.
Angewandte
Communications
[
11]
oxo complexes.
In contrast to the previously employed
(tris[2-(N-tetramethylguanidyl)ethyl]amine)
Similarly, treatment of 1 with PhIO in the presence of other
+
3
2+
TMG tren
redox-inactive metal ions, such as Y and Zn , results in the
generation of the characteristic absorption band at 600 nm
corresponding to the formation of 2-Y and 2-Zn, respectively.
It is notable that reactions of 1 with PhIO in the absence of
redox-inactive metal ions or in the presence of weaker Lewis
3
[8]
ligand, which enforced a trigonal-bipyramidal geometry at
the Co center stabilizing a S = 3/2 ground state, TAML can
provide access to a square-pyramidal cobalt(IV)-oxo center
that would be a prerequisite for the targeted S = 1/2 state.
Reactions of equimolar amounts of deprotonated TAML and
cobalt(III) acetylacetonate in tetrahydrofuran afforded the
previously reported purple [(TAML)Co ] anion, which is
obtained as a lithium salt Li[(TAML)Co ]·3(H O) (1)
containing three cocrystallized molecules of water (see the
Supporting Information (SI), Tables S1 and S2 and Figures S1
and S2). Complex 1 is paramagnetic with S = 1 (meff = 3.1 Bohr
magnetons) ground state, as expected for a Co ion in the
observed square-planar geometry. Cyclic voltammetry (CV)
measurements with 1 at 258C in CH CN give a reversible
oxidation wave centered at 1.00 V vs. saturated calomel
electrode (SI, Figure S3); coulometric measurements show
that the oxidation corresponds to a 1e process. The
reversibility of this oxidation wave at room temperature
suggests that a formal Co state is thermally and kinetically
IV
[13]
acidic metal ions, such as Mg(CF SO ) , Ca(CF SO ) , and
3
3
2
3
3 2
Sr(CF SO ) , does not form the 2-M species, thereby estab-
3
3 2
III
À
[12]
lishing the importance of the Lewis acidity of the redox-
III
inactive metal ions in trapping the 2-M intermediates. Never-
2
3
+
3+
theless, the absorption features of the 2-M (M = Sc , Ce ,
3
+
2+
Y , and Zn ) complexes are all identical irrespective of the
nature of the redox-inactive metal ions.
III
The nature of these blue species can be established by
a variety of spectroscopic techniques; characterizations were
performed mainly on 2-Sc, because it was the most stable
species among the trapped 2-M complexes. Thus, cold-spray
ionization time-of-flight mass spectrometry (CSI-TOF MS) of
2-Sc reveals two high-resolution ion peaks at a mass-to-charge
(m/z) ratio of 785.1010 and 803.1042, whose mass and isotope
distribution patterns correspond to respective formulations of
3
À
IV
1
6
accessible.
[(TAML)Co( O)(Sc)(CH CN) (CH OH) (CF SO )] (calcu-
3 2 3 2 3 3
1
6
In agreement with the electrochemical data, 1 is readily
oxidized in the presence of cerium ammonium nitrate (CAN)
in acetone at 58C to form a metastable blue species (2-Ce)
with a half-life (t ) of 20 min (SI, Figure S4, inset). The blue
color is associated with a band at 600 nm (e = 7200m cm )
with a shoulder near 730 nm (SI, Figure S4). We tentatively
assign these bands to be ligand-to-metal charge transfer
lated m/z = 785.1057) and [(TAML)Co( O)(Sc)(CH CN) -
3 4
(CF SO )] (calculated m/z = 803.1064) (SI, Figure S5). Upon
3
3
1
8
18
introduction of O into 2-Sc using PhI O, mass shifts from
785.1057 to 787.1015 and from 803.1042 to 805.1021 occur (SI,
Figure S5 inset), thereby demonstrating that an oxygen atom
from PhIO is incorporated into 2-Sc.
1
/2
À1
À1
In contrast to 1, which is EPR silent, the X-band EPR
(
LMCT) in origin, which presumably arise from transitions
spectrum of 2-Sc at 5.0 K shows a highly rhombic signal at g =
x
IV
from the amide nitrogens of TAML to the Co center in 2-
Ce; as expected, these bands are significantly blue-shifted in
the corresponding Co
4
treatment of 1 with iodosylbenzene (PhIO) in the presence
of Sc(CF SO ) also generates a blue species (2-Sc) with
2.57, g = 2.16, and g = 2.03 (Figure 1, inset). Additionally,
y
z
5
9
a Co hyperfine pattern is observed at g according to an A
y y
component of the hyperfine coupling tensor of 52 ꢀ 10 cm .
III
À4
À1
complex 1 [l = 510 nm (e =
max
À1
À1
À1
À1
000m cm ) and 650 (e = 1200m cm )]. Interestingly,
The observed large g-anisotropy (Dg = g Àg ) of 0.54 is
z
x
consistent with a cobalt-centered radical (S = 1/2) resulting
IV
[14]
from a low-spin Co configuration in the ground state of 2-
Sc; this is further corroborated by X-ray absorption and DFT
studies (see below). Spin quantification of the EPR signal can
account for 70% of the total cobalt spins present in solution,
thereby showing that 2-Sc represents the major product of the
3
3 3
absorption features indistinguishable from those of 2-Ce, but
-Sc is slightly more stable with a t1/2 of 35 min (Figure 1).
2
3
+
reaction of 1 with PhIO in the presence of Sc ion. EPR
spectra of other 2-M complexes were also measured and
[
15]
found to be identical to 2-Sc.
All our attempts to collect resonance Raman spectra
proved unsuccessful owing to the photodecomposition of 2-Sc
under laser irradiation. Hence, structural evidence for the
nature of 2-Sc was derived from X-ray absorption spectros-
copy (XAS) studies at the Co K-edge (Figure 2; SI, Figures S6
and S7). A frozen acetone solution of 2-Sc exhibits a K-edge
at 7721.5 eV, an energy value 0.9 eV higher than that
associated with its cobalt(III) precursor 1, thereby supporting
IV
metal-centered oxidation to the Co state. The principal
feature of the inner-sphere scattering peaks in the extended
X-ray absorption fine structure (EXAFS) spectra of 2-Sc at R’
ꢀ
1.40 ꢁ can be best fitted (SI, Table S3, fit 11) by considering
Figure 1. UV/Vis spectra of 2-Sc (gray line) observed in the reaction of
(black line, 0.25 mm) and PhIO (3.0 equiv) in the presence of
Sc(CF SO ) (20 equiv) in acetone at 58C. Inset shows the experimen-
two shells: an 0.7 O/N scatterer at 1.67 ꢁ (assigned to the
1
IV
Co ÀO unit) and a further shell of 4.3 O/N scatterers at
3
3 3
1
.86 ꢁ (corresponding to the N-donors of TAML). Fitting was
tal (gray line) and simulated (black line) X-band EPR spectrum of 2-Sc
0.5 mm) at 10 K. For simulation parameters see text.
(
also performed by considering only one shell of 5.0 O/N
1
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 10403 –10407