The valence electron configuration of a metal describes the relative ordering and properties of the highest-energy electrons - those that are available for bonding with other elements, and thus it is these which largely dictate the physicochemical properties of metal-containing materials. For much of the Periodic Table, some known principles can guide the design criteria needed to produce a given electron configuration of the element in question and result in desirable properties. The oxidation state of the metal - the (formal) number of electrons the metal has lost versus the neutral configuration, and the geometry of bound ligands are two of the most important features which can be controlled. This project seeks to develop the redox chemistry of the lanthanides, and early actinide (thorium, uranium, neptunium, plutonium) elements towards a deeper understanding of the design criteria which dictate the valence electron configurations of these metals in lower oxidation states.
Lanthanide and actinide elements (the f-block) occupy a position in the Periodic Table where the valence electrons in metal ions reside in highly angular 4f- and 5f-orbitals, which show poor radial extent and thus the bonding between these elements and others is typically quite weak. It is these characteristics which engender the unique physical properties typical of these metals (e.g. applications in magnetism and optical spectroscopy). Simply, a 3+ metal ion in a molecule may have f n valence electrons, where n is three less than the neutral configuration, and there is little which may be done to change the chemical properties of the remaining f-electrons substantially (though their physics may be altered).
Developments the chemistry of f-block redox chemistry over the last 20 years have produced examples of all the lanthanides (except Pm), as well as thorium, uranium, neptunium, and plutonium in the 2+ oxidation state, many of these for the first time. This remarkable pace of advancement has overturned many preconceptions about the redox properties of these elements, but has raised questions that this project seeks to address.
For some elements, the valence electron configuration of 2+ ions follows the example above with f n+1 valence electrons where n is three less than the neutral atomic configuration. Samarium, europium and ytterbium are examples of this and are often termed "traditional" 2+ lanthanide ions due to this behaviour. For other elements (such as cerium), all examples of molecular 2+ complexes feature an electron configuration described as: f n d1, where n is still three less than the neutral atomic configuration, and one electron resides in a d-orbital (for cerium 2+: f1 d1). As d-orbitals are fundamentally different to f-orbitals (larger radial extent, more diffuse, different magnetic properties), the physicochemical properties of cerium 2+ molecules (which have two metal valence electrons - f1 d1), should differ substantially from molecules with praseodymium 3+ which also features two metal valence electrons (f2). Some elements such as uranium have been shown to accommodate both types of electron configuration in the 2+ oxidation state: f n+1, and also f n d1. The underlying properties which drive the preference for one configuration over the other are poorly known, and is an under explored topic at the forefront of synthetic chemistry.
This project will produce examples across the f-block of metal 2+ compounds, determine their valence electron configurations, and use a combination of magnetic and spectroscopic techniques to identify the properties which direct the formation and stability of specific configurations. These results will have applications in the fields of spintronics and quantum technologies which rely on the control of spins by developing new routes to giant molecular spins, and produce fundamental advances in f-block coordination chemistry foundational to future advances in their reactivities.
|