Utility of tris(4-bromopyridyl) europium complexes as versatile intermediates in the divergent synthesis of emissive chiral probes

Article abstract of DOI:10.1039/c4dt00253a

The synthetic utility of europium complexes with three coordinated 4-bromopyridyl groups for chromophore elaboration has been assessed in palladium-catalysed Sonogashira coupling reactions, and in copper(i) mediated click reactions of the triazide derivative, generated in situ. The crystal structure of the Eu complex of a p-OMe-phenyl substituted triazole at 100 K is reported in which the pendant triazole sensitising moieties interdigitate in the solid-state lattice. The triazole complex can be separated into Δ and Λ enantiomers by chiral HPLC but is weakly emissive in methanol (ε 5.5 mM^-1 cm^-1; λ_exc 320 nm; 0.2%), contrasting with a set of four alkynyl-aryl derivatives which are one thousand times brighter and absorb strongly with broad absorption maxima in the range 332 to 360 nm. An enantiopure europium complex gives an intense CPL signal in solution that is the strongest yet reported.

Full text of DOI:10.1039/c4dt00253a

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Utility of tris(4-bromopyridyl) europium
complexes as versatile intermediates in the
divergent synthesis of emissive chiral probes†
Cite this: Dalton Trans., 2014, 43,
5721
Stephen J. Butler,a Martina Delbianco,a Nicholas H. Evans,a,b Andrew T. Frawley,a
Robert Pal,a David Parker,*a Robert S. Puckrina and Dmitry S. Yufita
The synthetic utility of europium complexes with three coordinated 4-bromopyridyl groups for chromo-
phore elaboration has been assessed in palladium-catalysed Sonogashira coupling reactions, and in
copper(I) mediated click reactions of the triazide derivative, generated in situ. The crystal structure of the
Eu complex of a p-OMe-phenyl substituted triazole at 100 K is reported in which the pendant triazole
sensitising moieties interdigitate in the solid-state lattice. The triazole complex can be separated into Δ
and Λ enantiomers by chiral HPLC but is weakly emissive in methanol (ε 5.5 mM−1 cm−1; λexc 320 nm;
ϕ 0.2%), contrasting with a set of four alkynyl–aryl derivatives which are one thousand times brighter and
absorb strongly with broad absorption maxima in the range 332 to 360 nm. An enantiopure europium
complex gives an intense CPL signal in solution that is the strongest yet reported.
Received 23rd January 2014,
Accepted 8th February 2014
DOI: 10.1039/c4dt00253a
www.rsc.org/dalton
examples to be found in contrast agent research, where Gd
complexes of octadentate ligands, with between one and four
Introduction
In the synthesis of metal coordination complexes, it is most peripheral functional groups, serve as intermediates in the
common to introduce the metal centre in the final step. This synthesis of higher molecular weight conjugates. Typically,
approach is often used in order to simplify the preparation these examples involve amide bond formation or coordination
and purification of ligand intermediates in non-polar media, of a pendant chelate to a second metal ion.7–10
and because many functionalisation reactions involve rather
In the development of emissive lanthanide optical probes,
harsh reactions conditions under which dissociation of the it is particularly attractive to devise a kinetically stable and syn-
metal ion may occur, e.g. extremes of pH. An alternative strat- thetically versatile precursor complex that can be modified
egy seeks to identify a robust metal coordination complex that easily, as this approach allows systematic changes in the struc-
can serve as a late intermediate in the synthesis of the target ture of the sensitising chromophore to be undertaken at the
complex, bearing an appropriate functional group (or groups) end of the synthesis. This divergent strategy contrasts with the
that can be modified or elaborated under relatively mild con- linear synthetic approaches usually adopted, wherein a single
ditions. Examples of this divergent approach can be found in sensitising moiety is engineered into the ligand at the begin-
several different areas of coordination chemistry with kineti- ning of the synthesis. It allows systematic variations to be
cally inert metal complexes. For example, Williams has used a made that permit an optimisation of the absorption properties
variety of Suzuki coupling reactions with iridium(III) complexes (λmax and ε), whilst retaining the common coordination
bearing aryl bromide functionality, either to extend ligand con- environment about the lanthanide ion that primarily deter-
jugation or create multimeric systems.1,2 In lanthanide mines the chemical stability of the complex and its resistance
systems, modular syntheses of heterometallic complexes have to excited state quenching.
been devised by using kinetically stable complexes of hepta-
Owing to their line-like emission spectra, large Stokes’
dentate and octadentate ligands, in a series of Ugi, Pd(0) or shifts and long-lived excited states, emissive lanthanide com-
Cu(I) catalysed ‘click’ reactions.3–6 There are also many plexes are proving to be excellent probes of biological systems
and fluids.11–19 Recently, a series of very bright Eu(III) phosphi-
nate and carboxylate complexes has been reported containing
pyridyl–alkynyl–aryl chromophores (Fig. 1).12 These bright
complexes (ε ∼ 60 mM−1 cm−1 and ϕem in the range 20 to
50%), possess broad absorption bands with λmax ∼ 330 to
350 nm, meaning they can be excited using lasers or light
emitting diodes at 337, 355, 365 or 375 nm, as required for use
aDepartment of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
E-mail: david.parker@dur.ac.uk
bDepartment of Chemistry, Lancaster University, Lancaster, LA1 4YB, UK
†Electronic supplementary information (ESI) available. CCDC 982687. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt00253a
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Fig. 1 Retrosynthetic analysis of tris-pyridyl–alkynyl–aryl phosphinate complexes.
in cellulo. The peripheral functionalisation of a second gene-
ration of complexes, has allowed for the staining of specific
cell organelles, and time-resolved microscopy and spectral
imaging studies have been investigated.13 A simple retrosyn-
thetic analysis of these systems (Fig. 1) reveals that the tris-
(4-bromopyridine) complex, [Eu·L1] can serve as a common
late intermediate and be used to prepare a wide range of ana-
logues, via palladium-catalysed C–C bond forming reactions to
sp2 or sp hybridised carbon centres.
To be used efficiently as stains and probes the complexes
should be amenable to structural modification, for example to
include substituents of varying steric demand, charge centres
and/or hydrogen bond donors/acceptors, to tune affinity and
gain selectivity. The triazole-forming copper-catalysed alkyne–
azide cycloaddition (CuAAC) reaction is an attractive chemical
reaction that allows such synthetic control, due to its wide
applicability.20 The tribromo-complex, [Eu·L1] can be easily
transformed into the corresponding triazide, owing to the
enhanced electrophilicity of the C–Br bond. Interestingly, only
two examples of lanthanide complexes incorporating pyridyl–
triazole–aryl acyclic chelating ligands appear in the literature
Fig. 2 Selected reported pyridyl–triazole–aryl lanthanide complexes.21,22
(Fig. 2).21,22 Critically, each report states that the complexes
have an absorption wavelength of 320 nm or more and, in the
latter example, sensitised excitation of Eu(III) has been
demonstrated.
Scheme 1 Synthesis of the trispyridyl–triazole–anisole Eu(III) complex, [Eu·L2].
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show that substitution of pyridyl–alkynyl–aryl chromophores
by pyridyl–triazole–aryl moieties allows the creation of a
lanthanide complex that can also be excited at the wavelengths
used in cellular studies. In addition to solution phase charac-
terisation, a single crystal structure determination is reported
of the triazole complex, [Eu·L2], as well as the characterisation
of a Eu complex, [Eu·L6], by circularly polarised luminescence
that reveals it to be the brightest enantiopure complex yet
observed; it possesses the strongest total CPL signal that has
been measured.
Results and discussion
Synthesis and characterisation of a triazole complex
Fig. 3 The molecular structure of [Eu·L2] (ellipsoids are drawn at the
50% probability level; one of the disordered components and the H
atoms are omitted for clarity: CCDC 982687).
A tris-pyridyl–triazole–anisole Eu(III) complex was prepared in
a two-step, one-pot sequence from the recently reported tris-
In this work, we exemplify the synthetic utility of [Eu·L1] in (4-bromopyridyl) complex, [Eu·L1] (Scheme 1).23 Substitution
a set of Sonogashira reactions that allows the synthesis of four of the bromide by azide was carried out at room temperature
different aralkynyl substituted complexes. In addition, we in DMF using excess sodium azide. The reaction was
Fig. 4 Views of the packing of [Eu·L2] in the crystal lattice (100 K), showing the channels in the lattice arising from π-stacking of the aryl-substituted
chromophores.
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quantitative and generated the intermediate triazide complex,
which was used directly without separation of the excess
sodium azide. The second step, the Cu-catalysed “click” cyclo-
addition occurred under mild conditions in aqueous DMF,
requiring the addition of sub-stoichiometric amounts of the
accelerant,
tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine
(TBTA).‡
The complex was characterized by 1H and 31P NMR spec-
troscopy and ES mass spectrometry (ESI). The presence of a
single peak in the 31P NMR spectrum is consistent with the
three-fold complex symmetry in solution. In addition, very
small single crystals were grown by evaporation of a water–
methanol solution of the complex, and were suitable for X-ray
crystallographic structure determination using synchrotron
radiation (Fig. 3). Even after taking account of the disorder
present in the structure, there is a clear loss in the C3 sym-
metry of the complex in the solid state, as was observed in the
structure of a closely related europium tris-alkynyl complex, in
which P-phenyl substituents replace the P-methyl groups in
[Eu·L2].12
Two “arms” of the molecule [Eu·L2] are essentially flat (the
absolute values of minimal torsion angles around the bonds,
connecting aromatic rings, are less than 8°) while the third
one is disordered over two positions and noticeably twisted –
the average torsion angles between pyridyl/triazole and tri-
azole/anisole rings are ca. 12° and 33° respectively. Such a
difference in the conformation of the pendant arms is prob-
ably induced by a different crystal environment. Indeed, the
two flat “arms” form extensive π⋯π interactions between
them, while the disordered arm is linked to adjacent mole-
cules only by weaker C–H⋯π and C–H⋯O interactions, which
operate independently of each other. The combination of
these intermolecular interactions results in a quite unusual
crystal packing, characterized by the presence of wide channels
along the crystallographic c-direction (Fig. 4: lower). The size
of the channels is approximately 13.5 Å × 7.5 Å and each one
contains a significant number of severely disordered solvent
molecules. This phenomenon partially explains the poor diffr-
acting properties of the crystals of [Eu·L2], for which the syn-
chrotron radiation source (Diamond) was used to aid data
acquisition.
Photophysical characterisations were subsequently under-
taken in methanol.§ Critically, the absorption spectrum
reveals that the complex has a broad absorption band at
320 nm, which allows for excitation of the complex at wave-
lengths >340 nm. The europium emission spectrum strongly
resembles that observed for [Eu·L1] and related C3 symmetric ara-
lkynyl complexes in solution, as expected (Fig. 5).12,13 The inter-
action between the chromophores evident in the packing diagram
may help to explain the low overall emission quantum yield of
Fig. 5 Upper: Absorption and emission spectra of [Eu·L2], (MeOH,
295 K); lower: CPL spectrum of the ΔJ = 1 region showing near mirror
image spectra of the Δ (blue: eluting first) and Λ (red) enantiomers (n.b.
photobleaching occurred during data acquisition, leading to a loss of
sensitivity: 10 scans were recorded (1 min each) and summed; [complex]
= 110 μmol).
0.2%, as the intermolecular π-stacking may enhance charge trans-
fer quenching of the intermediate triazole excited states.
The complex [Eu·L2] was analysed by chiral HPLC. The Δ
and Λ enantiomers were separated using a ChiralPAK ID
column, with isocratic MeOH as eluent. The separated isomers
were examined by CPL (λexc 320 nm) and near mirror image
spectra were observed. This particular complex (vide infra) is
1000 times less bright than the alkynyl analogues discussed
below, and was also rather sensitive to photobleaching during
CPL data acquisition, for which ten consecutive scans were
recorded and averaged. The sign and sequence of the CPL tran-
sitions observed were almost identical to those of [Eu·L1] and
its derivatives and analogues.23,24 The absolute configuration
of the enantiomeric complex that eluted first was therefore
deduced to be Δ and, by analogy, it must have an SSS configur-
ation at each chiral P centre and a λ configuration in each of
the three NCH2CH2N chelate rings. Considering that related
chiral Ln(III) complexes have been shown to give rise to large
circularly polarised luminescence (CPL)24 and that CPL spec-
troscopy and, in particular, microscopy remain under-
exploited,25 the ability to separate these enantiomers easily
when substituted with large groups at the 4-pyridyl position is
likely to be rather useful in the development of chiral probes
for future chiroptical studies.
‡Full conversion of the triazide complex to the product tris-triazole was observed
by LC-MS. The low isolated yield is associated with the efficiency of the chroma-
tographic purification steps.
§This complex was insoluble in pure water; hence, solution characterisation was
carried out in methanol.
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Scheme 2 Synthesis of the enantiopure complex [Eu·L6].
Fig. 6 Left: absorption and total emission spectrum of [Eu·L6]; right: circularly polarised emission spectra of (major/black) S-RRR-Λ-[Eu·L6], (MeOH,
295 K, 3 μM, 20 min; gem = +0.30 (708 nm); +0.17 (655 nm); −0.11 (598 nm)), and the enantiomeric Δ-complex (red/minor) with an R stereogenic
centre at C.
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reaction sequences. Given that, in the past, most sensitising
groups in europium complexes have been integrated at an
earlier stage in the synthetic route, the approach illustrated
here offers much greater scope for the creation of larger
families of complexes, as the route is inherently divergent. For
example, it can be anticipated that a wide range of functional-
ity may be incorporated into the aryl ring in each example,
allowing interaction with targets of interest. Considering that
the racemic complexes may be readily resolved by chiral HPLC,
the generation of a series of enantiopure complexes for CPL
investigations may also be envisaged.
Table 1 Selected photophysical data for Eu complexes (MeOH, 295 K)
Complex
λmax (nm)
ε (mM−1 cm−1
)
ϕem (%)
τEu (ms)
[Eu·L2]
[Eu·L3]
[Eu·L4]
[Eu·L5]
[Eu·L6]
320
332
355
331
355
5.50
56.4
55.0
56.6
55.1
0.2
47
43
44
55
1.33
1.15
1.13
1.10
1.05
Sonogashira coupling reactions of tris(4-bromopyridyl)
complexes
The carbon–bromine bond in 4-substituted pyridine deriva-
tives is activated towards oxidative addition by low-valent Pd
species. Indeed, the ligation of the pyridine lone pair to the
europium(III) centre in [Eu·L1] may enhance this reactivity, as
the europium can act as a charge sink stabilising any build up
of charge in the heterocyclic ring.26 Using a mixture of THF
and DMF, it was possible to solubilise the active palladium-
bisphosphinoferrocenyl catalyst and the europium complex
sufficiently to allow the alkyne coupling reaction to occur at
65 °C (Scheme 2). Using [Eu·L1] as the precursor, three
different alkynes were coupled, to give the complexes,
[Eu·L3–5], in 30–48% yield. Analysis of each Sonogashira reac-
tion by LC-MS revealed quantitative conversion of [Eu·L1] to
the desired tris-alkynyl product. However, loss of material in
purification of each complex by reverse-phase HPLC contribu-
ted significantly to the moderate yields obtained.
Experimental
X-Ray crystallography
The data for compound [Eu·L2] were collected at 100.0 K on a
Rigaku Saturn 724+ diffractometer at station I19 of the
Diamond Light Source synchrotron (undulator, λ = 0.6889 Å,
ω-scan, 1.0° per frame, Cryostream (Oxford Cryosystems) nitro-
gen open-flow cryostat) and processed using Bruker APEXII
software. The structure was solved by direct method and
refined by full-matrix least squares on F2 for all data using
SHELXTL29 and OLEX230 software. All non-disordered non-
hydrogen atoms were refined anisotropically, the hydrogen
atoms were placed in the calculated positions and refined in
riding mode. Disordered atoms were refined isotropically with
fixed SOF = 0.5. The channels in the structure of [Eu·L2]
contain a number of severely disordered solvent molecules
(139e/independent part of the unit cell) that were taken into
account by applying a MASK procedure in the OLEX2 program
package. Crystallographic data for the structure have been de-
posited with the Cambridge Crystallographic Data Centre as
supplementary publication CCDC 982687.
Using the analogous C-substituted complex, [Eu·L1b],
derived from S-lysine and 76% enantiomerically pure (see
ESI†),27 the enantiopure complex, [Eu·L6] was prepared. In
methanol solution, each of these Eu complexes is particularly
bright, with molar extinction coefficients of the order of
55 mM−1 cm−1 and overall emission quantum yields of 44 to
55% (Table 1 and Fig. 6). Such properties greatly accelerate the
acquisition of CPL data, and the CPL spectrum of [Eu·L6]
showed strong transitions in the ΔJ = 1 (589–600 nm) and ΔJ = 4
manifold (680 to 715 nm) in particular. The sequence and sign
of the observed transitions were consistent23–25 with formation
of a Λ overall complex helicity, so that the complete stereo-
chemical description of this complex is S-Λ-RRR-δδδ-[Eu·L6].
Given that the brightness, B, of this complex in methanol is
about 25 to 30 mM−1 cm−1 and that the observed gem values
are in the range 0.1 to 0.3 (Fig. 6), this complex gives the stron-
gest CPL signal yet observed for any lanthanide complex in
solution, allowing the spectrum to be acquired with good
signal intensity over a period of 20 minutes, using a 3 μM solu-
tion of the complex.
Crystal data for [Eu·L2]: C54H61EuN15O11P3 (M = 1341.05):
monoclinic, space group C2/c (no. 15), a = 59.156(4), b =
13.3370(10), c = 18.5660(10) Å, β = 102.310(3)°, V = 14 311.1(16)
Å3, Z = 8, T = 100.0(1) K, μ(synchrotron) = 0.926 mm−1, Dcalc
=
1.245 g mm−3, 56 723 reflections measured (1.36 ≤ 2Θ ≤
48.58), 11 499 unique reflections (Rint = 0.0892) were used in
all calculations. The final R1 was 0.0510 (7477 > 2σ(I)) and wR2
was 0.1309 (all data), GOF = 0.989.
NMR spectroscopy and mass spectrometry
1H, 13C, 19F and 31P NMR spectra were recorded in commer-
cially-available deuterated solvents on a Varian Mercury-200
(1H at 199.97 MHz, 13C at 50.29 MHz), Varian Mercury-400 or
Bruker Avance-400 (1H at 399.96 MHz, 13C at 100.57 MHz, 31P
at 161.94 MHz), Varian Inova-500 (1H at 499.77 MHz, 13C at
125.67 MHz,) or Varian VNMRS-700 (1H at 699.73 MHz, 31P at
283.26 MHz) spectrometer. Electrospray mass spectra were
recorded on a Waters Micromass LCT or Thermo-Finnigan
LTQ FT instrument operating in positive or negative ion mode
Summary and conclusions
A tris-pyridyl–triazole–aryl Eu(III) complex and a set of four as stated, with methanol as the carrier solvent. Accurate mass
strongly emissive aralkynyl analogues can be synthesised from spectra were recorded using the Thermo-Finnigan LTQ FT
a common tris(4-bromopyridine) intermediate in one-pot mass spectrometer.
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Alkyne synthesis: 3,4,5-trimethoxyphenylacetylene
tert-butyl (4-formyl-3,5-dimethoxyphenoxy)acetate (1.30 g,
4.4 mmol) in anhydrous dichloromethane (15 mL). The result-
ing red solution was stirred for 18 hours. The solvent was
removed under reduced pressure and the residue was stirred
in diethyl ether, filtered and concentrated under reduced
pressure. The crude product was purified by column chromato-
graphy (silica, neat hexane to 5 : 1 v/v hexane–ethyl acetate) to
give the product as a white solid (1.22 g, 62%); m.p. 122–124 °C;
1H NMR (400 MHz, CDCl3) δ 1.50 (9 H, s, H10), 3.79 (6 H, s,
H11), 4.52 (2 H, s, H7), 6.11 (2 H, s, H5), 7.17 (1 H, s, H2); 13C
NMR (101 MHz, CDCl3) δ 28.2, 55.8, 65.9, 82.7, 91.3, 93.1, 107.8,
130.9, 158.1, 160.1, 167.9; LRMS (ESI) m/z 451; (HRMS+) m/z
450.9771 [M + H]+ (C16H21O679Br2) requires 450.9756.
1,1-Dibromo-2-(3,4,5-trimethoxyphenyl)-ethylene
(1.41
g,
4.04 mmol) was dissolved in anhydrous THF (30 mL) and
cooled to −78 °C. n-Butyl lithium (2.5 M in hexane, 6.4 mL,
16.2 mmol) was added slowly and the mixture was stirred at
−78 °C under argon for 30 min. Water (8 mL) was added and
the mixture was extracted with EtOAc (2 × 30 mL). The combined
organic fractions were washed with half-strength brine solution
(30 mL), dried (MgSO4), and the solvent was removed under
reduced pressure. The residue thus obtained was subjected to
column chromatography (silica gel; neat hexane, then hexane–
EtOAc, 9 : 1 v/v) to give 3,4,5-trimethoxyphenylacetylene as a
white solid (0.53 g, 68%); m.p. 71–73 °C (lit.1 m.p. 70–71 °C);
1H NMR (400 MHz, CDCl3) δ 6.72 (2H, s, H4), 3.85 (3H, s, H8),
3.84 (6H, s, H7), 3.03 (1H, s, H1); 13C NMR (151 MHz, CDCl3)
δ 153.1 (C5), 139.3 (C6), 117.0 (C3), 109.4 (C4), 83.7 (C2), 76.2 (C1),
61.0 (C8), 56.2 (C7); (HRMS+) m/z 193.0845 [M + H]+ (C11H13O3
requires 193.0865); Rf = 0.45 (silica; hexane–EtOAc 4 : 1 v/v). The
spectral data were consistent with those reported previously.28
tert-Butyl (4-ethynyl-3,5-dimethoxyphenoxy)acetate
To a solution of tert-butyl [4-(2,2-dibromoethenyl)-3,5-dimethoxy-
phenoxy]acetate (0.797 g, 1.77 mmol) in dry THF (20 mL) under
argon at −78 °C, was added n-butyllithium (1.42 mL of a 2.5 M
solution, 3.54 mmol). The solution was stirred for 30 min when a
further 0.5 eq. of n-butyllithium was added. The mixture was
stirred for a further 15 min before water (20 mL) was added. THF
was removed under reduced pressure and the remaining aqueous
solution was extracted into dichloromethane (3 × 30 mL). The
organic layers were combined, dried over MgSO4, filtered and con-
centrated under reduced pressure. The crude product was puri-
fied by column chromatography (silica, hexane–ethyl acetate
8 : 1 v/v) to give the product as a white solid (0.248 g, 48%); m.
tert-Butyl (4-formyl-3,5-dimethoxyphenoxy)acetate
To a solution of 4-hydroxy-2,6-dimethoxybenzaldehyde (1.00 g,
5.49 mmol) in
a
dichloromethane–dimethylformamide
mixture (5 : 1, 36 mL) under argon was added potassium car-
bonate (1.52 g, 11.0 mmol) and tert-butyl bromoacetate
(1.62 mL, 2.14 g, 11.0 mmol). The mixture was stirred for 24 h
at which point the potassium carbonate was removed by
gravity filtration. The dichloromethane was removed under
reduced pressure and the dimethylformamide was removed by
vacuum distillation to give the product as a white solid (1.35 g,
83%), which was used without further purification; m.p.
130–132 °C; 1H NMR (400 MHz, CDCl3) δ 1.50 (9 H, s, H9),
3.86 (6 H, s, H10), 4.57 (2 H, s, H6), 6.08 (2 H, s, H4), 10.35 (1 H,
s, H1); 13C NMR (101 MHz, CDCl3) δ 28.2, 56.2, 65.7, 83.2, 91.0,
109.5 164.2, 164.4, 167.2, 187.9; LRMS (ESI) m/z 297 [M + H]+;
(HRMS+) m/z 297.1331 [M + H]+ (C15H21O6) requires 297.1338.
1
p. 107–108 °C; H NMR (700 MHz, CDCl3) δ 1.49 (9 H, s, H10),
3.49 (1 H, s, H1), 3.86 (6 H, s, H11), 4.53 (2 H, s, H7), 6.11 (2 H, s,
H5); 13C NMR (176 MHz, CDCl3) δ 28.2 (C9), 56.3 (C11), 65.9 (C7),
76.4 (C2), 82.9 (C9), 84.2 (C1), 91.2 (C5), 94.1 (C3), 160.2 (C6),
163.1 (C4), 167.6 (C8); LRMS (ESI) m/z 293 [M + H]+; (HRMS+)
m/z 293.1390 [M + H]+ (C16H21O6) requires 293.1389.
Ligand and complex synthesis
[Eu·L1]. The synthesis of the tris p-bromo Eu(III) complex
[Eu·L1] has been reported elsewhere.23 1H NMR (400 MHz,
CD3OD) δ 8.60 (1H, s, pyH), 7.97 (1H, s, pyCHN), 7.12 (1H, s, pyH),
4.36 (1H, s, NCHe′q), 0.54 (3H, s, CH3), −0.77 (1H, s, pyCH′N),
−1.37 (1H, s, NCHa′x), −2.23 (1H, s, NCHeq), −5.28 (1H, s, NCHax).
31P NMR (162 MHz, CD3OD) +39.8. m/z (HRMS+) 1020.851 [M +
H]+ (C27H3479Br3N6O6P3151Eu requires 1020.849). Rf = 0.31 (silica;
CH2Cl2–CH3OH–aq. ammonia: 82/15/3: v/v/v).
[Eu·L2]. CAUTION! Care should be taken using azides and
(poly)triazoles, particularly if scaling up the reaction con-
tert-Butyl [4-(2,2-dibromoethenyl)-3,5-dimethoxyphenoxy]acetate
To a solution of triphenylphosphine (4.62 g, 17.6 mmol) and ditions stated here.
tetrabromomethane (2.92 g, 8.8 mmol) in anhydrous dichloro-
To a solution of [Eu·L1] (5.0 mg, 4.9 μmol) in DMF
methane (15 mL) under argon at 0 °C was added a solution of (0.5 mL), NaN3 (3.8 mg, 59 μmol) was added. The reaction was
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heated to 70 °C, and monitored by LC-MS until full substi-
[Eu·L3]. To a stirred, degassed solution of the tris p-bromo
tution of the three bromines was complete (within 16 h). The Eu(III) complex [Eu·L1] (5 mg, 4.9 μmol) in a mixture of anhy-
reaction was cooled to RT, and 4-ethynylanisole (2.9 mg, drous THF–DMF (2 : 3 v/v, 500 μL) was added 3,4,5-trimethoxy-
22 μmol), TBTA (1.6 mg, 2.9 μmol), CuSO4·5H2O (0.7 mg, phenylacetylene (3.1 mg, 16.1 μmol) and triethylamine (14 μL,
2.9 μmol) in H2O (0.5 mL) and sodium ascorbate (1.2 mg, 98 μmol), and the solution was degassed (freeze–thaw cycle)
5.9 μmol) were added, and the progress of the reaction moni- three times. [1,1-Bis(diphenylphosphino)ferrocene]dichloro-
tored by LC-MS. Full conversion was observed within 1 h, with palladium(II) (1.2 mg, 1.47 μmol) and CuI (0.3 mg, 1.5 μmol)
the solution darkening from very pale yellow to pale green over were added and the resulting yellow solution was stirred at
this time. The reaction mixture was evaporated onto silica, and 65 °C under argon for 12 h. After this time the solution turned
the material purified by silica gel chromatography (silica, dark brown and LC-MS analysis revealed complete conversion
CH2Cl2–CH3OH 95 : 5 to 80 : 20; Rf = 0.15, CH2Cl2–CH3OH of starting material. The solvent was removed under reduced
90 : 10), then reverse phase HPLC (XBridge C18 10 cm 3.5 μm, pressure and the brown residue was purified by semi-prepara-
H2O–CH3OH with 0.1% HCOOH gradient elution), to give the tive RP-HPLC [gradient: 50–100% methanol in water (0.1%
tris-triazole complex as a white solid (1.0 mg, isolated yield 16%). formic acid) over 10 min; tR = 9.8 min] to give a white solid
δH (400 MHz, CD3OD) 9.34, 8.94, 7.96, 7.47, 7.11 (s + s +s +s (2.6 mg, 40%); LRMS (ESI) m/z 1355 [M + H]+, 678 [M + 2H]2+;
+s +s, ArCH and triazole CH) 7.90 (3H, br s, pyCH); 4.55 (3H, (HRMS+) m/z 1355.305 [M(151Eu) + H]+ (C60H67N6O15P3151Eu
br s, CHeq′); 3.89 (9H, s, OMe); 0.84 (9H, br s, PMe); −0.51 requires 1355.308); τMeOH = 1.15 ms and 0.91 ms (H2O);
em
(3H br s, CHpy′); −1.11 (3H, br s Hax′); −1.71 (3H, br s Heq);
Φ
MeOH
= 47 ( 15)%; εMeOH (332 nm) = 56 400 M−1 cm−1. tR
=
−4.95 (3H, br s, Hax). δP (162 MHz, CD3OD) +39.2. m/z (HRMS+) 9.8 min (50 to 100% MeOH in water
+ 0.1% formic
1304.296 [M + H]+ (C54H58N15O9P3151Eu requires 1304.297).
λmax (MeOH) = 320 nm (5500 M−1 cm−1); τEu (CH3OH)
1.33 ms; ϕem (MeOH) 0.2%.
acid).
RP-HPLC: tR = 9.7 min (XBridge C18 10 cm 3.5 μm, H2O–
CH3OH with 0.1% HCOOH [gradient elution, see Table below
for details], 1 mL min−1, λ = 320 nm, 293 K).
Triethyl 6,6′,6″-((S)-2-(4-(cyclohexanecarboxamido)butyl)-1,4,7-
triazacyclononane-1,4,7-triyl)tris(methylene)tris(4-bromo-
pyridine-6,2-diyl)tris(methylphosphinate), L1b
Details of the reverse phase HPLC eluant gradient:
Time/min
H2O (+0.1% HCOOH)
MeOH (+0.1% HCOOH) Curve
2.0
10.0
2.0
0.5
2.0
60
5
5
90
90
40
95
95
10
10
0
1
0
1
0
Chiral HPLC: tR = 22.7 min and 61.4 min (CHIRALPAK-ID,
4.0 mm × 250 mm, CH3OH, 1 mL min−1, 295 K). On the
semi-preparative column at 2.5 mL min−1, the complexes eluted
after 87 and 226 minutes. By analysis of the CPL emission sig-
nature with related complexes of established helicity, the first
eluted complex was assigned the Δ absolute configuration.23–25
(S)-N-(4-(1,4,7-Triazacyclononan-2-yl)butyl)cyclohexanecarboxa-
mide27 (75 mg, 0.24 mmol), was added to a solution of
(4-bromo-6-(ethoxy(methyl)phosphoryl)pyridin-2-yl)methyl
methanesulfonate (320 mg, 0.86 mmol) and K2CO3 (198 mg,
1.47 mmol) in dry CH3CN (15 mL) and stirred under argon
at 65 °C. After 6 h the solution was allowed to cool to RT
and filtered. The filtrate was concentrated and purified by
reverse phase HPLC (gradient: 10 to 100% methanol in
water [0.1% formic acid] over 10 min; tR = 8.5 min) to give
a yellow oil as a mixture of diastereoisomers, (65 mg, 25%);
δH (CDCl3) 8.20–7.82 (6H, m, py), 6.45 (1H, br, CONH),
4.17–3.87 (12H, m, POCH2 and H17–17′–17″), 3.23–2.59 (13H,
m, H2–3–5–6–8–9–13), 1.82–1.76 (9H, m, PCH3), 1.76–1.26 (26H,
m, CH3(Et) and H10–11–12–14–15–16–17); δP (CDCl3) + 37.76; m/z
(HRMS+) 1138.191 [M + H]+ (C44H6879Br3N7O7P3 requires
1138.193).
5728 | Dalton Trans., 2014, 43, 5721–5730
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Europium(III) complex of (S)-6,6′,6″-(2-(4-(cyclohexane-
carboxamido)butyl)-1,4,7-triazacyclononane-1,4,7-triyl)-
tris(methylene)tris(4-bromopyridine-6,2-diyl)tris
three times. [1,1-Bis(diphenylphosphino)ferrocene]dichloro-
palladium(II) (250 μg, 0.3 mmol) and CuI (60 μg, 0.3 mmol)
were added and the resulting yellow solution was stirred at
65 °C under argon for 12 h. LC-MS analysis at this time
revealed complete conversion of starting material. The solvent
was removed under reduced pressure and the brown residue
was purified by semi-preparative RP-HPLC [gradient: 50–100%
(methylphosphinate), [Eu·L1b
]
methanol in water (0.1% formic acid) over 15 min; tR
=
11.4 min] to give [Eu·L2] as a white solid (0.33 mg, 51%); LRMS
(ESI) m/z 1538 [M + H]+, 770 [M + 2H]2+; (HRMS+) m/z 769.7291
[M(153Eu) + 2H]2+ (C71H87N7O16P3153Eu requires 769.7322);
em
em
τMeOH = 1.05 ms; Φ
= 55 ( 15)%; Φ
= 10 ( 15)%; εMeOH
H2O
MeOH
(355 nm) = 55 100 M−1 cm−1
.
Triethyl 6,6′,6″-((S)-2-(4-(cyclohexanecarboxamido)butyl)-1,4,7-
triazacyclononane-1,4,7-triyl)tris(methylene)tris(4-bromopyri-
dine-6,2-diyl)tris(methylphosphinate) (10 mg, 8.8 μmol) was
dissolved in CD3OD (3 mL) and a solution of 0.1 M NaOH in
D2O (1 mL) was added. The mixture was heated to 90 °C under
argon and monitored with 31P-NMR [δP(reactant) = 38.8,
(δP(product) = 25.9]. After 5 h the solution was cooled to RT
and the pH was adjusted to 7 with HCl. Eu(AcO)3H2O (3.2 mg,
9.7 μmol) was added and the mixture heated to 65 °C overnight
under argon. The solvent was removed under reduced pressure
and the product purified by column chromatography (silica,
CH2Cl2–CH3OH–NH3 9 : 1 : 0.1) giving the title compound as a
white solid (5 mg, 50%); δP (CD3OD) 43.4, 40.7, 37.5; m/z
(HRMS+) 1203.995 [M + H]+ (C38H5379Br3153EuN7O7P3 requires
1203.995); Rf = 0.32 (silica, CH2Cl2–CH3OH–NH3, 9 : 1 : 0.1 (v/v)).
Acknowledgements
We thank the ERC (FCC 266804), CISbio Bioassays, the Royal
Society and EPSRC for support, and the Diamond Light Source
for an award of instrument time at the station I19 (MT 8682)
and the instrument scientists for their kind support.
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