Ligand-Centered Photocatalytic Hydrogen Production in an Axially Capped Rh₂(II,II) Paddlewheel Complex with Red Light

A new series of Rh₂(II,II) complexes with the formula cis-[Rh₂(DTolF)₂(bpnp)(L)]²⁺, where bpnp = 2,7-bis(2-pyridyl)-1,8-naphthyridine, DTolF = N,N'-di(p-tolyl) formamidinate, and L = pdz (pyridazine; 2), cinn (cinnoline; 3), and bncn (benzo[c]cinnoline; 4), were synthesized from the precursor cis-[Rh₂(DTolF)₂(bpnp)(CH₃CN)₂]²⁺ (1). The first reduction couple in 2-4 is localized on the bpnp ligand at approximately -0.52 V vs Ag/AgCl in CH₃CN (0.1 M TBAPF₆), followed by reduction of the corresponding diazine ligand. Complex 1 exhibits a Rh₂(δ*)/DTolF → bpnp(π*) metal/ligand-to-ligand charge-transfer (¹ML-LCT) absorption with a maximum at 767 nm (ε = 1800 M^-1 cm^-1). This transition is also present in the spectra of 2-4, overlaid with the Rh₂(δ*)/DTolF → L(π*) ¹ML-LCT bands at 516 nm in 2 (L = pdz), 640 nm in 3 (L = cinn), and 721 nm in 4 (L = bncn). Complexes 2 and 3 exhibit Rh₂(δ*)/DTolF → bpnp ³ML-LCT excited states with lifetimes, τ, of 3 and 5 ns, respectively, in CH₃CN, whereas the lowest energy ³ML-LCT state in 4 is Rh₂(δ*)/DTolF → bncn in nature with τ = 1 ns. Irradiation of 4 with 670 nm light in DMF in the presence of 0.1 M TsOH (p-toluene sulfonic acid) and 30 mM BNAH (1-benzyl-1,4-dihydronicotinamide) results in the production of H₂ with a turnover number (TON) of 16 over 24 h. The axial capping of the Rh₂(II,II) bimetallic core with the bpnp ligand prevents the formation of an Rh-H hydride intermediate. These results show that the observed photocatalytic reactivity is localized on the bncn ligand, representing the first example of ligand-centered H₂ production.