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Protonation and Muoniation Regiochemistry of [FeFe]-Hydrogenase Subsite Analogues
Jamie N.T. Peck, Joseph A. Wright, Stephen Cottrell, Christopher J. Pickett and Upali A.
Jayasooriya.
The mechanisms by which the hydrogenase enzymes catalyse the reversible reduction of
protons to dihydrogen are of intrinsic interest in the context of understanding the chemistry of
fascinating metallosulfur enzymes, and more widely in the context of a developing hydrogen
technology for energy transduction. Protonation of almost any diiron dithiolate complex
yields a bridging hydride as the thermodynamic product. Transient terminal hydrides have
also been observed, and result from the protonation of the vacant site offered by the rotated
bridging carbonyl. Protonation of the Fe2(pdt)(CO)4(PMe3)2 active site was monitored using
stopped flow IR, offering insights into the reaction from ~0.1 seconds onwards, indicating
that protonation proceeds to the Fe ̶ Fe bond.1 The reaction mechanism was exhaustively
modelled using DFT. This suggested the formation of a terminally bound hydride was
kinetically favoured.2 Single electron reduction of the protonated product gave a paramagnetic
species that retained its structure, as indicated by a combined IR, EPR and DFT study.3
In a complementary study using µSR spectroscopy and DFT, the addition of Mu to two di-
iron systems was investigated. This concerted addition of the muon and electron is analogous
to the two-step process of the protonation and reduction of the di-iron system. µSR
spectroscopy offers a unique time window, allowing observation of the reaction during the
first few nano-seconds. We find evidence to suggest Mu binds to the subsite terminally, which
is in agreement with the DFT mechanism.
[1] Wright, J. A., Pickett, C. J. Chem. Commun., 2009, 45, 5719 ̶ 5721.
[2] Lui, Caiping., Peck, J. N. T., Wright, J. A., Pickett, C. J., Hall, M. N., Eur. J. Inorg. Chem., 2011, 1080 ̶
1093.
[3] Jablonskyte, A., Wright, J. A., Fairhurst, S. A., Peck, J. N. T., Ibrahim, S. K., Oganesyan, V. S., Pickett, C. J.
J. Am. Chem. Soc., 2011, 18606 ̶ 18609.
Determining the regiochemistry of
mixed valent [Fe(I)Fe(II)]-hydrogenase
model complexes
Jamie N. T. Peck
University of East Anglia/STFC
Outline
• Introduction to the hydrogenase enzyme
• The Fe2(µ-pdt)(CO)4(PMe3)2 active site mimic and the current understanding of
the protonation mechanism
• Application of µSR can complement the existing data to help understand this
mechanism
Understanding the protonation mechanism of [FeFe]-hydrogenase
Understanding protonation mechanism is crucial for development of more efficient mimics
Ribbon representation of Clostridium
pasteurianum (Cpl) [FeFe]-
hydrogenase Shepard et al. PNAS
2010; 107, 10448-10453
The [FeFe]-hydrogenase active site
in native enzyme
H2 as an alternative energy vector
The [Fe2(µ-pdt)(CO)4(PMe3)2] model complex
IR of starting material
C. Liu, J. N. T. Peck, J. A. Wright, C. J. Pickett and M. B. Hall, Eur. J. Inorg. Chem, 2011, 1080-1093.
Understanding the protonation mechanism of the [Fe2(µ-pdt)(CO)4(PMe3)2]
model complex
J. A. Wright, C. J. Pickett, Chem. Comm, 2009, 5719-5721.
A 2 step reaction was proposed:
0.7s to 160s (0.2s intervals)
No direct evidence for terminal protonation
Protonation reaction monitored by stopped-
flow IR
DFT Calculations of the [FeFe]-Hydrogenase Model complexes
C. Liu, J. N. T. Peck, J. A. Wright, C. J. Pickett and M. B. Hall, Eur. J. Inorg. Chem, 2011, 1080-1093.
Initial coordination of the
acid onto CO in both
pathways
Pathway for terminal
protonation ~18 kcal/mol
lower than for protonation
at the Fe-Fe bond
Bridging pathway Terminal pathway
DFT Calculations of the [FeFe]-Hydrogenase Model complexes
C. Liu, J. N. T. Peck, J. A. Wright, C. J. Pickett and M. B. Hall, Eur. J. Inorg. Chem, 2011, 1080-1093.
Energy barrier for isomerisation from
kinetic product to thermodynamic
product is predicted to be ~8 kcal/mol
lower than for direct protonation at
the Fe-Fe bond
Mixed Valent [Fe(I)Fe(II)]-Hydrogenase Model Complexes
A. Jablonskyte, J. A. Wright, J. N. T. Peck, S. A. Fairhurst, S. K. Ibrahim, V. S. Oganesyan and C. J. Pickett. J. Am. Chem. Soc., 2011, 133, 18606–
18609.
g factor
Experiment 2.0066
DFT 2.0062
H
D
Difference IR spectrum of the reduction of the hydride species Isotropic EPR spectrum
Summary
• Stopped-flow IR suggests protonation to the Fe-Fe bond followed by
rearrangement to thermodynamic product
• IR and EPR confirm the basal/basal-transoid bridging hydride as the
thermodynamic product
• Mechanistic study of the reaction using DFT suggests an intermediate
is first formed where the proton is weakly bound to the CO, before
migrating to a terminal position on the Fe
• Simulated hyperfine constants associated with muonium bound to the
CO give best agreement with preliminary µSR data
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