Novel polyoxometalates: Is antimony the new molybdenum?

Polyoxometalates based on Mo, W or V have been known for a long time and present a diverse range of structures, with the [XMo₁₂O₄₀]ⁿ⁻ Keggin ions (X = P, Si ,…) perhaps the best known.¹ They are still subject to intense research with >4000 papers published in the past five years. Following on from our study² of aryl arsonic acids RAsO₃H₂, which are straightforward molecular species based on four-coordinate As(V), we became interested in the corresponding antimony compounds. Although aryl stibonic acids of nominal formula RSbO₃H₂ have been known for over 100 years,³ their composition has remained uncertain, as they form only amorphous solids, have complicated titration behaviour and only limited solubility. The presumption has been that they are polymeric, based on 5- or 6-coordinate Sb with Sb-O-Sb linkages, though direct evidence is sparse.⁴ Recently, it has been shown by Beckman that if very bulky R groups are used, then relatively simple dimers such as (2,6-Mes₂C₆H₃Sb₂O₂(OH)₄(Mes=mesityl) can be isolated, but these represent a special case.