Surface-active cobalt cage complexes: Synthesis, surface chemistry, biological activity, and redox properties

Aniphiphilic cobalt(III) cage complexes with bridgehead octyl, dodecyl and hexadecyl hydrocarbon chain substituents have been synthesized simply by co-condensation of formaldehyde and long chain aliphatic aldehydes with the tripodal cobalt(III) hexaamine complex, [Co(sen)]^3t {sen = 4,4′,4″-ethylidynetris(3-azabutan-1-amine)}. The synthetic methodology was also used to prepare a novel chiral surfactant by capping the Λ-(-)_D-[Co(sen)]³⁺ stereoisomer. The cobalt(III) cage complexes with octyl to hexadecyl substituents are all surface active and reduce the surface tension of water to levels approaching those of organic solvents. The dodecyl substituted cage complex forms aggregates in aqueous solution with a critical micelle concentration of (1.3 ± 0.1) × 10^-3 mol dm^-3 at 25.00°C. The surfactant cage complexes are biologically active and are lethal at millimolar levels to the tapeworm Hymenolepis diminuta, and the parasitic eukaryote, Tritrichomonas foetus, in vitro. The biological activity of these surfactants appears to involve insertion of the paraffin tail into the organism's exterior membrane and consequent incorporation of the highly charged head-group, which perturbs the normal membrane potential and leads to disintegration of the membrane and death of the organism. The cobalt(III) cage head-group of these surfactants also undergoes a chemically reversible one-electron reduction to the corresponding cobalt(III) cage complex and the construction of oriented films of such redox reagents should be feasible. The reduction potential of the cobalt(III)/(II) couple is shifted from -0.72 to -0.61 V (vs. saturated calomel electrode) by replacing a bridgehead hydrocarbon chain substituent with an alkoxy substituent. The shift in potential correlates with the electrochemical polar substituent constants of alkyl versus alkoxy chains.