Metal-directed synthesis of enantiomerially pure dimetallic lanthanide luminescent triple-stranded helicates

Article abstract of DOI:10.1021/ja9032204

(Figure Presented) The synthesis and photophysical evaluation of two enatiomerially pure dimetallic lanthanide luminescent triple-stranded helicates is described. The two systems, formed from the chiral (R,R) ligand 1 and (S,S) ligand 2, were produced as

Full text of DOI:10.1021/ja9032204

Published on Web 06/24/2009
Metal-Directed Synthesis of Enantiomerially Pure Dimetallic Lanthanide
Luminescent Triple-Stranded Helicates
Floriana Stomeo,† Christophe Lincheneau,† Joseph P. Leonard,† John E. O’Brien,†
Robert D. Peacock,§ Colin P. McCoy,‡ and Thorfinnur Gunnlaugsson*,†
School of Chemistry, Center for Synthesis and Chemical Biology, Trinity College Dublin, Dublin 2, Ireland, School
of Pharmacy, Queen’s UniVersity of Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, and Department
of Chemistry, Joseph Black Building, UniVersity of Glasgow, Glasgow G12 8QQ, U.K.
Received April 22, 2009; E-mail: gunnlaut@tcd.ie
The synthesis of novel architectures through the use of metal-
directed synthesis and structurally defined ligands is of great current
interest in supramolecular chemistry.1 To date, the use of transition-
metal ions to achieve such structures is well-documented.2 In
contrast, while the use of lanthanides for sensing and imaging
purposes is well-established,3 their use in metal-directed synthesis
of supramolecular systems has been much less explored.4 However,
some very elegant examples have recently been developed by
Bu¨nzli,5 Piquet,6 and several others.7 We have focused our research
on the formation of such f-based supramolecular structures and
recently demonstrated the formation of mixed f-d metal ion self-
assemblies,8 luminescent ternary complexes between sensitizing
antennae and f-metal ions on gold nanoparticles,9 and the formation
of highly ordered and chiral 1:3 metal/ligand self-assembled bundles
using f-metal ions.10 Herein we describe the use of the chiral ligands
1 (R,R) and 2 (S,S) (shown in Scheme 1) to form the novel,
enantiomerically pure, dinuclear triple-stranded helicates Eu2:13 and
Eu2:23 via Eu(III)-directed synthesis. These structures are, to the
best of our knowledge, among the first examples of such highly
stable, chiral dimetallic f-helicates11 that give rise to Eu(III)-centered
circularly polarized luminescence (CPL) upon excitation of the
naphthalene antennae.
1a and Figure S1 in the Supporting Information) demonstrated the
presence of C2 symmetry, while circular dichroism (CD) spectroscopy
confirmed the enantiomeric relationship of 1 and 2; the CD spectrum
of 1 gave rise to two negative bands centered at 230 and 298 nm
(Figure S2). Both 1 and 2 were complexed with Eu(III) triflate in a
ligand/metal ratio of 3:2 by refluxing in MeOH or CH3CN/CHCl3
followed by precipitation upon addition to diethyl ether, giving the
Eu2:13 and Eu2:23 complexes as off-white-colored powders in ∼50%
yield; elemental analysis confirmed the formation of the desired
1
products. The H NMR spectrum (400 MHz, CD3CN) of Eu2:13
(Figure 1b) confirmed the formation of a single product with a high
degree of symmetry. Moreover, Eu2:13 and Eu2:23 gave rise to identical
1H NMR spectra, demonstrating that the two were formed as a pair of
enantiomers.13 This was further confirmed by CD spectroscopy (Figure
S4). Eu2:13 was characterized using 1H-13C and 1H-15N heteronuclear
single-quantum correlation (HSQC) NMR spectroscopy (600 MHz,
CD3CN) (Figures S5 and S6), which showed that the methylene protons
of the spacer were equivalent, appearing as a singlet at 4.37 ppm; this
indicates that Eu2:13 was formed as a single, helical (rac) isomer.14
1
The formation of Eu2:13 was further analyzed by H NMR titration
(400 MHz) of 1 with Eu(III)(SO3CF3)3 in 1:1 (v/v) CD3CN/CDCl3.
The results demonstrate chemical shift changes and gradual broadening
of several resonances in the 1H NMR spectrum of 1 upon addition of
0 to 0.6 equiv of Eu(III), after which no significant changes occurred
(Figure S7). This confirmed the formation of a solution species with
2:3 stoichiometry (Figure S8), in which three ligands wrap around
two Eu(III) ions in an helical fashion, resulting in the formation of a
nine-coordinate environment for each ion, as depicted in Scheme 1.
Furthermore, we were able to determine the hydration states (q) of
both Eu2:13 and Eu2:23 by measuring their Eu(III) excited-state decays
in H2O and D2O, which indicated that q ≈ 0 (Tables S1 and S2 in the
Supporting Information) for both complexes, suggesting that each
Eu(III) ion is nine-coordinated within these dimetallic helicates.
Scheme 1. Synthesis of 1 (R,R), 2 (S,S), and the Corresponding
Dinuclear Triple-Stranded Helicates Eu2:13 and Eu2:23
Ligands 1 and 2 were designed to enable coordination to lanthanides
via each of the two pyridyl nitrogens and the 2,6-dicarboxylic amides.
Their synthesis was achieved in a few steps and in high yield from
commercially available starting materials (Scheme 1). The monopro-
tected 2,6-pyridinedicarboxylic acid 312 were reacted with the R and
S isomers of 1-(1-naphthyl)ethylamine using standard peptide-coupling
methodology to give the intermediates 4 and 5 in ∼80% yield, after
which deprotection of the benzyl ester using 10% Pd/C catalyst under
3 atm H2 yielded 6 and 7 in quantitative yields. Both were reacted
with 4,4′-diaminodiphenylmethane via a peptide-coupling reaction
using EDCI ·HCl, and 1 and 2 were isolated after aqueous workup in
∼80% yield. The 1H NMR spectrum (400 MHz, CD3CN) of 1 (Figure
Figure 1. 1H NMR spectra (400 MHz, CD3CN) of (top) 1 (R,R) and
(bottom) Eu2:13. Pyridyl proton peaks are labeled with *.
The spectroscopic properties of Eu2:13 and Eu2:23 were further
investigated in H2O, MeOH, CH3CN, and 1:1 (v/v) CH3CN/CHCl3.
The absorption spectra of Eu2:13 and Eu2:23 in MeOH showed the
presence of a broad band for the naphthalene moieties (Figures S9
† Trinity College Dublin.
‡ Queen’s University of Belfast.
§ University of Glasgow.
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10.1021/ja9032204 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
and S10). Excitation of this band gave rise to naphthalene-centered
emission with λmax ) 340 nm (Figure S14). Eu(III) emission was
also clearly evident upon excitation of the naphthalene antennae
or the pyridyl moieties (Figures S13 and S14). The total lumines-
cence spectrum for Eu2:23 (Figure 2) demonstrates the sensitization
of the 5D0 excited state by the six antennae and the deactivation to
the 7FJ (J ) 0-4) states, with narrow emission bands occurring at
590, 593, 613, 647, and 693 nm respectively. The fluorescence
excitation spectra of the antennae in MeOH and 1:1 (v/v) CH3CN/
CHCl3 also clearly demonstrated that both complexes successfully
Figure 3. (A) Overall changes in the Eu(III) emission of 2 in 1:1 (v/v)
CH3CN/CHCl3 using Job’s method of continuous variations. (B) Job’s plot
analyses for ∆J ) 1, 2, and 4, showing the formation of Eu2:23.
5
sensitized the D0 excited state (Figures S15-S18). The presence
Acknowledgment. We thank SFI, HEA (PRTLI Cycle 3 CSCB
funding), and TCD for financial support and Prof. Paul E. Kruger
(University of Christchurch) and Dr. Sally Plush for their help.
of the 5D0 f 7F0 band (Figure 2) in the emission spectrum suggests
that the local symmetry at the Eu(III) centers is C3 rather than D3,
which is the symmetry of the dimetallic helixes as a whole.
1
However, the H NMR spectra also suggest that D3 symmetry is
Supporting Information Available: Synthesis and characterization
of all novel compounds, Figures S1-S24, and Tables S1 and S2. This
material is available free of charge via the Internet at http://pubs.acs.org.
favored on the NMR time scale. Figure 2 also shows the CPL
spectra of Eu2:13 and Eu2:23, which have opposite signs and equal
magnitudes, confirming the enantiomeric nature of Eu2:13 and Eu2:
23, which is driven by asymmetric induction from 1 and 2. The
large values of the dissymmetry factor 2∆I/I (e.g., -0.23 for
the higher-energy component of the 593 nm transition of Eu2:13)
have the same sign and almost identical magnitude as those for
the corresponding monomeric complexes previously developed by
us.10 This implies not only that the absolute configurations of Eu2:
13 and Eu2:23 are the same as those of the corresponding monomers
(i.e., Eu2:13 has the Λ,Λ and Eu2:23 the ∆,∆ absolute configuration)
but also that the degrees of twist of the ligators away from
octahedral geometry must be very similar (within about (2°) for
the dimetallic and monomeric Eu(III) complexes. Thus, the CPL
spectra show that dimetallic Eu(III) triple-stranded homochiral
helicates are formed in solution for Eu2:13 and Eu2:23.11
References
(1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46,
72. Meyer, C. D.; Joiner, C. S.; Stoddart, J. F. Chem. Soc. ReV. 2007, 30,
1705. Nitschke, J. R. Acc. Chem. Res. 2007, 40, 103. Fujita, M.; Tominaga,
M.; Hori, A.; Therrien, B. Acc. Chem. Res. 2005, 38, 371.
(2) Goldup, S. M.; Leigh, D. A.; Lusby, P. J.; McBurney, R. T.; Slawin,
A. M. Z. Angew. Chem., Int. Ed. 2008, 47, 6999. Prikhod’ko, A. I.; Durola,
F.; Sauvage, J. P. J. Am. Chem. Soc. 2008, 130, 448. Chichak, K. S.;
Cantrill, S. J.; Pease, A. R.; Chui, S.-H.; Cave, G. W. V.; Atwood, J. L.;
Stoddart, J. F. Science 2004, 304, 1308.
(3) Deiters, E.; Song, B.; Chauvin, A. S.; Vandevyver, C. D. B.; Gumy, F.;
Bunzli, J. C. G. Chem.sEur. J. 2009, 15, 885. Leonard, J. P.; Nolan, C. B.;
Stomeo, F.; Gunnlaugsson, T. Top. Curr. Chem. 2007, 281, 1. Gunnlaugs-
son, T.; Stomeo, F. Org. Biomol. Chem. 2007, 5, 1999. Pandya, S.; Yu, J.;
Parker, D. Dalton Trans. 2006, 2757.
(4) Bu¨nzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53. Bu¨nzli, J.-C. G.; Piguet, C.
Chem. Soc. ReV. 2005, 34, 1048.
(5) Chauvin, A. S.; Comby, S.; Song, B.; Vandevyver, C. D. B.; Bu¨nzli, J.-
C. G. Chem.sEur. J. 2008, 14, 1726. (a) Albrecht, M.; Osetska, O.;
Fro¨hlich, R.; Bu¨nzli, J.-C. G.; Aebischer, A.; Gumy, F.; Hamacek, J. J. Am.
Chem. Soc. 2007, 129, 14178.
(6) Canard, G.; Koeller, S.; Bernardinelli, G.; Piguet, C. J. Am. Chem. Soc.
2008, 130, 1025. Riis-Johannessen, T.; Dupont, N.; Canard, G.; Bernar-
dinelli, G.; Hauser, A.; Piguet, C. Dalton Trans. 2008, 3661. Senegas, J. M.;
Koeller, S.; Bernardinelli, G.; Piguet, C. Chem. Commun. 2005, 2235.
(7) Examples include: Albrecht, M.; Liu, Y.; Zhu, S. S.; Schalley, C. A.;
Fro¨hlich, R. Chem. Commun. 2009, 1195. Gregolinski, J.; Starynowicz,
P.; Hua, K. T.; Lunkley, J. L.; Muller, G.; Lisowski, J. J. Am. Chem. Soc.
2008, 130, 17761. Hamacek, J.; Bernardinelli, G.; Filinchuk, Y. Eur.
J. Inorg. Chem. 2008, 3419. Ronson, T. K.; Adams, H.; Harding, L. P.;
Pope, S. J. A.; Sykes, D.; Faulkner, S.; Ward, M. D. Dalton Trans. 2007,
1006. Bonsall, S. D.; Houcheime, M.; Straus, D. A.; Muller, G. Chem.
Commun. 2007, 3676. Hoffart, D. J.; Loeb, S. J. Angew. Chem., Int. Ed.
2005, 44, 901. Argent, S. P.; Adams, H.; Riis-Johannessen, T.; Jeffery,
J. C.; Harding, L. P.; Mamula, O.; Ward, M. D. Inorg. Chem. 2005, 45,
3905. Bretonniere, Y.; Mazzanti, M.; Pecaut, J.; Olmstead, M. M. J. Am.
Chem. Soc. 2002, 124, 9012. Bretonniere, Y.; Mazzanti, M.; Wietzke, R.;
Pecaut, J. Chem. Commun. 2000, 1543. Lessmann, J. J.; Horrocks, W. D.
Inorg. Chem. 2000, 39, 3114.
Figure 2. Luminescence spectrum of Eu2:23 (black) and CPL spectra (×10)
of Eu2:13 (blue) and Eu2:23 (red) in MeOH. The ∆J ) 0 band is expanded.
(8) Se´ne´shal-David, K.; Leonard, J. P.; Plush, S. E.; Gunnlaugsson, T. Org.
Lett. 2006, 8, 2727. Se´ne´shal-David, K.; Pope, S. J. A.; Quinn, S.; Faulkner,
S.; Gunnlaugsson, T. Inorg. Chem. 2006, 45, 10040.
(9) Massue, J.; Quinn, S. E.; Gunnlaugsson, T. J. Am. Chem. Soc. 2008, 130,
6900. Plush, S. E.; Gunnlaugsson, T. Dalton Trans. 2008, 3801. Leonard,
J. P.; dos Santos, C. M. G.; Plush, S. E.; McCabe, T.; Gunnlaugsson, T.
Chem. Commun. 2007, 129. Nonat, A. M.; Quinn, S. J.; Gunnlaugsson, T.
Inorg. Chem. 2009, 48, 4646. Nonat, A. M.; Harte, A. J.; Sénéshal-David,
Leonard, J. P.; Gunnlaugsson, T. Dalton Trans. 2009, 4703.
The formation of Eu2:13 and Eu2:23 was also investigated in
1:1 (v/v) CH3CN/CHCl3 by observing the changes in their absorp-
tion and Eu(III) emission spectra upon variation of the amount of
Eu(III)(SO3CF3)3 at fixed concentrations of 1 and 2 (10 µM) after
24 h of equilibration. Significant changes were observed in the
absorption spectra, which were red-shifted to 320 nm (Figures S19
and S20), and in the Eu(III)-centered emission (Figures S21 and
S22), which was “switched on” for both systems within the addition
of ∼0.7 equiv of Eu(III) upon formation of both Eu2:13 and Eu2:
23. The 3:2 stoichiometry of these helicates was further confirmed
using Job’s method of continuous variations, where ꢀmax ) 0.6 was
determined from both the absorption (Figures S23 and S24) and
the Eu(III) emission (Figure 3).
(10) Leonard, J. P.; Jensen, P.; McCabe, T.; O’Brien, J. E.; Peacock, R. D.;
Kruger, P. E.; Gunnlaugsson, T. J. Am. Chem. Soc. 2007, 129, 10986.
(11) Albrecht, M.; Schmid, S.; Dehn, S.; Wickleder, C.; Zhang, S.; Bassett,
A. P.; Pikramenou, Z.; Frohlich, R. New J. Chem. 2007, 31, 1755. Cantuel,
M.; Bernardinelli, G.; Muller, G.; Riehl, J. P.; Piguet, C. Inorg. Chem.
2004, 43, 1840.
(12) Hamuro, Y.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc. 1997, 119,
10587.
(13) The Tb(III) complex of 1 was also formed. The 1H NMR spectrum (Figure
S3) showed the formation of a single species.
(14) Eu2L3 can be formed either as rac isomers (ΛΛ and ∆∆) or as a meso
isomer (Λ∆). In the meso form, the methylene protons would be
diastereotopic and give rise to two doublets in the 1H NMR spectrum. See:
Goetz, S.; Kruger, P. E. Dalton Trans. 2006, 1277.
In summary, we have developed novel, enantiomerially pure
dimetallic lanthanide luminescent triple-stranded helicates using
Eu(III)-directed synthesis. We are in the process of evaluating their
properties and developing related f-based helical structures.
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