Enhancement of electrocatalytic abilities toward CO2 reduction by tethering redox-active metal complexes to the active site

Tethering metal complexes, like [Ru(bpy)(2)Cl-2] (bpy = 2,2 '-bipyridine), which are redox-active at low reduction potentials and have the ability to transfer electrons to another complex, to a [Ni(cyclen)](2+) electrocatalyst enhanced the reduction of CO2 to CO at low overpotentials. The [Ni(cyclen)](2+) electrocatalyst was modified by tethering redox-active metal complexes via 4-methylpyridyl linkers. The redox-active metal complexes were reduced after CO2 bound to the active site. In controlled potential electrolysis (CPE) experiments in 95 : 5 (v/v) CH3CN/H2O, [{([Ru]pic)(4)cyclen}NiCl](5+) ([Ru](+) = {Ru(bpy)(2)Cl}(+); pic = 4-methylpyridyl) could be used to reduce CO2 into CO at a turnover frequency (TOF) of 708 s(-1) with a faradaic efficiency (FE) of 80% at an onset potential of -1.60 V vs. NHE. At the same time, this electrocatalyst was active at an onset potential of -1.25 V vs. NHE, which is the reduction potential of one of the bpy ligands of the [Ru](+) moieties, with FE = 84% and TOF = 178 s(-1). When the electrocatalysis was performed using [bn(4)cyclenNiCl]Cl (bn = benzyl) without tethered redox-active metal complexes, the TOF value was determined to be 8 s(-1) with FE = 77% at an onset potential of -1.45 V vs. NHE. The results show that tethering redox-active metal complexes significantly improves the electrocatalytic activities by lowering the potential needed to reduce CO2.