Reversible double oxidation and protonation of the non-innocent bridge in a nickel(II) salophen complex

Article abstract of DOI:10.1021/ic301684h

Substitution on the aromatic bridge of a nickel(II) salophen complex with electron-donating dimethylamino substituents creates a ligand with three stable, easily and reversibly accessible oxidation states. The one-electron-oxidized product is characterized as a nickel(II) radical complex with the radical bore by the central substituted aromatic ring, in contrast to other nickel(II) salen or salophen complexes that oxidize on the phenolate moieties. The doubly oxidized product, a singlet species, is best described as having an iminobenzoquinone bridge with a vinylogous distribution of bond lengths between the dimethylamino substituents. Protonation of the dimethylamino substituents inhibits these redox processes on the time scale of cyclovoltammetry, but electrolysis and chemical oxidation are consistent with deprotonation occurring concomitantly with electron transfer to yield the mono- and dioxidized species described above.

Full text of DOI:10.1021/ic301684h

Article
pubs.acs.org/IC
Reversible Double Oxidation and Protonation of the Non-Innocent
Bridge in a Nickel(II) Salophen Complex
David de Bellefeuille,† Mohammad S. Askari,† Benedikt Lassalle-Kaiser,‡ Yves Journaux,‡,§ Ally Aukauloo,‡
Maylis Orio,⊥ Fabrice Thomas,*,∥ and Xavier Ottenwaelder*,†
†Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec H4B 1R6, Canada
‡Institut de Chimie Molec
§Institut Parisien de Chimie Molec
́
ulaire et des Mater
́
iaux d’Orsay, UMR CNRS 8182, Universite
́
Paris-Sud, 91405 Orsay, France
́
ulaire, UMR CNRS 7201, Universite
́
Pierre et Marie Curie-Paris 6, 75252 Paris Cedex 05, France
⊥Laboratoire de Spectrochimie Infrarouge et Raman, UMR CNRS 8516, Universite
́
des Sciences et Technologies de Lille, 59655
Villeneuve d’Ascq Cedex, France
∥Dep
artement de Chimie Moleculaire, UMR CNRS 5250, Universite Joseph Fourier, 38041 Grenoble Cedex 9, France
́ ́ ́
S
* Supporting Information
ABSTRACT: Substitution on the aromatic bridge of a
nickel(II) salophen complex with electron-donating dimethy-
lamino substituents creates a ligand with three stable, easily
and reversibly accessible oxidation states. The one-electron-
oxidized product is characterized as a nickel(II) radical
complex with the radical bore by the central substituted
aromatic ring, in contrast to other nickel(II) salen or salophen
complexes that oxidize on the phenolate moieties. The doubly oxidized product, a singlet species, is best described as having an
iminobenzoquinone bridge with a vinylogous distribution of bond lengths between the dimethylamino substituents. Protonation
of the dimethylamino substituents inhibits these redox processes on the time scale of cyclovoltammetry, but electrolysis and
chemical oxidation are consistent with deprotonation occurring concomitantly with electron transfer to yield the mono- and
dioxidized species described above.
Scheme 1. Tuning the Redox Properties of an Aromatic
Ligand by Appropriate Substitution, from a Nickel(III)
Complex to a Delocalized Nickel(II) Radical Species to a
Pure Radical Species14
INTRODUCTION
■
Combining the redox properties of a metal ion and an organic
moiety constitutes an elegant methodology to access high
formal oxidation states when the metal ion alone is insufficient
to undergo all catalytic redox steps.1,2 In metalloenzymes,
organic cofactors such as the porphyrin of hemic systems3 and
tyrosine residues4 can thus be transiently oxidized to radicals
and promote efficient catalytic turnover. In chemical modeling,
a number of ligand types are redox-active (non-innocent) when
engaged within a complex,4,5 e.g., porphyrins,6 phenolates,7,8
catecholates,9 aminophenolates,10 phenylenediamines,11 ben-
zene-N-amidates,12−15 and diimines.16 In complexes with a
non-innocent ligand, the locus of oxidation (or reduction) is
dictated by the respective symmetry and energy of the frontier
orbitals of the metal ion and those of the organic framework.11
Thus, the oxidation site in a complex can be shifted from the
metal to the ligand by tuning the electronics of the ligand. Our
group previously investigated the oxidative chemistry of
nickel(II) phenylenediaminebis(oxamato) complexes (Scheme
1), where the aromatic ligand harbors substituents of various
electron-donating properties. The strongly electron-donating
dimethylamino (NMe2) substituents direct the oxidation site
toward the aromatic part of the ligand, whereas a metal-
centered oxidation process is observed for the parent,
unsubstituted complex.14 From a fundamental point of view,
identifying new types of metalloradical complexes will comple-
ment our knowledge of these intriguing species. In the present
paper, we investigate the oxidative chemistry of the nickel(II)
complex 1 of the novel salophen ligand 4,5-bis-
(dimethylamino)benzene-1,2-bis(3,5-di-tert-butylsalicylimine)
(5), which incorporates a (NMe2)2-substituted aromatic bridge.
We show that 1 undergoes two ligand-based, low-potential,
reversible oxidations to yield a phenylenediamine radical, 1+,
and an α-diiminoquinone species, 12+, both being stable for
hours (Scheme 2).
Received: July 31, 2012
© XXXX American Chemical Society
A
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Article
a
Scheme 2. Synthetic Route toward 1
Table 1. Crystallographic Data for 1 and [1H](SbF6)
1
[1H](SbF6)
CCDC number
empirical formula C40H56N4NiO2
fw 683.60
temperature (K) 110(2)
893505
893506
C40H57F6N4NiO2Sb
920.36
110(2)
wavelength (Å)
cryst syst
space group
a (Å)
1.54178
monoclinic
C2/c
1.54178
monoclinic
P21/c
23.9521(7)
15.1711(4)
10.3912(3)
90
15.5147(3)
9.0739(2)
32.0739(6)
90
b (Å)
c (Å)
α (deg)
β (deg)
101.883(2)
90
111.3800(10)
90
γ (deg)
volume (Å3)
a
3695.03(18)
4
4204.59(15)
4
(i) (CH2O)n, NaBH3CN, AcOH, 89%; (ii) H2, Pd/C, MeOH,
quantitative; (iii) HC(OMe)3, MeOH, 67%; (iv) Ni(OAc)2, CH2Cl2/
MeOH, 96%.
Z
density (calcd)
abs coeff
F(000)
1.229
1.454
1.049
6.201
1472
1896
RESULTS AND DISCUSSION
■
cryst size (mm3) 0.03 × 0.05 × 0.56
0.04 × 0.07 × 0.42
Synthesis and Structures of the Ligand and Neutral
Nickel(II) Complex. The synthesis of 5 is based on 4,5-
diamino-1,2-dinitrobenzene (2),17 a versatile precursor for
tetraminobenzene rings that are differently substituted at the
1,2 versus the 4,5 positions (Scheme 1).14,18,19 Metalation of 5
with nickel(II) acetate affords 1, a diamagnetic complex, as
evidenced by sharp NMR signals both in CDCl3 and in
dimethyl sulfoxide (DMSO)-d6. The aromatic protons H3 and
H6 of 1 are more shielded (7.02 ppm) than those in the MeO-
substituted (7.1 ppm)20 or Me-substituted (7.44 ppm)21
nickel(II) salophen complexes, indicating that the central ring
is electron-rich owing to the NMe2 substituents. The crystal
structure of 1 reveals the expected square-planar coordination
environment that is typical of nickel(II) in a salophen cavity
(Figure 1 and Table 1).20−22 A small puckering is induced by
index ranges
h = −28 → +28, k = −18 h = −18 → +18, k = −10
→ +17, l = −12 → +12 → +10, l = −38 → +38
reflns collected
16344 56704
independent
reflns
3334
7678
param
GOF
221
506
1.048
1.094
final R indices [I R1 = 4.14%, wR2 = 9.76% R1 = 3.826%, wR2 = 9.30%
> 2σ(I)]
R indices (all
data)
R1 = 6.82%, wR2 = 11.11% R1 = 4.68%, wR2 = 9.70%
largest diff peak
0.535 and −0.457 1.072 and −1.058
and hole (e/Å3)
nBu4NClO4 as the supporting electrolyte) by cyclic voltamme-
try, differential pulse voltammetry, and rotating-disk electrode
(RDE) voltammetry. All potentials are referenced versus the
Fc+/Fc couple.
The cyclovoltammogram (CV) of the free ligand 5 in
CH2Cl2 displays two oxidation waves at E1/21 = 0.07 V (ΔEp =
0.06 V) and E1/22 = 0.25 V (ΔEp = 0.06 V), each corresponding
to a one-electron process (Figure S1 in the Supporting
Information, SI). The first redox transfer is reversible, showing
that the stability of the cation is noteworthy. In contrast, the
a
c
low Ip /Ip value for the second wave indicates that the dication
is rather unstable, at least on the time scale of the experiment.
Both oxidations of 5 occur at much lower potentials than those
for the phenoxyl/phenol couples in the parent salophen ligand
(unsubstituted at the central ring, 0.74 V) and related salen
ligands involving di-tert-butylphenol moieties (0.65−0.80 V).23
Figure 1. ORTEP representation at 50% ellipsoid probability of 1 (i: x,
y, 0.5 − z). Hydrogen atoms were omitted for clarity. Selected bond
lengths: Ni1−N1, 1.8504(17) Å; Ni1−O1, 1.8475(14) Å.
1
2
E1/2 and E1/2 values for 5 are, however, within the range of
those measured for the oxidation of a series of (NMe2)2-
steric contact between the 6-tert-butyl substituents, as
evidenced by a ca. 13° angle between the average planes
defined by the central ring and each phenolate and a ca. 29°
angle between the average phenolate planes. Because of steric
contact between the methyl groups of distinct NMe2
substituents, the angle between the NMe2 groups and the
mean benzene plane is 55° in this solid-state structure, denoting
limited resonance.
substituted aromatic derivatives. For example, oxidation of
1
1,4,5,8-tetrakis(dimethylamino)naphthalene occurs at E1/2
=
2
−0.01 and E1/2 = 0.07 V, oxidation of 2,3,6,7-tetrakis-
1
2
(dimethylamino)naphthalene occurs at E1/2 = 0.08 and E1/2
= 0.16 V,24,25 and N,N,N′,N′-tetramethyl-p-phenylenediamine
exhibits oxidation potentials of −0.21 and 0.38 V, the first one
yielding the famous Wurster’s Blue salt.26 The electron transfers
̈
observed for 5 are thus ascribed to sequential oxidations on the
central tetraminobenzene ring into a radical cation and then a
closed-shell α-diiminoquinone dication.
Electrochemistry. The electrochemical behavior of the
ligand and complexes has been studied in CH2Cl2 (+0.1 M
B
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The CV of the nickel(II) complex 1 in CH2Cl2 (Figures 2
and S2 and S3 in the SI) displays three quasi-reversible one-
1+ is provided by its isotropic EPR spectrum in CH2Cl2 at 293
K (Figure 3). Under these conditions, the S = /2 signal is still
1
Figure 3. X-band EPR spectrum of the electrochemically generated 1+
in CH2Cl2. Black lines: experimental spectrum. Red lines: simulation
using the parameters given in the text. T = 293 K. Microwave power =
4 mW. Microwave frequency = 9.33 GHz. Modulation amplitude =
0.05 mT. Modulation frequency = 100 kHz.
Figure 2. CV traces of 1 mM CH2Cl2 solutions (+0.1 M nBu4NClO4)
of 1 () and electrogenerated 1+ (···) at a carbon working electrode.
Potentials are relative to the Fc+/Fc redox couple. Scan rate = 0.1 V/s.
T = 298 K.
1
electron oxidation waves at E1/2 = 0.12 (peak-to-peak
a1
c1
2
separation ΔEp = Ep 3 − Ep = 0.11 V), E1/2 = 0.24 V (ΔEp
= 0.11 V), and E1/2 = 0.97 V (ΔEp = 0.12 V). Typically,
symmetrical nickel(II) salen complexes with two di-tert-
butylphenolate moieties exhibit two quasi-reversible one-
electron oxidation waves above 0.4 V, which correspond to
the successive oxidations of the phenolate moieties into
observed at giso = 2.004, but a hyperfine splitting due to the
interaction of the electron spin with hydrogen and nitrogen
nuclei could be evidenced. From simulation of the spectrum,
the following isotropic hyperfine coupling (HFC) constants
1
were obtained: 13.7 MHz for six equivalent H nuclei, 17.7
1
MHz for six equivalent H nuclei, and 14.4 MHz for two
phenoxyl radicals.20−22,27−31 The low values of E1/2 and E1/2
equivalent 14N nuclei. The high HFC values to nitrogen atoms
rule out phenoxyl character for the radical species. Instead, the
HFC pattern indicates that the radical is delocalized over the
two NMe2 substituents without significant contribution of
either iminic nitrogen atoms or the nickel center.26,33 We
calculated the EPR parameters by density functional theory
(DFT) methods with the ORCA program package (see below
for more computational details). The calculated HFC constants
are A6H = 13.7 MHz, A6H = 16.7 MHz, and A2N = 12.9 MHz,
with a highly isotropic g matrix (g1 = 2.0018, g2 = 2.0047, and g3
= 2.0065; giso ∼ 2.004). The very good agreement between
experimental and theoretical EPR data confirms the assignment
of 1+ as a π-radical localized on the central bis(dimethylamino)-
benzene ring.34
1
2
of 1 advocate for a different assignment, e.g., successive
oxidations of the tetraaminobenzene central ring. This
hypothesis is further supported by the close similarity between
the oxidation potentials of 1 and the free ligand 5. The third
redox wave of 1 is likely centered on the phenolate moieties,
but it has not been further investigated because of the low
stability of the trication 13+.
For spectroscopic characterization, oxidized species 1+ and
12+ were prepared either by bulk electrolysis at a fixed potential
of 0.20 or 0.42 V, respectively, or by chemical oxidation using 1
equiv of acetylferrocenium (E° = 0.27 V) or 2 equiv of 1,1′-
diacetylferrocenium (E° = 0.49 V) in CH2Cl2, respectively.32
Given the proximity of the first two oxidation processes of 1, 1+
disproportionates to 1 and 12+ with an equilibrium constant of
ca. 1.0 × 10−3. A solution sample of 1+ would thus contain ∼3%
of each 1 and 12+. This phenomenon was taken into account
during analysis of the spectroscopic data. Both preparation
methods gave similar results, but the oxidized species were
more stable timewise under the chemical oxidation conditions.
For example, a solution of dication 12+, obtained via chemical
oxidation, was left standing for 4 h under an inert atmosphere.
Further reduction of this solution with ferrocene and analysis of
the products after chromatography indicated >95% recovery of
1, thus evidencing a noticeable stability of the dication.
The dication 12+ is EPR-silent in CH2Cl2 at both 20 and 100
K, consistent with a diamagnetic character of the doubly
oxidized ligand (see below).
DFT Calculations. DFT calculations were undertaken at the
B3LYP level of theory with the ORCA package in order to gain
insight into the geometric and electronic structures of the
oxidized species. The structures of the oxidized species 1+ and
12+ and protonated radical 1H2+, as well as the initial complexes
1 and 1H+ for comparison, were investigated by geometry
optimization. Except for the putative radical species 1H2+
(chemically irrelevant; see below for protonation studies), the
optimized structures were found to be symmetrical. A
reasonable agreement was found between the calculated and
experimental bond lengths for complexes 1 (Table 2 and Figure
S9 in the SI) and 1H+ (Table S1 and Figure S11 in the SI). In
the case of 12+, the electronic description is complicated by the
need to consider two spin states, singlet and triplet, hereafter
Electron Paramagnetic Resonance (EPR) Spectrosco-
py. The 25 K EPR spectrum of 1+ in CH2Cl2 consists of an
isotropic (S = 1/2) radical signal centered at giso = 2.004 (Figure
S4 in the SI). This suggests a lack of metal character in the
singly occupied molecular orbital (SOMO) of 1+ and argues
against a nickel(II) phenoxyl radical formulation because such
species are known to have higher giso values (typically 2.03 for
salen precursors involving 2,4-di-tert-butylphenolate moi-
eties).20−22,27−31 More information about the oxidation site in
1
3
denoted as 12+ and 12+, respectively (Figure S10 in the SI).
We calculated the energy gap between the optimized singlet
and triplet forms and found that the singlet was stabilized by
C
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Interestingly, the magnitude of the C10−N2 shortening (0.043
Å) is much larger than that for the C8−N1 bonds (0.019 Å),
suggesting that the radical character in 1+ resides on the 4,5-
diaminobenzene fragment rather than the 1,2,4,5-tetramino-
benzene fragment.
Table 2. Selected Experimental and Calculated Bond
Lengths (Å) for 1, 1+, and 12+
The energetically favored singlet spin state of 12+, 112+,
displays phenolic C−C and C−O bond lengths that are very
close to those calculated for 1 and 1+ and thus has
bis(phenolate) character (Table 2 and Figure S10 in the SI).
1
The bonds in the central ring of 12+ display the same trend
that was observed on going from 1 to 1+, i.e., α-diiminoquinone
character showing that double oxidation occurs on the electron-
rich bridge of 1. The angles between the NMe2 substituents and
nature
singlet
closed shell
triplet open
shell
closed shell
radical
bond
Ni−O
1(exptl) 1(calcd) 1+(calcd) 112+(calcd) 312+(calcd)
1
the central ring in both 1+ and 12+ are small (18 and 14°,
respectively) compared with that in 1 (55°), evidencing a
strong participation of the NMe2 groups in the resonance
stabilization of the oxidized species.36
1.848
1.850
1.310
1.438
1.379
1.412
1.369
1.419
1.419
1.420
1.310
1.419
1.394
1.387
1.428
1.382
1.420
1.883
1.885
1.299
1.444
1.382
1.418
1.371
1.420
1.430
1.417
1.305
1.414
1.398
1.395
1.440
1.393
1.408
1.881
1.884
1.291
1.446
1.378
1.425
1.366
1.426
1.440
1.403
1.317
1.395
1.383
1.407
1.462
1.427
1.365
1.890
1.895
1.280
1.448
1.373
1.435
1.360
1.432
1.457
1.386
1.364
1.364
1.379
1.411
1.500
1.472
1.325
1.869
1.875
1.287
1.449
1.378
1.421
1.380
1.407
1.444
1.412
1.309
1.407
1.377
1.413
1.460
1.420
1.358
Ni−N1
C1−O1
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C1−C6
C6−C7
C7−N1
N1−C8
C8−C9
C9−C10
C10−C10′
C8−C8′
C10−N2
The geometric structure of triplet 12+, 312+, is consistent with
diradical character (Table 2 and Figure S10 in the SI). A
comparison of the metrical parameters of 312+ with those of 1+
shows that some bonds within the phenolic rings are altered by
oxidation: in particular, C5−C6 is shortened by 0.019 Å, and
C4−C5 is lengthened by 0.015 Å. These features are indicative
35
3
of phenoxyl character for 12+, with the radical being equally
shared between the two peripheral rings of the molecule. In
agreement with this assumption, a comparison of the bond
3
lengths within the central radical rings of 1+ and 12+ does not
reveal any significant difference. Therefore, 312+ is better
described as a diradical involving both delocalized phenoxyl
moieties and a bis(dimethylamino)benzene π-radical unit.
Consistent with the metrical analysis, the calculated SOMO
of 1+ is essentially located on the bis(dimethylamino)benzene
fragment (75%) with a small contribution from the
salicylimines (25%; Figure 4a). Not surprisingly, the lowest
unoccupied molecular orbital (LUMO) of the singlet dication
112+ corresponds to the SOMO of 1+ (Figure 4b; 75%
bis(dimethylamino)benzene fragment and 25% salicylimines).
13.0 kJ/mol (including solvation terms). Therefore, the singlet
is likely the ground spin state of 12+.
Metrical analysis of the calculated 1 and 1+ structures
supports the locus of oxidation being on the central ring (Table
2 and Figure S9 in the SI). Examination of the C−C and C−O
bond lengths within the phenolic rings of 1 and 1+ reveals that
no significant change occurs upon oxidation, with the bond
lengths being similar within 0.01 Å and the N−Ni and O−Ni
bond lengths not varying by more than 0.002 Å. The C−C and
C−O bond lengths in the phenolic rings of 1+ are all in the
range expected for bis(phenolate) character. In contrast, clear
metrical changes are observed within the bis(dimethylamino)-
benzene fragment on going from 1 to 1+ (Table 2). The
shortening of the C10−N2 and C8−C9 bonds, concomitant
with the lengthening of C10−C10′ and C8−C8′, highlights a
quinoidal distribution of bond lengths within the tetramino-
benzene ring in 1+.35 This is strong evidence that oxidation of 1
occurs on the bridge, affording a phenylene π radical 1+.
3
The two SOMOs of the triplet diradical 12+ are shown in
Figure 4c. One has a main bis(dimethylamino)benzene
character (95%), with a small contribution of the salicylimines
(5%). The other has a main salicylimine character (85%), with
small contributions of the bis(dimethylamino)benzene central
ring (5%) and a nickel dxz orbital (10%).
The individual atomic contributions to the spin density
confirm the assignments made above (Table 3 and Figure S12
in the SI). In 1+, the spin density is found exclusively on the
central ring and the nitrogen atoms of the NMe2 substituents,
with negligible contribution from the phenolate or imine
moieties. In the case of 312+, the spin populations are essentially
the same on the central ring and NMe2 substituents as those in
1
3
Figure 4. Localized MOs: (a) SOMO of 1+; (b) LUMO of singlet 12+; (c) SOMOs for triplet diradical 12+. See text for fragment composition.
D
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1
Table 3. Individual Contributions of the Atoms to the Spin
Table 4. Electronic Transitions of 1+ and 12+ (Singlet, 12+;
a
3
3
Density in 1+ and Triplet 12+
Triplet, 12+)
atom
1+
312+
atom
1+
312+
experimental
TD-DFT
ν, cm−1
Ni
0
0.10
0.15
0
C5
C6
C7
C8
C9
C10
N2
0
0
a
(ε, M−1 cm−1
)
ν, cm−1 (f)
assignment
O
0
0.02
0.02
0.11
0
0.10
0
N1
C1
C2
C3
C4
0.02
0
1+
12000 (5410)
11592
(0.019)
βHOMO → βLUMO, IL
βHOMO−1 → βLUMO, IL
βHOMO−3 → βLUMO, IL
βHOMO → βLUMO, IL
βHOMO−2 → βLUMO, IL
βHOMO−3 → βLUMO, IL
βHOMO−7 → βLUMO, ML
0
0.07
0
18000 (15000)
22000 (27300)
18112
(0.120)
0
0.06
0
0
0.10
0.22
0.09
0.26
24409
(0.307)
0
0.13
a
See Table 2 for atom numbering.
112+ 6250 (1430)
5777
(0.102)
1+, with the addition of radical character on the phenolic
moieties, thereby confirming this species as a multisite diradical.
Electronic Spectra and Time-Dependent DFT (TD-
DFT) Calculations. Oxidation of 1 to 1+ and 12+ leads to
drastic changes in the electronic absorption spectra. In contrast
to 1, both orange-brown 1+ and brown 12+ exhibit intense
absorptions in the vis−near-IR (NIR) range (Figure 5 and
12000 (shoulder) 12991
(0.011)
15100 (13800)
18250 (15750)
312+
16674
(0.145)
18988
(0.171)
6920 (0.26)
βHOMO → βLUMO, IL
10316
(0.15)
βHOMO−1 → βLUMO, ML
a
IL = intraligand charge transfer. ML = metal-to-ligand charge transfer.
Figure 5. UV−vis−NIR spectra of 1 (a, black), 1+ (b, red), and 12+ (c,
blue) generated by spectroelectrolysis at −30 °C (1 mM CH2Cl2 and
0.1 M TBAPF6). (d) Extension of the spectra in the 1400−2800 nm
range as per chemical oxidation of 1 with Ac2Fc+ in CH2Cl2. The
asterisks denote solvent absorptions.
Table 4). The extinction coefficients of these absorptions are
reminiscent of charge-transfer (CT) bands. It is worth noting
that 1+ displays no significant absorption at wavenumbers lower
than 8000 cm−1 (Figure 5b), in sharp contrast with the
nickel(II) phenoxyl radical salen complexes, which typically
exhibit an intense intervalence charge-transfer (IVCT) band at
4300−4700 cm−1 (17−25 mM−1 cm−1).28−31 The red shift of
the vis−NIR features of 12+ (Figure 5c) compared with 1+
likely reflects an increase in the ligand oxidation state.
Figure 6. TD-DFT assignment of the main electronic excitations of 1+,
with fragment population of the relevant MOs. Fragments: Ni = nickel
atom; salicy = salicylimine moieties (phenolate rings + imines);
diamino = central ring + NMe2 substituents.
absorptions stem from βHOMO−1 → βLUMO and
βHOMO−3 → βLUMO transitions, respectively. The donor
orbitals are again ligand-centered, with βHOMO−1 being a π
system mainly located on the phenolate rings and βHOMO−3
being a π system delocalized over the whole salophen backbone
with dominant contributions of the phenolates and central ring.
TD-DFT calculations on 12+ were conducted by considering
both the singlet and triplet spin states (Table 4). Only the set
of excitations for the singlet species reproduced the
experimental bands, thus confirming the singlet nature of 12+.
A detailed analysis of the composition of the molecular orbitals
(MOs) involved in the transitions reveals that the absorptions
at 6250, 12000, and 15100 cm−1 stem from ILCT transitions,
while the band at 18 250 cm−1 corresponds to a metal-to-ligand
charge-transfer (MLCT) transition (Figure 7). In particular, the
The UV−vis−NIR spectrum of the radical cation 1+ is well
matched by the electronic excitations predicted by TD-DFT
calculations (Table 4). In particular, the calculations confirm
the absence of an intense IVCT transition in the NIR that
would have indicated a mixed-valence phenolate−phenoxyl
radical system.28−31 The three lowest-energy absorptions are
rising from interligand charge-transfer (ILCT) transitions
(Figure 6). In the βHOMO → βLUMO transition (HOMO
= highest occupied molecular orbital) at 11592 cm−1, the donor
orbital is a π system mainly located on the phenolate rings,
while the acceptor orbital (βLUMO) is principally located on
the central bis(dimethylamino)benzene fragment with a small
contribution from the phenolate moieties. The next two
E
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According to previous studies,28−31 and consistent with the
calculated oscillator strength of the 6920 cm−1 band, this
transition should give rise to a very intense band in the NIR, in
contrast to what is observed experimentally. This is again in
favor of a singlet description for 12+.
Protonation Studies. By virtue of its NMe2 substituents,
complex 1 can be protonated with a strong acid (HBF4 or
HSbF6). The process was monitored by UV−vis spectroscopy
(Figure 9) and yielded a complex with optical features similar
Figure 9. UV−vis titration of a CH2Cl2 solution of 1 by HBF4: (a, red)
before the addition of acid; (b, blue) after the addition of 1 equiv of
acid; (c, green) after the addition of 1 equiv of acid and 8 equiv of
NEt3. l = 1.000 cm. T = 298 K.
Figure 7. TD-DFT assignment of the main electronic excitations of
singlet 112+, with fragment population of the relevant MOs.
Fragments: Ni = nickel atom; salicy = salicylimine moieties (phenolate
rings + imines); diamino = central ring + NMe2 substituents.
to those of 1, although less intense. The addition of a base to
the acidic solution led to full recovery of the UV−vis features of
1, demonstrating the reversibility of the process. On a
preparative scale, using HSbF6 in methanol led to the isolation
of crystals of [1H](SbF6) that were suitable for X-ray
diffraction analysis. The crystal structure of [1H](SbF6)
confirmed that protonation occurred on a NMe2 group, with
hydrogen bonding to the other (Figure 10). Consequently, the
βLUMO and βHOMO of 112+ have essentially the same
character as those in 1+, but the relative energy of these orbitals
1
in 12+ differ greatly from the situation in 1+. The highest
oxidation state and nonradical nature of the central ring in 12+
contributes to stabilization of βLUMO and leads to an overall
red shift of the spectrum for 12+ compared with 1+.
3
In the hypothetical triplet diradical 12+, the main electronic
excitations arise from βHOMO → βLUMO and βHOMO−1
→ βLUMO transitions (Figure 8). Similarly to nickel(II)
phenoxyl radical salen complexes, the βHOMO → βLUMO
excitation corresponds to an IVCT, where both the donor and
acceptor orbitals have strong phenolate−phenoxyl character.
Figure 10. ORTEP representation at 50% ellipsoid probability of the
cationic part of [1H](SbF6). Hydrogen atoms, except the one located
between the NMe2 substituents, were omitted for clarity. Selected
bond lengths: Ni1−N1 = 1.851(3) Å; Ni1−N2 = 1.864(3) Å; Ni1−O1
= 1.862(2) Å, Ni1−O2 = 1.852(2) Å.
NMe2 groups are facing each other and are no longer related by
a C2 axis, as was the case in 1. The 1H+ molecule is more planar
than 1 in their respective solid-state structure, with only a 8.3°
angle between the carbon rings of each phenolate.
Protonation studies confirmed the involvement of the NMe2
substituents in the redox behavior of 1. The CV of 1H+
revealed a two-electron process (based on coulometry) with
Epa1 = 0.57 V and Epc1 = 0.21 V (Figure 11). The protonation-
Figure 8. TD-DFT assignment of the main electronic excitations of
triplet 312+, with fragment population of the relevant MOs. Fragments:
Ni = nickel atom; salicy = salicylimine moieties (phenolate rings +
imines); diamino = central ring + NMe2 substituents.
a1
induced increase by more than 300 mV of Ep compared with
F
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In order to confirm this oxidation scheme, we calculated the
EPR parameters for putative protonated (S = 1/2) radical
species 1H2+ by DFT methods (Figure S14 in the SI). The
computed EPR parameters are g1 = 1.990, g2 = 2.018, and g3 =
2.049 (giso ∼ 2.019) without any HFC higher than 3 MHz,
consistent with the location of the radical on the salicylimine
moieties (Figure S13 in the SI). The poor agreement between
these calculated values and the experimental data confirms that
protonated radical species 1H2+ is not realistic under the
experimental conditions.
UV−vis−NIR monitoring of the electrochemical oxidation of
1H+ (0.62 V, two electrons removed) showed the appearance
of absorption bands at 15100 and 18320 cm−1 indicating >95%
formation of 12+ (Figure S8 in the SI). All data thus suggest that
the radical dication 1H2+ is very acidic. It is easily deprotonated,
making oxidation of 1H+ a proton-coupled two-electron
transfer to afford 12+ (Scheme 3).
Figure 11. CV traces of 1 mM CH2Cl2 solutions (+0.1 M
nBu4NClO4) of 1H+ before () and after (···) electrolysis at 0.6 V
at a platinum working electrode. Potentials are relative to the Fc+/Fc
redox couple. Scan rates = 0.1 V/s. T = 298 K.
CONCLUSION
■
that of 1 is consistent with the NMe2 substituents being
significant contributors to the orbital from which the electron is
abstracted in 1. The large peak-to-peak separation, ΔEp = 0.36
V, indicates that a chemical reaction is coupled to electron
In summary, the two NMe2 substituents in salophen-type
nickel(II) complex 1 direct the first two oxidation processes
onto the central ring. Complex 1, together with radical 1+ and
α-diiminoquinone 12+, constitute a rare example in the
salophen family where the neutral part of the molecule, not
the phenolates, gets oxidized by one37 and two electrons
reversibly. In these nickel(II) complexes, ligand 5 is thus found
in three different oxidation states (2−, 1−, and 0) that are
reversibly accessible at low redox potentials. We are currently
studying the implications of the non-innocence of 5 onto the
reactivity of its complexes with several other metal ions.
transfer. The similarity between Ep of 1H+ and E1/2 of 1
suggests that a significant amount of dication 12+ (and likely
monocation 1+) is formed during oxidation of 1H+. In addition,
the CV recorded after complete electrolysis at 0.6 V is not
superimposable on that of 1H+ and displays an additional
feature at 0−0.3 V, which is reminiscent of the CV of 1 (Figure
11). These results suggest that oxidation of 1H+ affords a
transient acidic radical species that deprotonates readily to
c1
2
2
produce 1+. With the Epa1 value of 1H+ being higher than E1/2
EXPERIMENTAL SECTION
of 1+, the net result is a two-electron oxidation of 1H+ into 12+
■
Materials. Organics, solvents, and metal salts were obtained from
commercial sources and used as received unless indicated otherwise.
For oxidation studies, dichloromethane was degassed, dried over an
MBraun solvent purification system, and then stored over activated 4
Å molecular sieves in amber bottles inside an inert-atmosphere
glovebox (MBraun Labmaster) filled with a dry dinitrogen atmosphere
(less than 1 ppm in dioxygen and water). Compound 217 and 3,5-di-
tert-butylsalicylaldehyde38 were synthesized following literature
procedures.
and proton release in solution (Scheme 3).
Scheme 3. Protonation/Oxidation Processes from 1
Characterization. Elemental CHN microanalyses were performed
by the Microanalytical Service of the Institut de Chimie des Substances
1
Naturelles (CNRS, France). Solution H and 13C NMR spectra were
recorded on a Varian Innova 500 MHz instrument using
tetramethylsilane as the internal reference. Fourier transform infrared
(FT-IR) spectra were recorded on a Perkin-Elmer 882 spectropho-
tometer as KBr pellets. UV−vis−NIR solution spectra were recorded
on Varian CARY 5E (250−1400 nm) and 5000 (1400−3300 nm)
spectrophotometers. X-band EPR spectra were recorded on a Bruker
EMX Plus spectrometer controlled with Xenon software and equipped
with a Bruker teslameter. A Bruker nitrogen-flow cryostat connected to
a high-sensitivity resonant cavity was used for 100 K measurements.
An Oxford Instrument helium-flow cryostat connected to a dual-mode
resonant cavity was used to run experiments at 25 K. The spectra were
simulated using SIMFONIA software (Bruker). Cyclic voltammetry
curves were recorded on a CHI 620 potentiostat in a standard three-
electrode cell under an argon atmosphere. Electrochemical measure-
ments were performed at either 298 K for the nonoxidized complexes
or 243 K for the oxidized species to ensure as little decomposition as
possible. An AgNO3/Ag (0.01 M) reference electrode was used. All of
the potentials given in the text refer to the regular Fc+/Fc redox couple
used as the external reference. A vitreous carbon disk electrode (3 mm
diameter) or a platinum disk (2 μm diameter) polished with 1 mm
diamond paste was used as the working electrode. Both the carbon and
platinum working electrodes gave the same response for 1. In the case
The electrochemical oxidation of 1H+ at 0.62 V (i.e., at a
potential slightly higher than Epa1) was monitored by EPR
spectroscopy at 298 K. The EPR spectrum recorded at half-
electrolysis (one electron removed) consisted of a signal very
similar to the one of 1+, albeit much less intense (Figure S6 in
the SI), while two-electron oxidation (complete electrolysis)
resulted in a total quenching of the EPR signal. This behavior is
consistent with 12+ being the final oxidation product, with a
small amount of 1+ being transiently formed during electrolysis.
G
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of 1H+, we observed significant deposition at the carbon working
electrode, which was avoided by using a platinum one. Electrolysis was
performed on a PAR 273 potentiostat, under an argon atmosphere at
−40 °C. The setup for bulk electrolyses consisted of a carbon felt
electrode (for 1) or a large platinum grid (for 1H+), with the counter
electrode isolated by a salt bridge. RDE voltammetry was performed
with a Radiometer TCV101 speed control unit and an EDI 101
electrode.
mL) was stirred under a dihydrogen atmosphere at room temperature
either overnight under 40 bar of pressure in a stainless steel autoclave
or for 3 days under 40 psi of pressure in a Parr shaker. The suspension
was then filtered under an inert atmosphere over a pad of Celite that
was rinsed well with degassed methanol. Removal of the solvent
1
yielded a light-brown, air-sensitive solid. H NMR (DMSO-d6, 500
MHz): δ 3.34 (s, 12H), 4.07 (s, 4H), 6.20 (s, 2H).
4,5-Bis(dimethylamino)benzene-1,2-bis(3,5-di-tert-butylsalicyli-
mine) (5). After rinsing the Celite in the precedent synthesis (without
isolating the product), the methanolic solution of 4 was directly added
to a degassed solution of 3,5-di-tert-butylsalicylaldehyde38 (2.0 g, 8.5
mmol) and trimethylorthoformate (6.5 mL) in methanol (50 mL).
After reflux for 24 h under an inert atmosphere, the suspension was
cooled with an ice bath. The air-stable solid was filtered off, and excess
trimethylorthoformate was removed by twice-repeated redissolution in
a small amount of dichloromethane followed by evaporation to
dryness in a rotary evaporator. The orange solid was then collected
with cold methanol, filtered off, rinsed twice with cold methanol, and
dried under vacuum. Yield: 1.7 g, 67% over two steps. Anal. Calcd for
C40H58N4O2 (626.9 g/mol): C, 76.63; H, 9.33; N, 8.94. Found: C,
76.59; H, 9.43; N, 8.79. 1H NMR (CDCl3, 500 MHz): δ 1.30 (s, 18H,
t-Bu), 1.42 (s, 18H, t-Bu), 2.85 (s, 12H, NCH3), 6.73 (s, 2H, central
aromatic), 7.18 (d, 2H, 2.4 Hz, phenol), 7.38 (d, 2H, Hc, 2.4 Hz,
phenol), 8.62 (s, 2H, imine), 13.80 (s, 2H, OH). 13C NMR (CDCl3,
125 MHz): δ 29.47, 31.51, 34.16, 35.10, 41.47, 109.34, 118.63, 126.39,
127.45, 135.84, 136.99, 140.04, 144.61, 158.36, 162.34. FT-IR (KBr):
ν 1584, 1614 (CN) cm−1.
X-ray Crystallography. Crystallographic analysis was performed
using the microfocus Cu Kα source of a Bruker APEX-DUO
diffractometer. The frames were integrated with the Bruker SAINT
software package using a narrow-frame algorithm. Data were corrected
for absorption effects using the multiscan method (SADABS). The
structures were solved by direct methods and refined using the Bruker
APEX2 software package (SHELXL instructions).39 All non-hydrogen
atoms were refined with anisotropic thermal parameters. Hydrogen
atoms were generated in idealized positions, riding on the carrier
atoms with isotropic thermal parameters, except for H1N in
[1H](SbF6), which was located on the electronic density difference
map and allowed to refine with only a distance restraint from N3.
Computational Details. All theoretical calculations were
performed with the ORCA program package.40 Full geometry
optimizations were carried out for all complexes using the GGA
functional BP8641−43 in combination with the TZV/P44 basis set for
all atoms and by taking advantage of the resolution of the identity (RI)
approximation in the Split-RI-J variant45 with the appropriate
Coulomb fitting sets.46 Increased integration grids (Grid4 in the
ORCA convention) and tight self-consistent-field convergence criteria
were used. Solvent effects were accounted for according to the
experimental conditions. For that purpose, we used the CH2Cl2 (ε =
9.08) solvent within the framework of the conductor-like screening
(COSMO) dielectric continuum approach.47 The relative energies
were obtained from single-point calculations using the B3LYP48,49
functional together with the TZV/P44 basis set. They were computed
from the gas-phase-optimized structures as a sum of the electronic
energy, thermal corrections to free energy, and free energy of
solvation. Optical properties were also obtained from single-point
calculations using the hybrid functional B3LYP48,49 and the TZV/P44
basis set. Electronic transition energies and dipole moments for all
models were calculated using TD-DFT50−52 within the Tamm-Dancoff
approximation.53,54 To increase the computational efficiency, the RI
approximation55 was used in calculating the Coulomb term and at least
30 excited states were calculated in each case. g tensors and HFC
constants were obtained from single-point calculations employing the
hybrid functional B3LYP.48,49 The triply polarized core property basis
set CP(PPP)56 was applied for the metal, while the EPR-II57 basis set
was used for all remaining atoms. Special care was also taken to ensure
accurate results by increasing the size of the integration grid to 7
(ORCA convention) for the metal center.56
1: To a solution of 5 (313 mg, 0.5 mmol) in dichloromethane (5
mL) was added dropwise a methanolic solution (20 mL) of nickel(II)
acetate tetrahydrate (88.4 mg, 0.5 mmol). After 1 h of reflux, the
suspension was cooled. The precipitate was filtered off and rinsed
copiously with methanol. A pure material was obtained as a red
fraction upon short filtration over silica eluted by dichloromethane.
The fraction was then concentrated to ca. 5 mL and layered with
methanol to afford a red crystalline product. Yield: 328 mg, 96%. Anal.
Calcd for C40H56N4NiO2 (683.6 g/mol): C, 70.28; H, 8.26; N, 8.20.
1
Found: C, 70.24; H, 8.29; N, 8.15. H NMR (CDCl3, 500 MHz): δ
1.29 (s, 18H, t-Bu), 1.45 (s, 18H, t-Bu), 2.86 (s, 12H, CH3), 7.02 (s,
2H, central aromatic), 7.07 (d, 2H, 2 Hz, phenol), 7.35 (d, 2H, 2 Hz,
phenol), 7.96 (s, 2H, imine). 13C NMR (CDCl3, 125 MHz): δ 29.73,
31.30, 33.92, 35.91, 41.49, 102.67, 119.66, 126.35, 129.75, 136.43,
136.80, 140.39, 144.79, 152.12, 163.79. FT-IR (KBr): ν 1501, 1526,
1605 (CN) cm−1. Needle-shaped single crystals amenable for X-ray
crystallography were grown by the slow diffusion of a methanol layer
atop a dichloromethane solution of 1.
[1H](SbF6): Atop a solution of 1 (50 mg, 0.073 mmol) in
dichloromethane (5 mL) was carefully layered a solution of
HSbF6·6H2O (48 mg, 0.146 mmol) in 15 mL of methanol. The
mixture was allowed to diffuse and evaporate slowly, yielding a deep-
red crystalline solid that was washed with dichloromethane. Single
crystals were selected from this solid for X-ray diffraction studies.
Diacetylferrocenium hexafluoroantimonate (Ac2FcSbF6) was pre-
pared in the glovebox by stirring equimolar amounts of recrystallized
diacetylferrocene (Ac2Fc; 270.1 mg, 1.0 mmol) and AgSbF6 (343.62
mg, 1.0 mmol) in diethyl ether (10 mL) for 20 min. The dark-blue
precipitate was filtered off over a small plug of Celite and was rinsed
well with ether. The titled compound was then extracted off the plug
by dichloromethane, yielding solutions of ca. 7 mM concentration.
The titled compound can be isolated by rotary evaporation, but for
purity reasons, we preferred the use of freshly prepared solutions. The
exact concentration of these solutions was assessed by UV−vis
titration experiments of 1.
Syntheses. 4,5-Bis(dimethylamino)-1,2-dinitrobenzene (3). To a
degassed suspension of 217 (5.5 g, 27.7 mmol) and paraformaldehyde
(8.3 g, 276 mmol) in glacial acetic acid (50 mL) was added sodium
cyanoborohydride (8.75 g, 139 mmol) in small portions. The
suspension was then stirred overnight at room temperature under an
inert atmosphere. The solid was filtered off and rinsed with acetone.
The filtrate was evaporated to dryness, and dichloromethane and water
were added. The product was extracted two times, and the combined
organic phases were washed once with water, dried over sodium
sulfate, and evaporated to dryness. The resulting oil was dissolved in
50 mL of diethyl ether, filtered over cotton, and evaporated to a
minimum. A total of 500 mL of cold hexanes was added, and the
suspension was kept in a −20 °C freezer overnight to yield a dark-red
crystalline material that was filtered off and dried under vacuum. Yield:
6.25 g, 89%. Anal. Calcd for C10H14N4O4 (254.2 g/mol): C, 47.24; H,
5.55; N, 22.04. Found: C, 46.88; H, 5.53; N, 21.92. 1H NMR (CDCl3,
500 MHz): δ 2.87 (s, 12H), 7.21 (s, 2H). 13C NMR (CDCl3, 125
MHz): δ 40.41, 113.62, 136.43, 146.73. FT-IR (KBr): ν 1505, 1520,
1544, 1572, 1582 (NO2) cm−1.
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional electrochemical, EPR, UV−vis−NIR, and DFT data,
together with crystallographic CIF files (CCDC 893505 and
893506). This material is available free of charge via the
Internet at http://pubs.acs.org.
N1,N1,N2,N2-Tetramethylbenzene-1,2,4,5-tetramine (4). A suspen-
sion of 3 (1.02 g, 4 mmol) and 10% Pd/C (0.1 g) in methanol (80
H
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Inorganic Chemistry
Article
(24) Elbl-Weiser, K.; Krieger, C.; Staab, H. A. Angew. Chem., Int. Ed.
Engl. 1990, 29, 211−213.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dr.x@concordia.ca (X.O.), fabrice.thomas@ujf-
grenoble.fr (F.T.).
(25) Barth, T.; Krieger, C.; Neugebauer, F. A.; Staab, H. A. Angew.
Chem., Int. Ed. Engl. 1991, 30, 1028−1030.
(26) Elbl, K.; Krieger, C.; Staab, H. A. Angew. Chem., Int. Ed. Engl.
1986, 25, 1023−1024.
Author Contributions
All authors have given approval to the final version of the
manuscript.
(27) Shimazaki, Y.; Tani, F.; Fukui, K.; Naruta, Y.; Yamauchi, O. J.
Am. Chem. Soc. 2003, 125, 10512−10513.
(28) Storr, T.; Wasinger, E. C.; Pratt, R. C.; Stack, T. D. P. Angew.
Chem., Int. Ed. 2007, 46, 5198−5201.
Notes
(29) Shimazaki, Y.; Stack, T. D. P.; Storr, T. Inorg. Chem. 2009, 48,
8383−8392.
The authors declare no competing financial interest.
(30) Shimazaki, Y.; Arai, N.; Dunn, T. J.; Yajima, T.; Tani, F.;
Ramogida, C. F.; Storr, T. Dalton Trans. 2011, 40, 2469−2479.
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Sakaguchi, M.; Ogura, T.; Orio, M.; Shimazaki, Y.; Thomas, F.; Storr,
T. Chem.Eur. J. 2012, 18, 14117−14127.
ACKNOWLEDGMENTS
■
This work was funded by the CNRS (France) and by an
NSERC Discovery grant (Canada). X-ray diffraction data were
collected on a CFI-funded infrastructure (Canada).
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