of 5
1
A super
-
oxidized
radical
cationic
icosahedral boron
cluster
Julia M. Stauber
,
[a]
Josef Schwan
,
[b]
Xinglong Zhang,
[
b
]
Jonathan C. Axtell
,
[a
,c
]
Dahee
Jung,
[a
]
Brendon J.
McNicholas
,
[b
]
Paul H. Oyala,
[
b
]
Andrew J. Martinolich
,
[
b]
Jay R. Winkler
,
[b
]
Kimberly A. See
,
[
b
]
T
homas F.
Miller III,
*
[
b
]
Harry B. Gray
,
*
[b]
Alexander M. Spokoyny
*
[a
,
d
]
[a]
Dr. J. M. Stauber, Dr. J. C. Axtell, Dr. D. Jung, Prof. Dr.
A. M. Spokoyny*
Department of Chemistry and Biochemistry
,
University of California, Los Angeles
607 Charles E
.
Young Dr
.
E
.
, Los Angeles, CA 90095
(USA)
E
-
mail:
spokoyny@chem.ucla.edu
(A. M. S.)
[b]
Dr.
J. Schwan,
Dr. X. Zhang,
B. J. McNicholas,
Dr. P. H.
Oyala,
Dr. A. J.
Martinolich,
Prof. Dr.
J. R. Winkler,
Prof. Dr. K. A. See,
Prof. Dr. T. F. Miller
III
,*
Prof. Dr.
H. B. Gray*
Division of Chemistry and Chemical Engineering
,
California Institute of Technology
1200 East California Boulevard, Pasadena, CA
91125 (USA)
E
-
mail
:
tfm@caltech.edu
(T. F. M.),
hbgray@caltech.edu
(H. B. G.)
[
c
]
Dr. J. C. Axtell
The Dow Chemical Company
633 Washington St., Midland, MI, 48674
(USA)
[
d
]
Prof. Dr. A. M. Spokoyny
California NanoSystems Institute
,
University of
California, Los Angeles
570 Westwood Plaza, Los Angeles, CA 90095 (USA)
Abstract:
While the icosahedral
closo
-
[B
12
H
12
]
2
cluster does not
display reversible electrochemical behavior, perfunctionalization of
this species
via
substitution of all twelve B
H
vertices
with alkoxy or
benzyloxy (OR) substituents engenders reversible redox
chem
istry
,
providing
access to clusters in the dianionic, monoanionic, and neutral
forms.
Here,
we evaluated the electrochemical behavior of
the
electron
-
rich
B
12
(
O
-
3
-
methylbutyl)
12
(
1
) cluster
and discovered
that
a
new reversible redox ev
e
nt
that gives rise
to a
fourth electronic state
is
accessible
through one
-
electron oxidation
of the neutral
species
.
C
hemical
oxidation of
1
with
[N(2,4
-
Br
2
C
6
H
3
)
3
]
+
afforded the
isolable
[
1
]
+
cluster
,
which is the
first example of
an
open
-
shell cationic
B
12
cluster
in which the unpaired electron is
proposed to be
delocalized
throughout the
boron cluster
core
.
The oxidation of
1
is also
chemically reversible,
where
treatment of
[
1
]
+
with ferrocene resulted
in
its
reduction back to
1
.
The
identity of
[
1
]
+
is
supported
by EPR
,
UV
-
vis
, multinuclear NMR (
1
H
,
11
B
), and
X
-
ray photoelectron
spectroscopic
characterization
.
Polyhedral
boron clusters have provided a platform for rich
scientific
exploration with applications ranging from
material
s
science
to medicine.
[1
5]
The icosahedral
closo
-
[B
12
H
12
]
2
dianion
[6]
has been a focus of investigation on account of its
high
stability,
three
-
dimensi
onal
delocalized bonding,
resistance to
harsh chemical environ
ments,
and ease of electrophilic B
H
substitution
.
[7]
W
ork
describing
the one
-
electron oxidation of
[B
12
H
12
]
2
, however,
displayed
cluster decomposition and
dimerization, revealing the oxidative instability of th
is
species.
[8]
These findings were rationalized by
the
Wade
-
Mingos
2
n
+ 2
(
n
=
number of vertices)
electron
-
counting rule
s
, which predict
the
structure
s
of polyhedral molecules
based on the number of
skeletal electron pairs
(Figure 1A)
.
[9,10]
However, d
espite these
predictions
,
perf
unc
tionalization of
closo
-
[B
12
H
12
]
2
can
engender
reversible redox chemistry,
which
provides
ready access to stable
and isolable monoanionic,
hypocloso
-
[B
12
R
12
]
, and neutral,
hypercloso
-
B
12
R
12
derivatives
(Figure 1B)
.
[7,11
15]
S
tabilization of
these species
is attributed to
steric
protection of the
cluster
-
confined frontier orbitals
by bulky substituents,
and the electron
-
donor effects of
supporting groups
that supply extra electron
density
to
the electron
-
deficient
B
12
core
s
.
[2,7]
To our knowledge,
however,
dodecaborane
clusters
oxidized beyond the
hypercloso
state
have
never been
reported
.
This
redox boundary
poses
critical
questions
concerning
the oxidation limit of
icosahedral
boron
clusters
, which
is not only of fundamental interest,
but could
also have implications in the use of
B
12
-
based materials for energy
storage
and other
applications.
[2,16
18]
Figure
1.
A: Structures of polyhedral
closo
-
boron cluster
s
as predicted by the Wade
-
Mingos electron
-
counting rules.
[9,10]
B: Examples of redox
-
active,
perfunctionalized
closo
-
dodecaborate clusters
(
[
B
12
(OH)
12
]
2
,
[14]
[
B
12
(CH
3
)
12
]
2
,
[13]
and [
B
12
Cl
12
]
2
[11]
).
C:
Established redox chemistry for alkoxy and benzyloxy
-
substituted B
12
clusters,
[2,7,15,16,19,20]
and this work, which introduces a third, reversible redox event
for the B
12
(
O
-
3
-
methylbutyl)
12
(
1
)
derivative
.
2
As
previously established,
[2,7,15,16,19,20]
the redox potential of
alkoxy and benzyloxy
-
substituted B
12
clusters
([B
12
(OR)
12
])
can be
rationally
tuned as a function of
their
substituent
s
by greater than
1 V for the same redox event.
With this in mind,
in addition to the
high chemical, electrochemical, and structural stability of
icosahedral boron clusters
,
[1,2,5,16,21
23]
we envisioned
we could
tune
and expand
the
redox
potential window of
B
12
(OR)
12
derivatives
through judicious selection of
the supporting
substituents
. Surprisingly
,
during this work
,
we
discovered
a
previously unrecognized electrochemical
event
that allow
ed
us to
observe the first example of
a
super
-
oxidized
B
12
-
based radical
cation at
a
chemically
accessible potential
(
Figure
1
C
)
.
We identified the
B
12
(O
-
3
-
methylbutyl
)
12
(
1
)
derivative
,
originally reported by Hawthorne
et al.,
[7]
as a model cluster for
investigation due to the inherent steric protection provided by the
bulky
alkyl
groups, which we
predicted
would
provide extra
stability to a highly re
active,
cluster
-
based
radical
cation
.
Additionally,
the
inductive electron
-
donating
nature
of the 3
-
methylbutyl groups provides
1
with low potentials for the
[
1
]
2
/
and
[
1
]
/0
redox couples
,
[15]
which we
anticipated
would shift
a
[
1
]
0/
+
couple to a potential accessible using common chemical
oxidant
s
. Here, w
e report the successful
one
-
electron oxidation of
hypercloso
-
1
, yielding the [
1
]
+
radical
cation in isolable form
.
The
super
-
oxidized
[
1
]
•+
species
introduces a new redox state
of
B
12
clusters
previously perceived as inaccessible
,
and
represents
the
first
characterized
experimental
example
of a
boron cluster
-
based
three
-
electron violation of
the
Wade
-
Mingos
rules.
Figure
2
.
(A)
CV of
1
(100 mV/s). (B) CV of the [
1
]
0/
+
redox couple recorded at
variable scan rates. (C) Randles
-
Sevcik
plot of the CV data from (B). A DCM
solution of
1
(3 mM) with [TBA][PF
6
] (0.1 M) supporting electrolyte was used.
The
1
cluster was
synthesized
through per
-
O
-
alkylation
of
closo
-
[B
12
(OH)
12
]
2
with
1
-
bromo
-
3
-
methylbutane
followed by
sequential
two
-
electron chemical oxidation
with
Fe
(III)
.
[7,20]
N
eutral,
hypercloso
-
1
was isolated
in pure form
as
an air
-
stable
solid, and was
characterized by
1
H
and
11
B
NMR spectroscopy,
electrospray ionization mass spectrometry
(
ESI
-
MS(
+
)),
and
UV
vis spectroscopy
(SI
S
ection S2
.1
)
.
We
next
evaluated the electrochemical properties of
1
by
cyclic voltammetry
(CV)
. The CV of
1
(
Figure
2
A
)
displays two
reversible
,
one
-
electron
redox
events assigned to the
[
1
]
2
/
(
-
1.29 V) and
[
1
]
/0
(
-
0.71 V)
couples
(
vs.
Fc/Fc
+
)
with half
-
wave
potentials (
E
1/2
) that are consistent
with
those
previously
reported
.
[15]
However,
anodic scanning past the [
1
]
/0
couple
revealed
a third reversible redox event at
E
1/2
0.62 V, which was
not described in the original
report
.
[15]
We
considered
this
electrochemical event could be
attributed
to the
[
1
]
0
/
+
couple,
which is the first time to our knowledge that a 0/
+
B
12
-
based
redox
event has been
observed
for any
dodecaborane
cluster.
T
he scan rate dependent
behavior
of
this new redox event
indicated a
diffusion
-
controlled
reversible electrochemical
process as judged by a Randles
Sevcik analysis
[24]
(Figure
2
B
,
C)
of peak
current (
i
p
)
vs.
the square root of the scan rate (
ν
1/2
)
.
The
diffusion coefficient (
D
0
) determined from this study was
calculated
as
3.0 ± 0.3 × 10
-
6
cm
2
s
-
1
, which is consistent with
D
0
values reported for
related
[B
12
(OR)
12
]
clusters
[16]
and o
ther
molecular systems
[25]
of comparable size and shape
.
Before beginning our investigations into the preparation of
[
1
]
+
, we synthesized the
closo
-
[
1
]
2
cluster for its use to
benchmark the spectroscopic properties of 3
-
methylbutoxy
-
substituted clusters.
Closo
-
[
1
]
2
was prepared through a two
-
electron chemical reduction of
1
with Na/Hg amalgam in Et
2
O, and
the [Na(Et
2
O)]
2
[
1
] salt was characterized by UV
-
vis spectroscopy,
XPS, and
heter
o
n
uclear NMR spectroscopy (
1
H,
11
B). Efforts to
isolate the
hypocloso
-
[
1
]
cluster
in pure form
unfortunately have
thus far proven unsuccessful due to its
rapid
disproportionation to
1
and [
1
]
2
upon
formation (SI Section S8).
Scheme
1
.
P
rotocol for the preparation of [
1
][SbCl
6
].
The [N(2,4
-
Br
2
C
6
H
3
)
3
]
•+
triarylaminium radical
[26]
was
selected
as
a suitable reagent to
effect the
one
-
electron
chemical
oxidation of
h
ypercloso
-
1
due to its appropriate redox potential,
ease of preparation,
excellent
stability, and convenience of
handling.
[27]
T
reatment of
1
with
[N(2,4
-
Br
2
C
6
H
3
)
3
]
[
SbCl
6
]
in
dichloromethane elicited a color change from dark yellow to
yellow
brown immediately upon addition
(Scheme 1)
.
After
workup, the product of this reaction was analyzed by
1
H NMR
spectroscopy
(
SI
Figure S
7
)
,
which
revealed
the formation of new
and
broadened resonances consistent with the generation of a
paramagnetic species.
In line with these data, the
reaction
product is silent by
11
B NMR spectroscopy
(Figure
3
A)
, which
agrees well
with
the intended
one
-
electron oxidation of
the
diamagnetic
hypercloso
-
1
cluster
and
formation
of
the
paramagnetic
[
1
]
+
cation
, and suggests
the
spin
density
is
delocalized
throughout the boron atoms of the cluster core
.
3
Figure
3
.
(
A)
11
B{
1
H} NMR spectra of
[Na(Et
2
O)]
2
[
1
],
1
, and [
1
][
SbCl
6
]
.
(
B
)
X
-
band
CW
-
EPR
spectrum of
in
-
situ
generated [
1
]
+
. (C) Boron 1
s
XPS data of
[Na(Et
2
O)]
2
[
1
],
1
, and [
1
][SbCl
6
]
. (D) UV
-
vis spectra of
[
Na
(Et
2
O)]
2
[
1
]
,
1
, and
[
1
][SbCl
6
]
.
Electron
paramagnetic resonance (
EPR
)
spectroscopy was
used t
o further validate the paramagnetic
nature of [
1
]
+
. The
X
-
band
conti
nu
ous
-
wave (CW)
EPR spectrum of
in
-
situ
generated
[
1
]
+
(Figure
3
B) displays one
relatively
broad singlet
centered at
g
= 2.0083 that is
consistent with a doublet state
. The broadness
of this signal is due to extensive and
overlapping hyperfine
splitting patterns that result from the delocalization of a single
electron across all twelve boron nuclei of the cluster core.
[12,14]
Th
e
signal
broadness
and isotropic
g
-
factor
of this species
are
consistent with
EPR data
of
hyp
o
closo
-
dodecaborate radical
anion analogues that our group
[18
20]
and others
[11
14]
have
previously reported, and further support the presence of a
n
unpaired electron
that is delocalized throughout the
B
12
core
.
X
-
ray
photoelectron spectroscopy
(XPS)
has
proven to be a
valuable tool for assessing
the
charge state of
molecular and
extended network
boron cluster
systems
based on predictable
trends in boron 1
s
binding
energies.
[16,18
20]
The
XPS
measurements of
[
1
]
+
,
1
, and [
1
]
2
(Figure
3
C)
feature boron 1
s
region
s
that display
the presence of intact
molecular
clusters with
increasing
binding energ
ies
that are
consistent with
the
corresponding i
ncreased
redox
state of the cluster
s
. The highest
binding energy
was
observed for [
1
]
+
(190.3 eV)
,
and
suggests
that
this cluster is the most oxidized of the series,
further
substantiat
ing
the electron
-
deficient nature of the B
12
core, and
our formulation of this species as
a radical cation
.
The boron 1
s
binding energies
observed for
1
and [
1
]
2
of
189.4
and
188.5
eV,
respectively, agree well with
the
values
reported for other
hypercloso
and
closo
cluster derivatives
.
[16,18
20]
As
previously reported
, sequential oxidation of colorless
closo
-
[B
12
(OR)
12
]
2
species to the
hypercloso
form lead
s
to the
formation of intensely colored solutions marked by diagnostic
electronic absorption bands
observed by
UV
-
vis
spectr
oscopy
.
[7,15,19,20]
For the present system, the colorless
[
Na
(
Et
2
O
)
]
2
[
1
] salt
display
ed
negligible absorption
s
across the
visible region,
while
the UV
-
vis spectrum of
1
exhibits an
absorption located at λ
max
466 nm
that is
attributed to
charge
transfer from the peripheral oxygen atoms of the alkoxy
substituents to the
B
12
core
.
[2,7,19,20]
The [
1
][SbCl
6
] salt
displays
two major absorptions in the visible region at
λ
max
46
4
and
720 nm
(Figure
3
D)
.
Since t
he 720 nm
absorption is a diagnostic feature
of the radical cationic for
m
,
we performed
t
ime
-
dependent density
functional theory
(TD
-
DFT) calculations
(
vide infra
)
on the
optimized geometry of [
1
]
+
(SI Section S
9
.2
)
to investigate the
electronic transitions that give rise to this
defining
absorption
.
Figure
4
.
β
-
spin
m
olecular orbitals
of [
1
]
+
involved in the transitions
giving
rise
to
the 720 nm absorption observed in the UV
-
vis spectrum (Figure
3
D).
The TD
-
DFT calculations
reveal
ed
that t
he
720 nm
absorption is predominantly attributed to excitation
of
the
β
-
spin
electron from
the occupied orbitals (
MOs #319B, 320B, 321B
)
to
the
β
-
spin
LUMO orbital (
#324B
)
(
Figure
4
)
, which
involve charge
transfer from the peripher
al
oxygen atoms of the alkoxy
substituents to the B
12
cluster core.
These results together with
the spin density plot
calculated for
[
1
]
+
(S
I Section S9.4
) indicate
that
the unpair
ed
electron is
predominantly
delocalized over the
electronegative
oxygen
atom
s
as well as the B
12
cage
.
Scheme
2
.
P
rotocol for the reduction of [
1
][SbCl
6
] with
ferrocene
.
Having established the electrochemical reversibility of the
[
1
]
0/•+
redox event,
w
e
next assessed
its
chemical
reversibility
through
one
-
electron
reduction of
[
1
][SbCl
6
]
with ferrocene
(
Fc
)
(Scheme
2
)
.
T
reatment of
[
1
][SbCl
6
]
with
ferrocene resulted in
the
rapid
generation of
1
as assayed by
1
H and
11
B NMR
spectroscopic analys
e
s of the crude reaction mixture
(SI Section
S
6
)
.
The clean chemical
reversibility of this one
-
electron redox
process
is indicative that the
B
12
(O
-
3
-
methylbutyl
)
12
architecture
maintain
ed
its integrity
during the course of the oxidation of
1
to
[
1
]
+
without suffering any deleterious processes such as
degradation
or loss of alkoxy substituents, and that the [
1
]
+
species
is
in fact
a well
-
defined
and
intact cluster.
This work presents
the reversible electrochemical and
chemical oxidation of
hypercloso
-
1
, to provide the first example of
a super
-
oxidized,
spin
delocalized
B
12
-
based
radical cation
whose
identity
was substantiated by various spectroscopic
characterization
. The discovery of [
1
]
•+
provides an entry point into