The d 2 -hexahydride complex OsH 6 (P i Pr 3 ) 2 (1) promotes the activation of C−H bonds of 2,2′-bipyridines and related heterocycles. The study of the same reactions with the deuteride counterpart OsD 6 (P i Pr 3 ) 2 (1-d) reveals that the activation of the C−H bonds situated in the sterically less hindered positions is kinetically preferred. However, the isolated products are the result of the thermodynamic control of the reactions. Thus, reactions of 1 with 2,2′-bipyridine, 6-phenyl-2,2′-bipyridine, and 6-methyl-2,2′-bipyridine give the "rollover cyclometalation" products OsH 3 {κ 2 -C,N-[C 5 (R)H 2 N-py]}(P i Pr 3 ) 2 (R = H (2), Ph (3), Me (4)), whereas 3,5-dimethyl-6-phenyl-2,2′-bipyridine affords OsH 2 {κ 3 -C,N,C-[C 5 H 3 N-(Me) 2 py-C 5 H 4 ]}(P i Pr 3 ) 2 (5), containing a dianionic C,N,C-pincer ligand. The behavior of substrates pyridyl-benzimidazolium and -imidazolium is similar. Reaction of 1 with 3-methyl-1-(6-phenylpyridin-2-yl)-1H-benzimidazolium tetrafluoroborate leads to OsH 3 {κ 2 -C,C-[MeBzim-C 5 (Ph)H 2 N]}(P i Pr 3 ) 2 (6), bearing an anionic C py ,C NHCchelate. On the other hand, 3-methyl-1-(6-phenylpyridin-2-yl)-1H-imidazolium tetrafluoroborate yields [OsH 2 {κ 3 -C,N,C-(MeIm-py-C 6 H 4 )}(P i Pr 3 ) 2 ]BF 4 (7), containing a monoanionic C,N,C-pincer with a NHC-unit coordinated in an abnormal fashion. The reactivity pattern of these substrates is also observed with the d 4 -iridium-pentahydride IrH 5 (P i Pr 3 ) 2 (8), which has generated IrH 2 {κ 2 -C,N-[C 5 (R)H 2 N-py]}(P i Pr 3 ) 2 (R = H, (9), Ph (10)) and IrH{κ 3 -C,N,C-[C 5 H 3 N-(Me 2 )py-C 5 H 4 ]}(P i Pr 3 ) 2 (11). The osmium(IV)−carbon bonds display a higher degree of covalency than the iridium(III)−carbon bonds. In contrast to 2, the metalated carbon atom of 9 undergoes the addition of a proton of methanol to give [IrH 2 {κ 2 -N,N-(bipy)}(P i Pr 3 ) 2 ]BF 4 (12).
Reactions of polyhydrides OsH 6 (P i Pr 3 ) 2 (1) and IrH 5 (P i Pr 3 ) 2 (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MH n 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 OsH 3 {κ 2 -C,N-[C 5 RH 2 N-py]}(P i Pr 3 ) 2 (R = H (3), Me (4) Ph ( 5)). Reactions of 3 and 4 lead to the 8). Pentahydride 2 promotes C-H bond activation of 3 and the iridiumdihydride IrH 2 {κ 2 -C,N-[C 5 H 3 N-py]}(P i Pr 3 ) 2 (9) to afford the heterobinuclear pentahydride 10) and the homobinuclear tetrahydride 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 tuning the strength of the hydrogenhydrogen and metal-hydrogen interactions within the MH n units.
The hexahydride complex OsH 6 (P i Pr 3 ) 2 promotes the C–H bond activation of the 1,3-disubstituted phenyl group of the [BF 4 ] − and [BPh 4 ] − 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 κ 2 - C,N - or κ 2 - C,C -chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ 3 - 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 κ 2 - C,N - and κ 2 - 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 [BF 4 ] − inhibits the azolium reactions, [BPh 4 ] − 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.
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