A manganese(II) 4-hydroxycinnamate complex with the tripod ligand tris(2-benzimidazolylmethyl)amine

Article abstract of DOI:10.1515/znb-2008-0103

A six-coordinate manganese (II) complex with the tripod ligand tris(2-benzimidazolylmethyl)amine (ntb), with composition [Mn(ntb) (4-hydroxycinnamate)](4-hydroxycinnamate) · (DMF)_0.5 ·(H₂O)₃, was synthesized and characterized by elemental and thermal analyses, electrical conductivity, IR, and UV/vis spectral measurements. The crystal structure of the complex has been determined by the single-crystal X-ray diffraction. The Mn (II) cation is bonded to an ntb ligand and a 4-hydroxycinnamate ligand through four N atoms and two O atoms, giving a distorted octahedral coordination geometry. Cyclic voltammograms of the complex indicate a quasi-reversible Mn³⁺/Mn²⁺ couple. The X-band EPR spectrum of the complex exhibits a six-line manganese hyperfine splitting pattern with g = 2, A = 95, and confirms that the material is high-spin Mn(II).

Full text of DOI:10.1515/znb-2008-0103

A Manganese(II) 4-Hydroxycinnamate Complex with the Tripod Ligand
Tris(2-benzimidazolylmethyl)amine
a
b
a
a
Huilu Wu , Wei Ying , Jingkun Yuan , and Jian Ding
a
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070,
P. R. China
b
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
P. R. China
Reprint requests to Dr. Huilu Wu. E-mail: wuhuilu@163.com
Z. Naturforsch. 2008, 63b, 11 – 15; received August 24, 2007
A six-coordinate manganese (II) complex with the tripod ligand tris(2-benzimidazolyl-
methyl)amine (ntb), with composition [Mn(ntb) (4-hydroxycinnamate)](4-hydroxycinnamate) ·
(
DMF)0.5 · (H
2
O) , was synthesized and characterized by elemental and thermal analyses, electri-
3
cal conductivity, IR, and UV/vis spectral measurements. The crystal structure of the complex has
been determined by the single-crystal X-ray diffraction. The Mn (II) cation is bonded to an ntb lig-
and and a 4-hydroxycinnamate ligand through four N atoms and two O atoms, giving a distorted
octahedral coordination geometry. Cyclic voltammograms of the complex indicate a quasi-reversible
3+
2+
Mn /Mn couple. The X-band EPR spectrum of the complex exhibits a six-line manganese hy-
perfine splitting pattern with g = 2, A = 95, and confirms that the material is high-spin Mn(II).
Key words: Crystal Structure, Cylic Voltammetry, EPR, Manganese(II) Complex,
Tris(2-benzimidazylmethyl)amine
Introduction
synthesis and spectroscopic characterization of man-
ganese(II) complexes with ntb have appeared in
Model systems that mimic the active sites of metal- the literature [5 – 9]. Herein we report the synthe-
loenzymes are important not only for the understand- sis, crystal structure and properties of a novel man-
ing of enzyme mechanisms, but also for the devel-
ganese(II) complex with the tripodal tris(2-benzimid-
opment of small molecular weight biomimetic cata- azolylmethyl)amine and 4-hydroxycinnamate ligands.
lysts. The imidazole part of histine plays an important
role in the coordination of transition metals at the ac-
tive sites of numerous proteins. For example, super-
oxide dismutases (SOD) [1 – 2], which are contained
in microbes, plants and animals, protects cells against
oxygen toxicity [3 – 4], because these enzymes cat-
Experimental Section
Materials and physical measurements
All chemicals, of AR/GR quality, were used without fur-
ther purification.
−
C, H and N contents were determined using a Carlo Erba
alyze the conversion of superoxide (O2 ) to hydro-
1106 elemental analyzer. Metal contents were determined
gen peroxide and dioxygen via redox-active metals.
The tetradentate tripodal ligand tris(2-benzimidazolyl-
methyl)amine (ntb) (Fig. 1) may mimic the histidine
imidazole in coordination aspects. Reports on the
by EDTA titration. Thermal analyses were carried out un-
◦
−1
der an N
2
flow at a heating rate of 10 C min
on a
ZRY-2P thermal analyzer. The IR spectra were recorded in
−1
the 4000 – 400 cm region with a Nicolet FI-IR AVATAR
360 spectrometer using KBr pellets. Electronic spectra were
taken with a UV/vis spectrophotometer. Electrolytic con-
ductance measurements were made with a DDS-11A type
−
3
−3
conductivity bridge using a 10
mol dm
solution in
DMF at r. t. Electrochemical measurements were performed
with a LK98APLUS electrochemical analyzer under nitro-
gen atmosphere at 283 K. A glassy carbon working elec-
trode, a platinum-wire auxiliary electrode, and a saturated
Fig. 1. Structure of ntb.
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12
H. Wu et al. · A Manganese(II) 4-Hydroxycinnamate Complex
Table 1. Crystal data and structure refinement for off, washed with MeOH and absolute Et O, and dried in
2
[
(
Mn(ntb) (4-hydroxycinnamate)] (4-hydroxy-cinnamate) ·
vacuo. The dried precipitate was dissolved in DMF to give
a light yellow solution which was allowed to evaporate at
r. t. After four weeks a mass of light yellow crystals ap-
DMF)0.5 · (H
2
O)
3
.
Formula
C43.50H44.50MnN7.50O9.50
879.31
−
1
Molecular weight, g mol
Crystal size, mm3
Crystal system
Space group
a, A
b, A
c, A
α, deg
β, deg
γ, deg
peared, yield 0.62 g (70 %). – C43.50
H
44.50MnN7.50
O
9.50
0.33 × 0.30 × 0.27
(
879.31): calcd. C 59.42, H 5.10, N 11.95, Mn 6.25; found:
triclinic
C 59.74, H 5.31, N 11.73, Mn 6.20. – Λ
M
(DMF, 297 K):
¯
P 1
2
−1
˚
63.7 s · cm · mol
.
11.9360(12)
14.4348(16)
14.7519(15)
83.052(2)
89.970(2)
66.287(2)
2306.7(4)
2
˚
˚
X-Ray structure determination
Intensity data were collected using a Bruker Smart CCD
diffractometer with graphite-monochromated MoKα radia-
tion (λ = 0.071073 nm) at 293 K. Data reduction and cell
refinement were performed using the programs SMART and
SAINT [11]. The absorption corrections were carried out by
the empirical method. The structure was solved by Direct
Methods (SHELXTL) using all unique data [12]. The non-H
atoms in the structure were subjected to anisotropic refine-
ment. Hydrogen atoms were placed geometrically and treated
with the riding model. The crystal data and experimental pa-
rameters relevant to the structure determination are listed in
Table 1.
˚
3
Vol, A
Z
T, K
293 (2)
1.266
0.374
−
3
Dcalcd, g · cm
−
1
Absorption coefficient, mm
F (000), e
θ Range for data collection, deg
hkl Ranges
918
1.55 to 28.37
−15 ≤ h ≤ 15;
−
−
19 ≤ k ≤ 17;
19 ≤ l ≤ 17
2
Refinement method
Data / restraints / parameters
Full-matrix least-squares on F
11406 / 11 / 614
0.933
2
CCDC 627350 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
Goodness-of-fit on Fa
R1/wR2 [I ≥ 2σ(I)]
0.068/0.194
0.163/0.261
a
R1/wR2 (all data)
˚
−3
Large diff. peak and hole, e A
a
1.23/−0.37
w = 1/[σ Fo + (0.1402 P )2 + 0.0000 P ], where P = (Fo2
2
2
+
2
2
Fc )/3.
Result and Discussion
calomel (SCE) reference electrode were used in the three-
electrode measurements. The electroactive component was at
The manganese complex is soluble in DMF and
DMSO, but insoluble in water and organic sol-
vents, such as methanol, ethanol, benzene, petroleum
ether, trichloromethane etc. The elemental analyses
show that the composition is [Mn(ntb)(4-hydroxycinn-
amate)] (4-hydroxycinnamate)· (DMF)0.5 · (H2O)3. A
comparison of the molar conductance value shows
a 1 : 1 electrolyte similar to previously reported
data [13].
−
3
−3
a 1.0× 10 mol dm concentration with tetrabutylammo-
−
3
nium perchlorate (TBAP) (0.1 mol dm ) used as the sup-
porting electrolyte in DMF solution. The EPR spectra were
recorded with a Bruker 200D spectrometer with the X-band.
Preparation of tris(2-benzimidazolylmethyl)amine and its
manganese complex tris(2-benzimidazolylmethyl)amine
(
ntb)
Thermogravimetric analysis (TGA) of the man-
This compound was synthesized by the literature ganese(II) complex shows that it undergoes endother-
◦
method [10]. Yield: 19.5 g (60 %); m. p.: 274 – 275
(
sistent with the literature [10].
C
mic dehydration. The initial mass loss within the tem-
perature range 80 – 124 C is attributed to elimination
◦
1
271 C [10]). The infrared and H NMR spectra were con-
◦
of the hydrate H2O molecules. Differential thermal
analysis (DTA) also indicates that the dehydration pro-
cess appears as an endothermic peak. The mass losses
[
Mn(ntb) (4-hydroxycinnamate)](4-hydroxycinnamate) ·
(
DMF)0.5 · (H O)
2
3
◦
in the range 160 – 181 C are due to the loss of the
DMF molecules. The decomposition of the complex
starts at 273 C and is complete at ca. 610 C, yielding
MnO2 as the final product.
To a stirred solution of ntb (407.5 mg, 1 mmol)
in hot MeOH (20 mL) was added Mn(ClO
· 6H
361.8 mg, 1 mmol), followed by a solution of sodium-4-
hydroxycinnamate (372.4 mg, 2 mmol). A light yellow crys-
◦
◦
4
)
2
2
O
(
The molecular structure of the manganese(II) com-
talline product formed rapidly. The precipitate was filtered plex is shown in Fig. 2, selected bond lengths and an-
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H. Wu et al. · A Manganese(II) 4-Hydroxycinnamate Complex
13
Fig. 2. Molecular structure and atom
numbering of [Mn(ntb) (4-hydroxycinn-
amate)] (4-hydroxycinnamate) · (DMF)0.5
·
(
H
2
O)
3
with hydrogen atoms omitted for
clarity.
˚
Table 2. Selected bond lengths (A) and bond angles (deg).
cant elongation has been observed in other manganese
complexes of tripodal tetradentate ligands with benz-
imidazolylmethyl groups [7]. The average bond an-
gle (NA–Mn–NB) of the axial nitrogen atoms (NA =
Mn(1)–O(1)
Mn(1)–N(2)
Mn(1)–N(4)
Mn(1)–N(6)
Mn(1)–N(1)
2.323(4)
2.194(4)
2.206(4)
2.188(3)
2.543(3)
Mn(1)–O(2)
O(1)–C(33)
O(2)–C(33)
C(31)–C(32)
C(32)–C(33)
2.228(3)
1.237(8)
1.250(7)
1.324(1)
1.599(1)
N1), the manganese ion, and the basal nitrogen atoms
◦
(
NB = N2, N4, N6) is 71.3 , and the manganese ion
N(6)–Mn(1)–N(2)
N(6)–Mn(1)–N(4)
N(2)–Mn(1)–N(4)
N(6)–Mn(1)–O(2)
N(2)–Mn(1)–O(2)
N(4)–Mn(1)–O(2)
N(6)–Mn(1)–O(1)
N(2)–Mn(1)–O(1)
N(4)–Mn(1)–O(1)
O(2)–Mn(1)–O(1)
114.3(1)
104.6(1)
111.6(1)
131.5(1)
103.1(1)
88.03(1)
89.39(1)
95.85(1)
139.4(1)
56.04(1)
N(6)–Mn(1)–N(1)
N(2)–Mn(1)–N(1)
N(4)–Mn(1)–N(1)
O(2)–Mn(1)–N(1)
O(1)–Mn(1)–N(1)
C(33)–O(1)–Mn(1)
C(33)–O(2)–Mn(1)
C(32)–C(31)–C(25)
O(1)–C(33)–O(2)
C(31)–C(32)–C(33)
71.94(1)
71.08(1) is 0.704 A˚ above the basal plane N2–N4–N6. The
70.86(1)
4
-hydroxycinnamate ligand is accommodated at the
153.0(1)
148.9(1)
90.6(3)
open axial site without any significant change in the
pseudo-octahedral geometry of the complex (average
◦
94.7(4)
N –Mn(1)–N = 110.2 ). In the dichloro complex
B
B
112.0(1)
118.7(4)
113.7(1)
II
Mn (ntb)Cl2, a sixth ligand, the chloride anion, opens
one site of the trigonal basal plane to form a square
◦
basal plane (NB–Mn–NB = 143.1 ) [8]. When a sixth
gles are summarized in Table 2. The asymmetric unit ligand is coordinated to the metal complex of a tripo-
+
consists of a [Mn(ntb)(4-hydroxycinnamate)] cation, dal tetradentate ligand, the geometry of the three benz-
a 4-hydroxycinnamate anion, 0.5 molecule of DMF imidazole nitrogen atoms may be retained with the
and 3 molecules of water of crystallization. The tripo- complex changing its geometry from trigonal bipyra-
dal ntb ligand forms a pyramidal geometry with man- midal to partial trigonal pyramidal; alternatively, the
ganese, and the remaining open axial site is occupied geometry of the three benzimidazole nitrogen atoms
by a chelating bidentate 4-hydroxycinnamate anion. may change from trigonal basal to square basal to ac-
The manganese(II) ion is six-coordinate with a N4O2 commodate the new ligand with the complex changing
ligand donor set and a pseudo-octahedral structure. its geometry from trigonal bipyramidal to octahedral.
5
The bond length between the manganese ion and the The high-spin d manganese(II) ion has no crystal field
˚
apical nitrogen atom N(1)–Mn(1) is 2.543(3) A, which stabilization energy, so it could have various geome-
˚
is about 0.347 A longer than the bond lengths be- tries depending on the coordinated ligand. All the bond
tween the manganese ion and the basal nitrogen atoms lengths related to the manganese atom are comparable
˚
˚
(
2.188– 2.206 A, average = 2.196 A). This signifi- to the values observed in other complexes [14, 15].
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14
H. Wu et al. · A Manganese(II) 4-Hydroxycinnamate Complex
IR and UV/visible spectra
(CV) in DMF. The voltammogram shows only a sin-
gle reduction peak (Epc) at 0.343 V during the ca-
thodic potential scan. During the return anodic poten-
tial scan, just after the reduction peak, an anodic peak
In the free ligand ntb, a strong band is found at ca.
440 cm along with a weak band at 1460 cm
−
1
−1
1
.
By analogy with the assigned bands of imidazole, the
former is attributed to ν(C=N–C=C), while the lat-
ter is ν(C=N) [10, 16 – 18]. These bands are shifted
to higher frequencies by 10 cm for the complex,
which implies direct coordination of all four imine ni-
trogen atoms to Mn(II) as found for other metal com-
plexes with benzimidazoles [19]. The νas (COO) vi-
bration is assigned to the strong band at 1527 cm
whereas νs(COO) is attributed to the 1395 cm
peak, which suggest the presence of coordinated 4-
(
Epa) is observed at 0.487 V. The separation between
the cathodic and anodic peak potentials ∆Ep (∆Ep =
Epa − Epc) of 144 mV indicates a quasi-reversible re-
dox process assignable to the Mn(III)-Mn(II) couple
and E1/2 = (Epa + Epc)/2 is equal to 0.415 V [5, 8].
The free ntb ligand was proven to be not electroactive
over the range −1.2 to +1.2 V. According to previous
reports [7, 23 – 27], to be an effective mimic of super-
oxide dismutase, a transition metal complex must have
−
1
−
−
1
1
◦
1
−
a reduction potential below 0.65 V [E ( O2 − O2 )]
hydroxycinnamate [20 – 22]. The bands at 1701 and
◦
−
and above −0.33 V [E (O2 − O2 )] such that catal-
ysis can take place without toxic singlet oxygen be-
ing formed. Thus the redox potential of 0.415 V of the
complex shows that it has SOD activity.
−
1
1
445 cm
are attributed to ν(C=O) and ν(C–O)
indicative of free 4-hydroxycinnamate. The bands
−
1
present at 1640 cm may originate from the C=C
bond vibration of the α,β-unsaturated carboxylate
The X-band EPR spectrum of a single crystal was
measured at 285 K. The spectrum exhibits the typical
six-line hyperfine signal centered at g = 2 which is as-
−
1
groups. Medium bands near 1281 cm probably cor-
respond to ν(N–Ar). A broad band in the 3000–
−
1
3
300 cm
region may be ascribed to hydrogen-
55
sociated with the I = 5/2 nuclear spin of Mn. The ex-
perimental hyperfine coupling constant is equal to A =
bonded ν(O–H) and ν(N–H), and may also include
ν(C–H).
9
5 G and is of the same order as that found for other
DMSO solutions of the ligand ntb and its man-
ganese(II) complex show, as expected, almost identi-
cal UV spectra. The UV bands of ntb (284, 277 nm)
are only marginally blue-shifted (7 nm) in the com-
plex, which is clear evidence of C=N coordination to
manganese(II). The two absorption bands are assigned
mononuclear Mn(II) complexes [7 – 9]. This spectrum
confirms that the material is high-spin Mn(II). The
signal features are assignable to allowed transitions
(
∆ms = ± 1, ∆ml = ± 0).
Acknowledgement
∗
∗
to n → π and π → π (imidazole) transitions.
The authors acknowledge the financial support and a grant
from ‘Qing Lan’ Talent Engineering Funds by the Lanzhou
Jiaotong University and from the Middle-Young Age Science
Foundation of Gansu Province (grant no. 3YS061-A25-023).
Cyclic voltammogram and EPR spectrum
The electrochemical properties of the manganese
complex have been studied by cyclic voltammetry
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