Oxorhenium Phosphinophenolato Complexes with Model Peptide Fragments: Synthesis, Characterization, and Stability Considerations

The synthesis and characterization of a series of mixed-ligand oxorhenium(V) complexes containing the
o-diphenylphosphinophenolato ligand (HL) and model peptide fragments acting as the tridentate coligand are
reported. Thus, by reacting equimolar amounts of tiopronin, Gly-Gly, Gly-l-Phe, or glutathione (GSH) peptides
on the [(n-C4H9)4N][ReOCl3(L)] precursor in refluxing MeCN/MeOH or aqueous MeCN/MeOH mixtures, the
following complexes were obtained:  ReO{[SC(CH3)CONCH2COO][L]}[(n-C4H9)4N], 1, ReO{[H2NCH2CONCH2COO][L]}, 2, ReO{[H2NCH2CONCH(CH2C6H5)COO][L]}, 3, and ReO{[SCH2CH(NHCOCH2CH2CHNH2COOH)CONCH2COO][L]}Na, 4. The compounds are closed-shell 18-electron oxorhenium species adopting a
distorted octahedral geometry, as demonstrated by classical spectroscopical methods including multinuclear NMR.
X-ray diffraction analyses for 1 and 2 are also reported. By comparative stability studies of complexes 1−3
against excess GSH it was shown that complex 3 containing the bulky C6H5CH2 substituent adjacent to the
coordinated carboxylate group of Phe is the most stable complex.