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1. Understanding and exploitation of subtle interactions between f-block elements and hydrocarbons; control of small molecule activation chemistry for catalysis.

2. Unprecedented oxo-group reactivity of the uranyl ion, and control of metal-ligand multiple bonds.

 
 

 

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1.Understanding and exploitation of subtle interactions between f-block elements and hydrocarbons; control of small molecule activation chemistry for catalysis.

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.

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). In [X2U]2(μ-arene) 1.2 (X = N(SiMe3)2 for example) high (3.2 GPa) pressure forms genuine agostic interactions (dotted lines to C10) between the U and ligand C-H bonds (Angew. Chem. '15). The resulting control of subtle f-block ion – hydrocarbon interactions, and what they teach us about covalency in the bonding, is being developed to target selective hydrocarbon functionalisation in an ERC Advanced Grant.

New small molecule reductive activation: It is now recognised that close C-H contacts such as in 1.1 where X = N(SiMe2Ph)2) which were previously assumed unimportant, can shut down even reactions with powerful driving forces (such as the formation of σ-An-O bonds in 1.4). Our group has shown small molecule transformations that were in the past considered impossible for the f-block, e.g N2 reduction to 1.3; and have now made more than half of all known uranium dinitrogen compounds, and shown that spectroscopy is crucial fordetermining the extent of electron transfer (JACS '11). We showed the selective dimerisation of room temperature and pressure CO gas to form 1.4, a reaction not see in the d-block, and then demonstrated further unprecedented C-C and C-H bond formations to make 1.5 (Chem. Sci. '11). We also showed that unexpectedly energy-matched U-arene back-bonding in 1.2 could enable a mild, stoichiometric C-H functionalisation to form useful borylated arenes 1.6 (Nat. Chem. '12). This is a new mechanism for arene C-H functionalisation, not yet seen in the d-block.We have since exploited this π-interaction to control product selectivity in a new UX3 - catalysed alkyne cyclotrimerisation (Organometallics, '15).

In-depth studies of reduced, organo-neptunium: 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 behaviour 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.
Reasoning that the subtleties of f-block ion binding to these soft, neutral cyclic hydrocarbons could contribute to bonding and covalency understanding, we have now made and fully characterised new NpIII organometallics, revealing fascinating molecular and electronic structures (Nat. Chem. '16). Unlike uranium, organo-transuranic chemistry is significantly technically more challenging and neglected. Our studies on [(LAr)Np(Cl)] 1.7 and others (Chem. Sci. 2017), show NpIII can exhibit single molecule magnetism, and remarkably, 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.



We have been studying the preorganisation of two strongly reducing actinide centres for small molecule activation by binding two UIII or NpIII centres, using different anionic macrocycles. Compounds including 1.8 and the small molecule activation product 1.9, have enabled further collaborations with computational and transuranic scientists in the EU and US, and new 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 1.10 (JACS '16) and f-p-block (JACS '07) metal-metal bonded molecules. A complete set of six variants 1.10, enabled a combined experimental/computational bonding study, finding the strongest metallic bonding for U-Ni. This has since been exploited to switch the initiation of ring-opening polymerisations to make the renewable ester polylactide (Dalton Trans '17).

Reactive, labile NHCs: The international community has been extensively using NHCs (N-heterocyclic carbenes) as strongly bin ding ligands in late transition metal catalysis, but we have developed ligands that brought NHCs into f-block chemistry (Angew. Chem. '03). We demonstrated that the now-labile NHC group can be used in combination with the f-block centre for new small molecule activation and catalysis with redox-innocent rare earth complexes such as 1.11 (JACS '10, '11). Uniquely, the NHC here can react to deliver functional groups E, or CO2 to a metal-bound hydrocarbyl group. As an aside, we also demonstrated that these oxidation-resistant ligands can support high oxidation state palladium, enabling ligand control of catalytic C-H halogenation even in harsh conditions (JACS '09).

 

2.Unprecedented oxo-group reactivity of the uranyl ion, and control of 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 teh 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 2.1 (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 fn transuranic actinyl cations NpO2n+ and PuO2n+ which are thousands of times more radioactive, but known to have greater oxo-group reactivity than UVI 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 (2.2, Nature Chem. '12). Containing the shortest U-U bond yet reported, air-stable 2.2 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.
Arnold has now shown selective oxo-group metallation by cations from across the periodic table, from the proton (Angew. Chem. '12), to neptunium 2.3 and plutonium (Angew. Chem. '16). In-depth synthetic/electronic/computational studies have in many cases identified unusual electronic properties such as single molecule magnetism (JACS '13).
Most recently, she has shown that these electron transfers between actinyl salts and the rare earth ions do not even require complicated ligand architectures, just clever use of donor solvents. This latest breakthrough reports the controlled one or two-electron reduction of one or both uranyl oxos to form linear oligomers whose length (and therefore magnetic properties) is again controlled by donor solvent choice 2.4 (Angew. Chem. '17).

Unusual and reactive multiple bonds; thorium and cerium: Again, using simple ligands, Arnold developed a simple and general reaction to make metal nitrogen M=N double bonds, the first thorium complex containing two Th=NR imido ligands 2.5 (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. Arnold’s established expertise in unusual f-block metal ligand multiple bonding has led many others in the international f-block chemistry community to seek her expertise; for example, she is now collaborating on reactivity studies of the first terminal cerium oxo Ce=O complex (Inorg. Chem. '16).

 

 

 

 

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