Ligand-Centered Photocatalytic Hydrogen Production in an Axially Capped Rh 2 (II,II) Paddlewheel Complex with Red Light.

A new series of Rh 2 (II,II) complexes with the formula cis -[Rh 2 (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 -[Rh 2 (DTolF) 2 (bpnp)(CH 3 CN) 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 CH 3 CN (0.1 M TBAPF 6 ), followed by reduction of the corresponding diazine ligand. Complex 1 exhibits a Rh 2 (δ*)/DTolF → bpnp(π*) metal/ligand-to-ligand charge-transfer ( 1 ML-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 Rh 2 (δ*)/DTolF → L(π*) 1 ML-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 Rh 2 (δ*)/DTolF → bpnp 3 ML-LCT excited states with lifetimes, τ, of 3 and 5 ns, respectively, in CH 3 CN, whereas the lowest energy 3 ML-LCT state in 4 is Rh 2 (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 H 2 with a turnover number (TON) of 16 over 24 h. The axial capping of the Rh 2 (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 H 2 production.