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

Two heptanuclear discs, [Co7 (L1)6 (CH3 O)6 ]2+ (Co7 -L1) and [Co7 (L2)12 ](ClO4 )2 (Co7 -L2), have been prepared by the reaction of [Co(H2 O)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(H2 O)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 ⋅2H2 O (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.