Re(CO)3 complexes synthesized via an improved preparation of aqueous fac-[Re(CO)3(H2O)3]+ as an aid in assessing 99mTc imaging agents.: Structural characterization and solution behavior of complexes with thioether-bearing amino acids as tridentate ligands

Parallel studies of the preparation of Re and Tc-99m agents aid in interpreting the nature of tracer Tc-99m radiopharmaceuticals. Aqueous solutions of the fac-[Tc-99m(CO)(3)(H2O)(3)](+) cation are gaining wide use and are readily prepared, but such solutions of the fac-[Re(CO)(3)(H2O)(3)](+) cation (1) are not so easily accessible. Herein we describe a new, reliable, and straightforward preparation of aqueous solutions of 1, characterized by HPLC and ESI-MS. Treatment of solutions of 1 with thioether-bearing amino acids, AAH = S-methyl-L-cysteine (MECYSH), S-propyl-L-cysteine (PRCYSH), and methionine (METH), gave high yields of fac-Re(CO)AA complexes. X-ray crystallographic and NMR analyses indicated that MECYS-, PRCYS-, and MET- were bound in fac-Re(CO)(3)AA complexes as tridentate monoanionic ligands through amino, thioether, and a-carboxyl groups. In CD3OD, H-1 NMR spectra have broad signals but have two sets of signals at -10 degrees C, consistent with two isomers with different configurations at the pyramidal sulfur; these interconvert slowly on the NMR time scale at low temperatures. Indeed, the crystal structure of the fac-Re(CO)(3)(PRCYS) reveals a mixture of the two possible diastereoisomers. S-(Carboxymethyl)L-cysteine (CCMH2) and 1 gave two products, 5A (kinetically favored) and 5B (thermodynamically favored). X-ray crystallographic analyses of a crystal of 5B and of a 1:1 cocrystal of 5A and 5B showed that 5A and 5B are diastereoisomers with the CCMH- a-carboxyl group dangling. In addition to the amino and thioether groups, the S-(carboxymethyl) carboxyl group is coordinated, a feature that slows interconversion of diastereoisomers relative to the other fac-Re(CO)(3)AA complexes because interconversion can now occur only after the rupture of Re-ligand bonds. These N, O, and S tridentate adducts are quite stable, and the grouping has promise in Tc-99m(CO)(3) tracer development.