Predictive energetic tuning of C-Nucleophiles for the electrochemical capture of carbon dioxide.

This work maps the thermodynamics of electrochemically generated C-nucleophiles for reactive capture of CO2. We identify a linear relationship between the pKa, the reduction potential of a protonated nucleophile (E red ), and the nucleophile's free energy of CO2 binding ( Δ G b i n d ). Through synergistic experiments and computations, this study establishes a three-parameter correlation described by the equation Δ G b i n d = - 0.78 p K a + 4.28 E r e d + 20.95 for a series of twelve imidazol(in)ium/N-heterocyclic carbene pairs with an R 2 of 0.92. The correlation allows us to predict the Δ G b i n d of C-nucleophiles to CO2 using reduction potentials or pKas of imidazol(in)ium cations. The carbenes in this study were found to exhibit a wide range CO2 binding strengths, from strongly CO2 binding to nonspontaneous. This observation suggests that the Δ G b i n d of imidazol(in)ium-based carbenes is tunable to a desired strength by appropriate structural changes. This work sets the stage for systematic energetic tuning of electrochemically enabled reactive separations.

Chemical and Electrochemical Recycling of End-Use Poly(ethylene terephthalate) (PET) Plastics in Batch, Microwave and Electrochemical Reactors.

This work describes new methods for the chemical recycling of end-use poly(ethylene terephthalate) (PET) in batch, microwave and electrochemical reactors. The reactions are based on basic hydrolysis of the ester moieties in the polymer framework and occur under mild reaction conditions with low-cost reagents. We report end-use PET depolymerization in refluxing methanol with added NaOH with 75% yield of terephthalic acid in batch after 12 h, while yields up to 65% can be observed after only 40 min under microwave irradiation at 85 °C. Using basic conditions produced in the electrochemical reduction of protic solvents, electrolytic experiments have been shown to produce 17% terephthalic acid after 1 h of electrolysis at -2.2 V vs. Ag/AgCl in 50% water/methanol mixtures with NaCl as a supporting electrolyte. The latter method avoids the use of caustic solutions containing high-concentration NaOH at the outset, thus proving the concept for a novel, environmentally benign method for the electrochemical recycling of end-use PET based on low-cost solvents (water and methanol) and reagents (NaCl and electricity).

Metalloradical intermediates in electrocatalytic reduction of CO2 to CO: Mn versus Re bis-N-heterocyclic carbene pincers.

This work examines the relative reactivities of ReI and MnI tricarbonyl pyridine-2,6-bis-N-heterocyclic carbene pincers M(CO)3CNCBnX (M = Re, Mn and X = Cl and Br) towards catalysis for the electrochemical conversion of CO2 to CO. Unlike prior well-studied group VII catalysts, Mn(CO)3CNCBnX is extraordinarily active, while the new Re(CO)3CNCBnX complex surprisingly does not exhibit catalytic response. DFT calculations shed light on this puzzling behavior and show that the redox-active pyridine-2,6-bis-N-heterocyclic carbene ligand facilitates the reduction of the ground-state complexes; however, the extent of electronic delocalization in the reduced intermediates differs in the degree of metalloradical character. The highly-active Mn(CO)3CNCBnX complex proceeds through an intermediate with nucleophilic metalloradical character in which 66% of the unpaired electron spin resides on Mn. In contrast, Re(CO)3CNCBnX reduction proceeds through an intermediate with less metalloradical character in which only 38% of the unpaired spin is localized on Re with the remainder delocalized over the ligand. The energetic penalty of the electron delocalization of an electron on the ligand affects the M-CO bond strengths and related kinetic barriers. We discuss these observations in the context of turnover-enabling effects in CO2 reductions mediated by group VII NHC pincer molecular electrocatalysts.