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

Abstract

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 photoirradiation 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 ]À [3(H 2 O) 2 ]. ns. The laser pulse, the flash lamp, and the gate of the ICCD was synchronized using a digital pulse generator (DG535, Stanford Research Systems).

Dirhodium(II,II) Paddlewheel Complexes

“…Dirhodium compounds can be used as catalysts for hydrogenation of alkenes [106] (such reactions were proposed to proceed via heterolytic splitting of hydrogen by the dirhodium complex) or can be used as catalysts for transfer hydrogenations [107] , dynamic kinetic resolution of alcohols, DKR of homoallyl‐carboxylic acids [108] or as catalysts in other processes as hydroformylation of alkenes [109,274] or reduction of CO 2 with silanes [110] . Photochemical and thermal evolution of molecular hydrogen from water [111] can be catalyzed with dirhodium paddlewheel complexes with electron withdrawing carboxylate ligands [112] . Rh(II) to Rh(III) redox transitions of dirhodium complexes are used in reduction/oxidation processes (e. g. sulfide oxygenations catalyzed by Rh 2 (esp) 2 ( W1 , Figure 24) [275] or allylic oxidations of alkenes with hydrogen peroxide as an oxidant catalyzed by caprolactam dirhodium complex ( C1 , Figure 5) [113] or by Rh 2 (esp) 2 ( W1 , Figure 24)) [276] …”

Dirhodium Complexes as Panchromatic Sensitizers, Electrocatalysts, and Photocatalysts

“…More recently,K ataoka et al demonstrated that the paddlewheel Rh 2 (O 2 CR) 4 (H 2 O) 2 (R = CF 3 ,C Cl 3 )c omplexes are able to catalyze proton reduction with TON > 3000 in at wocomponent system. [21] Multiple-component photocatalytic systems, such as those shown in Scheme 1, provide an attractive avenue to convert solar energy to fuels. However, am ajor disadvantage in these schemesi st hat the charge transfer across each component is diffusion controlled.…”

“…More recently, Kataoka et al. demonstrated that the paddlewheel Rh 2 (O 2 CR) 4 (H 2 O) 2 (R=CF 3 , CCl 3 ) complexes are able to catalyze proton reduction with TON >3000 in a two‐component system [21] …”

Coordination-Induced Self-Assembly of a Heteroleptic Paddlewheel-Type Dirhodium Complex

“…Paddlewheel-type dirhodium (Rh 2 ) complexes have been attracting considerable interest owing to the basic understanding of their molecular geometries and electronic structures with metal-metal bonds [1] as well as their intriguing properties as antitumor agents [2][3][4], chemical sensors [5][6][7], and catalysts for various types of organic reactions [8][9][10][11] and photochemical and electrochemical hydrogen evolution from aqueous solutions [12][13][14]. A considerable number of experimental and theoretical studies have adequately evaluated the synthesis, molecular geometries, and electronic structures of homoleptic Rh 2 complexes with the same bridging carboxylate ligands, [Rh 2 (O 2 CR) 4 L 2 ] (O 2 CR = bridging carboxylate ligand, L = axial coordinated ligand) [1, [15][16][17][18][19][20][21].…”