Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and Mn3MO(OH) Clusters

Article abstract of DOI:10.1021/acs.inorgchem.8b02701

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn₃ and Fe₃ cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn₃MO₄], [Mn₃MO(OH)], and [Fe₃MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics.

Full text of DOI:10.1021/acs.inorgchem.8b02701

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer
Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and
Mn3MO(OH) Clusters
Davide Lionetti,† Sandy Suseno,† Emily Y. Tsui,† Luo Lu,§ Troy A. Stich,§ Kurtis M. Carsch,†,‡
Robert J. Nielsen,†,‡ William A. Goddard, III,†,‡ R. David Britt,§ and Theodor Agapie*,†
†Division of Chemistry and Chemical Engineering and ‡Materials and Process Simulation Center, California Institute of Technology,
Pasadena, California 91125, United States
§Department of Chemistry, University of California, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: The modulation of the reactivity of metal oxo
species by redox inactive metals has attracted much interest due to
the observation of redox inactive metal effects on processes
involving electron transfer both in nature (the oxygen-evolving
complex of Photosystem II) and in heterogeneous catalysis (mixed-
metal oxides). Studies of small-molecule models of these systems
have revealed numerous instances of effects of redox inactive metals
on electron- and group-transfer reactivity. However, the hetero-
metallic species directly involved in these transformations have
rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural
characterization of multiple series of heterometallic clusters based on Mn3 and Fe3 cores and described the effects of Lewis
acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis
acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn3MO4],
[Mn3MO(OH)], and [Fe3MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The
qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers
can affect the chemical reactivity of metal oxo species by modulating cluster electronics.
catalytic conditions, have been described.28−33 Sc3+ has been
1. INTRODUCTION
reported to affect the formation and reactivity of Fe(III)−
Lewis acidic, redox inactive metal ions are known to influence
(hydro) peroxo species.34,35 While the observation of
electron- and group-transfer processes in both natural and
synthetic systems.1−3 In biology, the preeminent example is
substantial effects of redox inactive metals on oxygen related
redox chemistry is well-demonstrated, structural character-
that of the redox inactive Ca2+ center found in the oxygen
ization of the precursor Lewis acid bound species active in
these transformations remains a challenge. In most cases,
reactive species are generated in situ by addition of large
excesses of Lewis acid additives to high-valent metal oxo
complexes. Although spectroscopic studies of the interaction of
metal oxo complexes with metal ions in solution have been
described in select cases, structural characterization of these
species has rarely been reported.17,36 The only crystallo-
graphically characterized high-valent metal oxo/redox inactive
metal ion adduct, a [Fe(IV)−O−Sc3+] complex,17 has recently
been reassigned as a [Fe(III)−(OH)−Sc3+] species based on
spectroscopic characterization and structural comparisons with
other Fe(III) and Fe(IV) complexes.37,38 Therefore, con-
clusions drawn from the majority of these studies must account
for both the effect of Lewis acid binding to metal−oxo species
evolving complex (OEC) of Photosystem II (PSII), a
Mn4CaOx cluster responsible for water oxidation to dioxygen
in plants, algae, and cyanobacteria. Although its role has not
been fully elucidated, Ca2+ is an essential cofactor in this
system.4,5 Redox inactive metals likewise affect the catalytic
behavior of heterogeneous mixed-metal oxides in reactions
such as water oxidation and dioxygen reduction.6−8 The
−
reactivity of inorganic oxidants (e.g., MnO4 , Cr2O72−) with
organic substrates can be modulated by Lewis acidic
additives.9−15 Redox inactive metals have been reported to
affect the rate of phosphine oxygenation by a MnV−oxo
complex.16 Lewis acidic, redox inactive metal ions show a
range of effects on the oxygen atom transfer (OAT) reactivity
of high-valent Fe−, Mn−, and Co−oxo species including rate
enhancement and shifts in reaction mechanism.17−25 Lewis
acid induced O−O bond cleavage or release of O2 from a
Fe(III)-bound peroxide species has also been reported.26,27
The effects of Lewis acids on group transfer reactions by high-
valent Mn, Fe, and V complexes, including those under
Received: September 21, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
and for the dynamic equilibrium processes responsible for
these interactions.
toward PMe3. This observation is consistent with the greater
oxidizing power of [MnIII2MnIV2O4] as indicated by its more
positive one-electron reduction potential (E1/2 = −0.70 vs
−0.94 V vs Fc+/0 for [MnIV3CaO4]). In contrast with this
model, [MnIV3ScO4] displayed only slow reactivity with PMe3,
generating an intractable mixture of metal-containing products,
Incorporation of Lewis acidic metal centers in well-defined
multimetallic complexes precludes the need for use of excess
Lewis acid additives by providing structurally unambiguous
precursors for reactivity studies. This is an appealing strategy
for such studies, as it enables a less ambiguous interpretation of
any observed heterometallic effects on reactivity. Recently,
studies of the effect of redox inactive additives on reactivity
related to oxygen or atom-transfer processes44−48 and on
photocatalytic water oxidation49 have been reported. Our
group has studied the effects of Lewis acidity of a redox
inactive metal center on the reduction potential of well-defined
heterometallic clusters.3,39−43 Tetrametallic complexes were
supported by a multinucleating ligand framework and
contained a Mn3 or Fe3 core, as well as a fourth, redox
inactive metal (M). Tetraoxo [Mn3MO4] cubane clusters, as
well as oxo-hydroxo [Mn3MO(OH)] (previously assigned as
[Mn3MO2] dioxo species, vide infra) and [Fe3MO(OH)]
clusters were investigated. A linear correlation was discovered
between the Lewis acidity of the redox inactive metal and the
reduction potential of the cluster,40,42,43 supporting the
plausibility of a potential-tuning role for redox inactive metals
in biological and heterogeneous systems. Although compar-
isons of these complexes have focused on their one-electron
reduction potentials (a thermodynamic parameter), these
clusters are well-suited to further exploration of the effects of
redox inactive metals on chemical reactivity. We have
previously reported initial studies of reactions of cubane
clusters with phosphines,50 including investigations into the
mechanism of OAT from these species. However, an extensive
structure−function study involving a broader range of cluster
structures containing redox-inactive metals of varying Lewis
acidity has not previously been carried out. Importantly, our
structurally characterized heterotetrametallic clusters provide
well-defined precursors for OAT reactions, and the availability
of structurally analogous complexes containing different redox
inactive metals allows for direct comparison of their effects.
Herein, we describe the qualitative assessment of OAT
reactivity with phosphine substrates (Scheme 1) of three
despite being a stronger oxidant than [MnIII2MnIV2O4] (E1/2
−0.24 vs −0.70 V vs Fc+/0, respectively; see Table 1).
=
These seemingly contradictory observations can be ration-
alized by considering the previously studied mechanism of
transfer of an oxygen atom from the cubane cluster to PMe3.
According to earlier computations,50 OAT involving cubane
clusters [MnIV3MO4] and [MnIIIMnIV2MO4] proceeds via
initial dissociation of a bridging acetate ligand from one of the
three core Mn centers. The MnIII center (high spin, d4) in
complexes [MnIIIMnIV2MO4] is more substitutionally labile
due to electrons occupying a Mn−O σ−antibonding orbital.
Thus, metal−carboxylate interactions in complexes
[MnIIIMnIV2MO4] are weaker than in the case of [MnIV3MO4]
clusters, in which all Mn centers are in the MnIV oxidation state
(d3). The difference in Mn oxidation states between
[MnIV3MO4] and [MnIIIMnIV2MO4] affects the kinetics of
ligand dissociation, and only indirectly the OAT process,
resulting in slower reactivity of [MnIV3ScO4] despite a larger
driving force for electron transfer.
On the basis of this interpretation, it was thus postulated
that one-electron reduction of other [MnIV3MO4] clusters,
resulting in generation of a kinetically labile MnIII center,
would provide more reactive precursors for OAT reactivity,
enabling more extensive comparisons of the effects of redox
inactive metal of varying Lewis acidity on this transformation.
One-electron reduction of [MnIV3MO4] complexes was
achieved using cobaltocene (CoCp2) or decamethylferrocene
(FeCp*2) as reductant. Reduction of [MnIV3CaO4] with
CoCp2 generated a highly unstable material, which could not
be isolated cleanly, and was therefore not investigated further.
Reduction of [MnIV3ScO4] and [MnIV3GdO4], however,
successfully yielded corresponding [MnIIIMnIV2ScO4] and
[MnIIIMnIV2GdO4] clusters as reported previously.39,40 Treat-
ment of [MnIIIMnIV2ScO4], with PMe3 (10 equiv) results in
clean conversion to a new species over 1.5 weeks at room
temperature as determined by proton nuclear magnetic
resonance (1H NMR, Figure S6). The reaction could be
accelerated by heating to 50 °C in benzene, resulting in full
conversion to the same product in 35 h (Figure S7). As a
control, heating of [MnIIIMnIV2ScO4] in the absence of PMe3
led to negligible (<10%) conversion. The product of reaction
of [MnIIIMnIV2ScO4] with PMe3 was characterized by single-
crystal X-ray diffraction (XRD) as the trioxo complex
[MnIII3ScO3] (Figure 2), analogous to the previously reported
reaction of [MnIII2MnIV2O4] to generate [MnIII4O3].50 The
average Mn−oxo and Mn−O(acetate) distances in
[MnIII3ScO3] are 1.88 and 2.10 Å, respectively, consistent
with the assignment of the Mn3 core as [MnIII]3. No signal for
trimethylphophine oxide (OPMe3), the expected byproduct of
OAT, was detected by 31P NMR spectroscopy during the
reaction. The OPMe3 generated in the reaction is thus
proposed to be bound to the nascent [MnIII3ScO3], at least in
solution under the reaction conditions. Formation of OPMe3
was however confirmed at the end of the reaction via gas
chromatography mass-spectrometry (GC-MS) following work-
up that included removal of unreacted, volatile PMe3 (see the
Experimental Section).
Scheme 1. Oxidation of Phosphines Investigated in the
Present Study
series of complexes: [Mn3MO4] cubanes and [Mn3MO(OH)],
and [Fe3MO(OH)] oxo-hydroxo complexes. The reassign-
ment of Mn3 oxo-hydroxo clusters, previously identified as the
corresponding [Mn3MO2] dioxo complexes, on the basis of
spectroscopic and computational data is also discussed.
2. RESULTS AND DISCUSSION
2.1. OAT Reactivity of [Mn3MO4] Cubanes (M = Ca2+,
Gd3+, Sc3+, Mn3+). The OAT reactivity of [MnIV3CaO4],
[MnIV3ScO4], and [MnIII2MnIV2O4] (Figure 1) with trialkyl-
phosphines has previously been described.50 Reaction of
[MnIII2MnIV2O4] with trimethylphosphine (PMe3, 2 equiv)
led to rapid generation of trioxo cluster [MnIII4O3], in which
the bottom oxide ligand is absent (Scheme 2). Cubane
complex [MnIV3CaO4] was instead found to be unreactive
B
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Left: Heterotetrametallic clusters investigated for OAT reactivity. Right: plots of cluster redox potentials vs pKa of M(aqua)n+ ion (as
measure of Lewis acidity) summarizing the previously reported redox inactive metal effects on redox processes of heterometallic clusters (red
squares: [Mn3MO4] cubane clusters;39−41 green circles: [Fe3MO(OH)] oxo-hydroxo clusters;42 blue diamonds: [Mn3MO(OH)] oxo-hydroxo
clusters,43 vide infra; the red square labeled as “Mn3+” corresponds to the [MnIIIMnIV3]/[MnIII2MnIV2] couple in the all-manganese cubane
cluster).41
Scheme 2. OAT Reactivity of [Mn3MO4] Cubane Clusters
Table 1. One-Electron Redox Potentials of Selected
[MnIV3MO4] and [MnIIIMnIV2MO4] Clusters
compound
E1/2 (V vs Fc+/0
)
redox couple
[MnIII2MnIV2O4]41
[MnIV3CaO4]41
[MnIV3ScO4]40
−0.70
−0.94
−0.24
MnIII2MnIV2/MnIII3MnIV
MMnIV3/MMnIIIMnIV
2
MMnIV3/MMnIIIMnIV
2
Figure 2. Left: truncated solid-state structure of [MnIII3ScO3]. Right:
relevant M−O and Mn−O bond distances in Å.
Sc3+) with PMe3 (10 equiv) at 50 °C in benzene led to
complete conversion of the starting material over 50 h to a
stable new product (heating in the absence of PMe3 did not
lead to detectable conversion). Although crystallographic
characterization of this product was unsuccessful, this species
was assigned as the trioxo cluster [MnIII3GdO3] on the basis of
the similarity of its 1H NMR features with those for structurally
characterized [MnIII3ScO3] (Figure S8). As a signal for OPMe3
was detected during the reaction by 31P NMR, the reaction was
monitored by NMR spectroscopy upon treating
[MnIIIMnIV2ScO4] with a smaller excess of PMe3 (3 equiv)
at 50 °C to improve accuracy of integration of NMR signal.
This experiment was performed in the presence of PPh3 (2
equiv) as an internal standard. A total of 1 equiv of PMe3 was
consumed throughout the reaction (67 h), and formation of
the same metal species was observed (1H NMR, Figure S9).
The reaction stoichiometry (1:1 [MnIIIMnIV2GdO4]/PMe3) is
Treatment of [MnIIIMnIV2GdO4], containing a significantly
less Lewis acidic Gd3+ center (pKa = 8.4 for Gd3+ vs 4.8 for
C
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Summary of OAT Reactivity of Heterometallic Mn3 and Fe3 Clusters
a
b
entry
compound
E1/2 (vs Fc+/0
)
Lewis acid
pKa51
substrate (equiv)
time
N.R.
N.R.
T (°C)
137
237
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
[MnIV3ScO4]
−0.24 V40
−0.94 V41
−0.70 V41
Sc3+
Ca2+
Mn3+
Sc3+
Sc3+
Gd3+
Gd3+
Y3+
4.8
12.9
0.1
4.8
4.8
8.4
8.4
8.6
8.6
PMe3 (2)
PMe3 (2)
PMe3 (2)
PMe3 (10)
PMe3 (10)
PMe3 (10)
PMe3 (10)
PEt3 (10)
PPh3 (10)
PEt3 (10)
PPh3 (10)
PEt3 (10)
PPh3 (10)
PEt3 (10)
PPh3 (10)
PPh3 (10)
PPh3 (10)
PMe3 (10)
PMe3 (10)
r.t.
r.t.
r.t.
r.t.
50 °C
r.t.
50 °C
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
[MnIV3CaO4]
[MnIII2MnIV2O4]
15 min
1.5 weeks
35 h
>2 weeks
50 h
15 min
30 min
3 h
[MnIIIMnIV2ScO4]
[MnIIIMnIV2ScO4]
[MnIIIMnIV2GdO4]
[MnIIIMnIV2GdO4]
[MnIII3YO(OH)]2
[MnIII3YO(OH)]2
[MnIII3CaO(OH)]
[MnIII3CaO(OH)]
[MnIIMnIII2YO(OH)]
[MnIIMnIII2YO(OH)]
[MnIIMnIII2CaO(OH)]
[MnIIMnIII2CaO(OH)]
[FeIII3LaO(OH)]
+0.42 V43
+0.42 V43
−0.07 V43
−0.07 V43
Y3+
Ca2+
Ca2+
Y3+
12.9
12.9
8.6
20 h
30 min
1 h
Y3+
8.6
Ca2+
Ca2+
La3+
Ca2+
Sc3+
La3+
Ca2+
12.9
12.9
9.06
12.9
4.8
36 h
>2 weeks
24 h
−0.08 V42
−0.48 V42
[FeIII3CaO(OH)]
[FeIIFeIII2ScO(OH)]
[FeIIFeIII2LaO(OH)]
[FeIIFeIII2CaO(OH)]
N.R.
14 h
130 h
270 h
9.06
12.9
r.t.
r.t.
PMe3 (10)
a
b
Fc+/0 = ferrocenium/ferrocene couple; only potentials for relevant reduction events included. N.R. = no reaction observed.
consistent with transfer of a single O atom equivalent during
the reaction, corroborating the assignment of the metal-
containing product as trioxo species [MnIII3GdO3]. Notably,
[MnIII3GdO3] is stable even in the presence of a large excess of
PMe3 (10 equiv), and formation of reduced clusters with <3
oxide ligands, the products of further OAT processes, was not
observed.
compounds across the entire series. The reactivity studies
described next were focused on the available oxo-hydroxo
complexes.
2.3. OAT Reactivity of [Mn3MO(OH)] Clusters (M =
Ca2+, Y3+). OAT reactivity of complexes [MnIII3MO(OH)]
and [MnIIMnIII2MO(OH)] was investigated using phosphines
as substrates (Scheme 3; Table 2, entries 8−15). Clusters
The observed rate of OAT from [MnIIIMnIV2MO4] clusters
to phosphine substrates correlates with the Lewis acidity of M
(Table 2, entries 1−7). To generate these correlations, we have
used the pKa of the M(H2O)n ion51 as a measure of Lewis
acidity; in our previous work, this scale of Lewis acidity has
enabled analogous correlations between Lewis acidity and
reduction potential. Analysis of the qualitative, relative rates of
OAT of [MnIIIMnIV2MO4] complexes with PMe3 as substrate
yields the following trend: [MnI II2MnI V2O4] >
[MnIIIMnIV2ScO4] > [MnIIIMnIV2GdO4].
Scheme 3. OAT Reactivity of Mn3 Oxo-Hydroxo Clusters
[MnIII3MO(OH)] and [MnIIMnIII2MO(OH)]
The decrease in OAT rate correlates with the progressively
higher pKa (i.e., lower Lewis acidity) across this series of
metals (pKa(M(H2O)n): Mn3+ = 0.1, Sc3+ = 4.7, Gd3+ = 8.4).51
Most notably, changing from Mn to Gd results in decrease in
the rate of OAT by more than 3 orders of magnitude.
2.2. Structural Reassignment of [Mn3MO2(H)] Clusters
(M = Na+, Sr2+, Ca2+, Zn2+, Y3+). Comparisons of the
structural parameters between initially reported Mn3MO2
clusters and more recently prepared Fe3MO(OH) show strong
similarities. Through a combination of EPR, XRD, and
computational studies, these Mn clusters were reassigned as
Mn3MO(OH), with an oxidation state of [MnIII3MO(OH)] as
[MnIII3] and of [MnIIMnIII2MO(OH)] as [MnIII2MnII]. A
detailed discussion of these data and their interpretation is
presented in Supporting Information, page S3. It should be
noted that the changes in assignment of the oxidation states
and identity of bridging ligands in clusters [MnIII3MO(OH)]
and [MnIIMnIII2MO(OH)] bear no effect on the conclusions
of earlier studies on these complexes regarding the effect of the
redox inactive metal on reduction potentials.43 All comparisons
within this series of complexes remain valid, as changes affect
D
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[MnIII3MO(OH)] and [MnIIMnIII2MO(OH)] displayed faster
reactivity with phosphines than that of cubane complexes
[MnIV3MO4] and [MnIIIMnIV2MO4]. Triethylphosphine
(PEt3) and triphenylphosphine (PPh3) were used, as PMe3
resulted in reactivity too rapid for comparisons to be possible
1
by H NMR spectroscopy. Treatment of [MnIIMnIII2CaO-
(OH)] with 10 equiv of PEt3 led to conversion, over 36 h, to a
product with very broad 1H NMR features (Figure S16),
typical of [LMnII X3] (X = −OAc or −OTf) species.52 This
3
observation is consistent with loss of the redox inactive metal
center following OAT. Similar to the case of cubane clusters,
triethylphosphine oxide (OPEt3), the product substrate
oxidation, was not observed spectroscopically during the
reaction. Removal of solvents and excess PEt3 in vacuo after
completion, followed by methanolysis and analysis by GC-MS
allowed for identification of the OPEt3 product.
[MnIIMnIII2CaO(OH)] also reacts with the weaker O-atom
acceptor PPh3, albeit much more slowly (>2 weeks; Figure
S17).
As expected, the more oxidized cluster [MnIII3CaO(OH)]
reacts more rapidly with both PEt3 and PPh3; full conversion
was achieved in 3 and 20 h, respectively (Figures S12 and
S13). Similar to [MnIIMnIII2CaO(OH)], OAT from
[MnIII3CaO(OH)] results in formation of reduced MnII
3
species whose structural characterization was unsuccessful.
Clusters displaying the more Lewis acidic Y3+ ion yield faster
reactivity. Reaction of [MnIIMnIII2YO(OH)] with PEt3 (10
equiv) proceeded to completion in 30 min (Figure S14),
whereas reaction with PPh3 was complete in 1 h (Figure S15).
Figure 3. Solid-state structure of [MnIII3YO(OH)]2. Five molecules
of outer sphere triflate anions and hydrogen atoms are omitted for
clarity.
As before, generation of MnII species was suggested by the
3
intermediate species during the reaction of [MnIIMnIII2YO-
broad 1H NMR features observed. Although isolation of
monomeric oxidized complex [MnIII3YO(OH)] has not been
achieved to date via either direct synthesis or transmetalation
from [MnIII3CaO(OH)], a dimeric form of this complex,
[MnIII3YO(OH)]2, has been prepared from precursor complex
LMn3(OAc)3 (1H NMR, elemental analysis). XRD studies
have only yielded a poor-quality structure (Figure 3), which is
nonetheless sufficient for basic connectivity determination. In
this complex, two [Mn3YO(OH)] cores are bridged by two
−OAc and one −OTf ligands, with five outersphere triflate
counteranions. [MnIII3YO(OH)]2 reacts rapidly with phos-
phine substrates: reaction with PEt3 (10 equiv/Y) was
complete in less than 15 min (Figure S10), and even the
more electron-deficient PPh3 led to complete reaction in 30
min (Figure S11).
1
(OH)] with PPh3. After 30 min, H NMR features consistent
with formation of a [Mn3MO] species were detected (Figure
S3),41 leading to the proposed generation of tetrametallic
monooxo species from [MnI I I 3 MO(OH)] and
[MnIIMnIII2MO(OH)] clusters prior to the second OAT
event and formation of the fully reduced products. Although
quantification of the phosphine oxide produced in these
processes was attempted in situ (31P NMR) as well as following
workup (see the Experimental Section for the workup
protocol), these procedures led to incomplete accounting of
phosphine oxide product even in most cases where the
stoichiometry of the reaction was well-established (e.g., the
reactivity of cubane clusters [MnIIIMnIV2MO4]), likely due to
strong interaction of the phosphine oxide product with metal-
containing byproducts. Thus, the stoichiometry of the reaction
of Mn oxo-hydroxo clusters [MnIII3MO(OH)] and
[MnIIMnIII2MO(OH)] (or their Fe counterparts, vide infra)
could not be conclusively assigned, except in the case of
[MnIIIMnIV2ScO4] which reacted to generate cleanly 1 equiv
of (O)PMe3 (31P NMR).
2.4. OAT Reactivity of [Fe3MO(OH)] Clusters (M =
Ca2+, La3+, Sc3+). Studies of the OAT reactivity of clusters
[FeIII3MO(OH)] and [FeIIFeIII2MO(OH)], isostructural to
Mn-containing complexes [MnIII3MO(OH)] and
[MnIIMnIII2MO(OH)], respectively, were carried out with
phosphine substrates in an analogous manner to those of
cubane and oxo-hydroxo Mn3 clusters (Scheme 4; Table 2,
entries 16−20). Treatment of clusters [FeIII3MO(OH)],
containing three FeIII centers, with electron-rich phosphines
such as PMe3 yielded complex mixtures. Therefore, OAT
behavior of [FeIII3MO(OH)] complexes was investigated with
the more electron deficient substrate PPh3. [FeIII3CaO(OH)]
displays minimal conversion in the presence of PPh3 (10 equiv,
Qualitative analysis of the relative OAT rates for clusters
[MnIII3MO(OH)] and [MnIIMnIII2MO(OH)] reveals the
following trend: [MnIII3YO(OH)]2 > [MnIIMnIII2YO(OH)]
> [MnIII3CaO(OH)] > [MnIIMnIII2CaO(OH)].
As observed for the cubane clusters, the complexes
containing more Lewis acidic Y3+ (pKa = 8.6) undergo faster
OAT processes than those containing Ca2+ (pKa = 12.9), in
some case by more than 2 orders of magnitude.51 Unlike their
cubane analogues, which displayed clean transfer of a single
oxygen atom to give trioxo clusters, oxo-hydroxo complexes
appear to transfer both oxygen atom equivalents to yield
[MnII ] species. It is probable the first OAT event (presumably
3
on the more sterically accessible μ2-bridge) relieves steric
congestion in the cluster, increasing flexibility and allowing
access to the second O-atom. Deprotonation of the hydroxide
may precede OAT, though independent deprotonation studies
were inconclusive. Formation of mono-oxo complexes en route
to the [MnII ] products is supported by the observation of
3
E
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
redox inactive metal. In both oxidation states, the clusters
containing the more Lewis acidic heterometal display faster
OAT to phosphine substrates. [FeIII3CaO(OH)] and
[FeIIFeIII2CaO(OH)], containing the least Lewis acidic Ca2+
(pKa = 12.9), react slower than clusters [FeIII3LaO(OH)] and
[FeIIFeIII2LaO(OH)] and [FeIIFeIII2ScO(OH)], respectively,
which contain stronger Lewis acidic metals (pKa: La3+ = 9.06,
Sc3+ = 4.8).
Scheme 4. OAT Reactivity of Fe3 Oxo-Hydroxo Clusters
[FeIII3MO(OH)] and [FeIIFeIII2MO(OH)]
2.5. Reactivity Trends. Access to structurally well-defined
tetraoxo and oxo-hydroxo clusters displaying both redox active
(Mn, Fe) and redox inactive metals has facilitated systematic
structure−reactivity studies (Table 2). Although effects of
Lewis acidic, redox inactive metals on redox processes such as
group transfer reactivity have been previously reported, to our
knowledge, this study is the first to employ heterometallic oxo
clusters for which an unambiguous structural assignment of the
OAT precursors is available. While in previous literature
reports redox inactive metal centers were included as additives
(often in suprastoichiometric amounts), generating the species
responsible for OAT in situ, the use of well-defined
heterometallic clusters simplifies the interpretation of the
observed reactivity. When redox inactive metal additives are
used in excess to generate adducts with high-valent metal oxo
species in situ, the effects of the Lewis acidity on the dynamic
equilibria involved in adduct formation must be accounted for
when analyzing the reactivity of the proposed adducts. In the
present systems, however, the redox inactive metal centers are
stoichiometrically incorporated in clusters that can be isolated
and structurally characterized. For each type of cluster reported
here, the first OAT step is qualitatively faster with increasing
Lewis acidity (decreasing pKa of metal aquo complex) of the
redox inactive, apical metal center in the cluster. For oxo-
hydroxo complexes, clusters in higher oxidation states display
faster OAT reactivity than that of their reduced counterparts.
The case of cubane clusters is more complex, as the high
kinetic barrier to dissociation of an acetate ligand from the all-
MnIV core in oxidized clusters [MnIV3MO4] inhibits the OAT
process (vide supra). Notably, in several classes of clusters, the
metal-containing species initially generated by the reaction of
the well-defined heterometallic precursors with phosphines
were observed to themselves undergo further reactivity with
phosphines. However, as neither the transiently generated
metal-containing species nor the final metal-containing
products of these processes could be cleanly isolated, no
further information on the reactivity of these systems can be
teased out from the available data. Nonetheless, the qualitative
analysis of the kinetics of the first step in this reactivity (i.e., the
reaction of the well-defined isolated clusters with phosphine
substrates) indicates that the choice of Lewis acid plays a
significant role in tuning the OAT reactivity of these clusters.
Comparison of the OAT activity of complexes
[MnI I I MnI V 2 GdO4 ], [MnI I MnI I I 2 YO(OH)], and
[FeIIFeIII2LaO(OH)], which contain heterometals with similar
pKa’s (Gd3+ = 8.4, Y3+ = 8.6, La3+ = 9.06), enables assessment
of the effect of various cluster architectures on reactivity.
Cubane cluster [MnIIIMnIV2GdO4] reacts slower (>2 weeks at
r.t.) with PMe3 than [FeIIFeIII2LaO(OH)] (120 h), which in
turn displays slower OAT activity than [MnIIMnIII2YO(OH)]
(30 min, PEt3). A similar reactivity ordering is observed
between clusters containing Ca2+ (pKa = 12.9) as the redox-
inactive metal; [MnIIMnIII2CaO(OH)] reacts faster (36 h)
than [FeIIFeIII2CaO(OH)] (270 h; the corresponding cubane
cluster, [MnIIIMnIV2CaO4], could not be isolated).
Figure S19). Conversely, [FeIII3LaO(OH)] reacts completely
with PPh3 (10 equiv) over 14 h, generating OPPh3 as
determined via 31P NMR (Figure S18). OAT reactivity of
reduced clusters [FeIIFeIII2MO(OH)] was studied using PMe3
as substrate. In the presence of a 10-fold excess of PMe3,
[FeIIFeIII2ScO(OH)] displayed complete conversion in 60 h,
slower than that for the more oxidized clusters. The OPMe3
product generated was observed during the reaction via 31P
NMR (Figure S20). Treatment of clusters [FeIIFeIII2LaO-
(OH)] and [FeIIFeIII2CaO(OH)] with 10 equiv of PMe3
resulted in slower reactivity, with full conversion in 130 and
270 h, respectively. As in the case with [FeIIFeIII2ScO(OH)],
generation of OPMe3 was confirmed via 31P NMR as the
reaction proceeded (Figure S21 and S22). Unfortunately,
characterization of the metal-containing products of these
transformations has thus far been unsuccessful. On the basis of
the observed reactivity of the Mn3 analogues [MnIII3MO-
(OH)] and [MnIIMnIII2MO(OH)], it is reasonable to expect
that OAT from complexes [FeIII3MO(OH)] and
[FeIIFeIII2MO(OH)] would also generate mono-oxo com-
plexes as well as reduced FeII species. However, due to the
3
similarity of the characteristic 1H NMR features of these
families of Fe3 compounds, deconvolution of the spectroscopic
data for the products of OAT has not been successful. Similar
to what was observed for Mn3 complexes, OAT reactivity of
Fe3 oxo-hydroxo complexes [FeIII3MO(OH)] and
[FeIIFeIII2MO(OH)] is affected by the Lewis acidity of the
F
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
transferred prior to use. Benzene-d6 was also purchased from
Cambridge Isotope Laboratories, Inc., dried over sodium/benzophe-
none ketyl, and vacuum transferred prior to use. Unless indicated
otherwise, all commercial chemicals were used as received.
L M n 3 ( O A c ) 3 , 5 2 [ M n I V 3 S c O 4 ] , 4 0 [ M n I V 3 G d O 4 ] , 3 9
[MnIIIMnIV2GdO4],39 [MnIII3CaO(OH)], [MnIIMnIII2YO(OH)],
[MnIIMnIII2CaO(OH)],43 [FeIII3LaO(OH)], [FeIII3CaO(OH)],
[FeIIFeIII2ScO(OH)], [FeIIFeIII2LaO(OH)], and [FeIIFeIII2CaO-
(OH)]42 were prepared according to previously published protocols.
1H and 31P NMR spectra were recorded on a Varian Mercury 300
spectrometer at room temperature. Elemental analyses were
performed by Robertson Microlit Laboratories, Ledgewood, NJ.
4.2. Synthesis of [MnIII3ScO3]. In an inert atmosphere glovebox,
[MnIV3ScO4] (13.5 mg, 0.0103 mmol, 1 equiv) was initially dissolved
in C6D6 in a J. Young NMR tube. To the wall of the J. Young tube was
added PMe3 (11 μL, 0.103 mmol, 10 equiv) quickly via a Hamilton
syringe. The J. Young tube was sealed and brought outside the
glovebox. The reaction mixture was heated to 50 °C using an oil bath.
The reaction was monitored over time by 1H NMR spectroscopy and
was complete after 30 h. The solvent and excess unreacted PMe3 were
removed under vacuum. The residue was washed with Et2O to
remove trace PMe3. The residue was resuspended in benzene and
then filtered. Crystals were obtained via vapor diffusion of Et2O into a
concentrated benzene solution to give the clean product in 56% yield.
1H NMR (C6D6, 300 MHz): δ 17.62 (v br), 10.69 (v br), 9.44 (br
overlapped), 9.05 (v br overlapped), 4.57 (br), 3.53 (br), 2.34 (br),
1.84 (br), 1.63 (br), 1.35 (m overlapped), −18.98 (v br) ppm. Anal.
Calcd for C63H48Mn3N6O12Sc ([MnIII3ScO3]) (%): C, 58.62; H,
3.75; N, 6.51. Found: C, 57.88; H, 3.70; N, 6.65.
The observed qualitative rate of OAT correlates not only
with the pKa of the redox-inactive metal incorporated in the
cluster but also with the reported one-electron reduction
potentials for the three series of clusters. Fe3 oxo-hydroxo
clusters display overall intermediate potentials in the three
series in the present study; the reduction potentials of Fe oxo-
hydroxo complexes are typically more positive than those of
cubane clusters and more negative than those of Mn oxo-
hydroxo complexes (Tables 1 and 2).40,42,43 In agreement with
the electrochemical data, [Fe3MO(OH)] complexes display
slower reactivity than more oxidizing [Mn3MO(OH)]
complexes and in turn undergo faster OAT than the less
oxidizing cubane clusters.
The exact mechanism of phosphine oxidation involving
electron transfer or concerted O atom transfer53 cannot be
conclusively assigned in all series of clusters based on the
available data. The faster OAT observed with clusters
displaying more Lewis acidic metals, M, could be a
consequence of a more electrophilic oxo motif, or an oxidant
that is more prone to undergo single electron transfer. As
indicated by computational studies, oxidation of phosphine by
cubane clusters [MnIIIMnIV2MO4] proceeds via concerted
oxygen-atom transfer rather than via electron transfer. This
assignment is consistent with the observed slower rate of OAT
to the more sterically hindered PEt3 (vs the less bulky PMe3
substrate). In the case of oxo-hydroxo clusters [Mn3MO-
(OH)] and [Fe3MO(OH)], however, a stepwise mechanism
involving reduction of the cluster species by the PR3 substrate
(i.e., an electron-transfer process) cannot be ruled out.
4.3. Synthesis of [MnIII3YO(OH)]2. To a suspension of
LMn3(OAc)3 (150 mg, 0.125 mmol, 1 equiv) in DME was added
Y(OTf)3 (67 mg, 0.125 mmol, 1 equiv). The mixture was stirred for 5
min then PhIO was added (58 mg, 0.263 mmol, 2.1 equiv). The
mixture was stirred for 1 h then filtered. The purple filtrate was
collected, and the solvent was removed under vacuum. The DME
fraction was recrystallized in CH3CN/Et2O to give clean product as
3. CONCLUSIONS
The Lewis acidity of redox inactive metal centers incorporated
in heterometallic Mn3 and Fe3 clusters was found to have a
substantial effect on the rate of oxygen atom transfer to
phosphine substrates. An important feature of these systems is
the stoichiometric incorporation a redox-inactive metal center.
Moreover, these precursors can be isolated and structurally
characterized. Qualitatively, within the same structural motif
and redox state, the rate of OAT of cubane [Mn3MO4] and
oxo-hydroxo [Mn3MO(OH)] and [Fe3MO(OH)] clusters
correlates with the Lewis acidity of the apical metal, similar to
previously observed one-electron reduction potentials of the
same series of clusters. OAT rates can be accelerated by
between one and 3 orders of magnitude, depending on the
nature of the cluster. The more Lewis acidic metals are
proposed to increase the electrophilicity of the oxo moieties,
promoting OAT. Higher oxidation state clusters further
exacerbate this effect, except in the case of all-MnIV cubane
clusters, which display slow ligand dissociation and, con-
sequently, slower OAT. Overall, this study demonstrates that
the OAT reactivity of the clusters can be tuned significantly by
choice of the Lewis acid.
1
precipitate in 71% yield. H NMR (CD2Cl2, 300 MHz): δ 77.49 (v
br), 67.83 (m overlapped), 52.93 (m br overlapped), 41.12 (v br),
38.51 (v br), 35.34 (v br), 25.17 (m br overlapped), 17.40 (br
overlapped), 8.3 (m br overlapped),−21.81 (m br overlapped),
−24.47 (d br overlapped), −28.05 (m br overlapped), −33.46 (m br
overlapped) ppm. Anal. Calcd for C132H96F18Mn6N12O40S6Y2
[MnIII3YO(OH)]2 (%): C, 44.89; H, 2.74; N, 4.76. Found: C,
44.89; H, 2.94; N, 4.57.
4.4. Protocol for Reactivity Studies of Metal-Oxo Clusters
with Phosphine Substrate. In an inert-atmosphere glovebox, a J.
Young NMR tube was charged with a solution (∼12−15 mM) of the
desired cluster complex in an appropriate NMR solvent (CD2Cl2,
CD3CN, or C6D6). Phosphine (PPh3, PEt3, or PMe3; 2−10 equiv)
was added neat via gas-tight microsyringe or as a stock solution in
1
deuterated solvent. Reaction progress was monitored via H and 31P
NMR as appropriate until full (>95%) conversion was observed by
disappearance of signal corresponding to starting material. Samples
requiring heating were kept at elevated temperature in mineral oil
baths. When phosphine oxide products were not observed during
reaction via 31P NMR, samples were returned to a glovebox, where
volatiles were removed in vacuo. On the benchtop, the resulting
residue was dissolved in dichloromethane, filtered through a short
silica plug, and further eluted with methanol. The resulting solution
was analyzed by GC-MS to confirm the presence of phosphine oxide
product.
4. EXPERIMENTAL SECTION
4.5. EPR Spectroscopy. Perpendicular-mode continuous-wave
(CW) X-Band (9.33−9.37 GHz) spectra were collected using a
Bruker E500 spectrometer with a superhigh Q resonator (SHQE).
Parallel-mode CW X-Band spectra (9.3 GHz) were collected using a
dual-mode cavity (ER 4116DM). All EPR spectra were collected
under nonsaturating conditions. Temperature control was maintained
with an Oxford Instruments model ESR900 helium-flow cryostat with
an Oxford ITC 503 temperature controller and an Oxford
Instruments CF935 helium-flow cryostat.
4.1. General Considerations. Unless otherwise specified, all
compounds were manipulated using a glovebox or standard Schlenk
line techniques with an N2 atmosphere. Anhydrous tetrahydrofuran
(THF) was purchased from Aldrich in 18 L Pure-Pac containers.
Anhydrous benzene, dichloromethane, and diethyl ether were purified
by sparging with nitrogen for 15 min and then passing under nitrogen
pressure through a column of activated A2 alumina (Zapp’s).
Methylene chloride-d2 and acetonitrile-d3 were purchased from
Cambridge Isotopes, dried over calcium hydride, and vacuum
G
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
4.6. Computational Protocol. Density functional theory (DFT)
Dreyfus fellow. We thank Dr. Michael K. Takase and Lawrence
M. Henling for assistance with X-ray crystallography. D.L.
gratefully acknowledges a graduate fellowship from the Resnick
Sustainability Institute at Caltech. K.C. and W.A.G. received
support from the NSF (CBET-1512759). The Bruker KAPPA
APEXII X-ray diffractometer was purchased via an NSF
Chemistry Research Instrumentation award to Caltech (CHE-
0639094). Computational resources included the Extreme
Science and Engineering Discovery Environment, which is
supported by National Science Foundation Grant ACI-
1548562.
calculations were carried out on complexes [LMnIII
2
MnIVCaO2(OTf)(DME)[OTf]2 (oxCa′), [LMnIII3CaO (OH)(OTf)-
(DME)][OTf]2 (oxCa′′), [LMnIII3O2Ca(OTf) (DME)[OTf]
(redCa′), and [LMnIIMnIII2CaO(OH) (OTf)(DME)][OTf]
(redCa′′ and redCa′′-noH) to elucidate the structure of complexes
[MnIII3CaO(OH)] and [MnIIMnIII2CaO(OH)]. Complexes oxCa′
and redCa′ were optimized using the solid-state structure without
outer-sphere solvent molecules as the initial geometry input whereas
complexes oxCa′′ and redCa′′ were modified by placing one triflate
in the vicinity (∼3.0 Å) of the bridging hydroxo motif for the initial
geometry input. To decrease computational expenses, a “truncated
ligand” approximation was employed in which the three carbon atoms
connecting the bipyridine motifs were spatially constrained, and the
remaining ligand scaffold connecting each carbon atom to the phenyl
backbone was substituted with three protons. This approximation is
expected not to influence bond parameters for the Mn3O2Ca core.
Schrodinger computational products Jaguar 8.454 and Maestro
were employed respectively to perform gas-phase geometry
optimization and measure the bond lengths and angles in the
optimized complexes. Calculations were accomplished with the basis
set/functional pairing of unrestricted dispersion-corrected B3LYP-
d355−58 and basis set lacvp** for nonmetals C, H, N, O, S, and F
along with basis set lacv3p** for Mn.59,60 Successful ground-state
optimizations were assessed by the forces on the all atoms residing
below 10−5 hartrees/bohr as well as total energy remaining unchanged
by 10−4 between the final SCF iterations. Oxidation states of clusters
oxCa′, oxCa′′, redCa′, and redCa′′ were tabulated through Mullikan
spin populations and inspection of the spin-density plots. For
structure redCa′′, wave functions with (redCa′′) and without
(redCa′′-noH) hydrogen bonding between triflate and the hydroxide
motif were isolated.
REFERENCES
■
(1) Fukuzumi, S. Prog. Inorg. Chem. 2009, 56, 49−154.
(2) Fukuzumi, S.; Ohkubo, K. Metal ion-coupled and decoupled
electron transfer. Coord. Chem. Rev. 2010, 254, 372−385.
(3) Tsui, E. Y.; Kanady, J. S.; Agapie, T. Synthetic Cluster Models of
Biological and Heterogeneous Manganese Catalysts for O-2
Evolution. Inorg. Chem. 2013, 52, 13833−13848.
(4) Umena, Y.; Kawakami, K.; Shen, J. R.; Kamiya, N. Crystal
structure of oxygen-evolving photosystem II at a resolution of 1 9
angstrom. Nature 2011, 473, 55−U65.
(5) Yano, J.; Yachandra, V. Mn4Ca Cluster in Photosynthesis: Where
and How Water is Oxidized to Dioxygen. Chem. Rev. 2014, 114,
4175−4205.
(6) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.;
Goodenough, J. B.; Shao-Horn, Y. Design principles for oxygen-
reduction activity on perovskite oxide catalysts for fuel cells and
metal-air batteries (vol 3, pg 546, 2011). Nat. Chem. 2011, 3, 647−
647.
(7) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.;
Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution
Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383−
1385.
(8) Zaharieva, I.; Najafpour, M. M.; Wiechen, M.; Haumann, M.;
Kurz, P.; Dau, H. Synthetic manganese-calcium oxides mimic the
water-oxidizing complex of photosynthesis functionally and structur-
ally. Energy Environ. Sci. 2011, 4, 2400−2408.
(9) Lau, T. C.; Mak, C. K. Oxidation of Alkanes by Barium
Ruthenate in Acetic-Acid - Catalysis by Lewis-Acids. J. Chem. Soc.,
Chem. Commun. 1993, 766−767.
(10) Lau, T. C.; Wu, Z. B.; Bai, Z. L.; Mak, C. K. Lewis-Acid
Catalyzed Oxidation of Alkanes by Chromate and Permanganate. J.
Chem. Soc., Dalton Trans. 1995, 695−696.
(11) Ho, C. M.; Lau, T. C. Lewis acid activated oxidation of alkanes
by barium ferrate. New J. Chem. 2000, 24, 587−590.
(12) Lai, S.; Lee, D. G. Lewis acid assisted permanganate oxidations.
Tetrahedron 2002, 58, 9879−9887.
(13) Yiu, S. M.; Wu, Z. B.; Mak, C. K.; Lau, T. C. FeCl3-activated
oxidation of alkanes by [Os(N)O3] (−). J. Am. Chem. Soc. 2004, 126,
14921−14929.
(14) Yiu, S. M.; Man, W. L.; Lau, T. C. Efficient catalytic oxidation
of alkanes by lewis acid/[Os-VI(N)CI4](−) using peroxides as
terminal oxidants. Evidence for a metal-based active intermediate. J.
Am. Chem. Soc. 2008, 130, 10821−10827.
(15) Du, H. X.; Lo, P. K.; Hu, Z. M.; Liang, H. J.; Lau, K. C.; Wang,
Y. N.; Lam, W. W. Y.; Lau, T. C. Lewis acid-activated oxidation of
alcohols by permanganate. Chem. Commun. 2011, 47, 7143−7145.
(16) Miller, C. G.; Gordon-Wylie, S. W.; Horwitz, C. P.; Strazisar, S.
A.; Peraino, D. K.; Clark, G. R.; Weintraub, S. T.; Collins, T. J. A
method for driving O-atom transfer: Secondary ion binding to a
tetraamide macrocyclic ligand. J. Am. Chem. Soc. 1998, 120, 11540−
11541.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02701.
■
S
Experimental procedures, NMR spectra, computational
details, EPR spectra, and X-ray crystallographic data
(PDF)
Accession Codes
CCDC 1035166 and 1035222 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: agapie@caltech.edu.
ORCID
Davide Lionetti: 0000-0002-4937-886X
Kurtis M. Carsch: 0000-0003-4432-7518
William A. Goddard, III: 0000-0003-0097-5716
R. David Britt: 0000-0003-0889-8436
Theodor Agapie: 0000-0002-9692-7614
■
Notes
The authors declare no competing financial interest.
(17) Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.
M.; Nam, W. Crystal structure of a metal ion-bound oxoiron(IV)
complex and implications for biological electron transfer. Nat. Chem.
2010, 2, 756−759.
ACKNOWLEDGMENTS
This work was supported by the California Institute of
Technology and the NIH R01 GM102687A (T.A.). T.A. is a
■
H
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(18) Morimoto, Y.; Kotani, H.; Park, J.; Lee, Y. M.; Nam, W.;
Fukuzumi, S. Metal Ion-Coupled Electron, Transfer of a Nonheme
Oxoiron(IV) Complex: Remarkable Enhancement of Electron-
Transfer Rates by Sc3+. J. Am. Chem. Soc. 2011, 133, 403−405.
(19) Park, J.; Morimoto, Y.; Lee, Y. M.; Nam, W.; Fukuzumi, S.
Metal Ion Effect on the Switch of Mechanism from Direct Oxygen
Transfer to Metal Ion-Coupled Electron Transfer in the Sulfoxidation
of Thioanisoles by a Non-Heme Iron(IV)-Oxo Complex. J. Am. Chem.
Soc. 2011, 133, 5236−5239.
(20) Lee, Y. M.; Bang, S.; Kim, Y. M.; Cho, J.; Hong, S.; Nomura,
T.; Ogura, T.; Troeppner, O.; Ivanovic-Burmazovic, I.; Sarangi, R.;
Fukuzumi, S.; Nam, W. A mononuclear nonheme iron(III)-peroxo
complex binding redox-inactive metal ions. Chem. Sci. 2013, 4, 3917−
3923.
of Cyclohexane and Benzene. J. Am. Chem. Soc. 2018, 140, 5798−
5804.
(36) Prakash, J.; Rohde, G. T.; Meier, K. K.; Jasniewski, A. J.; Van
Heuvelen, K. M.; Munck, E.; Que, L. Spectroscopic Identification of
an Fe-III Center, not Fe-IV, in the Crystalline Sc-O-Fe Adduct
Derived from [Fe-IV(O)(TMC)](2+). J. Am. Chem. Soc. 2015, 137,
3478−3481.
(37) Swart, M. A change in the oxidation state of iron: scandium is
not innocent. Chem. Commun. 2013, 49, 6650−6652.
(38) Prakash, J.; Rohde, G. T.; Meier, K. K.; Jasniewski, A. J.; Van
Heuvelen, K. M.; Munck, E.; Que, L. Spectroscopic Identification of
an FeIII Center, not FeIV, in the Crystalline Sc-O-Fe Adduct Derived
from [Fe(IV)(O)(TMC)]2+. J. Am. Chem. Soc. 2015, 137, 3478−
3481.
(21) Yoon, H.; Lee, Y. M.; Wu, X.; Cho, K. B.; Sarangi, R.; Nam, W.;
Fukuzumi, S. Enhanced Electron-Transfer Reactivity of Nonheme
Manganese(IV)-Oxo Complexes by Binding Scandium Ions. J. Am.
Chem. Soc. 2013, 135, 9186−9194.
(22) Nam, W.; Lee, Y. M.; Fukuzumi, S. Tuning Reactivity and
Mechanism in Oxidation Reactions by Mononuclear Nonheme
Iron(IV)-Oxo Complexes. Acc. Chem. Res. 2014, 47, 1146−1154.
(23) Park, J.; Morimoto, Y.; Lee, Y. M.; Nam, W.; Fukuzumi, S.
Unified View of Oxidative C-H Bond Cleavage and Sulfoxidation by a
Nonheme Iron(IV)-Oxo Complex via Lewis Acid-Promoted Electron
Transfer. Inorg. Chem. 2014, 53, 3618−3628.
(24) Hong, S.; Pfaff, F. F.; Kwon, E.; Wang, Y.; Seo, M. S.; Bill, E.;
Ray, K.; Nam, W. Spectroscopic Capture and Reactivity of a Low-Spin
Cobalt(IV)-Oxo Complex Stabilized by Binding Redox-Inactive Metal
Ions. Angew. Chem., Int. Ed. 2014, 53, 10403−10407.
(25) Hong, S.; Lee, Y. M.; Sankaralingam, M.; Vardhaman, A. K.;
Park, Y. J.; Cho, K. B.; Ogura, T.; Sarangi, R.; Fukuzumi, S.; Nam, W.
A Manganese(V)-Oxo Complex: Synthesis by Dioxygen Activation
and Enhancement of Its Oxidizing Power by Binding Scandium Ion. J.
Am. Chem. Soc. 2016, 138, 8523−8532.
(26) Bang, S.; Lee, Y. M.; Hong, S.; Cho, K. B.; Nishida, Y.; Seo, M.
S.; Sarangi, R.; Fukuzumi, S.; Nam, W. Redox-inactive metal ions
modulate the reactivity and oxygen release of mononuclear non-haem
iron(III)-peroxo complexes. Nat. Chem. 2014, 6, 934−940.
(27) Lionetti, D.; Agapie, T. How calcium affects oxygen formation.
Nature 2014, 513, 495−496.
(28) Dong, L.; Wang, Y. J.; Lv, Y. Z.; Chen, Z. Q.; Mei, F. M.; Xiong,
H.; Yin, G. C. Lewis-Acid-Promoted Stoichiometric and Catalytic
Oxidations by Manganese Complexes Having Cross-Bridged Cyclam
Ligand: A Comprehensive Study. Inorg. Chem. 2013, 52, 5418−5427.
(29) Zhang, Z.; Coats, K. L.; Chen, Z.; Hubin, T. J.; Yin, G.
Influence of Calcium(II) and Chloride on the Oxidative Reactivity of
a Manganese(II) Complex of a Cross-Bridged Cyclen Ligand. Inorg.
Chem. 2014, 53, 11937−11947.
(30) Chen, Z.; Yang, L.; Choe, C.; Lv, Z.; Yin, G. Non-redox metal
ion promoted oxygen transfer by a non-heme manganese catalyst.
Chem. Commun. 2015, 51, 1874−1877.
(31) Zhang, J.; Wang, Y.; Luo, N.; Chen, Z.; Wu, K.; Yin, G. Redox
inactive metal ion triggered N-dealkylation by an iron catalyst with
dioxygen activation: a lesson from lipoxygenases. Dalton Trans. 2015,
44, 9847−9859.
(32) Zhang, J.; Wei, W.-J.; Lu, X.; Yang, H.; Chen, Z.; Liao, R.-Z.;
Yin, G. Nonredox Metal Ions Promoted Olefin Epoxidation by
Iron(II) Complexes with H2O2: DFT Calculations Reveal Multiple
Channels for Oxygen Transfer. Inorg. Chem. 2017, 56, 15138−15149.
(33) Zhang, J.; Yang, H.; Sun, T.; Chen, Z.; Yin, G. Nonredox
Metal-Ions-Enhanced Dioxygen Activation by Oxidovanadium(IV)
Complexes toward Hydrogen Atom Abstraction. Inorg. Chem. 2017,
56, 834−844.
(34) Li, F. F.; Van Heuvelen, K. M.; Meier, K. K.; Munck, E.; Que,
L. Sc3+-Triggered Oxoiron(IV) Formation from O-2 and its Non-
Heme Iron(II) Precursor via a Sc3+-Peroxo-Fe3+ Intermediate. J. Am.
Chem. Soc. 2013, 135, 10198−10201.
(35) Kal, S.; Draksharapu, A.; Que, L. Sc3+ (or HClO4) Activation
of a Nonheme FeIII−OOH Intermediate for the Rapid Hydroxylation
(39) Lin, P. H.; Takase, M. K.; Agapie, T. Investigations of the Effect
of the Non-Manganese Metal in Heterometallic-Oxido Cluster
Models of the Oxygen Evolving Complex of Photosystem II:
Lanthanides as Substitutes for Calcium. Inorg. Chem. 2015, 54, 59−
64.
(40) Tsui, E. Y.; Agapie, T. Reduction potentials of heterometallic
manganese-oxido cubane complexes modulated by redox-inactive
metals. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 10084−10088.
(41) Kanady, J. S.; Tsui, E. Y.; Day, M. W.; Agapie, T. A Synthetic
Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in
Photosystem II. Science 2011, 333, 733−736.
(42) Herbert, D. E.; Lionetti, D.; Rittle, J.; Agapie, T. Heterometallic
Triiron-Oxo/Hydroxo Clusters: Effect of Redox-Inactive Metals. J.
Am. Chem. Soc. 2013, 135, 19075−19078.
(43) Tsui, E. Y.; Tran, R.; Yano, J.; Agapie, T. Redox-inactive metals
modulate the reduction potential in heterometallic manganese-oxido
clusters. Nat. Chem. 2013, 5, 293−299.
(44) Chantarojsiri, T.; Ziller, J. W.; Yang, J. Y. Incorporation of
redox-inactive cations promotes iron catalyzed aerobic C-H oxidation
at mild potentials. Chem. Sci. 2018, 9, 2567−2574.
(45) Reath, A. H.; Ziller, J. W.; Tsay, C.; Ryan, A. J.; Yang, J. Y.
Redox Potential and Electronic Structure Effects of Proximal
Nonredox Active Cations in Cobalt Schiff Base Complexes. Inorg.
Chem. 2017, 56, 3713−3718.
(46) Duan, P.-C.; Manz, D.-H.; Dechert, S.; Demeshko, S.; Meyer, F.
Reductive O2 Binding at a Dihydride Complex Leading to Redox
Interconvertible μ-1,2-Peroxo and μ-1,2-Superoxo Dinickel(II)
Intermediates. J. Am. Chem. Soc. 2018, 140, 4929−4939.
(47) Kumar, A.; Lionetti, D.; Day, V. W.; Blakemore, J. D. Trivalent
Lewis Acidic Cations Govern the Electronic Properties and Stability
of Heterobimetallic Complexes of Nickel. Chem. - Eur. J. 2018, 24,
141−149.
(48) Chantarojsiri, T.; Reath, A. H.; Yang, J. Cationic Charges Lead
to Inverse Free Energy Relationship for NN Bond Formation by
Mn(VI) Nitrides. Angew. Chem., Int. Ed. 2018, 57, 14037.
́
(49) Evangelisti, F.; More, R.; Hodel, F.; Luber, S.; Patzke, G. R.
3d−4f {CoII3Ln(OR)4} Cubanes as Bio-Inspired Water Oxidation
Catalysts. J. Am. Chem. Soc. 2015, 137, 11076−11084.
(50) Kanady, J. S.; Mendoza-Cortes, J. L.; Tsui, E. Y.; Nielsen, R. J.;
Goddard, W. A.; Agapie, T. Oxygen Atom Transfer and Oxidative
Water Incorporation in Cuboidal Mn3MOn, Complexes Based on
Synthetic, Isotopic Labeling and Computational Studies. J. Am. Chem.
Soc. 2013, 135, 1073−1082.
(51) Perrin, D. D. International Union of Pure and Applied Chemistry,
Commission on Equilibrium Data: Ionisation Constants of Inorganic
Acids and Bases in Aqueous Solution, 2nd ed.; Pergamon Press: Oxford
Oxfordshire ; New York, 1982; p xiii.
(52) Tsui, E. Y.; Kanady, J. S.; Day, M. W.; Agapie, T. Trinuclear
first row transition metal complexes of a hexapyridyl, trialkoxy 1,3,5-
triarylbenzene ligand. Chem. Commun. 2011, 47, 4189−4191.
(53) Xu, A.; Xiong, H.; Yin, G. Distinct Oxygenation Difference
between Manganese(IV) Hydroxo and Oxo Moieties: Electron
Transfer versus Concerted Oxygen Transfer. Chem. - Eur. J. 2009,
15, 11478−11481.
I
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(54) Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J.
R.; Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.;
Friesner, R. A. Jaguar: A high-performance quantum chemistry
software program with strengths in life and materials sciences. Int. J.
Quantum Chem. 2013, 113, 2110−2142.
(55) Becke, A. D. Density-functional thermochemistry. III. The role
of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(56) Becke, A. D. Density-functional exchange-energy approxima-
tion with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt.
Phys. 1988, 38, 3098−3100.
(57) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle-
Salvetti Correlation-Energy Formula into a Functional of the
Electron-Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988,
37, 785−789.
(58) Grimme, S. Semiempirical hybrid density functional with
perturbative second-order correlation. J. Chem. Phys. 2006, 124,
No. 034108.
(59) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.;
Gordon, M. S.; DeFrees, D. J.; Pople, J. A. Self-consistent molecular
orbital methods. XXIII. A polarization-type basis set for second-row
elements. J. Chem. Phys. 1982, 77, 3654−3665.
(60) Hehre, W. J.; Ditchfield, R.; Pople, J. A. SelfConsistent
Molecular Orbital Methods. XII. Further Extensions of Gaussian
Type Basis Sets for Use in Molecular Orbital Studies of Organic
Molecules. J. Chem. Phys. 1972, 56, 2257−2261.
J
DOI: 10.1021/acs.inorgchem.8b02701
Inorg. Chem. XXXX, XXX, XXX−XXX