Ligand-Centered Photocatalytic Hydrogen Production in an Axially Capped Rh<sub>2</sub>(II,II) Paddlewheel Complex with Red Light
Matthew T. Fortunato, Curtis E. Moore, Claudia Turró
Abstract
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.