Molecular Assembly of Metallacarboranes in Water:  Light Scattering and Microscopy Study

Abstract

Polyhedral metallacarboranes are used mainly as ion-pairing agents and recently have been recognized as potent inhibitors of HIV protease. They are characterized by exceptional hydrophobicity, rigid geometry, delocalized negative charge, ion-pairing behavior, and strong acidity of their conjugated acids. The completely novel phenomenon, association of these promising pharmaceutical tectons in aqueous solutions, is described here. The behavior of two structural types of metallacarboranes, [bis(1,2-dicarbollide)cobaltate(1-)] and bis[(3)-1,2-dicarbollylcobalt]-(3,6)-1,2-dicarbacanastide(2-)], in aqueous solution was studied by a combination of static and dynamic light scattering and microscopy methods. Spherical aggregates with radii of ca. 100 nm and fairly monodisperse nanostructures were found in aqueous solutions. The behavior of nanoaggregates is fairly complex and depends on the concentration and aging of the solutions. The particles are stabilized in the solution by counterions. The formation of larger clusters upon dilution of bis(1,2-dicarbollide)cobaltate(1-) solutions was observed. The secondary aggregation can be suppressed by addition of NaCl. Gel permeation chromatography measurements of sodium bis(1,2-dicarbollide)cobaltate(1-) show that the majority of matallacarborane molecules form nanoaggregates and only a small amount of the metallacarborane remains molecularly soluble or forms small oligomers.

The Chaotropic Effect as an Assembly Motif in Chemistry

“…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).…”

Studies of Multicenter and Intermolecular Dihydrogen B–H···H–C Bonding in [4,8,8′‐exo‐{PPh3Cu}‐4,8,8′‐(μ‐H)3‐commo‐3,3′‐Co(1,2‐C2B9H9)(1′,2′‐C2B9H10)]

“…[59][60][61][62][63] This bonding can be interpreted in terms of individual bond description and the molecular electrostatic potential distribution [64] (ESP, Figure 5, a). It can be seen that the ESP is negative in the apical region of carborane and above the phenyl rings, and positive in the region of H(C) atoms; the ESP distribution in the region of two dicarbollide anions is different.…”

Chiral separability of hydrophobic boron cluster anions with native cyclodextrins in water–methanol background electrolytes

“…First physicochemical study of the properties of cluster boranes was published in 2006 [14]. Poor attention paid to the analytical research of solid ionic and zwitterionic boron cluster compounds is documented by only 15 articles and single review [15] published till the end of 2002.…”