Dioxygen affinities of Schiff base cobalt(II) complexes with aza-crown or morpholino pendants

Article abstract of DOI:10.1007/s11243-010-9350-5

Schiff base Co(II) complexes, CoL¹₂-CoL ⁶₂with aza-crown or morpholino pendants were synthesized. The oxygenation constants (KO₂) of these complexes in MeOCH ₂CH₂OMe solution wer

Full text of DOI:10.1007/s11243-010-9350-5

Transition Met Chem (2010) 35:463–468
DOI 10.1007/s11243-010-9350-5
Dioxygen affinities of Schiff base cobalt(II) complexes
with aza-crown or morpholino pendants
•
Jian-zhang Li Lei Wei Fa-Mei Feng
•
•
•
Jun Zeng Sheng-ying Qin
Received: 30 January 2010 / Accepted: 9 March 2010 / Published online: 26 March 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Schiff base Co(II) complexes, CoL12–CoL26
with aza-crown or morpholino pendants were synthesized.
The oxygenation constants (KO2) of these complexes in
MeOCH2CH2OMe solution were measured over the range
of -5 to 25 °C, and the thermodynamic parameters (DH0,
DS0) for oxygenation were calculated based on these KO2
values. The effects of different substituents on the Schiff
base ligand with respect to the modulation of O2-binding
capability were explored. The results indicate that the
dioxygen affinities of the Co(II) complexes are much more
enhanced by aza-crown pendants than that by morpholino
pendants, and the O2-binding capabilities of the aza-crown
pendants complexes can also be enhanced by adding Na?
cations.
and most extensively investigated due to their structural
similarity to the metal sites found in biological systems [1].
Schiff base transition metal complexes have been one of the
most successful classes of synthetic oxygen carriers [2, 3].
In the past decades, a number of reports have appeared
on Schiff base complexes as models for oxygen carrying
metalloenzymes [4] and oxygenases [5], such as haemo-
globin and cytochrome P-450 enzymes, which play
important roles in the catalytic oxidation of various organic
compounds [1]. However, these complexes can easily
dimerize, and so lose activity upon absorption of dioxygen
[6]. Avdeef demonstrated that it is possible to form stable
oxygen adducts when these Co(II) complexes are modified
by appropriate substituents [7]. Thus, various substituents
have been employed to improve O2-binding capabilities of
the complexes [8, 9]. Among them, the crown ether-con-
taining Schiff bases have attracted much attention, due to
the interesting properties conferred by the hydrophobicity
of the outer ethylene groups and orderly arrangement of
inner oxygen atoms [10, 11]. Previous reports have indi-
cated that oxa-crowned Schiff base transition metal com-
plexes are good receptors for alkali cations, which bind in
the crown ether cavity [12, 13]. These complexes showed
much improved O2-binding activity due to the special
configuration and function of the crown ether ring [14, 15].
Co-complexation of a hard cation close to the transition
metal centre is believed to play an important role in mod-
ulating its oxygen-binding properties [16]. However, to the
best of our knowledge, very few studies on the dioxygen
affinities of aza-crown-substituted Schiff base complexes
have been reported to date. Recent studies have indicated
that crown ether-containing Schiff base complexes can also
show catalytic activity [17, 18]. These observations inspired
us to study the influence of pendant aza-crown substituents
on the O2-binding capabilities of Schiff base Co(II)
Introduction
Synthetic oxygen carriers are of great interest as models for
metalloenzymes involved in oxygen storage and transport.
Studies on dioxygen binding to transition metal complexes
are of intrinsic importance and provide opportunities to
address the fundamental chemistry of biological oxygen
carriers and oxygenases. The Co(II) complexes of Schiff
bases, such as Salen and its analogues, have been the first
J. Li (&) ꢀ L. Wei ꢀ F.-M. Feng ꢀ J. Zeng
Department of Chemistry, Key Laboratory of Green Catalysis
of Higher Education Institutes of Sichuan, Sichuan University
of Science and Engineering, Zigong, Sichuan 643000,
People’s Republic of China
e-mail: sichuanligong2006@hotmail.com
S. Qin
Department of Chemistry, Sichuan University, Chengdu,
Sichuan 610064, People’s Republic of China
123

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Transition Met Chem (2010) 35:463–468
X
X
O
O
CH2
N
Co
N
CH2
N
N
O
O
O
N
O
O
O
O
Co
O
N
O
N
O
CH2
O
N
CH2
O
X
X
CoL1
CoL4
,
,
X = Me; CoL2
X = Me; CoL5
,
,
X = Cl; CoL3
X = Cl; CoL6
,
,
X = NO2
X = NO2
2
2
2
2
2
2
Fig. 1 The structures of the Schiff base cobalt(II) complexes
complexes. Herein, the synthesis of Schiff base Co(II)
complexes with aza-crown pendants CoLn2 (n = 1–3) and
their analogues CoLn2 (n = 4–6; Fig. 1), and the study on
their dioxygen affinities are described. The influences of
pendant aza-crown ring substituents and the addition of
alkali cations on the dioxygen affinities of the Co(II)
complexes are also reported.
EtOH (20 cm3) was stirred for 4 h under N2 atmosphere at
80 °C, then cooled to room temperature. The yellow pre-
cipitate was filtered off and washed with EtOH. After
recrystallisation from EtOH, yellow crystals (2.54 g, yield
1
82%) were obtained. m.p. 95–96 °C. H NMR d (p.p.m.):
13.04 (s, 1H, OH, D2O exchangeable), 8.36 (s, 1H, N=CH),
7.57–6.82 (m, 7H, ArH), 3.71–3.52 (m, 6H, OCH2,
NCH2Ar), 2.81 (t, J = 5.5 Hz, 4H, NCH2), 2.26 (s, 3H,
CH3); I.r. (KBr, cm-1) mmax: 3239, 2961, 2857, 1621, 1601,
1500, 1255, 1194,1112; ESI–MS m/z: 311 (M??1);
(Found C 73.4, H 7.3, N 9.2. C19H22N2O2 calcd.: C 73.6, H
7.1, N 9.%).
Experimental
Melting points were determined on a Yanaco MP-500
micro-melting point apparatus and are uncorrected. IR
spectra were recorded on a Nicolet-1705X spectrometer.
1H NMR spectra were recorded on a Bruker AC-200 MHz
spectrometer using Me4Si as internal standard and CDCl3
as solvent. Mass spectra were obtained on a Finnigan LCQ
spectrometer. Metal contents were measured using an
IRIS-Advantage ICP emission spectrometer. The halogen
content was measured using the mercury titration method
[19, 20]. Other elemental analyses were performed on a
Carlo Erba 1106 elemental analyser. Molar conductances
were obtained on a DDS-11A conductivity meter in DMF
solutions (1.0 9 10-3 mol L-1). Molar Magnetic Suscep-
tibilities were obtained on a T3-200 magnetic balance at
25 °C.
Schiff base HL5
HL5 was prepared as described for HL4 except for the use of
4-chloroaniline instead of p-toluidine to give yellow solid,
1
yield 74%, m.p.144–145 °C. H NMR d (p.p.m.): 13.08
(s, 1H, OH, D2O exchangeable), 8.43 (s, 1H, N=CH),
7.52–6.88 (m, 7H, ArH), 3.70–3.54 (m, 6H, OCH2,
NCH2Ar), 2.82 (t, J = 5.3 Hz, 4H, NCH2). I.r. (KBr, cm-1
)
m
max: 3245, 2932, 2857, 1623, 1600, 1502, 1254, 1195,
1116; ESI–MS m/z: 332 (M??1); (Found C 65.5, H 5.6, N
8.3, Cl 10.6. C18H19N2O2Cl calcd.: C 65.4, H 5.8, N 8.5, Cl
10.7%).
The Schiff bases HL1–HL3 were synthesized according to
the literature [21], as was N-(3-formyl-4-hydro-xybenzyl)
morpholine [22]. All other analytical grade reagents were
purchased within China and used without further
purification.
Schiff base HL6
HL6 was prepared as described for HL4 except for the use
of p-nitroaniline instead of p-toluidine to give yellow solid,
yield 74%, m.p.162–164 °C. 1H NMR d (p.p.m.): 13.01 (s,
1H, OH, D2O exchangeable), 8.41 (s, 1H, N=CH), 7.57–
6.84 (m, 7H, ArH), 3.73–3.58 (m, 6H, OCH2, NCH2Ar),
Synthesis of Schiff bases HL4–HL6
2.84 (t, J = 5.2 Hz, 4H, NCH2). I.r. (KBr, cm-1) mmax
:
Schiff base HL4
3239, 2930, 2860, 1624, 1601, 1500, 1253, 1192, 1118;
ESI–MS m/z: 342 (M??1); (Found C 63.5, H 5.4, N 12.1.
C18H19N3O4 calcd.: C 63.3, H 5.6, N 12.3%).
A solution of N-(3-formyl-4-hydroxybenzyl)morpholine
(1.77 g, 8 mmol) and p-toluidine (0.87 g, 8 mmol) in
123

Transition Met Chem (2010) 35:463–468
465
Co 8.2.%. Km (S cm2 mol-1): 11.45, vM = 6.79 9 10-2
General method for preparation of the complexes
J mol-1 T-2, lm = 3.73 9 10-23 J T-1
.
A solution of the appropriate Schiff base (2.0 mmol) and
Co(AcO)2ꢀ4H2O (1.1 mmol) in EtOH (15 cm3) was stirred
for 2 h under a N2 atmosphere at 70 °C, and then cooled to
room temperature. The precipitate was filtered off and
washed with EtOH to give Schiff base Co(II) complex.
The pure product was obtained after recrystallization
from EtOH.
CoL62
Purple, 76% yield, m.p. 164–166 °C; I.r. (KBr, cm-1) mmax
:
2932, 2859, 1609, 1600, 1500, 1256, 1195, 1116; ESI–MS
m/z: 740 (M??1); (Found C 58.3, H 4.7, N 11.2, Co 7.8.
C36H36N6O8Co calcd.: C 58.5, H 4.9, N 11.4, Co 8.0%).
Km (S cm2 mol-1): 12.44, vM = 6.87 9 10-2 J mol-1 T-2
,
CoL12
lm = 3.75 9 10-23 J T-1
.
Purple, 84% yield, m.p. 197–199 °C; I.r. (KBr, cm-1) mmax
:
Oxygen uptake studies
2965, 2859, 1609, 1600, 1501, 1254, 1131, 1049; ESI–MS
m/z: 1038 (M??1); (Found C 67.2, H 6.2, N 5.4, Co 5. 8.
C58H66N4O10Co calcd.: C 67.1, H 6.4, N 5.4, Co 5.7%). Km
(S cm2 mol-1): 9.82. Molar magnetic susceptibility
The values of KO2 for complexes CoL12–CoL26 were
determined from the volumes of dioxygen absorbed at
equilibrium, as described in the literature [8]; these
experiments used diglyme (saturated with O2) as solvent,
plus 1.0 mol dm-3 of pyridine as axial ligand (B), complex
concentration 5 9 10-3 mol.dm-3, 9.7 9 104 Pa of O2
partial pressure. The equilibrium constant (KO2) was cal-
culated as follows:
vM = 7.19 9 10-2 J mol-1 T-2, magnetic moment lm
=
3.84 9 10-23 J T-1
.
CoL22
Purple, 84% yield, m.p. 207–209 °C; I.r. (KBr, cm-1) mmax
:
CoL þ B ꢀ CoLB
CoLB þ O2 ꢀ CoLBO2
2930, 2860, 1608, 1601, 1500, 1253, 1126, 1050; ESI–MS
m/z: 1079 (M??1); (Found C 62.2, H 5.6, N 5.4, Cl 6.5,
Co 5.6. C56H60N4O10Cl2Co calcd.: C 62.3, H 5.8, N 5.2,
½CoLBO2ꢁ
Cl 6.6, Co 5.5%). Km (S cm2 mol-1): 10.32, vM
=
KO2 ¼
½CoLBꢁ PO2
6.61 9 10-2 J mol-1 T-2, lm = 3.68 9 10-23 J T-1
.
where the complexes are expressed in terms of molarities,
and the dioxygen concentration is expressed as partial
pressure (PO2) in Torr. Thermodynamic parameters DH0
and DS0 for oxygenation were determined from variation
of KO2 over a range of temperatures. DH0 and DS0 were
calculated from the equations DG0 = -RTln KO2 and
CoL32
Purple, 82% yield, m.p. 189–191 °C; I.r. (KBr, cm-1) mmax
:
2930, 2861, 1609, 1600, 1501, 1251, 1127, 1048; ESI–MS
m/z: 1100 (M??1); (Found C 61.4, H 5.5, N 7.5, Co 5. 5.
C56H60N6O14Co calcd.: C 61.2, H 5.5, N 7.6, Co 5.4%). Km
(S cm2 mol-1): 10.75, vM = 7.08 9 10-2 J mol-1 T-2, lm =
3.81 9 10-23 J T-1.
0
TDS0 = DH - DG0.
Results and discussion
CoL42
Compared with the i.r. spectra of the free Schiff bases,
those of Co(II) complexes, CoL12–CoL26, were almost at the
same frequencies, except for the C=N stretching vibration
which was shifted 12–16 cm-1 to lower frequency and also
showed greater intensity compared to the free imine. The
absence of an OH stretching vibration (*3440 cm-1) in
the complexes indicated deprotonation of OH in the Schiff
base upon complex formation, suggesting the formation of
a Co–O coordination bond. The C–O stretching vibrations
in the crown ether ring for the complexes were at almost
the same frequencies as for the free Schiff bases. These
facts suggest that the cobalt atom only interacts with the
Ar–OH and CH=N groups [23]. The observed molar con-
ductances of the complexes in DMF solution
Purple, 79% yield, m.p. 192–194 °C; I.r. (KBr, cm-1) mmax
:
2962, 2855,1609,1600,1502, 1253, 1196, 1116; ESI–MS
m/z: 678 (M??1); (Found C 67.5, H 6.4, N 8.4, Co 8.9.
C38H42N4O4Co calcd.: C 67.4, H 6.2, N 8.3, Co 8.7%). Km
(S cm2 mol-1): 8.95, vM = 6.68 9 10-2 J mol-1 T-2, lm =
3.70 9 10-23 J T-1
.
CoL52
Purple, 78% yield, m.p. [ 270 °C; I.r. (KBr, cm-1) mmax
:
2930, 2854, 1611, 1600, 1502, 1252, 1197, 1114; ESI–MS
m/z: 719 (M??1); (Found C 60.0, H 5.2, N 8.0, Cl 9.7, Co
8.4. C36H36N4O4Cl2Co calcd.: C 60.2, H 5.0, N 7.8, Cl 9.9,
123

466
Transition Met Chem (2010) 35:463–468
(1.0 9 10-3 mol L-1) at 25 °C showed that they are non-
electrolytes [24]. The molar magnetic susceptibility vM and
magnetic moment lm values indicated that the cobalt has
three non-paired electrons in all of these complexes and is
therefore divalent. The ESI–MS mass spectra and ele-
mental analysis of the complexes indicated that HL1–HL6
all form complexes of stoichiometry CoL2. We therefore
suggest a structural formula for these Schiff base cobalt
complexes as shown in Fig. 1.
Dioxygen affinities of the complexes
The equilibrium constants (KO2) and thermodynamic param-
eters DH0 and DS0 for oxygenation of complexes CoL21–CoL62
Table 1 Equilibrium constants
and thermodynamic parameters
for oxygenation of CoL2
Complex
CoL12
B
T (°C)
Ions added
ln KO2 (mm-1
)
DH0 (kJ mol-1
)
DS0 (J K-1mol-1
)
Py
-5
0
/
-3.53
-3.79
-4.28
-4.96
-4.14
-3.21
-4.08
-3.68
-3.92
-4.37
-4.99
-4.31
-3.51
-4.25
-3.76
-3.99
-4.43
-5.04
-4.35
-3.61
-4.75
-5.31
-5.51
-5.88
-6.39
-5.78
-5.70
-5.76
-5.34
-5.53
-5.89
-6.39
-5.83
-5.76
-5.80
-5.40
-5.59
-5.94
-6.42
-5.86
-5.82
-5.85
-31.63
-147.36
/
10
25
10
10
10
-5
0
/
/
LiNO3
NaNO3
KNO3
CoL22
CoL32
CoL42
CoL52
CoL62
Py
Py
Py
Py
Py
/
-29.03
-28.25
-23.98
-23.20
-22.68
-138.92
-136.65
-133.64
-130.96
-129.55
/
10
25
10
10
10
-5
0
/
/
LiNO3
NaNO3
KNO3
/
/
10
25
10
10
10
-5
0
/
/
LiNO3
NaNO3
KNO3
/
/
10
25
10
10
10
-5
0
/
/
LiNO3
NaNO3
KNO3
/
/
10
25
10
10
10
-5
0
/
/
LiNO3
NaNO3
KNO3
/
/
10
25
10
10
10
/
/
LiNO3
NaNO3
KNO3
123

Transition Met Chem (2010) 35:463–468
467
CoL12
CoL22
CoL32
CoL42
CoL52
CoL62
are listed in Table 1. The data indicate that the dioxygen
affinities of these complexes are influenced greatly by both
the temperature and the structure of the Schiff base ligand.
Higher temperature results in a smaller oxygenation con-
stant. This may be due to the reduced solubility of O2 at
higher temperatures. Figure 2 shows the effect of tempera-
ture on the oxygen uptake of CoL12, which is a representative
example of oxygen uptake by these complexes at different
temperatures. As shown in Fig. 2, the maximum oxygen
uptakes of CoL12 are 0.83, 0.79, 0.75 and 0.69 at -5, 0, 10 and
25 °C, respectively. This suggests that the complex forms a
limiting 1:1 adduct with O2 [25], similar to other reported
examples [9]. A plot of lnKO2 versus 1/T is shown in Fig. 3.
There is a good linear relationship (R2 [0.99), which sug-
gests that the 1:1 adducts were almost completely formed
[26]. The dioxygen affinities of the complexes decrease in
the order CoL12 [ CoL22 [ CoL32 or CoL24 [ CoL25 [ CoL62
under the same conditions. The results can be attributed to
the effects of substituent, which could include both steric and
electronic effects. Although the steric effect seems likely to
be the more important, the electronic effect of electron-
donating groups can also enhance the electron density of the
cobalt atom and so facilitate the formation of the Co–O2
complex [8, 27]. Therefore, the dioxygen affinities of the
complexes which are substituted by an electron-donating
group (–CH3) on the aromatic ring are higher than those
containing electron-withdrawing groups (–Cl or –NO2). The
three complexes CoL21, CoL22 and CoL32 containing an aza-
crown pendant group show much more enhanced dioxygen
affinities than CoL24, CoL25 and CoL62 which possess a mor-
pholino pendant. This observation can be attributed to the
macrocycle effect of the crown rather than the electron-
donating effect of the nitrogen methylene (NCH2) on the
-3.5
-4.0
-5.5
-6.0
3.56
3.60
3.64
3.68
3.72
-3
1/T (10 K-1)
Fig. 3 The plot of lnKO2 versus 1/T at the range of -5 to ?10 °C
aromatic ring of the ligand, because the crown ring offers a
hydrophobic microenvironment and probably favours the
oxygen molecule’s approach to the central cobalt atom [28].
In addition, the aza-crown pendants offer larger steric hin-
drance around these Schiff base complexes than the mor-
pholino pendants. This may help to prevent the aza-crown
pendant complexes from dimerizing and so losing activity.
On comparing with other, Schiff base Co(II) complexes
reported in the literature [29], we note that the dioxygen
affinities of these complexes are also influenced by the
position of aromatic substituent. The oxygenation constants
of Schiff base complexes of 3-substituted groups (benzo-10-
aza-crown ether or morpholino) are bigger than those of 5-
substituted groups. Although the steric hindrance of 3-sub-
stituents is larger than that of 5-substituents, the hydrophobic
microenvironment of the complexes of 3-substituted groups
is likely to be superior to that of 5-substituted groups, since
the 3-substituted benzo-10-aza-crown ring is closer to the
metal coordination centre compared to the 5-substituted
analogue. These results indirectly illustrate that the dioxygen
affinities of these complexes are not controlled by a single
factor but by the integrated effect of several factors.
1.0
0.8
0.6
°
25 C
It is interesting that the addition of alkali metal salts
(metal nitrate/CoL2 = 2:1) to the reaction system can
enhance the O2-binding capability of CoL12, CoL22 and CoL23
but not of CoL42, CoL25 and CoL62. This suggests that the
alkali metal cations improve the O2-binding capability only
when they are coordinated within the crown ring of the
complexes. Table 1 indicates that the KO2 of complexes
containing aza-crown pendants is significantly enhanced by
adding excess NaNO3. However, the enhancement obtained
by adding an equal quantity of LiNO3 or KNO3 is very
°
10 C
°
0.4
0.2
0.0
0 C
°
-5 C
0
5
10
15
20
t /min
Fig. 2 The effect of temperature on oxygenation performance of
CoL12 n(O2)/n(CoL21) means mol O2 absorbed per mol of complex
CoL12, maximum oxygen uptake means mol O2 absorbed per mol of
complex CoL12 at oxygenation equilibrium
limited. This may be because the Li? ion (d = 1.36 A) is
˚
too small to match the cavity size of aza-15-crown-5
123

468
Transition Met Chem (2010) 35:463–468
?
(d = 1.7–2.2 A), whereas the K ion (d = 2.66 A) is too
˚
˚
3. Busch DH, Alcock NW (1994) Chem Rev 94:585
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Conclusions
Six Schiff base Co(II) complexes with aza-crown or mor-
pholino pendants were synthesized. The presence of a
pendant crown ether group in the Schiff base ligand sig-
nificantly improves the O2-binding capabilities of the
complexes. The addition of Na? cations enhances the O2-
binding capability of CoL12, CoL22 and CoL23 but not of
CoL42, CoL25 and CoL62. These results are explained in terms
of the macrocyclic effect of the benzoaza-crown ether ring,
which enlarges the steric hindrance around the Co(II)
centre, and so prevents the complexes form being oxidized
to the l-oxodimer and losing activity. The dioxygen
affinity of these complexes is influenced by several factors,
including the temperature, the electronic proprieties of the
substituent on the aromatic ring of the Schiff base ligand
and the macrocyclic effect of the crown ring.
17. Zeng W, Li JZ, Qin SY (2006) Inorg Chem Commun 9:10
18. Li JZ, Wang Y, Zeng W, Qin SY (2008) Supramol Chem 20:249
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20. Lalaneette RA (1972) Microchem J 17:665
21. Li JZ, Yan Y, Wei XY, Zhou B, Qin SY (2006) J Chem Res
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22. Wei XY, Li JZ, Zhou B, Qin SY (2004) Trans Met Chem 29:457
23. Zeng W, Li JZ, Mao ZH, Hong Z, Qin SY (2004) Adv Synth
Catal 346:1385
24. Geary WJ (1971) Coord Chem Rev 7:81
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Acknowledgments The authors gratefully acknowledge financial
support from the China National Natural Science Foundation (No:
20072025) and Key Project of Sichuan Province Education Office
(No: 2005D007).
28. Lu XX, Qin SY, Zhou ZY, Yam VW (2003) Inorg Chim Acta
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