Ionothermal Access to Defined Oligomers of Supertetrahedral Selenido Germanate Clusters

Supertetrahedral chalcogenido (semi)metalate clusters have been in the focus of inorganic and materials chemistry for many years owing to a variety of outstanding physical and chemical properties. However, a critical drawback in the canon of studying corresponding compounds has been the lack of control in assembling the supertetrahedral units, which have been known as either highly charged monomeric cluster anions or lower charged, yet extended anionic substructures of linked clusters. The latter is the reason for the predominance of applications of such materials in heterogeneous environment, or their solubilization by organic shielding, which in turn was unfavorable regarding the optical properties. Recently, we reported a partial alkylation of such clusters, which allowed for a significantly enhanced solubility at a marginal impact on the optical gap. Herein we showcase the formation of finite cluster oligomers of supertetrahedral architectures by ionothermal syntheses. We were successful in generating the unprecedented dimers and tetramers of the [Ge₄Se₁₀]^4- anion in salts with imidazolium-based ionic liquid counterions. The oligomers exhibit lower average negative charges and thus reduced electrostatic interactions between anionic clusters and cationic counterions. As a consequence, the salts readily dissolve in common solvents like DMF. Besides, the tetrameric [Ge₁₆Se₃₆]^8- anion represents the largest discrete chalcogenide cluster of a group 14 element. We prove that undestroyed cluster oligomers can be transferred into solution by means of electrospray ionization (ESI) mass spectrometry and provide a full set of characteristics of the compounds including crystal structures and optical properties.