Lanthanide contraction within a series of asymmetric dinuclear [Ln 2] complexes

Article abstract of DOI:10.1002/chem.201204451

A complete isostructural series of dinuclear asymmetric lanthanide complexes has been synthesized by using the ligand 6-[3-oxo-3-(2-hydroxyphenyl) propionyl]pyridine-2-carboxylic acid (H₃L). All complexes have the formula [Ln₂(HL)₂(H₂L)(NO₃)(py) (H₂O)] (Ln=La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14), Y (15); py=pyridine). Complexes of La to Yb and Y have been crystallographically characterized to reveal that the two metal ions are encapsulated within two distinct coordination environments of differing size. Whereas one site maintains the coordination number (nine) through the whole series, the other one increases from nine to ten owing to a change in the coordination mode of an NO₃^- ligand. This series offers a unique opportunity to study in detail the lanthanide contraction within complexes of more than one metal. This analysis shows that various representative parameters proportional to this contraction follow a quadratic decay as a function of the number n of f electrons. Slater's model for the atomic radii has been used to extract, from these decays, the shielding constant of 4f electrons. The average of O×××O distances within the coordination polyhedra shared by both metals and of the Ln×××Ln separations follow also a quadratic decay, therefore showing that such dependence holds also for parameters that receive the contribution of two lanthanide ions simultaneously. The magnetic behavior has been studied for all nondiamagnetic complexes. It reveals the effect of the spin-orbit coupling and a weak antiferromagnetic interaction between both metals. Photoluminescent studies of all the complexes in the series reveal a single broad emission band in the visible region, which is related to the coordinated ligand. On the other hand, the Nd, Er, and Yb complexes show features in the near-IR region due to metal-based transitions. Copyright

Full text of DOI:10.1002/chem.201204451

FULL PAPER
DOI: 10.1002/chem.201204451
Lanthanide Contraction within a Series of
Asymmetric Dinuclear [Ln2] Complexes
David Aguilꢀ,[a] Leonꢁ A. Barrios,[a] Verꢂnica Velasco,[a] Leticia Arnedo,[a]
Nfflria Aliaga-Alcalde,[b, c] Melita Menelaou,[a, d] Simon J. Teat,[e] Olivier Roubeau,[f]
Fernando Luis,[f] and Guillem Aromꢁ*[a]
ꢀ
Abstract:
A
complete isostructural
the coordination mode of an NO3
ligand. This series offers a unique op-
portunity to study in detail the lantha-
nide contraction within complexes of
more than one metal. This analysis
shows that various representative pa-
rameters proportional to this contrac-
tion follow a quadratic decay as a func-
tion of the number n of f electrons. Sla-
terꢀs model for the atomic radii has
been used to extract, from these
decays, the shielding constant of 4f
electrons. The average of O···O distan-
ces within the coordination polyhedra
shared by both metals and of the
Ln···Ln separations follow also a quad-
ratic decay, therefore showing that
such dependence holds also for param-
eters that receive the contribution of
two lanthanide ions simultaneously.
The magnetic behavior has been stud-
ied for all nondiamagnetic complexes.
It reveals the effect of the spin–orbit
coupling and a weak antiferromagnetic
interaction between both metals. Pho-
toluminescent studies of all the com-
plexes in the series reveal a single
broad emission band in the visible
region, which is related to the coordi-
nated ligand. On the other hand, the
Nd, Er, and Yb complexes show fea-
tures in the near-IR region due to
metal-based transitions.
series of dinuclear asymmetric lantha-
nide complexes has been synthesized
by using the ligand 6-[3-oxo-3-(2-hy-
droxyphenyl)propionyl]pyridine-2-car-
boxylic acid (H3L). All complexes have
the formula [Ln2(HL)
ACHTUNGTRENNUNG(H2O)] (Ln=La (1), Ce (2), Pr (3), Nd
2ACHTUNGTNERNUG(H2L)HCATUNGTRENN(UGN NO3)(py)-
(4), Sm (5), Eu (6), Gd (7), Tb (8), Dy
(9), Ho (10), Er (11), Tm (12), Yb (13),
Lu (14), Y (15); py=pyridine). Com-
plexes of La to Yb and Y have been
crystallographically characterized to
reveal that the two metal ions are en-
capsulated within two distinct coordi-
nation environments of differing size.
Whereas one site maintains the coordi-
nation number (nine) through the
whole series, the other one increases
from nine to ten owing to a change in
Keywords: contraction
nides luminescence
properties · X-ray diffraction
·
lantha-
magnetic
·
·
Introduction
netic properties,[7] in many respects more relevant than 3d
metals[8] in their potential role as part of molecular informa-
tion storage devices.[9–14] In contrast to the richness and plas-
ticity of their magnetic and optical properties, the metals of
the 4f period exhibit a more regular chemical behavior, in
part owing to the lack of strong consequences from small
differences in crystal-field effects. This has stimulated syn-
thetic chemists to face the challenge of constructing more or
The coordination chemistry of lanthanides is relevant to a
wide variety of fields of science and technology as a result
of the properties that arise from the peculiar electronic
structure of these metals. Among the most attractive phe-
nomena associated with 4f elements are these of photonic
nature.[1–6] These elements are also of interest for their mag-
[a] D. Aguilꢁ, Dr. L. A. Barrios, V. Velasco, L. Arnedo, Dr. M. Menelaou,
Dr. G. Aromꢂ
[d] Dr. M. Menelaou
Current address: Department of General and Inorganic Chemistry
School of Chemistry, Aristotle University of Thessaloniki
54124 Thessaloniki (Greece)
Departament de Quꢂmica Inorgꢁnica
Universitat de Barcelona
Diagonal 645, 08028 Barcelona (Spain)
E-mail: guillem.aromi@qi.ub.es
[e] Dr. S. J. Teat
Advanced Light Source, Berkeley Laboratory
1 Cyclotron Road, Berkeley, CA 94720 (USA)
[b] Dr. N. Aliaga-Alcalde
Instituciꢃ Catalana de Recerca I Estudis AvanÅats (ICREA)
Departament de Quꢂmica Inorgꢁnica
Universitat de Barcelona
[f] Dr. O. Roubeau, Dr. F. Luis
Instituto de Ciencia de Materiales de Aragꢃn (ICMA)
CSIC and Universidad de Zaragoza
Plaza San Francisco s/n, 50009 Zaragoza (Spain)
Diagonal 645, 08028 Barcelona (Spain)
[c] Dr. N. Aliaga-Alcalde
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204451.
Current address: ICREA—Institut de Ciꢄncia de Materials de
Barcelona (ICMAB-CSIC), Campus de la UAB
08193 Bellaterra (Spain)
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less comprehensive (quasi)isostructural series of complexes
that contain lanthanides. From the analysis of these series
emerges the conclusion that whenever differences in chemi-
cal reactivity occur, these mainly arise from the systematic
decrease in ionic radius that takes place when increasing the
atomic number throughout the series, a phenomenon that is
termed lanthanide contraction.[15] Very often, the result is a
decrease in coordination number (CN) as the ionic radius
shrinks (most frequently, the CN ranges from 7 to 10).[16,17]
Flexible bridging ligands often lead to polymeric arrange-
ments of lanthanides (1D, 2D, or 3D), with dramatic struc-
tural differences upon going from metal to metal as a result
of the slight changes in size.[18–20] In very few of the rather
complete series reported do all the complexes exhibit the
exact same molecular structure and space group. This occurs
almost exclusively with multidentate ligands that exhibit
enough flexibility to accommodate ions of different sizes
while encapsulating them within a fixed coordination envi-
ronment.[21–24] The seeming exception of the isostructural
Figure 1. Schematic representation of the structure of complexes
[Ln2(HL)2A(H2L)(NO3)(py)(H2O)] (1 to 15), emphasizing the difference
between coordinating sites 1 (CN=9) and 2 (CN=9 if NO3 is monoden-
G
E
ACHTUNGTRENNUNG
ꢀ
tate or 10 if it is bidentate).
pared, although its structure is not available. In these com-
pounds, one of the sites maintains the same nine-coordinate
environment throughout the series, whereas the other site
features a transition from CN=10 for the larger metals to
CN=9 for the smaller ions, facilitated by a transition from
chelating to monodentate of a nitrate ligand. This series pro-
vides a unique opportunity to investigate the structural con-
traction of lanthanides on molecules that simultaneously
feature both situations, that is, conservation and variation of
CN throughout the series. Such analysis is presented in this
paper together with a brief description of magnetic and opti-
cal properties.
series
ACHTUNGTRENNUNG
of
nonacoordinated
complexes
[LnACHTGUNTERN(NUG H2O)9]-
[25]
tion extended X-ray absorption fine structure (EXAFS) and
crystallographic studies to exhibit effective changes in coor-
dination number that result from varying partial occupancies
of some coordination sites.[26] In the special cases that are
isostructural, it is possible to examine the consequences of
the lanthanide contraction throughout the series. An accu-
ꢀ
rate analysis of the Ln O distances in one such family al-
lowed us to conclude that this contraction follows a quadrat-
ic decay as a function of the number of f electrons.[27] Such a
behavior was later shown[24] to follow from Slaterꢀs model
for calculating atomic radii.[28,29] For series of complexes that
contain different types of O-donor atoms bound to the
metal, a smooth behavior is obtained if the ensemble of the
Results and Discussion
Synthesis: The ligand H3L was obtained by the Claisen con-
densation of 2-hydroxyacetophenone and the asymmetric
species 6-(methoxycarbonyl)pyridine-2-carboxylic acid, as
previously reported.[34] This ligand was previously used in
our group for the synthesis of asymmetric dinuclear com-
plexes with the metals from the central segment of the lan-
ꢀ
Ln O distances is analyzed as a whole (e.g., by observing
ꢀ
the decay of the sum of Ln O distances for each complex).
Other parameters directly affected by the contraction were
found to exhibit the same quadratic decay, such as the sum
ꢀ
of the O O distances featured by the coordination polyhe-
thanide
[Tb2(HL)
series
(H2L)Cl(py)2]
ACTHNUGTRENN(UG NO3)(py)ACTHUNGTREN(NNUG H2O)] (6a) with almost identical structure (see
[Gd2(HL)
G
E
(16),
2ACHTUNGTRENNUNG(H2L)-
dron (i.e., the sum of the lengths of the edges).[24] The litera-
ture contains some series of dinuclear complexes of lantha-
nides, which usually contain fewer members.[30–33] From the
point of view of the single-ion structure, such series should
not be much more informative than mononuclear ones, con-
sidering that in these complexes both metals are structurally
equal. It would be interesting to analyze individually and
collectively the effect of lanthanide contraction of systems
that contain more than one noncrystallographically equiva-
lent metal. We report here an extensive quasi-isostructural
series of dinuclear lanthanide complexes with formulae
N
(17),
below). Asymmetric dinuclear complexes of 4f metals are
extremely less frequent than symmetric ones. These species
have been proposed as possible realizations of molecular-
spin-based quantum gates,[35] and in fact, complex 17 was
shown to fulfill many of the basic requirements.[36] The struc-
tural consistency in this peculiar reaction system appeared
as a good opportunity to generate an analogous sequence
for a systematic study on the lanthanide series. This structur-
al type is of interest because it contains metals in two dis-
tinct coordination environments, one that appears to accom-
modate larger metals than the other (Figure 1 and see
below). Thus, one outstanding question was whether or not
the scaffold formed by the various ligands of this molecular
arrangement would be capable of encapsulating any of the
4f homometallic pairs, independently of their varying sizes.
[Ln2(HL)2ACHTUNGTRENNUNG(H2L)ACHTUNGTRENNUNG(NO3)(py)CAHTNUGTREN(GUNN H2O)] (py=pyridine; Ln=La
(1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8),
Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Y (15); H3L=6-
[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic
acid) in which both metals from each molecule exhibit dis-
tinct coordination environments (see Figure 1). The ana-
logue of the complex containing Lu (14) has also been pre-
ꢀ
For this purpose, NO3 was chosen as the participant coun-
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Asymmetric Dinuclear [Ln2] Complexes
FULL PAPER
terion. Thus, the stoichiometric amounts of H3L and the cor-
could affect the formation or crystallization of the com-
plexes, given that these processes coexist with a series of
acid–base equilibria. This did not, however, allow us to
obtain 15 in crystalline form. Nevertheless, evidence for the
identity of the latter complex, as prepared in the conven-
tional manner, was obtained from elemental analysis, IR,
and mass spectrometry.
responding Ln
resulting solutions yielded, upon diffusion of diethyl ether
or toluene, crystals of the pertinent [Ln2(HL)2A(H2L)-
(NO3)(py)(H2O)] complex (Ln=Ce (2), Pr (3), Nd (4), Sm
ACHTUNGTRENNUNG(NO3)3 salt were stirred in pyridine and the
CTHUNGTRENNUNG
A
ACHTUNGTRENNUNG
(5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm
(12), Y (15)). The identity of all the molecules was establish-
ed by single-crystal X-ray diffraction (see below), and their
composition was consistent with results from microanalysis,
mass spectrometry, and bulk magnetization measurements.
All complexes exhibited virtually the same IR spectrum (see
the Supporting Information and Figure S1 therein). All at-
tempts to obtain crystals of the analogues with La (1), Yb
(13), and Lu (14) by using the above reaction failed. Howev-
Description of the structures: Complexes 1 to 13 and 15
crystallize in the monoclinic space group P21/c. All com-
pounds exhibit an [Ln2] molecule in the asymmetric unit in
addition to five molecules of pyridine (four in the case of
[Y2], 15). The compounds differ in the nature of an almost
always present additional solvent molecule (or fraction of it)
per asymmetric unit, which in most cases is pyridine, but can
also be H2O or Et2O (see Table 1 and cif files, available
from the Cambridge Crystallographic Data Centre; see Ex-
perimental Section). All compounds exhibit four asymmetric
units in the unit cell and display similar cell parameters
(Table 1). Compounds 1 to 13 and 15 are divided into two
groups, depending on the coordination mode of the terminal
er, yellow crystals of [La2(HL)
2ACHTUNGTRNE(NUGN H2L)ACHTUNTREGNN(GUN NO3)(py)ACHTUNGTREN(NUGN H2O)] (1)
and [Yb2(HL)2A(H2L)(NO3)(py)(H2O)] (13) were isolated
C
R
ACHTUNGTRENNUNG
when one and three equivalents, respectively, of HNO3 was
added to the reaction. Since these complexes feature two
different deprotonated forms of H3L (depending on whether
or not the carboxylic moiety maintains the proton), it was
thought that the presence of a small amount of nitric acid
Table 1. Crystal data and structure refinement for compounds 1–13 and 15.[a,b]
1·5py
2·6py
3·6py
4·6py
5·5py·H2O
6·5py·H2O
7·5py
T [K]
formula
Mr
l [ꢆ]
a [ꢆ]
b [ꢆ]
c [ꢆ]
b [8]
100
100
100
100
150
100
150
C75H60La2N10O19
1683.15
0.71073
14.9646(7)
15.6293(7)
33.8889(14)
110.799(2)
7409.6(6)
1.509
C80H65Ce2N11O19
1764.67
0.7749
14.867(2)
15.560(2)
34.323(3)
110.830(4)
7421.0(15)
1.579
C80H65N11O19Pr2
1766.25
0.7749
14.907(3)
15.574(3)
34.009(6)
111.316(8)
7355(2)
1.595
C80H65N11Nd2O19
1772.91
0.71073
14.514(1)
15.772(1)
35.797(3)
113.692(4)
7503.8(9)
1.569
C75H62N10O20Sm2
1724.05
0.7749
14.894(5)
15.586(5)
32.554(8)
111.183(11)
7046(4)
1.625
C75H62Eu2N10O20
1727.27
0.7749
14.813(3)
15.637(4)
32.594(6)
110.806(8)
7057(3)
1.626
C75H60Gd2N10O19
1719.83
0.71073
14.2665(11)
15.4250(8)
35.006(3)
109.791(7)
7248.4(9)
1.576
V [ꢆ3]
1 [gcmꢀ3
reflns
params
]
15134
1010
18408
1077
22491
1063
16524
955
11763
964
19770
970
12317
985
restraints
302
234
454
242
272
182
430
Rint
R1
wR2
0.0832
0.0518
0.1295
0.0586
0.0605
0.1480
0.0479
0.0536
0.1364
0.0687
0.0657
0.1607
0.0561
0.0907
0.2535
0.0508
0.0729
0.1642
0.1501
0.1172
0.2290
[c]
[d]
S
1.029
1.134
1.160
1.079
1.216
1.257
1.172
8·5py
9·5py
10·5py
11·5py
12·5py·Et2O
13·6py
15·4py
T [K]
formula
Mr
100
150
150
100
150
100
100
C75H60N10O19Tb2
1723.17
0.7749
14.255(4)
15.513(4)
34.078(8)
110.416(10)
7063(3)
1.621
C75H60Dy2N10O19
1730.33
0.71073
14.3307(10)
15.6882(7)
35.637(2)
110.910(5)
7484.4(7)
1.536
C75H60Ho2N10O19
1735.19
0.71073
14.3450(5)
15.6609(3)
35.6109(9)
110.211(2)
7507.6(4)
1.535
C75H60Er2N10O19
1739.85
0.7749
14.123(2)
15.400(2)
34.488(3)
110.659(4)
7018.6(15)
1.647
C79H70N10O20Tm2
1817.31
0.71073
14.3231(2)
15.6838(2)
35.5024(7)
109.857(2)
7501.1(2)
1.609
C80H65N11O19Yb2
1830.51
0.71073
14.1022(8)
15.3860(8)
35.172(2)
110.131(4)
7165.3(7)
1.697
C70H55N9O19Y2
1504.05
0.7749
14.108(7)
15.367(8)
34.70(2)
110.79(2)
7033(6)
1.420
l [ꢆ]
a [ꢆ]
b [ꢆ]
c [ꢆ]
b [8]
V [ꢆ3]
1 [gcmꢀ3
reflns
]
9440
10619
10634
17299
15325
14547
5953
params
restraints
Rint
974
359
0.0949
0.0912
0.2059
1.090
955
437
0.1656
0.0819
0.1646
0.960
937
169
0.0955
0.0453
0.0938
0.902
964
116
0.0524
0.0608
0.1472
1.067
1006
229
0.1090
0.0540
0.1135
0.982
1009
162
0.0907
0.0767
0.2076
1.058
901
561
0.1410
0.1376
0.3382
1.086
[c]
R1
wR2
[d]
S
[a] The structure of compound 6a has been reported previously.[33] [b] For all compounds, the crystal system is monoclinic, the space group is P21/c, and
Z=4. [c] R1 =S||Fo|ꢀ|Fc||/S|F¸ for I>2s(I). [d] wR2 ={S[w(F2ꢀF2)2]/S[w(F2)2]}1/2 for all data.
o
o
c
o
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G. Aromꢂ et al.
nitrate ligand of the [Ln2(HL)
(H2L)
N
(H2O)] mole-
The asymmetry of H3L causes both metals to be accommo-
dated in completely different coordination environments.
One of the sites (site 1) comprises one dipicolinate “ONO”
pocket and two b-diketonate “OO” units, in addition to one
molecule of pyridine and one of H2O, which complete a co-
ordination environment with CN=9. The protons of the
latter ligand were found to form hydrogen bonds with lattice
pyridine molecules. The other location (site 2) is formed by
two of the “ONO” units and one of the “OO” pockets,
whereas a didentate (group I) or monodentate (group II)
cule. For the group of lighter metals (from La to Eu (here-
after referred to as “group I”), 1 to 6), the ligand NO3ꢀ is di-
dentate and chelating, whereas for the rest of metals (in-
cluding Y and presumably Lu (hereafter referred to as
“group II”), 7 to 13, 15, and most likely 14), this donor acts
in a monodentate fashion. Figure 2 shows one representative
molecule for each of both types, and selected structural pa-
ꢀ
NO3 ligand completes nona- or decacoordination, respec-
tively (Figure 2). The decrease in the coordination number
as a result of the lanthanide contraction when no restrictions
prevent it is an established fact.[15,19,37] In the current series,
the transition occurs between the elements Eu and Gd.
However, the metals that mark the limits between groups
vary from system to system. It must also be emphasized that
our previously reported[34] polymorph of [Eu2(HL)
(H2L)-
ꢀ
(NO3)(py)
N
to what has been observed from the structure of the com-
ꢀ
plex reported here, 6, in which NO3 is h2. This means that
for this metal, both forms, 6 and 6a, are very similar in sta-
bility. This sort of “bistability” near the transition between
two structural types has been observed already for other Ln
series.[37] The heterogeneity of the ligand set in this family
causes the irregularity of the coordination polyhedra around
the metals. Continuous shape measures (CShMs)[38,39] have
been performed to elucidate the closest ideal figures ap-
proximated by these polyhedra (see Figure S2 and Tables S2
and S3 in the Supporting Information for a complete set of
distances to several model geometries). For site 1, the geom-
AHCTUNGTREGeNNNU tries have been found to approximate to either a spherical
capped antiprism (C4v symmetry; compounds 1 to 8, 11, 13,
and 15) or a spherical tricapped trigonal prism (D3h symme-
try; compounds 9, 10, and 12). Site 2 is more irregular in
nature since the distances of the coordination figures of this
site to any ideal polyhedron are always more than four
times larger than any of the best values observed for site 1.
Thus, for compounds of group I (decacoordinate), the clos-
est symmetry to site 2 is always C2v, with a preferred spheno-
corona geometry for complexes 2 to 4, and that of a tetrade-
cahedron for 1, 5, and 6. Site 2 of group II (nonacoordinate,
compounds 7 to 13 and 15) exhibits a closest resemblance to
the ideal figure that is termed “muffin”.
Figure 2. Povray molecular structure of [Eu2(HL)
2ACHTUNGERTN(UNGN H2L)ACHNUTRTGEG(NNUN NO3)(py)ACTHNUGTREN(NUGN H2O)]
(6, top) and [Ho2(HL)2A(H2L)(NO3)(py)(H2O)] (9, bottom), representing
C
R
ACHTUNGTRENNUNG
complexes of groups I and II, respectively (see text). Only crystallograph-
ically found hydrogen atoms on O carriers are represented.
rameters are listed in Table S1 in the Supporting Informa-
tion. The complex [Ln2(HL)2ACHTNUGTREN(NNGU H2L)ACHTUNTRGEN(GUNN NO3)(py)CAHTNUGTRENN(UGN H2O)] in all
compounds is a chiral species, which is accompanied by its
enantiomeric counterpart in the unit cell as generated by
symmetry. This molecule comprises two LnIII metals linked
by three H3L ligands in different degrees of deprotonation,
one as H2Lꢀ and two as HL2ꢀ. Thus each of these ligands
loses one proton from the b-diketone group upon coordina-
ꢀ
Analysis of the lanthanide contraction: Table 2 lists all Ln
X (X=O, N) bond lengths for complexes 1 to 13 and 15. It
can be seen that these distances follow the trend dictated by
the lanthanide contraction, that is, with increasing atomic
number Z, they decrease, as do the ionic radii of the metals.
From previous reports,[27] it was shown by analyzing the
decay of bond lengths that this contraction follows a quad-
ratic decay as a function of the number n of f electrons.
Later on,[24] the polynomial dependence of the lanthanide
contraction was associated with the model of Slater that
states that the atomic or ionic radii corresponds to the maxi-
mum of the radial part, rmax, of the outermost orbital of the
tion, whereas the carboxylic H+ is also absent from two of
ꢀ
ꢀ
them. The COOH proton that remains in the H2L moiety
was either determined crystallographically or inferred from
the establishment of hydrogen bonds with either one or two
lattice pyridine molecules. Each deprotonated ligand che-
lates both metals through its dipicolinate-like and b-diketo-
nate pockets, respectively, and bridges them with the O
atom that is common to both chelating moieties (Figure 1).
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Asymmetric Dinuclear [Ln2] Complexes
FULL PAPER
ꢀ
Table 2. Ln X bond lengths and Ln···Ln separations [ꢆ] for compounds 1–13 and 15.
La (1)
Ce (2)
Pr (3)
Nd (4)
Sm (5)
Eu (6)
Gd (7)
dLn1 (s)
2.493(3)
2.524(3)
2.541(3)
2.457(3)
2.455(3)
2.532(3)
2.579(3)
2.599(4)
2.786(4)
2.463(3)
2.577(3)
2.485(3)
2.693(3)
2.547(3)
2.503(3)
2.628(4)
2.767(4)
2.674(4)
2.739(4)
3.9590(4)
2.467(4)
2.513(4)
2.511(3)
2.444(3)
2.428(4)
2.520(3)
2.548(4)
2.581(4)
2.767(5)
2.438(4)
2.548(3)
2.461(4)
2.694(4)
2.517(4)
2.483(4)
2.588(5)
2.869(5)
2.635(5)
2.736(4)
3.9286(5)
2.445(3)
2.486(3)
2.482(3)
2.419(3)
2.412(3)
2.492(3)
2.545(3)
2.557(4)
2.737(4)
2.416(3)
2.534(3)
2.440(3)
2.671(3)
2.499(3)
2.470(3)
2.581(3)
2.771(4)
2.630(4)
2.714(3)
3.8941(6)
2.420(5)
2.483(4)
2.469(4)
2.405(4)
2.407(4)
2.488(4)
2.487(5)
2.534(5)
2.741(6)
2.419(5)
2.546(4)
2.435(4)
2.661(4)
2.486(4)
2.427(4)
2.544(5)
2.933(8)
2.595(6)
2.691(5)
3.8814(5)
2.422(10)
2.466(9)
2.408(9)
2.361(10)
2.388(10)
2.439(9)
2.478(11)
2.500(12)
2.720(14)
2.403(10)
2.468(9)
2.397(9)
2.651(9)
2.437(9)
2.454(9)
2.542(10)
2.674(11)
2.604(13)
2.656(12)
3.8197(13)
2.406(5)
2.447(5)
2.409(5)
2.352(5)
2.383(5)
2.430(4)
2.453(5)
2.500(6)
2.699(7)
2.390(5)
2.473(4)
2.391(5)
2.637(5)
2.430(5)
2.431(5)
2.519(6)
2.687(6)
2.581(6)
2.664(6)
3.8173(8)
2.375(11)
2.458(11)
2.416(10)
2.339(11)
2.372(12)
2.465(9)
2.444(12)
2.483(13)
2.700(14)
2.361(11)
2.437(9)
2.393(10)
2.631(10)
2.384(10)
2.373(10)
2.424(12)
–
ꢀ
O3
dLn1 (s)
ꢀ
O4
dLn1 (s)
ꢀ
O8
dLn1 (s)
ꢀ
O9
dLn1
(s)
(s)
(s)
ꢀ
O11
dLn1
ꢀ
O13
dLn1
ꢀ
O19
dLn1 (s)
ꢀ
N3
dLn1 (s)
ꢀ
N5
dLn2 (s)
ꢀ
O1
dLn2 (s)
ꢀ
O3
dLn2 (s)
ꢀ
O6
dLn2 (s)
ꢀ
O8
dLn2
(s)
(s)
(s)
(s)
ꢀ
O13
dLn2
ꢀ
O14
dLn2
ꢀ
O16
dLn2
ꢀ
O17
dLn2 (s)
2.538(13)
2.619(12)
3.8038(11)
ꢀ
N1
dLn2 (s)
ꢀ
N2
dLn1
(s)
ꢀ
Ln2
Tb (8)
Dy (9)
Ho (10)
Er (11)
Tm (12)
Yb (13)
Y (15)
dLn1 (s)
2.374(12)
2.445(11)
2.383(11)
2.336(11)
2.355(12)
2.463(9)
2.416(12)
2.482(13)
2.648(16)
2.375(11)
2.459(10)
2.367(10)
2.630(10)
2.359(10)
2.371(10)
2.399(12)
–
2.311(11)
2.435(8)
2.378(8)
2.323(9)
2.315(10)
2.455(7)
2.424(10)
2.466(10)
2.682(15)
2.358(9)
2.471(8)
2.359(9)
2.570(9)
2.374(9)
2.340(9)
2.396(11)
–
2.319(5)
2.407(5)
2.369(5)
2.304(6)
2.328(5)
2.431(4)
2.394(6)
2.447(6)
2.665(7)
2.348(5)
2.451(5)
2.354(5)
2.562(5)
2.365(5)
2.337(5)
2.375(5)
–
2.339(5)
2.415(5)
2.358(4)
2.290(5)
2.324(5)
2.394(4)
2.381(5)
2.437(6)
2.670(7)
2.339(4)
2.418(4)
2.339(4)
2.603(4)
2.331(5)
2.333(5)
2.380(5)
–
2.295(5)
2.386(4)
2.349(4)
2.296(4)
2.307(5)
2.414(4)
2.371(5)
2.423(5)
2.664(6)
2.344(5)
2.450(4)
2.342(4)
2.545(4)
2.352(4)
2.323(5)
2.359(5)
–
2.311(8)
2.400(7)
2.351(7)
2.273(8)
2.305(8)
2.401(7)
2.357(8)
2.419(8)
2.666(11)
2.328(7)
2.399(7)
2.327(7)
2.600(7)
2.304(7)
2.308(7)
2.362(8)
–
2.269(17)
2.402(15)
2.346(14)
2.282(14)
2.329(15)
2.433(13)
2.383(16)
2.434(18)
2.680(20)
2.313(14)
2.441(14)
2.317(14)
2.618(13)
2.291(15)
2.292(14)
2.392(16)
–
ꢀ
O3
dLn1 (s)
ꢀ
O4
dLn1 (s)
ꢀ
O8
dLn1 (s)
ꢀ
O9
dLn1
(s)
(s)
(s)
ꢀ
O11
dLn1
ꢀ
O13
dLn1
ꢀ
O19
dLn1 (s)
ꢀ
N3
dLn1 (s)
ꢀ
N5
dLn2 (s)
ꢀ
O1
dLn2 (s)
ꢀ
O3
dLn2 (s)
ꢀ
O6
dLn2 (s)
ꢀ
O8
dLn2
(s)
(s)
(s)
(s)
ꢀ
O13
dLn2
ꢀ
O14
dLn2
ꢀ
O16
dLn2
ꢀ
O17
dLn2 (s)
2.492(14)
2.610(13)
3.7854(13)
2.480(12)
2.591(11)
3.7709(9)
2.474(7)
2.581(7)
3.7570(5)
2.483(6)
2.590(5)
3.7405(5)
2.467(6)
2.562(5)
3.7354(4)
2.440(10)
2.554(10)
3.7245(7)
2.460(20)
2.568(18)
3.741(4)
ꢀ
N1
dLn2 (s)
ꢀ
N2
dLn1
(s)
ꢀ
Ln2
corresponding atom or ion. Within this model, the shielding
by inner electrons of the nuclear charge experienced by the
electrons of the outermost orbital is treated empirically by
the use of a screening constant s, which directly affects rmax
of each LnIII ion. In particular, the influence of adding 4f
electrons, known to be the cause of the contraction for their
poor screening ability, gives a polynomial expression of rmax
as a function of n.[28,29] This translates into the quadratic
decay of bond lengths with n. The contraction, however,
might affect some bond types differently than others, de-
pending on the constraints imposed by multidentate ligands.
Therefore, some authors have studied the decay by consider-
(for 5p electrons) of LaIII, respectively, and k is the screen-
ing constant for one 4f electron.
2
2
2
*
*
SmðnÞ¼ SmL þ mr0 þ ½mr0ð1ꢀkÞ=Z0 ꢁn þ ½mr0ð1ꢀkÞ =ðZ0 Þ ꢁn
¼ a þ bn þ cn2
ð1Þ
ꢀ
When dealing with the Ln X distances in the series re-
ported here, we considered that the metal size reduction
would have a concerted effect on the overall distance of the
(H3ꢀxL)ꢀx ligands to the metals. Thus, for each of the two
metals of the dinuclear complex (Ln1 and Ln2), the ensem-
ing the combined effect on the totality of bonds (Sd(Ln X)).[24]
ꢀ
ꢀ
For the sum Sm, of distances of an ensemble of m bonds, Sla-
terꢀs model for the lanthanide contraction yields the rela-
tionship [Eq. (1)] in which SmL is the sum of the atomic radii
of the m donor atoms considered (assumed to be constant),
r0 and Z0* are the ionic radius and effective nuclear charge
ble of Ln O distances that involves only these ligands was
ꢀ
considered as a whole, excluding the Ln N bonds for a
more homogeneous treatment. Figure 3 shows the variation
with n of the sums Smd
both sites) for complexes 1 to 13. The data could be fit satis-
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G. Aromꢂ et al.
Figure 3. Graph of Smd(Ln
versus the number of 4f electrons, n, for
ꢀ
O)
Figure 4. Graphs of Ln···Ln distances and average of various O···O dis-
tances (gray in inset) versus the number of 4f electrons (n) for complexes
1 to 13. The solid lines are best fits (weighted for the Ln···Ln distances)
to quadratic functions F(n)=a+bn+cn2, demonstrating that these pa-
rameters follow quadratic decays with n. Best-fit parameters: a=
3.9568(3), b=ꢀ0.0285(1), and c=0.00083(1) with R2 =0.9978; a=
2.809(4), b=ꢀ0.023(1), and c=0.00050(1) with R2 =0.9939, for d(Ln···Ln)
and average d(O···O), respectively).
sites 1 (circles) and 2 (squares) of complexes 1 to 13, with m=6 (corre-
sponding to the O atoms of the HL2ꢀ or H2Lꢀ ligands). The solid lines
are best weighted fits to the quadratic function S(n)=a+bn+cn2. Best-fit
parameters (in the 1/2 format): a=14.99(1)/15.26(1), b=ꢀ0.114(8)/
ꢀ0.106(8), and c=0.0032(6)/0.0024(6) with R2 =0.9946/0.9951.
factorily to a polynomial equation of second order (see the
legend of Figure 3 for best-fit coefficients) by employing a
weighted regression with a weighting factor of sꢀ2. From the
parameters obtained it is possible to calculate the screening
constant k, extracted as k=1+Z0*c/b from Equation (1),
which for Z0*=15.42[28] is found to be equal to 0.57 and 0.65
for Ln1 and Ln2, respectively. These numbers are close to
the commonly accepted value of k=0.69 and to the previ-
ously determined number for another series of Ln com-
plexes (k=0.64).[24] The difference in the value of the 4f-
electron screening constant obtained for metals in site 1 and
site 2 could point, experimentally, to a potential crystal-field
effect on the capacity of these electrons to shield the nuclear
charge. Another remarkable feature unveiled by this analy-
polyhedra of both sites (O3, O8, O13) were analyzed jointly
(Figure 4), which also resulted in a quadratic decay. Such
observation is the first manifestation of the lanthanide con-
traction on parameters affected by two lanthanide atoms si-
multaneously. This could also be probed by plotting the de-
pendence of the Ln···Ln distances on n (Figure 4), which ex-
hibits the same behavior.
Electrospray mass spectrometry (ESI-MS): Electrospray
mass spectrograms were recorded for compounds 1 to 15.
They showed several important peaks that correspond to
various ligand fragments, whereas weaker signals could be
identified that arose from the complexes. The experiments
were performed in methanol, in which the system was
shown to persist for the time necessary to collect the data
(see below). The poor intensity of the latter species versus
the fragments of the ligands is not surprising, since unsatu-
rated lanthanide complexes are known to possess weak ESI-
MS responses that result from the difficulty of solvated spe-
cies to transfer to the gas phase.[40] The ionization of the
clusters was achieved after losing the pyridine and H2O li-
sis is that one site exhibits systematically a larger Smd(Ln
ꢀ
O)
sum than the other (for m=6 in both cases), whereas the
gap is maintained practically constant throughout the series.
This difference in “cavity size” between both sites is un-
doubtedly the result of the structural constraints imposed by
the disposition of ligands within the assembly. Such a “geo-
metric component” imparted by the scaffold of the ligands
has an impact on the intercept of the S(n) curves, a. Parame-
ters b and c should almost exclusively be determined by the
factors predicted in Slaterꢀs model, as suggested by the fact
that they vary little from site 1 to site 2.
ꢀ
gands and the NO3 group, thus leading to general formulae
[Ln2(HL)
showed the addition of a second proton atom onto that spe-
cies, thus leading to [Ln2(HL)
(H2L)2]2+, or the addition of a
(H2L)]+. The mass spectrometric study also
2ACHTUNGTRENNUNG
ꢀ
The Ln X distances that involve the H2O and pyridine li-
ACHTUNGTRENNUNG
gands in site 1 were also analyzed independently (Figure S3
in the Supporting Information) and produced worse polyno-
mial fits, which is consistent with the proposed fact[24] that
individual bonds might exhibit a more irregular behavior.
Another way to evaluate the effect of the contraction is by
examining parameters that directly affect the volume of the
coordination polyhedron. It was shown previously that the
sum of the X···X distances (Sid(X···X)) that corresponds to the
edges of this polyhedron in mononuclear complexes also ex-
hibits a quadratic decay with n.[24] In the series 1 to 13, the
d(O···O) distances for the atoms shared by the coordination
sodium ion that substituted a proton, most likely from the
H2Lꢀ moiety, which led to [Ln2(HL)3]+Na+. Experimental
isotope patterns for these species are shown in Figures S4–
S18 in the Supporting Information for all complexes report-
ed, thereby confirming the formulation of each system. A
representative example of the signal given by these two dis-
tinctive fragments is shown in Figure 5 for the case of
[Er2(HL)2ACHTNUGRTEN(NNUG H2L)AHCTUNRTGEG(NNUN NO3)(py)ACHTNUGTREN(UNNG H2O)] (11), along with the simu-
lated diagrams. Especially important is the case of com-
pound 14 (Lu), for which no crystal data could be obtained.
The existence of the characteristic molecular peaks for this
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Asymmetric Dinuclear [Ln2] Complexes
FULL PAPER
lanthanoid has no orbital momentum (L=0), and thus it dis-
8
plays spin-only magnetism that results from a S7/2 state, and
a very weak magnetic anisotropy. In our previous work,[34]
the lack of orbital momentum in the related [Gd2] complex
16 was used to demonstrate the presence of weak antiferro-
magnetic coupling. Very low-temperature measurements
showed also the existence of a finite exchange coupling of
comparable magnitude in the complex [Tb2(HL)2-
AHCTUNGTRENNUNG
magnetic complexes prepared was investigated through vari-
able-temperature bulk magnetization measurements per-
formed on powdered microcrystalline samples. Figure 6
Figure 5. Selected fragments of the mass spectrogram (red lines) of
[Er2(HL)2ACHTUNGTRENNUNG(H2L)ACHTUNGTRENNUNG(NO3)(py)ACHTNUGTRENN(GUN H2O)] (11) as a representative example of the
complex series in this work. The simulated signals (black lines) are also
Figure 6. Curves of cMT versus T for compounds 2 to 13, cM being the
molar paramagnetic susceptibility per [Ln2] unit. The full red line repre-
sents the best fit of the experimental data for 7 (see text for details).
represented to show the isotopic distribution. Top: fragment [Ln2(HL)2-
ACHTUNGTRENNUNG CAHTUNGTRENNUNG .
(H2L)]+. Bottom: fragment [Ln2(HL)(H2L)2]2+
complex in the mass spectrometry confirms its successful
synthesis, as also shown by infrared spectroscopy and ele-
mental analysis.
shows the temperature dependence of cMT, cM being the
molar paramagnetic susceptibility. Experimental cMT values
measured at 300 K, as well as the calculated ones consider-
ing two isolated LnIII ions in the (2S+1)LJ ground state, are
also listed in Table 3. The majority of the measured values
at 300 K agree well with those expected at this temperature
for two uncoupled metals with all the Stark levels quasi-
equally populated. However, complexes 5 (Sm) and 6 (Eu)
deviate significantly from this behavior, which confirms the
Magnetic properties: The large unquenched orbital moment
of lanthanide ions converts them into an exquisite tool in
molecular magnetism. Recent years have garnered increased
interest as a result of the attractive behavior of many LnIII
complexes as single-molecule magnets (SMMs), as they ex-
hibit magnetic hysteresis at relatively high temperatures and
also high anisotropy barriers.[10,41–47] The electronic ground
state of 4fn ions are determined by electron–electron repul-
sion and the spin–orbit coupling, usually stronger than the
interaction with the crystal field. The latter interaction re-
moves the (2J+1)-fold degeneracy of each multiplet associ-
ated with a total angular momentum J, thus leading to a
manifold of substates known as Stark levels and to a net
magnetic anisotropy. At room temperature, all Stark levels
of the ground-state multiplet are usually populated. Howev-
er, as temperature decreases, the excited sublevels start de-
populating, thus leading to a variation of the cMT versus T
curve. In a LnIII dinuclear compound, the magnetic response
depends not only on this effect but also on the magnetic ex-
change between both ions. This magnetic exchange is almost
always weak, and is thus masked by the depopulation of the
Stark levels. However, the case of GdIII is special, since this
Table 3. Experimental cMT values for complexes 2–13 at T=300 K and
the calculated values corresponding to two isolated LnIII ions in the
(2S+1)LJ ground state.
(2S+1)LJ
gJ
cMT (calcd)[a]
cMT (exptl)[a]
Ce (2)
Pr (3)
2F5/2
3H4
4I9/2
6H5/2
7F0
6/7
4/5
8/11
2/7
–
1.60
3.20
3.28
0.18
0
15.76
23.64
28.34
28.14
22.96
14.30
5.14
1.88
4.07
3.55
1.17
3.45
16.80
24.11
28.22
27.99
22.74
12.72
5.97
Nd (4)
Sm (5)
Eu (6)
Gd (7)
Tb (8)
Dy (9)
Ho (10)
Er (11)
Tm (12)
Yb (13)
8S7/2
7F6
2
3/2
4/3
5/4
6/5
7/6
8/7
6H15/2
5I8
4I15/2
3H6
2F7/2
[a] In cm3 Kꢀ1 molꢀ1
.
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G. Aromꢂ et al.
influence of low-lying excited multiplets in these metals.[48]
The dependence of the cMT product on temperature pro-
vides information on the crystal field and exchange interac-
tions present in each complex. As argued above, complex 7,
with Gd, represents an especially simple situation, free from
the contributions of the spin–orbit and crystal-field interac-
tions. In this case, cMT shows a stable value from room tem-
perature down to 40 K. Below this temperature, it gradually
decreases, ultimately reaching a value of 15.28 cm3 Kmolꢀ1
at 2 K. Since no anisotropic effects can be invoked, this de-
cline can only be ascribed to the presence of weak antiferro-
magnetic interactions between the two GdIII ions. As men-
tioned above, an analogous response was found previously
for complex 16.[34] The cMT versus T curve was modeled
using an isotropic spin Hamiltonian (H=ꢀJS1S2), as well as
a temperature-independent paramagnetic (TIP) susceptibili-
ty of 6ꢇ10ꢀ4 cm3 Kꢀ1 molꢀ1.[49] The best fit was obtained for
J=ꢀ0.04(2) cmꢀ1 and g=2.05(1), which are in agreement
with the literature data[50] and with the values found for
16.[34] As expected, the magnetic responses of all the other
complexes follow a qualitatively different behavior. For
these compounds, cMT begins to decrease at as high as
300 K. As a general rule, this decrease can be attributed to
the depopulation of excited Stark sublevels that belong to
the ground multiplet of each lanthanide ion. The intramo-
lecular exchange interactions can lead to an additional drop
in cMT, analogous to that observed for complex 7. This be-
havior is clearly observed for complexes 2 (Ce), 8 (Tb), 9
(Dy), 10 (Ho), and 12 (Tm). As T decreases below 300 K,
cMT first decreases to an intermediate value, after which a
second, faster decline occurs. The first decrease can be at-
tributed to the depopulation of Stark sublevels, whereas the
second decline is most likely due to a weak antiferromagnet-
ic coupling between both LnIII ions. For complexes 3 (Pr), 4
(Nd), 11 (Er), and 13 (Yb), these two steps are not as clearly
separated, since cMT decreases continuously throughout the
whole temperature range. However, both contributions
might still be considered.
tion band with lmax in the range from 385 to 387 nm, which
is always accompanied by a shoulder of lower intensity be-
tween 405 and 408 nm (Figure S19 in the Supporting Infor-
mation). Both bands are directly related to the ligand (see
spectrum of H3L in Figure S19 in the Supporting Informa-
tion) and might be attributed to p–p* transitions as ob-
served in related systems.[51,52]
A slight redshift in absorption is observed for the com-
plexes with respect to the free ligand, which is indicative of
coordination between ligands and metal centers. The steady-
state luminescence of 1 to 15 and H3L were investigated in
DMF in the concentration 10ꢀ5 m. All the systems were re-
corded in the visible and/or near-IR regions. H3L exhibits a
broad emission band at 388 nm (Figure S21 in the Support-
ing Information). In turn, all complexes display a single
broad emission band in that region while no sharp lantha-
nide emissions were found. This indicates that, for the com-
plexes, luminescence in this region is exclusively due to the
coordinated ligand (Figure S21 in the Supporting Informa-
tion). On the other hand, only 4 (Nd), 11 (Er), and 13 (Yb)
displayed spectra in the near-IR region and showed features
that corresponded to metal-based transitions.[53,54] Figure 7
shows the emission spectra of 4 (Nd), 11 (Er), and 13 (Yb)
recorded from 800 to 1600 nm under the excitation of light
with l=385, 385, and 355 nm, respectively. The spectra in
all three cases show transitions with precedents in other lu-
minescent Yb, Nd, and Er compounds.[53,54] Thus, the above-
mentioned excitations resulted in narrow emission bands
Photoluminescent properties: Room-temperature absorp-
tion spectra of complexes 1 to 15 and of the free ligand H3L
were collected in DMF by using a concentration of 10ꢀ5 m.
A representative spectrum is shown on Figure S19 in the
Supporting Information, whereas all the data are listed on
Table S4 in the Supporting Information. The suitability of
this solvent was examined by performing the absorption
measurement on one of the complexes, [Eu2(HL)
2ACHTUNGERTN(NUNG H2L)-
A
ACHTUNGTRENNUNG
ure S20 in the Supporting Information). The absorption ex-
perienced only a very slow decline, thus demonstrating the
sufficient stability of the complexes in this solution during
the experiments. Likewise, the spectrum originally collected
in MeOH exhibited the same stability. Very similar values in
absorption maxima were observed in all systems. Within this
family of complexes, going from metal to metal has only
minor effects on the intensity and shift of the absorption
bands. Above 300 nm, the spectra exhibit a broad absorp-
Figure 7. Fluorescence emission spectra of complexes 4 (Nd, top), 11 (Er,
middle), and 13 (Yb, bottom) in 10ꢀ5 m DMF at room temperature, under
the excitation of light with l=385, 385, and 355 nm, respectively. Intensi-
ties are normalized. See text for further details.
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Asymmetric Dinuclear [Ln2] Complexes
FULL PAPER
A
2CATHNUGTRENU(NGN H2L)ACHTNURGTEG(NNUN NO3)(py)ACHTNUGTREN(NUNG H2O)] (4): A yellow solution of H3L (30.0 mg,
near 890, 1058, and 1331 nm for 4 (Nd), 1523 nm for 11
(Er), and 977 nm for 13 (Yb). These bands were assigned to
AHCTUNGTRENNUNG
4
4
4
4
the transitions F3/2! I9/2, F3/2! I11/2 (the most intense), and
was stirred for 2 h, and the resulting yellow solution was layered with tol-
uene. After two weeks, the complex was obtained as yellow crystals
(69%, 31.3 mg).
4
4
4
4F3/2! I13/2 for the [Nd2] complex, I13/2! I15/2 for the [Er2]
system, and F5/2! F7/2 for the [Yb2] complex.[52–54] Overall,
these transitions might be due to an antenna effect that has
been well described for other b-diketonate systems, in which
effective intramolecular energy transference from the coor-
dinated H3L ligand (as H2Lꢀ and HLꢀ) to the central lantha-
nide would lead to radiative emitting processes.[55]
2
2
E
2A
N
(H2O)] (5):
A yellow solution of H3L
AHCTUNGTRENNUNG
mixture was stirred for 2 h, and the resulting yellow solution was layered
with Et2O. After two weeks, the complex was obtained as yellow crystals
(81%, 37.2 mg).
A
2CATHNUGTRENU(NGN H2L)ACHTNURGTEG(NNUN NO3)(py)ACHTNUGTREN(NUNG H2O)] (6): A yellow solution of H3L (30.0 mg,
AHCTUNGTRENNUNG
Conclusion
was stirred for 2 h, and the resulting orange solution was layered with
Et2O. After two weeks, the complex was obtained as yellow crystals
(91%, 42.0 mg).
The first structurally characterized quasi-complete series of
dinuclear lanthanide complexes has been employed for the
study of the lanthanide contraction of two metal centers, si-
multaneously, within two different coordination environ-
ments. The quadratic nature of the decay in each site has
A
E
E
G
A yellow solution of H3L
AHCTUNGTRENNUNG
mixture was stirred for 2 h, and the resulting yellow solution was layered
with Et2O. After two weeks, the complex was obtained as yellow crystals
(90%, 41.8 mg).
ꢀ
been seen by analyzing the sum of all Ln O distances that
belong to the partially deprotonated H3L ligand, and suita-
bly evaluated with Slaterꢀs model of the atomic radii. Pa-
rameters that affect both LnIII ions, such as Ln···Ln or coor-
dination polyhedra O···O distances shared by both metals,
have been demonstrated to follow the quadratic decay due
to the contraction as well. The difference between cavity
size in both environments is maintained throughout the
series as a result of structural constrains imposed by the mo-
lecular arrangement. The use of this behavior to design het-
erometallic dinuclear lanthanide complexes, thus combining
two lanthanide ions with different ionic radii, is being inves-
tigated.
G
2CATHNUGTRENU(NGN H2L)ACHTNURGTEG(NNUN NO3)(py)ACHTNUGTREN(NUNG H2O)] (8): A yellow solution of H3L (30.0 mg,
AHCTUNGTRENNUNG
stirred for 2 h, and the resulting yellow solution was layered with Et2O.
After two weeks, the complex was obtained as yellow crystals (66%,
30.7 mg).
R
2ACHTUNGTRENGU(N H2L)(Cl)(py)ACHUTNTGERN(NUGN H2O)] (9): A yellow solution of H3L (30.0 mg,
AHCTUNGTRENNUNG
was stirred for 2 h, and the resulting orange solution was layered with
Et2O. After two weeks, the complex was obtained as yellow crystals
(71%, 33.2 mg).
A
2ACHTUNGTRENUNN(G H2L)AHCTUNGERTNG(NUN NO3)(py)ACHTGNUERTNN(UG H2O)] (10): A yellow solution of H3L
AHCTUNGTRENNUNG
mixture was stirred for 2 h, and the resulting yellow solution was layered
with Et2O. After two weeks, the complex was obtained as yellow crystals
(75%, 35.2 mg).
Experimental Section
A
2ACHTUNGTRENUNN(G H2L)AHCTUNGERTNG(NUN NO3)(py)ACHTGNUERTNN(UG H2O)] (11): A yellow solution of H3L
A
2ACHTUNGTRENNUNG(H2L)ACHTUNGTRENNUNG(NO3)(py)ACHTNUGTRENN(UGN H2O)] (1): Method 1: A yellow solution of
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
mixture was stirred for 2 h, and the resulting orange solution was layered
with toluene. After two weeks, the complex was obtained as yellow crys-
tals (59%, 27.8 mg).
The mixture was stirred for 2 h, and the resulting yellow solution was lay-
ered with Et2O. After two weeks, the complex was obtained as a yellow
solid (43%, 19.4 mg). Method 2: A yellow solution of H3L (30.0 mg,
0.11 mmol) in pyridine (10 mL) was added into a colorless solution of La-
A
2ACHTUNGTRENUNN(G H2L)AHCTUNGERTNG(NUN NO3)(py)ACHTGNUERTNN(UG H2O)] (12): A yellow solution of H3L
ACHTUNGTRENNUNG(NO3)3·6H2O (30.3 mg, 0.07 mmol) in pyridine (10 mL). The mixture was
AHCTUNGTRENNUNG
stirred for 30 min, when a colorless 0.13m solution of HNO3 in MeCN
(270 mL, 0.035 mmol) was added dropwise. The reaction was left stirring
for 2 h, and the resulting yellow solution was layered with Et2O. After
three weeks, small yellow crystals of 1 were collected (40%, 18.0 mg).
mixture was stirred for 2 h, and the resulting yellow solution was layered
with ether. After two weeks, the complex was obtained as yellow crystals
(15%, 7.9 mg).
A
2ACHTUNGTRENUNN(G H2L)AHCTUNGERTNG(NUN NO3)(py)ACHTGNUERTNN(UG H2O)] (13): A yellow solution of H3L
A
2ACHTUNGTRENNUNG(H2L)ACHTUNGTRENNUNG(NO3)(py)ACHTNUGTREN(NUNG H2O)] (2): A yellow solution of H3L (30.0 mg,
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
addition of 69% HNO3 (6.81 mL, 0.11 mmol), the mixture was stirred for
1 h, and the resulting yellow solution was layered with hexane. After four
weeks, small orange crystals of 13 were collected (7%, 3.0 mg).
stirred for 2 h, and the resulting orange solution was layered with Et2O.
After two weeks, the complex was obtained as red crystals (77%,
34.6 mg).
A
2ACHTUNGTRENUNN(G H2L)AHCTUNGERTNG(NUN NO3)(py)ACHTGNUERTNN(UG H2O)] (14): A yellow solution of H3L
A
2ACHTUNGTRENNUNG(H2L)ACHTUNGTRENNUNG(NO3)(py)ACHTNUGTREN(NUNG H2O)] (3): A yellow solution of H3L (30.0 mg,
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
mixture was stirred for 2 h, and the resulting orange solution was layered
with Et2O. After two weeks, the complex was obtained as an orange solid
(23%, 10.9 mg).
was stirred for 2 h, and the resulting yellow solution was layered with
Et2O. After two weeks, the complex was obtained as yellow crystals
(87%, 39.1 mg).
Chem. Eur. J. 2013, 00, 0 – 0
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G. Aromꢂ et al.
[Y2(HL)
0.11 mmol) in pyridine (15 mL) was added into a colorless solution of Y-
(NO3)3·6H2O (26.9 mg, 0.07 mmol) in pyridine (15 mL). The mixture was
(H2L)
N
U
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
ACHTUNGTRENNUNG
stirred under reflux for 2 h, and then cooled to room temperature. The
resulting orange solution was layered with ether. After two weeks, the
complex was obtained as orange crystals (39%, 16.9 mg).
Acknowledgements
Physical measurements: Variable-temperature magnetic susceptibility
data for compounds 2 to 12 and 14 were obtained on microcrystalline
samples by using a Quantum Design MPMS-XL SQUID magnetometer
either at the “Unitat de Mesures Magnꢄtiques” of the Universitat de Bar-
celona or at the SAI Physical Measurements of the University of Zarago-
za. Pascalꢀs constants were used to estimate diamagnetic corrections to
the molar paramagnetic susceptibility, and a correction was applied for
the sample holder. IR spectra were recorded on KBr pellets, in the range
4000–400 cmꢀ1, using a Thermo Nicolet Avatar 330 FTIR spectrometer.
Elemental analyses were performed using a Perkin–Elmer Series II
CHNS/O Analyzer 2400 at the Servei de Microanꢁlisi of the CSIC, Bar-
celona. Electronic absorption spectra were recorded with 1 nm resolution
using a Varian Cary 100 UV/Vis spectrophotometer in DMF at room
temperature (c=10ꢀ5 m). Fluorescence emission spectra were carried out
using Horiba Jobin–Yvon SPEX Nanolog-TM and Cary Eclipse spectro-
fluorimeters. The slits used were of 5 nm for [Nd2] (4) and [Yb2] (14) and
10 nm for [Er2] (11). Positive-ion ESI mass spectrometry experiments
were performed using a LC/MSD-TOF (Agilent Technologies) with a
dual source equipped with a lock spray for internal reference introduc-
tion at the Unitat dꢀEspectrometria de Masses (SSR) of the University of
Barcelona. The experimental parameters were capillary voltage 4 kV, gas
temperature 3258C, nebulizing gas pressure 15 psi, drying gas flow
7.0 Lminꢀ1, and fragmentor voltage ranging from 175 to 250 V. Internal
reference masses were m/z 121.05087 (purine) or 922.00979 (HP-0921).
Samples (microliters) were introduced into the source using an HPLC
system (Agilent 1100), using a mixture of H2O/CH3CN (1:1) as eluent
(200 mLminꢀ1).
G.A. thanks the Generalitat de Catalunya for the prize ICREA Academ-
ia 2008 and the ERC for a starting grant (258060 FuncMolQIP). The au-
thors thank the Spanish MCI through CTQ2009-06959 (N.A.A, D.A.,
V.V., L.B., and G.A.), and MAT2011-24284 (O.R.). The Advanced Light
Source is supported by the Director, Office of Science, Office of Basic
Energy Sciences of the U.S. Department of Energy under contract no.
DE-AC02-05CH11231.
[1] S. V. Eliseeva, J. C. G. Bunzli, New J. Chem. 2011, 35, 1165–1176.
[2] K. Binnemans, Chem. Rev. 2009, 109, 4283–4374.
[3] K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 2002, 102, 2347–2356.
[4] Y. Hasegawa, Y. Wada, S. Yanagida, H. Kawai, N. Yasuda, T. Naga-
mura, Appl. Phys. Lett. 2003, 83, 3599–3601.
[5] A. de Bettencourt-Dias, Dalton Trans. 2007, 2229–2241.
[6] B. S. Richards, Sol. Energy Mater. Sol. Cells 2006, 90, 1189–1207.
[7] C. Benelli, D. Gatteschi, Chem. Rev. 2002, 102, 2369–2387.
[8] G. Aromꢂ, E. K. Brechin, Struct. Bonding (Berlin) 2006, 122, 1–67.
[9] J. D. Rinehart, J. R. Long, Chem. Sci. 2011, 2, 2078–2085.
[10] N. Ishikawa, M. Sugita, T. Ishikawa, S. Koshihara, Y. Kaizu, J. Am.
Chem. Soc. 2003, 125, 8694–8695.
[11] L. Ungur, L. F. Chibotaru, Phys. Chem. Chem. Phys. 2011, 13,
20086–20090.
[12] G. Cucinotta, M. Perfetti, J. Luzꢃn, M. Etienne, P.-E. Car, A. Cane-
schi, G. Calvez, K. Bernot, R. Sessoli, Angew. Chem. Int. Ed. 2012,
51, 1606–1610.
X-ray crystallography: Crystals systematically suffer from deterioration
of crystallinity once out of their mother liquor, likely due to lattice sol-
vent loss. Single crystals were therefore selected and mounted directly
from their mother liquor using the oil-drop method and mounted as fast
as possible into the cold N2 stream on the goniometer. Data for com-
pounds 1·5py, 4·6py, and 13·6py were collected with MoKa radiation (l=
0.71073 ꢆ) using a Bruker APEX II QUAZAR diffractometer equipped
with a microfocus multilayer monochromator at 100 K. Data for com-
pounds 2·6py, 3·6py, 5·5py·H2O, 6·5py·H2O, 8·5py, 11·5py, and 15·4py
were collected at 100 K (150 K for 5·5py·H2O) using a Bruker APEX II
CCD diffractometer on the Advanced Light Source beamline 11.3.1 at
Lawrence Berkeley National Laboratory, from a silicon 111 monochro-
mator (l=0.7749 ꢆ). Data reduction and absorption corrections were
performed with SAINT and SADABS.[56] Data for compounds 7·5py,
9·5py, 10·5py, and 12·5py·Et2O were obtained at 150 K using an Oxford
Diffraction Excalibur diffractometer with enhanced MoKa radiation (l=
0.71073 ꢆ) at the X-ray diffraction and Fluorescence Analysis Service of
the University of Zaragoza. Cell refinement, data reduction, and absorp-
tion corrections were performed with the CrysAlisPro suite.[57] All struc-
tures were solved and refined on F2 with the SHELXTL suite.[58,59] All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed geometrically on their carrier atom and refined with a riding
model. In all cases, displacement parameters restraints were used to
refine some of the lattice solvent molecules. For 1·5py, 4·6py, 7·5py,
8·5py, 9·5py, 10·5py, 11·5py, and 15·4py void space with only diffuse elec-
tron density remaining at the end of the refinement were analyzed and
taken into account with SQUEEZE as implemented in the PLATON
package.[60, 61] Crystallographic and refinement parameters are summar-
ized in Table 1. Selected bond lengths and angles are given in Table 2
and Table S1 in the Supporting Information.
[13] J. Luzꢃn, R. Sessoli, Dalton Trans. 2012, 41, 13556–13567.
[14] L. Sorace, C. Benelli, D. Gatteschi, Chem. Soc. Rev. 2011, 40, 3092–
3104.
[15] P. DꢀAngelo, A. Zitolo, V. Migliorati, G. Chillemi, M. Duvail, P. Vi-
torge, S. Abadie, R. Spezia, Inorg. Chem. 2011, 50, 4572–4579.
[16] A. Bowden, S. J. Coles, M. B. Pitak, A. W. G. Platt, Inorg. Chem.
2012, 51, 4379–4389.
[17] Y. Fukuda, A. Nakao, K. Hayashi, J. Chem. Soc. Dalton Trans. 2002,
527–533.
[18] J. Cepeda, R. Balda, G. Beobide, O. Castillo, J. Fernꢈndez, A.
Luque, S. Pꢉrez-Yꢈnez, P. Romꢈn, D. Vallejo-Sꢈnchez, Inorg. Chem.
2011, 50, 8437–8451.
[19] Z. He, E.-Q. Gao, Z.-M. Wang, C.-H. Yan, M. Kurmoo, Inorg.
Chem. 2005, 44, 862–874.
[20] G.-L. Zhuang, X.-J. Kong, L.-S. Long, R.-B. Huang, L.-S. Zheng,
CrystEngComm 2010, 12, 2691–2694.
[21] P. V. Bernhardt, B. M. Flanagan, M. J. Riley, Aust. J. Chem. 2001, 54,
229–232.
[22] P. V. Bernhardt, B. M. Flanagan, M. J. Riley, Aust. J. Chem. 2000, 53,
229–231.
[23] K. Matsumoto, K. Suzuki, T. Tsukuda, T. Tsubomura, Inorg. Chem.
2010, 49, 4717–4719.
[24] M. Seitz, A. G. Oliver, K. N. Raymond, J. Am. Chem. Soc. 2007,
129, 11153–11160.
[25] A. Chatterjee, E. N. Maslen, K. J. Watson, Acta Crystallogr. Sect. B
1988, 44, 381–386.
[26] I. Persson, P. DꢀAngelo, S. De Panfilis, M. Sandstrom, L. Eriksson,
Chem. Eur. J. 2008, 14, 3056–3066.
[27] E. A. Quadrelli, Inorg. Chem. 2002, 41, 167–169.
[28] E. Clementi, D. L. Raimondi, W. P. Reinhardt, J. Chem. Phys. 1967,
47, 1300–1307.
CCDC-915325 (1), 915326 (2), 915327 (3), 915328 (4), 915329 (5), 915330
(6), 915331 (7), 915332 (8), 915333 (9), 915334 (10), 915335 (11), 915336
(12), 918743 (13), and 915337 (15) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
[29] J. C. Slater, Phys. Rev. 1930, 36, 57–64.
[30] J. Chakraborty, A. Ray, G. Pilet, G. Chastanet, D. Luneau, R. F.
Ziessel, L. J. Charbonniere, L. Carrella, E. Rentschler, M. S. El Fal-
lah, S. Mitra, Dalton Trans. 2009, 10263–10272.
&
10
&
www.chemeurj.org
ꢅ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Asymmetric Dinuclear [Ln2] Complexes
FULL PAPER
[31] P.-H. Lin, M. Leclere, J. Long, T. J. Burchell, I. Korobkov, R. Clꢉrac,
M. Murugesu, Dalton Trans. 2010, 39, 5698–5704.
[46] J. D. Rinehart, M. Fang, W. J. Evans, J. R. Long, Nat. Chem. 2011, 3,
538–542.
[47] R. Sessoli, A. K. Powell, Coord. Chem. Rev. 2009, 253, 2328–2341.
[48] Spectra and Energy Levels of Rare Earth Ions in Crystals (Ed: G. H.
Dieke), Wiley, New York (USA), 1968.
[49] A. Panagiotopoulos, T. F. Zafiropoulos, S. P. Perlepes, E. Bakalbas-
sis, I. Masson-Ramade, O. Kahn, A. Terzis, C. P. Raptopoulou,
Inorg. Chem. 1995, 34, 4918–4920.
[32] L. S. Natrajan, P. L. Timmins, M. Lunn, S. L. Heath, Inorg. Chem.
2007, 46, 10877–10886.
[33] J.-X. Xu, Y. Ma, D.-Z. Liao, G.-F. Xu, J. Tang, C. Wang, N. Zhou, S.-
P. Yan, P. Cheng, L.-C. Li, Inorg. Chem. 2009, 48, 8890–8896.
[34] D. Aguilꢁ, L. A. Barrios, F. Luis, A. Repollꢉs, O. Roubeau, S. J.
Teat, G. Aromꢂ, Inorg. Chem. 2010, 49, 6784–6786.
[50] L. E. Roy, T. Hughbanks, J. Am. Chem. Soc. 2006, 128, 568–575.
[51] Y. Luo, B. Chen, W. Wu, X. Yu, Q. Yan, Q. Zhang, J. Lumin. 2009,
129, 1309–1313.
[52] A. P. Bassett, S. W. Magennis, P. B. Glover, D. J. Lewis, N. Spencer,
S. Parsons, R. M. Williams, L. De Cola, Z. Pikramenou, J. Am.
Chem. Soc. 2004, 126, 9413–9424.
[53] N. M. Shavaleev, G. Accorsi, D. Virgili, Z. e. R. Bell, T. Lazarides,
G. Calogero, N. Armaroli, M. D. Ward, Inorg. Chem. 2005, 44, 61–
72.
[54] N. M. Shavaleev, R. Scopelliti, F. Gumy, J.-C. G. Bꢊnzli, Inorg.
Chem. 2009, 48, 2908–2918.
[55] K. Binnemans, in Handbook on the Physics and Chemistry of Rare
Earths, Vol. 35, Elsevier (Amsterdam), 2005, pp. 107–272.
[56] SAINT and SADABS, Bruker AXS Inc., Madison, Wisconsin, USA.
[57] CrysAlis PRO, Agilent Technologies, Yarnton, UK.
[58] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
[59] G. M. Sheldrick, SHELXTL, Bruker AXS Inc., Madison, Wisconsin,
USA.
[35] G. Aromꢂ, D. Aguilꢁ, P. Gamez, F. Luis, O. Roubeau, Chem. Soc.
Rev. 2012, 41, 537–546.
[36] F. Luis, A. Repollꢉs, M. J. Martꢂnez-Pꢉrez, D. Aguilꢁ, O. Roubeau,
D. Zueco, P. J. Alonso, M. Evangelisti, A. Camꢃn, J. Sesꢉ, L. A. Bar-
rios, G. Aromꢂ, Phys. Rev. Lett. 2011, 107, 117203.
[37] M. Estrader, J. Ribas, V. Tangoulis, X. Solans, M. Font-Bardꢂa, M.
Maestro, C. Dꢂaz, Inorg. Chem. 2006, 45, 8239–8250.
[38] A. Ruiz-Martꢂnez, S. Alvarez, Chem. Eur. J. 2009, 15, 7470–7480.
[39] A. Ruiz-Martꢂnez, D. Casanova, S. Alvarez, Chem. Eur. J. 2008, 14,
1291–1303.
[40] C. Piguet, J.-C. G. Bꢊnzli, G. Bernardinelli, G. Hopfgartner, S.
Petoud, O. Schaad, J. Am. Chem. Soc. 1996, 118, 6681–6697.
[41] M. T. Gamer, Y. Lan, P. W. Roesky, A. K. Powell, R. Clꢉrac, Inorg.
Chem. 2008, 47, 6581–6583.
[42] Y.-N. Guo, G.-F. Xu, P. Gamez, L. Zhao, S.-Y. Lin, R. Deng, J. Tang,
H.-J. Zhang, J. Am. Chem. Soc. 2010, 132, 8538–8539.
[43] S.-D. Jiang, S.-S. Liu, L.-N. Zhou, B.-W. Wang, Z.-M. Wang, S. Gao,
Inorg. Chem. 2012, 51, 3079–3087.
[60] A. L. Spek, PLATON,
A Multipurpose Crystallographic Tool,
[44] P.-H. Lin, T. J. Burchell, R. Clꢉrac, M. Murugesu, Angew. Chem.
2008, 120, 8980–8983; Angew. Chem. Int. Ed. 2008, 47, 8848–8851.
[45] P.-H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru, W. Wernsdorfer,
M. Murugesu, Angew. Chem. 2009, 121, 9653–9656; Angew. Chem.
Int. Ed. 2009, 48, 9489–9492.
Utrecht University, Utrecht (The Netherlands), 2008.
[61] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
Received: December 13, 2012
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢅ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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G. Aromꢂ et al.
Contractual obligation: The quadratic
decay of the lanthanide contraction is
manifested by using, for the first time,
a quasi-isostructural series of dinuclear
asymmetric complexes (see figure).
Crystallographic parameters of a total
of 13 structures have been analyzed to
confirm this trend, which also is
observed for parameters that receive a
simultaneous contribution from two
metals.
Lanthanides
D. Aguilꢀ, L. A. Barrios, V. Velasco,
L. Arnedo, N. Aliaga-Alcalde,
M. Menelaou, S. J. Teat, O. Roubeau,
F. Luis, G. Aromꢁ* ........... &&&& — &&&&
Lanthanide Contraction within a
Series of Asymmetric Dinuclear [Ln2]
Complexes
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Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!