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THOR: Organometallic Thorium Chemistry

Final Report Summary - THOR (THOR: Organometallic Thorium Chemistry)

Description of main results and achievements (Please see attached documents for the Schemes referenced throughout):

The scientific target of the project THOR is the cutting edge thorium (and related uranium) organometallic chemistry, especially towards synthesis, structural characterization, and reactivity study of thorium (and uranium) species containing An=E (M = Th, U; E = C, N, P) multiple bond. Using integration of structural, spectroscopic, and computational methodology to elucidate the electronic structure and bonding character of the An=E multiple bond, and driving our understanding of actinide chemistry to a higher level.

By supporting the project THOR, a mutual-beneficial skill and knowledge transfer can be built up between the researcher and the host, and a long standing collaborative link will be established, which can strengthen EU’s scientific competence.

Description of the work performed since the beginning of the project:

Since the start date of the project (01/06/2012), work has been carried out towards the objectives set in the Grant Agreement. The main effort has focussed on actinide (thorium and uranium) multiple bond chemistry.

1) In general, 18 novel thorium and uranium carbene complexes and derivatives have been synthesized and fully characterized, including X-ray crystallographic characterization. Bonding nature of the Th=C double was elucidated by DFT method for the first time.

2) A thorium carbene amide ketimide, which bears Th=Ccarbene, Th-Namide and Th-Nketimide bond in one molecule, was synthesized and thoroughly characterized. Comparative reactivity studies reveals that reactions occur preferentially at the Th-Nketimide bond rather than the Th=Ccarbene or Th-Namide bonds. This overturned the established view that metal-ketimide linkages are purely inert spectators.

3) A series of thorium carbene bis-alkyl complexes were synthesized and characterized. Reactivity studies of these complexes reveal novel and versatile reactivity of Th-Calkyl bond, which have not been observed before.

4) A landmark uranium(VI) carbene imido oxo complex was synthesized and characterized, which exhibits the unprecedented arrangement of three multiply bonded ligands to one metal where the coordinated heteroatoms derive from different element groups. Preliminary reactivity study revealed group transfer towards organic carbonyl substrates and produce a carbene uranyl ([O=U=O]2+) complex.

5) A cis-C=U=N uranium(IV) carbene imide complex was synthesized and characterized. As a rare and highly-desirable cis-analogue of uranyl ([O=U=O]2+), study of this complex will shed light on long-sought fundamental questions in actinide chemistry, such as the nature of inverse trans-influence (ITI), and the role of 5f orbital in actinide-ligand multiple bonding.
The details are described as below.

Description of main results and achievements:

1) Based on the bis(iminophosphorano)methane (BIPM) ligand, the nature of Th=Ccarbene double bond was studied using [ThBIPMTMS(ODipp)2] (1, Dipp = 2,6-diidopropylphenyl) as model complex. The Schrock-type nucleophilic carbene character of the Th=Ccarbene bond was confirmed by metallo-Wittig group transfer reaction (Scheme 1). DFT calculations suggested that both carbene lone electron pairs of Th=Ccarbene bond are entirely located on the C atom and the Th=Ccarbene is ionic in nature.

2) Thorium carbene amide ketimide (2) was synthesized and characterized. Reactions between 2 and unsaturated substrates take place at Th-Nketimide bond, in preference to supposedly more reactive Th=Ccarbene and Th-Namide bonds. The unprecedented reactivities of traditionally innocent Th-Nketimide linkage overturn the established view that metal-ketimide linkages are purely inert spectators (Scheme 2). These results open up a new horizon of reactivity for M-Nketimide linkages, and suggest that the role of the ketimide ligand in coordination and organometallic chemistry as a reactive functional group, instead of just being an inert supporting ligand, deserves full consideration.

3) Thorium carbene bis-alkyls (5, 6) containing both Th=Ccarbene and Th-Calkyl bonds were synthesized and versatile reactivity of the Th-Calkyl bond were unveiled, including C=O insertion, C-H activation, and tandem C-H/C-P activation (Scheme 3).

4) Uranium(VI) carbene imido oxo complex (12), which bears terminal U=C, U=N, and U=O double bond, was synthesized and characterized. 12 exhibits the unprecedented arrangement of three multiply bonded ligands to one metal where the coordinated heteroatoms derive from different element groups. 12 reacted with tert-butylisocyanate to produce a carbene uranyl ([O=U=O]2+) complex (13) (Scheme 4).

5) Uranium(IV) cis- carbene imide complex (14), which is a highly desirable cis- analogue of uranyl ([O=U=O]2+), was synthesized and characterized (Scheme 5). Studies of electronic structure and bonding nature of the cis-N=U=C moiety is still underway.

The expected final results and their potential impact and use;

The project THOR concentrated on thorium and uranium multiple bond chemistry and was proved fruitful. The outcomes of the project significantly expanded the boundary of actinide organometallic chemistry, highlighted by results such as: 1) the first unambiguous elucidation of the nature of Th=Ccarbene bond; 2) the first non-innocent M-Nketimide bond which overturned traditionally reactive sequence; and 3) the exceptional uranium carbene imido oxo complex which is a lankmark in metal-ligand multiple bond chemistry. By publications in highly impact peer-reviewed journal (e.g. two Angew. Chem. Int. Ed. already, at leat five more that are iminent and most likely more in the next two years) as well as attendance and presentation of poster in international academic conference (Dalton 2014, Royal Society of Chemistry, Warwick, UK, April 2014), the project THOR is demonstrated as successful and influential in chemistry community.

As the most important outcome of the project, a reliable set of methodologies to build An=E (An = Th or U; E = N, O, C) multiple bond, and extensive knowledge reserve of their nature and reactivity has been built up. They are not only highly important in perspective of basic research, but also in practical perspective. Nuclear power is the most reliable clean and carbon emission-free power source when we are facing the severe energy and environment crisis. Major doubt of public towards nuclear power industry is nuclear-safety and long-term influence of depleted fuel in environment. In natural environment and process of fuel-cycle, uranium-ligand multiple bond species are prevalent (such as uranyl species). On the other hand, as an abundant and low-radioactive element, thorium presents an opportunity to provide alternative fuel cycle, which is safer than the currently using uranium-cycle. Knowledge of An=E multiple bond chemistry produced by the project THOR can be highly important for designing of next generation thorium based nuclear power plant, as well as for environmental remediation of nuclear waste.