Methoxy-phenyl groups reduce the cytotoxicity and increase the aqueous solubility of phosphonium zwitterions and salts

The ability of phosphonium cations to act as intracellular transport vectors is well-established. Previous research has demonstrated that phosphonioalkylthiosulfate zwitterions, and -thioacetylalkylphosphonium salts are useful precursors for the formation of phosphonium-functionalised gold nanoparticles and enable the nanoparticles to be transported into cells for diagnostic and therapeutic purposes. In this report we describe the synthesis and characterisation of a series of phosphonioalkylthiosulfate zwitterions, and-thioacetylalkylphosphonium salts derived from the methoxy-phenylphosphines tris(2,4,6-trimethoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine and tri(4-methoxyphenyl)phosphine. The methoxyphenyl-substituted phosphonium compounds show greater solubility in aqueous systems than the corresponding phenyl derivatives and cytotoxicity studies reveal that the compounds are significantly less toxic than the related triphenylphosphonium derivatives. The solid-state structures of the tris(2,4,6-trimethoxyphenyl)- and tris(2,6-dimethoxyphenyl)-phosphoniopropylthiosulfate zwitterions have been investigated by single crystal X-ray crystallography. The differences in the molecular packing of the compounds may account for greater solubility of these zwitterions in aqueous solutions.