Activation of Small Molecules by the Metal-Amido Bond of Rhodium(III) and Iridium(III) (η 5 -C 5 Me 5 )M-Aminopyridinate Complexes.

We report the synthesis and structural characterization of five-coordinate complexes of rhodium and iridium of the type [(η5 -C5 Me5 )M(N^N)]+ (3-M+ ), where N^N represents the aminopyridinate ligand derived from 2-NH(Ph)-6-(Xyl)C5 H3 N (Xyl = 2,6-Me2 C6 H3 ). The two complexes were isolated as salts of the BArF anion (BArF = B[3,5-(CF3 )2 C6 H3 ]4 ). The M-Namido bond of complexes 3-M+ readily activated CO, C2 H4 , 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 C2 H4 , 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]CH2 CH2 N(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 C2 H4 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.