Nickel(II) Cyclen Complexes Bearing Ancillary Amide Appendages for the Electrocatalytic Reduction of CO₂

Substituted cyclen complexes of nickel(II) containing either one or two pendant amide groups were prepared as potential electrocatalysts for the reduction of CO2 to CO. Four complexes bearing two amide substituents with either tert-butyl ([Ni(DMCy2tBu)](PF6)2), mesityl ([Ni(DMCy2Mes)](PF6)2), 3,5-bis(trifluoromethyl)phenyl ([Ni(DMCy2CF3)](PF6)2), or pentafluorophenyl ([Ni(DMCy2C6F5)](PF6)) groups were all easily prepared and isolated without the need for column chromatography. Similarly, two other nickel(II) cyclen derivatives containing a single mesityl substituted amide pendant ([Ni(DMCyMes)(Cl)](PF6) and [Ni(TrMCyMes)(Cl)]Cl) were also prepared. X-ray crystal structures were obtained for each of these complexes and show that the pendant amides are bound to the nickel(II) center at the core of the cyclen complexes. The amides coordinate to the central metal via either the amide oxygen or amide nitrogen atoms depending on the electronic properties of the amide group. The ability of each of the six complexes to electrochemically reduce CO2 was surveyed by voltammetric and controlled potential electrolysis (CPE) experiments. As the functional groups on the amide arms become more electron donating, the ability of the complexes to electrochemically activate CO2 improves. Of the four complexes containing two amide groups, [Ni(DMCy2tBu)](PF6)2 and [Ni(DMCy2Mes)](PF6)2 show the highest Faradaic efficiencies and current densities for CO production in contrast to homologues that contain amides with fluorinated ancillary groups ([Ni(DMCy2CF3)](PF6)2 and [Ni(DMCy2C6F5)](PF6)). Ultimately, of the six cyclen complexes studied, the two that contain only a single pendant amide ([Ni(DMCyMes)(Cl)](PF6) and [Ni(TrMCyMes)(Cl)]Cl) proved to be the most active and efficient architectures for the electrocatalytic reduction of CO2 to CO. Both the [Ni(DMCyMes)(Cl)](PF6) and [Ni(TrMCyMes)(Cl)]Cl complexes were stable under the conditions of electrocatalysis and promoted the reduction of CO2 to CO with Faradaic efficiencies as high as 80%.