Controlled ligand deprotonation in lanthanide chelates with asymmetric semicarbazone/benzoylhydrazone or semicarbazone/thiosemicarbazone coordination spheres

Article abstract of DOI:10.1021/ic050556t

Asymmetric, potentially pentadentate ligands (H₂L³) are formed by subsequent condensation of a semicarbazide and benzoylhydrazine on 2,6-diacetylpyridine. Two equivalents of H₂L³ reacts with CeCl₃·7H₂O, Ce(SO₄)₂· 4H₂O, or EuCl₃·6H₂O under formation of [Ln^III(HL³)₂]⁺ cations (Ln = Ce, Eu) with exclusive deprotonation of the benzoylhydrazone ligand arms. The Ce ⁴⁺ ion of the sulfate salt is reduced during the reaction and forms 10-coordinate singly charged complex cations, the structure of which is identical to the product of the reaction of cerium(III) chloride. The exact position of deprotonation in the ligands is resolved by infrared spectroscopy, bond lengths considerations, and the hydrogen bonding in the solid-state structures of the products. A similar approach allows the synthesis of mixed semicarbazone/thiosemicarbazone ligands (H₂L⁴). The reaction of H₂L⁴ with Sm-(NO₃) ₃·6H₂O leads to the first structurally characterized lanthanide complex with thiosemicarbazone coordination. The solid-state structure of the 10-coordinate complex [Sm(HL⁴) ₂]NO₃·H₂O shows exclusive deprotonation of the thiosemicarbazone arms of the ligands. All isolated complexes are air stable and do not undergo ligand exchange reactions or hydrolysis in the presence of water.

Full text of DOI:10.1021/ic050556t

Inorg. Chem. 2005, 44, 5738−5744
Controlled Ligand Deprotonation in Lanthanide Chelates with
Asymmetric Semicarbazone/Benzoylhydrazone or Semicarbazone/
Thiosemicarbazone Coordination Spheres
Alexander Jagst,† Agustin Sanchez,‡ Ezequiel M. Vazquez-Lopez,§ and Ulrich Abram*,†
Institut fu¨r Chemie, Freie UniVersita¨t Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany,
Departamento de Quimica Inorganica, UniVersidade de Santiago de Compostela, E-15782
Santiago de Compostela, Galicia, Spain, and Departamento de Quimica Inorganica, UniVersidade
de Vigo, E-36200 Vigo, Galicia, Spain
Received April 11, 2005
Asymmetric, potentially pentadentate ligands (H2L3) are formed by subsequent condensation of a semicarbazide
and benzoylhydrazine on 2,6-diacetylpyridine. Two equivalents of H2L3 reacts with CeCl3
‚
7H2O, Ce(SO4)2‚4H2O, or
EuCl3
‚
6H2O under formation of [LnIII(HL3)2]+ cations (Ln
)
Ce, Eu) with exclusive deprotonation of the
benzoylhydrazone ligand arms. The Ce4 ion of the sulfate salt is reduced during the reaction and forms 10-
coordinate singly charged complex cations, the structure of which is identical to the product of the reaction of
cerium(III) chloride. The exact position of deprotonation in the ligands is resolved by infrared spectroscopy, bond
lengths considerations, and the hydrogen bonding in the solid-state structures of the products. A similar approach
allows the synthesis of mixed semicarbazone/thiosemicarbazone ligands (H2L4). The reaction of H2L4 with Sm-
+
(NO3)3
‚
6H2O leads to the first structurally characterized lanthanide complex with thiosemicarbazone coordination.
H2O shows exclusive deprotonation of the
The solid-state structure of the 10-coordinate complex [Sm(HL4)2]NO3
‚
thiosemicarbazone arms of the ligands. All isolated complexes are air stable and do not undergo ligand exchange
reactions or hydrolysis in the presence of water.
Introduction
A key requirement for these applications, however, is a
detailed knowledge of the coordination behavior of the metal
ions and ligands.3 This is not the case with most of the
hitherto applied complexes with citrate, phosphonate, or
butyrate ligands, which frequently undergo an in vivo ligand
exchange, for example, with plasma components.4 A promis-
ing approach to avoid undesired secondary reactions is the
use of bioconjugates with polydentate ligands, which form
kinetically inert complexes with a predictable reactivity. This
has successfully been demonstrated with tetraaza macro-
cycles.5
The increasing interest in the coordination chemistry of
lanthanides is focused on potential applications in catalysis,
advanced materials, and diagnostic medicine.1 Additionally,
certain group III and lanthanide elements possess radioactive
nuclides with a remarkable potential in nuclear medicine.2
This includes positron emitters such as 86Y and 152Tb,
â--emitters such as 99Y, 142Nd, 153Sm, or 177Lu, but also the
R-emitting isotope 149Tb, which allows therapeutic applica-
tions (radioimmunotherapy or palliative treatment of cancer)
under a strict control.
With this background, we were interested in the coordina-
tion chemistry of lanthanides with polydentate hydrazones,
semicarbazones, and thiosemicarbazones. Previous studies
indicated that semicarbazones derived from acetylpyridine
* To whom correspondence should be addressed. E-mail: abram@
chemie.fu-berlin.de.
† Freie Universita¨t Berlin.
‡ Universidade de Santiago de Compostela.
§ Universidade de Vigo.
(1) Aspinal, H. C. Chem. ReV. 2002, 102, 1977. (b) Wang, K.; Li, R.;
Cheng, Y.; Zhu, B. Coord. Chem. ReV. 1999, 190-192, 297. (c)
Choppin, G. R.; Peterman, G. R. Coord. Chem. ReV. 1998, 174, 283.
Aime, S.; Fasano, M.; Terreno, E. Chem. Soc. ReV. 1998, 27, 19. (d)
Alexander, V. Chem. ReV. 1995, 95, 273.
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A.; Arner, J. H.; Loeb, S. J.; Willis, C. J. Inorg. Chem. 1980, 19,
3553.
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5738 Inorganic Chemistry, Vol. 44, No. 16, 2005
10.1021/ic050556t CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/08/2005

Lanthanide Complexes with Asymmetric Ligands
IR (KBr) (νmax/cm-1): 3375, 3194 (s, N-H), 1697 (s, acetyl Cd
O), 1680 (s, CdO). H NMR (400 MHz, DMSO-d6): δ [ppm] )
or 2,6-diacetylpyridine (H2L1) act as neutral ligands.6 Cor-
responding benzoylhydrazones (H2L2) partially deprotonate,
but the number of positions of deprotonations could not be
predicted and their position in the complex molecule could
not be resolved by crystallography.7 Despite many attempts
to prepare stable thiosemicarbazone complexes of lan-
thanides,8 to our knowledge there exists no crystallographi-
cally characterized compound.
1
2.67 (s, 3H, [H3C-CdO]), 2.42 (s, 3H, [H3C-CdO]), 7.03-8.65
(8H, [arom. H]), 9.01 (s, 1H, [Ph-N-H), 10.03 (s, 1H, [N-H]).
FAB+-MS: m/z ) 297 ([M - H]+), 204 ([M - NH - Ph]+.
H2L3. HL3a (0.34 g,1.2 mmol) and benzoylhydrazine (0.16 g,
1.2 mmol) were suspended in about 30 mL of i-PrOH and heated
on reflux for 36 h. The pure product was filtered off from the hot
reaction mixture. Yield: 0.33 g, 66%. Anal. Calcd for C23H22N6O2
(414.5): C, 66.7; H, 5.4; N, 20.3. Found: C, 66.5; H, 5.6; N, 19.9.
IR (KBr) (νmax/cm-1): 3389, 3201 (s, N-H), 1689 (s, semicarbazone
1
CdO), 1653 (s, benzoylhydrazone CdO). H NMR (400 MHz,
DMSO-d6): δ [ppm] ) 2.42 (s, 3H, [H3C-C, semicarbazone]),
2.53 (s, 3H, [H3C-C, benzoylhydrazone]), 7.04-8.43 (aromat. CH,
13H,), 8.99 (s, 1H, [PhNH]), 9.96 (s, 1H, [semicarbazone NH]),
10.89 (s, 1H, [benzoylhydrazone NH]). FAB+-MS: m/z ) 415
([M - H]+), 105 ([Ph - CdO]+).
H2L4a. 2,6-Diformylpyridine (1.00 g, 7.4 mmol) was dissolved
in about 125 mL of hot H2O. 4-Phenylsemicarbazide (1.12 g, 7.4
mmol) was dissolved in 75 mL of H2O and slowly added at a
temperature of about 50 °C. After being stirred at this temperature
for 30 min, the formed precipitate was filtered off and redissolved
in 200 mL of hot CHCl3. An almost insoluble colorless solid
(disubstituted semicarbazone) was removed by filtration, and the
remaining solution was evaporated to dryness to give 970 mg (3.6
mmol, 49%) of H2L,4a which was used in the next step without
further purification. Anal. Calcd for C14H12N4O2 (268.27): C, 62.7);
In the present work, we report the synthesis and structural
characterization of lanthanide complexes with pentadentate
asymmetric ligand systems derived from 2,6-diacetylpyridine
or 2,6-diformylpyridine with mixed semicarbazone/benzoyl-
hydrazone and semicarbazone/thiosemicarbazone coordina-
tion sites.
Experimental Section
H, 4.5; N, 20.9. Found: C, 62.3; H, 4.3; N, 20.5. IR (KBr) (νmax
/
General Considerations. Infrared spectra were measured as KBr
pellets on a Shimadzu FTIR-spectrometer. FAB+ mass spectra were
recorded with a TSQ (Finnigan) instrument using a nitrobenzyl
alcohol matrix. Elemental analysis was determined using a Heraeus
vario EL elemental analyzer. NMR spectra were taken with a JEOL
400 MHz multinuclear spectrometer.
H2L3a. 2,6-Diacetylpyridine (1.63 g, 10.0 mmol) was dissolved
in about 50 mL of a hot EtOH/H2O mixture (v/v 1/1), and
4-phenylsemicarbazide (1.51 g, 10.0 mmol) in about 75 mL of H2O
was slowly added. After being heated on reflux for 1 h, the colorless
precipitate was filtered off and washed with EtOH. Yield: 2.75 g,
93%. When necessary, disubstituted byproduct can be removed
by recrystallization from CHCl3. Anal. Calcd for C16H16N4O2
(296.3): C, 64.9; H, 5.4; N, 18.9. Found: C, 64.6; H, 5.5; N, 19.1.
cm-1): 3382, 3201 (s, N-H), 1701 (s, formyl CdO), 1689 (s,
1
semicarbazone CdO). H NMR (400 MHz, DMSO-d6): δ [ppm]
) 7.06-8.67 (8H, [arom. H]), 8.10 (s, 1H, [H-C-CdN]), 9.11
(s, 1H, [PhNH), 10.00 (s, 1H, [H-CdO]), 11.19 (s, 1H, [NH]).
FAB+-MS: m/z ) 268 ([M]‚+), 148 ([M - C7H6NO]+), 119
([C7H5NO]+), 93 ([C6H7N]‚+)).
H2L4. H2L4a (0.30 g, 1.1 mmol) and 4-phenylthiosemicarbazide
(0.20 g, 1.2 mmol) were suspended in 30 mL of EtOH and heated
on reflux for 4 h. The bright yellow precipitate was filtered off
and washed with water. Yield: 0.45 g, 98%. Anal. Calcd for
C21H19N7OS (417.47): C, 60.4; H, 4.5; N, 23.4. Found: C, 60.4;
H, 4.4; N, 23.5. IR (KBr) (νmax/cm-1): 3382, 3313 (s, N-H), 1686
1
(s, semicarbazone CdO), 1596, 1542 (s, CdN). H NMR (400
MHz, DMSO-d6): δ [ppm] ) 7.04-8.43 (m, 15H [arom. H +
CHdN]), 9.06 (s, 1H, [OdC-NH-Ph]), 10.31 (s, 1H, [SdC-
NH-Ph]), 11.13 (s, 1H, [semicarbazone NH]), 12.11 (s, 1H,
[thiosemicarbazone NH]). FAB+-MS: m/z ) 417 ([M]‚+), 324
([M - C6H7N]+)]+, 265 ([M - C7H6N2S]+), 135 ([C7H5NS)]+, 119
([C7H5NO)]+, 93 ([C6N7N]‚+).
[Eu(HL3)2]Cl. EuCl3‚6H2O (44 mg, 0.12 mmol) was dissolved
in about 10 mL of EtOH, and H2L3 (100 mg, 0.24 mmol) was added.
A clear light-orange solution was obtained after adding 10 µL of
Et3N to the hot reaction mixture. After the mixture was heated on
reflux for 3 h, the volume of the solution was halved. A yellow
solid precipitated upon cooling. It was filtered off and washed with
2 mL of ethanol. Yield: 84 mg, 68%. Anal. Calcd for C46H42-
EuClN12O4 (1014.29): C, 54.5; H, 4.1; N, 16.6. Found: C, 53.9;
H, 4.4; N, 16.1. IR (KBr) (νmax/cm-1): 1658 (s, semicarbazone
CdO), 1600 (s, benzoyl-hydrazone CdO). FAB+-MS: m/z ) 979
([Eu(HL3)2]+ - H), 566 ([Eu(HL3)]+ - 2H).
(6) (a) Mital, S. P.; Singh, R. V.; Tandon, J. P. Synth. React. Inorg. Met.-
Org. Chem. 1982, 12, 269. (b) Thomas, J. E.; Palenik, G. J. Inorg.
Chim. Acta 1980, 44, L303. (c) Sommerer, S. O.; Westcott, B. L.;
Cundari, T. R.; Krause, J. A. Inorg. Chim. Acta 1993, 209, 101. (d)
Aghabozorg, H.; Palenik, G. J.; Palenik, R. C. J. Sci., Islamic Repub.
Iran 1991, 2, 103.
(7) (a) Xian-He, B.; Miao, D.; Lei, Z.; Xu-Bo, S.; Ruo-Hua, Z.; Clifford,
J. Inorg. Chim. Acta 2000, 308, 143. (b) Thomas, J. E.; Palenik, R.
C.; Palenik, G. J. Inorg. Chim. Acta 1979, 37, L459. (c) Christidis, P.
C.; Tossidis, I. A.; Paschalidis, D. G. Acta Crystallogr. 1999, C55,
707. (d) Pan, X.-Y.; Yan, S.-P.; Wang, G.-L.; Wang, H.-G.; Wang,
R.-J.; Yao, X.-K. Acta Chim. Sin. 1989, 47, 795. (e) Benson, M. T.;
Cundari, T. R.; Saunders, L. C.; Sommerer, S. O. Inorg. Chim. Acta
1997, 258, 127. (f) Paschalidis, D. G.; Tossidis, I. A.; Gdaniec, M.
Polyhedron 2000, 19, 2629. (g) Dan, J.; Seth, S.; Chakraborty, S. Acta
Crystallogr. 1989, 45, 1018. (h) Tambura, F. B.; Diop, M.; Gaye, M.;
Sall, A. S.; Barry, A. H.; Jouini, T. Inorg. Chem. Commun. 2003, 6,
1387. (i) Tambura, F. B.; Haba, P. M.; Gaye, M.; Sall, A. S.; Barry,
A. H.; Jouini, T. Polyhedron 2004, 23, 1191. (j) Sakamoto, M. Inorg.
Chim. Acta 1987, 131, 139.
(8) (a) Chakraborti, I. Orient. J. Chem. 2003, 19, 617 and refs therein.
(b) Agarwal, R. K.; Goel, N.; Sharma, A. K. J. Indian Chem. Soc.
2001, 78, 39 and refs therein. (c) Yang, Z.-Y.; Yang, R.-D. Synth.
React. Inorg. Met.-Org. Chem. 2002, 32, 1059 and refs therein. (d)
Wang, B.; Yang, S.; Daosen, J.; Zhou, L. Polyhedron 1993, 12, 1805.
(e) Garg, B. S.; Singh, S. R.; Basnet, R. B.; Singh, R. P. Polyhedron
1988, 7, 147.
[Ce(HL3)2]Cl. CeCl3‚7H2O (44 mg, 0.12 mmol) was dissolved
in about 10 mL of ethanol, and H2L3 (100 mg, 0.24 mmol) was
added. A clear orange-red solution was obtained after adding 10
µL of Et3N and 100 µL of H2O to the hot reaction mixture. After
the mixture was heated on reflux for 3 h, the volume was halved.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5739

Jagst et al.
Table 1. X-ray Structure Data Collection and Refinement Parameters
[Eu(HL3)2]Cl‚1.5iso-PrOH‚H2O [Ce(HL3)2]Cl
50.5H56ClN12O6.5Eu
[Ce(HL3)2]2(SO4)‚7EtOH
[Sm(HL4)2](NO3)‚H2O
formula
C
C46H42ClN12O4Ce
1002.49
orthorhombic
12.891(1)
21.859(2)
35.026(4)
90
C110H138N24O21SCe2
2444.74
monoclinic
23.169(5)
20.064(5)
25.288(5)
90
93.07(1)
90
11 739(5)
P21/n
C42H36N15O6S2Sm
1061.33
trigonal
49.76(5)
49.76(5)
13.85(3)
90
90
120
29 706(80)
R3h
18
Mw
1122.48
monoclinic
11.446(3)
31.751(7)
15.632(4)
90
102.78(1)
90
5541(2)
P21/n
crystal system
a/Å
b/Å
c/Å
R/deg
â/deg
90
90
9869(2)
Pbca
8
γ/deg
V/Å3
space group
Z
4
4
Dcalc/g cm-3
µ/mm-1
no. of reflns
no. independent
no. parameters
R1/wR2a
GOF
1.344
1.237
1.349
1.029
1.383
0.860
94 435
20 010
1475
0.0287/0.0674
0.979
1.068
0.997
48 520
10 795
650
0.0592/0.1115
1.019
46 252
11 563
577
0.0653/0.1146
0.865
11 364
11 364
586
0.0669/0.1558
0.975
temp/K
173
293
173
173
a R1 ) ∑(|Fo - Fc|)/∑|Fo|; wR2 ) [∑w(Fo - Fc )2/∑(wFo )]-1/2
.
2
2
2
Scheme 1. Synthesis of H2L3
An orange-red solid precipitated upon cooling. It was filtered off
and washed with 2 mL of EtOH. Yield: 40 mg, 33%. Anal. Calcd
for C46H42CeClN12O4 (1002.5): C, 55.1; H, 4.2; N, 16.7. Found:
C, 55.3; H, 4.1; N, 16.3. IR (KBr) (νmax/cm-1): 1664 (s, semicar-
bazone CdO), 1600 (s, benzoylhydrazone CdO). FAB+-MS: m/z
) 966 ([Ce(HL3)2]+ - H), 552 ([Ce(HL3)]+ - 2H).
[Ce(HL3)2]2(SO4). Ce(SO4)2‚4H2O (49 mg, 0.12 mmol) was
dissolved in about 20 mL of EtOH, and H2L3 (100 mg, 0.24 mmol)
was added. A pale yellow suspension was obtained after adding
30 µL of Et3N to the hot reaction mixture. After being heated on
reflux for 8 h, a colorless precipitate was filtered off. The product
precipitated in the form of orange-red crystals upon slow evapora-
tion of the mother liquor. Yield: 25 mg, 21%. More product could
be obtained by further concentration of the mother liquor, but this
material was contaminated impurities (e.g., (Et4NH)SO4) and gave
insufficient analytical results. Anal. Calcd for C92H84Ce2N24O12S1
(2030.1): C, 54.4; H, 4.2; N, 16.7. Found: C, 53.9; H, 3.9; N,
16.4. IR (KBr) (νmax/cm-1): 1666 (s, semicarbazone CdO), 1604
(s, benzoylhydrazone CdO), 1555 (s, CdN). FAB+-MS: m/z )
1064 ([Ce(HL3)2](SO4)+ - H), 966 ([Ce(HL3)2]+ - H), 552 ([Ce-
(HL3)]+ - 2H).
involved in hydrogen bonds. Their positions were derived from
the final Fourier maps and refined. More details on data collections
and structure calculations are contained in Table 1.
Additional information on the structure determinations has been
deposited with the Cambridge Crystallographic Data Centre.
[Sm(HL4)2](NO3). Sm(NO3)3‚5H2O (35 mg, 0.1 mmol) was
dissolved in about 25 mL of hot EtOH, and H2L4 (80 mg, 0.2 mmol)
was added. An orange-red suspension was obtained after adding
20 µL of Et3N to the hot reaction mixture. After the mixture was
heated on reflux for 3 h, the volume of the solution was halved.
The orange-red precipitate was filtered off and washed with EtOH.
Yield: 54 mg, 53%. Single crystals of the composition [Sm(HL4)2]-
(NO3)‚H2O were obtained from a two-phase system of the
mother liquor and n-hexane. Anal. Calcd for C42H36N15O5S2Sm
(1044.32): C, 48.3; H, 3.4; N, 20.1. Found: C, 47.8; H, 3.2; N,
19.9. IR (KBr) (νmax/cm-1): 3282, 3047 (s, N-H), 1658 (s,
semicarbazone CdO), 1600, 1554 (s, CdN). FAB+-MS: m/z )
986 ([M - H]+.
X-ray Crystallography. The intensities for the X-ray determina-
tions were collected on a SMART CCD (Bruker) with Mo KR
radiation (λ ) 0.71073 Å). Standard procedures were applied for
data reduction and absorption correction. Structure solution and
refinement were performed with SHELXS97 and SHELXL97.9
Hydrogen atom positions were calculated for idealized positions
and treated with the “riding model” option of SHELXL, except of
those H-atoms, which are bonded to nitrogen atoms and/or are
Results and Discussion
2,6-Diacetylpyridine benzoylhydrazone-4-phenylsemicar-
bazone, H2L3, is obtained in a two-step reaction. The
sequence of substitution (Scheme 1) is mandatory. The low
reactivity of the monosubstituted semicarbazone (HL3a)
requires a long reaction time in the second step. A synthesis
via a monosubstituted benzoylhydrazone is not recom-
mended, because it requires additional purification steps due
to the formation of disubstituted byproduct.
The reaction of 2 equiv of H2L3 and cerium trichloride in
ethanol yields an orange-red complex of the composition [Ce-
(HL3)2]Cl. The infrared spectrum of the compound shows
two ν(CdO) vibrations at 1664 cm-1 (semicarbazone) and
1600 cm-1 (benzoylhydrazone). This corresponds to batho-
chromic shifts of 25 and 53 cm-1 with respect to the values
(9) Sheldrick, G. M. SHELXS97 and SHELXL97 - programs for the
solution and refinement of crystal structures, University of Go¨ttingen,
Germany.
5740 Inorganic Chemistry, Vol. 44, No. 16, 2005

Lanthanide Complexes with Asymmetric Ligands
Scheme 2. Bonding Situations in the Side Arms of the Singly
Deprotonated Ligand {HL3}-
different bonding modes for the two parts of the pentadentate
ligands. This is supported by the bond lengths inside the
chelating functionalities (Table 2), which indicate delocal-
ization of π-electron density in the semicarbazone unit over
all C-N bonds, whereas solely the chelate ring bonds are
included in the benzoylhydrazone arms. This results in an
exclusive deprotonation of the benzoylhydrazone functional-
ity in H2L3, whereas the semicarbazone remains protonated
(see also Scheme 2). This bonding mode is supported by
the detection of hydrogen bonds between the semicarbazone
nitrogen atoms and the counterion Cl- in the solid-state
structure of [Ce(HL3)2]Cl. Details about these hydrogen
bonds are summarized in Figure S1 and Table S1 of the
Supporting Information.
Figure 1. The structure of the complex cation of [Ce(HL3)2]Cl.
Similar reaction conditions yield a yellow complex of the
composition [Eu(HL3)2]Cl when europium trichloride is used
as a metal source. Yellow crystals are obtained from a
2-propanol solution by slow evaporation of the solvent. The
complex cation is isostructural to [Ce(HL3)2]+ with identical
deprotonation positions in the coordinated ligand molecules
(Figure 3). All Eu-N and Eu-O distances are shorter than
the corresponding distances in the cerium complex, readily
explained by the smaller ionic radius of Eu3+. Nevertheless,
all previously discussed tendencies in bond lengths are
identical (Table 2). The coordination polyhedron of europium
can also be described to be intermediate between a bicapped
cube and a bicapped square antiprism with the pyridine
nitrogen atoms as caps.
The infrared spectrum of the complex shows almost similar
bathochromic shifts for the ν(CdO) vibrations in the
semicarbazone (31 cm-1) and benzoylhydrazone arms (53
cm-1). Figure 3 depicts a part of the hydrogen bond situation
in the solid-state structure. The formed network includes the
counterion Cl- and the solvent water and confirms the sole
deprotonation of the benzoylhydrazone site of the molecule.
N5 (benzoylhydrazone) acts as hydrogen acceptor, whereas
N36 (semicarbazone) is a hydrogen donor. Additional
Figure 2. Coordination polyhedron of cerium in [Ce(HL3)2]Cl.
in noncoordinated H2L3 and suggests coordination of both
functional groups to the metal. A peak at m/z ) 966 in the
FAB+ mass spectrum indicates coordination of two ligands.
Orange-red crystals of [Ce(HL3)2]Cl were obtained from
a two-phase system of ethanol and n-hexane. The crystal
structure (Figure 1) confirms the composition of the complex
as a singly charged cation. The cerium atom is 10-coordinate
by two of the singly deprotonated, pentadentate ligands. Its
coordination environment can best be described to be
intermediate between a bicapped cube and a bicapped square
antiprism, where the caps are formed by the pyridine nitrogen
atoms (Figure 2).
The bond lengths of the Ce-N(pyridine) bonds are in the
same magnitude as the Ce-N(azomethine) bonds in the
semicarbazone arms of the ligands. The shorter Ce-
N(azomethine) bonds in the benzoylhydrazone bonding sites
and the shorter Ce-O bonds in these arms strongly suggest
Table 2. Selected Bond Lengths (Å) and Angles (deg) in the [Eu(HL3)2]+ and [Ce(HL3)2]+ Cations
Ce
Eu
Ce
Eu
O(18)-Ln
2.592(8)
2.592(8)
2.775(9)
2.64(1)
2.764(9)
1.24(1)
1.21(1)
1.34(1)
1.40(1)
58.9(3)
2.446(4)
2.519(4)
2.663(5)
2.564(5)
2.611(5)
1.244(7)
1.244(7)
1.367(7)
1.400(6)
61.2(2)
O(28)-Ln
2.403(8)
2.403(8)
2.72(1)
2.760(9)
2.61(1)
1.310(1)
1.28(1)
1.36(1)
1.31(2)
59.1(3)
60.4(3)
2.361(4)
2.375(4)
2.617(5)
2.698(5)
2.559(5)
1.285(7)
1.277(7)
1.338(9)
1.306(8)
60.9(2)
O(48)-Ln
O(58)-Ln
N(1)-Ln
N(2)-Ln
N(4)-Ln
N(31)-Ln
N(32)-Ln
N(34)-Ln
O(18)-C(17)
O(48)-C(47)
N(2)-N(3)
O(28)-C(27)
O(58)-C(57)
N(3)-C(17)
N(5)-C(27)
O(28)-Ln-N(4)
O(58)-Ln-N(34)
N(5)-N(4)
O(18)-Ln-N(2)
O(48)-Ln-N(32)
N(31)-Ln-N(1)
58.3(3)
173.0(3)
60.2(3)
174.6(2)
61.5(3)
Inorganic Chemistry, Vol. 44, No. 16, 2005 5741

Jagst et al.
Table 4. Selected Bond Lengths (Å) and Angles (deg) in the Complex
Anion of [Ce(HL3)2]2(SO4)‚7EtOH
Ce(1)-O(18)
Ce(1)-O(28)
Ce(1)-O(48)
Ce(1)-O(58)
Ce(1)-N(34)
Ce(1)-N(1)
Ce(1)-N(2)
Ce(1)-N(4)
Ce(1)-N(31)
Ce(1)-N(32)
O(18)-C(17)
O(28)-C(27)
O(48)-C(47)
O(58)-C(57)
2.525(2)
2.413(2)
2.541(2)
2.413(2)
2.682(3)
2.790(2)
2.768(2)
2.706(3)
2.804(3)
2.794(3)
1.229(4)
1.271(4)
1.229(4)
1.269(4)
Ce(2)-O(88)
Ce(2)-O(118)
Ce(2)-O(78)
Ce(2)-O(108)
Ce(2)-N(94)
Ce(2)-N(64)
Ce(2)-N(61)
Ce(2)-N(62)
Ce(2)-N(92)
Ce(2)-N(91)
O(78)-C(77)
O(88)-C(87)
O(108)-C(107)
O(118)-C(117)
2.396(2)
2.414(2)
2.497(2)
2.541(2)
2.663(3)
2.673(3)
2.759(3)
2.758(2)
2.809(3)
2.840(3)
1.236(4)
1.275(4)
1.221(4)
1.278(4)
N(1)-Ce(1)-N(31)
O(18)-Ce(1)-N(2)
O(28)-Ce(1)-N(4)
O(48)-Ce(1)-N(32)
O(58)-Ce(1)-N(34)
179.2(1) N(61)-Ce(2)-N(91)
58.8(1) O(78)-Ce(2)-N(62)
59.3(1) O(88)-Ce(2)-N(64)
57.9(1) O(108)-Ce(2)-N(92)
59.6(1) O(118)-Ce(2)-N(94)
176.3(1)
58.7(1)
59.6(1)
58.3(1)
59.90(1)
Figure 3. The structure of the complex cation of [Eu(HL3)2]Cl‚H2O‚1.5
iso-PropOH with hydrogen bonds to the Cl- counterion and solvent water.
Table 3. Selected Hydrogen Bonds in [Eu(HL3)2]Cl [Å and deg]a
deprotonation of the ligands at the benzoylhydrazone arms.
They organize the complex molecule into a three-dimensional
network. Details are shown and listed in Figure S2 and Table
S2 of the Supporting Information. Selected bond lengths and
angles are contained in Table 4. The labeling of one of the
complex cations (Ce1) is identical to that in Figure 1, whereas
the same scheme has been applied for the cation with Ce2,
beginning with N61 for the first and N91 for the second
ligand. Additional hydrogen bonds are established with the
solvent ethanol molecules.
D-H‚‚‚A
d(D-H) d(H‚‚‚A) d(D‚‚‚A) -(DHA)
N(33)-H(33)‚‚‚Cl
N(36)-H(36)‚‚‚Cl
N(6)-H(6)‚‚‚O(81)#1
0.86
0.86
0.86
2.41
2
2.14
2.00(7)
1.91(6)
3.199(5)
3.224(5)
2.909(8)
2.773(7)
2.793(7)
3.194(5)
153.0
155.9
149.3
149.0(7)
165.0(7)
159.9(4)
N(3)-H(3N)‚‚‚O(81)#1 0.86(7)
O(81)-H(81A)‚‚‚N(5)
O(81)-H(81b)‚‚‚Cl
0.90(6)
0.729(5) 2.499(2)
a Symmetry transformations used to generate equivalent atoms: #1 x +
1, y, z.
The concept of ligands containing asymmetric arms can
be extended to other functional groups such as Schiff’ bases
or thiosemicarbazones. Thiosemicarbazones are under dis-
cussion as promising ligand systems for biomedical applica-
tions.10 Many complexes with main group and transition
metals were prepared and studied by X-ray crystallography.11
Structural data of corresponding lanthanide complexes to the
best of our knowledge are not available. This may be
understood by the low affinity of the “hard” lanthanide ions
to the “soft” sulfur atoms in thiosemicarbazones and has also
been observed for actinide elements, where the first crystal-
line thiosemicarbazonato complexes have been reported
recently.11 All of our attempts to isolate stable lanthanide
complexes with a potentially pentadentate ligand derived
from 2,6-diformylpyridine and 2 equivalents of 4-phenyl-
thiosemicarbazide failed up to now. Obviously, the presence
of two arms containing “soft” thiosemicarbazone sulfur atoms
does not allow the formation of stable complexes with “hard”
metal ions such as La3+, Sm3+, or Eu3+.
Figure 4. The structure of [Ce(HL3)2]2(SO4).
hydrogen bonds are formed between the water molecule and
the nitrogen atoms N3 and N6 of a neighboring complex
cation (Table 3).
The controlled deprotonation of H2L3 at the benzoylhy-
drazone arm was proven by a third example. Cerium(IV)
sulfate reacts with the mixed-functional ligand in ethanol
under reduction of the metal center and the formation of
[CeIII(HL3)2]2(SO4). The spectroscopic data of the complex
indicate a bonding situation close to that in the chloride.
Single crystals of the composition [Ce(HL3)2]2(SO4)‚
7EtOH were obtained by slow evaporation of a solution of
the complex in ethanol. Figure 4 illustrates that two of the
complex cations are connected by a hydrogen-bonded sulfate
anion. The hydrogen bonds are established by semicarbazone
NH-hydrogen atoms exclusively and confirm the defined
More successful were reactions with the asymmetric ligand
H2L4, which contains the “hard” oxygen donor atoms of one
semicarbazone arm. The ligand was synthesized by subse-
quent condensation of 4-phenylsemicarbazide and 4-phen-
ylthiosemicarbazide on 2,6-bisformylpyridine (Scheme 3).
(10) Klayman, D. L.; Scovill, J. B.; Bartosevich, J. F.; Bruce, J. J. Med.
Chem. 1983, 26, 39.
(11) (a) Campbell, M. J. M. Coord. Chem. ReV. 1975, 15, 279. (b) West,
D. X.; Padhye, S. B.; Sonawane, P. A. Struct. Bonding (Berlin) 1991,
76, 1. (c) Casas, J. S.; Garc´ıa-Tasende, M. S.; Sordo, J. Coord. Chem.
ReV. 2000, 209, 49.
5742 Inorganic Chemistry, Vol. 44, No. 16, 2005

Lanthanide Complexes with Asymmetric Ligands
Table 5. Selected Bond Lengths (Å) and Angles (deg) in
[Sm(HL4)2](NO3)‚H2O
Sm-O(18)
Sm-O(48)
Sm-N(1)
2.519(6)
2.528(8)
2.682(7)
2.677(8)
2.711(7)
1.263(9)
1.150(15)
Sm-S(28)
Sm-S(58)
Sm-N(31)
Sm-N(32)
Sm-N(34)
S(28)-C(27)
S(58)-C(57)
2.913(6)
2.956(4)
2.720(8)
2.681(8)
2.669(7)
1.758(8)
1.705(9)
Sm-N(2)
Sm-N(4)
O(18)-C(17)
O(48)-C(47)
O(18)-Sm-N(2)
N(1)-Sm-N(31)
N(4)-Sm-S(28)
58.7(2)
173.9(2)
63.1(2)
O(48)-Sm-N(32)
N(34)-Sm-S(58)
60.1(3)
60.7(2)
second step, but always causes the formation of disubstituted
byproduct in the first step. Due to its insolubility in
chloroform, the byproduct diformylpyridine bis-4-phenylsemi-
carbazone can be readily removed by recrystallization from
hot chloroform.
Samarium nitrate reacts with H2L4 in ethanol under
formation of an orange-red complex. Elemental analysis, IR,
and a peak at m/z ) 985 in the FAB+ spectrum indicate
coordination of two asymmetric, singly deprotonated ligands.
A bathochromic shift of the ν(CdO) vibrations by 28 cm-1
is observed. An X-ray structure analysis confirms the
composition of [Sm(HL4)2](NO3) and deprotonation of the
thiosemicarbazone unit. Figure 5 depicts the structure of the
complex cation.
Figure 5. The structure of the complex cation of [Sm(HL4)2](NO3)‚H2O.
Scheme 3. Synthesis of H2L4
As for the complexes with {HL3}-, the lanthanide ion in
[Sm(HL4)2]+ is 10-coordinate. The coordination polyhedron
does not fit with one of the expected regular polyhedra
(bicapped square cube, bicapped square antiprism). The
positions of the donor atoms can best be described to be
intermediate between these ideal arrangements. The capping
positions are occupied by the nitrogen atoms of the pyridine
rings. Details are depicted in Figure S3 of the Supporting
Information.
The formyl derivative HL4a is prepared in water, which is
necessary for an immediate precipitation of the monosub-
stituted product. The higher reactivity of the formyl group
(as compared to that of the corresponding acetyl compound)
leads to high yields and allows short reaction times in the
Selected bond lengths and angles are summarized in Table
5. The two Sm-S bonds of 2.913(6) and 2.956(6) Å are in
Figure 6. Packing diagram of [Sm(HL4)2](NO3)‚H2O with large channels along the z axis.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5743

Jagst et al.
the range of the corresponding values in dithiophosphato or
(partially) dithiocarbamato complexes of early and middle
lanthanides,12 where the Ln-S bonds are lengthened by the
restricting bite of the formed four-membered chelate rings
or sterical limitations, but are longer than Ln-thiolato bonds
to monodentate thiols or such with larger chelate rings.13,14
Hydrogen bonds between the H atoms of the semicarba-
zone arms and nitrate anions clearly indicate the sole
deprotonation of the thiosemicarbazone bonding sites (Table
S3 of the Supporting Information). These hydrogen bonds
arrange the complex molecules into a three-dimensional
network, in which channels are established along the z axis
(Figure 6). The channels have an approximate diameter of
16.2 Å and are not occupied by solvent molecules in the
studied single crystals. The solvent water molecule is
hydrogen bonded to one of the semicarbazone arms. The
solid-state structure of [Sm(HL4)2](NO3)‚H2O with an ex-
tended channel system results in an unusual low density (Dc
) 1.068 g/cm3) of the compound. The phenyl ring with the
carbon atoms C11-C16 (highlighted in brown) shows
exceptionally high thermal displacement parameters, which
cannot satisfactorily be explained by a low-quality X-ray
crystal structure or crystal artifacts. It results from the distinct
position of this phenyl ring, which points into the channels
of the structure (Figure 6). This allows the observed high
degree of thermal motion even at a low temperature such as
173 K.
All isolated complexes do not undergo hydrolysis or ligand
exchange reactions. They are indefinitely stable in ethanol/
water mixtures. In pure water, however, macroscopic amounts
of the complexes are insoluble, and tracer studies with
radioactively labeled compounds that will allow one to study
lower concentrations are planned for the future.
Conclusions
The examples of this paper demonstrate that a combination
of appropriate bonding sites in asymmetric ligands allows
one to control the deprotonation of such ligands and gives
access to hitherto unknown classes of lanthanide complexes.
[Sm(HL4)2](NO3) represents the first lanthanide complex with
a thiosemicarbazone coordination. The bonding to this
weakly coordinating functionality is directed by the strong
bonds to the semicarbazone site of the molecule. All isolated
complexes are air stable and do not undergo ligand exchange
reaction or hydrolysis in the presence of water. More detailed
studies concerning their behavior in biological media are
required to allow an evaluation of their potential in medical
procedures.
Studies with tripodal ligands of similar types, which will
allow one to encapsulate metal ions, are in progress in our
laboratory.
Acknowledgment. This work was kindly supported by
the DAAD (Germany) and M.C.T. (Spain) in the program
Acciones Integradas Hispano-Alemana.
(12) Abram, U.; Schulz Lang, E.; Bonfada, E. Z. Anorg. Allg. Chem. 2002,
628, 1873.
(13) Cambridge Crystal Structure Database, Vers. 5.25, update 2, April
2004, Cambridge.
(14) (a) Schumann, H.; Herrmann, K.; Muhle, S. H.; Dechert, S. Z. Anorg.
Allg. Chem. 2003, 629, 1184. (b) Froelich, N.; Hitchcock, P. B.; Jin
Hu, Lappert, M. F.; Dilworth, J. J. Chem. Soc. Dalton Trans. 1996,
1941. (c) Niemeyer. Eur. J. Inorg. Chem. 2001, 1969.
Supporting Information Available: Additional figures and
tables describing the hydrogen bonding and the coordination
polyhedra of the complexes. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC050556T
5744 Inorganic Chemistry, Vol. 44, No. 16, 2005