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 99mTc agents aid in interpreting the nature of tracer 99mTc radiopharmaceuticals. Aqueous solutions of the fac-[99mTc(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)3AA complexes. X-ray crystallographic and NMR analyses indicated that MECYS-, PRCYS-, and MET- were bound in fac-Re(CO)3AA complexes as tridentate monoanionic ligands through amino, thioether, and (alpha)-carboxyl groups. In CD3OD, 1H NMR spectra have broad signals but have two sets of signals at -10 °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)-Lcysteine (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- (alpha)-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)3AA 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 99mTc(CO)3 tracer development.