Experimental and Theoretical Study of Photochemical Hydrogen Evolution Catalyzed by Paddlewheel‐Type Dirhodium Complexes with Electron Withdrawing Carboxylate Ligands

The photochemical hydrogen evolution capabilities of paddlewheel‐type dirhodium complexes with electron withdrawing carboxylates, [Rh 2 (O 2 CR) 4 (H 2 O) 2 ] (R=CF 3 and CCl 3 for [2(H 2 O) 2 ] and [3(H 2 O) 2 ], respectively), were investigated and compared with that of [Rh 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ], ([1(H 2 O) 2 ]), which is the most effective hydrogen evolution catalyst (HEC) among rhodium complexes developed to date. Artificial photosynthesis (AP) systems with [2(H 2 O) 2 ] or [3(H 2 O) 2 ], [Ir(ppy) 2 (bpy)](PF 6 ), and TEA showed highly efficient hydrogen evolution activities; the turnover numbers (TON) of hydrogen evolution per Rh by the AP systems with [2(H 2 O) 2 ] and [3(H 2 O) 2 ] after 12 h of photo‐irradiation were 3334 and 3138, respectively. Experimental analyses and density functional theory (DFT) calculations afforded valuable insight into the hydrogen evolution mechanism of paddlewheel‐type dirhodium complexes; (i) hydrogen evolution activities of the AP systems with [2(H 2 O) 2 ] and [3(H 2 O) 2 ] were slightly lower than that of the AP system with [1(H 2 O) 2 ] despite one‐electron reduction potentials of [2(H 2 O) 2 ] and [3(H 2 O) 2 ] lie on the anode side than that of [1(H 2 O) 2 ], and (ii) two different pathways exist during the early stages in the photochemical hydrogen evolution by [2(H 2 O) 2 ] and [3(H 2 O) 2 ]. Moreover, the relative free‐energy diagrams estimated by DFT calculations clarified the energy profiles of the mechanism including the rate‐determining steps of the hydrogen evolution by [1(H 2 O) 2 ]−[3(H 2 O) 2 ].