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f-block hydrocarbon interactions: exploration; exploitation

Periodic Reporting for period 2 - f-ex (f-block hydrocarbon interactions: exploration; exploitation)

Okres sprawozdawczy: 2019-04-01 do 2020-09-30

Understanding, controlling, and predicting the subtle interactions that traditionally inert molecules such as dinitrogen, carbon dioxide and hydrocarbons form with metal cations is a major challenge in molecular science, and a key technology enabler in areas such as homogeneous catalysis, drug recognition, polymer properties, and metal recovery. The 4f-row, the rare earths, whose salts are as common as iodine and ten times less toxic than those of iron, offer great potential for the catalytic conversions of simple inert molecules needed for a sustainable, renewable-based future. Controlling the chemistry of the radioactive metals of the 5f-row, the actinides, underpins the safe management of our nuclear waste legacy.
We have used organometallic systems to study two types of poorly understood hydrocarbon interactions with f-block metal cations: arene binding which is stronger, yet controversial in terms of its electronic demands, and neutral hydrocarbon C-H bonding which is weaker, yet crucially reaction controlling.
f-ex set out a new way to experimentally measure and define these subtle hydrocarbon interactions. Further objectives include exploiting these for new inert hydrocarbon C-H bond cleavage, with the ultimate aim of viable, low-energy hydrocarbon functionalisations.
We extend our work to the transuranic elements, and exploit high pressure to interrogate bonding. The targets of this combined study offer high scientific impact by demonstrating fundamental bonding insight and ground-breaking structures and reactions.
Molecular f-block N2 complexes were considered unisolable until recently, even though Haber noted that uranium was better than Fe as a heterogeneous catalyst for the industrial fixation of atmospheric dinitrogen to ammonia in the Haber-Bosch process. We have now made over half of known uranium – N2 complexes, all simple complexes like 5 and used these simple systems to understand the complex, subtle back-bonding capacity of U.
Even though we have made most of the uranium N2 complexes now reported (six examples) further reactions of all these UX3-type complexes with substrates or electrons leads to rearrangement of X and loss of the N2.
We have now used a tetrakis(aryloxide) platform ligand to develop more robust letterbox–shaped bimetallic 5f metallacycles that can bind dinitrogen and catalyse its reduction and functionalisation to ammonia or secondary silylamines for the first time. No other catalyst, either homogeneous or heterogeneous, exists for the conversion of N2 to a secondary silylamine.

Arenes are another class of hydrocarbon that are not expected to bind to hard f-block centers. Yet d-block metal-arene complexes have applications in synthesis and catalysis, the understanding of graphite-intercalated metal ion behaviour in battery materials and in the search for new organic spintronic materials. We were the first to show the spontaneous, potassium-free arene-uranium binding. We have now shown this to be a ubiquitous process, and progress studies towards catalytic C-H borylation with earth abundant metals.

In transuranic work, we have found that the early actinide tetrakis(aryloxides) complexes of Th, U, and Np undergo a sudden shortening of the strong M-O bonds at high pressure. These bonds enter a more covalently-bonded regime at the transition through a greater interaction of the ligands with the 6d and 5f orbitals of the actinide. The pressure at which the transition occurs is metal-dependent: 3 GPa for the Th and U complexes, but 1.5 GPa for the Np complex. The DFT modelling shows that inter-ligand repulsion increases rapidly as the O-M-O bond angles are forced to vary with pressure. The effect is greatest for the Np complex, which has the shortest and most covalent M-O bonds at ambient pressure, and this is why its shortening occurs soonest. Importantly, the three compounds form an isomorphous series of crystal structures so that the differing transition pressures are truly a function of the metal, and not connected with differing crystal packing effects.
The nature of metal-ligand bonding in actinide complexes is still a controversial area; small but significant covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of these elements, and has been probed with a variety of experimental and theoretical techniques. Further, some of the experiments that set out to define covalency in actinide metal-ligand bonding provide answers that show the energy degeneracy of the orbitals.
Finally, the neptunium (IV) complex Np(OAr)4 is also a new compound, and a rare example of a Np(IV) coordination complex. An analogous amide has also been made and will be published in due course.
The real power in the actinide catalysts for N2 functionalisation is that they start from the most stable (and for Th, the group) oxidation state, which does not bind N2, suggesting we will be able to extrapolate these catalyses to other earth abundant, and non-radioactive metals.
Likewise, in the arene chemistry, we have been able to progress studies towards catalytic C-H borylation with earth abundant metals as a result.

Other pressure studies have included work on the pyramidal uranium(III) complexes with different heteroatom-substituted bulky ligands, using variable pressure to pick apart the potential reasons for the bending away from expected planar geometries. The pyramids are capable of inversion, all complexes exhibit modest weakening of the uranium-ligand bond in the planar TS, as evidenced by QTAIM and NLMO calcs. A manuscript describing the importance of d-orbital overlap vs weak and agostic ligand C-H interactions with the metals will be published soon.
We have developed unique, mono deoxygenative uranyl coupling strategies that rely simply on Lewis acid activation (e.g. with a group 1 salt) and silanes or boranes, into unique new ‘uber-uranyl’ tetracations [OUOUO]4+, also demonstrating for the first time that simple boranes are effective oxo-atom abstraction reagents. We can form these in either trans-,trans- linear, or trans-,cis- bent conformation depending on reagent.

Publications in prep>
Shephard, J. J. et al, High pressure and the control of covalency in actinide complexes M(OAr)4 (M = Th, U, Np).
Ochiai, T. et al Why UX3 uranium compounds are pyramidal.
Kerr, R. W. F. et al. Exceptional rate and switchability using bifunctional lanthanide-NHC catalysts for catalytic conversions of cyclic esters to macrocyclic or linear polymers.
Halliday, C. J. V. et al. Competition for arene binding by dinuclear f-block complexes with extended aromatic spacers.
Gray, S. J. et al. Dinuclear ceric siloxides: Highly active catalysts for anhydride/epoxide ring-opening copolymerisation.
diGiuseppe, A. et al. Ligand control of the excited state to enhance cerium photocatalytic carbon-halogen bond cleavage.
Cowie, B. E. et al. Selective oxo ligand functionalisation and substitution reactivity in an oxo/catecholate-bridged UIV/UIV pacman complex.
Beach, S. A. et al. Heterotrimetallic lanthanide complexes: weak antiferromagnetic coupling through terminal vanadyl oxo ligands.
Arnold, P. L. et al. Excited state structure and reactivity of uranyl photocatalysts for selective hydrocarbon oxidation..
Arnold, P. L. et al. Photocatalytic hydrocarbon oxidation by uranyl complexes controlled by ligand modulation; a combined experimental/computational study.