Pseudo-octahedral schiff base nickel(II) complexes: Does single oxidation always lead to the nickel(III) valence tautomer?

Article abstract of DOI:10.1002/ejic.200800410

With the aim of establishing correlations between the ligand structure and the oxidation site in nickel complexes from Schiff base ligands, five ligands and their nickel complexes have been synthesized. The prototypical asymmetric Schiff base ligand HL¹ contains both phenol and pyridine pendant arms with a pivotal imine nitrogen atom. Ligands HL^2-5 differ from HL¹ by either their phenolate para substituent, the hybridization of the pivotal nitrogen atom, and/or the N-donor properties of the pyridine moiety. The five complexes [Ni-(L^1-5)₂] are obtained by treating the corresponding ligands with 0.5 equiv. of Ni(OAc)₂·4H ₂O in the presence of NEt₃. Xray crystal-structure diffraction studies as well as DFT calculations reveal that [Ni(L ^1-5)₂] involves a high-spin nickel(II) ion within a pseudo-octahedral geometry. The two ligands are arranged in a meridional fashion when the pivotal nitrogen atom is an imine {as in [Ni(L^1-2) ₂] and [Ni(L^4-5)₂]}, while the fac isomer is preferred in [Ni(L³)₂] (amino pivotal nitrogen atom). [Ni(L¹)₂] is characterized by an oxidation potential at -0.17 V vs. Fc⁺/Fc. The one-electron-oxidized species [Ni(L ⁺)₂]⁺ exhibits an EPR signal at g = 2.21 attributed to a phenoxyl radical that is antiferromagnetically coupled to a high-spin Ni^II ion. [Ni(L²)₂] differs from [Ni(L¹)₂] by the phenolate para substituent (a tert-butyl instead of the methoxyl group) and exhibits an oxidation potential that is ca. 0.16 V higher. Compared to [Ni(L¹)₂]⁺ the cation [Ni(L²)₂]⁺ exhibits a SOMO that is more localized on the metal atom. The EPR and electrochemical signatures of [Ni(L³)₂]⁺ are similar to those of [Ni(L ¹)₂]⁺, thus showing that an imino to amino substitution compensates for a methoxy to tert-butyl one. Replacement of the pyridine by a quinoline group in [Ni-(L^4-5)₂] makes the complexes slightly harder to oxidize. The EPR signatures of the cations [Ni(L^4-5)₂]⁺ are roughly similar to those of the pyridine analogs [Ni(L^1-2)₂]⁺. The oxidation site is thus not significantly affected by changes in the N-donor properties of the terminal imino nitrogen atom. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800410

FULL PAPER
DOI: 10.1002/ejic.200800410
Pseudo-Octahedral Schiff Base Nickel(II) Complexes: Does Single Oxidation
Always Lead to the Nickel(III) Valence Tautomer?
Olaf Rotthaus,[a] Vanessa Labet,[a] Christian Philouze,[a] Olivier Jarjayes,*[a] and
Fabrice Thomas*[a]
Keywords: Phenoxyl / Radicals / Nickel / Valence tautomerism / Schiff bases
With the aim of establishing correlations between the ligand
structure and the oxidation site in nickel complexes from
Schiff base ligands, five ligands and their nickel complexes
have been synthesized. The prototypical asymmetric Schiff
base ligand HL1 contains both phenol and pyridine pendant
arms with a pivotal imine nitrogen atom. Ligands HL2–5 differ
from HL1 by either their phenolate para substituent, the hy-
bridization of the pivotal nitrogen atom, and/or the N-donor
properties of the pyridine moiety. The five complexes [Ni-
(L1–5)2] are obtained by treating the corresponding ligands
with 0.5 equiv. of Ni(OAc)2·4H2O in the presence of NEt3. X-
ray crystal-structure diffraction studies as well as DFT calcu-
lations reveal that [Ni(L1–5)2] involves a high-spin nickel(II)
ion within a pseudo-octahedral geometry. The two ligands
are arranged in a meridional fashion when the pivotal nitro-
gen atom is an imine {as in [Ni(L1–2)2] and [Ni(L4–5)2]}, while
the fac isomer is preferred in [Ni(L3)2] (amino pivotal nitro-
gen atom). [Ni(L1)2] is characterized by an oxidation potential
at –0.17 V vs. Fc+/Fc. The one-electron-oxidized species
[Ni(L1)2]+ exhibits an EPR signal at g = 2.21 attributed to a
phenoxyl radical that is antiferromagnetically coupled to a
high-spin NiII ion. [Ni(L2)2] differs from [Ni(L1)2] by the phe-
nolate para substituent (a tert-butyl instead of the methoxyl
group) and exhibits an oxidation potential that is ca. 0.16 V
higher. Compared to [Ni(L1)2]+ the cation [Ni(L2)2]+ exhibits
a SOMO that is more localized on the metal atom. The EPR
and electrochemical signatures of [Ni(L3)2]+ are similar to
those of [Ni(L1)2]+, thus showing that an imino to amino sub-
stitution compensates for a methoxy to tert-butyl one. Re-
placement of the pyridine by a quinoline group in [Ni-
(L4–5)2] makes the complexes slightly harder to oxidize. The
EPR signatures of the cations [Ni(L4–5)2]+ are roughly similar
to those of the pyridine analogs [Ni(L1–2)2]+. The oxidation
site is thus not significantly affected by changes in the N-
donor properties of the terminal imino nitrogen atom.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
shell ligand, or from the ligand, leading to an Mn+ open-
shell ligand. Practically, one-electron oxidation of these
CuII–phenolate complexes has been shown to afford nearly
all the time exclusively the Mn+–phenoxyl valence tauto-
mer.[2] Recently, nickel complexes of pro-radicals,[3] and es-
pecially pro-phenoxyl ligands, have also emerged in the lit-
erature.[4,5] In this case the ligand and metal redox-active
orbitals are closer in energy. Therefore, while one-electron
Introduction
Coordinated tyrosyl and phenoxyl radicals have been the
focus of increasing interest since the discovery of a CuII–
tyrosyl radical entity in the active site of galactose oxidase
ten years ago.[1] As this enzyme constitutes a protypical ex-
ample of the synergy between a metal atom and a phenoxyl
radical to perform oxidation reactions, several biomimetic
approaches to reproduce its active site have been developed
by chemists. This has led to the characterization of a large
number of copper(II) complexes involving coordinating re-
dox-active phenolate groups.[2] The localization of the oxi-
dation site in these complexes could, however, lead to dis-
crepancies as, in principle, electrons could be removed
either from the metal center, leading to an M(n+1)+ closed-
oxidation of the CuII–phenolate complexes affords Mn+
–
phenoxyl species, one-electron oxidation of the NiII–phen-
olate complexes affords either the M(n+1)+–phenolate or the
M
n+–phenoxyl valence tautomer.[3–4] Such compounds are
thus particularly useful to better understand the way in
which nature could favor either a radical or a high-valent
metal pathway to perform its oxidation reactions. They are
not only interesting from a fundamental point of view but
they are also valuable candidates for the design of molecu-
lar electronic devices and switches.[6]
The oxidative behavior of low-spin nickel complexes of
tetradentate Schiff bases has been investigated during the
last few years. The electronic structure of these complexes
in their one-electron-oxidized form has been elucidated only
[a] Département de Chimie Moléculaire, Chimie Inorganique
Redox Biomimétique (CIRE), UMR-5250, Université Joseph
Fourier
B. P. 53, 38041 Grenoble Cedex 9, France
Fax: +33-4-76514846
E-mail: Fabrice.Thomas@ujf-grenoble.fr
Supporting information for this article is available on the
WWW under http://www.eurjic.org/ or from the author.
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O. Rotthaus, V. Labet, C. Philouze, O. Jarjayes, F. Thomas
FULL PAPER
recently: In CH2Cl2 they exhibit an Ni2+–phenoxyl radical
character with partial delocalization of the SOMO onto the
metal orbitals,[4b,4d–4j] the extent of which depends on the
electronic properties of the phenolate para substituent.[4j] In
the presence of coordinating solvents such as methanol or
pyridine, the square-planar low-spin Ni2+–radical species
are converted into pseudo-octahedral Ni3+–phenolate com-
plexes.[4b,4e–4j,7]
These recent results would suggest that the oxidation site
could be predicted or even controlled in these nickel com-
plexes. In fact a direct correlation between the ligand struc-
ture and the oxidation site is still missing, and many ques-
tions remain unanswered: Is this behavior general for Schiff
base nickel complexes? Does the initial nickel(II) geometry
(and thus its spin state) affect the oxidation locus? What is
the influence of the donor atoms?
In order to clarify this situation, we herein examine a
unique series of pseudo-octahedral high-spin nickel com-
plexes synthesized from tridentate Schiff base ligands. This
series contrasts with previous ones as most of the one-elec-
tron-oxidized pseudo-octahedral Schiff base nickel com-
plexes are low spin and involve a tetradentate Schiff base
(pyridine or any bases and coordinating solvents are added
to complete the coordination sphere). We show that com-
plexes that belong to this series exhibit an unprecedented
oxidative behavior. In addition, an extreme versatility in
their oxidation locus makes them ideal candidates to de-
velop a new logic of work that consists in counterbalancing
Figure 1. Chemical formulae of the ligands used in this study.
HL1 was obtained by the treatment of 2-picolylamine
with
2-tert-butyl-6-formyl-4-methoxyphenol.
HL2,[8,9]
HL3,[9,10] and HL5[8a] have been described previously.
Treatment of 2-tert-butyl-6-formyl-4-methoxyphenol with
8-aminoquinoline affords HL4.
Structures of the NiII–Phenolate Complexes
The nickel(II) complexes [Ni(L1–5)2] have been obtained
the effect of one modification in the ligand structure by a by mixing 2 equiv. of HL1–5 and 1 equiv. of Ni(OAc)2·4H2O
second opposite one. We here demonstrate that by mod- in the presence of NEt3. Single crystals of [Ni(L2)2] and
ifying a single parameter in the ligand both the electro- [Ni(L5)2] were obtained by slow evaporation of methanol.
chemical and EPR properties of the one-electron-oxidized Crystals of [Ni(L1)2] could not be obtained by this method
nickel complex are affected. In contrast, modification of as the complex decomposes during solvent evaporation
two independent parameters affords a one-electron-oxid- (one week). Nevertheless, crystals from a decomposition
ized nickel complex that exhibits electrochemical and EPR product [Ni(L6)] could be obtained.
signatures similar to those of the initial compound.
The ORTEP diagrams of [Ni(L2)2] and [Ni(L5)2] are
shown in Figure 2, and selected bond lengths and angles
are listed in Table 1.
[Ni(L2)2] consists of a nickel(II) ion in a pseudo-octahe-
dral environment. The metal ion is surrounded by two de-
protonated ligands that coordinate through nitrogen atoms
from two pyridine rings [N(1) and N(3)] and two imine
groups [N(2) and N(4)], as well as two oxygen atoms O(1)
Results and Discussion
Design and Synthesis of the Ligands
Our strategy to obtain pseudo-octahedral Schiff base and O(2) from the phenolate groups. The two ligands are
nickel complexes was based on the use of 2 equiv. of a tri- arranged in a meridional fashion, with a cis orientation of
dentate ligand to coordinate a single metal ion.[5,8–10] The the phenolate moieties [the O(1)–Ni–O(2) angle is 97.5(1)°].
prototypical asymmetric Schiff base ligand HL1 (Figure 1) The imine nitrogen atoms are trans to one another with a
coordinates with two nitrogen atoms (from one pivotal im- N2–Ni–N4 angle of 177.68(8)°, whereas the pyridine nitro-
ine and one terminal pyridine group) as well as an oxygen gen atoms are cis to one another with an N1–Ni–N3 angle
atom (from the redox-active phenolate group). Modifica- of 92.84(7)°. The Ni–Npyridine bond lengths Ni–N(1) and
tions realized on HL1 concern the electronic properties of Ni–N(3) are 2.100(2) and 2.111(2) Å, respectively, the Ni–
the phenol para substituent (HL2), the hybridization of the Nimine bond lengths Ni–N(2) and Ni–N(4) are 2.027(2) and
pivotal nitrogen atom (HL3), and the N-donor properties 2.028(2) Å, respectively, whereas the Ni–Ophenolate bond
of the pyridine group (HL4–5). We did not investigate the lengths Ni–O(1) and Ni–O(2) are 2.021(2) and 2.003(2) Å,
effect of hybridization of the terminal nitrogen atom as it respectively. These bond lengths are typical for complexes
has been reported in nickel complexes of related tridentate involving a high-spin nickel(II) ion. This spin state is also in
ligands that a terminal N(CH3)2 group was only poorly co- agreement with the pseudo-octahedral geometry generally
ordinating.[5]
adopted by the nickel ion in this electronic configuration.
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Pseudo-Octahedral Schiff Base Nickel(II) Complexes
Table 1. Selected bond lengths [Å] and angles [°] for [Ni(L2)2],
[Ni(L5)2], and [Ni(L6)].
[Ni(L2)2]
[Ni(L5)2]
[Ni(L6)]
1.823(1)
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(2)–O(3)
Ni(2)–O(4)
Ni(2)–N(5)
Ni(2)–N(6)
Ni(2)–N(7)
Ni(2)–N(8)
2.021(2)
2.003(2)
2.100(2)
2.027(2)
2.111(2)
2.028(2)
2.003(4)
2.019(3)
2.019(1)
2.096(4)
2.028(4)
2.118(4)
2.008(4)
2.032(3)
2.017(4)
2.109(4)
2.003(4)
2.120(4)
88.5(1)
90.6(2)
170.9(2)
94.1(2)
92.8(2)
93.8(2)
91.3(2)
91.0(1)
170.8(2)
80.3(2)
173.3(2)
95.3(2)
94.9(2)
88.8(2)
79.8(2)
88.0(1)
91.0(2)
171.1(2)
94.4(2)
91.2(2)
96.0(2)
93.5(2)
90.6(2)
170.0(2)
80.1(2)
171.6(2)
93.9(2)
94.3(2)
88.8(2)
79.6(2)
1.822(1)
1.821(1)
1.917(1)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(2)
O(1)–Ni(1)–N(3)
O(1)–Ni(1)–N(4)
O(2)–Ni(1)–N(1)
O(2)–Ni(1)–N(2)
O(2)–Ni(1)–N(3)
O(2)–Ni(1)–N(4)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(4)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
N(3)–Ni(1)–N(4)
O(3)–Ni(2)–O(4)
O(3)–Ni(2)–N(5)
O(3)–Ni(2)–N(6)
O(3)–Ni(2)–N(7)
O(3)–Ni(2)–N(8)
O(4)–Ni(2)–N(5)
O(4)–Ni(2)–N(6)
O(4)–Ni(2)–N(7)
97.45(6)
166.93(6)
88.42(7)
84.87(6)
91.08(6)
87.42(7)
88.83(8)
168.15(6)
88.99(6)
79.53(7)
92.84(7)
101.16(7)
102.88(8)
177.68(8)
79.33(6)
96.60(6)
176.49(6)
94.77(6)
85.42(6)
167.40(6)
Figure 2. X-ray crystal structure of [Ni(L2)2] (a) and [Ni(L5)2] (b)
shown with 30% thermal ellipsoids. H atoms are omitted for clar-
ity. The crystal cell of [Ni(L2)2] contains two independent com-
plexes (only one complex is shown and was chosen arbitrarily).
83.51(6)
The crystal cell of [Ni(L5)2] contains two independent
complexes. The geometry was found to be very similar to
that adopted for [Ni(L2)2]. The phenolate oxygen atoms are
cis to one another with O(1)–Ni(1)–O(2) and O(3)–Ni(2)–
O(4) angles of 88.5(1) and 88.0(1)°, respectively, the imine
nitrogen atoms are trans to one another with N(1)–Ni(1)– O(4)–Ni(2)–N(8)
N(5)–Ni(2)–N(6)
N(3) and N(5)–Ni(2)–N(7) angles of 173.3(2) and 171.6(2)°,
N(5)–Ni(2)–N(7)
respectively, whereas the quinoline nitrogen atoms are cis
N(5)–Ni(2)–N(8)
to one another with N(2)–Ni(1)–N(4) and N(6)–Ni(2)–N(8)
N(6)–Ni(2)–N(7)
angles of 88.8(2) and 88.8(2)°, respectively. The Ni–Nimine
bond lengths Ni(1)–N(1), Ni(1)–N(3), Ni(2)–N(5), and
Ni(2)–N(7) are 2.019(4), 2.028(4), 2.017(4), and 2.003(4) Å,
respectively, whereas the Ni–Ophenolate bond lengths Ni(1)–
O(1), Ni(1)–O(2), Ni(2)–O(3), and Ni(2)–O(4) are 2.003(4),
2.019(3), 2.008(4), and 2.032(3) Å, respectively. They are
thus close to those obtained for [Ni(L2)2]. As expected, the
Ni–Nquinoline bonds Ni(1)–N(2), Ni(1)–N(4), Ni(2)–N(6),
and Ni(2)–N(8) are longer, with values of 2.096(4), 2.118(4),
2.109(4), and 2.120(4) Å, respectively. The Ni–Nquinoline
bond lengths in [Ni(L5)2] are quite similar to the Ni–
Npyridine bond lengths obtained for [Ni(L2)2]. They are also
much shorter than the Ni–Namine bond length reported for
a pseudo-octahedral nickel(II) complex of a tridentate
Schiff base ligand involving a terminal N(CH3)2 nitrogen
atom (2.318–2.463 Å).[5] The Ni–Nterminal bond, and conse-
quently the octahedral ligand field, is thus much stronger
when the terminal N-donor is a pyridine or a quinoline
group.
N(6)–Ni(2)–N(8)
N(7)–Ni(2)–N(8)
el(II) ion is coordinated by one amidate N(1), one tertiary
amine N(2), and one pyridine N(3) nitrogen atom as well
as a phenolate oxygen atom O(1). The Ni–O(1) bond length
is 1.823(1) Å; Ni(1)–N(1) is 1.822 Å, Ni(1)–N(2) is
1.821(1) Å, and Ni(1)–N(3) is 1.917(1) Å. They are much
The X-ray crystal structure of [Ni(L6)] {complex re-
sulting from the decomposition of [Ni(L1)2]} differs signifi-
Figure 3. X-ray crystal structure of [Ni(L6)] shown with 30% ther-
cantly from the former two complexes (Figure 3). The nick- mal ellipsoids. H atoms are omitted for clarity.
Eur. J. Inorg. Chem. 2008, 4215–4224
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O. Rotthaus, V. Labet, C. Philouze, O. Jarjayes, F. Thomas
The meridional versus facial preference in [Ni(L1–3)2] was
FULL PAPER
shorter than those obtained for [Ni(L2)2] and [Ni(L5)2], thus
showing that the nickel(II) atom is within a different spin studied by computing differences in energies for the mer-
configuration, i.e. S = 0. The O(1)–Ni(1)–N(1), O(1)–Ni(1)– and fac-isomeric pairs. Optimized geometries for the mer
N(2), N(1)–Ni(1)–N(2), and N(2)–Ni(1)–N(3) angles differ and fac structures are depicted in Figures 4 and 5. The opti-
only slightly from 90°, whereas the O(1)–Ni(1)–N(2) and mized bond lengths and angles of mer-[Ni(L2)2] were found
N(1)–Ni(1)–N(3) angles are close to 180°. The geometry to be in perfect agreement with the crystal structure. For
around the metal center is thus square-planar, in agreement both [Ni(L1)2] and [Ni(L2)2] the mer isomer was found to
with a low-spin configuration of the nickel ion. We did not be more stable than the fac one as observed experimentally.
further investigate the properties of [Ni(L6)] because of the The energy difference was calculated to be 12.9 kcalmol–1
low yield of crystallization, as well as the irreversibility of and 9.7 kcalmol–1 for [Ni(L1)2] and [Ni(L2)2], respectively.
the CV curves obtained from preliminary electrochemical In contrast, mer-[Ni(L3)2] is found to be 2.0 kcalmol–1 less
studies.
stable than cis,fac-[Ni(L3)2]. The energy reversal of more
The coordination modes of [Ni(L1–3)2] have also been in- than 10 kcalmol–1 makes the meridional coordination fa-
vestigated by calculations using the GAUSSIAN suite at vored in [Ni(L1)2] and [Ni(L2)2]. This is attributed to the
the DFT B3LYP/LANL2DZ level of theory.[8b,9,11]
rigidity of the deprotonated ligands L1 and L2.[9] Similar
results have been reported for the iron complex of HL1
where the mer isomer was more stable than the fac one by
8.5 kcalmol–1.[9] We have also investigated the cis versus
trans preference in fac-[Ni(L3)2] (Figure 5). The trans,fac-
[Ni(L3)2] isomer was found to be 5.2 kcalmol–1 more stable
than the cis,fac-[Ni(L3)2] isomer, which is ascribed to steric
hindrances between the tert-butyl groups of the phenolate
moieties.
Figure 4. Optimized structures for fac-[Ni(L1)2] (a), mer-[Ni(L1)2]
(b), fac-[Ni(L2)2] (c), and mer-[Ni(L2)2] (d).
Figure 5. Optimized structures for cis,fac-[Ni(L3)2] (a), trans,fac-
[Ni(L3)2] (b), and mer-[Ni(L3)2] (c).
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Pseudo-Octahedral Schiff Base Nickel(II) Complexes
Spectroscopic and Electrochemical Properties of the Nickel
Complexes
1
The five complexes [Ni(L1–5)2]+ exhibit H NMR spec-
troscopic resonances distributed over a wide spectral width
(δ = 0 to 50 ppm). They are thus paramagnetic, high-spin
NiII species, confirming that the nickel ion retains its
pseudo-octahedral geometry in solution. Despite being
paramagnetic, no signal could be observed in their X-band
EPR spectra. This paradox is explained by the fact that
high-spin nickel(II) complexes usually exhibit zero-field-
splitting parameters larger than the X-band quantum
(0.3 cm–1). They are consequently not detectable by conven-
tional EPR spectroscopy.[12]
The electronic spectra of [Ni(L1–5)2] are shown in Fig-
ure 6. The spectra of [Ni(L1–2)2] are characterized by an in-
tense absorption that results from a combination of π–π*
and charge-transfer transitions. This absorption is observed
at 438 nm (ε = 14600 –1 cm–1) for [Ni(L1)2] and 418 nm
(16000 –1 cm–1) for [Ni(L2)2]. This shift of λmax reflects the
lower electron-donating properties of the tert-butyl group
relative to the methoxy para substituent. In [Ni(L4–5)2] the
absorption is observed at an even longer wavelength
[529 nm (31000 –1 cm–1) and 508 nm (32000 –1 cm–1),
respectively] as a result of an extended conjugation between
the quinoline and the phenolate moiety. No such intense
absorption could be observed in the visible spectrum of
[Ni(L3)2], highlighting the contribution of the imino–phe-
nolate orbitals to this transition. The nickel(II) d–d transi-
tions could be observed as low intensity bands in the 540–
578 nm region for [Ni(L1–3)2], while they are hidden by
higher intensity transitions involving the ligand orbitals in
[Ni(L4–5)2].
The electrochemical behavior of [Ni(L1–5)2] has been
studied in CH2Cl2 by cyclic voltammetry (CV). All the po-
tential values are given relative to the Fc+/Fc reference elec-
trode.
The CV curves of [Ni(L1–5)2] exhibit two reversible oxi-
dation waves (Figure 7). Each redox process corresponds to
a one-electron transfer, which was confirmed by a coulo-
metric titration and a rotating disc electrode (RDE) amper-
ometric titration (Figures S1–S5). The corresponding po-
1
2
tentials are E1/2 = –0.17 V and E1/2 = 0.08 V for [Ni(L1)2]
1
2
and E1/2 = –0.01 V and E1/2 = 0.32 V for [Ni(L2)2]. The
higher values obtained for [Ni(L2)2] result from the lower
electron-donating properties of the tert-butyl group relative
to the methoxy group. A participation of the ligand in the
oxidation locus of the complexes is thus expected.
1
[Ni(L3)2] exhibits oxidation potentials at E1/2 = –0.20 V
and E1/2 = 0.09 V. These values are much lower than those
2
of [Ni(L2)2], in agreement with the electron-withdrawing ef-
fect of the conjugated imine. Most of all, they are very close
to those obtained for [Ni(L1)2] showing that the replace-
ment of the imine by an aminomethyl group efficiently
counterbalances the lower electron-donating properties of
the tert-butyl versus methoxy substituent.
Figure 6. Electronic spectra (233 K) of 0.05 m CH2Cl2 solutions
(+ 5 m TBAP) of: (a) [Ni(L1)2] (solid line), [Ni(L1)2]+ (dashed
line); (b) [Ni(L2)2] (solid line), [Ni(L2)2]+ (dashed line); (c) [Ni-
(L3)2] (solid line), [Ni(L3)2]+ (dashed line); (d) [Ni(L4)2] (solid line),
[Ni(L4)2]+ (dashed line); (e) [Ni(L5)2] (dotted line); [Ni(L5)2]+ (solid
line); path length is 1.000 cm; the samples were electrochemically
generated in CH2Cl2 + 0.1  TBAP solutions and diluted 20-fold
in CH2Cl2.
The CV curves of [Ni(L4)2] and [Ni(L5)2] are charac-
1
2
terized by two oxidation waves at E1/2 = –0.09 V, E1/2
=
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O. Rotthaus, V. Labet, C. Philouze, O. Jarjayes, F. Thomas
FULL PAPER
cess. It is also noticeable that the oxidation potentials are
lower than most of those reported for nickel complexes of
salen and reduced salen ligands.[4b,4d–4j]
Spectroscopic Properties of the One-Electron-Oxidized
Nickel Complexes
The one-electron-oxidized complexes [Ni(L1–5)2]+ have
been generated electrochemically in CH2Cl2 (+ 0.1 
TBAP) at 233 K.
Upon electrolysis, the brown solutions of [Ni(1)2] and
[Ni(2)2] take on an orange shade. The quantitative genera-
tion of the cations (Ͼ95%) was evident by RDE ampero-
metric titration: the anodic wave observed at –0.18 and
–0.03 V for [Ni(1)2] and [Ni(2)2], respectively, is progressively
replaced by a cathodic wave at the same potential (Fig-
ures S1–S5).
Figure 7. Cyclic voltammetry curves of 1 m solutions of the com-
plexes in CH2Cl2 containing 0.1  TBAP. (a) [Ni(L1)2], (b) [Ni-
(L2)2], (c) [Ni(L3)2] (d) [Ni(L4)2], (e) [Ni(L5)2]. Scan rate: 0.1 Vs–1,
T = 298 K {except [Ni(L3)2] recorded at 233 K}. The potentials are
referenced vs. the Fc/Fc+ couple.
The UV/Vis spectrum of [Ni(L1)2]+ is characterized by
an intense absorption band at 433 nm [12740 –1 cm–1] and
lower intensity broad transitions in the 500–700 nm region
(Table 3). The spectrum of [Ni(L2)2]+ also exhibits an in-
tense absorption band at a somewhat lower wavelength
(414 nm [6100 –1 cm–1]) and low intensity transitions in the
500–700 nm region (Figure 6, Table 3). The intense π–π*
transitions of a phenoxyl radical, if it is present, are ex-
pected to be found in the 390–440 nm region.[13] As the
initial complexes [Ni(L1)2] and [Ni(L2)2] already possess a
strong absorption band at 431 and 413 nm, respectively, it
is not possible to conclude unambiguously to a ligand-
based rather than metal-based oxidation process in these
cases.
1
2
0.16 V and E1/2 = 0.08 V, E1/2 = 0.40 V, respectively. They
are shifted according to the electronic properties of the phe-
nolate para substituent as in [Ni(L1)2] and [Ni(L2)2]. By
comparing the E1/2 values obtained for [Ni(L1)2] and those
obtained for [Ni(L4)2] it appears that the substitution of a
pyridine by a quinoline group makes the complexes slightly
harder to oxidize. The same observation prevails when the
E1/2 value obtained for [Ni(L2)2] is compared with that of
[Ni(L5)2]. In contrast, no such shifts are observed when the
oxidation potentials of the respective free ligands are com-
pared with one another (Table 2). The metal ion, in ad-
dition to the ligand, is thus involved in the oxidation pro-
The colorless CH2Cl2 solution of [Ni(L3)2] rapidly turns
blue upon one-electron oxidation. [Ni(L3)2]+ exhibits in-
tense absorption bands at around 390 nm as a shoulder
(5400 –1 cm–1), 595 nm (8900 –1 cm–1), 680 nm (shoulder,
4900 –1 cm–1), and Ͼ1050 nm (Ͼ4000 –1 cm–1) (Table 3).
These transitions resemble those reported for phenoxyl rad-
ical species obtained by electrochemical oxidation of either
the N,NЈ-bis{2,4-di-tert-butyl-phenol}ethylenediamine li-
gand or its copper(II) complex, and 2-imidazole-4,6-di-tert-
butylphenoxyl radicals.[14] These bands could thus be safely
attributed to the π–π* transitions of a coordinating phen-
oxyl radical.
Table 2. UV/Vis and electrochemical properties[a,b] of [Ni(L1–5)2].
1
2
Complex
λmax [nm] (ε [–1 cm–1])[c]
E1/2
E1/2
[Ni(L1)2]
[Ni(L2)2]
[Ni(L3)2]
[Ni(L4)2]
[Ni(L5)2]
438 (14600), 544 (360)
418 (16000), 540 sh (150)
310 (12910), 578 (26)
350 (27000), 529 (31000)
350 (30000), 508 (32000)
–0.176
–0.028
–0.204
–0.088
0.08
0.088
0.33
0.088
0.16
0.396
[a] In CH2Cl2 (+ 0.1  TBAP) at 298 K. Potential values in V vs.
the Fc+/Fc reference electrode. The E1/2 values were obtained from
differential pulse voltammetry (DPV) curves by adding half of the
pulse amplitude to the Ep value (pulse amp. 50 mV). The confi-
dence level is Ϯ0.005 V. [b] For the ligands HL1, HL2, HL3, HL4,
and HL5 the Ep values obtained from DPV curves are 0.307, 0.511,
0.215, 0.319, and 0.503 V, respectively. [c] In CH2Cl2 at 298 K.
The visible spectrum of [Ni(L4)2]+ is characterized by in-
tense absorption bands at 411 nm (9680 –1 cm–1) and
504 nm (20360 –1 cm–1), with a low-intensity broad transi-
tion at 800 nm (4000 –1 cm–1). The spectrum of [Ni(L5)2]+
Table 3. UV/Vis properties[a] of complexes [Ni(L1–5)2]+.
Complex
λmax [nm] (ε [–1 cm–1])
[Ni(L1)2]+
[Ni(L2)2]+
[Ni(L3)2]+
[Ni(L4)2]+
[Ni(L5)2]+
316 sh (6560), 433 (12740), 553 br (1380), 660 br (1080)
347 sh (5320), 398 sh (5420), 414 (6100), 523 br (1380), 588 br (1100)
390 sh (5400), 595 (8900), 680 sh (4900), Ͼ1050 br (Ͼ4000)
328 (18220), 411 (9680), 482 sh (19000), 504 (20360), 800 br (4000)
341 (25480), 448 sh (11720), 471 (17360), 505 sh (14100), 800 br (1900)
[a] In CH2Cl2 (+ 5 m TBAP) at 233 K. The samples were electrochemically generated in CH2Cl2 + 0.1  TBAP solutions and diluted
20-fold in CH2Cl2.
4220
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Eur. J. Inorg. Chem. 2008, 4215–4224

Pseudo-Octahedral Schiff Base Nickel(II) Complexes
exhibits absorption bands at 448 nm (11720 –1 cm–1), (axially elongated rather than axially compressed, Fig-
471 nm (17360 –1 cm–1), and 800 nm (1900 –1 cm–1) ure 9c).[4b,4e–4j,7] The ground state could thus be efficiently
(Table 3). The well-resolved 411 nm absorption in the vis- modulated by controlling pyridine ligation. The observed
ible spectrum of [Ni(L4)2]+, which is absent in the spectrum inversion in [Ni(L2)2]+ may result from the higher axial li-
of [Ni(L4)2], is typical for copper(II)- or nickel(II)-coordi- gand field provided by the rigid tridentate ligands.
nated phenoxyl radicals.
EPR spectroscopy has been used to probe more finely
the oxidation sites in [Ni(L1–5)2]+ and confirm the trends
deduced from UV/Vis spectroscopy.
The EPR spectrum of [Ni(L1)2]+ in frozen CH2Cl2 exhib-
its a dominant S = 1/2 signal at g = 2.21 (Figure 8a). It is
found to be nearly isotropic and identical to that reported
by Shimazaki et al. for an S = 1/2 complex involving a high-
spin nickel(II) complex coordinated to a phenoxyl radi-
cal.[4c] This signal is thus taken as evidence for an antiferro-
magnetic coupling between a high-spin NiII ion and a phen-
oxyl radical leading to an S = 1/2 system. Such a high
gaverage value for an S = 1/2 system has also been reported
recently for a complex involving a high-spin nickel(II) anti-
ferromagnetically coupled to a copper(II) center[15] or an
Figure 9. X-band EPR spectra of 1 m solution in CH2Cl2 con-
iminosemiquinone radical moiety.[3i]
taining 0.1  TBAP of: (a) [Ni(L2)2]+ (solid lines) and simulation
using parameters given in the text (dotted lines); (b) [Ni(L5)2]+ (so-
lid lines) and simulation using parameters given in the text (dotted
lines); (c) the pseudo-octahedral NiIII complex of a tetradentate
Schiff base ligand with two axially bound exogenous pyridine
rings.[4f] The chemical formula is shown at the top. T = 100 K,
microwave frequency 9.42 GHz, power 20 mW, modulation fre-
quency 100 kHz, amplitude 0.197 mT.
[Ni(L3)2]+ exhibits an EPR signal close to that observed
for [Ni(L1)2]+ (Figure 8b) although it is slightly more aniso-
tropic.[16] An electronic structure similar to that of [Ni-
(L1)2]+ thus prevails, despite the fact that the ligands differ
both by their phenolate para substituent and the pivotal
nitrogen atom.
It is important to remark that the EPR signatures of the
[Ni(L1–3)2]+ cations are correlated with the oxidation poten-
tial of the starting neutral compounds: The lower oxidation
potentials are indicative of an oxidation process that is li-
gand-centered, while the higher oxidation potentials are in-
dicative of a metal-centered oxidation process.
Figure 8. X-band EPR spectra of 1 m CH2Cl2 (+ 0.1  TBAP)
solutions of: (a) [Ni(L1)2]+, (b) [Ni(L3)2]+, (c) [Ni(L4)2]+. T = 100 K,
microwave frequency 9.42 GHz, power 20 mW, modulation fre-
quency 100 kHz, amplitude 0.197 mT.
The EPR spectra of [Ni(L4)2]+ and [Ni(L5)2]+ in frozen
CH2Cl2 compare with those of [Ni(L1)2]+ and [Ni(L2)2]+,
respectively, (Figures 8c and 9b). The global electronic dis-
The EPR spectrum of [Ni(L2)2]+ contrasts sharply with tribution is thus poorly affected by changes in the N-donor
that of [Ni(L1)2]+: The S = 1/2 signal was found to be properties of the pyridine group. The parameters obtained
clearly anisotropic with g values of g1 = 2.205, g2 = 2.113, by simulation of the spectrum of [Ni(L2)2]+ differ somewhat
and g3 = 2.053 (gav = 2.12, Figure 9a). In addition, a hyper- slightly from those obtained for [Ni(L5)2]+: The g values are
fine coupling resulting from the interaction of the electron g1 = 2.195, g2 = 2.103, and g3 = 2.058 (Figure 9b). As for
with two equivalent nitrogen atoms could be resolved in [Ni(L2)2]+, a hyperfine coupling resulting from the interac-
(2N)
the g3 component (A3
= 2.0 mT). This suggests that the tion of the electron with two equivalent nitrogen atoms
(2N)
contribution of the metal orbital to the SOMO is much could be resolved in the g3 component (A3
= 1.4 mT).
more significant in [Ni(L2)2]+ than in [Ni(L1)2]+. The order- This behavior is in agreement with changes in the donor
ing of g is also consistent with a ground state that is essen- properties of a coordinating nitrogen atom and indicates
[7i]
tially dx2–y2
.
This ground state is remarkable as most of that at least one nitrogen atom of the ligand remains coor-
the pseudo-octahedral NiIII complexes of tetradentate dinated to the metal ion after one-electron oxidation.
Schiff base ligands with two axially bound exogenous pyr- EPR spectra have also been recorded in a CH2Cl2/
idine rings (thus involving the same donor set, two Npy, CH3OH mixture, i.e. in the presence of a moderately coor-
two Nim and two OphO atoms) exhibit a dz2 ground state dinating solvent.[17]
Eur. J. Inorg. Chem. 2008, 4215–4224
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4221

O. Rotthaus, V. Labet, C. Philouze, O. Jarjayes, F. Thomas
FULL PAPER
The spectra were found to be similar to those obtained with partial delocalization of the SOMO onto the metal
in neat CH2Cl2 (Figure S6). This is an important result as orbitals. A tert-butyl to N(CH3)2 substitution[18] at the para
most of the one-electron-oxidized nickel–salen complexes position of the phenolate induces a small but detectable
are extremely sensitive to traces of coordinating solvents. In shift of the SOMO:[4j] The contribution of the metal atom
particular, CH3OH, which is known to bind tightly to the decreases from 6.6 to 1.8%. This substitution also results
nickel ion, promotes an intramolecular electron transfer in a very large drop of the oxidation potential (from 0.582
from the metal atom to the Schiff base ligand.[4b,4e–4i,7] to –0.144 V vs. Fc+/Fc). In the [Ni(L1–5)2] series a tert-butyl
Consequently, nickel(II)–phenoxyl radical complexes of to methoxy substitution is sufficient[17] to dramatically shift
tetradentate Schiff bases are always converted into nickel- the oxidation site from the metal atom to the ligand, i.e. to
(III) species in the presence of CH3OH. The metal ion in promote a “chemical valence tautomerism”.[3e] In addition
[Ni(L1–5)2]+ is already hexacoordinate. Binding of ad- this phenomenon is observed within a narrower range of
ditional low-affinity exogenous ligands is no longer pos- potentials {the oxidation potentials are –0.028 and
sible, thus preventing subsequent solvent-dependent valence –0.176 V for [Ni(L1)2] and [Ni(L2)2], respectively}.
tautomerism.
Pyridine or any exogenous ligand could be added to the
Changes in the phenolate para substituent or in the hy- nickel complexes of tetradentate Schiff bases in order to
bridization of the pivotal nitrogen atom thus dramatically obtain a pseudo-octahedral metal ion. Unfortunately, this
influence the EPR spectra of the cations. In contrast geometrical change is irremediably accompanied by a val-
changes in the N-donor properties of the imino terminal ence tautomerism making the oxidation locus strongly sol-
nitrogen atom (quinoline to pyridine substitution) do not vent-dependent: Axial coordination shifts the dz2 orbital to
significantly affect the EPR signature of the cations.
a higher energy, resulting in the transfer of an electron from
the metal center to the ligand. The oxidation state of the
nickel ion in the so-obtained pseudo-octahedral complex is
+III, and the ground state is dz2 [nickel(III) complexes that
Conclusions
We have described the oxidation behavior of five nickel- exhibit a dx2–y2 ground state exist but they are rare]. In con-
(II) complexes of tridentate Schiff base ligands [Ni(L1–5)2]. trast, [Ni(L1–5)2]+ exhibit an oxidation locus that is fully
In this series, the ligands differ from one another by the insensitive to the solvent. This is the result of an environ-
phenolate para substituent, the hybridization of the pivotal ment of the metal atom in [Ni(L1–5)2]+ that is already
nitrogen atom, and the N-donor properties of the terminal pseudo-octahedral, and the fact that solvent molecules are
pyridine group.
not able to displace one of the tridentate ligands (chelate
[Ni(L1)2] is characterized by an oxidation potential at effect). It is thus possible to cancel the “solvent-dependent
–0.17 V vs. Fc+/Fc. The one-electron-oxidized species valence tautomerism” simply by changing the denticity of
[Ni(L1)2]+ exhibits an EPR signal at g = 2.21 similar to the Schiff bases in the nickel complexes. Moreover, [Ni-
that reported for NiII–radical complexes of tripodal ligands (L2)2]+ and [Ni(L5)2]+ exhibit a dx2–y2 ground state, which is
(high-spin NiII antiferromagnetically coupled to a phenoxyl rather unusual. It may result from the higher axial ligand
radical).[4c] [Ni(L2)2] differs from [Ni(L1)2] by the phenolate field provided by two tridentate ligands compared to one
para substituent (tert-butyl instead of methoxy group) and tetradentate ligand plus two pyridine groups.
exhibits an oxidation potential that is ca. 0.16 V higher. The
The oxidative chemistry of apparently simple Schiff base
electrogenerated cation [Ni(L2)2]+ is characterized by a sig- nickel complexes is thus full of surprises. Complexes from
nificant contribution of a metal orbital to the SOMO and tridentate Schiff bases exhibit an unprecedented oxidative
an unusual dx2–y2 ground state. The oxidation site is thus behavior that contrasts sharply with that of more common
shifted from the ligand to the metal atom by lowering the tetradentate ones. A simple change in the denticity of the
electron-donating properties of the phenolate para substitu- ligand thus has strong consequences on the oxidation be-
ent. [Ni(L3)2]+ exhibits both an EPR and an electrochemi- havior of the nickel complexes. Actually, it is uncertain
cal signature similar to those of [Ni(L1)2]+ indicating that whether the [Ni(L1–5)2]+ series constitutes the exception that
the imino to amino substitution compensates exactly for the proves the rule or if it reflects a more general trend.
methoxy to tert-butyl one.
Further studies concerning the rearrangement mecha-
The influence of the pendant pyridine ring in [Ni(L1–2)2] nism of [Ni(L1)2] into [Ni(L6)], and the effect of the ligand/
has been studied by replacing it by a quinoline group metal stoichiometry on the nickel spin state are currently in
{[Ni(L4–5)2] compounds}. This change does not signifi- progress.
cantly affect the EPR signature of the cations, but makes
the complexes slightly harder to oxidize.
Finally, there are fundamental differences in the oxi- Experimental Section
dation behavior of the nickel complexes according to
whether they are synthesized from one tetradentate (low-
spin configuration of the metal ion) or two tridentate (high-
spin configuration of the metal ion) Schiff base ligands:
One-electron oxidation of square-planar nickel com-
Materials and Methods: All chemicals were of reagent grade and
were used without purification. CH2Cl2 was anhydrous (Ͼ99.8%)
and stored under argon. NMR spectra were recorded with a Bruker
AM 300 (1H NMR at 300 MHz) spectrometer. Chemical shifts are
quoted relative to tetramethylsilane (TMS). Mass spectra were re-
plexes of tetradentate Schiff bases affords radical species corded with a Thermofinningen (EI/DCI) or a Nermag R101C
4222 Eur. J. Inorg. Chem. 2008, 4215–4224
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Pseudo-Octahedral Schiff Base Nickel(II) Complexes
(FAB+) apparatus. Low-temperature UV/Vis spectra were recorded
with a Cary Varian 50 spectrophotometer equipped with a low-
temperature Hellma immersion probe (1.000 cm path length). The
temperature was set at 238 K with an RK8 KS Lauda cryostat. X-
band EPR spectra were recorded with a Bruker ESP 300E spec-
trometer equipped with a Bruker nitrogen flow cryostat. Spectra
were treated by using the WINEPR software and simulated by
using the Bruker SIMFONIA software. Electrochemical measure-
ments were carried out by using a CHI110 potentiostat. Experi-
ments were performed in a standard three-electrode cell under ar-
gon. An Ag/AgNO3 (0.01 ) reference electrode was used. All the
potentials given in the text refer to the regular Fc/Fc+ redox couple
used as an internal reference. A glassy carbon disc electrode (5 mm
diameter), which was polished with 1 mm diamond paste, was used
as the working electrode. The electrochemical behavior of the li-
gands and complexes was studied by cyclic voltammetry (CV) and
with a rotating disc electrode (RDE) in CH2Cl2 solutions contain-
ing 0.1  tetrabutylammonium perchlorate (TBAP) as the support-
ing electrolyte. Electrolysis was performed at 233 K by using a PAR
model 273 potentiostat and a carbon felt working electrode.
Preparation of the Ligands and Complexes
HL1: 2-Picolylamine (340 mg, 3.16 mmol) was added to a stirred
solution
of
3-tert-butyl-2-hydroxy-5-methoxybenzaldehyde
(658 mg, 3.16 mmol) in methanol (20 mL). The solution was heated
at reflux for 5 h. Removal of the solvent in vacuo yielded a dark-
red oil. Addition of pentane (1 mL) afforded a brown solid 15 h
later. Yield 820 mg (87%). C18H22N2O2 (298.38): calcd. C 72.46, H
1
7.43, N 9.39; found C 72.35, H 7.41, N 9.58. H NMR (CDCl3): δ
3
= 8.57 (d, 1 H, J = 4 Hz), 8.51 (s, 1 H), 7.39 (m, 1 H), 7.21 (m, 2
4
4
H), 6.99 (d, 1 H, J = 3.0 Hz), 6.64 (d, 1 H, J = 3.4 Hz), 4.94 (s,
2 H), 3.78 (s, 3 H), 1.42 (s, 9 H) ppm.
HL4: 8-Aminoquinoline (278 mg, 1.92 mmol) in methanol (10 mL)
was added to a stirred solution of 3-tert-butyl-2-hydroxy-5-meth-
oxybenzaldehyde (400 mg, 1.92 mmol) in methanol (15 mL). Two
drops of formic acid were added, and the solution was heated to
reflux for 4 h. Removal of the solvent in vacuo yielded a dark-red
oil. The crude product was purified by alumina column chromatog-
raphy with hexane/ethyl acetate (8:1). Yield 300 mg (46%).
C21H22N2O2 (334.41): calcd. C 75.42, H 6.63, N 8.83; found C
1
3
75.38, H 6.59, N 8.96. H NMR (CDCl3): δ = 8.98 (d, 1 H, J =
Crystal Structure Analysis: Crystals were mounted on a Kappa
CCD Nonius diffractometer equipped with graphite-monochro-
mated Mo-Kα radiation (λ = 0.71073 Å) and a cryostream cooler.
The collected reflections were corrected for Lorentzian and polar-
ization effects but not for absorption. Crystal structural solution
(direct method) and refinement (by full-matrix least squares on F)
was performed by using the teXsan analysis package. All non-hy-
drogen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms were generated in idealized positions, riding on
the carrier atoms, with isotropic thermal parameters. CCDC-
642613, -659639, and -659640 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/datarequest/cif.
3
3
2.7 Hz), 8.88 (s, 1 H), 8.17 (d, 1 H, J = 8.31 Hz), 7.69 (d, 1 H, J
= 8.19 Hz), 7.55 (m, 1 H), 7.46 (m, 2 H), 7.07 (d, 1 H, 4J = 2.9 Hz),
4
6.77 (d, 1 H, J = 3.1 Hz), 3.81 (s, 3 H), 1.49 (s, 9 H) ppm.
[Ni(L1)2]:
A methanolic solution (20 mL) of Ni(OAc)2·4H2O
(43 mg, 0.15 mmol) and 2 equiv. of triethylamine (90 µL,
0.62 mmol) were added to a stirred solution of HL1 (92 mg,
0.31 mmol) in methanol (20 mL). The solution was refluxed for 3 h.
After removal of half of the solvent, the solution was stored at
–20 °C overnight to afford a powder. Washings with cold methanol
led to [Ni(L1)2]. Yield 41 mg (42%). C36H42N4O4Ni (653.44): calcd.
C 66.17, H 6.48, N 8.57, Ni 8.98; found C 65.92, H 6.43, N 8.75,
Ni 9.02. MS: m/z = 653 [M + H]+.
[Ni(L2)2]: This complex was prepared in a similar manner in 40%
yield. X-ray quality crystals were obtained by slow evaporation of
the solvent from a concentrated solution of [Ni(L2)2] in MeOH.
C42H54N4O2Ni (705.60): calcd. C 71.49, H 7.71, N 7.94, Ni 8.32;
found C 71.70, H 7.62, N 8.03, Ni 8.10. MS: m/z = 705 [M + H]+.
Crystal Data
[Ni(L2)2]: Red prism (0.3ϫ0.2ϫ0.1 mm), Mr = 705.61 gmol–1,
monoclinic, space group P21/c, a = 10.291(1) Å, b = 22.160(4) Å,
c = 17.723(5) Å, α = 90, β = 96.2(3), γ = 90°, V = 3956(1) Å3, Z =
4, Dc = 1.185 gcm–3, T = 293 K, µ(Mo-Kα) = 5.29 cm–1. 59876
[Ni(L3)2]: This complex was prepared in a similar manner in 55%
yield. C42H58N4O2Ni (709.63): calcd. C 71.09, H 8.24, N 7.90, Ni
8.27; found C 71.22, H 8.29, N 8.02, Ni 8.12. MS: m/z = 709 [M
+ H]+.
reflections were collected. Of the 11799 unique reflections (Rint
0.16571), 6852 were observed [F Ն 2σ(F)] and used in the full-
matrix least-squares refinement of 442 variables. R = 0.0542, Rw
=
=
[Ni(L4)2]: This complex was prepared in a similar manner in 44%
yield. C42H58N4O2Ni (725.50): calcd. C 69.53, H 5.84, N 7.72, Ni
8.09; found C 69.82, H 5.69, N 7.96, Ni 8.13. MS: m/z = 731 [M
+ H]+.
0.0805, goodness of fit S = 0.746, max./min. residual peaks are
1.14/–0.61 eÅ3.
[Ni(L5)2]: Red platelet (0.4ϫ0.3ϫ0.02 mm), Mr = 1555.36 gmol–1,
¯
triclinic, space group P1, a = 15.514(4) Å, b = 15.742(2) Å, c =
[Ni(L5)2]: This complex was prepared in a similar manner in 39%
yield. C42H58N4O2Ni (777.66): calcd. C 74.13, H 7.00, N 7.20, Ni
7.55; found C 74.22, H 7.01, N 7.11, Ni 7.61. MS: m/z = 777 [M
+ H]+.
17.882(5) Å, α = 90.15(2), β = 102.48(2), γ = 101.37(2)°, V =
4176(2) Å3, Z = 2, Dc = 1.237 gcm–3, T = 150 K, µ(Mo-Kα) =
5.08 cm–1. 59876 reflections were collected. Of the 71767 unique
reflections (Rint = 0.21943), 7407 were observed [F Ն 2σ(F)] and
used in the full-matrix least-squares refinement of 991 variables. R
= 0.0703, Rw = 0.0740, goodness of fit S = 1.250, max./min. resid-
ual peaks are 0.83/–0.72 eÅ3.
Supporting Information (see footnote on the first page of this arti-
cle): Voltammetric curves at RDE and differential pulse voltam-
metries of [Ni(L1–5)2], and EPR spectra of [Ni(L1–5)2]+ in CH2Cl2/
CH3OH (1:1) solutions.
[Ni(L6)]: Red prism (0.4ϫ0.3ϫ0.1 mm), Mr = 699.48 gmol–1, tri-
clinic, space group P1, a = 9.2525(9) Å, b = 14.2426(4) Å, c =
15.476(1) Å, α = 116.572(4), β = 105.421(8), γ = 78.773(5)°, V =
1751.5(2) Å3, Z = 2, Dc = 1.326 gcm–3, T = 293 K, µ(Mo-Kα) =
6.05 cm–1. Of the 10101 unique reflections (Rint = 0.06128), 7324
were observed [F Ն 2σ(F)] and used in the full-matrix least-squares
refinement of 433 variables. R = 0.0434, Rw = 0.0599, goodness of
fit S = 1.217, max./min. residual peaks are 0.65/–0.48 eÅ3.
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We tried to simulate this spectrum by considering an axially
elongated octahedral NiIII species. The very-low-intensity
shoulder at 320 mT was then considered to be associated with
the dominant g = 2.21 signal. The g3 value (assimilated to gzz)
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[17]
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The complexes were found to be insoluble in neat CH3OH.
N(CH3)2 is strongly electron-donating as reflected by
a
σ+Hammet parameter of –1.17; by comparison, the tert-butyl and
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Received: April 22, 2008
Published Online: July 14, 2008
4224
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