Nonadiabatic Curve-Crossing Model for the Visible-Light Photoredox

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Nonadiabatic Curve-crossing Model for the Visible-Light Photoredox Catalytic Generation of Radical Intermediate via a Concerted Mechanism Wenjing Yang, Xuebo Chen, and Wei-Hai Fang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00601 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Catalysis

Nonadiabatic Curve-crossing Model for the Visible-Light Photoredox Catalytic Generation of Radical Intermediate via a Concerted Mechanism Wenjing Yang,†,‡ Xuebo Chen,†,* and Weihai Fang †,*

†

Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education,

Department of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China ‡

College of Material Science & Engineering, Taiyuan University of Technology, Shanxi 030024,

People’s Republic of China

Corresponding email for Xuebo Chen: [email protected] Corresponding email for Weihai Fang: [email protected].

KEYWORDS: photoredox catalysis; single electron transfer; nonadiabatic curve-crossing; C-H functionalization; ab initio calculations

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ABSTRACT: Photoredox catalysis relies on the excited-state single-electron transfer (SET) processes to drive a series of unique bond-forming reactions. In this work accurate electronic structure calculations at the CASPT2//CASSCF/PCM level of theory together with the kinetic assessment of SETs and intersystem crossing are employed to provide new insights into the SET initiation, activation, and deactivation by calculating the SET paths for a paradigm example of photoredox α-vinylation reaction mediated by iridium (III) catalysts. The concerted photocatalysis mechanism described by the nonadiabatic curve-crossing model, in essence of Marcus electron transfer theory, is first applied for the mechanistic description of the SET events in visible-light photoredox catalysis. The C-C bond functionalization has been revealed to take place in a concerted manner along an energy-saving pathway, in which the generated α-amino radical is unlikely independent existence but strongly depends on the mutual interaction with different substrates. These mechanistic insights offer a plausible picture for the excited-state SET-mediated chemical transformations that should be applicable to further studies of photoredox catalysis in organic chemistry.


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INTRODUCTION The increasing need for sustainable development prompts chemists to develop “green” synthetic methods that can be carried out in the same manner as plants utilizing sunlight. 1-2 Recently, significant progress has been made by MacMillan,3-8 Yoon,9-12 Stephenson,13-17 and other research groups18-24 for advocating visible-light-mediated photoredox catalysis in the field of organic synthesis. Upon irradiation, the metal-based photoredox catalysts can act as both a strong oxidant and a strong reductant in the excited state upon irradiation, which allows to convert visible light into chemical energy under exceptionally mild conditions.3-24 Although a wide variety of metal complexes have been examined as the photoredox catalysts for synthetic applications, the majority of them are ruthenium and iridium polypyridyl complexes,3 which exhibit a strong, broad absorbance in the visible light range. Many types of chemical reactions, which were the long-standing challenge in the field of organic synthesis, have been shown to be amenable to visible-light photocatalysis via SET between different substrates and the photoredox catalysts.3-24 As a paradigm example, the photoredox catalysts Ir(ppy)3 (1a) and its derivatives (1b and 1c) were applied in MacMillan’s laboratory to enable direct C-H vinylation of N-aryl tertiary amine (2) in combination with vinyl sulfone (3) to produce allylic amine (4) in high yield and with excellent olefin geometry control.8 The proposed mechanism for the C−H vinylation has been outlined in Scheme 1, which involves a sequence of the photoinitiated SET, the generation of αamino radical, the C-H functionalization, and the C-C bond formation. As emphasized by Yoon,10c the generation of organic radicals, amine radical cations, or radical ions was generally accepted as the crucial step in almost all the visible-light-mediated catalytic reactions reported to date.


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Scheme 1. Vinylation reaction through the combination of 2 and 3, mediated by the photoredox catalysts 1a and its derivatives 1b and 1c. The mechanism show here was proposed from experimental studies in the group of MacMillan.8

To generate these radical intermediates, however, the inert bonds of C-H or N-H have to be broken, which are highly endothermic (> 4.3 eV) and notably exceed the energy range of visible light. Although the shortage of energy is largely compensated by the simultaneous formation of base-hydrogen (B-H) bond through the proton abstraction reaction, the C-H functionalization proceeds smoothly in the visible-light photoredox catalysis of aza-Henry reaction mediated by 1b without the participation of base, where oxidative substrates may abstract hydrogen atom from the reductive substrates to accomplish the catalyst turnover.25 Indeed, the mechanistic understanding of visible-light photocatalysis reactions has seriously lagged behind the wealth of experimental investigations and remains largely elusive due to the lack of universal model from a


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theoretical viewpoint. As a successful attempt in our group, a multiconfigurational quantum chemical approach in conjunction with relativistic energy-adjusted ab initio pseudopotentials26 has been used to explore the reaction mechanism of enantioselective intramolecular enone [2+2] photo-cycloaddition reaction mediated by a chiral Lewis acid.27 In this work, we employ the combined CASSCF and CASPT2 computations together with the polarizable continuum solvation model (CASPT2//CASSCF/PCM) to provide first regulatory theory regarding the SET initiation, activation, and deactivation in the paradigm example of visible-light photoredox catalysis reaction.

METHODS The hybrid CASPT2//CASSCF/PCM method was used to calculate the vertical excitations and the minimum-energy profiles of SET events in the reductive quenching cycle followed by the proton transfer and the C-C coupling for the vinylation reaction. For comparison, the groundstate geometries of 1a, 1b and 1c photocatalysts in solution were re-optimized at the density functional theory (DFT)/PCM level followed by the singlet excited-state calculations at TDDFT/PCM level. The computational strategies of the orbital localization and configurational selection were applied to the multiconfigurational perturbation calculations in this work, which ensures the selected orbitals be closely related to the reactive process based on the numerous test calculations and careful examinations. The 6-31G* basis set was employed for the reaction center of substrates 2 and 3 of the 1a-2-3 complex, while a smaller STO-3G basis set was applied to the remaining atoms that are far away from the reaction center. To minimize the unbalanced optimization structures by using the mixed basis sets, computational tests for the dividing strategies were performed to set boundary at the appropriate -bonding region without losing the conjugative effect. To account for the relativistic effect of Ir, an energy-consistent scalar-


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relativistic

WB-adjusted

60-electron

core

pseudopotential26

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and

the

ECP60MWB

(8s7p6d2f1g)/[6s5p3d2f1g] basis set were applied to the Ir center in both the DFT-TDDFT and CASSCF calculations. Solvent effect was included using the polarizable continuum model for the dichloroethane matrix in all the optimizations and IRC computations of the isolated photocatalysts as well as the 1a-2-3 complex. The minimum energy pathways of SET were finally computed at the CASPT2//IRC/CASSCF(12e/11o)/PCM level of theory along the reaction coordinates, which will help gain insight into how this fundamentally important reaction takes place in solution. In this work, the computational errors for the vertical and adiabatic excitation energies were controlled to within an acceptable range (0.04–0.20 eV) as compared with the available experimental data.28,33 The reductive and oxidative SET rate constants for photocatalysts were calculated by using the formula of Marcus electron transfer theory34 while ISC rates were computed in the Condon approximation.35 All of the TDDFT, CASSCF, and IRC computations were performed with the Gaussian program package,36 while the CASPT2 calculations were conducted employing the Molcas program package.37 For more computational details see the sections S1, S2.2 and S2.3 of SI.

RESULTS AND DISCUSSION Origin of Visible-Light Absorption for the Photocatalysts of Iridium Complexes To explore the origin of visible-light absorption bands of catalysts 1a, 1b and 1c, the absorption spectra of the three catalysts were calculated at the CASPT2//CASSCF/PCM level of theory, as shown in Figure 1a-1c. The strongest band of 1a is located at 296-298 nm with a large oscillator strength (f = 0.8-1.2). This is in good agreement with the experimental measurements of an intense absorption centered around 285 nm.28 According to population analyses and charge translocation calculations, these bright spectra were brought by the CT from the  orbitals of the


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phenyl moiety to the * orbital of the pyridine group within the same ligand, hereafter referred as SSLCT (1*). Besides the SLCT transitions, other nine patterns investigated here are attributed to the photoinduced CT from the three d orbitals of the Ir metal center to the ligands, i.e., S0SMLCT (d*). These transitions overlap mutually, which results in a broad absorption band extending from 327 to 436 nm and peaking at 361 nm. The calculated spectra are in good SLCT

f

1.5

SLCT

1.5

1.2

1.2

0.9

0.9

f

MLCT

MLCT

0.6

SLCT

0.6

MLCT

MLCT

MLCT

MLCT 0.3

0.3

0.0

0.0

240

270

300

330

360

390

420

450

150

a

250

300

350

400

450

500

MLCT

TMLCT(3dπ*)-Min

0.9

MLCT

f

2.4

MLCT

MLCT

MLCT

1.6

hv 361nm

MLCT

e

hv 492nm

c

exp 5 7 -1 k p  1.310 ~5.710 s

0.8

δ-

δ+

0.0

150

180

210

240

270

300

330

360

390

420

450

480

510

540

λ / nm

0.0

c

π* orbital

d orbital

S0

0.0

120

700

SMLCT(1dπ*) TMLCT(3dπ*) S0

SMLCT(1dπ*)-Min/ STC(1dπ*/3dπ*)

3.2

650

λ / nm

FC of SMLCT(1dπ*)

SLCT

0.3

600

4.0

1.2

0.6

550

b

SLCT

1.5

200

λ / nm

ΔE/eV

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

ACS Catalysis

0.3

0.6

0.9

S0* 1.2

1.5

Intrinsic Reaction Coordinate/amu1/2 .Bohr

d

Figure 1. Absorption spectra for 1a(a), 1b(b) and 1c(c), the radiative relaxation pathway for 1a(d) in dichloroethane, and the assignment of various bands. The visible absorption bands that can be utilized directly in photoredox catalysis are highlighted in red. The results were obtained at the CASPT2//CASSCF/PCM level of theory.


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agreement with the experimentally observed band ranging from 350 to 500 nm with the maximum centered at 375-380 nm.28-29 The population analyses show that the electron pairs in the three degenerate d orbitals have the same probability to be promoted to the * orbitals of the three phenylpyridine ligands, resulting in a total of nine MLCT excited states. The strong coupling among these states extends the MLCT absorption band to the visible range. The maximum absorbance (361 nm) with the acceptable intensity (f = 0.38) is responsible for the visible absorption band that can be utilized directly in the photoredox catalysis (highlighted with red line in Figure 1). However, the TDDFT calculation predicts that the MLCT absorption band has a relatively narrow range,28,30 as compared with the experimental findings28-29 and the CASPT2//CASSCF/PCM calculated results that take into account dynamic correlation effect and coupling.31-32 Our calculation suggests that the strong coupling among various MLCT excitations associated with the mixed d- orbitals plays a fundamental role in improving the width and intensity for the visible absorption band of Ir(III) catalyst. This is the reason why the Ir(III) complex is widely used as the catalyst for the visible-light-induced redox reaction. As shown in Figure 1b and 1c, similarly strong MLCT absorption bands with a mixed character were found at 447 (1b) and 375 (1c) nm, which possess moderate oscillator strength (f = 0.2~0.3) (highlighted with red line in Figure 1). The calculated bands are close to the irradiation wavelengths (420 and 380 nm) found in the experiments.3,28 The presence of an electrondonating tert-butyl group in the bipyridine ligand of 1b notably reduces the energy level of the acceptor for the MLCT transition, i.e., the orbital of the substituted ligand, resulting in a redshifted absorption [361 (1a)  447 (1b) nm]. In contrast, a further structural modification by the introduction of electron-withdrawing F and CF3 groups in other two phenylpyridine ligands


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(1c) diminishes the electron donating ability of Ir3+ for the MLCT excitation. This accounts for the blue-shifted absorption (375 nm) as compared with that of 1b (447 nm). Radiative Relaxation Pathways of Photoredox Catalysts Upon irradiation of visible-light, the catalysts 1a, 1b and 1c are initially populated in the Frank–Condon (FC) region of the singlet metal-to-ligand charge transfer (SMLCT) state with the maximum absorbance (f = 0.38-0.44) in the extended bandwidth that is verified to be brought from the strong coupling among various MLCT excitations associated with the mixed three d and three * orbitals. As shown in Figure 1d, following through a downhill relaxation path, catalyst 1a rapidly decays to its minimum SMLCT-Min (0.28 eV below the FC point) that energetically matches with the MLCT triplet state at singlet-triplet crossing (STC) of STC(SMLCT/TMLCT). In addition, the spin-orbit-coupling (SOC) constant in the STC region is calculated to be ~50.0 cm-1, due to the effect of Ir heavy atom. The rate of intersystem crossing (kISC) is estimated to be 1.11012 s-1, allowing a fast decay to the low-lying emissive TMLCT(3d*) state. Careful comparisons reveal that there is almost no difference in structural parameters between the minimum-energy of TMLCT(3d*) state (TMLCT–Min) and ground state minimum but with large energy difference (>2.0 eV) (see Table S4 and Figure S2 in SI). This allows the effective ground state recovery of photo-catalyst in the repeated cycles of absorption-emission and photocatalysis. Meanwhile, none-radiative channel is largely closed due to lack of the energetically accessible crossing between TMLCT(3d*) and ground states. Therefore, the subsequent 475-596 nm phosphorescent emission with the lifetime of hundreds ns and 1-2 μs28,33 from the minimumenergy (TMLCT–Min) region is the most likely channel for the isolated catalysts 1a, 1b and 1c.


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Reduction Reaction of the Ir(III) Catalysts As shown in Figure 2, unlike the case of isolated catalysts, the relaxation process, as a preadjusted step of the reductive SET, can be triggered in presence of reductive tertiary amine 2 whose lone pair electrons are readily promoted to the unoccupied orbitals of TMLCT-Min for Ir catalyst. The computational tests reveal that the empty s orbital of Ir centre (i.e., 1a and 1b) or the lowest-lying ligand * orbital (i.e., 1c) is much easier to accept electron compared with the 4 SMLCT(1dπ*) T21CT S0

FC of SMLCT(1dπ*)

k ISC1  1.110 s STC(SMLCT/TMLCT) 1

12

3

k ISC2  2.310 s STC(T23CT/S0) 10

k et2  3.510 s CI(T21CT/T23CT)

k et1  7.110 s CI(TMLCT/T21CT) 5

SMLCT-Min

1

9

1

TMLCT(3dπ*) T23CT

1

S0-Max Barrier: 0.39 eV

T21CT-Min

T23CT-Min

TMLCT-Min

ΔE/eV

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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6 1

2

5 4

hv 361nm

7

2 3

8

9 10

40.2°

Ir(ppy)3(1a-)

amine(2+) vinyl sulfone(3)

deprotonated 2 protonated 3

1 Ir(ppy)3(1a)

α-amino radical

hv 492nm exp 5 7 -1 k p  1.310 ~5.710 s

0

S0 *

S0

E-allylic amine 0.0

0.7

1.4

2.1

2.8

3.5

4.2

4.9

5.6

6.3

7.0

7.7

8.4

9.1

9.8

10.5

11.2

11.9

12.6

Intrinsic Reaction Coordinate/amu1/2 .Bohr

Figure 2. Minimum-energy profiles of SET events in the reductive quenching cycle followed by the proton transfer and the C-C coupling for the vinylation reaction through the combination of 2 and 3 catalyzed by 1a. The results were obtained at the CASPT2//IRC/CASSCF(12e/11o)/PCM level of theory. The highlighted characteristic points of the 1a-2-3 complex are schematically shown with their key bond distances in Å. The kinetic data are also provided.


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vacant d orbital in TMLCT state (see section S1.3 in the SI). This possible participation of s orbital is mainly ascribed to penetration effect of the Ir3+ 6s orbital in the presence of relativistic effect. The MLCT excitation of Ir catalyst induces an opposite dipole moment distributed mainly in the +H1-C2-N3-

structural region of the amine 2 (see Figure 3 and Table S6 of SI). The

photoinduced dipole-dipole interaction between 1 and 2 leads to the enhanced electron donation ability of N3 with a concomitant initial activation of C-H bond (1.077  1.082 Å) in the decay from FC of SMLCT to TMLCT–Min for 1a-2-3 complex. Based on above structural changes and charge redistribution, the structural reorganization mainly associated with the planarization of amine 2 (C2N3C4C5 dihedral angle: 49.6  40.2) is triggered instantaneously by overcoming a medium sized barrier (0.28 eV). This promotes the catalytic systems to the conical intersection between TMLCT state and 1a-2 intermolecular charge transfer triplet state (T21CT), i.e., CI(TMLCT/T21CT), which provide an energetically accessible channel to allow the occurrence of

(1a)=0.71 (1b)=0.74 (1c)=0.32

ket(1a)=7.1105 ket(1b)=4.0106 ket(1c)=3.3107



CI(TMLCT/T21CT)

E (1a)= 0.28 E (1b)= 0.20 E (1c)= 0.27

ket2=3.5109 2=0.70

CI(T21CT/T23CT)

T21CT-Min

E

T23CT-Min

TMLCT-Min

kp=1.3105~5.7107

Scheme 2. A schematic drawing of the non-adiabatic crossing model for the vinylation reaction regulated by the reductive and oxidative SETs. The reorganization energy (λ1,2) and the Gibbs energy change of the SET reaction (E) are given in eV as well as the rate constant of electron transfer(ket) are provided in s-1.


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reductive SET from N3 lone pair of amine 2 to the lowest-lying ligand * orbital with some contributions from the s orbital of Ir centre. On the basis of the CASPT2//CASSCF/PCM calculated physical parameters shown in Scheme 2, the reductive SET rates are calculated to be 7.1105~3.3107 s-1 for three photocatalysts by using the formula of Marcus electron transfer theory.34 As an important consequence, the rate-determining step of the reductive SET takes place competitively over the phosphorescent emission of photo-catalysts from TMLCT-Min (kp=1.3105~5.7107 s-1).28,33 Moreover, the calculated SET rates for three photocatalysts correlate with the measured yields of the allylic amine (1a, 28~57%; 1b, 33%; 1c, 40~91%).8 The introduction of electron donating tertbutyl group for 1c notably stabilizes the electron acceptor of bipyridyl ligand centred * orbital, thereby reducing the vertical excitation energy from the TMLCT-Min of Ir catalyst to T21CT state and the corresponding reorganization energies (λ1). Meanwhile, the presence of electron withdrawing F or CF3 groups in the two other ligands of 1c facilitates the dispersion of excess negative charge when the reduction reaction of photocatalyst is triggered, leading to the thermally stable Ir(II) catalyst in its reduction state (see the section S2.1 in the SI). Accordingly, the rate of reductive SET is considerably accelerated for the optimal photocatalyst 1c compared with other two photo-catalysts (1a and 1b) based on the calculated SET rate and the charge translocation calculation of the reduction state of Ir(II) catalysts. Reduction State Relaxation of the Ir(II) Catalysts Once the reductive 21 SET is completed, a stable zwitterionic complex (1a--2+) is formed between 1a and 2 in the lowest-lying triplet T21CT state and has remarkable increase in dipole moment compared that in TMLCT(3d*) state (12.544.0 Debye). Because of the intermolecular


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coulomb interaction, the substrate 3 moves close to the 1a- anion, thereby resulting in the H1O6 hydrogen bond between 2+ and 3 with a concomitant planarization of the tertiary amine 2 (C2N3C4C5 dihedral angle: 40.223.0°). These structural reorganizations are actually the preadjusted step for the oxidative SET, in which one electron nonadiabatically jumps from Ir catalyst to the phenylethene * orbital of the substrate 3 with some contributions from the C9-S7 * orbital at the conical intersection between the T21CT and the 2-3 intermolecular charge transfer triplet state (T23CT), referred to as CI(T21CT/T23CT) hereafter. The oxidative SET (ket2 =3.5109 s-1) takes place more effectively compared with the former one (ket1= 7.1105 s-1). This is due to the formation of the extremely stable 2+-3- zwitterionic complex after the completion of 1a-3 SET, which causes the potential energy change (E) to become more negative compared with the positive values in the reductive SETs. As a result, the component contributions from exponential term can be improved significantly, thus speeding up the oxidative SET. The medium sized nonadiabatic couplings (37.9-49.3 cm-1) were found at the curve crossing regions of CI(TMLCT/T21CT) and CI(T21CT/T23CT). This corresponds to the moderate rates of reductive and oxidative SETs, which occur competitively with the lifetimes of phosphorescent emission in the subnanosecond region for Ir catalysts. It is obvious that the interaction of the two related diabatic states [i.e., TMLCT/T21CT for the reductive SET and T21CT/T23CT for oxidative SET] is not strong enough to allow passing through the intersection region along the adiabatic surface. Thus, the catalytic system undergoes the quantum jump, from one "adiabatic" surface to the other on passing through the intersection region via a non-adiabatic manner.


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1.093

ACS Catalysis

6

1

7

9 10

2

5

8

3

4

40.2°

1a-

2+

3

CI(TMLCT/T21CT)

1a- (Ir2+)-2+-3 δ-

δ+

δ+

Proton Transfer δ-

49.6°

1a

2

23.0°

2+

3 1a (Ir3+)-2+-3-

1a (*Ir3+)-2-3

TMLCT-Min

3CI(T21CT/T23CT)

S0 -0.0381 0.0630 1.0975 -0.3599 TMLCT -0.0512 0.0747 1.3321 -0.9209 C2—H1 Ir3+—L opposite dipole

ISC E-allylic amine

Concerted

hv

Charge redistribution

1a (Ir3+)-2-3

Concerted proton transfer C-S bond fission

Concerted C-C coupling Departure of leaving group

e

4.7°

α-amino radical

protonated 3

2+

Concerted C-H bond activation

S0-Max

3T23CT-Min

STC(T23CT/S0)

Figure 3. Summary of the photocatalytic cycle of the vinylation reaction based on the CASPT2//IRC/CASSCF(12e/11o)/PCM

calculations.

The

structures

at

the

highlighted

characteristic points are given along the reaction path of Figure 2 with their key bond distances (Å) and the C2N3C4C5 dihedral angles (°). Mulliken charge along the selective atoms/moieties of 1a-2-3 complex in S0-min and TMLCT-min were provided in the left dashed box.


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Concerted C-C Construction with the Departure of Protonated Leaving Group Starting the decay of T23CT state, the excess negative charge of the 3- moiety imposed by the oxidative SET is progressively transferred to the terminal C6H5SO2- group while H1-C2 bond is notably weakened along a downhill path. The remarkable H1-C2 bond activation triggers the H1 proton transfer towards the C6H5SO2 group with a concomitant significant elongation of C9-S7 bond, relaxing to the minimum of T23CT state, T23CT-Min. However, as shown in Figure 2, the continuously concerted reaction for the H1 proton transfer and the C9-S7 bond breaking leads to the remarkable increase in energy along a sharp uphill pathway of triplet state. Fortunately, the singlet-triplet crossing of STC(T23CT/S0) was determined between T23CT and ground states and schematically shown in Figure 3. Population analyses indicate that the singly occupied orbitals have evolved into the C2 and C9 at STC(T23CT/S0) with the partial departure of proton H1 and C6H5SO2- group, showing a diradical configuration at 3.5 Å C2-C9 distance. The ISC rate is calculated to be 2.3  1010 s-1 at STC(T23CT/S0), which allows an effective decay to the ground state via an intermolecular spin inversion. Following the occurrence of ISC from STC(T23CT/S0), another reaction channel of C2-C9 radical combination is triggered significantly in the ground state with the coexisting reactions of H1 proton transfer and the C9-S7 bond breaking. The possible procedures of stepwise C2-C9 bond construction and C9-S7 bond dissociation were inspected by test calculations, which lead to the sharp increase of ground-state energy (> 0.7~0.8 eV, see section S3 in the SI). The direct C2C9 bond construction can be sterically hindered without the effective departure of C6H5SO2leaving group. On the other hand, the direct C9-S7 or C2-H1 bond breaking is the energetic consumption process. Numerous computational tests show that the energy consumption of C2H1 and C9-S7 bonds dissociation is well compensated by the released energies of H1-O6 and


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C2-C9 bonds construction, which allows the concerted asynchronous H1 proton transfer, the C9S7 bond dissociation, and the C2-C9 bond formation to proceed smoothly in the ground state by overcoming a small barrier (0.39 eV with respect to the zero level of TMLCT-Min). In concerted asynchronous process, the deprotonated amine 2 always exhibits a triangle like arrangement with the coexisted leaving and phenylethylene groups. The sharp increase was repeatedly found along the ground-state path when three moieties slightly deviate from a triangle like arrangement. All lines of evidence tend to suggest that there are strong intermolecular interactions among the generated -amino radical and leaving proton H1 as well as the attacked species of phenylethylene group in the concerted asynchronous reaction. This indicates that the generated -amino radicals in visible light photocatalysis are not capable of independent existence but strongly rely on the mutual interaction with different substrates. Similarly, the sizeable barriers (> 2.0 eV) were found for the proton abstraction imposed by a base reagent (CH3COO-) in S0 and T21CT states without the participation of substrate 3 (see section S3 in the SI). This confirms that the concerted proton abstraction imposed by the substrate containing the leaving group provides the driving force for the C-H functionalization in the photo-catalyzed vinylation reaction. In the concerted asynchronous processes, the C9-C10 bond always remains the some single-bond character, which facilitates the attack of -amino radical, thus giving rise to the E-allylic amine with high yield.

CONCLUSION In this work, the concerted photocatalysis mechanism described by the nonadiabatic curvecrossing model, in essence of Marcus electron transfer theory, is established for interpreting the visible-light-mediated photoredox catalytic reactions. As far as we know, the non-adiabatic


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crossing model is the first applied in this work to interpret the excited-state SET mechanism in the visible-light-induced catalytic reactions. In another limit, a valence-bond configuration mixing theory was used by Shaik and his co- workers38 to treat organic reactions involving the adiabatic SET process. The catalytic efficiency can be improved in presence of electron donating group for the electron acceptor of ligand centred * orbital of Ir catalyst. This tunable photoredox properties notably promote the decreased reorganization energy associated with the nuclear deformation of amine substrate planarization along the reductive quenching pathway of Ir(III) catalyst, thereby accelerating the rate-determining SET. Apart from kinetic factor, the catalytic reaction is also regulated via the thermodynamic control by the introduction of electron withdrawing group in the other two ligands of Ir catalyst, which facilitates the dispersion of excess negative charge around the ligand of electron acceptor, thus producing the thermally stable Ir(II) catalyst in its reduction state. Most importantly, the C-C bond construction has been verified to proceed in a concerted fashion with the proton transfer and the departure of the leaving group. This ensures the occurrence of C-C bond functionalization in the mild condition along an energy-saving pathway, in which the generated α-amino radicals are not capable of independent existence but strongly rely on the mutual interaction with different substrates. These computational insights represent considerable advances in understanding the SET events of visible-light photoredox catalysis reaction and may help develop a mechanism-based design for the photocatalysts. SUPPORTING INFORMATION Computational details, calculation of vertical excitation, adiabatic excitation and emission energies, computational protocol of ground and excited-state reduction potentials, charge translocation calculations, and ISC rate constant calculation, rate constant calculations of the


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reductive and oxidative SETs are reported in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] [email protected] ORCID Xuebo Chen: 0000-0002-9814-9908 Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS We are grateful to the Natural Science Foundation of China (NSFC21725303 and NSFC21421003) for the financial support of this research.

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TABLE OF CONTENTS GRAPHIC


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Nonadiabatic Curve-Crossing Model for the Visible-Light Photoredox

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