Synthesis of Homoditopic Ligands with an Incrementable Rodlike Backbone

Article abstract of DOI:10.1002/ejoc.201601081

We describe the synthesis of architectures that consist of a symmetrical rodlike oligo(phenylene-ethynylene) (OPE) backbone of incrementable length connected to a pair of classical ligands for metal coordination. OPE spacers decorated with various end gro

Full text of DOI:10.1002/ejoc.201601081

DOI: 10.1002/ejoc.201601081
Full Paper
Homoditopic Platforms
Synthesis of Homoditopic Ligands with an Incrementable
Rodlike Backbone
Paul Demay-Drouhard,[a,b] Lise-Marie Chamoreau,[c] Régis Guillot,[d] Clotilde Policar,*[a,b] and
Hélène C. Bertrand*[a,b]
Abstract: We describe the synthesis of architectures that con- acetylene repeat units were quickly obtained through a bidirec-
sist of a symmetrical rodlike oligo(phenylene-ethynylene) (OPE) tional approach. Efficient further functionalization with useful
backbone of incrementable length connected to a pair of classi- coordinating groups were achieved. The resulting homoditopic
cal ligands for metal coordination. OPE spacers decorated with platforms are of interest in numerous fields ranging from supra-
various end groups and incorporating up to seven phenylene- molecular chemistry to materials science.
Introduction
resonance imaging (MRI)[9–11] and molecular recognition[12,13]
Ditopic ligands, that is, ligands that possess two distinct coordi-
due to their luminescent properties[14,15] as well as in model
nation sites,[1] are widely employed building blocks as they can
systems in which the spin–spin distances were measured by
lead to well-ordered supramolecular assemblies when coordi-
using pulsed electron paramagnetic resonance (EPR) meth-
nated to metal cations.[2] Starting from very simple molecules,
ods.[16–19]
complex and aesthetic architectures that display unusual prop-
The generation of ditopic platforms incorporating two li-
erties can be easily obtained through coordination-driven self-
gands connected to a central rigid molecular wire of increment-
assembly.[3] Hence, an impressive number of discrete metallacy-
ing length is needed for diverse applications. This has been
cles or cages have been prepared since the pioneering work of
achieved with bis-terpyridine (bis-Tpy) oligomers with ethynyl-
Lehn[4] and Sauvage.[5] In these molecular edifices, the ditopic
thiophene,[20] ethynylnaphthalene[21] or phenylene-ethynyl-
ligands often consist of two coordinating moieties connected
ene[22–24] repeat units. Oligo(phenylene-ethynylene) (OPE) spa-
to a rigid rodlike backbone. These “struts” can play the role of
cers[25–29] of incrementing length spin-labelled with GdIII com-
pillars in host–guest coordination cages[6] or organic linkers in
plexes of the ligand PyMTA (pyridine methylenediamine tet-
metal–organic frameworks (MOFs),[7] but they have numerous
raacetate), a derivative of dipicolinic acid (DPA), have also been
applications. For instance, metal complexes of bis-terpyridines
synthesized to act as molecular rulers for EPR-based distance
can form discrete dyads that can act as molecular devices.[8]
measurements.[30,31] Such systems, in which the ligand–ligand
Dinuclear lanthanide or MnII complexes of 1,4,7,10-tetraazacy-
distance can be easily tailored, are also very useful for control-
clododecane-1,4,7,10-tetraacetic acid (DOTA) and its counter-
ling the size of host–guest cavities or the porosity of MOFs.[32]
part missing an acetate arm (DO3A) linked through an aromatic
In this paper we describe an efficient strategy for building
spacer have also proved useful as contrast agents in magnetic
systems incorporating a central incrementable OPE rod con-
nected at both ends to a Tpy, PyMTA, DPA, DO3A or DOTA li-
gand. First, we present a method to rapidly generate OPE link-
ers with various end groups and then describe efficient tech-
niques to graft the aforementioned ligands. Discrete OPE
oligomers with up to 16 phenylene-acetylene repeat units,
[a] Département de Chimie, Ecole Normale Supérieure, PSL Research
University, UPMC Univ Paris 06, CNRS, Laboratoire des Biomolécules (LBM),
24 rue Lhomond, 75005 Paris, France
E-mail: helene.bertrand@ens.fr
equipped with diverse solubilizing moieties, have been pre-
pared by using different strategies that have been recently re-
viewed.[33] We have chosen the so-called bidirectional ap-
proach,[34,35] which is well suited to the synthesis of symmetri-
cal OPE linkers. To improve its efficiency, we decided to deriva-
tize only one phenyl ring out of two with small poly(ethylene
glycol) (PEG) chains so that the OPE spacer can be quickly elon-
gated (four repeat units added in one step) while maintaining
good solubility in organic solvents. This methodology is very
convergent as all the building blocks can be obtained in only a
few synthetic steps and OPE linkers with five or seven repeat
clotilde.policar@ens.fr
http://www.chimie.ens.fr/?q=en/accueil_umr_7203
[b] Sorbonne Universités, UPMC Univ. Paris 06, Ecole Normale Supérieure,
CNRS, Laboratoire des Biomolécules (LBM),
24 rue Lhomond, 75005 Paris, France
[c] Institut Parisien de Chimie Moléculaire, Sorbonne Universités,
UPMC Univ Paris 06, CNRS UMR 8232,
4 place Jussieu, 75252 Paris, France
[d] Institut de Chimie Moléculaire et des Matériaux d′Orsay,
Université Paris-Sud, UMR CNRS 8182, Université Paris-Saclay,
91405 Orsay, France
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201601081.
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units (i.e., up to 4.7 nm) are generated with the longest linear
sequence of five or six steps, respectively.
Results and Discussion
The central diethynyl building block was synthesized starting
from commercially available p-bromobenzaldehyde (1), which
was converted into the monoprotected diethynylbenzene 2 in
three steps (global yield: 81 %) according to a Sonogashira/
Corey–Fuchs sequence described by Nierengarten et al. with
slight modifications.[36] Indeed, under the conditions described,
a mixture of the desired alkyne 2 and the corresponding
bromoalkyne was obtained. The use of tBuLi as a stronger base
than LDA (for definitions of reagents, see the Experimental Sec-
tion) was necessary to convert the remaining bromoalkyne into
compound 2. This intermediate was then engaged in a Sonoga-
shira reaction with the diiodo building block 3, synthesized in
three steps under conditions previously described,[18] to afford
the expected coupling product 4. Compound 4 was depro-
tected with TBAF to give the OPE linker 5 equipped with eth-
ynyl end groups in gram amounts (Scheme 1). Its structure was
confirmed by X-ray crystallography, as yellow needles of OPE1-
diCCH 5 were obtained by slow evaporation from a 1:1 toluene/
DCM mixture (Figure 1A). This compound crystallizes in the P21
space group (monoclinic system), with two molecules per asym-
metric unit.
Figure 1. ORTEP drawings of (A) OPE1-diCCH 5 and (B) tetraester 23.
Hydrogen atoms have been omitted for clarity; ellipsoids are drawn at proba-
bility levels of 50 %.
tween compound 3 and 0.75 equiv. of the corresponding p-
ethynylbenzene, with limited formation of the dicoupled prod-
uct (between 5 and 10 %). Compounds 8 and 9 were then en-
gaged in a double Sonogashira coupling reaction with either
commercially available p-diethynylbenzene or OPE linker 5 to
obtain the corresponding linkers 10–13 in moderate-to-good
yields (Scheme 2).
Other useful functional groups can be generated. The reac-
tions of compounds 10 and 12 with bromoacetyl bromide in
the presence of NEt3 gave products 14 and 15, respectively,
equipped with a CH2Br moiety, which can react further with
various modules through nucleophilic substitution reactions.[18]
Compounds 7, 11 and 13 were oxidized to the corresponding
carboxylic acids 16, 17 and 18 in nearly quantitative yields by
using Oxone in DMF, as described previously for the corre-
sponding shorter OPE spacer (Scheme 2).[18]
OPE linkers 6, 7 and 10–18 were subsequently used for the
synthesis of homoditopic platforms. To illustrate the diversity of
the possible functionalizations, a selection of the described OPE
linkers were chosen to react with selected ligands. Terpyridine
19 equipped with a phosphonate group was synthesized in two
steps starting from p-tolyl-terpyridine by a radical bromination
with N-bromosuccinimide (NBS) and azobis-isobutyronitrile
(AIBN) followed by an Arbuzov reaction with P(OEt)3, according
to a procedure described by Winter et al.[37] The diethyl ester
of DPA 20, incorporating a bromo group in the para position,
was obtained from chelidamic acid by esterification and
bromination with PBr5 following a procedure of Takalo et al.[38]
According to the same procedure, reduction of the ester moie-
ties with NaBH4 followed by bromination with PBr3 gave the
corresponding tribrominated product. Reaction with ethyl
iminodiacetate and substitution with sodium azide, as de-
scribed by Tóth and co-workers, afforded the tetraethyl ester of
PyMTA 21 equipped with an azido moiety (Scheme 3).[39]
A double Horner–Wadsworth–Emmons reaction between
Scheme 1. Synthesis of OPE-diCCH 5.
Under the same conditions, OPE linkers 6 and 7 were synthe-
sized from compound 3 and 2.0 equiv. of the corresponding p-
ethynylbenzene equipped with an amine (commercially avail-
able) or an aldehyde group (synthesized in two steps), as de-
scribed previously.[18] Similarly, the iodinated coupling partners
8 and 9 were obtained by a mono-Sonogashira reaction be- terpyridine 19 and OPE1-diCOH 7 in the presence of NaH af-
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Scheme 2. Synthesis of compounds 6, 7 and 10–18.
composition, whereas no reaction was observed at room tem-
perature in the same solvent due to the poor solubility of the
reactants. The structure of tetraester 23 was confirmed by X-ray
crystallography: yellow crystals were grown from a deuteriated
chloroform solution. This molecule crystallizes in the P1 space
¯
group (triclinic system) with one of the ethyl groups being
slightly disordered. In contrast to linker 5, the five aromatic
rings are nearly coplanar: parallel sheets form in the packing
unit due to a combination of π stacking and hydrogen bonds
(Figure 1B).
The OPE1-bisDPA platform 24 was then obtained in nearly
quantitative yield by ethyl ester hydrolysis with LiOH followed
by treatment with ion-exchange resin Amberlite IR-120. Finally,
PyMTA tetraester 21 and linker 5 were connected by click
chemistry using CuI in MeCN, as proposed by Tóth et al.,[39] to
give octaester 25, which was deprotected to afford the OPE1-
bisPyMTA module 26 (Scheme 4).
The dicarboxylic acid terminated OPE linkers can be func-
tionalized through amidation reactions. Compounds 16–18
were treated with commercially available tri-tBu-DO3A in the
presence of HOBt and DCC or HATU as coupling agents to
afford the protected platforms 27–29. The smallest hexaester
Scheme 3. Synthesis of pyridine derivatives 19–21.
forded the corresponding OPE1-bisTpy 22 in good yield (80 %; 27 was then deprotected with TFA to give the bisDO3A module
Scheme 4). The use of KOtBu, as recommended by Winter et 30 after HPLC purification. Finally, to illustrate the diversity of
al.,[37] led to the recovery of the reactants. Next, a Sonogashira the functionalization methods available with these OPE linkers,
coupling between the diethyl ester of DPA 20[38] and OPE1- compounds 14 and 15 were treated with the previously
diCCH 5 in NEt3/DMF generated the corresponding product 23. reported tri-Pp-DO3A[18] to give platforms 31 and 32. The
Noteworthy, the use of NEt3/THF as solvent at reflux led to de- 2-phenylisopropyl (Pp) groups were deprotected with 2 % TFA
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Scheme 4. Synthesis of OPE1-bisTpy 22, OPE1-bisDPA 24 and OPE1-bisPyMTA 26.
Scheme 5. Synthesis of bisDO3A 30 and bisDOTAs 33–34.
in DCM to afford bisDOTA compounds 33 and 34 after HPLC
purification (Scheme 5).
Coordination chemistry studies of the described ditopic li-
gands will be performed with various transition-metal ions. In
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cal shifts (δ) are given in parts per million (ppm) and coupling con-
stants (J) in Hz. HRMS using electrospray ionization (ESI) was per-
formed at the Université Paris Sud in the Service de Spectrométrie
de masse of the ICMMO (Institut de Chimie Moléculaire et des Maté-
riaux d'Orsay). ESI-HRMS was also performed at UPMC (IPCM). Tpy
ligand 19 and ethyl esters of DPA 20 and PyMTA 21 were synthe-
sized as described previously.[37–39] p-Ethynylbenzaldehyde, tri-Pp-
DO3A, linkers OPE1-diNH2 6, OPE1-diCOH 7 and OPE1-diCO2H 16
were synthesized as described previously by us.[18] The following
abbreviations are used: Cy (cyclohexane), DCC (N,N′-dicyclohexyl-
carbodiimide), DIPEA (N,N′-diisopropylethylamine), DO3A (1,4,7,10-
tetraazacyclododecane-1,4,7-triacetic acid), DOTA (1,4,7,10-tetraaza-
cyclododecane-1,4,7,10-tetraacetic acid), EDTA (ethylenediamine-
tetraacetic acid), HATU {1-[bis(dimethylamino)methylene]-1H-1,2,3-
triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate}, HOBt (N-
hydroxybenzotriazole), LDA (lithium diisopropylamide), OPEG (di-
ethylene glycol monomethyl ether), TBAF (tetra-n-butylammonium
fluoride), TEA (triethylamine), TFA (trifluoroacetic acid).
preliminary experiments, the reaction of OPE1-bisDPA 24 with
Mn(ClO4)2·6H2O (2.5 equiv.) in a mixture of THF and H2O (1:1)
afforded a yellow precipitate, the very low solubility of which
in classical solvents precluded any further investigation. The re-
action of OPE1-bisPyMTA 26 with MnCl2 (2.5 equiv.) at con-
trolled pH afforded the expected bis-Mn complex as a yellow
solid (as shown by mass spectroscopy, see the Supporting Infor-
mation), which was soluble in water at pH 8. In the DO3A/DOTA
series, the formation of the bis-Mn complex of OPE1-bisDOTA
compound was also attested by mass spectroscopy.[18]
Conclusions
Various systems consisting of a pair of ligands connected
through an OPE wire of incrementing length have been effi-
ciently synthesized. The methodology employed in this work is
quick, convergent and provides linkers with various end groups
that can be easily further functionalized with useful ligands.
Such architectures could easily find applications in numerous
domains, from supramolecular chemistry to materials science.
The coordination chemistry of the ditopic ligands described
herein is currently being explored with various transition-metal
ions and the resulting homodinuclear complexes will be stud-
ied for their properties in different fields such as pulsed EPR-
based distance measurements and coordination-driven self-as-
sembly. Further investigations, such as the characterization of
the conductance of the OPE rods and of the electronic commu-
nication between metal centres in selected complexes and anal-
ysis of their mixed-valence character, would be of high interest.
Finally, adaptation of the methodology to the efficient synthesis
of disymmetrized heteroditopic ligands is currently being inves-
tigated.
General Procedure
A
—
Mono-Sonogashira Coupling:
Ph(OPEG)2I2 (3; 1.0 equiv.) and p-ethynylaniline or p-ethynylbenzal-
dehyde (0.75 equiv.) were dissolved in a mixture of dry THF and dry
TEA. Argon was bubbled through the solution for 15 min and then
[PdCl2(PPh3)2] (0.1 equiv.) and CuI (0.2 equiv.) were added. The re-
sulting suspension was stirred at room temp. for 48 h, a sat. aq.
solution of NH4Cl was added and the mixture was extracted with
DCM (3 ×). The combined organic layers were dried with Na2SO4,
filtered and concentrated. Column chromatography afforded the
corresponding products 8 and 9.
General Procedure B — Bis-Sonogashira Coupling: Iodide 8 or 9
(1.0 equiv.) and diyne 5 or p-diethynylbenzene (2.0 equiv.) were
dissolved in a mixture of dry THF and dry TEA. Argon was bubbled
through the solution for 15 min and then [PdCl2(PPh3)2] (0.1 equiv.)
and CuI (0.2 equiv.) were added. The resulting suspension was
stirred for 24 h at room temp., a sat. aq. solution of NH4Cl was
added and the mixture was extracted with DCM (3 ×). The com-
bined organic layers were dried with Na2SO4, filtered and concen-
trated. Column chromatography afforded the corresponding OPE
linkers 10–13.
General Procedure C — Synthesis of Dibromo OPE Linkers 14
and 15: Linker 10 or 12 (1.0 equiv.) was dissolved in dry DCM. The
resulting solution was cooled to 0 °C, and TEA (2.5 equiv.) was
added, followed by bromoacetyl bromide (2.5 equiv.) dissolved in
dry DCM dropwise. The resulting mixture was warmed to room
temp. for 3 h and washed with a sat. aq. solution of NaHCO3 (1 ×)
and a sat. aq. solution of NaCl (1 ×). The organic layer was dried
with Na2SO4, filtered and concentrated. Column chromatography
afforded the corresponding dibromo OPE linkers 14 and 15.
Experimental Section
General: Unless otherwise stated, all syntheses were performed un-
der inert atmosphere (argon or nitrogen). Reagents and chemicals
were used without further purification. Dry solvents [dichloro-
methane (DCM), MeCN, THF, dioxane, dimethylformamide (DMF)]
were purchased from commercial sources and used without further
purification. Triethylamine (TEA) was dried with CaH2, distilled un-
der argon and stored over 4 Å molecular sieves under argon. Pre-
parative column chromatography was carried out with silica gel (Si
60, 40–63 μm) unless otherwise stated. Analytical TLC analysis was
carried out on silica gel (60F-254) with UV visualization at 254 and
366 nm. Analytical HPLC measurements were performed with a Di-
onex Ultimate 3000 instrument using C18A ACE columns. Prepara-
tive HPLC was performed with a Waters 600 instrument using an
XBridgeTM Prep C18 OBDTM column. Gradients of MeCN in H2O,
both containing 0.1 % TFA, were employed. Products were moni-
tored by UV detection. 1H and 13C NMR spectra were recorded with
a 300 MHz spectrometer at Ecole Normale Supérieure (ENS) in the
General Procedure D — Synthesis of Dicarboxylic OPE Linkers
17 and 18: Linker 11 or 13 (1.0 equiv.) was suspended in dry DMF.
KHSO5·KHSO4·K2SO4 (Oxone®, 2.0 equiv.) was added and the result-
ing suspension was stirred at room temp. for 15 h. H2O was added
and the resulting precipitate was filtered, washed with H2O, taken
up in acetone and dried to afford the corresponding dicarboxylic
OPE linkers 17 and 18.
General Procedure E — Synthesis of tBu-Protected Bis-DO3A
Compounds 28 and 29: Linker 17 or 18 (1.0 equiv.) and HATU
Laboratoire des Biomolécules (LBM, UMR 7203) with solvent residu- (2.2 equiv.) were dissolved in dry DMF and stirred at room temp.
als as internal references [CDCl3: 7.26 ppm for 1H NMR and
77.16 ppm for 13C NMR; (CD3)2SO: 2.50 ppm for 1H NMR and
for 10 min. Tri-tBu-DO3A (2.5 equiv.) was added and the resulting
mixture was stirred at room temp. for 20 min. DIPEA (6.0 equiv.)
39.52 ppm for 13C NMR]. The following abbreviations are used: sin- was added and the resulting mixture was stirred at room temp. for
glet (s), doublet (d), triplet (t), multiplet (m) and broad (br.). Chemi- 48 h and concentrated. The residue was taken up in DCM, washed
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with H2O (1 ×) and a sat. aq. solution of NaCl (1 ×), dried with
Na2SO4, filtered and the solvents evaporated. Column chromatogra-
phy afforded the corresponding tBu-protected bisDO3A com-
pounds 28 and 29 as yellow sticky solids.
Acknowledgments
This work was financially supported by ANR MnHFPELDOR,
ANR-Deutsche Forschungsgemeinschaft (DFG) Chemistry (2011-
INTB-1010-01, Ph. D. scholarship to P. D.-D.).
General Procedure F — Synthesis of Pp-Protected BisDOTA
Compounds 31 and 32: Linker 14 or 15 (1.0 equiv.) and tri-Pp-
DO3A (2.5 equiv.) were mixed in dry MeCN. K2CO3 (10.0 equiv.) was
added and the resulting mixture was heated at 60 °C for 15 h and
concentrated. DCM was added and the mixture was washed with
H2O (1 ×) and a sat. aq. solution of NaCl (1 ×), dried with Na2SO4,
filtered and concentrated. Column chromatography afforded the
corresponding Pp-protected bisDOTA compounds 31 and 32 as yel-
low solids.
Keywords: Ligand design · N ligands · Synthetic methods ·
Rigid spacers · Supramolecular chemistry
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Yellow stick-like crystals of compound 23 suitable for X-ray crystal-
lography were obtained from a deuteriated chloroform solution. A
single crystal of the compound was selected, mounted onto a
CryoLoop and transferred in a cold stream of nitrogen gas. Intensity
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with a riding model. A model of disorder was introduced for a termi-
nal ethyl chain. The crystal data collection and refinement parame-
ters are given in Table S1 in the Supporting Information.
CCDC 1484299 (for 5) and 1485975 (for 23) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
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Received: September 2, 2016
Published Online: December 20, 2016
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