We focus 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 develop 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 safe manipulations of critical rare earth resources and civil nuclear waste.
- Sustainable catalysis with f-block complexes
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 metal centers are capable of rapid substrate exchange, electron transfer, and reorganization.
N2RR – catalytic dinitrogen reduction by electropositive metal complexes
We have made 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 ligands, and formation of a pocket, the controlled secondary amine synthesis is possible from N2 for the first time (for any metal catalyst) (Nature Chem. ’20). We are currently expanding this work to other electropositive and base metals, and working with spectroscopists at the Advanced Light Source (ALS) synchrotron beamlines to probe intermediates and mechanism.
Selective halocarbon functionalization by light absorbing, early Ln catalysts
Photoredox catalysis by lanthanide complexes remains relatively underexplored, especially with lanthanides other than cerium. We are using designed and organometallic ligands to enhance the light absorbance of early 4f-centers and elevate their excited state redox potentials to enable catalytic cleavage and functionalization of the most inert C-F and C-Cl bonds (Chem. Sci. ‘22). The halophilicity of the lanthanide is also key to these reactions, and
Bimetallic and bifunctional Ln catalysts
While dimerization of large lanthanide complexes is common, designed, bimetallic catalysts are very rare. Bifunctional catalysis is an important and growing area in controlled synthesis.
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), and di-CeIV complexes to develop single-component ring opening copolymerization catalysts for renewable polyester syntheses with very high activity (Organometallics ’21).
We collaborated on reactivity studies of the first terminal cerium oxo Ce=O complex (Inorg. Chem. ’16, Chem. Eur. J. ’19), and are now studying dinuclear cerium oxo complexes for sustainable oxidation catalysis.
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, including catalytic conversions of carbon dioxide (Chem. Sci. ’19), and most recently 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).
2. Lanthanide, actinide, transuranic organometallics – alkyl, aryl and carbocyclic complexes across the 4f and 5f series
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 metal-hydrocarbon weak interactions – agostics and pi interactions, and the strong f-element – carbon bonding. Current studies include a collaboration with Dow chemical on reactive metal alkyl chemistry, and transuranic organometallic studies.
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 are now finding that spontaneous arene binding is more common that we previously thought, due to energy-matched U-arene back-bonding. Since developing a new mild route to arene C-H functionalization (Nat. Chem. ’12) we have been studying catalyzed f-arene bond formation (Inorg. Chem. ’21). Stabilizing arene interactions have helped us study highly reduced, organometallic neptunium complexes (Nat. Chem. ’16). Current (Chem. Sci. ‘17) and future work is focusing on plutonium chemistry and the heavier transuranics, to provide new insight in the subtleties of f-element bonding. Current studies include extending these aromatic ligand studies into the transuranics, beyond plutonium.
- Electronic structures of strong, covalent, and multiple bonds
Covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of the actinides, but the ionic versus covalent nature of metal-ligand bonding in actinide complexes remains controversial. Our work to disturb the multiple bonding in the ubiquitous actinyl oxo ions, and to apply new experimental techniques to studying actinide-ligand bonding is contributing new knowledge.
Oxo reactivity and multiple bond manipulation with actinyl complexes
The uranyl ion is the most prevalent and stable form of uranium in the environment. In the textbooks, it is always linear with two strong axial oxo groups, and it was assumed to have no important oxo-reactivity. In collaboration with the Love group, we have spent many years disproving this using new molecular uranyl chemistry (Nature ’08). Beyond U, Np, Pu and Am can also form these linear, dioxo- ‘yl’ cations, but more valence f-electrons, making understanding actinyl bonding and oxo reactivity even more important.
With the standard U(VI) uranyl ion we have found unprecedented thermal hydrocarbon C-H bond cleavage (Nature Chem. ’10) and new photochemical C-H bond functionalization catalysis (ChemCatChem ’19).
We have developed oxo-bond chemistry to make a range of singly reduced U(V) uranyl complexes that were previously considered unisolable, including an air-stable dinuclear, silylated dimer (Nature Chem. ’12).
Selective oxo-group metalation by cations from across the periodic table, from the proton (Angew. Chem. ’12), to neptunium and plutonium (Angew. Chem. ’16), finding single molecule magnetism (JACS ’13) and making a new, linear ‘uber-uranyl’ [O-U-O-U-O]4+ cation (Chem. Sci. ’18).
In-depth synthetic/electronic/computational studies are possible in the simplest oxo-halide systems where we now control 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). Current studies are extending this chemistry to the transuranic ions.
Variable high pressure single crystal studies on simple molecules
We are studying how pressure can be a useful tool for manipulating energy landscapes and weak interactions. We have shown that at high (3.2 GPa) pressure, actinide organometallics in single crystals can undergo structural changes forming genuine agostic interactions (M – CH bonds), (Angew. Chem. ’15). Now, we are finding that pressure-induced M-O bond shortening in neptunium aryloxides involves a higher increase in covalency and metal f-orbital contribution, than for Th and U congeners,(Nature Commun. ’22). Most recently, we are starting to explain why actinide molecules form unexpectedly pyramidal compounds, rather than space-saving planar forms.(Nature Commun. ’22) This means the design of precise molecular, and thus electronic, structures for potential molecular qubit applications requires more than just ligand size control.