Synthesis, Assembly, and Sizing of Neutral, Lanthanide Substituted Molybdenum Blue Wheels {Mo90Ln10}

Abstract

Polyoxometalate Molybdenum blue (MBs) complexes typically exist as discrete multianionic clusters and are composed of repeating Mo building units. MB Wheels, such as {Mo176} and {Mo154} are made from pentagon-centered {Mo8} building blocks joined by equal number of {Mo1} units as loin, and {Mo2} dimer units as skirt along the ring edge and the ring sizes of the Mo-blue wheels are modulated by the {Mo2} units. Herein we report a new class of contracted lanthanide-doped MB structures that have replaced all the {Mo2} units with lanthanide ions on the ring edge with the general formula {Mo90+2nLn10-n} (n = 0, 1). We show three examples of this new decameric {Mo90Ln10} (Ln = La, Ce and Pr) framework synthesized by hydrothermal methods, and demonstrate that later Ln ions result in {Mo92Nd9}, conserving one {Mo2} linker unit in its structure, as a consequence of the lanthanide contraction. Remarkably the decameric {Mo90Ln10} compounds are the first examples of charge-neutral molybdenum wheels as confirmed by BVS and redox titrations. We detail our full synthetic optimization for the isolation of these clusters and complete characterization by X-ray, TGA, UV-vis and ICP studies. Finally we show that this fine-tuned self assembly process can be utilised to selectively enrich Ln-Mo Blue wheels for effective separation of lanthanide ions.