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Angewandte
International Edition
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Chemie
Accepted Article
Title:
Snapshots of a Migrating H-atom: Characterization of a Reactive
Fe(III) Indenide Hydride and its Nearly Isoenergetic Ring-
Protonated Fe(I) Isomer
Authors:
Marcus Walter Drover, Dirk J. Schild, Paul H. Oyala, and
Jonas Peters
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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content of this Accepted Article.
To be cited as:
Angew. Chem. Int. Ed.
10.1002/anie.201909050
Angew. Chem.
10.1002/ange.201909050
Link to VoR:
http://dx.doi.org/10.1002/anie.201909050
http://dx.doi.org/10.1002/ange.201909050
Snapshots of a Migrating
H
-
atom: Characterization of a Re
active
Fe(III) I
ndenide Hydride and its
Nearly
Isoenergetic
Ring
-
Protonated Fe(I) Isomer
Marcus W. Drover, Dirk J. Schild, Paul H. Oyala, and Jonas C. Peters*
Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena, California, 91125, United States
Supporting Information Placeholde
r
ABSTRACT:
W
e
report
the
characterization
of
a
n
S
= ½
iron
π
-
complex, [Fe(
η
6
-
IndH)(depe)]
+
(Ind = Indenide
(C
9
H
7
-
), depe = 1,2
-
bis
(diethylphosphino)ethane)
, which
results
via
C
-
H elimination from
a transient Fe
III
hydride,
[Fe(
η
3
:
η
2
-
Ind)(depe)H]
+
.
Owing to weak M
-
H/C
-
H bonds,
the
s
e
species
undergo
proton
-
coupled electron transfer
(PCET) to
release
H
2
through bimolecular recombination.
Mechanistic information, gained from stoichiometric as
well as computational studies
,
re
veal
the
open
-
shell
π
-
arene complex
to have
a BDFE
C
-
H
value
of
5
0
kcal
mol
-
1
,
roughly equal to
the
BDFE
Fe
-
H
of
its
Fe
III
-
H
precursor
(
Δ
G
o
≈ 0 between them)
.
Markedly, this reactivity
differs from
related
Fe(
η
5
-
Cp/
Cp*)
compounds, for which terminal
Fe
III
-
H
cations
are isolable and
have been structurally
characterized
, highlighting
the
effect of a benzannulated
ring (indene
)
. Overall, t
his study provides a structural,
thermochemical, and mechanistic foundation for the
cha
r-
acterization
of
indenide/indene
PCET precursors
and ou
t-
lines a
valuable
approach for the differentia
tion of
a
ring
-
versus
a
metal
-
bound H
-
atom
by way of
continuous
-
wave (CW)
and pulse
EPR
(HYSCORE)
spectroscopic
measurements.
INTRODUCTION
For decades, cyclopentadienyl (Cp, C
5
H
5
-
) and deriv
a-
tives thereof have been used as ligands to stabilize trans
i-
tion metal,
1
f
-
block,
2
and main
-
group
3
elements alike. This
ligand class is ubiquitous among the synthetic commun
i-
ty, no doubt a consequence of several alluring features,
including variable hapticity,
4
whereby multiple coordin
a-
tion modes can be accommodated, ranging fr
om
η
1
-
to
-
η
5
.
There has been a surge of renewed interest in cooperative
transformations with Cp
-
type ligands,
5
with protonation
at
such
“non
-
innocent”
rings
being
implicated in the co
n-
text of
hydride
or proton
-
coupled
-
electron
-
transfer
(PCET) pathways, for
instance in H
2
-
evolving or
di
nitr
o-
gen reduction reactions
(Chart 1A)
.
5,
6
M
etallocene
(Cp
2
’M)
and half
-
sandwich complexes
have been probed for decades in the context of ring prot
o-
nation (or hydride attack) chemistry.
7
For species that are
comparatively
stable, characterization data for complexes
where
in
a ring is “protonated” can be distinguished from
those
wherein
the “proton” instead resides on the metal as
a hydride ligand. In certain cases, the H
-
atom may shuttle
between the
se
two positions. For exam
ple,
the [Fe
III
-
H]
+
cation, [Fe
III
(
η
5
-
Cp*)(dppe)H]
+
(Cp* = C
5
Me
5
-
, dppe = 1,2
-
Chart 1.
Previous work highlighting
A
)
Cp* non
-
innocence for
small
-
molecule fixation,
B
) interest in assigning H
-
atom location
by XRD analysis, and
C
) the present study, featuring indene as a
supporting ligand.
B
C
Differentiating amongst hydride and ring protonated isomers for iron
P
P
R
R
R
R
Fe
I
H
H
This work: nearly isoenergetic isomers bearing a reactive H-atom
Co
H
+
P
P
R
R
R
R
Fe
H
III
+
Elimination
+
H
Rh
Br
N
N
P
P
R
R
R
R
Fe
H
II
Oxidation
P
P
Ph
Ph
Ph
Ph
Fe
H
+
III
I
II
BDFE
Fe(C)-H
= 50 kcalmol
-1
BDFE
C-H
< 29 kcalmol
-1
Δ
G
o
0
η
5
-C
5
Me
5
A
Proposed roles for Cp* non-innocence in reductive small molecule catalysis
[
H
NEt
3
]
+
H
-
H
Hydride transfer in HER
[
Fe
]
PCET in N
2
RR
H
H
2016
2017
-
N
N
H
[
Fe
]
N
N
Fe
H
II
Fe
IV
H
H
H
nearly isoenergetic
H
P
P
Ph
Ph
Ph
Ph
Fe
I
η
4
-C
5
Me
5
H
Fe
II
H
F/PF
5
+
+
η
4
-C
5
H
5
H
solid-state structure
2017
1992
+
P
P
Ph
Ph
Ph
Ph
Fe
H
II
η
5
-C
5
H
5
Oxidation
not observed
?
?
10.1002/anie.201909050
Accepted
Manuscript
Angewandte Chemie International Edition
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article
is protected
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