Preparation and structural studies of neutral oxorhenium(V) complexes with D-pencillamine methyl ester

Reaction of oxorhenium(V) gluconate with D-penicillamine methyl ester (methyl 3-sulfanylvalinate) yielded three neutral 1:2 complexes. For two complexes /1 and 2) the ¹H NMR spectrum in (CD₃)₂SO shows the presence of a deprotonated and a neutral nitrogen donor group in the ReO(NS)₂ system which is an unusual co-ordination mode for bidentate N,S ligands. Complex 1, [ReO{SC(CH₃)₂CH(CO₂CH₃)NH₂}{SC(CH₃)₂CH(CO₂CH₃)NH}], possibly containing in non-co-ordinating solvents an ester oxygen co-ordinated trans to the Re=O group, is able to bind a water molecule trans to the oxo core to give species 2. In aqueous solution the mixed-ligand complex [ReO{SC(CH₃)₂CH(CO₂CH₃)NH₂}{SC(CH₃)₂CH(CO₂)NH₂}] 3 was unexpectedly formed out of 1 and 2. It contains bidentate D-penicillamine methyl ester and tridentate D-penicillamine in a co-ordination geometry similar to that of the known D-penicillamine complex [ReO{SC(CH₃)₂CH(CO₂H)NH₂}{SC(CH₃)₂CH(CO₂)NH₂}] 4. Complexes 3 and 4 were structurally characterized by X-ray diffraction.