Summary:

• Complex [((Ad,MeArO)3mes)UIII] (1) is the first reported uranium electrocatalyst. It facilitates H2O reduction

to H2 by harnessing metal-ligand redox-cooperativity. • The proposed catalytic cycle and the redox cooperativity are supported by CV, EPR, XRD, and DFT analysis.

• Lanthanide complexes [((Ad,MeArO)3mes)Ln] (7–Ln) also catalyze H2O reduction, allow overpotential tuning,

and provide further mechanistic insight in underdeveloped f-element catalysis.

D.P. Halter acknowledges the Graduate School Molecular Science (GSMS) of the FAU for support. Funding by the German Federal Ministry of Education

and Research (BMBF 2020+ support codes 02NUK012C and 02NUK020C), the Joint DFG-ANR projects (ME1754/7-1, ANR-14-CE35-0004-01) and the FAU.

Acknowledgements:

• D. P. Halter, F. W. Heinemann, L. Maron, K. Meyer, Nat. Chem., 2018, 10, 259.

• D. P. Halter, F. W. Heinemann, J. Bachmann, K. Meyer, Nature, 2016, 530, 317.

• D. P. Halter, H. S. La Pierre, F. W. Heinemann, K. Meyer, Inorg. Chem., 2014, 53, 8418.

• H. S. La Pierre, H. Kameo, D. P. Halter, F. W. Heinemann, K. Meyer, Angew. Chem. Int. Ed., 2014, 53, 7154.

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

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

Literature:

Background:

Storage of unsteadily produced renewable energies, preferentially by electrocatalytic H2O reduction to H2,

is required to promote green energy. Due to its high reducing power, depleted (only weakly radioactive) 238U is an appealing material to catalyze H2O reduction, as recently shown by the first uranium based

electrocatalyst [(Ad,MeArO)3mes)U] (1). Despite the rich redox chemistry of uranium complexes, catalysis

remains scarce. An often discussed reason is that U complexes tend to undergo step-wise 1e– reactions,

whereas transition metal catalysis often proceeds through concerted 2e– pathways. This poster presents a

detailed analysis how metal-ligand redox-cooperativity enables catalysis via concerted 2e– reactivity with

uranium. Analogous lanthanide complexes were investigated to gain further insight in f-element catalysis.

resting state

active

catalyst

chelator enforces

d–bond

Low-valent

U(III) is a strong reductant

activating small molecules

Catalysis is very rare due to

prevailing 1 e– reactivity

Metal–ligand redox–

cooperativity is desired

Uranium–arene d–bonding

facilitates direct electronic

communication

•

•

•

•

Ligand

Design

Coordi-

nation

MO map illustrating

d–bond

2

1

Ln3+ / Ln2+ [V] E½ cat [V] kobs [M–1s–1]

La –3.08 –3.21 330

Ce –2.93 –2.99 350

Pr –2.96 –3.11 105

Nd –2.93 –2.94 20

Sm –2.60 –2.87 10

Gd –2.90 –2.95 50

Dy –2.86 –2.98 30

Er –2.87 –2.99 70

Yb –2.12 – –

• lanthanides are not radiotoxic

• easy access to compound series

trends in f–element electrocatalysis

• adjust overpotential by choice of the metal

Nd(III)-aquo complex

remained elusive in

uranium catalysis!

Nd(II) complex

active species in

lanthanide catalysis!

electrocatalytic H2O reduction with complexes 7–Ln

onset potential shifts with Ln3+ / Ln2+ couple

note: increasing Lewis acidity perturbs trends!

7–Nd–H2O 8–Nd

M–backbone M–OAr M–Ox MOOP

1 2.353 Å 2.169 Å – – 0.475

Å

2 2.703 Å 2.188 Å 2.106 Å – 0.023

Å

3 2.711 Å 2.173 Å 1.831 Å – 0.060

Å

5 2.056 Å 2.251 Å – – 0.880

Å

7–Nd 2.489 Å 2.186 Å – – 0.268

Å

7–Nd–H2O 2.489 Å 2.202 Å 2.479 Å – 0.265

Å

8–Nd 2.366 Å 2.237 Å – – 0.530

Å

U(II) 2.188 Å 2.236 Å – – 0.668

Å

clpx. strctr.

Proposed Mechanism

Structural parameters Electrocatalytic parameters

Lanthanide catalysts 7–Ln produce H2 via a 1

e– reactivity. The catalytic overpotential can be

adjusted by choice of the lanthanide.

Tafel plot by FOWA.

E½ values shift

under catalytic

conditions and were

determined from the

catalytic wave

Proposed Mechanism

EPR of a frozen reaction solution of

1 + H2O in toluene at 7.5 K

Convoluted: U(III) 1 and a

rhombic U(V), likely U–(OH)–(H)

g1 = 2.73, g2 = 1.83, g3 = 1.35

by synthesis

by EPR by GC-TCD

reaction profile based on computed

enthalpies of H2O reduction by 1.

(B3PW91 / 6-31G(d,p), RECPs, 32 e–)

DFT Analysis

DH (kcal mol–1)

U

–

H

OH

U–H2O U=O

TS-1

TS-2

SOMO

SOMO–1

SOMOS of TS-1

note: first reaction step is a 2 e– process

very unusual for uranium!

note: 2 e– for O–H cleavge

1 from U, 1 from ligand!

powder EPR of U(V)–oxo 3 at 94 K

evidence for ligand radical

active HER catalyst no HER activity

• redox active ligand

• 2 e– concerted

• fast

powder EPR of U(V)–oxo 3 at 7 K

metal centered electron, U(V) 5f 1

• –

• +

1

XRD: mes-radical (distorted backbone)

DFT: aryloxide radical

4 Yield: 40%

• innocent ligand

• 1 e– step-wise

• slow

Reactivity Studies

3

Catalytic activity of U(III) complex 1 is enabled

by metal-ligand redox-cooperativity as

evidenced by EPR, DFT, and reactivity studies.

5 6

3 3

10.2 ° 4.0 °

electrolysis for 300 s each, at different potentials

stable catalysis with 1, no activity of UI3, or 3

0.4 mol% of catalyst 1 in THF with 0.22M H2O:

overpotential reduction 0.5V, 25 times more H2

CV

TOF = 106 h–1

@ h =1.3 V

Electrolysis Tafel Plot

Tafel plot obtained by foot-of-the-wave analysis (FOWA)

1 1

3

1

performance

from

uranium

to the

lanthanides

The Role of Uranium-Arene Bonding

in H2O reduction Catalysis

Dominik P. Halter, Chad T. Palumbo, Frank W. Heinemann, William J. Evans,

Laurent Maron, Julien Bachmann, and Karsten Meyer

Friedrich-Alexander-Universität Erlangen-Nürnberg, Chair of General and Inorganic Chemistry,

Egerlandstraße 1 91058 Erlangen, Germany. Email: Dominik.Halter@fau.de Karsten.Meyer@fau.de

Reaxys PhD Prize Symposium 2019 – The Koepelkerk, Amsterdam, NL (October 3rd – 4th, 2019)

Take–Home

Messages U(III) activates H2O

via 2 e–

oxidative addition

Metal–ligand redox cooperativity

through covalent d-bonding is a

new and broadly applicable concept

to enable f-element catalysis

active site

electron “shuttle“

electron reservoir

Changing the

metal allows

reactivity tuning

A plethora of

reported f-element

mediated

reactions could

become catalytic

by following the

concept

introduced here

active catalyst

Redox active

ligand

inactive analogue

Redox innocent

ligand