Actinide coordination chemistry

Actinide Coordination Complexes for the (Electrocatalytic) Activation and Transformation of Small Molecules

In our efforts to activate small molecules of industrial and biological importance, we have turned our attention to coordinatively unsaturated, reactive uranium coordination complexes. For example, the chelating triazacyclononane (tacn), single N, and arene-anchored tris(aryloxide) ligands, (ArO)₃tacn^3–, (ArO)₃N^3–, (ArO)₃mes^3–, as well as the tetra(aryloxide) ligand (ArO)₄cyclen^4– and the monodentate ^Ad,Ad,MeArO^– have provided access to reactive coordination compounds of uranium in oxidation states II, III, IV, V, and VI with tailorable steric and electronic profiles. These complexes display a pronounced selectivity and reactivity in reactions with carbon dioxide, related small heteroallene molecules (COS and CS₂), N_xO and H₂O.

The stoichiometric and catalytic “disproportionation” of CO₂ to CO and CO₃^2– via reductive cleavage of CO₂ was accomplished. In analogy, we were able to isolate an entire series of chalcogenide mixed-carbonate complexes by reacting bridged chalcogenide complexes U‒E‒U (E = S, Se, Te) with CO₂, CS₂, and COS. The corresponding terminal chalcogenide complexes U≡E (E = O, S, Se, Te) were obtained more recently by reaction with H₂E and subsequent reduction.

With respect to fundamental understanding of actinide chemistry, the molecular and electronic structure of a rare oxidation state in U coordination chemistry, namely U(II), with a 5f ⁴ quintet ground state was stabilized by a mesitylene anchored tris(aryloxide) ligand. In K(crypt)[((Ad,MeArO)₃mes)U], the U(II) center is supported by d backbonding, a general motive of the (ArO)₃mes^3– ligand.

In analogy to the CO₂ reduction, a rare U(IV) hydroxo complex, [((^Ad,MeArO)₃mes)U(OH)], with a terminal OH functionality was synthesized by selective reduction of H₂O with the U(III) complex [((^Ad,MeArO)₃mes)U]. Mechanistic studies were performed to understand this fundamental reaction, establishing the basis of catalytic H₂O reduction for H₂ evolution. Comprehensive electrochemical studies revealed that the U(III) complex can be regenerated in a catalytic cycle combining chemical and electrochemical reaction steps to produce H₂ from water.

Complexation of the newly synthesized ligand (ArO)₄cyclen^4– provides remarkably rare water and air stable uranium(IV) and (V) complexes, with redox events ranging from tri- to hexavalent species and covering an electrochemical window of ~4V.

Reaction of the trivalent uranium complex [((^Ad,MeArO)₃N)U] with [Na(OCP)(dioxane)_2.5] or [Na(OCAs)(dioxane)₃] yielded the first example of a coordinated η¹-cyaarside ligand (CAs^–) and the unprecedented binding motif U–CP. The sterically demanding monodentate ^Ad,Ad,MeArO^– ligand gives access to activation of N₂O or NO and subsequent conversion of CO₂ into the solid carbonate TMS₂CO₃, in a closed synthetic cycle

For details on our actinide chemistry program, please check out the following selection of references:

H₂O Activation and Electrocatalysis

D.P. Halter, F.W. Heinemann, J. Bachmann and K. Meyer

Uranium-Mediated Electrocatalytic H₂ Production from Water
Nature 2016, 530, 317–321.

D.P. Halter, F.W. Heinemann, L. Maron and K. Meyer

The Role of Uranium-Arene Bonding in H₂O Reduction Catalysis

Nature Chemistry 2018, 10, 259–267.

D.P. Halter, C.T. Palumbo, J.W. Ziller, M. Gambicki, A.L. Rheingold, W.J. Evans and K. Meyer

Electrocatalytic H₂O Reduction with f-Elements: Mechanistic In-sight and Overpotential Tuning in a Series of Lanthanide Complexes

J. Am. Chem. Soc. 2018, 140, 2587–2594.

CO₂ Activation and Transformation

Castro-Rodriguez, H. Nakai, L. N. Zakharov, A.L. Rheingold and K. Meyer

A Linear, O-Coordinated eta1-CO₂ Bound to Uranium

Science 2004, 305, 1757–1759.

S.C. Bart, C. Anthon, F.W. Heinemann, E. Bill, N.M. Edelstein and K. Meyer

Carbon Dioxide Activation with Sterically Pressured Mid- and High-Valent Uranium Complexes

J. Am. Chem. Soc. 2008, 130, 12536–12546.

A.-C. Schmidt, A.V. Nizovtsev, A. Scheurer, F.W. Heinemann and K. Meyer
Uranium-Mediated Reductive Conversion of CO₂ to CO and Carbonate in a Single-Vessel Closed Synthetic Cycle

Chem. Comm. 2012, 48, 8634–8636.

O.P. Lam, S.M. Franke, F.W. Heinemann and K. Meyer
Reactivity of U-E-U (E = S, Se) Towards CO₂, CS₂ and COS: New Mixed-Carbonate Complexes of the Types U-CO₂E-U (E = S, Se), U-CS₂E-U (E = O, Se) and U-COSSe-U
J. Am. Chem. Soc. 2012, 134, 16877–16881.

Waldschmidt, C.J. Hörger, J. Riedhammer, F.W. Heinemann, C.T. Hauser and K. Meyer

CO₂ Activation with Formation of Uranium Carbonate Complexes in a Closed Synthetic Cycle

Organometallics 2020, 39, 1602–1611.

New and Unusual Oxidation States

H.S. La Pierre, H. Kameo, D.P. Halter, F.W. Heinemann and K. Meyer
Coordination and Redox Isomerization in the Reduction of a Uranium(III) Monoarene Complex
Angew. Chem. Int. Ed. 2014, 53, 7154–7157.

H.S. La Pierre, A. Scheurer, F.W. Heinemann, W. Hieringer and K. Meyer
Synthesis and Characterization of a Uranium(II) Monoarene Complex Supported by delta-Backbonding
Angew. Chem. Int. Ed. 2014, 53, 7158–7162.

Small Molecule Activation Chemistry and Unusual Reactivity

A.-C. Schmidt, F.W. Heinemann, W.W. Lukens Jr. and K. Meyer

Molecular and Electronic Structure of Dinuclear Uranium di-µ-Oxo Complexes with Diamond Core Structural Motifs

J. Am. Chem. Soc. 2014, 136, 11980–11993.

S.M. Franke, F.W. Heinemann and K. Meyer
Reactivity of Uranium(IV) Bridged Chalcogenido Complexes U^IV-E-U^IV (E = S, Se) with Elemental Sulfur and Selenium: Synthesis of Polychalcogenido-Bridged Uranium Complexes
Chem. Sci. 2014, 5, 942–950.

S.M. Franke, M.W. Rosenzweig, F.W. Heinemann and K. Meyer
Reactivity of Uranium(III) with H₂E (E = S, Se, Te): Synthesis of a Series of Mononuclear and Dinuclear Uranium(IV) Hydrochalcogenido Complexes”
Chem. Sci. 2015, 6, 275–282.

C.J. Hörger, F.W. Heinemann, E. Louyriac, M. Rigo, L. Maron, H. Grützmacher, M. Driess and K. Meyer
Cyaarside (CAs^–) and 1,3-Diarsaallendiide (AsCAs^2–) Ligands Coordinated to Uranium and Generated via Activation of the Arsaethynolate Ligand (OCAs^–)

Angew. Chem. Int. Ed. 2019, 58, 1679–1683.

Development of Custom-Tailored Ligands and Synthesis

J. Hümmer, F.W. Heinemann and K. Meyer

Uranium Tetrakis-Aryloxide Derivatives Supported by Tetraazacyclododecane: Synthesis of Air-Stable, Coordinatively-Unsaturated U(IV) and U(V) Complexes

Inorg. Chem. 2017, 56, 3201–3206.