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

Abstract

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.