Reduction of rheniumV oxo Schiff base complexes with triethylphosphine

Abstract

Pioneering techniques for therapeutic treatment of cancers involve targeting cancer sites with strong beta-emitting radionuclides, thereby destroying the cancer cells. This is achieved by coordinating the radioisotope to a very chemically stable environment and linking it to a specific biologically active targeting molecule, which interacts with particular cancer cells. Radioactive isotopes of rhenium possess characteristics of such a nuclide. The focus of our research is to investigate two possible pathways for the reaction of [ReOX(Schiff base)] with phosphine ligands, one a mono-substituted ReV complex and one a di-substituted ReIII complex. The preferred ReIII complex is lower in oxidation state and more kinetically inert or stable relative to ReV. For practical applications it is necessary to have an extremely stable in vivo radionuclide complex which can be conjugated to a suitable biological targeting agent.

The rigid sal2phen ligand, where Sal2phen is a tetradentate Schiff base ligand, was investigated to determine if the ReIII could be synthesized from the ReV starting complex [ReVOCl(Sal2phen)]. [ReVOCl(Sal2phen)] was reacted with triethylphosphine (PEt3) in attempts to yield the ReIII complex trans-[ReIII(PEt3)2(Sal2phen)][X]. Previous work indicated that the strongly reducing and strongly nucleophilic PEt3 might yield the ReV product from [ReVOCl(Sal2phen)]. The synthesized coordinated complex was reacted with an quaternary ammonium salt, ammonium hexaflurophosphate (NH4PF6), to induce crystallization of target compound [ReIII(PEt3)2(Sal2phen)][PF6].

Preliminary 1H-NMR, 31P-NMR, and infrared spectroscopy spectra indicate the formation of cis-[ReVO(PPh3)(Sal2phen)][X]. FTIR shows the presence of the Rhenium oxo group; 31P-NMR and 1H-NMR indicate the presence of ReV and a 1:1 PEt3 : Sal2phen complex. Single crystal x-ray diffraction, mass spectroscopy, and elemental analysis are additional methods of characterization.