Redox
Processes
Involving
Oxygen:
The Surprising
Influence
of
Redox-Inactive
Lewis
Acids
Davide
Lionetti,
Sandy
Suseno,
Angela
A. Shiau,
Graham
de Ruiter,
and Theodor
Agapie
*
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2024,
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ABSTRACT:
Metalloenzymes
with
heteromultimetallic
active
sites
perform
chemical
reactions
that
control
several
biogeochemical
cycles.
Transformations
catalyzed
by
such
enzymes
include
dioxygen
generation
and
reduction,
dinitrogen
reduction,
and
carbon
dioxide
reduction
−
instrumental
transformations
for
progress
in the
context
of
artificial
photosynthesis
and
sustainable
fertilizer
production.
While
the
roles
of the
respective
metals
are
of interest
in all
these
enzymatic
transformations,
they
share
a common
factor
in
the
transfer
of one
or multiple
redox
equivalents.
In light
of this
feature,
it is surprising
to find
that
incorporation
of redox-
inactive
metals
into
the
active
site
of
such
an
enzyme
is critical
to
its
function.
To
illustrate,
the
presence
of a redox-inactive
Ca
2+
center
is crucial
in
the
Oxygen
Evolving
Complex,
and
yet
particularly
intriguing
given
that
the
transformation
catalyzed
by
this
cluster
is a redox
process
involving
four
electrons.
Therefore,
the
effects
of redox
inactive
metals
on
redox
processes
−
electron
transfer,
oxygen-
and
hydrogen-atom
transfer,
and
O
−
O
bond
cleavage
and
formation
reactions
−
mediated
by
transition
metals
have
been
studied
extensively.
Significant
effects
of
redox
inactive
metals
have
been
observed
on
these
redox
transformations;
linear
free
energy
correlations
between
Lewis
acidity
and
the
redox
properties
of synthetic
model
complexes
are
observed
for
several
reactions.
In this
Perspective,
these
effects
and
their
relevance
to multielectron
processes
will
be
discussed.
KEYWORDS:
multimetallic
effects,
redox
reactions,
metal
ion-coupled
electron
transfer,
oxygen
transformations,
Lewis
acids,
bioinorganic
chemistry
1. INTRODUCTION
The
conversion
of small
molecules
such
as O
2
, N
2
, CO
2
, H
2
O,
and
H
2
into
value-added
chemicals
or liquid
energy
carriers
is
important
in
(i)
the
context
of
solar
energy
conversion
and
storage,
(ii)
the
efficient
use
of inexpensive
feedstocks,
and
(iii)
the
sustainable
generation
of
bulk
chemicals.
However,
transforming
these
small
molecules
into
useful
chemicals
is
challenging
due
to
the
number
of
e
−
/H
+
equivalents
transferred
and
in
terms
of
the
necessity
for
controlling
selectivity
and
the
performance
of
the
chemistry
at
low
overpotentials.
1
The
biological
catalysts
for
small
molecule
conversions
are
proteins
that
often
involve
complex
inorganic
active
sites:
Mn
4
Ca
in
Photosystem
II
(PSII)
for
O
2
evolution;
2
Cu
3
in laccase
and
CuFe
in cytochrome
c
oxidase
for
O
2
reduction;
3
Fe
4
Ni
or
CuMo
in
CO
dehydrogenase
(CODH)
for
CO
2
reduction;
4
Fe
8
, Fe
7
Mo,
or
Fe
7
V
in
nitrogenase
for
N
2
reduction;
5
Fe
6
or
FeNi
in
hydrogenase,
for
proton
reduction
and
hydrogen
oxidation.
6
In
many
of
these
gas-processing
enzymes,
two
or more
types
of metals
are
found
in
their
active
sites.
Accordingly,
the
interactions
between
different
metal
centers
and
their
influence
on
structure
and
reactivity
have
been
investigated
in
numerous
synthetic
systems
and
have
also
been
the
subject
of
several
reviews.
7
−
28
Among
the
enzymes
that
depend
on
heteromultimetallic
active
sites,
two
systems
are
unusual
for
the
involvement
of
metal
ions
that
are
redox-inactive,
namely
(i)
the
oxygen
evolving
complex
(OEC)
of Photosystem
II,
which
contains
a
Ca
2+
ion
and
catalyzes
the
oxidation
of H
2
O to
O
2
, and
(ii)
Cu/Zn-dependent
superoxide
dismutase
(Cu/Zn-SOD),
which
catalyzes
the
disproportionation
of
superoxide
(O
2
•−
)
to
H
2
O
2
and
O
2
at an
active
site
containing
a redox-inactive
Zn
2+
ion.
2,29
Because
both
enzymes
are
involved
in
oxygen
chemistry,
their
structural/activity
relationships
bear
great
relevance
for
energy
science
and
industrial
processes.
Not
Received:
October
31,
2023
Revised:
December
12,
2023
Accepted:
December
13,
2023
Published:
January
22,
2024
Perspective
pubs.acs.org/jacsau
© 2024
The Authors.
Published
by
American
Chemical
Society
344
https://doi.org/10.1021/jacsau.3c00675
JACS
Au
2024,
4, 344
−
368
This article is licensed under CC-BY-NC-ND 4.0
surprisingly,
many
investigations
have
probed
the
role
of these
redox-inactive
metal
ions
in
catalysis.
While
a primarily
structural
role
has
been
ascribed
to
Zn
2+
in
Cu/Zn-
SOD,
3,30
−
32
with
SOD
activity
being
largely
retained
upon
Zn
2+
loss,
30
the
role
of
Ca
2+
in
the
OEC
has
not
been
conclusively
elucidated.
Consequently,
synthetic
models
of the
OEC
have
been
targeted
with
the
aim
of
unraveling
fundamental
reactivity
relevant
to
biological
water
oxida-
tion.
33
−
35
Interest
in
the
effects
of
redox-inactive
metals
on
redox
processes
involving
oxygen
has
been
further
piqued
by
discoveries
in
the
field
of
heterogeneous
catalysis,
where
mixed-metal
oxides
containing
both
redox-active
and
-inactive
components
have
gained
significant
attention
as
electro-
catalysts
in
the
oxygen
evolution
reaction
(OER).
36
−
40
Moreover,
employing
multiple
different
metal
centers
in
discrete
complexes
can
likewise
result
in
new
physical
properties
and
reactivity
pathways
beyond
oxygen
chemis-
try.
41
−
46
As
a result,
aspects
of
the
effects
of
redox
inactive
metals
on
the
redox
reactivity
of transition
metals
with
oxygen
have
been
addressed
in several
types
of reactions,
and
this
field
has
seen
significant
increase
in research
activity.
47
−
55
Herein,
we
provide
an
overview
of the
literature
concerning
the
behavior
of
well-defined
molecular
systems
displaying
a
combination
of redox-active
and
redox-inactive
metals
in the
context
of
oxygen-related
chemistry.
Effects
of
redox-inactive
metals
on
electron
transfer
(ET),
oxygen-atom
transfer
(OAT),
hydrogen-atom
transfer
(HAT),
as
well
as
O
−
O
bond
cleavage
and
formation,
will
be
discussed
(Figure
1).
Because
of
their
inability
to
directly
participate
in
redox
processes,
the
basis
for
controlling
reactivity
with
redox-
inactive
metals
must
leverage
properties
such
as their
charge,
size,
polarizability,
and/or
coordination
number.
Given
the
complex
interplay
between
these
features,
many
(though
not
all)
studies
in this
area
have
shown
that
the
observed
effects
on
redox
properties
cannot
be
ascribed
to
a single
factor,
but
rather
to
the
broader
concept
of
Lewis
acidity
invoked
by
constructing
linear
free-energy
relationships
based
on
exper-
imental
properties
(e.g.,
reduction
potentials,
reaction
rates).
Because
Lewis
acidity
is
not
a well-defined
quantitative
parameter,
several
alternative
scales
have
been
developed
in
an
attempt
to
provide
a semiquantitative
measure
of
Lewis
acidity.
56,57
Methods
based
on
nuclear
magnetic
resonance
(NMR)
58,59
or
fluorescence
spectroscopy,
60,61
for
instance,
have
been
successfully
implemented
in
various
contexts.
A
scale
of
Lewis
acidity
based
on
electron
paramagnetic
resonance
(EPR)
measurements
has
also
been
developed
by
Fukuzumi
and
co-workers.
62
Our
work
has
relied
on
the
p
K
a
value
of metal
−
aqua
ions
as a measure
of Lewis
acidity
in part
due
to
the
availability
of
values
for
mono-,
di-,
and
trivalent
ions.
56,63
−
65
These
values
also
emerge
from
the
same
interplay
between
distinct
factors
that
makes
use
of
individual
parameters
often
inadequate
to explain
experimental
observa-
tions.
Overall,
useful
correlations
between
Lewis
acidity
and
various
types
of
electron-
and
group-transfer
reactivity
exist
across
many
transition
metal
compounds
of
varying
composition,
nuclearity,
and
complexity.
In this
Perspective,
correlations
between
observed
properties
and
Lewis
acidity
will
be
presented
using
the
methods
for
quantification
of
Lewis
acidity
described
in
the
original
publications,
including
in cases
where
other
parameters
(e.g.,
ionic
radius
or
charge)
were
found
to
provide
a more
satisfactory
correlation.
In
cases
where
such
correlations
were
not
originally
provided,
or in cases
where
a comparison
across
systems
originally
relying
on
different
Lewis
acidity
scales
would
be
particularly
insightful,
the
p
K
a
of
metal
−
aqua
ions
will
be
used
as the
measure
of Lewis
acidity.
Values
for
the
p
K
a
of a broad
series
of metal
−
aqua
ions
are
shown
in Table
1. The
majority
of these
values
were
obtained
from
the
extensive
lists
reported
by
Perrin;
63
wherever
possible,
values
extrapolated
to
I
= 0 were
chosen
to best
represent
thermodynamic
values.
It
should
be
noted,
however,
that
significant
variance
exists
for
the
p
K
a
of metal
−
aqua
ions
in different
sources,
especially
for
transition
metal
ions.
64,66,67
Therefore,
where
significantly
different
values
(
Δ
p
K
a
≥
0.5)
have
been
cited
in
other
literature
sources,
these
values
were
also
included
in the
table.
For
the
purposes
of
identifying
correlations
between
observable
properties
(reduction
potentials,
reaction
rates,
etc.)
and
Lewis
acidity
in this
Perspective,
the
Perrin
values
63
were
used
(with
the
exception
of the
values
for
K
+
, Cs
+
, and
Rb
+
, which
were
recently
extrapolated
on
the
basis
of
31
P NMR
experiments).
56
Nonetheless,
the
uncertainties
inherent
in
these
values
make
quantitative
interpretations
of
such
correlations
challenging,
and
these
relationships
will
therefore
be
discussed
here
primarily
in qualitative
terms.
Figure
1.
Lewis
acid
effects
on
redox
processes
involving
oxygen.
The
linear
correlation
between
the
p
K
a
of the
metal
aqua
ion
and
the
“properties”
of corresponding
metal
complexes
(center)
are
universally
found
among
many
transformations
that
involve
oxygen
including
(i)
electron
transfer
(top
left),
(ii)
oxygen-atom
transfer
(bottom
left),
(iii)
O
2
activation
and
cleavage
(top
right),
and
(iv)
O
−
O
bond
formation
(bottom
right).
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345
2. ELECTRON
TRANSFER
AND LEWIS
ACIDS
Lewis
acids
have
been
shown
to affect
reduction
potentials
and
rates
of
electron
transfer
in
transition
metal
compounds
(Figures
2 and
3).
These
effects
were
observed
either
upon
addition
of
excess
redox
inactive
metal
ion
to
solutions
of
transition
metal
complexes
displaying
a basic
site
for
binding
of
additional
metal
ions
or by
utilizing
well-defined
heterometallic
complexes.
One
of the
more
broadly
studied
effects
of Lewis
acids
on
electron
transfer
involves
the
conjugation
of redox-active
metal
complexes
to
crown-ethers
(Figure
2).
68
−
88
Extensive
studies
on
mono-
(
1
)
and
bimetallic
(
2
)
manganese
Schiff-base
complexes
first
appeared
in the
1990s
that
aimed
to elucidate
the
properties
of
manganese-dependent
enzymes
(including
the
OEC).
70
−
72
While
salen-type
related
ligands
have
been
shown
to be
useful
for
binding
or sensing
of a wide
variety
of
redox-inactive
metals
via
coordination
to O atom
donors
(e.g.,
phenoxide),
89
inclusion
of crown-ether
motifs
enables
stronger
binding
of
Lewis
acidic
ions
with
stoichiometric
control.
Exposing
complex
1
to
Li
+
, K
+
, Ca
2+
, or
Ba
2+
leads
to
incorporation
of a single
redox-inactive
metal
into
the
crown-
ether
moiety,
resulting
in a positive
shift
in reduction
potential
for
the
Mn
III
/Mn
II
redox
couple.
72
Nearly
identical
behavior
was
displayed
by
the
μ
-oxo
dimer
of
1
, although
the
observed
redox
couples
corresponded
to
the
two-electron
reduction
of
the
[Mn
III
2
O]
core
in
this
instance.
A
similar
effect
was
observed
in
a related
system
(
7
),
where
the
less
rigid
aminopropane
backbone
enables
isolation
of
a bridged
(
μ
-
O)
2
complex.
Incorporation
of redox-inactive
metal
ions
in this
Mn
IV
−
(O)
2
−
Mn
IV
species
leads
to positive
shifts
in potential
for
the
Mn
IV
2
/Mn
III
Mn
IV
and
Mn
III
Mn
IV
/Mn
III
2
redox
couples,
though
only
redox-inactive
metals
with
a narrow
range
of
Lewis
acidities
(
Δ
p
K
a
. =
∼
2.0)
were
investigated.
71
A similar
Table
1. p
K
a
of [M(H
2
O)
m
]
n
+
Species
M
n
+
p
K
a
a
M
n
+
p
K
a
a
M
n
+
p
K
a
a
Mn
3+
0.20
(
−
0.6)
b
Eu
3+
8.03
(8.6)
b
Ni
2+
9.86
Co
3+
0.66
(0.5)
b
(1.75)
c
Y
3+
8.04
Mn
2+
10.59
Fe
3+
2.19
Dy
3+
8.10
Mg
2+
11.40
Ga
3+
2.92
Tb
3+
8.16
Ca
2+
12.60
In
3+
3.54
Gd
3+
8.40
(9.78)
c
Sr
2+
13.18
Al
3+
4.90
Nd
3+
8.43
(9.0)
c
Li
+
13.11
(13.6)
b
(13.8)
c
Sc
3+
4.93
(4.3)
b
Co
2+
8.90
(9.7)
b
(9.85)
c
Ba
2+
13.40
Cu
2+
7.34
Zn
2+
8.96
Na
+
14.80
(13.9)
b
Lu
3+
7.66
(8.2)
b
La
3+
9.06
(8.5)
b
K
+
16.06
d
(14)
Pb
2+
7.78
Cd
2+
9.30
(10.1)
b
Cs
+
16.29
d
Yb
3+
8.01
Fe
2+
9.30
(6.8)
c
Rb
+
16.34
d
a
p
K
a
values
from
ref
63
except
where
otherwise
noted.
b
p
K
a
values
from
ref
67.
c
p
K
a
values
from
ref
64.
d
p
K
a
values
from
ref
56.
Figure
2.
Linear
correlation
between
the
reduction
potential
of crown-ether
supported
(multinuclear)
metal
complexes
and
the
Lewis
acidity
of
redox-inactive
metal
ions.
(A)
Selection
of crown-ether
supported
metal
complexes.
(B)
Observed
linear
correlation
is shown
for
complexes
1-M
(
μ
-oxo
dimer,
pink
diamonds),
72
2-M
(blue
dots),
68
3-M
(dark
green
dots),
78
4-M
(light
green
dots),
85
5-M
(purple
dots),
84
6-M
(salen-type
backbone,
teal
triangles),
86
6-M
(modified
backbone,
orange
triangles),
87
7-M
(two
redox
events,
maroon
and
gray
diamonds,
respectively),
71
and
8-M
(red
dots).
77
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346
set
of redox-inactive
metals
(Na
+
, K
+
, Ca
2+
, Sr
2+
, and
Ba
2+
) was
screened
in
structurally
related
cobalt
complexes
(
2-M
).
In
these
complexes,
a linear
correlation
between
the
p
K
a
of the
redox-inactive
metal
and
the
reduction
potential
of the
Co
II/I
redox-couple
was
observed,
though
the
narrow
range
of Lewis
acidities
once
again
limits
generalization
of this
relationship.
68
Related
nickel,
78
iron,
83
zinc,
85
palladium,
84
and
vanadyl
86,87
complexes
(
3-M
to
6-M)
have
been
shown
to display
similar
correlations
between
the
reduction
potential
of
the
complex
(typically
corresponding
to
a metal-centered
ET
process,
though
a ligand-based
one
for
Pd
and
Zn)
and
the
Lewis
acidity
of a wider
range
of cations
(e.g.,
Na
+
, Ca
2+
, Nd
3+
, Y
3+
).
Incorporating
a more
flexible
triamine-based
backbone
enables
coordination
of these
Schiff-base
ligands
to
the
uranyl
cation
([UO
2
]
2+
;
8
)
and
thereby
enables
the
extension
of
these
relationships
to
actinide
complexes.
The
Lewis
acidity
of the
incorporated
redox-inactive
metals
(K
+
, Na
+
, Ca
2+
, and
Nd
3+
)
linearly
modulates
the
reduction
potential
of the
U
VI
/U
V
redox
couple.
Notably,
the
reversibility
of
the
observed
electro-
chemical
processes
is also
affected
by
the
Lewis
acidity
of the
Figure
3.
Linear
correlation
between
the
reduction
potential
of (multinuclear)
metal
complexes
and
the
Lewis
acidity
of redox-inactive
metal
ions.
(A)
Observed
linear
correlation
is shown
for
Mn
3
MO
4
(
10
-
M
;
blue),
Mn
3
MO(OH)
(
11
-
M
;
green),
and
Fe
3
MO(OH)
(
12-M
;
red)
type
clusters;
Ln
= La,
Nd,
Eu.
Gd,
Tb,
Dy,
Yb,
Y. (B)
Linear
correlation
observed
for
the
rate
of electron
transfer
between
17
-
M
and
ferrocene.
(C)
Selection
of
metal
complexes
that
demonstrate
modulation
of the
reduction
potential
upon
interaction
with
redox-inactive
metal
ions.
Figure
3B
is reproduced
from
ref
149.
Copyright
2011
American
Chemical
Society.
JACS
Au
pubs.acs.org/jacsau
Perspective
https://doi.org/10.1021/jacsau.3c00675
JACS
Au
2024,
4, 344
−
368
347
redox-inactive
metal
ions,
with
evidence
of
downstream
chemical
reactivity
for
more
Lewis
acidic
divalent
and
trivalent
redox-inactive
cations.
77
Heterobimetallic
complexes
with
other
ligands
containing
phenolate
moieties
have
also
been
reported,
although
they
frequently
involve
a combination
of
two
redox-active
metals.
90
−
103
In
these
complexes,
some
effects
can
be
attributed
to
the
Lewis
acidity
of
a metal
ion,
but
their
properties
are
mainly
dominated
by
electrostatic
effects.
Notable
exceptions
are
the
rare-earth/alkali-metal
heterobime-
tallic
complexes
(
9
)
reported
by
Walsh
and
Schelter.
104
−
106
Binding
of
redox-inactive
metals
to
cerium
complexes
supported
by
binolate
ligands
modulates
the
reduction
potential
for
the
Ce
IV/III
couple.
Different
redox-inactive
metals
were
also
found
to
promote
formation
of chemical
oxidation
products
featuring
diverse
binding
modes
of the
Lewis
acidic
ions
as a function
of their
identity.
Our
group
has
extensively
studied
the
effects
of Lewis
acidity
on
reduction
potentials
in multimetallic
cluster
complexes
(
10
-
M
to
12
-
M
).
67,107
−
111
Some
of these
clusters
(
10-M
)
closely
match
the
structural
aspects
of the
OEC,
for
which
only
a few
well-defined
structural
examples
have
been
reported.
112
−
116
These
tetraoxido
clusters
display
a [Mn
3
CaO
4
] “cubane”
structural
motif,
with
the
redox-inactive
metal
situated
at the
apical
position
(
10-M
;
Figure
3).
108,109
In
these
compounds,
the
potential
for
the
Mn
IV
3
/Mn
III
Mn
IV
2
redox
couple
shifts
from
−
0.94
V (vs
the
ferrocenium/ferrocene
couple,
hereafter
denoted
Fc
+/0
) for
Sr
2+
to
+0.29
V for
Mn
3+
, and
correlates
linearly
with
the
Lewis
acidity
of the
corresponding
metal
ion.
The
linear
fit is observed
over
a wide
p
K
a
range
(
Δ
p
K
a
∼
15),
which
allows
a broader
demonstration
of
the
trend,
and
displays
a potential
shift
of
ca
.
0.1
V for
every
p
K
a
unit
(Figure
3A;
blue
dots).
Rather
than
Lewis
acidity,
computational
studies
of
the
OEC
incorporating
different
redox-inactive
cations
suggested
a correlation
between
the
reduction
potentials
of
the
clusters
and
the
charge
of
these
redox-
inactive
cations.
67
Notably,
the
protein
environment
that
encages
the
OEC
cluster
in Photosystem
II imposes
bonding
pressures
from
all
directions.
117,118
This
is in
contrast
with
synthetic
cubane
compounds,
where
the
carboxylate
bridges
do
not
place
a similar
constraint
on
the
apical,
redox-inactive
metal
center.
To
address
the
potential
effects
of
geometric
constraints
of
the
protein
environment
on
the
OEC
cluster,
synthetic
cubane
variants
with
chelating
amidate
ligands
were
prepared
(Figure
4).
Interestingly,
a caveat
to
the
correlation
between
the
reduction
potential
and
the
Lewis
acidity
trend
was
observed
in
these
systems.
In
a series
of
[Mn
4
O
4
] (
10-
Mn
)
and
[YMn
3
O
4
] (
10-Y
)
cubane
clusters,
119
our
group
showed
that
ligand
basicity
modulates
the
reduction
potentials
of
the
clusters
in a similar
fashion
to
reported
[Co
4
O
4
] and
[RuCo
3
O
5
] complexes.
120,121
However,
the
tris-amidate
substituted
YMn
3
O
4
(
10-Y(triam)
)
cluster
displays
a 140
mV
positive
shift
in
the
potential
for
the
[YMn
IV
3
]/
[YMn
IV
2
Mn
III
] couple
relative
to
the
tris-acetate
cluster
(
10-
Y
),
inconsistent
with
the
increased
basicity
of amidates
versus
acetates.
This
exception
can
be
explained
by
geometric
changes
in the
cluster
core
due
to
ligand
steric
constraints
−
chelating
tris-amidate
ligands
contract
the
Y
−
oxo
distances
by
roughly
0.1
Å
compared
to
the
those
in
the
tris-acetate
cluster,
decreasing
the
electron
density
available
at
the
Mn
centers.
Relevant
to the
cation
size
dependence
on
the
activity
of the
OEC,
these
results
suggest
geometric
constraints
may
cause
nonlinear
changes
in
reduction
potentials
and
reactivity,
engendering
more
pronounced
effects
than
those
predicted
on
the
basis
of its
lower
Lewis
acidity.
In other
words,
a larger
metal
may
behave
like
a smaller,
more
Lewis
acidic
one
of the
same
charge,
resulting
in
a correlation
of
properties
with
charge,
not
Lewis
acidity.
The
role
of
the
number
of
μ
-oxo
ligands
was
also
investigated
with
similar
tetranuclear
complexes
11
-
M
and
12
-
M
,
107,110
which
feature
a [M
3
(
μ
4
-O)(
μ
2
-OH)M’]
core
(M
= Mn,
Fe;
M’
= redox-inactive
metal;
Figure
3).
The
Lewis
acidity
of the
redox-inactive
metal
influences
the
potential
for
the
M
III
3
/M
III
2
M
II
couples,
which
shift
from
E
1/2
=
−
0.30
to
+0.42
V (vs
Fc
+/0
) for
M
= Mn
(Figure
3A,
green
dots)
and
−
0.49
to
+0.07
V for
M
= Fe
(Figure
3A,
red
dots)
as
the
Lewis
acidity
of
the
incorporated
redox-inactive
metal
increases.
Figure
4.
Plot
of
reduction
potential
vs
effective
basicity
for
a series
of
cubane
complexes.
(A)
Selection
of
metal-oxo
clusters.
(B)
Linear
correlation
between
reduction
potential
and
effective
ligand
basicity
in [Mn
4
O
4
] complexes,
including
10-Mn
(blue
circles).
Similar
trend
expected
in [YMn
3
O
4
] complexes,
including
10-Y
(red
circles),
with
the
exception
of trisamidate
complex
10-Y(triam)
due
to
geometric
effects.
Graph
adapted
from
ref
119.
Copyright
2019
American
Chemical
Society.
JACS
Au
pubs.acs.org/jacsau
Perspective
https://doi.org/10.1021/jacsau.3c00675
JACS
Au
2024,
4, 344
−
368
348