Sensitized lanthanide-ion luminescence with aryl-substituted N-(2-nitrophenyl)acetamide-derived chromophores

Article abstract of DOI:10.1002/hlca.200900108

The syntheses of the two tetraazamacrocyclic ligands L1 and L2 bearing a [(methoxy-2-nitrophenyl)amino]carbonyl chromophore, i.e., an N-(methoxy-2-nitrophenyl)acetamide moiety, together with their corresponding lanthanide-ion complexes are described. A combined spectroscopic (UV/VIS, ¹H-NMR), structural (X-ray), and theoretical (DFT) investigation revealed that the absorption properties of the chromophores were dictated by the extent of electronic delocalisation, which in turn was determined by the position of the MeO substituent at the aromatic ring. X-Ray crystallographic studies showed that when attached to the macrocycle, both isomeric forms of the N-(methoxy-2-nitrophenyl)acetamide unit can participate in coordination, via the C=O, to an encapsulated potassium cation. Luminescence measurements confirmed that such a binding mode also exists in solution for the corresponding lanthanide complexes (q ca. ≤1), with the para-MeO derivative allowing longer wavelength sensitization (λ_ex 330 nm).

Full text of DOI:10.1002/hlca.200900108

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Sensitized Lanthanide-Ion Luminescence with Aryl-Substituted
N-(2-Nitrophenyl)acetamide-Derived Chromophores
by Michael Andrewsa), Benjamin D. Warda), Rebecca H. Layeb), Benson M. Kariukia), and
Simon J. A. Pope*a)
a) School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff, UK CF10 3AT
(phone: þ 44-2920879316; fax: þ 44-2920874030; e-mail: popesj@cardiff.ac.uk)
b) Department of Chemistry, University of Sheffield, Sheffield, UK
Dedicated to Prof. Jean-Claude Bꢀnzli on the occasion of his 65th birthday
The syntheses of the two tetraazamacrocyclic ligands L1 and L2 bearing a [(methoxy-2-nitro-
phenyl)amino]carbonyl chromophore, i.e., an N-(methoxy-2-nitrophenyl)acetamide moiety, together
with their corresponding lanthanide-ion complexes are described. A combined spectroscopic (UV/VIS,
1H-NMR), structural (X-ray), and theoretical (DFT) investigation revealed that the absorption
properties of the chromophores were dictated by the extent of electronic delocalisation, which in turn
was determined by the position of the MeO substituent at the aromatic ring. X-Ray crystallographic
studies showed that when attached to the macrocycle, both isomeric forms of the N-(methoxy-2-
nitrophenyl)acetamide unit can participate in coordination, via the C¼O, to an encapsulated potassium
cation. Luminescence measurements confirmed that such a binding mode also exists in solution for the
corresponding lanthanide complexes (q ca. ꢀ 1), with the para-MeO derivative allowing longer
wavelength sensitization (lex 330 nm).
Introduction. – The rich and diverse synthetic organic chemistry associated with
functionalised aromatics can be exploited in the design of luminescent lanthanide
complexes [1]. Since lanthanide luminescence arises from forbidden f – f transitions, the
use of a sensitising chromophore is required to facilitate and optimise the emission
properties. From a purely energetic perspective, the choice of chromophore for visibly
emitting lanthanides such as EuIII and TbIII is far more limited than for near-IR emitters
such as NdIII, ErIII, and YbIII. However, the exploitation of lanthanide complexes in the
field of biological imaging, specifically confocal fluorescence microscopy [2], has
driven the rapid development of synthetic strategies aimed toward optimising
biocompatible optical and photophysical properties. Several research groups have
usefully exploited a wide range of organic aromatic chromophores which allow
reasonable long-wavelength sensitization of various emissive lanthanide ions. Such
species can be based upon functionalised aromatics such as xanthone [3], acridone [4],
pyrene [5], or quinoxaline [6] amongst other notable examples [7]. Chromophores with
red-shifted absorption properties can also encompass charge-transfer species based
upon transition metal complexes such as CrIII [8], RuII [9], ReI [10], OsII [11], IrIII [12],
and PtII [13]. More recently, Bꢀnzli and co-workers have used ꢀpush-pullꢁ aromatic
diketonato ligands incorporating electron-donating and electron-withdrawing units,
allowing the possibility of intra-ligand charge transfer (ILCT) excited states to be
ꢂ 2009 Verlag Helvetica Chimica Acta AG, Zꢃrich

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usefully exploited as a means of sensitisation [14]. In this context, simple nitrophenol-
derived aromatics represent a potentially useful class of comparable chromophores
[15]. Indeed poly-fluorinated nitrophenoxide ligands have been exploited in sensitized
near-IR emitting species [16]. Substituted acetophenone chromophores with varying
elements of charge-transfer character have also been employed to sensitise EuIII [17].
In this article, we describe the synthetic exploitation of two [(2-nitrophenyl)amino]-
carbonyl-derived chromophores i.e., N-(2-nitrophenyl)acetamide-derived chromo-
phores, within an azamacrocyclic framework yielding H2O-soluble lanthanide com-
plexes, together with their resultant sensitized VIS and near-IR emission.
Results and Discussion. – Synthesis and Characterisation of the Ligand Precursors
and Ligands. Prior to covalent attachment to the azamacrocycle, the chromophores
were derivatised as 2-chloroacetamides. Although it was possible to isolate the
nitrophenol-derived 2-chloroacetamide 1b (Scheme) by reaction of the 2-amino-3-
nitrophenol with 2-chloroacetyl chloride (the X-ray crystal structure of 1b (Fig. 1) is
discussed below), further reaction with base (either for the attempted methylation or
coupling with the macrocycle), not unexpectedly, yielded the intramolecularly cyclised
acetamide, identified by ESI-MS (m/z 194 ([M þ H]þ)) and 1H-NMR (unique
resonances for the CH2 and NH moieties at d(H) 4.64 and 9.99, resp.) rather than
the desired product. Consequently, for both isomers, the phenol groups were selectively
methylated with MeI in acetone in the presence of K2CO3 (Scheme). Subsequent
reaction of the methylated intermediates with 2-chloroacetyl chloride yielded the 2-
chloro-N-(2-methoxy-6-nitrophenyl)acetamide (1a) as a beige solid and the 2-chloro-
Scheme. Synthetic Pathway to the 2-Chloroacetamides

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Fig. 1. X-Ray-structure representation of 1b. Solvent molecules of crystallisation are omitted for clarity.
Probability ellipsoids set at 50%.
N-(4-methoxy-2-nitrophenyl)-2-chloroacetamide (2) as a dark yellow solid (2 has been
reported previously [18]; for the X-ray structure of 1a, see Fig. 2). Species 1a and 2
were characterised by ESI-MS which showed ion peaks for the parent ion Mþ (2) or
1
protonated parent ion [M þ H]þ (1a). In both cases, the H-NMR revealed the three
distinctive aromatic resonances together with two s for the acetamide CH2 group and
the MeO moiety. Since the spectra were obtained in CDCl3 solutions, it was also
possible to assign the amide resonance of 1a and 2, which showed dramatic differences
(NH of 1a at d(H) ca. 9.2 and of 2 at d(H) 11.5).
The 2-chloroacetamides 1a and 2 were then coupled with tri(tert-butyl) 1,4,7,10-
tetraazacyclododecane-1,4,7-triacetate [4] (which has previously been successfully
Fig. 2. X-Ray-structure representation of 1a. Solvent molecules of crystallisation are omitted for clarity.
Probability ellipsoids set at 50%.

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employed to access a large range of functionalised DO3A-derived macrocyclic ligands;
DO3A ¼ 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate), in the presence of one equiv.
of KI and base, to give the intermediate triester derivatives, E1 and E2 (for formulas,
see the corresponding L1 and L2), in moderate yield. Purification was achieved by
firstly removing unreacted triester by recrystallisation from toluene, and then by
alumina-based column chromatography, the eluent CH2Cl2 removing unreacted
chloroacetamide and CH2Cl2/MeOH 9 :1 furnishing the product E1 or E2. The triester
intermediates were characterised by a range of spectroscopic techniques. The ESI-MS
showed the expected peaks at m/z 723 for [M þ H]þ. In the aromatic region of the
1H-NMR spectra, there were minor shifts of the resonances when compared to the
precursor 2-chloroacetamides: assignment of the amide H-atoms again revealed a far
more deshielded NH environment for the para-isomer E2 than for E1.
Single crystals suitable for X-ray diffraction were obtained for both functionalised
triester intermediates E1 and E2 (Figs. 3 and 4). Although the data obtained for E1
were of poorer quality (due to a weak data set, a large asymmetric unit, and a
disordered t-Bu group), parameters associated with the provisional structure of E1 are
Fig. 3. X-Ray-structure representation of E1. Solvent molecules of crystallisation and counter ion are
omitted for clarity. Probability ellipsoids set at 50%.

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Fig. 4. X-Ray-structure representation of E2. Solvent molecules of crystallisation and counter ion are
omitted for clarity. Probability ellipsoids set at 50%.
provided to allow confirmation and comparison of the basic structure. Crystals were
grown from AcOEt/hexane solutions at ꢁ 208 or via vapor diffusion of Et2O into
concentrated CHCl3 solutions. Parameters associated with the data collection of E1 and
E2 are presented in Table 1. In both cases, the structural studies confirmed the
proposed formulations with the protected ligands isolated as their potassium
complexes, in accordance with previous reports on related triester [19] and triamide
[6] derivatives. Both structures show the potassium ion coordinated by all four N-atoms
of the macrocycle and by four C¼O O-atom donors, one of which is from the N-
arylacetamide moiety. For E1, the benzene ring is almost orthogonal to the amide group
and as a result almost perpendicular to the N4 plane of the macrocycle. The structure of
E2 revealed a twisted amide group which was not co-planar with the benzene ring and
may be a consequence of potassium coordination of the amide.
The differences in coplanarity between the amide group and benzene ring would be
expected to influence the degree of electronic conjugation throughout the system. It is
likely that this structural variation between E1 and E2 accounts for the significant
difference in the 1H-NMR chemical shift of the amide H-atom: the d(NH) of E2 was
shifted downfield by ca. 2 ppm suggesting a far more electron-deficient amide and,
therefore, a more conjugated chromophore.
In both cases, the complex cation is charge-balanced by a single iodide counter
anion (from KI). These X-ray structures are informative since we have previously
shown analogous sodium complexes to be useful benchmarks for predicting the
corresponding lanthanide coordination environment [19]. This is of particular use when
interpreting the luminescence behavior of the lanthanide complexes where an
understanding of the coordination sphere is of paramount importance. In these cases,
it would suggest that a lanthanide complex of these ligands would favor the eight-
coordinate binding mode where the amide C¼O participates in the first coordination
sphere.

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Finally, the tert-butyl ester! carboxylic acid deprotection step (CF3COOH,
CH2Cl2) [4] occurred without cleavage of the chromophore from the macrocyclic
framework, yielding the free triacid ligands L1 and L2. These hygroscopic species were
characterised by MS which showed clusters of peaks corresponding to ([M þ K]þ) and
the isotopomers of ([M þ Cu]þ) in each case, (presumably as a consequence of
scavenging trace Cu contamination within our mass spectrometry set-up). The
1H-NMR spectra again showed resonances attributable to the presence of the
macrocyle and appended chromophore and diagnostically established the absence of
the t-Bu resonances at d(H) ca. 1.3 – 1.4. The 13C{1H}-NMR spectra also revealed
the appropriate resonances associated with the ligands together with the presence of
CF3COOH, confirmed via characteristic qs observed at d(C) ca. 118 and 163. Previous
reports [19][20] have noted the formation of CF3COOH adducts following the
deprotection procedure described above. Accordingly, the CF3COOH adducts were
quantitatively established by elemental analyses which showed ca. two molecules of
CF3COOH per molecule of ligand, again in agreement with previous observations.
The lanthanide complex [Ln(L1)] was isolated as pale cream powder and [Ln(L2)]
as olive yellow powder; both were typically sensitive to moisture. High-resolution
accurate MS were obtained for each complex, confirming the isolation of the desired
species, and in each case showing the characteristic isotopic distribution representative
of the lanthanide.
An assessment of the absorption properties of the two chromophores revealed that
2 possessed a red-shifted absorption (lmax 373 nm, e 2100 molꢁ1 dm3 cmꢁ1) when
compared to 1a (lmax 323 nm, e 2200 molꢁ1 dm3 cmꢁ1). In each case, this long-
1
wavelength transition was assigned to an intraligand component, based upon a pp*
state. The reasons for the differing absorption behavior should be related to the overall
conjugation of the substituted aryl unit. The conjugation of the amide group may be
assessed, therefore, by considering a combination of structural and spectroscopic
handles. For the para-MeO derivatives [Ln(L2)], the more closely planar N-
arylacetamide unit appears to extend the conjugation of the chromophore, resulting
in a red-shifted absorption and an electron-deficient environment at the amide group
1
(as determined from H-NMR).
Density-Functional-Theory Calculations. Since the differing spectroscopic charac-
teristics of the chromophores arise from the ortho- vs. para-substitution, we undertook
a complementary density-functional-theory (DFT) study to probe the energetic
consequences of aryl substitution in these systems. Initially, the structure of the 2-
chloroacetamide 1bqm was computed and the metric parameters compared to those
obtained from the crystallographic study of 1b (Fig. 1). The bond lengths and angles
(Table 2) are generally in very good agreement with the measured X-ray data; this
comparison lends support to the reliability of the calculation methods applied in this
study. Interestingly, where the calculated model differs significantly from the X-ray
data is when considering the torsion angles associated with the amide and nitro
substituents. Both models minimise the steric clash between the amide and nitro groups
by twisting them out of the plane defined by the benzene ring. In contrast to the
calculations, the measured X-ray structural data shows that the molecule twists the
amide group significantly more (>608) than predicted, thereby allowing the nitro group
to reside closer to coplanarity with the benzene ring.

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Table 2. Comparison of Calculated and Measured Parameters for the Structure of 1b
Measured bond length [ꢄ]
Calculated bond length [ꢄ]
N(1)ꢁC(6)
N(1)ꢁO(2)
N(1)ꢁO(3)
O(1)ꢁC(2)
N(2)ꢁC(1)
Cl(1)ꢁC(8)
O(4)ꢁC(7)
N(2)ꢁC(7)
1.4711(17)
1.2215(15)
1.2344(15)
1.3491(15)
1.4177(15)
1.7787(13)
1.2323(16)
1.3340(16)
1.469
1.223
1.223
1.360
1.397
1.799
1.217
1.366
Measured bond angle [8]
Calculated bond angle [8]
O(2)ꢁN(1)ꢁO(3)
C(1)ꢁN(2)ꢁC(7)
C(7)ꢁC(8)ꢁCl(1)
122.99(11)
122.85(11)
114.84(9)
125.24
124.33
116.74
Measured torsion angle [8]
Calculated torsion angle [8]
O(4)ꢁC(7)ꢁC(8)ꢁCl(1)
C(6)ꢁC(1)ꢁN(2)ꢁC(7)
O(2)ꢁN(1)ꢁC(6)ꢁC(1)
170.9(1)
60.49(17)
19.98(17)
166.75
44.26
33.51
The model systems N-(2-methoxy-6-nitrophenyl)acetamide (3qm) and N-(4-me-
thoxy-2-nitrophenyl)acetamide (4qm) were studied by using DFT (B3PW91) methods;
the computed structures of 4qm and 3qm are displayed in Fig. 5. In 4qm, the amide and
nitro groups lie ca. coplanar to the benzene ring as would be expected for a fully
delocalised system. This observation is further supported by the fact that rotation of the
amide moiety has an activation barrier of ca. 10 kcal molꢁ1, consistent with the
disruption of a p-delocalised system. This is comparable with the calculated activation
barrier to rotation of ca. 5 kcal molꢁ1 reported by Kawauchi and co-workers [21] for a
system containing an aromatic ring on both sides of the amide linkage; the possibility of
delocalising the C¼O over a benzene ring, in addition to the N-based lone pair, is likely
to be the reason for the reduced activation barrier compared to the system described
herein. The ground-state structure of 3qm possessed a nonplanar arrangement of both
the amide and nitro groups (as observed in the calculated and measured structures of
1b), with the amide showing significantly more deviation from planarity compared to
the nitro group; although these groups are not perpendicular, the deviation from
Fig. 5. Calculated structures of 4qm (left) and 3qm (right)

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planarity is nevertheless expected to reduce the extent of the delocalisation. The likely
cause of the nonplanarity of the amide group is most probably due to the steric
repulsion imposed by the two ortho-substituents. The activation barrier associated with
rendering the amide planar with the ring is 11 kcal molꢁ1.
The energy gap between the HOMO and LUMO of 3qm and 4qm is affected by the
extent of p-delocalisation over the system, since the HOMO and LUMO are associated
with the aromatic ring and p-bonding to both the amide and nitro groups (Fig. 6). This
energy difference was, therefore, estimated by considering the relative energies of the
HOMO and LUMO in both 3qm and 4qm. In 4qm, the HOMO – LUMO gap is 3.46 eV
(359 nm), whereas in 3qm (where the p-delocalisation has been disrupted) the value
increases to 3.73 eV (333 nm). Obviously, this model system lacks the structural
complexity of the free ligands L1 and L2 or the complexes, but the difference (DE ꢃ
2200 cmꢁ1) is in reasonable agreement, in both direction and magnitude, with that
experimentally observed (from UV/VIS studies) for the macrocyclic triesters E1 and
E2 (2870 cmꢁ1). It is likely, therefore, that the absorption properties of the ortho- and
para-substituted chromophoric species are dependent upon the extent of p-delocalisa-
tion. This is a direct consequence of the steric accommodation of the amide substituent,
resulting in the electronically preferred planar arrangement of the amide group being
far more likely in the para isomer. Additionally, it is important to note that the amide
substitutent is likely to become even more electron-withdrawing when coordinated to
the strongly Lewis acidic lanthanide ions, although the act of coordination will also
effect the torsion angle of the N-arylacetamide.
Fig. 6. HOMO (left) and LUMO (right) of 3qm (top) and 4qm (bottom)
6
Luminescence Properties of the Complexes. The potentially emissive P7/2 state of
GdIII is at high UV energies (ca. 32000 cmꢁ1) [22]. The GdIII complexes were found to
be nonemissive at room temperature, but in EtOH at 77 K, they were luminescent.
Following excitation at 290 nm, the time-gated phosphorescence spectra of both
complexes [Gd(L1)] and [Gd(L2)] showed a broad, structureless emission with a

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maximum intensity at ca. 440 nm. In each case, the high-energy onset of the peak
allowed estimation of the triplet energy of the chromophore; ca. 23400 cmꢁ1 in both
cases and thus at sufficient energy to sensitise the lanthanide ions used in this study. The
NdIII, EuIII, and YbIII complexes each gave emission spectra characteristic of the
coordinated ion and consistent with its inclusion in the macrocyclic binding site. Time-
resolved measurements were obtained with H2O/D2O solutions of the complexes
(Table 3). The excitation wavelengths for each system were indirect irradiation (lex
355 nm) for NdIII and YbIII and both direct and indirect irradiation for EuIII (lex 396 and
320 nm). For each case, the decay profile was analysed as a single exponential
component, indicative of a single emissive species. The measured lifetimes can be
utilised to deduce the inner-sphere coordination environment, in terms of the degree of
lanthanide hydration, q (q ¼ A[(kH2O ꢁ kD2O) ꢁ B]: for EuIII, A ¼ 1.25 ms and B ¼
0.25 msꢁ1; for YbIII, A ¼ 1 ms and B ¼ 0.2 msꢁ1; for aqueous NdIII, q ¼ 130 (kH2O
ꢁ
kD2O) ꢁ 0.4) [23] presented in Table 3. From these values, we conclude that the amide
C¼O is coordinated to the lanthanide in each case as expected for eight-coordinate
ligands [22][23] of this type where q is ca. ꢀ 1. It is noteworthy that despite parameters
derived from related aminocarboxylate studies [23], both NdIII species possess a slightly
lower than expected q value. In fact, the values obtained in this study are more akin to
those reported for the Lehn-cryptand complex of NdIII (cf. tH2O ¼ 100 ns, q ¼ 0.46) [24]
where proximate CꢁH quenching modes are limited. Near-IR steady-state spectra
were obtained with excitation wavelengths between 320 and 370 nm. The NdIII complex
4
displayed near-IR emission dominated by a peak at 1058 nm (4F3/2 ! I11/2) (utilising a
4
4
longpass filter prevented the observation of the higher-energy F3/2 ! I9/2 transition)
with a weaker feature at ca. 1340 nm, corresponding to decay of the 4I13/2 transition. The
steady-state emission spectra of [Yb(L1)] and [Yb(L2)] (Fig. 7) gave spectral profiles
4
typical of YbIII with a peak at 975 nm (2F5/2 ! F7/2) together with a broadened, vibronic
component to low energy, below 10000 cmꢁ1.
Table 3. Luminescence Lifetimes of the Lanthanide Complexesa)b)
tH [ms]
tD [ms]
0.327
922
7.629
0.270
1507
8.611
qc)
[Nd(L1)]
[Eu(L1)]
[Yb(L1)]
[Nd(L2)]
[Eu(L2)]
[Yb(L2)]
0.095
429
0.776
0.085
530
0.6
1.2
1.0
0.6
1.2
0.9
0.802
a) Measurements obtained in H2O and D2O with lex 320 or 355 nm. b) Lifetimes are associated with a
ꢄ 10% error. c) Estimated inner-sphere hydration number.
Conclusions. – This article discusses the use of N-(nitrophenyl)acetamide-derived
chromophores as sensitisers in emissive lanthanide complexes. In comparison to simple
phenol-derived species, the chromophores presented here display moderately red-
shifted absorption bands between 300 and 350 nm allowing each chromophore to
sensitise a range of lanthanide ions in aqueous solution. Through experimental

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Fig. 7. Steady-state emission spectra of [Yb(L1)] (top) and [Yb(L2)] (bottom) in D2O (lex 330 nm)
spectroscopic studies and complimentary DFT calculations, attempts to rationalise the
differences in absorption properties between the two isomeric forms of the
chromophores were made. This investigation shows that the steric demands of the
nitro and aminocarbonyl substituent at the benzene ring cause varying disruptions in
the p-delocalisation throughout the chromophore and thus, the position of these
substituents at the benzene moiety dictates the HOMO – LUMO energy gap. In this
context, the extended conjugation of the para-MeO chromophore possesses a more
1
red-shifted IL p – p* transition allowing longer-wavelength sensitization.
The EPSRC are thanked for support (EP/E048390/1) together with the Universities of Cardiff and
Sheffield. The EPSRC MS National Service at Swansea is also gratefully acknowledged.
Experimental Part
1. General. UV/VIS Spectra: Jasco 570 spectrophotometer; lmax (e / molꢁ1 dm3 cmꢁ1) in nm. IR
Spectra: Varian 7000 FT-IR spectrometer; n˜max in cmꢁ1. NMR Spectra: Bruker DPX at 400 MHz; d in
ppm, J in Hz. ESI-MS: Waters LCT Premier XE in MeOH; in m/z (rel. %).
2. Precursors, Ligands, and Corresponding Lanthanide Complexes. 2.1. 2-Chloroacetamides: General
Procedure. A large excess (ca. 3 equiv.) of 2-chloroacetyl chloride was added under stirring to a soln. of
the substituted aniline in MeCN, and the mixture was stirred for 1 h. The solvent was evaporated, and
AcOEt was added. The org. phase was washed with H2O, dried (MgSO4), and concentrated: title
compounds in high yield (typically 95%). The 2-chloroacetamides were used without further purification.
2-Chloro-N-(2-methoxy-6-nitrophenyl)acetamide (1a): Pale yellow crystals. Yield 97%. UV/VIS: 323
(2173). IR (CHCl3): 1698 (C¼O), 1512 (NO2). 1H-NMR (CDCl3): 3.90 (s, MeO); 4.13 (s, CH2Cl); 7.12
(d, 1 arom. H); 7.28 (t, 1 arom. H); 7.48 (d, 1 arom. H); 9.2 (br. s, NH). ESI-MS (pos.): 245 ([M þ H]þ).
2-Chloro-N-(4-methoxy-2-nitrophenyl)acetamide (2): Dark yellow crystals. Yield 93%. UV/VIS: 373
(2091). IR (CHCl3): 1694 (C¼O), 1514 (NO2). 1H-NMR (CDCl3): 3.81 (s, MeO); 4.16 (s, CH2Cl); 7.18 (s,
1 arom. H); 7.64 (s, 1 arom. H); 8.58 (d, 1 arom. H); 11.5 (br. s, NH). ESI-MS (pos.): 244 (Mþ).
2.2. Protected Ligand Precursors: General Procedure. The 2-chloroacetamide 1a (1.5 equiv.) was
added to a mixture of the bromide salt of tris(1,1-dimethylethyl) 1,4,7,10-tetraazacyclododecane-1,4,7-
triacetate (1 equiv.), Et3N (3 equiv.), and KI (1 equiv.) in MeCN, and the mixture was heated to reflux
for 48 h. The soln. was filtered and concentrated, and the crude residue was purified by column
chromatography (alumina, CH2Cl2, then CH2Cl2/MeOH 9 :1).
Tris(1,1-dimethylethyl) 10-{2-[(2-Methoxy-6-nitrophenyl)amino]-2-oxoethyl}-1,4,7,10-tetraazacyclo-
dodecane-1,4,7-triacetate (E1): Brown oil. Yield 51%. UV/VIS: 220 (10799), 328 (709). IR (CHCl3):

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1727 (C¼O), 1682 (NC¼O), 1522 (N¼O). 1H-NMR (CDCl3): 1.33 (s, 3 COOCMe3); 1.90 – 3.74 (br. m,
24 H, CH2 ring, NCH2COO, NCH2CON); 3.84 (s, MeO); 7.08 (d, 1 arom. H); 7.19 (m, 1 arom. H); 7.36 (d,
1 arom. H); 9.21 (br. s, NH). ESI-MS (pos.): 723 ([M þ H]þ).
Tris(1,1-dimethylethyl) 10-{2-[(4-Methoxy-2-nitrophenyl)amino]-2-oxoethyl}-1,4,7,10-tetraazacyclo-
dodecane-1,4,7-triacetate (E2): Brown oil. Yield 67%. UV/VIS: 239 (87185), 362 (14267). IR (CHCl3):
1732 (C¼O), 1518 (N¼O). NMR (CDCl3): 1.35 (s, 3 COOCMe3); 1.88 – 3.68 (br. m, 24 H, CH2 ring,
NCH2COO, NCH2CON); 3.76 (s, MeO); 7.01 (d, 1 arom. H); 7.36 (s, 1 arom. H); 7.81 (d, 1 arom. H); 11.1
(br. s, NH). ESI-MS (pos.): 723 ([M þ H]þ).
2.3. Deprotection of Ligands: General Procedure. The protected ligand precursor was added to
CF3COOH/CH2Cl2 1:1, and the mixture was stirred at r.t. overnight. The solvent was evaporated, and the
residue was redissolved three times in MeOH to eliminate excess acid. The residue was then dissolved in
the minimum volume of MeOH and added dropwise under stirring to Et2O at 08. The precipitate was
filtered and dried to give the deprotected ligand.
10-{2-[(2-Methoxy-6-nitrophenyl)amino]-2-oxoethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic
Acid (L1): Yellow powder. 1H-NMR (D2O): 2.95 – 4.05 (br. m, 27 H, CH2 ring, NCH2COO, NCH2CON,
MeO); 7.32 – 7.50 (overlapping, 3 arom. H). 13C-NMR (101 MHz, D2O): 49.3 (br.); 55.1 (br.); 57.0
(MeO); 57.8; 115.2; 117.0; 117.8 (q, CF3COOH); 128.6; 145.6; 152.1; 154.5; 163.2 (q, CF3COOH); 174.5
(br., CO). ESI-MS (pos.): 593 ([M þ K]þ). Anal. calc. for C23H34N6O10 · (CF3COOH)2 (782.60): C 41.44,
H 4.64, N 10.74; found C 41.21, H 5.11, N 10.22.
10-{2-[(4-Methoxy-2-nitrophenyl)amino]-2-oxoethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic
Acid (L2): Yellow powder. 1H-NMR (D2O): 2.7 – 4.1 (br. m, 27 H); 3.81 (s, MeO); 7.30 – 7.49
1
(overlapping m, 3 arom. H); 7.51 (d, J ¼ 8.0, 1 arom. H). 13C-NMR (101 MHz, D2O): 49.4 (br.); 53.4
(br.); 57.0; 57.8; 62.0; 115.2; 117.8 (q, CF3COOH); 128.5; 145.6; 154.5; 163.2 (q, CF3COOH); 165.2;
174.7; 175.0 (CO). ESI-MS (pos.): 593 ([M þ K]þ), 617 ([M þ Cu]þ). Anal. calc. for C23H34N6O10
·
(CF3COOH)2 (782.60): C 41.44, H 4.64, N 10.74; found C 41.15, H 5.40, N 10.28.
2.4. Lanthanide Complexes: General Procedure. A mixture of the ligand and either Ln(OTf)3 or
Yb(NO3)3 was stirred in MeOH at 508 for 24 h. The soln. was concentrated and then added dropwise
under stirring to Et2O at 08. The precipitate was filtered, washed with Et2O, and dried to give the
lanthanide complex.
{10-{2-[(2-Methoxy-6-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}neodymium ([Nd(L1)]): Pale yellow powder. UV/VIS:
209 (8551), 327 (964). IR (nujol): 1600, 1645 (C¼O). ESI-MS (pos.): 694 ([M þ H]þ). HR-ESI-MS
(pos.): 694.1259 ([M þ H]þ, C23H32N6142NdO1þ0 ; calc. 694.1252).
{10-{2-[(2-Methoxy-6-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}europium ([Eu(L1)]): Pale yellow powder. UV/VIS: 210
(8870), 325 (1014). IR (nujol): 1599, 1648 (C¼O). ESI-MS (pos.): 703 ([M þ H]þ). HR-ESI-MS (pos.):
703.1383 ([M þ H]þ, C23H32151EuN6O1þ0 ; calc. 703.1373).
{10-{2-[(2-Methoxy-6-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}gadolinium ([Gd(L1)]): Pale yellow powder. UV/VIS: 210
(9381), 324 (1143). IR (nujol): 1601, 1635 (C¼O). ESI-MS (pos.): 707 ([M þ H]þ). HR-ESI-MS (pos.):
707.1397 ([M þ H]þ, C23H32151GdN6O1þ0 ; calc. 707.1401).
{10-{2-[(2-Methoxy-6-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}ytterbium ([Yb(L1)]): Pale yellow powder. UV/VIS: 214
(15378), 321 (1022). IR (nujol): 1598, 1630 (C¼O). ESI-MS (pos.): 722 ([M þ H]þ). HR-ESI-MS (pos.):
722.1520 ([M þ H]þ, C23H32N6O10170Ybþ; calc. 722.1522).
{10-{2-[(4-Methoxy-2-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}neodymium ([Nd(L2)]): Yellow powder. UV/VIS: 232
(11321), 340 (1000). IR (nujol): 1607, 1635 (C¼O). ESI-MS (pos.): 694 ([M þ H]þ). HR-ESI-MS (pos.):
694.1249 ([M þ H]þ, C23H32N6142NdO1þ0 ; calc. 694.1252).
{10-{2-[(4-Methoxy-2-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}europium ([Eu(L2)]): Yellow powder. UV/VIS: 233
(9212), 338 (1139). IR (nujol): 1603, 1638 (C¼O). ESI-MS (pos.): 703 ([M þ H]þ). HR-ESI-MS
(pos.): 703.1375 ([M þ H]þ, C23H32153EuN6O1þ0 ; calc. 703.1373).

Helvetica Chimica Acta – Vol. 92 (2009)
2171
{10-{2-[(4-Methoxy-2-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}gadolinium ([Gd(L2)]): Yellow powder. UV/VIS: 231
(11393), 338 (1182). IR (nujol): 1598, 1640 (C¼O). ESI-MS (pos.): 706 ([M þ H]þ). HR-ESI-MS (pos.):
706.1383 ([M þ H]þ, C23H32154GdN6O1þ0 ; calc. 706.1383).
{10-{2-[(4-Methoxy-2-nitrophenyl)amino]-2-(oxo-kO)ethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-
triacetato(3-)-kN1,kN4,kN7,kN10,kO1,kO4,kO7}ytterbium ([Yb(L2)]): Yellow powder. UV/VIS: 214
(13870), 340 (1164). IR (nujol): 1635, 1674 (C¼O). ESI-MS (pos.): 722 ([M þ H]þ). HR-ESI-MS
(pos.): 722.1521 ([M þ H]þ, C23H32N6O10170Ybþ; calc. 722.1522).
Structure Analysis and Refinement1). The structure was solved by direct methods with SHELXS-97
and was completed by iterative cycles of DF syntheses and full-matrix least-squares refinement. All non-
H-atoms were refined anisotropically, and difference Fourier syntheses were employed in positioning
idealised H-atoms and were allowed to ride on their parent C-atoms. All refinements were against F2 and
used SHELX-97 [25].
DFT. All calculations were performed on the Gaussian 03 suite of programs [26]. Calculations were
carried out with the hybrid B3PW91 functional, with the 6 – 31G(d,p) basis set for all atoms. Geometry
optimisations were initially performed without geometry restraints, followed by frequency calculations to
ascertain the nature of the resulting structure (minimum vs. transition state). Molecular images were
created with the Molekel program [27].
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Received March 24, 2009