Isolation and characterization of the first circular single-stranded polymetallic lanthanide-containing helicate

Article abstract of DOI:10.1039/b501399b

A thorough examination of the disassembly of bimetallic triple-stranded lanthanide helicates [Ln₂(Li)₃]⁶⁺ (stoichiometry S = m/n = 2/3 = 0.67, global complexity GC =m + n = 2 + 3 = 5) in excess of metals shows the competit

Full text of DOI:10.1039/b501399b

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Isolation and characterization of the first circular single-stranded
polymetallic lanthanide-containing helicate{
Jean-Michel Senegas,a Sylvain Koeller,a Ge´rald Bernardinellib and Claude Piguet*a
Received (in Cambridge, UK) 27th January 2005, Accepted 23rd February 2005
First published as an Advance Article on the web 9th March 2005
DOI: 10.1039/b501399b
systematically exploring the structures and stabilities of the poly-
metallic lanthanide complexes obtained with bis-tridentate recep-
tors connected by simple semi-flexible secondary amide linkers.§
A thorough examination of the disassembly of bimetallic triple-
stranded lanthanide helicates [Ln2(Li)3]6+ (stoichiometry S 5
m/n 5 2/3 5 0.67, global complexity GC 5 m + n 5 2 + 3 5 5) in
excess of metals shows the competitive formation of standard
linear bimetallic complexes [Ln2(Li)2]6+ (S 5 1.0, GC 5 4), and
circular trimetallic single-stranded helicates [Ln3(Li)3]9+ (S 5 1.0,
GC 5 6).
Although the decisive influence of the stoichiometric ratio S 5 m/n
on the final structure adopted by metallosupramolecular com-
plexes [MmLn], and its eventual control, have been the subject of
continual interest during the past 15 years,1 the factors affecting
the global complexity GC{ 5 m + n remain elusive despite their
crucial consequences on the issue of the assembly processes.2 The
alternative formation of a bimetallic triple-stranded helicate
[Ti2(L1)3]42 (S 5 m/n 5 0.67, GC 5 m + n 5 5) versus a
tetrametallic tetrahedral cluster [Ti4(L1)6,NMe4]72 (S 5 0.67,
GC 5 10, Fig. 1a),3 or of a trimetallic triple-stranded helicate
[Fe3(L2)3]6+ (S 5 1.0, GC 5 6) versus a pentametallic circular
double-stranded helicate [Fe5(L2)5,Cl]9+ (S 5 1.0, GC 5 10,
Fig. 1b)4 illustrates this challenge, while the thermodynamic
preference for the complexes displaying the maximum global
complexity in these systems has been assigned to counter-ion
templating effects.3–5
Inspired by the cyclic structures found for DNA in some
organisms (i.e. plasmid DNA),2 polymetallic d-block complexes
have been designed for exploring the much simpler interconversion
between bimetallic double-stranded helicates [M2(Li)2]z+ (S 5 1.0,
GC 5 4) and circular single-stranded helicates [Mn(Li)n]z+ (S 5 1.0,
GC 5 2n, n . 2).6 The small internal cavities found in these two-
dimensional circular helicates are not compatible with templating
processes, and a plethora of debatable intramolecular interstrand
interactions have been thus invoked for rationalizing the ultimate
thermodynamic preference for the cyclic structures.6 In this
context, intensive efforts are currently focused on the modelling
of sophisticated self-assembly processes with a minimum set of
reliable thermodynamic arguments.7 The design of new segmental
ligands programmed for the selective formation of multicompo-
nent metallosupramolecular architectures is thus becoming a
crucial issue for the validations of these predictive theories,8
and the ligands L3–L6 (Scheme 1) have been synthesized for
{ Electronic supplementary information (ESI) available: experimental part
for the synthesis of L3–L6 and of their complexes. Tables of bond
distances and interplanar angles in 1. Figures reporting detailed numbering
schemes, molecular structures and NMR data. See http://www.rsc.org/
suppdata/cc/b5/b501399b/
Fig. 1 Influence of the global complexity (GC) on the final structures of
[MmLn] supramolecular complexes. a) Linear triple-stranded helicate vs.
tetrahedral cluster (adapted from ref. 3) and b) linear triple-stranded
helicate vs. circular double-stranded helicate (adapted from ref. 4).
*Claude.Piguet@chiam.unige.ch
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communication, we focus on the mixing of stoichiometric amounts
of L3 (1 equiv.) and Eu(CF3SO3)3?5H2O (1 equiv.) in acetonitrile/
chloroform, which provided 84% of microcrystalline
[Eu3(L3)3(CF3SO3)9](H2O)4 after slow diffusion of diethyl ether.§
Recrystallization from acetonitrile/di-isopropyl ether gave very
fragile X-ray quality prisms of [Eu3(L3)3(CF3SO3)4(CH3CN)2-
(H2O)3](CF3SO3)5(CH3CN)9(H2O)2 (1)." The associated crystal
structure confirms the existence of [Eu3(L3)3(CF3SO3)4-
(CH3CN)2(H2O)3]5+ cations with GC 5 6, together with partially
disordered ionic triflates and non-coordinated solvent molecules.
The three europium atoms lie at the corners of an approximate
˚
equilateral triangle (length # 12.0 A), whose sides are defined by
the three helically wrapped ligand strands (Fig. 3).
Scheme 1 Chemical structures of the ligands L3–L6.
Each europium atom is nine-coordinated in strongly distorted
mono-capped square antiprismatic sites, the six donor atoms of the
two bound tridentate segments of the ligands L3 forming the basal
rectangular face (carboxamide oxygen and pyridine nitrogen
atoms) and one half of the upper capped face (benzimidazole
nitrogen atoms). Two monodentate triflate anions around Eu2
and Eu3, and one triflate and one water molecule around Eu1
complete the upper rectangular faces, while one additional bound
solvent molecule (water for Eu1 and acetonitrile for Eu2 and Eu3)
eventually caps these faces (Fig. 3). Neglecting the replacement of
the acetonitrile molecule bound to Eu2 or Eu3, with the water
molecule in Eu1 leads to an approximate D3 symmetry for the
circular single-stranded cation [Eu3(L3)3(CF3SO3)4(CH3CN)2-
(H2O)3]5+ (Fig. 4). Interestingly, we do not detect significant
intramolecular interstrand interactions, or strong templating effect,
which could rationalize the formation of the circular helicate.
However, the cyclic arrangement of the three helically wrapped
strands produces two half bowl-shaped cavities located at the
center of the cation, which are capped by the ionic triflates j and
Except for L5, for which the planned triple-stranded helicate
[Ln2(L5)3]6+ is formed quantitatively for a ratio Ln : L5 5 0.67, we
systematically detect intricate mixtures of slowly interconverting
1
complexes by using H NMR in acetonitrile/chloroform (1 : 1).
Parallel ESI-MS titrations recorded under the same conditions
(solvent and concentration) for Ln : Li (i 5 3–6) in the range 0.5–
1.0 strongly suggest the co-existence of three complexes,
[Ln2(Li)3]6+ (S 5 0.67, GC 5 5), [Ln2(Li)2]6+ (S 5 1.0, GC 5 4),
and [Ln3(Li)3]9+ (S 5 1.0, GC 5 6, Fig. 2). Although solid-state
and solution structures for the first two complexes, respectively
bimetallic triple-stranded helicates9 and bimetallic side-by-side
solvated complexes,10 have been firmly established for closely
related analogues, we are not aware of any structural information
reported for the latter complexes [Ln3(Li)3]9+.11
In order to maximize the formation of the oligomer with the
highest global complexity in solution, we have used an exact ratio
Ln : Li 5 1 combined with a high concentration of the
components. Crystallisation of the most charged cations is then
induced thanks to a stepwise decrease of the dielectric constant
resulting from the slow diffusion of volatile non-polar ether. In this
l
(Fig. S1, ESI).{ Only three weak intermolecular
Fig. 3 Perspective view along the pseudo-threefold axis, and partial
numbering scheme for the cation [Eu3(L3)3(CF3SO3)4(CH3CN)2(H2O)3]5+
…
(Eu1 Eu2
…
˚
11.9977(7) A,
˚
12.3217(8) A;
in the crystal structure of
…
Otf 5 CF3SO32).
1
5
5
˚
A
Fig. 2 ESI-MS spectrum recorded for a stoichiometric 1 : 1 mixture of
2
Eu2 Eu3
5
11.7933(8)
and Eu1 Eu3
L3 and Eu(CF3SO3)3?5H2O in CD3CN/CDCl3 (1 : 1). Otf 5 CF3SO3
.
2236 | Chem. Commun., 2005, 2235–2237
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Notes and references
{ The global complexity of a system strictly refers to the multiplying factor,
which correlates the final oligomer with its basic monomeric building block.
We thus exclude extra templating agents, specifically included within the
oligomeric form and absent in the monomer, for calculating GC.
§ The syntheses and characterizations of the ligands L3–L6 and of the
complex [Eu3(L3)3(CF3SO3)9](H2O)4 are given in the ESI.
" Crystal data: [Eu3(C40H38N8O2)3(CF3SO3)4(CH3CN)2(H2O)3]
¯
(CF3SO3)5(CH3CN)9(H2O)2, fw 5 4327.0, T 5 200 K, monoclinic, P1,
˚
Z 5 2, a 5 14.7345(11), b 5 21.8136(12), c 5 31.790(2) A, a 5 100.262(7),
3
˚
b 5 100.850(8), c 5 103.813(8)u, V 5 9475(1) A . 100621 measured
reflections, 34501 unique reflections (Rint 5 0.060), R 5 0.063, vR 5 0.065
for 2228 variables and 19896 contributing reflections (|Fo| . 4 s(Fo)).
Hydrogen atoms of the disordered water molecules O5w and O6w (with
population parameters of 0.50) were not observed nor calculated. CCDC
261626. See http://www.rsc.org/suppdata/cc/b5/b501399b/ for crystallo-
graphic data in CIF or other electronic format.
1 For reviews, see C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem.
Rev., 1997, 97, 2005; D. L. Caulder and K. N. Raymond, Acc. Chem.
Res., 1999, 32, 975; S. Leininger, B. Olenyuk and P. J. Stang,
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N. Fujita, T. Kusukawa and K. Biradha, Chem. Commun., 2001, 509;
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2 C. Piguet, J. Inclusion Phenom. Macrocycl. Chem., 1999, 34, 361;
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3 M. Scherer, D. L. Caulder, D. W. Johnson and K. N. Raymond,
Angew. Chem., Int. Ed., 1999, 38, 1588.
Fig. 4 View of the circular single-stranded cation [Eu3(L3)3(CF3SO3)4-
(CH3CN)2(H2O)3]5+ with the three helically wrapped strands represented
with CPK spheres of different colors.
(pyridine)CH??O(triflate) interactions are detected, but we cannot
definitively rule out their possible contribution to the stabilization
of the final pseudo-D3-symmetrical complex.
4 B. Hasenknopf, J.-M. Lehn, N. Boumediene, E. Leize and
A. vanDorsselaer, Angew. Chem., Int. Ed., 1998, 37, 3265.
Dissolution of 1 in CD3CN provides a clean paramagnetically-
shifted 1H NMR spectrum diagnostic for the existence of a single
highly symmetrical species in solution on the NMR timescale,
which is compatible with the solvated D3-symmetrical cation
[Eu3(L3)3]9+ (13 signals are observed for the protons, together with
diastereomeric methylenes, Fig. S2a, ESI).{ However, slow
thermodynamic re-equilibration leads to complicated mixtures of
[Eu3(L3)3]9+ and [Eu2(L3)2]6+ after a few hours (Fig. S2b, ESI).{
In conclusion, the assembly of the bimetallic triple-stranded
helicates [Ln2(Li)3]6+ (i 5 3–6, S 5 0.67) competes with the
formation of complexes with 1 : 1 stoichiometry and variable
global complexities [Lnn(Li)n]3n+ (n 5 2, GC 5 4 and n 5 3,
GC 5 6). The use of the exact stoichiometry S 5 1 combined with
high concentration favors the formation of the oligomer displaying
the maximum global complexity. Crystallisation of the resulting
mixture allows the isolation of the first circular single-stranded
lanthanide helicate. Particular attention is currently focused on the
5 M. Albrecht, I. Janser, J. Runsink, G. Raabe, P. Weis and R. Fro¨hlich,
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J. Hamacek and C. Piguet, Dalton Trans., 2004, 4096; A. Mulder,
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8 S. Floquet, M. Borkovec, G. Bernardinelli, A. Pinto, L.-A. Leuthold,
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J., 2004, 10, 1091; K. Zeckert, J. Hamacek, J.-P. Rivera, S. Floquet,
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factors controlling the interconversion process 3[Ln2(Li)2]6+
=
2[Ln3(Li)3]9+ in solution, because no obvious templating effects or
intramolecular interactions have been evidenced in the cyclic
complex.
10 N. Martin, J.-C. G. Bu¨nzli, V. McKee, C. Piguet and G. Hopfgartner,
Inorg. Chem., 1998, 37, 577.
11 Three circular polymetallic lanthanide clusters have been reported: J. Xu
and K. N. Raymond, Angew. Chem., Int. Ed., 2000, 39, 2745;
Y. Bretonnie`re, M. Mazzanti, J. Pe´caut and M. M. Olmstead, J. Am.
Chem. Soc., 2002, 124, 9012; H. Hou, Y. Wei, Y. Song, Y. Fan and
Y. Zhu, Inorg. Chem., 2004, 43, 1323. None of them matches the
structural criteria requested for a circular helicate, i.e. a discrete
supramolecular complex constituted by one or more covalent organic
strands wrapped about, and coordinated to a series of ions defining a
Jean-Michel Senegas,a Sylvain Koeller,a Ge´rald Bernardinellib and
Claude Piguet*a
aDepartment of Inorganic Chemistry, University of Geneva, 30 quai E.
Ansermet, CH-1211 Geneva 4, Switzerland.
E-mail: Claude.Piguet@chiam.unige.ch; Fax: +4122 379 6830;
Tel: +4122 379 6034
bLaboratory of X-ray Crystallography, University of Geneva, 24 quai E.
Ansermet, CH-1211 Geneva 4, Switzerland
two-dimensional regular polygon2–6
.
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Chem. Commun., 2005, 2235–2237 | 2237