Post on 25-May-2020
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