Oxorhenium(V) Complexes with the Non-Sulfur-Containing Amino Acid Histidine

Equivalent amounts of ReOX₃(OPPh₃)(Me₂S) (where X = Cl, Br) and l-histidine (l-hisH) in acetonitrile yield ReOX₂(l-his), in which the amino acid monoanion is N,N,O-tridentate. X-ray diffraction work on both compounds shows that the three donors occupy a face in a distorted octahedron and the carboxylate oxygen is coordinated trans to the ReO bond. The 2:1 complex [ReO(l-his)₂]I is obtained by reacting 2 equiv of l-histidine with ReO₂I(PPh₃)₂ in methanol in the presence of NaOCH₃. ¹H NMR spectroscopy indicates that these complexes contain one N,N,O-tridentate histidine anion coordinated as above and one N,N-bidentate histidine anion, whose carboxylate group is free. By refluxing ReOX₂(l-his) in methanol, the carboxylic groups esterify and two octahedral units condense into an oxo-bridged dinuclear complex {ReOX₂(l-hisMe)}₂O containing N,N-bidentate histidine methyl ester. The ORe−O−ReO backbone is approximately linear, and the two ReOX₂(l-hisMe) units are related by a 2-fold axis through the central oxygen. Crystals of {ReOBr₂(l-hisMe)}₂O consist of an ordered phase containing two of the possible diastereoisomers in a 1:1 ratio. ¹H NMR spectra of these crystals include two sets of signals, consistent with the presence of two isomers with C₂ symmetry, and the spectra of the nonrecrystallized material confirm that these are the only two isomers formed.