Binding of the Oxo-Rhenium(V) Core to Methionine and to N-Terminal Histidine Dipeptides

The ReOX 2 (met) compounds (X = Cl, Br) adopt a distorted octahedral structure in which a carboxylato oxygen lies trans to the Re=O bond, whereas the equatorial plane is occupied by two cis halides, an NH 2 , and an SCH 3 group. Coordination of the SCH 3 unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr 2 (D,L-met).½H 2 O and ReOBr 2 (D,L-met).½CH 3 OH contain only the syn isomer (S-CH 3 bond on the side of the Re=O bond), whereas ReOCl 2 (D-met) and ReOCl 2 (D,L-met) consist of the pure anti isomer. 1 H NMR spectroscopy shows that both isomers coexist in equilibrium in acetone (anti/syn ratio = 1:1 for X = Br, 3:1 for X = Cl). Exchange between these two isomers is fast above room temperature, but it slows down below 0 °C, and the sharp second-order spectra of both isomers at -20 °C were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX 2 (His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr 2 (His-Ala)]Br reveals the presence of distorted octahedral cations containing the Re= O 3 + core and a dipeptide coordinated through the histidine residue via the imidazole nitrogen, the terminal amino group, and the amide oxygen, the site trans to the Re=O bond being occupied by the oxygen. The alanine residue is ended by a protonated carboxylic group that does not participate in the coordination. The constant pattern of the 1 H NMR signals for the protons in the histidine residue confirms that the various dipeptides adopt a similar binding mode, consistent with the solid-state structure being retained in CD 3 OD solution.