Our research is focused on exploratory synthetic chemistry of the f-block that challenges preconceived ideas of their structure, reactivity, and bonding. We design and make metal compounds that can activate small, traditionally unreactive molecules such as hydrocarbons and dinitrogen, and develops these into innovative catalytic transformations. Where we can control and elicit unusual electronic structures in f-block molecules, we contribute to the understanding needed for the use of molecules in spintronics and molecule-based quantum information storage, and for the separations and manipulations of critical rare earth resources and civil nuclear waste.
1. Rare earth (group 3 and lanthanide) complexes for small molecule activation and sustainable catalysis
Many rare earths are more earth-abundant than 3d transition metals such as nickel, copper, and zinc, and their salts are less toxic by ingestion than most d-block salts, for example the LD50 for cerium trichloride is six times higher than that of iron trichloride, and similar to that of sodium chloride. The rare earth series provides tunability of size and Lewis acidity, and the lack of participation of d-orbitals in the predominantly ionic metal-ligand bonding allows for rapid substrate exchange and reorganization without geometric penalty that is needed for metal cation-controlled product formation and turnover. We are developing new catalysts for sustainable transformations of biorenewables that exploit these properties. Examples include carbon dioxide conversion, and polyester syntheses.
Selective methane C-H activation was demonstrated for an organo-lanthanide complex 25 years ago. However, to make this useful, and fully exploit these reactive metals for the transformation of small, inert molecules, we need to better understand both the weak interactions these metals form with them, and the fundamental nature of the f-element – ligand bond. We have introduced new techniques to study subtle f-block metal – CH bond interactions, and developed new strategies to compensate for the redox inactivity of the highly reactive rare earths.
Bifunctional and bimetallic catalysts: Bifunctional catalysis is an important and growing area in controlled synthesis. We have been studying the fundamental chemistry of the ‘incompatible’ pairing of Lewis acidic Ln ions with the strongly nucleophilic N-heterocyclic carbene (NHC) group, which is a poor ligand for lanthanides, for a variety of uses, including strong carbon-element bond cleavage (JACS ’10, ’11), and catalytic destruction of chemical warfare agent simulants (Chem. Sci. ’18) . However, our main focus is on biorenewable chemistry, including catalytic conversions of carbon dioxide (Chem. Sci. ’19). Most recently, we reported ultra-rapid, bifunctional cerium(III)–NHC catalysts for high molar mass polyesters, whose unusual polymerization mechanism enables the synthesis of macrocyclic polymer. For lactide, the monomer derived from corn starch, and the most widely used biodegradable polymer, we can achieve turnover frequencies of 864,000 per hour (ACS Catal. ’21).
We can readily make libraries of new bimetallic f-block complexes supported by robust tetraphenolate ligands that allow us to control intermetallic distance by simple replacement of the aromatic spacer. This has allowed us to combine pairs of rare earth cations that normally only have single electron chemistry, such as CeIII/IV which is an important oxidation catalyst in organic chemistry (Dalton Trans ’20). We have also used the Lewis acidity of the di-CeIV complexes to develop single-component ring opening copolymerization catalysts for renewable polyester syntheses with very high activity (Organometallics ’21).
2. Exploration of unusual electronic structure and reactivity across the actinide series
Dinitrogen activation and its development into catalysis: Our group has shown small molecule transformations that were in the past considered impossible for the f-block. We have now made more than half of all known uranium dinitrogen compounds, and shown that spectroscopy is crucial for determining the extent of electron transfer (JACS ’11). Most recently, we have now been able to make the first f-block catalysts for the conversion of dinitrogen to ammonia and bis(silyl)amines. Because of our use of a reactive proton in the TP ligands (see above) the controlled secondary amine synthesis is possible from N2 for the first time (for any metal catalyst) (Nature Chem. ’20)
Arene-complexes: Arene sandwich complexes such as bis(benzene)chromium have contributed to d-orbital bonding theory and played a historic role in d-block organometallic chemistry. Metal-arene complexes have applications in synthesis and catalysis, understanding of graphite-intercalated metal cation behavior in battery materials and in the search for new organic spintronic materials. However, arene binding by f-block cations is rarer and neither predictable nor understood. We have been developing showed that unexpectedly energy-matched U-arene back-bonding can enable a mild, homogeneous C-H functionalization to form useful borylated arenes (Nat. Chem. ’12). This is a new mechanism for arene C-H functionalization, not yet seen in the d-block.
We are now finding that spontaneous arene binding is more common that we previously thought, and finding ways to catalyse its formation (Inorg. Chem. ’21). We have also exploited this π-interaction to control product selectivity in a new UX3 – catalyzed alkyne cyclotrimerization (Organometallics, ’15).
We have made and fully characterized new Np(III) organometallics, revealing fascinating molecular and electronic structures (Nat. Chem. ’16). Unlike uranium, organo-transuranic chemistry is significantly technically more challenging and neglected. Our studies show Np(III) can exhibit single molecule magnetism, and is further reducible. But most importantly, significant covalency differences between the 4f- and 5f- analogues prove that fundamental Np organometallic chemistry can provide new insight in the subtleties of f-element bonding (Chem. Sci. ‘17).
High-pressure X-ray studies on f-block organometallics: We are studying how pressure can be a useful tool for manipulating energy landscapes and weak interactions. Single crystal diffraction analyses where close contacts with ligand C-H bonds, e.g. in UX3 (X = bulky monoanionic ligand) 1.1, are usually wrongly assigned as agostic (electron donation from the alkane C-H σ-electron pair to metal), but were able to show that at high (3.2 GPa) pressure genuine agostic interactions are formed between the U and ligand C-H bonds (Angew. Chem. ’15).
Unusual M-M bonding: We have been studying the pre-organization of two strongly reducing actinide centers for small molecule activation by binding two UIII or NpIII centers, using different anionic macrocycles, and extensive collaborations with computational and transuranic scientists in the EU, have uncovered new properties and reactivities (JACS ’14). To explore the more esoteric goals of M-M bonding in the f-block, we have looked at heterobimetallic f-d-block (JACS ’16) and f-p-block (JACS ’07) metal-metal bonded molecules. A set of six variants enabled a combined experimental/computational bonding study, finding the strongest metallic bonding for U-Ni. We showed how this could be used experimentally to switch the initiation of ring-opening polymerizations to make the renewable ester polylactide (Dalton Trans ’17).
3. New oxo-group reactivity of the uranyl ion, and control of f-block metal-ligand multiple bonds
Ten years ago, we started using Pacman-shaped N-donor macrocycles with collaborator Prof. Jason Love for binding the uranyl dication in a new desymmetrising mode. In the textbooks, the uranyl ion is the most prevalent and stable form of uranium in the environment. It is always linear with two strong axial oxo groups, and it was assumed to have no important oxo-reactivity. We have spent the last decade disproving this using new molecular uranyl chemistry.
Uranyl oxo reactivity: The Arnold/Love collaboration reported the first covalent bond forming reaction at the oxo group of the uranyl ion in 2008, here to silicon (Nature ’08). We have gone on to develop oxo-bond chemistry to make a range of singly reduced U(V) uranyl complexes that were previously considered unisolable. The new UV uranyl complexes also make much better models for the transuranic actinyl cations [NpO2]n+ and [PuO2]n+ which are thousands of times more radioactive, but known to have greater oxo-group reactivity than U(VI) uranyl.
We have shown how to control the oxo group activation by Lewis acid coordination (Inorg. Chem. ’15), enabling unprecedented thermal hydrocarbon C-H bond cleavage (Nature Chem. ’10), a new reactivity for the f-block that mimics transition metal oxo catalysts, hinting at new vistas in catalysis. She demonstrated oxo-rearrangement from the ubiquitous trans-dioxo to the previously unseen cis-dioxo geometry (Nature Chem. ’12). Containing the shortest U-U bond yet reported, the air-stable dinuclear complex also contradicts conventional wisdom that adjacent f1centres like this will disproportionate in nuclear waste solutions. As a result, theoreticians and spectroscopists in EU and US National labs are now studying these new actinyl motifs to inform our understanding of transuranic actinyl ion migration and aggregation in the environment and nuclear waste separations.
We have now shown selective oxo-group metalation by cations from across the periodic table, from the proton (Angew. Chem. ’12), to neptunium and plutonium (Angew. Chem. ’16), and can fuse two uranyl groups in a new, linear ‘uber-uranyl’ [O-U-O-U-O]4+ cation (Chem. Sci. ’18). In-depth synthetic/electronic/computational studies have in many cases identified unusual electronic properties such as single molecule magnetism (JACS ’13).
Perhaps most surprisingly, we have shown that these electron transfers between actinyl salts and the rare earth ions do not even require complicated ligand architectures, just careful choice of donor solvents. We are now controlling one and two-electron reduction of the uranyl oxos to form linear oligomers whose length (and therefore magnetic properties) is again controlled by donor solvent choice (Angew. Chem. ’17).
Unusual and reactive multiple bonds; thorium and cerium: Again, using simple ligands, we developed a simple and general reaction to make metal nitrogen M=N double bonds, the first thorium complex containing two Th=NR imido ligands (JACS ’15). The surprising cis M(=N)2 geometry contrasts with uranium’s linear structures, provide strong new evidence for one side of the long-running argument that thorium should behave more like a transition metal than an actinide, and suggesting it might participate in new hydrocarbon C-H bond activation chemistry. We are now collaborating on reactivity studies of the first terminal cerium oxo Ce=O complex (Inorg. Chem. ’16, Chem. Eur. J. ’19).