Andreas Suchopar

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).

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.

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.

Improved Synthesis, Structure, and Solution Characterization of the Cyclic 48-Tungsto-8-Arsenate(V), [H4As8W48O184]36−

Abstract We report on an improved synthesis and structural characterization of the cyclic 48-tungsto–8-arsenate(V) [H4As8W48O184]36− (1). The mixed lithium–potassium salt of this polyanion, K26.5Li9.5[H4As8W48O184]·90H2O (KLi-1), has been studied in the solid state by IR, single-crystal X-ray diffraction, and elemental analysis, and in solution by 183W NMR spectroscopy.