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.