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, [RhII 2(μ-O2CCH3)2(bpy)2](O2CCH3)2 (Rh2bpy2; bpy = 2,2′-bipyridine) and [RhII 2(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 [IrIII(ppy)2(dtbbpy)]+ (ppy = 2-phenylpyridine, dtbbpy = 4,4′-di-tert-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 RhIRhI 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 RhII 2-based complexes in photocatalytic H2 production application.