CN, followed by precipitation from dry ether. All complexes
were characterized using conventional methods. In the case
of the paramagnetic ions, the resonance corresponding to the
axial and the equatorial protons of the cyclen rings and the
R-methtylene spacer in the H NMR was substantially
shifted. In the case of 1.Eu, these appeared at δ 28.9, 3.7,
occupied by a water molecule15 as would be expected from
the analogues Eu(III), Gd(III), and Tb(III) complexes of 1,
giving rise to monocapped square antiprismatic geometry.
The depth of this cavity is 7.99 Å (distance from Na to the
center of morpholine oxygen atoms), and the width is 6.46
Å.
1
3
.45, 2.74, 2.51, 1.76, -5.78, -7.8, -12.7 ppm, when
The presence of the metal-bound water molecules in the
above lanthanide complexes was also observed by evaluating
the hydration number (q) for the Eu(III) and the Tb(III)
complexes of 1-4. Table 1 (q ( 0.3) lists these values, which
recorded in CD CN, indicating that the structure adopted a
3
square antiprismatic geometry in solution as the major
isomer. Similar results were observed for the other com-
plexes. This would suggest that the four arms of the cyclen
complexes would all be lining the “wall” of a cavity as the
lanthanide ions would be coordinated to the four nitrogens
of the cyclen ring and to the four oxygens of the carboxy
amides. Unfortunately, we were unable to grow crystals of
any of these lanthanide ion complexes that were suitable for
X-ray crystallographic analysis, despite attempting different
Table 1. Hydration Numbers (q) for Eu(III) and Tb(III)
Complexes of 1-4
k (H2O)
k (H2O)
k (D2O)
k (D2O)
no.
[ms]
[1/ms]
[ms]
[1/ms]
q ((0.3)
crystal growth techniques. However, we were able to obtain
1.Eu
1.Tb
0.41
1.40
3.06
0.57
1.62
0.49
1.59
2.47
0.72
3.27
1.77
0.61
2.02
0.63
1.26
2.00
0.54
2.18
2.69
1.82
2.64
0.79
0.50
1.86
0.46
0.37
0.55
0.38
1.34
0.77
1.04
0.91
0.91
1.12
0.94
the sodium complex of 1, 1.Na.12
2.Eu
3.Eu
3.Tb
4.Eu
4.Tb
The structure of 1.Na is shown in Figure 2, viewed from
the top phase, showing the high organization of the ion-
were obtained by using a luminescent method, where the
5
rate constant for the radiative decay (k) of the D
0
excited
5
state of Eu(III) and the D
measured in H O and D O, respectively.
4
excited state of Tb(III) are
1
,3,5,16
2
2
From these results it can be seen that these complexes all
have a single metal-bound water molecule. The correspond-
ing La(III) complexes, however, would have been expected
to have two such coordinated water molecules, due to their
1-4,17
higher coordination requirements.
The q-values of 1.Eu,
+
3.Eu, and 4.Eu, as well as 3.Tb and 4.Tb, were also
Figure 2. X-ray crystal structure of the Na complex of 1, when
viewed from the top of the cavity.
determined in 0.1 M NMe
4
Cl, and all gave q-values of ∼1,
-
indicating that Cl was not coordinating to the metal ion.
On the basis of the above results, we propose that all the
complexes are adopting a cavity-based conformation. We
also investigated the effect these “walls” would have on the
+
induced cavity. The use of Na as a substitute for lanthanide
ions in X-ray crystallography has previously been demon-
strated, as the ion is of similar size and has a similar
coordination requirement.13 The structure 1.Na has C
pKa
of the metal-bound water molecules. We could foresee
4
that this could have a considerable effect on (i) the ability
1
8
symmetry, adopting a monocapped square antiprismatic
of these complexes to cleave phosphodiesters, which is
highly depended on the pKa
3
conformation. The side view of this complex is shown in
s of any metal-bound water
molecules, and (ii) the relaxivity of the Gd(III) complexes.
The pKa
the graphical abstract, demonstrating that the morpholine
arms, indeed, line the wall of the cavity, with the ion placed
values were determined by potentiometric pH
titration against 0.1 M NMe4
1
4
in the center. Furthermore, the ninth coordination site is
OH aqueous solution with I )
4
.1 M (NMe Cl) at 37 °C and analyzed using the program
0
(12) SMART Software Reference Manual, version 5.054; Bruker Analyti-
a
SUPERQUAD, which gave the deprotonation constants pK .
cal X-ray Systems, Inc.: Madison, WI, 1998. Sheldrick, G. M. SHELXTL,
An Integrated System for Data Collection, Processing, Structure Solution
and Refinement; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 1998.
1
(14) H NMR (CDCl3) of 1 was also monitored upon titration with NaPF6.
+
Crystal data for 1: monoclinic, space group P21/c, a ) 10.2041 (12) Å, b
These preliminary titrations showed that upon introduction of Na , the
cyclen resonances were substantially broadened and the amide proton
resonance was shifted downfield.
(15) Parker, D.; Puschmann, H.; Batsanov, A. S.; Senanayake, K. Inorg.
Chem. 2003, 42, 8646.
(16) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker,
D.; Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. J. Chem.
Soc., Perkin Trans. 2 1999, 493.
3
)
10.6133 (12) Å, c ) 21.190 (2) Å, â ) 95.937 (2)°, U ) 2282.3 (5) Å ,
Z ) 2. A total of 25 760 reflections were measured for 4 < 2θ < 57, and
5
298 unique reflections were used in the refinement; the final parameters
were wR2 ) 0.1742 and R1 ) 0.0538 [I > 2σ(I)]. Crystal data for 1.Na:
tetragonal, space group P4/n, a ) b ) 13.5281 (11) Å, c ) 13.3754 (16)
3
Å, U ) 2447.8(4) Å , Z ) 2. A total of 20 249 reflections were measured
for 3 < 2θ < 57, and 2812 unique reflections were used in the refinement;
the final parameters were wR2 ) 0.1701 and R1 ) 0.0549 [I > 2σ(I)].
(17) Ref 9: This paper has also an X-ray crystal structure of 10-
coordinated La(III) tetraamide cyclen structure.
(13) Govenlock, L. J.; Howard, J. A. K.; Moloney, J. M.; Parker D.;
Peacock, R. D.; Siligardi G. J. Chem. Soc., Perkin Trans. 2 1999, 2415.
(18) This will be discussed in future publications.
Org. Lett., Vol. 6, No. 26, 2004
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