Proton-controllable fluorescent switch based on interconversion of polynuclear and dinuclear copper(II) complexes

Article abstract of DOI:10.1039/b613583h

The first reversible interconversion process between a one-strand polymeric copper(ii) complex {[Cu₂(L1)₂(ClO₄) ₂](ClO₄)₂}_n (1) and a dicopper(ii) helicate [Cu₂(L1-2H)

Full text of DOI:10.1039/b613583h

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Proton-controllable fluorescent switch based on interconversion of
polynuclear and dinuclear copper(II) complexes{
Hsueh-Ju Liu,a Yu-Hsin Hung,a Chang-Chuan Chou*b and Chan-Cheng Su*a
Received (in Cambridge, UK) 18th September 2006, Accepted 11th October 2006
First published as an Advance Article on the web 31st October 2006
DOI: 10.1039/b613583h
shift of the carbonyl n(CLO) at 1617 cm21 in the IR spectrum
The first reversible interconversion process between a one-
indicate that the carbonyl O-atom is the coordination site.9 The
strand polymeric copper(II) complex {[Cu2(L1)2(ClO4)2]
electronic spectrum of 1 is characterized by a broad d–d
absorption band at 708 nm in the CH3OH–CH3CN (1 : 1)
solution and in the solid state, indicative of a square pyramidal
stereochemistry around the copper(II) ion. A single crystal X-ray
diffraction analysis further supports the square pyramidal
geometry.{
(ClO4)2}n (1) and a dicopper(II) helicate [Cu2(L1–2H)2] (2),
proceeding via a deprotonation–protonation process, can
transduce fluorescence and function as a fluorescent switch
simply by introducing a one fiftieth equivalent of coumarine
343 anion, a fluorophore.
Polydentate ligands that can assemble dinuclear and polynuclear
complexes with unique structural motifs, such as rings,1 helicates,2
grids,3 cages,4 boxes,5 etc, have been widely studied. Further
development in this fascinating area has been directed at
controllable assembling/disassembling processes by an external
input (photon, electron, or proton) so as to form a molecular
switch. There are a few examples of monomer and oligomer
interconversions of copper(II) complexes.6–8 However, interconver-
sion between polynuclear copper(II) complexes and dicopper(II)
helicates has not been found to date. Herein, we report on the first
example of this sort of interconversion between copper(II)
complexes with a pyridyl-carboxamide ligand, N,N9-bis[(2-pyri-
dyl)methyl]isophthalamide L1, by a simple deprotonation–proto-
nation process. The employed ligand L1 contains two amide
groups and one benzene spacer and functions as a ditopic ligand
due to its potential carbonyl O-donor and amido N-donor binding
sites. Namely, upon the complexation of copper(II) ions, the
amido-N and pyridyl-N chelating sites would be favoured in the
deprotonated state, forming a planar 5-membered ring structure,
whereas the carbonyl-O and pyridine-N chealting sites would be
preferred in the protonated state, resulting in a puckered
7-membered metallocycle. Accordingly, such a structural rearran-
gement can easily be manipulated via a deprotonation–protonation
process (Fig. 1). These two complexes can be detected by UV-Vis
spectrometry and their bonding modes around the copper(II)
centers have been confirmed by X-ray diffraction analysis. The
spatial rearrangement on a fluorescent switch is described herein.
Ligand L1 was readily prepared by the reaction of 2-amino-
methylpyridine and isophthaloyl chloride in the presence of NEt3
to give a 90% yield. The reaction of L1 with [Cu(H2O)6](ClO4)2 in
acetonitrile generated the polymeric complex 1 [eqn (1)]. For
complex 1, the presence of the n(N–H) at 3343 cm21 and the red
2n½CuðH2OÞꢀ ðClO4Þ DCzDC2nLC1 A
6
2
(1)
ÈÂ
Ã
É
Cu2(L1)2ðClO4Þ2 ðClO4Þ2
n
ÈÂ
Ã
É
Â
Ã
Cu2ðL1Þ2ðClO4Þ2 ðClO4Þ2 n Cu2ðL1ꢁꢁ2HÞ2
(2)
(2)
n
Polymer
dimerðhelicateÞ
Upon the addition of 4 equivalents of [(CH3)4N]OH to a
CH3OH–CH3CN (1 : 1) solution of complex 1, dimeric helicates 2
are nearly quantitatively formed [eqn (2)]. The absence of n(N–H)
and the uncoordinated carbonyl n(CLO) at 1552 cm21 suggest that
the amide ligand binds to the copper through the amido N-atom.10
Unlike complex 1, the electronic spectrum of complex 2 shows a
broad d–d band at 608 nm with a shoulder at y847 nm in a
CH3OH–CH3CN (1 : 1) solution, indicative of a different
molecular geometry. A tetrahedrally distorted square-planar
CuN4 chromophore is suggested,11 consistent with the crystal
aDepartment of Chemistry, National Taiwan Normal University, Taipei,
116, Taiwan, R.O.C. E-mail: ccsu@ntnu.edu.tw; Fax: +886-2-29324249;
Tel: +886-2-29350749
bCenter for General Education, Chang Gung Institute of Technology,
Tao-Yuan, 333, Taiwan, R.O.C. E-mail: ccchou@mail.cgit.edu.tw;
Fax: +886-3-2118866; Tel: +886-3-2118999 ext. 5583
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b613583h
Fig. 1 Switchable chelating sites of protonated carbonyl-O and pyridyl-N
(L1) and deprotonated amido-N and pyridyl-N (L1–2H).
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Chem. Commun., 2007, 495–497 | 495

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structure of 2.§ The electronic spectra measured in the solid state
(diffusion reflectance spectrum) and in CH3OH–CH3CN (1 : 1) are
very similar, implying that the stereochemistry of the copper(II)
center does not change in solution. The absorption patterns of
complexes 1 and 2 are so much different that the presence of these
two complexes can easily be detected in UV-Vis spectra (Fig. 2,
curves a and b). Complex 2 is converted back to 1 upon the
addition of 4 equivalents of HClO4 and can also be shifted back
quantitatively to 2 by adding 4 equivalents of [(CH3)4N]OH,
exhibiting a reversible process. The structural transformation can
be visualized by the color change of the solution (blue « green).
Similarly, substituting RCOOH (R = CH3 and C6H5) for HClO4,
a new reversible process occurred with a new band appearing (lmax
674 nm for CH3COOH, curve c in Fig. 2, lmax 689 nm for
C6H5COOH, curve d in Fig. 2), suggesting that carboxylate anions
coordinate to the copper(II) ions of complex 1. Consequently,
complex 1 reacts with RCOONa to give the same d–d absorption
band as when complex 2 reacts with RCOOH. Since there is
essentially no reaction between RCOOH and complex 1, at a
molar ratio of 1 : 1, this confirms the interaction of RCOO2 with
Cu2+. Therefore, RCOOH provides not only a proton but also a
coordinated carboxylate anion.
The crystal structure of the cation of complex 1 indicates a one-
strand polymeric structure. The asymmetric unit, shown in Fig. 3
(1), consists of a Cu(II) ion coordinated to two portions of two
discrete L1 ligands through one O-atom of the carbonyl group and
one N-atom of the pyridine, and a perchlorate anion. The
copper(II) ions exhibit a nearly square pyramidal coordination
with t = 0.05.12 The structure of the neutral complex 2 is shown in
Fig. 3 (2), confirming the formation of a double-helical structure.
Each copper(II) center is bound by two pyridine and two amido-N
atoms, forming a distorted tetrahedral coordination geometry. The
coordination to two metal centers leads to interannular twisting
and the formation of a double-strand array. The bond lengths are
in the normal ranges. The two copper(II) centers are separated by
Fig. 3 Molecular structures of a portion of the cation of 1 (1) and the
˚
left-handed dicopper helicate 2 (2). Selected bond distances (A) for 1: Cu1–
N1 2.000(7), Cu1–N4 2.021(7), Cu1–O1 1.962(5), Cu1–O2 1.973(5), Cu1–
O3 2.297(6); bond angles (u) for 1: O1–Cu1–O2 174.8(2), O1–Cu1–N1
97.6(3), O2–Cu1–N1 84.8(3), O1–Cu1–N4 86.9(2), O2–Cu1–N4 90.1(2),
N1–Cu1–N4 171.7(3), O1–Cu1–O3 89.2(2), N1–Cu1–O3 92.3(3), N4–
˚
Cu1–O3 94.2(3). Selected bond distances (A) for 2: Cu1–N1 1.938(4),
Cu1–N2 2.007(4), Cu1–N3 2.002(4), Cu1–N4 1.939(4), Cu2–N5 1.932(4),
Cu2–N6 2.005(4), Cu2–N7 2.012(4), Cu2–N8 1.940(4); bond angles (u) for
2: N1–Cu1–N4 157.8(2), N2–Cu1–N4 106.5(2), N1–Cu1–N2 83.2(2), N3–
Cu1–N4 83.9(2), N1–Cu1–N3 102.2(2), N2–Cu1–N3 138.6(2), N5–Cu2–
N8 157.8(2), N5–Cu2–N6 83.5(2), N6–Cu2–N8 104.7(2), N5–Cu2–N7
103.0(2), N7–Cu2–N8 83.6(2), N6–Cu2–N7 141.5(2).
˚
4.8985(8) A, and the ligand twists around the copper–copper axis.
Due to the strong donor capability of the deprotonated amido-N
donors, it would be expected to have short Cu–Namido bond
lengths. Indeed, the average Cu–Namido bond length (1.937 ¡
˚
0.005 A) is considerably shorter than that of the Cu–Npyridine bond
˚
(2.007 ¡ 0.005 A). In complex 1, the weak axial Cu–OClO3 bond
introduced into this system because of its quenching properties
for 5-coordinated copper(II) complexes.6,13 The binding/releasing
of the fluorophore to/from the copper(II) ion may lead to the
quenching/revival of the fluorescent emission, thus functioning as a
switch.
˚
with a length of 2.297(6) A can readily be replaced by other neutral
or anionic ligands, for example, CH3COO2 or C6H5COO2, etc.
Taking advantage of such a conformational change, a fluorophore
with a carboxylate group, the coumarine 343 anion, was
In fluorescence quenching experiments (see ESI for details{), a
polymeric complex 1 solution in MeCN–MeOH (1 : 1) was added
to a coumarine 343, R343COOH, MeCN–MeOH (1 : 1) solution.
The emission band corresponding to the undissociated R343COOH
centered at lmax 491 nm was quenched only slightly when the
molar ratio was 1 : 1. As the molar ratio of complex 1 to
coumarine reached 100 : 1, the emission band shifted to lmax
480 nm, indicative of CuII–R343COO2 adduct formation.6 The
R343COO2 anion, produced by the dissociation of R343COOH,
bonded to complex 1 forming a CuII–R343COO2 adduct and
promoted the dissociation of R343COOH as the amount of CuII
increased, in spite of the very low Ka value (y1027 13
343COOH. Consequently, this confirms that the carboxylate
)
of
R
2
Fig. 2 UV-vis spectra of complex 2 (curve a), complex 1–ClO4 (curve
b), 1–CH3COO2 (curve c), and 1–C6H5COO2 (curve d).
group of coumarine 343 anion can interact with complex 1, similar
to CH3COO2 or C6H5COO2.
496 | Chem. Commun., 2007, 495–497
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In summary, we investigated the controllable behavior of a
simple pyridyl-carboxamide ligand L1 that forms two stable
complexes upon complexation with copper(II) ion via a deprotona-
tion–protonation process. While maintaining the same oxidation
state, the appearance of a reversible spatial rearrangement
demonstrates the advantage of employing such a ligand as a
proton-dependent switch in the control of molecular architectures.
In addtion, introducing a small quantity (1/50 equivalents) of
fluorophore with a carboxylate functional group, such as the
coumarine 343 anion, enhances the signal of the photo-switch.
Related studies regarding fluorescent switches with other metals,
such as nickel, iron, and cobalt, are currently in progress.
Financial support of the National Science Council of the
Republic of China is greatly appreciated. We also thank Mr T.-S.
Kuo for collecting the X-ray data. Our gratitude also goes to the
Academic Paper Edition Clinic, NTNU.
Fig. 4 Normalized fluorescent intensity, IF/Iave, of a bulk solution (50 ml)
of complex 2 (2 6 1025 M) with the addition of 1/50 equivalents
coumarine 343 anion solution in 5 cycles of proton abstraction/supply
expriments. A (lmax 480 nm): addition of HClO4 (8 6 1025 M, 0.2 ml per
cycle); B (lmax 471 nm): addition of [(CH3)4N]OH (8 6 1025 M, 0.2 ml
per cycle).
Notes and references
{ Crystal data for 1 squeezed 2Et2O: C40H36Cl4Cu2N8O20, M = 1217.65,
monoclinic, blue crystals, space group P21/c, a = 10.902(1), b = 17.764(2),
3
˚
˚
c = 15.503(2) A, a = 90u, b = 110.58u, c = 90u, V = 2810.7(5) A , Z = 2, Dc =
1.439 g cm23, F(000) = 1236, l(Mo-Ka) = 0.71073 A, 11329 reflections
˚
Since the stoichiometric reaction of [Cu2(L1–2H)2] with
R343COOH takes place as shown in eqn (3), the quenching effect
of R343COOH will be greatly reduced due to the emission from the
counter anion, R343COO2. Therefore, an effective way to enhance
the difference of the signals (IF/Iave) is to introduce a small quantity
(1/50 equivalents) of R343COO2 as the fluorophore and 4
equivalents of HClO4 as the proton and the counter anion,
ClO42, sources.
measured (Bruker Smart CCD diffractometer) in the h range 2.30 to 25.27u,
4794 unique (Rint = 0.1255), 335 parameters refined on F2 using 4794
reflections to final indices: Rf [I . 2s(I)] = 0.0814, Rw = 0.2138. CCDC
620454. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b613583h.
§ Crystal data for 2?2H2O: C40H36Cu2N8O6, M = 851.85, monoclinic,
green crystals, space group P21/n, a = 11.6029(2), b = 23.2657(4), c =
3
˚
˚
16.6550(3) A, a = 90u, b = 108.035(1)u, c = 90u, V = 4275.1(1) A , Z = 4,
Dc = 1.324 g cm23, F(000) = 1752, l(Mo-Ka) = 0.71073 A, 25181
˚
reflections measured (Bruker Smart CCD diffractometer) in the h range
2.04 to 25.34u, 7539 unique (Rint = 0.0537), 505 parameters refined on F2
using 7539 reflections to final indices: Rf [I . 2s(I)] = 0.0597, Rw = 0.1757.
CCDC 618879. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b613583h.
Â
Ã
z4nR343 COOH
n Cu2ðL1ꢁ2HÞ2 DCDCCA
Cu2(L1)2ðR343COOÞ2 ðR343COOÞ2
(3)
ÈÂ
Ã
É
n
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As shown in eqn (4), when the coumarine anion was delivered
to the helicate solution in a molar ratio of 1 : 50, an emission
with lmax 471 nm (due to free coumarine 343 anion)13 revealed
the ‘‘fluorescent on’’ condition. When 4 equivalents of HClO4 was
subsquently added, the 471 nm emission was quenched to a large
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adduct)6 appeared, representing the ‘‘fluorescent off’’ condition.
When 4 equivalents of [(CH3)4N]OH were added again for
deprotonation, an emission of lmax 471 nm was revived,
indicative of releasing R343COO2, namely, the reversed reaction
of eqn (4). Such an ON « OFF situation can be repeated a
number of times by a series of protonation–deprotonation
processes. The fluorescent outcome of the quenching/revival is
shown in Fig. 4.
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(4)
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Chem. Commun., 2007, 495–497 | 497