6-Nitro-2,2′;6′,2′′-terpyridine, terpyNO2 5c. Yield 12%, 1H
NMR δ 7.36 (m, 1H), 7.88 (m, 1H), 8.02 (t, 1H, 3JHH = 7.8 Hz),
moiety. In addition to showing this unusual coordination mode,
the ligands are also sensitizers of Eu(III) and Tb(III) luminescence
and detailed photophysical characterization is presented here.
3
3
8.18 (t, 1H, JHH = 7.8 Hz), 8.26 (m, 1H), 8.54 (d, 1H, JHH
=
3
8.0 Hz), 8.58 (m, 2H), 8.72 (d, 1H, JHH = 6.0 Hz), 9.01 ppm
3
(d, 1H, JHH = 9.5 Hz); 13C{1H} NMR δ 117.5, 121.9, 125.0,
Experimental section
126.1, 137.3, 140.6, 149.4, 153.6, 156.1, 156.5 ppm.
NMR spectra were recorded on a Bruker Avance DPX 300 spec-
trometer as well as Varian 400 and 500 MHz spectrometers. All
commercially obtained reagents were of analytical grade and
used as received. Solvents were dried by standard methods.
Lanthanide salts were either used as available or dried under
reduced pressure and heating and kept in a glove box under a
controlled atmosphere (O2 < 0.5 ppm, H2O < 2 ppm). Unless
otherwise indicated, all data were collected at a constant temp-
erature of 25.0 0.1 °C.
Bis(pyridin-2-ylmethyl)(6-nitro-pyridin-2-ylmethyl)amine,
tpaNO2 6. 0.087 g (0.8 mmol) 2-bromomethyl-6-nitropyridine
along with 0.5 g (3.6 mmol) K2CO3 and 0.8 g (0.8 mmol)
2-dipicolylamine were dissolved in 25 ml dry acetonitrile. The
reaction mixture was stirred at room temperature for 24 h. The
crude material was purified via column chromatography (SiO2,
1
CH2Cl2 : MeOH 9 : 1). Yield: 75%, H NMR (CDCl3) δ 3.93 (s,
3
4H), 4.03 (s, 2H), 7.16 (m, 1H, 2H), 7.55 (d, 1H, JHH = 8.0
3
1
3
Hz), 7.67 (dt, 1H, JHH = 7.6 Hz), 7.97 (t, H, JHH = 6.0 Hz),
8.07 (m, 1H), 8.55 ppm (m, 2H); 13C{1H} NMR δ 59.1, 60.2,
117.5, 122.3, 123.2, 128.9, 136.6, 140.6, 149.1, 159.2,
160.5 ppm.
Synthesis of ligands
2-Bromo-6-nitropyridine. 2-Bromo-6-aminopyridine (10 g,
57.80 mmol) was dissolved in 10 ml of concentrated sulfuric
acid (18.1 M) with the temperature maintained below 10 °C.
This mixture was added dropwise to a mixture of 30% hydrogen
peroxide and 30% fuming sulfuric acid while the temperature
was held below 5 °C. The solution was stirred at room tempera-
ture for 48 h. The crude product was then extracted using four
150 ml portions of methylene chloride. Subsequent evaporation
of methylene chloride followed by purification using flash
chromatography with dichloromethane : hexanes (5 : 2) yielded
73% of 2-bromo-6-nitropyridine, 2 (8.55 g, 42.08 mmol). 1H
NMR (300 MHz, CDCl3, TMS internal standard): δ 7.89 (2H,
m, 3J = 1.2, 6.9, 8.4 Hz), 8.22 ppm (1H, dd, 3J = 1.8, 7.2 Hz).
5a–5c: The syntheses of 5a, 5b and 5c follow the same proto-
col: stoichiometric quantities of the appropriate tin derivative,
2-tributyltin pyridine (a), 2-methyl-6-tributyltin pyridine (b) or
6-tributylstannyl-2,2′-bipyridine (c), and 2-bromo-6-nitropyri-
dine were mixed with 1 mol% of Pd(PPh3)4 and 2.5 equiv. of
LiCl. DMF or toluene was added under an inert atmosphere. The
mixture was heated to 110 °C overnight. The solvent was
removed under reduced pressure and the reaction mixture eluted
either with dichloromethane : methanol (9 : 1) (a and b) or
dichloromethane : ethyl acetate (4 : 1) (c) over silica gel to afford
the products.
Synthesis of metal complexes
All metal complexes were synthesized by dissolving equimolar
amounts of the ligand with the corresponding lanthanide nitrate
or triflate salt (Ln = Eu or Tb) in acetonitrile. The reaction was
stirred at room temperature for 24 h, filtered, the solvent
removed under reduced pressure and the obtained solid dried in
vacuum to give the metal complexes. For X-ray crystallography
suitable crystals were obtained by either solvent diffusion or
slow solvent evaporation methods. Since bulk complexes were
not isolated, yields were not determined.
X-ray crystallographic characterization
Crystal data, data collection and refinement details for all the
compounds are given in Table 1. Selected bond lengths are given
in Table 2. Suitable crystals were mounted on a glass fiber and
placed in the low-temperature nitrogen stream. Data were
collected on a Bruker SMART CCD area detector diffractometer
equipped with a low-temperature device,34 using graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å). Data were
measured using omega scans of 0.3° per frame and a full sphere
of data was collected, for a total of 1850 frames. Multi-scan
absorption corrections were applied. Cell parameters were
retrieved using SMART35 software and refined using SAINT-
Plus36 on all observed reflections. Data reduction and correction
for Lp and decay were performed using the SAINTPlus36 soft-
ware. Absorption corrections were applied using SADABS.37
The structures were solved by direct methods and refined by
least squares methods on F2 using the SHELXTL38 program
package. All atoms were refined anisotropically. The hydrogen
atoms were added geometrically and their parameters constrained
to the parent site. For coordinated water molecules and hydrox-
ide species, the hydrogen atoms could not be located on the
difference map or added geometrically and have been omitted,
although the formulae are correct. In the case of structure 5a, a
low completeness is observed, due to the small size of the
crystal. Molecular structure representations were created with
ORTEP39 for the thermal ellipsoid diagrams and with
6-Nitro-2,2′-bipyridine bpyNO2 5a. X-ray quality crystals
were isolated from acetonitrile. Yield: 60%, λmax (MeCN) = 231,
1
3
280 nm, H NMR (CDCl3): δ 7.38 (ddd, 1H, J = 1.2, 4.8, 1.8
3
3
Hz), 7.86 (ddd, 1H, J = 1.8, 4.5, 4.8 Hz), 8.12 (dd, 1H, J =
4.5, 4.8 Hz), 8.23 (dd, 1H, J = 0.9, 7.8 Hz), 8.54 (dd, 1H, J =
3
3
3
1.2, 8.1 Hz), 8.70 (dd, 1H, J = 1.2, 5.1 Hz), 8.82 ppm (dd, 1H,
3J = 0.9, 7,8 Hz); 13C{1H} NMR δ 117.9, 122.4, 125.5, 126.5,
137.9, 141.0, 149.9, 154.1, 156.5 ppm.
6′-Methyl-6-nitro-2,2′-bipyridine, MebpyNO2 5b. Yield: 41%,
λmax (MeCN) = 231, 287 nm, 1H NMR (CDCl3): δ 2.65 (s, 3H),
3
3
7.26 (s, 1H), 7.75 (dd, 1H, J = 7.8 Hz), 8.12 (dd, 1H, J = 7.8,
3
3
0.9 Hz), 8.22 (dd, 1H, J = 1.2, 8.1 Hz), 8.35 (d, 1H, J = 7.8
Hz), 8.85 ppm (dd, 1H, 3J = 1.2, 7.8 Hz); 13C{1H} NMR δ 24.6,
117.3, 118.8, 124.6, 126.1, 137.5, 140.4, 152.9, 156.4,
158.3 ppm.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 11212–11218 | 11213