Synthesis and Structural Characterization of ... - Jacobs University

Chapter 1
Introduction
mono-, di-, and tri-lacunary species. 1a They can be obtained from three of the Baker-Figgis
isomers, α, β and γ, by removing adjacent MO 6 octahedra. These lacunary species can act as
ligands towards the transition metal ions that can bind to the POMs at the vacant sites.
The plenary α-[XW 12 O 40 ] n− Keggin ion is stabilized under acidic conditions but it can lose
one or three adjacent MO 6 octahedra to form mono- and tri-lacunary species by increasing the
pH of the solution. The trilacunary species can be either A-α-, in which three corner-shared
octahedra have been lost ”[XW 9 O 34 ] n− ” (tetrahedral), or B-α, in which three edge-shared
octahedra have been lost ”[XW 9 O 33 ] n− ” (pyramidal), see Figure 1.6. Therefore, the type A
relates to the association of one W 3 O 10 and three W 2 O 6 groups around the central XO 4
tetrahedron and the type B to the association of three W 3 O 10 groups around the central XO 3
pyramid. It is noteworthy that only one type for the mono-lacunary α-[XW 11 O 39 ] n− isomer can
be obtained, due to the T d symmetry, by which the all twelve octahedra of the parent Keggin
ion are equivalent.
In contrast, the non-equivalent octahedra due to C 3v symmetry of the β isomer, implies the
formation of different types of lacunary species depending on the position and the type of
octahedra removed. Therefore, three types of the mono-lacunary species can be obtained (β 1 ,
β 2 , β 3 -[XW 11 O 39 ] n− ), that differ in the position of the vacant. The vacancy is located in the
opposite triad to the rotated one for β 1 (unconnected with the rotated triad) and in the belt for
β 2 (connected with the rotated triad), while β 3 is generated by the removal of one of the three
octahedra that are part of the rotated triad, see Figure 1.7. Two trilacunary species, A-β and B-
β can be obtained from the plenary β-[XW 12 O 40 ] n− Keggin by removal a triad, the opposite
triad to the rotated one (three corner-shared octahedra) for A-β type, and any triad (three edgeshared
octahedra) except for the rotated one for B-β type, see Figure 1.7.
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