Aggregation of metallacarborane mixtures in aqueous solutions: Iron bis(1,2-dicarbollide) as paramagnetic probe in aggregates of diamagnetic cobalt bis(1,2-dicarbollide)

• Metallacarboranes self-assemble at air/water interface and in solution. • Metallacarboranes produce mixed aggregates with FESAN as paramagnetic NMR probe. • Paramagnetic effects directly confirmed orientation of COSAN cluster in aggregates. • Rotation and dynamics of COSAN clusters are restricted in aggregates. Metallacarboranes are surface active inorganic boron cluster compounds, and they exhibit solution behavior comparable to organic amphiphiles. We compared the aggregation abilities of two anionic metallacarborane clusters, cobalt(III) bis(1,2-dicarbollide), COSAN, and iron(III) bis(1,2-dicarbollide), FESAN, in aqueous solutions using of NMR spectroscopy, isothermal titration calorimetry, tensiometry, and all-atom molecular dynamics simulations. Leveraging strong paramagnetic effect of FESAN, we used it as a probe in COSAN/FESAN mixed aggregates in order to understand their structure and dynamics. Calculations confirmed that both COSAN and FESAN clusters have the same propensity to form small aggregates with bound sodium counterions. It was confirmed that both COSAN and FESAN are moderately surface active, and the surface activity is monotonically decreasing even above the critical aggregation concentration, CAC. All the techniques confirmed that the aggregation process as well as the structure of the aggregates of COSAN, FESAN and their mixtures are practically the same. Consequently, FESAN is an effective paramagnetic probe. The thorough paramagnetic NMR study revealed that the dynamics of the aggregates in respect to clusters exchange is very fast. The paramagnetic induced shifts, PISs, did not provide any conclusive picture of the aggregate structure because of the rapid averaging. However, the investigation of the paramagnetic relaxation enhancements, PREs, clearly showed that the CH1,2 and CH1′,2′ groups of the metallacarborane clusters in the cisoid conformation are preferentially oriented inwards the aggregates, and the rotation and dynamics of the clusters are restricted.