A 4H+/4e- Electron-Coupled-Proton Buffer Based on a Mononuclear Cu Complex.

ABSTRACT

In this research article, we describe a 4H+/4e- electron-coupled-proton buffer (ECPB) based on Cu and a redox-active ligand. The protonated/reduced ECPB (complex 1 [Cu(8H+/14e-)]1+), consisting of CuI with 2 equiv of the ligand (catLH4 1,1'-(4,5-dimethoxy-1,2-phenylene)bis(3-(tert-butyl)urea)), reacted with H+/e- acceptors such as O2 to generate the deprotonated/oxidized ECPB. The resulting compound, (complex 5 [Cu(4H+/10e-)]1+), was characterized by X-ray diffraction analysis, nuclear magnetic resonance (1H-NMR), and density functional theory, and it is electronically described as a cuprous bis(benzoquinonediimine) species. The stoichiometric 4H+/4e- reduction of 5 was carried out with H+/e- donors to generate 1 (CuI and 2 equiv of catLH4) and the corresponding oxidation products. The 1/5 ECPB system catalyzed the 4H+/4e- reduction of O2 to H2O and the dehydrogenation of organic substrates in a decoupled (oxidations and reductions are separated in time and space) and a coupled fashion (oxidations and reductions coincide in time and space). Mechanistic analysis revealed that upon reductive protonation of 5 and oxidative deprotonation of 1, fast disproportionation reactions regenerate complexes 5 and 1 in a stoichiometric fashion to maintain the ECPB equilibrium.