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-2(II).(mu-O2CCH3)(2)(bpy)(2)](O2CCH3)(2) (Rh(2)bpy(2); bpy = 2,2'-bipyridine) and [Rh-2(II)(mu-O2CCH3)(2)(phen)(2)](O2CCH3)(2) (Rh(2)phen(2); phen = 1,10-phenanthroline) were synthesized, and their photocatalytic H-2 production activities were studied in multicomponent systems, containing [Ir-III(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 Rh(2)bpy(2) to Rh(2)phen(2) 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 mu M Rh(2)phen(2), 50 mu M PS, and 0.6 M SR. The electrochemical, luminescence quenching, and transient absorption experiments demonstrate that (RhRhI)-Rh-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 Rh(2)phen(2)-based multicomponent system, which exhibits an absorption centered at similar to 575 nm. This 575-nm intermediate may account for the much higher H-2 evolution efficiency of Rh(2)phen(2). Our work highlights the importance of N-based chelate ligands and opens a new avenue for pursuing more efficient Rh-2(II)-based complexes in photocatalytic H-2 production application.