Quinoline-based tetradendate nitrogen ligands stabilize the bis(-oxo) dinuclear manganese(iii,iii) core

Article abstract of DOI:10.1039/b705080a

Quinoline-based, tetradentate nitrogen ligands, N,N′-bis(2- quinolylmethyl)-N,N′-dialkyl-1,2-ethanediamine (alkyl = methyl, bqdmen; ethyl, bqdeen; isopropyl, bqdpen), have been investigated as the supporting ligands for the formation of bis(-oxo) dinuclea

Full text of DOI:10.1039/b705080a

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Quinoline-based tetradendate nitrogen ligands stabilize the bis(l-oxo)
dinuclear manganese(III,III) core†
Yuji Mikata,*a Haruka So,b Azusa Yamashita,b Ayano Kawamura,b Masahiro Mikuriya,c Koˆichi Fukui,d
Akio Ichimurae and Shigenobu Yano f
Received 3rd April 2007, Accepted 15th June 2007
First published as an Advance Article on the web 5th July 2007
DOI: 10.1039/b705080a
Quinoline-based, tetradentate nitrogen ligands, N,N ꢀ-bis(2-quinolylmethyl)-N,N ꢀ-dialkyl-1,2-
ethanediamine (alkyl = methyl, bqdmen; ethyl, bqdeen; isopropyl, bqdpen), have been investigated
as the supporting ligands for the formation of bis(l-oxo) dinuclear manganese complexes.
Bis(l-oxo)Mn2(III,III) complexes 1 and 2 were obtained for bqdmen and bqdeen, respectively, as
evidenced by X-ray crystallography, whereas bqdpen did not afford any manganese complexes due to its
steric bulk. Complexes 1 and 2 exhibit highly positive Mn2(III,III)/Mn2(III,IV) and Mn2(III,IV)/Mn2(IV,IV)
redox couples relative to the corresponding pyridine-ligated (l-O)2Mn2(III,III) complexes.
methyl groups into the pyridine rings of bispicen yields 6-
Mebispicen, which is a suitable ligand for (l-O)2Mn2(III,III)
Introduction
assembly.6,9 A similar observation was made in the tris(2-
pyridylmethyl)amine (tpa)/N,N-bis(6-methylpyrid-2-yl)methyl)-
N-(2-pyridylmethyl)amine (bpmpa) system.6,9 Since high-spin d4
manganese(III) exhibits Jahn–Teller elongation along the axial
ordinate, steric hindrance at the axial donor atom stabilizes the
(l-O)2Mn2(III,III) core.
In this paper, we report the preparation of quinoline-
based N4 ligands, N,N ꢀ-bis(2-quinolylmethyl)-N,N ꢀ-dialkyl-
1,2-ethanediamine (alkyl = methyl, bqdmen; ethyl, bqdeen;
isopropyl, bqdpen), and the evaluation of their ability to support
the (l-O)2Mn2(III,III) centre. A quinoline-based N6O1 ligand,
N,N,Nꢀ,Nꢀ -tetrakis(2-quinolylmethyl)-2-hydroxy-1,3-propanedi-
amine (Htqhpn), has been demonstrated to yield tetranuclear
manganese(II,III,III,II) complexes.17 Steric hindrance and the re-
duced basicity of the quinoline ring support low oxidation states
in the central bis(l-oxo)manganese unit.
Multinuclear metal complexes continue to attract interest not
only for their biological relevance1,2 but also for the develop-
ment of metal-based materials including single molecular mag-
nets (SMM).3 Among multinuclear manganese complexes, l-
oxodimanganese complexes are of particular interest because
the l-oxodinuclear structure is the minimum building unit for
the assessment of intramolecular metal–metal interactions that
contain the biologically abundant and magnetically important l-
oxo bridge. One such example includes the bis(l-oxo) dinuclear
manganese complex ligated by tetradentate nitrogen-donor (N4)
ligands, which has been proposed as a key intermediate for
functional modeling of the oxygen evolving complex (OEC) of
photosystem II.4
In addition to the structural arrangement of multinuclear
clusters, the control of oxidation state in oxo metal clusters
is of significant importance in both biological and magnetic
materials chemistry. Among various dinuclear l-oxo manganese
complexes formulated as Mn2(III,III), Mn2(III,IV) and Mn2(IV,IV),
there are very few examples of bis(l-oxo)dimanganese(III,III)
complexes.5–9 In contrast, a number of examples of Mn2(III,IV)
cores have been reported.2,10–16 A (l-O)2Mn2(III,IV) complex de-
rived from the pyridine-based ligand, N,N ꢀ-bis(2-pyridylmethyl)-
1,2-ethanediamine (bispicen), is one such example.10,11 On the
other hand, it was reported that the introduction of ortho-
aKYOUSEI Science Center, Nara Women’s University, Nara, 630-8506,
Japan. E-mail: mikata@cc.nara-wu.ac.jp
bDepartment of Chemistry, Faculty of Science, Nara Women’s University,
Nara, 630-8506, Japan
cSchool of Science and Technology, Kwansei Gakuin University, Gakuen 2-1,
Sanda, 669-1337, Japan
dDepartment of Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku,
Tokyo, 169-8555, Japan
Results and discussion
eDepartment of Chemistry, Faculty of Sciene, Osaka City University,
Sumiyoshi-ku, Osaka, 558-8585, Japan
Synthesis
f
Division of Material Science, Nara Women’s University, Nara, 630-8506,
The three N4 ligands, bqdmen,18 bqdeen and bqdpen, were
obtained from the reaction of 2-chloromethylquinoline hydrochlo-
ride with the corresponding N,N ꢀ-dialkylethylenediamines in
Japan
† CCDC reference numbers 297397–297399. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b705080a
3330 | Dalton Trans., 2007, 3330–3334
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◦
a
˚
acetonitrile in the presence of potassium carbonate. The resultant
products were characterized by 1H/13C NMR, ESI-MS and X-ray
crystallography (for bqdpen, see Fig. 1). The bqdmen ligand has
been utilized to prepare several metal complexes containing Fe(II),
Co(II), Pd(II), Cu(II), Ru(II) and Cr(0) ions.18–20
Table 1 Selected bond lengths (A) and angles ( ) for complex 1
Mn1 · · · Mn1*
Mn1–O1
Mn1–N1
2.6649(5)
1.8238(19)
2.169(2)
2.463(2)
93.83(9)
98.12(9)
98.21(8)
168.80(9)
96.99(8)
82.78(9)
91.32(9)
72.42(9)
O1 · · · O1*
2.492(2)
1.8251(19)
2.144(2)
2.439(2)
86.15(8)
167.95(9)
95.54(8)
95.21(9)
98.59(8)
72.22(8)
93.51(9)
159.83(7)
Mn1–O1*
Mn1–N2
Mn1–N4
Mn1–N3
Mn1–O1–Mn1*
O1–Mn1–N1
O1–Mn1–N3
O1*–Mn1–N1
O1*–Mn1–N3
N1–Mn1–N2
N1–Mn1–N4
N2–Mn1–N4
O1–Mn1–O1*
O1–Mn1–N2
O1–Mn1–N4
O1*–Mn1–N2
O1*–Mn1–N4
N1–Mn1–N3
N2–Mn1–N3
N3–Mn1–N4
a Symmetry operator (1 − y, 1 − x, 1/2 −z) is used to generate O1* and
Fig. 1 ORTEP plot (50% probability) for bqdpen. The atoms generated
by symmetry operations (1 − x, 1 − y, 2 − z) are marked with an asterisk.
Mn1*.
Manganese complexes incorporating these ligands were pre-
pared as follows. A methanol solution of the ligand and man-
ganese(II) perchlorate hexahydrate (1 eq.) was stirred at room
temperature open to air for 2 d. Pink precipitates formed and
were removed by filtration, and then hydrogen peroxide solution
was added to the filtrate. The resulting precipitate was collected
and recrystallized from DMF or acetonitrile with slow diethyl
ether diffusion to afford dark-red crystals. X-Ray crystallography
(Fig. 2 and 3 and Tables 1 and 2) revealed that the bqdmen and
bqdeen complexes contain a (l-O)2Mn2 core, but for bqdpen, dark-
brown precipitates of MnO2 were obtained after the addition of
hydrogen peroxide. This is indicative of the weak chelating ability
of the bqdpen to the metal centre due to the large steric hindrance
of the isopropyl groups.
Fig.
3
ORTEP plot (50% probability) for the cation portion of
[Mn(bqdeen)2(l-O)2](ClO4)2·H2O (2·H2O).
Structural descriptions
Fig. 2 and 3 show the cations from the molecular struc-
tures of [(l-O)2Mn2(bqdmen)2](ClO4)2·2DMF (1·2DMF) and [(l-
O)2Mn2(bqdeen)2](ClO4)2·H2O (2·H2O). The cation of complexes
1 and 2 is composed of a (l-O)2Mn2 core supported by two bqdmen
or bqdeen molecules. Complex 1 crystallizes in the P41212 space
group and the unit cell contains four molecules of 1 and eight
DMF molecules. The crystallographic 2-fold axis is found in the
centre of complex cation, relating the left half of the molecule to
the right half.
Based on the number of counter anions and bond lengths
around the metal centre, the oxidation state of the two manganese
centres was determined to be III,III. Valence bond sum (VBS)
analysis21 also supports this oxidation state assignment (VBS
Fig.
2
ORTEP plot (50% probability) for the cation portion of
[Mn(bqdmen)2(l-O)2](ClO4)2·2DMF (1·2DMF). The atoms generated by
symmetry operations (1 − y, 1 − x, 1/2 − z) are marked with an asterisk.
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Table 2 Selected bond lengths (A) and angles ( ) for complex 2
Mn1 · · · Mn2
Mn1–O1
Mn1–N1
2.6675(9)
1.824(2)
2.156(3)
O1 · · · O2
2.494(3)
1.821(2)
2.160(3)
Mn1–O2
Mn1–N2
Mn1–N3
Mn2–O1
2.434(4)
1.830(2)
Mn1–N4
Mn2–O2
2.445(3)
1.828(2)
Mn2–N5
Mn2–N7
2.145(4)
2.439(4)
Mn2–N6
Mn2–N8
2.150(3)
2.433(3)
Mn1–O1–Mn2
O1–Mn1–O2
O1–Mn1–N1
O1–Mn1–N3
O1–Mn2–N5
O1–Mn2–N7
O2–Mn1–N1
O2–Mn1–N3
O2–Mn2–N5
O2–Mn2–N7
N1–Mn1–N2
N1–Mn1–N4
N2–Mn1–N4
N5–Mn2–N6
N5–Mn2–N8
N6–Mn2–N8
93.77(11)
86.34(12)
96.70(12)
100.30(14)
165.27(16)
91.88(13)
165.54(16)
92.38(13)
96.45(12)
99.47(13)
82.23(13)
93.57(14)
73.04(14)
82.95(11)
95.08(13)
73.19(13)
Mn1–O2–Mn2
O1–Mn2–O2
O1–Mn1–N2
O1–Mn1–N4
O1–Mn2–N6
O1–Mn2–N8
O2–Mn1–N2
O2–Mn1–N4
O2–Mn2–N6
O2–Mn2–N8
N1–Mn1–N3
N2–Mn1–N3
N3–Mn1–N4
N5–Mn2–N7
N6–Mn2–N7
N7–Mn2–N8
93.92(11)
85.96(12)
165.69(16)
92.84(13)
98.21(11)
99.31(12)
98.26(13)
100.41(12)
165.90(15)
92.88(13)
73.17(15)
93.06(15)
162.22(12)
73.40(14)
93.87(13)
163.89(12)
value of 2.8 was obtained for all manganese atoms of 1 and 2).
Significant Jahn–Teller elongation for the two axial bonds between
the quinoline nitrogens and the manganese centre is evident
from the crystal structure. Since the quinoline nitrogen atoms in
bqdmen and bqdeen have a weaker coordinating ability than that
of pyridine ligands for electronic and steric reasons, the present
ligands effectively stabilize the Mn2(III,III) state. As compared
with the previously reported structures for (l-O)2Mn2(III,III)
complexes,6–9 complex 1 has the longest lengths for axial bonds
(2.463(2) A for Mn1–N3 and 2.439(2) A for Mn1–N4). The axial
bond distances of 1 and 2 are even longer than those for the central
(l-O)2Mn2(III,III) core of the tetramanganese complexes with the
quinoline-based Htqhpn ligand.17
Fig. 4 Variable-temperature magnetic moments (open circles) and mag-
netic susceptibilities (filled circles) of complexes 1 (a) and 2 (b). Solid lines
represent the best-fit of the data with the model described in ref. 22.
are at the extremes of the range of those (−86.5 to −100.5 cm−1)
for previously reported (l-O)2Mn2(III,III) complexes.
˚
˚
Electrochemistry
˚
The cyclic voltammograms of complexes 1 and 2 were measured
in acetonitrile (Fig. 5). The two redox couples for each complex
were found at E1/2 = 0.79 and 1.61 V (vs. Ag/AgCl) for 1 and 0.80
and 1.64 V for 2, which correspond to the Mn2(III,III)/Mn2(III,IV)
and Mn2(III,IV)/Mn2(IV,IV) couples, respectively. These potentials
are significantly high compared with those for (l-O)2Mn2(III,III)
complexes such as 6-Mebispicen (E1/2 = 0.57 and 1.34 V)6 or
bpmpa (E1/2 = 0.68 and 1.45 V)6 complexes. Hodgson and co-
workers have demonstrated that the stabilities of (l-O)2Mn2(III,III)
complexes are determined by steric and electronic effects.6 The
present results indicate that the combined steric and electronic
effects of the quinoline ring are suitable for stabilizing the (l-
O)2Mn2(III,III) core.
The Mn · · · Mn distance of 1 is 2.6649(5) A, which is the shortest
separation for a Mn2(III,III) complex ever reported. A p–p stacking
interaction between the two quinoline rings that occupy the axial
positions might account for this proximity.
Magnetic properties of complexes
Magnetic susceptibility measurements on complexes 1 and 2 were
performed (Fig. 4). At room temperature, both complexes show
a smaller magnetic moment per molecule (leff = 2.7 lB for 1
and 2.8 lB for 2) than the expected value (6.9 lB) for a high-
spin Mn2(III,III) system with no magnetic coupling. In addition,
at low temperature the magnetic moment significantly decreases,
indicating that a strong antiferromagnetic interaction is present in
both complexes. As in previous examples, the magnetic moment is
essentially constant below 50 K probably because of the existence
of a small amount of monomeric Mn(III) (or Mn(II)) impurities.
The curve fitting of temperature-dependent magnetic data for
these complexes was performed based on the isotropic spin-
exchange model.22 The curve fitting shown in Fig. 4 gave the
values of the exchange interaction parameter J for Mn · · · Mn, the
g value and the amount of paramagnetic Mn(III) impurity (p) as
−97.5 cm−1, 1.94 and 0.036 for complex 1 and −82.9 cm−1, 1.85 and
0.038 for complex 2, respectively. The J values of these complexes
Oxidation of the complexes
Preliminary experiments for the oxidation of the (l-O)2Mn2(III,III)
species by aqueous hydrogen chloride7 were also conducted. Upon
the addition of 1 N HCl into an acetonitrile solution of 1,
the weak band at 490 nm shifted to 400 nm with a 540 nm
shoulder, finally resolving to 410 and 590 nm absorptions. The
new long-wavelength absorbances are characteristic of high-valent
oxomanganese species. Bulk treatment of dark-red powder 1 with
acetonitrile–1 N HCl (1 : 1) afforded a green solid. The presence
of Mn2(III,IV) in this material was observed by EPR spectroscopy
3332 | Dalton Trans., 2007, 3330–3334
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7.2 Hz,4H), 2.716 (s, 4H), 3.903 (s, 4H), 7.469 (t, J = 6.9 Hz, 2H),
7.60–7.68 (m, 4H), 7.728 (d, J = 7.8 Hz, 2H), 7.965 (d, J = 8.4 Hz,
2H), 8.036 (d, J = 8.1 Hz, 2H). 13C NMR (CDCl3) d (ppm): 11.9,
48.5, 51.9, 61.1, 120.8, 125.8, 127.2, 127.4, 128.8, 129.1, 135.9,
147.3, 161.3. ESI-MS m/z = 399.2 ([M + H]+).
N,N ꢀ-Bis(2-quinolylmethyl)-N,N ꢀ-diisopropyl-1,2-ethanediamine
(bqdpen)
This ligand was prepared according to the procedure described for
the preparation of bqdeen using N,N ꢀ-diisopropylethylenediamine.
The product was recrystallized from methanol to give a colorless
crystalline material in 57% yield; mp 128–129 ◦C. 1H NMR
(CDCl3) d (ppm): 0.983 (d, J = 6.6 Hz, 12H), 2.576 (s, 4H), 2.899
(sept, J = 6.6 Hz, 2H), 3.843 (s, 4H), 7.482 (dt, J = 7.7, 1.2 Hz,
2H), 7.641 (d, J = 8.7 Hz, 2H), 7.664 (dt, J = 7.7, 1.5 Hz, 2H),
7.725 (d, J = 7.7 Hz, 2H), 7.933 (d, J = 8.7 Hz, 2H), 7.988 (d,
J = 8.4 Hz, 2H). 13C NMR (CDCl3) d (ppm): 18.1, 50.1, 51.3,
57.2, 120.8, 125.8, 127.3, 127.5, 128.8, 129.1, 136.1, 147.4, 162.5.
ESI-MS m/z = 427.2 ([M + H]+). Single crystals were obtained
from DMF.
[Mn2(bqdmen)2(l-O)2](ClO4)2 (1)
A mixture of bqdmen (371 mg, 1.0 mmol) and manganese(II)
perchlorate hexahydrate (362 mg, 1.0 mmol) in methanol (25 mL)
was stirred for 2 d exposed to air at room temperature. After
the resulting pink precipitate had been filtered off, 30% hydrogen
peroxide solution (1.2 mL) was added to the filtrate to afford
a red-brown precipitate. The product was filtered off, washed
with water, and dried in vacuo. Yield 197 mg (36%). The
compound was recrystallized from DMF–diethyl ether, affording
single crystals suitable for X-ray crystallography. Anal. calcd for
C48H54Cl2Mn2N8O11 (1·H2O) C, 52.42; H, 4.95; N, 10.30. Found: C,
52.7; H, 4.85; N, 10.3%. ESI-MS (m/z): 981.3 ([Mn2(bqdmen)2(l-
O)2](ClO4)+).
Fig. 5 Cyclic voltammograms of complexes 1 (a) and 2 (b) in acetonitrile.
measured in frozen acetonitrile solution, in which a typical 16-line
signal for dinuclear Mn2(III,IV) complex was exhibited (data not
shown). Further purification and characterization of the oxidized
species including the Mn2(IV,IV) complex are in progress in our
laboratory.
Experimental
[Mn2(bqdeen)2(l-O)2](ClO4)2 (2)
General information
The reaction was carried out under similar conditions to those
described for the synthesis of 1. Yield 27%. Single crystals suitable
for X-ray analysis were obtained from acetonitrile–diethyl ether.
Anal. calcd for C52H60Cl2Mn2N8O11 (2·H2O) C, 54.03; H, 5.41;
N, 9.69. Found: C, 54.3; H, 5.3; N, 9.4%. ESI-MS (m/z): 981.3
([Mn2(bqdmen)2(l-O)2](ClO4)+).
All reagents and solvents were used as received from commercial
sources. 1H NMR (300.07 Hz) and 13C NMR (75.00 Hz) spectra
were recorded on a Varian GEMINI 2000 spectrometer and
referenced to internal TMS or solvent signals. UV-Vis spectra
were measured on a Jasco V-700 spectrophotometer.
CAUTION: Perchlorate salts of metal complexes with organic
ligands are potentially explosive. All due precautions should be
taken.
X-Ray crystallography
Single crystals of bqdpen, [Mn2(bqdmen)2(l-O)2](ClO4)2·2DMF
(1·2DMF) and [Mn2(bqdeen)2(l-O)2](ClO4)2·H2O (2·H2O) were
covered with paraffin oil and mounted on a glass fiber. All of the
diffraction data were collected at 173 K on a Rigaku Mercury
CCD detector, with monochromatic Mo-Ka radiation, operating
at 50 kV/40 mA. Data were processed on a PC using CrystalClear
Software (Rigaku). All structures were solved by direct methods
(SIR-92) and refined by using full-matrix least-squares methods on
F2. All hydrogen atoms were found and treated as riding models.
Selected bond distances and angles for 1 and 2 are listed in Table 1
and 2, respectively, and crystallographic data are summarized in
Table 3.
N,N ꢀ-Bis(2-quinolylmethyl)-N,N ꢀ-diethyl-1,2-ethanediamine
(bqdeen)
A mixture of 2-chloromethylquinoline hydrochloride (1.07 g,
5.0 mmol), N,N ꢀ-diethylethylenediamine (291 mg, 2.5 mmol) and
potassium carbonate (2.08 g, 15 mmol) in acetonitrile (10 mL) was
refluxed for 48 h. After the solvent was removed under reduced
pressure, the residue was dissolved in chloroform and water and
the organic phase was separated, dried over Na2SO4, and then
evaporated to give bqdmen as a brown oil in 96% yield. 1H
NMR (CDCl3) d (ppm): 1.029 (t, J = 7.2 Hz, 6H), 2.590 (q, J =
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Table 3 Crystallographic data for bqdpen, [Mn2(bqdmen)2(l-O)2](ClO4)2·2DMF (1·2DMF) and [Mn2(bqdeen)2(l-O)2](ClO4)2·H2O (2·H2O)a
[Mn2(bqdmen)2(l-O)2]-
(ClO4)2·2DMF (1·2DMF)
[Mn2(bqdeen)2(l-O)2]-
(ClO4)2·H2O (2·H2O)
bqdpen
Formula
FW
C28H34N4
426.60
C54H66Cl2Mn2N10O12
1227.96
Tetragonal
P41212
12.9335(4)
12.9335(4)
32.6230(13)
90
90
90
5457.0(3)
4
1.495
C52H62Cl2Mn2N8O11
1155.88
Monoclinic
P21/a
16.018(6)
21.574(7)
17.179(6)
90
112.4191(11)
90
5488(3)
4
Crystal system
Space group
a/
Triclinic
¯
P1
7.2432(2)
8.8278(2)
9.6628(2)
81.368(6)
84.698(6)
74.897(6)
588.84(2)
1
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
D
calc/g cm−3
1.203
0.72
57.4
1.399
0.623
55.0
l/cm−◦1
0.633
55.0
2hmax
/
T/K
173
6342
2257
214
0.0390
0.1110
1.055
173
53797
6083
466
0.0503
0.1360
1.122
173
40797
9395
680
0.0827
0.2285
1.165
No. reflns collect.
No. reflns used
No. of params
Final R1 (I > 2r(I))
wR2 (all data)
GOF
Flack parameter
—
0.03(2)
—
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ꢁFo| − |Fcꢁ/ |Fo|. wR2 = [ w(Fo − Fc2)2/ w(Fo2)2]1/2
.
Electrochemistry
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experimental conditions.
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Acknowledgements
This work was supported by the Nara Women’s University
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