rH2O where p, q, r = [2,3,8] for 1 and [4,1,4] Polyoxometalates (POMs) are anionic molecular metal oxides constructed from W, Mo, V or Nb. They attract much attention due to their structures, electronic properties [1] and applications in catalysis, [2] magnetism, [3] as well as medicine [4] and molecular electronics. [5] The first polyoxometalate compound was reported by Berzelius [6] in 1826 but it was not until the 1930s that the XRay structure of this iconic compound, the Keggin ion, was first elucidated. [6] This ion has a tetrahedral symmetry with the general formula [XM12O40] n-, where X is a heteroatom (P, Si, S, Ge, As, Co, Fe) [7] with four O atoms completing the tetrahedral geometry. Investigations of the Keggin structure revealed four additional isomers, each resulting from the 60 o rotation of the four basic {M3O13} units, giving α, β, γ, δ and ε isomers as reported by Baker and Figgis, see Figure 1. [8] With the α and β -isomers, the four building blocks are linked together in a corner-shared fashion, whilst in the case of γ, δ and ε the corner-shared linkages are replaced by one, three and six edge-shared, respectively. [9] Since the first report of the most common α-and β-Keggin isomers, [10] many researchers have investigated their properties, [11] whilst others reported families of transition metal substituted derivatives of α-, β-and γ-isomers. [12] The first Keggin species containing an ε-core was reported almost 60 years later as a Rh-substituted oxomolybdenum(V) complex, [13] followed by the report of a mixed-valence Mo(V)/Mo(VI) isopolyanion [14] , the La and Ni-substituted oxomolybdenum ε-Keggin isomers [15] and recently the Bi substituted vanadiumbased ε isomer [16] , respectively. However, the only related δ-Keggin structures observed so far are not POM anions but cationic species e.g. the {Al13} cation [17] and the "reverse-Keggin" ions incorporating either p-block elements (Sb 5+ ) or first row transition metal ions (Co 2+ , Mn 2+ or Zn 2+ ). [18] Herein, we report the synthesis and characterization of the first members of the δ-Keggin polyanionic isomers to be isolated, compounds 1 and 2, with the general formula: TEAHpNaq[H2M12(XO4)O33 ( Supporting information for this article is given via a link at the end of the document.
