Tetranuclear copper(II) and nickel(II) complexes incorporating a new imidazole-containing ligand

Article abstract of DOI:10.1002/ejic.200400346

The ligating properties of a new tridentate imidazole- and phenol-containing ligand H₃L towards nickel(II) and copper(II) ions have been studied. The ligand generates, in the presence of air and base, iminosemiquinone radical anions, which are not stable in the presence of the metal ions. The amine bond >C-NH- of the parent ligand becomes oxidized to >C=N- bonds upon coordination with Ni^II or Cu^II. The imidazole N atom becomes deprotonated and the resulting imidazolate group acts as a bridging ligand to generate tetranuclear complexes [Cu^II ₄L′₄] and [Ni^II₄L′ ₄], where [L′]^2- is the dianionic new Schiff base formed after oxidation of the parent ligand. The square-planar tetranickel(II) complex is diamagnetic, as expected, whereas the four copper centres bridged by imidazolate anions in the complex [Cu^II₄L′ ₄] are antiferromagnetically exchange coupled (J = -49 cm ^-1), which has been supported by the EPR measurements. A simple magnetostructural correlation for the imidazolate-bridged copper(II) complexes is yet to emerge. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).

Full text of DOI:10.1002/ejic.200400346

FULL PAPER
Tetranuclear Copper(II) and Nickel(II) Complexes Incorporating a New
Imidazole-Containing Ligand
Soumen Mukherjee,[a] Thomas Weyhermüller,[a] Eckhard Bill,[a] and Phalguni Chaudhuri*[a]
Keywords: Tetranuclear complexes / Imidazolate bridging / Copper / Nickel / Magnetic properties
The ligating properties of a new tridentate imidazole- and
dianionic new Schiff base formed after oxidation of the par-
phenol-containing ligand H3L towards nickel(II) and cop- ent ligand. The square-planar tetranickel(II) complex is dia-
per(II) ions have been studied. The ligand generates, in the
presence of air and base, iminosemiquinone radical anions,
which are not stable in the presence of the metal ions. The
magnetic, as expected, whereas the four copper centres
bridged by imidazolate anions in the complex [CuII4LЈ4] are
antiferromagnetically exchange coupled (J = −49 cm−1),
amine bond ϾC−NH− of the parent ligand becomes oxidized which has been supported by the EPR measurements. A
to ϾC=N− bonds upon coordination with NiII or CuII. The im-
idazole N atom becomes deprotonated and the resulting im-
simple magnetostructural correlation for the imidazolate-
bridged copper(II) complexes is yet to emerge.
idazolate group acts as a bridging ligand to generate tetra- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
nuclear complexes [CuII4LЈ4] and [NiII4LЈ4], where [LЈ]2− is the
Germany, 2004)
Introduction
Imidazole (ImH) occurs in proteins as part of the side
chain of the amino acid histidine and constitutes all or part
of the binding sites of various transition metal ions such as
Ni2ϩ
,
Cu2ϩ or Zn2ϩ in
a
large number of
metalloproteins.[1Ϫ4] Particularly, copperϪimidazole inter-
actions are widely known in biological systems. As for ex-
ample, the conjugate base imidazolate ion (ImϪ) is present
as a bridging ligand between copper() and zinc() in
superoxide dismutase,[5] which catalyses the dispro-
portionation of the harmful superoxide radical anion.
In order to understand the unusual spectroscopic proper-
ties and the catalytic mechanisms of copper proteins, the
development of relatively simple synthetic models of the ac-
tive sites of copper proteins is of considerable interest. Since
imidazole is present in many natural enzymes and proteins,
we have been interested for some time in low molecular
weight copper complexes with imidazole[6,7] and imidazol-
ate-containing ligands which may serve as structural as well
as spectroscopic models for the active site of copper pro-
teins. As a continuation of this program, we report here a
new tridentate ligand, 2,4-di-tert-butyl-6-[(5-methyl-3H-im-
idazol-4-ylmethyl)amino]phenol (H3L) and its copper()
and nickel() complexes.
Complexes 1 and 2 have been characterised not only
structurally but also by magnetic susceptibility measure-
ments and EPR spectroscopy, in order to identify the ex-
change coupling for this tetracopper() complex 2 and
hopefully to add a new brick to the construction of the
correlation between structure and exchange coupling in im-
idazolate-bridged copper() complexes.
Results and Discussion
The ligand H3L has been prepared in a two-step process
involving first a Schiff-base condensation of 6-amino-2,4-
di-tert-butylphenol with the appropriate 5-methyl-3H-imi-
dazole-4-carbaldehyde and the subsequent reduction of the
Schiff base by NaBH4 in methanol. The ligand H3L precipi-
tates as a white solid in an aqueous medium. The ligand
was fully characterised by 1H, 13C NMR, and IR spec-
troscopy, mass spectrometry and microanalysis together
with its melting point determination (see Exp. Sect.). The
IR bands due to ν(OH), ν(NH), ν(CϪH of tert-butyl),
ν(CϭN), ν(CϭC) and ν(CϪO) are readily identified for the
free H3L which occur as strong peaks at ca. 3500, 3299,
2959, 1615, 1591 and 1233 cmϪ1, respectively, and serve as a
useful indication for complex formation. There is a distinct
This new ligand affords tetranuclear complexes, [LЈ4NiII
]
4
(1) and [LЈ4CuII4(THF)4] (2), where [LЈ]2Ϫ is the dianionic
Schiff-base form of the parent ligand H3L after oxidation.
[a]
Max Planck Institute for Bioinorganic Chemistry,
Stiftstraße 34Ϫ36, 45470 Mülheim an der Ruhr,
E-mail: chaudh@mpi-muelheim.mpg.de
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DOI: 10.1002/ejic.200400346
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S. Mukherjee, T. Weyhermüller, E. Bill, P. Chaudhuri
FULL PAPER
bathochromic shift typical for coordination of the depro- they were subjected to diffractometric studies. The molecule
tonated ligand to transition metal centres. The EI mass consists of a neutral tetranuclear nickel complex along with
spectrometric molecular peak at m/z ϭ 315 for H3L is con- 9 CH2Cl2 solvent molecules. The neutral molecule with its
sistent with the elemental analysis.
atom-labelling scheme is shown in Figure 2. Selected bond
A methanolic solution of the ligand, made basic by a few lengths and angles are listed in Table 1. Each of the nickel
drops of triethylamine, is susceptible to aerial oxidation and centres is in a distorted square-planar geometry with N2O
forms a radical, presumably an iminosemiquinone, as evi- coordination from the deprotonated ligand and the fourth
denced by the EPR spectrum (Figure 1) at ambient tem- donor atom being the nitrogen atom of the imidazolate ring
perature. A simulation of the spectrum yields the hyperfine originating from the adjacent NiN2O unit. Worth noting is
˚
coupling constants for the nitrogen atom and three protons the short C(8)ϪN(7) distance of 1.297(11) A with a ϾCϭ
to be aN ϭ 4.74 G, aH(1) ϭ 9.30 G and aH(2) ϭ 3.59 G. The NϪ character indicating oxidation of the ϾCH2ϪNHϪ
simulated g value of 2.0049 clearly shows that the ligand is bond of the ancillary ligand H3L. The average NiϪO (phe-
‘‘noninnocent’’ and forms radicals in air. This ligand radical nol), NiϪN (imine), NiϪN (imidazole) and NiϪN (adjac-
is not stable in the presence of a metal centre, which is ent imidazole) bond lengths of 1.856, 1.850, 1.891 and 1.850
˚
shown by the X-ray crystal structures of 1 and 2, con- A, respectively, are similar to those in other square-planar
firming the ligand to be present in an iminophenolate form NiII complexes.[8] The four nickel centres are arranged in
after complexation. The imine ϾCϭNϪ character of the the form of a ‘‘butterfly’’ with the Ni(1)ϪNi(1A)ϪNi(1B)
coordinated ligand [LЈ]2Ϫ is also exhibited in the solid-state and Ni(1C)ϪNi(1A)ϪNi(1B) planes forming the ‘‘wings’’
IR spectra for 1 and 2; a sharp strong band appears at 1600 of the butterfly (see b in Figure 2).[9] The dihedral angle
cmϪ1 for 1 and 1595cmϪ1 for 2.
between these two planes is 107.7°. All the adjacent Ni···Ni
˚
distances are equal with a value of 5.851 A.
Molecular Structure of [LЈ4Cu4 (THF)4] (2)
Although the analytical data unambiguously show the
presence of a CuLЈ core as the smallest unit in 2, the mag-
netic susceptibility data exhibit a strong temperature depen-
dence with effective moments as 1.52 µB at 290 K and 0.09
µB at 2 K, refuting the mononuclear formulation. Hence,
an X-ray analysis was undertaken to remove the doubts re-
garding connectivity. Unfortunately, crystals of 2 obtained
from
a THF/methanol mixture diffract X-rays very
weakly.[10] In spite of the high R factor and large standard
deviations due to the severity of the disorder and weakness
of the data collected from a poorly diffracting small crystal,
the crystal structure analysis of 2 confirmed its tetranuclear
nature. Because of its unacceptable quality, we are unable
to publish the crystal data in detail. It is not clear how
many THF molecules are present in the unit cell. The struc-
ture consists of distinct [LЈ4Cu4(THF)4] molecules (Fig-
ure 3).
Figure 1. EPR spectrum of the ligand H3L at room temperature in
the presence of triethylamine in methanol
Mass spectrometry in the ESI positive mode (in CH2Cl2)
is not very helpful in identifying the nuclearity of the
nickel() complex 1. The peak at m/z (%) ϭ 683.4 (100)
corresponds to the mononuclear species NiLЈ2. But for 2,
the CuII complex, peaks at m/z ϭ 374, 748, 1124 and 1496
with respective isotope distribution clearly show the tetra-
nuclear nature of the copper complex.
Selected interatomic distances and angles are given in
Table 2. As observed in 1, oxidation of the ligand occurs
˚
at the same position [C(8)ϪN(7) ϭ 1.31 A]. The structure
consists of a distorted square-pyramidal tetranuclear neu-
tral molecule with the Cu centres forming a ‘‘butterfly
structure’’ (Figure 3). A THF molecule occupies the fifth
position of the coordination site in each copper centre. The
dihedral angle between the plane defined by connecting
Cu(1)ϪCu(2)ϪCu(4) and Cu(3)ϪCu(2)ϪCu(4) is 150.1°.
The average CuϪO (phenol), CuϪN (imine), CuϪN (imid-
azolate) and CuϪN (imidazolate of adjacent ligand) bond
Complex 1, a tetrameric square-planar nickel() com-
plex, is diamagnetic as expected, as is evidenced from the
1
magnetic susceptibility as well as the H NMR spectrum.
1
In the H NMR spectrum, the ratio of protons tert-butyl/
tert-butyl/methyl/imine/aromatic/imidazole is 9:9:3:1:2:1,
which is to be expected from the oxidized deprotonated li-
gand (see Exp. Sect.).
˚
˚
˚
˚
lengths are 1.942 A, 1.942 A, 1.992 A and 1.948 A, respec-
tively, which are similar to other square-planar/pyramidal
CuII complexes.[6,7,11] It is observed that the Cu(1)ϪN(10)
bond is more elongated than it is for the other coordinating
atoms. This tendency is the same for the other copper
Molecular Structure of [LЈ4Ni4]·9CH2Cl2 (1)
From a dichloromethane/methanol (1:1) solvent mixture centres and is due to a JahnϪTeller effect which is present
orange red X-ray quality crystals of 1 were obtained and for d9 systems thus making 2 less symmetric than 1, a d8
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Complexes Incorporating a New Imidazole-Containing Ligand
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Figure 2.(a) ORTEP diagram of [Ni4LЈ4] 1; (b) view of 1 highlighting the butterfly nature of the four nickel centres; the ligand is denoted
only by the donor atoms joined by the curved lines
˚
Table 1. Selected bond lengths [A] and angles [°] for 1
Ni(1)ϪN(7)
Ni(1)ϪN(12A)
Ni(1)ϪN(10)
Ni(1)ϪO(1)
C(1)ϪO(1)
C(6)ϪN(7)
N(7)ϪC(8)
C(1)ϪC(2)
C(2)ϪC(3)
1.850(7)
1.891(6)
1.885(7)
1.856(5)
1.355(9)
1.391(10)
1.297(11)
1.396(11)
1.411(12)
1.368(13)
C(4)ϪC(5)
C(5)ϪC(6)
C(1)ϪC(6)
C(9)ϪN(10)
N(12)ϪC(11)
C(13)ϪN(12)
C(11)ϪN(10)
C(8)ϪC(9)
C(9)ϪC(13)
C(13)ϪC(14)
1.405(12)
1.405(11)
1.407(12)
1.381(10)
1.332(11)
1.362(10)
1.297(11)
1.447(11)
1.370(11)
1.489(11)
C(3)ϪC(4)
Ni(1A)ϪNi(1)ϪNi(1B)
O(1)ϪNi(1)ϪN(7)
N(7)ϪNi(1)ϪN(10)
N(7)ϪNi(1)ϪN(12A)
C(1)ϪO(1)ϪNi(1)
C(8)ϪN(7)ϪC(6)
Ni(1)ϪN(10)ϪC(11)
C(8)ϪN(7)ϪNi(1)
Ni(1)···Ni(1A)
Ni(1A)···Ni(1B)
77.8
86.5(3)
84.0(3)
174.6(3)
111.8(5)
129.8(7)
142.8(6)
117.3(6)
5.851
N(12A)ϪNi(1)ϪN(10)
N(12A)ϪNi(1)ϪO(1)
N(10)ϪNi(1)ϪO(1)
Ni(1)ϪN(10)ϪC(9)
C(11)ϪN(12A)ϪNi(1)
C(13)ϪN(12A)ϪNi(1)
C(6)ϪN(7)ϪNi(1)
94.1(3)
95.6(3)
170.2(3)
112.4(5)
123.8(5)
130.5(5)
112.8(5)
7.351
˚
system; Cu(1) is displaced by 0.11 A from the mean basal
plane of the three nitrogen atoms [N(7), N(10), N(42)] and
one oxygen atom [O(1)]. The dihedral angles between the
plane N(42)O(1)N(7)N(10) containing the Cu(1) ion and
the planes N(42)C(43)C(39)N(40)C(41) and N(10)-
C(9)C(13)N(12)C(11) describing the two imidazolate bridg-
ing groups are 47.2° and 82.2°, respectively. It is interesting
to note that the angle between the Cu(1)ϪNim(42) and
Cu(2)ϪNim(40) vectors is 142.2° and that between the
Magnetochemistry and EPR Studies with 2
The magnetic susceptibility data for a polycrystalline
sample of 2 was measured from 2 to 290 K in an applied
magnetic field of 1 T to characterise the nature and magni-
tude of the exchange interaction propagated by the bridging
ligands. The Heisenberg spin Hamiltonian used was in the
form
for an isotropic exchange coupling.
Cu(1)ϪNim(10) and Cu(4)ϪNim(12) vectors is 132.6°. The The experimental magnetic data were simulated using a le-
angle between the planes defined by Cu(1)O(1)N(7)N- ast-squares fitting computer program[12] with a full-matrix
(10)N(42) and Cu(2)O(31)N(37)N(40)N(72) is 139.1°.
diagonalisation of exchange coupling, Zeeman splitting,
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Figure 3.(a) Molecular structure of [Cu4LЈ4] 2; (b) view of 2 highlighting the butterfly dispositions of the copper centres
˚
energetically well-isolated ground state of total spin St ϭ 0,
discernible from the decline of effective moments at tem-
peratures below 100 K. The residual moment of 0.180 µB at
2 K is attributed to a monomeric (S ϭ 1/2) impurity (2.0%).
Monomeric species were also detected in weak abundance
in the EPR spectra, as will be mentioned later. The suscepti-
bility data could be simulated considering exchange coup-
ling between adjacent CuII pairs with local spins (S ϭ 1/2),
as shown in the inset in Figure 4. The multiplets |SASBStϾ
Table 2. Selected bond lengths [A] and angles [°] for 2
Cu(1)ϪN(7)
Cu(1)ϪN(42)
Cu(1)ϪN(10)
Cu(1)ϪO(1)
C(1)ϪO(1)
C(6)ϪN(7)
N(7)ϪC(8)
C(1)ϪC(2)
C(2)ϪC(3)
C(3)ϪC(4)
C(4)ϪC(5)
C(5)ϪC(6)
C(1)ϪC(6)
1.913(17) Cu(2)ϪN(37)
1.925(17) Cu(2)ϪN(72)
1.952(16) Cu(2)ϪN(40)
1.955(12) Cu(2)ϪO(31)
1.316(25) C(9)ϪN(10)
1.964(16)
1.957(16)
2.020(16)
1.965(13)
1.462
1.347
1.408
1.381
1.461
1.420
1.310
1.422
1.399
1.365
1.399
1.409
1.377
N(12)ϪC(11)
C(13)ϪN(12)
C(11)ϪN(10)
C(8)ϪC(9)
C(9)ϪC(13)
C(13)ϪC(14)
are labelled by the ‘‘pair spins’’, SA
(
) and SB
1.341
1.525
(
). The ground state is a singlet |SASBStϾ ϭ
|110Ͼ and the first excited state at energy 2J is a triplet
|111Ͼ (Figure 4). The states |011Ͼ, |000Ͼ and |101Ͼ are
degenerate at energy 4J and the highest state at energy 6J
is a quintuplet |112Ͼ. In zero field the multiplets remain
degenerate in energy with pure magnetic quantum numbers,
Cu(1)ϪCu(2)ϪCu(3) 89.8
Cu(2)ϪCu(3)ϪCu(4) 86.3
Cu(3)ϪCu(4)ϪCu(1) 89.8
Cu(4)ϪCu(1)ϪCu(2) 85.9
O(31)ϪCu(2)ϪN(37) 81.6
N(37)ϪCu(2)ϪN(40) 81.7
N(37)ϪCu(2)ϪN(72) 163.2
N(72)ϪCu(2)ϪO(31) 93.8
N(72)ϪCu(2)ϪN(40) 100.9
N(40)ϪCu(2)ϪN(31) 162.7
O(1)ϪCu(1)ϪN(7)
N(7)ϪCu(1)ϪN(10)
82.6
83.4
N(7)ϪCu(1)ϪN(42) 164.7
N(42)ϪCu(1)ϪN(10) 99.5
N(42)ϪCu(1)ϪO(1)
94.2
N(10)ϪCu(1)ϪO(1) 166.0
Cu(1)ϪN(10)ϪC(11) 146.14(16) Cu(2)ϪN(40)ϪC(41) 142.8(17)
Cu(1)ϪN(42)ϪC(41) 123.59(16) Cu(4)ϪN(12)ϪC(11) 123.89(16)
Cu(1)···Cu(2)
Cu(1)···Cu(4)
Cu(2)···Cu(3)
6.054
6.044
6.019
Cu(3)···Cu(4)
Cu(1)···Cu(3)
Cu(2)···Cu(4)
6.034
8.524
8.245
and axial single-ion zero-field interactions
,
if
necessary. The susceptibility data were corrected for diam-
agnetism (Pascal corrections), temperature-independent
paramagnetism (TIP) and the presence of paramagnetic
monomer impurities (P) in the following way: χcalcd. ϭ (1
Ϫ P)χ ϩ χTIP ϩ Pχmono
.
The nature of the µeff vs. T plot for 2 (Figure 4) shows a
behaviour typical for antiferromagnetic spin coupling. The
effective moment µeff decreases monotonically with the de-
crease in temperature; the values of µeff are 3.051 µB at
Figure 4. Plot of µeff vs. T for 2; the solid line represents the simu-
lated curve; insets: model used to simulate the spin coupling in 2
and corresponding spin states resulting from the spin Hamiltonian
290 K and 0.180 µB at 2 K. The magnetic data reveal an used (see text)
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Complexes Incorporating a New Imidazole-Containing Ligand
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according to their multiplicity. Projections of local spins Si that values up to 40 K were taken as line broadening, which
)
[13] reveal identical gt val-
occurs above this temperature. The best agreement was ob-
tained for I111T, describing the thermal population of the
first excited triplet state |111Ͼ, I111 T ഠ [exp(2J/kT)]/Z,
where Z ϭ 1 ϩ 3 exp(2J/kT) ϩ 7exp(6J/kT) ϩ 5 exp(12J/
kT). The solid line is a fit with J ϭ Ϫ42 cmϪ1, which is
close to the exchange coupling constant of J ϭ Ϫ49 cmϪ1
evaluated from the susceptibility measurements. Therefore,
particular non-Kramers resonance at B ϭ 480 mT, and
probably the whole complex EPR spectrum in the field
range 380Ϫ600 mT appears to arise from the first excited
triplet state |111Ͼ of the tetramer.
on the total spin St (
ues for all multiplets St ϶ 0. The simulation of the exper-
imental susceptibility data for the tetramer yielded an ex-
change coupling constant J ϭ Ϫ49 cmϪ1, g ϭ 2.022, a para-
1
magnetic impurity (P: S ϭ /2) of 2% and a temperature-
independent paramagnetism (TIP) of 240 ϫ 10Ϫ3 emu.
X-band EPR spectra for 2 in the solid state from 11.7 K
to 62.1 K are dominated by a strong almost isotropic signal
at g ϭ 2.08 which shows the usual Curie behaviour, I ϳ 1/T,
for the given temperature range. Therefore, the strong de-
rivative signal at g ഠ 2 is attributed to the monomeric para-
magnetic impurity (2%) present in the solid-state suscepti-
bility measurements. In addition to the prominent line there
are a number of weak signals in the measured field range
extending from 50 to 600 mT. Their appearance is typical
of integer spin manifolds arising for the exchange-coupled
CuII tetramer, because competing Zeeman and zero-field
splitting are expected due to dipolar coupling and aniso-
tropic exchange. As a consequence, the resonances are
widely spread over a large field range and the derivative
signals appear to be weak (not the intensity). On increasing
the temperature, changes in the relative intensities of these
In an imidazolate-bridged system, there can be two path-
ways used to explain the magnetic interaction, viz. the σ-
and the π-exchange pathways. Generally, it is agreed that
the π orbitals are not involved in the coupling.[7,14] It has
been stated that, since the magnetic orbitals are σ-anti-
bonding for square-planar and square-pyramidal com-
plexes, the relevant exchange pathway through the imida-
zolate bridge is of the σ-type. The imidazolate orbitals re-
sponsible for this sort of interaction are essentially parallel
to the NϪNЈ (or CϪC) direction.[7,11c]
Several studies have already been reported,[7,11c] in which
features are observed between 150 and 260 mT and between structural data have been used to find correlations between
380 and 600 mT (Figure 5), which is typical of integer spin structure and exchange coupling in imidazolate-bridged
signals from excited spin multiplets.
copper() complexes. Comparison of the numerical data
In order to elucidate the correlation between the EPR shows that a simple magnetostructural correlation for imid-
spectra and the spin multiplets of the coupled system, the azolate-bridged Cu complexes does not exist. However, as
change of intensity (I) of the derivative peak at 480 mT was the values of J are independent of θ, the angle between
scanned over the entire temperature (T) range. As the popu- the copper and imidazole coordination planes, a π-exchange
lation of the excited state in the first order is dependent on pathway can be discarded. It has been concluded from
the Boltzmann function of resonating levels, the product EHMO calculations[11c] that the extent of anti-ferromag-
intensity·temperature, I·T, was plotted versus temperature netic coupling will increase when the NϪN and other
(Figure 5b). The fading below 10 K and the strong rise at CuϪN bonds are parallel to the imidazolate
elevated temperatures prove that the signals arise from ex- carbonϪcarbon bond, thus favoring a σ-exchange pathway.
cited states and that the ground state is diamagnetic and Although the coupling constant for 2 is strong compared
EPR-silent, in accordance with the susceptibility findings. with the other complexes, the values of J cannot be strictly
For quantitative analysis, the experimental data were com- attributed to the values of α, the angle between the CuϪN
pared with the theoretical Boltzmann functions derived and NϪN vectors. This indicates that σ-superexchange can-
from the spin-coupling model (Figure 4). It is to be noted not be the only mechanism for the exchange coupling for
Figure 5. (a) EPR spectra of a powdered sample of 2 recorded as a function of temperature; (b) temperature dependence of the tempera-
ture-weighted intensity (I·T) of the EPR subspectra; the solid line is the calculated Boltzmann function I111·T with J ϭ Ϫ42 cmϪ1
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S. Mukherjee, T. Weyhermüller, E. Bill, P. Chaudhuri
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NMR field probe (Bruker 035M) and a microwave frequency coun-
ter HP 5352B.
this compound. The presence of ligandϪligand interactions
may also play a part in this type of exchange coupling. Re-
cently, a theoretical DFT study[15] on the magneto-struc-
tural correlation in imidazolate-bridged dicopper() com-
plexes showed that the exchange coupling constant J mainly
X-ray Crystallographic Data Collection and Refinement of the Struc-
tures: A single crystal of 1 was coated with perfluoropolyether,
picked up with glass fibres, and mounted on a Nonius Kappa-CCD
depends on the CuϪNϪC(Im) bond angle and is insensitive diffractometer equipped with a cryogenic nitrogen cold stream op-
erating at 100(2) K. Graphite-monochromated Mo-Kα radiation
(λ ϭ 0.71073 A) was used. Intensity data were corrected for Lor-
to the variation of the CuϪN(Im) distance and the dihedral
angle between the bridged imidazolate ring and copper co-
ordination planes.
˚
entz and polarisation effects. The intensity data set of 1 was cor-
rected for absorption with the program SADABS[16] The Siemens
SHELXTL software package[17] was used for solution, refinement,
and artwork of the structure, and neutral atom scattering factors
of the program were used. All structures were solved and refined
by direct methods and difference Fourier techniques. Non-hydro-
gen atoms were refined anisotropically, and hydrogen atoms were
placed at calculated positions and refined as riding atoms with iso-
tropic displacement parameters. Details of data collection and
structure refinements are summarized in Table 3. CCDC-236872
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html [or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: ϩ
44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk].
Concluding Remarks
The ligating properties of a new tridentate ligand, 2,4-di-
tert-butyl-6-[(5-methyl-3H-imidazol-4-ylmethyl)amino]-
phenol containing an imidazole N-donor atom together
with a phenol O-donor and amine N-donors towards the
nickel() and copper() centers are described. The imidaz-
ole N atom becomes deprotonated and the resulting imida-
zolate group serves as a bridging ligand to generate tetranu-
clear complexes of the said metal centres. Although the li-
gand generates the iminosemiquinone monoanion radical
very easily in the presence of a base and air, as evidenced
by EPR spectroscopy, the radical form could not be stabil-
ized in the presence of the metal centres. The CϪNH bonds
from the parent ligand have been oxidized to CϭN bonds
upon coordination with the NiII or CuII centres. The realiz-
ation that the present phenol as well as imidazole-contain-
ing ligand can generate polynuclear complexes highlights
the prospect of designing a generic ligand system, which
should form polynuclear species with other metal centres.
The imidazolate-bridged copper() complexes are, as ex-
pected, antiferromagnetically exchange-coupled yielding
Table 3. Crystal data and structure refinement for 1,
[Ni4LЈ4]·9CH2Cl2
1
Empirical formula
Formula mass
C76H100N12Ni4O4·9CH2Cl2
2244.85
100(2)
Temperature [K]
˚
Wavelength [A]
0.71073
tetragonal
Crystal system
Space group
Unit cell dimensions [A]
¯
P421/c (No. 114)
˚
a ϭ 20.74(3)
c ϭ 12.1538(10)
5227.9(12); 2
1.426
3
˚
diamagnetic ground states. No simple magneto-structural Volume [A ]; Z
Density (calcd.) [Mg/m3]
correlation has yet emerged for these imidazolate-bridged
complexes. The synthesis of more of these imidazolate-
bridged copper() complexes would therefore be warranted
in order to resolve this complex problem of magnetostruc-
tural correlation.
Absorp. coeff. [mmϪ1
]
1.220
F(000)
2324
θ range for data collection [°]
Index ranges
1.94Ϫ25.00
Ϫ31 Յ h Յ 32
Ϫ10 Յ k Յ 31
Ϫ18 Յ l Յ 17
39793
4608
SADABS
4591/16/289
1.048
R1 ϭ 0.0786, wR2 ϭ 0.2180
R1 ϭ 0.0982, wR2 ϭ 0.2366
Reflections collected
Independent reflections
Absorption correction
Data/restraints/parameters
Goodness-of-fit on F2
Final R int [I Ͼ 2σ(I)]
R int (all data)
Experimental Section
Materials and Physical Measurements: Commercial grade chemi-
cals were used for synthetic purposes, and solvents were distilled
and dried before use. Fourier transform IR spectroscopy on KBr
pellets was performed with a PerkinϪElmer 2000 FT-IR instru-
ment. Mass spectra were recorded either in the EI or the ESI (in
CH2Cl2) modes with a Finnigan MAT95 or 8200 spectrometer.
Magnetic susceptibilities of the polycrystalline samples were re-
corded with a SQUID magnetometer (MPMS, Quantum Design)
in the 2Ϫ290 K temperature range with an applied field of 1 T.
Ligand Synthesis
2,4-Di-tert-butyl-6-[(5-methyl-3H-imidazol-4-ylmethyl)amino]-
phenol (H3L): 2-Amino-4,6-di-tert-butylphenol (2.2 g, 10 mmol)
and 5-methyl-3H-imidazole-4-carbaldehyde (1.1 g, 10 mmol) were
Diamagnetic contributions were estimated for each compound by added to deaerated methanol (50 mL) and the resulting solution
the use of Pascal’s constants. 1H and 13C NMR spectra were re-
corded using a Bruker ARX 250, or DRX 400 spectrometer. The
spectra are referenced to TMS, using the 13C or residual 1H signals
of the deuterated solvents as internal standards. X-band EPR spec-
tra were recorded with a Bruker ESP 300E spectrometer equipped
was refluxed under argon for 2 h. The red solution was cooled and
NaBH4 was added portionwise until the solution turned a faint
yellow. The precipitation of a white solid was initiated by dropwise
addition of water. The white solid was collected by filtration and
dried under vacuum. Yield: 2.8 g (89%). M.p. 186 °C. C19H29N3O
with a helium flow cryostat (Oxford Instruments ESR 910), an (315.5): calcd. C 72.34, H 9.27, N 12.84; found C 71.0, H 9.2, N
4214  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org
Eur. J. Inorg. Chem. 2004, 4209Ϫ4215

Complexes Incorporating a New Imidazole-Containing Ligand
FULL PAPER
[5]
[6]
1
J. S. Richardson, K. A. Thomas, B. H. Rubin, D. C. Richard-
son, Proc. Natl. Acad. Sci. U. S. A. 1975, 72, 1349.
P. Chaudhuri, I. Karpenstein, M. Winter, C. Butzlaff, E. Bill,
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P. Chaudhuri, I. Karpenstein, M. Winter, M. Lengen, C.
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org. Chem. 1993, 32, 888 and references cited therein.
For example: D. J. White, N. Laing, H. Miller, S. Parsons, S.
Coles, P. A. Tasker, Chem. Commun. 1999, 2077.
12.8. H NMR ([D6]DMSO): δ ϭ 1.21 (s, 9 H), 1.31 (s, 9 H), 2.15
(s, 3 H), 4.02 (s, 2 H), 6.5 (t, 2 H), 7.4 (s, 1 H) ppm. 13C NMR
([D6]DMSO): δ ϭ 30.0 (s), 34.1 (s), 34.5 (s), 48.6 (s), 107.5 (s),
111.1 (s), 133.2 (s), 136.3 (s), 140.3 (s), 141.9 (s) ppm. IR (KBr):
ν˜ ϭ 3299 s, 2959 s, 1615 s, 1591 s, 1233 s cmϪ1. EI-MS: m/z ϭ
315 [Mϩ].
[7]
Complex 1 [Ni4LЈ4]: The ligand H3L (1 mmol, 0.315 g) was dis-
solved in 25 mL of deaerated methanol. Solid Ni(ClO4)2·6H2O
(1 mmol, 0.365 g) along with NEt3 (0.15 mL) were added to the
ligand solution. The resulting orange-red solution was refluxed un-
der argon for 1 h and then stirred in air for 0.5 h, whereupon a
microcrystalline orange-red solid precipitated. The solid was fil-
tered, dried in air. Yield: 0.21 g (58%). C76H100N12Ni4O4 (1480.5):
calcd. C 61.66, H 6.81, N 11.35, Ni 1586; found C 58.8, H 5.8, N
10.7, Ni 15.5. IR (KBr): ν˜ ϭ 2952, 1600, 1474, 1399, 1253, 1130,
861 cmϪ1. X-ray quality crystals for 1 [Ni4L4]·9CH2Cl2 were ob-
tained from a mixture of dichloromethane/methanol (1:1).
[8]
[9]
V. V. Pavlishchuk, S. V. Kolotilov, A. W. Addison, M. J. Pru-
shan, D. Schollmeyer, L. K. Thomspon, E. A. Goreshnik, An-
gew. Chem. Int. Ed. 2001, 40, 4734.
[10]
[11]
[C76H100N12Cu4 (C4H8O)4]·xTHF, monoclinic, space group
˚
˚
˚
P21/c, a ϭ 24.916(5) A, b ϭ 17.296(4) A, c ϭ 33.004(7) A, β ϭ
3
˚
91.72°, V ϭ 14216 A , Z ϭ 4, T ϭ 100(2) K, final R ϭ 0.151
for 10569 independent reflections.
[11a]
Selected references:
G. Ivarsson, B. K. S. Lundberg, N.
[11b]
Ingri, Acta Chem. Scand. 1972, 26, 3005.
Kolks, S. J. Lip-
pard, J. V. Waszczak, H. R. Lilienthal, J. Am. Chem. Soc. 1982,
104, 717. [11c] A. Bencini, C. Benelli, D. Gatteschi, C. Zanchini,
Complex 2 [Cu4LЈ4]: The ligand H3L was dissolved in 15 mL of
methanol. Solid Cu(CH3COO)2·4H2O (0.5 mmol, 0.10 g) was ad-
ded to the ligand solution, and the whole solution was made basic
by adding NEt3 (0.15 mL). The resulting light orange-red solution
was refluxed for 1 h and then cooled to room temperature, where-
upon a red-orange microcrystalline solid precipitated. The micro-
crystalline solid separated by filtration was recrystallised from a
tetrahydrofuran/methanol solvent mixture. Yield: 0.120 g (64%).
C76H100Cu4N12O4 (1499.9): calcd. C 60.86, H 6.72, Cu 16.95, N
11.21; found C 59.8, H 6.6, Cu 17.3, N 11.1. IR (KBr): ν˜ ϭ 2951,
[11d]
Inorg. Chem. 1986, 25, 398.
C. A. Salata, M. T. Youinou,
[11e]
C. J. Burrows, Inorg. Chem. 1991, 30, 3454.
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J. Aubin, M. S. Mashuta, R. A. Porter, J. F. Richardson, D. N.
Hendrickson, R. M. Buchanan, Inorg. Chem. 1996, 35, 3325.
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G. Tabbi, W. L. Driessen, J. Reedijk, R. P. Bonomo, N.
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E. Col-
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V. Staemmler, F. Birkelbach, C. Krebs, unpublished, Bochum,
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1595, 1470, 1398, 1257, 1126, 857, 647 cmϪ1
.
[12]
[13]
A. Bencini, D. Gatteschi, Electron Paramagnetic Resonance of
Exchange Coupled Systems, Springer-Verlag, Berlin 1990.
Acknowledgments
[14] [14a]
D. N. Hendrickson, M. S. Haddad, Inorg. Chem. 1978, 17,
[14b]
Financial support from the DFG (Priority Program Ch111/2-2) is
gratefully acknowledged. Thanks go to Mrs. H. Schucht and Mr.
A. Göbels for skilful technical assistance.
2622.
M. S. Haddad, E. N. Duester, D. N. Hendrickson,
[14c]
Inorg. Chem. 1979, 18, 141.
C. Benelli, R. K. Bunting, D.
Gatteschi, C. Zanchini, Inorg. Chem. 1984, 23, 3074.
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[17]
T. E. Creighton, in Proteins, Structures and Molecular Proper-
G. M. Sheldrick, University of Göttingen, Germany, 1994.
G. M. Sheldrick, SHELXL-97, Program for refinement of crys-
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versity of Göttingen, Germany, 1997.
Received April 29, 2004
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H. C. Freeman, in Inorganic Biochemistry (Ed.: G. L. Eich-
[2]
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R. J. Sundberg, R. B. Martin, Chem. Rev. 1974, 74, 471.
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Published Online September 2, 2004
Eur. J. Inorg. Chem. 2004, 4209Ϫ4215
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4215