Assembly and properties of heterobimetallic Co^II/III/Ca ^II 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 Ca^II ion and four manganese ions are required for function. In this report we describe the stepwise formation of two new heterobimetallic complexes containing Co^II/III and Ca^II ions and either hydroxo or aquo ligands. The preparation of a four-coordinate Co^II 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 [Co^IIMST]^- to form the five-coordinate [Co ^IIMST(OH₂)]^- complex that was used to prepare the Co^II/Ca^II complex [Co^IIMST(μ-OH ₂)Ca^II?15-crown-5(OH₂)]⁺ ([Co ^II(μ-OH₂)Ca^IIOH₂]⁺). [Co^II(μ-OH₂)CaOH₂]⁺ contained two aquo ligands, one bonded to the Ca^II 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. [Co^IIMST(OH₂)]^- was oxidized to form [Co ^IIIMST(OH₂)] that was further converted to [Co ^IIIMST(μ-OH)Ca^II?15-crown-5]⁺ ([Co ^III(μ-OH)Ca^II]⁺) in the presence of base and Ca^IIOTf₂/15-crown-5. [Co^III(μ-OH)Ca ^II]⁺ was also synthesized from the oxidation of [Co ^IIMST]^- with iodosylbenzene (PhIO) in the presence of Ca^IIOTf₂/15-crown-5. Allowing [Co^III(μ-OH) Ca^II]⁺ to react with diphenylhydrazine afforded [Co ^II(μ-OH₂)Ca^IIOH₂]⁺ and azobenzene. Additionally, the characterization of [Co^III(μ-OH) Ca^II]⁺ provides another formulation for the previously reported Co^IV-oxo complex, [(TMG₃tren)Co ^IV(μ-O)Sc^III(OTf)₃]²⁺ to one that instead could contain a Co^III-OH unit.