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.
ABSTRACT
Reactions of polyhydrides OsH6(PiPr3)2 (1) and IrH5(PiPr3)2 (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MHn unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal-ligand compounds OsH3{κ2-C,N-[C5RH2N-py]}(PiPr3)2 (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PiPr3)2H3Os{µ-[κ2-C,N-[C5RH2N-C5H3N]-N,C-κ2]}OsH3(PiPr3)2 (R = H (6), Me (7)), whereas 5 gives the pentahydride (PiPr3)2H3Os{µ-[κ2-C,N-[C5H3N-C5(C6H4)H2N]-C,N,C-κ3]}OsH2(PiPr3)2 (8). Pentahydride 2 promotes C-H bond activation of 3 and the iridium-dihydride IrH2{κ2-C,N-[C5H3N-py]}(PiPr3)2 (9) to afford the heterobinuclear pentahydride (PiPr3)2H3Os{µ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (10) and the homobinuclear tetrahydride (PiPr3)2H2Ir{µ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (11), respectively. Complexes 6-8 and 11 display HOMO delocalization throughout the metal-heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen-hydrogen and metal-hydrogen interactions within the MHn units.
