Assembly and Properties of Heterobimetallic Co~(II/III)/Ca~(II) Complexes with Aquo and Hydroxo Ligands

摘要

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,l-diyl)]- tris(2,4,6-trimethylbenzenesulfonamido, [MST]~(3-). Water binds to [Co~(II)MST]~- to form the five-coordinate [Co~(II)MST(OH_2)]~- complex that was used to prepare the Co~(II)/Ca~(II) complex [Co~(II)MST(μ-OH_2)Ca~(II)C15-crown-5(OH_2)]~+ ([Co~(II)(μ-OH_2)- Ca~(II)OH_2]~+). [Co~(II)(μ-OH_2)CaOH_2]~+ 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~(II)MST(OH_2)]~- was oxidized to form [Co~(III)MST(OH_2)] that was further converted to [Co~(III)MST(μ-OH)Ca~(II)C15-crown-5]~+ ([Co~(III)(μ-OH)Ca~(II)]~+) in the presence of base and Ca~(II)Otf_2/15-crown-5. [Co~(III)(μ-OH)Ca~(II)]~+ was also synthesized from the oxidation of [Co~(II)MST]~- with iodosylbenzene (PhIO) in the presence of Ca~(II)Otf_2/15-crown-5. Allowing [Co~(III)(μ-OH)Ca~(II)]~+ to react with diphenylhydrazine afforded [Co~(II)(μ-OH_2)Ca~(II)OH_2]~+ and azobenzene. Additionally, the characterization of [Co~(III)(μ-OH)Ca~(II)]~+ provides another formulation for the previously reported Co~(IV)-oxo complex, [(TMG_3tren)Co~(IV)(μ-O)Sc~(III)(Otf)_3]~(2+) to one that instead could contain a Co~(III)-OH unit.