All-inorganic ferric wheel based on hexaniobate-anion linkers.

Utilizing the inherent ability of Lindquist-type hexaniobate cluster-anions, [Nb 6 O 19 ] 8- , to serve as oxo-donor ligands in complexes with transition-metal cations, we report the synthesis and characterization of the first all-inorganic "ferric" wheel, Li 48 [(Nb 6 O 19 ) 8 Fe 8 (OH) 8 ]·88H 2 O, comprised of eight Fe atoms linked by eight hexaniobate cluster-anion ligands. Bond valence sum analysis of the X-ray structure and the synthesis conditions themselves indicate that the Fe atoms are in the +3 oxidation state. This is confirmed by magnetic susceptibility and electron paramagnetic resonance (EPR) measurements which indicate the presence of high spin ( S = 5/2) Fe(III) ions. In addition, magnetic susceptibility measurements reveal long-range superexchange antiferromagnetic interactions between the hexaniobate-ligand separated Fe 3+ ions ( J = -0.22 cm -1 ). More generally, the results suggest the use of hexaniobate cluster-anions as linkers in the synthesis of other two- or three-dimensional polyoxometalate framework structures.