Unraveling the condensation reactions of heterometallic {BiNb4} moieties into hybrid BixNby-oxo clusters with mass spectrometry

The most effective methodologies for the construction of polynuclear complexes usually take advantage of preorganized building blocks. Herein, we successfully developed a new versatile heterometallic {BiNb4} moiety, which can be applied for the assembly of less explored organic-inorganic hybrid polyoxoniobates. Through controlled condensation reactions, three {BiNb4} moieties can be assembled into a pair of isomeric Bi3Nb18 clusters at different temperatures; whilst four {BiNb4} moieties can be combined into two unique Bi4Nb16 and Bi4Nb18 tetramers in different solvents. Interestingly, we have used mass spectrometry to probe the above condensation reactions, which clearly confirmed a stepwise bottom-up assembly process by capturing the presence of bismuth subsalicylate precursors, salicylate coordinated intermediates, Nb incorporated {BiNb4} moieties and the final BixSiby-oxo clusters, respectively. Moreover, the packing modes of the {BiNb4} units induce different solution behaviors, and the {BiNb4} units in Bi4Nb16 can be recombined by post-synthetic recrystallization in acetonitrile to produce a new tetramer Bi4Nb18-R. Therefore, this work provides not only a useful {BiNb4} building unit but also the mechanism for their condensation, both of which will promote the future development of metal-oxo clusters.