Diolates

Diolates are dianionic ligands derived from diols by deprotonation of both hydroxyl groups. They are typically formed from vicinal diols and are often represented as O−–R–R′–O−, where the negative charges reside on the two oxygen atoms. In coordination chemistry, diolates commonly act as bidentate donors through the two oxygens, but they can also function as bridging ligands that connect two or more metal centers. This makes them versatile for stabilizing metal complexes, clusters, and extended networks.

Formation and structure

Diolates arise by treating a diol with a strong base, such as an alkali metal hydride, organolithium reagents, or metal alkoxides, which removes both protons. The resulting dianion can coordinate to metals as a chelating unit, often forming five- or six-membered rings, or can bridge multiple metals, supporting multi-nuclear assemblies. The geometry and conformational flexibility of the diol backbone influence the bite angle and overall connectivity of the resulting complex.

Coordination behavior and applications

As ligands, diolates are related to alkoxides but carry an extra negative charge and two donor sites. They can stabilize a wide range of oxidation states and metal centers, from early transition metals to lanthanides. Diolate ligands appear in homogeneous catalysis studies, in the synthesis of metal-oxo clusters, and in metal-organic frameworks where they act as multi-donor linkers. Their redox properties and denticity enable tuning of catalytic activity and structural features in solid-state materials.

Examples

Common examples include ethylene glycolato, derived from ethylene glycol, and related diolates from other vicinal diols. These ligands illustrate the general behavior of dianionic, two-oxygen donors in coordination chemistry.