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Synthesis and Characterization of Strong Cyclometalated Iridium Photoreductants for Application in Photocatalytic Aryl Bromide Hydrodebromination Jong-Hwa Shon, Steven Sittel, and Thomas S Teets ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02759 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019
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Synthesis and Characterization of Strong Cyclometalated Iridium Photoreductants for Application in Photocatalytic Aryl Bromide Hydrodebromination Jong-Hwa Shon, Steven Sittel, and Thomas S. Teets* University of Houston, Department of Chemistry, 3585 Cullen Blvd., Room 112, Houston, TX 77204-5003, USA. KEYWORDS. Photoredox catalysis; iridium; photochemistry; hydrodehalogenation; electron transfer; quenching ABSTRACT: A series of potent bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands is described. Structure-property analysis reveals that substituent modification of the NacNac ligands has a large effect on the ground-state IrIV/IrIII potential, which shifts cathodically as the NacNac is made more electron rich. In addition, the excited-state IrIV/*IrIII potentials are ca. 300–500 mV more negative than that of fac-Ir(ppy)3 (ppy = 2-phenylpyridine), indicating that these compounds have much more reducing excited states. Rate constants for excited-state electron transfer between these photosensitizers and benzophenone are ~2–3 times faster than facIr(ppy)3, demonstrating these complexes are both kinetically and thermodynamically more potent for excited-state electron transfer. We use these photosensitizers to optimize a simple reaction procedure for photocatalytic debromination of aryl bromide substrates, which requires only the photosensitizer, blue light, and an amine base, without silanes or other additives that are used in previously reported methods.
Introduction Visible-light photosensitizers, which can be organic molecules or metal-based coordination compounds, absorb light energy and convert it to a chemical potential via charge separation. The excitedstate electron-hole pair render the excited state both more oxidizing and more reducing than the ground state, enabling rich electrontransfer chemistry. Excited-state charge transport is important in renewable energy1–3 and catalysis applications,4 which span from small-molecule activation reactions and organic synthesis applications4–9 to the synthesis or degradation of large polymers.10,11 One of the most attractive features of photocatalysis in synthetic applications, often referred to as “photoredox catalysis,” is that generation of the reactive excited-state requires only input of light energy under otherwise ambient reaction conditions. As such, highly-reactive intermediates can be generated in a controlled fashion, and problems associated with incompatibility of redox reagents and temperature-induced decomposition can be circumvented. The “ambidextrous” nature of photosensitizers, i.e. the phenomenon that they are both stronger oxidants and reductants in their excited state, is best exemplified in [Ru(bpy)3]2+ (bpy = 2,2ʹbipyridine), the most well-studied metal-based photosensitizer. The excited-state redox potentials for [Ru(bpy)3]2+ are E(RuIII/*RuII) = −1.2 V vs ferrocenium/ferrocene (Fc+/Fc) (−0.8 V vs. SCE) and E(*RuII/RuI) = 0.4 V vs Fc+/Fc (0 V vs. SCE), allowing electrontransfer reactions to/from the redox partner. These pathways are respectively known as oxidative or reductive quenching, and both can be operative in catalytic transformations. Although there are many examples of photoredox reactions using [Ru(bpy)3]2+ as the catalyst, there are some limitations of this widely used sensitizer. Its modest reactivity, especially for reductive transformations, limits the types of transformations and the number of viable substrates, and
the poor photostability is detrimental to catalyst performance. More recently, researchers have recognized that cyclometalated iridium complexes are a class of photocatalysts more reactive and more stable than [Ru(bpy)3]2+ derivatives. Bernhard and coworkers have prepared an iridium complex, [Ir(dF(CF3)ppy)2(dtbbpy)]+ (df(CF3)ppy = 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine; dtbbpy = 4,4ʹ-di-tert-butyl-2,2ʹ-bipyridine), which has a similarly reducing and significantly more oxidizing excited state than [Ru(bpy)3]2+.12 This complex is a versatile photoredox catalyst currently used by many other groups for a variety of transformations.4 Neutral cyclometalated iridium complexes, in particular the archetypal compound fac-Ir(ppy)3 (ppy = 2phenylpyridine), have excited states that are more reducing than [Ru(bpy)3]2+ by almost 1 V.13 As a result, these compounds have been especially prominent for reductive transformations, allowing a larger substrate scope for reactions involving C–X (X = halogen, oxygen) cleavage. This principle is best exemplified by work from the Stephenson group, showing effective hydrodeiodination of a range of substrates using a combination of the photoreductant facIr(ppy)3 and sacrificial electron/hydrogen donors.14 In spite of these successes, it remains a challenge to perform visible-light photoredox catalysis on unactivated organobromide or organochloride substrates, which are at least 200 mV more difficult to reduce than their iodide counterparts.15 There have been some recent successes in engaging organobromide substrates in photoredox catalysis, although these do require an excess of silanol or silane additive.16,17 As expansive as the field of photoredox catalysis has become, there is comparatively little effort in catalyst development. Our group recognized that possibility that new types of photoredox catalysts, with even more reducing excited states than fac-Ir(ppy)3, could facilitate development of photoredox reactions on a wider range of substrates and with simpler reaction conditions. Along these lines,
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our group has recently reported three different complexes of the type Ir(ppy)2(NacNacR), where NacNacR is a N, Nʹ-diphenyl-βdiketiminate ligand with the 2- and 4-positions of the NacNac backbone varied (R = Me, NMe2, OEt).18 We showed in this work that these complexes, by virtue of the electron-rich NacNac ligands which destabilize the HOMO and shift the ground-state IrIV/IrIII potentials to more negative values, are more potent as photoreductants by ~300–400 mV when compared to fac-Ir(ppy)3. This work also showed us that the redox properties are strongly dependent on the NacNac substitution, motivating us to pursue substituent modifications at other positions of the NacNac framework to better understand structure-property relationships in this series of compounds and pursue improved next-generation photoreductants. In this study, we present a set of 10 wellcharacterized bis-cyclometalated iridium photoreductants with NacNac ancillary ligands, adding to the three already introduced by our group. Modifications to the N and Nʹ substituents of the NacNac, as well as methylation of the central 3-position of the NacNac backbone, are shown to significantly influence redox and photophysical properties. In addition, modifications of the cyclometalating (C^N) ligands on iridium are also explored as avenues to further optimize excited-state redox properties. The kinetics of excited-state electron transfer with the electron-acceptor benzophenone (BP) are determined from Stern−Volmer quenching experiments. The values of quenching rate are dependent on the driving force of the electron-transfer reaction, as predicted by Marcus theory. Finally, we use these strong photoreductants as catalysts for visible-light-promoted catalytic hydrodebromination reactions of aryl bromide substrates. This improved method operates with simple amine bases as the sole sacrificial reagent and does not require silane mediators. Supported by Stern-Volmer quenching studies with these substrates, the catalytic reactivity is consistent with an oxidative quenching mechanism for hydrodebromination, and illustrates the concept that these more reactive photocatalysts can enable transformations on a wide substrate scope with relatively simple conditions.
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dimers [Ir(C^N)2(μ-Cl)]2 (C^N= ppy, tbppy, and ppz). The reaction of alkali metal NacNac salts with the cyclometalated iridium dimer is complete in a couple of hours at room temperature, and the products are easily separated by precipitation using diethyl ether or n-hexane, without any column chromatography required.
Results Synthesis and structures of the iridium complexes. The chemical structures of all nine β-diketiminate (NacNac) ligand precursors and all 13 bis-cyclometalated iridium complexes prepared from these NacNac ligand precursors are shown in Chart 1. The three compounds 1–3 were reported in our previous study18 and the other ten compounds (4–13) are newly prepared. We sought several modifications to the structures of 1–3 as a means of further influencing and optimizing the key photosensitizer attributes. Complexes 4–9 incorporate one or more modifications onto the NacNac ligand, whereas 10–13 involve modification of the cyclometalating ligand. Several of the complexes include electrondonating substituents (Me, OMe, or NMe2) on the NacNac Nphenyl rings (4–7), with complex 7 also including a methyl group on the central backbone position. In 8 and 9 we installed the nonaromatic N and N′ substituents OMe (8) and cyclohexyl (9). Finally, in 10–13 we used cyclometalating ligands besides ppy, specifically 2-(4-tert-butyl-phenyl)-pyridine (tbppy, 10 and 11) and 1-phenylpyrazole (ppz, 12 and 13). To synthesize the iridium complexes, we used potassium or lithium salts of each modified NacNac ancillary ligand combined with chloride-bridged iridium
Chart 1 Structures of five of the compounds were verified by X-ray crystallography and are shown in Figure 1. Structures of complexes 5, 9, 10 and 12 are akin to the previously reported structures of 3,18 and several other cyclometalated iridium NacNac compounds from our group,19–21 which have approximate C2 symmetry with a planar NacNac core. On the other hand, the crystal structure of 11 is very similar to the structure of 2,18 with a nonplanar “puckered”
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conformation of the L2 ligand that results in loss of C2 symmetry. This phenomenon is only observed in the solid-state, with solution NMR spectra consistent with approximate C2 symmetry. We do not believe that this puckering behavior arises from the steric bulk of the dimethylamino group on the NacNac backbone, since the structures of iridium complexes with trifluoro-methyl-substituted NacNac ligands remain C2 symmetric.19
(CH3) onto the ligand backbone results in a ca. 140–190 mV cathodic shift in redox potential, as judged by comparing 2, 11, and 13 to 1, 10, and 12, respectively. We previously noted that complex 3, with the substitution of ethoxy groups onto the NacNac backbone, results in a ca. 100 mV of anodic shift of oxidation potential comparing to 1, the opposite effect of dimethylamino substitution. By comparing 6 and 7, which differ with respect to the substitution at the central 3-position of the NacNac backbone, we see a substantial cathodic shift of E(IrIV/IrIII) (ca. 230 mV) in 7, where the backbone is methylated. Other modifications involved alteration of the N-phenyl substituents on the NacNac, which likewise have significant effects on redox potentials. The effects of phenyl substituents generally track with the Hammet substituent constant σ, with more electron-donating substituents resulting in more cathodically shifted IrIV/IrIII potentials. Comparing compound 1 (−0.07 V; +0.33 V vs. SCE) to 4 (−0.11 V; +0.29 V vs. SCE) and 6 (−0.10 V; +0.30 V vs. SCE) reveals very modest effects of adding 4-methoxy (4) or 2,6-dimethyl (6) substituents to the NacNac phenyl rings. The effect is more pronounced in 4-dimethylaminosubstituted 5 (−0.23 V; +0.17 V vs. SCE). Replacing the phenyl rings with more electron-rich nonaromatic substituents also has demonstrable effects. Complex 8 with N-OMe substitution, E(IrIV/IrIII) = −0.15 V (+0.25 V vs. SCE), is modestly shifted relative to phenyl-substituted 1, and the most pronounced effect of all comes from replacing the aromatic substituents with cyclohexyl rings, which results in a potential of −0.39 V (+0.01 V vs. SCE) for complex 9. Switching from the ppy C^N ligand to other C^N ligand such as tbppy (10, 11) or ppz (12, 13) generally has a small impact on the oxidation potential; the largest perturbation is seen in complex 10, which is easier to oxidize by 60 mV when compared to 1, but in the rest of the pairs with the same NacNac and different C^N ligands the differences in IrIV/IrIII potential are no more than 30 mV. Sweeping cathodically, the reduction features of most complexes are irreversible, except 1 and 4, and the peak potentials for all first reduction events occur at very negative potentials, near −2.7 V (−2.3 V vs. SCE). Thus, the LUMO energy and the formally IrIII/IrII reduction potentials are not been perturbed much by changing the NacNac or C^N ligands.
Figure 1. Molecular structures of complexes 5, 9, 10, 11, and 12, determined by single-crystal X-ray diffraction. H atoms and solvent molecules are omitted.
Photophysical properties. UV-vis absorption spectra were recorded in dry acetonitrile solution and are overlaid in Figure 2. We have additionally recorded spectra of the free NacNac ligands, shown in Figure S2. All complexes exhibit strong 1(π→π*) transitions in the UV region (λ < 280 nm). Overlapped metal-to-ligand charge transfer bands, 1MLCT and 3MLCT, dominate the low-energy regions of the spectra. All complexes also have a band centered around 400 nm, tentatively ascribed to a NacNac π→π* transition as such pronounced bands are not present in isoelectronic complexes with other ancillary ligands.21,22 Furthermore, the position of this band is sensitive to the NacNac structure, and tends to red shift significantly as the ligand backbone becomes more electron rich via dimethylamino substitution (2, 11, and 13) or additional methylation (7). Significantly, cyclometalated iridium complexes with ppy and tbppy C^N ligand have intense visible absorption bands with ε > 5 × 103 M−1cm−1 at 400 nm and significant absorption intensity tailing out to 500−550 nm. This substantial visible absorption is a prerequisite for visible-light photosensitization and is comparable to many other iridium complexes commonly used in photoredox catalysis.4 One detriment to switching to the
Electrochemical properties. Having previously observed significant effects of NacNac backbone substituents on redox potentials, in particular the IrIV/IrIII couples which reflect relative HOMO energies, one of our primary interests in this study is to determine the effects of other NacNac modifications on the electrochemical properties of this photosensitizer library. As discussed below, the ground-state redox potentials are one of the two determinants of the excited-state redox potentials, so these trends in redox behavior provide key insight into how to control and optimize excited-state redox properties. The redox properties of the cyclometalated iridium complexes were measured via cyclic voltammetry in acetonitrile solution with a ferrocene internal reference. Overlaid voltammograms are collected in Figure S1, and Table 1 includes a summary of the IrIV/IrIII potentials. The first oxidation potentials are notably affected by the modified NacNac ligands. Installing dimethylamino (NMe2) groups instead of methyl
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Table 1. Summary of ground-state redox potentials, UV-vis absorption spectra, photoluminescence properties, and excited-state redox potentials. E(IrIV/IrIII) / V
E(IrIV/*IrIII) / V
emission, λ / nm
UV-vis Absorption
ΦPL
a
τ / μs
a
ET1 / eV
e
vs. Fc+/Fc
vs. Fc+/Fc (vs. SCE) g
λ / nm (ε/103 M–1cm–1)a
1
−0.07 (+0.33)
264 (39), 357 (12), 383 (11), 460 (sh) (1.8)
595
522, 554 (sh)
0.053
0.20
2.4
−2.5 (−2.1)
2
−0.26 (+0.14)
269 (27), 298 (21), 393 (6.1), 511 (1.9)
634
560, 594 (sh)
0.16
0.76
2.3
−2.6 (−2.2)
3
+0.03 (+0.43)
264 (62), 342 (24), 413 (sh) (6.1), 456 (sh) (3.2)
571
515, 540 (sh)
0.23
2.0
2.4
−2.4 (−2.0)
4
−0.11 (+0.29)
260 (14), 290 (sh) (8.7), 353 (3.0), 385 (sh) (2.9), 435 (0.5)
576
536, 575 (sh)
0.021
0.52
2.4
−2.5 (−2.1)
5
−0.23 (+0.17)
259 (14), 289 (sh) (7.2), 357 (sh) (3.4), 385 (3.8), 435 (0.5), 488 (sh) (0.3)
b
547, 589 (sh)
b
b
2.3f
−2.5 (−2.1)
6
−0.10 (+0.30)
259 (50), 287 (sh) (34), 308 (sh) (23), 365 (12), 391 (15), 500 (1.4)
605
553, 590 (sh)
0.18
0.53
2.3
−2.4 (−2.0)
7
−0.33 (+0.07)
258 (36), 288 (sh) (26), 319 (sh) (15), 358 (7), 391 (sh) (8.5), 421 (9.5), 536 (1.3)
623c
592, 633 (sh)
0.012c
0.63c
2.2
−2.5 (−2.1)
8
−0.15 (+0.25)
256 (23), 285 (sh) (13), 350 (4.7), 382 (sh, 4.0), 475 (sh) (0.8)
632
557, 600 (sh)
0.022
0.40
2.4
−2.6 (−2.2)
9
−0.39 (+0.01)
265 (36), 288 (sh) (26), 361 (10) 391 (Sh), 506 (2.4)
661
607, 658 (sh)
0.040
0.35
2.0
−2.4 (−2.0)
10
−0.13 (+0.27)
266 (17), 295 (sh) (10), 356 (4.8), 386 (4.2), 485 (0.5)
575
528, 566 (sh)
0.045
0.15
2.4
−2.5 (−2.1)
11
−0.27 (+0.13)
269 (15), 304 (sh) (10), 391 (2.3), 494 (0.6)
594
558, 593 (sh)
0.12
0.51
2.3
−2.6 (−2.2)
533
b
b
f
2.3
−2.4 (−2.0)
570
0.017
0.35
2.2f
−2.4 (−2.0)
293 K
12
−0.10 (+0.30)
245 (22), 307 (7.5), 308 (6.7)
b
13
−0.25 (+0.15)
240 (39), 302 (22), 400 (5.9)
637
a
77 K
d
(vs. SCE) g
a In MeCN unless otherwise noted. b Not luminescent at 293 K. c In THF. d In butyronitrile. e Determined from intersection point of UV-vis absorption and emission, unless otherwise noted. f Determined from first vibronic peak in 77-K emission spectrum. g Corrected from Fc+/Fc = 0.40 V vs. Standard calomel electrode.23
(b) 40
(a) 70
1
–1
40
8
in Chart 1.
9
20
–3
30
5
30
/ M cm
50
20 10 0
300
400
500 600 / nm
(c) 50
0
–1 –1 –3
×10
/ M cm
–1
20 10 300
400 / nm
500
600
300
(d) 40
–1
7
–3
6
30
0
10
700
40
/ M cm
4
3
–1
2
×10
×10
–3
–1
/ M cm
–1
60
×10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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400 / nm 10
500
11
12
600
13
30 20 10 0
300
400 / nm
500
600
Figure 2. UV-vis absorption spectra of (a) 1–3; (b) 4, 5, 8, and 9; (c) 6 and 7; (d) 10–13. All absorption spectra were recorded in acetonitrile.
cyclometalating ligand ppz is a decrease in visible absorption, and for 12 and 13 the absorption completely tails off before 500 nm and is very weak beyond 400 nm. In general, there are no significant differences between the spectra of the various NacNac free ligands, as shown in Figure S2 of the Supporting Information. Most NacNac ligands absorb in the near-UV region, with maxima between ca. 300 and 350 nm. The exception is L2H, which only absorbs high-energy light
