Large Improvement in the Catalytic Activity Due to Small Changes in the Diimine Ligands: New Mechanistic Insight into the Dirhodium(II,II) Complex-Based Photocatalytic H₂ Production

Two dirhodium­(II) complexes, [Rh^II₂(μ-O₂CCH₃)₂(bpy)₂]­(O₂CCH₃)₂ (Rh₂bpy₂; bpy = 2,2′-bipyridine) and [Rh^II₂(μ-O₂CCH₃)₂(phen)₂]­(O₂CCH₃)₂ (Rh₂phen₂; phen = 1,10-phenanthroline) were synthesized, and their photocatalytic H₂ production activities were studied in multicomponent systems, containing [Ir^III(ppy)₂(dtbbpy)]⁺ (ppy = 2-phenylpyridine, dtbbpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine) as the photosensitizer (PS) and triethylamine as the sacrificial reductant (SR). There is a more than 6-fold increase in the photocatalytic activity from Rh₂bpy₂ to Rh₂phen₂ just using phen in place of bpy. A turnover number as high as 2622 was obtained after 50 h of irradiation of a system containing 16.7 μM Rh₂phen₂, 50 μM PS, and 0.6 M SR. The electrochemical, luminescence quenching, and transient absorption experiments demonstrate that Rh^IRh^I is the true catalyst for the proton reduction. The real-time absorption spectra confirm that a new Rh-based species formed upon irradiation of the Rh₂phen₂-based multicomponent system, which exhibits an absorption centered at ∼575 nm. This 575-nm intermediate may account for the much higher H₂ evolution efficiency of Rh₂phen₂. Our work highlights the importance of N-based chelate ligands and opens a new avenue for pursuing more efficient Rh^II₂-based complexes in photocatalytic H₂ production application