A Study of the Interaction of Polydentate Bases with Complexes of Electropositive Metals

Abstract

As part of a long-range project to develop inexpensive, earth-abundant alternatives to the use of rare-earth metals in catalysts, it was discovered that an unusual color is generated when solutions of Ca[1,3-(SiMe3)2C3H3]2(thf)2 and terypyridine are mixed in toluene. As the Ca2+ center lacks valence electrons, the deep blue coloration almost certainly arises from a ligand-to-metal charge transfer phenomenon. It was further determined that the color arises with Li+ and K+ centers in addition to Ca2+, and with the [1-(SiMe3)C3H3]– and [N(SiMe3)2]– anions as well. The key requirements thus appear to be a Group 1 or 2 metal center, terpyridine, a noncoordinating solvent, and a ligand that contains delocalized π-electrons, such as the bulky amido or allyl ligands used here. All available information indicates that the terpyridine compounds being synthesized exist in a dynamic equilibrium in solution, and the terpyridines are weakly bound to the metal. Isolation of Ca(η1-C3H5)(η3-C3H5)(18-crown-6) demonstrates that it is possible to manipulate the hapticity of a ligand on a Group 2 center by adjusting the electron density on the metal with a suitably strong donor ligand. Further use of this capability could lead to the development of more active catalysts based on earth-abundant Group 2 metals.