Crystal structures and magnetic and luminescent properties of a series of homodinuclear lanthanide complexes with 4-cyanobenzoic ligand

Article abstract of DOI:10.1021/ic0602603

A series of homodinuclear lanthanide(III) complexes with the 4-cba ligand, [La₂(4-cba)₆(phen)₂(H₂O) ₆] (1) and [Ln₂(4-cba)₆(phen) ₂(H₂O)2] (Ln = Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), and Dy (7); 4-Hcba = 4-cyanobenzoic acid; phen = 1,10-phenanthroline), have been synthesized and structurally characterized by single-crystal X-ray diffraction. In 1, two water molecules bridge two nine-coordinated La ions, and six 4-cba ligands coordinate to the two La ions in terminal mode. In the isostructural complexes 2-7, two eight-coordinated Ln ions are connected by four bidentate 4-cba ligands, and another two 4-cba ligands terminate the two Ln ions. The variable-temperature magnetic properties of 2-7 have been investigated. Complex 7 shows a significant ferromagnetic interaction between Dy(III), while no magnetic interaction exists between Gd(III) ions in 6. In 2-5, the value of χ_MT decreases with decreasing temperature, but the magnetic interactions between the Ln(III) ions cannot definitely be concluded. Notably, the spin-orbit coupling parameters, λ, for Sm(III) (216(2) cm ^-1) and Eu(III) (404(2) cm^-1) have been obtained in 4 and 5, respectively. The strong fluorescent emissions of 4, 5, and 7 demonstrate that ligand-to-Ln(III) energy transfer is efficient and that the coordinated water molecules do not quench their luminescence by the nonradiative dissipation of energy.

Full text of DOI:10.1021/ic0602603

Inorg. Chem. 2006, 45, 6308−6316
Crystal Structures and Magnetic and Luminescent Properties of a
Series of Homodinuclear Lanthanide Complexes with 4-Cyanobenzoic
Ligand
Yan Li,†,‡ Fa-Kun Zheng,*,† Xi Liu,†,‡ Wen-Qiang Zou,†,‡ Guo-Cong Guo,*,† Can-Zhong Lu,† and
Jin-Shun Huang†
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and
Graduate School, Chinese Academy of Sciences, Beijing 100039, P. R. China
Received February 15, 2006
A series of homodinuclear lanthanide(III) complexes with the 4-cba ligand, [La2(4-cba)6(phen)2(H2O)6] (1) and
[Ln2(4-cba)6(phen)2(H2O)2] (Ln Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), and Dy (7); 4-Hcba 4-cyanobenzoic
acid; phen 1,10-phenanthroline), have been synthesized and structurally characterized by single-crystal X-ray
diffraction. In 1, two water molecules bridge two nine-coordinated La ions, and six 4-cba ligands coordinate to the
two La ions in terminal mode. In the isostructural complexes 2 7, two eight-coordinated Ln ions are connected by
four bidentate 4-cba ligands, and another two 4-cba ligands terminate the two Ln ions. The variable-temperature
magnetic properties of 2 7 have been investigated. Complex 7 shows a significant ferromagnetic interaction between
Dy(III), while no magnetic interaction exists between Gd(III) ions in 6. In 2 5, the value of MT decreases with
decreasing temperature, but the magnetic interactions between the Ln(III) ions cannot definitely be concluded.
Notably, the spin orbit coupling parameters,
, for Sm(III) (216(2) cm-1) and Eu(III) (404(2) cm-1) have been
)
)
)
−
−
−
ø
−
λ
obtained in 4 and 5, respectively. The strong fluorescent emissions of 4, 5, and 7 demonstrate that ligand-to-Ln(III)
energy transfer is efficient and that the coordinated water molecules do not quench their luminescence by the
nonradiative dissipation of energy.
Introduction
being very efficiently shielded by the fully occupied 5s and
5p orbitals and thus almost uninvolved in bonds between
neighboring lanthanide ions. In addition, the large orbital
contributions of most of lanthanide ions and the effects of
the crystal field, which make the explanation for the nature
The chemistry of lanthanide complexes is currently of great
interest because of their unique physicochemical properties
and various applications as functional materials.1 Especially,
the magnetic2 and luminescent properties3 of lanthanide
complexes have aroused much attention for decades. In the
magnetism area, lanthanide ions with large spin moments
should be natural choices for molecular magnetic materials,
but in most of these systems the small exchange-coupling
constants are problematic:4 this is the result of the 4f orbitals
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X. F.; Ai, X. C.; Sun, Y.; Wu, Y. S.; Zhang, J. P.; Wang, Y. Angew.
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* To whom correspondence should be addressed. E-mail: zfk@fjirsm.ac.cn
(F.-K.Z.); gcguo@ms.fjirsmac.cn (G.-C.G.).
† Fujian Institute of Research on the Structure of Matter, Chinese
Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
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1998, 33, 1517. (b) de Sa´, G. F.; Malta, O. L.; de Mello Donega´, C.;
Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr.
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Zukerman-Schpector, J.; Zinner, K. Coord. Chem. ReV. 2000, 196,
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6308 Inorganic Chemistry, Vol. 45, No. 16, 2006
10.1021/ic0602603 CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/08/2006

Lanthanide Complexes with 4-Cyanobenzoic Ligand
and magnitude of f-f interactions very difficult, embarrass
the study of lanthanide magnetic property in a way. Recently,
Ishikawa and co-workers proposed a new method to deter-
mine the crystal-field parameters for a series of dinuclear
phthalocyanine lanthanide complexes,4b,5 bringing some light
on this field. However, in contrast with the explanation for
the occurrence of ferro- or antiferromagnetic exchange for
transition metal ions,6 the situation for lanthanide ions is less
advanced, and the f-f interactions are much less investi-
gated.4,7 To further explore this field, another good choice
is to choose the homodinuclear complexes as good models
because they can provide valuable information regarding f-f
magnetic exchange.
and synthesized a series of homodinuclear lanthanide com-
plexes, [La2(4-cba)6(phen)2(H2O)6] 1 and [Ln2(4-cba)6(phen)2-
(H2O)2], where Ln ) Pr (2), Nd (3), Sm (4), Eu (5), Gd (6),
and Dy (7). Here, we report their syntheses, structures, and
magnetic and optical properties. It is worth noting that the
lanthanide complexes with the 4-cba ligand have not been
exploited.
Experimental Section
Materials and Instrumentation. All chemicals except Ln-
(NO3)3‚nH2O and Na(4-cba) were obtained from commercial
sources and used without further purification. A series of Ln(NO3)3‚
nH2O compounds were prepared by the reaction of Ln2O3 and nitric
acid in aqueous solution. The sodium salt of 4-Hcba, Na(4-cba),
was prepared by the reaction of 4-Hcba and NaOH in an equivalent
molar ratio in hot ethanol. Elemental analyses were performed on
a Vario EL III elemental analyzer. The FT-IR spectra were obtained
on a Perkin-Elmer Spectrum using KBr disks in the range of 4000-
400 cm-1. The magnetic susceptibilities of the title complexes were
measured with a Quantum Design MPMS-5S superconducting
quantum interference device (SQUID) magnetometer in the tem-
perature range of 2-300 K at a field of 1000 Oe, and diamagnetic
corrections were made using Pascal’s constants. Photoluminescence
analyses were performed on an Edinburgh Instrument F920
fluorescence spectrometer. All powder X-ray diffraction data were
collected using Rigaku Dmax2500PC powder diffractometer (Cu
KR radiation; 5° e 2θ e 60°).
Preparation of 1-7. A mixture of Ln(NO3)3‚nH2O (0.3 mmol),
4-cba (0.6 mmol), and phen (0.5 mmol) in a mixed solution of
ethanol (10 mL) and water (5 mL) was sealed into a 25 mL poly-
(tetrafluoroethylene)-lined stainless steel container under autogenous
pressure and then heated at 100 °C for 3 days and cooled to 30 °C
at 1 °C h-1. Prismatic single crystals suitable for X-ray analyses
were obtained by slow evaporation of the solvent after a few days.
Yield: 40% (based on La) for 1; 45% (based on Pr) for 2; 70%
(based on Nd) for 3; 65% (based on Sm) for 4; 50% (based on Eu)
for 5; 75% (based on Gd) for 6; 55% (based on Dy) for 7. Anal.
Calcd for 1: C, 53.26; H, 3.23; N, 8.63. Found: C, 53.90; H, 3.01;
N, 8.22. Anal. Calcd for 2: C, 55.59; H, 2.85; N, 9.01. Found: C,
55.23; H, 2.95; N, 9.09. Anal. Calcd for 3: C, 55.52; H, 2.85; N,
9.00. Found: C, 55.38; H, 2.99; N, 8.86. Anal. Calcd for 4: C,
54.82; H, 2.81; N, 8.88. Found: C, 54.71; H, 2.95; N, 8.86. Anal.
Calcd for 5: C, 54.75; H, 2.81; N, 8.87. Found: C, 54.35; H, 2.89;
N, 8.47. Anal. Calcd for 6: C, 54.40; H, 2.79; N, 8.82. Found: C,
54.16; H, 2.95; N, 8.79. Anal. Calcd for 7: C, 53.99; H, 2.77; N,
8.75. Found: C, 54.11; H, 2.81; N, 8.73. IR (KBr, cm-1): for 1
3441 (br), 2924 (m), 2853 (w), 2234 (s, ν (CN)), 1600 (s, νasym
(CO2)), 1589 (s, νasym (CO2)), 1554 (s), 1520 (w), 1426 (s, νsym
(CO2)), 1404 (vs, νsym (CO2)), 1356 (m), 1290 (w), 1142 (w), 1103
(w), 1020 (w), 874 (w), 848 (w), 788 (m), 729 (m), 697 (w), 667
(w), 569 (m), 546 (w); for 2 3437 (br), 2922 (w), 2852 (w), 2228
(s, ν (CN)), 1623 (s, νasym (CO2)), 1590 (s, νasym (CO2)), 1549 (s),
1516 (m), 1426 (s, νsym (CO2)), 1406 (vs, νsym (CO2)), 1346 (m),
1293 (m), 1142 (w), 1102 (w), 1018 (w), 864 (m), 844 (m), 780
(s), 728 (m), 693 (m), 637 (w), 566 (m), 545 (m); for 3 3436 (br),
Aromatic carboxylate coordination complexes, possessing
special physical properties and intriguing structural features,
have attracted increasing interest.8 In the reported complexes,
it has been proven by the aromatic carboxylate ligands
supporting significant magnetic interactions between the
lanthanide(III) ions7e,9 and supporting the strong lanthanide-
(III)-centered luminescent emission.10 In the above context,
we select 4-cyanobenzoic acid (4-Hcba) and 3-cyanobenzoic
acid (3-Hcba) with two coordination groups and an electron-
conjugated system to bind metal atoms to produce new
complexes, which may have novel magnetic and optical
properties. Compared to other carboxylic acids, the chemistry
of Hcba has only been investigated in a limited manner so
far.11 Our previous studies have mainly focused on transition
metal complexes with the Hcba ligand.12 Recently, we
employed 4-Hcba and 1,10-phenanthroline, as mixed ligands,
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Inorganic Chemistry, Vol. 45, No. 16, 2006 6309

Li et al.
2924 (w), 2853 (w), 2228 (s, ν (CN)), 1624 (s, νasym (CO2)), 1590
(s, νasym (CO2)), 1549 (s), 1516 (m), 1426 (s, νsym (CO2)), 1408
(vs, νsym (CO2)), 1346 (w), 1293 (m), 1143 (w), 1102 (w), 1018
(w), 856 (m), 844 (m), 781 (s), 728 (m), 693 (m), 637 (w), 567
(m), 545 (m); for 4 3441 (br), 2924 (w), 2854 (w), 2228 (s, ν (CN)),
1627 (s, νasym (CO2)), 1590 (s, νasym (CO2)), 1550 (s), 1517 (m),
1426 (s, νsym (CO2)), 1407 (vs, νsym (CO2)), 1346 (m), 1293 (m),
1143 (w), 1103 (w), 1018 (w), 865 (m), 843 (m), 780 (s), 728 (m),
693 (m), 638 (w), 567 (m), 545 (m); for 5 3452 (br), 3079 (w),
2927 (w), 2229 (s, ν (CN)), 1633 (s, νasym (CO2)), 1591 (s, νasym
(CO2)), 1550 (s), 1518 (m), 1426 (s, νsym (CO2)), 1409 (vs, νsym
(CO2)), 1346 (m), 1293 (m), 1143 (w), 1103 (w), 1018 (w), 865
(m), 844 (m), 781 (s), 728 (m), 693 (m), 638 (w), 567 (m), 546
(w); for 6 3443 (br), 3078 (w), 2920 (w), 2228 (s, ν (CN)), 1631
(s, νasym (CO2)), 1590 (s, νasym (CO2)), 1550 (s), 1517 (m), 1425
(vs, νsym (CO2)), 1410 (vs, νsym (CO2)), 1346 (m), 1293 (m), 1143
(w), 1103 (w), 1018 (w), 865 (m), 843 (m), 780 (s), 728 (m), 693
(m), 638 (w), 568 (m), 545 (w); for 7 3451 (br), 2924 (w), 2852
(w), 2229 (s, ν (CN)), 1635 (s, νasym (CO2)), 1591 (s, νasym (CO2)),
1551 (s), 1517 (m), 1426 (vs, νsym (CO2)), 1410 (vs, νsym (CO2)),
1346 (m), 1293 (m), 1143 (w), 1102 (m), 1018 (w), 866 (m), 843
(m), 780 (s), 728 (m), 693 (m), 638 (w), 568 (m), 545 (w). The
most characteristic peaks of IR (KBr) for Na(4-cba): 2232 (s, ν
(CN)), 1591 (s, νasym (CO2)), 1548 (s, νasym (CO2)), 1415 (sh, νsym
of the quadrangular faces is defined by one carboxylate
oxygen atom and three water oxygen atoms (O1W, O3WA,
O11, and O3W; symmetry code A -x, -y, -z) with a mean
deviation of 0.197 Å from the least-squares plane; the other
one, which is capped by one nitrogen atom (N2) from one
phen molecule, is defined by two oxygen atoms from two
crystallographically unique 4-cba ligands, one nitrogen atom
from the phen molecule and one coordinated water oxygen
atom (O2W, N1, O31, and O21) with a mean deviation of
0.0319 Å from the least-squares plane. The dihedral angle
between the two square faces is ca. 1.89(4)°. The La-N
distances are in the range of 2.723(2)-2.743(2) Å, in
agreement with those of the La(III) analogue with the phen
ligands,15 while the La-O distances are in the range of 2.458-
(2) and 2.743(2) Å; the longest La-O bonds involve the
bridging water oxygen atoms (La1-O3W ) 2.743(2) Å,
La-O3WA ) 2.632(2) Å; symmetry code A -x, -y, -z),
which are comparable to those found in the La2(C5H4NCO2)6‚
(H2O)4 complexes.16
Carboxylate groups exhibit only one coordination mode:
a terminal monodentate ligand. There are intramolecular
hydrogen bonds (O1W‚‚‚O32A ) 2.799(3) Å, O1W-
H1WA‚‚‚O32A ) 137(3)°; symmetry code A -x, -y, -z),
and the dimeric molecules are also connected by intermo-
lecular hydrogen bonds to give a one-dimensional (1D) chain
network along the a direction with distances of O1W‚‚‚O21B
) 2.940(3) Å, O2W‚‚‚O22B ) 2.746(3) Å, and O2W‚‚‚
O32B ) 2.750(3) Å and angles of O1W-H1WB‚‚‚O21B
) 163(3)°, O2W-H2WA‚‚‚O22B ) 173(3)°, and O2W-
H2WB‚‚‚O32B ) 162(3)° (symmetry code B 1 + x, y, z),
as depicted in Figure S2. Despite the existence of the terminal
monodentate 4-cba ligand, all four C-O distances of two
crystallographically independent carboxylate groups involved
with hydrogen bonds are almost equal (1.254(3)-1.265(3)
Å). Another carboxylate group is not concerned with
hydrogen bonds, and its C-O(coordinated) distance (1.282(3) Å)
is certainly slightly longer than the C-O(uncoordinated) distances
(1.233(3) Å). The nearest separation of La(III)‚‚‚La(III)
through hydrogen bonds is 6.9971(1) Å, which is obviously
longer than that through the bridging water oxygen atoms.
[Ln2(4-cba)6(phen)2(H2O)2] (2-7). Figure 2 shows the
molecular structure of 2. The molecular entity comprises a
center-related dinuclear [Pr2(CO2)4] unit. Four carboxylate
groups link a pair of Pr(III) atoms in the O,O′-bridging mode
to generate a paddle-wheel-like centrosymmetric dimer [Pr2-
(carboxylate-O,O′)4] with Pr(III)‚‚‚Pr(III) distances of 4.4018
(9) Å. The Pr(III) atom is coordinated by four O atoms from
four bridging 4-cba ligands, one O atom from a monodentate
4-cba, two N atoms from one chelating phen molecule, and
one coordinated water molecule. These eight coordination
atoms form a distorted square antiprism (Figure S3). One of
the quadrangular faces is defined by two nitrogen atoms from
one phen molecule, one terminal carboxylate oxygen atom,
and one water oxygen atom (N1, N2, O1W, and O31) with
the mean deviation of 0.168 Å from the least-squares plane;
(CO2)), 1403 (vs, νsym (CO2)) cm-1
.
Single-Crystal Structure Determination. The crystal structures
of the complexes were studied by single-crystal X-ray diffraction
analyses on a Rigaku Mercury CCD diffractometer equipped with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at 293-
(2) K. The intensity data sets were collected with the ω scan
technique and reduced by CrystalClear software.13 The structures
were solved by the direct methods and refined by full-matrix least-
squares techniques. Non-hydrogen atoms were located by difference
Fourier maps and subjected to anisotropic refinement. The hydrogen
atoms of the water molecule were located in difference Fourier
syntheses and refined with O-H distances restrained to a target
value of 0.96 Å and Uiso(H) ) 1.2Ueq(O), and the remaining
hydrogen atoms were calculated in idealized positions and allowed
to ride on their parent atom. All of the calculations were performed
by the Siemens SHELXTL, version 5, package of crystallographic
software.14 Pertinent crystal data and structure refinement results
for the complexes are listed in Table 1.
Results and Discussions
Description of the Structures. The X-ray crystallography
analyses revealed that 2-7 are isomorphous, so we will
choose 1 and 2 for detailed structural discussions.
[La2(4-cba)6(phen)2(H2O)6] (1). As shown in Figure 1,
the molecule of 1 consists of a centrosymmetric dimeric
[La2O2] unit. The two La(III) atoms are bridged by two water
molecules to generate an ideal parallelogram with a La(III)‚
‚‚La(III) distance of 4.5826(6) Å. The La(III) atom is nine-
coordinate, with three O atoms from three 4-cba, two N
atoms from one chelating phen molecule, and four coordi-
nated water molecules, to form a distorted monocapped
square antiprismatic coordination geometry (Figure S1). One
(13) CrystalClear, version 1.35; Software User’s Guide for the Rigaku
R-Axis and Mercury and Jupiter CCD Automated X-ray Imaging
System; Rigaku Molecular Structure Corporation: The Woodlands,
TX, 2002.
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tion Inc.: Madison, WI, 1994.
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(16) Moore, J. W.; Glick, M. D.; Baker, W. A., Jr. J. Am. Chem. Soc.
1972, 94, 1858.
6310 Inorganic Chemistry, Vol. 45, No. 16, 2006

Lanthanide Complexes with 4-Cyanobenzoic Ligand
Table 1. Crystal Data and Structure Refinement for Complexes 1-7
1
2
3
4
formula
fw
C72H52La2N10O18
1623.06
C72H44Pr2N10O14
1554.99
C72H44Nd2N10O14
1561.65
C72H44Sm2N10O14
1573.87
cryst dimensions (mm)
shape
0.18, 0.16, 0.10
prism
0.32, 0.20, 0.10
prism
0.25, 0.15, 0.10
prism
0.20, 0.18, 0.08
prism
color
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
colorless
P1h
pale green
P1h
pale purple
P1h
pale yellow
P1h
6.99710(10)
14.33860(10)
18.9494(2)
108.363(8)
97.397(7)
103.890(7)
1707.96(3)
1
8.308(2)
12.852(4)
16.419(5)
97.443(4)
101.952(4)
99.497(2)
1667.2(8)
1
8.297(5)
12.813(7)
16.426(9)
97.527(4)
102.014(7)
99.572(6)
1659.1(16)
1
8.2827(8)
12.8027(11)
16.4241(15)
97.762(2)
102.084(4)
99.682(4)
1652.4(3)
1
Z
F (g/cm3)
1.578
1.314
812
1.549
1.518
776
1.563
1.622
778
1.582
1.834
782
µ (mm-1
F(000)
)
θ range (deg)
reflns (measured)
Rint
3.01-25.02
3.17-25.03
3.17-25.03
3.14-25.03
10854
10571
10594
10601
0.0190
0.0180
0.0312
0.0281
data (obsd)/restraints/params
R1, R2 (obsd)
5639/6/473
0.0285, 0.0617
1.004
5532/14/448
0.0273, 0.0676
1.004
5305/2/450
0.0390, 0.0907
1.003
5305/2/448
0.0371, 0.0866
1.002
GOF on F2
largest and mean δ/σ
0.000, 0.000
1.051, -0.394
0.003, 0.000
0.539, -0.500
0.015, 0.000
0.327, -0.248
0.000, 0.000
0.484, -0.485
largest difference peak (e Å3)
5
6
7
formula
fw
C72H44Eu2N10O14
1577.09
C72H44Gd2N10O14
1587.67
C72H44Dy2N10O14
1598.17
cryst dimensions (mm)
shape
0.38, 0.30, 0.10
prism
0.40, 0.23, 0.10
prism
0.38, 0.25, 0.10
prism
color
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
colorless
P1h
colorless
P1h
colorless
P1h
8.2745(6)
12.8076(9)
16.4256(13)
97.838(2)
102.116(3)
99.720(3)
1650.6(2)
1
8.273(2)
12.796(4)
16.424(5)
97.903(2)
102.136(3)
99.775(3)
1647.9(8)
1
8.2525(7)
12.7935(12)
16.4146(12)
97.941(3)
102.068(2)
99.918(4)
1641.9(2)
1
Z
F (g/cm3)
1.587
1.957
784
1.600
2.070
786
1.616
2.333
790
µ (mm-1
F(000)
)
θ range (deg)
reflns (measured)
Rint
3.14-25.03
3.14-25.03
3.18-25.03
10510
10447
10387
0.0203
0.0168
0.0168
data (obsd)/constraints/params
R1, R2 (obsd)
5399/2/448
0.0269, 0.0644
1.007
5446/2/448
0.0234, 0.0589
1.002
5432/2/448
0.0232, 0.0583
1.006
GOF on F2
largest and mean δ/σ
0.000, 0.000
0.441, -0.493
0.001, 0.000
0.407, -0.641
0.000, 0.000
0.408, -0.972
largest difference peak (e Å3)
the other one is defined by four oxygen atoms from two
crystallographically unique 4-cba ligand (O11, O12A, O21,
and O22A; symmetry code A 1 - x, -y, 1 - z) with a
negligible mean deviation of 0.0005 Å from the least-squares
plane. The dihedral angle between the square faces is ca.
21.27(9)°, reflecting the distorted square antiprism. The bond
distances of Pr-N (Pr1-N1 ) 2.661(3) Å, Pr1-N2 )
2.675(3) Å are comparable to those of the previously reported
phen-containing Pr(III) complex.17 The Pr-O bond distances
fall in range of 2.405(2)-2.481(2) Å, which are similar to
those of the Pr(III) analogue.18,19
tate mode, a bidentate bridging mode in the syn-syn
configuration, as well as an unusual bidentate bridging
coordination mode in the syn-anti configuration, which, to
the best our knowledge, has not been encountered in 4-cba
complexes.
Likewise, supramolecular interactions also play an im-
portant role in the crystal packing and stabilization of 2.
There is a concurrence of hydrogen bonds and π-π stacking
interactions. First, there are strong intramolecular hydrogen
bonds resulting from the coordinated water molecule and
uncoordinated carboxylate O atom (O1W‚‚‚O32 ) 2.618-
(3) Å and O1W-H1WB‚‚‚O32 ) 162(3)°). Second, as
shown in Figure S4, face-to-face π-π stacking interactions
between rings R1 (atoms C1, C2, C3 C4, C12, and N1) and
The carboxylate groups are bound to the Pr(III) ions in
three different coordination fashions: a common monoden-
(17) Dong, N.; Zhu, L. G.; Xu, C. Chinese J. Struct. Chem. 1993, 12, 133.
(18) Xu, H. T.; Zheng, N. W.; Jin, X. L.; Yang, R. Y.; Wu, Y. G.; Ye, E.
Y.; Li, Z. Q. J. Mol. Struct. 2003, 655, 339.
(19) Junk, P. C.; Kepert, C. J.; Lu, W. M.; Skelton, B. W.; White, A. H.
J. Aust. J. Chem. 1999, 52, 459.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6311

Li et al.
Figure 3. Plots of øM and øMT vs T for 2.
(III) distances of 2-7 fall in the range of 4.401(2)-4.4252-
(9) Å, which are visibly shorter than that of 1 (4.5826(6)
Å). This is because the radius of the La(III) ion is larger
than those of other lanthanide(III) ions (namely, lanthanide
contraction). Another important reason is the anionic 4-cba
ligand has a higher affinity of bonding to the cationic
lanthanide ions than the neutral water molecule. As expected,
in all complexes, the Ln-O (terminal), Ln-O (bridging),
Ln-O (water), and Ln-N distances show a slight and
continuous decrease upon going from the larger La(III) to
the smaller Dy(III) cation, whereas the change of Ln(III)‚‚
‚Ln(III) distances is not obvious from 2 to 7.
Figure 1. Molecular structure of 1. All hydrogen atoms are omitted for
clarity, and the dashed line represents the intramolecular hydrogen bonds.
Magnetic Properties. In contrast with the effects of the
crystal field, the spin-orbital coupling in general plays a
more important role in the magnetism of lanthanide com-
plexes. This large spin-orbit coupling partly removes the
degeneracy of the 2S+1L group term of lanthanide ions, giving
2S+1LJ states, which further split into Stark levels by crystal
field perturbation. For most of free lanthanide ions, the
energy separation between the 2S+1LJ ground state and the
first excited state is so large that only the ground state is
thermally populated at room and low temperature, but this
situation is possibly exceptional for the Sm(III) and Eu(III)
ions in which the first excited state may be thermally
populated because of the weak energy separation. Thus, in
the series of dinuclear lanthanide complexes reported here,
the crystal field effect must be taken into account and the
possible thermal population of the higher states should also
be taken into consideration for Sm(III) and Eu(III) com-
plexes.
Figure 2. Molecular structure of 2. All hydrogen atoms are omitted for
clarity, and the dashed line represents the intramolecular hydrogen bonds.
R2 (atoms C4, C5, C6, C7, C11, and C12) from neighboring
phen molecules, with a dihedral angle of 2.3(2)° and a
centroid-to-centroid distance of 3.755 Å, generate a 1D
chained network along the b direction. The adjacent chains
are further connected to form a two-dimensional (2D)
network parallel to the ab plane through π-π stacking
between rings R2 and R3 (atoms C7, C8, C9, C10, N2, and
C11) from the adjacent-chain phen molecules, with a dihedral
angle of 2.5(2)° and a centroid-to-centroid distance of 3.646
Å. The nearest separation (6.3532(13) Å) of Pr(III)‚‚‚Pr(III)
via π-π stacking is longer than that via the bridging
carboxylate groups.
The range of Ln-O(terminal 4-cba), Ln-O(bridging 4-cba), Ln-
O
(water), Ln-N, and Ln‚‚‚Ln distances are given in Table 2.
[Pr2(4-cba)6(phen)2(H2O)2] (2). The variable-temperature
magnetic susceptibility, øM, and øMT are shown in Figure 3.
At 300 K, øMT is equal to 3.205 cm3 K mol-1, as expected
for two isolated Pr(III) ions (3.20 cm3 K mol-1). As the
temperature is lowered, the øMT value decreases more and
The dinuclear structures of 2-7, bridged by carboxylate
groups, are different from that of 1, bridged by water oxygen
atoms. The Ln(III) atom is eight-coordinate in 2-7, while
in 1 the La(III) atom is nine-coordinate. The Ln(III)‚‚‚Ln-
Table 2. Selected Bond Distances (Å)
Ln-O(terminal 4-cba)
Ln-O(bridging 4-cba)
Ln-O(water)
Ln-N
Ln‚‚‚Ln
1
2
3
4
5
6
7
2.458(2)-2.484(2)
2.427(2)
2.415(3)
2.387(3)
2.371(2)
2.577(2)-2.743(2)
2.468(2)
2.454(3)
2.432(3)
2.417(2)
2.723(2), 2.743(2)
2.661(3), 2.675(3)
2.636(4), 2.649(4)
2.616(4), 2.624(4)
2.604(3), 2.612(3)
2.590(2), 2.596(3)
2.563(2), 2.577(2)
4.5826(6)
4.4018(9)
4.401(2)
4.4156(6)
4.4214(4)
4.4252(9)
4.4145(4)
2.405(2)-2.481(2)
2.387(3)-2.461(3)
2.349(3)-2.429(3)
2.335(2)-2.417(2)
2.325(2)-2.408(2)
2.298(2)-2.382(2)
2.364(2)
2.342(2)
2.410(2)
2.387(2)
6312 Inorganic Chemistry, Vol. 45, No. 16, 2006

Lanthanide Complexes with 4-Cyanobenzoic Ligand
Figure 4. Plots of øM and øMT vs T for 3.
Figure 5. Plots of øM and øMT vs T for 4. The solid line is the calculated
more rapidly and reaches 0.099 cm3 K mol-1 at 2.0 K.
However, the whole profile of the øMT versus T curve is not
indicative of the nature of the interactions between the two
Pr(III) ions because of the crystal-field effect, which splits
the 9-fold degenerate 3H4 ground state into Stark levels, and
the values of øMT mainly depend on the populations of those
Stark levels. At room temperature, all the Stark levels from
the 9-fold degenerate 3H4 ground states are populated, such
that øMT is equal to the value expected for two free ions,
but as the temperature decreases, a progressive depopulation
of higher Stark levels occurs, so the value of øMT begins to
decrease.
curve over the whole temperature range.
Sm(III) ions. This behavior indicates that an antiferromag-
netic interaction possibly exists between Sm(III) pairs at low
temperature, although it is very weak. The øMT versus T data
have been analyzed on the basis of the equations deduced
from the Sm(III) ion in a monomeric system with free-ion
approximation,20a over the whole temperature range. The best
agreement between the experimental and calculated data
corresponds to the spin-orbit coupling parameter, λ ) 216-
(2) cm-1, and the Weiss-like constant, θ ) -11.32(78) K,
with an agreement factor R (R ) ∑[(øMT)obsd - (øMT)calcd
]2/∑(øMT)obsd2) of 3.1 × 10-4. The spin-orbit coupling
parameter λ is close to that of the Sm(III) mononuclear
complex (193 cm-1), but lower than the value deduced from
the spectroscopic data (λ ) 281 cm-1 with J ) 7/2).The
deviation of øMT with respect to the equation for the Sm-
(III) ion with free-ion approximation (θ ) -11.32(78) K)
is caused by the crystal field effect, as well as the possible
antiferromagnetic interactions between the two Sm(III) ions.
[Eu2(4-cba)6(phen)2(H2O)2] (5). Similar to 4, the 7F
ground term is split into seven states by spin-orbit coupling,
and the energy separation of this states are weak enough
that the excited states may be thermally populated at room
temperature and above. The øMT versus T data have also
been analyzed on the basis of the equations deduced from
the Eu(III) ion in the monomeric system with the free-ion
approximation,20a over the whole temperature range.
[Nd2(4-cba)6(phen)2(H2O)2] (3). A similar evolution is
observed in the case of 3. As illustrated in Figure 4, øMT is
equal to 3.286 cm3 K mol-1 at 300 K, which is consistent
with the value expected for two isolated Nd(III) ions (3.28
cm3 K mol-1), and decreases continuously to a value of 1.200
cm3 K mol-1 at 2 K. This is mainly because of the splitting
4
of the 10-fold degenerate I9/2 ground state by the crystal
field and the progressive depopulation of the higher energy,
as the temperature is lowered. Notably, the whole profile of
the øMT versus T curve is similar to that reported for
mononuclear and homodinuclear complexes.20 Therefore,
even if the magnetic interaction is present, it should be very
weak.
[Sm2(4-cba)6(phen)2(H2O)2] (4). The magnetic charac-
teristic of the Sm(III) complex is different from those of the
6
Pr(III) and Nd(III) complexes. The H ground term for the
free Sm(III) ions is split into six states by spin-orbit
coupling, and the spin-orbit coupling parameter is on the
order of 200 cm-1, so both the crystal-field effect and the
possible thermal population of the higher states should be
evaluated for the Sm(III) complex.
As can be seen from Figure 5, øMT is equal to 0.798 cm3
K mol-1 at 300 K, and decreases rapidly to a value of 0.086
cm3 K mol-1 at 2 K. The thermal variation of øMT is nearly
linear over the whole temperature range, which is similar to
that for the reported mononuclear complex. But, interestingly,
the value of 0.086 cm3 K mol-1 for øMT at 2 K is consistent
with the value of 0.089 cm3 K mol-1 predicted by theory
for one Sm(III) ion and obviously smaller than that for two
The least-squares fitting of the øMT versus T curve leads
to λ ) 404(2) cm-1 and θ ) 1.21(27) K with an agreement
factor R (R ) ∑[(øMT)obsd - (øMT)calcd]2/∑(øMT)obsd2) of 3.1
× 10-4. The spin-orbit coupling parameter λ is slightly
larger than that of Eu(III) mononuclear complex (362 cm-1),
and comparable to the value (379 cm-1) deduced from the
energy difference between the states F1 and F0, based on
the emission spectra (see below discussions on optical
spectroscopy).
By comparing the curves of øM and øMT versus T of 5 to
those of the mononuclear Eu(III) complex, it is interesting
to find that they are very similar. As shown in Figure 6, øM
smoothly increases over the 300-80 K temperature range
and then tends to a plateau. Below 25 K, øM starts to
increases again and more and more rapidly at low temper-
ature, reaching a value of 0.01218 cm3 mol-1. At 300 K,
7
7
(20) (a) Andruh, M.; Bakalbassis, E.; Kahn, O.; Trombe, J. C.; Porcher, P.
Inorg. Chem. 1993, 32, 2, 1616. (b) Hou, H. W.; Li, G.; Li, L. K.;
Zhu, Y.; Meng, X. R.; Fan, Y. T. Inorg. Chem. 2003, 42, 428.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6313

Li et al.
Figure 6. Plots of øM and øMT vs T for 5. The solid line is the calculated
curve over the whole temperature range.
Figure 8. Plots of øM and øMT vs T for 7.
) -2(6) × 10-4 cm3 mol-1. So 6 is paramagnetic, and there
is no interaction between Gd(III) ions.
[Dy2(4-cba)6(phen)2(H2O)2] (7). The Dy(III) ion belongs
to the heavy lanthanide ions, and the free-ion ground state,
6H15/2, is well separated from first excited state, so only the
ground state is thermally populated at room and low
temperature, which is very different from the situation of
Sm(III) because of the larger spin-orbit coupling for Dy-
(III), even though both of them have the same number of
localized unpaired f electrons.
As can be seen from Figure 8, it is worth noting that the
situation of 7 is very different from the above complexes.
For the latter complexes, the usual decrease in the value of
øMT is observed upon lowering the temperature, while for
the former complex 7 øMT versus T curves display a
minimum. At 300 K, øMT is equal to 28.54 cm3 K mol-1, as
expected for two isolated Dy(III) ions (28.34 cm3 K mol-1).
When the temperature is lowered, øMT increases very slowly
to 50 K (øMT ) 29.30 cm3 K mol-1) and then decreases
down to 14 K. At this temperature, it reaches a minimum of
28.77 cm3 K mol-1 and then increases dramatically as the
temperature is further lowered. The whole profile of the øMT
vs T curve is indicative of the occurrence of two conflicting
effects. On one hand, the decreases in øMT unambiguously
originate in the thermal depopulation of the highest Stark
levels resulting from the splitting of the free-ion ground state,
6H15/2, by the crystal field, and on the other hand, the
antagonist process must be the ferromagnetic interaction
between the Dy(III) ions. Interestingly, the ferromagnetic
interaction in 7 is obviously different from the situation in
the reported homodinuclear Dy(III) complex PcDyPcDyPc*
(Pc ) dianion of phthalocyanine, Pc* ) dianion of 2,3,9,-
10,16,17,23,24-octabutoxyphthalocyanine), 8, also with fer-
romagnetic interaction between Dy(III) ions.4b In 8, there is
only one inflection with Tmin ) 8 K in the whole profile of
the øMT versus T curve, while in 7, there are two inflections,
and the Tmin is 14 K, which is obviously larger than that of
8. So the presence of the stronger ferromagnetic interaction
between the Dy(III) ions in 7 can be concluded, despite a
longer Dy(III)‚‚‚Dy(III) distance, possible because of the
different exchange pathways from 8.
Figure 7. Plots of øM and øMT vs T for 6. The solid line is the calculated
curve over the whole temperature range.
øMT is equal to 2.461 cm3 K mol-1, obviously smaller than
the theoretical high-temperature limit (øMT)HT ) 4.50 cm3
K mol-1), and it decreased continuously to the value close
to zero (0.0244 cm3 K mol-1) at 2 K. Below 50 K, the
thermal variation of øMT is rigorously linear, with the slope
(øM)LT ) 0.0109 cm3mol-1, comparable to the two times that
for the mononuclear complex ((øM)LT ) 5.99 × 10-3
cm3mol-1). So the magnetic behavior of 5 is mainly caused
by single-ion properties, which is also reflected in the small
value of the Weiss-like constant θ (θ ) 1.21(27) K).
[Gd2(4-cba)6(phen)2(H2O)2] (6). The situation is simple
for 6. Since Gd(III) has an 8S7/2 ground state, which is almost
unsplit (0.1 cm-1) and also well separated from the first
excited state (about 32 000 cm-1), Gd(III) has a simple spin
system as the influence of the crystal field can be neglected.
The thermal variation of the magnetic susceptibility, øM,
and øMT are shown in Figure 7. øMT is almost constant with
small-scale range from 16.12 to 15.94 cm3 K mol-1 over
the whole temperature range, which is close to the theoretical
value for two isolated Gd(III) (15.86 cm3 K mol-1).
The almost unchanged value of øMT indicates that there
is no interaction between the two Gd(III) ions, which is also
reflected in the value of the Weiss constant θ. A nonlinear
fit via øM ) C/(T - θ) + ø0 over the whole temperature
range reveals a Curie-Weiss law behavior with a Curie
constant, C, of 15.99(1) cm3 K mol-1 and a Weiss constant,
θ, of -0.0063(22) K, and the background susceptibility ø0
Optical Spectroscopy. For each complex, the cyano ν-
(CtN) stretching vibration occurs at the almost same
6314 Inorganic Chemistry, Vol. 45, No. 16, 2006

Lanthanide Complexes with 4-Cyanobenzoic Ligand
Figure 10. Solid-state excitation (dashed line) and emission (full line)
spectra of Na(4-cba) and phen at room temperature.
Figure 9. Solid-state emission spectra of 4, 5, and 7 (λex ) 346, 357, and
350 nm, respectively) at room temperature.
present in most of all transitions. Interestingly, in the
emission of 5, the ligands-centered emission bond vanished
completely, as compared with the ligand-centered emission
of pure 4-Hcba and phen ligands (see Figure 10), while in
the emission of 4 and 7, the ligands-centered emission is
most quenched, indicating that the ligand-to-Ln(III) energy
transfer may be more efficient for 5. Notable radiationless
deactivation, caused by the coordinated water,23 can be
avoided to promote efficient fluorescent emission.
The use of phen-functionalized organic compounds in
promoting lanthanide ions emitting intense luminescence has
been reported.24 Actually, deduced from the absorption and
emission spectra of Na(4-cba) and phen (Figure 10), the
lowest-lying triplet excited energy level (ET) of Na(4-cba)
is close to that of phen. The ET level of phen can be estimated
to be at 21 640 cm-1. The lowest- lying excited energy levels
of Sm(III) Eu(III) and Dy(III) are 18 200, 17 500, and 21 100
cm-1, respectively.24a Thus, efficient ligand-Ln(III) energy
transfer can take place for complexes 4, 5, and 7; 4-cba
complexes may act as good candidates for efficient lumi-
nescent materials.
frequency, ca. 2230 cm-1, as that of the free Na(4-cba),
which suggests that the cyano group is not involved in
coordination. The asymmetrical and symmetrical stretching
frequencies in the carboxylate complexes, νasym(CO2) and
ν
sym(CO2), are sensitive to the carboxylate coordination
environments.21 The terminal carboxylate group in all
complexes does not have the typical character of unidentate
ligand because of the intervention of the strong bonds, which
leads to the difficult discrimination of carboxylate coordina-
tion modes. On the whole, both ν(CO2) in all complexes are
shifted to the higher frequencies relative to values of Na(4-
cba).
The luminescence of lanthanide complexes has also
attracted intensive research for their high color purity and
potentially high internal quantum efficiency.3,22 It is well-
known that the luminescence of lanthanide complexes from
the f-f transitions is usually generated via the “antenna
effect”, since the f-f transition is spin- and parity-forbidden.
So we should introduce suitable chromophores in the
lanthanide complexes, which absorb light and transfer
excitation energy to the lanthanide ions. Aromatic 4-cba and
phen ligands, possessing electron-conjugate systems, may
be good chromophores for the efficient luminescent sensi-
tization of lanthanide ions. We have measured the photolu-
minescence spectra of solid samples of 4, 5, and 7 at room
temperature upon photoexcitation at 346, 357, and 350 nm,
respectively, as shown in Figure 9.
Conclusions
In summary, the syntheses and crystal structures of a series
of homodinuclear lanthanide complexes containing the 4-cba
ligand have been reported, and their magnetic and lumines-
cent properties have been discussed. Two structural types
with parallelogramic [La2O2] and paddle-wheel-like [Ln2-
(CO2)4] molecular frameworks have been revealed. Ferro-
magnetic interactions can be unambiguously found in the
Dy(III) complex (7) from the profile of øMT versus T curve,
while the Gd(III) complex (6) is found to be paramagnetic,
even though both have comparable Ln(III)‚‚‚Ln(III) dis-
tances. It is worth noting that the ferromagnetic interaction
The emission peaks of 5 at 579, 592, 613, 650, and 695
nm can be assigned to 5D0 f 7FJ (J ) 0, 1, 2, 3, 4) transitions
5
7
of the Eu(III) ion, respectively. Notably, the D0 f F2
transition is much more intense than the 5D0 f 7F1 transition,
which reflects a low symmetry of the Eu(III) site, consistent
to the distorted square antiprism coordination environment.
The emission peaks of 4 at 563, 596, and 648 nm can be
assigned to 4G5/2 f 6HJ (J ) 5/2, 7/2, 9/2) transitions of the
Sm(III) ion, respectively, while the emission bands for 7 at
(23) (a) Haas, Y.; Stein, G. J. Phys. Chem. 1971, 75, 3677. (b) Stein, G.;
Wurzberg, E. J. Chem. Phys. 1975, 62, 208.
(24) (a) Quici, S.; Cavazzini, M.; Marzanni, G.; Accorsi, G.; Armaroli,
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4
6
480 and 574 nm can be assigned to F9/2 f HJ (J ) 15/2,
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Inorganic Chemistry, Vol. 45, No. 16, 2006 6315

Li et al.
between the Dy(III) ions in 7 is much stronger than that in
the reported homodinuclear Dy(III) complex with ferromag-
netic interaction. For the other four complexes (2-5), the
magnetic susceptibility, øMT, decreases when the temperature
is lowered. However, ferromagnetic or antiferromagnetic
interactions cannot be concluded because of the concurrence
of the effects of the crystal field and spin-orbital coupling.
Notably, a least-squares fitting of øMT versus T of 4 and 5,
deduced from equation for the Sm(III) and Eu(III) ions,
respectively, in monomeric system with free-ion approxima-
tion, leads to λ ) 216(2) cm-1 for Sm(III) and λ ) 404(2)
cm-1 for Eu(III). The emission spectra demonstrate that the
4-cba complexes, especially 5, might be good candidates for
efficient luminescent materials without the quenching effects
of coordinated water molecules.
Acknowledgment. We gratefully acknowledge the fi-
nancial support of the NSF for Distinguished Young Scientist
of China (20425104) and the NSF of Fujian Province
(A0420002 and 2005I017).
Supporting Information Available: Crystallographic data for
1-7 in CIF format, the figures of the coordination geometry of
La(III) and Pr(III), the chain structure of 1, and the layered structure
of 2, infrared spectra of Na(4-cba) and 1-7, excitation and emission
spectra of 4, 5, and 7, and XRPD diagrams of 3 and 6. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC0602603
6316 Inorganic Chemistry, Vol. 45, No. 16, 2006