Photo- and Electrocatalytic\nHydrogen Evolution by\nHeteroleptic Dirhodium(II,II) Complexes: Role of the Bridging and\nDiimine Ligands

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