Michael H. Dickman

6-Peroxo-6-Zirconium Crown and Its Hafnium Analogue Embedded in a Triangular Polyanion: [M6(O2)6(OH)6(γ-SiW10O36)3]18− (M = Zr, Hf)

We have synthesized and structurally characterized the unprecedented peroxo-zirconium(IV) containing [Zr6(O2)6(OH)6(gamma-SiW10O36)3]18- (1). Polyanion 1 comprises a cyclic 6-peroxo-6-zirconium core stabilized by three decatungstosilicate units. We have also prepared the isostructural hafnium(IV) analogue [Hf6(O2)6(OH)6(gamma-SiW10O36)3]18- (2). We investigated the acid/base and redox properties of 1 by UV-vis spectroscopy and electrochemistry studies. Polyanion 1 represents the first structurally characterized Zr-peroxo POM with side-on, bridging peroxo units. The simple, one-pot synthesis of 1 and 2 involving dropwise addition of aqueous hydrogen peroxide could represent a general procedure for incorporating peroxo groups into a large variety of transition metal and lanthanide containing POMs.

Nucleation Process in the Cavity of a 48‐Tungstophosphate Wheel Resulting in a 16‐Metal‐Centre Iron Oxide Nanocluster

The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH 1 through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe(III) centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mV s(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.

Self‐Assembly of a Heteropolyoxopalladate Nanocube: [PdII13AsV8O34(OH)6]8−

(Chemical Equation Presented) Not like the others: A molecular palladium oxide cluster was formed by self-assembly of palladium(II) and arsenic(V) using mild reaction conditions. The resulting heteropolypalladate [Pd II 13AsV 8O34(OH) 6]8- has a distorted cubic shape and edge lengths of about 1 nm. The thirteen PdII ions retain four-coordinate square-planar geometry, in marked contrast to all other known discrete polyoxometalates.

Heteropoly-13-palladates(II) [Pd(II)(13)(As(V)Ph)(8)O(32)](6-) and [Pd(II)(13)Se(IV)(8)O(32)](6-).

Two discrete anionic palladium(II)-oxo clusters have been prepared: [Pd(13)(As(V)Ph)(8)O(32)](6-) (1) and [Pd(13)Se(IV)(8)O(32)](6-) (2) were synthesized in one-pot self-assembly reactions of Pd(OAc)(2) with PhAsO(3)H(2) and SeO(2) and characterized by single-crystal X-ray analysis, IR, thermogravimetric analysis, elemental analysis, magnetic and electron paramagnetic resonance measurements, and electrochemistry.

Copper-, Cobalt-, and Manganese-Containing 17-Tungsto-2-Germanates

The sandwich-type tungstogermanates [Cu(3)(H(2)O)(B-beta-GeW(9)O(33)(OH))(B-beta-GeW(8)O(30)(OH))](12-) (1), [Co(H(2)O)(2){Co(3)(B-beta-GeW(9)O(33)(OH))(B-beta-GeW(8)O(30)(OH))}(2)](22-) (2), and [Mn(H(2)O)(2){Mn(3)(H(2)O)(B-beta-GeW(9)O(33)(OH))(B-beta-GeW(8)O(30)(OH))}(2)](22-) (3) were synthesized and characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, and infrared spectroscopy. Polyanion 1 is composed of two nonequivalent Keggin units, (B-beta-GeW(8)O(31)) and (B-beta-GeW(9)O(34)), linked to each other via three copper(II) ions in such a way that there is a plane of symmetry passing through both Ge atoms and the unique Cu atom, resulting in a sandwich-type structure with C(s) symmetry. On the other hand, the monomeric building blocks of 2 and 3 contain the same Keggin fragments as 1, but linked through three octahedrally coordinated Co(2+) or Mn(2+) ions. The major difference between complex 1 and complexes 2 and 3 is that the latter all lack a plane of symmetry due to a different orientation of the rotated triad. Magnetic measurements indicated antiferromagnetic exchange interactions between the three Cu(2+) ions in 1 and between the three Mn(2+) ions in 3. On the other hand, polyanion 2 possesses ferromagnetic interaction of the Co(2+) ions. The best least-squares fit values for 2 are J(z) = 7.9 cm(-1), J(z) = 3.1 cm(-1), J(y) = 2.4 cm(-1), g(z) = 6.77 cm(-1), and g(xy) = 4.15 (R = 2.6 x 10(-2)).

Organo-Ruthenium Supported Heteropolytungstates: Synthesis, Structure, Electrochemistry, and Oxidation Catalysis

The reaction of [Ru(arene)Cl(2)](2) (arene = benzene, p-cymene) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb(III), Bi(III)) in buffer medium resulted in four organo-ruthenium supported heteropolytungstates, [Sb(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (1), [Bi(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (2), [Sb(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (3), and [Bi(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (4), which have been characterized in solution by multinuclear ((183)W, (13)C, (1)H) NMR, UV-vis spectroscopy, electrochemistry, and in the solid state by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Polyanions 1, 2, and 4 crystallize in the triclinic system, space group P1 with the following unit cell parameters: K(5)Na(5)[Sb(2)W(20)O(70)(RuC(6)H(6))(2)] x 22 H(2)O (KNa-1), a = 12.1625(2) A, b = 13.1677(2) A, c = 16.0141(3) A, alpha = 78.9201(7) degrees, beta = 74.4442(8) degrees, gamma = 78.9019(8) degrees, and Z = 1; Cs(2)Na(8)[Bi(2)W(20)O(70)(RuC(6)H(6))(2)] x 30 H(2)O (CsNa-2), a = 11.6353(7) A, b = 13.3638(7) A, c = 16.7067(8) A, alpha = 79.568(2) degrees, beta = 71.103(2) degrees, gamma = 80.331(2) degrees, and Z = 1; Na(10)[Bi(2)W(20)O(70)(RuC(10)H(14))(2)].35H(2)O (Na-4), a = 15.7376(12) A, b = 15.9806(13) A, c = 24.2909(19) A, alpha = 92.109(4) degrees, beta = 101.354(4) degrees, gamma = 97.365(3) degrees, and Z = 2. Polyanions 1-4 consist of two (L)Ru(2+) (L = benzene or p-cymene) units linked to a [X(2)W(20)O(70)](14-) (X = Sb(III), Bi(III)) fragment via Ru-O(W) bonds resulting in an assembly with idealized C(2h) symmetry. Polyanions 1-4 are stable in solution as indicated by the expected (183)W, (13)C, and (1)H NMR spectra. The electrochemistry of 1-4 is described by considering the reduction and the oxidation processes. The nature of the arene in Ru(arene) has practically no influence on the formal potentials of the W-centers, which are more sensitive to the Sb or Bi hetero atoms. The results suggest that the respective Sb- and Bi derivatives have very different pK(a) values, with the reduced form of 1 being the most basic, thus permitting the observation of two well-developed voltammetric waves at pH 6. In contrast, the identity of the arene influences the oxidation processes, thus permitting to distinguish them. A strong electrocatalytic water oxidation peak is observed that is more positive than the one corresponding to the Ru(arene) oxidation process. Also a stepwise oxidation of the Ru(benzene) group could be observed at pH 3. The catalytic efficiency, on the other hand, of 1-4 toward the oxidation of n-hexadecane and p-xylene illustrated the effect of ruthenium substitution on the polyanion catalytic performance.

22-Isopolytungstate fragment [H2W22O74]14- coordinated to lanthanide ions.

The novel polyanions [Ln(2)(H(2)O)(10)W(22)O(71)(OH)(2)](8-) (Ln = La (1), Ce (2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb (8), Lu (9), Y (10)) have been synthesized by reaction of WO(4)(2-) and lanthanide ions in acidic aqueous medium. The low symmetry (C(i)) polyanion family [Ln(2)(H(2)O)(10)W(22)O(71)(OH)(2)](8-) consists of the isopolyanion [H(2)W(22)O(74)](14-) and two {Ln(H(2)O)(5)}(3+) supporting units. The [H(2)W(22)O(74)](14-) cluster, which consists of two undecatungstate {W(11)} fragments, acts as a tridentate ligand to two Ln(3+) ions. Polyanions 1-10 are isostructural, and the coordination number of the lanthanide ions correlates with their sizes. All compounds have been fully characterized in the solid state by Fourier transform infrared spectroscopy, single-crystal X-ray diffraction, thermogravimetric analysis, and elemental analysis. Single-crystal X-ray diffraction analyses show that 1-10 crystallize as sodium salts in the triclinic space group P1.

Dimerization of mono-ruthenium substituted α-Keggin-type tungstosilicate [α-SiW11O39RuIII(H2O)]5− to µ-oxo-bridged dimer in aqueous solution: synthesis, structure, and redox studies

We report the dimerization of a mono-ruthenium(III) substituted α-Keggin-type tungstosilicate [α-SiW11O39RuIII(H2O)]5− to a µ-oxo-bridged dimer [{α-SiW11O39Rum}2O]n− (m = III, n = 12; m = IV/III, n = 11; m = IV, n = 10). Single crystal X-ray structure analysis of Rb10[{α-SiW11O39RuIV}2O]·9.5H2O (triclinic, P, with a = 12.7650(6) A, b = 18.9399(10) A, c = 20.2290(10) A, α = 72.876(3)°, β = 88.447(3)°, γ = 80.926(3)°, V = 4614.5(4) A3, Z = 2) reveals that two mono-ruthenium substituted tungstosilicate α-Keggin units are connected through µ-oxo-bridging Ru–O–Ru bonds. Solution 183W-NMR of [{SiW11O39RuIV}2O]10− resulted in six peaks (−63, −92, −110, −128, −132, and −143 ppm, intensities 2 : 2 : 1 : 2 : 2 : 2) confirming that the µ-oxo bridged dimer structure is maintained in aqueous solution. The dimerization mechanism is presumably initiated by deprotonation of the aqua-ruthenium complex [α-SiW11O39RuIII(H2O)]5− leading to a hydroxy-ruthenium complex [α-SiW11O39RuIII(OH)]6−. Dimerization of two hydroxy-ruthenium complexes produces the µ-oxo bridged dimer [{α-SiW11O39RuIII}2O]12− and a water molecule. The Ru(III) containing dimer is oxidized by molecular oxygen to produce a mixed valence species [{α-SiW11O39RuIV-III}2O]11−, and further oxidation results in the Ru(IV) containing [{α-SiW11O39RuIV}2O]10−.

The structures of europium(III)- and uranium(IV) derivatives of [P5w3oo110]15-: Evidence for “cryptohydration”

The crystal structures of (Nh4)11.5K0.5[ Eu(OH2)P5W30O110] ·24 H2O, K5H5 [Eu(OH2)P5W30O110] ·31 H2O, and (NH4)11[U(OH2)P5Wn30O110 ·12 H2O have been determined. In each case, the anion has the overall virtual C5v symmetry previously observed for the sodium derivative, [NaP5W30110]14- The encrypted Eu3+ and U4+cations lie on the C5 axis, but are displaced further than the Na+ from the equatorial plane defined by the five phosphorus atoms. Only minor differences are observed between the structures of the two salts of the europium derivative, although solutions of these display31P NMR spectra with chemical shifts differing by 10 ppm, provisionally attributed to the effects of protonation of the anion, The most significant feature of the three new structures is the presence of a water molecule within the central cavity and coor-dinated to the Eu3+ or U4+ cation.The coordination spheres of the central cations can therefore be described as monocapped pentagonal antiprisms.

Carbonyl-ruthenium substituted alpha-Keggin-tungstosilicate, [alpha-SiW(11)O(39)Ru(II)(CO)](6-): synthesis, structure, redox studies and reactivity.

The carbonyl-ruthenium substituted undecatungstosilicate [alpha-SiW(11)O(39)Ru(II)(CO)](6-) () was isolated as a caesium salt and successfully characterized by using (183)W and (13)C NMR, elemental analysis, IR, UV-vis and cyclic voltammetry (CV). Polyanion represents the first example of a metal-carbonyl moiety being fully incorporated into the polyoxometalate (POM). As a result, the Ru(CO) moiety became redox active and was reversibly oxidized to the one-electron ruthenium(iii) derivative, [alpha-SiW(11)O(39)Ru(III)(CO)](5-). This Ru(III)(CO) moiety was unexpectedly stable in aqueous solution compared to the organo-ruthenium carbonyl derivatives and could be detected by using UV-vis and in situ IR coupled with electrolysis. The oxidized ruthenium(iii) derivative slowly released CO in aqueous solution, resulting in the aqua species [alpha-SiW(11)O(39)Ru(III)(H(2)O)](5-) and then the dimeric POM species by condensation. Furthermore, could be converted to the corresponding aqua polyanion by photo-irradiation.