Conrad Goodwin

Inorganic and Organometallic Chemistry


Transuranium bonding and covalency

The actinide elements sit at the bottom of the periodic table. All are radioactive, and are amongst the least understood of all elements. While thorium and uranium studies have become somewhat mainstream in academic laboratories, the study of the elements beyond uranium – the transuranium elements, has largely remained the purview of large facilities such as National Laboratories (like Los Alamos National Laboratory). This is due to the need for specialist handling and containment infrastructure for these highly radioactive and scarce materials.

Radiological safety concerns and material availability typically limit synthetic chemistry reaction scales in line with how far along the series the element lies. For example, while a reaction using 35 mg of [NpCl4(DME)2] might be "routine", moving to americium leads to limits of ~5 mg, and even less for Cf. This means that our experimentally-derived understanding of periodicity across the actinide series is hampered significantly compared to every other corner of the periodic table. Despite the difficulties in this chemistry, progress has been made in studying unique bonding environments such as the first Am–C or Am–Se bonds and helped to further our understanding of the periodicity in these elements. While tremendous historic insight has been made in contextualising the behaviour of these elements in solid-oxides and binary halides, there is still a plethora of unanswered questions regarding their solution and molecular chemistry.

High-temperature Single Molecule Magnets

A driving force during my time in the Mills group was the idea that the synthesis of a cationic Dy(III) complex with two, approximately linearly arranged ligands, might lead to an SMM with a large energy barrier to the reversal of magnetisation (ca. 2,000 cm–1). This culminated in the synthesis of an isolated dysprosocenium cation, [Dy(Cpttt)2][B(C6F5)4] [Cpttt = {C5H2(tBu)3}], which displays magnetic hysteresis at 60 K (liquid nitrogen boils at 77 K). This result is tantalizingly close to allowing molecular information storage at technologically feasible temperatures.

We furthered these studies by examining the effects of a highly axial ligand field on the heavy 4f ions (Gd–Yb, and diamagnetic Lu) and found that the unique geometry (2 linearly-disposed ligands without equatorial contacts) is important, but the constrained molecular vibrations inherent to the Cp {C5R5} ring are essential to mediating the high temperature relaxation in dysprosocenium.

Structure/electronic property correlations

Crystal field effects in 4/5f ions are much smaller than for d-block analogues, though still significant. During the last few years, we investigated several highly geometrically constrained 4/5f complexes utilising sterically demanding bis-silylamide ligands [vis, N**, {N(SitBuMe2)2}; and N††, {N(SiiPr3)2}] and then studied what the effects of these novel ligand geometries have on the electronic structure of the metal ions. Notable advances include the first trigonal-planar actinide complex [U(N**)3], as well as near-linear Ln(II) complexes, [Ln(N††)2], and their isoelectronic trigonal-planar analogues [K(2.2.2-crypt)][Ln(N**)3] (Ln = Sm, Eu, Tm and Yb).

Low-oxidation state lanthanide chemistry

The field of low-oxidation state f-block chemistry has undergone a resurgence in the last decade, with the availability of the +2 oxidation state having been discovered for every 4f element (save Pm), and also examples of U(II), Np(II), and Pu(II); as well as several new Th(III) complexes, and the discovery of Th(II). However, all of these examples (with the exception of two U(II) complexes) utilise substituted derivatives of the ubiquitous cyclopentadienide (Cp, {C5R5}) ligand. This limits the information we can obtain about how these new oxidation states affect metal-ligand interactions, because all examples use the same ligand-type!

By utilising bulky-silylamide ligands and bespoke synthetic routes to low-oxidation state complexes we hoped to synthesise new examples of non-traditional Ln(II) complexes. One such route is the use of a purpose-built and extremely rare reactor system (pioneered by the Evans group) that I constructed in the Mills Group lab which uses the high-temperature solid-state reaction between Ln metal and I2 (as in the above scheme) to routinely produce up to 20 g of LnI2.


Collaborators

  • Drs. Ping Yang and Enrique Batista
  • Dr. Nicholas Chilton and his group
  • Prof. Richard E. P. Winpenny
  • Prof. Eric J. L. McInnes
  • Prof. Stephen T. Liddle
  • Prof. William J. Evans
  • Dr. Floriana Tuna
  • Dr. Louise S. Natrajan
  • Prof. Stephen Hill
  • The Photon Science Institute - UoM


Chemistry Division, Los Alamos National Laboratory,
Los Alamos, N.M.
Office: 48-208 e: cgoodwin@lanl.gov
t: (505) 551-2837 | (505) 665-6931