Natalya V. Izarova

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.

Polyoxometalates Made of Gold: The Polyoxoaurate [AuIII4AsV4O20]8−†

Polyoxometalates (POMs, discrete nanosized metal oxo anions) are an increasingly fascinating object of study owing to their enormous structural diversity and unique combination of properties, such as thermal and oxidative stability, tunability of acidity and redox activity, and ability to activate easily accessible “green” oxidants (O2 and H2O2), thus making them attractive for applications in many different areas, for example, in catalysis, magnetism, nanotechnology, and medicine. Although this class of compounds was discovered as far back as 1826, until recently POMs were known predominantly for Group 5 and 6 addenda metals in high oxidation states (e.g. W, V). In 2008 we reported the synthesis, structure, and preliminary catalytic studies of [Pd13As8 O34(OH)6] 8 (Pd13As8), the first member of a novel subclass of polyoxoanions built exclusively of d Pd addenda metal ions. All Pd ions in Pd13As8 retain square-planar coordination geometry, in sharp contrast to all other known discrete POMs, which generally contain metal centers in octahedral environments. Even more recently we have shown the possibility of preparing other hetero-13-palladate derivatives, in which the {AsO4} 3 capping fragments in Pd13As8 are replaced by lone-pair-containing {SeO3} 2 and organic-functionalized {PhAsO3} 2 groups. We also demonstrated the existence of another structural type of polyoxopalladate, [Pd15P10O50] 20 (Pd15P10), which comprises 15 Pd addenda ions and adopts the shape of a star pentagon. We also discovered that the Pd12L8 heteropolypalladate shell in the above species can stabilize unusual coordination numbers and geometries for the encapsulated Pd ion, including unprecedented eight-fold cubic coordination. Wickleder et al. reported the solid-state d metal cluster anion [Pt12O8(SO4)12] 4 , which is composed of six dumbbell-shaped [Pt2] 6+ ions linked by oxo and sulfate bridges. There is great interest in soluble late-transition-metal oxo complexes as potential models of so-called MOx “suboxide clusters”, which have been proposed to be crucial intermediates in noble-metal-based heterogeneous catalytic oxidation systems. 6] Furthermore, noble-metal-based materials are important as catalysts in numerous industrially relevant processes and devices, including low-temperature and environmentally benign O2-based oxidations, reforming, automobile converters, and fuel cells. This background warrants an extension of our polypalladate(II) work to other d metal ions (e.g. platinum(II), gold(III)). The Au O bond is known to be weak and unstable, as seen with gold(III) oxide Au2O3, which decomposes at about 160 8C. Discrete gold(III) oxo complexes have been synthesized only recently, and all of them are stabilized by chelating organic nitrogen-donor ligands. Hill et al. have reported two Au-containing heteropolytungstates, in which the gold(III) ion is octahedrally coordinated and also has a terminal Au oxo bond. Very recent relativistic DFT computational studies performed by Bagno and Bini indicate that such a species is expected to be very unstable and should possess extremely unusual W and O NMR characteristics. The weakness and hence the high reactivity of the Au O bond provide much potential for gold oxo compounds as oxygen donors in oxidation reactions, such as the epoxidation of olefins. Herein we report the first example of a fully inorganic discrete heteropolyaurate [Au4As V 4O20] 8 (Au4As4, Figure 1a), which was synthesized in aqueous medium at room temperature and isolated as the hydrated sodium salt Na13[(H2O)4(NO3)2Na5{Au III 4As V 4O20}2]·39H2O (1). The polyanion Au4As4 was prepared by alkaline hydrolysis of hydrogen tetrachloroaurate H[AuCl4] followed by condensation of the obtained tetrahydroxogold(III) complex [Au(OH)4] in the presence of arsenate ions by decreasing the pH value of the reaction mixture from 11.5 to 7.8. It is well known that acidification of [Au(OH)4] solutions leads to the formation of insoluble Au(OH)3, which further undergoes aging processes including the release of water molecules and polymerization, thus resulting in the formation of colloidal particles with dimensions of 80 nm and larger. Apparently in our case, the arsenic ions act as protecting groups preventing condensation of the tetrameric Au4As4 polyanions to colloidal species, although a small amount of gold(III) hydroxide is [*] Dr. N. V. Izarova, Dr. N. Vankova, Prof. Dr. T. Heine, Prof. Dr. U. Kortz Jacobs University, School of Engineering and Science P.O. Box 750561, 28725 Bremen (Germany) Fax: (+ 49) 421-200-3229 E-mail: u.kortz@jacobs-university.de Homepage: http://www.jacobs-university.de/schools/ses/ukortz/

Polyoxopalladates Encapsulating Yttrium and Lanthanide Ions, [XIIIPdII12(AsPh)8O32]5− (X=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)

A series of novel yttrium- and lanthanide-containing heteropolyoxopalladates have been prepared and isolated as hydrated sodium salts, Na(5)[X(III)Pd(II)(12)(AsPh)(8)O(32)]y H(2)O (X=Y (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12), Lu (13); y=15-27). The polyanions [X(III)Pd(II)(12)(AsPh)(8)O(32)](5-) consist of a cuboid framework of twelve Pd(II) ions with eight phenylarsonate heterogroups located at the vertices and a central guest ion X. The compounds 1-13 have been prepared in a simple one-pot self-assembly reaction of Pd(CH(3)COO)(2), phenylarsonic acid and the respective salt of the element X in 0.5 M aqueous sodium acetate solution (pH 6.9), and characterized in the solid state by single-crystal X-ray diffraction, elemental and thermogravimetric (TGA) analyses, and IR spectroscopy. It was demonstrated that small, medium, and also large lanthanide ions can be incorporated in the center of the novel heteropolypalladate [X(III)Pd(II)(12)(AsPh)(8)O(32)](5-). The Ln-O bond lengths follow the expected trend decreasing from left to right in the lanthanide series. This indicates that the {Pd(II)(12)O(32)} shell can adjust to the coordination requirements of the encapsulated guest cation. Compounds 3 and 5 were selected for electrochemical studies. Their cyclic voltammetry in a lithium acetate buffer at pH 5.9 showed a Pd(0) deposition process on the glassy carbon electrode surface. Coulometry indicated that all Pd(II) centers were reduced to Pd(0). The film was stable and could be taken out of the deposition medium and characterized in pure pH 5.9 buffer. Magnetic susceptibility and EPR measurements were carried out on 5 and 6. The former was confirmed to be diamagnetic and the latter strongly paramagnetic with a S=7/2 ground state. DFT calculations for some of the polyoxometalates have been also performed.

Self-assembly of star-shaped heteropoly-15-palladate(II)

A novel heteropolyoxopalladate structural type has been discovered: the polyoxo-15-palladate(II) [Pd(0.4)Na(0.6) subsetPd(15)P(10)O(50)H(6.6)](12-) (Pd(15)) has been prepared in a one-pot self-assembly reaction and characterized by single-crystal XRD, elemental analysis, IR, TGA, (31)P NMR and electrochemistry; preliminary catalytic studies showed that Pd(15) is active as a catalyst in alcohol oxidation by dioxygen.

Polyoxopalladates Encapsulating 8-Coordinated Metal Ions, [MO8PdII12L8]n− (M = Sc3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Lu3+; L = PhAsO32–, PhPO32–, SeO32–)

A total of 16 discrete polyoxopalladates(II) [MO(8)Pd(II)(12)L(8)](n-), with a metal ion M encapsulated in a cuboid-shaped {Pd(12)O(8)L(8)} cage, have been synthesized: the phenylarsonate-capped series (1) L = PhAsO(3)(2-), M = Sc(3+) (ScPhAs), Mn(2+) (MnPhAs), Fe(3+) (FePhAs), Co(2+) (CoPhAs), Ni(2+) (NiPhAs), Cu(2+) (CuPhAs), Zn(2+) (ZnPhAs); the phenylphosphonate-capped series: (2) L = PhPO(3)(2-), M = Cu(2+) (CuPhP), Zn(2+) (ZnPhP); and the selenite-capped series (3) L = SeO(3)(2-), M = Mn(2+) (MnSe), Fe(3+) (FeSe), Co(2+) (CoSe), Ni(2+) (NiSe), Cu(2+), (CuSe), Zn(2+) (ZnSe), Lu(3+) (LuSe)). The polyanions were prepared in one-pot reactions in aqueous solution of [Pd(3)(CH(3)COO)(6)] with an appropriate salt of the metal ion M, as well as PhAsO(3)H(2), PhPO(3)H(2), and SeO(2), respectively, and then isolated as hydrated sodium salts Na(n)[MO(8)Pd(II)(12)L(8)]·yH(2)O (y = 10-37). The compounds were characterized in the solid state by IR spectroscopy, single-crystal XRD, elemental and thermogravimetric analyses. The solution stability of the diamagnetic polyanions ScPhAs, ZnPhAs, ZnPhP, ZnSe, and LuSe was confirmed by multinuclear ((77)Se, (31)P, (13)C, and (1)H) NMR spectroscopy. The polyoxopalladates ScPhAs, MnPhAs, CoPhAs, and CuPhAs were investigated by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS). Electrochemical studies on the manganese- and iron-containing derivatives demonstrated that the redox properties of the Mn(2+), Fe(3+), and Pd(2+) centers in the polyanions are strikingly influenced by the nature of the capping group. These results have subsequently been verified by density functional theory (DFT) calculations. Interestingly, electron paramagnetic resonance (EPR) measurements suggest that the coordination geometry around Mn(2+) is dynamically distorted on the EPR time scale (∼10(-11) s), whereas it appears as a static ensemble with cubic symmetry on the X-ray diffraction (XRD) time-scale (10(-15) s). The octacoordinated Cu(2+) cuboid is similarly distorted, in good agreement with DFT calculations. Interestingly, g(∥) is smaller than g(⊥), which is quite unusual, needing further theoretical development.

Synthesis and Biological Activity of Organoantimony(III)-Containing Heteropolytungstates

Three discrete organoantimony(III)-containing heteropolytungstates [(PhSb(III))(4)(A-α-Ge(IV)W(9)O(34))(2)](12-) (1), [(PhSb(III))(4)(A-α-P(V)W(9)O(34))(2)](10-) (2), and [{2-(Me(2)NCH(2)C(6)H(4))Sb(III)}(3)(B-α-As(III)W(9)O(33))](3-) (3) have been synthesized in one-pot reactions in aqueous medium using the appropriate lacunary heteropolyanion precursor and organoantimony(III) source. Polyanions 1-3 were isolated as hydrated salts, (NH(4))(12)[(PhSb(III))(4)(A-α-Ge(IV)W(9)O(34))(2)]·20H(2)O (1a), Rb(9)Na[(PhSb(III))(4)(A-α-P(V)W(9)O(34))(2)]·20H(2)O (2a), and Rb(3)[{2-(Me(2)NCH(2)C(6)H(4))Sb(III)}(3)(B-α-As(III)W(9)O(33))]·7H(2)O (3a). The compounds 1a-3a were fully characterized in the solid state using infrared (IR) spectroscopy, single-crystal XRD, and thermogravimetric and elemental analyses. The stability of 1-3 in aqueous solution was confirmed by multinuclear NMR ((1)H, (13)C, (31)P, and (183)W) spectroscopy. Preliminary studies on the biological activity of 1-3 showed that all three compounds might act as potent antimicrobial agents.

Bis[tetraruthenium(IV)]-Containing Polyoxometalates: [{RuIV4O6(H2O)9}2Sb2W20O68(OH)2]4– and [{RuIV4O6(H2O)9}2{Fe(H2O)2}2{β-TeW9O33}2H]−

The reaction of [Sb(2)W(22)O(74)(OH)(2)](12-) and [Fe(4)(H(2)O)(10)(β-TeW(9)O(33))(2)](4-) with (NH(4))(2)[RuCl(6)] in aqueous solution resulted in the novel ruthenium(IV)-containing polyanions [{Ru(IV)(4)O(6)(H(2)O)(9)}(2)Sb(2)W(20)O(68)(OH)(2)](4-) and [{Ru(IV)(4)O(6)(H(2)O)(9)}(2){Fe(H(2)O)(2)}(2){β-TeW(9)O(33)}(2)H](-), exhibiting two cationic, adamantane-like, tetraruthenium(IV) units {Ru(4)O(6)(H(2)O)(9)}(4+) bound to the respective polyanion in an external, highly accessible fashion.

3 d metal ions in highly unusual eight-coordination: the phosphate-capped dodecapalladate(II) nanocube.

Trapped in a noble cube: A novel family of noble metalates has been discovered in which a 3d metal ion M (M = Mn(II), Fe(III), Co(II), Cu(II), Zn(II)) is encapsulated by a 12 palladium-oxo cage {Pd(12)O(32)}, which is capped by eight phosphate groups. Such discrete nanocubes were further investigated by EPR spectroscopy, electrochemistry, and in homogeneous hydrogenation catalysis.

Noble metals in polyoxometalate chemistry: palladium-containing derivatives of the monolacunary Keggin and Wells-Dawson tungstophosphates.

We have prepared the three novel Pd(II)-containing tungstophosphates [Pd(2)(α-PW(11)O(39)H(0.5))(2)](9-) and two structural isomers of [Pd(2)(α(2)-P(2)W(17)O(61)H(n))(2)]((16-2n)-) via simple synthetic procedures and characterized their potassium salts by single-crystal X-ray diffraction, elemental analysis, and IR and multinuclear ((31)P and (183)W) NMR spectroscopy. This study sheds light on the long-standing question about the nature and structure of the actual products formed in the reaction of Pd(II) ions with monolacunary Keggin-type [α-XW(11)O(39)](n-) and Wells-Dawson-type [α(2)-P(2)W(17)O(61)](10-) heteropolytungstates.

The selenite-capped polyoxo-4 aurate(III), [Ausup(I)sup(I)sup(I)₄O₄(Sesup(I)sup(V)O₃)₄]⁴⁻

The discrete heteropolyaurate [Au(III)(4)O(4)(Se(IV)O(3))(4)](4-) (Au(4)Se(4)) represents only the second member of this class ever reported, and was synthesized via one-pot room temperature condensation in aqueous medium. was structurally characterized in the solid state and in solution by single-crystal XRD, TGA, FT-IR, (77)Se NMR, mass spectrometry and electrochemistry.