Surfactant behaviour of metallacarboranes. A study based on the electrolysis of water

Abstract

[3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻, [1]⁻, and its chloroderivatives have been described as displaying surfactant/aggregation properties. We have studied their behaviour as electrolytes in the water electrolysis process. The electrolysis experiments support the surfactant behaviour of these compounds. These conclusions have been drawn on the grounds of the intensity/voltage (I/V) curves of water splitting into H₂ and O₂ of aqueous solutions in which the electrolytes have been tested at the same concentration. The I/V curves have permitted us to map and group the different electrolytes studied in this work. Three differentiated zones have been observed: one for true electrolytes, NO₃⁻ and ClO₄⁻; a second one for intermediate electrolytes, BF₄⁻ and p-toluenesulfonate (PTS); and a third having the surfactant dodecylbenzenesulfonate (DBS), [1]⁻ and its chloroderivatives. The incorporation into the study of the chloroderivatives of [1]⁻ has allowed us to correlate molecular structure features with aqueous performance. The studied chloroderivatives perform better as electrolytes in accordance with the descending order of available B–H groups. This comes from the higher capacity to generate B–H⋯H–C_c dihydrogen bonds in the non- or less-halogenated molecules, considered one of the main reasons for the generation of the aggregates. In order to generate B–H⋯H–C_c dihydrogen bonds the H–C_c from the carborane cluster is needed. [B₁₂H₁₂]²⁻ was chosen to prove the hypothesis as it has B–H units but lacks H–C_c units. Consequently, it should not produce self-assembling motifs, as is the case. [B₁₂H₁₂]²⁻ has an aqueous behaviour similar to SO₄²⁻.