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

Two heptanuclear discs, [Co (L1) (CH O) ] (Co -L1) and Co (L2) (Co -L2), have been prepared by the reaction of Co(H O) 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(H O) with a mixture of the two ligands, having similar coordination ability but different shapes, resulted in the butterfly-structured tetranuclear cobalt cluster Co (L1) (L2) ⋅2H O (Co ). Electrospray ionization mass spectrometry (ESI-MS) analysis in real time revealed the {Co }→{Co }→{Co }→{Co } stepwise assembly when a single ligand was employed, but when both ligands were used only {Co } 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, [Co (L1) (L2) ] and [Co (L1) (L2) ] were the primary clusters observed at the beginning of the reaction, but [Co (L1) (L2) ] 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.