Abstract
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.
Highlights
The transition-metal-promoted activation of aromatic C−H bonds is one of the most relevant reactions in current chemistry,[1] due to the wide range of fields with which it is connected, ranging from organic[2] and organometallic[3] synthesis to catalysis[4] and materials science.[5]
The addition of the proton initially leads to the known trihydride-bis(dihydrogen) derivative [OsH3(η2H2)2(PiPr3)2]+, which loses molecular hydrogen and dimerizes to form the bimetallic cation [{OsH2(PiPr3)2}2(μ-H)3]+ in equilibrium with the deprotonated polyhydride (PiPr3)2H2Os(μ-H)3OsH(PiPr3)2.25 To prevent side products resulting from the formation of the OsH7 cation, the reactions of 1 with imidazolium salts are usually performed in the presence of triethylamine, including those where the imidazolylidene ligand acts as a chelating assistant.[26]
Treatment of toluene solutions of 1 with 1.0 equiv of the [BF4]− salt of 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium, in the presence of 15 equiv of triethylamine, under reflux leads to the cationic trihydride derivative 2 (Scheme 1) in 82% yield after 24 h, according to the 1H and 31P{1H} NMR spectra of the crude reaction product in dichloromethane-d2
Summary
The transition-metal-promoted activation of aromatic C−H bonds is one of the most relevant reactions in current chemistry,[1] due to the wide range of fields with which it is connected, ranging from organic[2] and organometallic[3] synthesis to catalysis[4] and materials science.[5]. The abstractor of the proton is a ligand of the metal coordination sphere or an external base.[7] In accordance with this sequence of events, the activation energy for the C−H bond rupture depends upon two factors: the stability of the σ-intermediate and the C−H bond dissociation energy of the coordinated bond.[8] Because in aromatic organic molecules the strengths of the different C(sp2)−H bonds are similar, the activation is mainly governed by the stability of the σ-intermediate, which is a function of the steric hindrance experienced by the coordinated C−H bond. The selectivity of C(sp2)−H bond activation in substituted aromatic arenes is kinetically controlled by steric factors.[9]
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