Elegant Approach to the Synthesis of a Unique Heteroleptic

Communication pubs.acs.org/Organometallics

Elegant Approach to the Synthesis of a Unique Heteroleptic Cyclometalated Iridium(III)-Polyoxometalate Conjugate Benjamin Matt,† Jamal Moussa,† Lise-Marie Chamoreau,† Carlos Afonso,† Anna Proust,*,†,‡ Hani Amouri,*,† and Guillaume Izzet† †

Institut Parisien de Chimie Moléculaire, IPCM, UMR CNRS 7201, Université Pierre et Marie Curie, UPMC Univ Paris 06, 4 Place Jussieu, Case 42, F-75005 Paris, France ‡ Institut Universitaire de France, 103 Boulevard Saint Michel, F-75005 Paris, France S Supporting Information *

ABSTRACT: A novel heteroleptic cyclometalated iridium(III) complex with one picolinic acid derivative bearing a pendant terminal alkynyl tether has been prepared following a new synthetic route. This pendant alkynyl tether can be further engaged in palladium C−C coupling reactions, allowing its grafting to a Keggin-type polyoxometalate and thus providing a unique iridio-POM conjugate.

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a Keggin-type polyoxometalate (POM) core, leading to an unprecedented Ir(III)−POM hybrid.

ecause of the increasing consumption in energy, direct sunlight conversion into chemical fuels as a green, renewable source of energy is receiving considerable attention. The design of new molecular functional artificial photosynthetic devices is indeed emerging as a major challenge. In these systems the photosensitizer is one of the key components, harvesting the solar radiation and photoinducing an electronic charge transfer. Ru(II)- and Os(II)-polypyridine complexes have been widely used as photosensitizers,1−3 but because of their outstanding photophysical properties, luminescent carbocyclometalated Ir(III) complexes have more recently exhibited a tremendous potential in a wide range of photonic applications, following the pioneering work of M. E. Thompson and S. R. Forrest et al.4−6 For example luminescent Ir(III) complexes with cyclometalated ligands such as 2-phenylpyridyl are used as emissive dyes in OLEDs,7,8 as sensitizers,9,10 as photocatalysts,11−13 and as biological probes.14−17 Among the huge number of reported complexes, neutral compounds of the general formula [(ppy)2Ir(L)] where L is a monoanionic bidentate ligand (Ppy, Acac, Pic, Pyrazol, ...) are of high importance since they show tremendous performances in various fields.18,19 Surprisingly such complexes bearing a functionalized ligand for further covalent grafting/linkage to a polymer, a solid surface, or even another simple molecule are rarely described.20−22 Thus developing synthetic methods for preparing neutral Ir(III) complexes with a pendant organic function is highly advisable. To the best of our knowledge, there is no example of a neutral Ir(III) complex bearing a single pendant terminal alkynyl unit that can be readily used in straightforward Sonogashira cross-coupling, like the one we report herein. As a proof of concept, it was covalently grafted to © 2011 American Chemical Society

Scheme 1. Synthetic Route to the Cyclometalated Ir(III) Complex 4a

a

Conditions: (i) Pd(PPh3)2Cl2, CuI, propylamine, trimethylsilylacetylene (52%); (ii) [Ir(ppy)2Cl]2, AgOTf, Cs2CO3 (94%); (iii) KF (97%).

POMs are a unique family of discrete metal oxide clusters with a huge range of elemental compositions, sizes, and molecular structures.23,24 Some of us have previously reported the covalent grafting of a cyclometalated ruthenium(II) polypyridine complex on disilylated Keggin- and Dawson-type POMs,25 while covalent linkage of ruthenium, organic, or metalloporphyrin chromophores to POMs has also been examplified by us and others.26−28 Indeed, POMs are attractive candidates for the elaboration of photochemical devices owing to their electron reservoir abilities.29 Our first systems exhibited a partial quenching of the luminescence of the ruthenium chromophore, which was attributed to a photoinduced Received: September 29, 2011 Published: December 12, 2011 35

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intramolecular electron transfer from the ruthenium to the POM. Nevertheless, these systems did not offer satisfactory results probably due to the very modest photophysical efficiency of the ruthenium chromophore. However, lifetime measurements of the MLCT excited state for the electrostatic hybrids confirmed the necessity of a covalent link to the POM since the photoinduced electron transfer rate was found to be much faster in covalent hybrids than in systems where POM and ruthenium complexes were associated via electrostatic interactions. We herein describe the synthesis of a novel neutral Ir(III) complex that possesses a pendant terminal alkynyl tether of formula [(ppy)2Ir(pic)] (4). This complex was successfully linked to a new organotin POM hybrid bearing an iodo-aryl moiety. Synthesis of the new Keggin POM-based platform [PW11O39{Sn(C6H4I)}]4‑ (named KSn[I]) was performed following a general procedure,30−33 through the reaction between the monovacant [PW11O39]7− and a slight excess of 1-iodo-4-(trichlorotin)benzene (1.5 equiv), in water. Increasing the pH to 3 by addition of aliquots of a 1 M KOH solution prevented the competitive formation of the complete [PW12O40]3−. After filtration of the solution, addition of tetrabutyl ammonium bromide led to the precipitation of the POM as a tetrabutyl ammonium (TBA) salt (denoted TBAKSn[I]). Its purity was attested by 1H and 31P NMR spectroscopy, elemental analyses, and ESI spectrometry. On the other hand, the related cyclometalated iridium complex 3 with one picolinic acid derivative ligand appended with a trimethylsilyl-protected alkynyl moiety was also prepared. To this end, the functionalized picolinic acid 2 was first prepared from 5-bromopyridine-2-carboxylic acid (1) with trimethylsilylacetylene, via Sonogashira cross-coupling under microwave irradiation, in dried and distilled propylamine. The pure compound could be obtained as a crystalline solid after recrystallization from hot acetonitrile. Then the synthesis of 3 involved preliminary chloride abstraction from the iridium dimer [Ir(ppy)2(μ-Cl)]2 (ppy = 2-phenylpyridiyl) with silver triflate in acetone. Subsequent addition of the filtrate to a suspension of 2 and anhydrous Cs2CO3 for 4 h provides the pure cyclometalated Ir(III) complex 3 nearly quantitatively (94%) after reaction workup. In contrast to the standard harsh reaction conditions followed to prepare neutral hetero/ homoleptic cyclometalated iridium(III) complexes consisting mainly in refluxing the reactants in high boiling point solvents such as DMF or 2-ethoxyethanol, we propose here a very mild synthetic pathway inspired by previous work achieved by some of us,34−37 which allowed us to prepare complex 3, which proved to be a key precursor to the final assembly TBAKSn[Ir]. Spectroscopic data are consistent with the proposed formula, and to confirm its molecular structure, single crystals were grown by slow diffusion of diethyl ether into a dichloromethane/acetone (50/50) mixture solution of 3 and an X-ray diffraction study was carried out. A perspective view of complex 3 is depicted in Figure 1. The structure shows that the iridium(III) center adopts a distorted octahedral environment with two cyclometalated ppy ligands and one picolinic acid derivative 2 acting as a bidentate ligand through one oxygen atom of the carboxylic acid function and the nitrogen of the pyridine moiety. The cyclometalated ppy ligands are disposed in such a way that the two nitrogen atoms are in a trans orientation and the two metalated ppy carbon centers are in a cis position to each other. Bond length distances and angles lie in the range of values of other related

Figure 1. Crystal structure of 3. Solvent molecules and hydrogen atoms were omitted for clarity.

reported X-ray structures.38−40 Interestingly the Ir1−N1 bond length (2.144(2) Å) is longer than Ir1−N2 (2.031(2) Å) and Ir1−N3 (2.039(2) Å) due to the strong trans influence of the metalated carbon atom. Scheme 2. Synthetic Route to the POM−Iridium Hybrida

a

Conditions: (iv) 1-iodo-4-(trichlorotin)benzene (95%); (v) 4, Pd(PPh3)2Cl2, CuI, NEt3 (78%).

To obtain the final cyclometalated Ir(III)-polyoxometalate assembly, quantitative trimethylsilyl group deprotection of complex 3 using KF in a THF/MeOH mixture at room temperature was first achieved to provide the terminal alkynyl compound 4. Then a Sonogashira C−C cross-coupling reaction between the cyclometalated iridium complex 4 and the new TBA-KSn [I] platform was performed by adapting the conditions that we previously developed.41,42 The Sonogashira coupling occurs at 80 °C in one hour under microwave activation, using 7 mol % of [Pd(PPh)3Cl2] and CuI as catalyst sources in DMF containing triethylamine (20 equiv) under an inert atmosphere. An excess of the iridium complex 4 (2 equiv) is necessary to get a total conversion of the polyoxometalate. After addition of a large excess of TBABr and precipitation by addition of diethyl ether, the crude compound is dissolved in dichloromethane and precipitated a second time to remove the remaining traces of noncoupled starting iridium(III) complex 4. The resulting POM−iridium(III) hybrid is isolated as a TBA salt (denoted TBA-KSn[Ir]) in good yield and characterized by 1 H and 31P NMR spectroscopy (Figure 2), elemental analyses, FT-IR spectroscopy, and ESI spectrometry. Combined 1D and 2D 1H NMR experiments (see the SI) and comparison with the 36

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(4) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (5) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (6) Nazeeruddin, M. K.; Klein, C.; Grätzel, M.; Zuppiroli, L.; Berner, D. In Molecular Engineering of Iridium Complexes and their Application in Organic Light Emitting Devices; Yersin, H., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA, 2008; p 363. (7) Sun, Y. R.; Giebink, N. C.; Kanno, H.; Ma, B. W.; Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908. (8) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.; Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790. (9) DeRosa, M. C.; Hodgson, D. J.; Enright, G. D.; Dawson, B.; Evans, C. E. B.; Crutchley, R. J. J. Am. Chem. Soc. 2004, 126, 7619. (10) Gao, R. M.; Ho, D. G.; Hernandez, B.; Selke, M.; Murphy, D.; Djurovich, P. I.; Thompson, M. E. J. Am. Chem. Soc. 2002, 124, 14828. (11) Metz, S.; Bernhard, S. Chem. Commun. 2010, 46, 7551. (12) Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131, 10875. (13) Fihri, A.; Artero, V.; Pereira, A.; Fontecave, M. Dalton Trans. 2008, 5567. (14) Lo, K. K. W.; Louie, M. W.; Zhang, K. Y. Coord. Chem. Rev. 2010, 254, 2603. (15) Lo, K. K. W.; Chung, C. K.; Zhu, N. Y. Chem.Eur. J. 2006, 12, 1500. (16) Caspar, R.; Cordier, C.; Waern, J. B.; Guyard-Duhayon, C.; Gruselle, M.; Le Floch, P.; Amouri, H. Inorg. Chem. 2006, 45, 4071. (17) Caspar, R.; Musatkina, L.; Tatosyan, A.; Amouri, H.; Gruselle, M.; Guyard-Duhayon, C.; Duval, R.; Cordier, C. Inorg. Chem. 2004, 43, 7986. (18) Tao, S. L.; Lai, S. L.; Chan, M. Y.; Lo, M. F.; Ng, T. W.; Lee, S. T.; Zhao, W. M.; Lee, C. S. J. Mater. Chem. 2011, 21, 4983. (19) You, Y. M.; Park, S. Y. J. Am. Chem. Soc. 2005, 127, 12438. (20) Kappaun, S.; Eder, S.; Sax, S.; Saf, R.; Mereiter, K.; List, E. J. W.; Slugovc, C. J. Mater. Chem. 2006, 16, 4389. (21) Whittle, V. L.; Williams, J. A. G. Inorg. Chem. 2008, 47, 6596. (22) Qin, T.; Ding, J.; Wang, L.; Baumgarten, M.; Zhou, G.; Mullen, K. J. Am. Chem. Soc. 2009, 131, 14329. (23) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (24) Pope, M. T. In Comprehensive Coordination Chemistry II, From Biology to Nanotechnology; McCleverty, J., Meyer, T. J., Eds.; Pergamon Press: Oxford, 2004; Vol. 4, p 635. (25) Matt, B.; Coudret, C.; Viala, C.; Jouvenot, D.; Loiseau, F.; Izzet, G.; Proust, A. Inorg. Chem. 2011, 50, 7761. (26) Elliott, K. J.; Harriman, A.; Le Pleux, L.; Pellegrin, Y.; Blart, E.; Mayer, C. R.; Odobel, F. Phys. Chem. Chem. Phys. 2009, 11, 8767. (27) Odobel, F.; Severac, M.; Pellegrin, Y.; Blart, E.; Fosse, C.; Cannizzo, C.; Mayer, C. R.; Eliott, K. J.; Harriman, A. Chem.Eur. J. 2009, 15, 3130. (28) Santoni, M. P.; Pal, A. K.; Hanan, G. S.; Proust, A.; Hasenknopf, B. Inorg. Chem. 2011, 50, 6737. (29) Sadakane, M.; Steckhan, E. Chem. Rev. 1998, 98, 219. (30) Knoth, W. H.; Domaille, P. J.; Farlee, R. D. Organometallics 1985, 4, 62. (31) Xin, F. B.; Pope, M. T. Organometallics 1994, 13, 4881. (32) Hussain, F.; Kortz, U.; Clark, R. J. Inorg. Chem. 2004, 43, 3237. (33) Bareyt, S.; Piligkos, S.; Hasenknopf, B.; Gouzerh, P.; Lacote, E.; Thorimbert, S.; Malacria, M. J. Am. Chem. Soc. 2005, 127, 6788. (34) Moussa, J.; Amouri, H. Angew. Chem., Int. Ed. Engl. 2008, 47, 1372. (35) Moussa, J.; Rager, M. N.; Chamoreau, L. M.; Ricard, L.; Amouri, H. Organometallics 2009, 28, 397. (36) Damas, A.; Moussa, J.; Rager, M. N.; Amouri, H. Chirality 2010, 22, 889. (37) Damas, A.; Ventura, B.; Axet, M. R.; Esposti, A. D.; Chamoreau, L. M.; Barbieri, A.; Amouri, H. Inorg. Chem. 2010, 49, 10762.

Figure 2. Enlargement of the 5−9 ppm region of the 1H NMR spectra of 4 (300 MHz, CD2Cl2) and TBA-KSn[Ir] (300 MHz, CD3CN). 1

H spectrum of the precursor allowed us to assign the 1H resonances of 4 and TBA-KSn[Ir] (Figure 2). We have described the synthesis under mild conditions of a novel neutral cyclometalated iridium(III) complex with an appended alkynyl tether that undergoes a Sonogashira crosscoupling reaction with a new Keggin-type polyoxometalate platform, leading to a unique Ir(III)−POM conjugate. Future work will involve the extension of these synthetic procedures to prepare a wider range of POM−Ir(III) hybrids, which would pave the way to functional photosynthetic devices using sunlight as a source of clean, renewable energy.

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ASSOCIATED CONTENT * Supporting Information General methods, details of the X-ray crystal structure determination, and crystallographic data in CIF format of 2 and 3, description of the synthesis of 1-iodo-4-(trichlorotin)benzene, 2, 3, 4, TBA-KSn[I], and TBA-KSn[Ir] together with their 1H, 13C (3, 4), and 31P (TBA-KSn[I], TBA-KSn[Ir]) NMR spectra, 1H NMR COSY spectrum of TBA-KSn[Ir], and ESI mass spectra of TBA-KSn[I] and TBA-KSn[Ir]. This material is available free of charge via the Internet at http://pubs.acs.org. S

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (A.P.); [email protected] (H.A.). Fax: (33)1-44-27-38-41.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the CNRS and the Ministère de la Recherche et de l’Enseignement Supérieur (B.M.).

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REFERENCES

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