Activation of Small Molecules by the Metal–Amido Bond of Rhodium(III) and Iridium(III) (η5‑C5Me5)M-Aminopyridinate Complexes
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We report the synthesis and structural characterization of five-coordinate complexes of rhodium and iridium of the type [(η5-C5Me5)M(N^N)]+ (3-M+), where N^N represents the aminopyridinate ligand derived from 2-NH(Ph)-6-(Xyl)C5H3N (Xyl = 2,6-Me2C6H3). The two complexes were isolated as salts of the BArF anion (BArF = B[3,5-(CF3)2C6H3]4). The M–Namido bond of complexes 3-M+ readily activated CO, C2H4, and H2. Thus, compounds 3-M+ reacted with CO under ambient conditions, but whereas for 3-Rh+, CO migratory insertion was fast, yielding a carbamoyl carbonyl species, 4-Rh+, the stronger Ir–Namido bond of complex 3-Ir+ caused the reaction to stop at the CO coordination stage. In contrast, 3-Ir+ reacted reversibly with C2H4, forming adduct 5-Ir+, which subsequently rearranged irreversibly to [Ir](H)(C(Me)N(Ph)−) complex 6-Ir+, which contains an N-stabilized carbene ligand. Computational studies supported a migratory insertion mechanism, giving first a β-stabilized linear alkyl unit, [Ir]CH2CH2N(Ph)–, followed by a multistep rearrangement that led to the final product 6-Ir+. Both β- and α-H eliminations, as well as their microscopic reverse migratory insertion reactions, were implicated in the alkyl-to-hydride–carbene reorganization. The analogous reaction of 3-Rh+ with C2H4 originated a complex mixture of products from which only a branched alkyl [Rh]C(H)(Me)N(Ph)– (5-Rh+) could be isolated, featuring a β-agostic methyl interaction. Reactions of 3-M+ with H2 promoted a catalytic isomerization of the Ap ligand from classical κ2-N,N′ binding to κ-N plus η3-pseudoallyl coordination mode.
