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

The ReOX2(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 NH2, and an SCH3
group. Coordination of the SCH3 unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction
studies reveal that the crystals of ReOBr2(d,l-met)·1/2H2O and ReOBr2(d,l-met)·1/2CH3OH contain only the syn
isomer (S−CH3 bond on the side of the ReO bond), whereas ReOCl2(d-met) and ReOCl2(d,l-met) consist of the
pure anti isomer. 1H 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 [ReOX2(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray
diffraction work on [ReOBr2(His-Ala)]Br reveals the presence of distorted octahedral cations containing the ReO3+ 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
the1H 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 CD3OD solution.

ReOX₂(met)（X=Cl、Br）类配合物呈现畸变八面体构型，其中羧基氧位于与Re=O键反式的位置，而赤道平面则由两个顺式卤原子、一个氨基（NH₂）与一个甲硫基（SCH₃）占据。甲硫基的配位会形成一个手性中心，进而生成两种非对映异构体（diastereoisomers）。X射线衍射（X-ray diffraction）研究表明，ReOBr₂(d,l-met)·1/2H₂O与ReOBr₂(d,l-met)·1/2CH₃OH的晶体仅含syn异构体（S-CH₃键位于Re=O键一侧），而ReOCl₂(d-met)与ReOCl₂(d,l-met)则仅存在anti异构体。氢核磁共振（¹H NMR）光谱显示，两种异构体在丙酮溶液中以平衡状态共存：当X=Br时，anti/syn比例为1:1；当X=Cl时比例为3:1。两种异构体间的异构化过程在室温以上速率较快，但在0℃以下会减缓；在-20℃时，两种异构体的清晰二级光谱已被完全归属。耦合常数与固态构象在溶液中得以保留的结论相符。以二肽（dipeptides）His-aa（aa=甘氨酸（Gly）、丙氨酸（Ala）、亮氨酸（Leu）及苯丙氨酸（Phe））为配体，可分离得到[ReOX₂(His-aa)]X（X=Cl、Br）型配合物。对[ReOBr₂(His-Ala)]Br的X射线衍射研究显示，其存在含Re=O³+核心的畸变八面体阳离子，二肽配体通过组氨酸残基的咪唑氮、末端氨基与酰胺氧进行配位，而与Re=O键反式的位点则由氧原子占据。丙氨酸残基末端为质子化的羧基，未参与配位。组氨酸残基质子的¹H NMR信号模式恒定，证实了各类二肽均采用相似的配位模式，这与固态结构在氘代甲醇（CD₃OD）溶液中得以保留的结论一致。