Bis-imine-cyclometalated macrocycles: synthesis, characterization and observation of solution behaviour

Abstract

A novel class of cyclometalated macrocycles [(Cp*Ir)₂(R–NC–C₆H₂–CN–R)₂]₂(pyrazine)₂·(OTf)₄ [R = Ph (4a), p-MeOC₆H₄ (4b), p-MeC₆H₄ (4c), p-ClC₆H₄ (4d), Me (4e)]; [(Cp*Rh)₂(R–NC–C₆H₂–CN–R)₂]₂(pyrazine)₂·(OTf)₄ [R = Ph (4a′), p-MeOC₆H₄ (4b′), p-MeC₆H₄ (4c′)] and [(Cp*Ir)₂(R–CN–C₆H₄–NC–R)₂]₂(pyrazine)₂·(OTf)₄ [R = Ph (5a), p-MeOC₆H₄ (5b)] was stepwise constructed through the double-site C–H activation of aromatic bis-imine substrates. The structures of binuclear complexes and tetranuclear macrocycles were confirmed by single-crystal X-ray diffraction. Isomers were found both in binuclear species and macrocyclic complexes. Flexible substrates led to the existence of isomers for binuclear species, yet gave no isomers after macrocyclic constructions; rigid ones, in contrast, led to isomers only for macrocyclic species. The isomers of tetranuclear macrocycles were thermodynamically stable to reversible transformation on a scale of days. Robust bonding and a certain degree of rigidity were invoked to explain the existence of isomers. This is the first example, to our knowledge, in which coordinated macrocycles containing half-sandwich Cp*M (M = Ir, Rh) fragments have been constructed, without a dynamic reversible process.