A facile iodide-controlled fluorescent switch based on the interconversion

between two- and three-coordinate copper(I) complexesw

Chang-Chuan Chou,* Hsueh-Ju Liu and Lucas Hung-Chieh Chao

Received (in Cambridge, UK) 5th May 2009, Accepted 17th September 2009

First published as an Advance Article on the web 5th October 2009

DOI: 10.1039/b908765f

The first paradigm of halide-controlled interconversion between

two- and three-coordinate copper(I) complexes, [Cu(LPh)](ClO4)

(1�ClO4) and [Cu(LPh)I] (2), where LPh = 1,3-bis-(3,5-dimethyl-

pyrazol-1-ylmethyl)-2-phenyl-2,3-dihydro-1H-perimidine, was

presented, which can result in reversible fluorescence changes.

Based on the flexible coordination numbers of transition metal

and the lability of non-covalent interactions, the issue of

reversible structural interconversion controlled by an external

input (photon, electron, proton and other chemical species)

has attracted considerable attention because it may offer some

feasible ideas for constructing functional molecules.1–4

Accordingly, we recently developed an effective supramolecular

fluorescent switch based on the proton-controlled assembling/

disassembling process between 1-D polymeric and helicate

dimeric copper(II) complexes.5 Herein, we present the first

example of halide-controlled reversible system based simply

on the interconversion between monomeric linear two-coordinate

copper(I) complex [Cu(LPh)](ClO4) 1�ClO4 and T-shaped

three-coordinate copper(I) complex [Cu(LPh)I] 2, which is

accompanied by a reversible change in fluorescence (ca. 5-fold

contrast fluorescence emission). The employed ligand LPh,

shown in Scheme 1, is a novel trans pyrazolyl chelator containing

a perimidine fluorophore because the pyrazolyl-derivatives are

suitable supporting ligands for stabilizing low coordinate

copper(I) complexes,6–8 and the perimidine nucleus could

provide a useful spectroscopic probe upon interacting with

the copper(I) center.

The mononucleating ligand LPh was prepared by treating

1,8-naphthalenediamine with 1 equivalent of benzaldehyde

and 2 molar equivalents of 1-(hydroxymethyl)-3,5-dimethyl-

pyrazole in a 63% yield. The crystal structure of LPh, shown in

Fig. S1,wz reveals that the puckered bridging methylene

H2C(7) produced a six-membered C4N2 heterocycle. The five

atoms N3, C7, C12, C13 and N4 were coplanar and may have

been involved in the conjugation framework of the perimidine.

The neutral LPh ligand featured only a single set of 1H and 13C

signals (Fig. S2w), which indicated a symmetrical structure in

solution.

Complexes 1�ClO4 and 2 were synthesized from the reaction

of the LPh ligand with [Cu(CH3CN)4](ClO4) and CuI, respectively,

in yields of 73% and 85%. Complex 2 can also be crystallized

from a mixed CH2Cl2 solution of 1�ClO4 and n-Bu4NI. The

elemental analyses and ESI-MS were consistent with the

proposed formulas. The structures of complex cation 1 (1+)

and 2 are shown in Fig. 1 and 2, respectively.z As expected, the

coordinated LPh in 1+ and 2 behaved as a trans-chelating

ligand which placed two coordinated pyrazole rings close to

coplanar. The dihedral angles of the two pyrazoles were

6.9(2)1 for 1+ and 19.9(3)1 for 2. On the whole, five carbon

atoms, C11, C12, C17, C24 and C27, and the copper center,

Cu1, were approximately situated on a pseudo-plane of

symmetry. Similar to LPh, both 1+ and 2 also displayed

symmetrical structures when the 1H and 13C spectra

Scheme 1 Formation and reversibility of complex cation 1+ and 2.

Fig. 1 Molecular structure of 1+ (hydrogen atoms have been omitted

for clarity). Selected bond distances (A) and bond angles (1): Cu1–N1

1.878 (3), Cu1–N6 1.875 (3), Cu1� � �N3 2.851 (3), Cu1� � �N4 2.816 (2),

C7–N3 1.395 (4), C6–N3 1.429 (4), C17–N3 1.450 (4), C13–N4 1.404

(4), C17–N4 1.454 (4), C18–N4 1.432 (4); N1–Cu1–N6 169.0 (1),

C6–N3–C7 122.1 (2), C7–N3–C17 116.1 (2), C6–N3–C17 119.9 (2),

C13–N4–C17 115.7 (2), C13–N4–C18 122.4 (2), C17–N4–C18 119.3

(2), N3–C17–N4 107.4 (2).

Center 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. 5583w Electronic supplementary information (ESI) available: Experimentaldetails, crystal structure of LPh and related 1H&13C NMR spectra,absorption and emission spectra. CCDC 722446–722448. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/b908765f

6382 | Chem. Commun., 2009, 6382–6384 This journal is �c The Royal Society of Chemistry 2009

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(Fig. S3–5w) were inspected, which is in agreement with the

solid state structures.

For cation 1+ (Fig. 1), the copper(I) ion was coordinated

by two pyrazole nitrogen atoms of the LPh with a

N(Pz0)–Cu–N(Pz0) bond angle of 169.0 (1)1 toward C17, which

formed a ten-membered metallocycle. The contacts of

Cu(I)� � �N(amine) were 2.816 and 2.851 A, indicating probable

weak N(amine)� � �Cu(I) interactions. The Cu(I)–N(Pz0)

distances of 1.875 and 1.878 A were in the normal range

of common two-coordinate copper(I)-pyrazole complexes

(1.87–1.88 A).7 Interestingly, the diastereotopic nature of the

methylenesH2C(6) andH2C(18) solely exhibited single peak at

d 5.96 in the 1H-NMR spectrum, indicating a remarkable

fluxional behavior for 1+ that was demonstrated further in

variable-temperature 1H-NMR spectroscopic analyses with a

coalescence temperature Tc = 295.6 K (S3w). Therefore 1+ is

unequivocally a new stereochemically nonrigid monomeric

two-coordinate copper(I) complex. The fluxional process of

1+ is under investigation.

For 2 (Fig. 3), the N(Pz0)–Cu–N(Pz 0) bond angle of

147.0(2)1 severely deviated from linearity due to the iodide

binding, which gave rise to a 13.01 increment of the dihedral

angle of the coordinated pyrazole rings. The Cu(I)–N(Pz0)

bond distances of 1.942 and 1.955 A were much longer than

those of the previous three-coordinate T-shape copper(I)-

pyrazole complexes (1.89–1.92 A).8 The terminal Cu(I)–I bond

distance of 2.7172(2) A was strikingly longer than that of a

trigonal planar anion [CuI3]2� at 2.55 A,9 which is a typical

T-shaped binding characteristic similar to the related copper(I)

complex [CuI(2,6-Me2-py)2] (2.66 A).10 Also, three noticeable

intramolecular C–H� � �I hydrogen bonds,11 as shown in Fig. 3

(C6� � �I = 3.902(6) A, H6B� � �I = 3.172(4) A, C6–H6B� � �I =131.9(4)1; C17� � �I = 3.908(6) A, H17A� � �I = 3.031(5) A,

C17–H17A� � �I = 1470.1(3)1; and, C18� � �I = 4.056(6) A,

H18A� � �I = 3.297(5) A, C18–H18A� � �I = 134.8(3)1), appears

to help stablize the iodide in 2. To the best of our knowledge,

complex 2 is the first example of a structurally characterized,

three-coordinate copper(I) halide complex with an N-donor

chelate ligand. In contrast to 1+, the diastereotopic nature of

the methylenes H2C(6) and H2C(18) in 2 displayed two sets of

doublets at d 5.93 and 5.78 (Fig. S5w) in CH2Cl2 solution,

indicating a rigid structure for 2.

The reversible reaction of 1+ and 2 were readily performed

and investigated by 1H NMR spectra. When complex 1�ClO4

reacted stoichiometrically with n-Bu4NI, neutral complex 2

was generated nearly quantitatively (Fig. S6w). On the other

hand, when complex 2 reacted stoichiometrically with

Ag(ClO4) to eliminate the coordinated iodide, complex

1�ClO4 was afforded again. If complex 1�ClO4 first reacted

with a stoichiometric amount of n-Bu4NI then a stoichiometric

amount of Ag(ClO4) was introduced, only the peaks of 1+ and

the n-butyl group were observed in the solution 1H NMR

spectrum (Fig. S7w), which manifested reversibility between

1+ and 2.

The fluorescence emission profiles originating from the

ligand-centered p–p* transitions of the perimidine moiety are

compared and shown in Fig. 3. The fluorescent properties of

LPh (lmax = 394 nm, Ff = 0.097) and of complexes 1�ClO4

(lmax = 380 nm, Ff = 0.005) and 2 (lmax = 394 nm, Ff =

0.031) in CH2Cl2 were examined when excited at l = 340 nm.

The ratio of fluorescence intensities were ca. 12.5 : 1.0 : 5.2

(for LPh : 1�ClO4 : 2, respectively). In fluorescence titration

experiments (see ESI for detailsw), complexes 1�ClO4 and 2 did

exhibit different degrees of quenching (Fig. S8–S10w).In addition, fluorescence enhancement took place upon intro-

ducing I� to 1+ (Fig. 4. Inset), indicating that 1+ changes

from an iodide-unbound two-coordinated species to an iodide-

bound three-coordinated species 2. By monitoring the change

in fluorescent spectra, the interconversion of 1+ and 2 was

reversible in the presence (ON) and absence (OFF) of I�,

giving an ON 2 OFF emission switch.

For complex 1�ClO4, a remarkable quenching effect and

substantial blue shift of 14 nm in the emission profile implies

an electronic perturbation in the delocalized p network of the

fluorophore, which is related to the existence of the weak

interaction between the lone pair electrons of the N(amine)

and the Cu(I) center because it could restrain the conjugation

of the perimidine nucleus and enlarge the energy levels of

p–p*. Besides, we were convinced that the influence of the

Cu(I)� � �N(amine) interaction in solution could be pronounced

Fig. 2 Molecular structure of complex 2. Selected bond distances (A)

and bond angles (1): Cu1–N1 1.955 (5), Cu1–N6 1.942 (5), Cu1� � �N3

2.896 (4), Cu1� � �N4 3.069 (5), Cu1–I1 2.7172 (9), C7–N3 1.399 (8),

C6–N3 1.435 (7), C17–N3 1.463 (7), C13–N4 1.386 (8), C17–N4 1.453

(7), C18–N4 1.445 (7); N1–Cu1–N6 147.0 (2), N6–Cu1–I1 109.4 (2),

N1–Cu1–I1 103.3 (2), C6–N3–C7 121.8 (5), C7–N3–C17 115.0 (4),

C6–N3–C17 119.3 (5), C13–N4–C17 116.3 (5), C13–N4–C18 122.5 (5),

C17–N4–C18 120.7 (5), N3–C17–N4 107.6 (4).

Fig. 3 Fluorescence spectra of ligand LPh, complexes 1�ClO4 and 2 in

CH2Cl2 excited at 340 nm at room temperature with a concentration

of 6.32 � 10 �5 M. Inset: the spectral change of fluorescence spectra

upon a gradual addition of n-Bu4NI (from 0.0 to 1.0 eq.) to complex 1.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 6382–6384 | 6383

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due to the wriggle of the perimidine framework, which may

result in an alternatively N(amine) atom approaching toward

the Cu(I) center.12 For complex 2, the emission profile partially

retrieved to that of ligand LPh, suggesting a less p–p*perturbation, because the N(amine)� � �Cu(I) interaction was

further weakened by the coordination of iodide. As shown in

Fig. 2, the average distance of N(amine)� � �Cu(I) was getting

longer at ca. 0.15 A.

The absorption spectra of LPh, 1�ClO4 and 2 are shown in

Fig. 4 and Fig. S11.w Prominent absorption peaks for these

compounds are indeed a little different each other, indicating

that the vibronic p–p* transitions of the perimidine nucleus in

1+ and 2 was really affected. For complex 2, a broad shoulder

was shown at 271 nm, which can be assigned to the Cu(I) to

pyrazole MLCT transition (ds* - p*).13 The absorption

envelope of the p–p* transition of the perimidine nucleus of

2 changed back to that of the free ligand to a large extent

because the perturbation of the p–p* transition is lessened

by the iodide coordination, which was consistent with the

emission spectra.

In addition to the influence of the electronic perturbation

imposed on perimidine, fluxionality could be also an

important factor for significant change in fluorescence.14

Therefore, it is rational that nonrigid complex 1�ClO4 has a

relatively low emission intensity with respect to the rigid ligand

LPh while the rigidification of 2 by an iodide binding leads to

an enhancement in emission intensity. For 2, the emission

intensity was not fully retrieved to that of LPh suggest that an

effect of the electronic perturbation is not entirely eliminated

and the motion of the chromophoric skeleton was not

completely ceased in solution.

In conclusion, a new fluorescent trans-chelator, LPh, and its

two- and three-coordinate copper(I) derivatives 1�ClO4 and 2

were synthesized and characterized. Also, the implementation

of controllable interconversion between linear two-coordinate

and T-shaped three-coordinate copper(I) complexes was first

realized, which may result in a chemical fluorescent switch

when an iodide is used as the modulator.15 For ligand LPh, the

emission decreases significantly with other divalent metal

ions like Ni2+, Cu2+, and Zn2+ with a probable M : L

stoichiometry of 1 : 2 (Fig. S12–14w), implying that different

coordination geometry could exist besides linear structure.

Further investigations are currently in progress.

Financial support of the National Science Council of the

Republic of China is greatly appreciated. We thank the

referee’s valuable comments. We thank Prof. I.-J. Chang

for helpful discussion. We also thank Mr. T.-S. Kuo and

Miss C.-H. He for assistance with collecting X-ray data and

variable-temperature NMR data, respectively.

Notes and references

z Crystal data for LPh: C29H30N6, M = 462.59, monoclinic, spacegroup P21/n, a = 10.7905(2), b = 19.1349(4), c = 13.3567(3) A, a =901, b = 113.474 (1)1, g = 901, V = 2529.6(1) A3, Z = 4, Dc =1.215Mg/m3, F(000) = 984, l(Mo-Ka) = 0.71073 A, 23 998 reflectionsmeasured (Bruker Kappa CCD diffractometer) in the y range 2.07 to25.021, 4448 unique (Rint = 0.0740), 317 parameters refined on F2

using 4448 reflections to final indices: Rf [I 4 2s(I)] = 0.0727, Rw =0.1932. E.A. results: calcd. C 75.30, N 18.17, H 6.54%; found C 75.37,N 18.16, H 6.60%. CCDC 722446. Crystal data for 1�ClO4�2CH3CN:C33H36ClCuN8O4, M = 707.69, monoclinic, space group P21/n, a =10.6294(2), b = 17.6905(3), c = 17.6917(3) A, a = 901, b =96.005(1)1, g = 901, V = 3308.5(1) A3, Z = 4, Dc = 1.421 Mg/m3,F(000) = 1472, l(Mo-Ka) = 0.71073 A, 23 078 reflections measured(Bruker Smart CCD diffractometer) in the y range 1.63 to 25.021, 5821unique (Rint = 0.0242), 426 parameters refined on F2 using 5821reflections to final indices: Rf [I4 2s(I)] = 0.0470, Rw = 0.1412. E.A.results of 1: calcd. C 55.68, N 13.43, H 4.83%; found C 55.59, N 13.89,H 4.63%. CCDC 726447. Crystal data for 2�CH2Cl2: C30H32CuCl2N6I,M = 737.96, monoclinic, space group P21/c, a = 13.2775(5), b =15.5804(6), c = 14.6968(6) A, a = 901, b = 99.069(1)1, g = 901,V = 3002.3(2) A3, Z = 4, Dc = 1.633 Mg/m3, F(000) = 1480,l(Mo-Ka) = 0.71073 A, 21 778 reflections measured (Bruker SmartCCD diffractometer) in the y range 1.55 to 25.031, 5312 unique (Rint =0.0391), 361 parameters refined on F2 using 5312 reflections to finalindices: Rf [I4 2s(I)] = 0.0497, Rw = 0.1476. E.A. results of 2: calcd.C 48.83, N 11.39, H 4.37%; found C 49.10, N 11.72, H 4.58%. CCDC726448.

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12 In a separate experiment, we have found such a phenomenon byusing an analogous ligand. Unpublished results.

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Macrocyclic Chem., 2008, 62, 99.15 Preliminary results for the binding of 1+ toward other halides

(F, Cl and Br) show that only bromide can make a T-shapedadduct similar to 2, whereas the fluoride and chloride cannot.

Fig. 4 Absorption spectra of ligand LPh (dash line), complexes

1�ClO4 (red line) and 2 (blue line) in CH2Cl2 solution at room

temperature.

6384 | Chem. Commun., 2009, 6382–6384 This journal is �c The Royal Society of Chemistry 2009

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