Stealth Amphiphiles: Self-Assembly of Polyhedral Boron Clusters

Abstract

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.

The Chaotropic Effect as an Assembly Motif in Chemistry

“…We further concluded that the chaotropic effect presents a generic driving force that accounts for the intrinsic affinity of chaotropic ions, prominently anions, for neutral organic or inorganic matter, including concave macrocyclic binding sites, lipid bilayers, membranes, and polymers 11, 12, 13, 16, 17, 18, 22, 24, 28, 40, 43, 44, 45, 46, 47, 48. It should be noted that indications for the affinity of anions to hydrophobic binding sites existed early in the literature81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103 but were in part attributed to a putative hydrophobic (or lipophilic) property of large anions, implying the hydrophobic effect as driving force.…”

“…The tendency of different boron clusters (B 10 H 10 2− , B 12 H 12 2− , B 12 H 11 SH 2− , 1‐carbadodecaborate, and cobalt bis(1,2‐dicarbollide)) to self‐assemble into aggregates and colloids in water has been detailed by Matějíček and co‐workers,11, 188, 190, 193 while the group of Bauduin and Diat have described the formation of charged nano‐colloids from POMs 15. The unusual behavior of anionic boron clusters can again be attributed to the chaotropic effect, which complements the classical hydrophobic effect as a supramolecular assembly and self‐assembly motif in water (Figure 10).…”

“…40 The behavior of POMs has also been related to their superchaotropic character 16, 18. The intrinsic self‐assembly tendency of both superchaotropic anions (borate clusters and POMs) accounts also for their high propensity to accumulate at the water–air surface; this leads to surfactant‐like behavior, such as a reduction of the surface tension 11, 40, 53, 190, 191. Accordingly, because they lack the polar head group/hydrophobic tail design of classical amphiphiles, they have also been classified as non‐classical surfactants or “intrinsic amphiphiles” 11, 40, 53, 190, 191…”

“…The chaotropic effect extends to molecular recognition events with macrocyclic hosts,10, 12, 21, 22, 23, 24, 25, 26, 27 biologically relevant interactions with proteins and peptides,13, 28, 29, 30, 31, 32 association to small organic molecules,10, 33 binding to membranes,17, 34, 35, 36, 37, 38 polymers,39 as well as colloids,15, 18, 40 and it manifests itself in solid‐state structures between superchaotropic ions and organic components 10, 12, 26, 41, 42. The terms “chaotropic effect” and “superchaotropic ions” have been readily absorbed by the chemical literature in different contexts 11, 12, 13, 15, 16, 17, 18, 22, 24, 28, 40, 41, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54. This relevance in several areas of chemistry reveals the chaotropic effect as a generic driving force, orthogonal to the hydrophobic effect.…”

Face‐Fusion of Icosahedral Boron Hydride Increases Affinity to γ‐Cyclodextrin: closo,closo‐[B21H18]− as an Anion with Very Low Free Energy of Dehydration

“…Isothermal titration calorimetry (ITC) was used to determine the absolute binding constants and to analyze the complexation thermochemistry (Figure ); access to this technique was essential, because the binding turned out to be too strong to employ NMR titrations for accurate determination of the binding constants ( K a ). An ITC dilution titration of B21 (1 mM) was performed first; no significant heat effect was observed, which excluded a competitive enthalpic effect due to micellization/de‐aggregation events (reported for ortho ‐COSANs), and the subsequent host‐guest titrations were performed at much lower B21 concentrations (0.1 mM). The ITC titration profiles were consistent with a 1 : 1 binding stoichiometry, consistent with the mass‐spectrometric study, in which also no 2 : 1 host‐guest complexes had been observed .…”