Mixed Polyanions

Ong, S.P., and Ceder, G. (2011) Novel mixed polyanions lithium-ion battery cathode materials predicted by high-throughput ah initio computations. 

Heteropolyatomic Zintl anions like the polyhomoatomic ones may also exist in solution. Actually, many heteronuclear compounds, which are not obtained as crystalline species, possibly due to the same causes discussed above for the homonuclear species, may be studied in solution. Multinuclear NMR spectroscopy has proved to be an excellent technique for such studies. Mixed polyanions containing atoms of the same group can apparently be prepared in solution practically in any proportion within the normal nuclearity range of the homonuclear species by dissolving the appropriate ternary alloys. Thus for instance, from sodium-thin-lead alloys in ethylenediamine solution the following series of cluster anions [SnxPb9 x] may be stabilized. 

Kim H, Park I et al (2012) New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries combined first principles calculations and experimental study. J Am Chem Soc 134 10369-10372... 

See colour insert.) The 3D sodium diffusion paths in the NaiFe3(P04)2(P207) structure and its activation barriers. (Reprinted with permission from H. Kim et al.. New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries Combined first principles calculations and experimental study, /. Am. Chem. Soc. 134,2012,10369-10372. Copyright 2012, American Chemical Society.)... 

For the preparation of spray-dried polyelectrolyte complexes, the polyanion was dissolved in dilute NH4HCO3 solution and mixed with the chitosan carbamate solution just before spray-drying. The excess NH4HCO3 decomposed thermally between 60 and 107 °C on the other hand, the carbamate function released carbon dioxide under the effect of the temperature at which the spray-drier was operated, thus regenerating chitosan at the moment of the polyelectrolyte microsphere formation (Fig. 5). 

Although no polyanions with nine metal atoms have so far been found in either vanadate(V) or molybdate(VI) solutions the [Mo4V5 O27]3- ion as well as its protonated form, [HMo4V5027]4 with pKa 2.5, have been identified in the mixed solutions (162, 165). 

In recent years, it has been shown (Douzou and Maurel, 1976) that some proteins can behave as polyanions or polycations, and the stability of their solid state might be endangered at lower salt concentration due to repulsive forces between protein molecules. Much more important is the problem of enzyme activity in crystals suspended in cooled mixed solvents as a consequence of cosolvent- and temperature-induced changes in salt concentration and therefore in electrostatic potentials. 

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

The synthesis of solid derivatives homo- and heteroatomlc polyanions has to date principally featured the simplest of techniques to obtain generally the least soluble and most stable of the Ions possible from either en or NHg. There is no question that many other species exist in solution if not as solids, as evidenced by Zlntl s results, numerous color changes observed during reactions of alloys with crypt solutions, several seml-quantltatlve analyses of either unsatisfactory crystals or solutions formed In mixed metal reactions (14,37), and new NMR signals from unknown species In a wide variety of systems (23,24,38). The solution chemistries alone deserve a great deal more attention. 

A second form of isomerism occurs with mixed metal polyanions, such as [PW8V404o]7, in which the relative positions of the W and V atoms may vary.56 That such positional isomers do exist has been amply demonstrated by NMR and ESR spectroscopy.57 The numbers of possible isomers increase with the lowering of skeletal symmetry, e.g. five for ar-[PWioV204o]5-, fourteen for /3-[SiMo2Wi0O4ol4. ... 

Kidoaki, S. and Yoshikawa, K. (1999) Folding and unfolding of a giant duplex-DNA in a mixed solution with polycations, polyanions and crowding neutral polymers. Biophys. Chem., 76, 133-143. 

Although the trivalent uranium species of the tetrapyrrolide polyanions display a very high reactivity, no evidence was found for the ability to interact with N2. However, the reduction of the trivalent [ [(—CH2 )5]4-calix[4]tetrapy-rrole U(dme)][K (dme)] with [K-(naphthalenide)] in DME afforded N2cleavage with formation of the mixed-valenceU(IV)-U(V) nitride below (Scheme 13) (36). The highly reactive species that performs the N2 cleavage in this case was not identified since reactions carried out under an Ar atmosphere promoted solvent deoxygenation, as well as depolymerization of polysilanol. 

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