Ligand-Centered Photocatalytic Hydrogen Production in an Axially Capped Rh2(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)(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 TBAPF(6)), followed by reduction of the corresponding diazine ligand. Complex 1 exhibits a Rh-2(delta*)/DTolF -> bpnp(pi*) metal/ligand-to-ligand charge-transfer ((ML)-M-1-LCT) absorption with a maximum at 767 nm (epsilon = 1800 M-1 cm(-1)). This transition is also present in the spectra of 2-4, overlaid with the Rh-2(delta*)/DTolF -> L(pi*) (ML)-M-1-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(delta*)/DTolF -> bpnp (ML)-M-3-LCT excited states with lifetimes, tau, of 3 and 5 ns, respectively, in CH3CN, whereas the lowest energy (ML)-M-3-LCT state in 4 is Rh-2(delta*)/DTolF -> bncn in nature with tau = 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.