Allosteric effects in binuclear homo- and heterometallic triple-stranded lanthanide podates

Article abstract of DOI:10.1021/ic301631n

This work illustrates a simple approach for deciphering and exploiting the various free energy contributions to the global complexation process leading to the binuclear triple-stranded podates [Ln₂(L9)]⁶⁺ (Ln is a trivalent lanthanide). Despite the larger microscopic affinities exhibited by the binding sites for small Ln³⁺, the stability constants measured for [Ln₂(L9)]⁶⁺ decrease along the lanthanide series; a phenomenon which can be ascribed to the severe enthalpic penalty accompanying the intramolecular cyclization around small Ln(III), combined with increasing anticooperative allosteric interligand interactions. Altogether, the microscopic thermodynamic characteristics predict β_1,1,1 ^La,Lu,L9/β_1,1,1^Lu,La,L9 = 145 for the ratio of the formation constants of the target heterobimetallic [LaLu(L9)] ⁶⁺ and [LuLa(L9)]⁶⁺ microspecies, a value in line with the quantitative preparation (>90%) of [LaLu(L9)]⁶⁺ at millimolar concentrations. Preliminary NMR titrations indeed confirm the rare thermodynamic programming of a pure heterometallic f-f′ complex.

Full text of DOI:10.1021/ic301631n

Article
pubs.acs.org/IC
Allosteric Effects in Binuclear Homo- and Heterometallic Triple-
Stranded Lanthanide Podates
Patrick E. Ryan,§ Gabriel Canard,*,§,⊥ Sylvain Koeller,§ Bernard Bocquet,§ and Claude Piguet*,§
§Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4,
Switzerland
⊥Faculty of Sciences de Marseille Saint Jer
ome, Avenue escadrille Normandie-Niemen, F-13397 Marseille Cedex 20, France
́ ̂
S
* Supporting Information
ABSTRACT: This work illustrates a simple approach for deciphering and exploiting the
various free energy contributions to the global complexation process leading to the
binuclear triple-stranded podates [Ln2(L9)]6+ (Ln is a trivalent lanthanide). Despite the
larger microscopic affinities exhibited by the binding sites for small Ln3+, the stability
constants measured for [Ln2(L9)]6+ decrease along the lanthanide series; a
phenomenon which can be ascribed to the severe enthalpic penalty accompanying
the intramolecular cyclization around small Ln(III), combined with increasing
La,Lu,L9
1,1,1
anticooperative allosteric interligand interactions. Altogether, the microscopic thermodynamic characteristics predict β
/
Lu,La,L9
β
= 145 for the ratio of the formation constants of the target heterobimetallic [LaLu(L9)]6+ and [LuLa(L9)]6+
1,1,1
microspecies, a value in line with the quantitative preparation (>90%) of [LaLu(L9)]6+ at millimolar concentrations. Preliminary
NMR titrations indeed confirm the rare thermodynamic programming of a pure heterometallic f-f′ complex.
rings in L7, instead of more demanding modifications of pyridine
rings in L5.10 Consequently, the tripodal ligand L8 represents a
rare case, in which three bent terdentate polyaromatic N-
heterocyclic binding units are coordinated to a single trivalent
lanthanide to give stable mononuclear [Ln(L8)]3+ podates.11
Despite the promising selectivity brought by the variable chelate
effect operating along the lanthanide series for this neutral N9
ligand,11 no attempt for exploiting this strategy in the design of
binuclear heterometallic podates has been considered, while
remarkable reports describe the systematic use of NNO or NOO
binding units for the complexation of lanthanide cations in
related mononuclear nine-coordinate podates.12 In this con-
tribution, we report on (i) the synthesis of an unprecedented (to
the best of our knowledge) compartmental bis-nine-coordinate
podate L9 and (ii) its reactivity with Ln(III) along the lanthanide
series with the ultimate goal of producing pure heterobimetallic
4f-4f′ microspecies under thermodynamic control.
INTRODUCTION
■
The stereochemical preference of numerous divalent and
trivalent d-block cations Mz+ (z = 2, 3) for octahedral geometry,
combined with the considerable chelate effect brought by rigid
bidentate 2,2′-bipyridine (L1)1 result in the systematic formation
of stable six-coordinated D3-helical [M(L1)3]z+ complexes with
predetermined structures (Scheme 1).2 Improved structural
control and stabilities are achieved by the connection of the
bidentate binding units to a covalent tripod in L2,3 a strategy
which has been further exploited for the selective complexation
of different cations in the C3-helical [FeHg(L3)]4+ complex,4 and
for the programming of redox-induced intramolecular metal
translocation in [Fe(L4)]2/3+.5 A strict geometrical analogy can
be drawn for trivalent 4f-cations, Ln3+, whose preference for
nine-coordination is satisfied by the complexation of three rigid
terdentate 2,2′;6′,2″-terpyridine ligands in the D3-helical [Ln-
(L5)3]3+ complexes.6 Because of the bent meridional tercoordi-
nation of L5 to Ln(III), the connection of three such terpyridine
units to a covalent tripod in L6 induces severe geometrical
limitations for the preparation of the putative mononuclear nine-
coordinated podates [Ln(L6)]3+, a challenge which remains
unsolved despite the exploration of the complexation properties
of L6 with Ln(III) = La, Pr, Eu, and Lu.7
In this context, the extended terdentate aromatic ligand L7
provides three crucial advantages over L5 for coordinating
lanthanides: (1) the stacked benzimidazole side arms control the
size of the metallic cavity in the target triple-helical nine-
coordinate complexes [Ln(L7)3]3+,8 (2) the associated selectiv-
ity can be tuned by a judicious choice of the substituents bound
to the noncoordinating nitrogen atoms of the benzimidazole
rings,9 and (3) the connection with a covalent tripod requires
straightforward synthetic transformations of terminal phenyl
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Podand L9 and
Its Complexes [Ln2(L9)](CF3SO3)6 (Ln = La, Lu). The ligand
L9 is prepared in 30 synthetic steps from the commercially
available compounds 1, 3, 7, and 17 according to a strategy
previously developed for L8 (Scheme 2).11 Two successive
amidation reactions produce the unsymmetrical bis-o-nitro-
areneamide key intermediate 11, which undergoes a reducing
cyclization to give 12. Four successive tedious, but efficient
reactions introduce a terminal nucleophilic thiol group in 16,
which is deprotonated and coupled under anaerobic conditions
Received: July 25, 2012
© XXXX American Chemical Society
A
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Scheme 1. Chemical Structures of the Ligands L1−L9 with Numbering Scheme for 1H NMR Studies
with the electrophilic trichloride 2113 to afford L9 in acceptable
yield.
Reaction of L9 (1 equiv.) with Ln(CF3SO3)3·xH2O (Ln = La,
Lu; x = 1−3; 2 equiv.) in acetonitrile followed by slow diffusion
of diethylether gave [La2(L9)](CF3SO3)6·16H2O and
[Lu2(L9)](CF3SO3)6·12H2O in fair yield (40−70%). The
The 1H NMR spectrum of L9 displays 30 signals (Supporting
Information, Table S1) in agreement with the existence of a 3-
fold axis, completed with three vertical symmetry planes (i.e., C3v
point group) responsible for the detection of enantiotopic
protons for the methylene groups of each strand (Figure 1a).
1
associated H NMR spectra still display 30 signals in line with
the regular wrapping of the three strands around the two metals
defining the 3-fold axis, but the diastereotopic methylene protons
(AB spin systems) point to the loss of the symmetry planes
B
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Scheme 2. Synthesis of the Ligand L9
induced by the helical arrangement of the ligand strands (i.e., C3
point group, Figure 1b,c). The complexation process affects all
1H NMR chemical shifts, with a special emphasis on the
considerable upfield shifts displayed by the singlets assigned to
the protons H5 (Δδ = −1.54 to −1.75 ppm), H13 (Δδ = −1.76
to −2.28 ppm) and H15 (Δδ = −1.71 to −2.23 ppm, Figure 1b,c
and Supporting Information, Table S1). This behavior is
diagnostic for the formation of a triple-helical arrangement of
the bound ribbons in the binuclear complexes, which puts these
protons in the shielding region of the benzimidazole rings of the
adjacent strands14 as previously reported for related protons in
the mononuclear diamagnetic triple-helical podates [Ln(L8)]3+
(Δδ = −0.83 to −1.46 ppm, Ln = La, Y, Lu).11
C
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Figure 1. Aromatic parts in the 1H NMR spectra of (a) the ligand L9 (CDCl3) and its complexes (b) [La2(L9)]6+ and (c) [Lu2(L9)]6+ (CD3CN, 298 K).
Figure 2. ESI-MS spectrum of [La2(L9)]6+ in CH3CN/CH2Cl2 (9:1) at 298 K. The series of peaks marked with an asterisk, *, corresponds to
[La2(L9)(HCOO)(CF3SO3)n](5−n)+ (n = 1−3), which result from a minor contamination of the sprayer with formic acid.
Electrospray ionization mass spectrometry (ESI-MS) spectra
recorded in acetonitrile/dichloromethane for [Ln2(L9)]-
(CF3SO3)6 (Ln = La, Eu, Lu) unambiguously confirm the
existence of the binuclear complexes in solution with the
detection of [Ln2(L9)]6+ together with its adducts [Ln2(L9)-
(CF3SO3)n](6−n)+ (Figure 2). Upon stepwise titrations of L9 (0.2
mM) with Ln(CF3SO3)3·xH2O, additional ESI-MS signals can
be assigned to the formation of the mononuclear precursors
D
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Figure 3. (a) Variation of absorption spectra and (b) corresponding variation of the molar extinctions at five different wavelengths observed for the
spectrophotometric titration of L9 (7·10−5 M) with La(CF3SO3)3·H2O (acetonitrile/dichloromethane (9:1) + 10−2 M NBu4ClO4, 298 K).
[Ln(L9)]3+ in default of metal. The intensities of the latter
Table 1. Cumulative Thermodynamic Formation Constants
log(βLmn,1,L9), Associated Intermolecular Microscopic
signals culminate around |Ln|tot/|L9|tot ≈ 1, followed by their
decrease and the growing up of the peaks produced by
[Ln2(L9)]6+ (Supporting Information, Tables S2−S4). No
other significant cationic species could be detected and the
complexation process can be summarized with the macroscopic
equilibria 1 and 2
a
b
Affinities, Intramolecular Microscopic Affinities, ,
c
d
Intermetallic Interactions, and Interligand Interactions
Obtained for [Ln2(L9)]6+ (Ln = La, Eu, Lu; CH3CN/CH2Cl2
(9:1) + 10−2 M NBu4ClO4, 298 K)
metal
Ln,L9
Ln3+ + L9 ⇌ [Ln(L9)]3+
2Ln3+ + L9 ⇌ [Ln2(L9)]6+
β
1,1
La
Eu
Lu
(1)
Ln,L9
log(β
log(β
)
)
8.89(4)
15.47(5)
6.9(2)
−39.1(9)
3.7(2)
−21(1)
−3.8(5)
−18(3)
4(3)
6.41(9)
12.80(8)
7.1(1)
5.57(1)
10.82(1)
9.63(2)
−54.9(1)
9.05(2)
−51.6(1)
−8.0(4)
−9(2)
−3(2)
−6(2)
−2.3(2)
13(1)
1,1
Ln,L9
2,1
log(f LNn3))
Ln,L9
β
(2)
2,1
Ln,N3
ΔG
log(f
/kJ·mol−1
−40.5(4)
8.0(1)
inter
Ln
)
Thermodynamic Behavior of the Complexes [Ln2(L9)]-
(CF3SO3)6 (Ln = La, Eu, Lu). The thermodynamic stability
macroconstants βLmn,1,L9 (eqs 1−2) were obtained by spectropho-
tometric titrations of L9 with [Ln(CF3SO3)3]·xH2O (Ln = La,
Eu, Lu) because the trans-trans → cis-cis conformational change
of the benzimidazole-pyridine scaffolds accompanying the
complexation process induces some significant differences in
the electronic structure,15 which are easily monitored in the UV
part of the absorption spectra (Figure 3a).16
N2O
Ln,N2O
inter
ΔG
/kJ·mol−1
−45.6(4)
−6.6(2)
−3(1)
−5(1)
−8(1)
Ln
prox
log(EM
)
ΔGLinntr,Na,p3rox/kJ·mol−1
ΔGLinntr,Na,d2iOst/kJ·mol−1
ΔGLinntr,Na,d2iOst‑n/kJ·mol−1
1(3)
N3,N3
Ln
log(u
)
−1.7(3)
10(2)
−0.9(1)
5(1)
N3,N3
ΔE
/kJ·mol−1
Ln
N2O,N2O
Ln
N2O,N2O
log(u
)
0.8(3)
−4(2)
0(2)
−1.3(1)
8(1)
−1.7(2)
10(1)
ΔE
/kJ·mol−1
The lack of isosbestic points suggests the existence of at least
three absorbing species in solution, whereas the two smooth end
points observed for |Ln|tot/|L9|tot = 1.0 and 2.0 (Figure 3b)
confirm the operation of equilibria 1 and 2. The spectrophoto-
metric data were satisfyingly fitted by using nonlinear least-
squares techniques17 to give the cumulative formation constants
collected in Table 1 (entries 1−2).
Ln
log(uLn,Ln
)
−0.9(8)
5(4)
−3(1)
16(8)
ΔELn,Ln/kJ·mol−1
−2(10)
a
b
Ln,N3
Ln
Ln,N2O
Ln
N2O
Ln,N3
ΔG
= −RT ln(f ) and ΔG
= −RT ln(f ). ΔG
=
intra,prox
inter
N3
inter
Ln,N2O
Ln Ln
−RT ln (EMprox N3
f
Ln Ln ), ΔG
= −RT ln (0.29EM
f
), and
intra,dist
prox N2O
c
Ln,N2O
intra,dist‑n
N3,N3
Ln Ln
ΔG
= −RT ln (0.69EM
f
Ln
). ΔELn,Ln = −RT ln(uLn,Ln).
prox N2O
N2O,N2O
d
N3,N3
Ln
N2O,N2O
).
LN
ΔE
= −RT ln(u
and ΔE
= −RT ln (u
Ln
One immediately notices the operation of an antielectrostatic
Ln,L9
Ln,L9/RT)
= e−(ΔG
> βLmu,1,L9) leading to a considerable
La,L9
m,1
Eu,L9
m,1
2,1
trend (i.e., β
> β
β
2,1
destabilization of the complexes [Ln(L9)]3+ and [Ln2(L9)]6+ for
the smaller lanthanide cations, the origin of which can be
approached by using the site-binding model.18 Using the van’t
Hoff equation with a standard concentration for the reference
chiral
2,1
Ln
2
= ω
ω2L,1n,L9(fNLn3 )3(fLn )3(EMpLrnox)2(EM
(e−ΔE /RT)3·(e−ΔE
)
dist‐n
Ln,Ln/RT
N2O
N3,N3
N2O,N2O
/RT)3(e−ΔE
)
Ln
Ln
state of cθ = 1 M,19 the thermodynamic formation macro-
(4)
Ln,L9
m,1
constants β
(eqs 3 and 4) can be written as the weighted
products of a limited set of “easily” interpretable thermodynamic
Ln,L9
stat
Ln,L9
The first contributions ΔG
= −RT ln(ωmch,niral·ω
)
m,n
correspond to the changes in rotational entropy occurring
when the reactants are transformed into products. It can be
describers.18
computed by using the symmetry numbers (σext, σint, and σchiral
)
Ln,L9
Ln,L9/RT)
1,1
β
= e−(ΔG
= ω chiral ω Ln,L9
controlling the statistical factors of the assembly ωcmh,niral·ωLmn,n,L9, as
soon as the point groups of the various partners are at hand.20 As
a working example, let us begin with the first part of eq 3 which
models the microconstant for the formation of the proximal-
[Ln(L9)]3+ microspecies, where the Ln3+ cation occupies the
internal nine-coordinate cavity produced by the three wrapped
1,1
2
N3,N3/RT 3
1,1,N3 1,1,N3(fLn )3(EMpLrnox) (e−ΔE
)
Ln
N3
ω1L,1n,,NL92O(fLn )3(EM
)
chiral
Ln
dist
2
+ ω
1,1,N2O
N2O
N2O,N2O/RT 3
(e−ΔE
)
Ln
(3)
E
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terdentate N3 binding units (Figure 4a). In this case
Scheme 3. Chemical Structures of the Ligands L10−L12
chiral
Ln,L9
ω
·ω
=12 (Supporting Information, Figure S1a),
1,1,N3
1,1,N3
Figure 4. Schematic structural representations of the microspecies
contributing to (a) [Ln(L9)]3+ and (b) [Ln2(L9)]6+ with pertinent
distances taken from the crystal structures of [Eu(L8)]3+ 11 and HHH-
[LaTb(L10)3]6+.24.
Ln
whereas the second term ΔGLinnte,Nr 3 = −RT∑i3= 1(f ) represents
N3
the sum of the free energies of metal-N3 binding unit
connections, which includes the desolvation/solvation processes
accompanying the complexation process.21 However, among the
three Ln-N3 binding connections occurring in proximal-[Ln-
(L9)]3+, only one is intermolecular and corresponds to the
association of two partners freely moving and statistically
dispersed in the condensed phase. The two remaining Ln-N3
coordination events are intramolecular, and the associated free
−3/2
LN3,N3
)
= (2 × 9/14)−3/2 = 0.69 (Figure 4c). Introducing
these ratios together with the statistical factors (computed in
Supporting Information, Figure S1) into eqs 3−4 eventually
yields eqs 5 and 6, in which only six microscopic thermodynamic
describers are involved.
Ln,N3
Ln Ln
Ln,L9
N3,N3 3
energy change ΔG
= −RT ln(EM
f ) additionally
intra
prox N3
β
= 12(EMpLrnox)2[(fLn )3(u
) + (fNLn2O)3(0.29)2
Ln
1,1
N3
depends on the effective molarities (EMLprnox) of the interacting
partners imposed by the reduced motion of the ligand strands.22
N2O,N2O 3
(u
) ]
(5)
Ln
N3,N3
Ln
N3,N3
Finally, ΔE
= −RT ln(u
) corresponds to the
Ln
Ln,L9
N3,N3 3
homocomponent interaction produced by the close location of
two N3 binding units connected to the same metal. This
approach can be repeated for the alternative distal-[Ln(L9)]3+
microspecies (second part of eq 3), where Ln3+ occupies the
remote nine-coordinate cavity produced by the three terminal
terdentate N2O binding units (Figure 4b). Since the binuclear
complex [Ln2(L9)]6+ exists as a single microspecies (Figure 4c),
β
= 72(fNLn3 )3(fLn )3(EMpLrnox)4(0.69)2(u
)
Ln
2,1
N2O
N2O,N2O 3
(u
) (uLn,Ln
)
(6)
Ln
Additional thermodynamic information is gained from the
spectrophotometric titrations of the ligands L7 and L11
(equilibria 7, n = 1−3), each mirroring one specific terdentate
binding site (N3 and N2O, respectively) found in L9, and of the
podands L8 and L12 (equilibrium 8), each mirroring one specific
nine-coordinate cavity found in L9 (N9 and N6O3, respectively),
with [Ln(CF3SO3)3]·xH2O (Ln = La, Eu, Lu) in the same
experimental conditions (Supporting Information, Table S5).
its thermodynamic modeling is much simpler with ΔELn,Ln
=
−RT ln(uLn,Ln) standing for the two metals bound in the adjacent
cavities (eq 4). If we roughly assume that the chains of atoms
constituting the chelate rings in [Ln2(L9)]6+ behave as freely
joint chains, Kuhn’s theory23 predicts that EM ∼ d−3/2, where d is
the distance separating the two connecting points involved in the
intramolecular process.
Ln3+ + nLk ⇌ [Ln(Lk)n]3+
Ln3+ + Lk ⇌ [Ln(Lk)]3+
β
Ln,Lk
(7)
1,n
Ln,Lk
Taking simplified representations for the various
β
(8)
1,n
[Lnm(L9)]3m+ complexes (Figure 4) with pertinent distances
11
measured in the crystal structures of [Eu(L8)]3+ and HHH-
Each thermodynamic formation constant is modeled with the
site binding model (Supporting Information, Figures S2−S4),
and the resulting set of 10 stability constants (eqs 5, 6, and
Supporting Information, eqs S1−S8) can be satisfyingly fitted by
using nonlinear least-squares techniques (Agreement Factors25
AFLn ≤ 0.075, Supporting Information, Table S5, and Figure S5)
to give the microscopic thermodynamic describers collected in
Table 1.
[LaTb(L10)3]6+ (see Scheme 3 for the chemical structure of this
ligand),24 we calculate LN3,N3 = 2 × 7 = 14 Å for the approximate
end-to-end chain length of the chelate ring controlling EMLprnox in
proximal-[Ln(L9)]3+ (Figure 4a), and LN2O,N2O = (2 × 7 + 2 × 9)
= 32 Å for EMLdinst in distal-[Ln(L9)]3+ (Figure 4b). Kuhn’s theory
then yields EMdLinst/EM = (LN2O,N2O/LN3,N3
)
= 0.29, and
Ln
−3/2
prox
this approach can be repeated for the binuclear podate
Ln
dist‑n
[Ln2(L9)]6+ where the missing effective molarity EM
The intermolecular microscopic affinities of the binding sites
for the entering metal provide the major driving forces to the
complexation process leading to [Ln2(L9)]6+ (Table 1, entries
characterizing the intramolecular ring closure around Ln2 in
Ln
prox
presence of Ln1 is given by EMLdinst‑n/EM
= (LNn 2O,N2O
/
F
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3−6 and Figure 5a). In line with the well-known electrostatic
effect, the free energy of intermolecular connection increases
two possible microspecies for the La/Lu pair (Supporting
Information, Figure S6) where La(III) occupies either the
proximal N9 coordination site (eq 9, [LaLu(L9)]6+ in Figure 6
Figure 6. Predicted (ΔEmix = 2ΔELu,La − (ΔELa,La + ΔELu,Lu) = 0)31
ligand distributions in the microspecies [La2−xLux(L9)]6+ during the
titration of [La2(L9)]6+ with [Lu2(L9)]6+ in absence of ligand
dissociation (lutetium mole fractions xLu = |Lu|tot/(|La|tot + |Lu|tot) =
0−1).
top) or the distal N6O3 coordination site (eq 10, [LuLa(L9)]6+ in
Figure 6 bottom).
La,Lu,L9
β
= 72(fNLa3)3(fLu )3(EMpLraox)2(EMpLruox)2(0.69)2
1,1,1
N2O
N3,N3 3 N2O,N2O 3
) (uLa,Lu
)
Figure 5. Representation of the thermodynamic contributions in kJ/mol
corresponding to (a) intermolecular and (b) intramolecular Ln-ligand
connections, and (c) interligand and (d) intermetallic interactions
responsible for the global complexation process leading to [Ln2(L9)]6+
(Ln = La, Eu, Lu; CH3CN/CH2Cl2 (9:1) + 10−2 M NBu4ClO4, 298 K).
(u
) (uLu
(9)
La
Lu,La,L9
β
= 72(fNLu3 )3(fLa )3(EMpLraox)2(EMpLruox)2(0.69)2
1,1,1
N2O
N3,N3 3 N2O,N2O 3
(u
) (uLa
) (uLu,La
)
(10)
Lu
along the lanthanide series (Figure 5a).26 Interestingly, the
effective molarities, EMLprnox, display the reverse order (Table 1,
entry 7), which results in a compensation effect and the
observation of intramolecular connection energies of similar
magnitude along the series (Table 1, entries 8−10 and Figure
5b). Reasonably assuming similar entropic contributions to the
effective molarities for Ln = La, Eu, and Lu,27 the drastic decrease
of EMLprnox can assigned to increasing enthalpic constraints within
the chelate rings induced by their coordination to smaller cations.
This effect is strengthened by concomitant anticooperative
interstrand and intermetallic interactions, which increase along
the lanthanide series (Table 1, entries 11−16 and Figure 5c,d).
The latter homocomponent interactions are responsible for the
unusual selectivity of this ligand for the smaller lanthanides
(Figure 5e).
All requested microscopic thermodynamic describers can be
found in Table 1, except for the heterometallic interactions uLa,Lu
and uLu,La. Following the concept of the mixing rule depicted in
eq 11,31 and which has been experimentally justified for closely
related bimetallic La/Lu helicates,32 we calculate uLa,Lu = uLu,La
=
Lu,La/RT)
1/2
e−(ΔE
= (uLa,LauLu,Lu
)
= 0.059 for [LaLu(L9)]6+ and
[LuLa(L9)]6+.
1
2
ΔELa,Lu = ΔELu,La
=
(ΔELa,La + ΔELu,Lu
)
(11)
Introducing the pertinent values of each describer into eqs 9 and
10 yields log(β1L,a1,,L1u,L9) = 14.36 and log(βL1,u1,,L1a,L9) = 12.20, from
which the speciation in solution in absence of ligand dissociation
can be foreseen (Figure 6).
Lu,La,L9
1,1,1
The ratio of the microsconstants βL1,a1,,L1u,L9/β
= 145
Thermodynamic Predictions and Selective Detection
of the Bimetallic [LaLu(L9)]6+ Microspecies. In line with
previous investigations,9b,28 the thermodynamic constants
collected in the Supporting Information, Table S5, for equilibria
(eq 7) show a steeper electrostatic trend along the lanthanide
series for the heterotopic N2O site in L11, as compared with the
N3 binding unit in L7; an observation at the origin of the design
of ligand L10 for the selective complexation of large Ln(III) in
the N9 coordination site and small Ln(III) in the N6O3
coordination site in the C3-symmetrical complexes HHH-
[Ln1Ln2(L10)3]6+.24,29 However, satisfying thermodynamic
modeling and predictions failed because of the HHH ↔HHT
isomerization processes operating in these binuclear helicates,30
a limitation which is overcome in the non-isomerizable podates
[Ln1Ln2(L9)]6+. Equations 9−10 thus predict the stability of the
indicates that the preferred microspecies with La(III) in the
proximal site always counts for more than 99% of the ligand
distribution in the heterometallic complexes, thus leading to high
selectivity for the formation of a single bimetallic f-f′ micro-
species. For a 10 mM total ligand concentration, the dissociation
of [Ln2(L9)]6+ is negligible (Ln = La, Lu; Supporting
Information, Figure S7) and the titration of [La2(L9)]6+ with
[Lu2(L9)]6+ indeed leads to the formation of the bimetallic
microspecies [LaLu(L9)]6+ as the single novel complex, which is
characterized by three 1H NMR signals with identical intensities
assigned to the diagnostic protons H5, H13, and H15 by using a
combination of scalar (COSY) and dipolar (NOESY) H−H
correlations (Figure 7).
The two extreme chemical shifts observed for H5 (low-field)
and H13 (high-field) in [LaLu(L9)]6+ (Figure 7b) are
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1
Figure 7. Aromatic parts of the H NMR spectra showing the signals for the protons H5, H13, and H15 during the titration of [La2(L9)]6+ with
[Lu2(L9)]6+ for (a) xLu = 0, (b) xLu = 0.35, and (c) xLu = 1.0 (CD3CN, 298 K total ligand concentration 10 mM).
reminiscent from those observed in [La2(L9)]6+ (H5, Figure 7a)
and in [Lu2(L9)]6+ (H13, Figure 7c), which rules out the
alternative assignment to [LuLa(L9)]6+. The same trend is
systematically reproduced for the signals of the aromatic protons
in [LaLu(L9)]6+. The absence of significant signals for the
alternative [LuLa(L9)]6+ microspecies suggests that the
application of the mixing rule for estimating ΔELa,Lu is an
acceptable approximation, as previously reported for related
heterometallic f-f′ helicates.32
each site (proximal and distal) prevents further exploitation of
these results for thermodynamic programming and predictions.
L9 thus appears to be the first multistranded receptor, for which
the various free energy contributions to the global complexation
procceses have been deciphered (Figure 5). As expected, the
driving force along the lanthanide series relies on the favorable
contribution of the binding events (both inter- and intra-
molecular), but the selectivity depends on the interligand
interactions, whereas the intermetallic interactions are marginal.
Equipped with these microscopic describers, thermodynamic
predictions are within the frame of reality, and the stability
constants computed for the heterometallic [LaLu(L9)]6+ and
[LuLa(L9)]6+ microspecies agree with preliminary solution
speciations obtained by NMR titrations. According to Figure 8,
the origin of the preference for the formation of [LaLu(L9)]6+
with La(III) occupying the proximal site and Lu(III) lying in the
distal site is spread over different factors, among which the
specific microscopic affinity of each site for each metal is
dominant. However, the physical roots of the successful
application of the mixing rule for estimating the heterometallic
interaction ΔELn1,Ln2 remain currently elusive since this
parameter combines simple electrostatic repulsion with changes
in solvation energies, both contributions being highly sensitive to
the exact structure and shape of the complexes.36 Further work
will focus on the isolation of pure heterobimetallic lanthanide-
CONCLUSION
■
Whereas the successive metal loading of adjacent binding sites in
single-stranded ligand is easily rationalized by using the classical
Ising model with a unique intermetallic ΔEM,M interaction,18,31,33
the case of multistranded receptors possessing different binding
sites and displaying mixed inter- and intramolecular connection
processes remains essentially ignored.34 To the best of our
knowledge, this challenge has been only approached once with
the thermodynamic study of triple-stranded helical ferric binders,
in which two Fe(III) cations are bound to hydroxamic binding
units incorporated into polyamide ribbons connected to a
standard tris-aminoethylamine (TREN) tripod.35 Depending on
the length of the spacers separating the binding sites, some minor
deviations from pure statistical binding could be detected,35b but
the lack of explicit consideration of different Fe(III) affinity for
H
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1H), 7.98 (t, 3J = 7.8 Hz, 2H), 7.81 (d, 1H, 4J = 1.9 Hz), 7.54 (dd, 3J = 8.1
Hz, 4J = 1.9 Hz, 1H), 7.33 (d, 3J = 8.1 Hz, 1H), 4.47 (s, 2H), 4.15 (dq, 2J
= 14.3 Hz, 3J = 7.2 Hz, 1H), 3.94 (s, 3H), 3.72 (dq, 2J = 14.3 Hz, 3J = 7.2
Hz, 1H), 3.42 (s, 3H), 1.25 (t, 3J = 7.2 Hz, 3H).
Preparation of 6-(Ethyl(4-(methoxymethyl)-2-nitrophenyl)-
carbamoyl)picolinic acid (6). A solution of LiOH (3.1 g, 73.8 mmol)
in water (35 mL) was dropwise added to a cooled (0 °C) solution of 6-
(ethyl(4-(methoxymethyl)-2-nitrophenyl)carbamoyl)picolinate (5, 5.5
g, 14.7 mmol) in methanol (40 mL). The resulting mixture was stirred at
0 °C for 3 h, poured into water (600 mL) and washed with
dichloromethane (4 × 100 mL). The aqueous phase was acidified at
pH = 2 with conc. hydrochloric acid, and extracted with dichloro-
methane (4 × 100 mL). The combined organic phases were dried over
Na2SO4, filtered and evaporated to dryness. The crude residue was
purified by column chromatography (Silicagel, CH2Cl2:CH3OH = 99:1)
to give 6 as a pale yellow solid (4.5 g, 12.5 mmol, yield 88%). 1H NMR
(CDCl3) δ/ppm: 8.16 (dd, 3J = 7.8 Hz, 4J = 1.2 Hz, 1H), 8.07 (dd, 3J =
7.8 Hz, 4J = 1.2 Hz, 1H), 7.97 (t, 3J = 7.8 Hz, 2H), 7.83 (d, 1H, 4J = 1.9
Hz), 7.56 (dd, 3J = 8.1 Hz, 4J = 1.9 Hz, 1H), 7.36 (d, 3J = 8.1 Hz, 1H),
4.47 (s, 2H), 4.15 (dq, 2J = 14.3 Hz, 3J = 7.2 Hz, 1H), 3.79 (dq, 2J = 14.3
Hz, 3J = 7.2 Hz, 1H), 3.40 (s, 3H), 1.25 (t, 3J = 7.2 Hz, 3H). 13C NMR
(CDCl3) δ/ppm: 12.6, 46.8, 58.8, 72.5, 123.7, 125.1, 129.2, 131.0, 132.1,
135.4, 139.6, 140.9, 144.0, 146.9, 151.6, 163.3, 165.1.
Preparation of N,N-Diethyl-N-(4-(4-(ethylamino)-3-nitroben-
zyl)-2-nitrophenyl)-N-(4-(methoxymethyl)-2-nitrophenyl)-
pyridine-2,6-dicarboxamide (9). 6-(Ethyl(4-(methoxymethyl)-2-
nitrophenyl)carbamoyl)picolinic acid (6, 1.13 g, 3.14 mmol) was
refluxed for 1.5 h with thionyl chloride (2.5 mL, 31.4 mmol) and N,N′
dimethylformamide (0.01 mL) in dichloromethane (20 mL). After
evaporation to dryness, the white residue was dried (10−2 Torr, 1 h),
dissolved in dry dichloromethane (50 mL), and added dropwise to 3,3′-
dinitro-4,4′-bis(N-ethylamino)diphenylmethane (8, 4.3 g, 12.6 mmol)
in dichloromethane (100 mL) containing diisopropylethylamine (0.8
mL, 4.7 mmol). The resulting mixture was refluxed under an inert
atmosphere for 12 h and evaporated to dryness. The orange solid was
dissolved in dichloromethane (150 mL), washed successively with half-
sat. NH4Cl (180 mL), water (2 × 50 mL), and brine (50 mL). The
organic phase was dried over Na2SO4, filtered and evaporated to
dryness. The crude solid was purified by column chromatography
(silicagel, CH2Cl2:CH3OH = 100:0→99:1) to give 9 as an orange solid
(1.59 g, 2.3 mmol, yield 74%). ESI-MS (CH2Cl2): m/z 686.3 ([M
+H]+), 708.6 ([M+Na]+), 1372.8 ([2M+H]+).
Preparation of 6-(N,N-Diethylcarbamoyl)pyridine-2-carbox-
ylic acid (10). Pyridine-2,6-dicarboxylic acid monomethyl ester (4,
11.60 g, 64.0 mmol) was refluxed for 1 h with thionyl chloride (50.9 mL,
700 mmol) and N,N′-dimethylformamide (0.06 mL) in dichloro-
methane (100 mL). After evaporation to dryness, the white residue was
dried (10−2 Torr, 1 h), dissolved in cooled dry dichloromethane (130
mL, 0 °C), and diethylamine (36.3 mL, 350 mmol) in dichloromethane
(20 mL) was slowly added. The resulting mixture was refluxed for 2 h,
then evaporated to dryness. The solid residue was partitioned between
dichloromethane (250 mL) and brine (800 mL). The organic phase was
separated, washed with water (250 mL), and evaporated to dryness. The
resulting solid was dissolved in 1 M aq. sodium hydroxide (400 mL) and
vigorously stirred for 10 min. The basic aqueous phase was washed with
dichloromethane (2 × 100 mL), acidifed to reach pH = 2 with conc.
hydrochloric acid, and the white precipitate was separated by filtration,
dried, and recrystallized from hot acetonitrile to give 10 (12.88 g, 58
mmol, yield 91%). 1H NMR (CDCl3) δ/ppm: 8.27 (dd, 3J = 7.8, 4J = 0.9,
1H), 8.06 (t, 3J = 7.8, 1H), 7.83 (dd, 3J = 7.8, 4J = 0.9, 1H), 3.59 (m, 2H),
3.30 (m, 2H), 1.29 (m, 3H), 1.21 (m, 3H).
Figure 8. Representation of the thermodynamic contributions in kJ/mol
corresponding to (a) intermolecular and (b) intramolecular Ln-ligand
connections, and (c) interligand and (d) intermetallic interactions
responsible for the global complexation process leading to [LaLu-
(L9)]6+ (violet) and [LuLa(L9)]6+ (orange) microspecies (CH3CN/
CH2Cl2 (9:1) + 10−2 M NBu4ClO4, 298 K).
containing podates displaying specific intermetallic communica-
tions for optical applications in molecular down- and
upconversion processes.37
EXPERIMENTAL SECTION
■
Chemicals were purchased from Strem, Acros, Fluka AG, and Aldrich,
and used without further purification unless otherwise stated. Ethyl-(4-
methyl-2-nitro-phenyl)amine (2),38 pyridine-2,6-dicarboxylic acid
monomethyl ester (4),39 3,3′-dinitro-4,4′-bis(N-ethylamino)-
diphenylmethane (8),40 1,5-dichloro-3-(2-chloro-ethyl)-3-methylpen-
tane (21)13 and the ligands L7,41 L8,11 L11,28 and L1210b were prepared
according to literature procedures. The trifluoromethanesulfonate salts
Ln(CF3SO3)3·xH2O (Ln = La, Eu, Lu, x = 1−3) were prepared from the
corresponding oxides (Aldrich, 99.99%).42 The Ln content of solid salts
was determined by complexometric titrations with Titriplex III (Merck)
in the presence of urotropine and xylene orange.43 Acetonitrile and
dichloromethane were distilled over calcium hydride. Thin layer
chromatography (TLC) used silicagel plates Merck 60 F254, and Fluka
silica gel 60 (0.04−0.063 mm) was used for preparative column
chromatography.
Preparation of Methyl 6-(ethyl(4-(methoxymethyl)-2-
nitrophenyl)carbamoyl)picolinate (5). Pyridine-2,6-dicarboxylic
acid monomethyl ester (4, 6.48 g, 35.8 mmol) was refluxed for 1 h
with thionyl chloride (25.0 mL, 358 mmol) and N,N′-dimethylforma-
mide (0.01 mL) in dry dichloromethane (80 mL) under an inert
atmosphere. After evaporation to dryness, the residual white solid was
dried (10−2 Torr, 1 h), dissolved in dry dichloromethane (20 mL) and
added dropwise to 2 (7.5 g, 35.8 mmol) in dichloromethane (20 mL) for
1 h. Diisopropylethylamine (6.1 mL, 35.8 mmol) in dichloromethane
(20 mL) was then added, and the resulting mixture was refluxed for 2 h.
The cooled organic phase was washed with half-sat. NaHCO3 (2 × 100
mL) and half-sat. NH4Cl (2 × 100 mL), dried over MgSO4, filtered and
evaporated to dryness. The crude residue was purified by column
chromatography (silicagel, CH2Cl2:CH3OH = 99:1) to give 5 as a
yellow solid (5.5 g, 14.7 mmol, yield 41%). 1H NMR (CDCl3) δ/ppm:
8.12 (dd, 3J = 7.8 Hz, 4J = 1.2 Hz, 1H), 8.05 (dd, 3J = 7.8 Hz, 4J = 1.2 Hz,
Preparation of N-(4-(4-(6-(Diethylcarbamoyl)-N-ethylpicoli-
namido)-3-nitrobenzyl)-2-nitrophenyl)-N,N-diethyl-N-(4-(me-
thoxymethyl)-2-nitrophenyl)pyridine-2,6-dicarboxamide (11).
6-(N,N-diethylcarbamoyl)pyridine-2-carboxylic acid (10, 2.2 g, 9.8
mmol) was refluxed for 1.5 h with thionyl chloride (2.9 mL, 40 mmol)
and N,N′-dimethylformamide (0.01 mL) in dichloromethane (60 mL).
After evaporation to dryness, the white residue was dried (10−2 Torr, 1
h), dissolved in dry dichloromethane (50 mL), and added to a
dichloromethane solution (70 mL) containing 9 (2.7 g. 3.93 mmol) and
I
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diisopropylethylamine (1.9 mL, 7.4 mmol). The resulting mixture was
refluxed under an inert atmosphere for 12 h and evaporated to dryness.
The residual solid was dissolved in dichloromethane (180 mL) and
washed successively with half-sat. NH4Cl (180 mL), water (50 mL), and
brine (50 mL). The organic phase was dried over Na2SO4, filtered, and
evaporated to dryness. The crude solid was purified by column
chromatography (silicagel, CH2Cl2:CH3OH = 100:0→98:2) to give 11
as a pale yellow solid (2.6 g, 3.1 mmol, 79%). ESI-MS (CHCl3/MeOH):
m/z 862.3 ([M+H]+).
(CDCl3) δ/ppm: 12.8, 14.3, 15.4, 15.5, 39.6, 39.8, 39.9, 40.6, 42.2, 42.8,
65.6, 110.1, 110.2, 110.3, 118.8, 120.0, 120.1, 122.4, 123.3, 124.9, 125.0,
125.1, 125.5, 125.6, 134.5, 134.8, 135.5, 136.2, 136.6, 137.9, 142.9, 143.0,
143.1, 149.3, 149.4, 149.8, 149.9, 150.2, 154.4, 168.5. ESI-MS (CH2Cl2/
MeOH 9:1): m/z 732.3 ([M+H]+).
Preparation of 6-(5-{2-[6-(5-Chloromethyl-1-ethyl-1H-ben-
zoimidazol-2-yl)-pyridin-2-yl]-1-ethyl-1H-benzoimidazol-5-yl-
methyl}-1-ethyl-1H-benzoimidazol-2-yl)-pyridine-2-carboxylic
Acid Diethylamide (14). A mixture of 6-(1-ethyl-5-{1-ethyl-2-[6-(1-
ethyl-5-hydroxymethyl-1H-benzoimidazol-2-yl)-pyridin-2-yl]-1H-ben-
zoimidazol-5-ylmethyl}-1H-benzoimidazol-2-yl)-pyridine-2-carboxylic
acid diethylamide (14, 2.0 g, 2.7 mmol), CH2Cl2 (80 mL) and thionyl
chloride (3.0 mL, 42 mmol) was stirred for 16 h at room temperature,
then poured into aqueous sat. NaHCO3 (650 mL). The aqueous layer
was extracted with dichloromethane (3 × 100 mL), and the combined
organic phases were washed with deionized water until neutral, dried
over Na2SO4, filtered, and evaporated to dryness. The resulting crude
compound was purified by column chromatography (silicagel, CH2Cl2/
MeOH 98:2 → 95:5) to afford 14 as a beige solid (1.95 g, 2.6 mmol,
yield 99%). 1H NMR (CDCl3) δ/ppm: 8.39 (d, 3J = 7.9 Hz, 1H), 8.30−
8.35 (m, 2H), 8.04 (t, 3J = 7.8 Hz, 1H), 7.93 (t, 3J = 7.8 Hz, 1H), 7.86 (d,
4J = 1.0 Hz, 1H), 7.74 (d, 4J = 1.0 Hz, 1H), 7.71 (s, 1H), 7.54 (dd, 3J = 7.8
Preparation of N,N-Diethyl-6-(1-ethyl-5-((1-ethyl-2-(6-(1-
ethyl-5-(methoxymethyl)-1H-benzo[d]imidazol-2-yl)pyridin-2-
yl)-1H-benzo[d]imidazol-5-yl)methyl)-1H-benzo[d]imidazol-2-
yl)picolinamide (12). N-(4-(4-(6-(diethylcarbamoyl)-N-ethylpicoli-
namido)-3-nitrobenzyl)-2-nitrophenyl)-N,N-diethyl-N-(4-(methoxy-
methyl)-2-nitrophenyl)pyridine-2,6-dicarboxamide (9, 1.77 g, 1.99
mmol) and activated metallic iron powder (2.77 g, 49.7 mmol) was
refluxed in ethanol/water/conc. hydrochloric acid (200 mL/60 mL/12
mL) for 16 h. Excess of metallic iron was filtered off and ethanol was
distilled under vacuum. Dichloromethane (150 mL) and Na2H2EDTA
(19 g, 49.7 mmol in 250 mL water) was added to the remaining solution.
The pH was adjusted to 7.0 with conc. aqueous ammonia. Conc.
hydrogen peroxide (30% in water, 5 mL) was added dropwise under
vigorous strirring, and the pH raised to 8.5 with conc. aqueous ammonia.
The organic layer was separated, and the aqueous phase was extracted
with dichloromethane (3 × 200 mL). The combined organic phases
were washed with water (200 mL), dried over Na2SO4, filtered, and
evaporated to dryness. The crude solid was purified by column
chromatography (silicagel, CH2Cl2:CH3OH = 100:0→97:3) to give 12
as a beige solid (1.2 g, 1.6 mmol, yield 81%). 1H NMR (CDCl3) δ/ppm:
8.37 (dd, 3J = 8.0 Hz, 4J = 1.0 Hz, 1H), 8.33 (dd, 3J = 7.8 Hz, 4J = 1.0 Hz,
1H), 8.32 (dd, 3J = 7.9 Hz, 4J = 1.1 Hz, 1H), 8.03 (t, 3J = 7.9 Hz, 1H),
7.93 (t, 3J = 7.8 Hz, 1H), 7.81 (d, 4J = 1.0 Hz, 1H), 7.74 (d, 4J = 1.0 Hz,
1H), 7.70 (d, 4J = 1.0 Hz, 1H), 7.54 (dd, 3J = 7.7 Hz, 4J = 1.1 Hz), 7.46 (d,
3J = 8.3 Hz, 1H), 7.35−7.39 (m, 3H), 7.24−7.27 (m, 2H), 4.72−4.82
Hz, 4J = 1.0 Hz, 1H), 7.43 (dd, 3J = 8.3 Hz, 4J = 0.5 Hz, 1H), 7.42 (dd, 3J
= 8.3 Hz, 4J = 1.5 Hz, 1H), 7.38 (d, 3J = 8.3 Hz, 1H), 7.37 (d, 3J = 8.3 Hz,
1H), 7.27 (t, 4J = 1.0 Hz, 1H), 7.25 (t, 4J = 1.0 Hz, 1H), 4.79 (s, 2H),
4.71−4.78 (m, 6H), 4.30 (s, 2H), 3.61 (q, 3J = 7.1 Hz, 2H), 3.35 (q, 3J =
7.1 Hz, 2H), 1.45 (t, 3J = 7.1 Hz, 3H), 1.32−1.38 (m, 6H), 1.29 (t, 3J =
7.1 Hz, 3H), 1.07 (t, 3J = 7.1 Hz, 3H). 13C NMR (CDCl3) δ/ppm: 12.8,
14.3, 15.3, 15.4, 39.5, 39.8, 39.9, 40.6, 42.2, 42.8, 47.2, 110.1, 110.2,
110.7, 120.0, 120.2, 120.6, 122.5, 124.5, 125.0, 125.7, 125.8, 132.3, 134.5,
134.8, 136.0, 136.7, 138.0, 138.2, 142.9, 143.1, 149.3, 149.7, 149.8, 150.1,
150.8, 154.5, 168.5; ESI-MS (CH2Cl2/MeOH 9:1): m/z 750.3 ([M
+H]+).
Preparation of Thioacetic acid S-[2-(6-{5-[2-(6-diethylcarba-
moyl-pyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-ylmethyl]-1-
ethyl-1H-benzoimidazol-2-yl}-pyridin-2-yl)-1-ethyl-1H-benzoi-
midazol-5-ylmethyl] Ester (15). 6-(5-{2-[6-(5-Chloromethyl-1-
ethyl-1H-benzoimidazol-2-yl)-pyridin-2-yl]-1-ethyl-1H-benzoimidazol-
5-ylmethyl}-1-ethyl-1H-benzoimidazol-2-yl)-pyridine-2-carboxylic acid
diethylamide (14, 2.0 g, 2.6 mmol) was dissolved in a suspension of
potassium thioacetate (1.5 g, 13.3 mmol) in acetone (40 mL) and
dichloromethane (40 mL). The mixture was refluxed for 16 h, and then
evaporated to dryness. The resulting solid was partitioned between
dichloromethane (400 mL) and water (300 mL). The aqueous phase
was separated and extracted with dichloromethane (3 × 100 mL). The
combined organic phases were washed with brine, dried over Na2SO4,
filtered, evaporated, and the residue was purified by column
chromatography (silicagel, CH2Cl2/MeOH 100:0 → 95:5) to afford
15 as a white solid (2.0 g, 2.5 mmol, yield 96%). 1H NMR (CDCl3) δ/
ppm: 8.37 (dd, 3J = 8.0 Hz, 4J = 1.0 Hz, 1H), 8.32 (dd, 3J = 7.8 Hz, 4J =
1.0 Hz, 1H), 8.30 (dd, 3J = 7.8 Hz, 4J = 1.0 Hz, 1H), 8.02 (t, 3J = 7.8 Hz,
1H), 7.92 (t, 3J = 7.8 Hz, 1H), 7.77 (d, 4J = 1.0 Hz, 1H), 7.73 (d, 4J = 1.0
Hz, 1H), 7.70 (d, 4J = 1.0 Hz, 1H), 7.53 (dd, 3J = 7.7 Hz, 4J = 1.0 Hz,
1H), 7.35−7.40 (m, 3H), 7.30 (dd, 3J = 8.4 Hz, 4J = 1.5 Hz, 1H), 7.26
(m, 1H), 7.24 (t, 4J = 1.5 Hz, 1H), 4.71−4.78 (m, 6H), 4.30 (s, 4H), 3.60
(q, 3J = 7.1 Hz, 3H), 3.35 (q, 3J = 7.1 Hz, 2H), 2.35 (s, 3H), 1.45 (t, 3J =
7.1 Hz, 3H), 1.30−1.35 (m, 6H), 1.28 (t, 3J = 7.1 Hz, 3H), 1.07 (t, 3J =
7.1 Hz, 3H). 13C NMR (CDCl3) δ/ppm: 12.8, 14.3, 15.3, 15.4, 15.5,
30.4, 34.0, 39.6, 39.8, 39.9, 40.6, 42.2, 42.8, 110.0, 110.2, 110.4, 120.0,
120.2, 120.4, 122.4, 124.6, 124.9, 125.0, 125.1, 125.6, 125.7, 132.3, 134.5,
134.8, 135.3, 136.5, 136.6, 137.9, 138.0, 143.0, 143.1, 143.2, 149.3, 149.4,
149.8, 149.9, 150.1, 150.5, 154.4, 168.5, 195.2. ESI-MS (CH2Cl2/
MeOH 9:1): m/z 790.3 ([M+H]+).
(m, 6H), 4.62 (s, 2H), 4.30 (s, 2H), 3.61 (q, 3J = 7.1 Hz, 2H), 3.41 (s,
3H), 3.35 (q, 3J = 7.1 Hz, 2H), 1.45 (t, 3J = 7.1 Hz, 3H), 1.35 (q, 3J = 7.1
Hz, 6H), 1.28 (t, 3J = 7.1 Hz, 3H), 1.07 (t, 3J = 7.1 Hz, 3H). 13C NMR
(CDCl3) δ/ppm: 12.8, 14.3, 15.3, 15.4, 39.5, 39.8, 39.9, 40.6, 42.2, 42.8,
57.8, 75.1, 110.0, 110.1, 110.2, 119.8, 120.0, 120.2, 122.4, 123.8, 124.9,
125.0, 125.5, 125.6, 132.9, 134.5, 134.8, 135.6, 136.5, 136.6, 137.9, 138.1,
142.9, 143.1, 143.3., 149.4, 149.5, 149.9, 150.1, 150.3, 154.5, 168.5. ESI-
MS (CH2Cl2/MeOH 9:1): m/z 746.5 ([M+H]+).
Preparation of 6-(1-Ethyl-5-{1-ethyl-2-[6-(1-ethyl-5-hydrox-
ymethyl-1H-benzoimidazol-2-yl)-pyridin-2-yl]-1H-benzoimida-
zol-5-ylmethyl}-1H-benzoimidazol-2-yl)-pyridine-2-carboxylic
Acid Diethylamide (13). A mixture of N,N-diethyl-6-(1-ethyl-5-((1-
ethyl-2-(6-(1-ethyl-5-(methoxymethyl)-1H-benzo[d]imidazol-2-yl)-
pyridin-2-yl)-1H-benzo[d]imidazol-5-yl)methyl)-1H-benzo[d]-
imidazol-2-yl)picolinamide (12, 1.2 g, 1.6 mmol) in acetic anhydride/
CH2Cl2 (25 mL/25 mL) and BF3·Et2O (1.0 mL, 8 mmol) was stirred for
16 h at room temperature, then poured into an ice-cooled aqueous 1 M
KOH solution (400 mL). The aqueous layer was extracted with CH2Cl2
(2 × 50 mL). The combined organic phases were washed with deionized
water until neutral, dried over Na2SO4, filtered, and evaporated to
dryness to afford the crude acetate, which was dissolved in methanol/2
M aq. KOH (100 mL/70 mL), and stirred for 12 h at room temperature.
The methanol was distilled under vacuum, the resulting solution was
poured into brine (500 mL) and extracted with CH2Cl2 (3 × 100 mL).
The combined organic phases were washed with deionized water until
neutral, dried over Na2SO4, filtered, and evaporated to dryness. The
resulting crude compound was purified by column chromatography
(silicagel, CH2Cl2/MeOH 97:3 → 95:5) to afford 13 as a white solid
(0.9 g, 1.2 mmol, yield 76%). 1H NMR (CDCl3) δ/ppm: 8.36 (dd, 3J =
8.0 Hz, 4J = 1.0 Hz, 1H), 8.23−8.27 (m, 2H), 7.95 (t, 3J = 7.9 Hz, 1H),
7.92 (t, 3J = 7.9 Hz, 1H), 7.77 (d, 4J = 1.0 Hz, 1H), 7.74 (d, 4J = 1.0 Hz,
1H), 7.69 (d, 4J = 1.0 Hz, 1H), 7.53 (dd, 3J = 7.7 Hz, 4J = 1.0 Hz, 1H),
7.40−7.44 (m, 2H), 7.37 (d, 3J = 8.4 Hz, 1H), 7.36 (d, 3J = 8.4 Hz, 1H),
7.24 (dd, 3J = 8.3 Hz, 4J = 1.4 Hz, 2H), 4.82 (s, 2H), 4.71−4.79 (m, 6H),
4.29 (s, 2H), 3.60 (q, 3J = 7.1 Hz, 2H), 3.35 (q, 3J = 7.1 Hz, 2H), 1.45 (t,
3J = 7.1 Hz, 3H), 1.26−1.37 (m, 9H), 1.07 (t, 3J = 7.1 Hz, 3H). 13C NMR
Preparation of 6-(1-Ethyl-5-{1-ethyl-2-[6-(1-ethyl-5-mercap-
tomethyl-1H-benzoimidazol-2-yl)-pyridin-2-yl]-1H-benzoimi-
dazol-5-ylmethyl}-1H-benzoimidazol-2-yl)-pyridine-2-carbox-
ylic Acid Diethylamide (16). Thioacetic acid S-[2-(6-{5-[2-(6-
diethylcarbamoyl-pyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-ylmethyl]-
1-ethyl-1H-benzoimidazol-2-yl}-pyridin-2-yl)-1-ethyl-1H-benzoimida-
zol-5-ylmethyl] ester (15, 1.1 g, 1.4 mmol) was dissolved in degassed
methanol (60 mL) containing conc. hydrochloric acid (37%, 5 mL). The
J
dx.doi.org/10.1021/ic301631n | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mixture was stirred and heated at 45 °C for 16 h under an inert
atmosphere. The methanol was then evaporated, and the solid residue
was partitioned between ethyl acetate (450 mL) and water (600 mL)
containing NaHCO3 (10.4 g, 123 mmol). The aqueous phase was
separated and extracted with ethyl acetate (3 × 100 mL). The combined
organic phases were washed with brine, dried over Na2SO4, filtered, and
evaporated to yield a pale yellow solid, which was purified by column
chromatography (silicagel, CH2Cl2/MeOH 98:2 → 95:5) to afford 16 as
a white solid (0.88 g, 1.2 mmol, 81%). 1H NMR (CDCl3) δ/ppm: 8.38
(dd, 3J = 8.0 Hz, 4J = 0.9 Hz, 1H), 8.32 (dd, 3J = 7.8 Hz, 4J = 1.0 Hz, 1H),
8.31 (dd, 3J = 7.9 Hz, 4J = 1.0 Hz, 1H), 8.02 (t, 3J = 7.9 Hz, 1H), 7.92 (t,
3J = 7.9 Hz, 1H), 7.77 (d, 4J = 0.8 Hz, 1H), 7.73 (d, 4J = 0.5 Hz, 1H), 7.70
(d, 4J = 0.5 Hz, 1H), 7.53 (dd, 3J = 7.7 Hz, 4J = 1.0 Hz, 1H), 7.42 (d, 3J =
8.4 Hz, 1H), 7.34−7.38 (m, 3H), 7.24−7.27 (m, 2H), 4.71−4.80 (m,
6H), 4.30 (s, 2H), 3.91 (d, 3J = 7.4 Hz, 2H), 3.60 (q, 3J = 7.1 Hz, 2H),
3.35 (q, 3J = 7.1 Hz, 2H), 1.83 (t, 3J = 7.4 Hz, 1H), 1.45 (t, 3J = 7.1 Hz,
3H), 1.25−1.36 (m, 9H), 1.07 (t, 3J = 7.1 Hz, 3H). 13C NMR (CDCl3)
δ/ppm: 12.8, 14.1, 14.3, 15.4, 15.5, 29.4, 39.5, 39.8, 39.9, 40.6, 42.2, 42.8,
110.0, 110.2, 110.5, 119.4, 120.0, 120.2, 122.4, 124.1, 124.9, 125.0, 125.1,
125.6, 125.7, 134.5, 134.8, 135.1, 136.1, 136.5, 136.6, 137.9, 138.1, 142.9,
143.0, 143.2., 149.3, 149.4, 149.8, 150.1, 150.4, 154.5, 168.5. ESI-MS
(CH2Cl2/MeOH 9:1): m/z 748.5 ([M+H]+).
(d, 2J = 14.8 Hz, 3H), 2.68−2.77 (m, 3H), 1.49−1.53 (m, 18H), 1.21−
1.30 (m, 3H), 1.23 (t, 3J = 7.2 Hz, 9H), 0.82−0.88 (m, 3H), 0.78 (t, 3J =
7.1 Hz, 9H), 0.71 (t, 3J = 7.1 Hz, 9H), 0.32 (s, 3H), 0.13−0.26 (m, 6H).
13C NMR (CD3CN) δ/ppm: 11.3, 12.9, 14.3, 14.7, 23.1, 29.4, 34.0, 35.6,
38.2, 39.2, 41.5, 41.9, 42.0, 42.7, 45.0, 111.3, 111.9, 112.6, 116.1, 119.6,
122.8, 124.8, 125.7, 126.0, 126.5, 126.9, 127.1, 127.5, 127.8, 133.1, 133.6,
134.3, 136.0, 136.1, 138.5, 139.6, 139.8, 142.6, 143.5, 145.6, 146.4, 147.0,
149.5, 149.6, 149.8, 151.1, 168.8. Elemental analyses: calcd for
La2C146H147N27O21S9F18·16H2O (MM = 3812.54) %C 45.99, %H
4.73, %N 9.92. Found %C 45.87, %H 4.29, %N 9.80.
1
[Lu2(L9)](CF3SO3)6·12H2O (yield 41%). H NMR (CD3CN) δ/
ppm: 8.38 (d, 3J = 8.2 Hz, 3H), 8.25 (t, 3J = 8.1 Hz, 3H), 7.92 (t, 3J = 8.1
Hz, 3H), 7.73 (dd, 3J = 8.0 Hz, 6H), 7.65 (dd, 3J = 8.6 Hz, 6H), 7.59 (d,
3J = 8.2 Hz, 3H), 7.31 (dd, 3J = 8.6 Hz, 4J = 1.6 Hz, 6H), 7.25 (d, 3J = 8.3
Hz, 3H), 7.94 (dd, 3J = 8.4 Hz, 4J = 1.5 Hz, 3H), 5.97 (d, 4J = 1.4 Hz,
3H), 5.46 (s, 3H), 5.44 (s, 3H), 4.78 (dq, 2J = 14.0 Hz, 3J = 6.8 Hz, 3H),
4.61 (dq, 2J = 14.6 Hz, 3J = 7.3 Hz, 3H), 4.52 (tt, 2J = 14.5 Hz, 3J = 7.3 Hz,
6H), 4.36 (dq, 2J = 14.6 Hz, 3J = 7.2 Hz, 3H), 4.28 (dt, 2J = 14.8 Hz, 3J =
7.4 Hz, 3H), 3.62 (d, 2J = 17.4 Hz, 3H), 3.50 (d, 2J = 17.4 Hz, 3H), 3.40−
3.27 (m, 6H), 2.75−2.63 (m, 6H), 2.62−2.52 (m, 3H), 1.52−1.44 (m,
24H), 1.42−1.22 (m, 3H), 1.04 (t, 3J = 7.1 Hz, 3H), 0.90 (td, 2J = 12.7
Hz, 3J = 5.8 Hz, 3H), 0.62 (t, 3J = 7.1 Hz, 9H), 0.36 (s, 3H), 0.25 (m,
6H). Elemental analyses: calcd for Lu2C146H147N27O21S9F18·12H2O
(MM = 3812.60) %C 45.8, %H 4.52, %N 9.92. Found %C 45.95, %H
4.26, %N 9.91.
Preparation of Ligand L9. All reactants and solvents were carefully
dried and degassed prior to be used. 6-(1-Ethyl-5-{1-ethyl-2-[6-(1-ethyl-
5-mercaptomethyl-1H-benzoimidazol-2-yl)-pyridin-2-yl]-1H-benzoi-
midazol-5-ylmethyl}-1H-benzoimidazol-2-yl)-pyridine-2-carboxylic
acid diethylamide (16, 1.39 g, 1.86 mmol) and 1,5-dichloro-3-(2-chloro-
ethyl)-3-methyl-pentane (21, 0.114 g, 0.53 mmol) were dissolved in a
suspension of cesium carbonate (0.61 g, 1.86 mmol) in freshly distilled
DMF (20 mL) under a nitrogen atmosphere. The mixture was stirred,
heated at 60 °C for 16 h, filtered, and evaporated to dryness. The
resulting solid was dissolved in dichloromethane (100 mL) and water
(100 mL). The organic phase was washed with water (50 mL) and brine
(50 mL), dried over Na2SO4, filtered, and evaporated to dryness to yield
a solid residue, which was purified by column chromatography (silicagel,
CH2Cl2/MeOH 98:2 → 92:8) to afford L9 as a pale yellow solid (0.65 g,
0.28 mmol, yield 53%). 1H NMR (CDCl3) δ/ppm: 8.37 (d, 3J = 7.8 Hz,
3H), 8.29 (d, 3J = 7.9 Hz, 6H), 7.97 (t, 3J = 7.9 Hz, 3H), 7.91 (t, 3J = 7.8
Hz, 3H), 7.72 (s, 6H), 7.69 (s, 3H), 7.53 (d, 3J = 7.8 Hz, 3H), 7.34−7.38
(m, 9H), 7.29 (dd, 3J = 8.4 Hz, 4J = 1.0 Hz, 3H), 7.22−7.26 (m, 6H),
Spectroscopic Measurements. Electronic spectra in the UV−vis
were recorded at 20 °C from solutions in CH3CN with a Perkin-Elmer
Lambda 900 spectrometer using quartz cells of 0.1 or 1 mm path length.
Spectrophotometric titrations were performed with a J&M diode array
spectrometer (Tidas series) connected to an external computer. In a
typical experiment, 25 mL of L9 (10−4 M) in CH3CN/CH2Cl2 (9:1) +
10−2 M NBu4ClO4 were titrated at 25 °C with a solution of
Ln(CF3SO3)3·xH2O (10−3 M) in the same solvent under an inert
atmosphere. After each addition of 0.10 mL, the absorbance was
recorded using Hellma optrodes (optical path length 0.1 cm) immersed
in the thermostatted titration vessel and connected to the spectrometer.
Mathematical treatment of the spectrophotometric data was performed
with factor analysis43 and with the SPECFIT program.17 1H and 13C
NMR spectra were recorded at 25 °C on a Bruker Avance 400 MHz and
Bruker DRX-500 MHz spectrometers. Chemical shifts are given in ppm
with respect to TMS. Pneumatically assisted electrospray (ESI-MS)
mass spectra were recorded from 10−4 M solutions on a Finnigan
SSQ7000 instrument or on an Applied Biosystems API 150EX LC/MS
System equipped with a Turbo Ionspray source. Elemental analyses
were performed by K. L. Buchwalder from the Microchemical
Laboratory of the University of Geneva. Least-squares fitting methods
were implemented in Excel and Mathematica.
3
4.71−4.74 (m, 18H), 4.28 (s, 6H), 3.80 (s, 6H), 3.60 (q, J = 7.1 Hz,
6H), 3.34 (q, 3J = 7.1 Hz, 6H), 2.25−2.28 (m, 6H), 1.37−1.45 (m, 15H),
1.24−1.34 (m, 27H), 1.06 (t, 3J = 7.1 Hz, 9H), 0.75 (s, 3H). 13C NMR
(CDCl3) δ/ppm: 12.9, 14.4, 15.4, 15.5, 24.4, 26.0, 29.8, 36.6, 36.8, 39.0,
39.7, 39.9, 40.0, 40.7, 42.3, 42.9, 110.1, 110.3, 110.5, 115.7, 120.0, 120.2,
120.3, 122.5, 124.8, 125.0, 125.1, 125.4, 125.7, 125.7, 125.9, 133.1, 134.6,
134.9, 135.2, 136.7, 138.0, 138.1, 138.3, 143.0, 143.2, 149.3, 149.4, 149.9,
150.0, 150.3, 154.5, 168.6. ESI-MS (CH2Cl2/MeOH 9:1): m/z 2352.9
([M+H]+), 1176.8 ([M+2H]2+). Elemental analyses: calcd for
C140H147N27O3S3·4H2O %C 69.36, %H 6.44, %N 15.60. Found %C
69.21, %H 6.19, %N 15.43.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Tables of H NMR chemical shifts, ESI-MS titrations, and
Preparation of the Complexes [La2(L9)](CF3SO3)6·16H2O and
[Lu2(L9)](CF3SO3)6·12H2O. A solution of Ln(CF3SO3)3·xH2O (Ln =
La or Lu, 21 μmol) in acetonitrile (4 mL) was added to a solution of
L9·4H2O (25.5 mg, 10.5 μmol) in dichloromethane (4 mL). The
resulting pale yellow mixture was stirred for 12 h, then evaporated to
dryness. The residue was dissolved in acetonitrile and diethyl ether was
slowly added to precipitate the complex. The resulting pale yellow
microcrystalline powders were collected by fitration and dried to give
[Ln2(L9)](CF3SO3)6·xH2O (Ln = La, x = 16; Ln = Lu, x = 12).
thermodynamic stability constants. Figures illustrating the
application of the site binding model, correlation between
calculated and experimental thermodynamic stability constants
and ligand distributions. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Gabriel.Canard@univ-amu.fr (G.C.), Claude.Piguet@
unige.ch (C.P.).
■
1
[La2(L9)](CF3SO3)6·16H2O. (yield 67%). H NMR (CD3CN) δ/
ppm: 8.30 (m, 6H), 8.11 (t, 3J = 8.1 Hz, 3H), 7.92 (d, 3J = 8.1 Hz, 3H),
3
4
7.70−7.75 (m, 6H), 7.64 (m, 6H), 7.37 (dd, J = 8.5 Hz, J = 0.8 Hz,
3H), 7.30 (dd, 3J = 8.7 Hz, 4J = 1.0 Hz, 3H), 7.19 (d, 3J = 8.3 Hz, 3H),
7.02 (dd, 3J = 8.3 Hz, 4J = 0.7 Hz, 3H), 6.18 (s, 3H), 5.98 (s, 3H), 5.96 (s,
3H), 4.69 (sext., 2J = 15.1 Hz, 3J = 7.2 Hz, 3H), 4.39−4.51 (m, 9H), 4.31
(sext., 2J = 15.1 Hz, 3J = 7.2 Hz, 3H), 4.18 (sext., 2J = 15.0 Hz, 3J = 7.2 Hz,
3H), 3.71 (d, 2J = 17.9 Hz, 3H), 3.54 (d, 2J = 17.9 Hz, 3H), 3.44 (d, 2J =
14.8 Hz, 3H), 3.35 (sext., 2J = 14.7 Hz, 3J = 7.1 Hz, 3H), 3.24 (sext., 2J =
14.7 Hz, 3J = 7.1 Hz, 3H), 2.81 (sext., 2J = 13.3 Hz, 3J = 7.1 Hz, 3H), 2.74
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Swiss National Science Foundation is
gratefully acknowledged.
■
K
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