Development of New Linking Chemistry with the fac-{Re(CO)3}+ Core for Eventual Applications in Radiopharmaceuticals in Imaging and Therapy

The facile labeling of biomolecules with a radionuclide is a key goal in radiopharmaceutical development. This study explores two different ligand systems for fac-[Re(CO)3L]+ complexes, that could be used in bioconjugation. The first approach uses a tridentate ligand having a sulfonamide linkage and modeled on previously evaluated fac-[Re(CO)3(N(SO2R)dpa)]PF6 complexes. The present goal was to develop new related sulfonamide complexes with more hydrophilic ligands designed to avoid the bioavailability problems that would plague the N(SO2R)dpa ligand system. A series of fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes with different R groups linked to the central nitrogen of a symmetric tridentate sulfonamides were synthesized with the aim of improving the favorable in vivo bioavailability. These compounds are characterized by NMR spectroscopy and by X-ray crystallography. The second approach using monodentate ligands led to the synthesis of several amidine complexes. The challenge of avoiding isomers of amidine complexes was overcome by using C2-symmetrical heterocyclic secondary amines with 6-membered and larger rings to create an amidine substituent bulkier than the amidine CCH3 group. Treatment of fac-[Re(CO)3(Me2bipy)(CH3CN)] BF4 with these amines in organic solvents yielded novel fac-[Re(CO)3(Me2bipy)(HNC(CH3)N(CH2)2Y)]BF4 complexes having only one isomer with the E configuration as established by solid-state and 1H NMR spectroscopic data. The combination of the high steric bulk and the C2-symmetry of the amidine substituents favors the E configuration exclusively. I extended the chemistry to smaller heterocyclic amines with 4- and 5-membered rings and found amidine formation reactions were faster. Moreover, I also showed that the amidine formation reactions were faster when the methyl group of fac-[Re(CO)3(Me2bipy)(CH3CN)]BF4 was replaced by a phenyl group. A series of fac-[Re(CO)3(Me2bipy)(HNC(C6H5)N(CH2)x)]+ complexes were synthesized, characterized and used in the comparison with the analogous fac-[Re(CO)3(Me2bipy)(HNC(CH3)N(CH2)x)]+ complexes in order to understand the properties of amidine complexes and to correlate the structural features with their behavior in solution. Furthermore, a new method employing the fac-[Re(CO)3(H2O)3]+ precursor successfully demonstrated the synthesis of fac-[Re(CO)3(Me2bipy)(amidine)]+ complexes in more aqueous conditions. This new method holds promise for use in biomedical studies