Synthesis and Structural Characterization of ... - Jacobs University

Chapter 1
Introduction
structures; typically, the addenda are molybdenum and tungsten, being the W the most
common due to the stability of the polyoxotungstates in solution. Keggin-type
heteropolyanions are known to be the most investigated POMs and with the most applications
found, mostly due to their properties as electron/ proton givers and acceptors. 1
1.3.1 Baker‐Figgis isomers
Keggin-type heteropolyanions can have five different rotational isomers known as Baker-
Figgis. 17 The structure illustrated in Figures 1.3-a and 1.4 with T d symmetry is called α isomer.
By a 60˚ rotation of one, two, three or all four triads, the isomers β, γ, δ and ε can be formed,
generating structures of C 3v , C 2v , C 3v and T d symmetry respectively, see Figure 1.5. 1a
It is noteworthy that the symmetry of β structure is reduced from T d to C 3v and the new
corner-shared W-O-W contacts between the rotated triad and the rest of the anion have two
features which may account for its lower stability than α isomer, shorter W … W separation
and more acute W-O-W angles than the α isomer. This results in an increasing of the
coulumbic repulsion and less favourable dπ-pπ interactions.
Isomers γ, δ and ε have some triads with edge-sharing which results in a decrease in the
dπ-pπ interactions, therefore, these isomers are less stable than the α and β ones. Although, γ,
δ and ε isomers are coulumbically-unfavourable and isomerize to α in aqueous solution, 18
derivatives of γ-structure have been reported.
Isomers α and β have no edge-sharing triads, therefore should have similar stability, but it
is experimentally observed that β is less stable and isomerizes to α in aqueous solution as well.
Nevertheless, the equilibria between α and β isomers of Keggin heteropolytungstates have
been studied confirming that α and β isomers are similar in energy. 19 Recently, it has been
proven by theoretical calculations that the β isomer is more polarizable than α one due to its
lower symmetry. Furthermore, the stability of the β isomer is dependent on the nature of the
central XO 4 group and the net charge of the cluster. 20
10