Polynuclear cations

Crown derivatives which are able to bind simultaneously to more than two metal ions have been synthesized such species have been termed multiloop crowns (Weber, 1982). Polynuclear cation receptors of this type provide yet another series of novel crown derivatives whose metal complexes tend to exhibit a range of unusual properties. Because of the diversity of the structures falling in this category, only a brief mention of this ligand type is presented here. 

The bulk of the systematic work has been carried out so far in this area. Even then most of it refers to the first period, and data for the second and third periods are needed. The predominance of hydroxo and polynuclear cationic species with this latter group will undoubtedly complicate the measurements. 

Hydrolysis is strong acid in which all hydroxo bridges are cleaved, followed by identification of the various mononuclear species and a determination of their molar ratios, may provide extremely valuable information. A straightforward example is the cleavage of the tetra-nuclear species Cr4(NH3)12(OH)66+, which yields Cr(H20)63+ and cis-Cr(NH3)4(H20)23+ in a ratio of 1 3 (40). Since it could be demonstrated at the same time that the polynuclear cation does not exhibit acid base properties in the pH region for terminally coordinated water, it was concluded that the only possible structure was 6 in Fig. 1, as later confirmed by a crystal-structure analysis (41). 

Metal-oxygen and metal-metal distances have been established for many oxo- and hydroxo-bridged dimers and polynuclear cations, for example (57)220 and several of type (58).217 Early work was on solids, but recently X-ray diffraction studies have established the geometries of a few such species in solution. EXAFS has proved valuable in establishing the geometry of the various di- and tri-nuclear forms of the various oxidation states of molybdenum in aqueous solution.23... 

The well known ability of antimony pentafluoride to oxidize some elements, such as sulfur or selenium, to the corresponding polynuclear cations [1] found its application in the synthesis of fluorinated materials. Reaction of sulfur with TFE, HFP, and PFIB in the presence of SbF5 was reported [ 129,130] ... 

Similarly, the nitride, carbide, cyanide, carboxylate, and carbonate salts of aluminum are unstable in aqueous solution. Aluminum salts of strong acids form solutions of the hydrated cation (see Hydrates). These solutions are acidic owing to the partial dissociation of one of the coordinated water molecules (equation 6), the p/fa of [A1(H20)6] + being 4.95 (see Acidity Constants). Note that this is quite similar to that of acetic acid. The second step in the hydrolysis reaction yields a dihydroxide species that undergoes condensation to form polynuclear cations (see Section 8). Antiperspirants often include an ingredient called aluminum chlorhydrate that is really a mixture of the chloride salts of the monohydroxide and dihydroxide aluminum cations. The aluminum in these compounds causes pores on the surface of the skin to contract leading to a reduction in perspiration. 

Exceptions to the simple monomeric cation structure are encountered when the bidentate ligand is an a,tt>-dinitrile. For example, the complex [Cu(CNCH2CH2CN)2]N03, first prepared in 1923 (250), has recently been shown to have a polynuclear cation made up of... 

In aqueous solution and in the absence of coordinating ligands, arsenic and antimony are present either as oxides, oxoanions, or their protonated forms such as As(OH)3. However, for bismuth a wide range of basic salts are known and various polynuclear cations have been proposed or... 

The polynuclear cation [(CeHe)3Co3(CO)2] has been reported from the reaction of Hg[Co(CO)4]a or Coa(CO)0 with benzene in the presence of an aluminum trihalide (66, 114). The cation is believed to have the structure (XXXI) analogous to (7r-C5H5)3Ni3(CO)a (289). Neutral polynuclear complexes of the type (arene)Co4(CO)g (arene = benzene, toluene, anisole, />-xylene, mesitylene, tetrahydronaphthalene) have been prepared by the reaction of (RC=CH)Co2(CO)e (R = H or Ph) with norbornadiene in the appropriate aromatic solvent or in some cases by simply warming Co4(CO)i2 with the arene (247, 365). The compounds are believed to have the structure (XXXII) derived from that of Co4(CO)i2 (435) by replacement of three apical CO groups by the arene. A normal coordinate analysis has been carried out on several of... 

The crystal structure of U6(S04)6(0H)404 (Fig. 17) contains both additional O and OH" anions that are not bonded to The U atoms and additional anions form the [U604(0H)4] polynuclear cations shown in Fig. 15b. The U atom coordination is a regular octahedron, whereas the O and OH anions are under each triangular face of the octahedron. The [U604(0H)4] complexes are linked into frameworks through sulfate tetrahedra. The ions are in square antiprismatic coordination. It is noteworthy that the compound is isostructural with its Ce analogue [89]. 

Pi.YAsuNov, A. V., and I. Greni hi . 1994. The temperature dependence of stability constants for the formation of polynuclear cationic species. Geochim. Cosmochim. Acta 58(17) 3561-82. 

Lean NOx-SCR with CH4 has been investigated on Co-modified synthetic ferrierite prepared in different conditions of ion exchange (temperature, time, precursor zeolite composition), A maximum NOx conversion (50% at 100% CH4 conversion) was obtained at 500°C. Activity and selectivity depend on the nature of Co species (from mononuclear to polynuclear cationic to oxidised phases) formed into ferrierite. The effect of side reactions such as uncatalysed and catalysed methane combustion on catalytic performance is discussed. 

The spaces between the natural layers can be enlarged to form pillared interlayered clays. This is carried out by ion exchanging the charge compensation cations with polynuclear metal ion hydro-complexes which are formed in hydrolysed solutions of polyvalent metal ions such as Al(III) or Zr(IV). The polynuclear cations dehydrate on calcination to create metal oxide clusters which act as pillars between the clay layers and create spaces of molecular dimensions. Example separations with pillared clays include the separation of oxygen and nitrogen, and the separation of isomers. 

A variety of polynuclear cations other than the Alj 3 Keggin ion have been used to pillar clays. Among them, we cite the zirconium tetramer [30,40,41,50,51], chromia polymers [52], bismuth [53], silicon [54], and niobium and tantalum halide clusters [55]. Of these, the most thoroughly studied have been zirconium polymers. 

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