Counterion-Induced Aggregation of Metallacarboranes

Abstract

The model of counterion-induced metallacarborane aggregation in water was verified on sodium bis(1,2-dicarbollide), one of the most studied examples of metallacarboranes. This case study introduces a novel approach in anionic boron cluster self-assembly that resembles the behavior of polyoxometalate nanoions. The key prerequisite of the counterion binding to the aggregates is the strongly uneven charge distribution of cobalt bis(dicarbollide) clusters. Simultaneously, the size of the counterion should fit into the void between the metallacarborane clusters within the aggregate. In consequence, compact pentamers with two bound sodium cations via Coulombic-driven assembly are formed. This behavior cannot be explained solely by the hydrophobic effect─the paradigm of metallacarborane aggregation in the current literature. The pentamers can merge into bigger nanostructures via the second aggregation mechanism that sheds light on the complex behavior of metallacarboranes in water. The proof-of-concept of counterion-induced metallacarborane aggregation and the proposed total description of the self-assembly via the two-process model are results of calorimetry modeling, quantum chemistry calculations, and molecular dynamics simulations.