Synthesis and spectroscopic studies on azo-dye derivatives of polymetallic lanthanide complexes: Using diazotization to link metal complexes together

Article abstract of DOI:10.1021/ja904362f

(Graph Presented) Heteronuclear tetrametallic lanthanide complexes have been synthesized from stable complexes by diazotization and azo-compound formation. Luminescence spectroscopy has been used to show that the complexes used as building blocks are stab

Full text of DOI:10.1021/ja904362f

Published on Web 07/06/2009
Synthesis and Spectroscopic Studies on Azo-Dye Derivatives of Polymetallic
Lanthanide Complexes: Using Diazotization to Link Metal Complexes
Together
Matteo P. Placidi,† Aaron Joseph L. Villaraza,† Louise S. Natrajan,† Daniel Sykes,†,‡
Alan M. Kenwright,§ and Stephen Faulkner*,†,‡
School of Chemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K., UniVersity of Oxford,
Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, U.K., and Department of Chemistry,
UniVersity of Durham, South Road, Durham DH1 3LE, U.K.
Received May 29, 2009; E-mail: Stephen.Faulkner@chem.ox.ac.uk
Scheme 1. Synthesis and Diazotization of Phenol Bridged
Lanthanide complexes have been widely used in a variety of
imaging and assay related applications. In particular, gadolinium
complexes have been used as contrast agents in magnetic resonance
imaging,1 while luminescent lanthanide ions are widely used in
bioassay2 and imaging.3 In such applications, it is important to be
able to control the local environment around the metal and form
kinetically stable complexes.4 For instance, in magnetic resonance
imaging a complex should allow close approach and rapid exchange
of water molecules, while retaining sufficient kinetic stability to
be excreted intact. The case is altered for luminescence applications,
since the excited states of lanthanide ions are quenched nonradia-
tively by vibrational harmonics of the O-H oscillator and it is
therefore desirable to exclude water molecules from the inner
sphere.5 Since electric dipole f-f transitions are Laporte forbidden
and have very low extinction coefficients, sensitizing chromophores
are often incorporated into the ligand structure: these can sensitize
emission from the lanthanide excited state and lead to dramatic
enhancement of the emission from the metal center.6 Energy transfer
from the chromophore to the metal is usually mediated via the
chromophore triplet state,7 though other mechanisms have also been
shown to be viable in some complexes.8,9
Bis-Macrocycles and Their Complexesa
Kinetically stable complexes can also be used as building blocks
for more complicated systems10 and can even be used in the
preparation of heterometallic lanthanide complexes containing
different lanthanide ions.11
a Reagents and Conditions: (a) Cs2CO3, MeCN reflux, 72 h; (b) tfa,
CH2Cl2, 48 h; (c) Ln(OTf)3, MeOH, 50 °C, 48 h; (d) 4-nitroaniline, NaNO2,
HCl, 0 °C, 0.5 h.
We now report how related bimetallic complexes can be linked
together using diazotization reactions that generate a chromophore
at the heart of the assembly, permitting visible light excitation of
the bound lanthanide ions.
As expected, azo dye formation results in a color change: the
[Ln2.4]- complexes are colorless, while [Ln2.5]- complexes are
deep red, owing to the increased conjugation between the phenolic
oxygen donor and the nitro group acceptor.
The synthesis of phenolic complexes suitable for reaction with
diazonium salts is outlined in Scheme 1. Reaction of the well-known
DO3A derivative12 (1) with 2-6-bis(bromomethyl)phenoxy acetate
(2) in the presence of cesium carbonate followed by aqueous
workup yielded 3. NMR revealed that cleavage of the phenoxy
acetate ester had occurred under the reaction conditions, obviating
the need for its subsequent removal.
All the complexes were found to be emissive in the near-IR
region of the spectrum, and data fitted well to single exponential
decays, suggesting that the conformers of the complex are in fast
exchange on the luminescence time scale, unlike the case of an
analogous complex with a 4-methyl substituent on the phenol
bridge, for which double exponential behavior has been observed.13
Time resolved luminescence spectroscopy was used to establish
the luminescence lifetimes of the complexes, and the number of
inner sphere water molecules in the ytterbium complexes was
determined from the luminescence decay constants in H2O and D2O
using the formula
Cleavage of the tert-butyl esters in trifluoroacetic acid unmasked
both lanthanide binding sites, and treatment with lanthanide triflate
salts resulted in the formation of bimetallic complexes [Ln2.4]-
(Ln ) Nd, Yb). Reaction of [Ln2.4]- with a 4-nitrophenyl diazo-
nium salt in aqueous solution resulted in the formation of azo dyes
[Ln2.5]- (Ln ) Nd, Yb). These were purified using size selective
dialysis tubing to separate any unreacted [Ln2.4]- from the product.
q ) 1.0(kH O - kD O - 0.1)
2
2
† University of Manchester.
‡ University of Oxford.
§ University of Durham.
where q is the number of bound water molecules at each ytterbium
ion, and kH O and kD O are the observed rate constants in H2O and
2
2
9
9916 J. AM. CHEM. SOC. 2009, 131, 9916–9917
10.1021/ja904362f CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
D2O, respectively (with units of µs-1).14 No values were calculated
based on neodymium luminescence owing to the large variations
in lifetime with ligand structure in such systems.2 The photophysical
properties of the lanthanide complexes are shown in Table 1.
conceivably as a result of steric crowding in the tetrametallic
complex inhibiting access by solvent to the metal center; indeed
qYb for Yb2.7 is the same as qYb for 9, while qYb for Yb2.5 is within
error of that for 8.
Table 1. Photophysical Properties of the Complexesa
compound
λ
em/nm
τH2O/ns
τD2O/ns
qYb
Nd2.4
Yb2.4
Nd2.5
Yb2.5
Nd2.7
Yb2.7
8
8
9
9
1055
980
1340
980
1055
980
980
1340
980
1340
98
1620
112
1410
102
1600
1950
140
1750
160
301
4510
355
4020
334
5310
5120
320
5170
400
0.3
0.4
0.3
0.2
0.3
Figure 1. Time-gated emission spectra of D2O solutions containing 8
(dashed line) and 9 (solid line): (a) in the period between 500 and 1000 ns
after the laser pulse; (b) from 3 to 6 µs after the laser pulse, normalized at
980 nm.
a All complexes exhibited emission following excitation at 337 nm.
For [Ln.5]- and 8 and 9, excitation at 420 nm gave lifetimes within
error of those given in the table.
The stability of systems such as these in the forcing conditions
inherent to diazotization reactions is further evidence for the kinetic
inertness of such complexes. This approach allows not only the
preparation of heterometallic systems containing different lanthanide
ions but also the simultaneous incorporation of a sensitizing
chromophore.
Having prepared and studied bimetallic systems, we turned our
attention to the preparation of heterotetrametallic arrays. Scheme
2 shows the synthetic strategy employed. The bis-macrocycle 7
was prepared using an established procedure as shown in Scheme
2.15 Complexation with the lanthanide triflate yielded the bimetallic
complexes Ln2.7. Reaction of Ln2.7 with sodium nitrite in dilute
hydrochloric acid followed by addition of [Ln′2.4]- yielded the
tetrametallic complexes 8 and 9.
Acknowledgment. The authors acknowledge support from the
Universities of Manchester, Oxford, and Durham and from the
EPSRC.
Scheme 2. Synthesis of Tetrametallic Complexesa
Supporting Information Available: Experimental procedures and
characterization data for synthesis of compounds and complexes,
absorption and (time-resolved) emission spectra of complexes. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Merbach, A. E. ; Toth, E. In ComprehensiVe Coordination Chemistry, 2nd
ed.; Ward, M. D., Ed.; Elsevier: 2004; Vol. 9, Chapter 19. Parker, D.;
Dickins, R. S.; Puschman, H.; Crossland, C.; Howard, J. A. K. Chem. ReV.
2002, 102, 1977.
(2) Hemilla, I. A. Applications of fluorescence in immunoassays; Wiley
Interscience: 1991. Faulkner, S.; Burton-Pye, B. P.; Pope, S. J. A. Appl.
Spectrosc. ReV. 2005, 40, 1. Bunzli, J. C. G.; Comby, S.; Chauvin, A. S.;
Vandevyer, C. D. B. J. Rare Earths 2007, 25, 257.
(3) Beeby, A.; Clarkson, I. M.; Faulkner, S.; Botchway, S.; Parker, D.; Williams,
J. A. G.; Parker, A. W. J. Photochem. Photobiol., B 2000, 57, 83.
Charbonniere, L.; Ziessel, R.; Guardigli, M.; Roda, A.; Sabbatini, N.;
Cesario, M. J. Am. Chem. Soc. 2001, 123, 2436. Weibel, N.; Charbonniere,
L. J.; Guardigli, M.; Roda, A.; Ziessel, R. J. Am. Chem. Soc. 2004, 126,
4888.
(4) Aime, S.; Botta, M.; Terreno, E. AdV. Inorg. Chem. 2005, 57, 173. Caravan,
P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV. 1999, 99,
2293.
(5) Bu¨nzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53. Bu¨nzli, J.-C. G.; Piguet, C.
Chem. Soc. ReV 2005, 34, 1048.
a Reagents and Conditions: (a) Cs2CO3, 1, MeCN, reflux, 72 h; (b) tfa,
CH2Cl2, 48 h; (c) Ln(OTf)3, MeOH, 48 h; (d) N2H4, Pd/C, MeOH, 1 h; (e)
NaNO2, HCl, Ln′2.4, NaOH, 0 °C, 0.5 h.
(6) Faulkner, S.; Perry, W. S.; Natrajan, L. S.; Sykes, D. Dalton Trans. 2009,
3890. Gunnlaugsson, T.; Leonard, J. P. Chem. Commun. 2005, 3114.
(7) Alpha, B.; Lehn, J. M.; Mathis, G. Angew. Chem., Int. Ed. Engl. 1987, 26,
266. Beeby, A.; Faulkner, S.; Parker, D.; Williams, J. A. G. J. Chem. Soc.,
Perkin Trans. 2 2001, 1268.
(8) Beeby, A.; Faulkner, S.; Williams, J. A. G. Dalton Trans. 2002, 1918.
(9) Silva, F. R. G. E.; Malta, O. L.; Reinhard, C.; Gudel, H. U.; Piguet, C.;
Moser, J. E.; Bunzli, J. C. G. J. Phys. Chem. A 2002, 106, 1670.
(10) Koullourou, T.; Natrajan, L. S.; Bhavsar, H.; Pope, S. J. A.; Feng, J.;
Narvainen, J.; Shaw, R.; Scales, E.; Kauppinen, R.; Kenwright, A. M.;
Faulkner, S. J. Am. Chem. Soc. 2008, 130, 2178.
(11) Pope, S. J. A.; Faulkner, S. J. Am. Chem. Soc. 2003, 125, 10526.
(12) Dadabhoy, A.; Faulkner, S.; Sammes, P. G. J. Chem. Soc., Perkin Trans.
2 2000, 2359.
(13) Pope, S. J. A.; Heath, S. L.; Kenwright, A. M.; Faulkner, S. Chem. Commun.
2003, 1550.
Both 8 and 9 were found to be luminescent; time-gated emission
spectra are shown in Figure 1. The short-lived signals arising from
neodymium emission dominate the early part of the luminescence
spectrum (Figure 1a), and the ratio of the peaks at 1340 and 1055
nm for the two isomers clearly indicates that the neodymium centers
are in different environments in the different complexes. Similarly,
the later part of the luminescence (Figure 1b) shows the ytterbium
signal after the overlapping neodymium emission has decayed. In
2
this case the fine structure of the F5/2-2F7/2 transition indicates
differences in the ytterbium environment between 8 and 9.
Luminescence lifetimes and values for qYb are shown in Table 1.
In this case lifetime measurements are less informative: 8 and 9
both exhibit low values for qYb despite the differences in ligand
denticity in the octadentate and heptadentate sites in the molecule,
(14) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker, D.; de
Sousa, A. S.; Williams, J. A. G. J. Chem. Soc., Perkin Trans. 2 1999, 493.
(15) Placidi, M. P.; Natrajan, L. S.; Sykes, D.; Kenwright, A. M.; Faulkner, S.
HelV. Chim. Acta 2009, in press (doi: 10.1002/hlca.200900141).
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