XPS and Chiroptical Studies on Lanthanide(III) Complexes of Hydroxylamine and Sodium D-Camphor-β-sulfonate

Article abstract of DOI:10.1081/SIM-120026546

Six new chiral complexes LnL₃ (Ln = La(III), Pr(III), Nd(III), Sm(III), Eu(III) and Gd(III)) with an optically active Schiff base ligand (HL) derived from sodium D-camphor-β-sulfonate and hydroxylamine were synthesized and characterized by elemental analyses, molar conductance, IR, XPS (X-ray photoelectron spectroscopy), electronic spectra, ¹H NMR, magnetic susceptibility measurements and thermal methods. It was found that the complexes behave as non-electrolytes in CH₃OH and in DMSO and possess the stoichiometric ratio of Ln:C:N:O:S = 0.33:10.1:1.0:4.0:1.0 which is consistent with the analytical data. Their dominant conformers were determined by CD spectra. A six-coordinated model is proposed for these complexes.

Full text of DOI:10.1081/SIM-120026546

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 10, pp. 1763–1774, 2003
XPS and Chiroptical Studies on Lanthanide(III)
Complexes of Hydroxylamine and Sodium
D-Camphor- -sulfonate
1
H.-Zh. Ma, B. Wang, and Q.-Zh. Shi
,2
1,*
2
1
College of Chemistry and Materials Science, Shaanxi Normal
University, Xian, P.R. China
2
Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department
of Chemistry, Northwest University, Xian, P.R. China
ABSTRACT
3
Six new chiral complexes LnL (Ln = La(III), Pr(III), Nd(III), Sm(III),
Eu(III) and Gd(III)) with an optically active Schiff base ligand (HL)
derived from sodium D-camphor-b-sulfonate and hydroxylamine were
synthesized and characterized by elemental analyses, molar conductance,
1
H
IR, XPS (X-ray photoelectron spectroscopy), electronic spectra,
NMR, magnetic susceptibility measurements and thermal methods. It
was found that the complexes behave as non-electrolytes in CH OH and
in DMSO and possess the stoichiometric ratio of Ln:C:N:O:S =
.33:10.1:1.0:4.0:1.0 which is consistent with the analytical data. Their
3
0
*
Correspondence: B. Wang, College of Chemistry and Materials Science, Shaanxi
Normal University, 710062 Xian, P.R. China; E-mails: wangbo@snnu.edu.cn,
wangbo@nwu.edu.cn.
1
763
DOI: 10.1081/SIM-120026546
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

1
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Ma, Wang, and Shi
dominant conformers were determined by CD spectra. A six-coordinated
model is proposed for these complexes.
Key Words: Chiral; D-Camphor-b-sulfonate; Hydroxylamine;
Non-electrolytes; CD spectra.
INTRODUCTION
Many Schiff bases and their metal complexes have been synthesized
[4,5]
[1–3]
and attracted much attention.
In our earlier papers,
we reported the
synthesis and properties of the chiral lanthanide complexes of D-camphor-
b-sulfonic acid, but no lanthanide optically active Schiff base complexes
derived from D-camphor-b-sulfonic acid and hydroxylamine have been
reported so far. In continuation of our study on chiral complexes of lan-
thanides, we report here the synthesis and characterization of some com-
plexes of this ligand (HL, Figure 1).
EXPERIMENTAL
Reagents and Solvents
All chemicals and solvents were analytically pure and used without
further purification. Sodium D-camphor-b-sulfonate was purchased from
Merck and used without further purification. Lanthanide chlorides were
prepared as follow: the requisite amount of Ln O was dissolved in a
2
3
minimum amount of conc. HCl. The solution was concentrated by evapo-
ration and the process was repeated thrice. Finally, the residue was dis-
solved in H O (5 mL) before use.
2
Preparation of NaL
To a solution of sodium D-camphor-b-sulfonate (5.0 g, 17.2 mmol)
in 50 mL methanol was added hydroxylamine hydrochloride (1.2 g,
Figure 1. Structure of HL.

Complexes of D-Camphor- -sulfonate
1765
17.2 mmol). The resulting solution was treated with 50% NaOH solution
under stirring until the pH was adjusted to 5.0 and refluxed for 1 h. The
yellowish precipitate thus obtained was separated by vacuum filtration
and recrystallized from anhydrous methanol and finally dried at 110 ꢀC
2
0
for 1 h, yield 4.5 g (85%): m.p. 252–254 ꢀC, [a]
molꢀL , DMSO).
= + 84–86ꢀ (c = 1.5
D
–
1
Preparation of Complexes
A solution of NaL (0.81 g, 3 mmol) and the lanthanide(III) chloride (1
mmol) in 40 mL anhydrous ethanol was refluxed in a water bath for 1 h.
After cooling to room temperature a precipitate was formed. The solid was
collected by vacuum filtration and purified by column chromatography
(
Al O , acetone/EtOH [1/3 (v/v) as eluent]. The solvent was then evapo-
2 3
rated under reduced pressure and the product dried at 110 ꢀC for 1 h, yields
8–90%.
7
Physical Measurements
The C, H, N and S contents were determined microanalytically on a
Perkin–Elmer 240C elemental analyzer. The lanthanide content was deter-
mined volumetrically by EDTA titration using xylenol orange as an indi-
cator. The molar conductance of the complexes in MeOH and in DMSO at
0.001 mol/L concentration was measured with a Shanghai DDS-IIA molar
conductor at 25 ꢀC. Room-temperature magnetic susceptibility measure-
ments were made by the Guoy method using Hg[Co(CNS) ] as calibrant.
Diamagnetic corrections were calculated from Pascal’s constants. Thermal
analyses were made under nitrogen in the range from room temperature and
4
800 ꢀC using a Thermoflex Q-1500D thermal analyzer. Electronic spectra of
DMSO solutions of the complexes in the 200–700 nm region were recorded
on a Shimazu 240 spectrophotometer. IR spectra (CsI pellets) were obtained
–
1
on a Nicolet-5DX spectrophotometer in the 200–4000 cm region. XPS was
taken on a Perkin–Elmer PHI-550 X-ray photoelectron spectrophotometer
using Mg-K radiation. The samples were finely pulverized and spread on a
a
gold holder as thin films. The background pressure of residual gases during
–
6
the measurements was lower than 10 Pa. The hemispherical electrostatic
analyzer was operated in the retarding mode with a pass energy of 20 eV.
The accuracy of the measured binding energy (BE) is ± 0.1 eV. The peak
areas for semiquantitative analyses were determined using a linear back-
ground. The C1s electron BE in benzene was used as an internal standard,
1
BE = 284.6 eV. The H NMR spectra were measured on a Bruker AC 80
spectrometer using DMSO-d as solvent. The chiral properties of the com-
6
plexes were investigated by circular dichroism (CD) spectra at 20 ꢀC on a

1
766
Ma, Wang, and Shi
Cary model 60 spectropolarimeter with a CD model 6001 accessary. The
sample cell was 1 cm and the slit was programmed for a spectral band width
of 1.5 nm. Cut-off was indicated when the dynode voltage reached 400 V.
Spectral measurements began at 200 nm, and the molar ellipticity ([q])
values were adjusted to an enantiomeric excesses of 100%.
RESULTS AND DISCUSSION
The synthesis of the salt of the ligand is given in Figure 2.
The formation of the complexes occurs by the following representa-
tive equation:
EtOH
3
NaL þ LnCl ꢀxH O ÀÀ ! LnL þ 3NaCl þ xH O
3
2
3
2
Physical and analytical data for the trivalent lanthanide complexes are
given in Table 1. The complexes are air-stable and soluble in CH OH,
3
chloroform, diethyl ether, EtOH, acetone, DMSO and dimethylformamide.
LM values in methanol and in DMSO indicate that the complexes behave
as non-electrolytes in solution. But the LM values in methanol are rela-
tively higher than in DMSO, and it is found that the LM values are
constant with time at room temperature. This phenomenon indicates that
the coordinated sphere or the coordination bond is stable in polar solvents
and no replacement reactions occured. The thermal analyses revealed that
all complexes are anhydrous.
Infrared Spectra
The main IR bands with their tentative assignments are listed in
–
1
Table 2. The bands at 3380–3250 cm for the salt of the ligand and its
complexes due to OH stretching frequencies remain almost unchanged,
which indicates that the OH group is not coordinated to the lanthanide(III)
–
1
ion. The salt of the Schiff base ligand exhibits a sharp band at 1640 cm
due to n(C=N). On complexation, this band shifts to a lower energy by
Figure 2. Scheme for the synthesis of the sodium salt of the ligand (NaL).

Complexes of D-Camphor- -sulfonate
1767

1
768
Ma, Wang, and Shi
–
Table 2. Principal IR bands of the ligand and the complexes (cm ).
1 a
n
(C=N) (–SO
n
as
n
s
Dn
–) (–SO
n
–) (Ln–O) (Ln–N)
n
Compounds n(OH)
3
–) (–SO
3
3
NaL
LaL3
3365 br 1640 s 1185 s
3358 br 1600 m 1211 s
1050 s
1051 m
3365 br 1602 m 1207 m 1052 m
135
160
155
160
155
155
153
515 m 320 m
510 w
PrL
NdL
3
325 m
518 m 324 w
514 m 318 w
3
3360 br 1603 m 1210 s
3366 br 1602 m 1200 s
3372 br 1595 m 1206 s
3370 br 1600 m 1200 s
1050 m
1045 w
1051 m
1047 m
SmL3
EuL3
GdL
520 w
513 w
322 w
321 w
3
a
s = sharp, m = medium, br = broad, w = weak.
–
1
3
8–45 cm , which might be due to a reduction in the electron density in
the azomethine group owing to coordination of the nitrogen atom to the
[
6]
metal ion. The nas and n modes of the –SO – group appear at 1200–
s
3
–
1
–1
1
211 and 1045–1052 cm , respectively, and the Dn value (155 cm ) is
consistent with unidentate coordination. Furthermore, several new bands
–
1
were observed at ca. 510 and 320 cm , some may be tentatively assigned
[7]
to n(Ln–O) and n(Ln–N) vibrations. Thus, it is apparent that the ligand
is attached to the metal ion by two coordination sites involving the sulfo-
nate oxygen and the azomethine nitrogen. The valence requirement of the
metal ion and the infrared data indicate that the three molecules of the
ligands are attached to the lanthanide(III) ion in a symmetric form.
Magnetic Moments and Electronic Spectral Studies
The paramagnetic behavior of the lanthanide(III) ion is consistent with
the presence of unpaired electrons. The room temperature magnetic moments
[
8]
of the complexes in Table 1 show little deviation from the literature values
1
/2
or values calculated using the equation: meff = x [J(J + 1)] b, thereby indi-
cating negligible participation of 4f electrons in bond formation.
The electronic spectra of the complexes are similar, but are different
from that of the salt of the ligand (Table 1). A strong peak was observed at
ca. 275 nm attributed to p–p* transition of the group –C=N. Compared with
the salt of the ligand, a blue-shift of about 7–16 nm was observed, which
[7]
indicates bond formation between the lanthanide(III) ion and the ligand.
XPS Spectra
The results of the XPS semi-quantitative analyses of the complexes are
in agreement with the elemental analysis data (Table 3, Figure 3). The N1s

Complexes of D-Camphor- -sulfonate
1769

1
770
Ma, Wang, and Shi
3
Figure 3. XPS spectra of NaL (1) and the complex EuL (2).
electron spectra of the complexes are symmetric and their deconvolution
revealed that one band was observed at ca. 401.5 eV which is related to
the salt of the ligand at 399.6 eV and is due to the C=N group. It was
found that after coordination the N1s BE value of C=N group shifted up
about 1.2 eV which indicates that the nitrogen atom in the C=N group
coordinates to the central metal ion and then leads to a decrease of the
electron density on the nitrogen atom, and its strength ratio is close to 3:1
which indicates the three coordinated ligands are in an identical chemical
environment. Another piece of information supporting the coordination of
the nitrogen atom in the C=N group is the C1s (C=N) BE which shows a
small shift from 281.3 to 281.8 eV in the complexes compared with that of
the salt of the ligand, indicating that the azomethine nitrogen is coordinated
to the metal ion. The S2p spectrum is also symmetric and bands at 165.5
eV for the salt of the ligand and at 165.4 eV for the complexes were
observed which can be assigned to the sulfur atom of the SO group. A fact
3
that the S2p binding energy remained unchanged indicates that the sulfur
atom in the SO group does not coordinate to the lanthanide ion. The O1s
3
electron spectra of the complexes were in an asymmetric form and three
bands were observed at ca. 533.3, 532.2 and 530.4 eV. In comparison, the
two asymmetric bands of the salt of the ligand at 533.0 and 530.5 eV

Complexes of D-Camphor- -sulfonate
1771
assigned to the SO3 group and OH group indicate that three chemical
environments arose from coordination. In view of the relative strength of
the bands at 532.2 eV and 530.4 eV (2:1), it can be deduced that the SO3
group coordinates to the metal ion through an oxygen atom in a unidentate
form and the OH group does not coordinate to lanthanide ion due to the
O1s BE value for the OH group (530.5eV) almost unchanged. On the
basis of the above discussions it may be concluded that the three ligands
coordinate to the lanthanide ion in a chemically identical coordination
mode. The atomic molar percentages in the complexes have also been
examined. It was found that the complexes possess the stoichiometric ratio
of Ln:C:N:O:S = 0.33:10.1:1.0:4.0: 1.0, which is consistent with the anal-
yses data.
1
H NMR Spectra
1
The H NMR spectra of the complexes show that only one set of
signals was observed for the hydrogen of the N–OH group and the hy-
drogen signal is shielded relative to the salt of the ligand by ca. 0.3–0.55
ppm (1H, s) (Table 4). Most of the other types of hydrogens (e.g., –CH3,
–CH2 , –CH< ) are also shielded to a varying extent. Although coordi-
–
nation usually involves deshielding of ligand hydrogens (especially of
coordinating groups) due to a decrease of electron density, a rigid chelate
1
Table 4. H NMR data of NaL and the complexes (DMSO-d
6
).
Compounds
NaL
Chemical shift (d, ppm)
12.2 (s, –SO
3
H, 1H), 9.82 (s, N–OH, 1H), 1.71
(
(
2
s, –CH< , 1H), 1.22 (m, J = 1.2Hz, –CH –, 8H), 0.83–1.15
t, J = 5.5Hz, –CH , 6H)
3
LaL
3
9.52 (s, N–OH, 1H), 1.68 (s, –CH< , 1H), 1.21
m, J = 1.1Hz, –CH –, 8H), 0.80–1.13 (t, J = 5.8Hz, –CH
9.50 (s, N–OH, 1H), 1.71 (s, –CH< , 1H), 1.20 (m, J = 1.3Hz,
CH –, 8H), 0.82–1.15 (t, J = 5.7Hz, –CH , 6H)
9.27(s, N–OH, 1H), 1.70 (s, –CH< , 1H), 1.21 (m, J = 1.5Hz,
CH –, 8H), 0.81–1.09 (t, J = 6.0Hz, –CH , 6H)
(
2
3
, 6H)
PrL3
–
2
3
NdL
3
–
2
3
SmL
3
9.32 (s, N–OH, 1H), 1.74 (s, –CH< , 1H), 1.23 (m, J = 1.2Hz,
CH –, 8H), 0.82–1.13 (t, J = 6.1Hz, –CH , 6H)
9.42 (s, N–OH, 1H), 1.70 (s, –CH< , 1H), 1.21 (m, J = 1.3Hz,
CH –, 8H), 0.82–1.11 (t, J = 6.0Hz, –CH , 6H)
–
2
3
EuL3
GdL
–
2
3
3
9.58 (s, N–OH, 1H), 1.72 (s, –CH < , 1H), 1.22 (m, J = 1.2Hz,
CH –, 8H), 0.80–1.14 (t, J = 5.9Hz, –CH , 6H)
–
2
3

1
772
Ma, Wang, and Shi
Table 5. CD spectra data of NaL and the complexes.
2
D
d
0
Conc.
(DMSO, molꢀL
l
max
(nm)
[a]
(ꢀ)
–
1
a
–3
b
c
Compounds
)
[q] ꢁ 10
De
NaL
LaL3
1.5
1.6
1.5
1.5
1.4
1.6
1.5
8.21
7.1
7.0
7.3
6.6
7.2
7.0
2.6
2.1
2.4
2.4
2.0
2.3
2.1
282
285
280
278
269
270
282
+ 84–86
+ 162.4
+ 168.6
+ 164.3
+ 144.8
+ 139.8
+ 182.2
PrL
NdL
3
3
SmL3
EuL3
GdL
3
a
q = molar ellipticity value.
e = molar extinction coefficient.
b
c
lmax = the maximum absorption wavelength.
d
a = specific rotatory degree.
structure (here octahedral) can cause anisotropic magnetic effects leading
to the observed shielding.
[9]
CD Spectra
A distinct Cotton effect in the CD spectra of the complexes was ob-
served (Table 5, Figure 4). The peak at ca.280 nm of the complexes is
diagnostic of the preferred conformation. In terms of the camphor chirality
rule, the conformer with the (S)-absolute configuration is dominant for
dichroic absorption similar to a positive effect in the salt of the ligand. The
Figure 4. CD spectra of the salt of the ligand and the complexes.

Complexes of D-Camphor- -sulfonate
1773
3
Figure 5. Suggested bonding model of LnL .
overall electronic situation is much the same in all these cases, as in-
dicated by the almost identical electronic spectra, and the complexes show
[10]
a smaller Cotton effect than the ligand.
CONCLUSION
On the basis of the above discussions, it is found that in the complexes
of the type LnL , the ligand is attached to the metal ion at two coordination
3
sites involving sulfonate oxygen and azomethine nitrogen. The three ligands
are coordinated to the lanthanide metal ion in a symmetry form and an
octahedral structure is proposed for these complexes (Figure 5).
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2
3
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6.
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9
Moeller, T. Comprehensive Inorganic Chemistry; Pergamon Press:
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Received May 26, 2002
Accepted July 2, 2003
Referee I: G. J. Grant
Referee II: E. A. Possum

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