Manipulating Clusters by Use of Competing N,O-Chelating Ligands: A Combined Crystallographic, Mass Spectrometric, and DFT Study - zeng's paper

Two heptanuclear discs, [Co7(L1)6(CH3O)6]2+ (Co7L1) and [Co7(L2)12](ClO4)2 (Co7-L2), have been prepared by the reaction of [Co(H2O)6](ClO4)2 with Schiff base HL1 (HL1= 2-methoxy-6-[(methylimino)methyl]phenolate) or benzimidazole derivative HL2 (HL2=(1H-benzo[d]imidazol-2-yl)methanolate), respectively, at room temperature. In contrast, the reaction of [Co(H2O)6](ClO4)2 with a mixture of the two ligands, having similar coordination ability but different shapes, resulted in the butterfly-structured tetranuclear cobalt cluster [Co4(L1)2(L2)4](ClO4)2·2H2O (Co4). Electrospray ionization mass spectrometry (ESI-MS) analysis in real time revealed the {Co1}!{Co2}!{Co4}!{Co7} stepwise assembly when a single ligand was employed, but when both ligands were used only {Co4} was obtained, an intermediate of the former reactions. Interestingly, ligand competition was evidenced in the assembly process of the reaction with two ligands; for example, [Co4(L1)4(L2)2]2+ and [Co4(L1)3(L2)3]2+ were the primary clusters observed at the beginning of the reaction, but [Co4(L1)2(L2)4]2+ was the final product. This observation suggests ligand exchange is taking place, and DFT calculations confirmed that these transformations were possible. By varying the ratio of ligands, the abundance of a certain species in the competitive reaction could be controlled.