Photo-assisted water oxidation by high-nuclearity cobalt-oxo cores: tracing the catalyst fate during oxygen evolution turnover

Abstract

Multi-nuclear cobalt cores have been proposed as molecular analogues of the natural oxygen evolving complex, enabling water oxidation for artificial photosynthesis schemes and the production of solar fuels. In particular, cobalt containing polyoxometalates (Co-POMs) display a record activity as water oxidation catalysts (WOCs) in terms of the turnover number, turnover frequency, and quantum yield, when combined in a light activated oxygen evolving cycle with Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine) as the photosensitizer. The unique behavior of high-nuclearity cobalt clusters is addressed herein by employing Co-POMs with Co ≥9 as molecular WOCs. The temporal dissection of the catalytic events is framed herein to investigate the initial photo-induced electron transfer (ET) occurring in the micro-to-millisecond time domain, and followed by the oxygen evolution kinetics taking place within a minute-to-hours regime. In particular, flash photolysis shows ET from the Co-POM to photogenerated Ru(bpy)₃³⁺ with well-behaved diffusional kinetics (bimolecular rate constants in the range k_ET = 2.1–5.0 × 10⁹ M⁻¹ s⁻¹) and counting up to 32 ET events in a 60 ms timeframe. The evolution of the Co-POMs is then traced under oxygenic conditions, where infrared and X-ray absorption spectroscopy (XAS) indicate that POM based structures are competent catalysts under the photo-assisted turnover regime.