Formation of novel 1-D chains by μ-amido bridging of dinuclear manganese(III)-Schiff base complexes

Article abstract of DOI:10.1002/ejic.200400198

Three novel neutral manganese(III) complexes of stoichiometry Mn(R-amsal-3,5-di-tBu)(H₂O)_n [H₃-R-amsal-3,5- di-tBu = 3-aza-4-(2-hydroxyphenyl)-N-(2-hydroxyphenyl)but-3-enamide; (R = H, 4-Me, 5-tBu)] have been synthesised u

Full text of DOI:10.1002/ejic.200400198

FULL PAPER
Formation of Novel 1-D Chains by µ-Amido Bridging of Dinuclear
Manganese(III)-Schiff Base Complexes
[a]
[b]
[a]
[b]
´
´
´
Manuel R. Bermejo,* Ana M. Gonzalez-Noya, Vıctor Abad, M. Isabel Fernandez,
[b]
[a]
[a]
´
Marcelino Maneiro, Rosa Pedrido, and Miguel Vazquez
Keywords: Amides / Manganese / Schiff bases / 1-D chains
Three novel neutral manganese(III) complexes of stoichi-
(MeOH)2]n 2 and [Mn2(5-tBu-amsal-3,5-di-tBu)2(MeOH)2]n 3
ometry Mn(R-amsal-3,5-di-tBu)(H2O)n [H3-R-amsal-3,5-di- with methanol as solvate. Their X-ray characterisation shows
tBu = 3-aza-4-(2-hydroxyphenyl)-N-(2-hydroxyphenyl)but-3-
enamide; (R = H, 4-Me, 5-tBu)] have been synthesised using
potentially trianionic and pentadentate Schiff-base ligands
containing an amide group. Complexes were thoroughly
characterised by elemental analysis, FAB mass spectrometry,
infrared spectroscopy, magnetic susceptibility measurements
that these complexes consist of infinite one-dimensional
chains with a repeating cyclic dimeric unit, where the amide
oxygen atom is bridging two neighbouring manganese(III
)
ions. Paramagnetic 1H NMR spectroscopy confirmed the
presence of MnIII in these complexes, while the cyclic vol-
tammetry studies suggest the possibility of obtaining polynu-
and molar conductivities. Recrystallisation of these com- clear MnIV complexes with this kind of ligand.
plexes from methanol yielded single crystals of [Mn2(amsal-
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
3,5-di-tBu)2(MeOH)2]n 1, [Mn2(4-Me-amsal-3,5-di-tBu)2-
Germany, 2004)
Introduction
and the Boucher procedure[11]). Nevertheless, the low solu-
bilities of these complexes in most common solvents have
prevented, until now, their structural characterisation by X-
ray diffraction. The knowledge of such structures should
improve understanding of ligand design and the synthesis
of high oxidation state polymeric manganese complexes.
In order to establish the structural make-up of these com-
plexes, we decided to investigate whether the introduction
of tert-butyl groups on the aromatic rings of the Schiff base
ligands would improve the solubility of the resulting com-
plexes and allow structural characterisation. We have de-
signed some new amide-derived Schiff base ligands in order
to prepare novel manganese() complexes, and in this
paper we report the results.
In recent years, considerable efforts have been focused on
the synthesis and characterisation of polynuclear manga-
nese complexes.[1] This is mainly because di- or multi-nu-
clear manganese sites involving oxygen or nitrogen donor
ligands are important in the function of enzymes such as
alkaline phosphatase,[2] pseudocatalases,[3] manganese-
based superoxide dismutase (MnSOD)[4] and the oxygen
evolving complex (OEC) of photosystem II.[5] Moreover,
the synthetic complexes have been shown to exhibit a vari-
ety of interesting structural and physical properties.[6]
Recently, we have directed our attention to the coordi-
nation chemistry of manganese with Schiff bases containing
amide groups. Our interest in this kind of ligands derives
from the known ability of ligands containing amide groups
to stabilise high oxidation states of metal ions when coordi-
nated with the deprotonated nitrogen atom.[7Ϫ9]
Results and Discussion
Previously, we succeeded in stabilising the neutral MnIII
complexes[10] with ligands derived from H3-amsal [H3-am-
sal ϭ 3-aza-4-(2-hydroxyphenyl)-N-(2-hydroxyphenyl)but-
3-enamide] by two methods (an electrochemical synthesis
Synthesis and Characterisation of the Ligands
The new unsymmetrical Schiff base ligands reported in
this study were prepared in a manner analogous (Scheme 1)
with the three-step method to similar ligands described by
us previously.[10]
[a]
´
´
´
Departamento de Quımica Inorganica, Facultade de Quımica,
Universidade de Santiago de Compostela,
The first step involves a carbodiimide amide coupling[12]
of N-benzyloxycarbonylglycine with 2-hydroxyaniline, 2-
hydroxy-4-methylaniline or 2-hydroxy-5-tert-butylaniline in
THF. This reaction proceeds in excellent yield (see Exp.
Sect.) and large quantities of amides 1a, 1b or 1c can be
15782 Santiago de Compostela, Spain
[b]
´
´
Departamento de Quımica Inorganica, Facultade de Ciencias,
Universidade de Santiago de Compostela,
27002 Lugo, Spain
Fax: (internat.) ϩ34-981-597-525
E-mail: qimb45@usc.es
3696
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400198
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µ-Amido Bridging of Dinuclear Manganese(III)-Schiff Base Complexes
FULL PAPER
coal as the catalyst.[13] This methodology yielded the de-
sired amines 2a, 2b or 2c in high yield and with high purity.
Amines 2a, 2b and 2c were then employed as the precursors
of the new ligands via conventional condensation reactions
with a commercially available substituted salicylaldehyde.
The obtained Schiff bases were satisfactorily characterised
by elemental analysis, mass spectrometry (Table 1), 1H
NMR (Table 2), 13C NMR and IR (Table 3) spectroscopy.
Mass Spectrometry, NMR and IR Spectroscopy
The mass spectra of the ligands each show the peak for
H3Lϩ at 100% intensity (see Table 1), in accord with the
proposed formulae.
The 1H NMR spectroscopic data for the ligands are
shown in Table 2. The spectra of H3-R-amsal-3,5-di-tBu,
recorded using [D6]DMSO as solvent, exhibit a singlet at
δ ഠ 8.6 ppm assigned to the imine hydrogen atom, cor-
roborating the formation of CϭN bonds. The aromatic pro-
tons lie in the range δ ϭ 6.50Ϫ8.03, while the presence of
a singlet at δ ഠ 4.5 indicates the presence of the methylene
group. Furthermore, all spectra show three singlets in the
region of 9.21Ϫ13.79 that disappear on deuteration. The
peak at higher field was assigned to the NH proton while
the two signals at lower field correspond to the OH protons.
The spectra of these Schiff bases also show two singlets (for
H3-amsal-3,5-di-tBu and H3-4-Me-amsal-3,5-di-tBu) or
Scheme 1. Reaction diagram for the preparation of the
SchiffϪbase ligands
prepared. The second step involves the removal of the Z three singlets (for H3-5-tBu-amsal-3,5-di-tBu) in the range
protecting group from the amine functionality by sacrificial δ ϭ 1.2Ϫ1.4, indicating the presence of two or three non-
hydrogenation using cyclohexene and palladium on char- equivalent tert-butyl groups, respectively.
Table 1. Analytical and other selected data for the ligands
Analysis (%)[a]
C
Ligand
Yield
Colour
M.p. (°C)
MS-EI (m/z)
H
N
H3-amsal-3,5-di-tBu
H3-4-Me-amsal-3,5-di-tBu
H3-5-tBu-amsal-3,5-di-tBu
85%
84%
87%
pale yellow
white
white
176
172
240
382
396
438
72.0 (72.3)
72.6 (72.7)
73.9 (74.0)
8.0 (7.9)
8.2 (8.1)
8.8 (8.7)
7.2 (7.3)
7.1 (7.1)
6.4 (6.4)
[a]
Found (calculated).
1
Table 2. H NMR spectroscopic data for the ligands in [D6]DMSO
Proton[a]
(a)
Ligand[b]
(b)
(c)
(d)
(e)
Harom
H3-amsal-3,5-di-tBu
9.91 (br)
9.73 (br)
9.71 (s)
9.30 (s)
9.21 (s)
9.30 (s)
4.51 (s)
4.51 (s)
4.52 (s)
8.63 (s)
8.52 (s)
8.64 (s)
13.70 (s)
13.72 (s)
13.79 (s)
6.71Ϫ8.03
6.50Ϫ7.81
6.73Ϫ7.92
H3-4-Me-amsal-3,5-di-tBu[c]
H3-5-tBu-amsal-3,5-di-tBu[d]
[a] See Scheme 2 for atom labelling. [b] Ligands show two singlets at δ ഠ 1.3 and 1.4 assigned to the tert-butyl groups. [c] Includes a singlet
[d]
at δ ϭ 2.10 assigned to the methyl group. Includes a singlet at δ ϭ 1.20 assigned to the tert-butyl group.
Table 3. Selected IR bands (in cmϪ1) for the ligands
Ligand
ν(OH)
ν(NH)
ν(amide I)
ν(CN)
ν(amide II)
H3-amsal-3,5-di-tBu
H3-4-Me-amsal-3,5-di-tBu
H3-5-tBu-amsal-3,5-di-tBu
3369 (m)
3381 (m)
3385 (m)
3104 (br)
3105 (br)
3115 (br)
1657 (s, br)
1662 (vs)
1661 (m)
1639 (sh)
1633 (s)
1628 (vs)
1595 (m)
1603 (m)
1585 (m)
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M. R. Bermejo et al.
FULL PAPER
The 13C NMR spectra of the ligands in [D6]DMSO show carbon atom (around 165 ppm) and a resonance assigned
signals corresponding to the aromatic carbon atoms (be- to the carbonyl carbon atom (around 168 ppm). Addition-
tween δ ϭ 115 and 160 ppm), a signal due to the imine ally, the methylene carbon and the methyl and tert-butyl
groups, where appropriate, give signals below δ ϭ 70 ppm.
The most significant IR bands for the ligands are re-
corded in Table 3. They have been assigned by comparison
with the IR spectra of the amine precursors and on the
basis of our knowledge in this field.[10a]
Synthesis and Characterisation of Manganese Complexes
The manganese complexes were prepared following a
variation of the Boucher method reported by us.[10c,14]
The brown powdery solid compounds were obtained in
high yield via this synthetic route, and appear to be stable
in the solid state and in solution. They are moderately sol-
uble in common organic solvents and soluble in polar ap-
rotic coordinating solvents such as DMF and DMSO.
Elemental analyses indicate that these manganese com-
plexes have the general stoichiometry Mn(R-amsal-3,5-di-
tBu)(H2O)4, suggesting that they are neutral manganese()
species in which the ligands are tri-deprotonated. Molar
conductivity measurements in 10Ϫ3  DMF solutions,
in the range 3Ϫ9 µS cmϪ1, show the non-electrolytic behav-
iour of these complexes.[15]
Mass Spectrometry and IR Spectroscopy
Figure 1. An ORTEP view of the crystal structure of [Mn2(amsal-
Fast atom bombardment (FAB) mass spectra contain
3,5-di-tBu)2(MeOH)2]n 1; ellipsoids are drawn at 50% probability;
peaks relating to fragments of the form [MnL]ϩ, indicating
ligand coordination to the metal centre. Furthermore, these
symmetry operations: Ј: Ϫx ϩ 1, y Ϫ 1/2, Ϫz ϩ 1/2; ЈЈ: Ϫx ϩ 1,
y ϩ 1/2, Ϫz ϩ 1/2
Figure 2. An ORTEP view of the crystal structure of [Mn2(4-Me-amsal-3,5-di-tBu)2(MeOH)2]n (2); ellipsoids are drawn at 50% probability;
symmetry operations: Ј: Ϫx ϩ 1, y ϩ 1/2, Ϫz ϩ 1/2; ЈЈ: Ϫx ϩ 1, y Ϫ 1/2, Ϫz ϩ 1/2
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µ-Amido Bridging of Dinuclear Manganese(III)-Schiff Base Complexes
FULL PAPER
spectra exhibit peaks due to the fragment [Mn2L2]ϩ, which X-Ray Diffraction Studies
can be attributed tentatively to the presence of dimeric/
polymeric species in the solid state.
Crystal Structures of Complexes 1, 2 and 3
The ligand coordination to the metal centre is also sup-
ported by the IR spectra, which are very similar for all the
complexes. Thus, a band at about 1620 cmϪ1, attributable
to ν(CN), shifted to lower energy compared with the free
ligands values, indicating the coordination of the ligand to
the metal through the imine nitrogen atom. This ν(CN)
band is strong and broad and it obscures the amide I and
amide II bands. Additionally, bands centred at ca. 3400
cmϪ1 can be observed in these spectra, in accordance with
the presence of coordinated and/or lattice water or meth-
anol.
Single crystals of [Mn2(amsal-3,5-di-tBu)2(MeOH)2]n
(1), [Mn2(4-Me-amsal-3,5-di-tBu)2(MeOH)2]n (2) and
[Mn2(5-tBu-amsal-3,5-di-tBu)2(MeOH)2]n (3) suitable for
X-ray diffraction studies were grown as detailed in the Exp.
Sect. These analyses show that 1, 2 and 3 have similar struc-
tures that consist of a polymeric chain of repeating dimeric
units. These dimeric units contain two ligand units, two
manganese centres and two molecules of methanol. Fur-
thermore, complexes 1, 2 and 3 contain methanol as solvate.
Complexes 1 and 2 crystallise in the orthorhombic space
group Pbca, while complex 3 crystallises in the monoclinic
space group C2/c. The ORTEP diagrams for 1 and 2 are
shown in Figure 1 and Figure 2, respectively. The ORTEP
view of the environment of the manganese atom for com-
plex 3 is shown in Figure 3. Experimental details are given
in Table 4 and selected bond lengths and angles in Table 5.
The geometry around each Mn atom can be described as
distorted octahedral, where the trianionic and pentadentate
ligand coordinates the MnIII centre with the two phenol O-
atoms [O(1), O(3)], the imine N-atom [N(2)] and the amide
N-atom [N(1)] occupying the equatorial positions (see Fig-
ure 3). The coordination sphere around each manganese
centre is then completed by a capping methanol molecule
[O(4)] and an amide O-atom [O(2Ј)] from the neighbouring
complex molecule. Thus, the amide oxygen atom [O(2)] of
one ligand is coordinated to the manganese ion of the
neighbouring molecule, acting as a bridge, which generates
an infinite one-dimensional chain MnϪOamide(L)Ϫ
Finally, the absence of a strong band at ca. 1100 cmϪ1
confirms the absence of perchlorate anions in these manga-
nese complexes.
Figure 3. An ORTEP view of the environment around the manga-
nese centre for [Mn2(5-tBu-amsal-3,5-di-tBu)2(MeOH)2]n (3); ellip-
soids are drawn at 50% probability
˚
MnϪO’amide(L) resulting in a Mn···Mn distance of 6.064 A
˚
˚
for 1, 6.120 A for 2, and 6.035 A for 3.
Table 4. Crystal data and details of refinement for the complexes 1, 2 and 3
1
2
3
Empirical formula
Molecular mass
Temperature [K]
Wavelength [nm]
Crystal system
Space group
C25H34MnN2O5
497.49
293(2)
0.71073
orthorhombic
Pbca
18.653(3)
8.8588(14)
32.470(6)
90.00
90.00
90.00
C26H36MnN2O5
511.52
293(2)
0.71073
orthorhombic
Pbca
19.572(3)
8.8073(12)
32.192(4)
90.00
90.00
90.00
C29H43MnN2O5
554.59
110(2)
0.71073
monoclinic
C2/c
37.646(10)
7.2783(19)
23.023(6)
90.00
108.256(5)
90.00
˚
a [A]
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
Z
8
10
0.511
8
Absorption coefficient [mmϪ1
Crystal size [mm3]
Reflections collected
Independent reflections
Final R int [I Ͼ 2σ (I)]
]
0.513
0.82 ϫ 0.42 ϫ 0.06
29618
5928 [R(int) ϭ 0.0577]
R1 ϭ 0.0711
wR2 ϭ 0.1818
R1 ϭ 0.1302
wR2 ϭ 0.2237
0.478
0.13 ϫ 0.17 ϫ 0.13
18487
6759 [R(int) ϭ 0.0344]
R1 ϭ 0.0525
wR2 ϭ 0.1044
R1 ϭ 0.1173
wR2 ϭ 0.1299
0.44ϫ 0.33 ϫ 0.18
34239
6310 [R(int) ϭ 0.0537]
R1 ϭ 0.0447
wR2 ϭ 0.1225
R1 ϭ 0.0910
wR2 ϭ 0.1662
R int (all data)
Eur. J. Inorg. Chem. 2004, 3696Ϫ3705
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M. R. Bermejo et al.
FULL PAPER
˚
˚
˚
A for 2 and 0.025 A for 3). These planes also present a
gauche conformation.
Table 5. Selected bond lengths [A] and angles [°] for the complexes
1, 2 and 3
These ligands are expected to show a high degree of plan-
arity owing to the conjugation and presence of only one sp3
atom in the chelate rings (named as C8 for 1, C9 for 2 and
C12 for 3 in Figures 1, 2 and 3, respectively). In fact, the
angles between calculated chelate planes present values of
0.81° and 4.43° for 1, 2.90° and 7.68° for 2, and the highest
values for 3 (10.87° and 14.91°).
1
2
3
Mn1ϪO3phenol
Mn1ϪO1phenol
Mn1ϪN1amide
Mn1ϪN2imine
Mn1ϪO2Јamide
Mn1ϪO4methanol
O2ϭCamide
1.881(3)
1.916(3)
1.939(4)
1.964(4)
2.321(3)
2.275(5)
1.247(6)
6.064
1.8913(18)
1.9198(19)
1.942(2)
1.963(2)
2.349(2)
2.281(2)
1.247(3)
6.120
1.875(2)
1.930(2)
1.933(3)
1.958(3)
2.242(3)
2.270(4)
1.248(4)
6.035
Mn1···Mn1Ј
O3ϪMn1ϪO1
O1ϪMn1ϪN1
O3ϪMn1ϪN2
N1ϪMn1ϪN2
O4ϪMn1ϪO2Ј
103.33(15)
82.87(15)
92.15(16)
81.64(16)
171.60(17)
104.01(8)
83.31(9)
91.21(9)
81.48(9)
173.34(8)
104.35(10)
83.44(11)
91.36(11)
80.84(12)
173.93(9)
It is well known that MnIII complexes are able to form
dimeric or polymeric species through different coordinative
bridges:[14,16] µ-Oxo, µ-phenoxo, µ-aquo, µ-carboxylate, µ-
cyanide, µ-thiocyanate, etc. µ-oxo and µ-phenoxo bridges
generate dimeric structures; µ-carboxylate, µ-cyanide and µ-
thiocyanate usually yield polymeric species, while the µ-
aquo ligand can generate both dimeric or polymeric struc-
tures. The distance Mn···Mn for the complexes reported
herein are longer than the corresponding distances in µ-
oxo, µ-phenoxo or µ-aquo MnIII complexes, and shorter
than the distances found for µ-carboxylate MnIII complexes.
On the other hand, ionic metal() complexes with amide
ligands where oxygen amide atoms behave as bridges have
been reported.[17] However, to the best of our knowledge,
the type of compound reported in the present case, with
MnIII, has not been described previously in the literature.
The bond distances in the equatorial plane MnϪN [rang-
Figure 4. Part of the polymeric 1ϪD chain of 3, showing the zigzag
disposition of the manganese centres (in violet)
It is interesting to note that these infinite chains are not
linear (see Figure 4), as the manganese atoms are situated
in a zigzag arrangement. The angles between neighbouring
manganese atoms (Mn1ЈϪMn1ϪMn1ЈЈ) are 93.85° for 1,
92.03° for 2 and 74.17° for 3. The head-to-tail arrangement
of the ligands allows coordination of the amide oxygen
atom of each unit to the metal ion of the next unit in the
chain.
˚
ing from 1.933 to 1.964 A] and MnϪO [in the range
˚
1.875Ϫ1.930 A] are typical of manganese() com-
plexes[16,18] and corroborate complete deprotonation of the
˚
ligands. The distances MnϪO4methanol [2.270Ϫ2.281 A] are
in agreement with other previous results,[18] and are longer
than the axial distances MnϪO2Јamide, except for complex
3. The distances O2ϭCamide (ca. 1.247) are longer than
those found for the related free ligands (ca. 1.235),[10a] as
expected. The axial MnϪO distances are considerably
longer than the equatorial MnϪO bond lengths quoted
above, indicating that the JahnϪTeller elongation expected
for a high-spin d4 manganese() ion is present along the
chain of these polymers.
The deviation from an ideal octahedral geometry is also
revealed by the range of angles observed around the metal
centre [from 80.84 to 104.35°], as well as by the interaxial
angle O4ϪMn1ϪO2Ј of ca. 173°. The slight deviation of
the equatorial plane from planarity further reinforces this
distortion (maximum deviation from the calculated least-
˚
squares plane formed by O1, N1, N2 and O3 is ca. 0.004 A
˚
˚
for 1, 0.009 A for 2 and 0.025 A for 3, with the manganese
˚
atoms lying out of this plane by ca. 0.011 A for 1, 0.006 Figure 5. Thermal dependence of 1/χM for 2
3700
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µ-Amido Bridging of Dinuclear Manganese(III)-Schiff Base Complexes
FULL PAPER
Magnetic Studies
pression that may be used to fit the experimental data de-
rives from Fisher’s expression:[19]
The study of room temperature magnetic properties for
the complexes shows that their magnetic moments (see Exp.
χT ϭ [Ng2β2S(S ϩ 1)/3k][(1 ϩ u)/(1 Ϫ u)] where
Sect.) are all close to the spin-only value of 4.9 BM, ex- u ϭ coth[Ϫ2JS(S ϩ 1)/kT] Ϫ [kT/Ϫ2JS(S ϩ 1)]
pected for a magnetically dilute octahedral high-spin d4
manganese() ion.
The best agreement between experimental and calculated
data from 300 to 5 K corresponds to J ϭ Ϫ0.3 cmϪ1 and
g ϭ 2.05 with an agreement factor R (ϭ Σ[(χMT)obsd.
(χMT)calcd.]2/Σ[(χMT)obsd.]2) of 3.4.10Ϫ4
Variable temperature magnetic susceptibility of the com-
plexes were measured in the temperature range 5Ϫ300 K
using a small applied field of 5000 G. Plots of the recipro-
cals of the magnetic susceptibilities against absolute tem-
perature are linear, so that all the complexes obey the
CurieϪWeiss law. An example is shown in Figure 5 for com-
plex 2.
The magnetic behaviours of complexes 1, 2 and 3 are
identical and can be illustrated by the thermal variation of
the χMT versus T plot for complex 2 as in Figure 6. The
χMT product is practically constant in the 300Ϫ100 K
range with an observed value (3.34 Ϯ 0.3 cm3 molϪ1 K)
Ϫ
.
This study indicates little or no antiferromagnetic interac-
tion between neighbouring metal centres in the solid state.
Therefore, we can interpret the magnetic properties of these
MnIII polymer complexes as ordinary paramagnetic[19] and
this is in agreement with the intermolecular Mn···Mn dis-
˚
tances of about 6 A.
1H NMR Spectroscopy
1
Paramagnetic H NMR spectra for the complexes were
corresponding to the value expected for an isolated high- recorded in [D6]DMSO at room temperature. The data are
spin MnIII cation. On further reduction of the temperature, presented in Table 6.
this value decreases smoothly down to 2.99 cm3 molϪ1
K
The spectra obtained for these complexes contain three
at 30 K, and then more steeply to reach a value of 1.17 cm3 or four proton resonances that lie outside the diamagnetic
molϪ1 K at 5 K.
region (δ ϭ 0Ϫ14). The interpretation of these data is based
We attempted theoretical calculations to reproduce, and on the findings of Pecoraro and co-workers[21] and also on
for comparison with, the experimental results. For an infi- our own results[10a,10b,22] for other MnIII complexes with
nite chain of 4/2 local spins the simpler analytical ex- related Schiff base ligands. The protons ortho to the ring
position occupied by a Mn-coordinated atom H3Ј, H6 and
H6Ј (see Scheme 2), were not observed.
Scheme 2. Hydrogen atom labelling of the ligands for the 1H
NMR studies
Two or three relatively sharp peaks are observed upfield
between δ ϭ Ϫ14 and Ϫ32 ppm, which we have assigned to
the H4 protons of the aldehyde and to H4Ј and H5Ј protons
Figure 6. Thermal dependence of χMT for 2; the full line (Ϫ) corre-
sponds to the best data fit
of the amine.[10a,10b,21,22] The comparison between the spec-
1
Table 6. H NMR and electrochemical data for the complexes
1H NMR
Redox potentials[a]
Complex
H4
H4Ј
H5Ј
ϪCH2Ϫ
35.90
Eox/V
Ered/V
Peak
1
2
3
Ϫ31.95
Ϫ31.50
Ϫ32.07
Ϫ14.80
na[b]
Ϫ25.90
Ϫ27.08
na
Ϫ
0.778
Ϫ
0.690
Ϫ
0.700
Ϫ0.278
Ϫ
Ϫ0.282
Ϫ
Ϫ0.286
0.602
R1
O2
R1
O2
R1
O2,R2
34.46
Ϫ18.42
30.42
[a]
[b]
Potentials vs. SCE. na ϭ not applicable.
Eur. J. Inorg. Chem. 2004, 3696Ϫ3705
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3701

M. R. Bermejo et al.
FULL PAPER
tra of these three complexes, with substitution of the H4Ј donor atoms, especially if an electron-donating substituent
and H5Ј positions of the amine ring, provides an indication were to be introduced.[7a,7b,8a,8b]
of the appropriate assignments. Clearly the upfield peaks at
We found that the complexes exhibit low current intensit-
δ ഠ Ϫ32 correspond to the H4 protons. The features at ies in their redox waves. This behaviour is attributed to the
δ ϭ Ϫ14.80 and Ϫ18.42 arise from H4Ј protons, while the polymeric nature of these complexes, which results in a
downfield resonances at δ ϭ Ϫ25.90 and Ϫ27.08 arise from lower diffusion coefficient.[26]
H5Ј protons.
Additionally, one broad peak observed downfield (δ ϭ
30Ϫ36) can be assigned to the aliphatic protons in the
methylene bridge between the imine and amide nitrogen
atoms.
Conclusion
It is well known that high-spin MnIII complexes in an
octahedral field show isotropically shifted 1H NMR spectra
arising from ligand protons.[23] Therefore, the data obtained
from these NMR studies serve to corroborate the formation
of these novel manganese() complexes.
Self-assembly of manganese ions with the novel unsym-
metrical and potentially trianionic pentadentate amide
group-containing Schiff base ligands, leads to neutral, poly-
meric manganese() complexes. The X-ray crystal struc-
tures of these complexes show that the oxygen amide atom
of these ligands can act as a bridge between neighbouring
manganese atoms and afford zigzag one-dimensional
chains, previously unreported for manganese() ions. In
addition, the observations reported here could be helpful
for the study of biological systems, in which relatively weak
but cooperative interactions between sets of metal ions and
linked amide groups are present.
Electrochemical Behaviour of the Complexes
Cyclic voltammograms of all the complexes were ob-
tained from dimethylformamide solutions at slow scan rates
of 0.02 V sϪ1 over a potential range from ϩ1 to Ϫ1 V. The
numerical data are given in Table 6 and a typical voltammo-
gram is shown in Figure 7.
Finally, it seems that this kind of ligand is suitable for
stabilization of MnIII complexes, and could have potential
in the synthesis of polynuclear MnIV complexes.
Experimental Section
General: Starting materials were purchased from Aldrich except
manganese() perchlorate hexahydrate (Fluka) and cyclohexene
(Panreac) and were used without further purification.
Elemental analyses were performed on a Carlo Erba EA 1108 ana-
lyser. NMR spectra were recorded on a Bruker WM-250 spec-
trometer using [D6]DMSO as solvent; δ ϭ chemical shift in ppm;
s ϭ singlet, d ϭ doublet, t ϭ triplet, m ϭ multiplet, br ϭ broad.
Infrared spectra were recorded as KBr pellets on a Bruker UFS-
66V spectrophotometer in the range 4000Ϫ600 cmϪ1. The bands
are reported as: vs ϭ very strong, s ϭ strong, m ϭ medium, w ϭ
weak, sh ϭ shoulder, and br ϭ broad. Mass spectrometry (Elec-
tronic Impact) was performed on a HewlettϪPackard 5988A mass
spectrometer, whilst fast atom bombardment (FAB) was done on a
Micromass AutoSpec mass spectrometer, employing m-nitrobenzyl
alcohol as the matrix. Magnetic susceptibility measurements in the
5Ϫ300 K temperature range were performed using a SQUID mag-
netometer operating at 5000 G. Room temperature magnetic sus-
Figure 7. Cyclic voltammetry for 3
All the complexes behave similarly. All voltammograms
display an irreversible process at negative potentials (quoted
as R1 in Table 6), which we assign to the reduction of
manganese() to manganese().[14,24]
On the other hand, a second redox process assigned to ceptibilities were measured using a Digital Measurement system
MSB-MKI, calibrated using tetrakis(isothiocyanato)cobaltate().
Conductivities were obtained at 25 °C from 10Ϫ3  solutions in
DMF on a Philips PW9526 instrument. Cyclic voltammetry was
performed by using an EG&G PAR model 273 potentiostat, con-
trolled by the EG&G PAR model 270 software, in conjunction with
a three-electrode cell, fitted with a purge gas inlet and outlet, con-
sisting of a graphite disc working-electrode, a saturated calomel
reference electrode and a platinum auxiliary electrode. Voltammog-
rams were obtained from ca. 10Ϫ3  dimethylformamide solutions
of the metal complexes containing 0.1  tetraethylammonium per-
the oxidation of manganese() to manganese() is present
at positive potentials for all the complexes (quoted as O2 in
Table 6). This redox wave is irreversible for 1 and 2, but for
3 presents an important rate of reversibility (R2, see Fig-
ure 7).
It is interesting to note that the presence of the methyl or
tert-butyl groups on the aromatic ring of the amine precur-
sor decreases this oxidation potential as it does with related
ligands.[25] Hence, it seems obvious that manganese()
could be stabilised by this kind of ligand, containing hard chlorate as supporting electrolyte.
3702  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org
Eur. J. Inorg. Chem. 2004, 3696Ϫ3705

µ-Amido Bridging of Dinuclear Manganese(III)-Schiff Base Complexes
FULL PAPER
Syntheses
C9H12N2O2 (180.0): calcd. C 60.0, H 6.7, N 15.6; found C 60.3, H
6.9, N 15.5.
Preparation of Schiff Base Ligands: Ligands were prepared accord-
ing to a three-step method previously reported by us[10] and which
is shown in Scheme 1.
2-Amino-N-(2-hydroxy-5-tert-butylphenyl)ethanamide (2c): Prep-
aration as described above, using 1c (2 g, 5.61 mmol) Yield 1 g
(80%). M.p. 210 °C. 1H NMR (250 MHz, [D6]DMSO): δ ϭ 0.91
(s, 9 H), 3.12 (s, 2 H), 6.55 (d, 1 H), 6.69 (d, 1 H), 8.03 (s, 1 H)
ppm. IR (KBr): ν˜ ϭ 3380 (vs, OH), 3270 (vs, NH2), 3065 (m, NH),
1650 (vs, amide I), 1610 (s, amide II) cmϪ1. EI-MS: m/z ϭ 222.1.
C12H18N2O2 (222.0): calcd. C 64.9, H 8.1, N 12.6; found C 64.6,
H 8.1, N 12.7.
First Step. Synthesis of Precursors 1a, 1b and 1c
N-(2-Hydroxyphenyl)-2-[(phenylmethoxy)carbonylamino]ethanamide
(1a): 2-Hydroxyaniline (2.61 g, 23.9 mmol) and a slight excess of
N,NЈ-dicyclohexylcarbodiimide (5.3 g, 25.7 mmol) was added to a
solution of N-benzyloxycarbonylglycine (5 g, 23.9 mmol) in THF
(100 mL). This mixture was stirred at room temperature for four
hours.[12] The insoluble N,NЈ-dicyclohexylurea was removed by fil-
tration and the solvent replaced by ethyl acetate (100 mL). Ad-
dition of petroleum ether (b.p. 40Ϫ60 °C) afforded 6.46 g (90%) of
Third Step. Preparation of Schiff Base Ligands: The Schiff-base li-
gands were prepared in a standard manner by the reaction of equi-
molecular quantities of 3,5-di-tert-butyl-2-hydroxybenzaldehyde
and the appropriate amines 2a, 2b or 2c, which is typified by the
preparation of H3-5-tBu-amsal-3,5-di-tBu. The amine 2c (0.5 g,
2.25 mmol) and the salicylaldehyde (0.53 g, 2.25 mmol) were dis-
solved in methanol (100 mL) and the solution was brought to re-
flux. The volume of the solution was reduced over a three hour
period to ca. 50 mL, using a DeanϪStark trap and was then al-
lowed to cool. The solid, which precipitated, was collected by fil-
tration, washed with diethyl ether (3 mL) and dried in vacuo. The
purity of the ligands was checked by elemental analysis, mass spec-
trometry (Table 1), 1H NMR (Table 2), 13C NMR and IR
(Table 3) spectroscopy.
1
the amine 1a. M.p. 172 °C. H NMR (250 MHz, [D6]DMSO): δ ϭ
4.06 (d, 2 H), 5.30 (s, 2 H), 6.96 (m, 1 H), 7.56 (m, 5 H), 7.91 (t, 1
H), 8.09 (d, 1 H), 9.31 (s, 1 H), 10.09 (s, 1 H) ppm. IR (KBr): ν˜ ϭ
3392 (s, OH), 3270 and 3075 (both s, NH), 1677 (vs, amide I), 1618
(s, amide II) cmϪ1. EI-MS: m/z ϭ 300.1. C16H16N2O4 (300.0):
calcd. C 64.0, H 5.4, N 9.3; found C 64.1, H 5.5, N 9.4.
N-(2-Hydroxy-4-methylphenyl)-2-[(phenylmethoxy)carbonylamino]-
ethanamide (1b): Preparation as described above, using 2-hydroxy-
4-methylaniline (2.95 g, 23.9 mmol). Yield 6.78 g (90%). M.p. 185
1
°C. H NMR (250 MHz, [D6]DMSO): δ ϭ 2.43 (s, 3 H), 4.07 (d,
Synthesis of the Complexes
2 H), 5.30 (s, 2 H), 6.82 (d, 1 H), 6.92 (s, 1 H), 7.60 (m, 5 H), 7.94
(t, 1 H), 7.97 (s, 1 H), 9.30 (s, 1 H), 9.99 (s, 1 H) ppm. IR (KBr):
ν˜ ϭ 3392 (s, OH), 3269 and 3076 (both s, NH), 1677 (vs, amide I),
1618 (s, amide II) cmϪ1. EI-MS: m/z ϭ 314.1. C17H18N2O4 (314.0):
calcd. C 65.0, H 5.8, N 8.9; found C 65.1, H 6.0, N 9.1.
All of the manganese complexes were obtained using the same pro-
cedure. A typical preparation is outlined below.
Mn(amsal-3,5-di-tBu)(H2O)4:
A solution of NaOH (0.03 g,
0.78 mmol) in water (3 mL) was added to a solution of the ligand
H3-amsal-di-tBu (0.1 g, 0.26 mmol) in methanol (15 mL). Manga-
nese() perchlorate hexahydrate (0.15 g, 0.41 mmol) in 5 mL of
methanol was then added and a rapid change of solution colour
was observed, from yellow to brown. (CAUTION: All perchlorates
are potentially explosive. Although no problems have been encoun-
tered during this study, care should be taken in the handling and
treatment of these materials). After stirring at room temperature in
the presence of air for 12 h, the solid formed was collected by fil-
tration, washed with hexane and dried in vacuo. Yield 80%.
C23H35MnN2O7 (505.9): calcd. C 54.6, H 6.9, N 5.5; found C 54.8,
H 6.7, N 5.3. MS(FAB): m/z (%) ϭ 434 (100, MnL), 869 (12,
Mn2L2). Magnetic moment µeff ϭ 5.0 B. M. Conductivity (in
N-(2-Hydroxy-5-tert-butylphenyl)-2-[(phenylmethoxy)carbonyl-
amino]ethanamide (1c): Preparation as described above, using 2-
hydroxy-5-tert-butylaniline (3.95 g, 23.9 mmol). Yield 6.4 g (75%).
1
M.p. 185 °C. H NMR (250 MHz, [D6]DMSO): δ ϭ 1.30 (s, 9 H),
3.88 (d, 2 H), 5.09 (s, 2 H), 6.80 (d,1 H), 6.96 (d, 1 H), 7.37 (m, 5
H), 7.71 (t, 1 H), 7.92 (s, 1 H), 9.15 (s, 1 H), 9.66 (s, 1 H) ppm. IR
(KBr): ν˜ ϭ 3390 (s, OH), 3268 (s, NH), 3077 (m, NH), 1678 (vs,
amide I), 1617 (s, amide II) cmϪ1
. EI-MS: m/z ϭ 356.1.
C20H24N2O4 (356.0): calcd. C 67.4, H 6.8, N 7.9; found C 67.5, H
6.9, N 7.8.
Second Step. Synthesis of precursors 2a, 2b and 2c
DMF) ΛM ϭ 3 µS cmϪ1
.
2-Amino-N-(2-hydroxyphenyl)ethanamide (2a): A solution of 1a
(2 g, 6.7 mmol) in ethanol (100 mL) was mixed with cyclohexene
(5 mL, in excess to the molar proportion required) and commercial
10% PdϪC catalyst (0.50 g; catalyst:substrate ratio 1:4 by
weight).[13] The mixture was refluxed for 15 min. The catalyst was
removed by filtration and washed with ethanol, and the combined
filtrates were evaporated under reduced pressure to give the depro-
tected amine 2a. Yield 1.05 g (95%). M.p. 148 °C. 1H NMR
(250 MHz, [D6]DMSO): δ ϭ 3.26 (s, 2 H), 6.77 (m, 1 H), 6.88 (s,
1 H), 6.89 (s, 1 H), 8.18 (d, 1 H) ppm. IR (KBr): ν˜ ϭ 3431 (s, OH),
3420 and 3252 (both s, NH2), 3083 (s, br, NH), 1649 (vs, amide I),
1614 (s, amide II) cmϪ1. EI-MS: m/z ϭ 166.0. C8H10N2O2 (166.0):
calcd. C 57.8, H 6.1, N 16.9; found C 57.5, H 6.2, N 16.6.
Recrystallisation of the dark brown powder from methanol by slow
evaporation afforded brown crystals of [Mn2(amsal-3,5-di-tBu)2-
(MeOH)2]n 1, suitable for X-ray diffraction studies.
Mn(4-Me-amsal-3,5-di-tBu)(H2O)2: Yield 65%. C24H33MnN2O5
(483.9): calcd. C 59.5, H 6.8, N 5.8; found C 60.1, H 6.6, N 5.5.
MS(FAB): m/z (%) ϭ 449 (100, MnL), 898.1 (16, Mn2L2). Magnetic
moment µeff ϭ 5.1 B. M. Conductivity (in DMF) ΛM ϭ 3 µS cmϪ1
.
Recrystallisation of the brown powder in methanol yielded crystals
of [Mn2(4-Me-amsal-3,5-di-tBu)2(MeOH)2]n 2, suitable for X-ray
analyses.
Mn(5-tBu-amsal-3,5-di-tBu)(H2O)4: Yield 70%. C27H43MnN2O7
(561.9): calcd. C 57.7, H 7.7, N 5.0; found C 58.1, H 7.5, N 5.0.
2-Amino-N-(2-hydroxy-4-methylphenyl)ethanamide (2b): Prep- MS(FAB): m/z (%) ϭ 490.3 (100, MnL), 981.6 (18, Mn2L2). Mag-
aration as described above, using 1b (2 g, 6.36 mmol). Yield 1.09 g netic moment µeff ϭ 4.9 B. M. Conductivity (in DMF) ΛM ϭ 9
(95%). M.p. 175 °C. 1H NMR (250 MHz, [D6]DMSO): δ ϭ 2.12 µS cmϪ1
(s, 3 H), 3.17 (s, 2 H), 6.49 (d, 1 H), 6.61 (d, 1 H), 7.49 (d, 1 H) Recrystallisation of the brown powder in methanol yielded crystals
ppm. IR (KBr): ν˜ ϭ 3356 (s, OH), 3278 (s, NH2), 3046 (s, br, NH), of [Mn2(5-tBu-amsal-3,5-di-tBu)2(MeOH)2]n 3, suitable for X-ray
1655 (vs, amide I), 1606 (s, amide II) cmϪ1. EI-MS: m/z ϭ 180.1. studies.
.
Eur. J. Inorg. Chem. 2004, 3696Ϫ3705
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3703

M. R. Bermejo et al.
FULL PAPER
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