DFT as a Powerful Predictive Tool in Photoredox Catalysis: Redox

Article pubs.acs.org/Organometallics

DFT as a Powerful Predictive Tool in Photoredox Catalysis: Redox Potentials and Mechanistic Analysis Taye B. Demissie,†,‡ Kenneth Ruud,†,‡ and Jørn H. Hansen*,‡ †

Centre for Theoretical and Computational Chemistry, ‡Department of Chemistry, UiT − The Arctic University of Norway, 9037 Tromsø, Norway

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 24, 2015 | http://pubs.acs.org Publication Date (Web): August 24, 2015 | doi: 10.1021/acs.organomet.5b00582

S Supporting Information *

ABSTRACT: Visible-light photoredox catalysis has come forth as a powerful activation mode in chemical synthesis, affording the development of a multitude of new strategies for molecular construction. However, detailed mechanistic knowledge of the various subprocesses involved is lacking, and new tools for addressing this are needed to drive innovation forward in the area. Herein, we describe predictions of groundand excited-state redox potentials of ruthenium and iridium photocatalysts using nonrelativistic and scalar relativistic zeroorder regular approximation density functional theory (DFT) methods. The computed redox potentials were correlated with experimental values and found to reproduce them well. Relativistic corrections were found to be important to reproduce experimental data. Moreover, the computational protocol allows us to estimate redox potentials that are not currently available in the literature or are difficult to determine experimentally. The mechanistic details of the photocatalyzed C−H functionalization of 1-methylindole with diethyl bromomalonate were also studied using the validated DFT method. We demonstrate how DFT can predict the experimentally observed redox behavior of common photocatalysts and mechanistic details of the C−H functionalization process. This work demonstrates that DFT can be a powerful tool for innovation and design in the field of visible-light photoredox catalysis by predicting redox properties and mechanistic behavior.

1. INTRODUCTION New methods enabling the construction of increasingly sophisticated molecules represent cornerstones in modern chemistry. Visible-light photoredox catalysis has recently emerged as a powerful reaction manifold for chemical synthesis.1,2 This activation mode enables the transformation of visible-light energy into chemical potential, a process reminiscent of photosynthesis in nature.3−5 As such, it could represent a greener and more sustainable direction toward solar-energy-driven synthetic processes.6,7 Furthermore, light activation represents a convenient and highly selective switch for controlling chemical reactions. Numerous applications have emerged, including polymerization,8 heterocycle construction,9 and, more recently, several transformations in conjunction with complementary modes of catalysis.10−14 One of the most exciting applications has been demonstrated in the area of complex molecule C−H functionalization, particularly in latestage structure diversification of medicinal agents (Scheme 1).15−18 The nature of the photocatalyst is of crucial importance in photocatalyzed processes. It is the central machinery that harvests visible-light energy through photon absorption. A metal-to-ligand charge-transfer process results, rendering the excited-state catalyst able to act as a powerful reductant or oxidant.1,2 As such, the photocatalyst plays a major role in determining the course of the reaction in these systems. In© XXXX American Chemical Society

Scheme 1. Photocatalytic Late-Stage Methylation of Voriconazole: A Powerful Approach to Structural Diversification for Drug Discovery15

depth studies of the redox properties, structures, and reactivities of such catalyst systems are therefore crucial in order to understand the fundamental aspects of photocatalyzed processes. Transition-metal complexes are utilized as catalysts in the majority of photocatalyzed reactions.15,16,18−21 Particularly common are complexes of ruthenium and iridium.15,18,19 The choice of the ligand−metal combination will govern the redox potentials (E0) in both the ground and excited states. For example, the ground-state reduction potentials for tris(bipyridine)ruthenium(II) (Ru(bpy) 3 2+ ) and tris(2,2′Received: April 2, 2015

A

DOI: 10.1021/acs.organomet.5b00582 Organometallics XXXX, XXX, XXX−XXX

Article


Organometallics bipyrazyl)ruthenium(II) (Ru(bpz)32+) are −1.34 V20 and −0.80 V,22,23 respectively, whereas their ground-state oxidation potentials are 1.29 V 20,22 and 1.86 V, 22 respectively. Furthermore, their excited-state reduction potentials are 0.79 V20 for Ru(bpy)32+*/1+ and 1.45 V22,24 for Ru(bpz)32+*/1+, all measured relative to the saturated calomel electrode (SCE) in acetonitrile (MeCN) solution. The latter complex is a potent oxidant in the excited state. Clearly, the nature of the ligand is crucial in determining the redox properties of such complexes. As such, photocatalysts can be tailored for particular applications. Electrochemistry has not been widely explored using quantum-chemical methods compared to other areas of chemistry, such as spectroscopy. In this study, we will consider the theoretical prediction of redox potentials (computational electrochemistry) of the common ruthenium and iridium photocatalysts shown in Figure 1. Cyclic voltammetry is a

Figure 1. Structures of the complexes studied. The different oxidation states considered for each complex are listed in Table 1. PC = photocatalyst.

it would constitute a practical and convenient tool for the study and design of new catalysts and reaction manifolds for chemical synthesis. The accuracy and reliability of DFT methods rely crucially on the choice of appropriate functionals and basis sets, as well as the treatment of solvent effects. In this regard, Roy et al.28 reported that the BP86 and PBE functionals provide better results for the redox potentials of iron dinuclear complexes compared to those calculated using B3LYP. They also reported that the influence of the basis set was minor compared to the choice of functional. A variety of approaches have been used for the theoretical determination of redox potentials in solution, such as estimations based on Koopmans’ theorem29 or adding gas-phase ionization energy to solvation free energy terms.25,30,31 The most commonly used protocol is the Born−Haber cycle (see Section 4.2 for details), which has been demonstrated to give reliable redox potentials for a variety of organic molecules as well as some transition metal complexes.25,27,28,32−34 The model used for the treatment of solvent effects is also a crucial factor to be considered when designing the computational protocol. A recent study of solvent effect models used for the prediction of redox potentials of organic molecules showed that the conductor-like screening model (COSMO)35 provides redox potentials in good agreement with experimental results.32 Furthermore, if there are heavy atoms in the molecular system, relativistic effects must also be considered.36 In this article, we report the application of DFT to the prediction of redox potentials of the heavy-atom photocatalysts shown in Figure 1. Moreover, we have applied DFT to study a synthetically powerful photocatalytic C−H functionalization reaction in the presence of such complexes. As such, we will demonstrate that DFT can be a powerful tool for the future design of photocatalysts with fine-tuned properties as well as for detailed elaboration of photocatalyzed reactions.

widely used experimental technique to determine such redox potentials. This method provides redox potentials with average errors as low as 0.01−0.02 V for reversible redox processes.25 However, for irreversible redox processes, the measurement is not straightforward.25,26 In cases where experimental techniques cannot easily generate reliable experimental E0 data (for instance, for organic radicals), state-of-the-art density functional theory (DFT) techniques provide an excellent alternative. DFT has recently been shown to produce accurate predictions of redox potentials for organic molecules,25,27,28 but it has not been explored in the context of heavy-metal photocatalysts. If such properties could be effectively predicted using DFT, then

2. RESULTS AND DISCUSSION 2.1. Molecular Geometries. Table 1 shows the selected optimized bond lengths of the eight complexes studied in different oxidation states using scalar-relativistic zeroth-order regular approximation (SC-ZORA) combined with the PBE-D3 functional and TZ2P basis set (see Section 4.2 for details). The facial isomers (N trans to C) for PC2 and PC8 and the pseudofacial isomers for PC3 and PC5 were considered. However, the meridional isomers (N trans to N and C trans to C) were also cross-checked energetically as well as in the redox potential calculations (see Figure S1 of the Supporting Information for details). In these complexes, the facial isomers

Table 1. Selected Bond Lengths (Å) of the Complexes Calculated at the SC-ZORA/PBE-D3/TZ2P Level in Acetonitrilea +1 M−N PC1 PC2 PC3 PC4 PC5 PC6 PC7 PC8 a

2.057 2.129 2.150 2.064 2.141 2.062 2.053 2.120

+2 M−C 2.015 2.026 2.028

2.016

M−N 2.063 2.133 2.159 2.071 2.147 2.073 2.058 2.124

+3 M−C 2.018 2.027 2.025

2.016

M−N 2.067 2.141 2.159 2.074 2.154 2.089 2.068 2.138

+4 M−C

M−N

M−C

2.015 2.028

2.146 2.176

2.006 2.015

2.026

2.162

2.012

2.012

2.139

2.006

All the bond lengths of the iridium complexes represent those between the central metal and the phenylpyridine ligand atoms. B


Article

Organometallics

Table 2. Comparison of Experimental, Nonrelativistic (NR/PBE-D3/TZ2P), and Scalar-Relativistic (SC-ZORA/PBE-D3/ TZ2P) Calculated Change in Gibbs Free Energies (kcal/mol) and Ground-State Redox Potentials (Volts)a ground state ΔG0 (soln, calcd) complex


2+/+

PC1 (Ru ) PC1 (Ru3+/2+) PC2 (Ir2+/+) PC2 (Ir3+/2+) PC2 (Ir4+/3+) PC3 (Ir2+/+) PC3 (Ir3+/2+) PC3 (Ir4+/3+) PC4 (Ru2+/+) PC4 (Ru3+/2+) PC5 (Ir2+/+) PC5 (Ir3+/2+) PC5 (Ir4+/3+) PC6 (Ru2+/+) PC6 (Ru3+/2+) PC7 (Ru2+/+) PC7(Ru3+/2+) PC8 (Ir2+/+) PC8 (Ir3+/2+) PC8 (Ir4+/3+)

E0 (calcd)

NR

ZORA

ΔG0 (exp)

NR

ZORA

−69.8 −130.7 −52.9 −47.1 −126.2 −62.0 −64.5 −138.1 −70.1 −130.5 −61.1 −65.4 −133.5 −60.5 −105.8 −85.1 −152.4 −51.9 −57.2 −132.9

−70.3 −128.0 −53.1 −48.5 −121.7 −63.4 −65.1 −133.6 −70.4 −130.7 −63.3 −65.0 −134.5 −64.1 −110.2 −88.1 −149.5 −55.6 −50.1 −128.9

−71.2 −131.9b

−1.40 1.24 −2.14 −2.39 1.04 −1.74 −1.63 1.56 −1.39 1.23 −1.78 −1.59 1.36 −1.81 0.16 −0.74 2.18 −2.18 −1.95 1.33

−1.38 1.12 −2.13 −2.33 0.85 −1.68 −1.61 1.37 −1.38 1.19 −1.69 −1.61 1.40 −1.65 0.38 −0.61 2.06 −2.02 −2.26 1.16

b

−48.9e −121.3g −69.4h −131.4h −70.8i −131.2i −70.1c −134.0c −63.6k −110.2d −86.5m −147.8m −53.2e −127.0e

E0 (exp) −1.34b (−1.36)c (−1.40)d 1.29b (1.26)c (1.29)d −2.31e (−2.19)f (−2.20)g 0.83g (0.77)f (0.72)e −1.42h (−1.42)c 1.27h (1.25)c −1.36i (−1.41)j (−1.35)j 1.26i (1.37)j (1.27)j −1.39c 1.38c −1.67k (−1.61)l (−1.61)j 0.35d (0.32)k (0.32)l −0.68m (−0.80)n (−0.765)o 1.98m (1.86)n (1.895)o −2.12e (−2.00)p 1.08e (1.29)p

ΔG0 (exp) is derived from the experimental redox potentials without parentheses. The redox potentials were corrected to E0(SCE) = 4.429 V [see Section 4.2]. The experimental ΔG0 values were calculated from the absolute redox potentials (4.429 V was added to the redox potentials listed in the table). Note that PC1, PC2, PC4, and PC6 were used as examples when discussing the results. Experimental values were taken from the following references. bRef 20. cRef 52. dRef 53. eRef 54. fRef 55. gRef 40. hRefs 45 and 56. iRef 57. jRef 45. kRef 59. lRef 58. mRef 60. nRefs 22 and 24. oRef 61. pRefs 55 and 62. a

are more stable than the meridional isomers. For instance, the facial isomer of PC8 with Ir3+ as a central metal is stabilized by 6.3 kcal/mol compared to the meridional isomer. The results also show that as the oxidation state increases, the metal-tonitrogen bonds elongate. For complexes involving phenylpyridine ligands, the metal−carbon bond lengths are relatively insensitive to the change in oxidation state, with the exception of the +4 oxidation state of the iridium complexes. These have slightly shorter M−C bond lengths than those in the lower oxidation state complexes. The Ir−N bond lengths are longer than the Ru−N bond lengths. For complexes involving both phenylpyridine (ppy) and bipyridine (bpy) ligands, the M− Nppy bonds are shorter than the M−Nbpy bonds. For instance, in Ir(ppy)2(bpy)+ (PC3), the Ir−Nppy bond, which is trans to Ir−Cppy, is 2.159 Å (the Ir−Nppy bond, which is trans to the Ir− Nbpy bond, is 2.051 Å), whereas the Ir−Nbpy trans to Ir−Cppy is 2.127 Å (the Ir−Nbpy bond, which is trans to Ir−Nppy, is 2.049 Å). The optimized structures are in good agreement with available experimental structural parameters. For example, in Ir(ppy)2(bpy)+ PC3, the experimental Ir−Nppy bond length, which is trans to the Ir−Nbpy bond, is 2.061 Å, and that of the Ir−Nbpy bond, which is trans to Ir−Cppy, is 2.154 Å,37 which can be compared, respectively, with the above results (see Supporting Information for details). 2.2. Ground-State Redox Potentials. The changes in solution Gibbs free energies and the ground-state redox potentials of the complexes were studied using nonrelativistic and relativistic methods. The results are presented together with experimental values in Table 2. The results show that the change in solvation Gibbs free energies obtained from calculations using SC-ZORA are generally in better agreement

with experiment than the nonrelativistic (NR) ones. For instance, the absolute differences between the experimental and nonrelativistically calculated solvation Gibbs free energies for PC2 (Ir4+/3+) and PC6 (Ru3+/2+) are found to be 4.9 and 4.4 kcal/mol, respectively, whereas that between those calculated by including scalar relativistic corrections are found to be 0.4 and 0.0 kcal/mol, respectively (Table 2). Also, the absolute differences between the experimental and nonrelativistically calculated oxidation potentials for these two complexes are 0.21 and 0.19 V, whereas those calculated by including scalar relativistic corrections are only 0.02 and 0.03 V, respectively. In the ruthenium complexes, there are structural changes of the molecular geometries upon oxidation. When an electron is removed from the highest occupied molecular orbital (HOMO, which is the oxidation center) of the Ru2+ complexes (metal dz2 orbital, with the exception of PC6), there is a larger structural change of the oxidized (Ru3+) complexes compared to their reduced (Ru2+ and Ru1+) counterparts. This causes the complexes to have a small energy gap (Eg) between the HOMO and the lowest unoccupied molecular orbital (LUMO). For instance, in PC1, the species with Ru1+ has an Eg value of 1.90 eV and with Ru2+, 1.80 eV, whereas the Ru3+ complex has 0.45 eV; in PC4, the complex with Ru1+ has an Eg value of 1.96 eV, with Ru2+, 1.87 eV, and with Ru3+, 0.25 eV, all calculated using SC-ZORA/PBE-D3/TZ2P. In these Ru3+ complexes, the HOMO is mainly composed of dxz and dyz orbitals of the metal, whereas the LUMO is composed of dx2−y2 and dz2 orbitals of ruthenium (see Figures S2−S5 of the Supporting Information). Compared to the lower oxidation states, the LUMO of the Ru3+ complexes is highly stabilized. Such a small Eg value is not observed for the iridium complexes; C


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Organometallics

Table 3. Comparison of Experimental and Calculated Values of Changes in Gibbs Free Energies (kcal/mol) and Ground-State Redox Potentials (Volts) of the Ruthenium Complexes Using SC-ZORA/PBE-D3/TZ2P and SC-ZORA/B3LYP-D3/TZ2Pa ΔG0 (soln, calcd) complexes PC1 PC1 PC4 PC4 PC6 PC6 PC7 PC7

2+/+

(Ru ) (Ru3+/2+) (Ru2+/+) (Ru3+/2+) (Ru2+/+) (Ru3+/2+) (Ru2+/+) (Ru3+/2+)

E0 (calcd)

PBE

B3LYP

ΔG (exp)

PBE

B3LYP

−70.3 −128.0 −70.5 −129.6 −64.1 −110.9 −88.1 −149.4

−67.6 −134.4 −67.3 −130.7 −62.0 −110.2 −82.1 −147.6

−71.2 −131.9b −70.8e −131.2e −63.6g −110.2d −86.5i −147.8i

−1.38 1.12 −1.37 1.19 −1.65 0.38 −0.61 2.05

−1.50 1.40 −1.51 1.24 −1.74 0.35 −0.87 1.97

0

b

E0 (exp) −1.34b (−1.332)c (−1.40)d 1.29b (1.354)c (1.29)d −1.36e (−1.41)f (−1.35)f 1.26e (1.37)f (1.27)f −1.67g (−1.61)h (−1.61)f 0.35d (0.32)g (0.32)h −0.68i (−0.80)j (−0.765)k 1.98i (1.86)j (1.895)k


a The redox potentials were corrected to E0(SCE) = 4.429 V [see Section 4.2]. The experimental ΔG0 values were calculated from the absolute redox potentials (4.429 V was added to the redox potentials listed in the table). Experimental values were taken from the following references. bRef 20. c Ref 88. dRef 53. eRef 57. fRef 45. gRef 59. hRef 58. iRef 60. jRefs 15 and 17. kRef 61.

for example, in PC2 for the Ir2+ complex, the Eg value calculated using SC-ZORA/PBE-D3/TZ2P is 2.30 eV, for the Ir3+ complex, 2.23 eV, and for the Ir4+ complex, 2.80 eV. The analysis of the electronic configurations also showed that there are competing electronic configurations for the Ru3+ complexes, resulting in different electronic states of the different displaced geometries during both geometry optimizations and frequency calculations due to their small Eg. We overcame these competing electronic configuration problems by specifically assigning the occupations for both the α and β electrons of the Ru3+ complexes in addition to the specification of the charges and multiplicities of the complexes. As such, the agreement between the calculated and experimental values was found to be satisfactory; see Table 2. For example, if the occupations are not specifically given, then E0(Ru3+/2+) for PC1 was found to be 1.76 V, whereas when the occupations (144α/ 143β for PC1(Ru3+)) are specifically given during both geometry optimizations and frequency calculations, an oxidation potential of 1.12 V was obtained, compared with an experimental value of 1.29 V.20 Such discrepancies were not observed for the complexes with +1 and +2 oxidation states of ruthenium as well as for all of the iridium complexes. The remaining errors for the redox processes involving Ru3+ could be due to the presence of stronger interactions with the counterions present in solution. We calculated additional redox potentials for the ruthenium complexes, obtaining decreasing Eg values as the oxidation number of the metal increases, using the B3LYP-D3 functional in order to check the accuracy of the PBE-D3 results. The results are listed in Table 3. In most of the complexes, the B3LYP-D3 results are not in better agreement with experiment than those calculated using PBE-D3. Previous studies by Roy et al.28 also showed that the BP86 and PBE functionals provide better results for the redox potentials of iron dinuclear complexes compared to those with B3LYP. The results from the two functionals in this study are in reasonable agreement (

DFT as a Powerful Predictive Tool in Photoredox Catalysis: Redox

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