Stable <i>exo-nido-</i>Metallacarboranes (M = Os^IV) that Incorporate <i>meta</i>-Carborane-Based Dicarbollide Ligands

Reactions of the [K]+ salts of the [nido-7,9-C2B9H12]- anion (2) and its C-phenylated derivative [7-Ph-nido-7,9-C2B9H11]- (4) with [OsCl2(PPh3)3] (3) proceed in benzene at ambient temperature with the formation of 16-electron chlorohydrido-Os(IV) exo-nido complexes, [exo-nido-10,11-{(Ph3P)2OsHCl}-10,11-(μ-H)2-7-R-7,9-C2B9H8] (5: R = H; 6: R = Ph), along with the small amounts of the charge-compensated nido-carboranes [nido-7,9-C2B9H11PPh3] (7) and [7-Ph-nido-7,9-C2B9H10PPh3] (8) as byproducts. However, when carried out under mild heating in ethanol, the reaction of 2 with 3 selectively afforded a 16-electron dihydrido-Os(IV) exo-nido complex [exo-nido-10,11-{(Ph3P)2OsH2}-10,11-(μ-H)2-7,9-C2B9H9] (9). Structures of both complexes 5 and 9 have been confirmed by single-crystal X-ray diffraction studies, which revealed that nido-carboranes in these species function as a bidentate dicarbollide ligands [7-R-nido-7,9-C2B9H10]2- linked to the Os(IV) center via two B−H···Os bonds involving adjacent B−H vertices in the upper CBCBB belt of the carborane cage. Thus, compounds 5 and 9 represent the first structurally characterized exo-nido-metallacarboranes based on meta-dicarbollide-type ligands. Variable-temperature 1H and 31P{1H} NMR experiments indicate that complex 9 is fluxional in solution and shows an unusual exchange between terminal Os−(H)2 and bridging {B−H}2···Os hydrogen atoms. Upon heating in d8-THF at 65 °C, complex 9 converts irreversibly to its closo isomer [2,2-(PPh3)2-2,2-H2-closo-2,1,7-OsC2B9H11] (13), which could thus be obtained as a pure crystalline solid. The structure of 13 has been established on the basis of analytical and multinuclear NMR data and a single-crystal X-ray diffraction study.巢式碳硼烷阴离子[nido-7,9-C₂B₉H₁₂]⁻（化合物2）及其C-苯基化衍生物[7-Ph-nido-7,9-C₂B₉H₁₁]⁻（化合物4）的钾盐与[OsCl₂(PPh₃)₃]（化合物3）在苯中室温下发生反应，生成16电子氯氢合锇(IV)外巢式（exo-nido）配合物[exo-nido-10,11-{(Ph₃P)₂OsHCl}-10,11-(μ-H)₂-7-R-7,9-C₂B₉H₈]（5：R=H；6：R=Ph），同时伴随少量电荷补偿型巢式碳硼烷副产物[nido-7,9-C₂B₉H₁₁PPh₃]（7）与[7-Ph-nido-7,9-C₂B₉H₁₀PPh₃]（8）。然而，当该反应在乙醇中温和加热时，化合物2与3的反应可选择性得到16电子二氢合锇(IV)外巢式配合物[exo-nido-10,11-{(Ph₃P)₂OsH₂}-10,11-(μ-H)₂-7,9-C₂B₉H₉]（9）。配合物5与9的结构均通过单晶X射线衍射（single-crystal X-ray diffraction）得以确认，结果表明，此类物种中的巢式碳硼烷以双齿二碳硼烷配体（dicarbollide）[7-R-nido-7,9-C₂B₉H₁₀]²⁻形式存在，通过两条B−H···Os键与Os(IV)中心配位，该键涉及碳硼烷笼上CBCBB环上相邻的B−H顶点。因此，化合物5与9是首例基于间位二碳硼烷型配体的经结构表征的外巢式金属碳硼烷。变温¹H与质子去耦³¹P{¹H}核磁共振（NMR）实验表明，配合物9在溶液中具有流变性，并展现出罕见的端基Os−(H)₂与桥连{B−H}₂···Os氢原子之间的交换过程。在氘代四氢呋喃（d₈-THF）中于65 ℃加热时，配合物9会不可逆地转化为其闭式（closo）异构体[2,2-(PPh₃)₂-2,2-H₂-closo-2,1,7-OsC₂B₉H₁₁]（13），该异构体可分离得到纯的结晶固体。配合物13的结构通过元素分析、多核核磁共振数据以及单晶X射线衍射研究得以确定。