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(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)⊂15-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)⊂15-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(3)tren)Co(IV)(μ-O)Sc(III)(OTf)(3)](2+) to one that instead could contain a Co(III)-OH unit.