Journal of Organometallic Chemistry 776 (2015) 51e59
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Facile synthesis and complete characterization of homoleptic and heteroleptic cyclometalated Iridium(III) complexes for photocatalysis Anuradha Singh, Kip Teegardin, Megan Kelly, Kariate S. Prasad, Sadagopan Krishnan, Jimmie D. Weaver* Department of Chemistry, Oklahoma State University, 107 Physical Science, Stillwater, OK 74078, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2014 Received in revised form 23 October 2014 Accepted 25 October 2014 Available online 12 November 2014
Herein we describe an improved synthesis for homoleptic iridium(III) 2-phenylpyridine based photocatalysts that allows rapid access to these compounds in good to high yields which have recently become a vital component within the field of catalysis. In addition, we synthesized a number of heteroleptic iridium(III) 2-phenylpyridine photocatalysts and report their photophysical and electrochemical properties. The emission energies span the range of 473e560 nm and reduction potentials from 2.27 V to 1.23 V and oxidation potentials ranging from 1.81 V to 0.69 V. Additionally, we provide the calculated excited state properties and comment on the role of these properties in designing catalytic cycles. © 2014 Elsevier B.V. All rights reserved.
Keywords: Iridium complexes Photocatalyst Synthesis
Introduction Cyclometalated iridium(III) complexes constitute an exceptional class of organometallic complexes that possess remarkable photophysical and photochemical properties [1,2]. Consequently, these complexes have been utilized in a number of applications in diverse fields. They have been especially important in the fields of organic light emitting diodes (OLEDs) [3e8], dye sensitized solar cells [9,10], sensing [11e14] and biology [15e17]. More recently, these complexes have been utilized as photocatalysts in the area of synthetic organic chemistry to perform unique chemical transformations that take place by catalytic removal (or addition) of an electron as well as by serving as photosensitizers [18e23]. Exploration of these complexes as photocatalysts is still in its infancy and will likely continue to expand and give rise to powerful chemical methodologies. Consequently, substantial quantities of complexes with a range of properties will be needed. Despite a rich history of exploration in the properties of these complexes, syntheses for many the facial homoleptic variants [1] are scattered, and often lack complete chemical, photophysical, and electrochemical characterization. Within our own research, we
* Corresponding author. Postal Mail: Oklahoma State University, 107 Physical Science, Stillwater, OK 74078, USA. Tel.: þ1 4057443966. E-mail address:
[email protected] (J.D. Weaver). http://dx.doi.org/10.1016/j.jorganchem.2014.10.037 0022-328X/© 2014 Elsevier B.V. All rights reserved.
have also utilized this type of an iridium photocatalyst. Recently, we have disclosed several photocatalytic reactions which utilize commercially available fac-tris-(2-phenylpyridine) iridium (Ir(ppy)3) in which the substrates are activated directly (or indirectly) via electron transfer or energy transfer from Ir(ppy)3 [24e27]. However, we often found our research was restricted by the photophysical and electrochemical properties of the limited number of commercially available photocatalysts. Herein, we report a simple synthetic method to access these complexes readily in sufficient quantities for use within the laboratory. Furthermore, we also report and discuss the photophysical and electrochemical properties of the synthesized complexes within the context of photoredoxcatalysis. The iridium complexes are tris-cyclometalated d-6, 18-electron complexes that are remarkably stable in the ground state. However, upon absorption of photons of the appropriate energy-in the blue region of the visible spectrum, the complexes undergo excitation (Scheme 1). Initially, an excited singlet state is produced but it rapidly relaxes to its long-lived, triplet state [28,29]. The triplet state has undergone a metal-to-ligand charge transfer and by virtue of charge transfer, it can serve as both a potent oxidant and reductant. By knowing the excited state redox potentials of E1/2(Irþ/ Ir*) and E1/2(Ir*/Ir) complexes, one can begin to rationally design novel chemical transformations provided the relevant potentials of substrates are known. With this goal in mind, we desired to develop a simple and robust method that would allow us to modulate the
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A. Singh et al. / Journal of Organometallic Chemistry 776 (2015) 51e59 Table 1 Optimization of reaction conditions.
1
*IrL3 represents the higher energy spin allowed excited state and 3*IrL3 represents the lowest spin forbidden excited state
Scheme 1. Energy and electron transfer processes in triscyclometalated iridium complexes.
redox potentials and triplet state energies of the iridium photocatalysts, allowing us to more fully explore possible synthetic transformations. A survey of the literature revealed that there are a number of reported synthetic methods that use more expensive Ir(acac)3 [30e33] or alternatively require two steps-forming the chlorobridged dimer and then subsequently adding the third ligand. This is done even in the case of the tris-homoleptic-cyclometalated complexes, which is less than ideal since it requires additional chemical steps. Furthermore, a stoichiometric chloride scavenger like AgOTf is often employed [34,35]. Konno reported microwave synthesis of tris-cyclometalated iridium complexes, but this required a large excess of ligand (50e100 equiv.) which limits the scope of the reaction to readily available ligands such as 2phenylpyridine [36]. Therefore, we set about to develop a general and simple synthesis that would allow us to acquire the facial homoleptic iridium complexes in high chemical yield via a simple and selective one step process.
Results and discussion In our initial attempt, a glass pressure vial was charged with IrCl3$nH2O, 2-phenylpyridine, Na2CO3 and water and heated at 200 C for 48 h [37]. Unfortunately, this reaction resulted in extremely low yield (99% yield (43 mg, 0.044 mmol) as orange solid. 1H NMR (400 MHz, Acetone-d6) d 8.35 (d, 2H, J ¼ 2.6 Hz), 8.32 (d, 2H, J ¼ 8.1 Hz), 7.99e7.92 (m, 4H), 7.91e7.87 (m, 2H), 7.83 (d, 2H, J ¼ 6.4 Hz), 7.24 (dd, 2H, J ¼ 6.4, 2.6 Hz), 7.17 (ddd, 2H, J ¼ 7.3, 5.8, 1.4 Hz), 7.02 (dd, 2H, J ¼ 8.0, 2.1 Hz), 6.32 (d, 2H, J ¼ 8.0 Hz), 4.09 (s, 6H), 1.32 (s, 18H). 13 C NMR (101 MHz, Acetone-d6) d 168.5, 168.1, 157.7, 151.5, 149.3, 147.3, 144.9, 143.9, 138.4, 131.6, 128.0, 123.4, 121.9, 119.9, 114.1, 111.6, 56.6, 34.2, 31.0. 31P NMR (162 MHz, Acetone-d6) d 130.19 to 157.39 (hept, J ¼ 707.94 Hz). Anal. calcd for C42H44F6IrN4O2P: C, 51.79; H, 4.55; N, 5.75. Found: C, 51.61; H, 4.38; N, 5.94. LC/MS (m/z) calculated for C42H44IrN4O2 829.31 found Mþ, 828.70. Financial interests The authors declare no competing financial interests. Acknowledgment Halliburton is gratefully acknowledged for its gift of a Parr Reactor. Oklahoma State University is gratefully acknowledged for startup funds. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2014.10.037. References [1] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura, F. Barigelletti, in: V. Balzani, S. Campagna (Eds.), Photochemistry and Photophysics of Coordination Compounds II, Springer, Berlin Heidelberg, 2007, pp. 143e203. [2] Y. You, W. Nam, Chem. Soc. Rev. 41 (2012) 7061e7084. [3] M.A. Baldo, D.F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151e154. [4] S. Jung, Y. Kang, H.-S. Kim, Y.-H. Kim, C.-L. Lee, J.-J. Kim, S.-K. Lee, S.-K. Kwon, Eur. J. Inorg. Chem. 2004 (2004) 3415e3423. [5] G.M. Farinola, R. Ragni, Chem. Soc. Rev. 40 (2011) 3467e3482.
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