Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

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′-[(1E,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.

C-alkylpyrogallol[4]arenes (PgCs) have been studied for their ability to form metal-organic nanocapsules (MONCs) through coordination to appropriate metal ions. Here we present the synthesis and characterization of an MnII/MnIII-seamed MONC in addition to its electrochemical and magnetic behavior. This MONC assembles from 24 manganese ions and 6 PgCs, while an additional metal ion is located on the capsule interior, anchored through the introduction of bridging nitrite ions. The latter originate from an in situ redox reaction that occurs during the self-assembly process, thus representing a new route to otherwise unobtainable nanocapsules.

Under suitable conditions, C-alkylpyrogallol[4]arenes (PgCs) arrange into spherical metal-organic nanocapsules (MONCs) upon coordination to appropriate metal ions. Herein we present the synthesis and structural characterization of a novel Fe II /Fe III-seamed MONC, as well as studies related to its electrochemical and magnetic behaviors. Unlike other MONCs which are assembled through 24 metal ions, this nanocapsule comprises 32 Fe ions, uncovering 8 additional coordination sites situated between the constituent PgC sub-units. The Fe II ions are likely formed by the reducing ability of DMF used in the synthesis, representing a novel synthetic route towards polynuclear mixed-valence MONCs.

Two structurally analogous Mn-seamed C-alkylpyrogallol[4]arene (PgC )-based metal-organic nanocapsules (MONCs) have been synthesized under similar reaction conditions and characterized by crystallographic, electrochemical, and magnetic susceptibility techniques. Both MONCs contain 24 Mn centers, but, somewhat surprisingly, marked differences in oxidation state distribution are observed upon analysis. One MONC contains exclusively Mn ions, while the other is a mixed-valence Mn/ Mn assembly. We propose that these disparate oxidation state distributions arise from slight differences in pH achieved during synthesis, a factor that may lead to many spectacular new MONCs (and associated host-guest chemistries).