RESUMO
The Dawson-type P(2)W(15)Nb(3)O(62)(9)(-) polyoxoanion-supported Re(CO)(3)(+) complex, [Re(CO)(3).P(2)W(15)Nb(3)O(62)](8)(-) (1), has been synthesized and characterized in two different counter-cation compositions. The [(n-C(4)H(9))(4)N](8)(8+) complex provides a highly soluble compound which exists as a single isomer in solution. The carbonyl stretching infrared frequencies suggest that the P(2)W(15)Nb(3)O(62)(9)(-) ligand serves as a strong electron donor to the Re(CO)(3)(+) fragment. The P(2)W(15)Nb(3)O(62)(9)(-) polyoxoanion-supported Ir(CO)(2)(+) complex [Ir(CO)(2).P(2)W(15)Nb(3)O(62)](8)(-) (2) has also been synthesized and characterized as its octakis(tetrabutylammonium), [(n-C(4)H(9))(4)N](8)(8+), salt. This compound was characterized by NMR and IR, results which demonstrate that 2 also exists as a single isomer in solution. The [Ir(CO)(2).P(2)W(15)Nb(3)O(62)](8)(-) complex is stable in the absence of water, but decomposes quickly in the presence of even 1 equiv of water. Attempted preparation of the analogous P(2)W(15)Nb(3)O(62)(9)(-) -supported Rh(CO)(2)(+) complex (3), while monitoring by (31)P NMR, revealed that this compound is unstable in solution at room temperature. In addition, we have discovered that added Na(+) can induce the formation of non-C(3)(v)() symmetry isomers of supported Re(CO)(3)(+) and Ir(CO)(2)(+) and, by inference, supported Ir(1,5-COD)(+). When Na(+) is removed from these systems by addition of Kryptofix[2.2.2], the non-C(3)(v)() isomers convert back to the single, C(3)(v)() isomer with heating, thereby providing a model system for the little studied mobility of M(CO)(n)()(+) cations across a soluble-oxide surface. When [Rh(CO)(2).P(2)W(15)Nb(3)O(62)](8)(-) is irradiated in the presence of hydrogen and cyclohexene a novel polyoxoanion-stabilized Rh(0)(n)() nanocluster is formed, results that bear a strong analogy to Yates' work studying atomically-dispersed Rh(CO)(2)(+) on solid Al(2)O(3).(10e) Yates and co-workers observe that Rh(CO)(+).Al(2)O(3) loses a CO upon photolysis, and that the resultant Rh(CO)(1)(+).Al(2)O(3) is reduced under H(2) to form Rh(0), which in turn yields Rh(0)(n)() clusters on Al(2)O(3)-a process that, intriguingly, is largely reversible if CO is readded. Also briefly discussed is other relevant literature of solid-oxide-supported Re(CO)(3)(+) and M(CO)(2)(+) (M = Ir, Rh), literature that makes apparent the potential significance of these complexes as EXAFS and other spectroscopic models of solid-oxide-supported M(CO)(n)()(+).
RESUMO
Reaction of acidified (pH approximately 7) sodium tungstate solutions with transition metal cations (Fe(3+), Ni(2+), Zn(2+), Co(2+)) leads to the formation of transition-metal-disubstituted Keggin-type heteropolytungstates with 3d-metal ions distributed over three different positions. A detailed investigation of the synthesis conditions confirmed that the complexes could equally be obtained using aqueous solutions of either Na(2)WO(4).2H(2)O (sodium monotungstate) at pH approximately 7, Na(6)[W(7)O(24)]. approximately 14H(2)O (sodium paratungstate A), or Na(10)[H(2)W(12)O(42)].27H(2)O (sodium paratungstate B) as starting materials. Three complexes, (NH(4))(6)Ni(II)(0.5)[alpha-Fe(III)O(4)W(11)O(30)Ni(II)O(5)(OH(2))].18H(2)O, (NH(4))(7)Zn(0.5)[alpha-ZnO(4)W(11)O(30) ZnO(5)(OH(2))].18H(2)O, and (NH(4))(7)Ni(II)(0.5)[alpha-ZnO(4)W(11)O(30)Ni(II)O(5)(OH(2))].18H(2)O were isolated in crystalline form. X-ray single-crystal structure analysis revealed that the solid-state structures of the three compounds consist of four main structural fragments, namely [MO(4)W(11)O(30)M'O(5)(OH(2))](n-) (Keggin-type, alpha-isomer) heteropolytungstates, hexaquo metal cations, [M''(OH(2))(6)](2+), ammonium-water cluster ions, [(NH(4)(+))(8)(OH(2))(12)], and additional ammonium cations and water molecules. The 3d metals occupy the central (tetrahedral, M) and the peripheral (octahedral, M') positions of the Keggin anion, as well as cationic sites (M'') outside of the polyoxotungstate framework. UV-vis spectroscopy, solution ((1)H, (183)W) and solid-state ((1)H) NMR, and also chemical analysis data provided evidence that the 3d-metal-disubstituted Keggin anions do not exist in solution but are being formed only during the crystallization process. Investigations in the solid state and in solution were completed by ESR, IR, and Raman measurements.