Photo- and Electrocatalytic Hydrogen Evolution by Heteroleptic Dirhodium(II,II) Complexes: Role of the Bridging and Diimine Ligands.

A series of heteroleptic Rh 2 (II,II) complexes, cis- [Rh 2 (μ-DPhF) 2 (μ-bncn) 2 ] 2+ ( 1 ; bncn = benzo[ c ]cinnoline), cis- [Rh 2 (μ-DPhF)(μ-OAc)(μ-bncn) 2 ] 2+ ( 2 ), and cis- [Rh 2 (μ-OAc) 2 (μ-bncn) 2 ] 2+ ( 3 ), is presented, and the excited state and redox properties of each complex was characterized for the photo- and electrocatalytic production of H 2 . The oxidation potentials shift anodically from 1 to 3 , consistent with a highest occupied molecular orbital (HOMO) with significant metal-ligand mixing, Rh 2 (δ*)/DPhF(π/nb). In contrast, modest differences in the first two bncn-localized reversible reduction potentials were observed in 1 - 3 . The lowest energy metal/ligand-to-ligand charge transfer ( 1 ML-LCT) transition, Rh 2 (δ*)/DPhF(π/nb) → bncn(π*), shifts from 633 nm in 1 to 553 nm in 2 , and the metal-to-ligand charge transfer ( 1 MLCT) Rh 2 (π*) → bncn(π*) absorption in 3 appears at 462 nm in CH 3 CN. Although the 3 ML-LCT excited state of 2 is shorter lived than that of 1 , 2.7 ns as compared to 19 ns, respectively, photocatalytic hydrogen generation is observed for the former upon 595 nm irradiation in the presence of 0.1 M TsOH ( p -tolylsulfonic acid) and 0.1 M BNAH (1-benzyl-1,4-dihydronicotinamide). The temperature dependence of the 3 ML-LCT lifetimes of 1 and 2 shows the presence of a thermally accessible deactivating state. In addition, the singly reduced intermediate, [ 2 ] - , is photoactive and able to generate hydrogen in the presence of TsOH. Importantly, the electrocatalytic currents generated by equimolar concentrations of 1 - 3 in CH 3 CN are nearly identical, consistent with a mechanism of catalysis that is localized on the bncn ligand and does not require a Rh-H hydride intermediate. This finding can be used to develop earth-abundant first-row transition metal complexes for photo- and electrocatalytic H 2 production.