Europium (III) complexes of amino acid-derived bis-imine-substituted phenanthroline ligands for phosphate recognition

Journal Pre-proofs Research paper Europium (III) complexes of amino acid-derived bis-imine-substituted phenanthroline ligands for phosphate recognition Yanfang Hu, P.S. Subramanian, Markus Albrecht PII: DOI: Reference:

S0020-1693(19)31926-7 https://doi.org/10.1016/j.ica.2020.119428 ICA 119428

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

9 December 2019 8 January 2020 8 January 2020

Please cite this article as: Y. Hu, P.S. Subramanian, M. Albrecht, Europium (III) complexes of amino acid-derived bis-imine-substituted phenanthroline ligands for phosphate recognition, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119428

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© 2020 Published by Elsevier B.V.

Europium

(III)

complexes

of

amino

acid-derived

bis-imine-substituted phenanthroline ligands for phosphate recognition Yanfang Hua, P. S. Subramanian b, and Markus Albrecht *a a

Institut for Organische Chemie, RWTH Aachen University, Aachen 52070,

Germany. b Central

Salt and Marine Chemicals Research Institute(CSIR-CSMCRI), Academy of

Scientific and Innovative Research (AcSIR), Bhavnagar 364002, Gujarat India.

Abstract Phenanthroline-based chiral hexadentate ligandsL1, L2 and L3, as well as their europium (III) complexes 1-3 are synthesized and characterized. The coordination compounds show strong red emission typical for europium (III) derivatives. Various anionic species, F-, Cl-, Br-, I-, SO42-, NO3-, NO2-, HPO42-, CO32-, HCO3- and OAc-, are employed to evaluate the chemo-sensor properties of the complexes using fluorescence spectroscopy. For complex 1, only the addition of HPO42- leads to quenching of luminescence. For the other complexes 2 and 3, NO2– and HPO42- exhibit a strong response in the emission properties. However, complexes 2 and 3 are found to be more sensitive for HPO42- than for NO2–. The corresponding binding constants (logKβ) of the adducts are determined using the Benesi-Hildebrand method. The described studies show that the europium(III) complexes of the hexadentate ligands L1-3 can be considered as highly specific sensors for the recognition ofphosphates with some competition by NO2- ions. While phosphate binding is strong in all 1

three examples, the sensitivity for nitrite increases in the order 1<2<3. The obtained results are remarkable because the differences are due to less sterically hindered substituents in the periphery of the ligandof 2 and 3, which enable the binding of the small nitrite while the bigger phosphate is bound less tightly.

* Corresponding author, Tel: +49 241 809 4678; Fax: +49 241 809 2385; E-mail: [email protected] Keywords: luminescence; europium (III); anion recognition; phosphate.

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2

1. Introduction Recently, selective recognition and sensing of anions has become one of the most active areas of supramolecular chemistry due to the important role of anions in biological, industrial, and environmental processes.[1-3] After long-term exploration and unremitting efforts, many detection techniques have been established to study molecular recognition processes involving anions.[4-6] Compared with the traditional analytical methods, such as titration,[7, 8] more modern analytics inductively coupled plasma-atomic

emission

spectroscopy,

electrochemical

methods,[9]

ion

chromatography or fluorescent sensing have received great attention because of their simple operation and high selectivity and sensitivity.[10, 11] Developing effective fluorescent probes for phosphate recognition and sensing is highly desirable because of the critical role of this anion in a range of biological processes. Much effort has been devoted to the development of luminescent chemosensors and with their help, phosphate can be recognized even in water or under physiological conditions.[12] As example, in 2014, an efficient and highly selective fluorescent sensor, a terpyridine-Zn(II) complex was successfully prepared which shows unprecedented fluorescence response (∼500 fold increase) to pyrophosphate in water at physiological pH.[13] Soon afterwards, a hemicryptophane cage revealing strong fluorescence was synthesized as an efficient and selective sensor for choline phosphate.[14] Recently, a novel fluorescent receptor was made by Schiff base condensation of 1-pyrenemethylamine with the vitamin B6 cofactor 3

pyridoxal, and a unique change in the fluorescence behaviour of its Zn(II) complex has been found upon its interacted with H2PO4– due to demetalation.[15]

Scheme 1. Europium(III) complexs 1-3used in this study. Among various luminescence sensors, probes based on lanthanide complexes are exceptional standouts due to their unique photo-physical properties such as narrow emission, large Stokes shifts and long luminescent lifetimes.[16-18] For example, the Eu3+ and Tb3+ complexes of DTTA ([4’-(2,4-dimethoxyphenyl)-2,2’:6’,2’’-terpyridine-6,6’’-diyl]bis(methylenenitrilo)tetrakis(acetic acid)), [Eu(DTTA)]3+ or [Tb(DTTA)]3+, were found to be highly water soluble and strongly luminescent with luminescence quantum yields of 10.0 or 9.9 %, and long luminescence life times of 1.38 or 0.26 ms.[19] Recently, the phenanthroline-based chiral ligand L1 and its luminescent europium complex 1 have been described, which recognize ATP and lead to selective quenching of luminescence.[20, 21] In this study, two related phenanthroline-based hexadentate 4

ligands L2/3as well as their europium complexes 2,3 are prepared in order to compare their photophysical behavior in the presence of anions with the one of complex 1. Hereby only the side chains of the amino acid residues are modified but the anion binding site at the metal ion is not directly altered. To evaluate those chemo-sensors, various anionic species, F-, Cl-, Br-, I-, SO42-, NO3-, NO2-, HPO42-, CO32-, HCO3- and OAc-, are tested as sodium salts. 2. Experimental section 2.1 Material and Characterizations All reagents were purchased from Alfa Aesar and Acros Organics, and were used directly without further purification. NMR spectroscopy was carried out with a Varian VNMRS 400 or 600 MHz spectrometer and the chemical shifts were expressed in ppm. IR spectroscopy was recorded using KBr pellets (1% w/w) on a PerkinElmer Spectrum 100 spectrometer. Mass spectrometry measurements were performed on a LTQ

Orbitrap

XL

(ESI). Elemental analysis was measured with a varioEL

Analyzer. UV-Vis measurements were carried out on a PerkinElmer Lambda 35 UV/Vis Spectrometer using PS/PMMA cuvettes (300-900 nm). For baseline calibration (auto zero) the corresponding solvents were measured as standard. Luminescence measurements at 360 nm were carried out on a PerkinElmer LS45 Fluorescence Spectrometer using quartz cuvettes. The slit sizes for emission and excitation are adjusted as 10.0/10.0 nm. 2.2 Preparation of 1,10-phenanthroline-2,9-dicarbaldehyde The synthetic procedure of this compound was reported previously.[22] 5

2.3

Prepartion

of

(2S,

2’S)-2,

2’-(((1,

10-phenanthroline-2,

9-diyl)bis(methanylyliden)) bis(azanylyliden))-bis(3-methyl-butanamid) (L1) L1 was synthesized as reported before.[20] 1H NMR (CDCl3,600 MHz): δ= 8.77 (s, 2H), 8.46 (d, J=8.4 Hz,2H), 8.37(d, J=8.4 Hz 2H), 7.92 (s, 2H), 6.62 (d, J=3.8 Hz, 2H),5.49 (d, J=3.8 Hz, 2H), 3.88 (d, J=4.3 Hz, 2H), 2.42 (m, 2H), 1.05 (d, J=6.8Hz, 6H), 0.99 (d, J=6.8Hz, 6H) ppm.13C NMR (CDCl3, 150 MHz): δ= 174.7, 164.1, 153.9, 145.6, 137.2, 130.0, 127.7, 120.8, 79.5, 33.0, 19.6, 17.7 ppm; IR (KBr): 3310 (br), 2963 (s), 2873 (m), 1666 (vs), 1585(s), 1498 (s),1463 (m), 1363 (s), 1321 (w), 1259 (vs), 1087 (vs), 1018(vs), 863 (vs), 796 (vs), 744(m). MS (ESI): m/z found (calcd): [M+H]+433.24 (433.23), [M+Na]+ 455.22 (455.22). Elemental analysis (%) calcd for C24H28N6O2•3H2O: C: 59.24, H: 7.04, N: 17.27; found: C: 59.77, H: 6.52, N: 17.04. 2.4

Preparation

of

(2S,

2’S)-2,

2’-(((1,10-Phenanthroline-2,9-diyl)bis(methanylyliden)) bis(azanylyliden))-bis(4-methyl-pentanamide) (L2) 1,10-Phenanthroline-2,9-dicarbaldehyde (0.12 g, 0.5 mmol, 1.0 eq.) and 4 Å molecular sieves were added into a mixture of l-leucine amide hydrochloride (0.25 g, 1.5 mmol, 3.0 eq.) and NaHCO3 (0.50 g, 6.0mmol, 12.0 eq.) in CH2Cl2 (50 mL). The mixture was heated to reflux at 50°C for 3 days. After filtration, the resulting mixture was extracted with a saturated solution of brine. The solvent was removed under vacuum and the product was isolated (0.14 g, yield: 60%). 1H NMR (CDCl3,600 MHz): δ= 8.82 (s, 2H), 8.50 (d, J=8.4 Hz, 2H), 8.39 (d, J=8.4 Hz, 2H), 7.94 (s, 2H), 6

6.65 (d, J=3.8 Hz, 2H), 5.46 (s, 2H), 4.16-4.14 (m, 2H), 1.89-1.86 (m, 6H), 0.98-0.94 (m, 12H) ppm.13C NMR (CDCl3, 150 MHz): δ= 175.5, 163.8, 154.1, 145.5, 137.1, 130.0, 127.7, 120.6, 72.1, 43.7, 24.2, 23.4, 21.4 ppm. IR (KBr): 3313 (br), 2960 (s), 2871 (m), 2160 (w), 2019 (w), 1947 (w), 1743 (w), 1667 (vs), 1586 (s), 1498 (m), 1463 (m), 1365 (m), 1259 (vs), 1087 (vs), 1017 (vs), 862(s), 795 (vs), 694 (m). MS (ESI): m/z found (calcd): [M+H]+ 461.27 (461.27), [M+Na]+ 483.25 (483.25). Elemental analysis (%) calcd for C26H32N6O2•HCl•2H2O•2CH3OH: C: 56.32, H: 7.60, N: 14.07; found: C: 56.97, H: 7.05, N: 13.12. 2.5

Preparation

of

(2S,

2’S)-2,2’-(((1,10-Phenanthrolin-2,9-diyl)bis(methanylylide-n)) bis(azanylyliden))-bis(3-phenylpropanamide) (L3) To a solution of 1, 10-phenanthroline-2,9-dicarbaldehyde (0.10 g, 0.4 mmol, 1.0 eq) dissolved in dichloromethane containing 4Å molecular sieves (1.5 g), l-phenylalanine amide (0.21 g, 1.3 mmol, 3.3eq) was added. The reaction mixture was stirred and heated at 50°C for 3 days. Subsequently, the reaction mixture was filtered and the solvent was removed under vacuum. The product was washed by water several times and dried (0.14 g, yield: 64%).

1

H NMR (CDCl3,600 MHz): δ=

8.38-8.33 (m, 6H), 7.89 (s, 2H), 7.38-7.03 (m, hidden under CDCl3), 6.66 (s, 2H), 5.70 (s, 2H), 4.22 (m, 2H), 3.44 (m, 2H), 3.15 (m, 2H) ppm.13 C NMR (CDCl3, 150 MHz): δ= 174.46, 164.65, 153.96, 145.67, 137.28, 129.98, 128.50, 127.77, 126.86, 120.80, 75.32, 41.21 ppm. IR (KBr): 3345 (w), 3311 (w), 3173 (w), 3029 (w), 2963 (s), 2162 (w), 2044 (w), 1950 (w), 1667 (vs), 1495 (s), 1450 (m), 1403 (m), 1259 (vs), 7

1084 (vs), 1016 (vs), 863 (s), 794 (vs), 696(vs). MS (ESI): m/z found (calcd): [M+H]+ 529.23 (529.23). Elemental analysis (%) calcd for C32H28H6O2N6•6H2O: C: 60.37, H: 6.33, N: 13.20; found: C: 60.32, H: 6.28, N: 13.19. 2.6 General procedure for the preparation of metal complexes Ligand (0.05 mmol, 1.0 eq.) was dissolved in methanol (25 mL) and europium (III) chlorid hexahydrate (0.06 mmol, 1.1 eq.) was added gradually. The reaction mixture was stirred over night at room temperature. During the process, a negligible amount of precipitate formed and was removed by filtration. Finally the resulting filtrate was evaporated under reduced pressure to obtain a solid powder. 1([EuL1Cl3]): L1 (22mg, 0.05 mmol) and [EuCl3]•6H2O (20mg,0.055 mmol) were used. MS (ESI): [L1+Eu+2Cl-]+ 655.086 (calc. 655.09), [L1+Eu+Cl-H]+ 619.109 (calc. 619.11). UV/Vis (methanol): λmax=360 and 432. IR (KBr): 3335 (br), 2641 (w), 2320 (m), 2227 (m), 2154 (w), 2113 (w), 2063 (w),

1996 (m), 1949 (w), 1626(vs),

1506 (s),1463(s), 1395 (vs), 1290 (s),1203(m), 1109 (vs), 1058 (s), 931(m), 870(vs), 811(m), 779(m). Elemental analysis (%) calcd for C24H28H6O2N6EuCl3•10H2O: C: 33.10, H: 5.55, N: 9.65; found: C: 32.16, H: 4.39, N: 9.02. 2 ([EuL2Cl3]): L2 (23mg, 0.05 mmol) and europium (III) chlorid hexahydrate (20mg, 0.055 mmol) were employed. MS (ESI): [L2+Eu+Cl-H]+ 647.153 (calc. 647.12). UV/Vis (methanol): λmax=321, 347 and 360. IR (KBr): 3319 (br), 2961 (vs), 2310 (w), 2195 (w), 2060 (w), 1997 (w), 1940 (w), 1626 (vs), 1504 (s), 1462 (s), 1429 (s), 1393 (vs), 1308 (m), 1260 (s), 1097 (vs), 925 (w), 871 (s), 800 (s), 756 (m), 696 (w). Elemental analysis (%) calcd for C26H32H6O2N6EuCl3•7H2O: C: 36.93, H: 5.44, N: 8

9.94; found: C: 36.78, H: 4.97, N: 9.69. 3 ([EuL3Cl3]): L3 (26mg, 0.049 mmol) and [EuCl3]·6H2O (20mg,0.055 mmol) were used.MS (ESI): [L3+Eu+2Cl-]+ 751.087 (calc. 751.09). UV/Vis (methanol): λmax=312 and 360.IR (KBr): 3784 (m), 3212 (br), 2503 (m), 2242 (m), 2182 (s), 2023 (w), 1990 (m), 1951(m), 1913 (w), 1855 (w), 1648 (vs), 1499 (s), 1352 (vs), 1261 (w) 1208 (m), 1101(s),

918

(w),

866

(s).

Elemental

analysis

(%)

calcd

for

C32H28H6O2N6EuCl3•15H2O: C: 36.36, H: 5.53, N: 7.95; found: C: 35.31, H: 4.38, N: 7.46. 3. Results and Discussion L1, and the europium complex 1 ([EuL1Cl3]), are prepared as reported earlier. Similar approaches are used for L2, L3 and 2([EuL2Cl3]) and 3([EuL3Cl3]).[20, 21] The characterization of the ligands and their europium (III) complexes were done using standard analytical techniques (Experimental section, SI). The normalized emission spectra of 1, 2 and 3 recorded in HEPES buffer at pH 7.4areshown in Figure S2. In aqueous buffer, the complexes are highly luminescent witha similar excitation pattern. The characteristic strong red europium (III) emission is observed around λ=615 nm in all spectra. This emission is attributable to a J=2 transition (5D0→7F2).[17, 22] Initially, 1 has been evaluated as a chemosensor for various anionic species by fluorescence spectrometry. Therefore, a solution of 1 (50μM, HEPES, pH=7.4) has been treated with the representative anions F-, Cl -, Br -, I -, SO42-, NO3 -, NO2-, HPO42-, CO32-, HCO3 -, and OAc- (50 equiv). As illustrated in Figure 1 (a), 1only responded 9

significantly to HPO42-, resulting in luminescence quenching and indicating binding of HPO42-.The dependence of luminescence quenching on the concentration of phosphate has also been investigated by adding 0.2 to 50 equiv Na2HPO4, as shown in Figure 1(b).

Fig.1 a) Relative luminescence intensities for a solution of 1 at 615 nm (5•10-5M) upon addition of the 50 eq. of F-, Cl -, Br -, I -, SO42-, NO3 -, NO2 -, HPO42-, CO32-, HCO3 -, and OAc- as sodium salts. b) Changes in the luminescence spectra obtained for a solution of complex 1(5×10-5 M) in 10 mM HEPES (pH = 7.4) upon addition of increasing amounts of HPO42-(sodium salt). Excitation wavelength was set at 360 nm. Based on this titration, the corresponding binding constant of 1 with HPO42- of logKβ=4.28±0.01 (Table 1), has been determined from the linear fit (Figure S3 (a)) using the Benesi–Hildebrand equation.[24, 25] 10

Binding Constants

1

2

3

log Kβ (HPO42-)

4.28±0.01

3.30±0.02

3.03±0.03

log Kβ (NO2-)

-

2.62±0.001

2.62±0.002

Table 1.The binding constants of 1, 2 and 3 with HPO42- (sodim salt) in HEPES buffer (10 mM, pH7.4) determined by luminescence spectroscopy Subsequently, 2 and 3 have been tested by the same methods as 1as shown in Figure 2 (a) and 3 (a). Unlike 1, they respond not only to HPO42-but also to NO2-. However, they are found to be more sensitive towardsHPO42-than toNO2-. For 2, as shown in Figure 2 (a), the intensity of luminescence descends after the addition of 50equiv of NO2-, however, the decrease is not remarkable. Furthermore, only 10.7 % of reduction of the signal intensity is observed when 10 equiv of NO2– are added. As for 3, the values of the relative intensity of the luminescence are 54, 73 and 93%, upon addition of 50, 10 or 5 equiv of NO2–, respectively.

11

Fig. 2 a) Luminescence intensity changes of 2 (5•10-5M) in the presence of various test anions (50 eq.) in HEPES-buffered (pH 7.4, 10 mM solution. b) Luminescence titrations of 2 (70 µM) upon addition of the increasing amounts of HPO42- in HEPES (excitation at 360 nm). b) Luminescence titration spectra (λex = 360 nm) of 2(60μM) with the increasing amounts of HPO42- (0−50 eq.) in 10 mM HEPES buffer (pH 7.4) solution. Titrations of 2 and 3 with HPO42-havealso been carried out (Figure 2 (b) and 3 (b)). Corresponding binding constants (log Kβ) of 3.30±0.02 (2) and 3.03±0.03 (3) have been determined to be lower than in case of 1 with the sterically most demanding isopropyl substituent.

12

Fig. 3 a) Luminescence intensity changes of 3 (5•10-5M) in the presence of various anions (50 eq.) in HEPES-buffered(pH 7.4, 10 mM solution. b) Luminescence titrations of 3 (70 µM) upon addition of the increasing amounts of HPO42- in HEPES (excitation at 360 nm). b) Luminescence titration spectra (λex = 360 nm) of 3 (60μM) with increasing amounts ofHPO42- (0−50 eq.) in 10 mM HEPESbuffer (pH 7.4) solution. 4. Conclusions A series of the phenanthroline-based chiral hexadentate ligands (L1, L2 and L3) and their europium ( Ⅲ ) complexes (1-3) were synthesized and characterized. The strong red emissions were observed in all the luminescence spectra of these complexes and their luminescence property was studied for the anion sensing. Except HPO42- and NO2-, the studied anions did not show any significant influence on the luminescence property. For Complex 1, a strong 13

response was detected only for hydrogen phosphate and the intensity of luminescence declined sharply. Complex 2 and 3 were also found sensitive to HPO42-. In the same time, NO2- was also found leading to a certain reduction of the intensity of their luminescence, however, the decreases were not dramatically. The selectivity of the quenching is not well understood but should be due to the specific binding of the respective anions. Furthermore, the corresponding binding constants (logKβ) of these complexes, which were 4.28±0.01, 3.30±0.02 and 3.03±0.03, respectively, were derived from the linear fit by using the Benesi–Hildebrand equation. The limits of detection 1, 2 and 3, were estimated to be 7.6 nM, 7.9 nM and 10.6 nM, respectively.

Acknowledgements Yanfang Hu acknowledges a scholarship by the CSC.

References [1] N. Busschaert, C. Caltagirone, W. Van Rossom, P.A. Gale, Chemical Reviews, 115 (2015) 8038-8155. [2] S. Kubik, Chemical Society Reviews, 39 (2010) 3648-3663. [3] P. Molina, F. Zapata, A. Caballero, Chemical Reviews, 117 (2017) 9907-9972. [4] D. Sharma, S.K. Ashok Kumar, S.K. Sahoo, Tetrahedron Letters, 55 (2014) 927-930. [5] N. Kaur, G. Kaur, U.A. Fegade, A. Singh, S.K. Sahoo, A.S. Kuwar, N. Singh, TrAC Trends in Analytical Chemistry, 95 (2017) 86-109. [6] S.K. Sahoo, G.-D. Kim, H.-J. Choi, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 27 (2016) 30-53. [7] S.L. Tobey, E.V. Anslyn, Journal of the American Chemical Society, 125 (2003) 14807-14815. [8] S.L. Tobey, B.D. Jones, E.V. Anslyn, Journal of the American Chemical Society, 125 (2003) 4026-4027. [9] T. Mizuno, W.-H. Wei, L.R. Eller, J.L. Sessler, Journal of the American Chemical Society, 124 14

(2002) 1134-1135. [10] X. Feng, Y. An, Z. Yao, C. Li, G. Shi, ACS Applied Materials & Interfaces, 4 (2012) 614-618. [11] D. Wu, A.C. Sedgwick, T. Gunnlaugsson, E.U. Akkaya, J. Yoon, T.D. James, Chemical Society Reviews, 46 (2017) 7105-7123. [12] K.A. Jolliffe, Accounts of Chemical Research, 50 (2017) 2254-2263. [13] S. Bhowmik, B.N. Ghosh, V. Marjomäki, K. Rissanen, Journal of the American Chemical Society, 136 (2014) 5543-5546. [14] D. Zhang, G. Gao, L. Guy, V. Robert, J.-P. Dutasta, A. Martinez, Chemical Communications, 51 (2015) 2679-2682. [15] Y. Upadhyay, T. Anand, L.T. Babu, P. Paira, G. Crisponi, A.K. Sk, R. Kumar, S.K. Sahoo, Dalton Transactions, 47 (2018) 742-749. [16] L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini, E. Tondello, Coordination Chemistry Reviews, 254 (2010) 487-505. [17] M.C. Heffern, L.M. Matosziuk, T.J. Meade, Chemical Reviews, 114 (2014) 4496-4539. [18] J.-C.G. Bünzli, Coordination Chemistry Reviews, 293-294 (2015) 19-47. [19] C. Song, Z. Ye, G. Wang, J. Yuan, Y. Guan, Chemistry – A European Journal, 16 (2010) 6464-6472. [20] S. Nadella, P.M. Selvakumar, E. Suresh, P.S. Subramanian, M. Albrecht, M. Giese, R. Fröhlich, Chemistry – A European Journal, 18 (2012) 16784-16792. [21] A. Stubenrauch Jan, C. Mevissen, F. Schulte Marie, S. Bochenek, M. Albrecht, S. Subramanian Palani, in:

Zeitschrift für Naturforschung B, 2016, pp. 1025.

[22] C.J. Chandler, L.W. Deady, J.A. Reiss, Journal of Heterocyclic Chemistry, 18 (1981) 599-601. [23] C. Yang, L.M. Fu, Y. Wang, J.P. Zhang, W.T. Wong, X.C. Ai, Y.F. Qiao, B.S. Zou, L.L. Gui, Angewandte Chemie International Edition, 43 (2004) 5010-5013. [24] H.A. Benesi, J.H. Hildebrand, Journal of the American Chemical Society, 71 (1949) 2703-2707. [25] T. Hayashita, S. Taniguchi, Y. Tanamura, T. Uchida, S. Nishizawa, N. Teramae, Y.S. Jin, J.C. Lee, R.A. Bartsch, Journal of the Chemical Society, Perkin Transactions 2, (2000) 1003-1006.

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Author statement: The work in this manuscript has been designed and supervised by M. Albrecht and P. S. Subramanian. The experimental work has been done by Y. Hu. A concept of the manuscript has been made by Y. Hu and finally the paper was written by M. Albrecht and P. S. Subramanian.

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