The self-assembly of metallacarboranes, a peculiar family of compounds exhibiting surface activity and resembling molecular-scale Pickering stabilizers, has been investigated by comparison to the micellization of sodium dodecylsulfate (SDS). These studies have shown that molecules without classical amphiphilic topology but with an inherent amphiphilic nature can behave similarly to classical surfactants. As shown by NMR techniques, the self-assembly of both metallacarboranes and SDS obey a closed association model. However, the aggregation of metallacarboranes is found to be enthalpy-driven, which is very unusual for classical surfactants. Possible explanations of this fact are outlined.

Anionic boron cluster compounds (ABCCs) are intrinsically amphiphilic building blocks suitable for nanochemistry. ABCCs are involved in atypical weak interactions, notably dihydrogen bonding, due to their peculiar polyhedral structure, consisting of negatively charged B-H units. The most striking feature of ABCCs that differentiates them from typical surfactants is the lack of head-and-tail structure. Furthermore, their structure can be described as intrinsically amphiphilic or aquaneutral. Therefore, classical terms established to describe self-assembly of classical amphiphiles are insufficient and need to be reconsidered. The opinions and theories focused on the solution behavior of ABCCs are briefly discussed. Moreover, a comparison between ABCCs with other amphiphilic systems is made focusing on the explanation of enthalpy-driven micellization or relations between hydrophobic and chaotropic effects. Despite the unusual structure, ABCCs still show self- and coassembly properties comparable to classical amphiphiles such as ionic surfactants. They self-assemble into micelles in water according to the closed association model. The most typical features of ABCCs solution behavior is demonstrated on calorimetry, NMR spectroscopy, and tensiometry experiments. Altogether, the unique features of ABCCs makes them a valuable inclusion into the nanochemisty toolbox to develop novel nanostructures both alone and with other molecules.

The anion [3,3'-Co(C2B9H11)2](-) ([COSAN](-)) produces aggregates in water. These aggregates are interpreted to be the result of C-H⋅⋅⋅H-B interactions. It is possible to generate aggregates even after the incorporation of additional functional groups into the [COSAN](-) units. The approach is to join two [COSAN](-) anions by a linker that can adapt itself to act as a crown ether. The linker has been chosen to have six oxygen atoms, which is the ideal number for K(+) selectivity in crown ethers. The linker binds the alkaline metal ions with different affinities; thus showing a distinct degree of selectivity. The highest affinity is shown towards K(+) from a mixture containing Li(+), Na(+), K(+), Rb(+) and Cs(+); this can be indicative of pseudo-crown ether performance of the dumbbell. One interesting possibility is that the [COSAN](-) anions at the two ends of the linker can act as a hook-and-loop fastener to close the ring. This facet is intriguing and deserves further consideration for possible applications. The distinct affinity towards alkaline metal ions is corroborated by solubility studies and isothermal calorimetry thermograms. Furthermore, cryoTEM micrographs, along with light scattering results, reveal the existence of small self-assemblies and compact nanostructures ranging from spheres to single-/multi-layer vesicles in aqueous solutions. The studies reported herein show that these dumbbells can have different appearances, either as molecules or aggregates, in water or lipophilic phases; this offers a distinct model as drug carriers.

This is the first experimental evidence that both self-assembly and surface activity are common features of all water-soluble boron cluster compounds. The solution behavior of anionic polyhedral boranes (sodium decaborate, sodium dodecaborate, and sodium mercaptododecaborate), carboranes (potassium 1-carba-dodecaborate), and metallacarboranes {sodium [cobalt bis(1,2-dicarbollide)]} was extensively studied, and it is evident that all the anionic boron clusters form multimolecular aggregates in water. However, the mechanism of aggregation is dependent on size and polarity. The series of studied clusters spans from a small hydrophilic decaborate-resembling hydrotrope to a bulky hydrophobic cobalt bis(dicarbollide) behaving like a classical surfactant. Despite their pristine structure resembling Platonic solids, the nature of anionic boron cluster compounds is inherently amphiphilic-they are stealth amphiphiles.

In search for biocompatible hydrophilic polymers suitable for preparation of delivery systems of boron cluster compounds with high loading capacity, we studied the interaction of metallacarborane sodium [3-cobalt(III) bis(1,2-dicarbollide)] with poly(2-ethyloxazoline) (PEOX) and with a double-hydrophilic block copolymer poly(ethylene oxide)-block-poly(2-ethyloxazoline) (PEO–PEOX) in aqueous solutions by a combination of scattering, microscopy, spectroscopy, and thermochemistry techniques. The paper is a contribution to our long-time study of novel hybrid nanostructures based on hydrophilic polymer–metallacarborane complexes. PEOX homopolymer interacts with metallacarborane, resulting in a water-soluble, negatively charged complex. In the case of diblock copolymer PEO–PEOX, both blocks interact with metallacarborane via dihydrogen bonds and participate in the formation of hybrid gel-like nanostructures in 0.1 M NaCl aqueous solutions, which are unique as compared to other boron cluster-containing polymeric systems. The stable spherical nanoparticles with high metallacarborane content do not adopt core/shell structure, which has been observed for other PEO-containing double hydrophilic block copolymers [Macromolecules 2009, 42, 4829], but the nanospheres are homogeneous. They contain intermixed PEO and PEOX blocks, which are cross-linked by metallacarborane molecules. The size of the nanospheres depends on a preparation protocol, while their inner structure does not. Besides the detailed study on PEO–PEOX/metallacarborane system, a high application potential of PEO–PEOX complexes with several metallacarborane-based drugs is also shown. The study clearly demonstrates that PEOX is suitable polymer for designing novel hybrid nanostructures.