Osmium- and Iridium-Promoted C–H Bond Activation of 2,2′-Bipyridines and Related Heterocycles: Kinetic and Thermodynamic Preferences

Abstract

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).

Recent Advances in Synthesis of Molecular Heteroleptic Osmium and Iridium Phosphorescent Emitters

“…Thus, the selectivity observed is the result of the trapping, by the heteroatom of a pyridyl ring, of the intermediate resultant from the activation of an ortho-CH bond of the other pyridyl unit, despite this position is sterically encumbered and therefore its activation is slow. [27] The orthometalated bipyridine ligands of complexes 8-10 undergo further cyclometalation promoted by a second molecule of 1. Complexes 8 and 9 afford the binuclear-hexahydrides 11 and 12.…”

Rollover Cyclometalation as a Valuable Tool for Regioselective C–H Bond Activation and Functionalization

“…C–H bond selectivity is the consequence of nitrogen trapping of the intermediate formed by C–H activation. Comparable considerations are likely true for the corresponding reaction of a pentahydride iridium complex [ 29 ].…”

“…The resulting electrophilic Os(IV) center promote the hydride-mediated heterolytic cleavage of the C–H bond. The C–H bond selectivity is considered the result of nitrogen trapping of the intermediate formed by C–H activation on the other ring [ 29 ].…”

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

“… 10 Although the latter is sterically hindered and therefore the last position being activated, the substituent thermodynamically abducts the ortho -activation product by coordination. 11 This is of central importance for the comprehension of catalytic organic reactions of ortho -CH functionalization. 12 Since a catalytic cycle represents the reaction pathway with the lowest activation energy and the ortho -metalation reaction has an activation energy higher than those of other C–H bond activations in the same ring, the o -CH bond activation should form part of the fast stage of the functionalization, the ortho -metalated intermediate being the resting state of the catalyst.…”