centers, in the form of four tetramers). The class of cobalt-containing POMs was pioneered by Baker and Pope. Meanwhile a large number of POMbased Co complexes with nuclearities ranging from 2 to 16 has been reported. Our group has reported a nona-cobaltcontaining polyanion capped by six antenna-like Co ions in the solid state, as well as several polytungstates containing smaller numbers of cobalt ions. Very recently it was shown that [Co4(H2O)2(PW9O34)2] 10 [9a] is a hydrolytically and oxidatively stable homogeneous water-oxidation catalyst. During the past decade many high-nuclearity transitionmetal-based coordination complexes with interesting electronic and magnetic properties have been prepared. Some cobalt derivatives with nuclearities ranging from 2 to 32 are also known. It is a challenge to encapsulate high-nuclearity magnetic cores in diamagnetic POM shells, in particular by using conventional, soft synthesis methods. We have now succeeded in preparing the tetrameric 36-tungsto-8-phosphate [{Co4(OH)3PO4}4(PW9O34)4] 28 (1), containing 16 cobalt(II) centers (Figure 1).

A Planar {Mn19(OH)12}26+ Unit Incorporated in a 60‐Tungsto‐6‐Silicate Polyanion

Polyoxometalates (POMs) are discrete metal–oxo anions of early transition-metals in high oxidation states (e.g. W, Mo, V) and they are usually synthesized in aqueous, acidic medium. Most classical POMs are based on edgeand corner-shared MO6 octahedra. However, the recently discovered POM subclass of noble metalates comprises linked square-planar MO4 units (M = Pd , Au). Lacunary (vacant) POMs can be considered as inorganic, multidentate ligands, and hence they are good candidates for the encapsulation of large, multinuclear dand f-block metal–oxo fragments, sometimes resulting in compounds with interesting magnetic properties. A pioneering result in this area was the synthesis of [Mn12(CH3COO)16(H2O)4O12] (Mn12) by Lis in 1980, which was shown to exhibit single-molecule magnet (SMM) behavior by Gatteschi s group 13 years later. During the past two decades many high-nuclearity, transition-metal based, coordination complexes with interesting electronic and magnetic properties have been prepared. High-nuclearity manganese complexes have been amongst the most studied in this class, and there are examples containing up to 84 manganese ions. 6] To date there are only a few high-nuclearity manganese– oxo-containing POMs, such as {[XW9O34]2[Mn III 4Mn II 2O4(H2O)4]} 12 (X = Si, Ge) and [Mn13Mn O12(PO4)4(PW9O34)4] 31 . Herein we report the synthesis and structure as well as the magnetic and electrochemical properties of a 19 manganese(II) center containing 60-tungsto-6-silicate, [Mn19(OH)12(SiW10O37)6] 34 (1), which was isolated as a hydrated sodium salt, Na34[Mn19(OH)12(SiW10O37)6]·115H2O (Na-1). Single-crystal X-ray diffraction revealed that polyanion 1 consists of a cationic {Mn19(OH)12} 26+ (Mn19) assembly stabilized by six dilacunary [a-SiW10O37] 10 units resulting in a structure with S6 point-group symmetry (Figure 1, top). To the best of our knowledge, 1 is the highest nuclearity manganesecontaining POM known to date. All 19 Mn ions lie in the same plane forming a hexagonal structure based on edgeshared MnO6 octahedra. The Mn II ions in Mn19 are connected by a total of twelve m3-hydroxo bridges, as determined by bond valence sum (BVS) calculations (Supporting Information, Table S1). The discrete Mn19 nanosheet (Figure 1, bottom) is held in place by six dilacunary [a-SiW10O37] 10

Cobalt, Manganese, Nickel, and Vanadium Derivatives of the Cyclic 48-Tungsto-8-Phosphate [H7P8W48O184]33−

The cobalt(II) containing tungstophosphate [Co(4)(H(2)O)(16)P(8)W(48)O(184)](32-) (1) has been synthesized by addition of Co(2+) ions to an aqueous solution of [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) and characterized by single-crystal XRD, IR, and UV-vis spectroscopy, elemental analysis, electrochemistry, and magnetochemistry. The novel polyanion 1 is a derivative of the superlacunary P(8)W(48) with four cobalt(II) ions coordinated to the rim of the central cavity and two additional cobalt(II) ions linked on the outside bridging neighboring polyanions. Using similar synthetic procedures, but adding a few drops of H(2)O(2), we isolated the manganese(II) derivative [Mn(4)(H(2)O)(16)(P(8)W(48)O(184))(WO(2)(H(2)O)(2))(2)](28-) (2) and its nickel(II) analogue [Ni(4)(H(2)O)(16)(P(8)W(48)O(184))(WO(2)(H(2)O)(2))(2)](28-) (3). Both polyanions have picked up two equivalents of tungsten resulting in the unprecedented {P(8)W(50)} host framework. We also made the vanadium(V) derivative [(VO(2))(4)(P(8)W(48)O(184))](36-) (4), with four tetrahedral vanadate groups grafted to the P(8)W(48) host. The voltammetric patterns associated with the W-centers in polyanions 1, 2, and 4 display enough distinct features allowing for a qualitative classification according to relative basicity of the reduced polyanions: 2 > P(8)W(48) > 1 > 4. The electrochemistry of 1 offers a new example for detection of the Co(2+) centers in a multicobalt containing polyanion. During a study of the Mn(2+) centers of 2 at pH 5, a film deposition is observed. The vanadium(V) centers of 4 are well-behaved in a pH 0.33 medium. Temperature and magnetic field dependence of the magnetic moment of 1-3 were performed on a SQUID magnetometer over the temperature range 1.8-250 K and field range 0-7 T. The results are consistent with the model of noninteracting 3d metal ions. Variable temperature (4-295 K) and variable frequency (34-413 GHz) EPR measurements support the magnetic susceptibility results. The zero-field splitting D and g values obtained for 1-3 are in agreement with those reported for high-spin Co(2+), Mn(2+), and Ni(2+) ions in axially distorted octahedral environments.

Synthesis, magnetism, and electrochemistry of the Ni14- and Ni5-containing heteropolytungstates [Ni14(OH)6(H2O)10(HPO4)4(P2W15O56)4]34- and [Ni5(OH)4(H2O)4(β-GeW9O34)(β-GeW8O30(OH))]13-.

The two Ni(2+)-containing heteropolytungstates [Ni14(OH)6(H2O)10(HPO4)4(P2W15O56)4](34-) (Ni14) and [Ni5(OH)4(H2O)4(β-GeW9O34)(β-GeW8O30(OH))](13-) (Ni5) have been successfully synthesized in aqueous, basic media under conventional reaction conditions, and they were characterized by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric and elemental analyses, electrochemistry, and magnetic studies. The cyclic voltammetry (CV) patterns of Ni14 and Ni5 showed chemically reversible multielectronic waves for slow scan time scales. For Ni14, an important acidity inversion effect between its reduced forms was observed. Magnetic studies revealed dominant ferromagnetic interactions among the nickel(II) ions in both polyanions.

Wheel-Shaped Cu20-Tungstophosphate [Cu20X(OH)24(H2O)12(P8W48O184)]25- Ion (X = Cl, Br, I) and the Role of the Halide Guest

We have synthesized the known [Cu(20)Cl(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))](25-) (1) and report here its bromide and iodide analogues, [Cu(20)Br(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))](25-) (2) and [Cu(20)I(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))](25-) (3). These polyanions were characterized in the solid state by IR spectroscopy and single-crystal X-ray diffraction. Magnetic susceptibility and magnetization data over 1.8-300 K show that the Cu(2+) ions in 1-2 are antiferromagnetically coupled, leading to a diamagnetic ground state. The effective exchange coupling constant J(eff) was estimated as approximately -3 K for both 1 and 2. Electron paramagnetic resonance measurements were made on 1 and 2 over 5-295 K at microwave frequencies of 9.5, 34, and 220 GHz. The observed (weak) signals were characteristic of randomly distributed Cu(2+) ions only, with g values and hyperfine constants typical of the unpaired electron in a 3d(x(2)-y(2)) orbital of Cu(2+). No signals attributable to the copper-hydroxo cluster were detected, supporting the conclusions from the magnetization measurements. DFT calculations were performed as well to obtain additional information on the anionic guest inside the cavity created by the copper-hydroxo cage related to electronic structure and energies of encapsulation. The polyanions 2 and 3 were also characterized by cyclic voltammetry (CV) in a pH 5 medium. Their CVs are composed by an initial two-step reduction of the Cu(2+) centers to Cu(0) through Cu(+), followed at more negative potential by the redox processes of the W centers. A comparison with the CV characteristics of the previously studied compound 1 indicates that the potential locations of the Cu or W waves of the three analogues do not depend significantly upon the identity of the central halide X. This observation is in accordance with conclusions of DFT calculations. The modified electrodes based on 2 and the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate triggers an efficient reduction of nitrate. To our knowledge, this is the first example of electrocatalytic nitrate reduction by a polyanion entrapped in room-temperature ionic liquid films.

Yttrium(III)-containing tungstoantimonate(III) stabilized by tetrahedral WO4(2-) capping unit, [{Y(α-SbW9O31(OH)2)(CH3COO)(H2O)}3(WO4)]17-.

The yttrium(III)-containing tungstoantimonate(III) [{Y(α-SbW(9)O(31)(OH)(2))(CH(3)COO)(H(2)O)}(3)(WO(4))](17-) (1) has been synthesized in a simple one-pot reaction of Y(3+) ions with [α-SbW(9)O(33)](9-) and WO(4)(2-) in a 3:3:1 molar ratio in 1 M LiOAc/AcOH buffer at pH 5.3. Polyanion 1 is composed of three (α-SbW(9)O(33)) units linked by three Y(3+) ions and a capping, tetrahedral WO(4)(2-) capping unit, resulting in an assembly with C(3v) symmetry. The hydrated ammonium-sodium salt of 1 was investigated in the solid state by single-crystal XRD, FT-IR spectroscopy, thermogravimetric and elemental analyses, and in solution by multinuclear NMR spectroscopy.

Synthesis and characterization of multinuclear manganese-containing tungstosilicates

The five manganese-containing, Keggin-based tungstosilicates [Mn(II)3(OH)3(H2O)3(A-α-SiW9O34)](7-) (1), [Mn(III)3(OH)3(H2O)3(A-α-SiW9O34)](4-) (2), [Mn(III)3(OH)3(H2O)3(A-β-SiW9O34)](4-) (3), [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-) (4), and [Mn(III)3Mn(IV)O3(CH3COO)3(A-β-SiW9O34)](6-) (5) were synthesized in aqueous medium by interaction of [A-α-SiW9O34](10-) or [A-β-SiW9O34H](9-) with either MnCl2 (1) or [Mn(III)8Mn(IV)4O12(CH3COO)16(H2O)4] (2-5) under carefully adjusted reaction conditions. The obtained salts of these polyanions were analyzed in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric analysis. The salts of polyanions 1, 2, and 4 were further characterized in the solid state by magnetic studies, as well as in solution by electrochemistry.

Multinuclear cobalt(II)-containing heteropolytungstates: structure, magnetism, and electrochemistry.

Interaction of the trilacunary Keggin polyanions [A-α-XW9O34](10-) (X = Si(IV), Ge(IV)) with Co(II) and phosphate ions in aqueous, basic media and under mild heating leads to the formation of the tetrameric, Co16-containing heteropolytungstates [{Co4(OH)3PO4}4(A-α-XW9O34)4](32-) (X = Si(IV), Ge(IV)). Both polyanions were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analyses. Furthermore, the electrochemical and magnetic properties of these isostructural polyanions were investigated.

Large Cation−Anion Materials Based on Trinuclear Ruthenium(III) Salts of Keggin and Wells-Dawson Anions Having Water-Filled Channels

Reaction of the trinuclear ruthenium(III) cation [Ru(3)O(OOCCH(3))(6)(CH(3)OH)(3)](+) with the Keggin-type [alpha-GeW(11)O(39)](8-), [alpha-SiW(11)O(39)](8-), and [alpha-SiMo(12)O(40)](4-) and the Wells-Dawson-type [alpha-P(2)W(18)O(62)](6-) polyanions in an aqueous, acidic medium resulted in plenary polyoxometalate-based materials K(2)Na[Ru(3)O(OOCCH(3))(6)(H(2)O)(3)][alpha-GeW(12)O(40)].10H(2)O (1), K(3)[Ru(3)O(OOCCH(3))(6)(H(2)O)(3)][alpha-SiW(12)O(40)].18H(2)O (2), K(3)[Ru(3)O(OOCCH(3))(6)(H(2)O)(3)][alpha-SiMo(12)O(40)].7H(2)O (3), and K(2)Na[Ru(3)O(OOCCH(3))(6)(H(2)O)(3)](3)[alpha-P(2)W(18)O(62)].26H(2)O (4), respectively. All four materials, 1-4, crystallize as sodium and/or potassium salts in the monoclinic space group P2(1)/n. Compounds 1-4 were characterized by IR, thermogravimetric analysis, and single-crystal/powder X-ray diffraction (XRD). The isolated solid-state frameworks, composed of cocrystallized trinuclear ruthenium cations and polyanions, exhibit nanosized voids filled with crystal waters. These water molecules can be removed reversibly upon heating under a vacuum, and powder XRD measurements demonstrated that the crystallinity of the compound was preserved. Sorption studies on ethanol and methanol were also performed.

Redox switching of polyoxometalate-methylene blue-based layer-by-layer films.

Iron-substituted crown-type polyoxometalate (POM) [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) has been successfully immobilized onto glassy carbon electrode surfaces by means of the layer-by-layer (LBL) technique employing the cationic redox active dye, methylene blue (MB). The constructed multilayers exhibit pH-dependent redox activity for both the anionic POM and the cationic dye moieties, which is in good agreement with their solution behavior. The films have been characterized by alternating current impedance, atomic force microscopy, and X-ray photoelectron spectroscopy, whereby the nature of the outer layer within the assemblies was found to have an effect upon the films behavior. Preliminary investigations show that the POM dye-based films show electrocatalytic ability toward the reduction of hydrogen peroxide, however, only when there is an outer anionic POM layer.