Dirhodium(II,II) Complexes as Panchromatic Single-Molecule Photocatalysts for Hydrogen Evolution

ConspectusThe growing global energy demand and climate change have prompted a shift from carbon-based fuels to sustainable energy sources. Hydrogen production by harnessing solar energy and using abundant proton sources represents an attractive approach to addressing this crisis. Single-molecule single-chromophore photocatalysts, capable of both absorbing the incident photon and catalyzing the chemical transformation, are able to circumvent energy losses present in multicomponent systems that require a photosensitizer and a catalyst, often employing additional redox relay molecules. The series of complexes derived from cis-[Rh₂(μ-DPhF)₂(μ-bncn)₂]²⁺ (1; DPhF = N,N'-diphenylformamidinate, bncn = benzo[c]cinnoline) discussed in this Account presents robust and air-stable single-molecule photocatalysts with panchromatic absorption from the ultraviolet spectral region to the near-infrared (NIR), with high turnover frequencies of ∼20 to 30 h^-1 under red light irradiation. For comparison, other single-molecule hydrogen-evolving photocatalysts reported to date exhibit low photocatalytic efficiency, are not operable in the visible or NIR regions, and are unstable under an ambient atmosphere.Through ground state photophysical characterization and theoretical calculations, the highest occupied molecular orbital (HOMO) in this class of complexes was assigned to be centered on the Rh₂(δ*)/Form(π/nb) MO, while the lowest occupied MO (LUMO) is localized on bncn(π*), with the lowest-energy absorption attributed to the HOMO → LUMO singlet metal/ligand-to-ligand charge transfer (¹ML-LCT) transition. Emission observed at 77 K was assigned to arise from the ³ML-LCT state with an estimated excited-state reduction potential of ∼+1.0 V vs Ag/AgCl, making these complexes strong oxidizing agents upon illumination. The ³ML-LCT lifetimes of these complexes at room temperature range from 1 to 33 ns and are influenced by the presence of a low-lying metal-centered (³MC) state.Experiments designed to elucidate the mechanism for photocatalytic proton reduction have shown that the parent Rh₂(II,II) molecule, [Rh₂], undergoes two sequential photon absorption and reduction events generating [Rh₂]^2-, thus storing two redox equivalents. The ability of the singly reduced complex, [Rh₂]^-, to absorb a photon and oxidize substrates in solution from its excited state to generate [Rh₂]^2- represents a critical step in the catalytic cycle. Both isolated [Rh₂]^- and [Rh₂]^2- species are able to produce hydrogen in acidic media, making multiple simultaneous pathways possible during photocatalysis; however, the latter was shown to be more efficient and is independent of photocatalyst concentration. The active site for these Rh₂ systems is localized on the bncn ligands without the formation of a Rh-H intermediate, under both electro- and photocatalytic conditions. During electrocatalysis, the bncn ligand acts as a proton relay for hydrogen evolution, supported by theoretical calculations suggesting interligand cooperativity for the formation of the H-H bond. The need for a Rh-H intermediate in the photocatalytic cycle has also been ruled out by coordinatively saturating the rhodium centers.This Account reviews the ground- and excited-state photophysical properties of Rh₂(II,II) single-molecule photocatalysts for hydrogen evolution. The insights into the photo- and electrocatalytic mechanisms will not only aid in improving the catalytic performance of these [Rh₂] systems but also provide a pathway to extend this reactivity to platforms composed of earth-abundant metals.