Technetium and Rhenium Schiff Base Compounds for Nuclear Medicine: Syntheses of Rhenium Analogues to 99mTc-Furifosmin.

Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential 99mTc diagnostic radiopharmaceuticals. The rhenium analogues to 99mTc-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[MIII(PR3)2(N2O2-Schiff base)]+ are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1 E,1' E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethyl-2,5-dihydrofuran-3-ol) (tmf2enH2) with the M(V) starting materials ( nBu4N)[TcOCl4] and ( nBu4N)[ReOCl4] gave the monomeric products trans-[TcOCl(tmf2en)] and trans-[ReOCl(tmf2en)], respectively. Reduction of in situ formed trans-[ReOCl(tmf2en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[ReIII(PR3)2(tmf2en)]+. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible ReIII/ReIV and ReIII/ReII redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf2en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt3)2(tmf2en)]PF6. A reversible TcIII/TcII redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible TcIII/TcIV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal X-ray crystallography, and cyclic voltammetry.