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.