Formation of Mono- and Polynuclear Luminescent Lanthanide Complexes based on the Coordination of Preorganized Phosphonated Pyridines

Article abstract of DOI:10.1021/acs.inorgchem.8b00666

A series of polynuclear assemblies based on ligand L (1,4,7-tris[hydrogen (6-methylpyridin-2-yl)phosphonate]-1,4,7-triazacyclononane) has been developed. The coordination properties of ligand L with Ln^III (Ln = La, Eu, Tb, Yb, Lu) have been stu

Full text of DOI:10.1021/acs.inorgchem.8b00666

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Formation of Mono- and Polynuclear Luminescent Lanthanide
Complexes based on the Coordination of Preorganized
Phosphonated Pyridines
Jeremy Salaam,†,‡ Lilia Tabti,†,‡,§ Sylvana Bahamyirou,†,‡ Alexandre Lecointre,‡ Oscar Hernandez Alba,∥
́
́
Olivier Jeannin,⊥ Franck Camerel,⊥ Sarah Cianferani,∥ Embarek Bentouhami,§ Aline M. Nonat,*,‡
́
and Loïc J. Charbonniere*,‡
‡LIMAA, IPHC, UMR 7178, Universite
̀
́
de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France
§LCIMN Laboratory, Department of Process Engineering, Faculty of Technology, University Ferhat Abbas, 19000 Set
́
if, Algeria
∥LSMBO, IPHC, UMR 7178, Universite
́
de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France
⊥Univ Rennes, CNRS, ISCR-UMR6226, F-35000 Rennes, France
S
* Supporting Information
ABSTRACT: A series of polynuclear assemblies based on ligand L (1,4,7-tris[hydrogen (6-
methylpyridin-2-yl)phosphonate]-1,4,7-triazacyclononane) has been developed. The
coordination properties of ligand L with LnIII (Ln = La, Eu, Tb, Yb, Lu) have been
studied in water (pH = 7.0) and in D2O (pD = 7.0) by UV-absorption spectrometry,
spectrofluorimetry, 1H and 31P NMR, DOSY, ESI-mass spectrometry, and X-ray diffraction.
This nonadentate ligand forms highly stable mononuclear complexes in water and provides
a very efficient shielding of the Ln cations, as emphasized by the very good luminescence
properties of the Yb complex in D2O, especially regarding its lifetime (τD O = 10.2 μs) and
2
quantum yield (ϕD O = 0.42%). In the presence of excess LnIII cation, polynuclar complexes
2
of [(LnL)2Lnx] stoichiometry (x = 1 and x = 2) are observed in solution. In the solid state, a
dinuclear complex of La could be isolated and structurally characterized by X-ray diffraction,
3+
unraveling the presence of strong hydrogen bonding interactions between a La(H2O)9
cation and the [LaL]3− complex.
Scheme 1. Polynuclear Lanthanide Assemblies with Ligands
L1, L2, and L2D
INTRODUCTION
■
Photonic up conversion (UC) is very appealing for
bioanalytical assays and theranostic applications since it allows
for the conversion of low energy photons in the near-infrared to
visible emission.1 Although this phenomenon is usually
observed in solid state materials2 and nanomaterials,3,4 a few
discrete molecular systems have proven their efficiency.5−9 In
particular, we have recently demonstrated that UC was also
possible with supramolecular systems in solution at room
temperature using lanthanide coordination complexes.7,8 In the
first example,7 visible up converted emission of Er3+ was
observed upon laser excitation of a sandwich [(L1Er)-F-
(L1Er)]+ assembly, in which the two Er centers were embedded
in a macrocyclic DOTA derivative (L1, Scheme 1), which holds
the dimeric structure by π−π stacking interactions and H-
bonding.10 In the second strategy, heteropolynuclear lanthanide
(Ln) assemblies have been developed in order to promote Yb
→Tb energy transfers. In this case, the tetraphosphonated
octadentate ligand L2 (Scheme 1) formed highly stable
mononuclear [LnL2] complexes which, in the presence of
excess Ln3+ salts, self-assembled into polynuclear [(LnL2)2Ln],
[(LnL2)2Ln2], and [(LnL2)2Ln3] species. The assembly is
driven by the strong polarization of the ligand due to the
electron-rich phosphonated pendant arms.11 Such engineering
offers a very good shielding of the Ln cations by the ligands and
very interesting photophysical properties in water and D2O. In
particular, the deuterated analogue YbL2D was used in the
Received: March 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00666
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Synthesis and Chemical Structure of Ligand L
oxide26 (4.00 g, 27.9 mmol) was dissolved in dichloromethane (100
mL) under an argon atmosphere. Ethyl chloroformate (8.0 mL, 83.7
mmol) was added, and the solution was stirred at r.t. for 30 min.
Triethyl phosphite (14.8 mL, 86.3 mmol) was added dropwise, and the
resulting solution was stirred at r.t. during 2 h. The resulting mixture
was washed with saturated NaHCO3, water, and brine, dried over
Na2SO4, filtered, and concentrated to dryness under vacuum.
Impurities were removed by distillation under reduced pressure.
Purification of the crude product was performed by column
chromatography over silica gel (CH2Cl2/MeOH gradient from 100:0
formation of [(YbL2D)2Tbx] complexes (x = 1−3), which
afforded characteristic visible UC emission of Tb, upon NIR
excitation of the Yb at 980 nm.
In our search for new systems with improved efficiency, we
turned our interest toward the development of chelates based
on the 1,4,7-triazacyclononane (tacn) macrocycle appended
with pyridine phosphonate units (Scheme 2). Lanthanide
complexes with ligands based on tacn substituted with
bipyridines,12 picolinate,13 8-hydroxyquinoline,14 and pyridyl
phosphinate15−17 have proven to be very efficient in the design
of highly stable mononuclear lanthanide complexes with very
interesting photophysical properties in the solid-state, in
solution, and even in cellulo18,19 or on cell membranes20 and
are ideal candidates for the development of biomedical imaging
probes. Derivatives with two-photon antenna are also appealing
for two-photon microscopy experiments in both the visible and
near-infrared spectral ranges.21
Introduction of 6-phosphonated-2-methylene bridged pyr-
idines on the nitrogen atoms is expected to lead to the
formation of two five-membered chelate rings per arm; a
nonadentate coordination suited to the protection of the first
coordination sphere of the Ln cations and a C3 symmetry with
the negatively charged phosphonate functions on one side of
the complexes, with the possibility to further develop
electrostatic interactions in excess of Ln cations and second
sphere interactions.11,22 Furthermore, phosphonated pyridines
are expected to provide highly stable lanthanide complexes
compared to their carboxylated analogues11,13,23 and efficient
sensitization of Tb3+, Eu3+, and, more interestingly, Yb3+.
In this study, we report the synthesis of the new ligand L
(Scheme 2) and a detailed analysis of its coordination
properties with Ln3+ ions by UV-absorption spectrometry,
spectrofluorimetry, 1H and 31P NMR, DOSY, ESI-mass
spectrometry, and X-ray diffraction.
1
to 97:3), yielding 3 (4.17 g, 57%) as an oil. H NMR (CDCl3, 400
MHz): δ 7.78 (m, 2H), 7.57 (td, J1 = 7.7 Hz, J2 = 1.6 Hz, 1H), 4.65 (s,
2H), 4.15 (m, 4H), 1.27 (t, J = 7.2 Hz, 6H) ppm. 13C NMR (CDCl3,
75 MHz): δ 157.7 (d, J = 22.8 Hz), 151.6 (d, J = 226.7 Hz), 137.2 (d, J
= 12.1 Hz), 127.2 (d, J = 24.9 Hz), 125.2 (d, J = 3.6 Hz), 63.2 (d, J =
5.9 Hz), 46.4, 16.4 (d, J = 5.9 Hz) ppm. 31P NMR (CDCl3, 162 MHz):
δ 10.0 ppm. IR (cm−1, ATR): ν 2983 (m), 1582 (m), 1478 (w), 1447
(m), 1392 (w), 1249 (s, νPO), 1182 (m), 1016 (s), 961 (s), 881 (m).
ESI+/MS: m/z = 270.064 ([M+Li]+, 100%). Anal. Calcd for
C10H15ClNO3P: C, 45.55; H, 5.73; N, 5.31. Found: C, 44.91; H,
5.71; N, 5.28.
Synthesis of Compound 4. A Schlenk tube containing 3 (2.00 g,
7.6 mmol), tacn (325 mg, 2.5 mmol), and flame-dried K2CO3 (1.75 g,
12.7 mmol), dissolved in anhydrous CH3CN, was stirred for 16 h at 50
°C. The remaining K2CO3 was removed by filtration, and the solvent
was removed under reduced pressure. The resulting residue was
dissolved in dichloromethane, washed with water and brine, dried over
Na2SO4, filtered, and concentrated to dryness under vacuum.
Purification of the crude product was performed by column
chromatography over alumina (CH2Cl2/MeOH gradient from
100:00 to 95:5), yielding compound 4 (1.18 g, 56%) as pale yellow
oil. 1H NMR (CDCl3, 500 MHz): δ 7.74 (t, J = 6.8 Hz, 3H), 7.68 (td,
J1 = 7.6 Hz, J2 = 5.9 Hz, 3H), 7.60 (d, J = 7.6 Hz, 3H), 4.23−4.11 (m,
12 H), 3.89 (s, 6H), 2.85 (s, 12H), 1.28 (t, J = 7.1 Hz, 18H) ppm. 13C
NMR (CDCl3, 125 MHz): δ 161.6 (br), 151.3 (d, J = 227.5 Hz), 136.5
(d, J = 12.3 Hz), 126.5 (d, J = 25.4 Hz), 125.7, 64.4, 63.0 (d, J = 5.9
Hz), 55.9, 16.5 (d, J = 5.9 Hz) ppm. 31P NMR (CDCl3, 202 MHz): δ
11.0 ppm. IR (cm−1, ATR): ν 2982 (w), 2906 (w), 1672 (w), 1582
(w), 1443 (m), 1391 (w), 1367 (w), 1254 (s, νPO), 1163 (m), 1016
(s), 961 (s), 790 (m), 760 (m), 564 (m), 532 (m). ESI+/MS: m/z =
811.34 ([M + H]+, 100%). Anal. Calcd for C36H57N6O9P3•H2O: C,
52.17; H, 7.15; N, 10.14. Found: C, 51.79; H, 6.96; N, 9.64.
EXPERIMENTAL SECTION
■
General Methods. Solvents and starting materials were purchased
from Aldrich, Acros, and Alfa Aesar and used without further
purification. Flash column chromatography was performed with silica
gel (10−63 μm, Macherey Nagel) or on C18 reversed phase silica
(Macherey Nagel). IR spectra were recorded on a PerkinElmer
Spectrum One spectrophotometer as solid samples, and only the most
significant absorption bands are given in cm−1. Elemental analysis and
mass spectrometry analysis were carried out by the Service Commun
Synthesis of Ligand L. Compound 4 (200 mg, 0.24 mmol) was
dissolved in 6 N HCl solution (30 mL) and was refluxed during 16 h.
The solvent was removed under vacuum, and ligand L (175 mg, 0.20
1
mmol, 90%) was obtained as a white powder. H NMR (D2O, 500
MHz): δ 8.21 (td, J1 = 7.9 Hz, J2 = 3.7 Hz, 3 H), 7.96 (t, J = 7.4 Hz, 3
H), 7.79 (d, J = 8.0 Hz, 3 H), 4.51 (s, 6 H), 3.27 (s, 12 H) ppm. 13C
NMR (D2O, 125 MHz): δ 153.1 (d, J = 194.4 Hz), 152.7 (d, J = 13.7
Hz), 143.3 (d, J = 10.0 Hz), 127.8 (d, J = 16.4 Hz), 127.5, 57.5, 50.1
ppm. 31P NMR (D2O, 202 MHz): δ 1.84 ppm. IR (cm−1, ATR): ν
2638 (m, br), 1628 (w), 1403 (w), 1147 (m), 925 (s), 812 (m), 692
(m), 532 (s). ESI+/MS: m/z = 643.15 ([M + H]+, 100%). Anal. Calcd
for C24H33N6O9P3•3HCl•3H2O: C, 35.77; H, 5.25; N, 10.43. Found:
C, 35.38; H, 5.08; N, 10.45.
d’Analyses of the University of Strasbourg. H, 31P, and 13C NMR
1
spectra and 2D COSY, NOESY, HSQC, and HMBC experiments were
recorded on Avance 300 and Avance 400 spectrometers operating at
300 and 400 MHz for 1H, respectively. Chemical shifts are reported in
ppm, with residual protonated solvent as internal reference.24 When
unspecified, coupling constants refer to H − H coupling. The given
pH values are corrected for the deuterium isotopic effects (pD =
apparent pH + 0.4).25
Synthesis and Characterization of the Ln3+ Complexes.
General Method. In order to control the strict 1:1 stoichiometry of
the complexes and to avoid excess of Ln cations in the formed
Synthesis of Diethyl (6-(Chloromethyl)pyridin-2-yl)phosphonate
(3). In a 250 mL round-bottom flask, 2-(chloromethyl)pyridine-1-
B
DOI: 10.1021/acs.inorgchem.8b00666
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
complexes, the complexes were prepared in D2O and the formations of
the complexes were monitored by 1H NMR. To a solution of ligand L
(ca. 40 mg) in D2O, a volume corresponding to half an equivalent of
Ln cation of a stock solution of LnCl3·xH2O in D2O was added. The
1H NMR spectrum of the solution was recorded, and the exact volume
corresponding to the missing half equivalent was added. A 0.1 M
NaOD solution was added to raise the pD to 7 and the complete
formation of the mononuclear complex was verified by 1H NMR.
Addition of HCl until pH = 3.5 resulted in the formation of a
precipitate. Complete precipitation was achieved by addition of a
THF/methanol mixture. The precipitate was isolated by centrifuga-
tion, dried under reduced pressure (2h, 40 °C), and purified (except
for the Lu and Tb complexes) by flash column chromatography on a
reverse phase C18 column (H2O/MeOH/TFA 0.1%) to give [LnL]
complex as a white powder.
hydrogen atoms were refined with anisotropic displacement
parameters. For the [EuL] structure, the H atoms of phosphonate
groups have been found in the electron density maps and refined as
idealized hydroxyl group with torsion from electron density (AFIX 147
instruction). H atoms on the less agitated water molecules have been
found in electron density map and refined with restraints on O···H
(DFIX 0.84 0.01) and H···H (DANG 1.34 0.01). The four most
agitated water molecules were squeezed. Their contribution to the
calculated structure factors was estimated following the BYPASS
algorithm,30 implemented as the SQUEEZE option in PLATON.31 The
new data set, free of the four highly agitated water molecules
contribution, was then used in the final refinement. For the [LaL]
structure, H atoms on water molecules are found in electron density
map and refined with restraints on O···H (DFIX 0.84 0.01) and H···H
(DANG 1.34 0.01). All the other hydrogen atoms localized on carbon
atoms were included in their calculated positions. Crystallographic
data for structural analysis of the mononuclear Eu and of the La
dinuclear complexes have been deposited with the Cambridge
Crystallographic Data Centre under CCDC nos. 1589355 and
1589357, respectively. Copies of this information are available free
of charge from the Web site (www.ccdc.cam.ac.uk).
[EuL]. From 456 μL of EuCl3·6H2O at 0.11 M (50 mmol) and 40.0
mg (50 mmol) of ligand L, one obtains 35.1 mg of [EuL]·
1
(CF3CO2H)2·2H2O (70%). H NMR (D2O, pD = 7.0, 400 MHz):
δ 8.42 (s, br, 3H), 7.53 (s, 3H), 7.30 (tr, br, 3H), 6.03 (d, br, 3J = 7.0
Hz), 3.72 (s, br, 3H), 1.03 (s, br, 3H), −1.72 (s, br, 3H), −2.82 (s, br,
3H), −5.61 (s, br, 3H) ppm. ESI+/MS: m/z = 791.1 ([(EuL)+H]+,
85%), 793.1 ([(EuL)+H]+, 100%). 19F NMR (D2O, 282 MHz): δ
−75.60 (s). Anal. Calcd for C24H30EuN6O9P3•2TFA•2H2O
(C28H36EuF6N6O15P3): C, 31.86; H, 3.44; N, 7.96. Found: C, 31.67;
H, 3.54; N, 8.21.
Physicochemical and Spectrophotometric Studies. Materials.
Distilled water was purified by passing through a mixed bed of ion-
exchanger (Bioblock Scientific R3−83002, M3-83006) and activated
carbon (Bioblock Scientific ORC-83005). All the stock solutions were
prepared by weighing solid products using an AG 245 Mettler Toledo
analytical balance (precision 0.01 mg). Metal cation solutions were
prepared from their chloride (LaCl3·7H2O, EuCl3·6H2O,TbCl3·6H2O,
YbCl3·6H2O, LuCl3·6H2O, 99.9%, Sigma-Aldrich), and their concen-
trations were determined by colorimetric titrations with EDTA (10−2
M, Merck, Titriplex III) according to standard procedures, using
xylenol orange as indicator.32 Sodium hydroxide (NaOH) and
hydrochloric acid (HCl) were used to adjust pH during titrations.
All the experiments described were repeated at least two times.
Spectroscopic measurements were performed with 10 × 10 mm2
quartz Suprasil certified cells (Helma Analytics). UV/vis absorption
spectra were recorded on a PerkinElmer lambda 950 or on a Specord
spectrometer from Jena Analytics. Steady state emission spectra were
recorded on an Edinburgh Instrument FLP920 spectrometer working
with a continuous 450 W Xe Lamp and a red sensitive R928
photomultiplier from Hamamatsu in Pelletier housing for visible
detection (230 to 900 nm) or a Hamamatsu R5 509-72 photo-
multiplier for the Vis-NIR part. All spectra were corrected for the
instrumental functions. For emission spectra upon UV excitation, a
399 nm cutoff filter was used to eliminate second order artifacts.
Phosphorescence lifetimes were measured on the same instrument
working in the Multi-Channel Spectroscopy (MCS) mode, using a
Xenon flash lamp as the excitation source. For short millisecond
lifetimes, the intensity decay was corrected from the lamp intensity
decay profile using a scattering solution of Ludox in water.
Luminescence quantum yields were measured according to conven-
tional procedures,33 with optically diluted solutions (optical density
<0.05), using [Ru(bipy)3Cl3] in water (Φ = 0.04)34 as reference for
Eu, a bipyridine Tb complex, [TbL(H2O)] in water (Φ = 0.31)35 as
reference for Tb, and cardiogreen (IR125) in MeOH (Φ = 0.078) for
Yb.36 The errors are estimated to be 10% on the lifetimes and 15% on
the luminescence quantum yields.
[YbL]. From 415 μL (50 mmol) of a 0.11 M solution of
YbCl3.6H2O and 40.0 mg (50 mmol) of ligand L, one obtains 22.6 mg
1
(32%) of [YbL]·5(CF3CO2H)·2H2O. H NMR (D2O, 300 MHz, pD
= 7.87): δ 23.00 (s, br, 3H), 11.69 (s, br, 3H), 10.94 (s, br, 6H), 5.90
(s, br, 6H), −5.45 (s, br, 3H), −6.60 (s, br, 3H), −17.18 (s, br, 3H)
ppm. 19F NMR (D2O, 282 MHz): δ −75.60 (s). Anal. Calcd for
C24H30N6O9P3Yb•5TFA•2H2O (C34H39F15N6O21P3Yb): C, 28.79; H,
2.77; N, 5.92. Found: C, 29.20; H, 3.08; N, 5.79. ESI+/MS: m/z =
814.1 ([(YbL)+H]+, 100%).
[LuL]. From 944 μL (51 mmol) of a 0.054 M solution of
LuCl3.6H2O and 40.9 mg (51 mmol) of ligand L, one obtains 27.9 mg
1
(44%) of [LuL]. H NMR (D2O, 400 MHz, pD = 6.3): δ 7.98 (td,
4
3
3JH−H = 7.6 Hz, JP−H = 3.7 Hz, 3H, H4Pyr), 7.73 (t, JH−H = 6.5 Hz,
3JP−H = 6.5 Hz, 3H, H2Pyr), 7.52 (d, JH−H = 7.5 Hz, 3H, H5pyr), 4.25
3
(AB system, δA = 4.70 ppm, δB = 3.81 ppm, JAB = 13.9 Hz, 6H), 3.37
(td, J = 12.8 Hz, J = 4.2 Hz, 3H), 2.75 (dd, J = 16.2 Hz, J = 5.2 Hz,
3H), 2.41 (dd, J = 12.9 Hz, J = 4.4 Hz, 3H), 2.27 (td, J = 13.8 Hz, J =
5.0 Hz, 3 H) ppm. ESI+/MS: m/z = 814.1 ([LuL-2H]+,100%); 837.1
([LuL-3H+Na]+, 40%). 31P NMR (D2O, 202 MHz, pD = 6.3): δ 12.86
ppm. Anal. Calcd for C2 4 H3 0 N6 O9 P3 Lu·7NaCl·4H2 O
(C24H38Cl7LuN6Na7O13P3): C, 22.25; H, 2.96; N, 6.49. Found: C,
22.04; H, 3.05; N, 6.32.
[TbL]. 46.2 mg (123.7 mmol) of TbCl3.6H2O in D2O (2 mL) were
1
added to 97.9 mg (122 mmol) of ligand. H NMR (D2O, 400 MHz,
pD = 7.96): δ 84.56 (s, br, 3 H), 32.84 (s, br, 3 H), 17.11 (s, br, 3 H),
−5.48 (s, br, 3 H), −8.40 (s, br, 3 H), −9.77 (s, br, 6 H), −14.53 (s,
br, 3 H), −70.05 (s, br, 3H). 31P NMR (D2O, 202 MHz, pD = 7.96): δ
−94.37 (br). ESI−/MS: m/z = 398.02 ([M-2H]2−, 100%); 797.04
([M-H]−, 74%). Anal. Calcd for C24H30N6O9P3Tb·3NaCl·7H2O
(C24H44Cl3TbN6Na3O16P3): C, 26.65; H, 3.91; N, 7.77. Found: C,
26.68; H, 4.10; N, 7.78.
X-ray Crystallography. Crystals suitable for X-ray diffraction were
obtained by slow evaporation of aqueous solutions. The crystals were
placed in oil, and a single crystal was selected, mounted on a nylon
loop, and placed in a low-temperature N2 stream. X-ray diffraction data
collection was carried out on a Bruker APEX II Kappa-CCD or a D8
VENTURE Bruker AXS diffractometer equipped with an Oxford
Cryosystem liquid N2 device, using Mo Kα radiation (λ = 0.71073 Å)
1H and 31P NMR Titrations. Aliquots of a stock solution of the
lanthanide trichloride salt in D2O ([Lu3+] = 5.4 × 10−2 M, [Yb3+] =
0.1 M, [Eu3+] = 0.1 M) were added to a ligand solution in D2O at pD
= 7 ([L] = 17 mM for Lu3+, [L] = 6.9 mM for Yb3+ and 110 mM for
Eu3+). After each addition, pD was adjusted to a value of 7.0 with 0.1
M NaOD solution in D2O. Changes after addition from 0 to 2.5 equiv
1
of metal were monitored by recording the H (400 MHz) and 31P
́ ́
at 150(2) K (Centre de diffractometrie X, Universite de Rennes 1,
spectra (162 MHz), respectively. In the case of Lu3+, diffusion
spectroscopy (DOSY) NMR studies were carried out in order to
measure the diffusion coefficient of the species present in solution. To
this aim, a second titration was performed by adding 0, 0.5, 1.2, 1.6,
and 2.0 equiv of a LuCl3·6H2O solution in D2O (5.4 × 10−2 M) to a
ligand stock solution (0.021 M) at pD = 7.0.
France). The Bruker SMART program was used to refine the values of
the cell parameters. Data reduction and correction absorption
(SADABS) were carried out using the Bruker SAINT programs. The
structures were solved by direct methods using the SIR97 program27
and then refined with full-matrix least-squares methods based on F2
(SHELX-97)28 with the aid of the WINGX program.29 All non-
C
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Inorganic Chemistry
Article
2F7/2 emission band of Yb.39 The luminescence intensity
gradually increases up to ca. 1 equiv of metal added and then
slowly decreases (Figure 1), which supports the formation of a
mononuclear complex together with polynuclear species under
these conditions.
Mass Spectrometry and Ion Mobility Mass Spectrometry. Mass
spectrometry and ion mobility experiments were performed on a
QTOF mass spectrometer (Synapt G2, Waters, Manchester, UK),
coupled to an automated chip-based nanoelectrospray device (Triversa
Nanomate, Advion, Ithaca, U.S.A.) operating in the positive ion mode.
The sampling cone and the extraction cone voltages were set to 40 and
4 V, respectively. Ions were efficiently thermalized with a constant
trapping voltage of 7 V and subsequently transferred into the TOF
analyzer.
Ion mobility (IM) experiments were performed under a constant
pressure of N2 of 3.3 mbar in the traveling wave IM cell. A constant
pressure of 1.4 × 10−3 mbar of He was applied to focus the ions prior
to IM separation. The IM wave height and velocity were 40 V and 950
m/s, respectively. Transfer collision energy was set to 10 V to extract
the ions from the IM cell to the TOF analyzer.
Spectrophotometric and Spectrofluorimetric Titrations. Titra-
tions were performed by stepwise addition of microvolumes of a 10−3
M stock solution of the lanthanide salt (LnCl3·6H2O, Ln = Eu, Tb,
Yb) at pH 7.0 (Tris-HCl 0.1 M) to 2 mL of ligand solution (5 × 10−5
M in Tris-HCl 0.1 M at pH = 7.0) in 1 cm quartz Suprasil cell. After
each addition, the UV−visible absorption spectra were recorded,
together with the emission spectra upon excitation of the ligand at λex=
267 nm. The software Hypspec V1.1.33 was used to determine the
coordination model and calculate the stability constants (log β) of the
formed species.37
Figure 1. Evolution of the emission spectra of a solution of L in 0.01
M Tris-HCl (4.0 × 10−5 M, pH = 7.0, λex = 267 nm) upon addition of
aliquots of YbCl3·6H2O (1.14 × 10−3 M). Inset: evolution of the
intensity at 976 nm as a function of the [Yb]/[L] ratio.
These observations are also supported by measurements
recorded with Eu3+ and Tb3+ cations (see Figures S4−S5) and
in particular by the evolution of the average luminescence
lifetimes recorded upon addition of Eu3+ and Tb3+, which
significantly shorten for [M]/[L] ratios above unity (Figure 2).
RESULTS AND DISCUSSION
■
Synthesis of Ligand L. Ligand L was obtained in four
synthetic steps starting from the commercially available 2-
chloromethylpyridine 1 as depicted in Scheme 2.
The pyridine-N-oxide 2 was first obtained according to a
literature procedure.26 Diethyl (6-(chloromethyl)pyridin-2-yl)-
phosphonate 3 was obtained by an adaptation of the procedure
described for the parent pyridine,38 using ethyl chloroformate
and triethylphosphite in CH2Cl2. Alkylation of 1,4,7-triaza-
cyclonane (TACN) in acetonitrile affords precursor 4, which is
hydrolyzed with 6 M hydrochloric acid to give ligand L as a
hydrated trihydrochloride salt.
Studies of the Coordination Properties of Ligand L
with Ln3+ Ions (Ln = La, Eu, Tb, Yb, Lu). UV−Visible
Absorption and Spectrofluorimetry. The coordination proper-
ties of L with Ln cations in water at pH = 7.0 (0.01 M Tris/
HCl) were investigated by monitoring the changes in UV−
visible absorption and steady-state emission spectra upon
addition of an increasing amount of metal. Considering that the
various Ln complexes formed during the experiments can be
present under a very large variety of protonation states but that
we have not been able to unambiguously ascertain these later
(for example by potentiometry experiments for which
precipitation was observed), the protonation states of the
complexes will not be specified, unless they are clearly defined
(see for example the X-ray crystal structure of the Eu complex
below). The absorption spectra of the ligand displayed broad
bands in the UV region characteristic of π → π* transitions of
the pyridinephosphonate moieties, with a maximum at 267 nm
(ε267 = 14 660 M−1 cm−1). Addition of metal salts caused a
hypochromic shift of the absorption spectra concomitant to a 3
nm bathochromic shift (see Figures S1−S3 for Ln = Yb, Tb,
and Eu). Similar evolutions were observed along the Ln3+
series, and in all cases, the presence of an inflection point near
one equivalent of the added Ln3+ suggested the formation of a
mononuclear complex together with other polynuclear species.
In the case of Yb3+, excitation in the ligand at 267 nm led to
Figure 2. Evolution of the average luminescence lifetimes (λex = 267
nm) measured for the titrations of a ligand solution by EuCl3·6H2O
(red triangles, λem = 616 nm) and TbCl3·6H2O (green diamonds, λem
= 545 nm) (in water, TRIS/HCl 0.01 M, pH = 7.0, [L] = 5.32 × 10−5
M for Eu and 4.76 × 10−5 M for Tb).
1H and 31P NMR Studies. In order to elucidate the structure
of the formed species, NMR titrations of ligand L with Ln =
La3+, Eu3+, Yb3+, and Lu3+ in D2O (pD = 7.0, 25 °C) were
1
performed. The H NMR spectra of the ligand were recorded
in the presence of 0 to 2.7 equiv of LuCl3, and they are shown
in Figure 3 (see Figure S6 for the full spectra). Between 0.1 and
0.9 equiv of Lu3+, two sets of signals are clearly observed, which
correspond to the free ligand and to the mononuclear [LuL]
complex, respectively. After addition of 1 equiv of Lu3+,
complete disappearing of the ligand peaks occurred, leaving a
single set of 8 signals in which 3 multiplets in the 7.4−8.0 ppm
region account for the aromatic protons, one signal is observed
at 3.84 ppm for one of the two diastereotopic CH2 protons of
the pendant arm, and four spin systems in the 2.22−3.38 ppm
region are assigned to the protons of the tacn skeleton. This
pattern is in very good agreement with a rigid mononuclear
complex with C3 symmetry. Evidence of the second methylene
proton atom at 4.76 ppm, hidden by the peak of residual
1
efficient sensitization of NIR Yb3+-centered luminescence by an
nondeuterated water, can be found by recording a H−1H
2
antenna effect and observation of the characteristic F5/2
→
COSY experiment (Figure S7). Between 1.2 equiv and 1.8
D
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signals were observed, all having the same integral within
experimental error (see Figure S8 for the integration of the
spectra recorded after addition of 1.6 equiv), therefore
suggesting either a unique species with four nonequivalent
phosphorus atoms or alternatively an equimolar mixture of two
species with two nonequivalent phosphorus atoms each,
possibly two diastereoisomeric species. At 1.9 equiv and
above, precipitation was observed together with the complete
disappearing of the NMR peaks.
Further details on the intermediate species were obtained by
1
recording the H and 31P DOSY (Diffusion Ordered Spectros-
copy) spectra in the presence of 0.5, 1.2, 1.6, and 2.0 equiv of
LuCl3, respectively. The corresponding figures are displayed in
the Supporting Information. For each species, the diffusion
t
coefficient was determined by using BuOH as an internal
reference. In the presence of 0.5 equiv of metal salt, two species
were clearly identified and the diffusion coefficients amount to
283 μm2 s−1 and 301 μm2 s−1 for L and [LuL], respectively
(Figure S9). Species with fast diffusion coefficients were also
observed at 732 μm2 s−1 and 1870 μm2 s−1, with a chemical
shift corresponding to water protons. The faster species is in
good agreement with the diffusion coefficient reported for
HOD in D2O (1902 μm2 s−1),40 and the other peak may
correspond to water molecules in interaction with the second
1
coordination sphere of the complex. As expected from H
1
NMR spectra, H DOSY data in the presence of 1.2 and 1.6
Figure 3. 1H NMR spectra of ligand L upon addition of 0 to 2.7 equiv
of LuCl3·6H2O (D2O, pD = 7, 400 MHz).
equiv were rather complex; however, neat information can be
obtained from 31P NMR DOSY (Figure 5). In these conditions,
equiv of added Lu3+ in the solution, the peaks assigned to
[LuL] disappeared and four sets of nine new protons were
observed. Above 1.8 equiv, a broadening of the signals was
observed, which was concomitant to the formation of a
precipitate.
Further insights into the structure of the species formed were
obtained by monitoring the same sample by 31P (Figure 4).
Figure 5. 31P DOSY of a solution of L in the presence of 1.6 equiv of
LuCl3 in D2O (25 °C, pD = 7).
three species could clearly be observed. The first species (with a
singlet at 12.64 ppm) had a diffusion coefficient of 205 μm2 s−1,
which is in good agreement with the value determined for
1
[LuL] by H DOSY. The two other species display very close
diffusion coefficients at 145 and 150 μm2 s−1, respectively,
which was indicative of the formation of adducts with larger
sizes, each adduct giving rise to a pair of singlets in 31P NMR.
Hence, they were expected to have similar size and
composition. One is characterized by two singlets at 10.00
and 7.26 ppm, respectively, and D = 150 μm2 s−1. The other
gave rise to two singlets at 15.45 ppm and 7.68 pm and a
diffusion coefficient D = 145 μm2 s−1 (Figure 5). The
hydrodynamic radii and volumes of the three species could
be obtained from the Stockes−Einstein equation and are
summarized in Table 1. From these data, the hydrodynamic
volumes calculated for the larger species were 2.5 and 2.8 times
larger than the volume determined for the [LuL] monomer. On
the basis of the stoichiometric proportions of the compounds in
solution ([Lu3+]/[L] = 1.5) and their hydrodynamic volumes,
we postulated that the two sets of 31P signals could be
Figure 4. 31P NMR spectra of ligand L upon addition of 0 to 1.9 equiv
of LuCl3·6H2O (D2O, pD = 7.0, 162 MHz).
Upon addition of 0.1 to 0.9 equiv of LuCl3, a strong deshielding
by ca. 9 ppm of the 31P singlet was observed, due to the
coordination of the phosphonate units. No changes occurred
between 0.9 and 1.2 equiv of added Lu3+ which, in line with
previous observations by 1H NMR, confirmed the large
predominance of the monuclear [LuL] species at these
stoichiometric ratios. Between 1.2 and 1.8 equiv of metal, the
peak assigned to [LuL] progressively broadened and four new
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Table 1. Diffusion Coefficients (D), Hydrodynamic Radii
(RH), Hydrodynamic Volumes (VH), the Corresponding
Chemical Shifts (δ), and the Postulated Composition of the
Species as Obtained from 31P NMR DOSY for a Solution of
L in the Presence of 1.6 equiv of LuCl3 in D2O (25 °C, pD =
7)
δ
31P (ppm)
3.75
D (μm2/s)
RH (Å)
VH (Å3)
Species (M:L)
301
205
150
145
5.80
8.51
11.6
12.0
817
2582
6538
7328
0:1
1:1
3:2
3:2
12.54
7.29/10.2
7.69/15.48
Figure 6. ESI-MS spectrum of a solution of L in water with 5 equiv of
EuCl3·6H2O (AcONH4 0.01 M, pH = 7). Inset. Arrival time
distributions obtained by ion mobility for the peak at 938.99 m/z
and the corresponding isotopic patterns of each peak.
attributed to two species with a [Lu3L2] composition in
agreement with the possibility of two diastereoisomers.
At 2.0 equiv, broadening of the signal occurred and the
determination of the diffusion coefficients was no longer
possible, presumably because of the formation of aggregates of
higher M:L stoichiometry.
Variable temperature NMR experiments were also performed
on a sample with [M]/[L] = 1.6. 1H and 31P spectra from 25 to
80 °C are displayed in Figure S10 and S11, respectively.
Significant sharpening of the 1H NMR signals was clearly
observed upon increasing the temperature, thereby suggesting
the occurrence of dynamic phenomena such as chemical
exchange and/or rearrangement leading to the progressive loss
of the four 31P signals.
Ion mobility mass spectrometry experiments on the same
sample were used to separate the ions at 940.97, 958.98, and
978.95 m/z as a function of their size. Ion mobility arrival time
distributions are presented in Figure 6 (inset) and in Figures
S20−S21. For each peak, two contributions could be clearly
evidenced, which correspond to [Eu4L2]2+ ([Eu4L2-4H]2+ at
938.99 m/z, [Eu4L2-6H+2Na]2+ at 957.00 m/z, [Eu4L2-7H
+2Na+K]2+ at 976.97 m/z) and [Eu2L]+ ([Eu2L-2H]+ at 938.99
m/z, [Eu2L-2H+H2O]+ at 957.00 m/z, [Eu2L-3H+Na+H2O]+
at 976.97 m/z) adducts. However, at this stage it was difficult to
conclude whether both species were simultaneously present in
solution or if aggregation of the [Eu2L]+ species/or dissociation
of the [Eu4L2]2+ species occurred in the source.
Speciation and Thermodynamic Stability Constants. In
light of these observations, a global analysis of the
spectrofluorimetric titration experiments was performed with
the Hypspec program by implementing a model with three
species with M:L stoichiometric ratios of 1:1, 3:2, and 4:2. In
both cases, such a model can nicely reproduce the evolution of
the emitted luminescence intensity as a function of the M/L
ratios (see Figures S22−S24). The overall stability constants
determined for Eu3+ and Tb3+ complexes are summarized in
Table 2. The calculated emission spectra of the ML, M3L2, and
1H and 31P NMR titrations were also performed with LaCl3,
YbCl3, and EuCl3, and the corresponding spectra are displayed
in Figures S12−S17. In all cases, the formation of a
mononuclear [LnL] complex with rigid C3 symmetry was
clearly observed and reached its maximum concentration upon
addition of 1 equiv of metal salt. However, different behaviors
were observed in excess of lanthanide ion. Addition of 1.2 equiv
of YbCl3 resulted in the formation of a nonsymmetrical [Ln3L2]
secondary species which became predominant at 1.6 equiv of
added metal and which was characterized by two sets of 9 peaks
1
in the H NMR spectrum (Figure S12). At 2.0 equiv, a third
species started to grow and above, broadening of the signals
was observed concomitantly to the formation of a precipitate in
the tube. In the case of Eu3+ and La3+, precipitation started to
occur as soon as 1.5 equiv of metal salts were added, also
suggesting the formation of new species with lower solubility.
Mass Spectrometry Analysis. In order to enlighten the
composition of the polynuclear species, a similar titration of the
ligand with 0 to 5 equiv of EuCl3 in water (ammonium acetate
0.01 M, pH = 7.0) was monitored by ESI-mass spectrometry.
Mass spectra (in positive mode) are presented in the
Supporting Information. From 0 to 1 equiv (Figure S18), the
ligand peaks at 643.16 m/z ([M + H]+), 665.15 m/z ([M +
Na]+), and 681.11 m/z ([M+K]+) decreased to form a pattern
with an isotopic peak at 791.07 m/z, which corresponds to the
[EuL-2H]+ species with the corresponding isotopic distribution
related to 151Eu(48%) and 153Eu(52%). Above 1 equiv and
under the same instrumental conditions, the mononuclear
complex remains the major species (Figure S19), possibly as a
result of the fragmentation of the polynuclear adducts in the
source, which renders the titration little informative. However,
a detailed examination of the spectra recorded in the presence
of 1.2 equiv and above, indicated the presence of a series of
peaks which could be attributed to polymetallic species (Figure
6).
Table 2. Overall Stability Constants of Eu3+ and Tb3+
Complexes with Ligand L ([L] ≈ 5 × 10−5 M, TRIS/HCl
0.01 M, pH = 7.0), λex= 267 nm
Species (MxLy)
Log βEu
Log βTb
1:1
3:2
4:2
12.9
38.6
46
12.9
36.6
42
M4L2 species (M = Eu, Tb), together with the evolution of the
concentrations of the species formed are displayed in Figures
S23−S25.
Assuming a minor variation of the thermodynamic stability
constant along the lanthanide series, this model was used to
analyze NMR data recorded with Lu3+. In particular, Figure 7
represents a comparison between the theoretical distribution
species (full lines) and the intensity of the experimental 31P
NMR signals (points) and shows a striking correlation between
31P NMR data and the previous model obtained from
spectrofluorimetric titrations.
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singlet at 12.51 ppm, −8.71 ppm, and 2.51 ppm, for the 31P
NMR spectra for Lu, Eu, and Yb, respectively.
Finally, the spectroscopic properties of the complexes were
measured in water and D2O and the corresponding absorption
and emission spectra are presented in Figure 8. Additional
parameters are presented in Table 3, and three points have
attracted our attention. At first, strictly monoexponential
luminescence decays confirm the presence of unique species
in solution. Second, the absence of inner-sphere water molecule
endorses the nonadentate coordination of the ligand and the
good protection of the metal inside the cavity. Third, the
luminescence properties of the Yb complex are rather
interesting in terms of lifetimes and quantum yields. In
particular, the lifetime measured in deuterated water is rather
long in comparison to typical values obtained for efficient Yb3+
emitters41−46 and longer than the one previously reported for
the tacn-based [Yb(thqtcn-SO3)] (8.63 μs).14 However, this
value remains significantly shorter than the values obtained for
YbL2 and YbL2D (>30 μs in D2O)8 and other per-deuterated
complexes in organic solvents.47−49
Surprisingly, despite the excellent shielding of the cations
from the solvent molecules (q ≈ 0 for all complexes),50 the
luminescence quantum yields of the Eu complex were rather
modest compared to those obtained with ligands possessing
carboxy pyridyl arms (picolinates)51,52 or phosphinate ones.53
Considering the good shielding and the long luminescence
lifetimes, it is surmised that these poor luminescence quantum
yields are to be linked with a modest ligand to metal energy
transfer.
Structural Studies of the Mono- and Polynuclear
Species. While isolating the mononuclear complexes, single
crystals suitable for X-ray diffraction analysis were obtained for
[EuL], which was isolated in acidic conditions as a mixture of
[EuL] and [EuLH]Cl species. The ORTEP diagram is
represented in Figure 9. Selected parameters, distances, and
angles are presented in Tables S1−S2. The metal ion is nine-
coordinated by three nitrogen atoms from the tacn ring, three
nitrogen atoms from the aromatic groups, and three oxygen
atoms from the phosphonate functions with a pseudo-C3
symmetry. The pyridinephosphonate pendant arms bind the
lanthanide in a helical fashion and both enantiomers (Δ and Λ)
are present in the solid state structures, therefore leading to
centrosymmetric space groups (P21/a). Similar structures have
previously been collected for tacn-based ligands with
pyridinecarboxylate,13 8-hydroxyquinoline pendant arms,14
and 6-carboxy-2,2′-bipyridine arms.12
Figure 7. Theoretical representation of the concentration of the
species formed (with log β1:1 = 12.9, log β3:2 = 38.6, log β4:2 = 46, full
lines) and evolution of the intensity measured by 31P NMR for the
three species: L (blue triangles), LM (red diamonds), L2M3 (green
plus signs) upon titration of ligand L with LuCl3.6H2O (D2O, pD =
7.0, 162 MHz). The M4L2 species was observed by ESI-MS and
spectrofluorimetric titrations.
This analysis also endorses our initial attribution of the two
sets of two peaks observed in the 31P NMR spectra between 1.3
and 1.8 equiv of Lu added, to two diastereoisomers with a 3:2
M:L stoichiometry and C3 symmetry. In conclusion, although
the identification of the polynuclear species was not
straightforward, a global analysis by NMR, mass spectrometry,
and spectrofluorimetric experiments converged toward the
evidence of the formation [Ln3L2], [Ln4L2] polynuclear species.
Synthesis and Spectroscopic Characterization of the
[LnL] Complexes. Considering the high stability of the [LnL]
complexes, it was envisaged to isolate the mononuclear
complexes of Eu3+, Tb3+, Yb3+, and Lu3+. In order to determine
the optimal conditions for their synthesis and purification, the
influence of the pH on a solution containing stoichiometric
amounts of L and EuCl3·6H2O was first monitored by
recording the Eu3+ emission spectra upon excitation in the
ligand absorption band (Figure S26). This titration indicated
that complete formation of the complex occurred above pH =
4.45 and once formed, the [EuL] complex remained stable in
the pH range from 12.04 to 1.98. Hence, the formation of the
desired mononuclear complexes was achieved at neutral pH
whereas their precipitation and purification was accomplished
under slightly acidic conditions (pH = 3.5), in order to avoid
the formation of polynuclear complexes. The complexes were
further purified by reverse phase chromatography on C18 and
analyzed by elemental analysis and ESI mass spectrometry,
showing expected isotopic patterns for [LnL-2H]+ species
(Figures S27−S29). The complexes were further characterized
1
by H and 31P NMR spectroscopy, except for Tb3+, each one
1
giving a set of 9 signals in H NMR (Figures S30−S32) and a
Figure 8. UV/vis absorption (dotted lines) and normalized emission spectra (λex = 265 nm, straight lines) of a D2O solution of [LnL] with Ln = Yb
(brown), Tb (green), and Eu (red).
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Table 3. Main Spectroscopic Parameters of the YbL, TbL, EuL, and LuL Complexes in Water and D2O at pH = 7.0
a
a
b
b
c
εH O (M−1cm−1
)
εD O (M−1cm−1
)
ϕH O (%)
ϕD O (%)
τH O (μs)
τD O (μs)
q
2
2
2
2
2
2
YbL
TbL
EuL
LuL
13050
14880
12260
12610
13300
15450
13870
0.15
51
0.42
52
2.8
2890
10.2
2980
0.0
−0.2
−0.2
2
8
1252
1375
a
b
c
λ = 265 nm. ϕ measured relative to [Ru(bipy)3Cl3]34 for Eu, to [TbL(H2O)]35 for Tb, and to IR12536 for Yb. Calculated according to ref 50.
interaction in which La2 faces the triazacyclononane macro-
cycles. The two motives are repeated along the c axis, at
distances of 5.716 Å (HH) and 8.461 Å (HT) between the two
lanthanum ions (Figure 11). In a column, all [La1L] complexes
Figure 11. Representation of the hydrogen bonding interactions
observed in the crystal structure of the [(LaL)La(H2O)9] complexes
Figure 9. ORTEP diagram of the complexes [EuL], [EuLH]Cl with
thermal ellipsoids at 50% probability.
(see text for the color code of H bonding interactions) and numbering
scheme of the H atoms used for the interpretation of the NMR
spectrum.
Interestingly, single crystals could be obtained upon cooling a
water solution of ligand L containing 1.5 equiv of lanthanum
chloride, originally heated at 80 °C. The crystal structure of the
[(LaL)La(H2O)9] species is represented in Figure 10.
display the same chirality (Δ or Λ). This chirality is inverted
between two adjacent columns. Within the two entities in a HH
conformation, the three water molecules coordinated to the
hydrated La2 atom and pointing toward the [La1L] complex
are held by a H-bond network in which the two proton atoms
of a water molecule are linked to one oxygen atom of a
phosphonate function coordinated to La1 and to an oxygen
atom not directly coordinated to La1 of an adjacent
phosphonate function (blue dashed lines in Figure 11).
The three water molecules coordinated in the equatorial
plane of La2 (relative to the pseudo-C3 axis) are also hydrogen
bonded to the three oxygen atoms of the phosphonate
functions of the neighboring [La1L] complex (black dashed
lines in Figure 11). The other H atoms of these water
molecules are bonded to an oxygen atom of a phosphonate
function of a [La1L] complex from an adjacent column
(isolated oxygen atoms in red in Figure 11).
Weak interactions are also observed between [La(H2O)9]
and a neighboring [La1L] complex in the HT configuration.
These interactions are relayed by three water molecules or by
the oxygen atom of a phosphonate function of a [La1L]
complex in a neighboring column (green dashed lines in Figure
11).
We hypothesized that this strong H bonding network may be
responsible for the high stability of the [Lu3L2] species
observed by NMR and spectroscopy in solution. For a
trinuclear complex in the THH configuration ([(LuL)Lu-
(LuL)] as schematized in Figure 11), one would expect the
observation of two peaks in the 31P NMR, depending on the
HH or HT interaction between the central Lu atom and the
Figure 10. Solid state structure of the [(LaL)La(H2O)9] complex
(ellipsoids drawn at 50% probability).
Structural parameters and selected distances and angles are
summarized in Tables S1 and S3. The structure is composed of
a nonacoordinated [LaL]3− mononuclear complex in which the
La3+ cation is coordinated inside the cavity of the ligand as for
the mononuclear [EuL] complex (noted La1) together with a
nonahydrated [La(H2O)]3+ complex (noted La2). Considering
the C3 symmetry of [La1L], the position of the phosphonate
allows us to define two kind of interactions between La1 and
La2, a head to head (HH) one in which the phosphonate
functions of [La1L] point toward La2 and a head to tail (HT)
H
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LuL complex. If one now assumes that the chirality of the LuL
complexes in the trinuclear complexes can be exchanged for
one of the LuL complexes, two pairs of peaks would be
expected for the [Δ(LuL)LuΔ(LuL)] and [Δ(LuL)LuΛ(LuL)]
diastereoisomeric species.
triazacyclononane backbone such as 7* and 7′, 7° and 7, or
8″ and 8. Such exchanges are typical of Δ ↔ Λ interconversion
processes associated with the concerted rotation of the pendant
arms in C3 symmetric LnL complexes. However, this
observation is particularly surprising for two reasons. The Δ
↔ Λ interconversion process could not be observed in the
isolated [LuL] complex, even by heating the solution up to 80
°C in D2O and is not observed in the NOESY spectrum for the
AB spin system of H4 and H5 protons of the remaining [LuL]
complex. Then, if such an isomerization has already been
observed for lanthanide complexes with 1,4,7-triazacyclononane
derivatives such as H4nota54 and H3bpatcn,55 they always
correspond to ligands with decreased denticity (CN = 6 for
H4nota and 8 for H3bpatcn), whereas isomerization was not
reported for tacn ligands with functionalized pyridines
displaying CN = 9. On the opposite, enantiopure Eu and Tb
complexes with trispyridylphosphinate triazacyclononane could
be isolated, either using chiral HPLC17 or directly synthesized
thanks to the inclusion of a stereogenic center on the tcn ring.56
A possible explanation of the observed exchange is that the
interconversion occurs between the diastereomeric pairs,
exchanging the [Δ(LuL)LuΔ(LuL)] into the [Λ(LuL)LuΛ-
(LuL)] (and similarly with [Λ(LuL)LuΔ(LuL)] and [Δ(LuL)-
LuΛ(LuL)]). Considering that the Δ ↔ Λ interconversion of
[LuL] is slow on the NMR time scale, the exchange between
the diastereomeric pairs would require a dissociation process.
Unfortunately, within a dyad such as depicted in Figure 11,
distances between the two mononuclear LuL complexes are too
high to expect any correlations by 2D NOESY NMR.
To further confirm this hypothesis, a mixture containing 1.4
1
equiv of LuCl3·6H2O per ligand L was analyzed by H, COSY,
NOESY, and ROESY NMR experiments. The attribution of the
signals is presented in Figure 12. In this spectrum, five families
1
Interestingly, H NMR spectra of mixed species have been
recorded by addition of 1 equiv of LuCl3 on the mononulclear
[YbL] complex and by addition of 1 equiv of YbCl3 and [LuL]
(Figure S35), and they indicate no decoordination and no ion
exchange at room temperature and upon heating up to 80 °C,
therefore confirming the formation of kinetically inert
complexes.
CONCLUSION
■
In conclusion, the assembly of pyridinephosphonate units into
the nonadentate tripodal ligand L allows the synthesis of highly
stable monomeric lanthanide complexes that are isostructural
along the series. Moreover, such complexes are highly stable in
water and display luminescent properties, due to an efficient
sensitization of Eu, Tb, and Yb ions by the pyridinephosph-
onate antenna. More importantly, such a scaffold is responsible
for relatively low vibrational quenching, as emphasized by the
very good luminescence properties of the Yb complex in D2O
(τD2O = 10.2 μs and ϕD2O = 0.42%). Furthermore, using a
combination of UV-absorption spectrometry, spectrofluorim-
1
Figure 12. H−1H COSY spectra of ligand L upon addition of 1.4
equiv of LuCl3.6H2O (D2O, pD = 7.0, 400 MHz) and peak assignment
(see numbering in Figure 11).
of 9 signals were observed and their attribution could be
partially performed by looking at H−1H correlations in the
1
COSY experiment (Figure 12). The first family (1−9, in black)
corresponds to the mononuclear [LuL] complex. A nuclear
Overhauser effect was observed between H1 and H4/5 (see
Figure 11 for H numbering); however, dipole−dipole
interactions between other species could not be observed.
Their attribution was possible with the examination of the
ROESY spectrum, based on the 1x, 4/5x correlations (x = ′, ′′, °,
Figure S33). No correlation could clearly be distinguished for
the fourth family (1*−9*, in blue). Another striking point in
the examination of the NOESY spectrum (Figure S34) is the
observation of exchange correlations between, on the one hand,
the two protons 4 and 5 corresponding to the AB system and
on the other hand, between proton atoms from the
1
etry, H and 31P NMR, DOSY, ESI-mass spectrometry, and X-
ray diffraction, we demonstrated that this nonadentate ligand is
a good building block for the formation of highly stable
polynuclar complexes of [(LnL)2Lnx] stoichiometry (x = 1 and
x = 2). Such architectures are very interesting for studying
intermetallic communication and energy transfer within
heteropolynuclear species and can potentially lead to very
appealing photon up conversion properties. Future works are
engaged in this direction.
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Article
(3) Liu, X.; Qiu, J. Recent Advances in Energy Transfer in Bulk and
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00666.
(4) Haase, M.; Schafer, H. Upconverting Nanoparticles. Angew.
̈
Chem., Int. Ed. 2011, 50 (26), 5808−5829.
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́ ́
Eliseeva, S. V.; Petoud, S.; Hauser, A.; Piguet, C. Taming Lanthanide-
Centered Upconversion at the Molecular Level. Inorg. Chem. 2016, 55
(20), 9964−9972.
Evolution of the UV−visible absorption and emission
spectra of a solution of L upon addition of Yb3+, Tb3+,
1
1
and Eu3+ (Figures S1−S5); H NMR, 31P NMR, and H
̈
(6) Hyppanen, I.; Lahtinen, S.; Aaritalo, T.; Makela, J.; Kankare, J.;
̈
̈
̈
̈
DOSY experiments with Lu:L mixtures (Figures S6−
Soukka, T. Photon Upconversion in a Molecular Lanthanide Complex
in Anhydrous Solution at Room Temperature. ACS Photonics 2014, 1
(5), 394−397.
S11); H and 31P NMR titrations of ligand L with Yb3+
1
(Figures S12−S13), Eu3+ (Figures S14−S15), and La3+
(Figures S16−S17); ESI-MS titrations of L with Eu3+ and
ionic mobility chromatograms (Figures S18−S21);
calculated spectra and species distribution diagrams for
the spectrofluorimetric titrations of L with Eu3+ (Figures
S22−S23) and Tb3+ (Figures S24−S25); pH titration of
(7) Nonat, A.; Chan, C. F.; Liu, T.; Platas-Iglesias, C.; Liu, Z.; Wong,
̀
W.-T.; Wong, W.-K.; Wong, K.-L.; Charbonniere, L. J. Room
Temperature Molecular up Conversion in Solution. Nat. Commun.
2016, 7, 11978.
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̀
Charbonniere, L. J. Upconverted Photosensitization of Tb Visible
Emission by NIR Yb Excitation in Discrete Supramolecular
Heteropolynuclear Complexes. J. Am. Chem. Soc. 2017, 139, 1456−
1459.
̀
(9) Charbonniere, L. J. Bringing Upconversion down to the
Molecular Scale. Dalton Trans. 2018, Advance Article, doi: 10.1039/
1
a solution of Eu:L 1:1 (Figure S26); ESI-MS and H
NMR spectra of isolated [EuL] (Figures S27−S31);
[LuL] (Figures S28−S30); [YbL] (Figures S29−S32);
crystallographic data for [EuL] and [LaL] structures
1H
(Tables S1−S3); 2D
NMR spectra of a L:Lu 1:1.4
mixture (Figures S33−S34) (PDF)
C7DT04737A.
̀
(10) Nonat, A.; Liu, T.; Jeannin, O.; Camerel, F.; Charbonniere, L. J.
Accession Codes
Energy Transfer in Supramolecular Heteronuclear Lanthanide Dimers
and Application to Fluoride Sensing in Water. Chem. - Eur. J. 2018, 24,
3784−3792.
CCDC 1589355 and 1589357 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(11) Souri, N.; Tian, P.; Lecointre, A.; Lemaire, Z.; Chafaa, S.; Strub,
J.-M.; Cianferani, S.; Elhabiri, M.; Platas-Iglesias, C.; Charbonniere, L.
J. Step by Step Assembly of Polynuclear Lanthanide Complexes with a
Phosphonated Bipyridine Ligand. Inorg. Chem. 2016, 55 (24), 12962−
12974.
AUTHOR INFORMATION
̀
(12) Charbonniere, L.; Ziessel, R.; Guardigli, M.; Roda, A.; Sabbatini,
■
N.; Cesario, M. Lanthanide Tags for Time-Resolved Luminescence
Microscopy Displaying Improved Stability and Optical Properties. J.
Am. Chem. Soc. 2001, 123 (10), 2436−2437.
Corresponding Authors
*E-mail: aline.nonat@unistra.fr.
*E-mail: l.charbonn@unistra.fr.
(13) Nocton, G.; Nonat, A.; Gateau, C.; Mazzanti, M. Water Stability
and Luminescence of Lanthanide Complexes of Tripodal Ligands
Derived from 1,4,7-Triazacyclononane: Pyridinecarboxamide versus
Pyridinecarboxylate Donors. Helv. Chim. Acta 2009, 92 (11), 2257−
2273.
ORCID
Sarah Cianferani: 0000-0003-4013-4129
́
Aline M. Nonat: 0000-0003-0478-5039
Loïc J. Charbonniere: 0000-0003-0328-9842
̀
́
(14) Nonat, A.; Imbert, D.; Pecaut, J.; Giraud, M.; Mazzanti, M.
Author Contributions
Structural and Photophysical Studies of Highly Stable Lanthanide
Complexes of Tripodal 8-Hydroxyquinolinate Ligands Based on 1,4,7-
Triazacyclononane. Inorg. Chem. 2009, 48 (9), 4207−4218.
(15) Walton, J. W.; Di Bari, L.; Parker, D.; Pescitelli, G.; Puschmann,
H.; Yufit, D. S. Structure, Resolution and Chiroptical Analysis of Stable
Lanthanide Complexes of a Pyridylphenylphosphinate Triazacyclono-
nane Ligand. Chem. Commun. 2011, 47 (45), 12289−12291.
(16) Starck, M.; Pal, R.; Parker, D. Structural Control of Cell
Permeability with Highly Emissive Europium(III) Complexes Permits
Different Microscopy Applications. Chem. - Eur. J. 2016, 22 (2), 570−
580.
†J.S., L.T., and S.B. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the French Centre National de la
Recherche Scientifique and the University of Strasbourg. The
authors thank Lionel Allouche (Service commun de RMN,
Universite de Strasbourg) for the acquisition and the
́
interpretation of the DOSY experiments and Jean-Marc Strub
for his help when recording mass spectra. The authors thank
(17) Frawley, A. T.; Pal, R.; Parker, D. Very Bright, Enantiopure
Europium(III) Complexes Allow Time-Gated Chiral Contrast
Imaging. Chem. Commun. 2016, 52 (91), 13349−13352.
(18) Walton, J. W.; Bourdolle, A.; Butler, S. J.; Soulie, M.; Delbianco,
M.; McMahon, B. K.; Pal, R.; Puschmann, H.; Zwier, J. M.; Lamarque,
L.; Maury, O.; Andraud, C.; Parker, D. Very Bright Europium
Complexes That Stain Cellular Mitochondria. Chem. Commun. 2013,
49 (16), 1600−1602.
(19) Butler, S. J.; Delbianco, M.; Lamarque, L.; McMahon, B. K.;
Neil, E. R.; Pal, R.; Parker, D.; Walton, J. W.; Zwier, J. M. EuroTracker
(R) Dyes: Design, Synthesis, Structure and Photophysical Properties
of Very Bright Europium Complexes and Their Use in Bioassays and
Cellular Optical Imaging. Dalton Trans. 2015, 44 (11), 4791−4803.
́
GIS IBiSA and Region Alsace for financial support in
purchasing a Synapt G2 HDMS instrument.
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