Structural diversity in neodymium bipyrimidine compounds with near infrared luminescence: From mono- and binuclear complexes to metal-organic frameworks

Article abstract of DOI:10.1021/ic800967x

Treatment of Nd(NO₃)₃ with 2,2′-bipyrimidine (bpm) afforded the mononuclear adduct [Nd(NO₃)₃(bpm)(MeOH) ₂] (1) after recrystallization from MeOH, while reactions of hydrated NdCl₃ and various β-diketonates in the presence of bpm gave the binuclear compounds [{Nd(dbm)₃(THF)}₂(μ-bpm)] (2) and [{Nd(bta)₃(MeOH)}₂(μ-bpm)]·bpm (3·bpm) (Hdbm = dibenzoylmethane, Hbta = 4,4,4-trifluoro-1-phenyl-1,3-butanedione) and the one-dimensional coordination polymer [Nd(tta)₃(μ-bpm) ·MeOH]_∞ (4·MeOH) (Htta = 2- thenoyltrifluoroacetone). The crystal structures of 2-4 demonstrate that the bpm molecule can act as a planar bridging ligand between two lanthanide ions as large as Nd³⁺. Luminescence measurements revealed that near-IR emission from neodymium can be obtained after excitation of either the bpm or the β-diketonate ligand, and that an energy transfer occurs from the β-diketonate group to the bpm molecule.

Full text of DOI:10.1021/ic800967x

Inorg. Chem. 2008, 47, 10398-10406
Structural Diversity in Neodymium Bipyrimidine Compounds with Near
Infrared Luminescence: from Mono- and Binuclear Complexes to
Metal-Organic Frameworks
Gae¨l Zucchi,*,† Olivier Maury,‡ Pierre Thue´ry,† and Michel Ephritikhine†
CEA, IRAMIS, SCM, Laboratoire Claude Fre´jacques, CNRS URA 331,
91191 Gif-sur-YVette, France, and Laboratoire de Chimie, UMR 5182 CNRS-ENS Lyon,
46 alle´e d’Italie, 69364 Lyon Cedex 07, France
Received May 28, 2008
Treatment of Nd(NO3)3 with 2,2′-bipyrimidine (bpm) afforded the mononuclear adduct [Nd(NO3)3(bpm)(MeOH)2] (1)
after recrystallization from MeOH, while reactions of hydrated NdCl3 and various ꢀ-diketonates in the presence of
bpm gave the binuclear compounds [{Nd(dbm)3(THF)}2(µ-bpm)] (2) and [{Nd(bta)3(MeOH)}2(µ-bpm)] · bpm (3· bpm)
(Hdbm ) dibenzoylmethane, Hbta ) 4,4,4-trifluoro-1-phenyl-1,3-butanedione) and the one-dimensional coordination
polymer [Nd(tta)3(µ-bpm) · MeOH]∞ (4· MeOH) (Htta ) 2-thenoyltrifluoroacetone). The crystal structures of 2-4
demonstrate that the bpm molecule can act as a planar bridging ligand between two lanthanide ions as large as
Nd3+. Luminescence measurements revealed that near-IR emission from neodymium can be obtained after excitation
of either the bpm or the ꢀ-diketonate ligand, and that an energy transfer occurs from the ꢀ-diketonate group to the
bpm molecule.
Introduction
emission bands combined with relatively long excited-state
lifetimes that result from intraconfigurational 4f-4f transitions.
In addition, the color of the emission can easily be tuned
from the UV to the near-infrared (NIR) region of the elec-
tromagnetic spectrum by a proper choice of the metal ion.10
Luminescence in the NIR is of interest for medical applica-
tions, as biological tissues are transparent in this spectral
range,11 or for telecommunications. In particular, materials
incorporating Nd3+ and Er3+ ions are well-suited because
their emission lines are relevant to the telecommunication
window at 1.34 and 1.54 µm, respectively; thus, they are
used for optical amplification for integrated optics,12-14 and
Nd3+ is widely employed in solid-state lasers.15 It has also
been demonstrated that these IR-emitters have a great poten-
tial in photonic devices such as light-emitting diodes.12,16-20
Much attention is currently paid to materials incorporating
lanthanide ions which, in view of their flexible coordination
geometry and unique optical and magnetic features, are quite
appropriate for engineering solids with tunable structural and
physical properties. Lanthanide-based systems find applica-
tions as light emitters,1-4 (photo)catalysts,5-7 and magnetic
materials.8,9 Regarding emission properties, the huge interest
devoted to lanthanide ions is due to their extremely narrow
* To whom correspondence should be addressed. E-mail: gael.zucchi@
cea.fr.
†
CEA.
ENS Lyon.
‡
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10398 Inorganic Chemistry, Vol. 47, No. 22, 2008
10.1021/ic800967x CCC: $40.75  2008 American Chemical Society
Published on Web 10/11/2008

Structural DiWersity in Neodymium-Bipyrimidine Complexes
Among the materials investigated, coordination polymers
with metal-organic frameworks have emerged as a new class
with intriguing structural diversity and physical properties.
Although lanthanides have often been regarded as unsuitable
for the design of coordination polymers because of the lack
of directional bonds making geometries difficult to predict
and control, such hybrid materials have become a field of
thorough investigations. Materials with unusual topologies21
such as three-dimensional porous solids22-25 or with unique
luminescent or magnetic properties have been synthe-
sized.26-28 The design and construction of such materials
require a precise knowledge of the structure and behavior
of the components, especially the key role of the organic
linker in the structural assembly and the physicochemical
properties. A number of lanthanide-based coordination
polymers have been built up with a variety of carboxylic
acids22 or anionic oxygen ligands such as phosphates and
phosphonates.29-32 In contrast, molecules with nitrogen
donors have been much less considered as associating ligands
for the construction of metal organic frameworks with
f-elements.33-35 In particular, 2,2′-bipyrimidine (bpm), whose
numerous and various complexes of d transition metals have
received great attention for their photocatalytic and magnetic
properties,36,37 is found in a relatively limited number of
structurally characterized polynuclear lanthanide compounds:
a few heterometallic 4f-d systems,38-40 the homobinuclear
complexes [{(C5Me5)2Eu}2(µ-bpm)],41 [{Ln(NO3)3(bpm)}2(µ-
bpm)] (Ln ) Eu, Tb)42 and those of general formula [{Ln(ꢀ-
diketonate)3}2(µ-bpm)] (Ln ) Eu, Tb),43-47 and the one-
dimensional arrays [Ln(hfac)3(µ-bpm)]∞ (Ln ) Eu,48 Gd, and
Nd;49 hfac ) hexafluoroacetylacetonate). In the search of
NIR luminescent neodymium complexes, we studied the sys-
tems composed of Nd3+ ions, bpm, and nitrate or ꢀ-diketo-
nate ligands; these latter are commonly used as antenna
ligands to overcome the low intensity of Laporte forbidden
4f-4f transitions, but their behavior in the presence of the
bpm ligand has never been characterized. We have isolated
a series of compounds showing diverse nuclearity and dimen-
sionality, including a rare example of a structurally charac-
terized mononuclear lanthanide complex with the bpm ligand,
and of binuclear and one-dimensional polymeric compounds
of a light lanthanide ensured by bridging bpm ligands. Here
we present the synthesis and crystal structure of these
complexes and their luminescent properties in the solid state.
Experimental Section
Materials and Methods. Reagents and solvents were purchased
from Aldrich and used without further purification except the
tetrahydrofuran (THF) used for the synthesis of 1 that was dried
by standard methods and distilled before use. [Nd(dbm)3(H2O)2]
was synthesized by a previously described synthetic procedure.50
IR samples were prepared as KBr pellets, and the spectra were
recorded with a Perkin-Elmer FTIR 1725X spectrometer. Elemental
analyses were performed by Analytische Laboratorien at Lindlar
(Germany).
Synthesis of [Nd(NO3)3(bpm)(MeOH)2] (1). A 50 mL round-
bottom flask was charged with bpm (292 mg, 1.80 mmol) and
Nd(NO3)3 ·6H2O (609 mg, 1.8 mmol), and THF (10 mL) was
condensed in it. After stirring for 1 h at room temperature, the white
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Inorganic Chemistry, Vol. 47, No. 22, 2008 10399

Zucchi et al.
Table 1. Crystal Data and Structure Refinement Details
1·MeOH
2
3·bpm
4
empirical formula
C11H18N7NdO12
584.56
monoclinic
P21/n
8.3511(7)
15.6224(9)
16.0769(13)
90
105.005(4)
90
2025.9(3)
4
C
106H88N4Nd2O14
C78H56F18N8Nd2O14
1959.79
C32H18F9N4NdO6S3
M (g mol-1
)
1930.28
monoclinic
P21/c
9.7576(5)
15.2085(9)
29.2564(14)
90
92.128(3)
90
4338.6(4)
2
965.92
monoclinic
C2/c
22.7458(14)
16.6276(10)
22.4826(15)
90
114.619(6)
90
7730.2(9)
8
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
triclinic
j
P1
9.8931(4)
12.3048(7)
16.4237(10)
101.910(3)
103.016(3)
90.116(3)
1903.53(18)
1
Z
Dcalcd (g cm-3
)
1.917
2.639
1156
1.478
1.254
1968
1.710
1.464
974
1.660
1.595
3800
µ (Mo KR) (mm-1
F(000)
)
reflns collected
indep reflns
40482
3840
93639
8208
51768
7203
72350
7298
obsd reflns [I > 2σ(I)]
Rint
3350
0.028
7101
0.022
6414
0.076
4765
0.057
params refined
283
568
542
496
R1
wR2
S
0.025
0.056
1.045
-0.86
0.41
0.022
0.056
1.046
-0.57
0.30
0.033
0.075
1.050
-1.25
0.47
0.055
0.145
1.061
-0.98
0.90
∆Fmin (e Å-3
)
)
∆Fmax (e Å-3
powder was filtered off, washed with THF (3 × 5 mL) and dried
under vacuum. Crystallization from methanol gave light purple
crystals of 1·MeOH (480 mg, 48%). IR (KBr): 3460 cm-1 (νOH);
1574 and 1560 cm-1 (νc-c bpm); 1470, 1305, 1027 cm-1 (νNO). The
crystals were found to desolvate upon drying under vacuum to give
a purple powder of 1. Anal. Calcd for C10H14N7O11Nd: C, 21.74;
H, 2.55; N, 17.35%. Found: C, 21.83; H, 2.67; N, 17.79%.
Synthesis of [{Nd(dbm)3(THF)}2(µ-bpm)] (2). A 50 mL round-
bottom flask was charged with Hdbm (213 mg, 0.95 mmol) in
ethanol (10 mL) and aqueous 1 M NaOH (0.95 mL) was added.
The solution was stirred at room temperature for 15 min and bpm
(50 mg, 0.32 mmol) in ethanol (10 mL) and NdCl3 ·6H2O (113
mg, 0.32 mmol) were successively added. A white precipitate
immediately formed and after stirring for 1 h, it was filtered off.
Greenish-yellow crystals of 2 were obtained by slow diffusion of
pentane (10 mL) into a THF solution (5 mL) of the crude reac-
tion product (252 mg, 41%, based on Nd). Anal. Calcd for
Synthesis of [Nd(nta)3(bpm)]∞ (5). By following the same pro-
cedure as for the preparation of 2, 3, and 4, the nta compound (Hnta
) 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione) was synthesized
from NdCl3 ·6H2O (113 mg, 0.32 mmol), Hnta (252 mg, 0.95
mmol), aqueous 1 M NaOH (0.95 mL), and bpm (50 mg, 0.32
mmol). Pale purple crystals were obtained by cooling a hot THF
solution of the crude reaction product (305 mg, 88%). Anal. Calcd
for C50H30F9N4O6Nd: C, 54.25; H, 2.73; N, 5.06%. Found: C, 54.43;
H, 2.96; N, 4.89%. IR (KBr): 1614 cm-1 (νc-o); 1570, 1559 cm-1
(νc-c bpm).
Crystallographic Data Collection and Structure Determina-
tion. The data were collected at 100(2) K on a Nonius Kappa-
CCD area detector diffractometer51 using graphite-monochromated
Mo KR radiation (λ 0.71073 Å). The crystals were introduced in
glass capillaries with a protecting “Paratone-N” oil (Hampton
Research) coating. The unit cell parameters were determined from
ten frames, then refined on all data. The data (combinations of ꢁ-
and ω-scans giving complete data sets up to θ ) 25.7°) were
processed with HKL2000.52 The structures were solved by direct
methods (1·MeOH, 2 and 4) or by Patterson map interpretation
(3·bpm) with SHELXS-97, expanded by subsequent Fourier-
difference synthesis, and refined by full-matrix least-squares on F2
with SHELXL-97.53 Absorption effects were corrected empirically
with the program SCALEPACK.52 All non-hydrogen atoms were
refined with anisotropic displacement parameters. The hydrogen
atoms bound to oxygen atoms were found on Fourier-difference
maps when present, and all the others were introduced at calculated
positions. All were treated as riding atoms with a displacement
parameter equal to 1.2 (OH, CH, CH2) or 1.5 (CH3) times that of
the parent atom. Some voids in the structure of 4 indicate the
presence of unresolved solvent molecules; the available volume
amounts to approximately 90 Å3 per asymmetric unit (i.e., per Nd
atom), and it may correspond to one methanol molecule (which
was however not added to the crystal chemical formula). Crystal
data and structure refinement parameters are given in Table 1. The
C
106H88N4O14Nd2: C, 65.95; H, 4.59; N, 2.90%. Found: C, 65.46;
H, 4.20; N, 2.97%. IR (KBr): 1595 cm-1 (νc-o); (νc-c bpm) not
identifiable because of absorption of dbm.
Synthesis of [{Nd(bta)3(MeOH)}2(µ-bpm)]·bpm (3 ·bpm). By
following the same procedure as for the preparation of the dbm
derivative, the binuclear bta compound was synthesized from
NdCl3 ·6H2O (113 mg, 0.32 mmol), Hbta (205 mg, 0.95 mmol),
aqueous 1 M NaOH (0.95 mL) and bpm (50 mg, 0.32 mmol).
Recrystallization from methanol afforded pale purple crystals
of 3 · bpm (210 mg, 33% based on Nd). Anal. Calcd for
C78H56F18N8O14Nd2: C, 47.37; H, 2.85; N, 5.67%. Found: C, 47.57;
H, 3.08; N, 5.68%. IR (KBr): 3460 cm-1 (νOH); 1614 cm-1 (νc-o);
1580, 1575, 1561 cm-1 (νc-c bpm).
Synthesis of [Nd(tta)3(bpm)]∞ (4). By following the same pro-
cedure as for the preparation of 2 and 3, the coordination polymer
was synthesized from hexahydrated NdCl3 (113 mg, 0.32 mmol),
Htta (211 mg, 0.95 mmol), aqueous 1 M NaOH (0.95 mL) and
bpm (50 mg, 0.32 mmol), and isolated as an off-white powder.
Crystals of 4·MeOH were grown from a saturated methanol solution
(240 mg, 78%). Anal. Calcd for C32H18F9N4O6S3Nd: C, 39.42; H,
1.86; N, 5.75%. Found: C, 39.20; H, 2.09; N, 5.61%. IR (KBr):
1614 cm-1 (νc-o); 1575, 1560 cm-1 (νc-c bpm). These data are
consistent with the absence of methanol after drying under vacuum.
(51) Kappa-CCD Software; Nonius BV: Delft, The Netherlands, 1998.
(52) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(53) Sheldrick, G. M. SHELXS-97 and SHELXL-97; University of Go¨ttin-
gen: Go¨ttingen, Germany, 1997.
10400 Inorganic Chemistry, Vol. 47, No. 22, 2008

Structural DiWersity in Neodymium-Bipyrimidine Complexes
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complex
1·MeOH
Nd-N(1)
2.655(2)
Nd-N(2)
2.645(2)
2.5366(18)
2.557(2)
2.5737(19)
2.4855(18)
50.67(6)
Nd-O(1)
2.5256(18)
2.5047(18)
2.5673(17)
2.4856(19)
61.31(6)
Nd-O(2)
Nd-O(4)
Nd-O(7)
Nd-O(10)
N(1)-Nd-N(2)
O(4)-Nd-O(5)
Nd-O(5)
Nd-O(8)
Nd-O(11)
O(1)-Nd-O(2)
O(7)-Nd-O(8)
50.78(6)
49.72(5)
molecular plots were drawn with SHELXTL54 and ORTEP-3/POV-
Ray.55
Luminescence Spectra. All measurements were performed on
dried crystals after being crushed in a mortar. The luminescence
spectra in the solid state and in solution were recorded on a Horiba-
Jobin Yvon Fluorolog-3 spectrofluorimeter, equipped with a three
slit double grating excitation and emission monochromator with
dispersions of 2.1 nm/mm (1200 grooves/mm). The steady-state
luminescence was excited by unpolarized light from a 450 W xenon
CW lamp and detected at an angle of 90° for diluted solution
measurements or at 22.5° for solid state measurements by a red-
sensitive Hamamatsu R928 photomultiplier tube. Spectra were
reference corrected for both the excitation source light intensity
variation (lamp and grating) and the emission spectral response
(detector and grating). Uncorrected near-infrared spectra were
recorded using a liquid nitrogen cooled, solid indium/gallium/
arsenide detector (850-1600 nm).
Figure 1. View of [Nd(NO3)3(bpm)(MeOH)2] (1). Carbon-bound hydrogen
atoms are omitted. Displacement ellipsoids are drawn at the 50% probability
level.
and 2.60(1) Å are, respectively, 0.08 and 0.03 Å larger than
the Eu-N distance corresponding to the terminal bpm ligand
in the europium complex; the difference of 0.05 Å between
the Nd-N(bpm) and Nd-N(bipy) distances could be related
to the weaker Lewis basicity of the bpm versus bipy ligand.
The IR spectrum of 1 exhibits a strong absorption as a
quasi-symmetric doublet at 1574 and 1560 cm-1, diagnostic
of a terminal chelating bpm ligand;58-60 the bands at 1470,
1305 and 1027 cm-1 are characteristic of bidentate nitrate
groups.42,61
Absorption Spectra. Solid UV-vis and near-infrared spectra
were recorded from Teflon cells with Suprasil 300 quartz windows,
using a Perkin-Elmer Lambda 950 instrument and PE Winlab
software.
The synthesis of 1 is in contrast with that of the dinucl-
ear complexes [{Ln(NO3)3(bpm)}2(µ-bpm)] (Ln ) Eu, Tb)
whichweretheonlyisolableproductsfromtheLn(NO3)3 ·6H2O/
bpm reaction system, whatever the reagent ratio, the solvent,
and the crystallization method used.42 Reaction of NdCl3 with
3 equiv of dibenzoylmethane in a basic ethanolic solution
and in the presence of 1 equiv of bpm immediately afforded
a yellow precipitate which was dissolved in THF, and yellow
greenish crystals of [{Nd(dbm)3(THF)}2(µ-bpm)] (2) were
obtained in 41% yield by slow diffusion of pentane into the
solution. The centrosymmetric crystal structure of 2 is shown
in Figure 2, and selected bond lengths and angles are listed
in Table 3. The structure of complex 2 differs from that of
the europium analogue [{Eu(dbm)3}2(µ-bpm)]43 by the co-
ordination of a supplementary THF molecule to the larger
Nd3+ ion. The metal center is nine coordinate, in a distorted
monocapped square antiprismatic configuration; the two
square faces are defined by the O(1C), O(2A), O(2B), O(2C)
and O(1), O(1A), O(1B), N(2′) atoms and N(1) is in capping
position. The Nd-N(1) and Nd-N(2′) distances of 2.7611(16)
and 2.7454(15) Å are 0.1 Å larger than those measured in
the mononuclear complex 1; such a difference was found
between the Eu-N distances of the bridging and terminal
bpm ligands in [{Eu(NO3)3(bpm)}2(µ-bpm)].42 The average
Results and Discussion
Synthesis and Characterization of the Complexes. Tre-
atment of Nd(NO3)3 · 6H2O with 1 equiv of bpm in THF led
to the precipitation of an off-white powder. Pale purple
crystals of [Nd(NO3)3(bpm)(MeOH)2] · MeOH (1 · MeOH)
were obtained by crystallization from methanol in 48% yield.
After [{Ln(NO3)3(bpm)}2(µ-bpm)] (Ln ) Eu, Tb),42 1 is a
new lanthanide nitrate complex with a bpm ligand and also
a rare example of a lanthanide complex with bpm as a
terminal ligand. 1 and the recently reported [Tb(hfac)3(bpm)-
(H2O)] compound49 are the only mononuclear lanthanide
compounds comprising bpm to have been crystallographi-
cally characterized. A view of 1 is shown in Figure 1 and
selected bond lengths and angles are listed in Table 2. The
ten-coordinate metal is in a distorted bicapped square anti-
prismatic environment, the square bases defined by the N(2),
O(1), O(4), O(11) and O(2), O(5), O(8), O(10) atoms, and
the N(1) and O(7) atoms in capping positions. The average
Nd-O(NO3) of 2.54(2) Å is identical to that measured in
[Nd(NO3)3(bipy)2] (bipy ) 2,2′-bipyridine);56 these Nd-O
distances are 0.08 Å larger than the average Eu-O distance
measured in [{Eu(NO3)3(bpm)}2(µ-bpm)], reflecting the
variation in the radii of the Nd3+ and Eu3+ ions.57 The
average Nd-N(bpm) and Nd-N(bipy) distances of 2.650(5)
(58) Be´re´zovsky, F.; Harem, A. A.; Triki, S.; Sala Pala, J.; Molinie´, P.
Inorg. Chim. Acta 1999, 284, 8.
(59) Hong, D. M.; Chu, Y. Y.; Wei, H. H. Polyhedron 1996, 15, 447.
(60) Julve, M.; Verdaguer, M.; De Munno, G.; Real, J. A.; Bruno, G. Inorg.
Chem. 1993, 32, 795.
(61) Fawcett, J.; Platt, A. W. G.; Vickers, S.; Ward, M. D. Polyhedron
2004, 23, 2561.
(54) Sheldrick, G. M. SHELXTL, version 5.1; Bruker AXS Inc.: Madison,
WI, 1999.
(55) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(56) Bower, J. F.; Cotton, S. A.; Fawcett, J.; Russell, D. R. Acta
Crystallogr., Sect. C 2000, 56, e8.
(57) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10401

Zucchi et al.
Figure 2. View of [{Nd(dbm)3(THF)}2(µ-bpm)] (2). Hydrogen atoms are
omitted. Displacement ellipsoids are drawn at the 50% probability level.
Symmetry code: ′ ) 1 -x, 2 - y, -z.
Figure 3. View of [{Nd(bta)3(MeOH)}2(µ-bpm)] (3). Fluorine atoms and
carbon-bound hydrogen atoms are omitted. Hydrogen bonds are shown as
dashed lines. Displacement ellipsoids are drawn at the 30% probability level.
Symmetry codes: ′ ) 1 - x, 1 - y, 1 - z; ′′ ) 2 - x, 1 - y, 2 - z.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complexes 2
and 3·bpma
2
3·bpm
Nd-N(1)
2.7611(16)
2.7454(15)
2.4506(13)
2.3687(13)
2.4300(13)
2.4077(13)
2.3971(13)
2.4879(13)
2.6322(13)
58.74(4)
2.756(3)
2.678(3)
2.4190(19)
2.4162(19)
2.4270(19)
2.4490(19)
2.467(2)
2.434(2)
2.551(2)
59.56(7)
71.56(7)
70.34(6)
67.99(7)
Nd-N(2′)
Nd-O(1A)
Nd-O(2A)
Nd-O(1B)
Nd-O(2B)
Nd-O(1C)
Nd-O(2C)
Nd-O(1)
N(1)-Nd-N(2′)
O(1A)-Nd-O(2A)
O(1B)-Nd-O(2B)
O(1C)-Nd-O(2C)
70.70(4)
71.20(4)
69.45(4)
a Symmetry codes: ′ ) 1 - x, 2 - y, -z for complex 2; ′ ) 1 - x, 1 -
y, 1 - z for complex 3.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for Complex 4a
Nd-N(1)
2.769(5)
2.803(5)
2.453(5)
2.439(4)
2.446(5)
57.72(14)
71.14(17)
68.61(16)
Nd-N(2′)
2.808(5)
2.703(5)
2.395(4)
2.451(5)
2.454(4)
58.43(14)
68.96(14)
171.23(15)
Nd-N(3)
Nd-N(4′′)
Nd-O(1A)
Nd-O(2A)
Figure 4. Partial view of [Nd(tta)3(µ-bpm)]∞ (4). Fluorine and hydrogen
atoms are omitted. Displacement ellipsoids are drawn at the 20% probability
level. Symmetry codes: ′ ) 1 - x, 2 - y, 1 - z; ′′ ) 1/2 - x, 3/2 - y, 1
- z.
Nd-O(1B)
Nd-O(2B)
Nd-O(1C)
Nd-O(2C)
N(1)-Nd-N(2′)
O(1A)-Nd-O(2A)
O(1C)-Nd-O(2C)
N(3)-Nd-N(4′′)
O(1B)-Nd-O(2B)
N(2′)-Nd-N(3)
a Symmetry codes: ′ ) 1 - x, 2 - y, 1 - z; ′′ ) 1/2 - x, 3/2 - y, 1 -
z.
Nd-O(dbm) distance of 2.42(4) Å can be compared with
that of 2.39(2) Å in [Nd(dbm)3(phen)].62 The Nd · · · Nd′
distance is 7.1831(3) Å.
Changing Hdbm with Hbta in its reaction with NdCl3 in
the presence of bpm led to the formation, after crystallization
from methanol, of pale purple crystals of [{Nd(bta)3(MeOH)}2-
(µ-bpm)] · bpm (3 · bpm). A view of 3 · bpm is shown in
Figure 3 and selected bond lengths and angles are listed in
Table 3. The Nd atom is here again in a distorted mono-
capped square antiprismatic environment with N(1) in
capping position. However, all three ꢀ-diketonate ligands
labeled A-C (instead of two in 2, labeled A and B) are
spanning edges between the two square faces which are
defined by the O(1), O(1C), O(2A), O(2B) and N(2′), O(1A),
Figure 5. View of the polymeric arrangement in complex 4. Hydrogen
atoms are omitted. Atoms are represented as spheres of arbitrary radii.
O(2C), O(1B) atoms. The planar bpm ligand is not sym-
metrically chelated, the Nd-N(1) distance being 0.08 Å
larger than Nd-N(2′). The average Nd-O(bta) distance of
2.435(18) Å is identical to the Nd-O(dbm) distance in 2;
the Nd · · · Nd′ separation is 7.0885(5) Å.
Since the crystal structures of 2 and 3 demonstrated that
the bpm molecule was capable of bridging Nd3+ ions in the
presence of ꢀ-diketonates, the supposed mononuclear struc-
(62) Sun, L. N.; Zhang, H. J.; Meng, Q. G.; Liu, F. Y.; Fu, L. S.; Peng,
C. H.; Yu, J. B.; Zheng, G. L.; Wang, S. B. J. Phys. Chem. B 2005,
109, 6174.
10402 Inorganic Chemistry, Vol. 47, No. 22, 2008

Structural DiWersity in Neodymium-Bipyrimidine Complexes
Figure 6. Solid state absorption spectra of 1 (purple), 2 (blue), 3 (orange), 4 (green), and 5 (black) (5% in MgO).
purple crystals composed of polymeric chains of [Nd(tta)3(µ-
bpm) ·MeOH]∞ (4·MeOH); views of 4 are shown in Figures
4 and 5 and selected bond lengths and angles are listed in
Table 4. The metal center is in a distorted bicapped square
antiprismatic environment, with the two square faces defined
by the N(1), O(1C), O(2A), O(2B) and N(4′′), O(1A), O(1B),
O(2C) atoms, and N(2′) and N(3) in capping positions. The
average Nd-N and Nd-O distances of 2.77(4) and 2.44(2)
Å, respectively, are quite similar to those measured in
complexes 2 and 3; these values can be compared with those
of 2.70(3) and 2.44(6) Å in [Nd(tta)3(bipy)].63
The IR spectrum of 4 exhibits a very asymmetric doublet
at 1575 and 1560 cm-1, characteristic of a bridging bpm
ligand; this peak is masked in 2 by those of the dbm ligand.
The IR spectrum of 3 · bpm shows, in addition to the ab-
sorption bands at 1575 and 1561 cm-1, a peak at 1580 cm-1
corresponding to the free bpm molecule. From the IR
spectrum (asymmetric doublet at 1570 and 1559 cm-1), it is
likely that the compound isolated as a white powder by using
the ꢀ-diketone Hnta, the elemental analysis of which cor-
responds to the formula [Nd(nta)3(µ-bpm)] (5), adopts a
polymeric crystal structure similar to that of 4, with bridging
bpm ligands.
The distinct crystal structures of [{Nd(dbm)3(THF)}2(µ-
bpm)] (2) and [{Eu(dbm)3}2(µ-bpm)],43 where the lanthanide
ions are respectively nine- and eight-coordinate, can be
accounted for by the larger size of the Nd3+ versus Eu3+
ion.57 This explanation also holds for the comparison of the
structures of [Nd(tta)3(µ-bpm) · MeOH]∞ (4 · MeOH) and
[{Eu(tta)3}2(µ-bpm)],43 where the metal centers are respec-
tively ten- and eight-coordinate. It is noteworthy that the
formation of the polymeric compound 4 is preferred to that
of a binuclear complex of the type [{Nd(tta)3(S)n}2(µ-bpm)]
Figure 7. Top: emission spectrum of 1 in the near-IR (λexc ) 270 nm);
bottom: emission spectra in the UV-visible range (λexc ) 270 nm) of 1 at
298 K in the solid state (purple) and in a DMF/MeOH (4:1) solution (black),
and in frozen solution (red).
ture of [Nd(tta)3(bpm)] appeared questionable.44,45 Indeed,
crystallization of this complex from methanol gave pale
(63) Leipoldt, J. G.; Bock, L. D. C.; Basson, S. S.; Laubscher, A. E. J. Inorg.
Nucl. Chem. 1976, 38, 1477.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10403

Zucchi et al.
Figure 8. Energy level diagram for 1 in the solid state (left) and energy transfers occurring in 1-5.
(S ) solvent molecule), similar to 2 and 3; the reasons for
this difference are not obvious. The syntheses of the binuclear
complex 2 and of the polymeric compound 4 (and presum-
ably 5), which are respectively nine and ten coordinate, are
also in contrast with those of the mononuclear complexes
[Nd(dbm)3(phen)]62 and [Nd(tta)3(bipy)];63 in these latter,
the greater electron richness of the metal center, because of
the more basic character and electron-donating capacity of
bipy or phen than bpm, would impede the coordination of a
supplementary ligand.
Photophysical Properties. The solid state absorption
spectra of the compounds (5% in MgO) are shown in Figure
6. They all exhibit the typical parity-forbidden narrow
absorption bands from the 4I9/2 ground-state to various excited
states of the Nd3+ ion and spin-allowed broad bands cor-
responding to π-π* transitions within the electronic states
of the ligands. These spectra clearly show the absorption
bands of the bpm and ꢀ-diketonate ligands. The spectrum
of 1 shows the absorption band due to bpm with a maximum
at 249 nm. This band also appears for the other complexes,
at 249 nm for 2 and 3 and at 262 nm for 4 and 5. The
ꢀ-diketonate ligands have absorption maxima at 354 nm
(shoulder at 395 nm), 335 nm, 344 nm (shoulder at 366 nm),
and 348 nm (shoulder at 372 nm) in 2, 3, 4, and 5, res-
pectively.
The assignment of the triplet emission has been established
from variable temperature luminescence experiments in a 4:1
DMF: MeOH mixture. At room temperature, the emission
spectrum shows only one broad band with a maximum at
310 nm, while the spectrum recorded at 77 K shows an
emission band with a maximum at 431 nm which is similar
to that observed in the solid state, in addition to the broad
band centered at 300 nm. This assignment is further
reinforced by the results we have obtained with the analogous
terbium complex for which excitation at 270 nm afforded
an intense terbium emission with no other emission band.
Phosphorescence from the bpm is not observed for this
complex; this indicates a relatively efficient energy transfer
to the Tb(III) excited level.64 These measurements definitely
demonstrate that the broad band observed on the emission
spectrum of 1 is due to the phosphorescence of bpm. The
low efficiency of the bpm-to-Nd3+ energy transfer is
explained by the difference in energy between the triplet state
4
of the bpm and the F3/2 excited-state of Nd3+ that is about
12000 cm-1. This value is too high to afford an efficient
energy transfer.65 Furthermore, the relatively low emission
intensity of the Nd(III) ion in this complex is also due to
the presence of O-H oscillators in the first coordination
sphere that efficiently quench the luminescence by a non-
radiative energy transfer pathway. The data obtained above
from absorption and emission measurements enable the
establishment of the energy level diagram for 1 (Figure 8,
left).
The emission spectra of complexes 2-5 in the visible and
near-IR regions are shown in Figure 9. These spectra show
well-structured near-IR emission bands of Nd3+ when excited
at 260-270 nm within the bpm π-π* bands, or after
When excited at 270 nm within a π-π* transition of bpm,
1 shows the characteristic (although weak) narrow near-IR
emission bands from the 4F3/2 excited-state to the 4IJ manifold
with maxima at 910, 1061, and 1338 nm for J ) 9/2, 11/2,
and 13/2, respectively (Figure 7, top). This demonstrates that
bpm can sensitize the Nd3+ luminescence in the near-IR. The
spectrum also shows two bands which are due to emission
from the first excited singlet (maximum at 300 nm) and triplet
(structured band with maxima at 416 and 436 nm, and a
shoulder at 460 nm) states of the bpm (Figure 7, bottom).
(64) The results on the Tb(III) complex will be published elsewhere.
(65) Steemers, F. J.; Verboom, W.; Reinhoudt, D. N.; van der Tol, E. B.;
Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 9408.
10404 Inorganic Chemistry, Vol. 47, No. 22, 2008

Structural DiWersity in Neodymium-Bipyrimidine Complexes
tonate ligands are excited and reflects an energy transfer from
the singlet state of the ꢀ-diketonates to the triplet state of
bpm. As the emission of the triplet states of the dbm, bta,
tta, and nta ligands of the [Ln(ꢀ-diketonate)3(H2O)2] com-
plexes have been located at 490,66,67 470,68 525,69 and 57170
nm, respectively, the ꢀ-diketonates-to-bpm energy transfer
is made possible by the position of the triplet state of the
bpm that lies at an energy localized in-between the excited
levels of the ꢀ-diketonates (Figure 8, right). Complex 2
behaves in a different way since excitation of the dbm ligands
at 353 nm results in a broad structured band between 380
and 570 nm. This band is due to phosphorescence from the
bpm and dbm ligands, as shown in Figure 10 where we have
reported the emission spectra of [Nd(dbm)3(H2O)2],66,67 1,
and 2. Thus, an energy transfer occurs from the dbm to the
bpm, but it is not as efficient as it is for complexes 3-5.
Furthermore, excitation at 270 nm does not induce any
phosphorescence from bpm. As yet, we have not been able
to explain this different behavior experimentally observed
for 2. Finally, the dbm-bpm system is a better sensitizer than
the hfac-bpm one for Nd3+ as luminescence from this ion in
2 has been detected in a methanol solution at room
temperature, while no emission was observed for the hfac
derivative49 (Supporting Information).
Conclusion
We have described the X-ray crystal structures of four
neodymium(III) complexes with bpm and nitrates or various
ꢀ-diketonate ligands that are of interest for luminescence
studies. These systems exhibit a remarkable structural diver-
sity, as we have obtained one mononuclear and two
dinuclear complexes and one coordination polymer by
using identical synthetic procedures and stoichiometric
quantities. They show that bpm can both act as a bridging
or a terminal ligand in lanthanide chemistry. Investigation
of the photophysical properties of compound 1 allowed
the localization of the excited singlet and triplet states of
bpm and demonstrated that this ligand could sensitize the
Nd3+ luminescence in the near-IR. However, the Nd(III)
emission intensity is low in 1 because of the poorly
Figure 9. (a) Visible emission spectra of 2 (blue, λexc ) 270 nm), 3 (orange,
λexc ) 270 nm), 4 (green, λexc ) 270 nm), and 5 (black, λexc ) 260 nm);
(b) visible emission spectra of 2 (blue, λexc ) 353 nm), 3 (orange, λexc
)
370 nm), 4 (green, λexc ) 370 nm), and 5 (black, λexc ) 333 nm); (c) near-
IR emission spectra of 2 (blue, λexc ) 370 nm), 3 (orange, λexc ) 370 nm),
4 (green, λexc ) 343 nm), and 5 (black, λexc ) 370 nm) in the solid state.
4
efficient energy transfer from bpm to the F3/2 level and
the presence of O-H oscillators in the first coordination
sphere that deactivate the Nd(III) luminescence. In systems
containing both bpm and ꢀ-diketonate ligands, an energy
(66) Hebbink, G. A.; Klink, S. I.; Oude Alink, P. G. B.; van Veggel,
F. C. J. M. Inorg. Chim. Acta 2001, 317, 114.
(67) Yang, L.; Gong, Z.; Nie, D.; Lou, B.; Bian, Z.; Guan, M.; Huang, C.;
Lee, H. J.; Baik, W. P. New J. Chem. 2006, 30, 791.
(68) Voloshin, A. I.; Shavaleev, N. M.; Kazalov, V. P. J. Lumin. 2000,
91, 49.
(69) Malta, O. L.; Brito, H. F.; Menezes, J. F. S.; Gonc¸alves e Silva, F. R.;
Alves, S., Jr.; Farias, F. S., Jr.; de Andrade, A. V. M. J. Lumin. 1997,
75, 255.
(70) Carlo, L. D.; De Mello Donega´, C.; Albuquerque, R. Q.; Alves Jr, S.;
Menezes, J. F. S.; Malta, O. L. Mol. Phys. 2003, 101, 1037.
(71) Banerjee, S.; Hueber, L.; Romanelli, M. D.; Kumar, G. A.; Riman,
R. E.; Emge, T. J.; Brennan, J. G. J. Am. Chem. Soc. 2005, 127, 15900.
(72) Kumar, G. A.; Riman, R. E.; Diaz Torres, L. A.; Banerjee, S.;
Romanelli, M. D.; Emge, T. J.; Brennan, J. G. Chem. Mater. 2007,
19, 2937.
Figure 10. Normalized emission spectra of 1 (purple, λexc ) 270 nm), 2
(blue, λexc ) 353 nm), and [Nd(dbm)3(H2O)2] (dark green, λexc ) 353 nm).
excitation within the ꢀ-diketonate π-π* bands around 350
nm. Compounds 3-5 also exhibit broad emission bands
between 370 and 580 nm with a similar envelope whatever
the excitation wavelength used. This emission revealed that
the bpm excited triplet state is populated when the ꢀ-dike-
(73) Romanelli, M.; Kumar, G. A.; Emge, T. J.; Riman, R. E.; Brennan,
J. G. Angew. Chem., Int. Ed. 2008, 47, 6049.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10405

Zucchi et al.
transfer was found to occur between the ꢀ-diketonate and
bpm ligands as phosphorescence from bpm is observed
when the ꢀ-diketonate groups are excited. Thus, our work
shows that the bpm ligand not only acts as a connector
between neodymium ions but also plays an important role
in the sensitization of the Nd(III) luminescence. Quantita-
tive data on these luminescent systems need to be de-
termined to further discuss their efficiency as near-IR
emitters. To date, solid state studies of the luminescent
properties of molecular neodymium complexes are rela-
tively scarce. Some efficient systems involving chalco-
genide clusters have been reported.71-73 It would be of
interest to draw a comparison between these latter and
our systems.
Acknowledgment. We thank the CNRS and CEA for
financial support.
Supporting Information Available: Figure with the emission
spectrum of 2 in methanol (PDF). Tables of crystal data, atomic
positions and displacement parameters, anisotropic displacement
parameters, bond lengths and bond angles in CIF format. This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.
IC800967X
10406 Inorganic Chemistry, Vol. 47, No. 22, 2008