NH NMR Shifts of New Structurally Characterized fac-[Re(CO)(3)(polyamine)](n+) Complexes Probed via Outer-Sphere Hydrogen-Bonding Interactions to Anions, Including the Paramagnetic [(ReBr6)-Br-IV](2-) Anion

作者：Perera Theshini; Marzilli Patricia A; Fronczek Frank R; Marzilli Luigi G^*

来源：Inorganic Chemistry, 2010, 49(12): 5560-5572.

DOI：10.1021/ic100386c

fac-[Re-l(CO)(3)L](n) complexes serve as models for short-lived fac-[Tc-99m(l)(CO)(3)L](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)(3)L](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-Me(2)dipn (N,N-dimethyldipropylenetriamine), aepn [N-2-(aminoethyl)-1,3-propanediamine], trpn [tris-(3-aminopropyl)amine], and tren [tris-(2-aminoethyl)amine]. In DMSO-d(6), 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-d(3), 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 upheld 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.

• 出版日期2010-6-21