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 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 [(CoMST)-M-II](-) to form the five-coordinate [(CoMST)-M-II(OH2)](-) complex that was used to prepare the Co-II/Ca-II complex [(CoMST)-M-II(mu-OH2)Ca(II)C15-crown-5(OH2)](+) ([Co-II(mu-OH2)-(CaOH2)-O-II](+)). [Co-II(mu-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. [(CoMST)-M-II(OH2)](-) was oxidized to form [(CoMST)-M-III(OH2)] that was further converted to [(CoMST)-M-III(mu-OH)Ca-II subset of 15-crown-5](+) ([Co-III(mu-OH)Ca-II](+)) in the presence of base and (CaOTf2)-O-II/15-crown-5. [Co-III(mu-OH)Ca-II](+) was also synthesized from the oxidation of [(CoMST)-M-II](-) with iodosylbenzene (PhIO) in the presence of (CaOTf2)-O-II/15-crown-5. Allowing [Co-III(mu-OH)Ca-II](+) to react with diphenylhydrazine afforded [Co-II(mu-OH2)(CaOH2)-O-II](+) and azobenzene. Additionally, the characterization of [Co-III(mu-OH)Ca-II](+) provides another formulation for the previously reported Co-IV-oxo complex, [(TMG(3)tren)Co-IV(mu-O)Sc-III(OTf3](2+) to one that instead could contain a Co-III-OH unit.