Versatile chemical transformations of benzoxazole based ligands on complexation with 3d-metal ions

Article abstract of DOI:10.1021/ic202493z

Two benzoxazoles derivative ligands were synthesized from the condensation of 3,5-di-tert-butyl-o-benzoquinone (DTBBQ)with ethanolamine or 1,3-diamino-2-hydroxypropane in methanol. Condensation of DTBBQ with ethanolamine gives the expected 5,7-di-tert-but

Full text of DOI:10.1021/ic202493z

Article
pubs.acs.org/IC
Versatile Chemical Transformations of Benzoxazole Based Ligands
on Complexation with 3d-Metal Ions
Olga Iasco,† Ghenadie Novitchi,†,‡ Erwann Jeanneau,† Jean Bernard Tommasino,† Nans Roques,§,⊥
and Dominique Luneau*,†
†Laboratoire des Multimater
́ ́
iaux et Interfaces (UMR 5615), Universite Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France
‡Institute of Chemistry, Academy of Sciences of Moldova, Academiei, str. 3 MD-2028 Chisinau, Moldova
§LCC (Laboratoire de Chimie de Coordination), CNRS, 205 route de Narbonne, F-31077 Toulouse, France
⊥Universite
́
de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
S
* Supporting Information
ABSTRACT: Two benzoxazoles derivative ligands were synthesized from the
condensation of 3,5-di-tert-butyl-o-benzoquinone (DTBBQ) with ethanolamine or
1,3-diamino-2-hydroxypropane in methanol. Condensation of DTBBQ with
ethanolamine gives the expected 5,7-di-tert-butyl-2-methylenhydroxylbenzoxazole
(HL1) while with 1,3-diamino-2-hydroxypropane it gives (2-hydroxyethyl-2-{2,4-
bis(1,1-dimethylethyl)-1-phenol-6 amino}-2{5,7-di-tert-butyl-benzoxazole})
(H2L2) with only one benzoxazole ring instead of the symmetric bis-benzoxazole
derivative. The structure of HL1 and H2L2 were confirmed by NMR-spectroscopy
and X-ray diffraction on a single crystal for HL1. The reaction of HL1 with CuCl2
gives a mononuclear [CuII(HL1)2Cl2] (1) complex for which the crystal structure
shows that HL1 is preserved. In contrast, upon reaction with nickel(II), cobalt(II),
and manganese(II) H2L2 is further oxidized and transformed in new ligands HL3 in
mononuclear complexes [MII(L3)2] (M = NiII (2); M = CoII (3)) and H2L4 in
tetranuclear complex [MnII (HL4)4Cl4] (4) as found from the crystal structures of complexes 2−4. Electrochemical studies for
4
complexes 2 and 3 evidence complicated redox properties. [MnII (HL4)4Cl4] (4) has a cubane-like structure with a “4 + 2”
4
fashion The magnetic susceptibility of 4 is well fitted considering one Mn---Mn interaction Ja(MnII-MnII) = −0.50(1) cm−1 with g
= 2.00(7).
and amines.38−43 We have previously reported that the reaction
INTRODUCTION
■
of ethanolamine with DTBBQ gives 5,7-di-tert-butyl-2-methyl-
enhydroxylbenzoxazole. This ligand was formed in situ in a one
pot reaction with a mixture of Cu(II) and Ln(III) salts to give
[Ln4Cu8] systems.42 When 1,3-diamino-2-hydroxypropane was
reacted with DTBBQ instead of getting the corresponding
symmetric bis-benzoxazole ligand we got an asymmetric ligand
with only one benzoxazole moiety. Synthesis of the ligands,
chemical transformations, and their 3d-metal complexes with
structural, electrochemical, and magnetic properties studies are
presented in this work.
The design of ligands and metal complexes with redox active
properties has been an important field of investigation in
coordination chemistry for many decades1−10 and is still very
active as shown by the recent dedicated forum in Inorganic
Chemistry.11−15 The development of molecular systems with
such redox activity found direct interest as model in
bioinorganic studies,16−21 paramagnetic labeling,22−25 or in
catalytic applications.26−30 An emblematic example is the work
on semiquinone and catechol and derivatives.3−5,8,9,14 Mono-
nuclear13,31,32 and dinuclear complexes of copper33−35 as well
as mononuclear36 and dinuclear16−19 iron with pro-radical
catecholate and semiquinone ligands have been identified as
intermediates in catalytic oxidations of catechols and phenol
ligands. A main property associated with redox active ligands is
the metal−ligand electron transfer. This may afford systems
exhibiting valence tautomerism accompanied by reversible
changes in magnetic and optical properties that are particularly
attractive if one thinks of photoswitchable devices.7−10,37
In this context and also regarding our interest in single-
molecule magnets (SMM), we investigated the coordination
ability of benzoxazoles formed by condensation/oxidation
reaction between 3,5-di-tert-butyl-o-benzoquinone (DTBBQ)
EXPERIMENTAL SECTION
■
Materials. All chemicals and solvents were purchased from Aldrich
and used without further purification. Manipulations were performed
under aerobic conditions using chemicals and solvents as received.
Synthesis. HL1 (5,7-di-tert-butyl-2-methylenhydroxylbenzoxa-
zole). Reaction between 3,5-di-tert-butyl-o-benzoquinone (DTBBQ)
and ethanolamine was carried out under aerobic conditions at room
temperature (Scheme 1). Ethanolamine (0.12 mL, 2 mmol) was added
to a solution of DTBBQ (0.44 g, 2 mmol) in methanol (50 mL). The
Received: November 28, 2011
Published: January 10, 2012
© 2012 American Chemical Society
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Article
Scheme 1. Synthesis of HL1
Table 1. Crystal Data and Structure Refinement Parameters for Compounds HL1 and 1−4
HL1
[CuII(HL1)2Cl2] (1)
[NiII(L3)2] (2)
[CoII(L3)2] (3)
[MnII4(HL4)4Cl4] (4)
formula
fw
C16H23NO2
261.35
C32H46Cl2CuN2O4
657.18
C62H82NiN4O6
1038.07
monoclinic
P2/n
C62H82CoN4O6
1038.25
triclinic
C124H164Cl4Mn4N8O16
2384.23
monoclinic
C2/c
cryst. syst.
space group
a (A)
monoclinic
P2/c
monoclinic
P2/n
P1
9.9461(8)
40.599(2)
12.2510(10)
90
10.1542(6)
11.812(8)
27.947(2)
90
13.2714(9)
14.8637(6)
17.3337(9)
90
10.4757(9)
10.8700(10)
27.854(2)
83.040(6)
80.938(6)
67.407(8)
2885.4(5)
2
18.756(2)
35.459(4)
19.164(2)
90
b (A)
c (A)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
111.900(9)
90
96.970(5)
90
104.710(6)
90
90.642(8)
90
4590.0(6)
12
3327(2)
4
3307.2(3)
2
12745(2)
4
ρcalcd (g/cm3)
1.135
1.312
1.042
1.195
1.243
−1
μ (mm )
0.584
0.854
0.339
0.350
0.533
T (K)
100(2)
0.0468
100(2)
0.0517
293(2)
110(2)
100(2)
a
R
0.0885
0.0913
0.0793
b
Rw
0.1351
0.1038
0.1352
0.2189
0.1651
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. Rw2 = {Σ[w(Fo − Fc2)2]/[w(Fo )2]}1/2 and [I > 2σ(I)].
resulting light green solution was stirred for 2−3 h. The final product
HL1 was isolated by chloroform extraction with followed chromatog-
raphy on silica gel. Yield 29%. Anal. Calcd for C16H23NO2: C, 73.53;
H, 8.87; N, 5.36. Found: C, 74.06; H, 9.02; N, 5.25%. IR/cm−1: νmax
3199 (O−H), 2956, 2905, 2868 (C−H), 1578 (CN), 1481 (C
2 were collected by filtration. Yield: 32% (based on Ni). Anal. Calcd
for C62H82N4NiO6: C, 71.74; H, 7.96; N, 5.40. Found: C, 71.84; H,
7.87; N, 5.67%. IR/cm−1: νmax 2958, 2908, 2872 (C−H), 1678 (C
O); 1605 (CN), 1494, 1445 (CC), 1366, 1309 (CH3).
[CoII(L3)2] (3). The same procedure as for 2 was used with
Co(CH3COO)2·6H2O (0.125 g, 0.5 mmol) and H2L2 (0.5 g, 1
mmol). After 1−2 days green-blue intense needle-like crystals of 3
were collected by filtration. Yield: 36% (based on Co). Anal. Calcd for
C62H82N4CoO6: C, 71.72; H, 7.96; N, 5.40. Found: C, 71.83; H, 8.02;
N, 5.52%. IR/cm−1: νmax 3369 (O−H), 2958, 2904, 2869 (C−H),
1672 (CO); 1623 (CN), 1498, 1459, 1441 (CC), 1359 (CH3).
[MnII4(HL4)4Cl4] (4). Solid MnCl2·4H2O (0.2 g, 1 mmol) was
added to a stirred solution of H2L2 (0.5 g, 1 mmol) in acetonitrile (20
mL) at room temperature. The resulting solution was stirred for 1 h;
then it was filtered and left undisturbed to concentrate slowly by
evaporation. After 5−7 days colorless needle-like crystals of 4 were
collected by filtration. Yield: 25% (based on Mn). Anal. Calcd for
C124H164Cl4Mn4N8O16: C, 62.47; H, 6.93; N, 4.70. Found: C, 62.32;
H, 6.85; N, 4.89%. IR/cm−1: νmax 3191 (O−H), 2959, 2907, 2871 (C−
H), 1607, 1572 (CN), 1458 (CC), 1404, 1365 (CH3).
Physical Measurements. NMR spectra were recorded at 400 and
300 MHz on a Bruker DRX spectrometer in DMSO−D6 solution. The
chemical shifts were referred to TMS using the residual signals from
the solvent.
C), 1403, 1361 (CH3).
H2L2 (2-hydroxyethyl-2-{2,4-bis(1,1-dimethylethyl)-1-phenol-6
amino}-2{5,7-di-tert-butyl-benzoxazole}). The reaction between
DTBBQ and 1,3-diamino-2-hydroxypropane was carried out under
aerobic conditions in refluxed methanol. To a solution of DTBBQ
(0.44 g, 2 mmol) in methanol (50 mL) was added 1,3-diamino-2-
hydroxypropane (0.09 g, 1 mmol). The resulting light green solution
was stirred and refluxed for 2−3 h. After cooling H2L2 was isolated as
a white precipitate which was filtered, washed with small amount of
methanol, and dried under vacuum. Yield 39%; mp 200 °C. Anal.
Calcd for C31H46N2O3: C, 75.26; H, 9.37; N, 5.66. Found: C, 74.84;
H, 9.50; N, 5.71%. IR/cm−1: νmax 3369 (O−H), 2954, 2908, 2867 (C−
H), 1589, 1573 (CN), 1481 (CC), 1403, 1357 (CH3).
[CII(HL1)2Cl2] (1). The copper(II) complex 1 could be obtained
starting from the ligand HL1 or by a one pot reaction as it is described
below. To a stirred solution of DTBBQ (0.44 g, 2 mmol) in
acetonitrile (15 mL) at room temperature was added the equivalent
quantity of ethanolamine (0.12 mL, 2 mmol). After 15 min of stirring,
the solid CuCl2·2H2O (0.17 g, 1 mmol) was added to the reaction
mixture. The resulting reddish solution turned dark green and was
filtered. The filtrate was left undisturbed to concentrate slowly by
evaporation. After one day green needle-like crystals of 1 were
collected by filtration and washed with a small amount of MeCN.
Yield: 42% (based on Cu). Anal. Calcd for C32H46Cl2CuN2O4: C,
58.48; H, 7.06; N, 4.26. Found: C, 58.74; H, 7.07; N, 4.57%. IR/cm−1:
νmax 3252 (O−H), 2958, 2869 (C−H), 1609, 1584 (CN), 1440
(CC), 1402, 1363 (CH3).
IR spectra were recorded in the solid state on a NICOLET
spectrophotometer in the 400−4000 cm−1 range.
Magnetic susceptibility data (2−300 K) were collected on
powdered samples using a SQUID magnetometer (Quantum Design
MPMS-XL), applying magnetic fields of 0.1 T. All data were corrected
for the contribution of the sample holder and the diamagnetism of the
samples estimated from Pascal’s constants.44,45 Magnetic data analyses
were carried out including temperature independent paramagnetism,
impurity contributions (ρ), and intermolecular interactions (zJ′).
Minimization of magnetic susceptibility, derived from analytical
calculation of energy levels associated with the spin Hamiltonians
presented in the text, was carried out with Visualizeur-Optimiseur for
[NiII(L3)2] (2). Solid Ni(CH3COO)2·6H2O (0.125 g, 0.5 mmol)
was added to a stirred solution of H2L2 (0.5 g, 1 mmol) in acetonitrile
(20 mL) at room temperature. The resulting solution was stirred for 1
h then it was filtered and left undisturbed to concentrate slowly by
evaporation. After 1−2 days, intense green-blue needle-like crystals of
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Matlab, using nonlinear least-squares of the Levenberg−Marquardt
algorithm.46,47
resulting in the formation of a Schiff-base (Scheme 1). Then
the oxazole cyclization occurs because of successive oxidation
(Scheme 1) as previously reported.38,39
Single crystals of HL1 suitable for X-ray analysis were grown
from chloroform solution. HL1 crystallizes in the monoclinic
P2/c space group. The crystal structure consists of discrete
molecules arranged in three symmetrically independent sites.
One molecule of HL1 is presented in Figure 1.
Electrochemical measurements were performed using an AMEL
7050 all-in one potentiostat, using a standard three-electrode setup
with a glassy carbon electrode, platinum wire auxiliary electrode and
SCE (saturated calomel electrode) as the reference electrode. The
complex solutions in CH2Cl2 were 1.0 mM, 2 mM, and 0.1 M in the
supporting electrolyte n-Bu4NPF6. Under these experimental con-
ditions, the ferrocene/ferricinium couple, used as an internal reference
for potential measurements, was located at E1/2 = 0.421 V.
X-ray Data Collection and Structure Refinement. Diffraction data
were collected at 100.0(1) K using a Gemini A Ultra diffractometer
from Agilent Technologies Ltd. (Mo Kα graphite-monochromated
radiation, λ = 0.71073 Å) equipped with a CCD camera and controlled
by the CrysAlisPro Software (Agilent Technologies, Version
1.171.34.49). A summary of the crystallographic data and structure
refinement is given in Table 1. An analytical absorption correction was
applied by modeling the crystal habit.48 The crystals were placed in the
cold stream of an Oxford Cryosystems open-flow nitrogen cryostat
with a nominal stability of 0.1 K.49 All the structures were solved by
direct methods using the SIR97 program50 and refined against F2 using
the CRYSTALS program.51 All non-hydrogen atoms were refined
anisotropically, whereas the hydrogen atoms were placed in ideal,
calculated positions, with isotropic thermal parameters riding on their
respective carbon atoms. The graphical manipulations were carried out
with the DIAMOND52 program. Selected bond distances and angles
Table 2. Selected Bond Distances for 1−4
[CuII(HL1)2Cl2] (1)
[NiII(L3)2] (2)
Cl1−Cu1
2.284 (2)
2.283 (2)
2.502(2)
2.523(2)
2.069(2)
2.021(3)
N2−Ni1
O3−Ni1
N1−Ni1
2.071(4)
1.996(4)
2.074(5)
Cl2−Cu1
O2−Cu1
O3−Cu1
N1−Cu1
N2−Cu1
Figure 1. Crystal structure of HL1 (top) and [CuII(HL1)2Cl2] (1)
(down). C−H hydrogen atoms are omitted for clarity.
Bond lengths and angles in the three HL1 independent
molecules are similar and consistent with distances and angles
found in our previously reported heterometallic [Ln4Cu8]
clusters.42 Distances for H2C−OH bonds are in the 1.398−
1.400 Å range, which is in agreement with a single bond
between methylene carbon and hydroxyl group. The crystal
packing of HL1 is achieved through OH···N hydrogen bonds
(2.755−2.794 Å) which give rise to a one-dimensional (1D)
helical chain (Supporting Information, Figures S1, S2).
The reaction of DTBBQ with ethanolamine in presence of
CuCl2·2H2O leads to the formation of the mononuclear
copper(II) coordination compound [CuII(HL1)2Cl2] (1). It
could be also synthesized by reaction of HL1 with
CuCl2·2H2O. 1 crystallizes in the monoclinic space group
P2/n. The crystal structure consists of the mononuclear
complexes (Figure 1). The copper(II) ion is coordinated by
two nondeprotonated ligands HL1 and two chloride ions. The
metal center has a strong elongated cis-octahedral environment
with a basal plane made of two nitrogen atoms from HL1 (Cu−
N bond lengths of 2.021; 2.069 Å) and two chloride ions (Cu−
Cl bond lengths of 2.283; 2.284 Å). The apical positions are
occupied by two oxygen atoms from HL1 with Cu−O bond
lengths of 2.502 and 2.523 Å. The axial distortions are typical
for copper(II) complexes and caused by the Jahn−Teller
effect.54
[CoII(L3)2] (3)
[MnII4(HL4)4Cl4] (4)
O3−Co1
O4−Co1
O7−Co2
O12−Co2
N1−Co1
N2−Co1
N3−Co1
N4−Co1
N5−Co2
N6−Co2
N7−Co2
N8−Co2
2.002(4)
1.996(4)
2.000(5)
2.017(4)
2.160(5)
2.136(5)
2.130(6)
2.138(5)
2.199(5)
2.137(5)
2.204(6)
2.133(5)
N1−Mn1
2.213(5)
2.229(5)
2.284(5)
2.179(4)
2.39(2)
N2−Mn1
N4−Mn2
N3−Mn2
Cl1−Mn1
Cl2−Mn2
Mn1−O3
Mn1−O7
Mn2−O3
Mn2−O7
Mn1---Mn2
Mn1---Mn2′
2.40(2)
2.198(4)
2.143(4)
2.169(4)
2.170(4)
3.361(4)
3.504(7)
3.731(3)
3.705(8)
a
a
Mn1--- Mn1′
a
Mn2---Mn2′
a
Mn1′ and Mn2′ hold for position: (1 − x, y, 0.5 − z).
for 1−4 and HL1 are listed in Table 2, Supporting Information, Table
S1.
RESULTS AND DISCUSSION
■
HL1 (5,7-Di-tert-butyl-2-methylenhydroxylbenzoxazole) was
synthesized in methanol by a condensation reaction between
3,5-di-tert-butyl-o-benzoquinone (DTBBQ) and ethanolamine
H2L2 (2-hydroxyethyl-2-{2,4-bis(1,1-dimethylethyl)-1-phe-
nol-6 amino}-2{5,7-di-tert-butyl-benzoxazole}) was obtained
in methanol by reaction of DTBBQ with 1,3-diamino-2-
hydroxypropane and has been characterized by NMR and IR-
spectroscopy. Unexpectedly, this reaction does not result in the
in aerobic conditions53 as it is well evidenced from 1H and 13
C
NMR spectroscopy (Supporting Information, Figure S3). In a
first step the amino group of ethanolamine reacts with the less
hindered carbonyl group from the first position of DTBBQ
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Scheme 2. Chemical Transformation in the Synthesis of H2L2
bis-benzoxazole analogue of HL1. The condensation of two
amino groups of 1,3-diamino-2-hydroxypropane with carbonyl
groups of two molecules of DTBBQ occurs as expected, but the
following oxidation process results in the formation of only one
benzoxazole cycle (Scheme 2). On the basis of previously
reported works38−41 the possible mechanism for the formation
of H2L2 is presented in Scheme 2.
1H NMR spectra (400 MHz) of H2L2 in d6-DMSO shows 13
lines with well-defined hyperfine splitting. The NMR spectrum
contains additional lines and is not consistent with the bis-Schiff
base HLa (Scheme 2) or symmetrical bis-benzoxazole
compound. The two-dimensional (2D) NMR spectroscopy
1
was used to clarify the structure of H2L2. According to H
NMR spectra the peaks in region of 1.1−1.5 ppm correspond
to four methyl groups (with integral ratio of 3:3:3:3)
(Supporting Information, Figure S4a). In 1H−1H COSY
spectra these four signals do not have any cross picks. The
spectral position of these signals and the absence of any
superfine splitting allow assignment of these signals to tert-butyl
groups from two magnetically non equivalent positions of two
different aromatic rings, in agreement with condensation of two
DTBBQ. The 1H NMR spectra in the aromatic region
(Supporting Information, Figure S4b) indicates the presence
of two different di-tert-butyl substituted aromatic rings in H2L2.
According to the structure of HL1 and other literature
examples38,39,41,55,56 the oxazole cyclization can be expected;
however, the presence of two di-tert-butyl aromatic rings
suggests that only one moiety of expected Schiff base was
cyclized. A similar case with unsymmetrical benzoxazole
cyclization has been reported for the reaction of 2-
hydroxybenzaldehyde and 4,6-diaminoresorcinol in methanol.43
On the other hand it has been reported that the condensation
of DTBBQ with ethylenediamine or 2,2-dimethylpropylenedi-
amine under aerobic conditions in acetonitrile only gives the
symmetrical Schiff bases or their reduced amino form but no
benzoxazole cyclization occurs in these conditions.
1
Figure 2. H,1H COSY NMR spectra (400 MHz) of H2L2 in D6-
DMSO solution. Blue lines correspond to expanded regions. Signal at
3.35 ppm corresponds to residual solvent H2O. (see Supporting
Information, Figure S4 for aromatic and tert-butyl regions).
The formation of H2L2 can be explained as a multistep
process that involves a sequence of consecutive chemical
transformations (Scheme 2). A similar mechanism was
previously proposed for the interaction of DTBBQ with
primary amines.38,39 As for HL1 the first step is the formation
of symmetrical Schiff base because of the reaction of two amino
groups of 1,3-diamino-2-hydroxypropane with the less hindered
carbonyl groups of DTBBQ (Scheme 2a). In the following step,
the solvated Schiff base undergoes a keto−enol tautomer-
ism57−59 (Scheme 2b). Then the deprotonation and intra-
molecular reorganization occur (Scheme 2c). Finally the
benzoxazole cyclization leads to the formation of H2L2
(Scheme 2d).
Two aromatic rings in H2L2 are connected by a flexible
-NH−CH2−CH(OH)- chain which can have different
conformations because of the free rotation around -HaHbC-
(sp3)---(sp3)CHc(OH). The analysis of the hyperfine splitting
constants JHaHc and JHbHc correlated to the theoretical
simulation of NMR spectra indicates that the most probable
conformation in H2L2 corresponds to values of dihedral angles
of 28.37° and 151.81° for HcCCHb and HcCCHa respectively
Thereby in 3.0−6.5 ppm region of 1H−1H COSY spectra five
signals with chemical shift δH(ppm) 6.226(d); 5.005(dd),
4.836(t); 3.671(m), 3.476(m) and equal integral ratios are
presented. The presence of the cross picks correlated with these
five signals allows to assign them to protons of HO−C, O−
CH; NH-C and two magnetically not equivalent N−CH2
protons as it is presented in Figure 2. The proposed
3
3
1
assignments are consistent with H−13C HSQC (Figure 3)
and 1H−13C HMBC (Supporting Information, Figure S5)
NMR experiments and confirm the structure of H2L2.
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in a cis-octahedral (O2N4) environment (Figure 5). The central
atom in 3 has the same coordination mode (Supporting
Figure 5. Coordination mode of HL3 in 2.
Figure 3. 1H,13C HSQC NMR spectra of H2L2 in D6-DMSO solution.
Signal at 3.35 ppm corresponds to residual solvent H2O.
Information, Figure S7). The bond lengths analysis in the
structure of 2 and 3 evidence the chemical transformation of
the initial ligand H2L2 into a new HL3 one during the reaction
with cobalt(II) and nickel(II) (Scheme 3). In compound 2, the
bond lengths of 1.29 and 1.21 Å found for N1−C15 and C16−
O2 respectively in compound 2 both correspond to double
bonds in contrast with the single bonds observed in the initial
H2L2. The distances of 1.221−1.325 Å were found for the same
bonds in the Co(II) analogue (3). The oxidation of H2L2 upon
the reaction with cobalt(II) and nickel(II) is also confirmed by
appearance of the CO band (ν(CO)= 1675 cm−1) and
disappearance of the O−H large band in 3500−3000 cm−1
region in the IR spectra of 2 and 3 (Supporting Information,
Figure S8). The catalytic oxidation of H2L2 into a HL3 in
presence of cobalt(II) and nickel(II) is shown in Scheme 3.
Electrochemistry studies were carried out on complexes 2
and 3. For 3, cyclic voltammetry in CH2Cl2 solution exhibits
two monoelectronic reversible reduction waves (Figure 6, B
and C) with half wave potentials at E1/2 = −0.63 V/SCE and
E1/2 = −0.99 V/SCE which were determined by Rotating Disc
Electrode and Cyclic Voltammetry (Table 3). They are
followed by a last irreversible reduction at a peak potential
value Ep = −1.53 V/SCE (D in Figure 6). The same reduction
sets of peaks are observed with complex 2. According to these
observations, the two consecutive reduction waves B and C can
be attributed to consecutive reduction of the coordinated
ligands HL3 in the complex leading to the formation of
successive stable anion radical and dianion species in
nonaqueous solvent (Scheme 4).
(Supporting Information, Figure S6). With this conformation
the relative positions of the OH and NH are appropriated for
the coordination of H2L2 to the metal centers as a tridentate
chelating ligand. Following the coordination ability of this new
asymmetric bensoxazole ligand (H2L2) was studied using
different metal ions.
[MII(L3)2] (M = Ni (2), Co(3)) were obtained in
acetonitrile as green-blue intense needle-like crystals by the
reaction of H2L2 with nickel(II) or cobalt(II) acetate in a 2:1
ratio. Complexes 2 and 3 (Figure 4, Supporting Information,
Upon oxidation, the cyclic voltammetry of complex 3 shows
a supplementary monoelectronic reversible oxidation wave with
a half wave potential E1/2 = 0.29/SCE (A in Figure 6). The
linear dependence of the peak current Ip versus the square root
of the scan rate potential √V between 0.05 and 0.3 V/s
demonstrates a diffusion-controlled process. Moreover, the
peak separation ΔEp = Epox − Epred is equal to 0.11 at 0.05 V/s
which is typical of a slow heterogeneous electron transfer on
the GC electrode. The voltammogram under stationary
conditions recorded during the forward−backward coulometry
on the peak A shows that the oxidized and reduced form are
stable in solution under an inert atmosphere in the time scale of
Figure 4. Molecular structure of 2. Methyl groups and hydrogen
atoms are omitted for clarity.
Figure S7) crystallize in monoclinic P21/n and triclinic P1 space
groups respectively. Because of the similarity of the molecular
structure for the complexes 2 and 3 we will describe hereafter
only the nickel analogue. The structure of 2 consists of the
neutral mononuclear complex of nickel(II). The central atom is
located on the 2 axis and coordinates with two ligands by two
nitrogen atoms (N1, N2) and one oxygen atom (O3) resulting
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Scheme 3. Catalytic Oxidation of H2L2 into HL3 in Presence of Nickel(II) or Cobalt(II) Salts
monoclinic space group C2/c. The crystal structure consists of
cubane-like discrete molecules (Figure 7).
Figure 6. Cyclic voltammetry of 2 and 3 at GC electrode (3 mm
diameter) in CH2Cl2 at 0.05 V/s.
Figure 7. Molecular structure of 4 with cubane “4 + 2” cluster core.
Methyl groups and hydrogen atoms are omitted for clarity.
a
Table 3. Electrochemical Data for 2 and 3
The reaction of H2L2 with manganese(II) ions causes a more
significant modification in the structure of the initial ligand
compared with cobalt(II) and nickel(II) based compounds.
Indeed, the X-ray crystal structure of 4 shows that, in the
manganese(II) complex, a new ligand H2L4 containing a
second benzoxazole ring and an additional OH group is
obtained (Figure 8). The further oxidation of HL3 into H2L4
anodic peak potential (E1/2
)
cathodic peak potential (E1/2)
b
b
c
c
2
3
1.00
1.71
1.30
1.86
−0.74 (−0.71) −1.02 (−0.97)
c c
b
b
c
0.35 (0.29) −0.67 (−0.63) −1.05 (−0.99)
a
Peak potential (V) and half wave potential E1/2 recorded in CH2Cl2 at
293 K with a glassy carbon electrode, 0.1 M n-Bu4NPF6 as supporting
electrolyte; all potentials are versus SCE, scan rate 0.05 V s−1.
b
c
Irreversible system. Reversible system.
Scheme 4. Redox Process Proposed for 2 (M = NiII) and 3
(M = CoII)
the electrolysis. By comparison, no oxidation peak exists in this
potential region for the complex 2. These results are
characteristic of the redox process cobalt(III)/cobalt(II) in
the complex 3, which is interesting for future catalytic system
studies.
Finally, two large and irreversible waves are recorded at larger
positive potential (E and F in Figure 6) at 1.71 V/ECS and 1.86
V/ECS.
In this last similar potential domain, we observe two new
successive monoelectronic and irreversible oxidation waves at
1.00 V/SCE and 1.30 V/SCE for complex 2. The oxidation
waves are associated with new reduction peaks of weak
intensity at 0.28 V/ECS and 0.50 V/ECS respectively. This
behavior is characteristic of a complicated oxidation mecha-
nism.
Figure 8. Coordination mode of H2L4 in 4.
could be ascribed to a stronger catalytic activity of manganese-
(II) ions. After the formation of HL3 (Scheme 3) the
cyclization of the second benzoxazole ring takes place (Scheme
5g) and the intermediate ketone in the equilibrium with water
gives the bis-hydroxyl compound (Scheme 5h), as may be
observed for pyridine-ketones.60−62 A similar formation of two
benzoxazole rings was reported when the condensation product
[MnII (HL4)4Cl4] (4) was also obtained from H2L2 as
4
colorless needle-like crystals when H2L2 was reacted with
MnCl2·4H2O in a 1:1 ratio in acetonitrile. It crystallizes in
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Scheme 5. Catalytic Oxidation of HL3 into H2L4 in Presence of Manganese(II) Salts
of DTBBQ with 2,2-dimethylpropylenediamine was reacted
with PdCl2 in methanol.20
S3S4, S1S4 may be considered nearly equivalent (Ja = J12 = J23
=
J
34 = J14), we neglected the magnetic interactions between S1S3
The coordination mode of H2L4 in 4 is presented in Figure
8. The core of the cluster is formed by four manganese(II) ions
arranged in a cubane-like “4 + 2” fashion.63 Central atoms are
bridged together by alkoxo groups of four monodeprotonated
H2L4 ligands. The geometry of manganese(II) ions is square-
based pyramidal with two bridging alkoxo groups and two
nitrogen atoms of H2L4 in the basal plane and a chlorine ion in
the apical position with bond lengths of 2.143−2.170 Å, 2.179−
2.284 Å, and 2.390−2.395 Å for Mn−O, Mn−N, and Mn−Cl,
respectively.
and S2S4 (J13 = J24 = 0) to simplify the following calculations.
Thus, the final expression of Hamiltonian used to describe the
exchange interactions in 4 is
H = − 2Ja(S1S2 + S2S3 + S3S4 + S1S4)
A similar approach has been widely used for magnetic
characterization of homo- and heterometallic spin clusters like
[Cr4(μ3-O)2],64 [V4(μ3-O)2],65 [Fe4(μ3-O)2],66−71
[Fe2Mn2(μ3-O)2],72 [Cr2Mn2(μ3-O)2],72 [Cr2Fe2(μ3-O)2]66
including homovalence [MnIII4(μ3-O)2] and mixed-valence
The magnetic properties studies of 4 are shown in Figure 9
under the form of χM and χMT versus T plot. At room
[MnIII2MnII (μ3-O)2]71−81 clusters.
2
A satisfactory fit of the experimental data for compound 4
(Figure 9) was obtained with parameters Ja(MnII-MnII) =
−0.50(1) cm−1; g = 2.00(7) and zJ = 0.0 (fix) cm−1 where zJ
holds for intermolecular interactions. The value for magnetic
interactions is consistent with previously reported alkoxo
bridged MnII−MnII.
CONCLUSION
■
In summary, we report the synthesis and characterization of
two benzoxazoles derivative ligands resulting from the
condensation of 3,5-di-tert-butyl-o-benzoquinone (DTBBQ)
with ethanolamine or 1,3-diamino-2-hydroxypropane in meth-
anol. With ethanolamine we obtained the expected methyl-
enhydroxylbenzoxazole (HL1). In contrast with 1,3-diamino-2-
hydroxypropane the condensation of DTBBQ occurs with the
two amino groups but the successive process results in a
benzoxazole ring only on one side to give H2L2 instead of the
symmetric bis-benzoxazole derivative. The structure of HL1
and H2L2 was confirmed by NMR-spectroscopy and X-ray
diffraction. Their ability to coordinate was checked for both
ligands. The reaction of HL1 with CuCl2 gives rise to a
mononuclear [CuII(HL1)2Cl2] (1) complex, while the use of a
mixture of copper(II) and lanthanide (III) salts results in
heterometallic dodecanuclear complexes [Ln4Cu8], as we have
previously reported.42 The structure of the HL1 does not
change upon the reaction with CuII or CuII/LnIII ions as shown
by X-ray crystal structure. The reaction of H2L2 with nickel(II),
cobalt(II), and manganese(II) salts were also studied. The X-
ray crystal structure show that during reaction with transition
metal ions the initial H2L2 undergoes versatile chemical
transformation to give HL3 in mononuclear complexes
[MII(L3)2] (M = NiII (2); M = CoII (3)) and H2L4 in the
Figure 9. χMT and χM versus T plot for 4. The solid lines indicate the
best fit of the data with the theoretical model with the parameters
indicated in the text.
temperature, the value of the χMT product (H = 0.1 T) is 16.88
cm3 K mol−1 (Figure 9) which is lower than the value of 17.50
cm3 K mol−1 expected for four noninteracting manganese(II)
ions (S = 5/2, g = 2). Upon cooling χMT decreases
continuously down to 1.97 cm3 K mol−1 at 2 K, indicating
dominant antiferromagnetic MnII−MnII interactions within the
tetranuclear complex.
The general spin-Hamiltonian we used to describe the
isotropic exchange interactions within the tetranuclear complex
is given by the following expression:
H = − 2(J S S + J S S + J S S + J S S
1 4
14
1 2
2 3
3 4
12
23
34
tetranuclear complex [MnII (HL4)4Cl4] (4). These results
4
+ J S S + J S S )
demonstrate that benzoxazole based ligands as H2L2 possess
interesting redox activity and can be selectively oxidized using
different transition metal ions. We believe that these results give
some new insights in the domain of benzoxazole based
compounds, which is complicated, quite unpredictable, and
still not very well studied but give promising perspectives.
1 3
2 4
24
13
In this case S1=S2=S3=S4 = 5/2. As described above, the
molecular structure of 4 consists of a “4 + 2” cubane-like
arrangement of manganese(II) ions63 (Figure 7). While the
magnetic exchange interactions between each pair S1S2, S2S3,
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ASSOCIATED CONTENT
■
S
* Supporting Information
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2011.
Crystallographic refinement details for HL1 and 1−4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
(28) Sakaguchi, S.; Nishiwaki, Y.; Kitamura, T.; Ishii, Y. Angew.
Chem., Int. Ed. 2001, 40, 222−224.
Corresponding Author
*E-mail: luneau@univ-lyon1.fr.
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Commun. 2001, 2352−2353.
ACKNOWLEDGMENTS
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■
O.I. thanks the “French Ministry of Education” for Ph.D. grant.
G.N. thanks the Marie Curie Actions (FP7-People-IIF) for
financial support. D.L. thanks the Laboratoire des Champ
́ ́
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