Electronic effects of redox-active ligands on ruthenium-catalyzed water oxidation

doi:10.1016/S1872-2067(23)64497-3

Abstract

Abstract:

The process of water oxidation presents a significant challenge in the development of artificial photosynthetic systems. This complexity arises from the necessity of charge accumulation, involving four electrons and four protons, and O-O bond formation. The strategy of using redox-active ligands in conjunction with metals is recognized as an effective approach for managing this charge accumulation process, attracting considerable attention. However, the detailed mechanisms by which the electronic effect of the redox-active ligands affect the reactivity of the catalytic centers remain ambiguous. In this study, the electronic effect of a series of mononuclear Ru complexes furnished with redox-active ligands ([(L_R^N5‒)Ru^III-OH]⁺, R = OMe, 3a; Me, 3b; H, 3c; F, 3d; CF₃, 3e) was examined on water oxidation. A correlation was observed between redox potentials and substituent constants (σ_para), indicating that different successive redox pairs are influenced by electron effects of varying intensities. Particularly, the ligand-centered oxidation (E {[(L^N5-)^+•Ru^IV=O]²⁺/[(L^N5-)Ru^IV=O]⁺}) shows a greater dependence than the metal-centered PCET oxidations, E(Ru^III-OH/Ru^II-OH₂) and E(Ru^IV=O/Ru^III-OH). The critical intermediate, [(L^N5-)^+•Ru^IV=O]²⁺, formed through ligand-centered oxidation, triggers O-O bond formation via its reaction with water. The rate constants of this crucial step can be effectively modulated by the substituents of the ligand. This study provides intricate insights into the role of the redox-active ligand in regulating the water oxidation process and further substantiates the effectiveness of the synergistic interaction of redox ligands and metals in controlling the multi-electron catalytic process.

Key words: Artificial photosynthesis, Water oxidation, Redox-active ligand, Ru^IV=O intermediate, Substituent effect

Jing Shi, Yu-Hua Guo, Fei Xie, Ming-Tian Zhang, Hong-Tao Zhang. Electronic effects of redox-active ligands on ruthenium-catalyzed water oxidation[J]. Chinese Journal of Catalysis, 2023, 52: 271-279.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(23)64497-3

https://www.cjcatal.com/EN/Y2023/V52/I9/271

Figures/Tables 10

Fig. 1. (a) Redox ligand-assisted water nucleophilic attacked on RuIV=O in water oxidation catalysis. (b) This study: the core structures of the designed RuIV=O intermediates with different substituents to explore the relationship between the electron effect and catalytic activity.

Scheme 1. Synthetic procedure and structures of the ruthenium (III) complexes [(LRN5-)RuIII-OH]+ (R = OMe, 3a; Me, 3b; H, 3c[67]; F, 3d; CF3, 3e).

Fig. 2. Crystallographic structures of 2a (CCDC 1977765), 2b (CCDC 1977762), 2d (CCDC 1977769), and 2e (CCDC 1977767). The thermal ellipsoids are displayed at a 30% probability. Hydrogen atoms and solvents have been omitted for clarity.

Fig. 3. CVs (a) and DPVs (b) of 1.0 mmol/L 3a-3e. They were obtained using a glassy carbon (GC) electrode in 0.1 mol/L nBu4NPF6 propylene carbonate solution.

Table 1 Summary of redox potentials (vs. Fc+/0) for Ru-based complexes 3a-3e in propylene carbonate a.

Complexe	R	σ_para^b	Ru^III/II	Ru^IV/III	L^+•Ru^IV/LRu^IV
3a	OCH₃	-0.27	-0.30	0.44	0.60
3b	CH₃	-0.17	-0.27	0.56	0.80
3c	H	0	-0.23	0.63	0.90
3d	F	0.06	-0.22	0.64	0.88
3e	CF₃	0.54	-0.16	0.78	1.07

Fig. 4. Hammett correlation between redox potentials and substituent constants σpara for Ru catalysts 3a-3e in PC.

Fig. 5. CVs at 100 mV/s scan rate (a) and DPVs (b) of 1.0 mmol/L 3a-3e obtained via GC electrode in 0.1 mol/L PBS (pH = 7.0).

Fig. 6. Correlation between overpotential and σpara (a), correlation between kobs and σpara (b), correlation between kobs and onset potential (c), and correlation between pKa of [(LRN5-)RuIII-OH2]2+ and σpara (d) for Ru catalysts 3a-3e in 0.1 mol/L PBS (pH = 7.0).

Table 2 Summary of electrochemical data, determined pKa values, and catalytic parameters for Ru catalysts 3a-3e in 0.1 mol/L PBS (pH = 7.0).

Complexe	R	E_onset(V vs. NHE)	Overpotential (mV)^b	k_obs(s^-1)	pK_a
3a	OCH₃	0.93^a	113	0.52	6.52
3b	CH₃	0.97	153	0.72	6.28
3c	H	1.02	203	1.26	6.03
3d	F	1.03	213	1.60	5.85
3e	CF₃	1.12	303	3.47	5.33

Fig. 7. (a) Energy barrier diagram of the O-O bond forming step via H2O nucleophilic attacking [(LRN5-)+?RuIV=O]2+ intermediate. (b) Correlation between the activation Gibbs free energies (ΔG?) and σpara.

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