Assembly and Properties of Heterobimetallic CoII/III/CaII Complexes with Aquo and Hydroxo Ligands

The use of water as a reagent in redox-driven reactions is advantageous because it is abundant and environmentally compatible. The conversion of water to dioxygen in photosynthesis illustrates one example, in which a redox-inactive CaII ion and four manganese ions are required for function. In this report we describe the stepwise formation of two new heterobimetallic complexes containing CoII/III and CaII ions and either hydroxo or aquo ligands. The preparation of a four-coordinate CoII synthon was achieved with the tripodal ligand, N,N′,N″-[2,2′,2″-nitrilotris­(ethane-2,1-diyl)]­tris­(2,4,6-trimethylbenzenesulfonamido, [MST]3–. Water binds to [CoIIMST]− to form the five-coordinate [CoIIMST­(OH2)]− complex that was used to prepare the CoII/CaII complex [CoIIMST­(μ-OH2)­CaII⊂15-crown-5­(OH2)]+ ([CoII(μ-OH2)­CaIIOH2]+). [CoII(μ-OH2)­CaOH2]+ contained two aquo ligands, one bonded to the CaII ion and one bridging between the two metal ions, and thus represents an unusual example of a heterobimetallic complex containing two aquo ligands spanning different metal ions. Both aquo ligands formed intramolecular hydrogen bonds with the [MST]3– ligand. [CoIIMST­(OH2)]− was oxidized to form [CoIIIMST­(OH2)] that was further converted to [CoIIIMST­(μ-OH)­CaII⊂15-crown-5]+ ([CoIII(μ-OH)­CaII]+) in the presence of base and CaIIOTf2/15-crown-5. [CoIII(μ-OH)­CaII]+ was also synthesized from the oxidation of [CoIIMST]− with iodosylbenzene (PhIO) in the presence of CaIIOTf2/15-crown-5. Allowing [CoIII(μ-OH)­CaII]+ to react with diphenylhydrazine afforded [CoII(μ-OH2)­CaIIOH2]+ and azobenzene. Additionally, the characterization of [CoIII(μ-OH)­CaII]+ provides another formulation for the previously reported CoIV–oxo complex, [(TMG3tren)­CoIV(μ-O)­ScIII(OTf)3]2+ to one that instead could contain a CoIII–OH unit.