Synthesis and characterization of N-2-aryl-1,2,3-triazole based iridium complexes as photocatalysts with tunable photoredox potential

Abstract

N-2-Aryl chelated 1,2,3-triazole-Ir(III) complexes with various substituents were prepared for the first time. These photoactive Ir(III) complexes were characterized by X-ray crystallography and their redox potentials were evaluated. This study revealed a new class of photocatalysts with tunable photoredox potentials.

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The cationic Ir(III) polyimine complexes have been widely applied in various areas,¹ including live cell imaging,² electroluminescent materials,³ water oxidation etc.⁴ Recently, [Ir(ppy)₂(bpy)]⁺ and [Ir(ppy)₂(dtbbpy)]⁺ have gained tremendous attention as efficient photocatalysts in promoting organic transformations.^5,6 The fast growth of photocatalytic research led to strong needs for new systems with structural novelty and potentially new reactivity. Herein, we report the application of N-2-aryl-1,2,3-triazoles as ligands for the synthesis of novel N-aryl chelated Ir(III) photocatalysts with tunable redox potentials.

After the discovery of “click chemistry”, 1,2,3-triazole (TA) became one of the most important heterocycles in chemical research.⁷ However, the studies regarding the binding ability of N-2-substituted triazole toward a metal cation are relatively rare. During the past several years, our group has been working on developing new synthetic methods toward triazole functionalization⁸ while investigating the coordination ability of various triazole derivatives.⁹ Inspired by recent success on Ir(III) promoted photocatalysis, we become interested in studying the TA binding ability with Ir(III) cations and hope to develop a new class of photoactive complexes with the potential to further extend the reaction scope of photocatalysis.

In 2009, Schubert and co-workers reported the first triazole coordinated Ir(III) complexes using N1-substituted 1,2,3-triazole as the C^N ligand.¹⁰ Later, De Cola and co-workers reported the electroluminescence properties of similar Ir(III) complexes (Scheme 1A).¹¹ Although these and other pioneering studies¹² revealed a good coordination ability of TA ligands with Ir(III), no photocatalytic reactivity has been reported. More importantly, as mentioned by Schubert, only C-aryl-TA chelated Ir(III) complexes could be produced. The attempts to form N-aryl-TA chelating complexes were unsuccessful due to either the poor complex stability or the formation of unidentified byproducts.

Scheme 1 New Ir(III) complexes with 1,2,3-triazole ligands.

Among various triazole derivatives, one particularly interesting compound is the fluorescence active N-2-aryl triazoles (NATs).^7b As revealed by the crystal structures, the N-2-aryl group holds a perfect co-planar conformation with the triazole ring, resulting in a strong fluorescence emission, whereas the N-1-isomer exhibits no emission.¹³ Moreover, substitution on the N-2-aryl group exhibited a strong influence on the fluorescence emission wavelength and intensity (Scheme 1B). Based on these results, we postulated that the N-2-aryl-triazole may be used as a ligand to prepare novel N-aryl-chelated Ir(III) complexes by avoiding the potential triazole decomposition (via loss of N₂) and providing specific coordination sites.¹⁴ To test our hypothesis, N-2-aryl-triazoles were prepared and used for Ir(III) complex formation. The general synthetic route is summarized in Fig. 1.¹⁵

Fig. 1 General synthetic route and X-ray crystal structures of N-aryl-chelated TA-Ir complexes: (a) 2-ethoxyethanol, 140 °C, N₂, 24 h; (b) 2,2′-bispyridine (bpy, 2.5 eq.), 1,2-ethanediol, 120 °C, N₂, 20 h, followed by anion exchange with NH₄PF₆; (c) 2-picolinamide (2.6 eq.), Na₂CO₃ (11 eq.), 2-ethoxyethanol, 140 °C, N₂, 20 h.

As expected, treating triazole 1 with IrCl₃·xH₂O gave the corresponding chloro-bridged iridium(III) dimer 2 in excellent yields (generally >80%). Subsequent reactions with either a neutral N^N ligand (condition b) or an anionic N^N ligand (condition c) gave the corresponding cationic TA-Ir (C-1) or neutral TA-Ir (N-1) complexes. Both complexes were characterized by X-ray crystallography. Notably, unlike N-1-aryl triazoles, the N-2-aryl triazoles showed a strong coordination ability, forming stable Ir(III) complexes, which could be easily purified by column chromatography without decomposition. With this synthetic method, various TA-Ir complexes were prepared. Good to excellent yields were obtained in most cases, which warranted further applications of N-2-aryl triazoles as the C^N ligand for iridium photocatalysts.

As shown in Scheme 2A, the general mechanism of Ir-photocatalysis is initiated from the promotion of Ir(III) to the excited state Ir(III)* by visible light, followed by oxidative or reductive quenching through single-electron transfer. The resulting Ir(IV) or Ir(II) will be reduced or oxidized to regenerate the ground state Ir(III). The redox potential for each step is a crucial factor in catalyst design, simply because oxidation/reduction reactions require good matching of the redox potentials between catalysts and substrates. Compared to 2-phenylpyridine (ppy) ligands, N-2-aryl triazoles are more electron-deficient. Thus, higher oxidation potentials of the corresponding iridium complexes are expected.^5c To evaluate the photophysical properties of these new TA-Ir complexes, UV-Vis absorption and fluorescence emission were examined.

Scheme 2 (A) The general mechanism for Ir(III) photocatalysis; (B) TA-Ir complexes prepared from N-2-aryl triazoles.

Similar to other literature reported systems,^15a the neutral complex N-1a (with the anionic N^N ligand) gave a very weak fluorescence emission and almost no photocatalytic reactivity. On the other hand, compared with [(ppy)₂Ir(bpy)]PF₆, the TA-Ir cationic complexes [(tapy)₂Ir(bpy)]PF₆ showed better absorption of the blue light and a stronger fluorescence emission in the visible light region (Fig. 2, see detailed spectral comparison in ESI†).

Fig. 2 UV-Vis (left) and fluorescence (right) spectral comparison.

The fluorescence emission and redox potentials of some representative TA-Ir complexes are summarized in Table 1.

Table 1 Redox potentials of selected iridium complexes^a,b

	R^1	R^2	λ em (nm)	Φ	τ (ns)	E 1/2 (V)
						(Ir^4+/3+)	(Ir^4+/3+*)	(Ir^3+/2+)	(Ir^3+*^/2+)
a S1 = [(ppy)2Ir(bpy)]PF6. All potentials are given versus the saturated calomel electrode (SCE). Measurements were performed at room temperature in acetonitrile using an internal standard Fc/Fc^+ redox couple (0.40 V vs. SCE). b PF6^− is used as the counter anion. c The quantum yields were calculated relative to Ru(bpy)3(PF6)2 (Φ = 0.062 in ACN). d Excited-state lifetime.
S1	—	—	566			1.28	−0.91	−1.38	0.81
C-1a	H	H	530	0.32	266	1.54	−0.80	−1.34	1.00
C-1b	F	H	503	0.39	358	1.67	−0.80	−1.33	1.14
C-1c	OMe	H	550	0.02	30	1.37	−0.88	−1.35	0.90
C-1d	F	F	501	0.29	270	1.66	−0.82	−1.33	1.15
C-1e	OMe	F	554	0.02	32	1.37	−0.87	−1.35	0.89
C-1f	F	OMe	508	0.21	172	1.61	−0.83	−1.32	1.12
C-1g	OMe	OMe	558	0.01	18	1.25	−0.90	−1.37	0.85

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The data in Table 1 revealed a clear substituent effect on the Ir^4+/3+ redox potential. First, with more electron-deficient phenyl-triazole (pta) ligand, C-1a gave a higher Ir^4+/3+ redox potential (1.54 V) than [Ir(ppy)₂(bpy)]PF₆ (S1, 1.28 V). The introduction of electron withdrawing groups (such as F, C-1b) at the R¹ position (N-2 aryl) further increased the E_1/2 (Ir^4+/3+) to 1.67 V. In contrast, complex C-1c (with the electron donating OMe) gave a lower Ir^4+/3+ redox potential (1.37 V), which was still higher than ppy complex S1. A similar trend was also observed in the Ir³⁺*^/2+ redox potential, though with a smaller variation. However, substitution at the R² position indicated little influence on the redox potential (e.g.C-1b, 1.67 V vs.C-1d, 1.66 V), suggesting that the electronic effect influence from the ring that is directly touching the metal center is much more important. Similarly, introducing EWG at the R¹ position (C-1b) helped to increase the excited-state lifetime and quantum yield while introducing EDG (C-1c) impaired the excited-state lifetime and quantum yield.

The reduction potential E_1/2 (Ir^3+/2+) of all tested C-1 TA-Ir complexes were almost the same even with different C^N ligands. This is likely due to the fact that Ir(III) reduction is more related to the metal–ligand charge transfer (MLCT) through the π* orbital of the N^N ligand (bpy).¹ To fully elucidate the ligand effect on the redox potential, we prepared triazole-pyridine (tapy)¹⁶ as a new type of the N^N ligand to coordinate with Ir cations. The reaction between the chloro-bridged iridium dimer 2a and N-2 tapy gave messy mixtures with no desired complex isolated. Interestingly, a much cleaner reaction was obtained with N-1-tapy. Although growing a single crystal is unsuccessful at this moment, the TA-Ir complexes with N-1-tapy as the N^N ligand have been successfully prepared and characterized by ¹H, ¹³C, ¹⁹F NMR and HRMS. The fluorescence emission and redox potentials of these complexes were then determined as shown in Table 2.

Table 2 Redox potentials of iridium complexes formed from the tapy ligand^a,b

	R^1	λ em (nm)	Φ ^c	τ ^d (ns)	E 1/2 (V)
					(Ir^4+/3+)	(Ir^4+/3+*)	(Ir^3+/2+)	(Ir^3+*^/2+)
a S1 = [(ppy)2Ir(bpy)]PF6; S2 = [(ppy)2Ir(dtbbpy)]PF6. b ^,c,dSame conditions as in Table 1 applied.
S1	—	566			1.28	−0.91	−1.38	0.81
S2	—	560	0.20	175^d	1.25	−0.96	−1.48	0.73
C-2	—	546	0.12	110	1.37	−0.90	−1.48	0.79
C-3a	H	462, 486	0.06	217	1.61	−0.94	−1.45	1.10
C-3b	F	458, 482	0.07	375	1.75	−0.82	−1.41	1.16
C-3c	OMe	508	0.01	108	1.37	−1.07	−1.43	1.01

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Interestingly, compared to complex S1 (with the ppy N^N ligand), complex C-2 (with the tapy N^N ligand) resulted in a clear increase of the Ir^3+/2+ reduction potential, from 1.38 V to 1.48 V, similar to complex S2 (with the dtbbpy N^N ligand). Notably, the Ir^3+/2+ reduction potential remained almost the same when changing the C^N ligand to pta. These results highlighted the excellent viability of these new triazole-based Ir photocatalysts: using pta as the C^N ligand to adjust the Ir^4+/3+ oxidation potential while using tapy as the N^N ligand to tune the Ir^3+/2+ reduction potential.

Two typical photocatalytic reactions^6c,17 were performed to prove the feasibility of these new TA-Ir complexes and good catalytic reactivities were observed as shown in Fig. 3.

Fig. 3 TA-Ir as effective new photocatalysts.

In conclusion, we herein report the synthesis and characterization of N-2-aryl-1,2,3-triazole-Ir(III) complexes (TA-Ir). Various complexes have been prepared and their photophysical properties were evaluated. Tunable redox potentials were achieved by varying substituents on either the pta C^N ligand or the tapy N^N ligand, which indicated a promising future for these new photocatalysts. Investigation on challenging photocatalytic reactions using this new class of photocatalysts is currently ongoing in our lab.

We thank the NSF (CHE-1362057) and NSFC (21228204) for the financial supports.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR, UV, fluorescence spectra, cyclic voltammetry and X-ray data with CCDC numbers. CCDC 1012360 and 1010939. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qo00281d

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