[MW ₁₂ O ₄₄ ] clusters: unprecedented central heteroatoms atomically dispersed in the eight coordination state bridging the 1 : 12 polyoxometalate family of Keggin and Silverton

A general synthetic protocol is developed to afford a series of [MW12O44] (M = Ni2+, Co2+ and Fe3+) clusters with diverse central heteroatoms, by employing [W12O44]16- as the structure-directing precursor. The structures of the [MW12O44] clusters are definitively confirmed by single crystal X-ray diffraction (XRD). The central heteroatoms are monodispersed and capture the "empty" cavity of [W12O44]16- with an 8 coordination number state, as demonstrated by the combination of single crystal structure extended X-ray absorption fine structure (EXAFS) fitting analysis and wavelet transform EXAFS (WTEXAFS). The chemical state of the heteroatoms is confirmed by analyses of high resolution synchrotron radiation photoelectron spectroscopy (HR-SRPES), high resolution X-ray photoelectron spectroscopy (HR-XPS) and EXAFS. The exact components of the [MW12O44] clusters and their thermal stability are also investigated by inductively coupled plasma-mass spectrometry (ICP-MS) and thermal gravimetric analyses (TGA). The powder XRD patterns indicate their phase purity. Moreover, the newly discovered [MW12O44] clusters bridge the Keggin and Silverton polyoxometalate family structures and so unify the 1 : 12 hetero-polyoxometalates.