The lithium salt of p-tolylstibonic acid contains the anion [LiH 3 (p-MeC 6 H 4 Sb) 12 O 28 ] 4− , in which the Li + is fully enclosed within an {Sb 12 O 28 } cage with γ Keggin ion geometry; the idealized overall formula is Li 5 [LiH 3 (p-MeC 6 H 4 Sb) 12 O 28 ]Br·18H 2 O. In contrast, Ba 2+ induces the formation of the unprecedented {Sb 14 O 34 } bowl-shaped polyoxostibonate [BaH 10 (p-MeC 6 H 4 Sb) 14 O 34 ]·4H 2 O. Compounds were characterized by single-crystal X-ray crystallography and by ESI-MS.
■ INTRODUCTIONWe have recently reported the structures of sodium and potassium salts of arylstibonic acids, which for several different e x a m p l e s r e v e a l e d p o l y o x o s t i b o n a t e a n i o n s [M 2 H 10−x (RSb) 12 O 30 ] x− (M = Na, K) with a distortedhexagonal-antiprismatic array of Sb atoms and with the M cations attached in two different sites to framework O atoms within the hexagonal channel. 1,2 This structural arrangement is conserved for a variety of aryl R groups, crystallized with several different cations. 3 ESI-MS studies showed that the cluster units retained their integrity in MeCN solution. 1,2 Recently, other polyoxometalates incorporating organoantimony have been described, particularly from the groups of Beckmann 4 and Baskar. 5 We herein demonstrate that the size of the cation is important in templating the polyoxometalate formation by showing that the smaller Li + provides an Sb 12 species with a more closed geometry, while the larger Ba 2+ gives a more open Sb 14 framework. Cronin et al. have recently reviewed 6 the importance of cations (mainly organic ones) in controlling the synthesis of molybdenum and tungsten polyoxometalates, and the present work extends this concept to polyoxostibonates. As an interesting historical note, in 1922, Schmidt 7 noted that arylstibonic acids showed different neutralization behavior with LiOH compared to that with NaOH or KOH. Only now has the reason for this become apparent.
■ EXPERIMENTAL SECTIONp-Tolylstibonic acid was prepared using the method of Doak and Steinman 8 and freed from Na + cations as described earlier. 2 ESI-MS was carried out on a Bruker MicrOTOF instrument, operating under standard conditions in negative ion mode, with samples made up in MeCN immediately before infusion. Assignment of ions was aided by matching the characteristic patterns generated by the 121 Sb (57%) and 123 Sb (42%) isotopes. Peaks are reported as the m/z values with the greatest intensity in the isotopic envelope. EDAX measurements on single crystals were performed on a Hitachi S-4700 field emission scanning electron microscope. The reported compounds do not melt at <330°C but rather undergo apparent evolution of water followed by gradual darkening. Yields are not quoted for the identified products, since these are hard to quantify. The initial powders represent essentially quantitative recovery of the solids added but are mixtures of species (as shown by ESI-MS) and have an unknown level of solvent incorporation even after drying in vacuo over P 2 O 5...