Reactions of an Osmium-Hexahydride Complex with Cytosine, Deoxycytidine, and Cytidine: The Importance of the Minor Tautomers

Complex OsH6(PiPr3)2 (1) deprotonates cytosine
to give molecular hydrogen and the d4-trihydride derivative
OsH3(cytosinate)­(PiPr3)2 (2), which in solution exists as a mixture
of isomers containing κ2-N1,O (2a) and
κ2-N3,O (2b) amino-oxo and κ2-N3,N4 (2c) imino-oxo tautomers. The major isomer 2b associates with the minor one 2c through N–H···N
and N–H···O hydrogen bonds to form [2b·2c]2 dimers, which crystallize from
saturated pentane solutions of 2. Complex 1 is also able to perform the double deprotonation of cytosine (cytosinate′)
to afford the dinuclear derivative (PiPr3)2H3Os­(cytosinate′)­OsH3(PiPr3)2 (3), where the anion is coordinated κ2-N1,O and κ2-N3,N4 to two different OsH3(PiPr3)2 metal
fragments. The deprotonation of deoxycytidine and cytidine leads to
OsH3(deoxycytidinate)­(PiPr3)2 (4) and OsH3(cytidinate)­(PiPr3)2 (5), respectively, containing the anion κ2-N3,N4 coordinated.
Dimer [2b·2c]2 and dinuclear
complex 3 have been characterized by X-ray diffraction
analysis.

配合物OsH₆(三异丙基膦(PiPr₃))₂（1）可对胞嘧啶（cytosine）进行去质子化反应，生成氢气与d⁴-三氢化物衍生物OsH₃(胞嘧啶根(cytosinate))(PiPr₃)₂（2）。该衍生物在溶液中以异构体混合物形式存在，包含三种互变异构体（tautomers）：采用κ2-N1,O配位模式的2a、采用κ2-N3,O配位模式的2b（二者均为氨基-氧代型）以及采用κ2-N3,N4配位模式的2c（亚氨基-氧代型）。
主要异构体2b与次要异构体2c通过N–H···N与N–H···O氢键发生缔合，形成[2b·2c]₂二聚体（dimers）；该二聚体可从2的饱和戊烷（pentane）溶液中结晶析出。
配合物1同样可实现胞嘧啶的双重去质子化，得到双去质子化胞嘧啶根（cytosinate′），生成双核（dinuclear）衍生物(PiPr₃)₂H₃Os(双去质子化胞嘧啶根)OsH₃(PiPr₃)₂（3）。在此结构中，该阴离子以κ2-N1,O与κ2-N3,N4两种配位模式，分别与两个不同的OsH₃(PiPr₃)₂金属单元配位。
对脱氧胞苷（deoxycytidine）与胞苷（cytidine）进行去质子化反应，分别得到OsH₃(脱氧胞苷酸根)(PiPr₃)₂（4）与OsH₃(胞苷酸根)(PiPr₃)₂（5），两种配合物中的阴离子均以κ2-N3,N4模式配位。
二聚体[2b·2c]₂与双核配合物3均通过X射线衍射分析（X-ray diffraction analysis）完成了结构表征。