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

A series of heteroleptic Rh₂(II,II) complexes, <i>cis-</i>[Rh₂(μ-DPhF)₂(μ-bncn)₂]²⁺ (<b>1</b>; bncn = benzo[<i>c</i>]cinnoline), <i>cis-</i>[Rh₂(μ-DPhF)(μ-OAc)(μ-bncn)₂]²⁺ (<b>2</b>), and <i>cis-</i>[Rh₂(μ-OAc)₂(μ-bncn)₂]²⁺ (<b>3</b>), is presented, and the excited state and redox properties of each complex was characterized for the photo- and electrocatalytic production of H₂. The oxidation potentials shift anodically from <b>1</b> to <b>3</b>, consistent with a highest occupied molecular orbital (HOMO) with significant metal-ligand mixing, Rh₂(δ*)/DPhF(π/nb). In contrast, modest differences in the first two bncn-localized reversible reduction potentials were observed in <b>1</b> - <b>3</b>. The lowest energy metal/ligand-to-ligand charge transfer (¹ML-LCT) transition, Rh₂(δ*)/DPhF(π/nb) <i>→</i> bncn(π*), shifts from 633 nm in <b>1</b> to 553 nm in <b>2</b>, and the metal-to-ligand charge transfer (¹MLCT) Rh₂(π*) <i>→</i> bncn(π*) absorption in <b>3</b> appears at 462 nm in CH₃CN. Although the ³ML-LCT excited state of <b>2</b> is shorter lived than that of <b>1</b>, 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 (<i>p</i>-tolylsulfonic acid) and 0.1 M BNAH (1-benzyl-1,4-dihydronicotinamide). The temperature dependence of the ³ML-LCT lifetimes of <b>1</b> and <b>2</b> shows the presence of a thermally accessible deactivating state. In addition, the singly reduced intermediate, [<b>2</b>]^-, is photoactive and able to generate hydrogen in the presence of TsOH. Importantly, the electrocatalytic currents generated by equimolar concentrations of <b>1</b> - <b>3</b> in CH₃CN 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 H₂ production.