Mechanistic Study of Acetate-Assisted C−H Activation of 2-Substituted Pyridines with [MCl₂Cp*]₂ (M = Rh, Ir) and [RuCl₂(p-cymene)]₂

Reactions of 2-substituted pyridines HL with [MCl₂Cp*]₂ (M = Ir, Rh) and [RuCl₂(p-cymene)]₂ have been carried out in the presence and absence of sodium acetate. 2-Phenylpyridine (HL1) is cyclometalated easily to form [MCl(L1)(ring)] 1a−c (M = Rh, Ir, ring = Cp*; M = Ru, ring = p-cymene). However, in the case of 2-acetylpyridine (HL2) sp³ CH activation occurs cleanly with rhodium to form N,C chelate complex [RhCl(L2)Cp*] 2b, but the reactions with iridium and ruthenium give unseparable mixtures of products. The N,C cyclometalated products [MCl(L2)(ring)] 2a−c (M = Ir, Rh, ring = Cp*; M = Ru, ring = p-cymene) have been independently prepared from the lithium enolates of 2-acetylpyridine. Notably, in the absence of acetate, [RhCl₂Cp*]₂ shows no reaction with 2-acetylpyridine, whereas [IrCl₂Cp*]₂ and [RuCl₂(p-cymene)]₂ react to form equilibrium mixtures of the starting materials and N,O chelate complexes 4a,c, respectively. In the presence of KPF₆ the N,O chelate complexes [MCl(HL2)(ring)][PF₆] 4a,c,d (M = Ir, ring = Cp*; M = Ru, ring = p-cymene, mesitylene) can be isolated. These are not intermediates en route to the N,C cyclometalated products. These results suggest that for CH activation to occur under these mild conditions acetate must coordinate to the metal prior to coordination of the ligand.