Trends in NMR chemical shifts and ligand mobility of TcO(V) and ReO(V) complexes with aminothiols

Detailed H-1 and C-13 NMR studies have been conducted in a series of oxotec hnetium and oxorhenium complexes with aminothiol ligands ([SNS] [S], [SNN] [S], [SNNS]) designed as potential radiopharmaceuticals. The results of the se studies, in combination with others in the literature show that the oxom etal core creates an anisotropic environment and affects the chemical shift s of the coordinated ligand backbone in a consistent way. Protons oriented towards the oxygen appear deshielded relative to their geminals oriented aw ay from the oxygen. In addition, the direction of a side chain (towards or away from the oxometal core) on the ligand backbone is shown to have a majo r effect on chemical shifts. The fluxional mobility of the ligand in comple xes of the [SNS] [S] type was also studied by NMR and the free energy of ac tivation Delta G(C)(not equal) for the conformational inversion of the liga nd was calculated from the temperature dependence of the carbon chemical sh ifts. Delta G(C)(not equal) was found to depend on the orientation of the s ide chain present on the coordinated nitrogen. The energy barrier for the i nversion is larger for the oxorhenium complexes than for the analogous oxot echnetium complexes. (C) 2000 Elsevier Science Inc. All rights reserved.