Bioinspired Active Site with a Coordination-Adaptive Organosulfonate Ligand for Catalytic Water Oxidation at Neutral pH

Abstract

Many enzymes use adaptive frameworks to preorganize substrates, accommodate various structural and electronic demands of intermediates, and accelerate related catalysis. Inspired by biological systems, a Ru-based molecular water oxidation catalyst containing a configurationally labile ligand [2,2':6',2″-terpyridine]-6,6″-disulfonate was designed to mimic enzymatic framework, in which the sulfonate coordination is highly flexible and functions as both an electron donor to stabilize high-valent Ru and a proton acceptor to accelerate water dissociation, thus boosting the catalytic water oxidation performance thermodynamically and kinetically. The combination of single-crystal X-ray analysis, various temperature NMR, electrochemical techniques, and DFT calculations was utilized to investigate the fundamental role of the self-adaptive ligand, demonstrating that the on-demand configurational changes give rise to fast catalytic kinetics with a turnover frequency (TOF) over 2000 s^-1, which is compared to oxygen-evolving complex (OEC) in natural photosynthesis.

Figure 1

(a) Rearrangement of first and second coordination spheres during enzymatic catalysis. (b) Transition from S₂ to S₃ during the oxygen-evolving Kok cycle (O_x stands for OH or O). (c) Proposed self-adaptive configurations during water oxidation catalysis by Ru-tds. Axial ligands are omitted for clarity.

Scheme 1. Structures of Water Oxidation Catalysts Discussed in the Paper and Ru-tds

Figure 2

Single-crystal structures of (a) Ru^II(tds-κ-N³O)Py₂ and (b) Ru^II(tds-κ-N³O²)Py₂ with thermal ellipsoids at 50% probability. Hydrogen atoms and solvent molecules are omitted for clarity. (c) VT ¹H NMR spectra of Ru-tds in CD₃OD/D₂O (v/v = 4/1).

Figure 3

(a) CVs without background subtraction of 0.13 mM Ru-tds and Ru-bda at pH 7.0 in a 0.1 M phosphate buffer solution containing 1% CF₃CH₂OH, scan rate = 100 mV s^–1, working electrode: BDD. (b) Potential vs pH diagram for Ru-tds in aqueous buffer solutions containing 1% CF₃CH₂OH, in which the potentials of Ru^V/IV were measured at a current of 3.7 μA from their LSVs. (c) CVs without background subtraction of Ru-tds at different potential windows, working electrode: BDD. (d) Negative-scan CV without background subtraction from 1.3 V, working electrode: BDD.

Scheme 2. Proposed Water Oxidation Pathways of (a) Adaptive Catalysis by Ru-tds and (b) Semiadaptive Catalysis by the Analogue Catalyst^,

Axial ligands are omitted for clarity.

Figure 4

(a) CVs without background subtraction of 0.13 mM Ru-tds in a 0.1 M phosphate buffer solution in H₂O and D₂O, (pH = 7 and pD = 7.87) containing 1% CF₃CH₂OH, scan rate = 20 mV s^–1, working electrode: BDD. (b) Schematic diagram of potential anion and cation effects for water oxidation. (c) CVs without background subtraction of 0.13 mM Ru-tds in phosphate buffer solution with various concentrations (pH 7.1) containing 1% CF₃CH₂OH, scan rate = 20 mV s^–1, working electrode: BDD; inset: enlargement of the 1.0–1.3 V range (upper) and a plot of i_cat/i_p vs [phosphate] (bottom). Ionic strength kept at 0.5 M with NaClO₄. (d) CVs without background subtraction of 0.13 mM Ru-tds in 0.1 M KPi, NaPi, and LiPi, working electrode: BDD; inset: comparison of i_cat/i_p at different potentials, scan rate = 20 mV s^–1.

Figure 5

Energy profiles of ligand exchange on Ru^IV at pH 7.0 using H₂PO₄^– as the base. The units of energies are kcal mol^–1.

Figure 6

(a) 1st and 200th CV scans without background subtraction of Ru-tds at pH 7.0 in a 0.1 M phosphate buffer solution containing 1% CF₃CH₂OH, scan rate = 100 mV s^–1, [Ru-tds] = 0.13 mM, working electrode: BDD; inset: enlargement of the 0.2–1.4 V range. (b) Controlled potential electrolysis (1.7 V) in a 0.1 M phosphate buffer solution containing 1% CF₃CH₂OH with Ru-tds (0.13 mM, red) and only with the electrode (gray), working electrode: GC; inset: bubble formation on the electrode surface.