Chiroptical Probing of Lanthanide-Directed Self-Assembly Formation Using btp Ligands Formed in One-Pot Diazo-Transfer/Deprotection Click Reaction from Chiral Amines

Article abstract of DOI:10.1002/chem.201504257

A series of enantiomeric 2,6-bis(1,2,3-triazol-4-yl)pyridines (btp)-containing ligands was synthesized by a one-pot two-step copper-catalyzed amine/alkyne click reaction. The Eu^III- and Tb^III-directed self-assembly formation of these ligands was studied in CH₃CN by monitoring their various photophysical properties, including their emerging circular dichroism and circularly polarized luminescence. The global analysis of the former enabled the determination of both the stoichiometry and the stability constants of the various chiral supramolecular species in solution.

Full text of DOI:10.1002/chem.201504257

DOI: 10.1002/chem.201504257
Communication
&
Synthetic Methods
Chiroptical Probing of Lanthanide-Directed Self-Assembly
Formation Using btp Ligands Formed in One-Pot Diazo-Transfer/
Deprotection Click Reaction from Chiral Amines**
Joseph P. Byrne,[a] Miguel Martínez-Calvo,[a, c] Robert D. Peacock,[b] and
Thorfinnur Gunnlaugsson*[a]
btp ligands can form stable complexes with f-metal ions;[8]
however, only a few examples of f-btp systems have been de-
veloped to date.[9] Many beautiful examples of luminescent
lanthanide (LnIII) complexes with terdentate ligands have been
reported.[10] Due to the remarkable photophysical properties of
many of the lanthanides and building on pioneering work by
Bünzli, Piguet and co-workers,[11] we[12] and others[13] have de-
veloped various amide derivatives of dipicolinic acid (dpa), syn-
thesized from chiral amines, and used these in LnIII-directed
synthesis of supramolecular architectures. Moreover, chiral LnIII
complexes exhibit interesting chiroptical properties, which can
be capitalized upon for sensing and imaging.[13a,14] The btp li-
gands can be considered as structural analogues of these dpa
derivatives, because the triazole units are considered an iso-
stere for amide bonds.[15] With this in mind, we set out to de-
velop a convenient methodology for introducing chirality into
the btp motif in a one-pot synthesis from optically pure
amines. Herein, we demonstrate such synthesis, using a-aryl
amines, with the formation of ligands 1–3 (Scheme 1) and the
subsequent use of chiral ligands to form optically active lumi-
nescent self-assembly structures through LnIII-directed synthe-
sis, the formation of which was both probed and quantified,
among other techniques, by circular dichroism (CD) spectros-
copy, in a manner recently developed within our laboratory.[16]
This communication will focus the characterization and photo-
Abstract: A series of enantiomeric 2,6-bis(1,2,3-triazol-4-
yl)pyridines (btp)-containing ligands was synthesized by
a one-pot two-step copper-catalyzed amine/alkyne click
reaction. The EuIII- and TbIII-directed self-assembly forma-
tion of these ligands was studied in CH3CN by monitoring
their various photophysical properties, including their
emerging circular dichroism and circularly polarized lumi-
nescence. The global analysis of the former enabled the
determination of both the stoichiometry and the stability
constants of the various chiral supramolecular species in
solution.
The development of highly organized self-assembly structures
from simple starting materials is of great interest in supra-
molecular and materials chemistry. Recently, a new class of
highly versatile and synthetically adaptable ligands based on
the 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) motif have emerged
as candidates for use in such self-assembly formations.[1] Syn-
thesized by the CuAAC click reaction, the btp motif has been
shown to coordinate a wide range of metals and have diverse
applications including, but not limited to, catalysis,[2] enzyme
inhibition,[3] luminescence-based molecular logic,[4] and nano-
materials.[5] The wide scope of CuAAC has allowed a wide
range of functionalities to be introduced into btp arms, using
various azide substrates.[6] We have recently reported the use
of simple non-chiral btp ligands to give discrete d-metal ion
complexes and metallogels.[7] We have also shown that such
[a] Dr. J. P. Byrne, Dr. M. Martínez-Calvo, Prof. T. Gunnlaugsson
School of Chemistry and Trinity Biomedical Sciences Institute
Trinity College Dublin, University of Dublin, Dublin 2 (Ireland)
E-mail: gunnlaut@tcd.ie
[b] Dr. R. D. Peacock
School of Chemistry, Joseph Black Building
University of Glasgow, Glasgow, G12 8QQ (UK)
[c] Dr. M. Martínez-Calvo
Current address: Departamento de Química Orgµnica and
Centro Singular de Investigación en Química Biolóxica e
Materiais Moleculares (CIQUS)
Universidade de Santiago de Compostela
15782 Santiago de Compostela (Spain)
[**] btp=2,6-Bis(1,2,3-triazol-4-yl)pyridines.
Supporting information and ORCID(s) from the author(s) for this article is
available on the WWW under http://dx.doi.org/10.1002/chem.201504257.
Scheme 1. Structures of chiral btp ligands 1–3.
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physical properties of ligands 1 and their LnIII complexes (Ln=
Eu, Tb). Details of the interactions of ligands 2 and EuIII are
given in Supporting Information.
A one-pot amine/alkyne CuAAC methodology has been pre-
viously reported,[17] but relies on the relatively unstable TfN3 as
a diazo-transfer reagent. In contrast, a salt of imidazole-1-sulfo-
nyl azide, ImSO2N3·HCl, was reported as a more robust shelf-
stable diazotransfer agent,[18] and consequently Stonehouse
and co-workers employed this to form triazoles,[19] whereas Xie
and Maisonneuve presented a route to form triazoles from al-
Figure 2. X-ray crystal structures of the S,S- (left) and R,R-enantiomers (right)
of ligand 1, shown with thermal ellipsoids at 50%. Hydrogen bonding be-
tween triazoles and adjacent pyridines is shown by a dashed line. Hydrogen
atoms have been omitted for clarity. Selected crystallographic and refine-
ment data: 1a (CCDC 1019214): a=10.0935(15), b=27.370(4),
c=36.577(6) Š; Z=16; GOF R1, wR2=1.045, 0.0392, 0.1032; Flack=0.1(2).
1b (CCDC 1019215): a=10.0910(15), b=27.262(4), c=36.585(6); Z=16;
GOF, R1, wR2=1.045, 0.0481, 0.1287; Flack=0.3(2). CCDC 1019214 and
1019215 contain the supplementary crystallographic data for this paper.
These data are provided free of charge by The Cambridge Crystallographic
Data Centre.
[20]
dehydes and amines also using ImSO2N3. However, it has re-
cently been reported that ImSO2N3·HCl is less stable than origi-
nally thought,[21] and ImSO2N3·H2SO4 was proposed as an alter-
native. Our work presented herein makes use of ImSO2N3·H2SO4
in a modular and mild one-pot diazo-transfer/deprotection
click reaction, which has two steps, catalyzed by CuII and CuI,
respectively. The reaction mixture underwent dramatic color
change during the course of the reaction, allowing it to be
monitored by the naked eye, as shown in Figure 1 for the for-
Figure 1. Schematic representation of synthesis of chiral btp ligands 1.
(a) Cyan mixture of amine, CuSO4·5H2O, ImSO2N3·H2SO4, and K2CO3 in CH3OH
(t=0 h). (b) Lilac suspension upon formation of the azide (t=5 h). (c) Dull
yellow suspension after addition of sodium ascorbate and degassing mixture
(t=5.5 h). (d) Orange reaction mixture upon completion of CuAAC reaction
(t=18 h).
Figure 3. (a) CD spectra of ligands 1a and b; and (b) CD spectra of the EuIII
and TbIII complexes ([Ln·(1)3](CF3SO3)3). (c) UV/Vis absorbance spectrum of
1a. (d) UV/Vis absorbance spectrum of [Eu·(1a)3](CF3SO3)3. All data recorded
in CH3CN at room temperature.
mation of 1a (S,S) and 1b (R,R) (from S- and R-amines, respec-
tively). The initial mixture, a bright cyan color, changed into
a lilac suspension after stirring for 5 h in the presence of
K2CO3, corresponding with the conversion of the amine to
azide (as was monitored by TLC; this color change was not ob-
served if insufficient amount of base was added). Subsequent
addition of sodium ascorbate (to reduce CuII!CuI) and degass-
ing of the reaction mixture resulted in formation of a dull pale
yellow suspension, Figure 1(c), while addition of the protected
dialkyne, 2,6-bis(TMS-ethynyl)pyridine, gave an orange color
upon successful formation of 1 after 18 h.[22] After aqueous
workup and precipitation from CH3OH or flash chromatogra-
phy, 1a and b were isolated in moderate overall yields of ap-
proximately 50%, with retention of stereochemistry, as was
confirmed by X-ray crystallography (Figure 2) and circular di-
chroism (CD; Figure 3a), in which opposite ellipticities were
seen for the enantiomers, 1a displaying positive signals cen-
tered at 300 and 236 nm, which correspond closely to the
main absorbance bands (see below). Both enantiomers of
ligand 1 crystallized in the orthorhombic C2221 space group,
Figure 2. The structures were identical, but mirror images of
each other. There were two molecules in the asymmetric unit
of the structure and, as a result of nonclassical triazolyl hydro-
gen bonding to the pyridyl nitrogen of the adjacent molecule,
a double helix structure occurred in the solid state. These hy-
drogen bonds had CÀH···N distances ranging from 2.458(2)–
2.608(3) Š (Table S2 in the Supporting information). The btp
motifs of the two molecules were nearly orthogonal to each
other (858).
LnIII complexes were prepared by reacting
1
with
Ln(CF3SO3)3 in 3:1 stoichiometry in CH3OH and isolated as
solids upon ether diffusion, using both EuIII and TbIII. The TbIII
complexes were highly luminescent in CH3CN solution upon
excitation of the ligand (antenna), displaying characteristic
green TbIII emission, demonstrating successful sensitization
of the TbIII 5D0 excited state. The quantum yield of
[Tb·(1b)3](CF3SO3)3 was determined as 67Æ12% by a relative
method, compared to Cs3[Tb·(dpa)3].[23] The EuIII analogue was
also emissive, giving rise to metal-based emission in the red;
however, in contrast to the TbIII, the quantum yield was signifi-
cantly less, which is consistent with our previous observation-
s.[8a] The hydration states, or the numbers of metal-bound
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water molecules (q), were determined for both complexes by
measuring their lanthanide centered luminescence lifetimes
upon excitation of the ligand in both H2O and D2O. The results
showed a q of zero for both pair of enantiomers indicating
that they were coordinatively saturated in solution; confirming
the 3:1 stoichiometry (Table S3 in the Supporting Information).
The UV/Vis absorbance spectra of ligands 1a and b were
measured in CH3CN, showing a band centered at 300 nm, with
a slight shoulder at 283 nm, as well as a broader band with
a maximum at 230 nm, Figure 3(c). The UV/Vis spectra of the
LnIII complexes of 1 resembled those of the ligands, but the
band centered at 300 nm underwent a redshift to 315 nm; the
spectrum of [Eu·(1a)3](CF3SO3)3 is shown in Figure 3d. The cor-
responding CD spectra of the [Ln·(1)3](CF3SO3)3 complexes is
shown in Figure 3b; these display significantly different fea-
tures to those seen for the ligands. Furthermore, the CD spec-
tra of the complexes were, within error, nearly identical for
both EuIII and TbIII complexes demonstrating that both adopt-
ed the same geometry in solution. The dramatic differences in
the CD spectra of the ligands versus those seen for the LnIII
complexes are the result of exciton coupling between the
chromophores in adjacent ligands, brought together by the
metal-directed self-assembly formation.[24] The spectra of the
complexes of 1a showed a large positive exciton couplet with
a Davydov splitting of approximately 27 nm, representing
a Cotton effect splitting of the 315 nm-centered absorbance
band, a broad negative band centered at 260 nm, with
a shoulder at 267 nm, as well as a positive band centered at
242 nm. The complexes of 1b gave equal and opposite spectra
of those seen for 1a.
Upon ligand excitation (of both 1a and b), a broad ligand-
centered fluorescence band (max. at 335 nm) was observed. As
was referred above, the ligands were capable of sensitizing the
characteristic LnIII luminescence. For the EuIII complexes, emis-
sion bands were observed at l=593, 617, 650, and 694 nm,
5
7
arising from the D0! FJ transitions (J=1–4), whereas for TbIII
complexes emission bands were observed at l=487, 541, 580,
5
7
and 619 nm, which represent the D4! FJ transitions (J=6–3).
7
Emission related to the 5D0! F0 transition in the complexes
5
7
[Eu·(1)3](CF3SO3)3 was not observed, and the D0! F2 transition
7
was quite weak compared to the 5D0! F1 transition. The
former observation is consistent with the tris-btp complex
adopting D3 symmetry in solution.[25] Moreover, the chirality of
the lanthanide complexes was further probed by using circular
polarized luminescence (CPL). All the LnIII complexes exhibit
CPL spectra with each pair of enantiomer displaying signals of
equal magnitude and opposite sign for each band characteris-
tic of a specific f–f electronic transition (Figure 4a and b). This
demonstrates that the chirality of the ligands is transferred to
the LnIII complexes. These measurements further confirmed the
enantiomeric and chiral nature of these compounds. However,
the LnIII complexes possess quite low luminescence dissymme-
try factors, glum, compared to previous work by Muller[13a] and
by our group,[12c,f,g] suggesting that the twist away from sym-
metrical arrangement is small. For instance, glum for the most
5
7
5
7
intense transitions in the EuIII complex, D0! F1 and D0! F2,
are À0.016 and +0.023 for 1a. The dissymmetry factor for the
5D0! F3 transition of the TbIII complexes was the largest ob-
7
served, with a value of +0.085 for 1a and À0.081 for 1b.
These values are comparable with chiral organic fluoro-
Figure 4. (a–b) Circularly polarized luminescence spectra of both enantiomers and total luminescence spectra of (a) [Eu·(1)3](CF3SO3)3 and (b) [Tb·(1)3](CF3SO3)3
in CH3CN at room temperature. (c) CD titration data upon addition of a solution of Eu(CF3SO3)3 to a solution of ligand 1b in CH3CN ([1b]ꢀ1”10À5 m), spectra
at 0.0, 0.3, 0.5, and 1.0 equivalents are highlighted. (d) Experimental (”) and calculated (c) binding isotherms at various wavelengths. (e) Recalculated spec-
tra of various Eu(1)n stoichiometries. (f) Calculated speciation distribution diagram.
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phores[26] (see Table S4 in the Supporting Information for full
details).
band (e.g., a negative couplet for 1b); a minor band of oppo-
site sign to the original spectrum centered at 260 nm also de-
veloped. Spectral changes were largely complete upon addi-
tion of 0.5 equivalents, as can be seen from the binding iso-
therms at various wavelengths (Figure 4d); the final spectra
matching those observed for the isolated complexes shown in
Figure 3b. These data were fitted to the same theoretical
models used above using non-liner regression analysis and
gave excellent fits (c.f. Figure 4d). In all cases, binding con-
stants could be determined that complemented those which
were seen in both the absorption and the emission spectra dis-
cussed above (Table S5 in the Supporting Information), that is,
logb1:1 =7.7, logb1:2 =14.9, and logb1:3 =21.0. Furthermore,
these results allowed the calculation of a speciation distribu-
tion diagram for each of the complex stoichiometries formed
(for example Figure 4 f; see the Supporting Information for the
fitting of the titration data of 1a, as well as data for TbIII titra-
tions and data for titrations of 2 with EuIII). Because significant
changes occurred in the CD spectra for each Ln(1)n stoichiome-
try, the recalculated spectra of these stoichiometries can be
used as a “fingerprint” for each species in solution. Figure 4e
demonstrates this, and the potential for employing CD titra-
tions to probe the metal-directed self-assembly of supramolec-
ular systems—a phenomenon not much employed to date.
In summary, by using a one-pot two-stage reaction strategy,
we demonstrated the convenient synthesis of enantiomers of
chiral btp ligands 1–3 from chiral amines and a protected bis-
alkyne, with retention of stereochemistry. Focusing on 1a and
b, both were fully characterized, including by X-ray crystallog-
raphy. The EuIII and TbIII complexes of these were prepared, ex-
hibiting characteristic LnIII-centered luminescence. The chiral
nature of the ligands was probed further using CD and CPL
spectroscopies. Conventional spectroscopic titrations and data
fitting gave reasonably high global stability constants for the
1:1, 1:2, and 1:3 ligand/metal stoichiometric self-assemblies in
solution. Furthermore, by carrying out CD titrations, we were
also able to probe the formation of each of these species in
solution by observing the chiroptical spectroscopic changes.
This allowed us to identify the fingerprint spectra for each of
the stoichiometries and determine the stepwise stability con-
stants, which were comparable to those obtained by conven-
tional methods, allowing a new approach to probing self-as-
sembly formation in real time.
To investigate the formation of the LnIII complexes of 1, the
changes in the UV/Vis absorption, fluorescence emission, and
the LnII-centered emission were all monitored, upon addition
of Ln(CF3SO3)3, in CH3CN solution at room temperature. Upon
the addition of either EuIII or TbIII, the absorbance band of 1,
centered at 300 nm, was redshifted by 15 nm, whereas the
more absorbing band at 231 nm was only shifted slightly ap-
proximately 3 nm with concomitant hypochromic effect (an ex-
ample is shown in Figure 5a). All spectral changes were com-
plete upon addition of 0.5 equivalents. Simultaneously, the
ligand fluorescence was quenched, and the characteristic LnIII-
centered emission spectra were sensitized, reaching a maxi-
mum at 0.5 equivalents (see the Supporting Information).
Figure 5. (a) UV/Vis absorption and (b) TbIII-centered luminescence titration
results upon addition of a solution of Tb(CF3SO3)3 to ligand 1a in CH3CN
([1a]ꢀ1”10À5 m). Spectra at 0.0, 0.3, 0.5, and 1.0 equivalents are highlight-
ed. Insets show experimental (”) and calculated (c) binding isotherms.
Global stability constants of each species were estimated by
fitting both the UV/Vis absorbance and the Ln luminescence
data using the non-linear regression analysis program ReactLab
Equilibria (Jplus Consulting Pty Ltd), using the following equili-
bria: Ln+1$Ln(1) (logb1:1); Ln+2(1)$Ln(1)2 (logb1:2); Ln+
3(1)$Ln(1)3 (logb1:3). For the absorbance data of 1a, upon ad-
dition of Eu(CF3SO3)3 in CH3CN, values of logb for the various
equilibria were found to be 6.7, 13.4, and 20.9, respectively,
whereas for the luminescence titrations, the binding constants
for 1a in these three equilibria were determined as 7.0, 13.8,
and 19.0 respectively, with very similar results for the R,R-enan-
tiomer (Table S5 in the Supporting Information; experimental
binding isotherms and calculated fitting curves are shown in
Supporting Information as well).
Experimental Section
Given the remarkable changes observed between the CD
spectra of the ligands and those of their corresponding com-
plexes (c.f. Figure 3), with the occurrence of bisignate Cotton
effects for both complexes, we also carried out CD titrations
upon the addition of Ln(CF3SO3)3 solution with ligands 1. The
overall CD changes are shown in Figure 4c. The ligand spec-
trum featured bands centered at 300 and 235 nm of equal
signs. Throughout the course of the titration, minimal changes
were observed in the band centered at 235 nm; however, the
band at higher wavelength (300 nm) underwent a redshift of
15 nm and exhibited an intense Cotton effect, giving a bi-
signate CD couplet of the same sign as the respective ligand
General experimental procedure: To a suspension of ImSO2N3·H2SO4
(0.24 mmol, 65 mg), K2CO3 (0.40 mmol, 55 mg), and CuSO4·5H2O
(0.04 mmol, 10 mg) in CH3OH (1.7 mL) was added the chiral amine
(0.20 mmol). Significant color change was observed after 5 h (6 h
for 4), then 0.5 mL of an aqueous solution of sodium ascorbate
(0.09 mmol, 18 mg) and K2CO3 (0.20 mmol, 28 mg) was added
along with 0.75 mL tBuOH, and the reaction mixture degassed
with argon. 2,6-Bis(TMS-ethynyl)pyridine (0.11 mmol, 30 mg) in
DMF (0.3 mL) was added by syringe, and the reaction mixture was
stirred at room temperature for 18 h. Reaction mixture was con-
centrated, aqueous EDTA/NH4OH (1:9, pH 9) solution added, and
product was extracted into CH2Cl2. Product was isolated upon tritu-
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This research was supported by the Irish Research Council for
Science, Engineering & Technology (IRCSET) and the Irish Re-
search Council (IRC; PhD Scholarship J.P.B.), University of
Dublin and Science Foundation Ireland (SFI;SFI PI Awards 10/
IN.1/B2999 and 13/IA/1865), for funding. We thank Dr. Oxana
Kotova and Dr. Jennifer Jones for useful discussions.
Keywords: chirality · circular dichroism · click chemistry ·
lanthanides · 1,2,3-triazoles
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[25] Relative intensities of bands related to various transitions in EuIII com-
plexes give insight into the geometry of the metal ion. The absence of
7
the 5D0! F0 emission band is consistent with the absence of an inver-
sion center in the structure. Of the possible point groups for a 1:3 tris-
chelate complex, only D3 symmetry fulfills this criterion. This phenom-
enon is discussed in the following references: R. D. Peacock, Struct.
Bonding (Berlin) 1975, 22, 83; A. S. Souza, Y. A. R. Oliveira, M. A. Couto
dos Santos, Opt. Mater. 2013, 35, 1633. Moreover, the intensity of the
7
hypersensitive electric dipole transition band (5D0! F2) is determined
by geometry, being absent at octahedral geometry and non-zero for
lower symmetries. In this case, this band is of lower intensity than the
7
5D0! F1, which is relatively unusual; ligand-polarization theory sup-
ports the assignment of trigonal antiprismatic geometry about the
metal ion, in which the ligators are closest to octahedral positions. See
S. F. Mason, R. D. Peacock, B. Stewart, Mol. Phys. 1975, 30, 1829 for fur-
ther discussion of ligand polarization theory.
[26] G. Muller, Dalton Trans. 2009, 9692.
Received: October 22, 2015
Published online on December 7, 2015
Chem. Eur. J. 2016, 22, 486 – 490
www.chemeurj.org
490
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