Chemistry of rhenium(I) tricarbonyl complexes of biomedical relevance
fac-[ReI(CO)3L]n complexes serve as models for short-lived fac-[99mTcI(CO)3L] imaging tracers. Dangling groups on L, needed to achieve desirable biodistribution, complicate the NMR spectra, which are not readily understood. In fac-[ReI(CO)3L]+ with less complicated L, NH groups (exo-NH) projecting toward the L face sometimes showed an upfield shift attributable to steric shielding of the exo-NH group from the solvent by the chelate rings. To investigate whether exo-NH groups in six-membered rings exhibit the same effect and whether the presence of dangling groups alters the effect, we prepared new fac-[Re(CO)3L]n complexes that allow direct comparisons of exo-NH shifts for six-membered vs. five-membered chelate rings. The use of anions as probes, including the new use of the [ReBr6]2– anion as a paramagnetic outer-sphere H-bonding shift reagent, establishes that these NH protons are not well solvated. Lack of solvation, induced by chelate ring bulk, accounts for the upfield shift. To evaluate syntheses of fac-[Re(CO)3L]+ complexes in organic solvents, we treated fac-[Re(CO)3(CH3CN)3]PF6/BF4 in acetonitrile with triamine ligands (L). When L had two primary or two tertiary terminal amine groups, the expected fac-[Re(CO)3L]+ complexes formed. Treatment of fac-[Re(CO)3(CH3CN)3]+ with various tridentate amine ligands has produced several novel compounds, which most likely arise from reaction of the coordinated nitrile with ligand terminal amines. The new compounds advance our understanding of the spectral and structural properties of Re analogues of 99mTc radiopharmaceuticals. In fac-[Re(CO)3(5,5¢-Me2bipy)(HNC(CH3)NHR)]BF4 complexes, the monodentate amidine ligand adopts the E, E¢, and Z, but not the Z¢ configuration in solution. Both amidine CN bonds have double-bond character, leading to slow isomerization on the NMR time scale. The equilibrium favors the E¢ isomer as NHR rises above a threshold size or when stacking can occur. The structural characterization of fac-[Re(CO)3L]+ complexes bearing novel ligands having a central sulfonamide group and two pyridine rings has revealed that the central N of the tertiary sulfonamide group binds to Re. These are among the few structurally characterized complexes with a tertiary (neutral) sulfonamide bound to a metal. We show that a sulfonamide can be used to conjugate the {Re(CO)3}+ unit to a porphyrin. The new ligands may be used eventually in 99mTc imaging.
iminoether, amidine, polyamine, rhenium tricarbonyl, sulfonamide
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