A Terminal Fe
III
-Oxo in a Tetranuclear Cluster: Effects of Distal
Metal Centers on Structure and Reactivity
Christopher J. Reed
,
Theodor Agapie
*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
Tetranuclear Fe clusters have been synthesized bearing a terminal Fe
III
-oxo center stabilized by
hydrogen bonding interactions from pendant
tert
-butyl amino pyrazolate ligands. This motif was
supported in multiple Fe oxidation states, ranging from [Fe
II
2
Fe
III
2
] to [Fe
III
4
]; two oxidation
states were structurally characterized by single crystal X-ray diffraction. The reactivity of the
Fe
III
-oxo center in proton coupled electron transfer (PCET) with X–H (X = C, O) bonds of various
strengths was studied in conjunction with analysis of thermodynamic square schemes of the
cluster oxidation states. These results demonstrate the important role adjacent metal centers have
on modulating the reactivity of a terminal metal-oxo.
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*
Corresponding Author: agapie@caltech.edu.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Experimental Procedures and Supplimentary Data (PDF)
Crystallographic data files (CIFs)
The authors declare no competing financial interests.
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Published in final edited form as:
J Am Chem Soc
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Terminal metal-oxo moieties are invoked as key intermediates in both natural and synthetic
catalysts of mid-first-row transition metal ions (Mn, Fe, and Co).
1
For example in
photosynthesis, water is oxidized in photosystem II by a CaMn
4
O
5
cluster known as the
oxygen evolving complex (OEC);
2
numerous computational studies of the catalytic
mechanism have proposed a high-valent Mn-oxo playing a key role in O–O bond formation.
3
Similarly, a number of synthetic water oxidation catalysts employing various multinuclear
scaffolds have been reported, where a terminal metal-oxo is implicated as a key intermediate
(Figure 1).
1e
–
g
,
4
Studies of synthetic transition metal-oxo complexes have been integral for understanding
these reactive moieties in catalytic systems.
1a
,
5
However, there is a paucity of literature
concerning multinuclear complexes bearing well-characterized terminal metal-oxo motifs.
6
In a rare example where the effects of a neighboring metal oxidation state on a terminal
metal-oxo could be interrogated, Que and coworkers reported that the spin state of an Fe
IV
-
oxo center would change depending on the oxidation state of a neighboring Fe in a
μ
2
-O
bridged bimetallic complex (L’
2
OFe
2
(OH)(O)
2+/3+
).
6c
The authors demonstrated that
structural and spin-state changes due to reduction of this secondary Fe leads to a thousand-
fold activation of the [Fe
2
] complex towards C–H oxidation.
To gain further insights into these multimetallic effects, our group has examined well-
defined tetranuclear clusters of Fe and Mn, which facilitate intramolecular oxygen atom
transfer reactions; however, a terminal metal-oxo intermediate could not be observed.
7
Inspired by reports of mononuclear terminal metal-oxo motifs stabilized by second
coordination sphere hydrogen bonding interactions,
8
our group has previously used this
strategy to access a terminal Mn
III
–OH moiety as part of a [Mn
4
] cluster.
9
Herein, we
describe the synthesis, structural characterization, and reactivity studies of clusters bearing a
Reed and Agapie
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terminal Fe
III
-oxo motif, stabilized by
tert
-butyl-amino-pyrazolates, to probe the
significance of a multinuclear scaffold on structural and reactivity aspects of a terminal
metal-oxo.
Treating the reported
LFe
3
(OAc)(OTf)
2
cluster (
−
OTf, triflate = trifluoromethane
sulfonate)
10
with three equivalents of potassium
tert
-butyl-amino-pyrazolate (KPzNHtBu)
and iodosylbenzene (PhIO), followed by addition of iron (II) triflate bis-acetonitrile
(Fe(OTf)
2
• 2 MeCN) and excess potassium hydroxide in tetrahydrofuran produces the
neutral [Fe
II
3
Fe
III
] cluster,
1
(Scheme 1). Single crystal X-ray diffraction (XRD) studies of
1
reveal a structure similar to our previously reported [Mn
4
] cluster bearing a terminal
hydroxide ligand (Figure 2A);
9
the apical metal displays a trigonal bipyramidal geometry,
with the terminal hydroxide ligand hydrogen bonded to each amino-pyrazolate (N–O
distances of 2.826(1), 2.765(1), 2.789(1) Å for
1
). The relatively short distance between the
apical Fe and the interstitial
μ
4
-O (Fe4–O1), 1.837(1) Å, is consistent with an Fe
III
in the
apical position of the cluster, with the remaining Fe centers being Fe
II
.
7b
,
11
The electrochemistry of the [Fe
4
] hydroxide clusters in THF features three quasi-reversible
events assigned to the [Fe
II
3
Fe
III
]
→
[Fe
II
2
Fe
III
2
] (−1.53 V; all potentials vs. Fc/Fc
+
),
[Fe
II
2
Fe
III
2
]
→
[Fe
II
Fe
III
3
] (−0.68 V), and [Fe
II
Fe
III
3
]
→
[Fe
III
4
] (−0.10 V) redox couples
(Figure S36). Each of the corresponding oxidation states of the cluster could be isolated
(Scheme 1). Mössbauer spectra of the oxidized clusters
2
,
3
, and
4
are consistent with
oxidations occurring at the Fe
II
centers in the tri-iron core and the Fe–OH moiety remaining
Fe
III
(Figures 2C, S42, S46, and S47).
Access to a terminal Fe
III
-oxo moiety was achieved by deprotonation of the [Fe
II
2
Fe
III
2
]
hydroxide cluster,
2
, with potassium
tert
-butoxide (KOtBu; Scheme 1). The resulting
compound,
5
, was crystallographically characterized (Figure 2B); deprotonation of the
hydroxide ligand leads to structural changes to the apical Fe in
5
. The Fe4–O2 distance
contracts to 1.817(2) Å, compared to the distances in
1
(1.937(1) Å) and the precursor
2
(1.907(3) Å); this bond length matches closely with the structurally characterized Fe
III
-oxo
complexes reported by Borovik and Fout.
8e
,
8h
,
8i
Compound
6
, prepared by deprotonating
3
,
also displays a short Fe4–O2 distance (1.795(8) Å). Furthermore, the apical Fe-
μ
4
-O
distance (Fe4–O1) elongates to 1.965(2) Å in
5
and 2.049(7) Å in
6
, from 1.890(3) Å in
2
and 1.948(2) Å in
3
, which is consistent with a greater trans influence exerted by the
terminal oxo ligand. The M
ӧ
ssbauer spectra of
5
and
6
are consistent with the [Fe
III
2
Fe
II
2
]
and [Fe
III
3
Fe
II
] oxidation state assignments, respectively (Figure 2D and S54). The
quadrupole doublet assigned to the apical Fe
III
-oxo centers in
5
and
6
have parameters
distinct from the other previously reported data for
[(H
3
beau)Fe(O)]
2-
, and most other
terminal Fe-oxo complexes (Table 1).
8e
,
12
Further spectroscopic studies of these Fe
III
-oxo
clusters are underway to understand the source of their atypical M
ӧ
ssbauer parameters.
Terminal Fe
III
-oxo complexes are rare, and typically stabilized through hydrogen bonding
interactions.
8e
,
8h
,
8i
,
13
The structures of
5
and
6
display comparable hydrogen bonding
distances to other structurally characterized Fe
III
-oxo complexes,
[(H
3
beau)Fe(O)]
2-
and
[N(afa
Cy
)
3
Fe(O)]
+
, along with similar equatorial Fe–N distances (Table 1). However, the
μ
4
-O distances in
5
(1.965(2) Å) and
6
(2.049(7) Å) are significantly shorter than the Fe–N
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distances for the amine trans to the oxo in the mononuclear systems (~2.27 Å). This is likely
a result of greater ligand flexibility in the mononuclear systems; the geometry of these Fe
III
-
oxo complexes display greater deviations from ideal trigonal bipyramidal geometry
compared to the apical Fe in
5
and
6
, based on a structural index parameter (
τ
; ideal trigonal
bipyramidal geometry = 1.0). For the clusters reported here, the rigid geometry of the
pyrazolate ligands prevents significant distortion of the apical Fe out of the equatorial plane.
The hydroxide ligand in
2
was determined to be very basic in THF (p
K
a
= 30.1; Table S1).
Analogous equilibrium studies were performed on
3
and, as expected, oxidation of the
cluster reduces the basicity of the Fe
III
-oxo moiety (p
K
a
= 23.0 for
3
; Table S2). Attempts to
deprotonate
4
with various bases, even at low temperatures, only resulted in decomposition,
so a p
K
a
value for this oxidation state was not measured. These data were combined with
electrochemical information for clusters 1 (vide supra) and 5 (Figure S38), to produce
thermodynamic square schemes according to equation 1 (Figure 3):
14
BDE
O−H
= 23.06
E
° + 1.37p
K
a
+
C
(1)
Similar to our previously reported studies on [Fe
3
Mn] hydroxide and aquo clusters, the bond
dissociation enthalpy of the O–H bond (BDE
O–H
) increases upon oxidation of the distal Fe
centers, ranging from 72 kcal/mol in
1
to 84 kcal/mol in
3
.
15
The three distal Fe oxidation states have a dramatic effect on the reactivity of the Fe
III
-oxo
center through modifying the p
K
a
and BDE
O–H
values. For example,
5
is incapable of
performing proton coupled electron transfer (PCET) reactions
16
,
17
with substituted phenols
over a range of phenol BDE
O–H
values (79 – 85 kcal/mol); only proton transfer to generate
2
is observed as expected from the combination of low BDE
O–H
for
1
and high p
K
a
of
2
(Figure 3, Table 2 and Figure S13). Oxidation of the remote Fe centers in
6
and
7
enables
PCET reactivity with these phenols (Figures S14 and S16), resulting in the formation of
2
and
3
, respectively.
31
P NMR and GC/MS analyses suggest that
7
is capable of transferring an oxygen atom to
trimethylphoshine (PMe
3
), where the other Fe
III
-oxo clusters display no reaction towards the
phosphine on similar timescale (see SI). The difference in reactivity is likely due to the low
reduction potentials of
5
and
6
precluding efficient oxygen atom transfer reactivity. A more
oxidizing cluster, through oxidations of the distal Fe centers,
7
can undergo OAT.
The kinetics of C–H activation by these clusters was investigated. The reaction between
5
and 9,10-dihydroanthracene (DHA; BDE
C–H
= 78 kcal/mol)
14c
displays an expected first
order dependence on substrate concentration, with an overall second order rate constant of
87 M
−1
s
−1
, and a considerable kinetic isotope effect (KIE) of 7 with
d
4
-DHA. These data
are consistent with a rate-limiting C–H bond activation for the PCET process to form
1
and
anthracene. The second-order rate constants between
5
and C–H bonds of varying BDE
C–H
and p
K
a
values were measured and display a linear dependence of the PCET reaction rate on
the p
K
a
of the organic substrate (Figure 4), suggesting either a concerted or stepwise p
K
a
-
driven process.
18
Reactions between DHA and
6
or
7
produce the corresponding hydroxide-
clusters and anthracene in yields comparable to
5
(Table S3) indicating PCET processes, but
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complex kinetics precluded the determination of rate constants and further insights into the
mechanism of these reactions.
Overall, this report offers a rare systematic study of the effects of neighboring redox active
metals on structural and reactivity aspects of a terminal metal-oxo. Because it is part of a
cluster, the reactivity of the terminal metal-oxo motif can be tuned without changing the
formal redox state of the metal supporting it; however, redox events at distal centers have
significant effect on the acidity and BDE of the corresponding O-H bond. Clearly, the cluster
as an assembly is essential for reactivity beyond the structural aspects of the isolated metal-
oxo motif. Further development of multinuclear model systems is necessary to fully
understand the nature and amplitude of these effects.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
ACKNOWLEDGMENT
This research was supported by the NIH (R01-GM102687B) and the Dreyfus Teacher-Scholar Program (T.A.).
C.J.R. thanks the Resnick Sustainability Institute at Caltech for a fellowship. We thank Dr. Mike Takase and Larry
Henling for assistance with crystallography, Prof. Jonas Peters for use of his group’s M
ӧ
ssbauer spectrometer, and
the Dow Next Generation Educator Fund for instrumentation.
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