Polyoxometalates, often considered as soluble metal oxides, have long attracted interest because of their large field of applications, especially in the domain of heterogeneous catalysis. [1] The famous Keggin ion [a-PMo 12 O 40 ] 3À was isolated nearly 200 years ago. Isomerization formally results from successive 608 rotations of the four basic Mo 3 O 13 groups.
Although the five isomers a, b, g, d, and e of the Keggin structure have been postulated, only the a [2] and b [3] isomers of PMo 12 O 40 have been structurally characterized to date. However, the e-Keggin structure has been encountered in related compounds which are either polyoxocations, with an Al III 12 core and a central tetrahedral Al III , Ga III , or Ge III[5] center, or polyoxoions with Mo V 12 and V V 12 cores. In the case of Mo and V derivatives, the highly negatively charged structure is stabilized by electrophilic capping groups. The Mo V 12 O 40 skeleton has been crystallographically characterized in four polyoxometalates: the [(C 5 Me 5 Rh III ) 8 -(Mo V 12 O 36 )(Mo VI O 4 )] 2 complex [7] has a central Mo VI O 4 2À tetrahedron and eight Rh III capping centers, the [NaMo 16 (OH) 12 O 40 ] 7À[8, 9] and [H 2 Mo 16 (OH) 12 O 40 ] 6À[8] polyoxometalates are stabilized by four capping Mo VI O 3 units and have a central cavity encapsulating a sodium cation and two protons, respectively. Finally, the most recent example is the [Mo 12 O 30 (OH) 10 H 2 {Ni(H 2 O) 3 } 4 ] cluster [10] with two central protons and four Ni II capping centers. These compounds highlight the capacity of the e-{Mo 12 O 40 } core to encapsulate various guests. We report here the synthesis and characterization of the first e-Keggin cation with a central phosphorous atom, stabilized by four {La(H 2 O) 4 } 3 capping groups. This cation was isolated in the three different salts 1, 2 and 3: [e-PMo 12 O 36 (OH) 4 {La(H 2 O) 4 } 4 ]Br 5 ¥ 16 H 2 O 1 [e-PMo 12 O 36 (OH) 4 {La(H 2 O) 2.5 Cl 1.25 } 4 ] ¥ 27 H 2 O 2 K 3 [e-PMo 12 O 36 (OH) 4 {La(H 2 O) 4.25 Cl 0.75 } 4 ][a-PMo 12 O 40 ] ¥ 28 H 2 O 3 Compound 1 is the bromide salt of the [e-PMo 12 O 36 -(OH) 4 {La(H 2 O) 4 } 4 ] 5 polyoxocation. Compounds 2 and 3 have chloride ions directly bound to the capping La 3 centers; 2 is a neutral compound while 3 has an [a-PMo 12 O 40 ] 5À ion as the counterion.
Compounds 1 and 2 were characterized by 31 P NMR, IR, and UV/Vis spectroscopy, elemental analysis, potentiometric amenable to scale-up, geared to provide access to analogues, and involves only one protecting-group manipulation.