Howard T. Evans

Bio: Howard T. Evans is an academic researcher from Georgetown University. The author has contributed to research in topics: Crystal structure & Tetrahydrate. The author has an hindex of 4, co-authored 6 publications receiving 246 citations.

X-Ray crystallographic and tungsten-183 nuclear magnetic resonance structural studies of the [M4(H2O)2(XW9O34)2]10– heteropolyanions (M = COII or Zn, X = P or As)

Abstract: The crystal structures of K10[Co4(H2O)2(PW9O34)2]·22H2O (1) and isomorphous K10[Zn4(H2O)2(AsW9O34)2]·23H2O (2) have been determined {Mo-Kα radiation, space group P21/n, Z= 2; (1)a= 15.794(2), b= 21.360(2), c= 12.312(1)A, β= 91.96°, R= 0.084 for 3 242 observed reflections [I 3σ(I)]; (2)a= 15.842(4), b= 21.327(5), c= 12.308(4)A, β= 92.42(4)°, R= 0.066 for 4 675 observed reflections [F 3σ(F)]}. The anions have crystallographic symmetry and non-crystallographic symmetry very close to 2/m(C2h). Each consists of two [XW9O34]9– moieties [α-B isomers; X = P (1) or As (2)] linked via four CoIIO6 or ZnO6 groups. Two Co or Zn atoms each carry a water ligand. The 183W n.m.r. spectra of the anions [Zn4(H2O)2(XW9O34)2]10–(X = P or As) confirm that the anions retain 2/m symmetry in aqueous solution. Homonuclear coupling constants between 183W atoms are 5.8–9.0 Hz for adjacent WO6 octahedra sharing edges, and 19.6–25.0 Hz for octahedra sharing corners.

Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo7O24]6–, in the ammonium and potassium tetrahydrate salts

Abstract: The crystal structures of the isomorphous salts MI6[Mo7O24],4H2O (M = NH4 or K) have been refined by three-dimensional X-ray diffraction methods. Unit cell dimensions of these monoclinic compounds, space group P21/C with Z= 4, are, ammonium salt: a= 8·3934 ± 0·0008, b= 36·1703 ± 0·0045, c= 10·4715 ± 0·0011 A, β= 115·958°± 0·008°; and potassium salt: a= 8·15 ± 0·02, b= 35·68 ± 0·1, c= 10·30 ± 0·02 A, β= 115·2°± 02°.By use of multiple Weissenberg patterns, 8197 intensity data (Mo-Kα radiation) for the ammonium compound and 2178 (Cu-Kα radiation) for the potassium compound were estimated visually and used to test and refine Lindqvist's proposed structure in the space group P21/c. Lindqvist's structure was confirmed and the full matrix least-squares isotropic refinement led to R 0·076 (ammonium) 0·120 (potassium), with direct unambiguous location of the cations and water molecules in the potassium compound.

Crystal structures of [NEt3H]5[XCoIIW11O39]·3H2O (X = P or As)

Abstract: The orthorhombic crystal structures of [NEt3H]5[XCoIIW11O39]·3H2O for X = P and As have been determined with data collected at room temperature, and for X = P at –100 °C, using Mo-Kα radiation. For the latter the space group is Pna21, a= 21.670(11), b= 14.805(4), c= 20.393(5)A and Z= 4. The structure consists of chains of α-Keggin-type molecules joined by W–O–links aligned in the a-axis direction. The Co/W occupancy at the link is disordered, with 61% Co on one side and 39% on the other. Further probable disorder, by lamellar merohedral twinning on (001) and by misorientation of the triethylammonium ions, has obscured the ethyl groups and the water molecules. In polarized light the crystals are deep wine-red normal to the chains (in the b direction), but nearly colourless in the a and c directions. The structure of the arsenate is similar to that of the phosphate.

Polyoxometalates from heteropoly “brown” precursors. A new structural class of mixed valence heteropolytungstates, [(XO4)WIV3WVI17O62Hx]n−

Abstract: Reduction of the α-Keggin anion [Xn+O4W12O36][8 − n]− (X = H22+, B3+, Si4+) by six electrons results in the known tungsten “brown” species [Xn+O4(H2O)3WIV3WVI9O33][8 − n]− in which three W atoms have been reduced from WVI to WIV, forming a metal–metal bonded triad. The WIV atoms have terminal water coordinated in place of terminal oxo groups. Additional tungstate can condense onto these water molecules in aqueous solution between pH = 4 and 6.5 to form the species reported here, [(XO4)WIV3WVI17O62Hx]y−. The boron derivative (X = B3+) is more stable than the metatungstate (X = H22+), both of which have been characterized by elemental analysis, 183W NMR and X-ray crystal structure analysis. Eight additional tungstate groups condense in the form of a partial Keggin structure containing two triads and one dyad which is rotated 60° relative to a hypothetical α-isomer.

Synthesis, structures and applications of electron-rich polyoxometalates

Abstract: Ever since the discovery and development of polyoxometalates (POMs), it has been known that they can exist in electron-rich reduced forms of different archetypes, structural flexibilities and functionalities. There are now reliable synthetic strategies for electron-rich POMs — materials that have unique and potentially useful catalytic, electronic and magnetic properties. This Review covers the synthesis and applications of these reduced species, and also highlights their differences and advantages relative to fully oxidized POMs. More than 200 reduced POM structures are described in this Review, with emphasis placed on how reduction influences POM structure, function and properties. The metals in polyoxometalates need not be in their highest oxidation states. Indeed, polyoxometalates can exist in reduced forms, and several different metals can be incorporated into various structural archetypes. This Review describes the synthesis and characterization of these complexes, along with their topical catalytic, electronic and biological properties.

Increasing the Nuclearity of Magnetic Polyoxometalates. Syntheses, Structures, and Magnetic Properties of Salts of the Heteropoly Complexes [Ni3(H2O)3(PW10O39)H2O]7-, [Ni4(H2O)2(PW9O34)2]10-, and [Ni9(OH)3(H2O)6(HPO4)2(PW9O34)3]16-

Abstract: The rational synthesis and the structural and magnetic characterization of three different nickel clusters encapsulated in Keggin trivacant fragments are presented. The three complexes show how it is possible to increase the nuclearity of the clusters (from 3 and 4 to 9) by slightly changing the synthetic conditions. These three anionic clusters crystallize as mixed salts of K+ and Na+. The trimeric complex [Ni3(H2O)3PW10O39H2O]7- (Ni3) crystallizes in the triclinic space group P1 (a = 10.896(6) A, b = 12.869(5) A, c = 20.373(6) A, α = 94.67(6)°, β = 101.12(8)°, γ = 110.72(8)°, Z = 2) and presents a ferromagnetic triangular cluster. The tetranuclear complex [Ni4(H2O)2(PW9O34)2]10- (Ni4) crystallizes in the monoclinic space group P21/n (a = 11.824(5) A, b = 16.551(6) A, c = 21.074(5) A, β = 100.38(2)°, Z = 2) and presents a ferromagnetic rhomb-like cluster. The nonanuclear complex [Ni9(OH)3(H2O)6(HPO4)2(PW9O34)3]16- (Ni9) crystallizes in the monoclinic space group P21/c (a = 30.589(10) A, b = 12.521(6) A,...