Inorganic Chemistry
Article
were distilled over CaH . The lanthanide perchlorate salts (Ln = La,
2
Nd, Sm, Eu, Yb, Lu) were prepared from the corresponding oxides
(Rhodia and Aldrich, 99.99%) and dried according to published
procedures. Caution! Perchlorate salts are potentially explosive and
should be handled carefully in small quantities. The Ln(III) content in
solid salts was determined by complexometric titrations with Titriplex
III (Merck) in the presence of urotropine and xylene orange.
Compounds 2a and 4a were synthesized according to literature
1
7
Synthesis of L2. For L2, 42 mg (0.2 mmol) of 4-methoxy-2,6-
1
8
pyridine-dicarboxylic acid monomethyl ester (5a) was dissolved in
0 mL of CH Cl . After adding 5 μL of DMF and 150 μL of SOCl
2
1
2
2
(10 equiv), the mixture was refluxed for 2.5 h. The excess of SOCl2
was evaporated, and the residue was dried under vacuum. After
1
5
dissolving in CH Cl , a mixture of triamine 6 (15 mg, 0.05 mmol)
2
2
and triethylamine (50 μL) was added dropwise to this solution. The
reaction was stopped after 16 h of mixing at rt. The organic solvent
was evaporated. The solid residue was partitioned between CH Cl
2
2
and saturated aqueous NaHCO . The latter was then extracted twice
3
by CH Cl . The combined organic phases were evaporated, and the
2
2
Figure 5. Excitation (blue) and emission (λ = 350 nm) spectra of
ex
12+
12+
residue was purified on a chromatographic column (SiO , MeOH/
2
tetranuclear [Sm L3 ] (green) and [Nd L3 ] (red) complexes in
4
4
4
4
CH Cl , 0−1% (v/v)). Finally, the isolated L2 was dried under
2
2
acetonitrile.
1
vacuum (31 mg, 70% yield). L2: H NMR (600 MHz, CDCl ) δ ppm
3
=
4.00 (s, 18H, CH ), 5.61 (s, 1H, CH), 5.68 (s, 1H, CH), 7.41−7.47
3
(m, 6H, CH), 7.74 (s, 3H, CH), 7.88 (s, 3H, CH), 8.03 (s, 3H, CH),
9.99 (s, 3H, CH). 13C NMR (150 MHz, CDCl ) δ ppm = 52.7, 53.4,
3
1
2+
Tetranuclear species [Ln LX ] are formed with L2 under
54.45, 57.01, 111.19, 114.83, 117.18, 117.57, 124.54, 135.85, 142.45,
4
4
1
46.7, 149.34, 152.76, 161.94, 165.51, 169.22. ESI-MS m/z: calc. for
stoichiometric conditions along the lanthanide series, but those
formed with larger cations are relatively stable with respect to
the transformation into trinuclear species, which occurs only at
higher metal excess. This observation can be explained by a
better fit of the coordinating strands with larger Ln(III) cations,
where the coordination sphere is completed with one water
molecule per metal ion (CN = 10). This coordination
environment is evidenced with the crystal structures of
+
+
2+
[
L2 + H ] , 879.3; obs., 879.7; calc. for [L2 + 2H] , 1758.5; obs.,
1
758.2.
Synthesis of L3. 5b (147 mg, 0.41 mmol) was solubilized in dry
chloroform under argon. Thionyl chloride (0.3 mL; 4.1 mmol) was
added slowly; then, 20 μL of distillated DMF was added. The reaction
was stirred at 45 °C. After 3 h, the mixture was cooled to room
temperature, and thionyl chloride was evaporated. The obtained oil
was solubilized in 15 mL of dry chloroform, under argon. Thirty
milligrams of 6 and 200 μL (1.46 mmol) of Et N were added. The
reaction mixture was stirred at rt overnight. The solvent was
evaporated under vacuum. The oil was solubilized in dichloromethane
1
2+
12+
[
Eu L2 ] and [La L2 ] , which also show one perchlorate
3
4
4
4
4
12+
anion captured inside the tetranuclear assembly. [Ln L2 ] is
4
4
still formed with smaller lanthanides, but it is readily
transformed in trinuclear complexes. In case of L3, the
tetranuclear species are observed only for larger cations La−
Eu, but the tetranuclear complexes with La(III) and Nd(III) are
particularly stable in excess of both ligand and metal. Since the
coordination moieties are identical for both ligands, this
difference is tentatively attributed to the destabilizing effect of
pending PEG groups attached to coordinating pyridine
moieties. In addition, the smaller size of the lanthanides
destabilizes the tetranuclear species, and only more compact
trinuclear complexes are observed. It can be concluded that
thermodynamic stabilities of different species evolve across the
lanthanide series and are mediated by the substituents on
coordinating pyridyldicarbonyl moieties. The spectroscopic
characterization of tetranuclear complexes in acetonitrile shows
that both triptycene-based ligands can be used for the
sensitization of lanthanides emitting not only in the visible
region but also in the near-infrared. Interestingly, complexes
with Sm(III) exhibit emission in both regions.
and washed with a saturated aqueous NaHCO solution. The organic
3
layers were combined, dried with anhydrous Na SO , filtered, and
2
4
1
evaporated under vacuum to give ligand L3 (100 mg, 95%). H NMR
(
600 MHz, CD CN) δ ppm = 1.42 (t, 9H, CH ), 3.26 (s, 9H, CH ),
3
3
3
3
6
.43 (m, 6H, CH ), 3.53 (m, 6H, CH ), 3.56 (m, 6H, CH ), 3.62 (m,
2 2 2
H, CH ), 3.82 (t, 6H, CH ), 4.33 (t, 6H, CH ), 4.42 (q, 6H, CH ),
2
2
2
2
5.6 (s, 1H, CH), 5.7 (s, 1H, CH), 7.41 (dd, 3H, CH), 7.45 (d, 3H,
CH), 7.74 (d, 3H, CH), 7.88 (d, 3H, CH), 7.99 (d, 3H, CH), 9.98 (s,
3
5
1
1
H, NH). 13C NMR (600 MHz, CD CN) δ ppm = 14.52, 52.78,
3
4.54, 58.87, 62.91, 69.55, 69.68, 70.99, 71.06, 71.4, 72.56, 111.62,
15.25, 117.32, 117.65, 124.71, 136.02, 142.66, 146.91, 149.54, 152.93,
+
+
62.13, 165.08, 168.42. ESI-MS m/z: calc. for [L3 + H ] , 1317.5;
2+
obs., 1317.6; calc. for [L3 + 2H] , 659.5; obs., 659.3.
Characterization of the Ln(III) Complexes with L2 and L3.
The Ln(III) complexes were prepared by adding a solution of Ln(III)
perchlorate salts in CD CN to an equivalent amount of L2 dissolved in
3
CDCl /CD CN (50/50 v/v). The mixture was allowed to equilibrate
3
3
The solid compounds were isolated after diffusing t-BME into the
above solutions by filtration of precipitates followed by drying under
Finally, the tetranuclear complexes are generally transformed
9
+
vacuum. The complexes were redissolved in CH
CN or CD CN for
3 3
into well-defined trinuclear complexes [Ln LX ] in metal
3
2
further analyses by mass spectrometry and NMR.
excess. Their NMR spectra are compatible with a D3-
symmetrical sandwich-like structure.
Spectroscopic and Physical Measurements. Electrospray
ionization mass spectra (ESI-MS) of Eu(III) complexes were recorded
on a Finnigan SSQ7000 instrument with the optimized ionization
temperature (120−180 °C) from acetonitrile solutions. High-
resolution spectra were recorded on a 4 GHz MaXis ultra-high-
resolution QTOF mass spectrometer from Bruker Daltonics
EXPERIMENTAL SECTION
Solvents and Starting Materials. Chemicals were purchased
from Acros Organics, Fluka AG, and Aldrich and used without further
purification unless otherwise stated. Acetonitrile and dichloromethane
■
1
13
(Germany). H and C spectra including 2D experiments (COSY,
G
Inorg. Chem. XXXX, XXX, XXX−XXX