Anion dependent self-assembly of "tetra-decker" and "triple-decker" luminescent Tb(III) salen complexes

Article abstract of DOI:10.1021/ja051292c

The Schiff base complexes [Tb₃L₄(H₂O)₂]Cl and [Tb₃L₃(OAc)₂Cl] both have unusual multi-decker architectures formed via intramolecular π-π interactions between phenylene units. Cop

Full text of DOI:10.1021/ja051292c

Published on Web 05/10/2005
Anion Dependent Self-Assembly of “Tetra-Decker” and “Triple-Decker”
Luminescent Tb(III) Salen Complexes
Xiaoping Yang and Richard A. Jones*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, 1 UniVersity Station A5300,
Austin, Texas 78712
Received March 1, 2005; E-mail: rajones@mail.utexas.edu
Polynuclear lanthanide complexes with distinct magnetic and
luminescent properties are currently of interest for use in applica-
tions involving the fabrication of novel materials and as probes in
biological systems.1 For example, Eu3+ and Tb3+ are attractive
5
5
luminescent centers due to their long-lived D0 and D4 excited
states and their large Stokes’ shifts. However, the photophysical
properties of these ions depend markedly on their environments.
For efficient emissions, chromophoric ligands are often employed
to transfer absorbed energy efficiently to the lanthanide ion. They
should also be capable of protecting the Ln(III) center from solvent
molecules which can quench emissions. A variety of multidentate
cyclic and acyclic ligands designed to encapsulate lanthanides are
known.2 Recently, the use of phthalocyanines and porphyrins by
Ishikawa and co-workers enabled the synthesis of several so-called
“double-decker” or “triple-decker” lanthanide complexes, described
as “stacked π-conjugate molecules”.3 Since a multi-decker frame-
work could be effective in keeping solvent and water molecules
away from Ln(III) centers, it seemed reasonable to assume that
enhanced luminescent properties could be achieved with these types
of architectures. As part of our ongoing interest in luminescent
lanthanide complexes,4 we describe here the synthesis, structures,
and photophysical properties of two multi-decker trinuclear Tb-
(III) complexes (1 and 2) which are formed with the Schiff base
ligand N,N′-bis(5-bromo-3-methoxysalicylidene)phenylene-1,2-di-
amine (H2L). Although numerous lanthanide derivatives of salen-
type ligands have been described, many are poorly characterized
with speculative structures proposed on the basis of elemental
compositions and spectroscopic data.5 Few structures of polynuclear
lanthanide derivatives of salen-type ligands are known.6 This paper
describes definitive structural details of the interactions between
Tb(III) and salen-type Schiff base ligands. These interactions occur
between both N2O2 and O2O2 ligand donor sets and the metals.
Furthermore, intramolecular π-π interactions between aryl groups
lead to unusual multi-decker stacked configurations.
Reaction of H2L with TbCl3‚6H2O (4:3) in an acetonitrile/
methanol mixture resulted in the formation of the trimetallic tetra-
decker complex [Tb3L4(H2O)2]Cl (1) in 40% yield.7 A view of the
cationic moiety of 1 and skeletal view of the N and O donor
framework are shown in Figure 1. The two outer Tb3+ ions, Tb(1)
and Tb(3), have similar nine-coordinate environments comprising
the N2O2 donor set of the outer L group, the O2O2 set of one inner
L group and one H2O molecule. The central Tb(2) ion has an eight-
coordinate pseudo-square-based antiprismatic geometry formed by
the two N2O2 donor sets of the internal L ligands. The phenolic
oxygen atoms of the interior L group are bridging, while those of
the outer L are monodentate. The Tb-Tb separations are similar
at 3.884 and 3.872 Å for Tb(1)-Tb(2) and Tb(2)-Tb(3), respec-
tively. The valence requirements for 1 are satisfied by the presence
of a single uncoordinated Cl- anion.
Figure 1. (a) Crystal structure of 1 showing general ligand configurations.
(b) Skeletal view of 1 showing locations of the N and O donor atoms.
The self-assembly process of Tb3+/L multi-decker systems
appears to be anion dependent. Thus, if the reaction between TbCl3‚
6H2O and H2L is conducted in the presence of Zn(OAc)2‚2H2O,
the triple-decker complex [Tb3L3(OAc)2Cl] (2) is produced (Figure
2).8,9 We assume that OAc- groups are able to coordinate effectively
to one Tb3+ ion and prevent coordination of the fourth L ligand. A
discussion of the structural differences between 1 and 2 is provided
in the Supporting Information. A key feature in both 1 and 2 is the
presence of intramolecular π-π stacking interactions between
phenylene units. The distances range from 3.491 to 3.962 Å. These
interactions may add to the stability of these multi-decker archi-
tectures. To coordinate effectively to two Tb3+ ions, the inner L
ligands in both 1 and 2 are virtually planar. In contrast, the outer
L ligands adopt angular configurations in which the two salicylal-
dehyde rings of the Schiff base are pinned back into a wing-like
formation. The dihedral angles between these rings in 1 are 116.7
and 121.3°. In 2, where the steric restraints are less severe, the
dihedral angle for the analogous outer L is 146.5°.
9
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J. AM. CHEM. SOC. 2005, 127, 7686-7687
10.1021/ja051292c CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
CN and CD3OD are much higher than those in CH3OH. This
suggests a strong interaction between the complexes and CH3OH,
which eventually leads to some decomposition (as evidenced by
the NMR data). The absence of typical Tb3+ ion excitation bands
in the excitation spectra and the ligand-centered luminescence in
the emission spectra of the 1 and 2 indicates that the ligand-to-
metal energy transfer takes place efficiently.11
Acknowledgment. We thank the Robert A. Welch Foundation
(Grant F-816) for support.
Supporting Information Available: Details of the synthesis and
characterization of H2L, 1 and 2, a discussion of the structural
differences between 1 and 2, views of the crystal structures of 1 and 2,
view of the structure of the monolanthanide complex [LnZnL(NO3)3],
1H NMR spectra of 1 in CD3CN, UV-vis spectra of free H2L and 1 in
CH3CN, excitation and emission spectrum of 1 in CH3OH. Crystal-
lographic files in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. A view of the molecular structure of 2. Hydrogen atoms are
omitted for clarity. X1A‚‚‚X1B 3.955 Å, X1C‚‚‚X1D 3.593 Å.
References
(1) (a) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes;
VCH: Weinheim, Germany, 1995. (b) Piguet, C.; Bu¨nzli, J.-C. G. Chem.
Soc. ReV. 1999, 28, 347. (c) Piguet, C. Chimia 1996, 50, 144-153.
(2) (a) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993,
123, 201-228. (b) Guerriero, P.; Tamburini, S.; Vigato, P. A. Coord.
Chem. ReV. 1995, 139, 17-243.
(3) (a) Ishikawa, N.; Iino, T.; Kaizu, Y. J. Am. Chem. Soc. 2002, 124, 11440-
11447. (b) Ishikawa, N.; Kaizu, Y. J. Phys. Chem. 1996, 100, 8722-
8730. See also: (c) Pan, N.; Bian, Y.; Fukuda, T.; Yokoyama, M.; Li,
R.; Neya, S.; Jiang, J.; Kobayashi, N. Inorg. Chem. 2004, 43, 8242-
8244.
(4) (a) Yang, X.-P.; Kang, B.-S.; Wong, W.-K.; Su, C.-Y.; Liu, H.-Q. Inorg.
Chem. 2003, 42, 169-179. (b) Yang, X.-P.; Su, C.-Y.; Kang, B.-S.; Fong,
X.-L.; Xiao, W.-L.; Liu, H.-Q. J. Chem. Soc., Dalton Trans. 2000, 3253-
3260.
(5) (a) Costes, J.-P.; Dupuis, A.; Laurent, J.-P. Inorg. Chim. Acta 1998, 268,
125-130. (b) Costes, J.-P.; Laussac, J.-P.; Nicode`me, F. J. Chem. Soc.,
Dalton Trans. 2002, 2731-2736. (c) Liu, G.; Na, C.; Liu, B.; Mao, K.
Polyhedron 1990, 9, 2019-2022. (d) Chen, H.; Archer, R. D. Inorg. Chem.
1994, 33, 5195-5202.
Figure 3. The emission spectra of free H2L, 1, and 2 in CH3CN.
Concentrations: 1.2 × 10-6 M (H2L); 5.6 × 10-5 M (1); 5.2 × 10-5
M
(6) (a) Cai, Y.-P.; Ma, H.-Z.; Kang, B.-S.; Su, C.-Y.; Zhang, W.; Sun, J.;
Xiong, Y.-L. J. Organomet. Chem. 2001, 628, 99-106. (b) Hogerheide,
M. P.; Boersma, J.; Konten, G. V. Coord. Chem. ReV. 1996, 155, 87-
126. (c) Xie, W.; Heeg, M. J.; Wang, P. G. Inorg. Chem. 1999, 38, 2541-
2543.
(2).
Molar conductivity studies in CH3CN confirmed a 1:1 electrolyte
for 1, while 2 was neutral, in accordance with the solid-state
structures. 1H NMR spectra of 1 and 2 in CD3CN contain multiple
broad peaks ranging from -60 to +60 ppm (Figure S6, Supporting
Information) and remain unchanged over a month-long period.
(7) Crystal data: (1) [Tb3L4(H2O)2]‚Cl‚9CH3OH‚4H2O, C97H112Br8ClN8O31
-
Tb3, M ) 3037.42, monoclinic, space group P2(1)/c, a ) 22.764(5) Å, b
) 19.015(4) Å, c ) 29.285(6) Å, â ) 104.78(3)°, V ) 12257 (4) Å3, Z
) 4, Dc ) 1.621 g cm-3, µ(Mo KR) ) 4.412 mm-1, F(000) ) 5776, T
) 153 K, R1 ) 0.0845, wR2 ) 0.1694 for 27 837 independent reflections
with a goodness of fit of 0.976. The crystal of 1 was sealed in a glass
capillary along with the solution.
1
However, in CD3OD, H NMR spectra develop additional peaks
over this time frame, suggesting that a slow decomposition process
takes place in this solvent. Both 1 and 2 exhibit green luminescence
in the solid state. In solutions of CH3CN, CH3OH, and CD3OD,
the free ligand H2L exhibits strong absorption bands at 235, 280,
and 335 nm. These maxima are all red-shifted on metal ion
coordination. Excitation of the ligand-centered absorption bands
of both 1 and 2 produces the typical emission bands of the Tb(III)
ion (5D4 f 7Fn transitions, n ) 6, 5, 4, and 3; Figure 3), while the
ligand-centered 1π-π* emission was not detected. The fluorescence
quantum yields (Φem) of 1 and 2 in CH3CN are 0.153 and 0.181,
respectively.10 The quantum yield of 1 is slightly lower than that
of 2, probably due to the coordination of two water molecules which
can quench lanthanide luminescence. With the same absorbance
value of 255 nm for both 1 and 2, the emission intensities in CH3-
(8) Crystal data: (2) [Tb3L3(OAc)2Cl]‚2C2H5OH‚H2O‚(C2H5)2O, C78H78Br6-
ClN6O20Tb3, M ) 2411.14, monoclinic, space group P2(1)/n, a ) 20.211-
(4) Å, b ) 22.089(4) Å, c ) 20.519(4) Å, â ) 114.69(3)°, V ) 8323 (3)
Å3, Z ) 4, Dc ) 1.919 g cm-3, µ(Mo KR) ) 5.507 mm-1, F(000) )
4656, T ) 153 K, R1 ) 0.0814, wR2 ) 0.1519 for 17 651 independent
reflections with a goodness of fit of 1.015. The crystal of 2 was sealed in
a glass capillary along with the solution.
(9) Reaction of Ln(NO3)3‚6H2O, Zn(OAc)2‚2H2O, and H2L produced the
heterobimetallic complexes, [ZnLnL(NO3)3] (Ln ) Eu, Nd): Yang, X.-
P.; Jones, R. A. Unpublished data.
(10) Fluorescence quantum yields were determined by using quinine sulfate
(Φem ) 0.546 in 0.5 M H2SO4) as standard for the Tb3+ complex: Meech,
S. R.; Philips, D. J. J. Photochem. 1983, 23, 193-217. Values of relative
emission intensities (298 K) at 546 nm for 1 and 2: CH3CN (12.5, 14.8),
CH3OH (1.0, 1.3), CD3OD (9.8, 3.1). Concentrations were adjusted to
give the same absorbance value at 255 nm for all samples.
(11) Petoud, S.; Cohen, S. M.; Bu¨nzli, J.-C. G.; Raymond, K. N. J. Am. Chem.
Soc. 2003, 125, 13324-13325.
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