Polyoxometalates, aromaticity

Mylonas, A., A. Hiskia, and E. Papaconstantinou (1996). Contribution to water purification using polyoxometalates. Aromatic derivatives, chloroacetic acids. J. Mol. Cat. A Chem., 114 191-200. 

The Hiickel approach (Section 10.3.1) can be used to treat the type I polyoxometalates as aromatic systems in which the d . orbitals on each of the transition metal vertices overlap to form molecular orbitals. Since these orbitals (Figure 17a) have two nodes, the resulting aromaticity can be called binodal aromaticity. Such binodal aromaticity is much weaker than anodal or uninodal orbital aromaticity since the metal atom vertices furnishing the orbitals participating in the delocalization are much farther apart being separated by M-O-M bridges rather than direct M-M bonds. 

The concept of binodal orbital aromaticity in reduced early transition-metal polyoxometalates may be related to their classification as mixed valence compounds. Robin and Day divide mixed valence compounds into the following three classes ... 

The aldol reaction of cyclic ketones and acetone with aromatic aldehydes were carried out in combination with triflic acid in water at 25°C [250]. Other chiral primary-tertiary diamine catalyst such as compound 167 (20 mol%) was used in combination with solid polyoxometalate acid support (6.67% mol) in the aldol reaction between dihydroxyacetone (149a) and aromatic aldehydes in NMP as solvent at 25°C to afford mainly iyn-aldol products in good yields (59-97%) and high diastereo- and enantioselectivities (78-99% de, 84-99% ee). The combination of catalyst 167 with triflic acid was used in the reaction of acyclic ketones and a-hydroxyketones 8 with aromatic aldehydes also with good results [251]. Simple chiral diamine 168 (10 mol%) in the presence of Iriflic acid (20 mol%) was applied as catalyst in the reaction between acetone and cyclohexanone with aromatic aldehydes in water at 25°C, giving aldol adducts 4 in low yields (15-58%) and moderate diastereo- and enantioselectivities (50-98% de, 45-93% ee) [252]. 

Topological aspects of aromaticity in selected inorganic and organic compounds including polyhedral boranes and polyoxometalates. 

In recent years the fundamental ideas of Huckel molecular orbital theory, the Huckel rule, and other aspects of aromaticity have been extended to polyhedral three-dimensional inorganic structures regarded as aromatic like the two-dimensional aromatic hydrocarbons. Such an extension of Huckel molecular orbital theory requires recognition of its topological foundations so that they can be applied to three-dimensional structures as well as two-dimensional structures. In this connection graph theoretical methods can be used to demonstrate the close analogy between the delocalized bonding in two-dimensional planar aromatic systems such as benzene and that in three-dimensional deltahedral boranes, and carboranes. Related ideas can be shown to be applicable for metal carbonyl clusters, bare post-transition metal clusters, and polyoxometallates. ... 

Figure 14 Types of venex orbitals participating in the delocalization of aromatic systems of various types as classified by their nodalities. (a) The anodal sp hybrid unique internal orbitals of a B-H vertex in the deltahedral boranes. (b) The uninodal p orbital of a C-H vertex in planar polygonal aromatic hydrocarbons such as benzene, (c) The binodal d y orbital of a (p -0)5M0 vertex in polyoxometalates...

The topology of the overlap of the d y orbitals in Type I polyoxometalate structures can be described by a graph G whose vertices and edges correspond to the vertices and edges, respectively, of the macropolyhedron (octahedron or cubocta-hedron). Equation (26) (Section 3.2) can then be applied to these binodal aromatic systems to relate the spectrum of G... 

The concept of binodal orbital aromaticity in reduced early transition metal polyoxometalates may be related to their... 

ESR studies on the one-electron reduced polyoxometalates M60 9" and XMi204o" suggest class II mixed valence species." " Although such species are delocalized at accessible temperatures, they behave as localized systems at sufficiently low temperatures this behavior is similar to that of semiconductors. This behavior is in accord with the much smaller overlap (i.e., lower P in equation 26) of the metal dxy orbitals associated with binodal orbital aromaticity as compared with the boron sp hybrid anodal internal orbitals in the deltahedral boranes or the carbon uninodal p... 

Han Z, Zhao Y, Peng J, Feng Y, Yin J, Liu Q (2005) The electrochemical behavior of Keggin polyoxometalate modified by tricyclic, aromatic entity. Electroanalysis 17 1097-1102... 

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