Low-load Θ-metallacarboranes as safe and efficient photoredox catalysts for UVA-driven oxidation in water

<h2>Summary</h2> Transition metal (TM) ions 4d⁶ or 5d⁶ configurations, typically used in photoredox catalysis, are often rare and expensive. In contrast, abundant first-row TM ions tend to favor metal-centered excited states, limiting their practical photocatalytic use. Here, we present two efficient 3d⁶ and 3d⁵ photoredox catalysts, [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ and [3,3′-Fe(1,2-C₂B₉H₁₁)₂]⁻, with identical structures and charges (−1). Despite different electronic configurations, both catalysts deliver comparable performances in alcohol and alkene oxidation in water, even at low catalyst loadings (0.01 mol %). The catalysts are designed to operate in water without additional cosolvents, thanks to the surfactant properties of both θ-metallacarboranes. The impact of irradiating wavelength, explored through quantum chemistry, rationalizes their photocatalytic outcomes. Remarkably, 3d⁵ or 3d⁶ systems can match or outperform heavier TM analogs when supported by boron-based ligands, though selectivity differences highlight the role of metal electronic structure. UVA irradiation significantly boosts [o-COSAN]⁻ activity, offering new design insights for earth-abundant photoredox catalysts.