From Low to High Oxidation States - New Oxidative Routes to Lanthanide Multiple Bonds

The lanthanides are fourteen chemically related elements which exhibit diverse optical and magnetic properties that have facilitated their employment in a myriad of technological applications. They are defined by their highly polarised bonding, which is dominated by electrostatic attractions, and their marked tendency to exhibit the +3 oxidation state in the vast majority of their complexes.

As a direct consequence of the predominantly electrostatic bonding in these systems, multiply-bonded lanthanide chemistry is underdeveloped in comparison to corresponding d-block chemistry and is limited to double bonds to carbon to date. Given the catalytic applications of d-transition metal complexes exhibiting double bonds, the development of lanthanide-multiple bond chemistry is a long-term synthetic target and it is proposed that with judiciously selected supporting ligands and synthetic routes terminal lanthanide-element double bonds with a variety of p-block elements are accessible. These complexes will exhibit properties that contrast with and complement examples from well-understood d-block chemistry.

In this proposal, cerium(IV) complexes exhibiting terminal multiple bonds to nitrogen, phosphorus and chalcogens (O, S, Se, Te) are targeted. Cerium is unique amongst the lanthanides in having a readily accessible +4 oxidation state. Complexes of Ce(IV) are strong oxidising agents and this property has been utilised in synthesis with ubiquitous reagents such as ceric ammonium nitrate (CAN). Given that CAN has some selectivity and solubility issues, the Ce(IV) complexes developed herein will be investigated for their viability as alternatives to CAN in synthetic chemistry.

The Ce(IV) complexes targeted in this proposal will be prepared from Ce(II) synthon precursors. Divalent lanthanide chemistry, with metals in a formal + 2 oxidation state, was previously limited to only a handful of lanthanides but recent major advances in this field have now allowed the isolation and study of divalent chemistry for all lanthanides with the exception of radioactive promethium. The Ce(II) synthons in this proposal will be interesting complexes in their own right with distinctive and unique oxidation potentials. Therefore they will be investigated as alternatives to the widely-used one electron reducing agent samarium(II) diiodide in synthesis. It is important to expand this area in order to realise new synthetic methodologies.

All complexes prepared herein will be analysed by advanced analytical and computational techniques. This will give fundamental insight into terminal cerium multiple bonds and although the properties of these complexes cannot be accurately predicted, they promise to be highly reactive and interesting. The target Ce(IV) complexes will exhibit enhanced reactivity in comparison to their early d-transition metal counterparts as they will additionally be strong oxidising agents. Detailed analysis will determine if these complexes hold promise in materials chemistry and catalysis, as archetypal Ce(IV) complexes have previously shown.

It is expected that results generated from these studies will be worthy of publication in leading international journals such as Science, Nature Publishing Group, Journal of the American Chemical Society and Angewandte Chemie. These fundamental studies will not just be confined to textbook examples because results from this research will have implications for physics, engineering and materials science.

The award of this grant will give the principal investigator the means to train post-graduate and -doctoral workers with the rare ability to handle highly air sensitive f-element complexes, recently highlighted as a major skills shortage by government and the nuclear and fine chemical industries. This research will be communicated to the public by outreach activities as its benefit to society and the environment are favourable for the public perception of science.

This research programme provides novel routes to terminal unsupported lanthanide-imido, -oxo and -phosphinidene bonds. Complexes containing these bonds have not been isolated to date but have long been synthetic targets as closely related multiply bonded complexes of early d-block metals have found a myriad of applications in industrially relevant synthetic transformations and catalysis. Such processes are relevant to the preparation of fine chemicals including pharmaceuticals. Hence the expansion of lanthanide-element multiple bond chemistry will benefit parties in academia, industry and society as a whole.

As the multiply bonded Ce(IV) complexes targeted in this proposal represent a new class of materials, their properties and industrial applications cannot be accurately gauged in advance, hence the primary impact of this research will be in academia. It can be predicted with some certainty that these complexes will be strong one electron oxidising agents, as are other Ce(IV) complexes in the literature. The potential advantages of multiply bonded Ce(IV) complexes over widely used Ce(IV) reagents such as ceric ammonium nitrate (CAN) can be realised by significant investigations outside the timescale of this First Grant scheme.

Should these studies prove fruitful, Ce(IV) complexes generated in this research programme may have industrially relevant advantages over CAN in selected synthetic transformations where CAN is unselective and insoluble in the reaction solvent system. Furthermore, Ce(II) synthons generated in this scheme of work will be useful one electron reducing agents, with reduction potentials and solubilities that are distinct from that of samarium(II) diiodide, which is used extensively in C-C bond-forming reactions. Therefore the potential advantages of Ce(II) synthons over samarium(II) diiodide will give impact.

Together, Ce(II) synthons and Ce(IV) complexes synthesised herein are potentially useful reagents in synthetic chemistry. These may be used in future industrial processes to prepare essential chemicals that benefit society. Novel reagents and methodologies are crucial for the continued development of this discipline, given their integral part in synthesising new molecules in the pharmaceutical and fine chemical industries. A lack of new reagents will slow progression in these fields and will be detrimental to future synthetic developments.

Detailed analytical and computational analysis of Ce(IV) complexes generated in this study will provide insight into their bonding orbital compositions. Given that lanthanide-element multiple bonds are to date limited to carbon, the study of lanthanide double bonds to nitrogen, phosphorus and chalcogens will provide a step-change in our understanding of multiply-bonded lanthanide systems. These fundamental studies will be of interest to the academic community but have real-world relevance. Given the numerous applications of Ce(IV) complexes in catalysis, materials science and photonic devices that are already in industrial operation, the expansion of Ce(IV) chemistry is vital to realise future applications in these areas.

In terms of future industrial and societal benefit, this research is timely as it targets synthetic, catalytic and materials applications of relatively abundant and sustainable lanthanides as precious metal prices continue to rise and resources become more limited. Training researchers to handle air-sensitive f-elements will also be of benefit to the UK chemical industry and economy as there is a recognised skills shortage in this area.