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

Two heptanuclear discs, [Co-7(L1)(6)(CH3O)(6)](2+) (Co-7-L1) and [Co-7(L2)(12)](ClO4)(2) (Co-7-L2), have been prepared by the reaction of [Co(H2O)(6)](CIO4)(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)](CIO4)(2) with a mixture of the two ligands, having similar coordination ability but different shapes, resulted in the butterfly-structured tetranuclear cobalt cluster [Co-4(L1)(2)(L2)(4)](CIO4)(2)2H(2)O (Co-4). Electrospray ionization mass spectrometry (ESI-MS) analysis in real time revealed the {Co-1}->{Co-2}->{Co-4}->{Co-7} stepwise assembly when a single ligand was employed, but when both ligands were used only {Co-4} 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-4(L1)(4)(L2)(2)](2+) and [Co-4(L1)(3)(L2)(3)](2+) were the primary clusters observed at the beginning of the reaction, but [Co-4(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.