Transmetalation for Flexible Pendant-Armed Schiff-Base Macrocyclic Complexes Influenced by Halide Effects.

Three 46-membered [2 + 2] pendant-armed Schiff-base macrocyclic dinuclear CdII and CuII complexes (2a, 2b, and 3b) and one 23-membered [1 + 1] CuII macrocycle 4a were prepared from the template-directed condensation reactions between 1,2-bis(2-aminoethoxy)-ethane and extended Cl-dialdehyde in the presence of CdX2 and CuX2 (X = Cl and Br), in which halide effects play important roles in the organization of final macrocyclic complexes in addition to the dominant influence of metal cations. Transmetalation was intensively studied among these CdII and CuII complexes with large and flexible macrocyclic ligands, including two previously synthesized dinuclear ZnII macrocycles (1a and 1b). Our results indicate that ZnII → CuII and CdII → CuII transmetalation proceeds more quickly than that from CdII to ZnII, and all the processes are found to be irreversible. It is noted that a [2 + 2] heterodinuclear CdIIZnII macrocyclic intermediate could be detected by ESI-MS together with [2 + 2] homodinuclear CdII and ZnII macrocyclic complexes. Furthermore, distinct halide behavior was observed in the in situ CdII → CuII and ZnII → CuII metal-ion exchange. That is to say, [2 + 2] macrocycles (1a and 2a) could be converted to [1 + 1] macrocycles 4a and 4b under the reflux condition in the case of CuCl2, accompanied by the configurational transformation from half-folded dinuclear ZnII and CdII to unfolded CuII macrocyclic skeleton. In contrast, CuBr2 leads to a highly efficient transmetalation to corresponding [2 + 2] dinuclear CuII complex 3b from both 1b and 2b no matter the experimental condition used.