Predictions, synthetic strategy, and isolation of a linear tetrametallic triple-stranded lanthanide helicate

Article abstract of DOI:10.1002/anie.200503040

(Figure Presented) Complex twists: Following theoretical thermodynamic predictions, it is seen that the tetrametallic triple-stranded lanthanide helicate [Ln₄L₃]12⁺ (see structure) dominates the speciation in solution at m

Full text of DOI:10.1002/anie.200503040

Communications
Self-Assembly
DOI: 10.1002/anie.200503040
Predictions, Synthetic Strategy, and Isolation of a
Linear Tetrametallic Triple-Stranded Lanthanide
Helicate**
Kornelia Zeckert, Josef Hamacek, Jean-Michel Senegas,
Natalia Dalla-Favera, SØbastien Floquet,
GØrald Bernardinelli,* and Claude Piguet*
Dedicated to Professor Jean-Claude G. Bünzli
The majority of self-assembled linear multi-stranded helicates
are bimetallic, with some extensions toward trimetallic
analogues,[1] whilst those with four or more metal centers
remain rare.[2] Although the regular arrangement of several
metal ions in a single direction, thus forming a discrete chain,
may induce novel and unusual electrochemical,[3] photophys-
[*] Dr. G. Bernardinelli
Laboratory of X-ray Crystallography
University of Geneva
24 Quai E. Ansermet, 1211 Geneva 4 (Switzerland)
Fax: (+41)22-379-6108
E-mail: gerald.bernardinelli@cryst.unige.ch
Dr. K. Zeckert,[+] Dr. J. Hamacek, Dr. J.-M. Senegas, N. Dalla-Favera,
Dr. S. Floquet,[++] Prof. Dr. C. Piguet
Department of Inorganic
Analytical and Applied Chemistry
University of Geneva
30 Quai E. Ansermet, 1211 Geneva 4 (Switzerland)
Fax: (+41)22-379-6830
E-mail: claude.piguet@chiam.unige.ch
[+] Present address: Institut für Anorganische Chemie
der Universität Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
[
++] Present address: Institut Lavoisier, IREM, UMR 8637
UniversitØ de Versailles
45 Avenue des Etats-Unis, 78035 Versailles (France)
[**] This work was supported through grants from the Swiss National
Science Foundation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
7954
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7954 –7958

Angewandte
Chemie
ical,[4] or magnetic[5] properties, multimetallic linear helicates
have been mainly investigated for their structural aspect,
which is indebted to their undeniable aesthetic appeal. The
recent expansion of (supra)molecular chemistry toward the
nanosciences dramatically revealed the very limited quanti-
tative predictions available for the design of large multi-
component assemblies,[6] despite the remarkable reports of
major successes based on qualitative symmetry matching and
stereochemical restrictions.[7] In this context, the assembly of
simple mono-dimensional multimetallic helicates can bring
some valuable contributions to the fundamental understand-
ing and rationalization of thermodynamically driven multi-
component complexation processes. In two seminal reports,[8]
Lehn and co-workers first proposed a thermodynamic model
that accounted for the formation of trimetallic double-
stranded helicates. Scatchard plots based on the classical
two-parameter site-binding model,[9–11] which considers the
total free energy of formation of the final complex as the sum
of the connection energies of each metal ion to a single
receptor modulated by intermetallic pair interactions, sug-
gested the occurrence of thermodynamic complexation
processes driven to completion by positive cooperativity—a
landmark that deeply influenced the rationalization of metal-
losupramolecular self-assembly processes for more than a
decade.[10] The recent reanalysis of these original data by
Ercolani indicates that helicate formation cannot be modeled
with pure intermolecular interactions and entropic correc-
tions must be considered for adequately rationalizing intra-
molecular metal–ligand connections, which occur during the
self-assembly processes.[11] An explicit treatment, which
combines metal–ligand connections (intra- and intermolecu-
lar) and pair interactions (intermetallic and interligand) has
interligand (uLL) interactions that occur in the [MmLn] micro-
species.[12] Obviously, the reliable estimation of these four
parameters requires a large amount of independent macro-
scopic constants that characterize comparable complexation
events, a situation which is met when the thermodynamic
equilibria relevant to the formation of the bimetallic
[Eu2(L1)3]6+ and [Eu2(L2)3]6+ and trimetallic [Eu3(L3)3]9+
triple-stranded helicates are simultaneously considered
(only two different tridentate binding units N2O and N3 are
involved in these complexes; Scheme 1). A complete set of
microscopic parameters f , fENu, uEuEu, uLL, and ceff results,[13]
Eu
N2O
and Equation (2) can be then 3used to predict the stability
constant of the unknown tetrametallic homologue
[Eu4(L4)3]12+ [Eq. (3)].
Eu,L4
4,3
6
6
12
4:33
6
b
¼ 2 ðfNEuOÞ ðfNEuÞ ðuLL
Þ
ðuEuEu
Þ
ðceff
Þ
ð3Þ
2
3
We calculate log(bE4,u3,L4) = 42.5 for the saturated triple-
stranded helicate [Eu4(L4)3]12+ [Eq. (3)], which is much larger
than log(b3E,u3,L4) = 35.5, log(bE3,u2,L4) = 27.4, and log(b4E,u2,L4) = 31.5
predicted for its decomposition products, respectively, in
excess of ligand ([Eu3(L4)3]9+
) or in excess of metal
([Eu3(L4)2]9+ and [Eu4(L4)2]12+; detailed calculations are
given in the Supporting Information). The associated pre-
dicted distribution curves for a total ligand concentration
been recently proposed, and the macroscopic formation
M,L
m,n
constant
b
of any supramolecular complex [MmLn]
[Eq. (1)] can be modeled with four microscopic parameters
[Eq. (2), extended site-binding model]:[12]
m M þ n L Ð ½MmLnŠ bmM,,nL
ð1Þ
ð2Þ
m n
b
¼ s wm,n
ðfMi ,L
Þ
m nÀmÀnþ1 ðcei ff
Þ
ðu
Þ
ðu
Þ
Y
Y
Y
Y
00
000
M,L
m,n
M,L
chir
MM
ij
LL
kl
i¼1
i¼1
i<j
k<l
1) fMi ,L represents the intermolecular microscopic affinity
constant that characterizes the connection of a metal M to the
MM
ij
binding site i of a ligand L (including desolvation); 2) u
=
MM
exp(ÀDE /RT) is the Boltzman factor that accounts for the
ij
MM
intermetallic interaction DE
(in terms of free energy),
ij
which occurs when two metal centers occupy two adjacent
LL
kl
LL
kl
binding sites i and j of the same ligand; 3) u = exp(ÀDE
/
RT) is the Boltzman factor that accounts for the interligand
LL
kl
interactions DE , which results from the connection of two
binding sites k and l to the same metal center; 4) cieff
=
M,L
i,intra
exp((DS
ÀDSMi,in,Lter)/R) is the “effective concentration”
used for the correction of the entropy change observed
between inter- and intramolecular binding processes; 5) the
M,L
chir
term s wm,n represents the degeneracy of the microscopic
state, which must be evaluated for each microspecies by using
Scheme 1. Structures of the ligands L1–L4, with the common N3 and
N2O tridentate binding sites highlighted and the numbering scheme
used for the 1H NMR spectroscopic analysis given.
standard statistical methods.[9–12] Finally, the two last products
Q
Q
’’ and
’’’ run over all the pairs of intermetallic (uMM) or
Angew. Chem. Int. Ed. 2005, 44, 7954 –7958ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7955

Communications
j L4 j tot = 10À3
m
show that [Eu4(L4)3]12+ corresponds to
> 95% of the solution species for the stoichiometric ratio
Eu/L4 = 1.33, despite its high 12 + charge (see the Supporting
Information). With this encouraging prediction in mind, we
embarked upon a program to prepare the first stable
tetrametallic lanthanide-containing triple-helical complex.
Our ultimate goal is concerned with the exploration of the
exact dependence of uMM and ceff on the intersite separation d,
which is responsible for 1) the potential programming of local
intermetallic dielectric constants (through DEMM / 1/er d) and
2) the quantitative description of preorganization in self-
assembled processes (through ceff / 1/dn).[6,11,12] Herein, we
present the synthetic strategy, isolation, and structural and
thermodynamic characterizations of the first tetrametallic
lanthanide triple-stranded helicate [Eu4(L4)3]12+. Following
the original synthetic strategy developed for the preparation
of L3,[14] we attempt to react the unsymmetrical o-nitro-
aminoarene 3[14] with the diacid 2 (Scheme 2).[15]
However, 3 proved to be such a poor nucleophile that it
does not react with the activated dicarbonyl chloride 2-
(COCl)2 (Scheme 2, left). The remarkable C-shaped molec-
ular structure of 3 observed in the solid state[16] may help to
rationalize its poor nucleophilicity because 1) one face of the
incriminated secondary ethyl amine N1 is blocked by the
distal diethylcarboxamido group bound to the pyridine ring
and 2) the H1 proton is involved in an intramolecular
Figure 1. Perspective view of the C-shaped molecular structure of the
o-nitroaminoarene synthon 3 with partial numbering. Ellipsoids are
shown at the 50% probability level.
secondary amine to react with 2-(COCl)2 to give the bis(o-
nitroaminoarene) 4, which can be further successfully con-
verted into L4 (Scheme 2, right).
ESI-MS
titrations
of
L4
(10À3 m)
with
[Ln(CF3SO3)3]·xH2O (x = 3–5) in acetonitrile (Ln = La, Eu,
Lu) are dominated by the signals of the saturated species
[Ln4(L4)3(CF3SO3)n](12Àn)+ (n = 3–9) together with minor
signals (< 10%) that arise from the unsaturated intermediates
[Ln3(L4)3(CF3SO3)n](9Àn)+ (excess of ligand, Ln/L4 < 1.33) and
[Ln3(L4)2(CF3SO3)n](9Àn)+ and [Ln4(L4)2(CF3SO3)n](12Àn)+
(excess of metal, Ln/L4 > 1.33; see the Supporting
Information). Parallel 1H NMR titrations in CD3CN/
CD2Cl2 (9:1, j L4 j tot = 8 ” 10À3 m) confirm this behavior
À
hydrogen bond with the adjacent nitro group N1 H1···O1
À
(N1···O1: 2.634(3) Š, a N1 H1···O1: 130(2)8; Figure 1).
Removing the terminal N,N-diethylcarboxamide pyridyl
group in 1 sufficiently improves the nucleophilicity of the
with the stepwise conversion of free L4 into [Ln4(L4)3]12+
,
which largely dominates the speciation in solution (only
very weak signals are detected for some low-symmetrical
intermediates; see the Supporting Information). The only
species detected in solution for Ln/L4 = 1.33 are
1
[Ln4(L4)3]12+, and their H NMR spectra are compatible
with D3-symmetry (six equivalent half-ligand strands that
lead to 15 different CH signals, an enantiotopic central
methylene signal for H16,16’, and six diastereotopic
methylene protons H17,17’–H22,22’; Figure 2). Finally,
preliminary-batch spectrophotometric titrations show
complicated variations of the absorption spectra for Eu/
L4 in the range 0.1–4.0 (j L4 j tot = 10À4 m, CH3CN + 5%
CH2Cl2; see the Supporting Information), which can be
satisfyingly fitted with the four absorbing complexes
[Eu3(L4)3]9+
[Eu4(L4)2]12+
,
[Eu4(L4)3]12+
The associated macroscopic constants
,
[Eu3(L4)2]9+,
and
.
log(bE3,u3,L4) = 36.2(1.9), log(bE4,u3,L4) = 42.1(1.9), log(b3E,u2,L4) =
27.9(1.4), and log(bE4,u2,L4) = 32.0(1.7) match our predictions
within experimental errors.
The diffusion of diethyl ether into a concentrated
solution of [Ln4(L4)3]12+ in methanol provides white
microcrystals in 70–85% yield; the elemental
analyses were shown to be compatible with
[Ln4(L4)3](CF3SO3)12·xH2O·yCH3OH (Ln = La: x = 8.1,
Scheme 2. Synthetic strategy for the preparation of L4.
7956
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7954 –7958

Angewandte
Chemie
analysis of the 11 successive helical portions delimited by the
12 parallel facial planes containing the coordinated donor
atoms (see Figure 3b and the Supporting Information) shows
the usual tightening of the helical pitch about the complexed
metal centers, and its relaxation within the intermetallic
domains (see the Supporting Information). The total length of
the helix amounts to 31 Š (3.1 nm according to modern
semantics) for 2.22 turns (Figure 3c), which translates into an
average helical pitch of 14.0 Š, comparable with 13.6 Š
previously reported for the trimetallic analogous complex
[Eu3(L3)3]9+.[14]
We thus conclude that the incorporation of an additional
central tridentate N3 segment into L3 to form L4 is not
deleterious for the extension of the helicate series and, in
agreement with theoretical thermodynamic predictions, the
tetrametallic complex [Eu4(L4)3]12+ indeed dominates the
speciation in solution for concentrations within the millimolar
range. Its isolation in the solid state eventually demonstrates
the formation of the expected linear triple-stranded helicate.
Interestingly, the considerable intermetallic separation in
these complexes limits destabilizing intramolecular interca-
tionic repulsions, thus allowing the preparation of stable
highly charged polymetallic cations. Although further work is
required to address the exact dependence of uMM and ceff on
the intersite separation, our novel synthetic strategy for the
stepwise extension of the ligand strand offers remarkable
perspectives for the design of higher homologues, a crucial
Figure 2. 1H NMR spectrum of the D3-symmetrical complex
[La4(L4)3]12+ in CD3CN (298 K, numbering given in Scheme 1).
y = 1.6; Ln = Eu: x = 5.0, y = 2.3; Ln = Lu: x = 9.6, y = 4.7; see
the Supporting Information). Their ESI-MS and 1H NMR
spectra obtained after redissolution in acetonitrile are iden-
tical with those obtained during the original titration
processes (except for an extra peak in the 1H NMR
spectrum corresponding to CH3OH), and slow recrystalliza-
tion from methanol/diethyl ether gives prisms of
[Eu4(L4)3](CF3SO3)12·5H2O·5CH3
OH (5) suitable for X-ray diffrac-
tion studies.[17] The crystal struc-
ture of
the expected
5
is composed of
tetrametallic
[Eu4(L4)3]12+ cations together
with 12 non-coordinated triflate
anions and 10 interstitial solvent
molecules. The pseudo-D3-sym-
metrical
triple-helical
cation
[Eu4(L4)3]12+ shows only minor
disorder that affects two periph-
eral ethyl groups, and its molecular
structure (Figure 3a) can be best
described as the packing of two
terminal {EuN6O3} and two central
{EuN9} pseudo tricapped trigonal
prismatic building blocks along the
approximate threefold axis defined
by the metal ions, as previously
reported for [Eu3(L3)3]9+.[14] The
À
À
Eu N and Eu O bond lengths are
standard, as are the bond angles
(see the Supporting Information).
The four europium atoms are
aligned along the pseudo threefold
axis (aEu1···Eu2···Eu3: 170.71(1),
aEu2···Eu3···Eu4: 172.03(1)8) and
regularly spaced by approximately
Figure 3. Perspective views of the cation [Eu4(L4)3]12+ perpendicular to the pseudo threefold axis in the crystal
structure of 5. a) View of the metallic environments (hydrogen atoms have been omitted for clarity);
b) schematic representations of the 12 facial planes containing the donor atoms; c) CPK view of the three ligand
9.3Š
(Eu1···Eu2:
9.312(1),
Eu2···Eu3: 9.054(1), Eu3···Eu4:
9.405(1) Š; Figure 3a). A detailed strands intertwined (represented with different colors).
Angew. Chem. Int. Ed. 2005, 44, 7954 –7958ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7957

Communications
[16] Crystal data for 3: C28H32N6O6, Mr = 548.7, T= 200 K, triclinic,
point for unraveling reliable long-range intermetallic param-
¯
eters (through uMM) and long-range preorganization effects
(through ceff).
P1, Z = 2, a = 8.1231(7), b = 11.7582(10), c = 15.4242(15) Š, a =
72.580(10), b = 77.868(10), g = 89.854(10)8, V= 1367.9(2) Š3;
16181 measured reflections, 5504 unique reflections (Rint
=
0.065), R = 0.039, wR = 0.038 for 364 variables and 2764
contributing reflections (j Fo j > 4s(Fo)). CCDC-282004 contains
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Received: August 26, 2005
Published online: November 17, 2005
Keywords: helical structures · lanthanides ·
.
polymetallic complexes · self-assembly
[17] Crystal data for 5: Eu4C278H288N54O52S12F36, Mr = 6894.2, T=
¯
200 K, triclinic, P1, Z = 2, a = 16.9598(9), b = 29.7098(17), c =
34.8898(15) Š, a = 83.449(6), b = 88.614(6), g = 88.695(7)8, V=
17457(2) Š3; 113988 measured reflections, 50309 unique reflec-
tions (Rint = 0.086), R = wR = 0.085 for 3719 variables and 23103
contributing reflections (j Fo j > 3s(Fo)). CCDC-282005 contains
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[1] a) C. Piguet, G. Bernardinelli, G. Hopfgartner, Chem. Rev. 1997,
97, 2005; b) M. Albrecht, Chem. Rev. 2001, 101, 3 457; c) M. J.
Hannon, L. J. Childs, Supramol. Chem. 2004, 16, 7.
[2] a) J.-M. Lehn, A. Rigault, Angew. Chem. 1988, 100, 1121; Angew.
Chem. Int. Ed. Engl. 1988, 27, 1095; b) K. T. Potts, M.
Keshavarz-K, F. S. Tham, K. A. Gheysen Raiford, C. Arana,
H. D. Abruæa, Inorg. Chem. 1993, 32, 5477; c) A. Marquis-
Rigault, A. Dupont-Gervais, A. Van Dorsselaer, J.-M. Lehn,
Chem. Eur. J. 1996, 2, 1395; d) C.-C. Wang, W.-C. Lo, C.-C. Chou,
G.-H. Lee, J.-M. Chen, S.-M. Peng, Inorg. Chem. 1998, 37, 4059;
e) R. Krämer, I. O. Fritsky, H. Pritzkow, L. A. Kovbasyuk, J.
Chem. Soc. Dalton Trans. 2002, 1307; f) C. J. Matthews, S. T.
Onions, G. Morata, L. J. Davis, S. L. Heath, D. J. Price, Angew.
Chem. 2003, 115, 3274; Angew. Chem. Int. Ed. 2003, 42, 3166.
[3] a) B. R. Serr, K. A. Andersen, C. M. Elliott, O. P. Anderson,
Inorg. Chem. 1988, 27, 4499; b) K. T. Potts, M. Keshavarz-K, F. S.
Tham, H. D. Abruæa, C. Arana, Inorg. Chem. 1993, 32, 4422;
c) K. T. Potts, M. Keshavarz-K, F. S. Tham, H. D. Abruæa, C.
Arana, Inorg. Chem. 1993, 32, 4436; d) K. T. Potts, M. Kesha-
varz-K, F. S. Tham, H. D. Abruæa, C. Arana, Inorg. Chem. 1993,
32, 4450.
[4] J.-C. G. Bünzli, C. Piguet, Chem. Rev. 2002, 102, 1897, and
references therein.
[5] a) R. ClØrac, F. A. Cotton, K. R. Dunbar, C. A. Murillo, I.
Pascual, X. Wang, Inorg. Chem. 1999, 38, 2655; b) J. K. Berra,
K. R. Dunbar, Angew. Chem. 2002, 114, 4633; Angew. Chem. Int.
Ed. 2002, 41, 4453.
[6] a) L. J. Prins, D. N. Reinhoudt, P. Timmerman, Angew. Chem.
2001, 113, 2446; Angew. Chem. Int. Ed. 2001, 40, 2382; b) A.
Mulder, J. Huskens, D. N. Reinhoudt, Org. Biomol. Chem. 2004,
2, 3409, and references therein.
[7] For recent reviews, see: a) D. L. Caulder, K. N. Raymond, Acc.
Chem. Res. 1999, 32, 975; b) S. Leininger, B. Olenyuk, P. J. Stang,
Chem. Rev. 2000, 100, 853; c) B. J. Holliday, C. A. Mirkin,
Angew. Chem. 2001, 113, 2076; Angew. Chem. Int. Ed. 2001, 40,
2022; d) M. Fujita, M. Tominaga, A. Hori, B. Therrien, Acc.
Chem. Res. 2005, 38, 371.
[8] a) T. M. Garrett, U. Koert, J.-M. Lehn, J. Phys. Org. Chem. 1992,
5, 529; b) A. Pfeil, J.-M. Lehn, J. Chem. Soc. Chem. Commun.
1992, 838.
[9] a) B. Perlmutter-Hayman, Acc. Chem. Res. 1986, 19, 90; b) M.
Borkovec, J. Hamacek, C. Piguet, Dalton Trans. 2004, 4096.
[10] For a recent review, see: C. Piguet, M. Borkovec, J. Hamacek, K.
Zeckert, Coord. Chem. Rev. 2005, 249, 705.
[11] G. Ercolani, J. Am. Chem. Soc. 2003, 125, 16097.
[12] a) J. Hamacek, M. Borkovec, C. Piguet, Chem. Eur. J. 2005, 11,
5217; b) J. Hamacek, M. Borkovec, C. Piguet, Chem. Eur. J. 2005,
11, 5227.
[13] fENuO = 5.9, fNEu = 5.6, log(uEuEu) = À2.4, log(uLL) = À0.9, and log
2
3
(ceff) = À0.9; taken from Table S2 in reference [12b].
[14] S. Floquet, N. Ouali, B. Bocquet, G. Bernardinelli, D. Imbert, J.-
C. G. Bünzli, G. Hopfgartner, C. Piguet, Chem. Eur. J. 2003, 9,
1860.
[15] M. Elhabiri, R. Scopelliti, J.-C. G. Bünzli, C. Piguet, J. Am.
Chem. Soc. 1999, 121, 10747.
7958
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7954 –7958