A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity

Article abstract of DOI:10.1021/jacs.9b03157

Tetranuclear Fe clusters have been synthesized bearing a terminal Fe^III-oxo center stabilized by hydrogen-bonding interactions from pendant (tert-butylamino)pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [Fe^II₂Fe^III₂] to [Fe^III₄]; 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 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 of distal metal centers in modulating the reactivity of a terminal metal-oxo.

Full text of DOI:10.1021/jacs.9b03157

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Communication
A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects
of Distal Metal Centers on Structure and Reactivity
Christopher Reed, and Theodor Agapie
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03157 • Publication Date (Web): 14 May 2019
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A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal
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Christopher J. Reed, Theodor Agapie*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United
States
Supporting Information Placeholder
ABSTRACT: Tetranuclear Fe clusters have been synthesized
bearing a terminal FeIII-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
[FeII FeIII2] to [FeIII4]; two oxidation states were structurally char-
2
acterized by single crystal X-ray diffraction. The reactivity of the
FeIII-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 oxi-
dation states. These results demonstrate the important role adjacent
metal centers have on modulating the reactivity of a terminal metal-
oxo.
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 CaMn4O5 cluster known as the
oxygen evolving complex (OEC);2 numerous computational stud-
Figure 1. Multinuclear catalysts with proposed terminal metal-
ies 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 impli-
cated as a key intermediate (Figure 1).1e-g, 4
oxo intermediates (top), and structurally characterized terminal
FeIII-oxo complexes (bottom)
tert-butyl-amino-pyrazolates, to probe the significance of a multi-
nuclear scaffold on structural and reactivity aspects of a terminal
metal-oxo.
Studies of synthetic transition metal-oxo complexes have been
integral for understanding these reactive moieties in catalytic sys-
tems.1a, 5 However, there is a paucity of literature concerning mul-
tinuclear 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 FeIV-oxo center
would change depending on the oxidation state of a neighboring Fe
in a μ2-O bridged bimetallic complex (L’2OFe2(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 [Fe2] complex towards C–H oxidation.
Treating the reported LFe3(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 tetrahy-
drofuran produces the neutral [FeII FeIII] cluster, 1 (Scheme 1). Sin-
3
gle crystal X-ray diffraction (XRD) studies of 1 reveal a structure
similar to our previously reported [Mn4] cluster bearing a terminal
hydroxide ligand (Figure 2A);9 the apical metal displays a trigonal
bipyramidal geometry, with the terminal hydroxide ligand hydro-
gen 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 μ4-O (Fe4–O1), 1.837(1) Å, is con-
sistent with an FeIII in the apical position of the cluster, with the
remaining Fe centers being FeII.7b, 11
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 ob-
served.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 MnIII–OH moiety as part of a [Mn4] cluster.9 Herein, we
describe the synthesis, structural characterization, and reactivity
studies of clusters bearing a terminal FeIII-oxo motif, stabilized by
The electrochemistry of the [Fe4] hydroxide clusters in THF fea-
tures three quasi-reversible events assigned to the
[FeII FeIII]→[FeII FeIII2] (-1.53 V; all potentials vs. Fc/Fc+),
3
2
[FeII FeIII2]→[FeIIFeIII3] (-0.68 V), and [FeIIFeIII3]→[FeIII4] (-0.10
2
V) redox couples (Figure S36). Each of the corresponding oxida-
tion states of the cluster could be isolated (Scheme 1). Mössbauer
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Scheme 1. Synthesis of [Fe4] clusters. (Inset) 1,3,5-triarylbenzene ligand platform (L3-) and tert-butyl amino pyrazolate ligand
(PzNHtBu-).
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spectra of the oxidized clusters 2, 3, and 4 are consistent with oxi-
dations occurring at the FeII centers in the tri-iron core and the Fe–
OH moiety remaining FeIII (Figures 2C, S42, S46, and S47).
structurally characterized FeIII-oxo complexes, [(H3beau)Fe(O)]2-
and [N(afaCy)3Fe(O)]+, along with similar equatorial Fe–N dis-
tances (Table 1). However, the μ4-O distances in 5 (1.965(2) Å) and
6 (2.049(7) Å) are significantly shorter than the Fe–N 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 mononu-
clear systems; the geometry of these FeIII-oxo complexes display
greater deviations from ideal trigonal bipyramidal geometry com-
pared to the apical Fe in 5 and 6, based on a structural index param-
eter (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.
Access to a terminal FeIII-oxo moiety was achieved by deproto-
nation of the [FeII2FeIII2] 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
FeIII-oxo complexes reported by Borovik and Fout.8e, 8h, 8i Com-
pound 6, prepared by deprotonating 3, also displays a short Fe4–
O2 distance (1.795(8) Å). Furthermore, the apical Fe-μ4-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
The hydroxide ligand in 2 was determined to be very basic in
THF (pKa = 30.1; Table S1). Analogous equilibrium studies were
performed on 3 and, as expected, oxidation of the cluster reduces
the basicity of the FeIII-oxo moiety (pKa = 23.0 for 3; Table S2).
Attempts to deprotonate 4 with various bases, even at low temper-
atures, only resulted in decomposition, so a pKa value for this oxi-
dation state was not measured. These data were combined with
electrochemical information for clusters 1 (vide supra) and 5 (Fig-
ure S38), to produce thermodynamic square schemes according to
equation 1 (Figure 3): 14
Mӧssbauer spectra of 5 and 6 are consistent with the [FeIII2FeII ]
2
and [FeIII3FeII] oxidation state assignments, respectively (Figure 2D
and S54). The quadrupole doublet assigned to the apical FeIII-oxo
centers in 5 and 6 have parameters distinct from the other previ-
ously reported data for [(H3beau)Fe(O)]2-, and most other terminal
Fe-oxo complexes (Table 1).8e, 12 Further spectroscopic studies of
these FeIII-oxo clusters are underway to understand the source of
their atypical Mӧssbauer parameters.
BDEO–H = 23.06 E° + 1.37 pKa + C
(1)
Similar to our previously reported studies on
Terminal FeIII-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
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BDEO–H values. For example, 5 is incapable of performing proton
coupled electron transfer (PCET) reactions16,17 with substituted
phenols over a range of phenol BDEO–H values (79 – 85 kcal/mol);
only proton transfer to generate 2 is observed as expected from the
combination of low BDEO–H for 1 and high pKa of 2
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Figure 3. Thermodynamic cycles to evaluate the BDEO–H values of
the hydroxide clusters 1 – 3. Reduction potentials (horizontal lines)
are references to Fc/Fc+. pKa values (vertical lines) are based on
relative pKa values of cationic acids in THF. Diagonal lines are the
BDEO–H values calculated from these parameters according to the
Bordwell equation (eq 1). Approximate values (~) have been ex-
trapolated from the Bordwell equation.
Figure 2. Crystal structures of tetranuclear Fe hydroxide cluster, 1
(A), and oxo cluster, 5 (B). Ellipsoids shown at the 50% probability
level with solvent molecules, and hydrogen atoms (except for N–H
moieties) omitted for clarity. (C) Mӧssbauer spectrum of 2 (black
dots) with simulated parameters: (i) δ = 1.12 mm/s, |ΔEq| = 3.20
mm/s (solid blue), (ii) δ = 1.10 mm/s, |ΔEq| = 2.76 mm/s (dashed
blue), (iii) δ = 0.52 mm/s, |ΔEq| = 0.81 mm/s (orange), (iv) δ = 0.41
mm/s, |ΔEq| = 2.17 mm/s (green). (D) Mӧssbauer spectrum of 5
(black dots) with simulated parameters: (i) δ = 1.12 mm/s, |ΔEq| =
3.14 mm/s (solid blue), (ii) δ = 1.10 mm/s, |ΔEq| = 2.87 mm/s
(dashed blue), (iii) δ = 0.52 mm/s, |ΔEq| = 1.13 mm/s (orange), (iv)
δ = 0.43 mm/s, |ΔEq| = 3.04 mm/s (green).
(Figure 3, Table 2 and Figure S13). Oxidation of the remote Fe
centers in 6 and 7 enables PCET reactivity with these phenols (Fig-
ures S14 and S16), resulting in the formation of 2 and 3, respec-
tively.
31P NMR and GC/MS analyses suggest that 7 is capable of trans-
ferring an oxygen atom to trimethylphoshine (PMe3), where the
other FeIII-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.
[Fe3Mn] hydroxide and aquo clusters, the bond dissociation en-
thalpy of the O–H bond (BDEO–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 FeIII-oxo center through modifying the pKa and
Table 1. Selected Bond Distances and Angles, Structural Index Parameter, and Mӧssbauer Parameters of Reported FeIII-Oxo
Complexes
5
6
[(H3beau)Fe(O)]2- 8e
[N(afaCy)3Fe(O)]+ 8h
1.817(2)
1.795(8)
1.813(3)
1.806(1)
Fe–O (Å)
2.104(2), 2.098(2),
2.093(2)
2.100(8), 2.085(9),
2.087(9)
2.030(4), 2.060(4),
2.082(4)
2.049(1), 2.049(1),
2.052(1)
Fe–Nequatorial (Å)
1.965(2) (L=O2-)
2.647, 2.717, 2.685
96.3, 92.8, 92.0
0.14
2.049(7) (L=O2-)
2.718, 2.790, 2.750
93.6, 97.5, 96.3
0.22
2.271(4) (L=NR3)
2.732, 2.702, 2.686
103.3, 99.7, 100.8
0.42
2.276(1) (L=NR3)
2.641, 2.645, 2.673
102.6, 103.1, 103.1
0.45
Fe–Ltrans (Å)
N–O (H-bond; Å)
∠Nequatorial–Fe–O (°)
Fe–N|N’|N’’equatorial (Å)
Structural Index
0.9
0.8
0.5
0.4
-
Parameter (τ)a
Mossbauer parameters
(mm/s)
δ = 0.43, |ΔEq| = 3.04
δ = 0.47, |ΔEq| = 2.53
δ = 0.30, |ΔEq| = 0.91
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a τ = [Σ (∠Nequit.–Fe–N’equit.) - Σ (∠Nequit.–Fe–O)]/90
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Table 2. Reactivity of the [Fe4]-Oxo Clusters, 5 - 7.
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ASSOCIATED CONTENT
1
2
3
4
Supporting Information
BDE
Reactivity Observeda
(kcal/mol)
5
6
7
The Supporting Information is available free of charge on the ACS
Publications
(FeII2FeIII
)
(FeII
(FeIII
)
4
2
website.
FeIII
)
3
5
9,10-dihy-
droanthra-
cene
6
7
8
9
Experimental Procedures and Supplimentary Data (PDF)
Crystallographic data files (CIFs)
78
PCET
PCET
PCET
AUTHOR INFORMATION
fluorene
82
82
-
PCET
PT
PCET
PCET
NR
PCET
PCET
OAT
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2,4,6-tBu3-
PhOH
Corresponding Author
*agapie@caltech.edu
PMe3
NR
Notes
aPT = proton transfer, PCET = proton-coupled electron transfer
(based on cluster products), OAT = oxygen atom transfer, NR = no
reaction observed. bSecond-order rate constant.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The kinetics of C–H activation by these clusters was investi-
gated. The reaction between 5 and 9,10-dihydroanthracene (DHA;
BDEC–H = 78 kcal/mol)14c displays an expected first order depend-
ence 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 d4-DHA. These data are consistent with a rate-lim-
iting C–H bond activation for the PCET process to form 1 and an-
thracene. The second-order rate constants between 5 and C–H
bonds of varying BDEC–H and pKa values were measured and dis-
play a linear dependence of the PCET reaction rate on the pKa of
the organic substrate (Figure 4), suggesting either a concerted or
stepwise pKa-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
complex kinetics precluded the determination of rate constants and
further insights into the mechanism of these reactions.
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 crys-
tallography, Prof. Jonas Peters for use of his group’s Mӧssbauer
spectrometer, and the Dow Next Generation Educator Fund for in-
strumentation.
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