Lanthanide complexes with diethyl(2-oxopropyl) phosphonate and diethyl(2-oxo-2-phenylethyl) phosphonate ligands

Article abstract of DOI:10.1016/j.jallcom.2007.04.155

Ln³⁺ complexes with the phosphonate ligands: diethyl(2-oxopropyl) phosphonate [CH₃C(O)CH₂P(O)(OC₂H₅)₂] (OPP) and diethyl (2-oxo-2-phenylethyl) phosphonate [C₆H₅C(O)CH₂P(O)(OC₂H₅)₂] (DEPP) were synthesized and characterized. Complexation studies and spectral characterizations are performed. Based on the Nd³⁺ absorption spectra, using computer-assisted target factor analysis, the stability constants of Nd-OPP complexes in solution were determined to be log β₁₁ = 2.8 ± 0.4 and log β₁₂ = 5.0 ± 0.7. Luminescence intensities, lifetimes, τ, and quantum yields, Φ, of the Eu³⁺ and Tb³⁺ ions in the complexes were measured. The most intense luminescence was observed in the case of the Tb/DEPP system. Emission enhancements were observed for Tb³⁺ and Eu³⁺ complexes in acetonitrile solutions, which occurred as a result of sensitized luminescence. These observed luminescence intensities were over two orders of magnitude larger than those for the uncomplexed Ln(III) ions. The complexes of Ln/L/NO₃, obtained in oil forms from ethanol at room temperature, were characterized by elemental analysis, IR spectroscopy, and luminescence lifetime measurements. The composition of the complexes studied is suggested to be LnL₂(NO₃)₃, with monodentate coordination by the phosphoryl group.

Full text of DOI:10.1016/j.jallcom.2007.04.155

Journal of Alloys and Compounds 451 (2008) 395–399
Lanthanide complexes with diethyl(2-oxopropyl) phosphonate
and diethyl(2-oxo-2-phenylethyl) phosphonate ligands
a,∗
a
a
b
Z. Hnatejko , S. Lis , G. Pawlicki , G. Meinrath
a
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, Pozna n´ 60-780, Poland
b
RER Consultants, Schiessstattweg 3a, D-94032 Passau, Germany
Available online 19 April 2007
Abstract
Ln3+ complexes with the phosphonate ligands: diethyl(2-oxopropyl) phosphonate [CH
3
2 2 5 2
C(O)CH P(O)(OC H ) ] (OPP) and diethyl (2-oxo-
2
-phenylethyl) phosphonate [C C(O)CH P(O)(OC ] (DEPP) were synthesized and characterized. Complexation studies and spectral
6
H
5
2
2 5 2
H )
3
+
characterizations are performed. Based on the Nd absorption spectra, using computer-assisted target factor analysis, the stability constants
of Nd-OPP complexes in solution were determined to be log β11 = 2.8 ± 0.4 and log β12 = 5.0 ± 0.7. Luminescence intensities, lifetimes, τ, and
3
+
3+
quantum yields, Φ, of the Eu and Tb ions in the complexes were measured. The most intense luminescence was observed in the case of the
3
+
3+
Tb/DEPP system. Emission enhancements were observed for Tb and Eu complexes in acetonitrile solutions, which occurred as a result of
sensitized luminescence. These observed luminescence intensities were over two orders of magnitude larger than those for the uncomplexed Ln(III)
ions.
The complexes of Ln/L/NO , obtained in oil forms from ethanol at room temperature, were characterized by elemental analysis, IR spectroscopy,
3
and luminescence lifetime measurements. The composition of the complexes studied is suggested to be LnL
by the phosphoryl group.
2 3 3
(NO ) , with monodentate coordination
©
2007 Elsevier B.V. All rights reserved.
Keywords: Absorbance; Lanthanide ions; Luminescence; Phosphonate ligands
1
. Introduction
stable compositions as LnL2(NO3)3 were isolated for the lan-
thanides series La–Eu. [10].
Phosphonates are important in biology and in living sys-
The purpose of this work is to characterize the complexes of
the OPP and DEPP ligands with lanthanide nitrates in acetoni-
trile solution and in oil forms, using complexation studies and
spectroscopic characterizations.
tems as effective complexing agents [1]. Their complexes with
3
+
Ln ions have been extensively investigated with regard to
their structures and their various practical applications [2–4].
A series of lanthanide phosphonate complexes, containing P O
and O C–N groups, closely related to ␤-diketones, have been
used for the extraction and separation of these ions [5,6].
2. Experimental
The Ln(NO3)3 salts were obtained by dissolving lanthanide oxides (spectro-
scopically pure, prepared in Laboratory of Rare Earths, Faculty of Chemistry,
Adam Mickiewicz University, Pozna n´ ) in nitric acid (p.a.). Stock solutions
for the spectroscopic study were prepared by dissolving them and the lig-
ands in acetonitrile (Aldrich, 99.93%). The commercially available ligands:
diethyl(2-oxopropyl) phosphonate and diethyl(2-oxo-2-phenylethyl) phospho-
nate (Aldrich) were used without further purification (Fig. 1).
Ln(III) compounds type of phosphoro-azo derivatives of the
ꢀ
␤
-diketones undoped and doped with ␣␣ dipyridyl have been
recently reported in [7], and the interactions of the ligands
with the lanthanide ions were confirmed both by IR and
electronicspectra. Thesynthesis, structures, andcatalyticbehav-
ior of the complexes bis(diphenylphosphino)methane dioxide
The 1:2 complexes were prepared by mixing together stoichiometric quanti-
ties of lanthanide nitrates and R–C(O)CH2P(O)(OEt)2 (where R = CH3 or C6H5)
in ethanol. The reaction mixtures were stirred for 1 h at room temperature. The
solvent was evaporated in air, and the oil products were washed with isopropanol
and dried under vacuum.
[
Ph2P(O)CH2P(O)Ph2] with Ln(OTf3)3 [8] and Ln(NO3)3 [9]
have been studied. Complexes of (EtO)2P(O)CH2P(O)(OEt)2
with lanthanide nitrates have also been characterized, and their
3
+
The absorption spectra of Nd complexes were measured on a Shimadzu
UV-2401 PC spectrophotometer and used for the threshold bootstrap computer-
assistedtargetfactoranalysis(TBCAT)previouslydescribedin [11]. Thespectra
∗
Corresponding author.
E-mail address: zbychuh@amu.edu.pl (Z. Hnatejko).
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.04.155

3
96
Z. Hnatejko et al. / Journal of Alloys and Compounds 451 (2008) 395–399
Fig. 1. Structures of the studied ligands.
of sample acetonitrile solutions with varying molar ratios of the components
were recorded with spectral slits of 0.5 nm and with 0.2 nm steps using cuvettes
having 4 cm path lengths. Four repeated scans were averaged to reduce the
3
+
spectral noise. Interactions of Nd with the OPP ligand were studied using
the absorption bands at 550–615 and 705–773 nm corresponding to the transi-
4
4
2
4
4
4
tions I9/2– G5/2, G7/2 (λmax ∼ 581 nm) and I9/2– F7/2, S3/2 (λmax ∼ 736 nm).
Luminescence measurements were made on a Perkin Elmer MPF3 spectrofluo-
rimeter. All samples were prepared by mixing appropriate amounts of Eu or
Tb and the ligands in scaled flasks of 5 ml volume. The luminescence lifetime
and excitation spectra of the D0– F0 transition of Eu were measured using
a detection system, consisting of a nitrogen laser pumping a tunable dye laser
3
+
3
+
5
7
3+
(
using the P3 CDOMAT dye in acetonitrile), as described previously in ref. [12].
Experiments were conducted at room temperature.
The IR spectra of the ligands and complexes were recorded on a Bruker
infrared spectrophotometer IFS 66v/S using the KBr pellet technique. The ele-
mental analyses of the complexes were carried out with an elemental analyzer,
VARIO EL III, Elementar. Experimental data were in good agreement with the
theoretical values; found: C, 23.66; H, 4.63; N, 5.81; calculated: C, 23.57; H,
4
.21; N, 5.89; for LaL2(NO3)3 where L is OPP, and found: C, 36.89; H, 4.55;
N, 4.28; calculated: C, 35.30; H, 4.57, N, 4.57% for TbL2(NO3)3.C3H8O where
1
3
31
L is DEPP. C and P NMR spectra were recorded on a VARIAN Mercury
00 in 5 mm tubes at room temperature. The spectra were referenced to the
3
1
3
31
corresponding solvent signals ( C) or externally to 85% aqueous H3PO4 ( P).
3
. Results and discussion
The Nd(III) absorption spectra of the systems showed
a deceasing of the molar absorption coefficients, ε, and a
bathochromic shift of absorption bands with the addition of
4
4
the ligand, as presented in Fig. 2. The bands I9/2– G5/2,
3+
Fig. 2. The absorption spectra of Nd ion, in Nd(III)-OPP system (molar ratio
2
4
4
4
G7/2 (λmax ∼ 581 nm) and I9/2– F7/2, S3/2 (λmax ∼ 736 nm),
4
4
2
4
4
4
from 1:0 to 1:6) in range of I9/2– G5/2, G7/2 (a) and I9/2– F7/2, S3/2 (b)
transitions.
showing the largest changes after ligand addition, were ana-
lyzed using the TB CAT [11]. This analysis (30 experimental
spectra) revealed the existence of two species ML and ML2
of the Nd/OPP complexes. Their stability constants were
calculated as log β11 = 2.6 ± 0.4 and log β12 = 4.9 ± 1.0 for
4
4
2
the I9/2– G
,
5/2
G7/2 transition and log β11 = 2.9 ± 0.5 and
4 4 4
log β12 = 4.7 ± 0.6 for the I9/2– F7/2, S3/2 transition, respec-
tively. The log β values obtained from measurements concerning
the combined Nd3 spectra were as follows: log β11 = 2.8 ± 0.4
and log β12 = 5.0 ± 0.7, respectively.
+
Fig. 3 shows the absorbance and excitation spectra for the
Tb/DEPP system. The absorption spectra of the ligands, under
study (also their complexes with Eu3 and Tb ions) showed
one or two bands at about 210 and 260 nm, respectively, in
the range 200–400 nm. No significant bathochromic shifts were
observed upon complexation.
+
3+
All excitation spectra were recorded by monitoring the
Eu(III) luminescence at 618 nm and the Tb(III) luminescence
at 548 nm. The complexes of Eu3 and Tb
+
3+
are char-
acterized by similar excitation spectra and have the same
maximum in their respective excitation spectra. Excitation of
the Eu and Tb systems at λ = 270 nm in CH3CN solutions
3
+
3+
Fig. 3. Absorption spectra of DEPP (a), and Tb-DEPP (b), and excitation spectra
of Tb-DEPP (c) and Tb-OPP (d) recorded in CH3CN solution.
resulted in characteristic emissions with the following transi-

Z. Hnatejko et al. / Journal of Alloys and Compounds 451 (2008) 395–399
397
Fig. 4. Emission spectra of the Tb/DEPP (1:3) systems in CH3CN solution,
Fig. 5. Plots of the Eu3+ luminescence intensity vs. [L/Ln3+] in the Eu3+/L
(L = OPP and DEPP) systems.
−5
C
3+ = 4 × 10 M.
Tb
Table 1
The luminescence parameters for the Eu and Tb3+ systems
that the enhanced Eu3+ and Tb3+ emission observed in the com-
plexes under study were due to energy transfer from the ligands
3
+
System (1:3 molar ratio)
λex (nm)
Ilum
Φlum (%)a
3+
to the Ln ions (antenna effect). The energy transfer was more
effective for the Tb3 complexes compared to that in the Eu
+
3+
Eu/OPP
Eu/DEPP
Tb/OPP
Tb/DEPP
300
270
300
270
0.04
0.45
0.09
1
–
7.7
–
complexes. The luminescence quantum yields of the complexes
2+
(
Table 1) were measured [13] using Ru(byp)3 (Φ = 0.028 in
11.1
water) and quinine sulphate (Φ = 0.577 in 0.1 M H2SO4) as stan-
a
Excitation in the ligand absorption; Ru(byp)32+ (Φ = 0.028 in aerated water)
dards [14,15]. Following the trend in the emission spectra, the
and quinine sulphate (Φ = 0.577 in 0.1 M H2SO4).
3+
quantum yield for the Tb complex was much higher than the
corresponding value for the Eu3 complex.
+
5
7
5
7
5
7
tions: λ = 595 nm ( D0– F1), 618 nm ( D0– F2), and 697 nm
The selective excitation spectra of D0– F0 of europium ion
were measured. The ground state and the excited state are non-
degenerate. Sincetheligandfieldcannotfurthersplitthesestates,
5
5
7
5
7
(
(
D0– F4) for europium, and λ = 493 nm ( D4− F ), 548 nm
6
7
5
7
5
7
D4– F ) 589 nm ( D4– F4), and 625 ( D4– F3) for terbium.
5
3
+
As shown in the spectra (Fig. 4), for the terbium ion, the most
only a single electronic transition is allowed for each Eu envi-
ronment. The excitation spectra for different molar ratios of
Eu(NO3)3 and DEPP in CH3CN are shown in Fig. 6.
5
7
intense emission can be attributed to the D4– F transition. For
5
5
7
the europium ion, the D0– F2 transition was the most intense.
The spectra of the Eu–L and Tb–L complexes formed in the
acetonitrile solution showed a large increase in the Ln(III) lumi-
nescence for the M:L 1:2 molar ratio (Fig. 5). The intensities of
the DEPP complexes were larger than those of the OPP com-
plexes (Table 1). At 394 and 368 nm excitation, the Eu–L and
Tb–L systems showed very weak luminescence (Fig. 4).
Deconvolution of the spectra shows peaks with maxima at
579.16 (free metal ion), 579.55 (M:L 1:1), 579.98 (M:L 1:2,
1:3,), and 580.02 nm (1:4). The luminescence decay times were
obtained by fitting the decay curves, from the maxima, to a sin-
gle exponential function. For the free Eu3 (as Eu(NO3)3) in
CH3CN solution, the luminescence exhibited a faster decay with
a lifetime of 468.3 ␮s. The 1:1 Eu/DEPP system showed a lumi-
nescencelifetimeequalto958.1 ␮s. Theluminescencemeasured
+
This very weak luminescence of the Eu3 and Tb ions,
+
3+
together with the observed excitation spectra (Fig. 3), indicates
Fig. 6. The Eu selective excitation spectra in the range of 7F0– D0 transition recorded in room-temperature for various Eu/DEPP ratios, C 3+ = 0.001 M;
3
+
5
Eu
Eu3+/L = 1:0 (1), 1:1 (2), 1:2 (3).

3
98
Z. Hnatejko et al. / Journal of Alloys and Compounds 451 (2008) 395–399
Table 2
The IR data for the OPP and DEPP ligands and their complexes with La and
Tb ions
−1
frequencies of the ligands (ꢀν(PO) = 53–69 cm ) support the
notion that coordination is via the phosphoryl oxygen atom. This
is in agreement with the data for other lanthanide complexes
3
+
3
+
Ligand OPP
La/OPP system
ꢀν
[
16,17].
The IR spectrum of the nitrate ion shows bands: ν2 (out-of-
plane deformation), ν (doubly degenerate stretch), ν (doubly
1
1
–
–
–
–
–
–
–
–
716
257.9
C
P
O
O
1721.8
1204.6
C
P
O
O
+5.8
−53.3
3
4
2522.1 w
2308.8 w
2058.2 v.w
1982 v.w
1498 s
1308.7 s
817.5 m
743.2 m
NO3 ␯1+3
NO3 ␯1+3
NO3 2␯1
NO3 2␯1
NO3 ␯3
NO3 ␯3
NO3 ␯2
NO3 ␯4
NO3 ␯4
P–O–C
P–O–C
P–O–C
C–P
+213.3
degenerate in-plane bending), and combination bands: ν1 + ν3
and ν1 + ν4 [18]. Methods to discriminate between ionic, biden-
tate, andmonodentatenitrategroupshavebeenreported[18–20].
The presence, in the studied complexes, of the bands for ν(NO)
+189.3
+37.1
−
1
around 2300–2520, 1300–1500, and 700–740 cm , in addition
−1
−1
to the high values of ꢀν3 (∼200 cm ) and ꢀν4 (∼40 cm
,
Table 2), can be taken to be an indication of the bidentate nature
of the nitrate ion. This is in accord with the data for alternative
complexes [10,17].
706.1 v.w
1
1
8
1
7
7
052.3
026.5
21.1
444.5
89.3
P–O–C
P–O–C
P–O–C
C–P
C–P
C–P
1049.0
1029.8
828.2
1446.4
796.7
−3.3
+3.3
+7.1
+1.9
+7.4
−1.7
The ligands under study, contained intense C O vibrational
peaks at 1716 and 1682 cm for the OPP and DEPP ligands,
respectively. The absence in the IR spectra of a low-frequency
shift in the C O bands suggests that there is coordination only
by the P O groups in the systems studied.
−1
C–P
C–P
71.9
770.2
Ligand DEPP
Tb/DEPP system
ꢀ␯
The La(OPP)2(NO3)3 complex is diamagnetic. Table 3 shows
the chemical shifts in the 13C and P NMR spectra of solutions
of the OPP ligand and the La(OPP)2(NO3)3 complex in CD3OD.
The 31P NMR spectrum of the complex exhibits one singlet,
indicating the homogeneity of sample.
1
1
–
–
–
681.6
253.8
C
P
O
O
1683.7
1184.5
2519.1 w
2304.7 w
1979.6 v.w
C
P
O
O
+2.1
31
−69.3
NO3 ␯1+3
NO3 ␯1+3
NO3 2␯1
NO3 ␯1+4
NO3 ␯3
NO3 ␯3
NO3 ␯2
NO3 ␯4
NO3 ␯4
P–O–C
P–O–C
P–O–C
C–P
+214.4
1771.3 v.w
In the 13C NMR spectrum of the La(OPP)2(NO3)3 complex,
there was a significant shift of the signals associated with the
carbon atom directly bonded to the phosphorus atom (CH2–P),
but there was no change of the position of the signal due to
the carbon atom of the CO group. This indicates there is direct
–
–
–
–
1494.5 s
1299.7 s
816.4 m
743 m
+194.8
+38.4
704.6 v.w
1
1
8
1
7
7
054.4
026.6
36.2
448.7
85.4
P–O–C
P–O–C
P–O–C
C–P
C–P
C–P
1051.5
1029.7
841.6
1449.2
774.7
−3.3
+3.1
+5.2
+0.5
−10.7
–8.6
3
+
participation of the PO group in the coordination of the La
ion, and it confirms the results from the IR spectra indicating
that the ligands under study are monodentate.
C–P
C–P
51.8
743.2
4. Conclusion
for the 1:2 and 1:3 systems exhibited exponential decays with
lifetimes of equal to 1145.5 and 1177.2 ␮s, respectively. For
the 1:4 system, the luminescence lifetime was determined to be
We have performed a detailed study of the spectroscopic
properties of lanthanide(III) ions with the organophosphorus
ligands, OPP and DEPP. In solution, the ligands form M:L and
M:L2 complexes with the Ln3 ions. The excitation and emis-
sion spectra show that energy transfer from the ligands to the
+
1
205.1 ␮s. This confirms that the ML and ML2 complexes were
formed in acetonitrile solutions.
3+
3+
The IR spectra of the free ligands and their coordination
compounds were recorded. The main changes in the spectra
were related to the ν(NO) and ν(PO) bands. The IR spectra of
the free ligands and their complexes, in KBr tablets, contained
Eu and Tb ions occurred.
Counter ions can play a significant role in the formation of
complexes of the type LnL2(NO3)3. Based on the infrared and
13
31
NMR spectra ( C and P) one can conclude, that the phos-
phoryl groups of the two organophosphorus ligands and three
−
1
absorption bands at 1253–1258 and 1184–1204 cm , respec-
tively, due to the P O valence vibrations (Table 2). The lower
ν(PO) frequencies of the complexes compared with analogous
−
NO3 ions, whichactasbidentateligands, chelatethelanthanide
ions.
Table 3
NMR data for the OPP ligand and La(OPP)2(NO3)3 complex in CD3OD solution
Compounds
δ(31P)
δ(13CH3CO)
δ(13
C
O)
δ(13CH2P)
δ(O13CH2)
δ(CH213CH3)
Ligand
Complex
20.87
21.32
31.52
31.58
201.59
202.19
42.15, 43.86
58.34
64.10
64.44
16.64
16.57

Z. Hnatejko et al. / Journal of Alloys and Compounds 451 (2008) 395–399
399
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1