Sequential phenolate oxidations in octahedral cobalt(III) complexes with [N2O3] ligands

Article abstract of DOI:10.1002/ejic.201200171

Three six-coordinate cobalt(III) complexes containing electron-rich phenolato pentadentate [N₂O₅] ligands were synthesized and characterized, namely, [Co^III(L¹)(MeOH)] (1), [Co ^III(L²)(MeOH)] (2) and [Co^III(L ³)(MeOH)] (3), where L¹, L² and L³ are the triply deprotonated, triply negative form of(E)-6,6'-[({2-[(3,5-di-tert- butyl-2-hydroxybenzylidene)amino]phenyl}azanediyl)bis(methylene)]bis(2, 4-di-tert-butylphenol), (E)-6,6'-[({3-[(3,5-di-tert-butyl-2-hydroxybenzylidene) amino]naphthalen-2-yl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol) and (E)-6,6'-[({2-[(2-hydroxy-3-methoxybenzylidene)amino]phenyl}azanediyl) bis(methylene)]bis(2,4-di-tert-butylphenol), respectively. Crystal structures were obtained for 1-3 and reveal a hexacoordinate cobalt(III) ion bound to the [N₂O₃] donors of each ligand and a methanol molecule occupying the sixth position. The complexes exhibited comparable electronic behavior dominated by phenolate→cobalt charge transfer processes and four redox-accessible states involving three distinct phenolato/phenoxyl radical couples and a fourth process associated with the Co^II/Co ^III couple. The redox processes were cycled 30 times without major decomposition at the surface of the electrode for 1 and 2, indicating that the oxidized species should be substitutionally inert and do not degrade significantly upon cycling. Electronic-structure DFT calculations on models 1' and 2' favor the generation of localized phenoxyl radicals and suggest distinctive oxidation sequences associated to the nature of the ligands. Copyright

Full text of DOI:10.1002/ejic.201200171

Job/Unit: I20171
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FULL PAPER
DOI: 10.1002/ejic.201200171
1
Sequential Phenolate Oxidations in Octahedral Cobalt(III) Complexes with
[N2O3] Ligands
Marco M. Allard,[a] Fernando R. Xavier,[a] Mary Jane Heeg,[a] H. Bernhard Schlegel,[a] and
Cláudio N. Verani*[a]
Keywords: Cobalt / Electrochemistry / Redox-active ligands / DFT calculations
6
Three six-coordinate cobalt(III) complexes containing elec-
tron-rich phenolato pentadentate [N2O5] ligands were syn-
thesized and characterized, namely, [CoIII(L1)(MeOH)] (1),
[CoIII(L2)(MeOH)] (2) and [CoIII(L3)(MeOH)] (3), where L1, L2
and L3 are the triply deprotonated, triply negative form of
(E)-6,6Ј-[({2-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]-
phenyl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol),
(E)-6,6Ј-[({3-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]-
naphthalen-2-yl}azanediyl)bis(methylene)]bis(2,4-di-tert-but-
ylphenol) and (E)-6,6Ј-[({2-[(2-hydroxy-3-methoxybenzyl-
idene)amino]phenyl}azanediyl)bis(methylene)]bis(2,4-di-
tert-butylphenol), respectively. Crystal structures were ob-
tained for 1–3 and reveal a hexacoordinate cobalt(III) ion
bound to the [N2O3] donors of each ligand and a methanol
molecule occupying the sixth position. The complexes exhib-
ited comparable electronic behavior dominated by phenol-
ateǞcobalt charge transfer processes and four redox-access-
ible states involving three distinct phenolato/phenoxyl radi-
cal couples and a fourth process associated with the CoII/CoIII
couple. The redox processes were cycled 30 times without
major decomposition at the surface of the electrode for 1 and
2, indicating that the oxidized species should be substitution-
ally inert and do not degrade significantly upon cycling.
Electronic-structure DFT calculations on models 1Ј and 2Ј
favor the generation of localized phenoxyl radicals and sug-
gest distinctive oxidation sequences associated to the nature
of the ligands.
21
26
31
11
16
Introduction
system containing redox-active ligands and metal centers.[6]
An added complication to study iron(III) complexes in
solution is the relative lability of the metal ion, which war-
rants the use of more inert diamagnetic transition metals.
Therefore, in this study we investigate the use of the co-
balt(III) ion associated with an [N2O3] platform aiming to
describe the structural and electrochemical behavior of the
resulting complexes.
The presence of an inert and low-spin ion prevents anti-
ferromagnetic coupling between the metal atom and the re-
sulting phenoxyl radicals allowing investigation of the se-
quence by which the phenolates are oxidized. We compare
three ligands, HL1 or (E)-6,6Ј-[({2-[(3,5-di-tert-butyl-2-hy-
droxybenzylidene)amino]phenyl}azanediyl)bis(methylene)]-
bis(2,4-di-tert-butylphenol), HL2 or (E)-6,6Ј-[({3-[(3,5-di-
tert-butyl-2-hydroxybenzylidene)amino]naphthalen-2-yl}-
azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol) and
HL3 or (E)-6,6Ј-[({2-[(2-hydroxy-3-methoxybenzylidene)-
amino]phenyl}azanediyl)bis(methylene)]bis(2,4-di-tert-
butylphenol), respectively. Three six-coordinate CoIII com-
plexes containing electron-rich phenolato pentadentate
[N2O5] ligands were synthesized and characterized, namely,
The notion that redox chemistry is carried out primarily
by the metal centers of coordination complexes has long
been challenged,[1] and it has been unequivocally estab-
lished that ligands can act as electron reservoirs. Current
concerns in the field involve the incorporation of Earth-
abundant metals along with ligand design to accomplish
direct or supported catalytic substrate transformations. This
goal depends on the understanding of optimal geometrical
parameters that determine redox-based activity. Our group
has been investigating the relationship between geometrical
and redox factors in phenol-rich systems containing tri-
valent iron[2] and cobalt[3] ions. We have shown that penta-
coordinate iron(III) species seem to stabilize ligand-based
redox cycling, whereas the 3d6 electronic configuration of
cobalt(III) ions reinforce hexacoordinate complexes. In re-
cent reports we have shown that (i) in five-coordinate
iron(III) complexes, the electrochemical behavior is heavily
dependent on ligand modifications,[4] and that (ii) cobalt
can form hexacoordinate phenoxazinylato ligands capable
of six ligand-based redox processes.[5] These works have re-
iterated some of the many challenges of using an open-shell
56
61
66
71
76
36
41
46
51
[CoIII(L1)(MeOH)] (1), [CoIII(L2)(MeOH)] (2) and [CoIII
-
(L3)(MeOH)] (3), as shown in Scheme 1, to investigate vari-
ations in the resulting complexes, such as in (i) their struc-
tural geometry, (ii) the stability of radical formation,
(iii) the redox reversibility and identification of the specific
redox centers.
[a] Department of Chemistry, Wayne State University
5101 Cass Ave., Detroit, MI 48202, USA
Fax: +1-313-577-8822
E-mail: cnverani@chem.wayne.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200171.
Eur. J. Inorg. Chem. 0000, 0–0
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M. M. Allard, F. R. Xavier, M. J. Heeg, H. B. Schlegel, C. N. Verani
FULL PAPER
Scheme 1. Complexes [CoIII(L1)(MeOH)] (1), [CoIII(L2)(MeOH)] (2) and [CoIII(L3)(MeOH)] (3).
with [M + H]+ peaks at m/z = 817.6, 867.7 and 735.5 for 1,
81
Results and Discussion
2 and 3, respectively. Besides the expected bands associated
with the ligands, the IR spectra of 1–3 display a band of an
azomethine group C=N observed at around 1600 cm–1. The 106
absence of perchlorate anions in the IR spectra supports
the neutrality of these species.
Synthetic Approach
The selected ligands H3L1 and H3L2 were synthesized by
treatment of their respective diaminoarylene precursors
with 2,4-di-tert-butyl-6-(chloromethyl)phenol under basic
conditions as previously reported.[4b,4c] The ligands were
obtained with overall yields of 40–50% and characterized
by 1H NMR spectroscopy, electrospray ionization mass
spectrometry (ESIMS), and IR spectroscopy. It is worth
86
91
96
Molecular Structures
Complexes 1, 2, and 3 crystallized from methanol solu-
mentioning that the ligand moiety is oxidized upon com- tions and consist of discrete mononuclear neutral units. The 111
plexation with the metal center, thus resulting in a C=N ORTEP diagrams for 1–3 can be seen in Figure 1 showing
bond on the monosubstituted amine for 1 and 2. This pro- a perspective view of each complex along with the selected
cess has been detailed previously.[4b,4c] Compound 3 incor- atom numbering schemes The three complexes are structur-
porates a Schiff base C=N bond in the ligand by design. ally similar six-coordinate metal complexes, each of which
The ligand H3L3 was synthesized in two steps to allow for with an [N2O3] trianionic and pentadentate ligand and a 116
the introduction of different substituted phenolate groups methanol molecule occupying the sixth position. Selected
(see Figure S1, Supporting Information). All complexes bond lengths and bond angles are listed in Table 1. The
were obtained as dark brown powders upon treatment of observed Co–N bonds lengths are ca. 1.86 Å for N1 in the
the respective ligand dissolved in dry dichloromethane with azomethine group and 1.97 Å for tertiary N2.
hexahydrated cobalt(II) perchlorate dissolved in dry meth-
For the discussion of the Co–O bond lengths, as well as 121
101 anol. The complexes yielded good elemental analyses and the DFT section, we will use the nomenclature indicated
diagnostic ESI mass spectra in the positive mode (ESI+) in Scheme 1; phenolates A and AЈ refer to the chemically
Figure 1. ORTEP diagram of [CoIII(L1)(MeOH)] (1) (left), [CoIII(L2)(MeOH)] (2) (center), and [CoIII(L3)(MeOH)] (3) (right).
2
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Phenolate Oxidations in Octahedral Cobalt(III) Complexes
Table 1. Selected bond lengths [Å] and angles [°].
[CoIIIL1(MeOH)] (1)
[CoIIIL2(MeOH)] (2)
[CoL3(MeOH)] (3)
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(3)
Co(1)–O(4)
Co(1)–N(1)
Co(1)–N(2)
N(2)–C(14)
N(1)–C(7)
1.883(2)
1.932(2)
1.905(0)
1.994(1)
1.864(7)
1.980(6)
1.5025(19)
1.309(2)
1.421(4)
1.468(2)
86.35(4)
86.19(5)
89.08(5)
82.13(5)
86.55(5)
95.29(5)
91.72(5)
93.65(5)
86.43(5)
93.52(5)
95.35(5)
93.68(5)
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(3)
Co(1)–O(4)
Co(1)–N(1)
Co(1)–N(2)
N(2)–C(17)
N(1)–C(7)
1.888(1)
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(3)
Co(1)–O(5)
Co(1)–N(1)
Co(1)–N(2)
N(2)–C(14)
C(7)–N(1)
1.8741(15)
1.9181(15)
1.9057(15)
1.9833(15)
1.8799(18)
1.9722(18)
1.516(3)
1.305(3)
1.409(3)
1.493(3)
86.96(7)
87.73(7)
170.36(7)
84.83(6)
90.28(7)
93.25(7)
176.36(7)
93.68(7)
85.98(8)
177.45(7)
94.71(7)
91.05(7)
1.950(1)
1.892(1)
2.010(1)
1.868(2)
1.974(2)
1.471(3)
1.302(2)
1.428(2)
1.511(2)
86.32(6)
86.28(6)
88.34(6)
88.15(6)
87.36(6)
95.91(7)
89.85(6)
92.44(7)
86.88(7)
94.02(6)
91.47(7)
93.22(6)
N(1)–C(8)
N(2)–C(13)
N(1)–C(8)
N(2)–C(18)
N(1)–C(8)
N(2)–C(21)
O(1)–Co(1)–O(3)
O(2)–Co(1)–O(1)
O(4)–Co(1)–O(3)
O(2)–Co(1)–O(4)
O(1)–Co(1)–O(4)
N(1)–Co(1)–O(1)
N(2)–Co(1)–O(4)
N(1)–Co(1)–O(3)
N(1)–Co(1)–N(2)
O(3)–Co(1)–N(2)
O(2)–Co(1)–N(1)
O(2)–Co(1)–N(2)
O(1)–Co(1)–O(3)
O(2)–Co(1)–O(1)
O(4)–Co(1)–O(3)
O(2)–Co(1)–O(4)
O(1)–Co(1)–O(4)
N(1)–Co(1)–O(1)
N(2)–Co(1)–O(4)
N(1)–Co(1)–O(3)
N(1)–Co(1)–N(2)
O(3)–Co(1)–N(2)
O(2)–Co(1)–N(1)
O(2)–Co(1)–N(2)
O(1)–Co(1)–O(5)
O(1)–Co(1)–O(2)
O(3)–Co(1)–O(2)
O(2)–Co(1)–O(5)
O(3)–Co(1)–O(5)
N(1)–Co(1)–O(2)
N(1)–Co(1)–O(5)
O(2)–Co(1)–N(2)
N(1)–Co(1)–N(2)
O(1)–Co(1)–N(2)
O(3)–Co(1)–N(2)
N(2)–Co(1)–O(5)
identical moieties appended to the tertiary N2, and phenol- the notion that steric effects, rather than electronic effects
ate B refers to the phenolato moiety connected to the azo- predominate in these complex systems with a low-spin co-
126 methine N1. The Co–OphenolateA and Co–OphenolateAЈ bond balt(III) ion.
161
lengths reach 1.90–1.95 Å, whereas Co–OphenolateB is
shorter at 1.88 Å. The three compounds display structures
Redox Properties
consistent with having a low-spin cobalt(III) atom.[3b,7]
In complex 1 the cobalt center can be considered pseudo-
131 octahedral, with all twelve angles to the adjacent donor
atoms ranging from 82.13(5) to 93.68(5)° as compared to
the 90° required for a perfect octahedron. The longest bond
to cobalt(III) is Co–O(4) at 1.994 Å, consistent with a pro-
tonated methanol molecule. The shortest bond to CoIII is
136 Co–N(1) at 1.864 Å, with N(1) being the Schiff base nitro-
gen atom [C=N, N(1)–C(7) 1.309 Å]. The phenolates at-
tached to N(2) are trans to one another (O3 and O2), where
Co–O(2) is 1.932 Å. The Co–O(3) bond is slightly shorter at
1.905 Å. The dihedral angle of 1.4° between the azomethine
141 phenolate B and the phenylene moiety [C(8)–N(1)–C(7)–
C(6)] is indicative of extensive π-conjugation.
The cyclic voltammograms (CV) of 1, 2, and 3 were mea-
sured in dichloromethane by using TBAPF6 (TBA = tetra-
butylammonium) as electrolyte to evaluate the redox poten-
tials in which metal- and ligand-based redox processes take 166
place, as well as to assess the ability of these complexes
to withstand extensive redox cycling necessary for eventual
catalytic activity. The CVs for 1–3 are shown in Figure 2,
while half-wave potentials (E1/2), peak potential separations
(ΔE) and peak current ratios (|ipc/ipa|) are summarized in 171
Table 2. The cathodic waves observed near –0.65 V vs. Fc+/
Fc are similar for 1–3 and are assigned to the CoIII/CoII
couple, based on literature values[7d,7e,7g,7h] for related sal-
ophen-type complexes.
Complex 2 is structurally related to 1, where the pseudo-
octahedral cobalt ion is bound to an [N2O4] donor set. The
longest bond to cobalt(III) is Co–O(4) at 2.010 Å, with the
146 protonated methanol molecule. The Co–N(1) bond with the
azomethine nitrogen atom is 1.868 Å long, whereas the Co–
N(2) bond with the tertiary nitrogen atom equals 1.974 Å.
The Co–Ophenolate bonds show similar trends as in 1.
Finally, in complex 3 similarities to 1 and 2 are observed,
151 but the replacement of tert-butyl groups by a single meth-
oxy group in the position 2 of the ring B decreases the steric
bulk and allows for small rearrangements that lead to a less
planar framework and a slightly shorter Co–O(1) bond of
1.874 Å This difference seems small if considering the pres-
156 ence of an electron-withdrawing group. On the other hand,
the Co–O(2) bond length is 1.918 Å, thus shorter than in
Figure 2. Cyclic voltammetry of [CoIII(L1)(MeOH)] (1), [CoIII(L2)-
(MeOH)] (2) and [CoIII(L3)(MeOH)] (3) in dichloromethane by
the equivalent phenolate rings A for 1 and 2 and reinforcing using TBAPF6. Potentials are given vs. Fc/Fc+.
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M. M. Allard, F. R. Xavier, M. J. Heeg, H. B. Schlegel, C. N. Verani
FULL PAPER
Table 2. Redox parameters for 1–3.[a]
CoIII/CoII
E1/2 (ΔE; |ipc/ipa|)
L–3/L–2·
E1/2 (ΔE; |ipc/ipa|)
L
–2·/L–··
L
–··/L···
E1/2 (ΔE; |ipc/ipa|)
E1/2 (ΔE; |ipc/ipa|)
1
2
3
–0.65
(0.16; 0.95)
–0.67
(0.12; 0.65)
–0.63
(0.13; 0.80)
0.23
(0.14; 0.90)
0.17
(0.12; 0.95)
0.21
(0.12; 0.60)
0.63
(0.16; 1.1)
0.57
(0.11; 0.90)
0.65
(0.14; 0.80)
1.0V
(0.15; ca. 0.65)
0.92
(0.14; ca. 0.60)
0.98
(0.20; 0.35)
[a] E1/2 and ΔE in Volt. Values of |ipc/ipa| are approximate. Measurements in 1ϫ10–3 m dichloromethane solutions, 0.1 m TBAPF6 (Fc+/
Fc: E1/2 = 0.47 V, ΔE = 0.12 V, |ipc/ipa| = 0.95). Scan rates are 100 mV/s.
176
In spite of this accepted attribution, these waves seem Electronic Behavior
unusually reversible in nature, and considerable contri-
The UV/Vis spectra of compounds 1–3 were taken in 221
1.0ϫ10–3 m DCM solutions and are shown in Figure 3. In-
tense πǞπ * intraligand bands were observed in the UV
region, whereas phenolateǞCoIII charge-transfer bands
were found between 450 and 750 nm (22000–13000 cm–1).
Although exact attributions are difficult, the low-energy 226
bands around 750 nm (ca. 1000 Lmol–1 cm–1) are tentatively
ascribed to pπphenolate (out-of-plane)Ǟdσ*CoIII contributions,
whereas the more intense bands at around 450 nm
(ca. 9000 Lmol–1 cm–1) are most likely attributed to
pπphenolate (in-plane)Ǟdσ*CoIII transitions.[10] When this phen- 231
olate(out-of-plane)ǞCoIII band was compared in octahedral
species with [(Namine)2(Npyridine)2(Ophenolate)2] donor sets,[3b]
where the phenolates were either unsubstituted or con-
tainied tert-butylphenolates like species 1 and 2, this LMCT
band shifted bathochromically from 430 to 470 nm. How- 236
ever, no clear trend can be drawn for the contribution of
the methoxy-substituted phenolate present in 3. The com-
parison of spectroscopic changes associated with tert-butyl-
and halogeno-substituted phenolates, as well as their elec-
tron-acceptor capabilities, will be the topic of an upcoming 241
study from our group.
bution from the phenylene ring to the composition of the
LUMO is expected. At first glance, the anodic side of the
voltammograms resembles that of their gallium and iron
181 counterparts in equivalent [N2O3] environments,[4c] in which
the generation of three phenoxyl radicals was established.
We observe three anodic waves for 1 and 2 centered at ap-
proximately 0.20 V (ΔE = 0.13 V), 0.60 V (ΔE = 0.15 V),
and 1.0 V (ΔE = 0.16 V) associated with the phenolate/
186 phenoxyl couple.[1e,2c,4,8] The amplitudes of these waves
suggest that they are successive one-electron oxidations, and
the two first processes appear to be reversible/quasirevers-
ible, with appropriate peak-potential separations and peak-
current ratios, when compared to the standard Fc/Fc+ cou-
191 ple (ΔE = 0.12 V; |ipc/ipa| = 0.95). The last ligand-based oxi-
dation at ca. 1.0 V is usually the least reversible process.
This is explained by considering that phenolate oxidation
leads to phenoxyl groups where the ring displays uncharged
carbonyl-like nature that elongates the Co–O bonds. Com-
196 plex 3 shows the same general features seen for 1 and 2 but
in general displaying less reversibility. If the third process is
avoided, two consecutive one-electron oxidations are avail-
able, and cyclability of these processes can be evaluated.
Redox cyclability of multiple electrochemical processes is
201 fundamental for potential uses in multielectronic catalysis.
In previous papers we have shown dependence between cy-
cling and the geometry adopted by the central atom. Octa-
hedral [FeIII/phenolate] systems tend to show limited redox
cyclability, whereas five-coordinate FeIII systems display re-
206 versible processes with minor decay after 50 cycles. Square-
planar [CuII/phenolate] systems were also investigated by
our group[9] and seem to support reversibility when the phe-
nolates are connected to amines, rather than azomethine
groups. Redox cycling in species 1–3 was attempted; the two
211 ligand-centered processes along with the metal-based pro-
cess were cycled thirty times. Minimal decomposition at the
surface of the electrode was observed for 1 and 2, as indi-
cated by a nearly perfect superposition of the first and last
CVs. (see Figures S3, S4) Complex 3 shows considerable de-
216 composition, in good agreement with the observed ΔE and
|ipc/ipa| values (Figure S5). These results suggest that redox
cycling of the phenolate/phenoxyl couple is viable in some
octahedral cobalt(III) species.
Figure 3. Selected features in the UV/Vis spectra of 1.0ϫ10–3
dichloromethane solutions of 1, 2 and 3.
m
4
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Phenolate Oxidations in Octahedral Cobalt(III) Complexes
Spectroelectrochemical Behavior
for the first ligand oxidation is reached. Unlike in the re-
ductive process, the initial spectrum of the parent species 266
shows an increased intensity for the band at 470 nm. Other
diagnostic new bands are observed at 400 nm along with a
broad band at 755 nm. These bands are characteristic of
the presence of a phenoxyl radical. The band at 470 nm
overlaps with the original phenolateǞCoIII LMCT process 271
and has been assigned to a πǞπ* transition of the phenoxyl
radical,[11] whereas the band at 755 nm is attributed to a
phenolateǞphenoxyl intravalent charge transfer. Species 1
was also treated with oxidative bulk electrolysis and ana-
lyzed by EPR spectroscopy in DCM at 120 K (shown as an 276
inset in Figure 4c). The expected lack of signal characteris-
tic of a 3d6 low-spin species with S = 0 is replaced by a
distinctively sharp signal at 3320 G with g = 2.02 and line
width of ca. 40 G typical of a coordinated phenolate.[12]
Spectroelectrochemical measurements were performed
for 1 in order to investigate the pattern of changes in the
246 electronic properties of the cobalt(III) complexes under oxi-
dative and reductive conditions. Although this study is pri-
marily focused on the investigation of ligand oxidation,
spectroelectrochemical reduction was also probed for com-
pound 1, as shown in Figure 4a. By applying a potential of
251 –0.80 V vs. Fc/Fc+, a negative potential was generated to
reduce the cobalt(III)/(II) couple seen at –0.65 V vs. Fc/Fc+.
The LMCT absorption band at 470 nm decreased along
with other less intense transitions in the visible portion of
the spectrum in agreement with a CoIII + e– Ǟ CoII redox
256 process. Additionally, isosbestic points were observed below
400 nm and at 465 nm close to the original phenolateǞcob-
alt(III) LMCT process. A new absorption band appeared at
440 nm and was attributed to an intraligand process, likely
involving a phenolateǞimine charge transfer. The first oxi-
261 dation of the ligand in complex 1 was investigated in detail
by the application of a potential at +0.50 V vs. Fc/Fc+ to
the cell and is shown in Figure 4b. As in the reductive pro-
cess, the selected potential assures that the anodic potential
Electronic Structure Calculations
281
Aiming to gain understanding on the electronic proper-
ties of 1 and 2, we used the comparable models 1Ј and 2Ј,
where the tert-butyl groups appended to the phenolate rings
were replaced by hydrogen atoms (H) to make the calcula-
tions more practical. Calculations were performed by using 286
the B3PW91/6-31G(d) level of theory[13] and included sol-
vent effects (DCM) by using the IEF-PCM polarizable con-
tinuum model.[14] We were interested in the comparison be-
tween the experimental structural, electronic and redox
properties observed for 1 and 2, as well as in the sequence 291
in which the phenolates are oxidized. The geometries for
models 1Ј and 2Ј are in good agreement with the experimen-
tally solved molecular structures for 1 and 2. It is note-
worthy that since there are no unpaired electrons from Co-
III, no magnetic interactions between any phenoxyl radical– 296
CoIII metal center can be expected. The discussions will fol-
low the nomenclature introduced earlier, in which the vici-
nal phenolate rings A and AЈ are associated to the tertiary
nitrogen atom and the phenolate B is linked to the azo-
methine group.
301
Frontier orbitals are directly associated with the reactiv-
ity of complexes[3a,4,9–10,15] and can be useful to predict and
explain the behavior of complex molecules. For compounds
1 and 2 we expect that the oxidative redox chemistry will
be associated with the highest occupied molecular orbitals 306
HOMO, HOMO-1, and HOMO-2. The calculated energies
for these orbitals and the MO ladders are similar in models
1Ј and 2Ј, as indicated in Figure 5. The HOMOs have essen-
tially the same energy, whereas the LUMO for 1Ј is ca.
5 kcal/mol higher than that for 2Ј resulting in a HOMO – 311
LUMO gap of ca. 72 kcal/mol for 1Ј and ca. 67 kcal/mol
2Ј.
The composition of the HOMO for 1Ј involves a contri-
bution of ca. 77% from the phenolate AЈ, while the
HOMO-1 shows ca. 74% of the imino–phenol B, and the 316
HOMO-2 is predominantly based on phenolate A at ca.
78% (Table S1, Figure S6). Similarly, 2Ј shows the HOMO
as ca. 78% of AЈ, HOMO-1 as 67% of B and HOMO-2 as
Figure 4. Spectroelectrochemical features for 1 in 1.0ϫ10–3
dichloromethane solutions: (a) reduction, (b) oxidation, (c) EPR
spectrum.
m
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Job/Unit: I20171
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Date: 21-06-12 15:57:59
Pages: 11
M. M. Allard, F. R. Xavier, M. J. Heeg, H. B. Schlegel, C. N. Verani
FULL PAPER
Figure 5. MO diagrams for [CoIII(L1(–H))(MeOH)] (1Ј) (left) and [CoIII(L2(–H))(MeOH)] (2Ј) (right).
78% of A (Tables S1 and S2 and Figure S7). The lowest
Table 3. Mulliken charge and Mulliken spin analysis.[a]
unoccupied LUMO for 1Ј is ca. 61% based on the phenyl- 321
ene bridge moiety, and similarly the LUMO of 2Ј is ca. 68%
based on the naphthalene bridge.
Mulliken
spin
Mulliken
charge
Co–O
[Å]
C(ring)–O
[Å]
[1Ј]0 (S = 0)
CoIII
Ring A
Ring AЈ
Ring B (C=N)
It is noteworthy that the usually expected LUMO would
be composed of the remaining two empty eg*-like orbital
of the low-spin cobalt(III) ion, but in both models those 326
metal-centered orbitals contribute too heavily to the
LUMO+1 and LUMO+2.
–
–
–
–
1.05
–0.43
–0.45
–0.28
1.89
1.88
1.87
1.33
1.32
1.29
[1Ј]+ (S = 1/2)
Interested in the redox behavior of 1 and 2, we have mod-
CoIII
0.04
0.10
0.88
0.00
1.08
–0.29
0.16
eled the following compounds [1Ј]0 Ǟ [1Ј]+ Ǟ [1Ј]2+
Ǟ
Ring A
Ring AЈ
Ring B
1.85
1.89
1.86
1.33
1.28
1.30
[1Ј]3+ and [2Ј]0 Ǟ [2Ј]+ Ǟ [2Ј]2+ Ǟ [2Ј]3+ equivalent to the 331
expected radical-containing species [CoIII(L)MeOH]0
–0.22
Ǟ
[CoIII(L·)MeOH]+ Ǟ [CoIII(L··)MeOH]2+ Ǟ [CoIII(L···)-
MeOH]3+ observed experimentally by cyclic voltammetry
for both species. These results are shown in Table 3 and
Figure 6. We confirmed that each of the oxidative redox 336
processes is ligand-centered, as indicated by Mulliken spin
densities near zero and by the constant Mulliken charge at
ca. 1.10. It is convenient to visualize these processes by
using spin-density plots. Figure 6 (top) and Table 3 indicate
that the first oxidation, leading to [1Ј]+, is located on phe- 341
nolate ring A, while the second oxidation, associated with
[1Ј]2+, is on the chemically similar AЈ center. The last oxi-
dation, linked to [1Ј]3+ occurs on the azomethine-connected
phenolate B. Interestingly, for the [2Ј] redox series [Figure 6
(bottom)], the first oxidation, [2Ј]+, is similarly located on 346
phenolate A, but the second oxidation, associated with
[2Ј]2+, is now on ring B. This leaves the third oxidation,
[1Ј]3+ centered on ring AЈ. Compound 1 behaves in fashion
similar to that of our previously published[4a] high-spin
[1Ј]2+ (S = 2/2)
CoIII
0.04
1.01
0.99
0.01
1.11
0.33
0.31
–0.15
Ring A
Ring AЈ
Ring B
1.88
1.88
1.85
1.28
1.28
1.30
[1Ј]3+ (S = 3/2)
CoIII
0.03
1.01
1.01
0.93
1.15
0.37
0.36
0.43
Ring A
Ring AЈ
Ring B
1.88
1.87
1.88
1.28
1.28
1.27
[2Ј]0 (S = 0/2)
CoIII
–
–
–
–
1.05
Ring A
Ring AЈ
Ring B
–0.43
–0.45
–0.28
1.89
1.88
1.87
1.33
1.32
1.29
[2Ј]+ (S = 1/2)
CoIII
0.04
0.08
0.90
0.0
1.09
–0.30
0.18
Ring A
Ring AЈ
Ring B
1.85
1.90
1.86
1.33
1.28
1.30
–0.22
[2Ј]2+ (S = 2/2)
iron(III) species with the same ligand.
351
CoIII
0.05
0.06
0.96
0.78
1.12
–0.26
0.24
0.29
Ring A
Ring AЈ
Ring B
1.84
1.90
1.88
1.34
1.28
1.27
Conclusions
In this study we synthesized and characterized a series
of octahedral cobalt(III) complexes coordinated to [N2O3]
ligands and an MeOH occupying the sixth position. Species
[2Ј]3+ (S = 3/2)
CoIII
0.03
1.01
1.01
0.76
1.15
0.37
0.36
0.33
Ring A
Ring AЈ
Ring B
1.88
1.87
1.88
1.28
1.28
1.27
[CoIII(L1)(MeOH)] (1), [CoIII(L2)(MeOH)] (2) and [CoIII
-
356
(L3)(MeOH)] (3) show similar structural, spectroscopic and
[a] B3PW91/6-31G(d) IEF-PCM (dichloromethane).
redox properties. The electronic spectra are characterized
6
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Phenolate Oxidations in Octahedral Cobalt(III) Complexes
Figure 6. Spin-density plots of [CoIII(L1(–H))(MeOH)] (1Ј) (top) and [CoIII(L2(–H))(MeOH)] (2Ј) (bottom).
the reference Ag/AgCl electrode freshly prepared and calibrated vs.
by phenolateǞcobalt charge transfers, and their redox pro-
cesses are heavily influenced by the ligand. Cyclic voltam-
361 metry indicates that there are three accessible oxidations as-
sociated with the formation of the phenoxyl radical, and
one reduction. In spite of these similarities, subtle geometric
and electronic differences appear to lead to distinctive oxi-
dative sequences. Computational calculations have helped
366 elucidate the nature and location of these oxidations, while
for the phenylene-bridged 1 the phenolate oxidation se-
ferrocene.[16] A platinum wire was used as an auxiliary electrode,
and a vitreous carbon disk was used as a working electrode for the
cyclic voltammetry experiments. For the bulk electrolysis experi-
ment a vitreous carbon basket was used. Concentrations of the
analytes were 1.0ϫ10–3 m, and all supporting electrolytes were
0.1 m, except where otherwise stated. The Fc/Fc+ couple in CH2Cl2
was found to be 0.47 V (ΔE = 0.14 V), in good agreement with
literature data.[2c] First-derivative X-Band EPR spectra of
1.0ϫ10–3 m DCM solutions of 1 were performed with a Bruker
391
396
quence is given as A followed by AЈ followed by B, the ESP 300 spectrometer by using liquid nitrogen as the coolant.
naphthalene-bridged 2 shows a sequence A followed by B
followed by AЈ. We anticipate the rich redox chemistry ex-
X-ray Structural Determinations for Complexes 1, 2, and 3: Diffrac-
tion data were measured with a Bruker X8 APEX-II kappa-geome-
371 hibited by these species, with four one-electron responses, try diffractometer with Mo radiation and a graphite monochroma- 401
tor. Frames were collected as a series of sweeps with the detector
at 40 mm and 0.3 between each frame. Frames were recorded for
10 s. APEX-II[17] and SHELX-97[18] software were used in the col-
lection and refinement. Table 4 summarizes the data. Crystals of
[C52H73N2O4Co1] (1) appeared as dark rhomboids, and a sample
approximately 0.25ϫ0.20ϫ0.20 mm was used for data collection.
A total of 3162 frames were collected at 100 K, yielding 44204 re-
flections, of which 11752 were independent. The asymmetric unit
consists of one neutral complex, including 1 equiv. of coordinated
methanol. Crystals of [C57H79N2O5Co1] (2) appeared as dark rods,
and the fragment used was approximately 0.14ϫ0.14ϫ0.12 mm.
A total of 117784 reflections were recorded, of which 13041 were
unique. The asymmetric unit contains one cobalt complex with one
coordinated methanol molecule and one methanol solvate mole-
cule. Crystals of [C45.5H60N2O5Co1Cl1] (3) appeared as dark
rhomboids, and the sample used was approximately
0.20ϫ0.28ϫ0.16 mm. A total of 89973 reflections were recorded,
of which 21069 were unique. The asymmetric unit contains two
independent molecules, each associated with a coordinated meth-
to be an important electron reservoir motif towards our ul-
timate goal of multimetallic catalytic assemblies.
406
411
416
Experimental Section
Materials and Methods: Reagents and solvents were used as re-
ceived from commercial sources, unless otherwise noted. Dichloro-
methane and methanol were distilled from CaH2. Infrared spectra
were measured from 4000 to 400 cm–1 as KBr pellets with a Tensor
376
381
386
1
27 FTIR spectrophotometer. H NMR spectra were measured by
using Varian 300 and 400 MHz instruments. ESI (positive mode)
spectra were measured with a single-quadrupole Waters ZQ2000
mass spectrometer by using an electrospray/APCI or ESCi source.
Experimental assignments were simulated on the basis of peak po-
sition and isotopic distributions. Elemental analyses were per-
formed by Midwest Microlab, Indianapolis, IN. Cyclic voltam-
metric experiments were performed by using a B.A.S. 50 W voltam-
metric analyzer. A standard three-electrode cell was employed with
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Date: 21-06-12 15:57:59
Pages: 11
M. M. Allard, F. R. Xavier, M. J. Heeg, H. B. Schlegel, C. N. Verani
FULL PAPER
Table 4. Crystal data.[a]
1
2
3
Empirical formula
M
Space group
a [Å]
b [Å]
c [Å]
[C52H73N2O4Co]
849.05
P1
[C57H79N2O5Co]
931.15
P21/c
13.4355(8)
20.8538(13)
19.0011(12)
[C45.5H60N2O5CoCl]
808.35
P1
¯
¯
12.5503(3)
14.0106(3)
14.4339(4)
88.9690(10)
86.1680(10)
71.0990(10)
2395.78(10)
2
15.4747(3)
15.7986(3)
18.8968(4)
106.3500(10)
90.7000(10)
106.0900(10)
4238.93(15)
4
α [°]
β [°]
99.932(2)
γ [°]
V [Å3]
5244.0(6)
4
Z
T [K]
λ [Å]
100(2)
0.71073
1.177
0.403
4.07
100(2)
0.71073
1.179
0.375
4.85
100(2)
0.71073
1.268
0.514
5.10
Dcalcd. [gcm–3]
μ [mm–1]
R(F) [%][a]
Rw(F) [%][a]
10.08
9.80
14.20
[a] R(F) = Σ||Fo| – |Fc||/Σ|Fo| for IϾ2σ(I); Rw(F) = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2 for IϾ2σ(I).
2
2 2
421
anol molecule and one half dichloromethane solvate molecule.
CCDC-869371 (for 1), -869372 (for 2), and -869373 (for 3) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Preparation of the Ligand H3L2: 2,3-diaminonaphthalene (0.25 g,
1.6 mmol), 2,4-di-tert-butyl-6-(chloromethyl)phenol (1.2 g,
461
466
471
4.7 mmol) and triethylamine (0.50 g, 4.7 mmol) in dichloromethane
(80 mL) were refluxed at 40 °C for 48 h. After confirmation of reac-
tion by TLC, the solution was extracted with 5% aqueous Na2CO3
solution (3ϫ 50 mL). The solution was dried with MgSO4, filtered,
and the solvent was removed by rotatory evaporation to obtain
the crude ligand. The ligand was recrystallized from acetonitrile to
obtain a light brown crystalline powder. Yield 0.66 g (51.4%). M.p.
426
431
436
441
Spectroelectrochemistry: Spectroelectrochemical data were per-
formed at room temperature by employing an optically transparent
thin-layer cell, composed of two glass slides sandwiched against
each other and equipped with a U-shaped flat platinum working
electrode that extended to the outside for electrical contact. The
inner sides of these slides were coated with indium tin oxide (ITO)
(8–2 Ω/sq). A second platinum wire was used as the counter elec-
trode, and the Ag/AgCl wire was the reference electrode. The cobalt
complex was dissolved in dichloromethane and purged with argon
before being introduced into the cell through capillary interaction.
The electrochemical potentials applied to the cell were –800 and
+500 mV vs. Fc/Fc+ for individual measurements of the reductive
and oxidative processes, respectively. The selected potentials assure
that the respective reduction occurred at a more negative potential
than that of the cobalt(III)/(II) couple, and more positive than the
anodic potential for the first ligand oxidation. These potentials
were achieved by using a B.A.S. 50 W potentiometer, and the re-
sulting spectra were collected by using a Varian Cary 50 apparatus.
178–181 °C. IR (KBr): ν = 3638 (br., O–H), 3477 (s, N–H), 2960
˜
(s, C–H, alkane), 1595 (s, C=C aryl), 1286 (s, C–N), 1200 (s, C–O),
879 (br., C–H, phen) cm–1. 1H NMR (400 MHz, CDCl3): δ = 1.24–
1.35 (m, 54 H, multiple tert-butyl), 3.94–4.47 (m, 6 H, CH2), 7.03–
7.65 (m, 12 H, aryl) ppm. MS (ESI+ in CH2Cl2): m/z = 813.8 for
[C55H76N2O3 + H]+.
Preparation of the Ligand H3L3: (E)-2-[(2-Aminophenylimino)-
methyl]-6-methoxyphenol (2.7 g, 8 mmol), 2,4-di-tert-butyl-6-
(chloromethyl)phenol (2.2 g, 9 mmol) and triethylamine (2.4 g,
24 mmol) were refluxed in dichloromethane (80 mL) for 48 h. The
reaction solution was extracted three times with water by using a
separatory funnel, dried with Na2SO4, and the solvent was removed
by rotatory evaporation. The purple residue was recrystallized from
acetonitrile by keeping the solution at 0 °C for 24 h to form an
476
481
orange crystalline precipitate. M.p. 87–90 °C. IR (KBr): ν = 3600
˜
Electronic Structure Methods: Electronic structure calculations
were carried out with the Gaussian 09 suite of programs[19] by using
Density Functional Theory (DFT). The B3PW91/6-31G(d) level of
theory[13] was employed throughout. Geometries were fully opti-
mized without symmetry constraints, and stationary points were
verified by frequency analysis. Solvent effects in dichloromethane
were estimated by using the IEF-PCM polarizable continuum
model.[14] Molecular orbitals were plotted by using GaussView.
Cartesian coordinates of all optimized structures are provided as
Supporting Information.
1
(br., O–H), 2958 (s, C–H, alkane), 1615 (s, C=N, imine) cm–1. H
NMR (400 MHz, CDCl3): δ = 1.21–1.42 (m, 36 H, multiple tert-
446
451
butyl), 4.25 (s, 4 H, CH2), 4.68 (s, 3 H, OCH3), 6.89–7.29 (m, 11 486
H, aryl), 8.87 (s, 1 H, HC=N) ppm. MS (ESI+ in MeOH): m/z =
677.5 [C44H58N2O4 + H]+.
Preparation [CoIIIL1(MeOH)] (1): H3L1 (1.0 g, 1.3 mmol) was dis-
solved in anhydrous methanol and treated with NaOCH3 (0.2 g,
3.93 mmol) under argon. To the resulting solution was added
[Co(ClO4)2]·(H2O)6 (0.47 g, 1.31 mmol), and the mixture was
heated at 50 °C and stirred for 30 min. The solvent was removed
by rotatory evaporation, and the product was crystallized from
MeOH/DCM (1:2) to form single crystals suitable for X-ray analy-
sis. Yield: 0.66 g (60%). Suitable X-ray crystals were also obtained
by precipitating the complex with water and subsequent recrystalli-
491
496
Syntheses
Caution! Some procedures involve perchlorate salts for the synthesis
of neutral complexes. Although we have not experienced problems,
caution in the handling of these samples is required.
456
Preparation of the Ligand H3L1: The ligand H3L1 was synthesized
zation from MeOH/diethyl ether. Yield 75%. IR (KBr): ν = 2949–
˜
as previously reported.[4c]
2865 (C–H, tert-butyl), 1599–1475 (C=C, ring stretch), 1270 (C–O,
8
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/KAP1
Date: 21-06-12 15:57:59
Pages: 11
Phenolate Oxidations in Octahedral Cobalt(III) Complexes
stretch), 880–749 (C–H, bend out of plane) cm–1. 1H NMR
501 (400 MHz, [D6]DMSO): δ = 0.86 (s, 18 H, tert-butyl), 1.00 (s, 18
H, tert-butyl), 1.24 (s, 9 H, tert-butyl), 1.62 (s, 9 H, tert-butyl), 3.86
ences of the U.S. Department of Energy (DOE-BES) through the
SISGR-Solar Energy Program (Grant DE-FG02-09ER16120 to 561
C. N. V. and H. B. S.). DOE support to M. M. A. is acknowledged.
(d, 2 H, CH2), 4.86 (d, 2 H, CH2), 6.46 (s, 2 H, aryl), 6.53 (s, 2 H, Computer time allocated at the WSU – Grid System for the DFT
aryl), 6.88 (t, 1 H, aryl), 7.08 (t, 1 H, aryl), 7.10 (s, 1 H, aryl), 7.28 calculations is also acknowledged. We are grateful to Dr. Maurício
(s, 1 H, aryl), 7.53 (s, 1 H, N=CH), 7.65 (d, 1 H, aryl), 8.04 (d, 1 Lanznaster and Camille Imbert for partial preliminary experimen-
506
H, aryl) ppm. MS (ESI+ in MeOH): m/z = 817.6 [CoIIIL1 + H]+.
C52H73CoN2O4 (848.49): calcd. C 73.56, H 8.67, N 3.30; found C
73.61, H 8.71, N 3.28.
tal results and to Jason Sonk for help with the execution of the
calculations.
566
Preparation of [CoIII(L2)(MeOH)] (2): A Schlenk flask was flushed
with nitrogen for 5 min, then H3L2 (0.20 g, 0.25 mmol) and sodium
methoxide (0.040 g, 0.74 mmol) were dissolved in dry DCM
(80 mL). Dry MeOH (30 mL) was added by cannulation. [Co-
(ClO4)2]·(H2O)6 (0.073 g, 0.20 mmol) was added to a small round-
bottom flask, and purged with nitrogen. Dry MeOH (ca. 10 mL)
was added to dissolve the metal salt, and the solution was purged
again. The methanolic cobalt perchlorate solution was transferred
slowly with a cannula to the ligand solution. The ligand solution
turned from a milky color to a deep brown color as the cobalt
perchlorate solution was added. The mixture was stirred under ni-
trogen for 2 h, then under oxygen for 24 h. The reddish solution
was concentrated to a brown solid by rotary evaporation. The
crude complex was washed with water, and recrystallized by slow
concentration from methanol/diethyl ether (1:1) at room tempera-
[1] a) W. I. Dzik, J. I. van der Vlugt, J. N. H. Reek, B. de Bruin,
Angew. Chem. 2011, 123, 3416–3418; Angew. Chem. Int. Ed.
2011, 50, 3356–3358; b) W. Kaim, Inorg. Chem. 2011, 50, 9752–
9765; c) R. H. Crabtree, New J. Chem. 2011, 35; d) P. J. Chirik,
K. Wieghardt, Science 2010, 327, 794–795; e) F. Thomas, in
Stable Radicals, John Wiley & Sons, Ltd., Chichester, West Sus-
sex, UK, 2010, pp. 281–316.
[2] a) R. Shakya, M. M. Allard, M. Johann, M. J. Heeg, E.
Rentschler, J. M. Shearer, B. McGarvey, C. N. Verani, Inorg.
Chem. 2011, 50, 8356–8366; b) F. D. Lesh, R. Shanmugam,
M. M. Allard, M. Lanznaster, M. J. Heeg, M. T. Rodgers, J. M.
Shearer, C. N. Verani, Inorg. Chem. 2010, 49, 7226–7228; c) C.
Imbert, H. P. Hratchian, M. Lanznaster, M. J. Heeg, L. M.
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[3] a) R. Shakya, S. S. Hindo, L. Wu, M. M. Allard, M. J. Heeg,
H. P. Hratchian, B. R. McGarvey, S. R. P. da Rocha, C. N. Ver-
ani, Inorg. Chem. 2007, 46, 9808–9818; b) R. Shakya, C. Im-
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McGarvey, M. Allard, H. B. Schlegel, C. N. Verani, Dalton
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[4] a) M. M. Allard, J. A. Sonk, M. J. Heeg, B. R. McGarvey,
H. B. Schlegel, C. N. Verani, Angew. Chem. 2012, 51, 3178–
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vey, C. N. Verani, Inorg. Chem. 2007, 46, 72–78; c) M. Lanznas-
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955–957.
511
516
521
526
531
571
576
581
586
591
596
ture to give small crystals. Yield 0.21 g (99%). IR (KBr): ν = 2955
˜
(s, C–H, alkane), 1604 (s, C=N), 1586 (s, C=C, aryl), 1262 (s, C–
N), 1202 (s, C–O), 865 (br., C–H, phen) cm–1. 1H NMR (400 MHz,
[D6]DMSO): δ = 0.82 (s, 18 H, tert-butyl), 0.86 (s, 18 H, tert-butyl),
1.26 (s, 9 H, tert-butyl), 1.64 (s, 9 H, tert-butyl), 3.96 (d, 2 H, CH2),
4.97 (d, 2 H, CH2), 6.45 (s, 2 H, aryl), 6.41 (s, 2 H, aryl), 6.46 (t,
1 H, aryl), 7.14 (t, 1 H, aryl), 7.30 (s, 1 H, aryl), 7.36 (d, 1 H, aryl),
7.59 (d, 1 H, N=CH), 7.71 (s, 1 H, aryl), 8.1 (2, 1 H, aryl), 8.62 (2,
1 H, aryl) ppm. MS (ESI+ in MeOH): m/z = 867.7 for [CoIIIL2 +
H]+. C56H75CoN2O4 (898.51): calcd. C 74.80, H 8.41, N 3.12;
found C 73.05, H 8.66, N 3.12.
Preparation of [CoIIIL3 (MeOH)] (3): H3L3(1.30 g, 1.9 mmol) was
dissolved in methanol (300 mL) in a 500 mL round-bottomed flask,
and [Co(ClO4)2]·(H2O)6 (0.80 g, 1.7 mmol) was added in the pres-
ence of excess triethylamine. A brown solution immediately formed
and was refluxed for 60 min. The solution was then cooled in an
ice bath, and the methanol was removed by rotary evaporation.
After drying, the product was recrystallized from dichloromethane
and yielded crystals suitable for X-ray analysis. Yield: 70%. IR
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stretch), 1246 (C–O, stretch), 890–739 (C–H, bend out of plane)
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(s, 18 H, tert-butyl), 1.00 (s, 18 H, tert-butyl), 3.15 (d, 3 H, CH3),
3.84 (d, 2 H, CH2), 3.88 (s, 3 H, CH3), 4.87 (d, 2 H, CH2), 5.74 (s,
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6.87 (t, 1 H, aryl), 6.97 (d, 1 H, aryl), 7.12 (t, 1 H, aryl), 7.64 (s, 1
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Acknowledgments
This research was made possible by the Division of Chemical Sci-
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. M. Allard, F. R. Xavier, M. J. Heeg, H. B. Schlegel, C. N. Verani
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Received: February 21, 2012
Published Online: ᭿
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Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20171
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Date: 21-06-12 15:57:59
Pages: 11
Phenolate Oxidations in Octahedral Cobalt(III) Complexes
Sequential Ligand Oxidations
676
M. M. Allard, F. R. Xavier, M. J. Heeg,
H. B. Schlegel, C. N. Verani* .......... 1–11
Sequential Phenolate Oxidations in Octa-
hedral Cobalt(III) Complexes with [N2O3]
Ligands
Keywords: Cobalt
/
Electrochemistry
/
681
686
Redox-active ligands / DFT calculations
Octahedral cobalt(III) complexes coordi-
nated to [N2O3] ligands and MeOH display
apparently similar structural, spectroscopic
and redox properties. Two compounds be-
have as electron reservoirs with four revers-
ible 1-electron ligand-based oxidations and
one metal-based reduction. In spite of these
similarities, subtle geometric and electronic
differences lead to distinctive oxidative se-
quences.
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
11