Oxoselenido clusters of the lanthanides: Rational introduction of oxo ligands and near-IR emission from Nd(III)

Article abstract of DOI:10.1021/ja054261q

Reactions of Ln(SePh)₃ with SeO₂ in THF give octanuclear oxoselenido clusters with the general formula (THF) ₈Ln₈O₂Se₂(SePh)₁₆ (Ln = Ce, Pr, Nd, Sm). In this isomorphous series,

Full text of DOI:10.1021/ja054261q

Published on Web 10/21/2005
Oxoselenido Clusters of the Lanthanides: Rational
Introduction of Oxo Ligands and Near-IR Emission from
Nd(III)
Santanu Banerjee,† Louise Huebner,† Michael D. Romanelli,† G. Ajith Kumar,‡
Richard. E. Riman,‡ Thomas J. Emge,† and John G. Brennan*,†
Contribution from the Department of Chemistry and Chemical Biology, and Materials Science
and Engineering, Rutgers, the State UniVersity of New Jersey, 610 Taylor Road, Piscataway,
New Jersey 08854-8087
Received June 28, 2005; E-mail: bren@ccbmail.rutgers.edu
Abstract: Reactions of Ln(SePh)3 with SeO2 in THF give octanuclear oxoselenido clusters with the general
formula (THF)8Ln8O2Se2(SePh)16 (Ln ) Ce, Pr, Nd, Sm). In this isomorphous series, the eight Ln(III) ions
are connected in the center by a pair of µ3-O2- ligands and µ5-Se2- ligands, with 14 bridging and two
terminal selenolate ligands capping the cluster surface. Thermal decomposition at 700 °C of the Nd
compound in vacuo led to the formation of a phase mixture of NdSe2, Nd2Se3, and Nd2O3. Near-IR emission
experiments on the (THF)8Nd8O2Se2(SePh)16 and the fluorinated thiolate compound (DME)2Nd(SC6F5)3
demonstrate that clusters with oxo ligands are not only highly emissive, but also they emit at wavelengths
not found in conventional oxides.
Introduction
the delivery of high Ln concentrations. Cluster compounds, with
their solubility in organic solvents, their relative absence of high-
Emission of light from lanthanide ions is a fundamentally
important process with a continuously expanding range of
applications in contemporary electronic devices, from TV
screens to lasers and optical fibers.1-5 Control of emission
intensity is often elusive, with competitive processes such as
upconversion, photon splitting, or nonradiative (vibronic)
quenching often detracting from ideal performance.6 While
introduction of Ln into solid-state oxide materials has long been
facile, the incorporation of near-IR emissive lanthanide ions into
unconventional materials (i.e., organic polymers) remains chal-
lenging due to the intrinsic properties of Ln sources and the
various technological barriers associated with solids processing.
The utility of solid-state Ln compounds as Ln doping sources
is moderated by the insolubility of these materials in apolar
matrixes. Molecular Ln sources are soluble, but suffer from the
tendency of these compounds to contain ligands with high
energy vibrational modes (i.e., ligands with C-H or O-H
functional groups) that vibrationally relax emissive states.7
Further, the voluminous nature of most ligand systems precludes
energy vibrational modes, and the relatively high concentration
of Ln ions/unit volume, are promising sources of lanthanide
ions in the preparation of emissive materials.
In Ln cluster synthesis, addition of E (E ) S/Se/Te) to
solutions of Ln(EPh)3 has been a reliable synthetic approach to
a wealth of compounds with E2- and EE2- ligands, heterome-
tallic and heterochalcogen species.8-17 Most recently, an Er10
cluster with both surface and internal Er atoms was found to
exhibit exceptional emission at 1.54 µm.18 Subsequent studies
have revealed that Er cluster compounds can be used to deliver
high concentrations of emissive Er ions into apolar matrixes.19
Unfortunately, the air sensitivity of LnxEy clusters presents
an obstacle in composite materials synthesis. Air stable ligands
such as O2- are potentially useful, but the chemistry of lan-
thanide oxo cluster compounds is complicated by the fact that
oxo compounds20-29 can react with water to form hydroxides.
(8) Freedman, D.; Emge, T. J.; Brennan, J. G. J. Am. Chem. Soc. 1997, 119,
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(10) Melman, J. H.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1999, 38, 2117.
(11) Freedman, D.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1999, 38, 4400.
(12) Freedman, D.; Melman, J. H.; Emge, T. J.; Brennan, J. G. Inorg. Chem.
1998, 37, 4162.
† Department of Chemistry and Chemical Biology.
‡ Materials Science and Engineering.
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15900
J. AM. CHEM. SOC. 2005, 127, 15900-15906
10.1021/ja054261q CCC: $30.25 © 2005 American Chemical Society

Oxoselenido Clusters of the Lanthanides
A R T I C L E S
Experimental Section
Oxo compounds without OH ligands are rare, often isolated from
reactions of air-sensitive Ln complexes with “adventitious”
water. More recently, an elegant series of oxo clusters have been
prepared in aqueous environments.30-33 In these aqueous
preparations, aqua and hydroxide ligands are also present in
the products, and such functional groups unfortunately quench
low energy emissions. Methods for introducing oxo ligands that
do not concomitantly afford the possibility of introducing OH/
OH2 ligands are essentially nonexistent.
General Methods. All syntheses were carried out under high purity
nitrogen (WELCO Praxair), using conventional drybox or Schlenk
techniques. Solvents (Aldrich) were purified with a dual column Solv-
Tek Solvent Purification System. Lanthanides, SeO2, and Hg were
purchased from Strem. HSC6F5 was purchased from Aldrich. PhSeSePh
was purchased from Aldrich and recrystallized from hexane. Hg(SC6F5)2
was prepared according to the literature procedure.40 Melting points
were taken in sealed capillaries and are uncorrected. IR spectra were
taken on a Thermo Nicolet Avatar 360 FTIR spectrometer and were
recorded from 4000 to 600 cm-1 as a Nujol mull on NaCl plates.
Electronic spectra were recorded on a Varian DMS 100S spectrometer
with the samples in a 0.10 mm quartz cell attached to a Teflon stopcock.
Powder diffraction spectra were obtained from Bruker AXS D8
Advance diffractometer using Cu KR radiation. Elemental analyses were
performed by Quantitative Technologies, Inc. (Whitehouse Station, NJ).
The compounds are air-sensitive and are particularly unstable when
isolated from the mother liquor. They appear to lose lattice solvent
more rapidly than anything we have yet encountered, to the extent that
mounting crystals for diffraction analysis by our conventional methods
was impossible for the Sm compound.
Synthesis of (THF)8Ce8(µ3-O)2(µ5-Se)2(SePh)16‚6THF (1). Ce
(0.140 g, 1.0 mmol), PhSeSePh (0.47 g, 1.5 mmol), and Hg (0.024 g,
0.11 mmol) were combined in THF (ca. 30 mL), and the mixture was
stirred until all of the metal was consumed (1 day) to give a yellow
solution with a small amount of greenish-yellow solid at the bottom of
the flask. The solution was filtered, and SeO2 (0.11 g, 1.0 mmol) was
added to the filtrate. The resulting mixture was stirred for 1 h at 55
°C, during which the SeO2 dissolved to give a brownish-yellow solution
that was filtered, reduced in volume under vacuum to ca. 15 mL, and
layered with hexane (15 mL) to give yellow plate-shaped crystals (0.33
g, 68%) that turn yellow-brown between 75 and 80 °C, darken between
100 and 150 °C, melt at 180 °C, and remain dark brown from 200 to
350 °C. IR: 3043 (m), 2967 (s), 2943 (s), 2914 (s), 2728 (w), 1567
(m), 1468 (s), 1374 (m), 1293 (w), 1246 (w), 1170 (w), 1071 (m),
1018 (m), 901 (w), 727 (m), 680 (w) cm-1. UV-vis: no well-defined
absorption maximum was observed from 300 to 750 nm when the
compound was dissolved in pyridine. Anal. Calcd for C152H192Ce8O16-
Se18: C, 37.9; H, 4.02. Found: C, 35.7; H, 3.36.
Synthesis of (THF)8Pr8(µ3-O)2(µ5-Se)2(SePh)16‚6THF (2). As for
1, Pr (0.14 g, 1.0 mmol), PhSeSePh (0.47 g, 1.5 mmol), and Hg (0.024
g, 0.11 mmol) in THF (ca. 30 mL) gave an olive-green solution with
a small amount of green solid that was removed by filtration. SeO2
(0.110 g, 1 mmol) was added to the filtrate, the resulting mixture was
stirred for 1 h at 55 °C, after which all of the SeO2 was dissolved. The
lime-green solution was filtered and reduced in volume under vacuum
to ca. 15 mL, and layered with hexane (15 mL) to give green rod-
shaped crystals after 2 days (0.35 g, 64%) that turn light brown between
75 and 80 °C, darken between 120 and 150 °C, melt at 185 °C, and
remain dark brown from 200 to 350 °C. IR: 3043 (m), 2956 (s), 2926
(s), 2722 (w), 1567 (m), 1444 (m), 1374 (m), 1293 (w), 1252 (w),
1176 (w), 1071 (m), 1024 (m), 908 (w), 733 (w), 680 (w) cm-1. UV-
vis: The compound does not display absorption maximum from 300
to 750 nm when dissolved in pyridine. Anal. Calcd for C152H192Pr8O16-
Se18: C, 37.8; H, 4.01. Found: C, 37.1; H, 3.16.
Of the technologically important near-IR emission sources,
Nd, Tm, and Er stand out. The emission of Er at 1.54 µm is
used extensively in telecommunications because glass fibers are
transparent to this wavelength.34 Both Tm and Nd are alluring
additions to optical fiber manufacture because they can ef-
fectively broaden the available bandwidth that can be amplified
optically. Nd is the more complicated emission source of the
three, with a cascade of emission energies6,35 that are either
rarely (1.34 µm) or never (1.81 µm) observed either from
molecular Nd sources or from metal oxides.36 Of these transi-
tions, 1.34 µm is relevant to the telecommunications window.
Chemical modification intended to enhance Nd emission, such
as Nd to fluorescein,37 porphyrin,38 or terphenyl39 sensitizers,
remains a challenging synthetic goal.
In the obvious extension of chalcogenido cluster chemistry
to oxo systems, reactions of Ln(EPh)3 with elemental oxygen
have yet to give crystalline oxo products in any significant yield.
In this work, we demonstrate the utility of SeO2 as an alternative,
soluble source of oxo ligands, in reactions with Ln(SePh)3 that
give a homologous series of crystalline oxoselenido cluster
compounds. The thermolysis chemistry of these clusters has
been investigated, and the near-IR emission properties of the
Nd cluster compound are reported and compared to the novel
molecular fluorothiolate (DME)2Nd(SC6F5)3.
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Trans. 2001, 1931.
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J. L.; Day, V. W.; Belot, J. A. Inorg. Chem. 2004, 43, 6203.
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Fundamentals and Technology; Academic Press: New York, 1999.
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Berlin, 1989.
Synthesis of (THF)8Nd8(µ3-O)2(µ5-Se)2(SePh)16‚6THF (3). As with
2, Nd (0.144 g, 1.0 mmol), PhSeSePh (0.468 g, 1.5 mmol), and Hg
(0.024 g, 0.11 mmol) in THF (ca. 30 mL) gave a blue-colored solution
with a small amount of pale blue solid. The solution was filtered, SeO2
(0.110 g, 1 mmol) was added, the mixture was filtered, reduced in
volume under vacuum to ca. 15 mL, and layered with hexane (15 mL)
to give blue rod-shaped crystals after 2 days (0.41 g, 69%) that turn
light brown between 75 and 80 °C, darken between 120 and 150 °C,
melt at 185 °C, and remain dark brown from 200 to 350 °C. IR: 3043
(36) Kaminski, A. A. Crystalline Lasers-Physical Processes and Operating
Scheme; CRC Press: New York, 1996.
(37) Wolbers, M. P. O.; Van Veggel, F. C. J. M.; Peters, F. G. A.; Van Beelen,
E. S. E.; Hofstraat, J. W.; Geurts, F. A. J.; Reinhoudt, D. N. Chem.-Eur.
J. 1998, 4, 772.
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Williams, J. A. G.; Maupin, C. L.; Riehl, J. P.; Siligardi, G. Chem. Commun.
2000, 1183.
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9
J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005 15901

A R T I C L E S
Banerjee et al.
Table 1. Summary of Crystallographic Details for 1-4
compound
1
2
3
4
empirical formula
C
149H186O15.25Ce8Se18
C
149H186O15.25Pr8Se18
C
152H192O16Nd8Se18
C141H170O13.25Sm8Se18
fw
4763.22
P2(1)/c
16.804(2)
16.718(2)
29.929(3)
91.455(2)
8124(2)
2
1.947
-173
0.71073
6.280
4768.74
P2(1)/c
16.803(2)
16.732(2)
28.875(3)
91.403(2)
8116(1)
2
1.951
-173
0.71073
6.444
4850.26
P2(1)/c
17.126(1)
16.941(1)
28.758(2)
91.299(1)
8341(1)
2
1.931
-73
0.71073
6.425
4699.05
P2(1)/n
17.833(6)
21.011(7)
22.611(8)
102.181(6)
8304(5)
2
1.879
20
0.71073
6.776
0.0385
0.0958
space group
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
D(calcd) (g/cm-3
temp (°C)
λ (Å)
)
abs coeff (mm-1
)
R(F)a [I > 2σ(I)]
Rw(F2)a [I > 2σ(I)]
0.0448
0.1150
0.0389
0.0913
0.0404
0.0926
2
2
2
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|; Rw(F2) ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2. Additional crystallographic details are given in the Supporting Information.
(m), 2955 (s), 2920 (s), 2716 (w), 1567 (w), 1444 (m), 1374 (m), 1287
(w), 1246 (w), 1153 (w), 1059 (w), 1012 (w), 728 (m), 680 (w) cm-1
) 84.394(1)°, γ ) 88.361(1)°, V ) 1581(2) Å3, Z ) 2. The compound
is structurally similar to an earlier reported Er derivative.41
.
Anal. Calcd for C144H176Nd8O14Se18: C, 36.7; H, 3.77. Found: C, 36.1;
H, 3.60. The compound also crystallizes as found for the Sm cluster 4:
X-ray diffraction; unit cell P2(1)/n, with a ) 17.869(4) Å, b ) 20.734-
(4) Å, c ) 22.661(4) Å, â ) 101.67(4)°, V ) 8222(1) Å3 at -120 °C.
Thermolysis and X-ray Powder Diffraction Measurements. A
sample of 3 was placed in a quartz tube that was then sealed under
vacuum. The end of the tube with the sample was placed in an oven,
and the temperature was raised (20 °C/min) to 650 °C, and held at this
temperature for 5 h. The other end of the thermolysis tube was kept in
liquid nitrogen during the experiment. Black powder formed, and an
X-ray powder diffraction pattern was obtained by scanning from 20 to
80 °C. The colorless liquid that condensed in the cold part of the tube
was dissolved in acetonitrile and analyzed by GC/MS.
Synthesis of (THF)8Sm8(µ3-O)2(µ5-Se)2(SePh)16‚4THF (4). Method
A: Sm (0.150 g, 1.0 mmol), PhSeSePh (0.468 g, 1.5 mmol), and Hg
(0.024 g, 0.11 mmol) were combined in THF (ca. 30 mL), and the
mixture was stirred until all of the metal was consumed (2 days) to
give a yellow-orange solution with a small amount of yellow-orange
solid. The solution was filtered, and SeO2 (0.110 g, 1 mmol) was added
to the filtrate. The resulting mixture was stirred for 1 h at 55 °C to
give an orange-colored solution that was filtered, reduced in volume
to ca. 15 mL, and layered with hexane (15 mL) to give yellow-orange
plate-shaped crystals (0.27 g, 59%). Method B: Sm (0.40 g, 2.7 mmol),
PhSeSePh (1.25 g, 4.0 mmol), and Hg (0.029 g, 0.12 mmol) were
combined in THF (ca. 45 mL), and the mixture was stirred for 1 day
until all of the metal was consumed to give an orange-colored solution.
SeO2 (0.11 g, 1.0 mmol) was added to the solution. The resulting
mixture was stirred for 2 days at room temperature, giving an orange-
colored solution. The solution was allowed to settle for a few days
whence some gray precipitate appeared. The solution was filtered (42
mL) and layered with hexane (43 mL) to give yellow-orange plate-
shaped crystals after 2 days (0.98 g, 62%). These crystals turn a red-
orange color between 75 and 80 °C, darken around 130 °C, melt
between 170 and 190 °C, and turn dark brown from 275 to 350 °C.
IR: 2924 (s), 2726 (w), 1573 (w), 1460 (m), 1377 (m), 1295 (w), 1260
X-ray Structure Determination of 1-4. Data for 1-4 were
collected on a Bruker Smart APEX CCD diffractometer with graphite
monochromatized Mo KR radiation (λ ) 0.71073 Å) at 100 K. Crystals
of 1-3 were immersed in Paratone oil and examined at low temper-
atures, whereas 4 lost solvent too quickly for this procedure, and a
crystal of 4 had to be mounted in a glass capillary along with THF.
The data were corrected for Lorenz effects and polarization, and
absorption, the latter by a multiscan (SADABS)42 method. The
structures were solved by Patterson or direct methods (SHELXS86).43
All non-hydrogen atoms were refined (SHELXL97)44based upon Fobs
.
2
All hydrogen atom coordinates were calculated with idealized geom-
etries (SHELXL97). Scattering factors (fo, f′, f′′) are as described in
SHELXL97. Crystallographic data and final R indices for 1-4 are given
in Table 1. An ORTEP diagram45,46 for the isomorphous core of 1-4
is shown in Figure 1. Significant bond geometries for 1-4 are given
in Tables 2 and 3. Complete crystallographic details for 1-4 are given
in the Supporting Information.
Optical Characterization. Absorption measurements were carried
out with compounds dissolved in THF using a double beam spectro-
photometer (Perkin-Elmer Lambda 9, Wellesley, MA) in a 1 cm cuvette
using THF as the reference solvent at approximately 0.01 M. The
emission spectra of the Nd samples were recorded by exciting the
sample with the 800 nm band of a laser diode in the 45° excitation
geometry. The emission from the sample was focused onto a 1 m
monochromator (Jobin Yvon, Triax 550, Edison, NJ) and detected by
(m), 1094 (w), 1068 (w), 1020 (m), 732 (m), 690 (w), 666 (w) cm-1
UV-vis: There was no resolved LMCT absorption maximum from
.
300 to 750 nm when dissolved in pyridine. Anal. Calcd for C144H176
-
Sm8O14Se18: C, 36.4; H, 3.73. Found: C, 35.1; H, 3.40.
Synthesis of (DME)2Nd(SC6F5)3 (5). Nd (0.14 g, 1.0 mmol) and
Hg(SC6F5)2 (0.90 g, 1.5 mmol) were combined in DME (ca. 20 mL),
and the mixture was stirred overnight until the metal flakes were
completely consumed and elemental mercury was visible in the bottom
of the flask. The pale blue/green solution was filtered away from the
mercury (0.25 g, 82%), and layered with hexane (ca. 20 mL) to give
near colorless pale blue crystals (0.92 g, 96%) that turn red at 140 °C
and melt at 145 °C. IR: 2949 (s), 2926 (s), 2844 (s), 2725 (w), 2394
(w), 2345 (w), 1883 (w), 1707 (w), 1619 (m), 1568 (w), 1508 (s), 1461
(s), 1380 (s), 1257 (m), 1187 (w), 1082 (m), 1030 (s), 960 (s), 866 (s),
721 (w) cm-1. Anal. Calcd for C26H20F15NdO4S3: C, 33.9; H, 2.20.
Found: C, 34.7; H, 2.55. X-ray diffraction: unit cell P1h, with a )
8.5954(5) Å, b ) 12.7731(7) Å, c ) 15.2844(9) Å, R) 71.244(1)°, â
(41) Melman, J.; Rhode, C.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 2002,
41, 28.
(42) Bruker-ASX. SADABS, Bruker Nonius area detector scaling and absorption
correction, v2.05; Bruker-AXS Inc.: Madison, WI, 2003.
(43) Sheldrick, G. M. SHELXS86, Program for the Solution of Crystal Structures;
University of Go¨ttingen: Germany, 1986.
(44) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refinement;
University of Go¨ttingen: Germany, 1997.
(45) Johnson, C. K. ORTEP II, Report ORNL-5138; Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
(46) Sheldrick, G. M. SHELXTL (XP). Version 6.14. Bruker-AXS, Inc.:
Madison, WI, 2000.
9
15902 J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005

Oxoselenido Clusters of the Lanthanides
A R T I C L E S
the conformations of one bridging selenolate yield two different
space groups and thereby two distinct crystallographic phases
for the nearly isomorphic pair. Second, there is a distinctly
nonlinear change in the bond length between Ln(2) and Se(6),
an interaction that can arguably be described as a bond. As the
lanthanide contraction progresses, average bond lengths between
Ln and O2-, Se2-, O(THF), and SePh all decrease accordingly,
with the exception of the Ln(2)-Se(6) separation, which varies
from 3.39 Å (Ce) to 3.34 Å (Nd) to 3.63 Å (Sm), presumably
in response to an increase in ligand-ligand repulsions as the
Ln coordination spheres contract.
Thermal treatment of the Nd compound was examined to
establish the identity of the solid-state phases that form upon
thermal decomposition. Pyrolysis of 3 at 700 °C under vacuum
gave a black solid, and X-ray powder diffraction analysis of
the product indicated the presence of crystalline Nd2Se3,47
NdSe2,48 and Nd2O3 (JCPDS 21-0579) phases (reaction 2). There
was no indication of any oxoselenido phase. GCMS analysis
of the volatile products identified Ph2Se and THF in the
condensate.
Figure 1. ORTEP diagram of the (THF)8Ln8O2Se2(SePh)16 core, with the
C and H atoms removed for clarity. Labeled atoms are mentioned in the
text. Primed atoms are related to unprimed ones by inversion symmetry.
a thermoelectrically cooled InGaAs detector. The signal was intensified
with a lock-in amplifier (SR 850 DSP, Stanford Research System,
Sunnyvale, CA) and processed with a computer controlled by the
Spectramax commercial software (GRAMS 32, Galactic Corp., Salem,
NH). To measure the decay time, the laser beam was modulated at
500 Hz by a chopper and the signal was collected on a digital
oscilloscope (model 54520A, 500 MHz, Hewlett-Packard, Palo Alto,
CA). While these compounds lose lattice solvent quickly at room
temperature, the elemental analyses indicate that coordinated solvents
stay bound to the Ln over periods far longer than the time required for
emission measurements.
(THF)8Ln8Se2O2(SePh)16 f
NdSe2 + Nd2Se3 + Nd2O3 + 8Ph2Se + 8THF + ?? (2)
Near-IR properties of the Nd cluster 3 and (DME)2Nd(SC6F5)3
(5) were evaluated. Absorption spectra of 3 and 5 are shown in
Figure 2 with the standard spectral assignments. Most of the
spectral bands in the UV-vis-NIR are well resolved with their
characteristic spectral width as in other solid-state materials.
Results
4
The oscillator strength of the hypersensitive transition I9/2
f
The early lanthanide benzeneselenolates reduce SeO2 to form
oxoselenido clusters (reaction 1) that crystallize as the THF
adducts (THF)8Ln8Se2O2(SePh)16 in 60-70% isolated yield (Ln
) Ce (1), Pr (2), Nd (3), Sm (4)). Low-temperature structural
characterization of all four compounds reveals a nearly isomor-
phous cluster core, with 1 and 2 crystallizing with more lattice
THF than is found for 4, and cluster 3 crystallizing in both space
groups. Figure 1 shows an ORTEP diagram of the common
structure for 1-4, and Table 2 gives a list of significant bond
geometries for 1-4. The general structure consists of a
hexanuclear group of Ln connected by two trigonal planar O2-
and two trigonal bipyramidal Se2- anions. Two opposite sides
of this core are capped by Ln(III) ions, and the entire group is
encapsulated with 14 µ2SePh and two terminal SePh ligands
bound to the Ln that do not interact with O2-. There are three
distinct Ln coordination environments, with two eight coordinate
central Ln (THF, O2-, 2Se2-, 4µSePh), four oxo bound seven
coordinate Ln (THF, O2-, Se2-, 4µSePh), and two seven
coordinate Ln that do not interact with the oxo ligands (THF,
Se2-, SePh, 4µSePh). Each Ln coordinates a neutral THF donor.
4G5/2 appears to be greater in 3 relative to 5 and is smaller
comparable to other reported Nd organic complexes,49-52 and
the spectrum of 3 is more complicated, as befits a compound
with three chemically distinctive Nd environments. The emission
spectra of 3 and 5 are compared in Figure 3, with the
characteristic Nd3+ emission transition scheme shown in the
inset. For the Nd3+ ion, the only excited J manifold that is not
4
relaxed predominantly by multiphonon relaxation is the F3/2
4
state. The pump photons at 800 nm populate the F3/2 excited
4
state, and relaxation to the IJ (J ) 9/2, 11/2, 13/2, and 15/2)
manifold gives rise to the four emission bands. The observed
emission wavelengths for these compounds are 927, 1078, 1360,
and 1843 nm in 3 and 897, 1071, 1347, and 1824 nm in 5. The
measured fluorescence branching ratios of these transitions are
4
4
4
4
in the order I11/2> I13/2 > I9/2 > I15/2 as is typical for Nd-
based compounds.53 In addition, numerous spectral differences
can be noted between 3 and 5. Most notably, 5 has lower
emission intensity but also does not exhibit the peak splitting
demonstrated by 3.
Lifetimes of the emissions were extracted from exponential
fits of the respective fluorescence decay curves (see Supporting
4
Information). For the F3/2f4I11/2 emissions, the decay times
8Ln(SePh)3 + SeO2 f
(THF)8Ln8Se2O2(SePh)16 + 8PhSeSePh + ?? (1)
(47) Flahaut, P. J.; Guittard, M.; Patrie, M.; Pardo, M. P.; Golabi, S. M.;
Domange, L. Acta Crystallogr. 1965, 19, 14.
(48) Doert, T.; Graf, C. Anorg. Allg. Chem. 2005, 631, 1101.
(49) Oczko, G. J. Alloys Compd. 2000, 300, 414.
(50) Lis, S. J. Alloys Compd. 2000, 300, 88.
There are two structural features that distinguish the two
different clusters. First is the direction of the phenyl group
attached to the bridging Se atom Se(3), which, in 4, is
perpendicular to that for 1 or 2. The Ln(1)-Se(3)-C(7)-C(8)
torsion angle for 1 and 2 is -138°, and for 4 it is -40°. The
subtle differences in amount of solvated THF in the lattice and
(51) Gubina, K. E.; Shatrava, J. A.; Ovchynnikov, V. A.; Amirkhnov, V. M.
Polyhedron 2000, 19, 2203.
(52) Legendziewicz, J.; Oczko, G.; Wiglusz, R.; Amirkhanov, V. J. Alloys
Compd. 2001, 323, 792.
(53) Kaminski, A. A. Laser Crystals-Their Physics and Properties; Springer:
Berlin, 1989.
9
J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005 15903

A R T I C L E S
Banerjee et al.
Sm
Table 2. Average and Range of Bond Lengths [Å] and Angles [deg] for (THF)8Ln8O2Se2(SePh)16 Phases
Ln
f
Ce
Pr
Nd
Ln-O (oxide)
Ln-O (THF)
2.26(5)a/2.23-2.31
2.52(2)/2.48-2.54
3.07(5)/3.04-3.15
3.08(7)/2.94-3.20
3.394
2.24(4)/2.21-2.29
2.50(2)/2.47-2.53
3.05(5)/3.02-3.14
3.06(7)/2.91-3.18
3.392
3.88(4)/3.85-3.92
92(9)/77-102
167(9)/161-174
89(8)/78-99
120(4)/117-124
74(2)/71-75
2.24(4)/2.21-2.29
2.49(2)/2.46-2.51
3.05(6)/3.01-3.15
3.04(7)/2.90-3.19
3.430
3.86(4)/3.82-3.90
93(10)/77-102
167(9)/160-173
90(8)/77-100
119(4)/117-124
74(2)/72-76
2.21(4)/2.18-2.25
2.47(2)/2.44-2.49
3.03(6)/2.98-3.03
3.01(6)/2.89-312
3.627
3.82(6)/3.77-3.88
93(11)/77-104
165(8)/159-171
90(9)/77-101
119(5)/116-124
75(3)/71-77
Ln-Se (selenide)b
Ln-Se (Ph)c
Ln(2)-Se(6)
Ln-Lnd
3.91(4)/3.88-3.96
<chgrow;lp;4q>Ln-Se-Ln (Se2- cis)
Ln-Se-Ln (Se2- trans)
Ln-Se-Ln (SePh)e
92(9)/77-102
168(9)/162-174
90(7)/78-99
Ln-O-Ln (oxide)f
120(6)/117-124
73(2)/71-75
Se-Ln-O (Se2-, O2-
)
a The ESD of the mean is enclosed in parentheses. b The second Ln(2)-Se(1) bond to the 8-coordinate Ln(2) is noticeably longer at about 3.15 Å; the
other selenido bonds are about 3.05 Å. c The longest Ln-Se bond is Ln(2)-Se(6) (∼3.5 Å), so this bond is excluded from the average (see text). d The next
closest Ln‚‚‚Ln distance is Ln(1) ‚‚‚Ln(4) at ∼4.4 Å. e Angles Sm(3)′-Se(6)-Sm(2) and Sm(1)-Se(6)-Sm(2) (91.1° and 89.2°) are not included in the
average. f For the Ln-O-Ln angles, the sum is nearly 360° indicating a planar trigonal coordination geometry of the oxide O atom, and the Ln(3)-O(1)-
Ln(4) is always ∼125° and the other two are always ∼117°.
Table 3
Discussion
phonon
frequency (cm-
While lanthanide oxo cluster compounds have always held
1
host
lifetime (
µ
s)
)
ref
the promise of tremendous applications in both chemical and
spectroscopic areas, the synthesis of such materials has always
been challenging due to the tendency of LnO to react with water
(reaction 3) to form hydroxide. Indeed, much of the chemistry
leading to the description of crystalline oxo compounds has
resulted from what is usually described as the incorporation of
adventitious water into a reaction mixture. The present series
of compounds is important as the first rational synthesis of
hydroxy-free lanthanide clusters with oxo ligands, in reactivity
that parallels the reductive generation of chalcogenido anions.
sulfide
110
410
233-313
406-550
540-610
800
450-700
450-700
450-700
900
500
200-400
400
77
77
60
77
77
selenide
tellurite
germanate
ZBLA fluoride glass
LaF3
yttrium aluminum garnet
53, 58, 59
57
250
obtained are 186 and 111 µs, respectively, for 3 and 5. In 3, a
decay time of 78 µs was obtained for the 925 nm emission,
and in 5 a decay time of 130 µs was obtained for the 1350 nm
emissions. The average decay times of the 4F3/2 level for 3 and
5 are 55 and 60 µs, respectively. Because of the noisy signal,
no decay curve was observed for the 4F3/2f4I15/2 and 4F3/2f4I9/2
XLnO + HOH T XLn(OH)2
(3)
The selection of SeO2 as an oxygen source is not exactly an
obvious first choice reagent. In fact, the chemistry of O2,
MoO42-, H2O, Ph2CO, HgO, and Ph2CO was explored briefly
before SeO2, and while all clearly reacted with Ln(SePh)3, SeO2
was the first reactant to give a reproducible, crystalline oxo
cluster product. Incorporation of both oxo and selenido ligands
adds a new dimension to the Ln cluster field and is not surprising
given that Se readily displaces SePh. It should be noted that
analogous reactions with TeO2 give oxo clusters without
tellurido ligands, and this work is still in progress.
The geometries about the oxo ligands are at first surprising.
In these materials, some of the most oxophilic metals in the
periodic chart are bonding three to O2- and five to Se2-. Of
the LnO clusters currently in the literature, there is only one
with µ3 oxo ligands, and the structure of this cluster is likely to
have been influenced significantly by the steric requirements
of the polydentate ancillary ligands.26 All other Ln compounds
with oxo ligands have O2- with Ln4, Ln5, and Ln6 coordination
spheres, all in the presence of ligand systems that are viewed
as classically more competitive for access to the Ln than are
the ancillary SePh present here. Presumably the coordination
geometry about O2- in 1-4 is a reflection more of the physical
properties of the ancillary SePh anions, which, because of their
tendency to bond at right angles using selenium p orbitals, would
find it increasingly difficult to bridge Ln ions that were brought
closer together by higher coordinate O. With the geometry about
the oxo ligands nearly trigonal planar (with Ln-O-Ln angles
averaging 119.6° (1), 119.5° (2), and 119.2° (4)), these Ln are
spaced further apart than would be Ln bound to four, five, or
transitions in 3 or the F3/2f4I13/2 transition in 5. With this
4
fluorescence decay time, the quantum yield of 4F3/2f4I11/2
emission can be estimated from the ratio of the fluorescence
decay time (τfl) to radiative or “natural” decay time (τr). With
a calculated radiative decay time of 1187 µs following the
Judd-Ofelt procedure,54,55 a quantum efficiency of 16% is
obtained for 3. In 5, the radiative quantum efficiency obtained
is 9% corresponding to a radiative decay time of 1398 µs. The
effective spectral bandwidth (fwhm) of the 4F3/2f4I11/2 emission
band was 38 nm in 3 and 59 nm in 5, which is higher than that
of the commercial laser glass LHG-8 (35 nm) or in Nd:YAG
(30 nm).56,57 The stimulated emission cross sections of the
4
principal F3/2f4I11/2 emission bands are estimated to be 3.04
× 10-20 cm2 for 3 and 1.61 × 10-20 cm2 for 5, which are
comparable to the value of 1.35 × 10-20 cm2 from the well-
known host LaF3: Nd.58,59 The 4F3/2f4I13/2 emission band shows
a spectral width of 39 nm in 3 and 31 nm in 5, which is smaller
than that of Nd-doped tellurite glass (51 nm).60 The stimulated
emission cross section of this band is estimated to be 0.30 ×
10-20 cm2, which is in the range of the reported values of Nd-
doped tellurite and silicate glass.
(54) Judd, B. R. Phys. ReV. B 1962, 127, 750.
(55) Ofelt, G. S. J. Chem. Phys. 1962, 37, 511.
(56) Reisfeld, R.; Jorgensen, C. K. Lasers and Excited States of Rare Earths;
Springer-Verlag: New York, 1977.
(57) Kumar, G. A.; Lu, J.; Kaminskii, A. A.; Ueda, K. I.; Yagi, H.; Yanagitani,
T.; Unnikrishnan, N. V. IEEE J. Quantum Electron. 2004, 40, 747.
(58) Fan, T. Y.; Kokta, M. R. IEEE J. Quantum Electron. 1989, 25, 1845.
(59) Stouwdam, J. W.; van Veggel, F. C. J. M. V. Nano Lett. 2002, 2, 733.
(60) Shen, S.; Jha, A.; Zhang, E.; Wilson, S. J. C. R. Chimie 2002, 5, 921.
9
15904 J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005

Oxoselenido Clusters of the Lanthanides
A R T I C L E S
Figure 2. Absorption spectrum of (THF)8Nd8O2Se2(SePh)16 and (DME)2Nd8(SC6F5)3 measured in THF.
ligand coordination.62 The emission spectra of 3 and 5 are
different in terms of the Stark level splitting, broadening, and
the intensity, and all of these depend on the coordination
environment of the emitting Nd ion and the number of Nd ions
in the compound. Compound 5 shows less emission intensity
and quantum efficiency mainly due to the lower ionic concen-
tration (1.26 × 1021 Nd ions/cc in 5 and 1.92 × 1021 Nd ions/
cc in 3) and consequent energy transfer quenching. The reported
111-186 µs emission lifetimes are typical of low phonon energy
hosts, where the emission lifetimes of various classes of low
phonon energy hosts are found to range from 100 to 800 µs
(Table 3). The calculated radiative lifetimes are comparable to
the reported values in organic compounds,63 whereas the
fluorescence decay time of 186 µs and 16% quantum yield are
considered to be the highest reported values among Nd organic
complexes. Earlier, Hasegawa et al. obtained a decay time of
13 µs and a quantum yield of 3.2% in Nd(bis-perfluorooctane-
sulfonylimide)3,64 and in all other reports65-71 the quantum yields
Figure 3. Comparison of the Nd3+ emission spectra in the two compounds
with the spectroscopic schemes of the observed transitions in the inset.
six coordinate oxo ligands, and this Ln-Ln separation allows
for SePh to bridge more effectively. This steric rationalization
also accounts for the higher coordination at the significantly
larger Se2-. Alterations in structure with introduction of a
stronger Lewis base solvent or the replacement of SePh with
SPh/TePh will eventually prove interesting.
obtained are in the range of 10-2-10-5
.
In addition to concentration quenching, the major factors
reducing the fluorescence quantum yield in Nd systems are
multiphonon relaxation of the ceramic cluster core and vibra-
tional quenching by nearby organic ligands. For Nd3+ ions, one
principal channel for nonradiative multiphonon relaxation is
through 4F3/2f4I15/2, which is emission at 1843 nm. The energy
Thermal decomposition of the Nd cluster compound led to
the observation of a series of microcrystalline Ln solid-state
phases, including Nd2O3, Nd2Se3, and NdSe2. There was no
evidence for the formation of any ternary oxoselenido phases.
Such products are not necessarily impossible synthetic targets
when starting from molecular sources, but further work in both
precursor design and thermolysis conditions will be required if
they are to be targeted. Commercially important Ln2OxEy phases
have been identified in the thermolysis of (THF)6Ln4S9I2, but
only when the displaced THF was not trapped immediately upon
dissociation.61 Clearly, reaction conditions and precursor identi-
ties will have to be examined systematically if single-phase
oxochalcogenido solids are to be approached from molecular
sources.
gap of this band in the present Nd complexes is 5868 cm-1
,
and, assuming the highest vibrational frequencies of Nd-O and
Nd-Se are ca. 700 and 400 cm-1, respectively,72 8 and 15
phonons are required to bridge the 4F3/2T4I15/2 energy gap. The
(62) Jorgenson, C. K. Modern aspects of Ligand Filed Theory; North Holland:
Amsterdam, 1971.
(63) Hasegawa, Y.; Murakoshi, K.; Wada, Y.; Kim, J. H.; Nakashima, N.;
Yamanaka, T.; Yanagida, S. Chem. Phys. Lett. 1996, 260, 173.
(64) Hasegawa, H.; Okhubo, T.; Sogabe, K.; Kawamura, Y.; Wada, Y.;
Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed. 2000, 39, 357.
(65) Slooff, L. H.; Polman, A.; Klink, S. I.; Hebbink, G. A.; Grave, L.; van
Veggel, F. C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W. Opt. Mater. 2000,
14, 101.
(66) Wolbers, M. P. O.; van Veggel, F. C. J. M.; Hofstraat, J. W.; Geurts, F. A.
J.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 1997, 2275.
(67) Wolbers, M. P. O.; van Veggel, F. C. J. M.; Snellink-Ruel, B. H. M.;
Hofstraat, J. W.; Geurts, F. A. J.; Reinhoudt, D. N. J. Chem. Soc., Perkin
Trans. 1998, 2141.
The near-IR emission properties of the Nd cluster 3 and
thiolate 5 are consistent with Nd ions bound to heavier S- and
Se-based anions. All of the absorption and emission bands in 3
show considerable red shift that is attributed to the higher
nephelauxetic effect in 3 due to the increased covalency and
(68) Klink, S. I.; Hebbink, G. A.; Grave, L.; van Veggel, F. C. J. M.; Reinhoudt,
D. N.; Slooff, L. H.; Polman, A.; Hofstraat, J. W. J. Appl. Phys. 1999, 86,
1181.
(69) Hebbink, G. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Eur. J. Org.
Chem. 2001, 4101.
(70) Hasegawa, Y; Murakoshi, K.; Wada, Y.; Kim, J. H.; Nakashima, N.;
Yamanaka, T.; Yanagida, S. Chem. Phys. Lett. 1996, 248, 8.
(71) Wada, Y.; Okubo, T.; Ryo, M.; Nakazawa, T.; Hasegawa, Y.; Yanagida,
S. J. Am. Chem. Soc. 2000, 122, 8583.
(72) Berg, D. J.; Burns, C. J.; Andersen, R. A.; Zalkin, A. Organometallics
1989, 8, 1865.
(61) Melman, J. H. Ph.D. Thesis, Rutgers University. UMI, Order No.
DA3117623 (2004). From: Diss. Abstr. Int., B 2004, 64, 6084.
9
J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005 15905

A R T I C L E S
Banerjee et al.
in the Ph (Se) ligands encapsulating the cluster core. The SePh
are also considered to be weakly bound to the Ln, and these
ligands are distanced by the relatively long Nd-Se bond lengths.
It seems reasonable to assume that the multitude of SePh ligands
have detracted from optimal quantum efficiency. With direct
coordination of Nd3+ to heavier atoms, the coupling between
the energy levels is less efficient and the Franck-Condon factor
for the relaxation process is reduced, effectively increasing the
lifetime. Thus, for this reason, both compounds are more
emissive in the near-IR spectrum, particularly when compared
to the current literature.
Absolute relationships that correlate structure and quantum
efficiencies (QE) are still elusive. For Er, both the molecular
thiolate (DME)2Er(SC6F5)3 and the decanuclear cluster
Figure 4. Schematic representation of the vibrational quenching of the
4F3/2 emission in Nd3+ by the vibrational modes of CH and OH functional
groups. The energy differences between the interacting levels of Nd3+ and
the vibrational groups are labeled with A, B, C, and D where ∆EA ) 489
75
(THF)8Er10S6Se12I6 were found to be similarly emissive (QE
) 78% and 75%, respectively), while QE dropped dramatically
in heterometallic (py)8M2Er4Se6(SePh)4 (M ) Cd, Hg).76 In the
present Nd experiments, the fluorinated thiolate compound 5 is
shown to be the most emissive monometallic Nd compound
reported to date, and cluster 3, with a significant number of
hydrocarbon functionalized selenolate ligands, has even brighter
emission properties, with the highest quantum efficiency
reported for a discrete Nd compound, and a high concentration
of Nd ions that should be useful when doping into apolar
matrixes. Further developments in the areas of oxo and non-
oxo clusters are currently under investigation.
cm-1, ∆EB ) 435 cm-1, ∆EC ) 565 cm-1, and ∆ED ) 1432 cm-1
.
probability of such a high order process is limited, and the result
4
is that the F3/2f4I15/2 transition is radiative, contrary to other
high phonon energy hosts where the band at 1843 nm could
not be observed. It seems reasonable to assume that the
multitude of SePh ligands have detracted from optimal quantum
efficiency. Elimination of SePh ligands will presumably improve
quantum efficiency, and experiments along those lines are in
progress.
The fluorescence quenching mechanism of various Nd3+
emission bands by the presence of OH (3450 cm-1) and CH
(2950 cm-1) vibration is schematically shown in Figure 4. The
second-, third-, and fourth-order CH bond vibrational frequen-
cies are almost in resonance (energy difference is of the order
of 435-565 cm-1) with the emission bands at 1825 (4F3/2f4I15/2),
1077 (4F3/2f4I11/2), and 890 nm (4F3/2f4I9/2), and hence there
are chances of energy transfer by the Forster mechanism,73 the
extent of which depends directly on the overlap of the emission
spectrum of Nd with the absorption spectrum of OH/CH and
inversely on the sixth power of the separation between Nd and
vibrating species. Quenching due to OH overtones primarily
Conclusions
Oxo ligands are readily introduced into lanthanide cluster
chemistry by the reduction of SeO2 with Ln(SePh)3. The
relatively strong Ln-O bond does not negatively impact Ln
emission properties. The investigation into the emission
properties of the molecular thiolate compound (DME)2Nd-
(SC6F5)3 and the oxyselenido cluster (THF)8Nd8O2Se2(SePh)16
reveals emission at 925 and 1077 nm. Furthermore, for the first
time, emission at 1.35 and 1.83 µm has been observed from a
molecular Nd(III) source.
4
influences the F3/2f4I13/2 band. Our detailed energy transfer
simulation74 shows that in both molecules 3 and 5 there is strong
fluorescence quenching (84% in 3 and 91% in 5) and the major
sources of this quenching are (1) concentration quenching due
to multipolar energy transfer interactions between Nd ions and
(2) the presence of CH bonds. In both complexes, the Nd-Nd
separation is always less than that of Nd-CH, and hence the
major contribution of the fluorescence quenching comes from
the interaction between Nd ions.
In the present compounds, O-H is completely absent, and
C-H is present, but in an apparently limited sense. In the
fluorinated thiolate, C-H units are found only on the DME
ligand that coordinates weakly though Nd-OR2 bonds. In cluster
3, there are C-H bonds in the weakly bound THF ligand and
Acknowledgment. This work was supported by the National
Science Foundation under Grant No. CHE-0303075, and the
APEX diffractometer was obtained under Grant No. CHE-
0091872. Support from the New Jersey State Commission on
Science and Technology is gratefully acknowledged by R.E.R.
Supporting Information Available: X-ray crystallographic
files in CIF format for the crystal structures of 1-4. Fluores-
cence decay curves and exponential fits. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA054261Q
(75) Kumar, G. A.; Riman, R. E.; Banerjee, S.; Kornienko, A.; Brennan, J. G.
Chem. Mater. 2005, 17, 5130.
(76) Kornienko, A.; Banerjee, S.; Kumar, G. A.; Riman, R.; Emge, T. J.;
Brennan, J. G. J. Am. Chem. Soc. 2005, 127, 14008.
(73) Forster, T. Ann. Phys. 1948, 2, 55.
(74) Kumar, G. A.; Riman, R. E.; Diaz-Torres, L. A.; Banerjee, S.; Romanelli,
M.; Brennan, J. G., manuscript in preparation.
(77) Optical Fiber Amplifiers-Materials, DeVices, and Applications; Sudo, S.,
Ed.; Artech House Inc.: Norwood, MA, 1997.
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15906 J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005