We suggest that the paucity of PTMs is due to challenges in stabilizing clusters against agglomeration, without resorting to capping-ligands which can block catalyst active-sites. We recently reported that Mo(VI)/Rh(III)based, Anderson-type polyoxometalates can be controllably heterogenized, protected against sintering, even at 200 o C, and employed catalytically for a representative, gas-phase oxidation reaction, under thermally demanding conditions. Our approach was to site the POMs within sizematched pores of a hierarchically porous, metal-organic framework (MOF) such that periodic arrays of reactantaccessible, yet immobile, clusters are presented. Related work has been described for Keggin POMs, with the organized POMs then being deployed as heterogeneous catalysts for condensed-phase oxidation reactions. We reasoned that similarly useful arrays of agglomerationimmune PTMs could be obtained by starting with preisolated POMs. The obtained arrays could then be deployed as heterogeneous catalysts for reduction reactions.