NH NMR Shifts of New Structurally Characterized <i>fac</i>-[Re(CO)₃(polyamine)]^<i>n</i>+ Complexes Probed via Outer-Sphere Hydrogen-Bonding Interactions to Anions, Including the Paramagnetic [Re^IVBr₆]²⁻ Anion

fac-[ReI(CO)3L]n complexes serve as models for short-lived fac-[99mTcI(CO)3L]n imaging tracers (L = tridentate ligands forming two five-membered chelate rings defining the L face). Dangling groups on L, needed to achieve desirable biodistribution, complicate the NMR spectra, which are not readily understood. Using less complicated L, we found that 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. Our goal is to advance our ability to relate these spectral features to structure and solution properties. 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 versus five-membered chelate rings. New complexes were structurally characterized with the following L: dipn [N-3-(aminopropyl)-1,3-propanediamine], N′-Medipn (3,3′-diamino-N-methyldipropylamine), N,N-Me2dipn (N,N-dimethyldipropylenetriamine), aepn [N-2-(aminoethyl)-1,3-propanediamine], trpn [tris-(3-aminopropyl)amine], and tren [tris-(2-aminoethyl)amine]. In DMSO-d6, the upfield exo-NH signals were exhibited by all complexes, indicating that the rings sterically shield the exo-NH groups from bulky solvent molecules. This interpretation was supported by exo-NH signal shift changes caused by added halide and [ReBr6]2− anions, consistent with outer-sphere hydrogen-bond interactions between these anions and the exo-NH groups. For fac-[Re(CO)3(dipn)]PF6 in acetonitrile-d3, the exo-NH signal shifted further downfield in the series, Cl− > Br− > I−, and the plateau in the shift change required a lower concentration for smaller anions. These results are consistent with steric shielding of the exo-NH groups by the chelate rings. Nevertheless, despite its size, the shape and charge of [ReBr6]2− allowed the dianion to induce large upfield paramagnetic shifts of the exo-NH signal of fac-[Re(CO)3(dipn)]PF6. This dianion shows promise as an outer-sphere hydrogen-bonding paramagnetic shift reagent.