Azolium Control of the Osmium-Promoted Aromatic C–H Bond Activation in 1,3-Disubstituted Substrates

The hexahydride complex OsH6(PiPr3)2 promotes the C–H bond activation of the 1,3-disubstituted phenyl group of the [BF4]− and [BPh4]− salts of the cations 1-(3-(isoquinolin-1-yl)­phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)­phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium­(IV) derivatives bearing κ2-C,N- or κ2-C,C-chelating ligands, a cationic dihydride-osmium­(IV) complex stabilized by a κ3-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium­(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ2-C,N- and κ2-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF4]− inhibits the azolium reactions, [BPh4]− favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C–H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium­(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.六氢配合物OsH6(PiPr3)₂可介导阳离子1-(3-(异喹啉-1-基)苯基)-3-甲基咪唑鎓与1-(3-(异喹啉-1-基)苯基)-3-甲基苯并咪唑鎓的[BF4]−及[BPh4]−盐中1,3-二取代苯基的C-H键活化反应。该反应可选择性生成配位有双齿C,N-或C,C-螯合配体的中性与阳离子型三氢合锇(IV)衍生物、由三齿C,C,N-钳形配体稳定的阳离子型二氢合锇(IV)配合物，以及由两个三氢合锇(IV)结构单元构成的双金属六氢配合物。该双金属六氢配合物的金属中心由兼具双齿C,N-与双齿C,C-螯合结构的桥联配体分隔，可实现金属中心间的电子通讯。所获产物种类多样且生成选择性优异，这源于唑鎓基团在C-H键活化过程中的核心作用，以及不同唑鎓基团间存在显著的行为差异。唑鎓基团的作用由反应所用盐的阴离子种类决定，而其反应行为则取决于自身为咪唑鎓(imidazolium)还是苯并咪唑鎓(benzimidazolium)结构：[BF4]−会抑制唑鎓参与活化过程，而[BPh4]−则可促进唑鎓参与该活化反应。与去质子化后生成的苯并咪唑卡宾不同，由咪唑鎓取代基去质子化得到的咪唑卡宾会以异常配位模式进行配位，将苯基的C-H键活化位点导向2号位。该阳离子型二氢合锇(IV)钳形配合物的氢配体之间存在强烈的量子力学交换耦合。此外，该盐在光激发下可发射红色磷光，且对1-苯乙醇脱氢生成苯乙酮、1,2-苯二甲醇脱氢生成1-异苯并呋喃酮均表现出显著的催化活性。该双金属六氢配合物在1,2,3,4-四氢异喹啉与醇类的脱氢反应中展现出金属间的催化协同效应。