Introduction of Niobium in the Chemistry of Octahedral Chalcogenide Clusters: Synthesis and Detailed Study of Compounds Based on Condensed and Discrete {Re₅NbQ₈} (Q = S or Se) Cores

It is known that niobium practically does not form cluster chalcogenide compounds of the {M₆(μ₃-Q₈)} type, which are widespread in the chemistry of group 6 and 7 metals. This work reports the preparation of a series of polymeric and discrete niobium-containing heterometallic clusters based on the {Re₅Nb(μ₃-S₈)} and {Re₅Nb(μ₃-Se₈)} cores. The compounds were prepared by the high-temperature reaction between rhenium and niobium dichalcogenides in a KCN melt. The 1D polymers K₅[Re₅NbQ₈(CN)₅] (Q = S or Se), which were formed as a result of the reaction, crystallize in the structural type of K₆[Mo₆Se₈(CN)₅], similar to the previously reported heterometallic clusters K₆[Re_6-<i>x</i>Mo_<i>x</i>Q₈(CN)₅] (<i>x</i> = 2-3). The polymers were solubilized to form discrete anionic clusters [Re₅NbQ₈(CN)₆]^4-. The structure and properties of the new clusters were investigated using a combination of X-ray diffraction analysis, UV/vis spectroscopy, high-resolution electrospray mass spectrometry, cyclic voltammetry, and DFT calculations. Among other features, the compounds showed high electrochemical activity, being able to form three redox states in solution with reversible transitions. It was found that redox potentials of the isoelectronic octahedral clusters demonstrate a strong cathodic shift in the sequence [Re₅OsSe₈(CN)₆]^3- > [Re₆Se₈(CN)₆]^4- > [Re₅MoSe₈(CN)₆]^5- > [Re₅NbSe₈(CN)₆]^6-, illustrating the effect of systematic changes in the composition of octahedral cluster cores on their properties.