of 25
Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer
Reactivity of Heterometallic Mn
3
MO
4
Cubane and Fe
3
MO(OH) and
Mn
3
MO(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, California Institute of Technology, 1200 East
California Boulevard, MC 127-72, Pasadena, California 91125, United States
Department of Chemistry, University of California, Davis, California 95616, United States
§
Materials and Process Simulation Center, California Institute of Technology, Pasadena, California
91125, United States
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 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
Mn
3
and Fe
3
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
[Mn
3
MO
4
], [Mn
3
MO(OH)], and [Fe
3
MO(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.
*
Corresponding Author
agapie@caltech.edu.
Present Addresses
If an author’s address is different than the one given in the affiliation line, this information may be included here.
Author Contributions
All authors have given approval to the final version of the manuscript.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website. This material is available free of charge via
the Internet at
http://pubs.acs.org
. Crystal data have been deposited with the Cambridge Crystallographic Database as CCDC numbers
1035166 and 1035222.
Experimental procedures, NMR spectra, computational details, EPR spectra, and X-Ray crystallographic data (PDF).
Cartesian coordinates (XYZ)
The authors declare no competing financial interests.
HHS Public Access
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. Author manuscript; available in PMC 2019 June 17.
Published in final edited form as:
Inorg Chem
. 2019 February 18; 58(4): 2336–2345. doi:10.1021/acs.inorgchem.8b02701.
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Graphical-abstract
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1. INTRODUCTION
Lewis acidic, redox inactive metal ions are known to influence electron- and group-transfer
processes in both natural and synthetic systems.
1
3
In biology, the preeminent example is
that of the redox inactive Ca
2+
center found in the oxygen evolving complex (OEC) of
Photosystem II (PSII), a Mn
4
CaO
x
cluster responsible for water oxidation to dioxygen in
plants, algae, and cyanobacteria. Although its role has not been fully elucidated, Ca
2+
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. MnO
4
, Cr
2
O
7
2–
) 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 Mn
V
–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
O
2
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 under
catalytic conditions, have been described.
28
33
Sc
3+
has been reported to affect the
formation and reactivity of Fe(III)-(hydro)peroxo species.
34
,
35
While the observation of
substantial effects of redox inactive metals on oxygen related redox chemistry is well
demonstrated, structural characterization 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 crystallographically characterized high-
valent metal oxo/redox inactive metal ion adduct, a [Fe(IV)–O–Sc
3+
] complex,
17
has
recently been reassigned as a [Fe(III)–(OH)–Sc
3+
] species of spectroscopic characterization
and structural comparisons with other Fe(III) and Fe(IV) complexes.
37
,
38
Therefore,
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conclusions drawn from the majority of these studies must account for both the effect of
Lewis acid binding to metal–oxo species and for the dynamic equilibrium processes
responsible for these interactions.
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 processes
44
48
and on photocatalytic water oxidation
49
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 Mn
3
or
Fe
3
core, as well as a fourth, redox inactive metal (M). Tetraoxo [Mn
3
MO
4
] cubane clusters,
as well as oxo-hydroxo [Mn
3
MO(OH)] (previously assigned as [Mn
3
MO
2
] dioxo species,
vide infra
) and [Fe
3
MO(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 comparisons 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 series of complexes: [Mn
3
MO
4
] cubanes and [Mn
3
MO(OH)],
and [Fe
3
MO(OH)] oxo-hydroxo complexes. The reassignment of Mn
3
oxo-hydroxo clusters,
previously identified as the corresponding [Mn
3
MO
2
] dioxo complexes, on the basis of
spectroscopic and computational data is also discussed.
2. RESULTS AND DISCUSSION
2.1 OAT Reactivity of [Mn
3
MO
4
] Cubanes (M = Ca
2+
, Gd
3+
, Sc
3+
, Mn
3+
).
The OAT reactivity of
[Mn
IV
3
CaO
4
]
,
[Mn
IV
3
ScO
4
]
, and
[Mn
III
2
Mn
IV
2
O
4
]
(Figure 1) with
trialkylphosphines has previously been described.
50
Reaction of
[Mn
III
2
Mn
IV
2
O
4
]
with
trimethylphosphine (PMe
3
, 2 equiv) led to rapid generation of trioxo cluster
[Mn
III
4
O
3
]
, in
which the bottom oxide ligand is absent (Scheme 2). Cubane complex
[Mn
IV
3
CaO
4
]
was
instead found to be unreactive towards PMe
3
. This observation is consistent with the greater
oxidizing power of
[Mn
III
2
Mn
IV
2
O
4
]
as indicated by its more positive one-electron
reduction potential (
E
½
= –0.70 vs. –0.94 V vs. Fc
+/0
for
[Mn
IV
3
CaO
4
]
). In contrast with
this model,
[Mn
IV
3
ScO
4
]
displayed only slow reactivity with PMe
3
, generating an
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intractable mixture of metal-containing products, despite being a stronger oxidant than
[Mn
III
2
Mn
IV
2
O
4
]
(
E
½
= –0.24 vs. –0.70 V vs. Fc
+/0
, respectively; see Table 1).
These seemingly contradictory observations can be rationalized by considering the
previously studied mechanism of transfer of an oxygen atom from the cubane cluster to
PMe
3
. According to earlier computations,
50
OAT involving cubane clusters
[Mn
IV
3
MO
4
]
and
[Mn
III
Mn
IV
2
MO
4
]
proceeds via initial dissociation of a bridging acetate ligand from
one of the three core Mn centers. The Mn
III
center (high spin, d
4
) in complexes
[Mn
III
Mn
IV
2
MO
4
]
is more substitutionally labile due to electrons occupying a Mn–O
σ
antibonding orbital. Thus, metal–carboxylate interactions in complexes
[Mn
III
Mn
IV
2
MO
4
]
are weaker than in the case of
[Mn
IV
3
MO
4
]
clusters, in which all Mn centers are in the
Mn
IV
oxidation state (d
3
). The difference in Mn oxidation states between
[Mn
IV
3
MO
4
]
and
[Mn
III
Mn
IV
2
MO
4
]
affects the kinetics of ligand dissociation, and only indirectly the OAT
process, resulting in slower reactivity of
[Mn
IV
3
ScO
4
]
despite a larger driving force for
electron transfer.
Based on this interpretation, it was thus postulated that one-electron reduction of other
[Mn
IV
3
MO
4
]
clusters, resulting in generation of a kinetically labile Mn
III
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
[Mn
IV
3
MO
4
]
complexes was achieved using cobaltocene (CoCp
2
) or
decamethylferrocene (FeCp*
2
) as reductant. Reduction of
[Mn
IV
3
CaO
4
]
with CoCp
2
generated a highly unstable material, which could not be isolated cleanly, and was therefore
not investigated further. Reduction of
[Mn
IV
3
ScO
4
]
and
[Mn
IV
3
GdO
4
]
, on the other hand,
successfully yielded the corresponding
[Mn
III
Mn
IV
2
ScO
4
]
and
[Mn
III
Mn
IV
2
GdO
4
]
clusters as reported previously.
39
,
40
Treatment of
[Mn
III
Mn
IV
2
ScO
4
],
with PMe
3
(10
equiv) results in clean conversion to a new species over 1.5 weeks at room temperature as
determined by proton nuclear magnetic resonance (
1
H 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
[Mn
III
Mn
IV
2
ScO
4
]
in the absence of
PMe
3
led to negligible (<10%) conversion. The product of reaction of
[Mn
III
Mn
IV
2
ScO
4
]
with PMe
3
was characterized by single-crystal X-ray diffraction (XRD) as the trioxo
complex
[Mn
III
3
ScO
3
]
(Figure 2), analogously to the previously reported reaction of
[Mn
III
2
Mn
IV
2
O
4
]
to generate
[Mn
III
4
O
3
]
.
50
The average Mn–oxo and Mn–O(acetate)
distances in
[Mn
III
3
ScO
3
]
are 1.88 and 2.10 Å, respectively, consistent with the assignment
of the Mn
3
core as [Mn
III
]
3
. No signal for trimethylphophine oxide (OPMe
3
), the expected
byproduct OAT, was detected by
31
P NMR spectroscopy during the reaction. The OPMe
3
generated in the reaction is thus proposed to be bound to the nascent
[Mn
III
3
ScO
3
]
, at least
in solution under the reaction conditions. Formation of OPMe
3
was however confirmed at
the end of the reaction via gas chromatography mass-spectrometry (GC-MS) following
workup (see Experimental Section).
Treatment of
[Mn
III
Mn
IV
2
GdO
4
]
, containing a significantly less Lewis acidic Gd
3+
center
(p
K
a
= 8.4 for Gd
3+
vs. 4.8 for Sc
3+
)with PMe
3
(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
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absence of PMe
3
did not lead to detectable conversion). Although crystallographic
characterization of this product was unsuccessful, this species was assigned as the trioxo
cluster
[Mn
III
3
GdO
3
]
on the basis of the similarity of its
1
H NMR features with those for
structurally characterized
[Mn
III
3
ScO
3
]
(Figure S8). As a signal for OPMe
3
was detected
during the reaction by
31
P NMR, the reaction was monitored by NMR spectroscopy upon
treating
[Mn
III
Mn
IV
2
ScO
4
]
with a smaller excess of PMe
3
(3 equiv) at 50 °C to improve
accuracy of integration of NMR signal. This experiment was performed in the presence of
PPh
3
(2 equiv) as an internal standard. A total of 1 equiv of PMe
3
was consumed throughout
the reaction (67 h), and formation of the same metal species was observed (
1
H NMR, Figure
S9). The reaction stoichiometry (1:1
[Mn
III
Mn
IV
2
GdO
4
]
/PMe
3
) is consistent with transfer
of a single O-atom equivalent during the reaction, corroborating the assignment of the metal-
containing product as trioxo species
[Mn
III
3
GdO
3
]
. Notably,
[Mn
III
3
GdO
3
]
is stable even
in the presence of a large excess of PMe
3
(10 equiv.), and formation of reduced clusters with
<3 oxide ligands–the products of further OAT processes–was not observed.
The observed rate of OAT from
[Mn
III
Mn
IV
2
MO
4
]
clusters to phosphine substrates
correlates with the Lewis acidity of M (Table 2, Entries 1–7). To generate these correlations,
we have used the p
K
a
of the M(H
2
O)
n
ion
51
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
[Mn
III
Mn
IV
2
MO
4
]
complexes with PMe
3
as substrate yields the following trend:
[Mn
III
2
Mn
IV
2
O
4
]
>
[Mn
III
Mn
IV
2
ScO
4
]
>
[Mn
III
Mn
IV
2
GdO
4
]
The decrease in OAT rate correlates with the progressively higher p
K
a
(i.e. lower Lewis
acidity) across this series of metals (p
K
a
(M(H
2
O)
n
): Mn
3+
= 0.1, Sc
3+
= 4.7, Gd
3+
= 8.4).
51
Most notably, changing from Mn to Gd results in decrease in the rate of OAT for more than
three orders of magnitude.
2.2 Structural Reassignment of [Mn
3
MO
2
(H)] Clusters (M = Na
+
, Sr
2+
, Ca
2+
, Zn
2+
, Y
3+
).
Comparisons of the structural parameters between initially reported Mn
3
MO
2
clusters and
more recently prepared Fe
3
MO(OH) show strong similarities. Through a combination of
EPR, XRD and computational studies, these Mn clusters were reassigned as Mn
3
MO(OH),
with oxidation state of
[Mn
III
3
MO(OH)]
as [Mn
III
3
] and of
[Mn
II
Mn
III
2
MO(OH)]
as
[Mn
III
2
Mn
II
]. A detailed discussion of these data and their interpretation is presented in SI,
page S3. It should be noted that the changes in assignment of the oxidation states and
identity of bridging ligands in clusters
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(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 compounds across the entire series. The reactivity
studies described next were focused on the available oxo-hydroxo complexes.
2.3 OAT Reactivity of [Mn
3
MO(OH)] Clusters (M = Ca
2+
, Y
3+
).
OAT reactivity of complexes
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(OH)]
was
investigated using phosphines as substrates (Scheme 3; Table 2, entries 8–15). Clusters
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[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(OH)]
displayed faster reactivity with phosphines
than cubane complexes
[Mn
IV
3
MO
4
]
and
[Mn
III
Mn
IV
2
MO
4
]
. Triethylphosphine (PEt
3
)
and triphenylphosphine (PPh
3
) were used, as PMe
3
resulted in reactivity too rapid for
comparisons to be possible by
1
H NMR spectroscopy. Treatment of
[Mn
II
Mn
III
2
CaO(OH)]
with 10 equiv. PEt
3
led to conversion, over 36 h, to a product with very broad
1
H NMR
features (Figure S16), typical of [LMn
II
3
X
3
] (X =
OAc or
OTf) species.
52
This observation
is consistent with loss of the redox inactive metal center following OAT. Similar to the case
of cubane clusters, triethylphosphine oxide (OPEt
3
), the product substrate oxidation, was not
observed spectroscopically during the reaction. Removal of solvents and excess PEt
3
in
vacuo
after completion, followed by methanolysis and analysis by GC-MS allowed for
identification of the OPEt
3
product.
[Mn
II
Mn
III
2
CaO(OH)]
also reacts with the weaker O-
atom acceptor PPh
3
, albeit much more slowly (>2 weeks; Figure S17).
As expected, the more oxidized cluster
[Mn
III
3
CaO(OH)]
reacts more rapidly with both
PEt
3
and PPh
3
; full conversion was achieved in 3 and 20 h, respectively (Figures S12 and
S13). Similar to
[Mn
II
Mn
III
2
CaO(OH)]
, OAT from
[Mn
III
3
CaO(OH)]
results in formation
of reduced Mn
II
3
species whose structural characterization was unsuccessful. Clusters
displaying the more Lewis-acidic Y
3+
ion yield faster reactivity. Reaction of
[Mn
II
Mn
III
2
YO(OH)]
with PEt
3
(10 equiv.) proceeded to completion in 30 min (Figure
S14), whereas reaction with PPh
3
was complete in 1 h (Figure S15). As before, generation
of Mn
II
3
species was suggested by the broad
1
H NMR features observed. Although isolation
of monomeric oxidized complex
[Mn
III
3
YO(OH)]
has not been achieved to date via either
direct synthesis or transmetallation from
[Mn
III
3
CaO(OH)]
, a dimeric form of this
complex,
[Mn
III
3
YO(OH)]
2
, has been prepared from precursor complex
LMn
3
(OAc)
3
(
1
H
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
[Mn
3
YO(OH)] cores are bridged by two
OAc and one
OTf ligands, with five outer-sphere
triflate counteranions.
[Mn
III
3
YO(OH)]
2
reacts rapidly with phosphine substrates: reaction
with PEt
3
(10 equiv./Y) was complete in less than 15 minutes (Figure S10), and even the
more electron deficient PPh
3
led to complete reaction in 30 minutes (Figure S11).
Qualitative analysis of the relative OAT rates for clusters
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(OH)]
reveals the following trend:
[Mn
III
3
YO(OH)]
2
>
[Mn
II
Mn
III
2
YO(OH)]
>
[Mn
III
3
CaO(OH)]
>
[Mn
II
Mn
III
2
CaO(OH)]
As observed for the cubane clusters, the complexes containing more Lewis acidic Y
3+
(p
K
a
= 8.6) undergo faster OAT processes than those containing Ca
2+
(p
K
a
= 12.9), in some case
by more than two 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 [Mn
II
3
] species. It is probable the
first OAT event (presumably on the more sterically accessible μ
2
-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 [Mn
II
3
]
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products is supported by the observation of intermediate species during the reaction of
[Mn
II
Mn
III
2
YO(OH)]
with PPh
3
. After 30 minutes,
1
H NMR features consistent with
formation of a [Mn
3
MO] species were detected (Figure S3),
41
leading to the proposed
generation of tetrametallic monooxo species from
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(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
(
31
P 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
[Mn
III
Mn
IV
2
MO
4
]
),
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
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(OH)]
(or their Fe counterparts,
vide infra
) could
not be conclusively assigned, except in the case of
[Mn
III
Mn
IV
2
ScO
4
]
which reacted to
generate cleanly 1 equiv. of (O)PMe
3
(
31
P NMR).
2.4 OAT Reactivity of [Fe
3
MO(OH)] Clusters (M = Ca
2+
, La
3+
, Sc
3+
).
Studies of the OAT reactivity of clusters
[Fe
III
3
MO(OH)]
and
[Fe
II
Fe
III
2
MO(OH)]
,
isostructural to Mn-containing complexes
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(OH)]
,
respectively, were carried out with phosphine substrates in an analogous manner to those of
cubane and oxo-hydroxo Mn
3
clusters (Scheme 4; Table 2, Entries 16–20). Treatment of
clusters
[Fe
III
3
MO(OH)]
, containing three Fe
III
centers, with electron rich phosphines such
as PMe
3
yielded complex mixtures. Therefore, OAT behavior of
[Fe
III
3
MO(OH)]
complexes was investigated with the more electron deficient substrate PPh
3
.
[Fe
III
3
CaO(OH)]
displays minimal conversion in the presence of PPh
3
(10 equiv., Figure
S19). Conversely,
[Fe
III
3
LaO(OH)]
reacts completely with PPh
3
(10 equiv.) over 14 hours,
generating OPPh
3
as determined via
31
P NMR (Figure S18). OAT reactivity of reduced
clusters
[Fe
II
Fe
III
2
MO(OH)]
was studied using PMe
3
as substrate. In the presence of a 10-
fold excess of PMe
3
,
[Fe
II
Fe
III
2
ScO(OH)]
displayed complete conversion in 60 hours –
slower than for the more oxidized clusters. The OPMe
3
product generated was observed
during the reaction via
31
P NMR (Figure S20). Treatment of clusters
[Fe
II
Fe
III
2
LaO(OH)]
and
[Fe
II
Fe
III
2
CaO(OH)]
with 10 equiv PMe
3
resulted in slower reactivity, with full
conversion in 130 h and 270 h, respectively. As in the case with
[Fe
II
Fe
III
2
ScO(OH)]
,
generation of OPMe
3
was confirmed via
31
P NMR as the reaction proceeded (Figure S21
and S22). Unfortunately, characterization of the metal-containing products of these
transformations has thus far been unsuccessful. Based on the observed reactivity of the Mn
3
analogues
[Mn
III
3
MO(OH)]
and
[Mn
II
Mn
III
2
MO(OH)]
, it is reasonable to expect that
OAT from complexes
[Fe
III
3
MO(OH)]
and
[Fe
II
Fe
III
2
MO(OH)]
would also generate
mono-oxo complexes as well as reduced Fe
II
3
species. However, due to the similarity of the
characteristic
1
H NMR features of these families of Fe
3
compounds, deconvolution of the
spectroscopic data for the products of OAT has not been successful. Similar to what was
observed for Mn
3
complexes, OAT reactivity of Fe
3
oxo-hydroxo complexes
[Fe
III
3
MO(OH)]
and
[Fe
II
Fe
III
2
MO(OH)]
is affected by the Lewis acidity of the redox
inactive metal. In both oxidation states, the clusters containing the more Lewis acidic
heterometal display faster OAT to phosphine substrates.
[Fe
III
3
CaO(OH)]
and
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[Fe
II
Fe
III
2
CaO(OH)]
, containing the least Lewis acidic Ca
2+
(p
K
a
= 12.9), react slower
than clusters
[Fe
III
3
LaO(OH)]
and
[Fe
II
Fe
III
2
LaO(OH)]
and
[Fe
II
Fe
III
2
ScO(OH)]
,
respectively, which contain stronger Lewis acidic metals (p
K
a
: La
3+
= 9.06, Sc
3+
= 4.8).
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 p
K
a
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
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-Mn
IV
core in oxidized clusters
[Mn
IV
3
MO
4
]
inhibits the OAT process (
vide supra
). Notably, in several classes of clusters,
the metal-containing species generated by the reaction of the well-defined heterometallic
precursors with phosphines were observed to themselves undergo reactivity with
phosphines. However, as neither the transiently-generated metal-containing species nor the
final metal-containing products of these processes could be conclusively 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
[Mn
III
Mn
IV
2
GdO
4
]
,
[Mn
II
Mn
III
2
YO(OH)]
,
and
[Fe
II
Fe
III
2
LaO(OH)]
, which contain heterometals with similar p
K
a
’s (Gd3
+
= 8.4, Y3
+
= 8.6, La3
+
= 9.06), enables assessment of the effect of various cluster architectures on
reactivity. Cubane cluster
[Mn
III
Mn
IV
2
GdO
4
]
reacts slower (>2 weeks at r.t.) with PMe
3
than
[Fe
II
Fe
III
2
LaO(OH)]
(120 h), which in turn displays slower OAT activity than
[Mn
II
Mn
III
2
YO(OH)]
(30 min, PEt
3
). A similar reactivity ordering is observed between
clusters containing Ca2
+
(p
K
a
= 12.9) as the redox-inactive metal;
[Mn
II
Mn
III
2
CaO(OH)]
reacts faster (36 h) than
[Fe
II
Fe
III
2
CaO(OH)]
(270 h; the corresponding cubane cluster,
[Mn
III
Mn
IV
2
CaO
4
]
, could not be isolated).
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The observed qualitative rate of OAT correlates not only with the p
K
a
of the redox-inactive
metal incorporated in the cluster, but also with the reported one-electron reduction potentials
for the three series of clusters. Fe
3
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, the
[Fe
3
MO(OH)]
complexes display slower reactivity than the more
oxidizing
[Mn
3
MO(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 transfer
53
–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
[Mn
III
Mn
IV
2
MO
4
]
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 PEt
3
(vs. the less bulky PMe
3
substrate).
In the case of oxo-hydroxo clusters
[Mn
3
MO(OH)]
and
[Fe
3
MO(OH)]
, on the other hand,
a stepwise mechanism involving reduction of the cluster species by the PR
3
substrate (i.e. an
electron-transfer process) cannot be ruled out.
3. CONCLUSIONS
The Lewis acidity of redox inactive metal centers incorporated in heterometallic Mn
3
and
Fe
3
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 [Mn
3
MO
4
] and oxo-hydroxo [Mn
3
MO(OH)] and [Fe
3
MO(OH)]
clusters correlates with the Lewis acidity of the apical metal, similarly to previously
observed one-electron reduction potentials of the same series of clusters. OAT rates can be
accelerated by between one and three 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-Mn
IV
cubane clusters, which display slow ligand dissociation and,
consequently, slower OAT. Overall, this study demonstrates that the OAT reactivity of the
clusters can be tuned significantly by choice of the Lewis acid.
EXPERIMENTAL SECTION
General Considerations.
Unless otherwise specified, all compounds were manipulated using a glovebox or standard
Schlenk line techniques with an N
2
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 minutes
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